Phase-controlled growth of 2D crystals of the MB_2T_4 family via a flux-assisted method

Domain: physics
Published: 2026-01-14
Editorial outcome: major_revision

Abstract
VanderWaalsmaterialsoftheMB T family(M=transitionmetalorrare-earth
2 4
metal,B=BiorSb,T=Te,Se,orS)haveattractedwideinterestfortheirexotic
topologicalandmagneticproperties,aswellaspotentialspintronicapplica-
tions.However,thedirectgrowthof2DternaryMB T remainschallenging
2 4
duetomultiplecompetingphasesandcomplexatomicarrangements.Here,
wereportaflux-assisted,phase-controlledgrowthstrategytodirectlygrowsix
distinct2DMB T crystals.UsingMB T bulkpowderasprecursor,itsdis-
2 4 2 4
solutionintothecosolventensuresstoichiometriccontrol,while
thermodynamic-kineticequilibriumsuppressesphaseseparation.Taking
MnSb Te asanexample,weobtainedhighlyorderedseptuplelayers,with
2 4
superconductingquantuminterferencedevice(SQUID)andreflectivemag-
neticcirculardichroism(RMCD)measurementsconfirminglayer-dependent
ferromagnetismandCurietemperature(T )rangingfrom12.3to33.7K.This
c
strategyprovidesaneffectiverouteforsynthesizingcomplexlayeredcrystals
andoffersversatileplatformsforadvancingspintronicapplications.

Introduction
With the pursuit of understanding and researching certain funda- membersofMB T familymaterials,suchasMnSb Te andMnBi Te ,
2 4 2 4 2 4
mentalphysicaltopics,suchasquantumanomalousHalleffect1,axion possessesthesurfaceDiracconestructure,showingtheirpromising
electrodynamics2,topologicalconductivity3,andMajoranafermions2,4, potentialfortopologicalresearchandapplications1,5,15.Forinstance,it
tremendouseffortshavebeendevotedtoexploringsuitablematerials isreportedthatalargemagneticgapofupto90meVat1Khasbeen
platformstheoreticallyandexperimentally1,3,5–9.Recently,anewkind observed in MnBi Te , which is significantly enhanced by a self-
4 7
of van der Waals layered MB T family materials has acquired con- organizedalternatingsequenceofMnBi Te septuplelayers(SL)and
2 4 2 4
siderableattentionduetotheirexoticpropertiescontainingtopolo- Bi Te quintuplelayers(QL)1,inspiringthedesignofdevicestoachieve
2 3
gicalelectronicstatesand(anti)ferromagnetismsimultaneously2,7,10–13.
dissipation-lesstransportinthefuture16,17.
Specifically,layer-dependentmagneticbehaviorshavebeenpredicted Despite the superior and attractive properties of the MBT
2 4
and observed in MnBi Te single crystals, where odd-layer samples family,thedirectpreparationoftheirultrathinsingle-crystalflakesis
2 4
indicate uncompensated ferromagnetic properties while even-layer stillrestrictedduetotheircomplexseptuple-atomstructures,varieties
crystalsdisplayantiferromagneticfeaturesbelow25K14.Besides,both of phases, and strong interlayer van der Waals forces18–20. Recent
theoretical simulation and angle-resolved photoemission spectro- attemptstoobtainseveral-layerMBT familysamplesonlyrelyonAu-
2 4
scopy (ARPES) results show thatthe electronic structureof specific assisted exfoliation, Al O -assisted exfoliation, and molecular beam
2 3
epitaxy (MBE)10,19,21–28. Other methods, such as evaporation-rate- control. Consequently, the cosolvent of NaCl–KCl with a proper
controlled chemical vapor deposition (CVD), have been adapted to meltingpointandgoodchemicalinertnessisthebestchoiceforsyn-
growmultilayeredMnBi Te andMnBi Te .However,duetothedif- thesizingMnSb Te nanocrystals.
2 4 4 7 2 4
ficulty in controlling precursor distribution, thermodynamics, and Figures1d,erevealtheevolutionofthechemicalcompositionand
kinetics,theeffectivephase-controlledgrowthroutesandpatternsof reactionprocesswithgrowthtemperature,respectively,furtherela-
these MB T family materials are still not fully understood, which boratingonthemechanismoftemperatureinphasecontrol.Allthe
2 4
restrictsthewidespreadexplorationandapplication29.
Herein,wedevelopedtheflux-assistedphase-controlledgrowth statistical data in Fig. 1d are collected from the EDS results of 156
methodtoobtainMnSb Te nanosheetsdirectly.MnSb Te powderis triangularorhexagonalsamples.Itisobviousthatmaximumyieldwith
2 4 2 4
directlyutilizedasprecursorstobedissolvedintotheNaClandKCl the stoichiometric ratio ~1:2 of Mn: Sb can be obtained at 700°C.
mixtureflux,ensuringthestoichiometryratioanduniformdistribu- According tothe MnSb Te phase diagram,647°Cisthe formation
2 4
tion. Then reaction thermodynamics and kinetics are regulated by ~650°C,onlya tinyfractionofprecursorpowderdissolvesintothe
temperature, together with the ratio of flux. An in-situ heating moltenfluxmixture,whichreducestheprobabilityofthreetypesof
experimentandenergydispersiveX-rayspectroscopy(EDS)mapping precursoratomsencounteringeachothercomparedwiththemeeting
analysis prove the mechanism of crystallization. The thickness of oftwokindsofatoms,resultinginthenucleationofbi-elementcom-
MnSb Te nanosheetscanbeadjustedtoasthinas2.4nm,andthe poundsandphaseseparation.Atthisrange,thethermodynamicsof
2 4
lateralsizeistensofmicrons.Thenaberration-correctedtransmission thetargetreactionarefavorable,andthemainfactorleadingtophase
electronmicroscope(AC-TEM)imagesshowthecrystalstructurewith separationisthelackofcompatibilityinkinetics,asshownintheblue
SL-atomarrangement,indicatingthecapabilityofthephasecontrolin partinFig.1e.Withthetemperaturerisingto700°C,thedissolved
2DMnSb Te .Other 2DMBT familycrystals,including MnSb Te , precursoratomsdramaticallyincreaseandtendtocrystallizeintothe
2 4 2 4 4 7
MnBi Te ,MnBi Te ,MnBi Se ,andMnBi Se canbeobtainedviaa targetnanosheets,exhibitingboththermodynamicandkineticcom-
2 4 4 7 2 4 4 7, patibility. Once the growth temperature is higher than 730°C, the
similar strategy, which provides promising platforms to investigate reactionpathsaredominatedbytemperatureratherthanthediffusion
theirmagneticpropertiesandapplications.

Results
Flux-assistedgrowthstrategyandprocess
The growth process and phase control are shown in Fig. 1a. First,
MnSb Te powderissynthesizedandthentestedbyEDSinSupple-
2 4
mentaryFig.1.Then,amixtureof1:1molarratioofKClandNaClas
cosolvent with MnSb Te powder is placed between two pieces of
2 4
mica30.Next,thesemicaaretightlypressedbetweentheself-designed
setup,whichconsistsoftwo304stainlesssteelplates(10cminlength,
2cminwidth,and0.3cminthickness).Screwholesaremachinedinto
theplatestoallowformechanicalfastening.Whenallscrewsarefully
tightened,thesetupcangenerateapressureofabout1.7×105Pa,as
measuredbythethin-filmsensorinSupplementaryFig.3.However,
duetothesizeoftheprecursorandcosolventparticles,thestacked
micacannotachieveperfectnanoscalecontact,leavingtheconfined
spacepartiallyopen.Afterloadingitintoatubefurnace,thesetupwas
heatedunderacontinuousArflow.Whenthetemperaturegoesupto
650°C,themixedsaltsgraduallymeltintoaliquidstatetodissolvethe
precursorpowderstoichiometrically.Itcanalmostmaintainastable
and evenly distributed atomic ratio compared with the traditional
methods, which provide the primary conditions for phase control.
Molten flux may be squeezed out, and any volatile species formed
insidecanescapewiththecarrierArgasthroughthemicaedges.Then
theacquisitionofthetargetphasecanbeachievedthroughcareful
temperature control. Notably, at high temperatures over 760°C,
MnSb Te powder is quite easy to decompose and separate into
2 4
Sb Te andMnTephases.Incontrast,whenthesynthesistemperature
2 3
is below the melting temperature of the flux mixture, no obvious
reactionoccurs.ThemeltingpointofthemixedsaltofequimolarKCl
andNaClis~650°Caccordingtothedifferentialscanningcalorimeter
(DSC)resultinFig.1b.Additionally,itisfoundthattheendothermic
peaksaroundtheredregionforthemixtureoffluxandprecursors
(massratioof2:1)aresimilartothatfortheNaCl–KClflux,indicating
thefactthatthemeltingofthefluxandthedissolutionoftheprecursor
arenearlysimultaneous.Besides,ninekindsoffluxeswithdifferent
meltingpointshavebeenappliedandlistedinFig.1c,wherethecyan
regionindicatesthatthetemperatureistoohightopreventthephase
separation.Asforthefluxcandidateslocatedintheblueregion,such
as NaI, NaI–KCl, and LiCl, together with KI, are prone to inducing
deliquescence reactions easily in the air, leading to the growth of
antimonyoxyhalides31.MnCl andTeintheredpartmayprovidean
2 2
excess of Mn or Te sources, which goes against the composition
control. Consequently, the cosolvent of NaCl–KCl with a proper
meltingpointandgoodchemicalinertnessisthebestchoiceforsyn-
thesizingMnSb Te nanocrystals.
2 4
Figures1d,erevealtheevolutionofthechemicalcompositionand
reactionprocesswithgrowthtemperature,respectively,furtherela-
boratingonthemechanismoftemperatureinphasecontrol.Allthe
statistical data in Fig. 1d are collected from the EDS results of 156
triangularorhexagonalsamples.Itisobviousthatmaximumyieldwith
the stoichiometric ratio ~1:2 of Mn: Sb can be obtained at 700°C.
According tothe MnSb Te phase diagram,647°Cisthe formation
2 4
temperatureforMnSb Te 32.However,whenthefluxbeginstomeltat
2 4
~650°C,onlya tinyfractionofprecursorpowderdissolvesintothe
moltenfluxmixture,whichreducestheprobabilityofthreetypesof
precursoratomsencounteringeachothercomparedwiththemeeting
oftwokindsofatoms,resultinginthenucleationofbi-elementcom-
poundsandphaseseparation.Atthisrange,thethermodynamicsof
thetargetreactionarefavorable,andthemainfactorleadingtophase
separationisthelackofcompatibilityinkinetics,asshownintheblue
partinFig.1e.Withthetemperaturerisingto700°C,thedissolved
precursoratomsdramaticallyincreaseandtendtocrystallizeintothe
targetnanosheets,exhibitingboththermodynamicandkineticcom-
patibility. Once the growth temperature is higher than 730°C, the
reactionpathsaredominatedbytemperatureratherthanthediffusion
of reactant atoms, promoting the generation of thermodynamically
stableSb Te andMnTephases,asshownintheredpartinFig.1e.
2 3
Asforthethicknessoftheobtainedsamples,synthesistempera-
ture, and the ratio of cosolvents/precursors play a crucial role, as
showninSupplementaryFig.7.Itisfoundthattheminimumthickness
detected by AFM increases with the synthesis temperature rising,
which may be attributed to the increased concentration of the dis-
solved precursors in flux at high temperatures, leading to thermo-
dynamicallyrapidverticalcrystallization,whichshowssimilartrendto
thepreviousdiscussion31.Besides,thethicknesscanalsobeeffectively
tunedbytheratioofcosolventsandprecursors.Itisfoundthatthinner
samples can usually be formed when the percentage of precursors
decreases.This is because in such a situation, there areinsufficient
atoms available to facilitate both lateral and vertical expansion31,
causingsmallandthinnersamples.Figure1fshowsthetypicalmor-
phologyoftheobtainednanosheetsinalargeview,wherethelateral
sizeofthesamplescanbeuptotensofmicrons.Moreopticalimages
withdifferentthicknessesareshowninSupplementaryFigs.8,9,and
10.TheminimumthicknessforMnSb Te is2.4nminFig.1g,equiva-
2 4
lenttothe2-layerthickness,withtheinsetshowingthecorresponding
heightprofile.EDSmappingovera~450μmregioninFig.1hshows
elementaldistributions ofdifferentnanosheets. Histogram in Fig. 1i
displays the Mn:Sb ratios for 44 samples grown at 700°C, where
~72.7% are close to the ideal 1:2 stoichiometry, while the remainder
likelycorrespondstophase-separationproducts.

Phase-controlmechanismanduniversality
To better reveal the mechanism of the flux-assisted precipitation
process, in-situ heating experiments were designed and conducted
withtheopticalmicroscopeasanobservationandrecordingmedium
showninFig.2a.SupplementaryMovie1andopticalimagesinSup-
plementaryFig.13recordeverystepofthemeltingandprecipitation
process.Besides,ex-situSEMobtainedat500,700,and760°Cisused
toverifythechemicalspeciesateachstep.First,belowthemelting
pointoftheflux,allprecursorsandfluxmaintainasolidstate.Then,as
thetemperatureisincreasedto650°C,fluxpowderbeginstomeltinto
fluidandfloweasilyonthesubstrate.Meanwhile,theprecursorsdis-
solveintotheliquidcosolventstoformthefinalfluxmixture,asshown
inSupplementaryFig.17.Withthevolatilizationofthecosolvent,the
oversaturatedfluxwillgenerateultrathinMnSb Te crystals.Themain
2 4
products generated at 700 °C are MnSb Te single crystals. If the
2 4
temperature continuously goes up, MnSb Te flakes will further
2 4
decomposeandremelt.MnTenanosheetsthenprecipitatefromthe
mixturewhiletherestpartturnsintoMn 1−x Sb 2 Te 4−x (0≤x≤1).More
ex-situ EDS images obtained at 500, 650, 680, 700, 730, 760, and
780°C are shown in Supplementary Fig. 15–21, which indicates the
stateoftheproductsateachtemperaturestageindetail.
Tofurthercertifythephaseseparationproductsofflux-assisted
precipitation, AC-TEM is conducted to characterize the final pro-
ducts.Figures2b,cshowtheEDSmappingofthephase-separated
area, where Sb Te samples are grown along the edges of MnTe
2 3
crystalsinamutuallyinterlockedmanner.Figures2d,edisplaythe
high-angleannulardark-fieldimaging(HAADF)imagesofSb Te and
2 3
MnTe,respectively.IntheSb Te area,alltheatomsshowequivalent
2 3
brightnessduetothesimilaratomicnumbers(Z)valueofSbandTe.
Asacontrast,thestrongcontrastinFig.2efortheMnTearearesults
fromtheZdifferenceofMnandTe.Toassesstheuniversalityofthe
phasecontrolstrategyinpreparingvarious2DMB T familymate-
2 4
rials,MnSb Te ,MnBi Te ,MnBi Te ,MnBi Se ,andMnBi Se bulk
4 7 2 4 4 7 2 4 4 7
crystalsareappliedasprecursorstocrystallizeinto2Dsampleswith
differentmixedfluxes.AsshownintheinsetofFig.2f,bothMnBi Te
2 4
andMnBi Se displaySLstructure,whileMnSb Te ,MnBi Te and
2 4 4 7 4 7,
MnBi Se showalternatingQL/SLstacking33.Figure2falsoillustrates
4 7
the crystallization conditions for the listed materials, where 2D
MnBi Te andMnBi Te canonlybeobtainedunderastrictgrowth
2 4 4 7
condition (~590°C) with 0.4mol NaCl and 0.6mol NaI as flux
(meltingpointof~575°C)duetothepoorthermalstability34.Forthe
rest, NaCl–KCl mixed flux can satisfy their growth conditions.
Althoughthesespeciescanprecipitatefromthe mixedfluxovera
widetemperaturerange,themostappropriategrowthtemperatures
forMnSb Te ,MnBi Se ,andMnBi Se arequitehardtoobtainpure
4 7 2 4 4 7
phaseproducts(660,750,and730°C,respectively).Theupperand
lower whiskers in Fig. 2f indicate the maximum and minimum
achievable synthesis temperatures, respectively. The obtained 2D
MB T family materials illustrate triangular or hexagonal shapes
2 4
with lateral domain sizes of tens of microns as shown in Fig. 2g.
Figure 2h shows the AFM results of the grown 2D MB T family
2 4
materials,wherethedetectedminimumthicknessfor2DMnSb Te ,
4 7
MnBi Te ,MnBi Te ,MnBi Se ,andMnBi Se is~2.8,6.4,8.9,1.9and
2 4 4 7 2 4 4 7
2.8nm, respectively. EDS test further confirms the chemical stoi-
chiometricratioofthegrown2DnanosheetsinFig.2i,closetothe
ideal chemical composition. Detailed characterizations using EDS,
AFM,Raman,andTEMareshowninSupplementaryFigs.25–27.The
successfulpreparation of2D MB T family crystalsproves the fea-
2 4
sibility of flux-assisted precipitation for multi-element complex
materials.

StructuralcharacterizationofMnSb Te
2 4
AC-TEM is further conducted to analyze the crystal structures and
atomicarrangementsoftheas-grownnanosheets.Weselectedthree
samplesofdifferentthicknessesforcross-sectionalTEMimaging.As
shown in Fig. 3a, the low-magnification images display the initial
sample thicknesses of approximately 49, 98, and 200nm, with the
samples sandwiched between the substrates and the deposited Pt
layer.Duetothepoorstabilityofthematerial,surfacedegradation
occurred during transfer and FIBpreparation,resulting in asmaller
effectivethickness.Theatomicarrangementsoftheabplanearealso
analyzedbyAC-TEM.TheuniformelementdistributionofMn,Sb,and
TeontheabplanewithsharpedgesinFig.3bdemonstratestheche-
micaluniformity.Thestoichiometryratioofthesampleisindicatedin
Fig. 3c, closeto 1:2:4, proving the feasibility ofcomponent control.
Figure 3d presents cross-sectional HAADF images of MnSb Te
2 4
nanosheets,inwhichnostaggeredlayerssuchasSb Te areobserved,
2 3
demonstrating effective control over phase purity. Complementary
cross-sectional AC-TEM analyses for the other 2 thinner samples in
Supplementary Fig. 29 consistently reveal a well-ordered SL-type
stackingwithoutanyindicationofQL-typephaseseparation,indicat-
ingthatthestackingorderremainsunaffectedbythicknessandcor-
roboratingtheeffectivenessofthephase-controlledgrowthstrategy.
Figure3eshowstheHAADFimagesalongthec-axis,indicatingasingle-
crystalnature.TheinsetinFigs.3d,edisplaystheselectedareaelec-
trondiffraction(SAED)result,wheretheindexedlatticeplaneiscon-
sistent with the lattice parameters. The atomic structure and
correspondingannotationsareillustratedintheimage,wherethepink
middleatomiclayersrepresenttheMnlayerduetoitssmalleratomic
number.TheresultsofAC-TEMfurthervalidatethehighqualityofthe
samples, concurrently showcasing the capability of this method in
phase control.

MagneticpropertiesofMnSb Te
2 4
SQUID and RMCD are further conducted to study the magnetic
propertiesoftheobtainednanosheets.Theprevioussimulationand
experiment results illustrate thata transition from paramagnetic to
antiferromagnetic occurs below a certain temperature in pure bulk
MnSb Te , even though the value of Néel temperature (T ) is
2 4 N
controversial21,35. In a single-layer MnSb Te , Mn atoms provide the
2 4
out-of-planenon-zeromagneticmoments.Inmultiple-layersamples,
spinsofMninadjacentlayerspointinoppositedirections,showing
A-typeantiferromagneticbehaviors26,35.However,Fig.4a,bdisplaythe
out-of-plane magnetization-temperature (M–T) and magnetization-
magnetic field (M–H) curves, where the ferromagnetic transition
occursaround25K,supportedbytheslopechangeintheM–Tcurves
atthispoint.M–Hcurvesat5,15,and25Kdisplayapparenthysteresis
loops.Whenthetemperatureincreases,theloopsshrinkandfinally
disappear. Even though the SQUID signals are obtained from many
nanosheets,theystillprovidestrongevidenceforthejudgmentofthe
ferromagneticpropertiesoftheobtainedsamples.Topreciselychar-
acterize the magnetic properties of one single nanosheet, RMCD is
further conducted on MnSb Te nanosheets with different thick-
2 4
nesses,asFig.4c,d,whereRMCDsignalsatdifferenttemperaturesare
collected. For the sample with a thickness of 4.8nm (~4 layers) in
Fig.4c,thehysteresisloopsstarttoappearbelow17Kandgradually
enlargeasthetemperatureisloweredto2K,whichisconsistentwith
the traditional ferromagnetic behavior and the SQUID results. As for
the17.3nm(~14layers)sampleinFig.4d,asimilartransitionoccursat
~33K.Thereasonwhytheobtainednanosheetsexhibitferromagnet-
ism may be the inevitable substitution between Mn and Sb atoms,
whichhasbeenwidelyreportedinthepreviousliterature21,23,35–39.The
effectsofvariouskindsofsubstitutiontypesbetweenMnandSbon
magneticpropertieshavebeenwellstudied,wherethereplacementof
SbbyMn–Sbantisitesmayintroduceanothermagneticmomentinthe
antiferromagnetic system, making MnSb Te a ferromagnet below
2 4
theT40.
c
Notably,theHAADFimagesoftheabplanefromanothersample
showthatdarkdotsexistontheperiodicbackground,inwhichthe
substitution between Mnand Sb contributes to these atomic impu-
ritiesinFig.4e.Generally,duringthepreparationprocess,itonlytakes
several seconds to achieve crystallization from flux; hence, atomic
misalignments will be inevitable. The substitution defects are also
observedinthesamplessynthesizedbyothermethods,suchassolid-
statereactionroute,chemicalvaportransportmethod,andmolecular
beam epitaxy, induced by rapid cooling and metastable
thermodynamics21,35–37.Meanwhile,thesubstitutionofMnwithadja-
cent Sb atoms introduces negligible lattice stress reflected by the
similardiffractiondatafromtheinsetsinFigs.3eand4e.Therefore,
thesmalllatticedistortionenergyfurtherdiminishesthedrivingforce
for crystalrecovery38. Besides,DensityFunctionalTheory (DFT) cal-
culations show that Mn–Sb antisite substitution has the lowest for-
mation energy (−0.1492eV) compared to Sb–Te substitution,
demonstratingthatsuchsubstitutionisthermodynamicallyfavorable
in Supplementary Fig.32. Furthermore, the M–T curvesofthe Che-
micalVaporTransport(CVT)-grownbulksamplesalsoexhibitafer-
romagneticsignalwithT ~27KinSupplementaryFig.31,whichisclose
c
tothevalueaboveratherthanshowingantiferromagneticbehavior,
therebyprovidingfurtherevidencethatMn–Sbantisitesubstitutionis
present.Hence,weattributetheoriginofmagnetismtotheatomic
misalignment. Other RMCD data from the samples with different
thicknesses are illustrated in Supplementary Fig. 33. T from these
c
sampleswithdifferentthicknessesbyRMCDisstatisticallyanalyzedin
Fig.4f,wheretheremnantRMCDsignalsareextractedfromhysteresis
loopsandfittedbythefunctionalformA(1−T/T)β(A,T andβare
c c,
simultaneousparameters).ItisobviousthatthedetectedT fallsfrom
c
33.7to12.3Kasthethicknessofthesamplesdecreases,similartothe
regularityinplentyof2DmagnetssuchasFe GeTe .Thisisbecause
3 2
the weakening of interlayer exchange interactions with thickness
drops,makingithardtoresistthermalfluctuations41–44.However,as
showninSupplementaryFig.34,MnSb Te exhibitsanon-monotonic
2 4
coercivity(H)-thicknesstrendat2K,incontrasttoconventional2D
c
ferromagnets such as Fe GeTe and Cr Ge Te , where H typically
3 2 2 2 6 c
increasesmonotonicallywithdecreasingthicknessowingtoenhanced
magneticanisotropy41,45.InMnSb Te ,althoughreduceddimension-
2 4
alityisalsoexpectedtoenhanceanisotropy,ferromagnetismislargely
mediatedbyMn–Sbantisitedefects.Thedensityofsuchdefectsmay
decreaseinthinnerflakes,weakeningFMexchangeinteractionsand
loweringthemagneticmomentdensity.In addition,ultrathin flakes
aremoresusceptibletosurfaceandedgedisorderaswellasdegra-
dation, which destabilize magnetic domains and suppress
anisotropy26,41,45–48.Thecompetitionamongtheseeffectsgivesrisetoa
maximumH atintermediatethickness,atrendconsistentwithpre-
c
viousobservationsinmechanicallyexfoliatedMnSb Te .
2 4
In summary, we developed a universal flux-assisted phase-con-
trolledstrategywithpropermixedsaltsasafluxtosuccessfullyobtain
6 kinds of ultrathin MB T family crystals. Besides, the phase-
2 4
separation mechanism has been thoroughly investigated via in-situ
andex-situmethods,makingthethermodynamicandkineticcontrol
ofphaseandthicknessin2Dmaterialseffective.Thestrategymaybe
inspiring for the controlled synthesis of other complex and multi-
element 2D compounds. Additionally, in the ultrathin MnSb Te
2 4
nanosheets,itisfoundthatthesubstitutionofMnatomsandSbatoms
canintroduceferromagneticbehaviorwithtunableT from~12.3to
c
33.7K as the thickness increases, paving a promising way to study
fundamentalphysicsanddesignexoticdissipationlessnanodevices.

Methods
SynthesisofbulkMB T crystals
2 4
BulkMB T crystalsweresynthesizedviaacarefullycontrolledCVT
2 4
method. MnSb Te is described here as a representative example.
2 4
High-purity elemental precursors, including manganese, antimony,
and tellurium were weighed in a stoichiometric ratio of 1:2:4. The
mixturewassealedinaquartzampouleunderhighvacuum(~10−5Torr)
topreventoxidation.Toensurecompleteprecursormixingandsup-
presstheformationofsecondaryphasesincoolerregions,thesealed
ampoule was fully positioned within the high-temperature zone of
tubefurnace.Thethermalprocessincludedseveralkeystages:First,
thetemperaturewasrapidlyrampedto900°Cin1handmaintained
for 2h. Then, the system was cooled to 647°C over 0.5hours, 
establishingconditionssuitableforcontrollednucleationandcrystal
growth.Thetubewasthenheldat647°Cfor48htofacilitatecrystal
growth.Finally,thefurnacewascooledtoroomtemperatureover2h,
yielding bulk MnSb Te crystals. Other MB T -family crystals were
2 4 2 4
synthesized following the same CVT procedure, with appropriate
adjustmentstoprecursorstoichiometryandgrowthtemperature,as
summarizedbelow: For MnSb Te crystals, Mn, Sb, and Te were
4 7
weighed with a molar ratio of 1:4:7. The system was first heated to
900°Cin1handthencooledfrom900°Cto628°Cover0.5hand
held at628°C for 48h. For MnBi Te crystals,Mn, Bi, and Te were
2 4
weighed with a molar ratio of 1:2:4. The system was first heated to
900°C and held for 2h, then cooled from 900°C to 590°C over
40minandheldat590°Cfor72h.ForMnBi Te crystals,Mn,Bi,and
4 7
Tewereweighedwithamolarratioof1:4:7.Thesystemwasfirstheated
to900°Candheldfor2h,thencooledfrom900°Cto580°Cover
45minandheldat580°Cfor72h.ForMnBiSe crystals,Mn,Bi,andSe
2 4
wereweighedwithamolarratioof1:2:4.Thefurnacetemperaturewas
rampedto750°Cwithin45minandmaintainedfor48h.ForMnBi Se
4 7
crystals,Mn,Bi,andSewereweighedwithamolarratioof1:4:7.The
furnacetemperaturewasrampedto725°Cwithin40minandmain-
tained for 48h. After the growth stage, all samples were cooled to
roomtemperaturebeforeremovalfromthefurnace.

Synthesisof2DMB T crystals
2 4
All2DMBT -familynanosheetsweresynthesizedusingaflux-assisted
2 4
growthmethodstartingfrombulkMB T precursors.2DMnSb Te is
2 4 2 4 2 4
describedbelowasanexample.ForthegrowthofMnSb Te nanosh-
2 4
eets,~2mgofbulkMnSb Te crystalpowderwasmixedwith~4mgofa
2 4
NaCl–KClcosolventwithanequimolarcomposition.Themixturewas
finelygroundtoensurehomogeneousmixing.Thenthepowderswere
uniformlyspreadbetweentwofreshlycleavedmicasubstrates,form-
ingamica-precursor-micasandwichconfiguration.Next,themicawas
placedbetweentheself-designedequipmentmadeofstainlesssteel
andcompressedtightlybytighteningthescrews.Then,theequipment
washeatedto700°Cin25minandmaintainedfor3minwith100sccm
Arasprotectivegas.Finally,thesamplescanbetakenoutafterthe
furnace and equipment cool down naturally. Other MBT family
2 4
nanosheetsweresynthesizedfollowingthesameflux-assistedproce-
dure,withappropriateadjustmentstothesaltcomposition,growth
temperature,andgasatmosphere,assummarizedbelow:
For2DMnSb Te ,~2mgMnSb Te bulkpowdermixedwith~4mg
4 7 4 7
of equimolar NaCl–KCl salts. The assembly was heated to 660°C
within20minandheldfor3minunder100sccmAr.
For2DMnBiTe ,~2mgMnBi Te bulkcrystalsmixedwith~4mg
2 4 2 4
ofNaCl–NaImixedsaltwithaNaCl:NaImolarratioof2:3.Theassembly
washeatedto590°Cwithin20minandheldfor3minunderamixed
100sccmAr/H atmosphere.
2
For2DMnBi Te ,~2mgMnBi Te bulkpowdersmixedwith~4mg
4 7 4 7
ofaNaCl–NaImixedsalt(NaCl:NaI=2:3).Theassemblywasheatedto
585°C within 20min and held for 3min under 100 sccm Ar/H
2
mixedgas.
For2DMnBiSe ,~2mgMnBi Se bulkpowdersmixedwith~4mg
2 4 2 4
of an equimolar NaCl–KCl salt. The assembly was heated to 750°C
within25minandheldfor3minunder100sccmAr.
For2DMnBi Se,~2mgMnBi Se bulkcrystalsmixedwith~4mg
4 7 4 7
of an equimolar NaCl–KCl salt. The assembly was heated to 730°C
within25minandheldfor3minunder100sccmAr.

AC-TEMcharacterization
CrystalstructureswereanalyzedbyAC-TEM,amongwhichthesam-
plestestedalongthec-axiswerepreparedbypoly(methylmethacry-
late)(PMMA)(950PMMAA4,MicroChem)-assistedmethod.First,the
samples were spin-coated with PMMA and then etched with 3%
hydrofluoric acid. Afterward, the peeled-off PMMA/sample was
washedwith deionized water and then transferred ontothecopper
TEM grids. Lastly, PMMA was removed by acetone. As for the pre-
paration of cross-section samples, MnSb Te nanosheets were first
2 4
transferred onto SiO /Si substrates by the PMMA-assisted method.
2
Afterthat,thepickedMnSb Te crystalwasplatedwitha~100nmPt
2 4
layerandthencutalongthepredetermineddirectiontoexposethe
cross-sectioncrystalplanebyFIB(HeliosG4).Bothc-axisorientedand
cross-sectional AC-TEM measurements were carried out using a FEI
ThemisZmicroscopeoperatedatanacceleratingvoltageof300kV.

RMCDmeasurement
TheRMCDmeasurementswereperformedusingtheAttocubeclosed-
cyclecryostat(attoDRY2100),withtemperaturesdownto2Kandout-
of-planemagneticfieldsupto9T.Thelinearlypolarizedlightofthe
633-nm He–Ne laser was modulated between left and right circular
polarizationbyaphotoelasticmodulator(PEM)andfocusedontothe
samplethroughahighnumericalaperture(NA=0.82)objective.The
reflected light was detected by a photomultiplier tube (THORLABS
PMT1001/M).Themagneticreversalundertheexternalmagneticfield
wasdetectedbytheRMCDsignaldeterminedbytheratioofthea.c.
component of PEM at 50.05kHz and the a.c. component of the
chopper at 779Hz, measured using a two-channel lock-in amplifier
(ZurichHF2LI).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors have demonstrated phase-controlled growth of the MB₂T₄ family using a flux-assisted method. The creative approach to growing the MB₂T₄ family and the comprehensive study of various flux types, precursor-to-flux ratios, and growth temperatures are highly intriguing and exhibit significant novelty, making this work suitable for publication in a high-impact journal such as Nature Communications. However, the current manuscript places excessive emphasis on the structural analysis of the synthesized materials, while the study on the flux-based growth method, which is the core aspect of this research, appears insufficient. Strengthening the discussion and providing more experimental details related to the growth method would more effectively highlight the novelty of this work. Additionally, the manuscript somewhat lacks sufficient data to fully support the claims made in the study. To address this, the authors are requested to provide additional experimental evidence and responses to the key comments outlined below. Based on these considerations, I recommend a Major Revision.\nComments\n1. Please provide a more detailed explanation of the flux-assisted growth method. The manuscript lacks sufficient information on the self-designed flux environment using stainless steel mentioned by the authors. It is unclear whether the medium gas in the flux growth system is air or an inert atmosphere such as Ar. Please specify this information. Additionally, as the temperature increases in this flux growth system, precursors and flux chemicals may vaporize and increase the growth pressure. To help readers understand the exact growth conditions, please provide details on the vapor pressures of each chemical and clarify the pressure range under which the synthesis occurs.\n2. The synthesized MB₂T₄ flakes in Figure 1f appears to be well-aligned, suggesting that epitaxial growth have occurred on the mica substrate. Therefore, despite using a flux-assisted method, the influence of the mica substrate seems to be highly important. To investigate this substrate effect, please provide data on how the growth behavior changes when using other substrates, such as SiO₂ or sapphire, instead of mica.\n3. Figure 2 presents various MB₂T₄ family phases formed under different growth conditions, primarily analyzed using SEM EDS. However, structural characterization methods such as XRD or Raman spectroscopy would more effectively support the claims made in the study. Since the synthesized MB₂T₄ materials appear to have sufficient thickness and coverage for XRD and Raman spectroscopy measurements, please include structural analysis using these techniques for the different phases.\n4. The most critical aspect of this study is phase control achieved through precise growth conditions. However, to substantiate this claim, it is essential to ensure sample uniformity. Please analyze the phase distribution of MB₂T₄ flakes grown over a several-hundred-micrometer range and provide data in main figure.\n5. Figure 3e presents a well-resolved cross-sectional TEM image that clearly shows the atomic structure and stacking order of MnSb₂Te₄. As recent studies have reported various property changes in 2D materials depending on stacking order, it would be beneficial for readers if the manuscript provided additional information on whether stacking order varies with sample thickness.\n6. Magnetic moment measurements in Figure 4a and 4b, obtained via SQUID, should be quantified. Please express the values in emu per gram or emu per sample area to ensure clarity and facilitate comparison.\n7. The M-T curve in Figure 4a presents only the zero-field-cooled (ZFC) data. Typically, a more comprehensive analysis of magnetic materials involves comparing both ZFC and field-cooled (FC) data, which provides deeper insight into the material's magnetic properties. Please include field-cooled (FC) data and compare it with the ZFC data to offer a more thorough characterization of the magnetic behavior.\nMinor Comments\nFigures in the supplementary section should be labeled as Fig. Sx or Supplementary Fig. x instead of Fig. x for clarity.", "reviewer_2": "This work has reported a refined flux-assisted phase-controlled growth strategy, elucidated the underlying phase-selection mechanisms, and successfully synthesized six different 2D MB₂T₄ family materials. Recently, MB₂T₄ family materials have attracted intensive attention due to their fundamental magnetic, electric, and topological properties. However, the controlled synthesis of thin-layer target materials has always posed great difficulties. Here, the authors achieved the growth of thin-layer 2D MB₂T₄ family materials through a strategy of precise atomic-level control of precursors, matching selection of cosolvents, and coordinated equilibrium of thermodynamics/kinetics. Moreover, the phase-separation process was revealed using STEM and in-situ observation, and its growth mechanism was clearly characterized and explained. Finally, the MnSb₂Te₄ system was selected for a detailed study. SQUID and RMCD characterizations further verified the ferromagnetism caused by the substitution of Mn - Sb, and its Tc can reach up to 33.7 K with the increase in the number of layers. I think the strategy to control phase selection is effective and feasible. The magnetic property study also further explains the relationship between the micro-composition and the macro-properties of MnSb₂Te₄. In general, this work is interesting and has good novelty. I can recommend the publication after addressing the following concerns:\n1. The use of crystal powders as precursors can ensure the accuracy of atomic-level stoichiometric coefficients. Could the authors elaborate on this in more detail and provide more definite evidence to prove the melting process of the crystal powders and the cosolvents together?\n2. It is noted that the endothermic peak positions in the DSC curves are similar for two different powders, but there are obvious differences between the two curves before melting. Please explain in detail the sources of the differences in the DSC curves. In addition, the DSC data of single MnSb₂Te₄ crystal powder needs to be provided as support to further verify the melting - precipitation process.\n3. The authors have elaborated that the magnetic properties of odd-layer and even-layer samples of this material should be different. Please infer the specific number of layers of the RMCD-tested samples.\n4. The authors have elaborated on the phase-control factors in the sample growth process. For the MnSb₂Te₄ material, how to control its growth thickness, size, etc. Please explain in detail.\n5. It is suggested that the authors should compare the lattice constants of MnSb₂Te₄, MnSb₄Te₇, MnTe, and Sb₂Te₃ to rule out the possibility of Mn-doped Sb₂Te₃.\n6. Based on the STEM results, can the authors infer the number of Mn-Sb mutually substituted atoms in the synthesized MnSb₂Te₄ crystals and what is the percentage of the substitution?", "reviewer_3": "In this manuscript, the authors synthesized the MBT family using a phase-controlled strategy with a mixed salt flux. They also characterized the magnetic properties of MnSb₂Te₄ within the MBT family. Materials such as MnSb₂Te₄ and MnBi₂Te₄ have garnered significant attention due to their exotic topological properties and magnetic properties, which make them highly promising for various studies and applications. Notably, the direct synthesis of these materials as nanosheets with high crystallinity is intriguing. So, this paper is adequate to be published in Nature Communications. However, before publication, the authors need to have a clear explanation for several comments.\nComments are listed below,\n1. In the 1st paragraph of the 3rd page and method section, the authors explained the synthesis method using a self-designed equipment. I think the explanation for this part is insufficient. Could you elaborate more on this part? For example, detailed conditions of MnSb2Te4 synthesized by CVT method, the structure of the synthesis equipment, the possibility of using substrates other than mica (if synthesis is possible only with mica, why is that), etc.\n2. In the 3rd paragraph of the 4th page, authors argued that the reason the minimum thickness increases with increasing temperature is because rapid vertical crystallization occurs. Why is vertical growth preferred over lateral growth?\n3. In Fig. 4, It was shown that the curie temperature decreases as the thickness decreases. It was said that this shows a similar trend to other two-dimensional materials. However, while two-dimensional magnetic materials such as Fe3GeTe2 have an increasing coercive field at low temperatures as the thickness decreases, why does the coercive field of MnSb2Te4 decrease at low temperatures when comparing Fig. 4c and Fig. 4d?\n4. In the 1st paragraph of the 9th page, the reason why MnSb2Te4 shows FM behavior instead of A-type AFM behavior is explained as the change in magnetic properties due to the substitution of Mn and Sb. Can you explain why Mn and Sb are better substituted than Sb and Te, which are more similar in size? And does MnSb2Te4 powder synthesized by CVT show A-type AFM characteristics?"}, "author_response": "RESPONSE TO REVIEWERS’ COMMENTS\nReviewer #1 (Remarks to the Author):\nThe authors have demonstrated phase-controlled growth of the MB₂T₄ family using a\nflux-assisted method. The creative approach to growing the MB₂T₄ family and the\ncomprehensive study of various flux types, precursor-to-flux ratios, and growth\ntemperatures are highly intriguing and exhibit significant novelty, making this work\nsuitable for publication in a high-impact journal such as Nature Communications.\nHowever, the current manuscript places excessive emphasis on the structural analysis\nof the synthesized materials, while the study on the flux-based growth method, which\nis the core aspect of this research, appears insufficient. Strengthening the discussion\nand providing more experimental details related to the growth method would more\neffectively highlight the novelty of this work. Additionally, the manuscript somewhat\nlacks sufficient data to fully support the claims made in the study. To address this,\nthe authors are requested to provide additional experimental evidence and responses to the\nkey comments outlined below. Based on these considerations, I recommend Major\nRevision.\nThank you for your valuable feedback and constructive comments. We appreciate the\nrecognition of the novelty of our work on the flux-assisted growth of the MB T family.\n2 4\nIn response to your concern, we have strengthened the discussion on the flux-based\ngrowth method by providing more detailed experimental data for the growth\nmechanism, which should better highlight the novelty of our approach. Your\nsuggestions have greatly helped improve the quality and completeness of our\nmanuscript. We have addressed all concerns and made the necessary revisions, as\noutlined below.\n1. Please provide a more detailed explanation of the flux-assisted growth method. The\nmanuscript lacks sufficient information on the self-designed flux environment using\nstainless steel mentioned by the authors. It is unclear whether the medium gas in the\n1\nflux growth system is air or an inert atmosphere such as Ar. Please specify this\ninformation. Additionally, as the temperature increases in this flux growth system,\nprecursors and flux chemicals may vaporize and increase the growth pressure. To help\nreaders understand the exact growth conditions, please provide details on the vapor\npressures of each chemical and clarify the pressure range under which the synthesis\noccurs.\nResponse: Thank you for your constructive advice. First, we provide a detailed\nschematic illustration of the setup and sample growth process in Fig.R1a. The setup\nconsists of two stainless steel plates (10 cm × 2 cm × 0.3 cm) with threaded holes for\nfastening. During growth, MnSb Te crystal powders are first placed onto a mica\n2 4\nsubstrate. A eutectic mixture of NaCl and KCl in a 1:1 molar ratio is then evenly\ndistributed on mica substrate. Another piece of mica is placed on top to cover both the\nprecursor and the mixed salts. The setup is fastened with screws, and the applied\npressure is regulated by the number of screw turns. The setup is then loaded into the\ntube furnace, and argon gas is introduced to purge atmospheric oxygen. The system is\nheated to 700 °C within 20 minutes, held at that temperature for 3 minutes, and then\nallowed to cool naturally to room temperature with a constant Ar flow rate of 50 sccm\nthroughout the process. The use of Ar is crucial for removing oxygen from the system\nbecause MnSb Te nanosheets are highly sensitive to air exposure. Noted that when the\n2 4\ntemperature rises to the melting point of mixed salts ~ 650 °C, MnSb Te crystal powder\n2 4\nwill dissolve into the molten salts to form a uniform flux, then hold at the target\ntemperature, the flux will flow around the mica and vaporize slowly, finally, the sample\ncan be precipitated and grown. After cooling, we can get samples from both inner\nsurfaces of mica. Fig.R1b illustrates the reaction states at different temperatures during\nthe growth process. At low temperatures, the mixed salts do not melt and thus fail to\nform a flux, preventing sample formation. In the temperature range of 650-720 °C, the\ntarget samples precipitate from the flux. At temperatures above 760 °C, phase\nseparation occurs dramatically, leading to the formation of MnTe as a secondary phase.\nreactants at different temperatures and confirming that the target samples precipitate\nfrom the flux. Fig.R1d presents SEM images of samples quenched from different\ntemperatures, which match well with the in-situ observations in Fig.R1c. At around\n700 °C, triangular-shaped samples are precipitated. As the temperature increases further,\nphase separation occurs, resulting in the formation of MnTe and Mn Sb Te structures.\nx 2 4\nFig.R1. Schematic illustration of the flux-assisted growth process. a. Components of\nthe growth setup and steps of the growth process. b. Schematic diagram of the growth\nprocess, showing the states of the reactants at different temperatures: powders remain\nsolid at low temperatures; at ideal growth temperatures, the target product precipitates\nfrom the flux; at higher temperatures, impurity phases form. c. In-situ observation of the\ngrowth process at 500 °C, 700 °C, and 760 °C. d. SEM images of samples grown at\ndifferent temperatures.\nBesides, the growth process is conducted in an open system in tube furnace. As for the\nmechanical pressure of the setup, when all screws are fully tightened, it can reach\n3\napproximately 1.7 × 105 Pa, as measured by a thin-film pressure sensor shown in\nFig.R2a. However, due to the size of the precursor particles and cosolvent powder\nparticles, it is impractical for the two stacked mica to achieve true nanoscale contact.\nConsequently, the confined space is not totally sealed. So, when the powders melt, some\nflux can be squeezed out of the confined space. And any volatile species generated\ninside can be carried away by the carrier Ar gas through the edges of the mica, as shown\nin Fig.R2b. Besides, according to phase diagrams, NaCl-KCl eutectic mixture (1:1\nmolar ratio) melts at approximately 650 °C and is still in a liquid phase at 700 °C [1].\nAnd MnSb Te also indicates a mixed phase of liquid and MnTe at the growth\n2 4\ntemperature of 700 °C [2]. Hence, we emphasize that during all growth processes, any\nvolatile species released from the precursors can be easily carried away by the carrier\ngas, as the setup is an open system. While localized pressure variations may occur\nwithin the mica, caused by phenomena such as precursor melting or thermal expansion\nof the stainless steel, the growth remains relatively stable during the crystal growth\nstage.\nWe have added a description of the self-designed setup in the main text, provided a\nmore detailed explanation of the growth process, and clarified potential\nmisunderstandings regarding the growth pressure in blue color. Relevant apparatus\nimages, growth process details, and pressure test data are included in the Supplementary\nInformation as Supplementary Fig.2 and Supplementary Fig.3 are shown.\nN on a 1 cm2 area. b. Schematic illustration of the confined but non-sealed space\nbetween mica during the growth process. As the flux melts and forms, the applied\nmechanical pressure squeezes the molten flux out of the confined space. Then, volatile\nproducts are carried away by the Ar carrier gas through the open edges. c. Phase\ndiagram of the NaCl-KCl system, showing that the mixture is predominantly in the\nliquid phase at 700 °C [1]. d. Phase diagram of the MnSb Te system, indicating the\n2 4\npresence of a liquid phase coexisting with MnTe above 647 °C [2].\n2. The synthesized MB T flakes in Figure 1f appears to be well-aligned, suggesting\n2 4\nthat epitaxial growth has occurred on the mica substrate. Therefore, despite using a\nflux-assisted method, the influence of the mica substrate seems to be highly important.\nTo investigate this substrate effect, please provide data on how the growth behavior\nchanges when using other substrates, such as SiO or sapphire, instead of mica.\nsubstrate plays a critical role in guiding the growth behavior of MB T flakes. As shown\n2 4\nin Fig.1f, the flakes grown on mica exhibit a high degree of in-plane alignment, which\n5\nstrongly suggests epitaxial or quasi-epitaxial growth. This is likely due to the atomically\nflat surface and surface symmetry of mica, which provides a favorable template for the\noriented growth of MnSb Te . MnSb Te crystallizes in the R-3m space group with a\n2 4 2 4\nthreefold rotational symmetry. And the surface of fluorophlogopite, due to slight lattice\ndistortion, also exhibits a reduced threefold symmetry [3, 4]. This symmetry\ncompatibility may allow MnSb Te to adopt energetically favorable orientations during\n2 4\ngrowth. In addition to symmetry matching, the in-plane lattice constant of MnSb Te is\n2 4\n4.2385 Å, while that of fluorophlogopite is 9.2123 Å. A supercell comparison reveals\nthat a 9 × 9 MnSb Te domain (9 × 4.2385 = 38.1465 Å) closely matches with\n2 4\nfluorophlogopite supercell (36.849, 37.217, and 37.228 Å in different directions),\nresulting in a lattice mismatch of only ~ 2.4-3.4%, as shown in Fig.R3a. This relatively\nsmall mismatch, combined with symmetry compatibility, facilitates aligned growth of\nMnSb Te via epitaxy. A similar symmetry-guided alignment mechanism was\n2 4\npreviously demonstrated in our group’s work on α-In Se (R-3m space group) growth,\n2 3\nwhere the lattice and symmetry match with fluorophlogopite enabled the formation of\nparallel-oriented flakes with preferred orientations at 0° and 60° [3].\nFig.R3. Schematic illustration of MnSb Te alignment on fluorophlogopite mica. a. The\n2 4\ncrystal structure along the c-axis direction of fluorophlogopite, among which 3 different\nalignment types are shown to match with the 9 × 9 MnSb Te domain. b. The view along\n2 4\nthe c-axis direction of MnSb Te .\n2 4\nTo further study the substrate effect, we performed experiments using other common\n6\nsubstrates, including sapphire and SiO /Si. As shown in Fig.R4a and R4b, respectively,\n2\non these substrates, the samples resulted in randomly oriented and isolated thick crystals,\nrather than the well-aligned ultrathin flakes observed on mica. For sapphire, the in-\nplane lattice constant is 4.7580 Å, which exhibits a large mismatch with that of\nMnSb Te . In addition, the sapphire surface is artificially cut, producing atomic terraces\n2 4\nrather than being atomically flat, thereby preventing effective symmetry matching\nduring growth [5]. For SiO /Si, it is amorphous and lacks any obvious lattice symmetry,\n2\nmaking epitaxial alignment impossible. Furthermore, under identical growth conditions,\nMnSb Te samples grown on sapphire and SiO /Si substrates tend to exhibit larger\n2 4 2\nthicknesses, but lower areal densities compared to those synthesized on\nfluorophlogopite mica. This difference is primarily attributed to surface topography\namong the substrates. The atomically flat surface of fluorophlogopite offers minimal\nkinetic barriers for lateral precursor diffusion and flux spreading, thereby facilitating\ngrowth of thin nanosheets. In contrast, the sapphire substrate typically presents terrace\nstructures, which act as diffusion barriers and prevent precursor migration. Meanwhile,\nfor SiO /Si, the amorphous and defect-rich surface suppresses mass transport efficiency.\non SiO , shown as the yellow circled part in Fig.R4a, which further hinders the fast\n2\nmass transport. In Fig.R4c, we observed that the samples exhibit well-aligned domains\nin certain areas. However, misaligned domains are also present. This misalignment may\nbe attributed to the rapid growth rate of the samples. In our in-situ observation\nexperiments, the nucleation and growth process were completed within only seconds.\nSuch a short growth time may lead to misorientation. Additionally, intrinsic defects\nwithin MnSb Te may further promote local misorientation of the grown domains.\n2 4\nThe results of MnSb Te growth on different substrates and the corresponding analysis\n2 4\nwere added to the Supplementary Information as Supplementary Fig.11 and 12.\n2 4\nat 700 °C for 3 minutes. Due to the brittle nature of the SiO substrate, the screws were\n2\nnot tightened firmly. a. Samples grown on SiO /Si, b. Sample grown on sapphire\n2\nsubstrate, c. Sample grown on mica substrate. The scale bar is 20 μm.\n3. Figure 2 presents various MB T family phases formed under different growth\n2 4\nconditions, primarily analyzed using SEM EDS. However, structural characterization\nmethods such as XRD or Raman spectroscopy would more effectively support the\nclaims made in the study. Since the synthesized MB T materials appear to have\n2 4\nsufficient thickness and coverage for XRD and Raman spectroscopy measurements,\nplease include structural analysis using these techniques for the different phases.\nResponse: We sincerely appreciate your insightful comments and constructive\nsuggestions, which have significantly contributed to enhancing the clarity and\nrobustness of our manuscript.\ncorresponding characteristic peaks.\nWe performed Raman spectroscopy on six representative MB T -family samples to\n2 4\nidentify their vibrational fingerprints across different compositions. Fig.R5a and\nFig.R5d present the Raman spectra of MnBi Te and MnBi Te , respectively. Both\n4 7 2 4\nsamples exhibit an obvious E3 mode near 100 cm-1. However, the A3 mode in\ng 1g\nMnBi Te shows a blue shift to approximately 140 cm-1, whereas the A3 peak appears\n2 4 1g\naround 136 cm-1 in MnBi₄Te₇, consistent with previous literature reports [6]. For the\nMnSb Te and MnSb Te in Fig.R5b and R5e, the E2 and E3 modes remain\n4 7 2 4 g g\nrelatively unchanged, located at approximately 87 cm-1 and 119 cm-1, respectively. A\ndistinct A3 peak is observed at 139 cm-1 in MnSb Te , while in MnSb Te , this mode\n1g 2 4 4 7\nis much less pronounced and only appears as a small shoulder in one spectrum [7-9].\nNotably, the MnSb Te sample exhibits a characteristic A2 mode of Sb Te ,\n4 7 1g 2 3\ncorresponding to out-of-plane vibrations of the Sb-Te bond. This observation is\n9\nconsistent with the presence of Sb Te -type quintuple layers in the MnSb Te crystal\n2 3 4 7\nstructure. In the case of MnBi Se -based samples in Fig.R5c and R5f, the Raman\n2 4\nsignals are generally much weaker, likely due to the lower structural stability of the\nselenide phases. Although specific reference spectra for MnBi Se and MnBi Se are\n2 4 4 7\nlimited in the literature, several distinguishable peaks can still be identified. Both\nphases exhibit peaks near 156, 171, and 252 cm-1, while MnBi Se additionally shows\n2 4\na vibration at 122 cm-1, and MnBi Se exhibits a mode at 131 cm-1. For comparison, the\n4 7\nreference peaks of Bi Se , namely 131 cm-1 (in-plane Bi-Se vibration) and 174 cm-1\n2 3\n(out-of-plane Bi-Se bond vibration), are overlapped with some peaks of MnBi Se and\n2 4\nMnBi Se [10]. Despite overlapping spectral features, these differences suggest\n4 7\ncompositional and structural distinctions between the two selenide phases.\nWe have added Raman test data and corresponding analysis of different samples in the\nSupplementary Information as Supplementary Fig.26.\nFig.R6. XRD test for the obtained 6 samples. Due to the inevitable formation of\nsecondary phases during the synthesis process and the requirement for a relatively large\nsample volume in XRD measurements, the presence of impurity peaks is unavoidable.\nIn the XRD pattern, the standard peaks of the target phase are highlighted with colored\nstripes.\n2 4\nXRD. To minimize the strong background signal from mica, the as-grown crystals were\ntransferred from mica to SiO /Si substrates before XRD analysis. As shown in Fig.R6,\n2\neach panel highlights the target phase’s characteristic XRD peaks with colored stripes\nand index crystal planes. The bottom curve in each figure corresponds to the standard\nreference pattern of the target compound, while the other 2 gray curves indicate the\npossible diffraction peaks from secondary phases. Noted, there is no available CIF\nreference pattern for MnBi Se . However, its characteristic peaks can be identified by\n4 7\ncomparison with the distinct features of MnSe and Bi Se . Due to the thermodynamic\n2 3\ncomplexity and narrow phase stability windows of these compounds, phase separation\nduring synthesis is difficult to avoid, which increases the probability of obtaining\nmixed-phase products, making the XRD peaks complex. Additionally, the structural\ninstability of these compounds, especially in ultrathin form, makes them susceptible to\noxidation or degradation during the transfer process [6, 8, 11-13]. As shown in Fig.R7,\ntaking the relatively stable MnSb Te as example, the sample was transferred to SiO /Si\n2 4 2\nwithin one hour after growth. Optical microscopy reveals that ultrathin nanosheets had\nalready degraded into aggregated particles, while slightly thicker samples exhibited\ndifferent colors at the edges, indicating signs of degradation. These two reasons also\nexplain the presence of complex peaks in the XRD measurements.\nFig.R7. Morphological degradation of MnSb Te after transfer from Mica to SiO /Si.\n2 4 2\nThe red circles mark the degradation part.\nConsidering that XRD measurements reflect signals from all samples on the substrate,\nthe detection of secondary phases is unavoidable, particularly given the high\n11\nsusceptibility of the samples to oxidation during sample preparation. To obtain more\naccurate structural information of the synthesized MB T samples, we carefully\n2 4\ntransferred and selected representative flakes for TEM analysis, which allows localized\ncharacterization of individual crystals. The results are shown in Fig.R8. The measured\nlattice spacings and diffraction patterns are consistent with the expected R-3m layered\nstructures reported in literature and crystallographic databases. Specifically, for\nMnBi Te and MnBi Te , the observed lattice of 0.22 nm corresponds well to the (110)\n2 4 4 7\nplanes, matching the d-spacing values calculated from the standard CIF files. Similarly,\nMnSb Te displays the d-spacing of 0.21 nm assigned to the (110) plane, which is\n4 7\nconsistent with the MnSb Te reference structure. For MnBi Se , a lattice spacing of\n4 7 2 4\n0.20 nm was observed, corresponding to crystal plane (220). In MnBi Se , the d-\n4 7\nspacing of 0.21 nm and the SAED pattern with a (100) spacing of 0.37 nm are observed,\nconsidering the structural similarity with other species. In addition, the chemical\ncompositions determined by EDS are closely aligned with the stoichiometries. For\ninstance, MnBi Te was measured as MnBi Te and MnBi Te as MnBi Te .\n2 4 2.06 4.13 4 7 3.91 7.21\nBoth reflect relatively high compositional accuracies. Notably, the slightly elevated Te\ncontent may be attributed to surface-adsorbed Te, which has also been observed in\nCoTe nanosheets [14].\nSupplementary Fig.27.\n2 4\ntypes of samples, where the sharp edges of the samples illustrate the good crystal quality.\nb and c. HRTEM images and SAED patterns of these five samples respectively. The\nordered atomic arrangement and index lattice spacing both confirm the synthesized\nsamples. The scale bars are 500 nm in a, 2 nm in b, and 2 nm-1 in c. d. EDS results for\neach sample.\n4. The most critical aspect of this study is phase control achieved through precise\ngrowth conditions. However, to substantiate this claim, it is essential to ensure sample\nuniformity. Please analyze the phase distribution of MB T flakes grown over a several-\n2 4\nhundred-micrometer range and provide data in the main figure.\nResponse: Thank you for your valuable comment. We fully agree that verifying phase\nuniformity is essential to support the claim of phase-controlled growth. To enrich the\ndataset, we performed elemental mapping over a large area (~ 450 microns) using EDS,\nas shown in Fig.R9a. The SEM image and the corresponding EDS elemental maps of\n2 4\nflakes. In Fig.R9b, we statistically analyzed the elemental ratios of 44 different flakes.\nAs shown, approximately 72.7% of the samples match the expected stoichiometry of\nMn:Sb: Te = 1:2:4, while Mn-rich and Mn-deficient variants account for 11.4%\nand15.9%, respectively. Despite careful temperature control, phase-separated products\nsuch as Mn-rich/poor species still inevitably appear. We further conducted composition\nstatistics across 112 samples synthesized at different temperatures to examine the effect\nof growth temperature on phase uniformity. As shown, the distribution of sample\ncompositions varies significantly with temperature, while at 700 ℃, we can get the best\nyield. This observation reflects the metastable nature of this material family. Due to its\ntendency for solid-state phase transitions, phase separation is difficult to avoid entirely.\nNevertheless, our data demonstrates that the synthesis conditions employed here can\nachieve a high probability of forming the desired MnSb Te phase.\n2 4\nWe put the EDS mapping in the main figure as part of Fig.1 while the others are added\nin the Supplementary Information as Supplementary Fig.4. Besides, we have carefully\nrevised the manuscript to refine the language regarding phase control, and we have\nremoved descriptions such as “precise control” to better reflect the experimental\nlimitations and physical nature of the system.\nFig.R9. SEM image and EDS analysis of MnSb Te flakes in a large view. a. EDS\n2 4\nmapping of a large-area MnSb Te sample. b. Composition statistics of 44 analyzed\n2 4\n14\nflakes: approximately 72.7% correspond to stoichiometric MnSb Te , while Mn-rich\n2 4\nand Mn-poor variants account for 11.4% and 15.9%, respectively. c. Statistical analysis\nof 156 samples synthesized at different temperatures indicates that the highest yield of\nthe target MnSb Te phase is achieved at 700 °C.\n2 4\n5. Figure 3e presents a well-resolved cross-sectional TEM image that clearly shows the\natomic structure and stacking order of MnSb Te . As recent studies have reported\n2 4\nvarious property changes in 2D materials depending on stacking order, it would be\nbeneficial for readers if the manuscript provided additional information on whether\nstacking order varies with sample thickness.\nResponse: Thank you for your insightful advice. We agree that stacking order plays a\ncritical role in determining the physical properties of 2D materials. In response, we\ntested 3 samples with various thicknesses to study the stacking order with thickness.\nFig.R10. Cross-section ACTEM analysis for samples of different thicknesses. a.\nmagnification SEM image showing the overall shape of the FIB-prepared sample. c.\nHigh-resolution TEM image of the sample region. The sample has a total thickness of\n~ 49 nm, but only the bottom ~ 15 nm exhibits well-preserved lattice, while the upper\nportion appears amorphous, likely due to surface degradation and FIB damage. d. EDS\nspectrum collected from the sample, showing the distribution of Sb. e. HAADF image\nfrom the area near the bottom interface, showing well-ordered atomic stacking with a\nseptuple layer (SL)-type structure. f. HAADF image near the top surface. The topmost\nregion 1 is amorphous, and a transition zone 2 with preserved lattice is observed. The\nlattice constant in this region is comparable to the septuple layer (SL) region, suggesting\nthe distortion may be caused by a slightly tilted cutting angle. g. Morphology and FIB\ncutting direction of sample 2. h. TEM image showing a sample thickness of ~ 98 nm. I\nand j. High-resolution images from different regions of sample 2, both confirming\nconsistent SL-type stacking. k. Surface morphology and cutting direction of sample 3.\nl. FIB image showing the thickness of ~ 200 nm with visible surface non-uniformity,\nlikely due to oxidation. m and n. Atomic-resolution images from two distinct regions,\nboth showing regular SL-type stacking without any sign of phase separation.\nIn the early stages of sample preparation, we selected a relatively thick triangular flake\n(sample 3, shown in Fig.R10k) for cross-section TEM analysis in case the sample get\noxidation or degradation. As shown in Fig.R10l, the sample exhibits a thickness of ~\n200 nm with an uneven surface and amorphous outer layers, suggesting oxidation\nduring post-growth handling. Despite this, SL-type stacking is consistently observed in\ntwo randomly selected regions, confirming the structural uniformity and absence of\nphase separation in Fig.R10m and R10n. In contrast, sample 1 in Fig.R10a was thinner\n(~ 49 nm), but only the bottom ~ 15 nm preserved lattice information, with the upper ~\n30 nm degraded into an amorphous layer, likely caused by transfer, protective coating,\nand FIB processes. However, as seen in Fig.R10e, the bottom layers near the substrate\nexhibit clear SL stacking, with no sign of quintuple layer (QL-type) phase separation.\nmay result from local defects, as it was not observed in other samples or regions. In\nFig.R10f, the topmost region has become amorphous (region 1), while a transition zone\n(region 2) still retains lattice alignment. The lattice constant measured here remains\nconsistent with that of the SL structure, possibly due to non-perpendicular FIB slicing.\nSample 2 (shown in Fig.R10g) has a thickness of ~ 100 nm. Although its cutting\ndirection differs from that of samples 1 and 3, the SL-type stacking is still consistently\nobserved in different regions, as seen in Fig.R10i and R10j without any misalignment\nor phase separation. In summary, cross-section STEM analysis across all three samples\nwith various thicknesses confirms that the internal crystal structure consists of well-\nordered SL-type stacking without the emergence of quintuple layer (QL)-type phase\nseparation, supporting the success of phase-controlled growth. However, degradation\nand surface amorphization are commonly observed in the 3 samples, likely due to their\nintrinsic fragility and unavoidable oxidation during sample preparation. Overall, no\nsignificant change in stacking order is observed with varying thickness.\nFig.R11. SEM and AFM images of thin MnSb Te samples.\n2 4\nNotably, we initially attempted to select relatively thin MnSb Te flakes (~ 12.4 nm) for\n2 4\nFIB sample preparation to investigate their cross-sectional structure. However, these\nultrathin samples exhibited poor stability. As shown in Fig.R11, SEM screening after\ntransfer revealed that the thin flakes had undergone significant degradation, with the\nsurfaces appearing largely as aggregated particles. Due to this instability, we opted to\nuse slightly thicker flakes for FIB processing and cross-sectional analysis.\nWe have added the related content of Fig.R10 to the Supplementary Information as\nby thickness” in the main text.\n6. Magnetic moment measurements in Figure 4a and 4b, obtained via SQUID, should\nbe quantified. Please express the values in emu per gram or emu per sample area to\nensure clarity and facilitate comparison.\nResponse: Thank you for your suggestion. As the magnetic measurements were\nconducted on flakes grown directly on mica substrates, it is not feasible to determine\nthe exact sample mass. Therefore, normalizing the magnetic moment by sample area\n(in emu·m-2) is a more appropriate approach in this context. The substrate area used for\nSQUID measurements is well defined (0.5 cm × 0.5 cm), and we specifically selected\nregions with high flake coverage. To estimate the coverage rate, we applied an AI-\nassisted image recognition method to analyze three different regions as shown in\nFig.R12a, yielding an average flake coverage of approximately 17.80%. Based on this\nestimation, the magnetic moment was normalized by the total flake-covered area and\npresented in emu·m-2. The revised values and units have been updated in Fig.R12b,\nR12c, and Figure 4 accordingly. We have updated the figure in the main manuscript in\nFigure. 4 and also added into the Supplementary Information as Supplementary Fig.30.\n2 4\ncoverage on mica substrate. Three different regions were analyzed to estimate the\naverage flake coverage, which was determined to be ~ 17.80%. b. Temperature-\ndependent magnetization curve. c. Magnetic hysteresis curve. Scale bar: 50 μm.\n7. The M-T curve in Figure 4a presents only the zero-field-cooled (ZFC) data. Typically,\na more comprehensive analysis of magnetic materials involves comparing both ZFC\nand field-cooled (FC) data, which provides deeper insight into the material's magnetic\nproperties. Please include field-cooled (FC) data and compare it with the ZFC data to\noffer a more thorough characterization of magnetic behavior.\nResponse: Thank you for the constructive comments. To provide a more comprehensive\nanalysis of the magnetic behavior of MnSb Te , we have included both ZFC and FC\n2 4\nmagnetization curves measured under an applied field of 500 Oe. Both curves reveal a\nwell-defined magnetic transition at approximately 25 K, confirming the ferromagnetic\nnature of the phase transition from paramagnetic to ferromagnetic. However, the\nmagnetic response upon cooling or warming exhibits characteristic differences,\nespecially near 2 K. In the FC process, the sample is cooled from the paramagnetic state\nto under 500 Oe. As the temperature decreases below 25 K, the field progressively\naligns spins along its direction, while the suppression of thermal fluctuations enhances\nlong-range ferromagnetic order when cooling down[15-18]. This results in a monotonic\nincrease in magnetization with cooling, approaching gradual saturation at 2 K. This\nbehavior is consistent with typical ferromagnetic-like ordering such as Fe GeTe [19].\n3 2\nIn contrast, the ZFC process involves cooling the sample to low temperature in the\nabsence of an external field, leading to spins frozen in random orientations, making the\nmagnetization moment of ZFC smaller than that of FC at 2 K. Upon subsequent\nwarming in a magnetic field, thermally activated domain wall motion and unblocking\nof spin clusters allow spins to gradually align, producing an initial rise in magnetization.\nAs the temperature approaches ~ 25 K, thermal agitation disrupts spin order, resulting\nin a decrease in magnetization. The small difference between FC and ZFC curves below\n19\nthe transition temperature likely originates from magnetic anisotropy and weak\ninterlayer exchange, consistent with observations in other van der Waals magnets such\nas CrI [20-22].\nconditions, measured under a 500 Oe external field. The magnetic transition occurs near\nT ~ 25 K. The inset shows the first derivative of the magnetization curve (dM/dT),\nc\nhighlighting the transition point more clearly.\nMinor Comments\nFigures in the supplementary section should be labeled as Fig.Sx or Supplementary\nFig.x instead of Fig.x for clarity.\nResponse: Thank you for pointing this out. We have revised the figure labels in the\nsupplementary section to \" Supplementary Fig.x \" throughout the manuscript and figure\ncaptions to ensure clarity and consistency.\nOverall, we thank the Reviewer for the valuable reviewing efforts. We hope we have\nconvincedly addressed all the comments raised by the Reviewer and our revised\nmanuscript meets the criteria for publication in Nature Communications.\nThis work has reported a refined flux-assisted phase-controlled growth strategy,\nelucidated the underlying phase-selection mechanisms, and successfully synthesized\nsix different 2D MB T family materials. Recently, MB T family materials have\n2 4 2 4\nattracted intensive attention due to their fundamental magnetic, electric, and topological\nproperties. However, the controlled synthesis of thin-layer target materials has always\nposed great difficulties. Here, the authors achieved the growth of thin-layer 2D MB T\n2 4\nfamily materials through a strategy of precise atomic-level control of precursors,\nmatching selection of cosolvents, and coordinated equilibrium of\nthermodynamics/kinetics. Moreover, the phase-separation process was revealed using\nSTEM and in-situ observation, and its growth mechanism was characterized and\nexplained. Finally, the MnSb Te system was selected for a detailed study. SQUID and\n2 4\nRMCD characterizations further verified the ferromagnetism caused by the substitution\nof Mn-Sb, and its Tc can reach up to 33.7 K with the increase in the number of layers.\nI think the strategy to control phase selection is effective and feasible. The magnetic\nproperty study also further explains the relationship between the micro-composition\nand the macro-properties of MnSb Te . In general, this work is interesting and has good\n2 4\nnovelty. I can recommend the publication after addressing the following concerns:\nResponse: Thank you very much for your positive evaluation and constructive\ncomments. We are pleased that you recognized the novelty and significance of our work,\nparticularly the refined flux-assisted phase-controlled growth strategy and the\ninvestigation of the MnSb Te system. In response, we have revised the manuscript to\n2 4\nprovide a more detailed and comprehensive discussion.\n1. The use of crystal powders as precursors can ensure the accuracy of atomic-level\nstoichiometric coefficients. Could the authors elaborate on this in more detail and\nprovide more definite evidence to prove the melting process of the crystal powders and\nthe cosolvents together?\nMnSb Te as the precursor is essential for precise stoichiometric control at the atomic\n2 4\nlevel. In contrast to elemental or oxide powders, which can cause side reactions or\nuneven reactivity at high temperatures, the crystalline precursor fixes the Mn:Sb:Te\nratio before growth begins. To verify this, we heat a mixture of MnSb Te powder and\n2 4\nNaCl-KCl cosolvents to 680 °C, which is above the eutectic point. Then the setup was\nrapidly quenched for SEM observation. As shown in Fig.R14, elemental mapping\nreveals a uniform distribution of Na, K, and Cl, confirming that the cosolvents formed\na homogeneous molten flux during the reaction. More importantly, Mn, Sb, and Te are\nalso homogeneously distributed within the flux region, providing direct evidence that\nthe precursor powder fully dissolves and participates uniformly in the flux environment.\nIn addition, DSC results of both the salt mixture alone and in combination with the\nprecursor powders show closely matched melting points, indicating synchronized\nmelting behavior. Together, these results validate that the use of MnSb Te powder,\n2 4\ncombined with a homogeneous molten salt medium, enables stoichiometric control. We\nhave included the images and analysis in the Supplementary Information as\nSupplementary Fig.17.\nFig.R14. EDS mapping of the reaction process at 680 ℃. It is obvious that KCl and\nNaCl present a uniform state, which proves that the selected cosolvents can form an\neven flux mixture at high temperature. Additionally, the Mn, Sb, and Te EDS mapping\n22\nimages provide solid evidence that the bulk precursors can be directly dissolved into\nthe molten flux.\n2. It is noted that the endothermic peak positions in the DSC curves are similar for two\ndifferent powders, but there are obvious differences between the two curves before\nmelting. Please explain in detail the sources of the differences in the DSC curves. In\naddition, the DSC data of single MnSb Te crystal powder needs to be provided as\n2 4\nsupport to further verify the melting - precipitation process.\nResponse: Thank you for your constructive comment. As shown in Fig.R15a, the DSC\ncurves for the NaCl-KCl cosolvent alone and for the mixture of cosolvent with\nMnSb Te precursors both display a pronounced endothermic peak at around 660 °C,\n2 4\ncorresponding to the melting point of the NaCl-KCl eutectic [1]. Importantly, in the\nprecursor-containing mixture, no additional endothermic signal attributable to the\nmelting or decomposition of MnSb Te is detected. This indicates that once the\n2 4\ncosolvent melts, the MnSb Te precursor dissolves directly into the flux rather than\n2 4\nmelting independently. The small differences between the two DSC curves before the\neutectic peak are mainly due to variations in the specific heat capacity (Cp) of the two\nsystems. To further elucidate the thermal behavior of MnSb Te , we performed both\n2 4\nthermogravimetric analysis (TGA) in Fig.R15b and DSC measurements in Fig.R15c\non phase-pure crystalline MnSb Te powder. The TGA curve shows a gradual mass loss\n2 4\nbeginning from ~ 450 °C, likely due to the slow evaporation of residual Te. A sharp\nmass loss event is observed around ~ 690 °C, indicating significant thermal\ndecomposition of MnSb Te . Correspondingly, the DSC curve in Fig.R15c exhibits a\n2 4\nsmall endothermic peak around ~ 620 °C, which may be attributed to a solid-state\ntransition or the melting of a trace impurity phase such as Sb Te . Notably, no strong\n2 3\nmelting peak is observed near 660 °C, further supporting the hypothesis that MnSb Te\n2 4\ndoes not melt as a discrete phase but rather decomposes and dissolves into the molten\nflux upon heating in Fig.R15a. In addition, the DSC curve of MnSb Te powder also\n2 4\nshows two minor endothermic features at ~ 420 °C and ~ 860 °C. The former likely\n23\ncorresponds to the melting or evaporation of residual elemental Te, while the latter may\nbe associated with the decomposition of Mn-Te binary phases. All thermal analyses\nwere conducted under flowing argon without spatial confinement, allowing volatile\ndecomposition products to escape.\nFig.R15. Thermal analysis of precursor powder. a. DSC curves of pure cosolvent\n(NaCl-KCl) and the mixture of cosolvent with MnSb Te precursor. b.\n2 4\nThermogravimetric analysis (TGA) of phase-pure MnSb Te powder, showing gradual\n2 4\nand sharp mass loss with increasing temperature. c. DSC curve of MnSb Te powder.\n2 4\n3. The authors have elaborated that the magnetic properties of odd-layer and even-layer\nsamples of this material should be different. Please infer the specific number of layers\nof the RMCD-tested samples.\nResponse: Thank you for your valuable suggestion. The number of layers plays a\ncritical role in determining the magnetic properties of MnSb Te according to the\n2 4\nprevious report. Based on high-resolution cross-sectional STEM imaging, the thickness\nof a single septuple layer (SL) is measured to be approximately 1.22 nm in Fig.R16.\nIn our study, the RMCD-tested samples exhibit AFM-measured thicknesses of\napproximately 4.8 nm, 6.9 nm, 9.4 nm, and 17.3 nm. It should be noted that AFM-based\nthickness measurements of 2D materials are typically larger than the intrinsic interlayer\nspacing due to surface adsorbates, ambient oxidation, or tip-sample convolution effects.\nFor example, although monolayer graphene has a theoretical thickness of ~ 0.34 nm, its\nmeasured step height by AFM is often ~ 0.5 nm [23, 24]. Taking this into account, we\n24\ncarefully estimated the layer numbers of our MnSb Te samples. The measured values\n2 4\ncorrespond to 4, 5, 8, and 14 septuple layers, respectively. These values have been\nclearly annotated in the main text.\nFig.R16. Cross-sectional AC-TEM image of MnSb Te , revealing a single septuple\n2 4\nlayer thickness of approximately 1.22 nm. Based on this reference, the RMCD-tested\nsamples with AFM-measured thicknesses of 4.8 nm, 6.9 nm, 9.4 nm, and 17.3 nm\ncorrespond to 4, 5, 8, and 14 septuple layers, respectively.\n4. The authors have elaborated on the phase-control factors in the sample growth\nprocess. For the MnSb Te material, how to control its growth thickness, size, etc.\n2 4\nPlease explain in detail.\nResponse: Thank you for your constructive comment. The thickness of MnSb Te\n2 4\nnanosheets can be effectively tuned by two key parameters: synthesis temperature and\nthe precursor-to-cosolvent mass ratio, as illustrated in Fig.R17a, the minimum sample\nthickness measured by AFM increases with synthesis temperature. This trend can be\nattributed to the enhanced solubility of precursors in the molten flux at higher\ntemperatures, which promotes vertical crystallization due to thermodynamic driving\nforces [25, 26]. In Fig.R17b, the precursor concentration also significantly affects\nthickness. When the precursor-to-cosolvent ratio is low, thinner nanosheets are obtained.\nThis is because a higher proportion of cosolvent improves atom mobility and prolongs\nthe evaporation time of the molten salt, allowing atoms to diffuse further and favor\nlateral growth. In contrast, a higher precursor loading leads to supersaturation, which\n25\naccelerates nearby vertical nucleation and results in thicker flakes. Therefore, by\ncarefully optimizing the temperature and precursor/cosolvent ratio, we can reliably\ncontrol the MnSb Te crystal thickness. In our work, the minimum thickness achieved\n2 4\nis ~ 2.4 nm, corresponding to approximately two septuple layers.\nAs shown in Fig.R17c, the effect of the cosolvent-to-precursor ratio on sample size is\ngenerally modest. At a fixed growth temperature, varying the ratio produces only minor\nchanges in the lateral size. Under extreme conditions, however, an insufficient amount\nof cosolvent can limit mass transport, favor vertical growth and resulting in thicker but\nsmaller flakes. Conversely, an excessively high cosolvent content can dilute the\nprecursor concentration, leading to smaller nanosheets due to an inadequate atomic\nsupply. Fig.R17d shows the influence of growth temperature. Between 680 °C and\n700 °C, the lateral dimensions remain largely unchanged. At higher temperatures,\nhowever, the nanosheets tend to become thicker and slightly smaller. This may be due\nto thermodynamically driven vertical growth at high temperatures, which promotes\natomic alignment along the out-of-plane direction and slightly reduces the lateral\ndimensions [27-29].\nThe evolution of thickness and lateral size with temperature and precursor-to-flux ratio\nhas been added to the Supplementary Information as Supplementary Fig.7 and\nSupplementary Fig.8.\nratio. a. Evolution of sample thickness with growth temperature. b Evolution of sample\nthickness with precursor molar ratio. c. Morphology of samples obtained at different\nprecursor-to-cosolvent ratios. d. Morphology of samples synthesized at different\ngrowth temperatures. Scale bar: 20 μm.\n5. It is suggested that the authors should compare the lattice constants of MnSb Te ,\n2 4\nMnSb Te , MnTe, and Sb Te to rule out the possibility of Mn-doped Sb Te .\n4 7 2 3 2 3\nResponse: Thank you for the constructive suggestion. We systematically compared the\nlattice parameters and structural features of MnSb Te , MnSb Te , MnTe, and Sb Te .\n2 4 4 7 2 3\nAs shown in the diffraction patterns, all four samples exhibit similar interplanar spacing\n27\nalong the (001) direction and show 3-fold rotational symmetry, resulting in subtle\ndifferences in diffraction signals. Therefore, lattice fringe measurements in TEM may\nnot reliably distinguish between Mn-doped Sb Te and MnSb Te . However, further\n2 3 2 4\nanalysis using AC-TEM and HAADF imaging reveals distinct Z-contrast differences.\nMnTe displays noticeable bright-dark contrast due to the significant atomic number\ndifference between Mn and Te atoms. In contrast, the other samples show uniform\ncontrast at the surface, which is expected since the topmost atomic layers are composed\nof Sb and Te, whose atomic numbers are relatively close. Moreover, EDS elemental\nmapping was employed to distinguish between phases, providing further confirmation\nof the chemical identity of the samples.\nFig.R18. Crystal structure characterization of MnSb Te , MnSb Te , MnTe, and Sb Te .\n2 4 4 7 2 3\n6. Based on the STEM results, can the authors infer the number of Mn-Sb mutually\nsubstituted atoms in the synthesized MnSb Te crystals and what is the percentage of\n2 4\n28\nthe substitution?\nResponse: Thank you for your thoughtful suggestion. High-resolution STEM analysis\nindicates that the surface termination of MnSb Te is composed of Sb and Te atoms.\n2 4\nBecause these two elements have similar atomic numbers, they are expected to produce\na uniform contrast in Z-contrast STEM images. In our measurements, however, we\nobserve distinct variations in the contrast, with certain atomic columns appearing\nnoticeably darker. This contrast variation suggests substitution by lighter atoms. Given\nMn’s substantially lower atomic number, its occupation of Sb sites (Mn-Sb antisite\ndefects) would account for the observed dark contrast [12].\nBy statistically counting the bright and dark atomic columns at the surface, we estimate\nthe Mn-Sb antisite defect concentration to be about 1.3% (81 out of 6,216 columns).\nIt should be noted that this value mainly reflects the near-surface region; the defect\ndensity in the bulk could be different. Cross-section STEM images did not show clear\nantisite features, which may be due either to a genuinely low bulk defect concentration\nor to technical factors such as local defocus or electron beam-induced damage. Overall,\nwhile this estimate is limited to the surface region, it provides useful insight into the\ndefect landscape of MnSb Te .\n2 4\n.\nFig.R19. HAADF-STEM images of MnSb Te viewed along the c-axis. The left panel\n2 4\nshows the overall atomic arrangement, where we can see the dark spots. The right panel\nhighlights Mn–Sb antisite defects, indicated by yellow dashed circles.\nWe greatly appreciate the Reviewer’s valuable efforts and constructive feedback. We\n29\nhope that our responses have adequately addressed all the comments, and we also hope\nthat the revised manuscript is now suitable for publication in Nature Communications.\nIn this manuscript, the authors synthesized the MBT family using a phase-controlled\nstrategy with a mixed salt flux. They also characterized the magnetic properties of\nMnSb Te within the MBT family. Materials such as MnSb Te and MnBi Te have\n2 4 2 4 2 4\ngarnered significant attention due to their exotic topological properties and magnetic\nproperties, which make them highly promising for various studies and applications.\nNotably, the direct synthesis of these materials as nanosheets with high crystallinity is\nintriguing. So, this paper is adequate to be published in Nature Communications.\nHowever, before publication, the authors need to have a clear explanation for several\ncomments.\nResponse: We sincerely appreciate your recognition of our work and your positive\nassessment of its significance. We also thank you for your thoughtful and constructive\ncomments, which are valuable in helping us further improve the quality and clarity of\nthe manuscript. Below, we provide detailed responses to each of your concerns.\n1. In the 1st paragraph of the 3rd page and method section, the authors explained the\nsynthesis method using self-designed equipment. I think the explanation for this part is\ninsufficient. Could you elaborate more on this part? For example, detailed conditions\nof MnSb Te synthesized by CVT method, the structure of the synthesis equipment, the\n2 4\npossibility of using substrates other than mica (if synthesis is possible only with mica,\nwhy is that), etc.\nResponse: Thank you for the insightful comments. First, we provide more details about\nthe equipment and synthesis process. The self-designed setup consists of two 304\nstainless steel plates, each measuring 10 cm in length, 2 cm in width, and 0.3 cm in\nthickness. Screw holes are machined into the plates to allow for mechanical fastening,\nas shown in Fig.R20. During the actual experiment, MnSb Te crystal powder is first\n2 4\nplaced onto a mica substrate. A eutectic mixture of NaCl and KCl in a 1:1 molar ratio\nis then uniformly distributed over the crystal powder to serve as the flux. Another mica\n31\nsheet is placed on top to cover both the precursor and the mixed salts. The entire setup\nis tightly secured using screws, with the applied pressure controlled by the number of\nscrew rotations. Notably, when all screws are fully tightened, it can reach approximately\n1.7 × 10⁵ Pa, as measured by a thin-film pressure sensor. Then, the setup is loaded into\na tube furnace, and argon gas is introduced to purge atmospheric oxygen. The system\nis rapidly heated to 700 °C within 20 minutes, held at that temperature for 3 minutes,\nand then allowed to cool naturally to room temperature with a constant Ar flow rate of\n50 sccm in the whole process. The use of Ar is crucial for removing oxygen from the\nsystem, as MnSb Te nanosheets are highly sensitive to air exposure. Notably, when the\n2 4\ntemperature reaches the melting point of the mixed salts (~ 650 °C), the MnSb Te\n2 4\ncrystal powder dissolves into the molten salts, forming a uniform flux. Upon holding at\nthe target temperature, the flux spreads over the mica substrate and gradually vaporizes,\nallowing the sample to precipitate and grow. After cooling, the setup is disassembled to\nretrieve the sample, which forms within the confined space between the two mica sheets,\nadhering to both inner surfaces. Fig.R20b illustrates the reaction state at different\ntemperatures. At low temperature, the mixed salts do not melt and thus fail to form a\nflux, preventing sample formation. In the temperature range of 650-720 °C, the target\nsamples precipitate from the flux. At temperatures above 760 °C, phase separation\noccurs dramatically, leading to the formation of MnTe as a secondary phase. Fig.R20c\nshows in-situ observation of the growth process, revealing the state of the reactants at\ndifferent temperatures and confirming that the target samples precipitate from the flux.\nFig.R20d presents SEM images of samples quenched from different temperatures,\nwhich correlate well with the in-situ observations in Fig.R20c. At around 700 °C,\ntriangular-shaped samples are precipitated. As the temperature increases further,\nphase separation occurs, resulting in the formation of MnTe and Mn Sb Te structures.\nx 2 4\ngrowth setup and steps of the growth process. b. Schematic diagram of the growth\nprocess, showing the states of the reactants at different temperatures: powders remain\nsolid at low temperatures; at optimal temperatures, the target product precipitates from\nthe flux; at higher temperatures, impurity phases form. c. In-situ observation of the\ngrowth process at 500 °C, 700 °C, and 760 °C. d. SEM images of samples grown at\ndifferent temperatures.\nTo verify that the precursor powder can dissolve into the molten cosolvent during the\ngrowth process, we heat a mixture of MnSb Te powder and NaCl-KCl cosolvents to\n2 4\n680 °C, which is above the eutectic point [1]. Then the setup was rapidly quenched for\nSEM observation. As shown in Fig.R21, elemental mapping reveals a uniform\ndistribution of Na, K, and Cl, confirming that the cosolvents formed a homogeneous\nmolten flux during the reaction. More importantly, Mn, Sb, and Te are also\nhomogeneously distributed within the flux region, providing direct evidence that the\ncrystal precursor fully dissolves and participates uniformly in the flux environment. In\n33\naddition, DSC results of both the salt mixture alone and in combination with the crystal\nprecursor show closely matched melting points, indicating synchronized melting\nbehavior.\nFig.R21. EDS mapping of the reaction process at 680 ℃. It is obvious that KCl and\nNaCl present a uniform state, which proves that the selected cosolvents can form an\neven flux mixture at high temperature. Additionally, the Mn, Sb and Te EDS mapping\nimages provide solid evidence that the bulk precursors can be directly dissolved into\nmolten flux.\nBulk MnSb Te crystals were synthesized via a carefully controlled CVT method, as\n2 4\nillustrated in Fig.R22. High-purity elemental precursors: manganese, antimony, and\ntellurium were weighed in a stoichiometric ratio of 1:2:4. The mixture was sealed in a\nquartz ampoule under high vacuum (~ 10-5 Torr) to prevent oxidation. To ensure\ncomplete precursor mixing and suppress the formation of secondary phases in cooler\nregions, the sealed ampoule was fully positioned within the high-temperature zone of a\nsingle-zone horizontal tube furnace. The thermal process included several key stages:\nFirst, the temperature was rapidly ramped to 900 °C and maintained for 0.5 hours. This\nhigh-temperature stage enabled thorough reaction and homogenization of the\nprecursors, promoting the formation of a uniform intermediate phase. Subsequently, the\nsystem was cooled to 650 °C over 2 hours, establishing conditions suitable for\n34\ncontrolled nucleation and crystal growth. The tube was then held isothermally at 650 °C\nfor 48 hours to facilitate crystal growth. Final cooling to room temperature over 2 h,\nwhich relieves internal stress and maintains structural integrity. The obtained MnSb Te\n2 4\ncrystals exhibit well-defined layered morphology as Fig.R22c is shown. The EDS\nmapping results show that no obvious impure phases exist while the point scanning\nanalysis tells the accurate stoichiometric ratio of ~ 1:2:4 in Fig.R22d.\nFig.R22. Schematic and experimental details of the CVT process for synthesizing\nMnSb Te bulk crystals. a. Schematic of the quartz ampoule placement in the furnace,\n2 4\nwith the entire ampoule positioned within the high-temperature zone to ensure uniform\nheating and prevent low-temperature side reactions. b. Heating profile used in the CVT\nprocess, showing key stages including ramping, holding, and cooling. c. Optical\nmicroscopy image of the as-grown MnSb Te crystals, showing their layered\n2 4\nmorphology. d. The mapping results show that no obvious impure phases exist while\nthe point scanning analysis tells the accurate stoichiometric ratio of ~ 1:2:4.\nTo further study the substrate effect, we performed growth experiments using other\nsubstrates, including sapphire and SiO /Si. As shown in Fig.R23a and R23b\n2\nrespectively, growth on these substrates results in the formation of randomly oriented\nand isolated thick crystallites, and we were not able to obtain well-aligned ultrathin\nflakes like those on mica. For sapphire, the in-plane lattice constant is 4.7580 Å, which\nexhibits a large mismatch with that of MnSb Te . In addition, the sapphire surface is\n2 4\n35\nartificially cut, producing atomic terraces rather than being atomically flat, thereby\npreventing effective symmetry matching during growth. For SiO , it is amorphous and\n2\nlacks any defined lattice symmetry, making epitaxial alignment impossible.\nFurthermore, under identical growth conditions, MnSb Te samples grown on sapphire\n2 4\nand SiO substrates tend to exhibit larger thicknesses but lower areal densities\n2\ncompared to those synthesized on fluorophlogopite mica. This difference is primarily\nattributed to surface topography among the substrates. The atomically flat surface of\nfluorophlogopite offers minimal kinetic barriers for lateral precursor diffusion and flux\nspreading, thereby facilitating growth of thin nanosheets [3, 5]. In contrast, the sapphire\nsubstrate typically presents terrace structures, which act as diffusion barriers and\nconfine precursor migration [30]. Meanwhile, for SiO , the amorphous and defect-rich\n2\nsurface suppresses mass transport efficiency. Moreover, the chemically reactive nature\nof the flux may induce partial surface etching on SiO , shown as the yellow circled part\n2\nin Fig.R23a, which further hinders the fast mass transport. In Fig.R23c, we observed\nthat the samples exhibit well-aligned domains in certain areas. However, misaligned\ndomains are also present. This misalignment may be attributed to the rapid growth rate\nof the samples. In our in-situ observation experiments, the nucleation and growth\nprocess were completed within only seconds. Such a short growth time may lead to\nmisorientation. Additionally, intrinsic defects within MnSb Te may further promote\n2 4\nlocal misorientation of the grown domains [31-33].\n2 4\nconditions at 700 °C for 3 minutes. Due to the brittle nature of the SiO substrate, the\n2\nscrews were not tightened firmly. a. Samples grown on SiO /Si, b. Sample grown on\n2\nsapphire substrate, c. Sample grown on mica substrate. The scale bar is 20 μm.\nAs for the well alignment of MnSb Te on mica in Fig.R23c. This is likely due to the\n2 4\natomically flat and surface symmetry of mica, which provides a favorable template for\nthe oriented growth of MnSb Te . MnSb Te crystallizes in the R-3m space group with\n2 4 2 4\na threefold rotational symmetry, and the surface of fluorophlogopite, due to slight lattice\ndistortion, also exhibits a reduced threefold symmetry [3-5]. This symmetry\ncompatibility may allow MnSb Te to adopt energetically favorable orientations during\n2 4\ngrowth. In addition to symmetry matching, the in-plane lattice constant of MnSb Te is\n2 4\n4.2385 Å, while that of fluorophlogopite is 9.2123 Å. A supercell comparison reveals\nthat a 9 × 9 MnSb Te domain (9 × 4.2385 = 38.1465 Å) closely matches with\n2 4\nfluorophlogopite supercell (36.849, 37.217, and 37.228 Å in different directions),\nresulting in a lattice mismatch of only ~ 2.4-3.4%, as shown in Fig.R24a. This\n37\nrelatively small mismatch, combined with symmetry compatibility, facilitates aligned\ngrowth of MnSb Te via epitaxy. A similar symmetry-guided alignment mechanism was\n2 4\npreviously demonstrated in our group’s work on α-In Se (R-3m space group) growth,\n2 3\nwhere the lattice and symmetry match with fluorophlogopite enabled the formation of\nparallel-oriented flakes with preferred orientations at 0° and 60° [3, 32, 33].\nWe have added to the main text a description of the growth setup: “The self-designed\nsetup consists of two 304 stainless steel plates, each measuring 10 cm in length, 2 cm\nin width, and 0.3 cm in thickness. Screw holes are machined into the plates to allow for\nmechanical fastening.” Additional images and data related to the growth process,\nsubstrate effects, and precursor preparation have been included in the Supplementary\nInformation as Supplementary Fig.1, 2, 3, 11 and 12.\nFig.R24. Schematic illustration of MnSb Te alignment on fluorophlogopite mica. a.\n2 4\nThe crystal structure along the [001] direction of fluorophlogopite, among which 3\ndifferent alignment types are shown to match with the 9 × 9 MnSb Te domain. b. The\n2 4\nview along the [001] direction of MnSb Te . We can still observe certain regions where\n2 4\nthe alignment is not ideal, which is attributed to imperfect epitaxial matching and the\npresence of defects induced by excessively rapid growth in the flux environment.\n2. In the 3rd paragraph of the 4th page, authors argued that the reason the minimum\nthickness increases with increasing temperature is because rapid vertical crystallization\noccurs. Why is vertical growth preferred over lateral growth?\nthat the minimum flake thickness of MnSb Te increases with increasing growth\n2 4\ntemperature, as illustrated in Fig.R25. This trend reflects a shift in growth mode from\nlateral expansion to vertical accumulation, which can be attributed to both\nthermodynamic and kinetic factors that govern crystal growth in van der Waals layered\nsystems. At lower temperatures, the growth process is primarily kinetically dominated.\nAdatoms have limited surface mobility and insufficient energy to overcome the\nnucleation barrier in the vertical direction. As a result, growth is constrained to lateral\ndirections, producing atomically thin, well-dispersed 2D flakes, as shown in Fig.R25a\nand R25c (e.g., at 680 °C) [25, 29]. As the temperature increases, the growth\nenvironment becomes thermodynamically dominated. Enhanced surface diffusion\nallows adatoms to more efficiently reach energetically favorable binding sites, and the\nthermal energy becomes sufficient to overcome the vertical nucleation barrier.\nConsequently, vertical growth becomes preferred, leading to the formation of thicker\nflakes (at 720 °C) [26, 27, 29]. At even higher temperature such as 780 °C, it may\ndestabilize the desired phase and promote the formation of secondary phases, such as\nSb Te or MnTe.\n2 3\nWe have added the discussion on the effect of growth temperature on the growth process\nto the Supplementary Information as Supplementary Fig.8.\n2 4\nillustration of the temperature effect on the minimum flake thickness. At lower\ntemperatures, the growth process is kinetically dominated, leading to the formation of\nthinner flakes. As the temperature increases, the process becomes thermodynamically\ndominated, resulting in thicker flakes. Further temperature increases lead to the\nformation of undesired secondary phases due to excessive thermal energy. b. Plot\nshowing the minimum thickness of MnSb Te flakes obtained at different synthesis\n2 4\ntemperatures. c. Optical microscopy images of MnSb Te flakes synthesized at 680 °C,\n2 4\n720 °C, and 780 °C, respectively, showing the morphology evolution with increasing\ntemperature.\n40\n3. In Fig.4, It was shown that the curie temperature decreases as the thickness decreases.\nIt was said that this shows a similar trend to other two-dimensional materials. However,\nwhile two-dimensional magnetic materials such as Fe GeTe have an increasing\n3 2\ncoercive field at low temperatures as the thickness decreases, why does the coercive\nfield of MnSb Te decrease at low temperatures when comparing Fig.4c and Fig.4d?\n2 4\nResponse: We sincerely appreciate the reviewer’s insightful comments. We have\ncarefully analysed the evolution of coercive field H as a function of both temperature\nc\nand sample thickness. As shown in Fig.R26a, H decreases monotonically with\nc\nincreasing temperature for all thicknesses and vanishes near the Curie temperature. This\ntrend is consistent with other two-dimensional ferromagnets and can be attributed to\nthermal activation: as temperature rises, magnetic anisotropy becomes insufficient to\ncounteract thermal fluctuations, facilitating magnetization reversal and reducing H .\nc\nFig.R26b reveals a non-monotonic trend in the H as a function of thickness at 2 K,\nc\ncharacterized by a peak at intermediate thicknesses (~6-8 nm). This behavior suggests\nthe presence of two competing effects that jointly determine the magnetic hardness of\nMnSb Te . In conventional 2D ferromagnets such as Fe GeTe and Cr Ge Te [34, 36],\n2 4 3 2 2 2 6\na monotonic increase in H with decreasing thickness is typically observed in Fig.R26c,\nc\nwhich is attributed to the enhancement of magnetic anisotropy and domain stabilization\nin thinner flakes due to reduced dimensionality. This trend originates from intrinsic\nmagnetism and strong spin-orbit coupling effects that become increasingly dominant in\nthe 2D limit [15-17, 35, 36]. The reason is that MnSb Te exhibits a fundamentally\n2 4\ndifferent ferromagnetic origin. Its ferromagnetism arises from Mn-Sb antisite defects\nthat disrupt the native A-type antiferromagnetic interlayer coupling and induce\nferromagnetic exchange. As the sample thickness decreases, the density of these antisite\ndefects also decreases. This reduction leads to weaker overall FM exchange interactions\nand lower magnetic moment density in ultrathin flakes. At the same time, thinner\nMnSb Te flakes suffer from increased surface and edge disorder, enhanced exposure\n2 4\nto ambient degradation, and finite-size effects, all of which destabilize magnetic\n41\ndomains and suppress magnetic anisotropy [12, 24, 37-39]. As for the thicker flakes,\nalthough the antisite density is higher, magnetic anisotropy is lower due to the reduced\ninfluence of surfaces, moderate magnetic anisotropy and confinement, resulting in\nmoderate coercivity. The combination of these two opposing effects leads to a\nmaximum coercive field at intermediate thickness, where the balance between\nsufficient antisite-induced ferromagnetism and enhanced dimensionality anisotropy is\noptimized. Besides, similar non-monotonic behavior has been observed in\nmechanically exfoliated MnSb Te samples, where the coercive field increases with\n2 4\ndecreasing thickness down to an intermediate regime and subsequently drops for\nultrathin flakes in Fig.R26c [24].\nTo improve the clarity of our discussion, we have revised the main text to distinguish\nbetween two phenomena: the temperature dependence of H , which is consistent with\nc\nother 2D ferromagnets, and the thickness dependence of H , which deviates from\nc\nFe GeTe and Cr Ge Te due to the defect-induced ferromagnetic nature of MnSb Te\n3 2 2 2 6 2 4\n[34-36, 24, 37-41]. The coercivity evolution thus reflects a balance between enhanced\nanisotropy in thin flakes and the suppression of FM ordering due to reduced antisite\ndefect density and low stability. Besides, we have added the figure into the\nSupplementary Information as Supplementary Fig.34.\nFig.R26. Evolution of coercive field H with temperature and thickness. a.\nc\nTemperature-dependent H curves for MnSb Te samples of various thicknesses. b.\nc 2 4\nThickness-dependent H values measured at 2 K for MnSb Te samples. c. Comparison\nc 2 4\nof H -thickness trends among MnSb Te , Fe GeTe and Cr Ge Te at 2 K from\nc 2 4 3 2 2 2 6\n42\nreferences [24, 34, 36].\n4. In the 1st paragraph of the 9th page, the reason why MnSb Te shows FM behavior\n2 4\ninstead of A-type AFM behavior is explained as the change in magnetic properties due\nto the substitution of Mn and Sb. Can you explain why Mn and Sb are better substituted\nthan Sb and Te, which are more similar in size? And does MnSb Te powder\n2 4\nsynthesized by CVT show A-type AFM characteristics?\nResponse: Thank you for your valuable comments. To explain the origin of the\nferromagnetic behavior observed in our MnSb Te samples, we conducted first-\n2 4\nprinciples calculations. As shown in Fig.R27, DFT calculations reveal that Mn-Sb\nantisite substitution, where Mn2+ replaces Sb3+ in the lattice, exhibits a negative\nformation energy of -0.1492 eV, indicating it is thermodynamically favorable. In\ncontrast, Sb-Te antisite substitution (both top Te and down Te) shows a positive\nformation energy of 0.9080 and 1.3335 eV, respectively, making both processes highly\nunfavorable. The preference for Mn-Sb antisite formation can be rationalized by ionic\nsize compatibility and charge balance considerations. As Fig.R27c shows, the ionic\nradius of Mn2+ is approximately 0.83 Å, while that of Sb3+ is 0.76 Å, a relatively small\nmismatch, which supports facile cation-cation substitution with minimal lattice\ndistortion. In contrast, the ionic radius of Te2- is 2.21 Å, which is significantly larger\nthan either Mn2+ or Sb3+, making cation-anion exchange unfavorable. Furthermore, Te2-\ntypically occupies chalcogenide positions with distinct bonding environments and\nvalence states, further disfavoring substitution with trivalent or divalent cations. This\ntheoretical insight is well supported by other reports [20, 37-39]. These antisites\nintroduce excess Mn magnetic moments and enhance ferromagnetic exchange\ninteractions, thus destabilizing the ideal A-type antiferromagnetic interlayer coupling\nand resulting in a net FM ground state.\n2 4\nsubstitution types. b. Mn-Sb antisites exhibit negative formation energy (-0.1492eV),\nwhereas Sb-Te top and Sb-Te down substitutions have much higher, positive values\n(0.9080 eV and 1.3335 eV respectively), indicating that only Mn-Sb exchange is\nthermodynamically favored. c. Radius of different ions in MnSb Te .\n2 4\nWe also performed SQUID test on bulk MnSb Te powder, as shown in Fig.R28. The\n2 4\nM-T curve of the CVT-grown powder exhibits a clear magnetic transition at T ~ 27K,\nc\nclosely matching the value observed in exfoliated 2D flakes. Both ZFC and FC curves\ndisplay the typical separation expected for a ferromagnetic system. In the FC process,\nspins progressively align along the field direction upon cooling below T , producing a\nc\nmonotonic increase in magnetization toward low temperatures; in contrast, the ZFC\n44\ncurve starts from a lower magnetization due to randomly oriented spins frozen in zero\nfield at 2 K, and increases upon warming as domain walls unblock and spins align,\nbefore decreasing again near T due to thermal disorder [16-18]. This behavior confirms\nc\na transition from a paramagnetic to a robust ferromagnetic state, rather than an\nantiferromagnetic ground state, and is consistent with previous reports on defect-\nmediated ferromagnetism in MnBi Te and MnSb Te . Based on our CVT-grown\n2 4 2 4\nMnSb Te samples, the synthesized two-dimensional flakes, and previously reported\n2 4\nstudies, the formation of Mn-Sb antisite defects in this system appears to be nearly\nunavoidable. Such defects perturb the intrinsic antiferromagnetic ground state, driving\nthe system toward a ferromagnetic configuration [24, 37-39].\nFig.R28. M-T curve of CVT-grown MnSb Te powders, showing a clear ferromagnetic\n2 4\ntransition near 27 K.\nWe have added to the main text a description noting that the formation energy of Mn-\nSb antisites is lower than that of Te-Sb substitutions. In addition, when discussing the\nSQUID results of nanosheets, we have included a comparative discussion with powder\nsamples: the M-T curves of the Chemical Vapor Transport (CVT)-grown bulk samples\nalso exhibit a ferromagnetic signal with T ~27 K in Supplementary Fig.31, which is\nc\n45\nclose to the value above rather than showing antiferromagnetic behavior. The remaining\ndata and discussion have been included in the Supplementary Information as\nSupplementary Fig.31 and Supplementary Fig.31.\nWe greatly appreciate the Reviewer’s time and insightful feedback, which have been\ninvaluable in improving our work. We have carefully considered and addressed each\npoint, and we sincerely hope that the revised manuscript may now be suitable for\nconsideration in Nature Communications.\n46"}, {"round": 2, "reviewer_comments": {"reviewer_1": "I have carefully reviewed the revised version of the manuscript entitled “Phase-Controlled Growth of MB2T4 Family Crystals via Flux-Assisted Method”. I am impressed by the authors’ comprehensive and rigorous responses to all reviewer comments. The revisions have substantially strengthened the manuscript by providing detailed clarification of the flux-assisted growth mechanism, additional structural characterizations (Raman, XRD, TEM), systematic substrate and thickness-dependent studies, and expanded magnetic measurements including normalized magnetization values and ZFC–FC comparisons.\nThe authors have convincingly demonstrated phase-controlled synthesis across the MB2T4 family, clarified the role of substrate and growth conditions, and provided robust evidence linking antisite defects to ferromagnetic behavior in MnSb2Te4. Taken together, the manuscript now presents a complete and compelling study of significant novelty and importance.\nGiven the thorough and satisfactory revisions, I am fully satisfied with the current version of the manuscript. I recommend its acceptance in its present form for publication in Nature Communications.", "reviewer_2": "All the comments have been well addressed, and I am now able to recommend publication.", "reviewer_3": "The authors addressed my comments adequately, so I don't have further comments."}, "author_response": ""}]

Decision letter