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Magnetic phase transitions in some Nd=Fe=M=Co=B magnetic materials (M-V, Mo or Re)
Journal of Magnetism and Magnetic Materials 104-107 (1992) 1193- 1194 North-Holland
Magnetic phase transitions in some N d - F e - M - C o - B materials (M = V, Mo or Re)
magnetic
M. Jurczyk t, G.K. Nicolaides and K.V. Rao Department of Condensed Matter Physics, Royal Institute of Technology, S-IO0 44 Stockholm, Sweden The effect of the substitution of V, Mo and Re for iron in R15Fe62.3_xMxColsBT.7 (R = Nd or Didymium) magnetic materials for 0 < x < 3.8 has been investigated by X-ray, magnetothermomagnetic and ac susceptibility techniques. The compounds crystallize in a tetragonal structure of P42/mnm type. Based on these measurements we present the phase diagrams for the spin arrangement in the R - F e - M - C o - B systems. The observed dependence of the spin re-orientation temperature, TSR, to the various substituent elements, suggests that crystal field effects are the primary cause for controlling TsR. The tetragonal intermetallic compounds R2Fe14 B ( R - r a r e earth) have been intensively studied in the recent years. Some of them (R = Nd and Pr) have outstanding magnetic properties at r o o m temperature. However, their temperature coefficient of remanence a, and coercivity/3, limit applications for a wide temperature range. In order to obtain better thermomagnetic properties, efforts have been centered on the substitution with Co and other elements in the N d F e - B system. Partial replacement of Fe by Co increases the Curie temperature, unfortunately at the same time reduces iHc significantly [1]. On the other hand, substitution with small amounts of nonmagnetic elements such as V and M o in N d - F e - C o - B is found to raise the anisotropy field [2-4] and therefore increase iHc without decreasing B r substantially [5]. The above manipulation method reduces both c~ and /3. Recently it has been reported that refractory elements V and Mo enhance i n c in a large temperature range and inhibit oxidation in the Co containing N d F e - B - t y p e sintered magnets [5]. The magnet of Nd16Fe72V4B s which has a coercivity as high as = 17 kOe consists of three phases: N d 2 F e l a B , a Nd-rich phase and V2FeB 2 instead of N d l + , F e a B 4 which is contained in the conventional N d - F e - B magnets [6]. The corrosion resistance of N d - F e - V - C o - B magnet is improved by blocking the selective oxidation of the Nd rich phase thanks to the network of V2FeB 2 compounds and to the N d - C o precipitates which enhance its stability [6]. It is well known that below 135 K Nd15Fe77.3B7.7 magnets, because of spin re-orientation, are not suitable for application as hard magnets [1]. In our present studies we investigate the magnetic phase transitions in R15Fe62.3_xMxColsB7. 7 systems w h e r e R = Nd, Didymium (a mixture of 80% Nd, 15% Pr and 5% Ce) and M = V, Mo, Re. i Permanent address: Institute of Molecular Physics, Polish Academy of Sciences, 60-179 Poznafi, Poland.
The alloys with composition NdlsFe77.3B7. 7 and R15Fe62.3_xMxColsB7. 7 ( R = N d , Didymium; M = V, Mo, Re) were prepared using the method described in our earlier work [2-4]. The Curie temperatures, To were determined from magnetothermogravimetric data obtained in the temperature range of 300-1200 K. The spin re-orientation p h e n o m e n o n was studied using ac susceptibility technique. The ac susceptibility of the samples was measured during warming runs in the temperature range 4.2-320 K, in an ac magnetic field Hrm s = 1 0 e , and at a frequency f - - 3 0 0 Hz using a conventional mutual impedance method and a twophase PAR-5206 lock-in amplifier to determine both the in-phase, X' and out-of-phase X" components of Xac at various temperatures. The results obtained for the R15Fe62.3_xMxCOlsB7.7 alloys, where R = Nd, Didymium and M = V, Mo, Re, in the present study are presented in table 1 and plotted in figs. 1 and 2. AI of the studied systems
Table 1 Curie and spin re-orientation temperatures for the some R - F e - M - C o - B magnetic materials Composition
Tc (K)
TsR (K)
NdI5Fe77.3B7. 7 Nd 15Fe62.3COI5B7.7
589 723
135 127
Nd 15Fe6o.sV1.sCo15B7.7 Nd 15Fe60.1V2.2Co15B7.7 Nd 15Fess.sV3.8Co 15B7.7
712 706 694
118 116 114
Nd15Fe61.2MoHColsB7. 7 Nd 15Fe58.5M03.8C015 By.7
704 690
119 110
Nd 15Fe61.~Reo.sCo 15B7.7 Nd 15Fe 60.8Re 1.5Co 15B 7.7
705 698
121 117
Didymium 15Fe62.3CoI5B7. 7 Didymium 15Fe61.4V0.9CO 15B7.7
715 708 700
97 97 97
Didymium 15Fe61.4Moo.9Co 15B 7.7
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
1194
7©
M. Jurczyk et al. / Magnetic phase transitions
-I NdlaFe°a'a-×V~C°'5[177***** x : 0
0,04
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= 2.2 = 3.8
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1
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Temperature
7
i
100
I
200
(K)
D i d y m i u m lsFe61.4To.9Coi,~Bv.v
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= =
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T e l n p e r a L u r e (K) Fig. 1. Temperature dependence of the real part of the ac susceptibility for (a) NdlsFe62.3 xVtCoI5B7.7and (b) Didymium 15 F e 6 t . 4 C o t 5 T 0 . 9 B 7.7. exhibit below room temperature a spin reorientation similar to that of pure Nd2Fel4 B. As mentioned earlier, the magnetic structure of NdzFeI4B is not the same at all temperatures. Below 135 K the easy axis rotates from the c-axis to that of a cone. The value of angle between the resulting easy direction and the c-axis is about 30 ° at 0 K [1]. Spin re-orientation phenomena have been explained in terms of an interplay of the single ion anisotropy and the N d - F e sublattice exchange interactions [7]. Givord et al. [8] have proposed an alternative explanation. They
have interpreted the low-temperature behavior of Nd2Fel4B in terms of a noncollinear arrangement of the magnetic moments brought about by different anisotropies at the two Nd sites [8]. As has been demonstrated before, ac susceptibility constitutes a good technique to detect spin re-orientation transitions at cryogenic temperatures [9]. For example, the results of low-temperature ac susceptibility for NdlsFe62.3 xgxCOl5B7.7 and Didymium15Fe61.4ColsTo.gB7.7 are shown in fig. l. As shown in the figure, these compounds possess a single spin rcorientation temperature, which is indicated by the appearence of maxima in X'c(T). It is interesting to note that for the neodymium system, the easy-axis to cone transition temperature (TsR) is found to decrease with increasing content of the M elements (M = V, Mo or Re), but for the didymium system TSR remains unchanged; (see also table 1). The phase diagram for NdlsFe62.3 xVxCO15B7.7system is shown in fig. 2. The dominant region in this diagram is that of the axial spin arrangement. Similar behaviour was observed for other refractory elements: Mo or Re in a neodymium system. Generally, the spin re-orientation temperature TsR decreases for all the cases of possible substitutions. The observed changes in TSR bear no direct relationship to the anisotropy of the transition metal sublattices. This suggests that crystal field effects might be the primary cause of the changes in TsR. Boltich and Wallace [10] have shown that the substituent dependence of TSR can be explained by the crystal field theory, using the single-ion model. Specifically, changes in TsR can be related to changes in exchange field j Hexch [ and hence related to the magnetic moment of the transition metal sublattices. This research has been supported by the Swedish agency STU. References
iooo
[1] [2] [3] [4] [5]
disordered •
e - - - - e
__
500
4
Ca
axial
-e-
•
on 0
~-, 0
i , ,rl
i , , ~ 1 l
al
, i , i i , i , i , , t7-1 r I , I I , I ! 2 3 4
x i n NdlsFe6z.~_xVxColsB7.7 Fig. 2. Diagram of spin configuration types observed in the Nd15Fe62.3_xVxColsB7.7
system.
K.H.J. Buschow, Mater. Sci. Rep. 1 (1986) 1. M. Jurczyk, J. Magn. Magn. Mater. 73 (1988) 199, 367. M. Jurczyk, IEEE Trans. Magn. MAG-24 (1988) 1942. M. Jurczyk, J. Less-Common Met. 158 (1990) 117. S. Hirosawa, H. Tomizawa, S. Mino and A. Hamamura, IEEE Trans. Magn. MAG-26 (1990) 1960. [6] M. Sagawa, P. Tenaud, F. Vial and K. Hiraga, IEEE Trans. Magn. MAG-26 (1990) 1957. [7] E.B. Boltich and W.E. Wallace, Solid State Common. 55 (1985) 529. [8] D. Givord, H.S. Li and R. Perrier, Solid State Commun. 51 (1984) 857. [9] M. Jurczyk, G.K. Nicolaides and K.V. Rao, J. Magn. Magn. Mater. 94 (1991) L6. [10] E.B. Boltich and W.E. Wallace, J. LEss-Common Met. 126 (1986) 35.
Magnetic phase transitions in some N d - F e - M - C o - B materials (M = V, Mo or Re)
magnetic
M. Jurczyk t, G.K. Nicolaides and K.V. Rao Department of Condensed Matter Physics, Royal Institute of Technology, S-IO0 44 Stockholm, Sweden The effect of the substitution of V, Mo and Re for iron in R15Fe62.3_xMxColsBT.7 (R = Nd or Didymium) magnetic materials for 0 < x < 3.8 has been investigated by X-ray, magnetothermomagnetic and ac susceptibility techniques. The compounds crystallize in a tetragonal structure of P42/mnm type. Based on these measurements we present the phase diagrams for the spin arrangement in the R - F e - M - C o - B systems. The observed dependence of the spin re-orientation temperature, TSR, to the various substituent elements, suggests that crystal field effects are the primary cause for controlling TsR. The tetragonal intermetallic compounds R2Fe14 B ( R - r a r e earth) have been intensively studied in the recent years. Some of them (R = Nd and Pr) have outstanding magnetic properties at r o o m temperature. However, their temperature coefficient of remanence a, and coercivity/3, limit applications for a wide temperature range. In order to obtain better thermomagnetic properties, efforts have been centered on the substitution with Co and other elements in the N d F e - B system. Partial replacement of Fe by Co increases the Curie temperature, unfortunately at the same time reduces iHc significantly [1]. On the other hand, substitution with small amounts of nonmagnetic elements such as V and M o in N d - F e - C o - B is found to raise the anisotropy field [2-4] and therefore increase iHc without decreasing B r substantially [5]. The above manipulation method reduces both c~ and /3. Recently it has been reported that refractory elements V and Mo enhance i n c in a large temperature range and inhibit oxidation in the Co containing N d F e - B - t y p e sintered magnets [5]. The magnet of Nd16Fe72V4B s which has a coercivity as high as = 17 kOe consists of three phases: N d 2 F e l a B , a Nd-rich phase and V2FeB 2 instead of N d l + , F e a B 4 which is contained in the conventional N d - F e - B magnets [6]. The corrosion resistance of N d - F e - V - C o - B magnet is improved by blocking the selective oxidation of the Nd rich phase thanks to the network of V2FeB 2 compounds and to the N d - C o precipitates which enhance its stability [6]. It is well known that below 135 K Nd15Fe77.3B7.7 magnets, because of spin re-orientation, are not suitable for application as hard magnets [1]. In our present studies we investigate the magnetic phase transitions in R15Fe62.3_xMxColsB7. 7 systems w h e r e R = Nd, Didymium (a mixture of 80% Nd, 15% Pr and 5% Ce) and M = V, Mo, Re. i Permanent address: Institute of Molecular Physics, Polish Academy of Sciences, 60-179 Poznafi, Poland.
The alloys with composition NdlsFe77.3B7. 7 and R15Fe62.3_xMxColsB7. 7 ( R = N d , Didymium; M = V, Mo, Re) were prepared using the method described in our earlier work [2-4]. The Curie temperatures, To were determined from magnetothermogravimetric data obtained in the temperature range of 300-1200 K. The spin re-orientation p h e n o m e n o n was studied using ac susceptibility technique. The ac susceptibility of the samples was measured during warming runs in the temperature range 4.2-320 K, in an ac magnetic field Hrm s = 1 0 e , and at a frequency f - - 3 0 0 Hz using a conventional mutual impedance method and a twophase PAR-5206 lock-in amplifier to determine both the in-phase, X' and out-of-phase X" components of Xac at various temperatures. The results obtained for the R15Fe62.3_xMxCOlsB7.7 alloys, where R = Nd, Didymium and M = V, Mo, Re, in the present study are presented in table 1 and plotted in figs. 1 and 2. AI of the studied systems
Table 1 Curie and spin re-orientation temperatures for the some R - F e - M - C o - B magnetic materials Composition
Tc (K)
TsR (K)
NdI5Fe77.3B7. 7 Nd 15Fe62.3COI5B7.7
589 723
135 127
Nd 15Fe6o.sV1.sCo15B7.7 Nd 15Fe60.1V2.2Co15B7.7 Nd 15Fess.sV3.8Co 15B7.7
712 706 694
118 116 114
Nd15Fe61.2MoHColsB7. 7 Nd 15Fe58.5M03.8C015 By.7
704 690
119 110
Nd 15Fe61.~Reo.sCo 15B7.7 Nd 15Fe 60.8Re 1.5Co 15B 7.7
705 698
121 117
Didymium 15Fe62.3CoI5B7. 7 Didymium 15Fe61.4V0.9CO 15B7.7
715 708 700
97 97 97
Didymium 15Fe61.4Moo.9Co 15B 7.7
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
1194
7©
M. Jurczyk et al. / Magnetic phase transitions
-I NdlaFe°a'a-×V~C°'5[177***** x : 0
0,04
AA~A ooeo.
7
X
*,~.". ..**~ .
= 2.2 = 3.8
X
)
a
0.02
"><
i ....
0.00
I
i
~
i
i
..... i
i
i
0
i
i
i
i
I
50
I
I
I
]
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I
0.04
I
I
I
~
I ~ [
1
,
I
I
]
150
Temperature
7
i
100
I
200
(K)
D i d y m i u m lsFe61.4To.9Coi,~Bv.v
I
0
7 0.02
(o AAAAA T ooooo T
E ~
= =
V Mo
~
.
.
.
.
0.00
"><.
0
50
lOO
150
200
T e l n p e r a L u r e (K) Fig. 1. Temperature dependence of the real part of the ac susceptibility for (a) NdlsFe62.3 xVtCoI5B7.7and (b) Didymium 15 F e 6 t . 4 C o t 5 T 0 . 9 B 7.7. exhibit below room temperature a spin reorientation similar to that of pure Nd2Fel4 B. As mentioned earlier, the magnetic structure of NdzFeI4B is not the same at all temperatures. Below 135 K the easy axis rotates from the c-axis to that of a cone. The value of angle between the resulting easy direction and the c-axis is about 30 ° at 0 K [1]. Spin re-orientation phenomena have been explained in terms of an interplay of the single ion anisotropy and the N d - F e sublattice exchange interactions [7]. Givord et al. [8] have proposed an alternative explanation. They
have interpreted the low-temperature behavior of Nd2Fel4B in terms of a noncollinear arrangement of the magnetic moments brought about by different anisotropies at the two Nd sites [8]. As has been demonstrated before, ac susceptibility constitutes a good technique to detect spin re-orientation transitions at cryogenic temperatures [9]. For example, the results of low-temperature ac susceptibility for NdlsFe62.3 xgxCOl5B7.7 and Didymium15Fe61.4ColsTo.gB7.7 are shown in fig. l. As shown in the figure, these compounds possess a single spin rcorientation temperature, which is indicated by the appearence of maxima in X'c(T). It is interesting to note that for the neodymium system, the easy-axis to cone transition temperature (TsR) is found to decrease with increasing content of the M elements (M = V, Mo or Re), but for the didymium system TSR remains unchanged; (see also table 1). The phase diagram for NdlsFe62.3 xVxCO15B7.7system is shown in fig. 2. The dominant region in this diagram is that of the axial spin arrangement. Similar behaviour was observed for other refractory elements: Mo or Re in a neodymium system. Generally, the spin re-orientation temperature TsR decreases for all the cases of possible substitutions. The observed changes in TSR bear no direct relationship to the anisotropy of the transition metal sublattices. This suggests that crystal field effects might be the primary cause of the changes in TsR. Boltich and Wallace [10] have shown that the substituent dependence of TSR can be explained by the crystal field theory, using the single-ion model. Specifically, changes in TsR can be related to changes in exchange field j Hexch [ and hence related to the magnetic moment of the transition metal sublattices. This research has been supported by the Swedish agency STU. References
iooo
[1] [2] [3] [4] [5]
disordered •
e - - - - e
__
500
4
Ca
axial
-e-
•
on 0
~-, 0
i , ,rl
i , , ~ 1 l
al
, i , i i , i , i , , t7-1 r I , I I , I ! 2 3 4
x i n NdlsFe6z.~_xVxColsB7.7 Fig. 2. Diagram of spin configuration types observed in the Nd15Fe62.3_xVxColsB7.7
system.
K.H.J. Buschow, Mater. Sci. Rep. 1 (1986) 1. M. Jurczyk, J. Magn. Magn. Mater. 73 (1988) 199, 367. M. Jurczyk, IEEE Trans. Magn. MAG-24 (1988) 1942. M. Jurczyk, J. Less-Common Met. 158 (1990) 117. S. Hirosawa, H. Tomizawa, S. Mino and A. Hamamura, IEEE Trans. Magn. MAG-26 (1990) 1960. [6] M. Sagawa, P. Tenaud, F. Vial and K. Hiraga, IEEE Trans. Magn. MAG-26 (1990) 1957. [7] E.B. Boltich and W.E. Wallace, Solid State Common. 55 (1985) 529. [8] D. Givord, H.S. Li and R. Perrier, Solid State Commun. 51 (1984) 857. [9] M. Jurczyk, G.K. Nicolaides and K.V. Rao, J. Magn. Magn. Mater. 94 (1991) L6. [10] E.B. Boltich and W.E. Wallace, J. LEss-Common Met. 126 (1986) 35.