Effects of Heat Treatment on the Magnetic Phase Transition
and Magnetocaloric Properties of Mn
1þAs
1xSb
xHirofumi Wada
1, Chie Funaba
2;*1and Tetsuya Asano
2;*21
Department of Physics, Kyushu University, Fukuoka 812-8581, Japan
2Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan
We studied effects of heat treatment on the magnetic transition and the magnetocaloric effects (MCEs) of Mn1þAs1xSbxwithx¼0:1and 0.25. It is found that sintering at an appropriate temperature gives a sharp magnetic transition and hence giant MCEs for Mn1þAs1xSbx. In the bulk samples slowly cooled from a liquid state, free Sb was precipitated, which gives rise to compositional deviation. Quenching from a liquid state suppresses the precipitation of Sb. Subsequent annealing is effective to improve homogeneity, as a result, the sample undergoes a sharp magnetic transition. The optimum conditions of heat treatments of the present system for a sharp magnetic transition are discussed.
(Received October 11, 2005; Accepted November 28, 2005; Published March 15, 2006)
Keywords: magnetic phase transition, heat treatment, magnetic entropy, magnetocaloric effect
1. Introduction
The magnetocaloric effects (MCEs) mean the isothermal entropy change or the adiabatic temperature change caused by a magnetic field. Magnetic refrigeration is based on the magnetocaloric nature of ferromagnetic materials. The compounds with large MCEs are of particular interest, because they can be applied to magnetic refrigerant materi-als. Recently, we have found giant MCEs of MnAs1xSbx near room temperature.1,2)MnAs with the hexagonal NiAs-type structure is a ferromagnet with the Curie temperature,
TC, of 317 K.3) This compound undergoes a first-order magnetic transition (FOMT) atTC. The transition is accom-panied by a structural transformation from the hexagonal NiAs-type structure to the orthorhombic MnP-type structure. The magnetic entropy change caused by a magnetic field,
SM, of MnAs exceeds 30 J/K kg in a field change of 5 T. This value is about twice as large as that of Gd5Si2Ge2 and MnFeP0:45As0:55, which are known as materials with huge
MCEs.4,5) The adiabatic temperature change,T
ad, reaches 13 K in a field change of 5 T. Thermodynamically, SM is related to the temperature derivative of magnetization, M, through the Maxwell relation,
SM¼ ZH
0
dM dT
H
dH: ð1Þ
In the FOMT, jdM=dTj is quite large at around TC, which gives a largeSMvalue.
The substitution of Sb for As stabilizes the NiAs-type structure. Although the FOMT is suppressed, the magnetic transition of MnAs1xSbxis still sharp for0:05x0:30. The Curie temperature can be tuned between 220 and 318 K without any significant reduction of MCEs.1,2)These results indicate that MnAs1xSbx is a possible candidate for magnetic refrigerant materials near room temperature.
This paper presents effects of heat treatment on the magnetic transition of Mn1þAs1xSbx. The samples of
MnAs1xSbxare usually prepared by a solid-vapor reaction. In our previous studies, the samples were sintered at 800C.2,6) We found that a small amount of Sb was precipitated as a secondary phase, when the equiatomic composition of Mn1:00As1xSbxis used as an initial concen-tration in the sample preparation.6) This fact suggests that MnAs1xSbx forms a single phase with the NiAs-type structure in the Mn-rich region from the stoichiometry, similarly to the binary Mn–Sb system. In the latter system, excess Mn atoms are considered to occupy the interstitial site at random.7)We believe that some of Mn atoms must occupy the interstitial site to keep the NiAs-type structure in the ternary system, too. In such nonstoichiometric compounds, heat treatments often have strong influences on magnetic properties. In fact, we observed double peaks in the specific heat curve of MnAs0:75Sb0:25 at around TC, suggesting compositional inhomogeneity in the sample.
The first purpose of the present study is to investigate optimum conditions of heat treatment for the appearance of giant MCEs in Mn1þAs1xSbx. We have examined various heat treatments and found that sintering conditions play a crucial role in the magnetic transition.
The samples obtained by sintering are generally porous and brittle. They are easily broken into powders. This makes it difficult to measure bulk properties, such as thermal conductivity and electrical resistivity, accurately. Another purpose of this study is to produce bulk ingots of Mn1þ
As1xSbx. For this purpose, the samples were melted and then subjected to various heat treatments. In this paper, we will report magnetic properties of both sintered samples and bulk samples of Mn1þAs1xSbx.
2. Experiments
Polycrystalline samples of Mn1:02As0:9Sb0:1 and Mn1:05 As0:75Sb0:25 were synthesized in this study. Here, Mn is slightly enriched from the equiatomic composition to prevent precipitation of Sb, as described previously. Powder of Mn (3 N), As (6 N) and Sb (5 N) weighted in a required proportion were sealed in an evacuated quartz tube covered inside by carbon. The tube was heated at 600C for three days
*1Undergraduate Student, Kyoto University
*2Graduate Student, Kyoto University
Special Issue on Research on New Magnetic Properties and Applications to Materials Science in Itinerant-Electron Systems
followed by annealing atT1¼800or 1000C for four days. This is a preliminary reaction process. For sintering, the resultant products were pulverized, mixed, sealed again and heated at T2 for another seven days. The sintering temper-ature,T2, was varied from 600 to 900C. Bulk ingots were obtained in the following way. After a preliminary reaction, the products were pulverized, mixed and sealed in a quartz tube together with a small amount of Ar gas. To prevent a reaction between the sample and a quartz tube, we used a carbon crucible. The samples were melted at 1000C. After
that, some samples were quenched into iced water, others were cooled down to room temperature in the furnace. Finally, the samples were annealed atT3¼700or 800C for four days. Samples were checked by X-ray diffraction measurements. The composition analyses were made for Mn1:02As0:9Sb0:1 bulk samples with electron probe micro-analysis (EPMA). Magnetic measurements were performed with a commercial superconducting quantum interference device (SQUID) magnetometer. The magnetic entropy change, SM is usually estimated from eq. (1). For MnAs1xSbx, however, we found thatSMin a field change of 1 T is almost proportional todM=dTat a magnetic field of 1 T. Approximately, 14107Wb m/kg K of dM=dT corresponds to 0.85 J/K kg ofSM. Thus, we measuredM–T curves at 1 T for rough estimation ofSM. For some samples, we evaluatedSMfrom eq. (1).
3. Results
3.1 Sintered samples
Figure 1(a) shows the temperature dependence of M of Mn1:02As0:9Sb0:1 samples with T1¼800C and T2¼700, 800 and 900C. All the samples were confirmed to be a single
phase with the NiAs-type structure by X-ray diffraction. Nevertheless, the magnetic behavior strongly depends onT2. The magnetization of the sample sintered at 700C shows
gradual temperature variation, while the sample sintered at 900C exhibits a quite sharp transition. This is clearly seen in
thedM=dTvs.Tcurves depicted in Fig. 1(b). The maximum value of jdM=dTj for T2¼900C is about 234
107Wb m/kg K, which is four times as large as that for
T2 ¼800C. The Curie temperature also depends onT2. The
TCdetermined from the peak position ofdM=dTvs.Tplots is 271, 268 and 302 K for T2¼700, 800 and 900C, respec-tively.
The M–T and dM=dT–T curves of Mn1:05As0:75Sb0:25 sintered at various T2 are illustrated in Figs. 2(a) and (b), respectively. The preliminary reaction was done at 800C.
The samples sintered at600CT
2800Cwere of single phase with the NiAs-type structure. Although the sample sintered at 900C had the same structure, some of X-ray
diffraction peaks were split into two peaks, indicating phase segregation. We found that the ingots were melted during sintering at 900C. Correspondingly, the sample with T
2¼
900C shows a broad magnetic transition. The dM=dT–T
curve for T2¼800C exhibits two peaks at around TC. On the other hand, the transition for T2¼700C is extremely sharp. The maximum value of jdM=dTjreaches 284
107Wb m/kg K. These results have revealed that sintering temperature has strong impacts on the magnetic transition of
Mn1þAs1xSbx. The optimum sintering temperature for a sharp transition depends on the Sb content.
We have also examined magnetic behavior of Mn1þ
-As1xSbx for which a preliminary reaction was done at T1 ¼1000C. However, no quantitative differences were observed between the M–T curves for T1 ¼1000C and those forT2¼800C. In Figs. 3(a) and (b), theM–T curves and thedM=dT–T curves of the samples with T1¼1000C for x¼0:1and 0.25 are displayed together with those with
T1 ¼800C. Here, sintering was done at optimum temper-atures (900C for x¼0:1 and 700C for x¼0:25). The
Curie temperatures of the samples with T1 ¼1000C are exactly the same as those with T1¼800C. The maximum values ofjdM=dTjforT1¼1000C are comparable to those for T1 ¼800C. These results indicate that the magnetic transition is only sensitive toT2in Mn1þAs1xSbx.
The magnetic entropy change, SM, in a field change of 1 T of Mn1:05As0:75Sb0:25 with T2¼700C was estimated from eq. (1), which is shown in Fig. 4 as a function of temperature. In this figure, previous results of Mn1þ
-As0:75Sb0:25 with T2¼800C are also shown.6)The figure indicates that the peak value of jSMj of the newly synthesized compound exceeds those of previous samples
M
/ Wb m kg
-1
T
/ K
T2
700°C 800°C 900°C
(a)
µ0H=1T×
4
π
×10
-7Mn
1.02As
0.9Sb
0.1220
240
260
280
300
320
340
0
20
40
60
80
100
T
/ K
-dM/dT
/ Wb m kg
-1
K
-1
Mn
1.02
As
0.9Sb
0.1µ0H=1T
T2
700°C 800°C 900°C
(b)
×4π×10-7220
240
260
280
300
320
340
0
5
10
15
20
25
[image:2.595.314.542.75.441.2]by a factor of 1.4. Although the peak value of jSMj of Mn1:05As0:75Sb0:25is a little smaller than that of MnAs, 30 J/ K kg, it is still in a high level of the MCEs. These results have demonstrated the new synthesis method improved magneto-caloric properties of MnAs1xSbx.
3.2 Bulk samples
To obtain bulk samples, a melting process is necessary. The ingots of Mn1:02As0:9Sb0:1were obtained from solutions either by quenching into iced water or by slow (furnace) cooling. The cooling rate of the latter heat treatment is approximately 100C/h. X-ray diffraction measurements
have shown that these samples are not of single phase but contain Sb as a secondary phase. So, we annealed these samples at T3¼800C for four days. In the X-ray diffraction patterns of the annealed samples, the impurity peak of free Sb was still detected, but its intensity is much reduced for the annealed sample after quenching. Figures 5(a) and (b) show the secondary electron images of annealed samples. Energy dispersive spectra revealed that the bright part is precipitated Sb. In the image of the annealed sample after slow cooling, we see large precipitates of Sb, while precipitation of Sb is much suppressed in the annealed
sample after quenching. The compositions of the main phase of various Mn1:02As0:9Sb0:1 determined by EPMA are listed in Table 1.
T / K
-dM/dT
/ Wb m kg
-1
K
-1
×
4
π×
10
-7(b)
µ0H=1TT2
600°C 700°C 800°C 900°C
Mn
1.05As
0.75Sb
0.25180
200
220
240
260
0
5
10
15
20
25
30
T / K
M
/ Wb m kg
-1
T2
600°C 700°C 800°C 900°C
×
4
π×
10
-7(a)
µ0H=1TMn
1.05As
0.75Sb
0.25180
200
220
240
260
0
20
40
60
80
100
120
Fig. 2 (a) The M–T curves and (a) the dM=dT–T curves of Mn1:05 As0:75Sb0:25sintered at600CT2900Cafter a preliminary reaction at 800C.
T / K
M
/ Wb m kg
-1
Mn1.05As0.75Sb0.25
Mn1.02As0.9Sb0.1
T1=800°C T1=1000°C
T1=800°C T1=1000°C
×4π×10-7
(a)
µ0H=1T
180 200 220 240 260 280 300 320 340 0
20 40 60 80 100 120
T / K
-dM/dT
/ Wb m kg
-1 K
-1 Mn1.02As0.9Sb0.1
T1=800°C T1=1000°C
Mn1.05As0.75Sb0.25
T1=800°C T1=1000°C
×4π×10-7
(b)
µ0H=1T
180 200 220 240 260 280 300 320 340 0
5 10 15 20 25 30
Fig. 3 (a) The M–T curves and (b) the dM=dT–T curves of Mn1:02 As0:9Sb0:1and Mn1:05As0:75Sb0:25, for which a preliminary reaction was done atT1¼1000and 800C. Sintering was done at optimum temper-atures (900C forx¼0:1and 700C forx¼0:25).
T
/ K
-∆
S
M/ J kg
-1
K
-1
Mn1.05As0.75Sb0.25 (T2=700°C)
Mn1.05As0.75Sb0.25 (T2=800°C)
Mn1.03As0.75Sb0.25 (T2=800°C)
0-1 T
160
180
200
220
240
260
0
5
10
15
20
25
30
Fig. 4 Temperature dependence of the magnetic entropy change,SM, in a
[image:3.595.55.285.72.448.2] [image:3.595.313.540.74.429.2] [image:3.595.314.542.504.683.2]We found that the Sb content of the samples is less than the initial composition. Previously, we analyzed the composition of Mn1:03As0:75Sb0:25 sintered at 800C by EPMA.6) The chemical analyses indicated that the exact composition is Mn1:02As0:74Sb0:24, being very close to the initial composi-tion. This is consistent with the facts that no trace of free Sb was detectable in the X-ray diffraction patterns of sintered Mn1þAs1xSbx in the present study. These results suggest that a melting process gives rise to compositional deviation from the initial concentration in Mn1þAs1xSb.
The M–T anddM=dT–T curves of Mn1:02As0:9Sb0:1 bulk samples are shown in Figs. 6(a) and (b), respectively. The as-slowly-cooled sample shows a broad magnetic transition, while a relatively sharp transition is observed in the as-quenched sample. For both cases, subsequent annealing
makes the transition sharper. In particular, the annealed sample after quenching undergoes a quite sharp magnetic transition. The peak value ofjdM=dTjis comparable to that of the sample sintered at the optimum temperature. Similar results were also obtained for Mn1:05As0:75Sb0:25 bulk samples (annealed at 700C), which are illustrated in Figs. 7(a) and (b). In the figures, the results of the sintered sample are also shown. Compared with x¼0:1, effects of quenching and subsequent annealing on a magnetic transition is more significant for x¼0:25. Furthermore, the Curie temperature of the annealed sample after quenching is in agreement with that of the sintered sample.
4. Discussion
The present study has revealed that heat treatments have significant influences on the magnetic transition of Mn1þAs1xSbx. Sintering at an appropriate temperature leads to a quite sharp transition and hence largeSMin the present system. The optimum sintering temperature becomes lower with increasingx. For bulk samples, quenching from a liquid state and subsequent annealing is effective to obtain a
[image:4.595.56.282.72.429.2]Fig. 5 Secondary electron images of (a) Mn1:02As0:9Sb0:1 annealed after slow cooling and (b) that annealed after quenching.
Table 1 Compositions of Mn1:02As0:9Sb0:1compounds after various heat treatments determined from EPMA.
Heat treatment Mn As Sb
Annealed at 800C after
1.06 0.887 0.072
slow cooling
As quenched 1.05 0.886 0.085
Annealed at 800C after
1.04 0.919 0.064
quenching
T / K
M
/ Wb m kg
-1
as s. c. as q.
a. at 800°C after q.
a. at 800°C after s. c.
(a)
µ0H=1T
Mn1.02As0.9Sb0.1
×4π×10-7
240 260 280 300 320 0
20 40 60 80 100 120
T / K
-dM/dT
/ Wb m kg
-1 K
-1
as s. c. as q.
a. at 800°C after q. a. at 800°C after s. c.
(b)
µ0H=1TMn1.02As0.9Sb0.1
×4π×10-7
240 260 280 300 320 0
5 10 15 20 25 30
[image:4.595.313.540.75.441.2] [image:4.595.46.291.505.581.2]sharp transition. In this case, a small amount of free Sb was precipitated, which causes compositional deviation from the initial concentration in the main phase. These results suggest that the Mn1þAs1xSbx compounds are formed peritecti-cally. In the binary Mn–Sb system, Mn1þSb with the
NiAs-type structure exists in a wide concentration range of
0:040:44.8) While MnAs is a congruent compound, Mn1þSb is formed by a peritectic reaction at 840C.9)When
the sample with lowis cooled from a liquid state, Mn1þ0Sb with0> first appears and finally Mn
1þSb is formed by
consuming the liquid completely. Although no phase diagram of the ternary system has been reported, we believe that Mn1þAs1xSbx is formed in a similar manner. In sintering, a reaction occurs in the solid phase, which causes no precipitation of Sb. Thus, the exact composition is close to the initial concentration. Judging from the magnetic tran-sition, the sample becomes more homogeneous, as T2 is higher. WhenT2is higher than the solidus line, the sample is partially melted and the magnetic transition becomes broad. This is the case of Mn1:03As0:75Sb0:25 annealed at T2¼
900C. The optimum T2 of x¼0:1 is higher than that of
x¼0:25. This is consistent with the fact that the melting
point of MnAs is higher than that of Mn1þSb. The bulk
samples experienced a liquid state. If the samples had been cooled very slowly, we would have a single phase. Indeed, Gover et al. obtained single crystals of Mn1þAs1xSbx by the Bridgeman method.10) In our case, free Sb was precipi-tated in the slowly cooled sample. This is because our cooling rate (100K/h) is too fast to realize an equilibrium state during the cooling. Furthermore, this cooling rate gives rise to compositional inhomogeneity in the main phase. As a result, the magnetic transition is broadened. On the other hand, quenching suppresses the precipitation of Sb. Further annealing at a high temperature below the solidus line is effective to improve compositional homogeneity in the main phase. These features are often observed in an incongruent system.
The present results indicate that Mn1þAs1xSbxshows a quite sharp magnetic transition, when the optimum heat treatment is done. Recently, Ishikawa et al. measured the temperature dependence of lattice parameters of our Mn1:02As0:9Sb0:1 sample annealed after quenching.11) They observed discontinuities in the temperature dependence of lattice parameters, a and c at TC. The lattice parameter a shrinks, whilecexpands atTCwith increasing temperature. In a very narrow temperature range nearTC, the coexistence of two phases,i.e., a ferromagnetic phase and a paramagnetic phase was confirmed. No structural transformation was observed at TC. These results clearly indicate that the compound undergoes a FOMT at TC without a change of crystal structure. Gover et al. also reported a FOMT of MnAs1xSbxsingle crystals withx0:5.8)Therefore, giant MCEs of the present system originate in a FOMT. The origin of a FOMT of MnAs1xSbx is debatable. Long time ago, Bean and Rodbell explained a FOMT of MnAs by assuming strong volume dependence of the exchange interaction within a localized moment model.12)Quite recently, von Rankeet al. discussed a MCE of MnAs1xSbx on the basis of this model.13)They successfully explained the giantS
M of the compounds with0x0:3. However, it is now recognized that a localized moment model is insufficient to describe magnetic properties of metallic materials. Another approach is an itinerant electron theory of metamagnetism. Yamada has shown that a first-order ferromagnetic to paramagnetic transition can take place in the framework of the theory of itinerant electron magnetism, in which spin fluctuations are renormalized, when band parameters satisfy certain condi-tions.14) This model has been successfully applied to Co(S1xSex)2and La(Fe1xSix)13systems.15)However, early band calculations on MnAs did not give positive results on the conditions.16) To elucidate the origin of the FOMT, further studies on electronic structures of MnAs1xSbx are strongly desired.
In conclusion, we have revealed that the magnetic transition of Mn1þAs1xSbx strongly depends on the heat treatment. When sintered at an optimum temperature, the sample shows an extremely sharp transition. For bulk samples, quenching from a liquid state and subsequent annealing leads to a FOMT. These results suggest that Mn1þAs1xSbx is an incongruent compound. As described in Ref. 6), the Curie temperature of the present system strongly depends not only on the Sb content but also on the
T
/ K
×
4
π×
10
-7M
/ Wb m kg
-1
as s. c.
a. at 700°C after s. c.
a. at 700°C after q.
sintered at at700°C
(a)
µ0H=1TMn
1.05As
0.75Sb
0.25180
200
220
240
260
0
20
40
60
80
100
120
T
/ K
-dM/dT
/ Wb m kg
-1
K
-1
×
4
π×
10
-7as s. c.
a. at 700°C after s. c.
a. at 700°C after q.
sintered at at700°C
Mn
1.05As
0.75Sb
0.25(b)
µ0H=1T180
200
220
240
260
0
5
10
15
20
25
30
[image:5.595.56.284.69.444.2]excess Mn content. Nevertheless, we obtained reproducible results by sintering the sample at the optimum temperatures for both x¼0:1 and 0.25, as shown in Figs. 3(a) and (b). This is quite important for practical applications.
Acknowledgments
The authors are indebted to Prof. K. Fukamichi for his continual encouragement until now. We are also grateful to Prof. S. R. Nishitani for useful discussion. This work was supported by New Energy and Industrial Technology Development Organization (NEDO), by Kansai Research Foundation for technology promotion and by Iketani Science and Technology Foundation.
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