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The existence region and the magnetic and electrical properties of MnSb
J. Phys. Chem. Solids
Pergamon Press 1968. Vol. 29, pp. 347-355.
Printed in Great Britain.
THE EXISTENCE REGION AND THE MAGNETIC AND ELECTRICAL PROPERTIES OF MnSb I. TERAMOTO* Philips Research Laboratories,
and A. M. J. G. VAN RUN
N.V. Philips’ Gloeilampenfabrieken, (Received
24August
Eindhoven-Netherlands
1967)
Abstract-The existence region of the ferromagnetic metallic compound MnSb is discussed. A description is given of Curie temperature and lattice parameter measurements carried out on samples equilibrated at several temperatures between 400 and 800°C and quenched. A relation was found between the Curie temperature and the composition of the MnSb phase, which proved to be independent of the annealing temperature. Density measurements showed that in non-stoichiometric MnSb the excess Mn-atoms occupy interstitial sites. For single crystals of different compositions the resistivity and the Seebeck coefficient are given as a function of the temperature. 1. INTRODUCTION MANGANESE
antimonide is a ferromagnetic compound which crystallizes in the hexagonal NiAs structure. Transition-metal pnictides of this type frequently have a broad existence region extending from the equiatomic composition to the metal side of the phase diagram. According to Guillaud[ 11 the magnetic transition of MnSb, between ferromagnetic and paramagnetic states occurs at a temperature between 363 and 587”K, depending on the composition. In order to reveal composition-sensitive properties in detail, single-phase samples of the appropriate compositions are required. The composition limits of stable MnSb, however, have not been established accurately in the published phase diagrams [2,3]. The present investigation was undertaken to determine quantitatively the effect of the composition of MnSb on the magnetic and electrical properties. 2. EXPERIMENTAL PROCEDURES
Powder samples of MnSb with compositions ranging from 38 to 55 at.% Sb, were prepared *Present address: Research Laboratory, Matsushita Electronics Corporation, Takatsuki, Osaka, Japan.
347
as follows. Mixtures of appropriate quantities of spectroscopically pure manganese and antimony were sealed in evacuated silica tubes and heated for about 16 hr at a temperature slightly higher than 9OO”C,followed by quenching in water. The reaction products obtained were finely ground in a mortar and sealed in evacuated silica tubes for further heat treatment to establish thermal equilibrium at the desired temperatures. Single crystals of MnSb were grown by the Bridgman method from the melt. Since the method involves the normal-freezing process, the composition of a specimen cut from an asgrown crystal is not accurately known, and it even may contain a second phase. Single-phase crystals with definite compositions were prepared as follows. A crystal of MnSb is annealed at a constant high temperature T, with a relatively large amount of MnSb powder of known composition A. At equilibrium the MnSb crystal is forced to accept the composition A. In order to obtain single-phase MnSb saturated with Mn or Sb at a given temperature T, the crystal is annealed with MnSb + Sb and MnSb + Mn 14951. The phases present in the samples were
I. TERAMOTO
348
and A. M. J. G. VAN RUN
identified by powder X-ray analysis and the unit cell dimensions were obtained in the same way. The magnetic measurements were made with a vibrating-sample magnetometer in fields up to 16 kOe and at temperatures ranging from 4 to approximately 670°K. From these measurements the atomic saturation moments, the ferromagnetic Curie temperatures, the asymptotic Curie temperatures and the Curie constants were determined. The electrical resistivity and the Seebeck coefficient of single crystals were measured from 80°K up to a temperature higher than the Curie temperature. The pycnometer method was used for determining the density of some of the powder samples. 3. RESULTS AND DISCUSSIONS
3.1. Curie temperature The Curie temperatures T, of samples equilibrated at 700°C and quenched were determined from the saturation magnetization versus temperature curves with an accuracy of ?YK (Fig. 1). The closed circles (0) re-
i
5
50
4.66474849
-at Fig. 1. Curie temperature
51
52
%Sh
T, as a function of composition.
present powders prepared from weighed amounts of manganese and antimony, and the open circles (0) are results for single crystals annealed with powders of known compositions. The open squares (0) represent singlephase compositions saturated with Mn or Sb, The Curie temperature changes with the composition in the single-phase region whereas it remains constant in the two-phase regions. The intersections of the extrapolated Curie temperature versus composition curve in the existence region with the horizontals in the two-phase regions represent the stability limits at the temperature at which the two-phase equilibrium is frozen in. From differential thermal analysis experiments (Fig. 2 (A)) it may be concluded that at the Sb-rich side the equilibrium is frozen in almost immediately upon quenching. At the Mn-rich side, however, thermal effects could not be detected by differential thermal analysis. It is therefore possible that the equilibrium at this side is frozen in at a lower temperature. The Curie temperature is proved to be independent of annealing temperature by the following experiment. Powders with the same composition, i.e. 49.0 at.% Sb, were equilibrated at several temperatures between 400 and 700°C and quenched. A Curie temperature of about 573°K was observed for all samples within the experimental error. The same experiment with powders of the composition 46-O at.% Sb in the temperature range 500800°K gave a Curie temperature of about 390°K. Consequently the Curie temperature is a function of composition only. The existence region as a function of temperature has been determined by Curie temperature measurements on quenched twophase powder samples which had been annealed at 800, 700, 600, 500 and 400°C for 5, 15, 70, 70 and 200 days, respectively. The composition of the MnSb phase can be found from the observed Curie temperature using the curve of Fig. 1. The results are given in Fig. 2. At the Sb-rich side they (0) agree well with the results of differential thermal analysis
EXISTENCE
REGION AND MAGNETIC
ELECTRICAL 1, (‘WIIO500
1 8
I
PROPERTIES OF MnSb
349
I
600 la (‘Cl t 500
I Mn
36
36
40
42 -at
44
46
40
50
b&l
52
5.4
‘1, Sb
Fig. 2. Partial phase diagram of the Mn-Sb system as determined from Curie temperature measurements (0,O) and differential thermal analysis (A).
measurements, indicating that the two-phase equilibria are frozen in immediately upon quenching. At the Mn-rich side the results (0) for the temperature range from 820 to 600°C are almost identical. This may indicate that the equilibrium MnSb-Mn,Sb has frozen in at about 550°C upon quenching. The dashed line could then represent the limit composition of MnSb at the Mn-rich side above 500°C. The discrepancy between our results and those of Guillaud (Fig. 1 (X)) can be ascribed to the fact that he did not account for the separation of second phase during very slow cooling (3 days). The existence region obtained properly
explains the difference in Curie temperature between the slowly cooled and the quenched samples reported by Takei et a!.[63 as being a consequence of the temperature dependence of the limit composition at the Sb-rich side. 3.2. Lattice parameters The unit cell dimensions a, c and c/a of the NiAs structure measured on powders equilibrated at 700°C and quenched are shown in Fig. 3 as a function of composition. For the compositions of 44.0 and 52-O at.% Sb a small amount of a second phase, Mn,Sb and Sb respectively, was detected. A linear change in lattice parameters with composition is found
350
I. TERAMOTO and A. M. J. G. VAN RUN
1.4204220 5790
I
I.410 4,2105780 !4oL, 4200 5770 13904.1905760 13804180 5750 070 4170 5.7&I om41Ml
sic?0
13504150 5720 13404uo 5no 13304130 5.700 1172L) 4120 5690 44
45
46
47
48
-at
49
50 51 O/6Sb
52
Fig. 3. Lattice parameters a and c, axial ratio c/u, and unit cell volume D = &z*c~3 as a function of composition.
(Vegard’s law). Figure 3 shows a close correspondence with Fig. 1. In the literature determinations of the lattice parameters of MnSb have dealt almost exclusively with the so-called stoichiometric composition [6,8]. According to the existence region given above, however, the stoichiometric composition falls outside the existence region at 700°C. The lattice parameters extrapolated to the stoichiometric composition (Fig. 3), i.e. a = 4.125, c = 5.799 and c/a = l-406, are in fair agreement with the results obtained by Willis and Rooksby[7] at room temperature, namely a = 4.128 and c = 5789. Takei et al.[6] obtained a = 4.130, c = 5.789 and c/a = 1.402 for a slowly cooled sample and for a quenched sample a = 4.153, c = 5.771 and c/u = 1.390. Comparison of their results with the present ones also indicates that the limit composition at the Sb-side shifts
to the stoichiometric one at lower temperatures. The slow cooling of one of the samples used by Takei et al. will allow the corresponding amount of free antimony to diffuse into the MnSb phase, giving a smaller value of a and a larger value of c. The unit cell volume increases with increasing Mn content, as is shown in the upper part of Fig. 3. 3.3. Other magnetic properties The susceptibility per gram, xQ, was measured as a function of temperature above the Curie temperature for single-phase compositions equilibrated at 700°C and quenched. The relations obtained between xs-’ and temperature T are shown in Fig. 4. For the 49 at.% Sb composition the paramagnetic behaviour can be described well by the Curie-Weiss law. With increasing excess Mn concentration the curve becomes less linear.
EXISTENCE REGION AND MAGNETIC ELECTRICAL
PROPERTIES OF MnSb
351
4 WI0
400
4.547
500
550
600 -T(OK)
650
Fig. 4. Reciprocal paramagnetic susceptibility xg? (per gram) as a function of temperature and composition.
From the straight parts found near the Curie temperature, the asymptotic Curie temperatures, T,, and the Curie constants per gram, C,, are obtained. The T, values agree with the ferromagnetic Curie temperatures T, within ten degrees. The effective number of Bohr magnetons per manganese atom, np, calculated from the Curie constant C,, and the effective moments from the ferromagnetic saturation extrapolated to O”K, nf, are given in Table 1. Table I. Magnetic properties of homogeneous MnSb equilibrated at 700°C and quenched. T, is the Curie temperature; np and nf are the effective moments per Mn atom deduced from the Curie constant and from the ferromagnetic saturation, respectively Composition (at.% Sb)
T,
nP
nf
(“K)
(PB)
(PB)
46.0 47.0 48.0 49.0 49.7 50.0
390 488 517 573 592 (600) *
*Extrapolated
values.
4.11 4.20 4.21 4.21 (4.21)*
2.45 2.82 2.98 3.28 (3.55) *
The nP value changes only slightly with composition and agrees with that obtained by Serres[9], while the change of nf with composition is found to be much greater. An extrapolation of the atomic saturation moment vs. composition curve gives approximately 3.55 pE at the stoichiometric composition, which is in agreement with the results of Hirone et al.[ lo] 3.4 pE and of Guillaud[l] 3.53 ,..+ The results obtained by Takei et a/.[6], who reported 3.53 pB and 3.37 pB for the slowly cooled and the quenched MnSb are again accounted for as respectively, resulting from a shift of the limit composition with temperature. In samples with a composition of 46 at.% Sb, an irreversible increase of the susceptibility was observed when the measurement was repeated from room temperature up to 400°C. The result is shown in Fig. 5, where, in the fifth measurement, the sample was cooled after being held at 400°C for 30 min. The deviation of the reciprocal susceptibility vs. temperature curve from the Curie-Weiss law becomes more marked with the heat treatment repetition. The curves in the high-temperature region indicate an additional asymptotic Curie temperature of about 550°K. This irreversible
352
I. TERAMOTO and A. M. J. G. VAN RUN
400
450
550
650
600 -
!m
T(OK)
Fig. 5. Irreversible behaviour of MnSb with acomposition of 46 at.% Sb during repeated measurements of the susceptibility as a function of temperature.
behaviour is caused by the precipitation of a small amount of a second phase, Mn,Sb, which changes the composition of the MnSb phase. This is concluded from the following facts. 1. The additional asymptotic Curie temperature observed in Fig. 5 is very close to that of Mn,Sb. 2. A single-phase sample with the composition 46 at.% Sb quenched from 700°C and having a Curie temperature of 390”K, was annealed at 350°C for a few days, followed by quenching. The Curie temperature was now observed to be 45°K higher than before the heat treatment. This suggests that a Mn-richer
second phase, Mn,Sb, precipitated during the low-temperature heat treatment, consequently increasing the concentration of Sb in the MnSb phase. 3. A direct observation of the precipitation of the second phase is possible. A single-phase single crystal equilibrated at 700°C and quenched had the Curie temperature of 335°K and consequently a composition of 45.5 at.% Sb. This crystal had been annealed at 350°C for a few days and quenched. Subsequent microscopic observation of the crystal, polished parallel and perpendicular to the crystallographic c-axis, clearly showed that the crystal
Fig. 6. Photomicrographs of an originally single-phase, single crystal with a composition of 45.5 at. % Sb, after annealing at 350°C for a few days: (a) 11to c-axis; (b)l to c-axis.
[Facing page 3521
EXISTENCE REGION AND MAGNETIC ELECTRICAL PROPERTIES OF MnSb was no longer homogeneous but included a number of precipitates, which were not observed before the heat treatment. The photomicrographs (Figs. 6(a) and 6(b)) show that the precipitates have a needle shape parallel to the c-axis.
3.4. Electrical resistivity and Seebeck coeficient Figure 7 gives typical curves of the electrical resistivity p as a function of temperature, measured on single crystals of MnSb in the direction perpendicular to the crystallographic c-axis. The curves show an abrupt change of slope at a temperature TcP, in such a way that the resistivity increases with increasing temperature up to T,, and becomes practically X10
353
independent of temperature above T,. The break temperature T,, is approximately equal to the ferromagnetic Curie temperature T, within the limits of experimental error. The behaviour shown can be explained in terms of a spin-disorder scattering mechanism[ 1 l] in the metallic compound, a mechanism which has proved useful in explaining data on other ferromagnetic compounds with the NiAs structure [ 121. The residual resistivity at 0°K differs considerably from sample to sample. The crystals, however, had been forced to accept the composition of the MnSb powders in the presence of which they were annealed. This change of composition adversely influences the quality of the crystals. This may also explain the fact that the residual resistivities of our samples
-4
Fig. 7. Resistivity p as a function of temperature.
I. TERAMOTO and A. M. J. G. VAN RUN
354
are considerably higher than values reported in the literature [ 12, 131. The Seebeck coefficients (Yas a function of temperature are shown in Fig. 8. The sign is always negative, being opposite to that of the Hall coefficient measured by Nogami et ~21.1141. The curves are similar in behaviour to the resistivity curves shown in Fig. 7; with increasing temperature the Seebeck coefficient increases in magnitude up to the Curie temperature, where it reaches a maximum value. 3.5. Density measurements For a better understanding of the composition-dependent properties of MnSb described above, a knowledge of the predominant type of point defects involved in non-stoichiometric MnSb is required. Since this compound has a broad existence region, a comparison of the density of samples of different compositions is sufficient for the present purpose. The density was measured with a pynco-
meter at room temperature on two MnSb powder samples, of compositions 46 and 49 at.% Sb, which had been equilibrated at 700°C and quenched. The results are shown in Table 2. For comparison, values of the density Table 2. Densities of MnSb with the compositions 46.0 at.% Sb and 49.0 at.% Sb and their ratio. Calculated values are deduced assuming Mn-interstitials, Sb-vacancies and antistructure for the point defects caused by non-stoichiometry Composition (at.% Sb) 46.0 49.0 Ratio
Calculated density Observed density
Mn-int.
Sb-vat.
Anti-str.
7.075 6.920 1.022
6.027 6.647 0.907
6.509 6.782 0.960
6.946 6.842 1.015
calculated from the observed lattice parameters for three possible types of point de-
-20
fNiP”& t -15
- II
-5
Fig. 8. Seebeck coefficienta
-TCW as a function of temperature.
EXISTENCE REGION AND MAGNETIC ELECTRICAL
authors wish to thank Dr. C. Haas and Dr. W. Albers for helpful discussions. Thanks are due to Dr. G. van Aller for carrying out the electrical measurements, to Mr. J. Verberkt for performing the differential thermal analysis and to Miss A. Reitsma for carrying out some of the magnetic measurements.
REFERENCES 1. GUILLAUD
C., Thesis, University of Strassbourg
(1943);Ann.dePhys.4,671(1949).
I
I I
C
I
2. HANSEN M., Constitution of Binary Alloys, 2nd edn. (1958). 3. MURAKAMI T. and HATTA A., Sci. Rep. Tohoku Uniu.22,88(1933). 4. ALBERS W., .I. them. Phys. 43,4401(1965). 5. ALBERS W. and ATEN A. C., Philips Res. Rep. 20, 556 (1965).
6. TAKE1 W. J., COX D. E. and SHIRANE G., Phys. Reu.129,2008 (1963). 7. WILLIS B. T. M. and ROOKSBY H. P., Proc. phys. Sot. B67,290 (1954). 8. LOTGERING F. K. and GORTER E. W., J. Phys. Chem. Solids 3,238
NiAs
0
0
M&c?/
3.55
Acknowledgements-The
fects, Mn-interstitials, Sb-vacancies, and antistructure (i.e. Mn on Sb sites) are included. This comparison shows that almost all the excess manganese atoms in the homogeneous MnSb occupy interstitial sites. This is consistent with the fact that the unit cell volume increases with increasing Mn content, as is shown in Fig. 3. Crystallographically this means a continuous change from the NiAs structure in the direction of the N&In structure (Fig. 9). The
c
PROPERTIES OF MnSb
Ni$n
Metalloid
Fig. 9. Unit cell of the NiAs and of the Ni,In structure.
(1957).
9. SERRES A., .I. Phys. Radium, Paris 8, 146 (1947). 10. HIRONE T., MAEDA S., TSUBOKAWA I. and TSUYA N.,J.phys. Sot. Japan 11,1083 (1956). T. and DEKKER A. J., Il. van PESKI-TINBERGEN Physica29,917(1963).
12. ALBERS W. and HAAS C.,Proc.
occurrence of interstitial metal atoms in the NiAs structure was assumed by Roberts[ 151 to explain the native disorder of stoichiometric MnBi.
Int. Conf. Physics of Semi-conductors, Paris (1964); ALBERS W. and HAAS C.,Phys. Lett. 8,300 (1964). 13. SUZUOKA T., J. phys. Sot. Japan 12,1344 (1957). 14. NOGAMI M., SEKINOBU M. and DOI H., Jap. J. appl. Phys. 3,572 (1964). 15. ROBERTS B. W., Phys. Rev. 104,607 (1956).
Pergamon Press 1968. Vol. 29, pp. 347-355.
Printed in Great Britain.
THE EXISTENCE REGION AND THE MAGNETIC AND ELECTRICAL PROPERTIES OF MnSb I. TERAMOTO* Philips Research Laboratories,
and A. M. J. G. VAN RUN
N.V. Philips’ Gloeilampenfabrieken, (Received
24August
Eindhoven-Netherlands
1967)
Abstract-The existence region of the ferromagnetic metallic compound MnSb is discussed. A description is given of Curie temperature and lattice parameter measurements carried out on samples equilibrated at several temperatures between 400 and 800°C and quenched. A relation was found between the Curie temperature and the composition of the MnSb phase, which proved to be independent of the annealing temperature. Density measurements showed that in non-stoichiometric MnSb the excess Mn-atoms occupy interstitial sites. For single crystals of different compositions the resistivity and the Seebeck coefficient are given as a function of the temperature. 1. INTRODUCTION MANGANESE
antimonide is a ferromagnetic compound which crystallizes in the hexagonal NiAs structure. Transition-metal pnictides of this type frequently have a broad existence region extending from the equiatomic composition to the metal side of the phase diagram. According to Guillaud[ 11 the magnetic transition of MnSb, between ferromagnetic and paramagnetic states occurs at a temperature between 363 and 587”K, depending on the composition. In order to reveal composition-sensitive properties in detail, single-phase samples of the appropriate compositions are required. The composition limits of stable MnSb, however, have not been established accurately in the published phase diagrams [2,3]. The present investigation was undertaken to determine quantitatively the effect of the composition of MnSb on the magnetic and electrical properties. 2. EXPERIMENTAL PROCEDURES
Powder samples of MnSb with compositions ranging from 38 to 55 at.% Sb, were prepared *Present address: Research Laboratory, Matsushita Electronics Corporation, Takatsuki, Osaka, Japan.
347
as follows. Mixtures of appropriate quantities of spectroscopically pure manganese and antimony were sealed in evacuated silica tubes and heated for about 16 hr at a temperature slightly higher than 9OO”C,followed by quenching in water. The reaction products obtained were finely ground in a mortar and sealed in evacuated silica tubes for further heat treatment to establish thermal equilibrium at the desired temperatures. Single crystals of MnSb were grown by the Bridgman method from the melt. Since the method involves the normal-freezing process, the composition of a specimen cut from an asgrown crystal is not accurately known, and it even may contain a second phase. Single-phase crystals with definite compositions were prepared as follows. A crystal of MnSb is annealed at a constant high temperature T, with a relatively large amount of MnSb powder of known composition A. At equilibrium the MnSb crystal is forced to accept the composition A. In order to obtain single-phase MnSb saturated with Mn or Sb at a given temperature T, the crystal is annealed with MnSb + Sb and MnSb + Mn 14951. The phases present in the samples were
I. TERAMOTO
348
and A. M. J. G. VAN RUN
identified by powder X-ray analysis and the unit cell dimensions were obtained in the same way. The magnetic measurements were made with a vibrating-sample magnetometer in fields up to 16 kOe and at temperatures ranging from 4 to approximately 670°K. From these measurements the atomic saturation moments, the ferromagnetic Curie temperatures, the asymptotic Curie temperatures and the Curie constants were determined. The electrical resistivity and the Seebeck coefficient of single crystals were measured from 80°K up to a temperature higher than the Curie temperature. The pycnometer method was used for determining the density of some of the powder samples. 3. RESULTS AND DISCUSSIONS
3.1. Curie temperature The Curie temperatures T, of samples equilibrated at 700°C and quenched were determined from the saturation magnetization versus temperature curves with an accuracy of ?YK (Fig. 1). The closed circles (0) re-
i
5
50
4.66474849
-at Fig. 1. Curie temperature
51
52
%Sh
T, as a function of composition.
present powders prepared from weighed amounts of manganese and antimony, and the open circles (0) are results for single crystals annealed with powders of known compositions. The open squares (0) represent singlephase compositions saturated with Mn or Sb, The Curie temperature changes with the composition in the single-phase region whereas it remains constant in the two-phase regions. The intersections of the extrapolated Curie temperature versus composition curve in the existence region with the horizontals in the two-phase regions represent the stability limits at the temperature at which the two-phase equilibrium is frozen in. From differential thermal analysis experiments (Fig. 2 (A)) it may be concluded that at the Sb-rich side the equilibrium is frozen in almost immediately upon quenching. At the Mn-rich side, however, thermal effects could not be detected by differential thermal analysis. It is therefore possible that the equilibrium at this side is frozen in at a lower temperature. The Curie temperature is proved to be independent of annealing temperature by the following experiment. Powders with the same composition, i.e. 49.0 at.% Sb, were equilibrated at several temperatures between 400 and 700°C and quenched. A Curie temperature of about 573°K was observed for all samples within the experimental error. The same experiment with powders of the composition 46-O at.% Sb in the temperature range 500800°K gave a Curie temperature of about 390°K. Consequently the Curie temperature is a function of composition only. The existence region as a function of temperature has been determined by Curie temperature measurements on quenched twophase powder samples which had been annealed at 800, 700, 600, 500 and 400°C for 5, 15, 70, 70 and 200 days, respectively. The composition of the MnSb phase can be found from the observed Curie temperature using the curve of Fig. 1. The results are given in Fig. 2. At the Sb-rich side they (0) agree well with the results of differential thermal analysis
EXISTENCE
REGION AND MAGNETIC
ELECTRICAL 1, (‘WIIO500
1 8
I
PROPERTIES OF MnSb
349
I
600 la (‘Cl t 500
I Mn
36
36
40
42 -at
44
46
40
50
b&l
52
5.4
‘1, Sb
Fig. 2. Partial phase diagram of the Mn-Sb system as determined from Curie temperature measurements (0,O) and differential thermal analysis (A).
measurements, indicating that the two-phase equilibria are frozen in immediately upon quenching. At the Mn-rich side the results (0) for the temperature range from 820 to 600°C are almost identical. This may indicate that the equilibrium MnSb-Mn,Sb has frozen in at about 550°C upon quenching. The dashed line could then represent the limit composition of MnSb at the Mn-rich side above 500°C. The discrepancy between our results and those of Guillaud (Fig. 1 (X)) can be ascribed to the fact that he did not account for the separation of second phase during very slow cooling (3 days). The existence region obtained properly
explains the difference in Curie temperature between the slowly cooled and the quenched samples reported by Takei et a!.[63 as being a consequence of the temperature dependence of the limit composition at the Sb-rich side. 3.2. Lattice parameters The unit cell dimensions a, c and c/a of the NiAs structure measured on powders equilibrated at 700°C and quenched are shown in Fig. 3 as a function of composition. For the compositions of 44.0 and 52-O at.% Sb a small amount of a second phase, Mn,Sb and Sb respectively, was detected. A linear change in lattice parameters with composition is found
350
I. TERAMOTO and A. M. J. G. VAN RUN
1.4204220 5790
I
I.410 4,2105780 !4oL, 4200 5770 13904.1905760 13804180 5750 070 4170 5.7&I om41Ml
sic?0
13504150 5720 13404uo 5no 13304130 5.700 1172L) 4120 5690 44
45
46
47
48
-at
49
50 51 O/6Sb
52
Fig. 3. Lattice parameters a and c, axial ratio c/u, and unit cell volume D = &z*c~3 as a function of composition.
(Vegard’s law). Figure 3 shows a close correspondence with Fig. 1. In the literature determinations of the lattice parameters of MnSb have dealt almost exclusively with the so-called stoichiometric composition [6,8]. According to the existence region given above, however, the stoichiometric composition falls outside the existence region at 700°C. The lattice parameters extrapolated to the stoichiometric composition (Fig. 3), i.e. a = 4.125, c = 5.799 and c/a = l-406, are in fair agreement with the results obtained by Willis and Rooksby[7] at room temperature, namely a = 4.128 and c = 5789. Takei et al.[6] obtained a = 4.130, c = 5.789 and c/a = 1.402 for a slowly cooled sample and for a quenched sample a = 4.153, c = 5.771 and c/u = 1.390. Comparison of their results with the present ones also indicates that the limit composition at the Sb-side shifts
to the stoichiometric one at lower temperatures. The slow cooling of one of the samples used by Takei et al. will allow the corresponding amount of free antimony to diffuse into the MnSb phase, giving a smaller value of a and a larger value of c. The unit cell volume increases with increasing Mn content, as is shown in the upper part of Fig. 3. 3.3. Other magnetic properties The susceptibility per gram, xQ, was measured as a function of temperature above the Curie temperature for single-phase compositions equilibrated at 700°C and quenched. The relations obtained between xs-’ and temperature T are shown in Fig. 4. For the 49 at.% Sb composition the paramagnetic behaviour can be described well by the Curie-Weiss law. With increasing excess Mn concentration the curve becomes less linear.
EXISTENCE REGION AND MAGNETIC ELECTRICAL
PROPERTIES OF MnSb
351
4 WI0
400
4.547
500
550
600 -T(OK)
650
Fig. 4. Reciprocal paramagnetic susceptibility xg? (per gram) as a function of temperature and composition.
From the straight parts found near the Curie temperature, the asymptotic Curie temperatures, T,, and the Curie constants per gram, C,, are obtained. The T, values agree with the ferromagnetic Curie temperatures T, within ten degrees. The effective number of Bohr magnetons per manganese atom, np, calculated from the Curie constant C,, and the effective moments from the ferromagnetic saturation extrapolated to O”K, nf, are given in Table 1. Table I. Magnetic properties of homogeneous MnSb equilibrated at 700°C and quenched. T, is the Curie temperature; np and nf are the effective moments per Mn atom deduced from the Curie constant and from the ferromagnetic saturation, respectively Composition (at.% Sb)
T,
nP
nf
(“K)
(PB)
(PB)
46.0 47.0 48.0 49.0 49.7 50.0
390 488 517 573 592 (600) *
*Extrapolated
values.
4.11 4.20 4.21 4.21 (4.21)*
2.45 2.82 2.98 3.28 (3.55) *
The nP value changes only slightly with composition and agrees with that obtained by Serres[9], while the change of nf with composition is found to be much greater. An extrapolation of the atomic saturation moment vs. composition curve gives approximately 3.55 pE at the stoichiometric composition, which is in agreement with the results of Hirone et al.[ lo] 3.4 pE and of Guillaud[l] 3.53 ,..+ The results obtained by Takei et a/.[6], who reported 3.53 pB and 3.37 pB for the slowly cooled and the quenched MnSb are again accounted for as respectively, resulting from a shift of the limit composition with temperature. In samples with a composition of 46 at.% Sb, an irreversible increase of the susceptibility was observed when the measurement was repeated from room temperature up to 400°C. The result is shown in Fig. 5, where, in the fifth measurement, the sample was cooled after being held at 400°C for 30 min. The deviation of the reciprocal susceptibility vs. temperature curve from the Curie-Weiss law becomes more marked with the heat treatment repetition. The curves in the high-temperature region indicate an additional asymptotic Curie temperature of about 550°K. This irreversible
352
I. TERAMOTO and A. M. J. G. VAN RUN
400
450
550
650
600 -
!m
T(OK)
Fig. 5. Irreversible behaviour of MnSb with acomposition of 46 at.% Sb during repeated measurements of the susceptibility as a function of temperature.
behaviour is caused by the precipitation of a small amount of a second phase, Mn,Sb, which changes the composition of the MnSb phase. This is concluded from the following facts. 1. The additional asymptotic Curie temperature observed in Fig. 5 is very close to that of Mn,Sb. 2. A single-phase sample with the composition 46 at.% Sb quenched from 700°C and having a Curie temperature of 390”K, was annealed at 350°C for a few days, followed by quenching. The Curie temperature was now observed to be 45°K higher than before the heat treatment. This suggests that a Mn-richer
second phase, Mn,Sb, precipitated during the low-temperature heat treatment, consequently increasing the concentration of Sb in the MnSb phase. 3. A direct observation of the precipitation of the second phase is possible. A single-phase single crystal equilibrated at 700°C and quenched had the Curie temperature of 335°K and consequently a composition of 45.5 at.% Sb. This crystal had been annealed at 350°C for a few days and quenched. Subsequent microscopic observation of the crystal, polished parallel and perpendicular to the crystallographic c-axis, clearly showed that the crystal
Fig. 6. Photomicrographs of an originally single-phase, single crystal with a composition of 45.5 at. % Sb, after annealing at 350°C for a few days: (a) 11to c-axis; (b)l to c-axis.
[Facing page 3521
EXISTENCE REGION AND MAGNETIC ELECTRICAL PROPERTIES OF MnSb was no longer homogeneous but included a number of precipitates, which were not observed before the heat treatment. The photomicrographs (Figs. 6(a) and 6(b)) show that the precipitates have a needle shape parallel to the c-axis.
3.4. Electrical resistivity and Seebeck coeficient Figure 7 gives typical curves of the electrical resistivity p as a function of temperature, measured on single crystals of MnSb in the direction perpendicular to the crystallographic c-axis. The curves show an abrupt change of slope at a temperature TcP, in such a way that the resistivity increases with increasing temperature up to T,, and becomes practically X10
353
independent of temperature above T,. The break temperature T,, is approximately equal to the ferromagnetic Curie temperature T, within the limits of experimental error. The behaviour shown can be explained in terms of a spin-disorder scattering mechanism[ 1 l] in the metallic compound, a mechanism which has proved useful in explaining data on other ferromagnetic compounds with the NiAs structure [ 121. The residual resistivity at 0°K differs considerably from sample to sample. The crystals, however, had been forced to accept the composition of the MnSb powders in the presence of which they were annealed. This change of composition adversely influences the quality of the crystals. This may also explain the fact that the residual resistivities of our samples
-4
Fig. 7. Resistivity p as a function of temperature.
I. TERAMOTO and A. M. J. G. VAN RUN
354
are considerably higher than values reported in the literature [ 12, 131. The Seebeck coefficients (Yas a function of temperature are shown in Fig. 8. The sign is always negative, being opposite to that of the Hall coefficient measured by Nogami et ~21.1141. The curves are similar in behaviour to the resistivity curves shown in Fig. 7; with increasing temperature the Seebeck coefficient increases in magnitude up to the Curie temperature, where it reaches a maximum value. 3.5. Density measurements For a better understanding of the composition-dependent properties of MnSb described above, a knowledge of the predominant type of point defects involved in non-stoichiometric MnSb is required. Since this compound has a broad existence region, a comparison of the density of samples of different compositions is sufficient for the present purpose. The density was measured with a pynco-
meter at room temperature on two MnSb powder samples, of compositions 46 and 49 at.% Sb, which had been equilibrated at 700°C and quenched. The results are shown in Table 2. For comparison, values of the density Table 2. Densities of MnSb with the compositions 46.0 at.% Sb and 49.0 at.% Sb and their ratio. Calculated values are deduced assuming Mn-interstitials, Sb-vacancies and antistructure for the point defects caused by non-stoichiometry Composition (at.% Sb) 46.0 49.0 Ratio
Calculated density Observed density
Mn-int.
Sb-vat.
Anti-str.
7.075 6.920 1.022
6.027 6.647 0.907
6.509 6.782 0.960
6.946 6.842 1.015
calculated from the observed lattice parameters for three possible types of point de-
-20
fNiP”& t -15
- II
-5
Fig. 8. Seebeck coefficienta
-TCW as a function of temperature.
EXISTENCE REGION AND MAGNETIC ELECTRICAL
authors wish to thank Dr. C. Haas and Dr. W. Albers for helpful discussions. Thanks are due to Dr. G. van Aller for carrying out the electrical measurements, to Mr. J. Verberkt for performing the differential thermal analysis and to Miss A. Reitsma for carrying out some of the magnetic measurements.
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(1943);Ann.dePhys.4,671(1949).
I
I I
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2. HANSEN M., Constitution of Binary Alloys, 2nd edn. (1958). 3. MURAKAMI T. and HATTA A., Sci. Rep. Tohoku Uniu.22,88(1933). 4. ALBERS W., .I. them. Phys. 43,4401(1965). 5. ALBERS W. and ATEN A. C., Philips Res. Rep. 20, 556 (1965).
6. TAKE1 W. J., COX D. E. and SHIRANE G., Phys. Reu.129,2008 (1963). 7. WILLIS B. T. M. and ROOKSBY H. P., Proc. phys. Sot. B67,290 (1954). 8. LOTGERING F. K. and GORTER E. W., J. Phys. Chem. Solids 3,238
NiAs
0
0
M&c?/
3.55
Acknowledgements-The
fects, Mn-interstitials, Sb-vacancies, and antistructure (i.e. Mn on Sb sites) are included. This comparison shows that almost all the excess manganese atoms in the homogeneous MnSb occupy interstitial sites. This is consistent with the fact that the unit cell volume increases with increasing Mn content, as is shown in Fig. 3. Crystallographically this means a continuous change from the NiAs structure in the direction of the N&In structure (Fig. 9). The
c
PROPERTIES OF MnSb
Ni$n
Metalloid
Fig. 9. Unit cell of the NiAs and of the Ni,In structure.
(1957).
9. SERRES A., .I. Phys. Radium, Paris 8, 146 (1947). 10. HIRONE T., MAEDA S., TSUBOKAWA I. and TSUYA N.,J.phys. Sot. Japan 11,1083 (1956). T. and DEKKER A. J., Il. van PESKI-TINBERGEN Physica29,917(1963).
12. ALBERS W. and HAAS C.,Proc.
occurrence of interstitial metal atoms in the NiAs structure was assumed by Roberts[ 151 to explain the native disorder of stoichiometric MnBi.
Int. Conf. Physics of Semi-conductors, Paris (1964); ALBERS W. and HAAS C.,Phys. Lett. 8,300 (1964). 13. SUZUOKA T., J. phys. Sot. Japan 12,1344 (1957). 14. NOGAMI M., SEKINOBU M. and DOI H., Jap. J. appl. Phys. 3,572 (1964). 15. ROBERTS B. W., Phys. Rev. 104,607 (1956).