Room temperature spin-polarizations of Mn-based antiferromagnetic nanoelectrodes
Toyo Kazu Yamada and Amadeo L. Vazquez de Parga
Citation: Applied Physics Letters 105, 183109 (2014); doi: 10.1063/1.4901047
View online: http://dx.doi.org/10.1063/1.4901047
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APPLIED PHYSICS LETTERS 105, 183109 (2014)

Room temperature spin-polarizations of Mn-based antiferromagnetic
nanoelectrodes
Toyo Kazu Yamada1,a) and Amadeo L. Vazquez de Parga2
1

Graduate School of Advanced Integration Science, Chiba University, 1-33, Yayoi-cho, Inage-ku,
Chiba-shi 263-8522, Chiba, Japan
2
Instituto Madrile~no de Estudios Avanzados en Nanociencia IMDEA-Nanociencia and Dep. Fisica de laMateria Condensada, Universidad Aut
onoma de Madrid, Cantoblanco, 28049 Madrid, Spain(Received 31 August 2014; accepted 21 October 2014; published online 6 November 2014)
Antiferromagnets produce no stray field, and therefore, a tip electrode made of antiferromagnetic
material has been considered to be the most suitable choice to measure such as magnetoresistance(MR) through single isolated magnetic nanoparticles, molecules, and ultrathin films. Spinpolarizations (P) of antiferromagnetic 3-nm, 6-nm, and annealed 3-nm Mn films grown on W tips
with a bcc(110) apex as well as bulk-NiMn tips were obtained at 300 K by measuring MR in ultrahigh vacuum by means of spin-polarized scanning tunneling microscopy using a layerwise antiferromagnetically stacking bct-Mn(001) film electrode. The Mn-coated tips with coverages of 3 and 6 nm
exhibited P values of 1 6 1% and 3 6 2%, respectively, which tips likely contain a- or strained Mn.
With a thermal assist, the crystalline quality and the magnetic stability of the film could increase. The
annealed tip exhibited P ¼ 9 6 2%. The bulk-NiMn tips exhibit spin polarizations of 0 or 6 6 2%
probably depending on the chemical species (Mn or Ni) present at the apex of the tip. Fe-coated W
C 2014 Author(s). All article
tips were used to estimate the bct-Mn(001) film spin polarization. V
content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0
Unported License. [http://dx.doi.org/10.1063/1.4901047]

The miniaturization of electronic devices has been considered to be a key solution to realize novel nanodevices
with low energy consumption, low cost, and high performance to cater to the increasing demands of data storage andretrieval. Unique electron-spin properties have been
observed in exotic materials at atomically small-scale, yielding applications such as electric-field control spintronics
with ultrathin magnetic films,1–3 1-nm-size single organic
molecular magnetoresistance sensors,4–6 and graphene-based
spintronics.7
Scanning tunneling microscopy (STM) is considered as a
powerful tool to precisely measure spin information, e.g.,
spin-polarization or magnetoresistance (MR), of 1-nm-size
nanomagnets or atomically thin magnetic films.3–5,8 Figure1(a) shows a sketch of STM magnetic junctions, in whichelectrode (1) is the magnetic tip and electrode (2) is the magnetic sample. Ferromagnetic (FM) Fe-coated W tips havebeen used only for studying nanomagnets with higher
anisotropies (e.g., Fe thin-films on W(110)9 and Co bilayer
islands on Cu(111)4,10) or antiferromagnetic (AFM) samples(e.g., Mn(001) ultrathin films11–14 and Cr(001) surfaces15).
However, if the magnetic nanostructure under study has asmall magnetic anisotropy, the spin structure may be easily
affected by external magnetic fields,16 and therefore, the useof FM tips should be avoided because they produce a considerable stray field.17,18 As opposed to ferromagnets, antiferromagnets exhibit no stray field, and therefore, a tip made of
AFM material has been considered to be the best choice toimage single isolated magnetic nanoparticles or single magnetic molecules. Tips made of AFM material have alreadya)

Email: toyoyamada@faculty.chiba-u.jp. Tel.: 81-432903915.

0003-6951/2014/105(18)/183109/5

been fabricated and used in STM experiments in recent years,
e.g., bulk-Cr,19,20 Cr-coated,18,21,22 and Mn-coated W tips.23
The fabrication techniques of Cr-based AFM tips have been

FIG. 1. (a) STM magnetic junctions. The STM tip probe detects spinpolarization as well as the magnetoresistance (spin-polarized conductance)
through the junction. A ferromagnetic (FM) tip has a stray field, but an antiferromagnetic (AFM) tip has no stray field. (b) A simple model of electron
spin tunneling between two magnets through a spacer gap. Grey arrows
denote magnetization vectors. Black arrows indicate up and down spins.
Here, we assume that 3 up-spins exist for the electrode (1) and 3 up- and 1
down-spins exist for the electrode (2) at the Fermi energy.

105, 183109-1

C Author(s) 2014
V

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183109-2

T. K. Yamada and A. L. Vazquez de Parga

investigated in detail. In one such study, the spin polarization of
the apex of the Cr/W tip was measured to be 10% at 300 K.22
However, the MR as well as spin polarization of Mn-based
AFM tip electrodes has not been examined in detail.
In this study, quantitative 300-K spin polarization valuesof Mn-based AFM tip electrodes were obtained by measuring MR values with a spin-polarized STM setup in ultrahigh vacuum (UHV). Mn-coated W and bulk-NiMn tips
were used in our experiments. Fe-coated W tips11,12,25 wereused to estimate the sample spin polarization.
Thin bct-Mn(001) films epitaxially grown on Fe(001)whiskers with Curie temperatures higher than 300 K wereused as magnetic samples.24 A clean and atomically flat bccFe(001) surface was obtained on single crystal Fe whiskers
via several cycles of Arþ sputtering and annealing up to
870 K. Mn(001) ultrathin films were grown on clean Fe(001)surfaces whose temperatures were maintained at 370 K during the Mn deposition.
Mn films grown on the Fe(001) thicker than four layers
have an equivalent bct structure with in-plane and out-ofplain atomic lattice distances of 0.287 and 0.165 nm, respectively (c/a ¼ 1.15).24 Although intermixing was observed up
to the third layer, the films thicker than four layers consist of
only pure Mn atoms. Also, each Mn layers above the fourthlayer have the same local density of states.24 Concentrationof impurities on the Mn(001) surface was less than 1%.
Since the contrast in the STM spectroscopy maps may haveits origin in different local density of states, different chemical species, and different spin polarizations, the bct-Mn(001)films thicker than four layers are considered to be one of thetest samples to check spin contrast.11–14 In this study, weused fifth (“5”) and sixth (“6”) layers of the Mn films tocheck the spin contrast.
The W tips were electrochemically etched from W wires(diameter 0.3 mm, purity 99.9%) in air using aqueous KOH,
and subsequently, the tip was rinsed with hot water andtransferred into the load lock chamber of our STM setup.
Once in UHV, the apex of the W tip was sputtered with Neþ
in UHV and annealed at temperatures up to 2000 K to obtaina clean W apex. Fe-coated W tips were prepared by depositing 5-nm Fe on a clean W tip with a radius of 10–30 nm at
300 K in UHV following the method described in Ref. 25.
Mn-coated W tips were prepared by depositing Mn on aclean and blunt W tip in UHV at 300 K. The post-annealing
process of the Mn-coated W tip involved contacting a heaterfilament (10 W) for 5 min at a point at 3 mm below the tip
apex. NiMn tips were electrochemically etched (4 V, 20 mA)
from NiMn polycrystalline rods (diameter 0.5 mm) using
0.5M HCl aq.
STM and scanning tunneling spectroscopy (STS) experiments were performed at 300 K in UHV with a base pressureof <8  109 Pa. The STS measurements were performed bydisconnecting the feedback loop, i.e., fixing the tip-sample separation (z) and measuring the tunneling current at each pixel ofthe corresponding topographic image by varying the sample
voltage around the Fermi level.26 The differential conductivity(dI/dV) curves were obtained by numerical differentiation of
the corresponding I(V) curves. Magnetoresistance (MR) measurements were obtained as discussed below. Figure 1(b) shows
an example of the electron spin transport through a gap

Appl. Phys. Lett. 105, 183109 (2014)

between two magnets. Grey arrows indicate magnetizations.
Black arrows indicate up and down spins. Here, we assume
that 3 up-spins exist for the electrode (1) and 3 up- and 1
down-spins exist for the electrode (2) at the Fermi energy, i.e.,electrode (1) exhibits only a minority spin peak at the Fermi
energy, thereby indicating 100% spin polarization. In the caseof parallel coupling, the electron spin transfer probability
through the gap is 3  3 þ 1  0 ¼ 9. On the other hand, when
the coupling is antiparallel, the minority and majority spinstates of electrode (2) exhibit a reversal, and the transfer probability is consequently 3  1 þ 0  3 ¼ 3, i.e., the conductance(G) depends on the type of magnetic coupling. In the case of27
1
1
Fig. 1(b), we obtained an MR of ðG1"#  G"" Þ=G"# ¼ 67%,
wherein the spin polarizations of electrodes (1) and (2) were100% and 50%, respectively.
First, we obtained the spin-resolved dI/dV maps with the
W tip covered by a 5-nm Fe film. Figure 2(a) shows a crosssectional model of the Mn(001) films, which were grown on
an Fe(001) whisker substrate at 370 K. Due to the growthmode, several Mn layers are exposed at the surface, as can beseen in the STM topographic image shown in Fig. 2(b). Figure2(c) shows the spectroscopic dI/dV map measured simultaneously with the topographic image shown in panel (b) with an
Fe-covered W tip. The black and white contrast between adjacent Mn terraces is due to the AFM layered structure, and thecontrast reflects the spin-dependent electron tunneling probability between the magnetic tip and sample, as shown in Fig.
1(b), i.e., the contrast in the dI/dV map indicates that the tip
electrode has a Curie temperature that is higher than 300 K andalso that the MR as well as spin polarization are not zero at
300 K.26,28 From this contrast, we determine that the 5-nm Fe
film on the W tip can be used to measure MR at 300 K.
Figures 3(a)–3(d) show topographic images and thecorresponding spin-resolved dI/dV maps obtained with

FIG. 2. (a) Model of a spin-polarized STM setup at the surface of layerwise
antiferromagnetic stacking bct-Mn(001) layers grown on Fe(001) whisker
with a FM tip. Arrows indicate directions of spin polarization vectors.
Numbers denote the stacking number of the Mn monolayers at each position.
Interlayer spacing of the Mn films thicker than fourth layer is 0.165 nm.24 A
hidden Fe step was also observed on the surface of Mn films as a 0.02 nm
step,14 where spin polarization vectors rotate 180 within 0.5 nm. (b) and (c)An STM image (Vs ¼ 0.5 V, I ¼ 0.5 nA, 99  99 nm2) and an STS image(a dI/dV map at þ0.1 V), respectively, which were obtained with an Fe-coated
W tip positioned on the surface of the Mn films on the Fe(001) whisker.
Numbers “5,” “6,” and “7” denote the local thickness in number of Mn layers
at each position. Step heights are about 0.165 nm. Even and odd layers show
dI/dV values of about 1.30 and 1.05 nA/V, respectively, (MR ¼ 24%).

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183109-3

T. K. Yamada and A. L. Vazquez de Parga

FIG. 3. (a)–(d) Spin-polarized STM images and spectroscopy maps obtainedon a 5.5 ML Mn film grown on an Fe(001) with AFM Mn-coated W tips andbulk-MnNi tips, where fifth (“5”) and sixth (“6”) layers are exposed on thesurface. Numbers denote the local thickness in number of Mn layers. (a) A
topographic image (30  9.5 nm2, VS ¼ 0.5 V, I ¼ 0.5 nA) and a spinresolved dI/dV map at þ0.2 V obtained from the same area with a 3-nm Mn/W
tip. At the middle, a hidden Fe step is observed (step height 0.02 nm). dI/dV
values obtained from the fifth and sixth layers are about 0.88 and 0.90 nA/V,
respectively (MR ¼ 2%). (b) A topographic image (120  60 nm2, VS ¼ 0.5 V,
I ¼ 0.5 nA) and a spin-resolved dI/dV map at þ0.2 V obtained from the same
area with a 6-nm Mn/W tip. Fifth and sixth layers are observed as a terrace
and islands, respectively. dI/dV values obtained from the fifth and sixth
layers are about 2.18 and 2.13 nA/V, respectively (MR ¼ 2%). (c) A topographic image (104  36 nm2, VS ¼ 0.5 V, I ¼ 0.5 nA) and a spin-resolved
dI/dV map at þ0.2 V obtained from the same area with an annealed 3-nm
Mn/W tip. Fifth and sixth layers are observed as a terrace and islands,
respectively. dI/dV values obtained from the fifth and sixth layers are about
1.23 and 1.29 nA/V, respectively (MR ¼ 5%). (d) A topographic image(40  16 nm2, VS ¼ 0.5 V, I ¼ 0.5 nA) and a spin-resolved dI/dV map at
þ0.2 V obtained from the same area with a MnNi tip. dI/dV values obtained
from the fifth and sixth layers are about 1.74 and 1.79 nA/V, respectively(MR ¼ 3%). (e) Experimentally obtained MR values as a function of thesample bias. The value of 0 eV corresponds to the Fermi level. Data
were obtained using 5-nm Fe- (dark blue), 3-nm Mn- (white), 6-nm Mn(yellow), annealed 3-nm Mn-coated W tips (light blue), and the bulk-NiMn
tip (red).

(a) 3-nm, (b) 6-nm, (c) annealed 3-nm Mn-coated W, and (d)bulk NiMn tips, wherein two terraces with thicknessesof five and six Mn monolayers (ML) are observed. We

Appl. Phys. Lett. 105, 183109 (2014)

performed measurements with about three to five different
tips for each case. In the spin-resolved dI/dV map obtained
from this area, we can clearly observe the black-white contrast between the 5- and 6-ML sections, as in the case of the
Fe-coated W tip (Fig. 2). Although with all type of tips, we
get magnetic contrast, the amplitude changes from tip to tip,and in Figure 3 we reproduced the maps with higher magnetic contrast for each type of tip. These spin-resolved dI/dV
maps clearly show that these AFM tip electrodes are able todetect MR at 300 K.
Quantitatively, the MR value of each tip was obtainedby examining the differential conductivity curves. Figure3(e) shows the obtained MR values as a function of bias voltage, and the corresponding values are listed in Table I.31 The
5-nm Fe film electrode exhibits an MR of 23 6 1%. The primary point of interest is that the MR is considerably smallerin the AFM tip electrodes in comparison with that in the Fecoated tip. In particular, the Mn-coated tip with a coverageof 3 nm exhibits an extremely low MR value of 1 6 1%. The
MR slightly increases to 2 6 1% when the thickness of theMn film increases to 6 nm. These low values make these tips
unsuitable for spin-resolved measurements due to the low
signal-to-noise ratio.
The crystallographic ordering of Mn has the tendency of
imitating the ordering of the substrate atomic structure. For
example, well-ordered bcc(001) Mn films can be formed on
the substrate of bcc-Fe(001) with thicknesses of more than
10 ML;24 further Mn films grown uniformly on fcc-Ag(111)exhibit a hexagonal lattice symmetry.29 It is well known thatthe apex of W tips heated at high temperature in UHV
recrystallizes presenting the close-packed (110) face,25 andtherefore, the magnetic structures of Mn on the W tip may be
examined in the light of studies of Mn on W(110). The Neeltemperatures of bulk a-Mn and b-Mn are considerably lessthan 300 K, while fcc c-Mn and bcc d-Mn exhibit Neel temperatures greater than 300 K. Mn layers grow pseudomorphically on a W(110) single crystal at 300 K,30 and highly
strained bcc d-Mn layers have been observed with thicknesses of up to 3 ML, and three-dimensional islands (a- ord-Mn) start to grow for thicker coverages, which indicatesthat gentle annealing assists the phase transition from a-Mn
or strained bcc-Mn to ordered bcc-Mn, thereby producing ahigher MR. From Fig. 3(e), we note that the MR of the 3-nm
Mn film increases to 6 6 1% after annealing.
Bulk NiMn alloy, which has a CuAu-I type facecentered tetragonal structure with lattice parameters of
a ¼ 3.714 Å and c ¼ 3.524 Å, is an antiferromagnet. Each Mn
atom has a large magnetic moment of approximately 4 lB
and is antiferromagnetically aligned orthogonal to the c-axis
TABLE I. Magnetoresistance (MR) through the junctions and spin polarizations of the tip electrodes (Ptip).
300-K MR junctions

MR (%)

Ptip (%)]

Mn(001)/vac./Fe film (5 nm)
Mn(001)/vac./Mn film (3 nm)
Mn(001)/vac./Mn film (6 nm)
Mn(001)/vac./annealed Mn film
Mn(001)/vac./bulk-NiMn

23 6 1
161
261
661
461

31(Ref. 25)
161
362
962
662

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183109-4

T. K. Yamada and A. L. Vazquez de Parga(i.e., either [001] or [110]), and the Ni atoms exhibit smaller
magnetic moments with values below 0.6 lB. These values
are good for atoms in the bulk of the alloy; the size anddirection of the magnetic moments as well as the spin polarization for surface atoms are unknown. For the MnNi tips,
we observed MR values of 4 6 1%, but we were unable tocontrol the choice of atomic species (Mn or Ni) present at
the apex of the tip, which leads to a very unstable MR, i.e.,the MR switches between 0% and 4 6 1%, with these low
values these tips are not a good choice for spin-polarizedmeasurements at RT.
The spin polarization values of these Mn-based AFM tipelectrodes can be obtained by using the experimentally
1obtained MR values via the expression MR ¼ ðG1"#  G"" Þ=27
1
G"# ¼ 2Psample Ptip =ð1 þ Psample Ptip Þ. Since the spin polarization of the 5-nm Fe film covering the W tip (PFe–tip ) is
known to be 31%,25 the spin polarization of the sample
Mn(001) films can be obtained by using the expression
Psample ¼ MRFeð5nmÞ =ðPFe–tip ð2MRFeð5nmÞ ÞÞ ¼ 4162%. Thus,
the spin polarization of the AFM tips can be obtained via theexpression PAFM–tips ¼ MRAFM–tips =ðPsample ð2MRAFM–tips ÞÞ.
The obtained spin polarizations of the 3- and 6-nm Mn film
tips are 161% and 362%, respectively. The annealed tip
has a spin polarization of 962%, which is comparable to thespin polarization of Cr-coated W tips, which is 10%.22
Further, we note that at 300K, AFM Cr- and Mn-coated W
tips have spin polarizations that are smaller by a factor of 4
to 5 with respect to that of the FM Fe-coated W tip. The
bulk-NiMn tips exhibit spin polarizations of 0% or 561%,
and the polarizations probably depend on the chemical species (Mn or Ni) present at the apex of the tip.
In this study, quantitative 300-K spin polarization valuesof Mn-based AFM tip electrodes were obtained by measuring MR with a spin-polarized STM setup in ultrahigh vacuum (UHV). Mn-coated W and bulk-NiMn tips were used in
the experiments. All the tips exhibit MR at 300 K, and theMR values of the W tips covered by Mn films with thicknesses of 3 and 6 nm exhibit MR values of only 1 6 1% and
2 6 1%; however, increasing the crystalline quality of theMn film by annealing increases the MR to 6 6 1%. We
obtained spin polarizations of 1 6 1%, 3 6 2%, and 9 6 2%
for 3-nm, 6-nm, and annealed 3-nm Mn-coated W tips,
respectively. The results demonstrate that the crystallographic quality of the films is crucial to obtain stable MR as
well as spin polarization at 300 K. The annealed tip has aspin polarization comparable to that the Cr-coated W tips,
which value is 10%.22 It is observed that at 300 K, AFM
Cr- and Mn-coated W tips exhibit spin polarizations that aresmaller by a factor of 4 to 5 with respect to that for the FM
Fe-coated W tip. Bulk-NiMn tips exhibit an MR of 4 6 1%
and spin polarization of 6 6 2%, but it can switch to 0%,
which is probably because the choice of chemical species at
the apex of the tip cannot be controlled.
Kind guidance for chemical vapor deposition for Fe
whiskers by Dr. D. T. Pierce is acknowledged. We thankProfessor Dr. I. V. Shvets and Dr. S. F. Ceballos for helpful
discussions for assistance in the preparations of MnNi tips.
Careful measurements were realized with the support ofProfessor Dr. H. van Kempen. We thank Dr. L. Gerhard andAppl. Phys. Lett. 105, 183109 (2014)

Mr. N. K. M. Nazriq for carefully reading our manuscript. This
work was supported by JSPS KAKENHI Grant Nos. 23681018
and 25110011, Japan Science and Technology Agency (JST)Improvement of Research Environment for Young Researchers:
Chiba University Young Research-Oriented Faculty Member
Development Program in Bioscience Areas, Chiba Univ. Global
COE Program: Advanced School for Organic Electronics, Casio
Science Promotion Foundation, Asahi Glass Foundation,
Hatakeyama Culture Foundation, The Association for the
Progress of New Chemistry, Iketani Science and Technology
Foundation, The Noguchi Institute, Ozawa Yoshikawa
Memorial Electronics Foundation, Research Foundation for theElectrotechnology of Chubu, The Nakajima Foundation, The
Futaba Electronics Memorial Foundation, and the Shimadzu
Foundation. Spanish government through the MICINN Project
FIS2012-18847 and the Comunidad the Madrid through Project
NANOBIOMAGNET S2009/MAT1726.
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Appl. Phys. Lett. 105, 183109 (2014)
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All data were obtained using the same bias voltage (Vs ¼ 0.5 V) and tunneling current (I ¼ 0.5 nA) to stabilize the distance between the tip andsample before disconnecting the feedback loop to acquire the dI/dV curves
in order to avoid the dependence of the magnetic contrast on the tipsample separation.11 Since the spin polarization values for Mn(001) are
well characterized and almost constant above the Fermi level,11 we measured magnetoresistance values between þ0.2 to þ0.4 eV.
30

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