Synthesis of Ferromagnetic Alloys Semiconductor–Ferromagnet in the CdAs2–MnAs System

Article abstract of DOI:10.1134/S0036023620080112

Abstract: Semiconductor–ferromagnet alloys in the CdAs₂–MnAs system were synthesized in evacuated ampules. Differential thermal analysis (DTA), X-ray powder diffraction analysis, differential scanning calorimetry, and scanning electron microscopy showed that this system is eutectic (of the acicular type), and the coordinates of the eutectic are (6 ± 0.5 mol % MnAs, T_melt = 614 ± 1°C). The liquidus lines constructed based on the thermal events of melting in DTA are 40–50°C higher than the lines constructed based on the thermal events of crystallization, which is due to the tendency of CdAs₂ toward glass transition. The synthesized alloys are ferromagnetic with T_C = 315 K, and the magnetization of them increases with increasing MnAs content. The alloys with nanoinclusions ≤40 nm of the ferromagnetic phase MnAs were produced by crystallization from melts at high cooling rates (≤100 deg/s). They have a higher Curie temperature of T_C = 353 K and a negative magnetoresistance of ΔR = 2% at 300 K in a saturation magnetic field of 4500 Oe, which is of practical interest for creating magnetic granular spintronic structures.

Full text of DOI:10.1134/S0036023620080112

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2020, Vol. 65, No. 8, pp. 1219–1225. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Zhurnal Neorganicheskoi Khimii, 2020, Vol. 65, No. 8, pp. 1092–1098.
PHYSICOCHEMICAL ANALYSIS
OF INORGANIC SYSTEMS
Synthesis of Ferromagnetic Alloys Semiconductor–Ferromagnet
in the CdAs –MnAs System
S. F. Marenkin , A. I. Ril’ *, I. V. Fedorchenko , and V. V. Kozlov
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia
2
a, b
a,
a
c
a
b
National University of Science and Technology “MISiS”, Moscow, 119991 Russia
Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Moscow, 119991 Russia
c
*e-mail: ril_alexey@mail.ru
Received February 21, 2020; revised March 12, 2020; accepted March 27, 2020
Abstract—Semiconductor–ferromagnet alloys in the CdAs –MnAs system were synthesized in evacuated
2
ampules. Differential thermal analysis (DTA), X-ray powder diffraction analysis, differential scanning calo-
rimetry, and scanning electron microscopy showed that this system is eutectic (of the acicular type), and the
coordinates of the eutectic are (6 ± 0.ꢀ mol % MnAs, T = 614 ± 1°C). The liquidus lines constructed based
melt
on the thermal events of melting in DTA are 40–ꢀ0°C higher than the lines constructed based on the thermal
events of crystallization, which is due to the tendency of CdAs toward glass transition. The synthesized alloys
2
are ferromagnetic with T = 31ꢀ K, and the magnetization of them increases with increasing MnAs content.
C
The alloys with nanoinclusions ≤40 nm of the ferromagnetic phase MnAs were produced by crystallization
from melts at high cooling rates (≤100 deg/s). They have a higher Curie temperature of T = 3ꢀ3 K and a neg-
C
ative magnetoresistance of ΔR = 2% at 300 K in a saturation magnetic field of 4ꢀ00 Oe, which is of practical
interest for creating magnetic granular spintronic structures.
Keywords: spintronics, semiconductors, granular structures, giant magnetoresistance
DOI: 10.1134/S0036023620080112
INTRODUCTION
various nonmagnetic metals or insulators that can
form systems of the above types with the ferromagnet.
The main spintronic materials are nanolayers The giant magnetoresistance is higher if the nonmag-
superlattices) of magnets and nonmagnets, obtained netic layers or the matrix are made of highly conduc-
(
by molecular beam epitaxy [1, 2], that exhibit giant tive materials [12–14].
magnetoresistance. An intensely studied alternative to
superlattices can be magnetically ordered heteroge-
neous systems (magnetic granular structures). Such
structures comprise a nonmagnetic matrix and ferro-
magnetic nanoinclusions [3–7]. Such structures are of
interest because they can be produced by simpler
methods than molecular beam epitaxy. The properties
of magnetic granular structures depend [8–11] on the
ratio between the compositions of the matrix and the
ferromagnet, their particle sizes, and also the electrical
conductivity of the materials of the matrix and the fer-
romagnet. For successful performance, the matrix and
the ferromagnet of a magnetic granular structure
should have sharp interfaces without chemical inter-
action or diffusion. Such properties can be exhibited
by phases in systems with a eutectic, a peritectic, or
a wide immiscibility region. However, to obtain nano-
structures, eutectic systems are more suitable because
Because of the high mobility of charge carriers,
semiconductors were proposed to be used [1ꢀ–17].
The object of investigation was taken to be the CdAs –
2
MnAs system, which consists of a semiconductor and
a ferromagnet. The triangulation of the ternary system
Cd As –MnAs–CdAs showed a high probability of
3
2
2
the formation of a eutectic in the CdAs –MnAs sys-
2
tem [18]. The compound CdAs is a semiconductor
2
with a band gap of 0.9 eV at 300 K (1.14 eV at 0 K) and
shows a unique anisotropy of optical and electrical
properties. Cadmium diarsenide crystallizes in the
tetragonal system in the space group I4 22. The unit
1
cell parameters are a = 7.9ꢀ4 Å and c = 4.678 Å. In
CdAs , along with Cd–As bonds, As–As bonds form,
2
which determines the ability of this compound to glass
transition [19–21].
Manganese arsenide, a well-known ferromagnet
they can crystallize at high cooling rates. The ferro- with T = 313–318 K and a magnetic moment of 3.4 μ ,
C B
magnetic components in these systems are typically 3d is an intermetallic and has a high electrical conductiv-
4
–1
–1
elements or their compounds with the Curie tempera- ity of σ ~ 2 ×10 Ω cm at 300 K [22–24]. The fer-
ture T above room temperature. The matrix can be romagnetic phase MnAs crystallizes in the hexagonal
C
1219

1220
MARENKIN et al.
out in sealed Stepanov quartz vessels evacuated to
–2
a pressure of 10 Pa, the sample weight was 1 g, the
heating and cooling rates were 4 deg/min, the accu-
racy of determining the temperature of thermal events
was ±1 deg. The magnetic structure transformation in
MnAs was investigated with a TA Instruments Q20
differential scanning calorimeter in the temperature
range 293–3ꢀ3 K at heating and cooling rates of
1
0 deg/min, the sample weights were 2–4 mg, and the
accuracy of measuring the temperatures of thermal
events was ±1 deg. The microstructural analysis was
conducted with an Ipiquant optical metallographic
microscope (magnification ×400) and a Jeol JSM-
6
480 scanning electron microscope (SEM) with an
Oxford Instruments X-Max energy dispersive X-ray
analyzer (EDX); the accuracy of determining the
composition was ±0.1 at %. The microstructures were
prepared by cutting ingots into plates and polishing the
plates with a SiC abrasive while sequentially decreas-
ing the abrasive grain size from 20 to 0.1 μm. The mag-
netic measurements were made with an S600X mag-
netometer (Cryogenic, UK); the electrical conductiv-
ity was measured by four-terminal sensing with an
accuracy of ±0.ꢀ% at 300 K.
Fig. 1. CdAs –MnAs alloy ingot.
2
system (space group P6 /mmc) with the unit cell
3
parameters a = 3.72 Å and c = ꢀ.71 Å. At T , this phase
C
RESULTS AND DISCUSSION
transforms to the paramagnetic phase crystallizing in
the orthorhombic system (space group Pnma) with the
unit cell parameters a = 3.72 Å, b = 3.66 Å, and c =
The samples after the synthesis were dense ingots
with the minimum porosity (Fig. 1). According to the
XRD data, the samples are two-phase. The XRD
6
.37 Å [22]. An interesting, practically important
peculiarity of manganese arsenide is that it shows the
colossal magnetocaloric effect [2ꢀ].
patterns show peaks assigned to the phases CdAs
2
(
space group I4 22) and MnAs (space group P63/mmc)
1
(
Fig. 2a). The analysis of the peaks of CdAs and the
2
EXPERIMENTAL
alloys containing 6 and 10 mol % MnAs found no
changes in their positions at large angles (Fig. 2b). The
Alloys were obtained in evacuated ampules both
calculation of the parameters of the phases CdAs and
MnAs in the alloys gave the following results: a =
2
from presynthesized compounds CdAs and MnAs,
2
and by the direct interaction of elemental substances.
Quartz ampules lined with pyrolytic carbon were used
to prevent the interaction with the melt. No less than
7
.9ꢀ41 Å and c = 4.6778 ± 0.00ꢀ Å, and a = 3.7367 Å and
c = ꢀ.6976 ± 0.006 Å) for CdAs and MnAs, respec-
2
tively. These data agree with the published parameter
1
ꢀ
ꢀ samples at concentration intervals of no more than
value [19, 21]. The analysis of the microstructures
mol % were prepared to study the CdAs –MnAs sys-
2
demonstrated that the CdAs –MnAs system is char-
2
tem. The precursors were Cd (99.999%), As
99.999%), and Mn (99.998%). The synthesis was car-
acterized by the acicular type of the eutectic. Figure 3
shows the acicular inclusions of MnAs along and
across the growth direction. The composition of the
eutectic was found with the Jeol JSM-6480 scanning
electron microscope with the Oxford Instruments X-
Max energy dispersive X-ray analyzer. The
SEM/EDX determined that the composition of the
eutectic is (2 at % Mn, 33.4 at % Cd, 64.6 at % As) or
(
ried out at a temperature 10 deg higher than the melt-
ing point of MnAs. To prevent the transfer of highly
volatile components from the reaction zone, the
ampules were placed in the isothermal; zone of the
vertical electric oven [10, 18]. The alloys were identi-
fied and analyzed by X-ray powder diffraction analysis
(
XRD) and differential thermal analysis (DTA). The
(
94 mol % CdAs , 6 mol % MnAs) The results of the
2
microstructures were studied with an optical micro-
scope and an electron microscope. The XRD was
made at the Shared Equipment Center, Kurnakov
Institute of General and Inorganic Chemistry, Rus-
sian Academy of Sciences, Moscow, Russia, with a
XRD and the microstructural studies were supple-
mented with the DTA data. Table 1 presents the tem-
peratures of the thermal events of melting and cooling.
Figure 4 shows the liquidus and solidus lines of the
system.
Bruker D8 powder diffractometer (CuK radiation,
λ = 0.1ꢀ40 nm, recording range 2θ = 10°–90°, inte-
α
The DSC studies of the magnetic and electrical
gration time 1 s, step size 0.014 deg). DTA was carried properties were performed on crystallized alloys at
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY
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SYNTHESIS OF FERROMAGNETIC ALLOYS SEMICONDUCTOR–FERROMAGNET
a)
1221
(
MnAs (P63/mmc) 00-028-0644 PDF-2 ICDD
CdAs –MnAs
2
CdAs (I4 22) 01-072-0639 PDF-2 ICDD
I
2
1
As (R3m) 01-072-1048 PDF-2 ICDD
MnAs
4
5 mol % MnAs
0 mol % MnAs
0 mol % MnAs
2
1
6
mol % MnAs
CdAs2
1
0 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
2θ, deg
I
(b)
Kα2
Kα2
10 mol % MnAs
6
0
mol % MnAs
mol % MnAs
54
51
52
53
2θ, deg
Fig. 2. Experimental X-ray powder diffraction patterns: (a) CdAs , MnAs, and CdAs –MnAs alloys; and (b) CdAs and CdAs –
2
2
2
2
MnAs alloys at large angles.
–2
ordinary (~1 × 10 deg/s) and high (annealing-level, at annealing-level rates, it decreases to ≤40 nm. Figure ꢀ
2
illustrates the results of the DSC of the alloy contain-
~
1 × 10 deg/s) cooling rates. The size of inclusions of
–2
the ferromagnetic phase MnAs in the alloys crystallized ing 20 mol % MnAs at a cooling rate of 1 × 10 deg/s,
at ordinary cooling rates was several micrometers, and at which there is a thermal event at 31ꢀ K, correspond-
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 6ꢀ No. 8 2020

1222
MARENKIN et al.
CdAs2
CdAs2
MnAs
MnAs
20 μm
20 μm
(a)
(b)
Fig. 3. Microstructure of the alloy of the eutectic composition 94 mol % CdAs –6 mol % MnAs (a) perpendicular to and (b) in
2
the crystal growth plane.
950
900
850
wt %
0
1.2 2.4 3.7 5.0 6.3 7.6
6
30
20
L
6
L + MnAs
СdAs
2
+ L
СdAs
e
6
10
614
2
+ MnAs
6
00
800
0
2
4
6
8
10 12
MnAs
СdAs
2
mol %
L
750
L + MnAs
700
650
600
614
СdAs + MnAs
e
2
(±1°C)
0
6
20
40
60
80
100
MnAs
СdAs2
mol %
Fig. 4. Phase diagram of the CdAs –MnAs system.
2
–2
ing to the magnetic structure transformation of the samples crystallized at cooling rates of ~1 × 10 and
ferromagnetic (hexagonal) to the paramagnetic (ort-
horhombic) phase of MnAs. In the samples crystal-
lized at a high cooling rate, the thermal event of the
transformation vanishes.
2
~
1 × 10 deg/s, respectively. The T of the samples dif-
C
fer: 315 and 353 K, respectively. The temperature
dependences of the electrical conductivity showed
that the alloys containing MnAs, unlike CdAs , have
2
The shapes of the hysteresis loops at 300 K (Fig. 6)
metallic conduction. The alloys crystallized at anneal-
ing-level cooling rates have a negative magnetoresis-
tance of ΔR = 2% at 300 K in a saturation magnetic
field of 4500 Oe. With a further increase in the mag-
netic field, the magnetoresistance reverses sign and
show that alloys in the CdAs –MnAs system are soft
2
ferromagnets with low coercivity. Their magnetization
increases with increasing their manganese arsenide
content. Figure 7 presents the temperature depen-
dences of the magnetic susceptibility of the alloy con-
taining 20 mol % MnSb. Curves 1 and 2 describe the becomes positive.
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SYNTHESIS OF FERROMAGNETIC ALLOYS SEMICONDUCTOR–FERROMAGNET
CONCLUSIONS
1223
The results of XRD, DTA, and microstructural
studies confirmed the eutectic type of the interaction
in alloys of cadmium and manganese arsenides. The
coordinates of the eutectic are 6 ± 0.5 mol % MnAs,
Heating
T
= 614 ± 1°C; the type of the eutectic is acicular.
melt
The analysis of the changes in the positions of the
–2
peaks of CdAs and MnAs in the XRD patterns and
2
the SEM/EDX data showed a low mutual solubility of
these phases (≤1 mol %). According to the DTA data,
the temperatures of the thermal events of melting and
heating differ by 40–50 deg; i.e., the crystallization of
melts of cadmium arsenide with manganese arsenide
is accompanied by a significant supercooling because
of the tendency of cadmium diarsenide to glass transi-
tion. The results of DSC and magnetic measurements
demonstrated the effect of the inclusion size on the
caloric and magnetic properties. Once the ferromag-
–4
Cooling
netic phase MnAs becomes nanosized, T increases to
C
3
53 K, and the thermal event of the magnetic structure
transformation of MnAs vanishes. These data seem to
be valid because the mass transfer and, consequently,
the coarsening of the ferromagnet are complicated by
the semiconductor matrix.
300
310
Т, К
320
330
FUNDING
This work was carried out within the State Assignment
on Fundamental Research to the Kurnakov Institute of
Fig. 5. DSC curves of the 80 mol % CdAs –20 mol % MnAs
2
2
alloy cooled at a rate of ~1 × 10 deg/s.
Table 1. Temperature events in alloys in the CdAs –MnAs system according to DTA data
2
DTA effects, °C
MnAsconcentration,
No. of sample
heating
cooling
mol %
eutectic
liquidus
eutectic
liquidus
1
2
3
4
5
6
7
8
9
0
3
–
621
618
–
–
491
540
–
614
614
615
615
616
615
610
616
618
615
617
–
550
538
533
536
540
545
541
550
555
539
560
–
6
8
619
625
632
642
667
697
758
847
877
933
540
551
560
580
620
671
751
822
854
910
10
12
14
20
30
45
65
80
100
10
11
12
13
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 65 No. 8 2020

1224
MARENKIN et al.
100 mol % MnAs
5
5 mol % CdAs –45 mol % MnAs
2
100
80 mol % CdAs –20 mol % MnAs
2
1
50
2
3
–4000
–2000
0
2000
4000 Н, Oe
–50
–100
Fig. 6. Concentration dependence of the specific magnetization on the external field for CdAs –MnAs alloys at 300 K and (1) 100,
2
(2) 45, and (3) 20 mol % MnAs.
100
REFERENCES
1
2
1
. P. Grünberg, R. Schreiber, Y. Pang, et al., Phys. Rev. B
3
9, 4828 (1989).
https://doi.org/10.1103/PhysRevB.39.4828
2
. M. N. Baibich, J. M. Broto, A. Fert, et al., Phys. Rev.
Lett. 61, 2472 (1988).
https://doi.org/10.1103/PhysRevLett.61.2472
50
3. C. P. Poole, Jr., and F. J. Owens, Introduction to Nano-
technology (Wiley, New York, 2003).
TC = 315 K
TC = 353 K
4
. C. Lamberti and G. Agostini, Characterization of Semi-
conductor Heterostructures and Nanostructures (Elsevier
Science BV, Amsterdam, 2013).
0
5
. E. K. Naimi and O. I. Rabinovich, Crystallogr. Rep.
310
320 330 340 350
360
370
5
6, 486 (2011).
T, K
https://doi.org/10.1134/S1063774511030199
6
. S. A. Gridnev, Y. E. Kalinin, A. V. Kalgin, et al., Phys.
Solid State 57, 1372 (2015).
Fig. 7. Temperature dependence of the magnetic suscepti-
bility of the 80 mol % CdAs –20 mol % MnAs alloy in a
2
–2
https://doi.org/10.1134/S1063783415070136
magnetic field of 100 Oe, cooled at rates of (1) ~1 × 10
2
and (2) ~1 × 10 deg/s.
7. Kalavathy Santhi, Dhanapal Kumarsan, Naryanan
Vengidusamy, et al., J. Magn. Magn. Mater. 433, 202
(
2017).
General and Inorganic Chemistry, Russian Academy of
Sciences, and supported in part by the Presidium of the
Russian Academy of Sciences, program no 15.1: Scientific
Principles of Producing Novel Functional Materials.
https://doi.org/10.1016/j.jmmm.2017.03.020
8. N. Samarth, Solid State Phys. 58, 1 (2004).
https://doi.org/10.1016/S0081-1947(04)80009-2
9. M. Tanaka, S. Ohya, and P. N. Hai, Appl. Phys. Rev. 1,
0
11102 (2014).
https://doi.org/10.1063/1.4840136
CONFLICT OF INTEREST
10. S. F. Marenkin, A. N. Aronov, I. V. Fedorchenko, et al.,
The authors declare that they have no conflicts of
interest.
Inorg. Mater. 54, 1187 (2018).
https://doi.org/10.1134/S0020168518120105
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY
Vol. 65
No. 8
2020

SYNTHESIS OF FERROMAGNETIC ALLOYS SEMICONDUCTOR–FERROMAGNET
1225
1
1. O. I. Rabinovich, J. Alloys. Compd. 586 (Suppl. 1), 19. L. Červinka and A. Hrubý, Acta Crystallogr., Sect. B
S258 (2014).
26, 457 (1970).
https://doi.org/10.1016/j.jallcom.2013.03.214
https://doi.org/10.1107/S0567740870002650
1
2. Y. Xu, D. D. Awschalom, and Ju. Nitta, Handbook of
2
0. Yu. M. Basalaev, A. V. Kopytov, A. S. Poplavnoi, et al.,
Spintronics (Springer, Dordrecht, 2016).
Semiconductors 51, 783 (2017).
https://doi.org/10.1007/978-94-007-6892-5
https://doi.org/10.1134/S1063782617060057
13. Vinod Kumar Joshi, Eng. Sci. Technol. 19, 1503 (2016).
21. L. Červinka and P. Holba, Phys. Chem. Glasses: Eur. J.
https://doi.org/10.1016/j.jestch.2016.05.002
Glass Sci. Technol. Part B 57, 201 (2016).
https://doi.org/10.13036/17533562.57.5.002
1
4. V. E. Borisenko, A. L. Danilyuk, and D. B. Migas,
Spintronics (Laboratoriya znanii, Moscow, 2017) [in
Russian].
22. H. Okamoto, Bull. All Phase Diagr. 10, 549 (1989).
https://doi.org/10.1007/BF02882414
15. S. F. Marenkin, A. D. Izotov, I. V. Fedorchenko, and
V. M. Novotortsev, Russ. J. Inorg. Chem. 60, 295 23. A. Continenza, S. Picozzi, W. T. Geng, and A. J. Free-
(
2015).
man, Phys. Rev. B: Condens. Matter 64, 085204
(2001).
https://doi.org/10.1103/PhysRevB.64.085204
https://doi.org/10.1134/S0036023615030146
1
6. S. F. Marenkin, I. V. Fedorchenko, A. D. Izotov, and
M. G. Vasil’ev, Inorg. Mater. 55, 865 (2019).
https://doi.org/10.1134/S0020168519090061
24. G. A. Govor, Phys. Solid State 57, 871 (2015).
https://doi.org/10.1134/S1063783415050121
17. N. M. Shchelkachev and V. G. Yarzhemsky, Inorg.
2
5. S. F. Marenkin, A. V. Kochura, A. D. Izotov, and
M. G. Vasil’ev, Russ. J. Inorg. Chem. 63, 1753 (2018).
https://doi.org/10.1134/S0036023618140036
Mater. 54, 1093 (2018).
https://doi.org/10.1134/S0020168518110110
18. A. I. Ril, I. V. Fedorchenko, S. F. Marenkin, et al.,
Russ. J. Inorg. Chem. 62, 976 (2017).
https://doi.org/10.1134/S0036023617070191
Translated by V. Glyanchenko
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 65 No. 8 2020