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Field-induced transitions in intermetallic compounds Mn2−xCoxSb (x⩽0.15)
PHYSICA 1
Physica B 177 (1992) 119-122 North-Holland
Field-induced Mn,_,Co,Sb T. Kanomataa,
transitions (x G 0.15)
Y. Hasebe”,
in intermetallic
T. Kaneko’,
compounds
S. Abeb and Y. Nakagawab
“Faculty of Engineering, Tohoku Gakuin University, Tagajo, Miyagi 985, Japan bInstitute for Materials Research, Tohoku University, Katahira, Sendai 980, Japan
and Mn, 8,Co,,,,Sb were studied in magnetic fields up to 150kOe. Field-induced transitions in Mq,r Co,,,Sb Antiferromagnetic (AF)-ferrimagnet~c (Fr) field-induced transitions in Mn, 85Co,,,Sb and intermediate (I)-Fr transitions in Mn, ,,Co,,,Sb are of the first order. Temperature variations of transition fields were determined.
1. Introduction The compounds Mn,_,Co,Sb(x 6 0.37) crystallize in the tetragonal Cu,Sb-type structure [I]. Mn,Sb is a fe~imagnet with a Nitel temperature of 550 K. The spin ordering of the ferrimagnetic (Fr) state for Mn,Sb is an antiparallel arrangement of unequal magnetic moments associated with two kinds of manganese atoms (Mn,, Mn,,) present in a unit cell as shown in fig. 1, where
the Mn, and Mn,, atoms have magnetic moments of 2.1~~ and 3.9~~) respectively [2]. For Mn,_,Co,Sb, all of the compounds with the composition O.lO
2. Experimental
I d
(x = 0.09 and The compounds Mn,_,Co,Sb 0.15) were prepared by the ceramic method 131. X-ray diffraction studies showed that all diffraction lines could be indexed with the tetragonal Cu,Sb-type structure. The lattice parameters of Mn,_,Co,Sb (x = 0.09 and 0.15) agree well with those previously reported by Kanomata et al. [4]. According to the results of Miissbauer [5] and neutron diffraction [6] investigations, the Co atoms preferentially occupy site I in Mn,Sb. Magnetization was measured by using a vibrating sample magnetometer in fields up to 150 kOe which were generated by a water-cooled solenoid of the Bitter type at the High Field Laboratory, Tohoku University.
lr!!iLL 0
---i-
a
-T
l
-a-
l
Mn, (site I I
0
Mnl
(SiteIt)
@I Sb Fig. 1. Crystal structure of Mn,Sb. 0921-4526/92/$05.00 0
1992 - Elsevier Science Publishers B.V. All rights reserved
T. Kanomata et al. I Field-induced
120
3. Results and discussion
Figure 2 shows the magnetization (T versus field at various temperatures for Mn,,,,Co,,,,Sb. We observe the field-induced metamagnetic phase transitions from the AF to the Fr state. The field-induced transition fields, Her and Hcl, are defined as shown with arrows in the figure. According to the results of neutron diffraction investigation of Mn 1,80Co,.,,Sb [6], the Fr and AF phases have spin structures of the Mn,Sb and Mn,As types, respectively. Therefore, it is considered that the first-order magnetic phase transition shown in fig. 2 takes place between the spin
transitions in Mn2_,Co,Sb
structures of the Mn,As and Mn,Sb types. Figure 3 shows the temperature dependence of magnetization for Mn,.,,Co,,,,Sb in various fields. In the temperature range from 120 to 200 K in a field of 150 kOe, the magnetization is -20% higher than the saturation magnetization of Mn,Sb in the same temperature range. Magnetization curves of Mn,.,,Co,.,,Sb at various temperatures are shown in fig. 4. Figure 5 shows the temperature dependence of magnetization for Mn,,,,Co,,,,Sb in various fields. There appears a magnetization of -lOemu/g for Mn,,,,Co,,,,Sb at low field, suggesting the appearance of a new phase (intermediate one, I).
Mn,.,,CBosSb
60
1 I
H
Fig. 2. The magnetization tures for Mn, &oO ,,Sb.
0
150
100
50
50
(T versus
100 H
(kOe) field at various
I
tempera-
Fig. 4. The magnetization tures for Mn, ,,Co, ,,,Sb.
150
(kOe)
c versus
field at various
tempera-
I
50 -
60
Mn,,%,Sb . H=lSOld)e a H=lOOkOe ” Hz 50kOe oH=20kOe
-
0 H=7.6kOe
0
0 0
rrm_..o.n ,.,.oo-d 100
0
I
I 200
300
400
Oc. 0.3 00
so 500
T(K)
Fig. 3. The magnetisation fields for Mn, BsCo,, ,,Sb.
(T versus
T(K)
temperature
in various
Fig. 5. The magnetization fields for Mn,,,,Co,,,,Sb.
w versus
temperature
in various
T. Kanomata et al. I Field-induced
With further increase of the field, the magnetization of the compound increases abruptly. This corresponds to the first-order transition from the I to the Fr state. In the temperature range from 5.9 to 135 K in a field of 150 kOe, the magnetizations of Mn,,,,Co,,,,Sb are -30% higher than the saturation magnetization of Mn,Sb in the same temperature range; Her - HEl at 5.9 K is 71.4 kOe. By extrapolating the magnetization versus temperature curves at 150 kOe in the Fr state to 0 K in figs. 3 and 5, averaged magnetic moments are found to be 2.2b and -2.2/1, for Mn,,,,Co,.,,Sb and Mn,,$o,.,,Sb, respectively. The increase of moment by substitution of Mn for Co is explained by assuming that nonmagnetic Co atoms occupy site I and the magnetic moments of Mn, and Mn,, atoms have values equal to those of Mn,Sb in the pseudobinary system Mn,_,Co,Sb. The temperature variations of the observed transition fields are shown in figs. 6 and 7 for Mn,,,,Co,,,,Sb and Mn,,,,Co,,,,Sb, respectively. The values of dH,,ldT just below the AF-Fr transition temperature T, are estimated to be - 1.42 and -0.86 kOe/K for Mn,,,,Co,.,,Sb and Mn,,,, Co,,,,Sb, respectively. The total entropy change AS at T, can be calculated from the following thermodynamic ex-
OOW
200 1 (K)
Fig. 6. Transition field versus temperature for Mn,,,,CoO,,,Sb. Hc+ for increasing field and H,, for decreasing field.
transitions in Mn, _,Co,Sb
121
T(K)
Fig. 7. Transition field versus temperature for Mn,,,,Co,,,,Sb. Her for increasing field and H,, for decreasing field.
pression AS = -Aa(dH,/dT)
,
in which Au is the change in magnetization at the magnetic transition. AS of Mn,.,,Co,,,,Sb and Sb are estimated to be 0.17R and Mn,.,,Coo.o, O.llR (R is the gas constant) using the values of dH,,ldT and Au obtained in this study, respectively. According to the results of neutron diffraction for Mn 1.80C00,20Sb[6], the AF-Fr phase transition is accompanied by a change in the magnetic moments of the Mn atoms. In particular, the magnetic moments on the Mn,, atoms are found to be -3.3b just below T, and -2.4~~ just above T,. Then, the magnetic contribution in AS is estimated to be -O.lBR, which is comparable to the values of AS mentioned above. Thus, the main source of the entropy change at T, is considered to be ascribed to the magnetic contribution, especially to that from the magnetic moment fluctuation at the Mn,, site. Kittel [7] and Jarrett [8] discu$sed the firstorder AF-Fr phase transition of Mn,_,Cr,Sb by a two-sublattice model assuming that the net exchange interaction along the c-axis between the magnetic moments of the two sublattices changes from ferromagnetic to antiferromagnetic as the crystal lattice contracts thermally. However, these models based on the molecular field theory do not allow the I phase observed in this study. The magnetic moment on the Mn,, atom
122
T. Kanomata et al. I Field-induced
changes discontinuously at T, in the compound Mn,,,,Co,,,,,Sb as mentioned above. This fact is in contradiction with the constant sublattice magnetization through the transition assumed in the Kittel model, Recently, Chonan et al. [93 calculated the band structure of Mn,Sb by a self-consistent augmented plane wave method. According to their calculation, the density of states consists of three parts: bonding and antibonding d-p bands and a nonbonding d-band with the Fermi level lying in the nonbonding band width is about 3-4 eV, which is fairly wide. Therefore, these authors concluded that the d electrons of Mn atoms should be treated not as localized electrons, but as itinerant electrons. Thus, it is conthe magnetic properties of sidered that MrrP,Co,Sb should be discussed in terms of the itinerant-electron magnetism. General theories of magnetic transitions in an itinerant-electron system were proposed by Moriya and Usami [lo], and Isoda [ll] on the basis of spin fluctuation. However, the transitions from the I to the Fr state and from the AF to the Fr state ob-
transitions in Mnz_,Co,Sb
served for Mn,_,Co,Sb do not occur in both the theories mentioned above. A further experimental and theoretical investigation of the magnetic transition will be necessary.
References and H. Ido, J. Appl. Phys. 55 (1984) [II T. Kanomata 2039. PI M.K. Wilkinson, N.S. Gingrich and C.G. Shull, J. Phys. Chem. Solids 2 (1957) 289. H. Yoshida and T. Kaneko, J. 131 T. Suzuki, T. Kanomata, Appl. Phys. 67 (1990) 4816. Y. Hasebe, T. Ito, H. Yoshida and T. 141 T. Kanomata, Kaneko, J. Appl. Phys. 69 (1991) 4642. G.R. Mackay and W. Leiper, J. Magn. 151 C. Blaauw, Magn. Mater. 8 (1978) 240. and T. Kanomata, J. Magn. 161 M. Ohashi, Y. Yamaguchi Magn. Mater., submitted. 171 C. Kittel, Phys. Rev. 120 (1960) 335. Phys. Rev. 134 (1964) 942. 181 H.S. Jarrett, A. Yamada and K. Motizuki, J. Phys. Sot. 191 T. Chonan, Jpn. 60 (1991) 1638. and K. Usami, Solid State Commun. 23 1101 T. Moriya (1977) 935. 1111 M. Isoda, J. Phys. Sot. Jpn. 53 (1984) 3587.
Physica B 177 (1992) 119-122 North-Holland
Field-induced Mn,_,Co,Sb T. Kanomataa,
transitions (x G 0.15)
Y. Hasebe”,
in intermetallic
T. Kaneko’,
compounds
S. Abeb and Y. Nakagawab
“Faculty of Engineering, Tohoku Gakuin University, Tagajo, Miyagi 985, Japan bInstitute for Materials Research, Tohoku University, Katahira, Sendai 980, Japan
and Mn, 8,Co,,,,Sb were studied in magnetic fields up to 150kOe. Field-induced transitions in Mq,r Co,,,Sb Antiferromagnetic (AF)-ferrimagnet~c (Fr) field-induced transitions in Mn, 85Co,,,Sb and intermediate (I)-Fr transitions in Mn, ,,Co,,,Sb are of the first order. Temperature variations of transition fields were determined.
1. Introduction The compounds Mn,_,Co,Sb(x 6 0.37) crystallize in the tetragonal Cu,Sb-type structure [I]. Mn,Sb is a fe~imagnet with a Nitel temperature of 550 K. The spin ordering of the ferrimagnetic (Fr) state for Mn,Sb is an antiparallel arrangement of unequal magnetic moments associated with two kinds of manganese atoms (Mn,, Mn,,) present in a unit cell as shown in fig. 1, where
the Mn, and Mn,, atoms have magnetic moments of 2.1~~ and 3.9~~) respectively [2]. For Mn,_,Co,Sb, all of the compounds with the composition O.lO
2. Experimental
I d
(x = 0.09 and The compounds Mn,_,Co,Sb 0.15) were prepared by the ceramic method 131. X-ray diffraction studies showed that all diffraction lines could be indexed with the tetragonal Cu,Sb-type structure. The lattice parameters of Mn,_,Co,Sb (x = 0.09 and 0.15) agree well with those previously reported by Kanomata et al. [4]. According to the results of Miissbauer [5] and neutron diffraction [6] investigations, the Co atoms preferentially occupy site I in Mn,Sb. Magnetization was measured by using a vibrating sample magnetometer in fields up to 150 kOe which were generated by a water-cooled solenoid of the Bitter type at the High Field Laboratory, Tohoku University.
lr!!iLL 0
---i-
a
-T
l
-a-
l
Mn, (site I I
0
Mnl
(SiteIt)
@I Sb Fig. 1. Crystal structure of Mn,Sb. 0921-4526/92/$05.00 0
1992 - Elsevier Science Publishers B.V. All rights reserved
T. Kanomata et al. I Field-induced
120
3. Results and discussion
Figure 2 shows the magnetization (T versus field at various temperatures for Mn,,,,Co,,,,Sb. We observe the field-induced metamagnetic phase transitions from the AF to the Fr state. The field-induced transition fields, Her and Hcl, are defined as shown with arrows in the figure. According to the results of neutron diffraction investigation of Mn 1,80Co,.,,Sb [6], the Fr and AF phases have spin structures of the Mn,Sb and Mn,As types, respectively. Therefore, it is considered that the first-order magnetic phase transition shown in fig. 2 takes place between the spin
transitions in Mn2_,Co,Sb
structures of the Mn,As and Mn,Sb types. Figure 3 shows the temperature dependence of magnetization for Mn,.,,Co,,,,Sb in various fields. In the temperature range from 120 to 200 K in a field of 150 kOe, the magnetization is -20% higher than the saturation magnetization of Mn,Sb in the same temperature range. Magnetization curves of Mn,.,,Co,.,,Sb at various temperatures are shown in fig. 4. Figure 5 shows the temperature dependence of magnetization for Mn,,,,Co,,,,Sb in various fields. There appears a magnetization of -lOemu/g for Mn,,,,Co,,,,Sb at low field, suggesting the appearance of a new phase (intermediate one, I).
Mn,.,,CBosSb
60
1 I
H
Fig. 2. The magnetization tures for Mn, &oO ,,Sb.
0
150
100
50
50
(T versus
100 H
(kOe) field at various
I
tempera-
Fig. 4. The magnetization tures for Mn, ,,Co, ,,,Sb.
150
(kOe)
c versus
field at various
tempera-
I
50 -
60
Mn,,%,Sb . H=lSOld)e a H=lOOkOe ” Hz 50kOe oH=20kOe
-
0 H=7.6kOe
0
0 0
rrm_..o.n ,.,.oo-d 100
0
I
I 200
300
400
Oc. 0.3 00
so 500
T(K)
Fig. 3. The magnetisation fields for Mn, BsCo,, ,,Sb.
(T versus
T(K)
temperature
in various
Fig. 5. The magnetization fields for Mn,,,,Co,,,,Sb.
w versus
temperature
in various
T. Kanomata et al. I Field-induced
With further increase of the field, the magnetization of the compound increases abruptly. This corresponds to the first-order transition from the I to the Fr state. In the temperature range from 5.9 to 135 K in a field of 150 kOe, the magnetizations of Mn,,,,Co,,,,Sb are -30% higher than the saturation magnetization of Mn,Sb in the same temperature range; Her - HEl at 5.9 K is 71.4 kOe. By extrapolating the magnetization versus temperature curves at 150 kOe in the Fr state to 0 K in figs. 3 and 5, averaged magnetic moments are found to be 2.2b and -2.2/1, for Mn,,,,Co,.,,Sb and Mn,,$o,.,,Sb, respectively. The increase of moment by substitution of Mn for Co is explained by assuming that nonmagnetic Co atoms occupy site I and the magnetic moments of Mn, and Mn,, atoms have values equal to those of Mn,Sb in the pseudobinary system Mn,_,Co,Sb. The temperature variations of the observed transition fields are shown in figs. 6 and 7 for Mn,,,,Co,,,,Sb and Mn,,,,Co,,,,Sb, respectively. The values of dH,,ldT just below the AF-Fr transition temperature T, are estimated to be - 1.42 and -0.86 kOe/K for Mn,,,,Co,.,,Sb and Mn,,,, Co,,,,Sb, respectively. The total entropy change AS at T, can be calculated from the following thermodynamic ex-
OOW
200 1 (K)
Fig. 6. Transition field versus temperature for Mn,,,,CoO,,,Sb. Hc+ for increasing field and H,, for decreasing field.
transitions in Mn, _,Co,Sb
121
T(K)
Fig. 7. Transition field versus temperature for Mn,,,,Co,,,,Sb. Her for increasing field and H,, for decreasing field.
pression AS = -Aa(dH,/dT)
,
in which Au is the change in magnetization at the magnetic transition. AS of Mn,.,,Co,,,,Sb and Sb are estimated to be 0.17R and Mn,.,,Coo.o, O.llR (R is the gas constant) using the values of dH,,ldT and Au obtained in this study, respectively. According to the results of neutron diffraction for Mn 1.80C00,20Sb[6], the AF-Fr phase transition is accompanied by a change in the magnetic moments of the Mn atoms. In particular, the magnetic moments on the Mn,, atoms are found to be -3.3b just below T, and -2.4~~ just above T,. Then, the magnetic contribution in AS is estimated to be -O.lBR, which is comparable to the values of AS mentioned above. Thus, the main source of the entropy change at T, is considered to be ascribed to the magnetic contribution, especially to that from the magnetic moment fluctuation at the Mn,, site. Kittel [7] and Jarrett [8] discu$sed the firstorder AF-Fr phase transition of Mn,_,Cr,Sb by a two-sublattice model assuming that the net exchange interaction along the c-axis between the magnetic moments of the two sublattices changes from ferromagnetic to antiferromagnetic as the crystal lattice contracts thermally. However, these models based on the molecular field theory do not allow the I phase observed in this study. The magnetic moment on the Mn,, atom
122
T. Kanomata et al. I Field-induced
changes discontinuously at T, in the compound Mn,,,,Co,,,,,Sb as mentioned above. This fact is in contradiction with the constant sublattice magnetization through the transition assumed in the Kittel model, Recently, Chonan et al. [93 calculated the band structure of Mn,Sb by a self-consistent augmented plane wave method. According to their calculation, the density of states consists of three parts: bonding and antibonding d-p bands and a nonbonding d-band with the Fermi level lying in the nonbonding band width is about 3-4 eV, which is fairly wide. Therefore, these authors concluded that the d electrons of Mn atoms should be treated not as localized electrons, but as itinerant electrons. Thus, it is conthe magnetic properties of sidered that MrrP,Co,Sb should be discussed in terms of the itinerant-electron magnetism. General theories of magnetic transitions in an itinerant-electron system were proposed by Moriya and Usami [lo], and Isoda [ll] on the basis of spin fluctuation. However, the transitions from the I to the Fr state and from the AF to the Fr state ob-
transitions in Mnz_,Co,Sb
served for Mn,_,Co,Sb do not occur in both the theories mentioned above. A further experimental and theoretical investigation of the magnetic transition will be necessary.
References and H. Ido, J. Appl. Phys. 55 (1984) [II T. Kanomata 2039. PI M.K. Wilkinson, N.S. Gingrich and C.G. Shull, J. Phys. Chem. Solids 2 (1957) 289. H. Yoshida and T. Kaneko, J. 131 T. Suzuki, T. Kanomata, Appl. Phys. 67 (1990) 4816. Y. Hasebe, T. Ito, H. Yoshida and T. 141 T. Kanomata, Kaneko, J. Appl. Phys. 69 (1991) 4642. G.R. Mackay and W. Leiper, J. Magn. 151 C. Blaauw, Magn. Mater. 8 (1978) 240. and T. Kanomata, J. Magn. 161 M. Ohashi, Y. Yamaguchi Magn. Mater., submitted. 171 C. Kittel, Phys. Rev. 120 (1960) 335. Phys. Rev. 134 (1964) 942. 181 H.S. Jarrett, A. Yamada and K. Motizuki, J. Phys. Sot. 191 T. Chonan, Jpn. 60 (1991) 1638. and K. Usami, Solid State Commun. 23 1101 T. Moriya (1977) 935. 1111 M. Isoda, J. Phys. Sot. Jpn. 53 (1984) 3587.