PHYSICAL REVIEW MATERIALS 8, 023401 (2024)

Single crystal growth of FeRh from AuPb flux
Nikola Subotić ,1,2,3,* Miwako Takahashi,1 Takashi Mochiku,4 Yoshitaka Matsushita ,4 Takanari Kashiwagi,1Osamu Takeuchi,1 Hidemi Shigekawa,1 and Kazuo Kadowaki2
1

Department of Pure and Applied Science, University of Tsukuba, 1-1-1 Tenoudai, Tsukuba, Ibaraki 305-8573, Japan
2
Institute for Quantum Material Research (IQMR), 3441-19, Kurihara, Tsukuba, Ibaraki 305-0001, Japan
3
Montenegrin Science Promotion Foundation (PRONA), Ul. Marka Radovića br. 149, 81000 Podgorica, Montenegro
4National Institute for Material Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan(Received 20 October 2023; accepted 18 January 2024; published 13 February 2024)
The FeRh compound has been known for a long time as an itinerant magnet with a peculiar first-orderantiferromagnetic (AFM)-to-ferromagnetic (FM) transition near room temperature. Although a lot of work has
been done, the origin of the physical properties associated with the AFM ↔ FM transition is still an ongoingdebate and needs deeper investigation using good single crystals. Here, we report on the single-crystal growth of
FeRh from the AuPb flux and confirm it by x-ray crystallographic methods such as Laue diffraction, four-circle
diffractometer measurements, and electron probe microanalyzer elemental analysis. The temperature dependenceof magnetization in our single crystals below the AFM ↔ FM transition shows an anomalous cascadelike
multiple transition behavior, which is obviously very different from the reported results previously. It is only
such high-quality single crystals grown here that will pave the way for a comprehensive understanding of thelongstanding issues of the FeRh compound.
DOI: 10.1103/PhysRevMaterials.8.023401
I. INTRODUCTION

The intermetallic compound FeRh crystalizes into the
CsCl (Pm-3m; B2 type) crystal structure [1]. Despite having a simple, high-symmetry crystal structure, FeRh exhibits
complex magnetic, structural, thermodynamical, mechanical,
and electrical behaviors. One of the most peculiar examples
is the occurrence of the first-order ferromagnetic (FM)-toantiferromagnetic (AFM) transition ∼350 K upon cooling[1]. Although the transition was reported more than 80 years
ago [1], its origin is still under ongoing debate [2–9]. The
transition is accompanied by the lattice expansion which
varies from 0.3 to 1% depending on the sample preparation[6,7,10]. At first, the crystal structure change through thetransition was reported to be like the α-γ transformation of
Fe (magnetostructural transition) [1], while the later results
stated that the crystal structure remains the same [10] (magnetoelastic transition). Due to the FM-AFM transition, giant
magnetocaloric [11,12], elastocaloric [13], barocaloric [14],magnetostrictive [15], and magnetoresistive [16] effects occur. For the giant magnetocaloric effect, the entropy change
is moderate, but the adiabatic temperature change holds thehighest record value among all known room-temperaturemagnetocaloric materials [17]. Also, the transition exhibits atemperature memory effect [18]. Moreover, it was recently
shown that the FeRh alloy doped with Ni exhibits a strikingly high value of the Thomson coefficient due to the steep
change of the Seebeck coefficient during the transition [19],again, the highest value among all known materials nearroom temperature. Furthermore, FeRh has been reported to
exhibit a topological Hall effect that is protected by a nonzero*

Corresponding author: nikolasubotic95@gmail.com

2475-9953/2024/8(2)/023401(8)Berry phase in real space [20]. Additionally, the magnetic
structure of the AFM phase is reported to be a so-called
G-type structure, which has the DO3, Fe3 Al-type symmetry(F m-3m), known as the symmetry of the atomic order in the
Heusler alloy Cu2 MnAl, which is favorable for the formationof topological states [21]. These properties make the FeRhcompound uniquely intriguing and its application broad, suchas magnetic refrigeration [14,22–24], heat-assisted magnetic
recording [25,26], multiferroic devices [27,28], AFM memory [29,30], micromachine and shape memory devices, andmedicine [6].
Although FeRh is such an attractive material with many
peculiar physical properties, it appears that the binary phase
diagram has not been established well [31]. First, according
to Swartzendruber [31], it is not known at which temperature
the FeRh alloy melts for almost all ranges of the composition. Second, the order-disorder transition temperature is also
not exactly known. The high-temperature disordered phase isoften referred to as the γ phase, which crystalizes into theface-centered cubic (fcc) crystal structure (the same as Rh andγ Fe), while the low-temperature ordered phase (the phase ofinterest) has a CsCl-type ordered structure that forms directlyfrom the γ phase through the order-disorder transition [31].
The ordered phase undergoes a usual second-order paramagnetic (PM)-to-FM (which is often referred to as the α  phase in
the literature) transition ∼400 °C at equiatomic composition.
As the α  phase is further cooled, it transforms into an AFM
phase referred to as the α  phase without accompanying crystal structural change.
The usual sample preparation process is, first, melting at
a temperature above Tm ∼ 1600 ◦ C, followed by long-term
annealing at a temperature below the order-disorder transition between fcc and CsCl-type ordered structure (∼1300 ◦ C[31]) to reduce the volume fraction of the γ phase to a

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PHYSICAL REVIEW MATERIALS 8, 023401 (2024)

negligible amount. However, recent tenacious and beautiful
work [32–35] demonstrated the opposite: the volume fractionof the γ phase can be as high as 14%, even though the samples
were annealed for 2 weeks [33]. Moreover, it was observedthat the microstructure is responsible for the drastic changeof the AFM-FM transition temperature, not the compositionof α  as previously thought [6,7,33]. This is a clear indicationthat the binary phase diagram Fe-Rh [7,31], which was constructed without considering the microstructure effect, is notadequate (that might be the reason why the binary phase diagram FeRh was shrouded in mystery for a long time in the firstplace). Accordingly, the physical properties of FeRh known so
far in the literature turn out to be mixed alloy properties of twophases, ordered CsCl type, and the γ phase. Therefore, theintrinsic properties of the single phase are still under the coverof a dark veil of mystery, and one must be prudent enough tounderstand how physical, chemical, and thermodynamic properties of FeRh are related to the presence of foreign phases,
for example, the γ phase [32–35], or the degree of disordersuch as vacancies [10] or antisite doping. Answers to suchquestions could only be provided by examining high-quality
single crystals and comparing the results with their polycrystal/polyphase counterparts, as suggested in Ref. [7]. In theliterature, there is a report on the single-crystal growth of
the FeRh compound along with the neutron diffraction and the
Mössbauer study [36,37]. However, it is not clearly described
how the formation of the γ phase was avoided in their crystal
growth process only by annealing the melt-quenched FeRhat 1200 °C for a day or so [37], whose process in principle
is the same as the one described in the aforementioned work
[32–35].
To shed light on this interesting issue, we have recently
developed an approach to prepare single-phase and singlecrystal FeRh using AuPb flux. The crystal habits of FeRh,obtained from the growth attempts described in this paper, areplatelike (up to 200 × 300 ×10 µm), cubic (up to 400 × 400 ×
400 µm), and needlelike (up to 100 × 100 × 1500 µm). The
quality of single crystals was examined by x-ray diffraction
measurements and its composition by electron probe microanalyzer (EPMA) measurements. To further characterize thephysical properties of our single crystals, the temperaturedependence of magnetization was measured.
II. EXPERIMENTAL METHODS
A. Preliminary investigation

An AuPb flux was chosen for the single-crystal growthof FeRh for several reasons. First, it was noted that, during
the crystal growth of RhPb2 [38,39], the Fe crucible reacts
with the RhPb melt, making Pb a suitable choice as a flux.
Second, from the rich Pb part at a higher temperature of theternary phase diagram Au-Pb-Rh, AuPb4 Rh5 crystals tend togrow at the surface of the boule, making the extraction of thecrystals easier [40]. Third, the solubility of Fe in the Au canreach up to 74 at. % at 1173 ◦ C [41], which could promote thecrystal growth of FeRh. Last, the melting temperature of the
AuPb alloy can be as low as 212 ◦ C for a Pb concentration of
85 at. % [42], clearly indicating that the melting temperatureof FeRh could be lowered well below the order-disorder transition, avoiding the formation of the γ phase. Raw materialsof Au and Pb were 99.9% purity in shots, while Rh and Fe
were 99.9% purity in powder form (300 mesh), purchasedfrom Furuuchi Chemical Co, Japan.
Before the crystal growth, as a preliminary test procedure,a corresponding amount of the material was melted in an
evacuated quartz tube by the flame torch to observe the distribution of Fe inside the boule and to estimate the melting
temperatures. It was noted, for a higher concentration of Fe
than Rh, a higher degree of crystal precipitation was observedat the surface of the boule. After the flame torch treatment,
for all melting experiments, the melted ingot was put intoanother evacuated quartz tube that was heat treated at various
temperature profiles depending on the initial composition.
Due to the possibility of precipitation of FeRh crystals at
the surface of the boule, an infrared mirror furnace [38,39]
was used for the preliminary melting experiment because thesurface of the boule during the growth can be monitored indetail with a camera. The initial composition of the preliminary melting experiment was Au : Fe : Pb : Rh = 2:2:4:1. An
additional reason why Fe : Rh = 2:1 was chosen is to see if
it is possible to grow completely ordered FeRh crystals in
the Fe-rich side of the phase diagram. As expected, at higher
temperatures, the precipitation of the edge of a cube from themelt was observed. As the temperature decreased, the cube
dissolved. The crystal that was formed at the upper part ofthe boule at a higher temperature was probably an Fe-richcompound since the upper part of the boule was magnetic.
At the lower part of the boule, after the melting experiment,
the formation of AuPb2 Rh2 was observed.
B. Compositions investigated and temperature profiles

The preliminary melting experiment provided crucial information: For the investigated ratio, at higher temperatures,
the Fe compound, possibly FeRh, forms, and as the temperature is decreased, the formation of AuPb2 Rh2 becomes
more favorable. Thus, to avoid the formation of AuPb2 Rh2 forthe 2:2:4:1 ratio, the growth temperature should be >800 °C,
the approximate melting temperature of AuPb2 Rh2 [40]. In
addition, FeRh : AuPb2 = 1:x should be explored as well sinceuseful information could be obtained and the formation of
AuPb2 Rh2 could be suppressed. In Table I, the investigated

TABLE I. The composition of the melting experiments.
Molar ratios
Melting experiment

Au

Fe

Pb

Rh

Batch

Quartz tube placement

A
B
C

2
2
4

2
1
1

4
4
8

1
1
1

A
B1 , B 2
C1 , C 2

Vertically
Horizontally
Horizontally

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SINGLE CRYSTAL GROWTH OF FeRh FROM AuPb FLUXPHYSICAL REVIEW MATERIALS 8, 023401 (2024)

FIG. 1. The temperature profiles for the different batches. FC
and Q stand for the furnace cooling and quenching processes,
respectively.

compositions are summarized. The prepared quartz tubes
were put in a regular muffle furnace with a programabletemperature profile. As shown later, the growth is seriously
affected by how the quartz tube is placed in the furnace.
In Fig. 1, the temperature profiles for A, B, and C melting
experiments are shown.
III. RESULTS
A. Au:Fe:Pb:Rh = 2:2:4:1 ratio

The results of batch A are shown in Fig. 2. The grown boule
contained three parts. The upper part, which was rich in Fe;

the lower part, mostly AuPb2 flux; and the bridgelike structure
connecting the upper part of the boule with the lower one. A
possible reason for this kind of boule structure may be that theevaporated Pb could not pass through the Fe-rich part of theboule, resulting in the separation of the upper part of the boulefrom the lower as the pressure of the Pb vapor increased.
The surface of the upper part of the boule contained a
mixture of hexagonal AuPb2 Rh2 crystals and FeRh. Going
deeper inside the upper part of the boule, the formation of
FeRh becomes more favorable. Due to the high density of nucleation, the intergrowth of FeRh crystals frequently occurs,
resulting in interesting shapes and morphology [Fig. 3(a)].
Moreover, in some cases, the layer growth of the FeRh crystal,with clear terraces and edges, can be seen by scanning electron
microscopy (SEM) [Fig. 3(b)]. The size of the crystals here
varies from 10 to 50 µm. It is interesting to note that, if thedimensions of the crystal are in this range, it is easy to separate
them by applying a small amount of force, while the opposite
is true for crystals having a much smaller size.
In the middle part of the boule in Fig. 2, at its surface
as well as inside, crystal formation is observed. Since
the FeRh : AuPb ratio is smaller, the nucleation density
is also smaller, resulting in cubiclike crystal habits with
few intergrowths happening. At the surface, triangular andplatelike crystals could be observed. In this part, mostly FeRh

FIG. 2. Results of batch A. (a) The grown boule inside the quartz tube is shown. The color code represents the image of the Fe concentration[red means higher Fe concentration while blue means lower Fe concentration based on electron probe microanalyzer (EPMA) measurements].
In addition to the picture of the whole boule, the representative parts of the boule are shown: (b)–(d) are obtained from the upper part, (e)–(g)are obtained from the middle part, and (h)–(j) are obtained from the lower parts of the boule. As the Fe concentration decreases, the crystal
habit becomes clearer due to the lower density of nucleation. The conglomeration growth (c) was often observed in the upper part of the boule,
while crystals with clear shiny faces as seen in (e) and (h) in the rich AuPb flux part can be observed. Those crystals tend to precipitate to thesurface as expected [40]. In the second column of the pictures, the EPMA composition images are shown, where the brighter parts correspondto AuPb flux, while the darker parts correspond to FeRh. The composition of FeRh is very close to the 1:1 ratio within the accuracy of ±1
at. %. In the far-right column, the extracted crystals are shown. The extraction of those crystals is relatively simple. The boule is immersed indiluted nitric acid (10%). Diluted nitric acid softens the AuPb flux from where, either mechanically or by vibration, the FeRh crystals can beseparated.
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FIG. 3. Many small single crystals grown together at the upper part of the boule that is shown in Fig. 2. (a) The congregation of FeRh
crystals. (b) Clear edges, steps, and terraces of FeRh crystals can be observed. The composition investigated by electron probe microanalyzer(EPMA) was Fe : Rh = 1:1 within an experimental error of ±1 at. %.

crystals grow. The lower part of the boule contained only a
few FeRh crystals with no intergrowth. The crystals shown in
Fig. 2 tend to grow on the surface and have a platelike habit.
Like the upper part of the boule, the growth of AuPb2 Rh2crystals with a hexagonal crystal habit was observed. It can beobserved that AuPb flux plays an important role in the crystal
formation of FeRh.
B. Au:Fe:Pb:Rh = 2:1:4:1 and Au:Fe:Pb:Rh = 4:1:8:1 ratios

The results of batch B2 are shown in Fig. 4. Comparedwith batch A, the quartz tube was placed horizontally, as
shown in Fig. 4(a). One part of the surface of the boule isshown in Fig. 4(b). The precipitation, bulk, and platelike andneedlelike crystals were observed. In Fig. 4(c), some of thecrystals extracted from the surface part of the boule are shown.
As before, the crystals have been extracted from the flux byusing diluted nitric acid. Due to the proximity of the growing
crystals to each other, seldom intergrowth is observed. In
Fig. 4(d), the needlelike crystals up to 1.5 mm in length are
shown. Those crystals were extracted from the middle part ofthe boule. In Fig. 4(e), the extracted crystals from the bottom
part of the boule are shown. The intergrowth compared withthe single-crystal growth becomes more favorable.
Batches B1 and C2 yielded crystals with clear crystal habit,
mostly platelike and cubiclike, forming at the surface of theboule, but the size is significantly different from the previous
batches. Instead, crystal sizes with dimensions of 50 × 50 ×
50 µm for cubic shaped ones and 100 × 100 × 10 µm forplatelike ones were found. Batch C1 did not yield any FeRh
crystals except the ones formed due to the quenching process,as shown in Fig. 4(b).
C. EPMA and XRD analysis

The quality of the single crystal was evaluated by x-ray
diffraction, and the results are shown in Fig. 5. The x-raytransmission Laue diffraction pattern of the single crystal isshown in Fig. 5(a), and the backscattering electron imageof the single crystal with a thin rectangular shape of 300
× 200 × 20 µm in size is shown in Fig. 5(b), on which

the diffraction experiment was done. Figure 5(c) shows theω-scan rocking curve of the 110 Bragg reflection measured
using a four-circle diffractometer (AFC-7R, Rigaku with Mo
Kα radiation). The measured crystal is cubic in shape withdimensions of 25 × 25 × 25 µm. This crystal was extracted
from batch A, from the middle part of the boule. The peak
profile was well fitted with a Lorentz function, and the full
width at half maximum (FWHM) was 0.26 °. The FWHM
is almost the same as the resolution width, indicating thatthe quality of the sample is quite high. With this sample,

FIG. 4. The results of batch B2 . (a) The grown boule after themelting experiment. (b) One part of the surface of the boule. FeRh
crystals with flat surfaces are clearly observed. Small shiny dots
are found on the surface of FeRh crystals that have formed due toquenching. (c) Some of the FeRh crystals extracted from the surfaceof the boule. The surface of crystals is degraded if it is exposed tonitric acid for a longer time. (d) Needlelike crystals extracted fromthe inner part of the boule. (e) One of the hopper crystals of FeRhthat usually forms at the bottom part of the boule. The underlying
grid in the picture represents a 1 mm scale.

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measurement spot was a flat surface without AuPb flux andcarbon tape residue. The average values are ∼50% for both
Fe and Rh concentrations.
D. Magnetization

FIG. 5. The results of x-ray diffraction (XRD) analysis. (a) A
typical photograph of the transmission Laue diffraction pattern obtained from the crystal shown in (b) using a W target (40 kV and
20 mA). The collimated x-ray beam was shone perpendicularly tothe platelike single crystal of FeRh, and the imaging plate was put
behind the single crystal ∼5 cm apart from the crystal. (c) The ω
scan rocking curve of the 110 reflection.

structural analysis has been done with 281 reflections whose
lattice spacing was d < 0.625 A. The result confirmed that the
crystal structure is in a completely ordered phase of the CsCl
type with the lattice parameter being a = 2.9891(7)Å. The
value coincides with the reported value for the AFM phase of
the Fe-Rh alloy [32]. The Laue pattern certainly has a fourfold
symmetry, suggesting that the flat plane of the single crystal isthe (100) plane. The crystal has a well-developed rectangularshape with shiny surfaces, which intersect each other at a rightangle, as expected from cubic symmetry. This crystal is thesame one shown in Fig. 2(j). The data reduction yielded Rint =0.071, indicating that the absorption correction was adequate.
In Table II, the results of the EPMA analysis are given,and in Fig. 6, the SEM images of the measured samples areshown. The acceleration voltage was 20 keV, and the beam
spot diameter was as small as 1 µm. The averaged values ofFe and Rh concentrations are shown along with the calculated
standard deviation. The measured spots were chosen carefully
and measured repeatedly 10–14 times, and the results wereaveraged to obtain the most reliable data. The criterion for the

In Fig. 7, the temperature dependence of magnetization
measured from 10 to 400 K in 1 T magnetic field applied
to the [1,0,0] direction is shown. The measured sample issample No. 1 listed in Table II. First, the temperature was
set to 10 K, and then a magnetic field of 1 T was applied.
As shown in Fig. 7, the temperature dependence shows two
anomalous kink behaviors. The minimum is reached ∼130 K,
while the change of the slope can be found at 284 K [fieldcooling (FC)] and 301 K [field warming (FW)] within theso-called AFM phase. Furthermore, it is interesting to pointout that significant temperature hysteresis can be observed ina wide temperature range. The saturation magnetization above
TFM↔AFM is 178 emu/g, which corresponds to 5.04 μB per
FeRh molecule, while at lower temperatures, it is 66 emu/g,
corresponding to 1.88 μB . All such anomalous magnetization
behaviors cannot be explained by the simple AFM orderedstate as previously considered [6,7] but may suggest the consecutive occurrence of at least two magnetic phases.
IV. DISCUSSION

The results of batch A might seem chaotic, but several
important things were noticed. First, obvious phase separation
happens. Fe tends to migrate to the surface of the boule, modifying the local composition. Second, the surface of the boule
plays an important role in the formation of the clear crystal
habit of FeRh with no intergrowth in contrast with the crystals
extracted from the inner and upper parts of the boule. Moreover, the FeRh : AuPb flux ratio affects the crystal growth aswell. This becomes obvious by comparing the surfaces of theupper, middle, and lower parts of the boule. On the surface
lower part of the boule, only one FeRh single crystal can beobserved, which is an ideal condition for growing big-sized
crystals. Third, even though the ratio of Fe : Rh at the upper
part of the boule is at least two times bigger, the formationof Fe-doped FeRh crystals is not observed. This observationstrongly suggests that the ordered FeRh phase line of Fe : Rh
= 1:1 may exist in the ternary phase diagram. The line might
have also existed in the binary phase diagram, but due to thelow diffusion of the solid, the separation may not occur.
To reproduce the crystal growth condition that occurred on
the surface of the lower part of the boule, the quartz tube
was placed horizontally (B and C melting experiments), thesame amounts of Fe and Rh were used, and the amount of

TABLE II. The results of EPMA measurement of FeRh single crystals are shown in Fig. 6. The number of measured points (N), the size,
and the batch from where the crystals were taken are shown.
Sample No.
1
2
3
4

Fe

Rh

Size

N

Batch

50.1 ± 0.8%
50.4 ± 1.3%
50.9 ± 1.1%
49.6 ± 1.4%

49.9 ± 0.9%
49.6 ± 1.1%
49.1 ± 1.1%
50.4 ± 1.3%

100 × 100 × 1500 µm
120 × 120 × 700 µm
400 × 400 × 400 µm
200 × 300 × 10 µm

10
14
13
12

B2
B2
B2
A

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FIG. 6. The scanning electron microscopy (SEM) images of single crystals of FeRh on which electron probe microanalyzer (EPMA)
measurements were done. Clear crystal habits with sharp edges can be observed. The scale in the photo (white bar shown in the lower part ofthe picture) represents 100 µm.

the AuPb2 flux was increased in the case of the C melting
experiment. The results, as shown, are much improved fromthe crystal growth point of view. Needlelike, platelike, andbulklike crystals could be observed, being larger in size thanthe previous melting experiment A. This is concrete evidence
that, for high-quality and bigger size crystal growth, the larger
surface of the boule is crucial, which might be a unique feature

FIG. 7. Temperature dependence of magnetization M(T) of FeRhin 1 T magnetic field applied to [1,0,0] direction between 10 and 400
K. The arrows denoted as FW and FC mean that the curves were obtained in the warming process and the cooling process, respectively.

of AuPb flux [40]. Another important point is that the phaseseparation is significantly reduced, allowing crystals to growin all regions of the grown boule.
Combining the results of the melting experiments, a roughestimate can be made at which FeRh regions can form in
the quaternary phase diagram of Au-Fe-Rh-Pb. The melting
temperature of FeRh for 2:1:4:1 and 4:1:8:1 ratios are ∼850
and 580 °C, respectively, since only small cubiclike crystals
have been grown. Also, for batch B2 , FeRh crystals form due
to quenching. Accordingly, the ratios between 2:1:4:1 and
4:1:8:1 should also result in FeRh formation. Therefore, it
can be concluded that FeRh can be grown in a surprisingly
wide temperature range and AuPb flux concentration. This is
the starting point of further investigation, i.e., optimization of
FeRh crystal growth. Regarding the Au-rich part of the phase
diagram, one can ask if FeRh crystals or maybe some newcompound forms. For example, it was noted from the 4:1:8:1
ratio that, at lower temperatures, Au-doped RhPb4 crystals
form.
Another intriguing question to ask is to what extent Au
has a dominant role in the formation of the single-phase,single-crystal FeRh. Since the phase diagrams of the binary
compounds FeX, with X = Ni, Ir, Pd, Pt, etc., are surprisingly
very similar to the case of FeRh, the answer to this question

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can be obtained rather directly by growing FeX compounds
and investigating whether FeX compounds can be formed outof Au-Pb flux or not. As can be seen, by EPMA measurements, Au was not detected in our single crystals of FeRh
within our detection level of 1 at. %, whereas Au-doping in
FeRh up to several at. % of Au has been reported to strongly
suppress and decrease TFM↔AFM [7,43,44]. These latter results
certainly contradict our results. However, to compromise andsolve this inconsistency, it is possible to think that only thedoping of the γ phase with Au may occur but not in theα phase since our sample is a pure single crystal with theα phase only. This could explain the absence of Au in our
samples.
The preliminary magnetization study has been carriedout by using a SQUID magnetometer (Quantum Design
MMPMS) on a few single crystals obtained from different
batches grown in different ampoules. All the results are consistent and in very good agreement, but they are very different
from the ones reported previously [6,7], as was pointed out
earlier in Fig. 7 as an example of apparently different behaviors. It may not be a surprise since our sample does notcontain the γ phase, which on the contrary remained in thesamples used in the previous works as the primary impurity
phase, which seems to be extremely difficult to remove by
thermodynamical procedures only.
The most striking difference in the magnetization behavior
shown in Fig. 7 is the temperature dependence of magnetization: It exhibits strange two-kink behaviors at TAF1 = 130 K
and TAF2 = 302 K for the heating process, while TAF1 smears
out and TAF2 = 284 K for the cooling process. The temperature dependence of magnetization strongly indicates theexistence of some additional magnetic phase transitions in
the AFM phase, suggesting nontrivial complicated orderedstructures. Although the detailed mechanism of such complicated magnetic phases remains unknown and needs deeper
study in both experimental and theoretical works to fully
understand them, it may be worthwhile pointing out that
the Rh moment may play an important role, as discussed
in the mechanism of the AFM ↔ FM phase transition, where
the presence or absence of the magnetic moment of Rh in
the AFM phase is crucial. If Rh carries a magnetic moment,
then the AFM structure cannot be simple, resulting in the
more complicated structure being under ongoing debate [45].
Finally, we would like to comment on the possible explanation
for why our results are so different from previous ones. The
reason may be due to the pressure effect which stabilizes the
AFM phase [46] through the remaining γ phase [33] as animpurity phase with different lattice parameters. This can beeasily verified by measuring the magnetization of our single
crystals of FeRh under pressure. The temperature dependenceof magnetization shown in Fig. 7 indicates a kink at ∼130 K
and increases as the temperature decreases and finally saturates at low temperatures. This discovery strongly suggests
that, <130 K, a new magnetic phase becomes stable. Thisshould be confirmed further by other experimental techniquessuch as neutron diffraction. Growing crystals that are suitable

for neutron diffraction is underway. It is certainly a great challenge since hopper crystal growth was observed [Fig. 4(e)].
Another intriguing point is the magnetic saturation moment
value, which corresponds to 5 μB /f.u. of FeRh, indicating aconsiderably larger value than all values reported previously[6,7], except in Ref. [47]. All these facts cast serious intriguingquestions about how the phase transformation TFM↔AFM canoccur, what sort of magnetically ordered phases emerge in the
single-phase single crystals, and what kind of mechanism isdriving them. Considering the hypersensitivity of the physicalproperties of FeRh to doping and impurities, all kinds ofmeasurements must be redone to establish intrinsic propertiesof a unique and rare system of FeRh using newly grown single
crystals, which is currently in progress.
V. CONCLUSIONS

Single crystals of a famous and peculiar FeRh compound
have been successfully grown from the unique AuPb flux.
The method is surprisingly simple and elegant. Furthermore,
the results are very transparent and reproducible, as shown
above. The choice of the AuPb flux may be unorthodox, but
the obtained single crystals are indeed pure and single-phasesingle crystals without any impurities within the limit of thedetection level, as confirmed by x-ray single-crystal analysis as well as EPMA measurements. This widely opens thepossibility to thoroughly reinvestigate the properties of FeRh,
whose measurements have been done on unexpectedly impure
materials in previous studies, and to deeply reconsider thefundamental properties of pure FeRh, especially on the mechanism of the AFM ↔ FM phase transition as well as the newly
emerging complicated magnetic phases. As shown in Fig. 7
in magnetization measurements as an example, the results
are strikingly different from the previous ones. More detailed
results on magnetic, transport, and thermodynamical measurements exhibiting unique and unprecedented properties are in
progress and will be reported separately. Furthermore, oursingle crystals will provide an excellent opportunity to explore how physical, chemical, and metallurgical properties areaffected by impurities, foreign atoms, vacancies, etc. Finally,
AuPb flux might be useful for growing single crystals fromother systems similar to Fe-Rh, such as FePt, FePd, FeIr,
FeCo, FeNi, FeCr, and FeV, which is likely to spark further
research.
ACKNOWLEDGMENTS

The corresponding author would like to thank Abdou Hassanien and Kaveh Delfanazari for their kind support anduseful suggestions. Also, the corresponding author would liketo thank Ministry of Education, Culture, Sports, Science andTechnology (MEXT) for the financial support as a MEXT
scholarship student. This paper is supported by the CROSSlab and is part of a research proposal: Magnetic and crystal structure analysis of FeRh single crystals, Proposal No.
2023B0151.

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NIKOLA SUBOTIĆ et al.

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