Supplementary Information
Tetragonal phase of epitaxial room-temperature antiferromagnet
CuMnAs
P. Wadley,1, 2, ∗ V. Novák,1 R. P. Campion,2 C. Rinaldi,3, 1 X. Martı́,1, 4, 5 H. Reichlová,1, 4
J. Železný,1 J. Gazquez,6 M. A. Roldan,7, 8 M. Varela,7, 8 D. Khalyavin,9 S. Langridge,9
D. Kriegner,10 F. Máca,11 J. Mašek,11 R. Bertacco,3 V. Holý,4 A. W. Rushforth,2
K. W. Edmonds,2 B. L. Gallagher,2 C. T. Foxon,2 J. Wunderlich,1, 12 and T. Jungwirth1, 2
1
Institute of Physics ASCR, v.v.i., Cukrovarnická 10, 162 53 Praha 6, Czech Republic
2
School of Physics and Astronomy, University of Nottingham,
Nottingham NG7 2RD, United Kingdom
3
LNESS-Dipartimento di Fisica del Politecnico di Milano Via Anzani 42, 22100 Como, Italy
4
Faculty of Mathematics and Physics, Charles University in Prague,
Ke Karlovu 3, 121 16 Prague 2, Czech Republic
5
Department of Materials Science and Engineering,
University of California, Berkeley, California 94720, USA
6
Institut de Cincia de Materials de Barcelona, ICMAB-CSIC, Bellaterra, Spain
7
Departamento de Fsica Aplicada III,
Universidad Compluense de Madrid, Madrid, Spain
8
Materials Science & Technology Division,
Oak Ridge National Laboratory, USA
9
ISIS, Rutherford Appleton Laboratory,
Harwell Science and Innovation Campus,
Science and Technology Facilities Council,
Oxon. OX11 0QX, United Kingdom
10
Institute of Semiconductor and Solid State Physics,
University Linz, Altenbergerstr. 69, A-4040 Linz, Austria
11
Institute of Physics ASCR, v.v.i.,
Na Slovance 2, 182 21 Praha 8, Czech Republic
12
Hitachi Cambridge Laboratory, Cambridge CB3 0HE, United Kingdom
1
S.1.
SURVEY OF MAGNETIC COUNTERPARTS OF COMMON SEMICON-
DUCTOR COMPOUNDS
In Table S1 we show a survey of the magnetic counterparts of the most common II-VI
and III-V compound semiconductors, and of the related I-VI-III-VI and II-V-IV-V families,
in which Mn (Eu) acts as a group-II atom and Fe (Gd) as a group-III element. The table
illustrates that AFM ordering occurs much more frequently than FM ordering. Yet, only a
few of these AFM compounds have Néel temperatures above room temperature. In MnTe,
TN = 323 K is presumably still too low to allow for room-temperature applications of the
material in spintronics. MnSiN2 appears as an attractive candidate material which should
also allow for the application of common molecular beam epitaxy techniques for the synthesis
of high quality films. The natural mineral CuFeS2 is more challenging from the perspective
of the epitaxial growth because of the vastly different vapor pressures of S and transition
metals. Another limiting factor is that both MnSiN2 and CuFeS2 might be the only highTN AFMs in their respective semiconductor compound families. The search for other high
temperature AFM semiconductors has recently resulted in a report of the semiconducting band structure of alkali-metal I(a)-Mn-V compounds and of the successful synthesis of
single-crystal LiMnAs by molecular beam epitaxy.4 In contrast to the other common semiconductor compound families, many of the I(a)-Mn-V semiconductors are room-temperature
AFMs.21,23 While favorable from the perspective of their electronic band structure and magnetic characteristics, the utility of I(a)-Mn-V materials in devices may represent a challenge
due to the high reactivity and diffusivity of the I(a) alkali metal elements. In our work we
focus on the stable I(b)-Mn-V compounds with high Néel temperature, in particular on the
tetragonal CuMnAs epitaxial films.
2
II-VI
TC (K) TN (K)
III-V
TC (K) TN (K)
MnO
122
37
FeN
100 1
38
MnS
152
39
FeP
115
40
MnSe
173
37
FeAs
77
41
MnTe
323
37
FeSb
100-220
42
EuO
67
37
GdN
37
EuS
16
37
GdP
15
43
72
EuSe
5
37
GdAs
19
44
EuTe
10
37
GdSb
27
45
490
47
I-VI-III-VI
II-V-IV-V
CuFeO2
11
46
CuFeS2
825
37
CuFeSe2
70
48
CuFeTe2
254
49
MnSiN2
TABLE S1: Comparison of FM Curie temperatures (TC ) and AFM Néel temperatures (TN ) of
II-VI, I-VI-III-VI, III-V, and II-V-IV-V magnetic semiconductors.
S.2.
SQUID MEASUREMENT OF THE Fe CAPPED CuMnAs SAMPLES
In order to verify the origin of the observed shift in the hysteresis loops it is necessary to
exclude the influence of the residual flux in the superconducting magnet. The QD MPMS
XL7 magnetometer used has a residual field of <5 Oe, which is already significantly lower
than the observed shifts (∼30 Oe at room temperature). In addition to this a magnet reset
was performed at the measurement temperature before each measurement. This involves
a controlled quenching of the superconducting magnet, which alleviates issues caused by
cooling in a large field before measurement. To determine the residual field a palladium
reference sample was put through a comparable measurement. The results are shown in
Figure S1. The observed shift is repeatedly ∼2.5 Oe for the control sample, which is
an order of magnitude lower than the observed exchange bias shift in the Fe/CuMnAs
heterostructures.
Additional support for the interfacial exchange bias origin of the observed shift is given by
3
Fe/CuMnAs structures heated above 450 K. As described in the main text, the Fe/CuMnAs
interface permanently alters at approximately 450 K, which was observed with temperature
dependent x-ray reflectivity measurements. Figure S2a displays the obtained curve. While
the magnitude of the saturated moment is largely unaffected, the observed shift in the loop
after cooling in the same field is now negligible. If the observed shift in samples cooled from
430 K was due to residual field one would expect to see a comparable shift here. A similar
result is obtained for samples which are field cooled from temperatures lower than 430 K.
Again if the observed shift was due to residual field we would expect it to be insensitive to
the temperature from which it is cooled.
The SQUID figure shown in the main text depicts loops taken after training of the
sample (the magnetic field has been repeatedly swept from one field direction to the other).
Figure S2b shows the first (virgin) and second loops of a Fe/CuMnAs samples for positive
(black) and negative (red) field cool directions. After the first loop the exchange bias remains
stable. This is expected for an exchange bias system50 . In contrast a residual field in the
superconducting magnet would be expected to decrease with increasing number of field
sweeps at lower applied field.
Figure S1: SQUID measurements on Pd reference sample after cooling from 400 K in a
1000 Oe field. Squares and circles represent two repeats of the same measurement.
4
Figure S2: SQUID measurements on the Fe/CuMnAs bilayer films (a) after cooling from
450 K showing absence of exchange bias. (b) Training effect in a Fe/CuMnAs bilayer film after
cooling from 430 K.
S.3.
TRANSPORT MEASUREMENTS ON CuMnAs
As Figure S3 shows the resistivity at 4 K is about 90 µΩ.cm rising to about 160 µΩ.cm
at 300 K. These values are compatible with the semi-metallic-like band structure suggested
by the GGA+U calculations, with strongly suppressed density of states around the Fermi
energy. The measured Hall coefficient of 6 × 10−8 Ω.cm/T interpreted in a single carrier
5
model would give a carrier density of 1.1 × 1022 cm−3 . But this is probably misleading as the
band structure is not simple. If the material is a semimetal a single carrier interpretation
is inappropriate, and can greatly overestimate the number of carriers if (as the calculations
indicate) the numbers of hole-like and electron-like carriers are comparable.
Figure S3: Transport in CuMnAs. (a) shows the temperature dependence of resistivity and
(b) the Hall resistivity as a function of applied magnetic field.
∗
37
Electronic address: [email protected]; Corresponding author
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