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Magnetization reversal in SmCo5 single crystal
1029
Journal of Magnetism and Magnetic Materials 31-34 (1983) 1029-1030 M A G N E T I Z A T I O N R E V E R S A L IN S m C o s S I N G L E C R Y S T A L T. S H I B A T A a n d T. K A T A Y A M A Electrotechnical Laboratory, Umezono, Sakuramura, Ibaraki 305, Japan
The process of magnetization reversal in some single crystal spheres of SmCo5 are studied between 80 and 300 K. It is proposed that the coercivity depends on the surface condition of the single crystal spheres and on the grain boundary circumstance of the SmCo5 particles in the Sin-Co sintered materials.
1. Introduction
A large number of experimental studies have been carried out on magnetic properties in a series of Sm-Co sintered magnets [11. However, a clear explanation has not been made for a mechanism of their high coercivity. In general, intrinsic coercive force iHc in SmCos permanent magnet is small compared to the anisotropy field 2 k / M s , and the i n c, depends markedly on the previous magnetizing fields H m [2,3]. Becker [2,3] suggested that coercivity is dominated by domain-wall nucleation. On the other hand, Zijlstra [4,5] concluded that, at least in the SmCo5 powder, the demagnetization process takes place by wall motion and the coercivity is determined by pinning rather than by nucleation of a wall. The coercivity was observed with a single particle of a few micron size. In the experiments, the effects of particle shape and surface condition are difficult to determine. However, in an experiment using a bulk single crystal, it is much easier to control the shape effect and surface condition of the specimen. We report some magnetic properties and experimental results on the magnetization reversal process in a SmCo5 single crystal. 2. Experiment
A recrystallized AI20 3 crucible coated with BN was placed within the tubular Ta susceptor in order to get a wide temperature uniformity. An induction furnace with a 10-turn work coil, 400 kHz frequency, and 15 kW power was used to melt the ingots in Ar gas atmosphere, During the Bridgman drop, output of the rf generator was held constant by use of a PID control system [6]. Since SmCo5 is a peritectic phase and the vapor pressure of Sm at the melting temperature is very high, the starting material, which contains excess Sm, was prepared in an arc-melting furnace. The purities of the metals used for the growth of SmCo5 were 99.9%. The size of the single crystals obtained by this technique was about 13 mm in diameter and 20 mm in length. The single crystals were defined by their bulk magnetic anisotropy, metallographic examination and back-reflection X-ray Lane pattern taken on polished faces. They were annealed at I I00°C for 5 h and were quenched to room temperature in pure Ar atmosphere.
Several spheres were made from each single crystal boule and were electropolished carefully in a solution of CrO3-H3PO 4. 3. Results and discussion
Magnetization curves were measured along the easy and hard axes between 80 and 300 K by means of a vibrating sample magnetometer. The magnetocrystaUine anisotropy constant K] was determined by the method of Sucksmith and Thompson. The temperature dependence of K l, the anisotropy field H A, and the saturation magnetization 4~rMs of the SmCos single crystals are shown in fig. 1.4~rMs, Kj and H A increase with decreasing temperature. Particularly, the variation of K] with temperature for this specimen is remarkable. The hysteresis loop of a SmCo5 sphere along the easy axis at room temperature is shown in fig. 2. In this figure, H d represents the demagnetization line. The point A repre~ sents the field ( - l . l kOe) where the magnetization reverses from 9.66 to - 9 . 6 6 kG after magnetization in fields from I1 to 17 kOe. Similarly, the points B (0.6 kOe), C, D, E and Frepresent the fields where magnetization reversals occur after magnetization in fields of 10, 9, 8, 7 and 6 kOe, respectively. It is important to note that A and A~ are symmetric with respect to the origin (0), but, other pairs are not symmetry with each other. The dependence of H.~fr on Hmer~ saturates at
x ~' ~_ 2a ~ 20 v ~ 18 "~ 16 ~" 1,L ~, ~g 12 < 0~
40..,I
SmCo5
10.~
-%. 0%%., ,9.8
:~
300 200
9.6
oo
400
\
,oo 2;0 TemperLture ( K )
g
"%
100 ~t
an0
Fig. I. Temperature dependence of Kl, 4~'M, and HA of the SmCo5 single crystal quenched from 1100°C to room temperature.
0 3 0 4 - 8 8 5 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d
T. Shibata and T. Katayama / Magnetization reversal in SmCo 5
1030
47~M (kG) A
SmCos at R T.
8
4
i -5
i
i -3
-4
BC
12,
i -2
D E F J'4d/
/
y,,, ,
11
2
3
i
4
| !
¢
/it
DIB1 C1
F1
A1
-I 2
Fig. 2. Hysteresis loops of the SmCo5 single crystal sphere at room temperature. Hd is the demagnetization line.
room temperature at values of Hmeff above 8 kOe and at 80 K at those of above l0 kOe, respectively, as shown in fig. 3. In this figure, the upper black spot and the one with open circle ((D) represent values of the//neff where magnetization reversals at 80 K occur after magnetization in a field of 14 kOe at room temperature, respectively. The value of //neff in the second measurement becomes lower than that of the first one ( Q ) on the same Hme f f. It may be considered that, in the cases of A and A 1 in fig. 2, the magnetization reversals occur by nucleation of a domain wall and its propagation from a certain defect point. As discussed by Becker, crystal defects such as nonmagnetic inclusion, bits, corners, inhomogeneities of magnetization and anisotropy, local lattice misorientations, and so on, may behave as the nuclea-15 SmCos
..~
®
(Quenched at 1100°C) 0 - 1 0 .~f $---* o
~: -5 .~-"
-2'o
I
I
-15
-lo
I
,t.~-'~
80 K
/
~2
0 5
5
R .T.
10
15
20
tion and pinning sites. If the magnetization of the sphere is fully saturated in one direction, the demagnetizing fields at' the defects are expected to be equal in both senses of magnetization. Accordingly, it is considered that the magnetization reversals at A and A~ are independent of any previous magnetizing field, and their reversing behavior is symmetric. Their experimental results support the nucleation model of Becket. On the other hand, in the case of B - F and B I - F I in this figure, the magnetization reversals occur by domain-wall propagation from a small reverse domain enclosed by a locally pinned domain wall at a certain defect point. Therefore, each reversal field of magnetization depends on the behavior of the pinning sites. From the result in fig. 3 it is considered that the reason for the increase in the Hneff in the measurement at 80 K is attributable to the increase of the crystalline anisotropy constant (see fig. 1) at low temperatures and the maintenance of the saturation magnetization up to 80 K. Numerous rectangular hysteresis loops were observed similar to the small-particle experiment, and they were discussed in terms of the nucleation model and the p i n n i n g model. These results may be useful for understanding the relatively low coercivity of R - C o magnets with high anisotropy.
Hmef f (kOe)
References eOe ~
10
0
15 Fig. 3. Dependence of Hn~f on H ~ t t at room temperature and 80 K. Hneff and Hmerf represent the effective field of magnetization reversal and the effective magnetizing field, respectively.
[1] T. Shibata, T. Katayama and T. Tsushima, J. Appl. Phys. 49 (3) (1978). [2] J.J. Becker, J. Appl. Phys. 39 (1968) 1270. [3] For example, J.J. Becker, J. Appl. Phys. 41 (1970) 1055. [4] H. Zijlstra, Philips Tech. Rev. 31 (1970) 40. [5] H. Zijlstra, J. Appl. Phys. 41 (1970) 4881. [6] T. Katayama and T. Shibata, J. Crystal Growth 24/25 (1974) 396.
Journal of Magnetism and Magnetic Materials 31-34 (1983) 1029-1030 M A G N E T I Z A T I O N R E V E R S A L IN S m C o s S I N G L E C R Y S T A L T. S H I B A T A a n d T. K A T A Y A M A Electrotechnical Laboratory, Umezono, Sakuramura, Ibaraki 305, Japan
The process of magnetization reversal in some single crystal spheres of SmCo5 are studied between 80 and 300 K. It is proposed that the coercivity depends on the surface condition of the single crystal spheres and on the grain boundary circumstance of the SmCo5 particles in the Sin-Co sintered materials.
1. Introduction
A large number of experimental studies have been carried out on magnetic properties in a series of Sm-Co sintered magnets [11. However, a clear explanation has not been made for a mechanism of their high coercivity. In general, intrinsic coercive force iHc in SmCos permanent magnet is small compared to the anisotropy field 2 k / M s , and the i n c, depends markedly on the previous magnetizing fields H m [2,3]. Becker [2,3] suggested that coercivity is dominated by domain-wall nucleation. On the other hand, Zijlstra [4,5] concluded that, at least in the SmCo5 powder, the demagnetization process takes place by wall motion and the coercivity is determined by pinning rather than by nucleation of a wall. The coercivity was observed with a single particle of a few micron size. In the experiments, the effects of particle shape and surface condition are difficult to determine. However, in an experiment using a bulk single crystal, it is much easier to control the shape effect and surface condition of the specimen. We report some magnetic properties and experimental results on the magnetization reversal process in a SmCo5 single crystal. 2. Experiment
A recrystallized AI20 3 crucible coated with BN was placed within the tubular Ta susceptor in order to get a wide temperature uniformity. An induction furnace with a 10-turn work coil, 400 kHz frequency, and 15 kW power was used to melt the ingots in Ar gas atmosphere, During the Bridgman drop, output of the rf generator was held constant by use of a PID control system [6]. Since SmCo5 is a peritectic phase and the vapor pressure of Sm at the melting temperature is very high, the starting material, which contains excess Sm, was prepared in an arc-melting furnace. The purities of the metals used for the growth of SmCo5 were 99.9%. The size of the single crystals obtained by this technique was about 13 mm in diameter and 20 mm in length. The single crystals were defined by their bulk magnetic anisotropy, metallographic examination and back-reflection X-ray Lane pattern taken on polished faces. They were annealed at I I00°C for 5 h and were quenched to room temperature in pure Ar atmosphere.
Several spheres were made from each single crystal boule and were electropolished carefully in a solution of CrO3-H3PO 4. 3. Results and discussion
Magnetization curves were measured along the easy and hard axes between 80 and 300 K by means of a vibrating sample magnetometer. The magnetocrystaUine anisotropy constant K] was determined by the method of Sucksmith and Thompson. The temperature dependence of K l, the anisotropy field H A, and the saturation magnetization 4~rMs of the SmCos single crystals are shown in fig. 1.4~rMs, Kj and H A increase with decreasing temperature. Particularly, the variation of K] with temperature for this specimen is remarkable. The hysteresis loop of a SmCo5 sphere along the easy axis at room temperature is shown in fig. 2. In this figure, H d represents the demagnetization line. The point A repre~ sents the field ( - l . l kOe) where the magnetization reverses from 9.66 to - 9 . 6 6 kG after magnetization in fields from I1 to 17 kOe. Similarly, the points B (0.6 kOe), C, D, E and Frepresent the fields where magnetization reversals occur after magnetization in fields of 10, 9, 8, 7 and 6 kOe, respectively. It is important to note that A and A~ are symmetric with respect to the origin (0), but, other pairs are not symmetry with each other. The dependence of H.~fr on Hmer~ saturates at
x ~' ~_ 2a ~ 20 v ~ 18 "~ 16 ~" 1,L ~, ~g 12 < 0~
40..,I
SmCo5
10.~
-%. 0%%., ,9.8
:~
300 200
9.6
oo
400
\
,oo 2;0 TemperLture ( K )
g
"%
100 ~t
an0
Fig. I. Temperature dependence of Kl, 4~'M, and HA of the SmCo5 single crystal quenched from 1100°C to room temperature.
0 3 0 4 - 8 8 5 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d
T. Shibata and T. Katayama / Magnetization reversal in SmCo 5
1030
47~M (kG) A
SmCos at R T.
8
4
i -5
i
i -3
-4
BC
12,
i -2
D E F J'4d/
/
y,,, ,
11
2
3
i
4
| !
¢
/it
DIB1 C1
F1
A1
-I 2
Fig. 2. Hysteresis loops of the SmCo5 single crystal sphere at room temperature. Hd is the demagnetization line.
room temperature at values of Hmeff above 8 kOe and at 80 K at those of above l0 kOe, respectively, as shown in fig. 3. In this figure, the upper black spot and the one with open circle ((D) represent values of the//neff where magnetization reversals at 80 K occur after magnetization in a field of 14 kOe at room temperature, respectively. The value of //neff in the second measurement becomes lower than that of the first one ( Q ) on the same Hme f f. It may be considered that, in the cases of A and A 1 in fig. 2, the magnetization reversals occur by nucleation of a domain wall and its propagation from a certain defect point. As discussed by Becker, crystal defects such as nonmagnetic inclusion, bits, corners, inhomogeneities of magnetization and anisotropy, local lattice misorientations, and so on, may behave as the nuclea-15 SmCos
..~
®
(Quenched at 1100°C) 0 - 1 0 .~f $---* o
~: -5 .~-"
-2'o
I
I
-15
-lo
I
,t.~-'~
80 K
/
~2
0 5
5
R .T.
10
15
20
tion and pinning sites. If the magnetization of the sphere is fully saturated in one direction, the demagnetizing fields at' the defects are expected to be equal in both senses of magnetization. Accordingly, it is considered that the magnetization reversals at A and A~ are independent of any previous magnetizing field, and their reversing behavior is symmetric. Their experimental results support the nucleation model of Becket. On the other hand, in the case of B - F and B I - F I in this figure, the magnetization reversals occur by domain-wall propagation from a small reverse domain enclosed by a locally pinned domain wall at a certain defect point. Therefore, each reversal field of magnetization depends on the behavior of the pinning sites. From the result in fig. 3 it is considered that the reason for the increase in the Hneff in the measurement at 80 K is attributable to the increase of the crystalline anisotropy constant (see fig. 1) at low temperatures and the maintenance of the saturation magnetization up to 80 K. Numerous rectangular hysteresis loops were observed similar to the small-particle experiment, and they were discussed in terms of the nucleation model and the p i n n i n g model. These results may be useful for understanding the relatively low coercivity of R - C o magnets with high anisotropy.
Hmef f (kOe)
References eOe ~
10
0
15 Fig. 3. Dependence of Hn~f on H ~ t t at room temperature and 80 K. Hneff and Hmerf represent the effective field of magnetization reversal and the effective magnetizing field, respectively.
[1] T. Shibata, T. Katayama and T. Tsushima, J. Appl. Phys. 49 (3) (1978). [2] J.J. Becker, J. Appl. Phys. 39 (1968) 1270. [3] For example, J.J. Becker, J. Appl. Phys. 41 (1970) 1055. [4] H. Zijlstra, Philips Tech. Rev. 31 (1970) 40. [5] H. Zijlstra, J. Appl. Phys. 41 (1970) 4881. [6] T. Katayama and T. Shibata, J. Crystal Growth 24/25 (1974) 396.