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Effect of cobalt nanoclusters on magnetization processes in CoB amorphous alloys
ELSEVIER
Journal of magnetism and magnetic materials
Journal of Magnetism and Magnetic Materials 196-197 (1999) 175-176
Effect of cobalt nanoclusters on magnetization processes in Co-B amorphous alloys A. Gonzfilez a'*, A. Zern b, A.
Hernando
a
alnstituto de Magnetismo Aplicado, P.O. Box 155 Las Rozas, 28230 Madrid, Spain bMPI fur Metallforschung, Heisenbergstrasse 1, 70569 Stuttgart, Germany
Abstract The magnetization processes in Co80B2o and CovsB25 amorphous alloys have been studied by means of their hysteresis loops and switching curves at room temperature. This analysis is correlated with the structural characterization made by HRTEM which showed different microstructure in both compositions. In particular, it is proposed that the presence of cobalt nanoclusters in the composition with stoichiometry departing from CoaB, plays a main role in the magnetic behavior. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Magnetization process; Wall pinning; Cobalt nanoclusters
The amorphous metallic glasses have been extensively studied in the last two decades [1]. However, their structure remains to be a subtle problem provided that they do not produce sharp diffraction peaks. Particularly, amorphous Co-B alloys display a complex crystallization behavior with a variety of precipitated phases, some of them metastable. Within the composition range 20-25% boron, the tendency to the formation of CoaB leads to an early segregation of the excess of cobalt. In the case of CosoBzo, and for very low annealing temperatures, it is found that the structure consists of cobalt nanoclusters embedded in an amorphous matrix. These clusters, which are not present in Co75B25, show a peculiar microscopic structure. They are amorphous in the sense that they do not present sharp peaks in the X-ray diffraction pattern. However, HRTEM observations have shown that they exhibit short-range order broken in a few atomic planes. These results will be an object of a future publication [2]. In this paper, we focus on the magnetization processes of these two compositions, with
* Corresponding author. Tel.: + 34-1-6304278; fax: + 34-1630 1625; e-mail: [email protected].
the aim of illustrating the differences in the microstructure mentioned above. As the size of the cobalt clusters present in the sample with 20% boron (20-30 nm) is smaller than the exchange correlation length, they are expected to affect the magnetization process. Amorphous ribbons of compositions Cos0B2o and Co75B25 were produced by the melt-spinning technique. The samples were submitted to successive annealings to study the evolution of the material. The annealing temperature was always lower than the crystallization temperature. From calorimetry analysis and HRTEM observations [2], we can conclude that these treatments lead to the relaxation of the amorphous phase in the case of C075B25 , and to the appearance of the cobalt nanoclusters in the case of Cos0B20. The number of precipitated clusters increases with the annealing temperature and time. The field dependence of the magnetization was measured by means of inductive methods at a frequency of 120 Hz. In order to obtain the switching curves of the samples, a series of minor loops were registered varying the maximum applied field from 4 0 e down to 0. The points that make up these curves represent the highest value of magnetization versus the maximum applied field for each minor loop.
0304-8853/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 7 1 2-4
176
A. Gonzgtlez et al. /Journal of Magnetism and Magnetic' Materials 196-197 (1999) 175-176 10
10
H
0.5
C 2;
°8: 2o
C07~B:,,:,
[
~ 0.5
O0
I ~ CoszB2o -0.5
:1
-1.0 2
0
--~ --~ ~
as-cast 335"C, 3min 350°C, lOmin
i 2
2;
---c--- as-cast 335°C, 3rain + 330°C, 30min 330°C, 60min 350°C. lOmin
tl
s . . . . . . ive i treatments
O0 00
05
10 '
15 i
20
H (Oe)
H (Oe)
Fig. 1. Hysteresis loops of CosoB20 and CovsB25 in the as-cast state and after successive annealings.
Fig. 2. Switching curves of C08oB2o and C075B25 in the as-cast state and after successive annealings.
The hysteresis loops and switching curves are plotted in Figs. 1 and 2, respectively, for both compositions in the as-cast and annealed states. As can be seen in Fig. 1, the as-cast CosoB2o loop presents a higher squareness character than the as-cast C07sB2s loop, which is more round. This difference is better reflected in the switching curves, see Fig. 2. Thus, in the switching curves of CoaoB2o it is possible to define a field H~ that separates the magnetization process in two parts. For fields higher than H~ almost the whole material is magnetized to saturation. Whereas for fields lower than Hs the magnetization falls down very quickly, meaning that only a very small part of the magnetization can follow the alternating field. The value of this field increases with the annealing temperature or time, i.e., with the number of cobalt clusters. On the other hand, the switching curves of C0~5B25 decrease continuously towards zero, and after a first relaxation of the amorphous phase, they do not change upon further annealings. The magnetization process in C08oB2o is characterized by the presence of defects (cobalt nanoclusters that are amorphous in a sense discussed above) which act as
pinning centres to the wall displacements [3,4]. Whereas the walls can move uniformly through the material in the case of C075B25, the amorphous matrix of which exhibits a homogeneous structure. It is possible to compare the contribution of the defects to the wall pinning in both compositions, which should be described by the product of coercivity times the saturation magnetization. We obtain that for the as-cast samples the contribution in C080Bzo is twice as much that of Co75B2s. Besides, this product increases with the annealing treatments in the CosoB2o sample: while the coercive field of Co75B25 remains constant (He ~ 0.20e), it grows continuously for Co8oB2o (from 0.3 to 0.60e).
References
[t] R. Hasegawa, R. Ray, J. Appl. Phys. 50 (1979) 1586. [2] A. Zern, J. Appl. Phys. (1999) in press. [3] A. Hernando, M. Vazquez, T. Kulik, C. Prados, Phys. Rev. B 51 (1995) 3581. [4] S. Chikazumi. Physics of Magnetism, Krieger, New York, 1964, p. 285.
Journal of magnetism and magnetic materials
Journal of Magnetism and Magnetic Materials 196-197 (1999) 175-176
Effect of cobalt nanoclusters on magnetization processes in Co-B amorphous alloys A. Gonzfilez a'*, A. Zern b, A.
Hernando
a
alnstituto de Magnetismo Aplicado, P.O. Box 155 Las Rozas, 28230 Madrid, Spain bMPI fur Metallforschung, Heisenbergstrasse 1, 70569 Stuttgart, Germany
Abstract The magnetization processes in Co80B2o and CovsB25 amorphous alloys have been studied by means of their hysteresis loops and switching curves at room temperature. This analysis is correlated with the structural characterization made by HRTEM which showed different microstructure in both compositions. In particular, it is proposed that the presence of cobalt nanoclusters in the composition with stoichiometry departing from CoaB, plays a main role in the magnetic behavior. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Magnetization process; Wall pinning; Cobalt nanoclusters
The amorphous metallic glasses have been extensively studied in the last two decades [1]. However, their structure remains to be a subtle problem provided that they do not produce sharp diffraction peaks. Particularly, amorphous Co-B alloys display a complex crystallization behavior with a variety of precipitated phases, some of them metastable. Within the composition range 20-25% boron, the tendency to the formation of CoaB leads to an early segregation of the excess of cobalt. In the case of CosoBzo, and for very low annealing temperatures, it is found that the structure consists of cobalt nanoclusters embedded in an amorphous matrix. These clusters, which are not present in Co75B25, show a peculiar microscopic structure. They are amorphous in the sense that they do not present sharp peaks in the X-ray diffraction pattern. However, HRTEM observations have shown that they exhibit short-range order broken in a few atomic planes. These results will be an object of a future publication [2]. In this paper, we focus on the magnetization processes of these two compositions, with
* Corresponding author. Tel.: + 34-1-6304278; fax: + 34-1630 1625; e-mail: [email protected].
the aim of illustrating the differences in the microstructure mentioned above. As the size of the cobalt clusters present in the sample with 20% boron (20-30 nm) is smaller than the exchange correlation length, they are expected to affect the magnetization process. Amorphous ribbons of compositions Cos0B2o and Co75B25 were produced by the melt-spinning technique. The samples were submitted to successive annealings to study the evolution of the material. The annealing temperature was always lower than the crystallization temperature. From calorimetry analysis and HRTEM observations [2], we can conclude that these treatments lead to the relaxation of the amorphous phase in the case of C075B25 , and to the appearance of the cobalt nanoclusters in the case of Cos0B20. The number of precipitated clusters increases with the annealing temperature and time. The field dependence of the magnetization was measured by means of inductive methods at a frequency of 120 Hz. In order to obtain the switching curves of the samples, a series of minor loops were registered varying the maximum applied field from 4 0 e down to 0. The points that make up these curves represent the highest value of magnetization versus the maximum applied field for each minor loop.
0304-8853/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 7 1 2-4
176
A. Gonzgtlez et al. /Journal of Magnetism and Magnetic' Materials 196-197 (1999) 175-176 10
10
H
0.5
C 2;
°8: 2o
C07~B:,,:,
[
~ 0.5
O0
I ~ CoszB2o -0.5
:1
-1.0 2
0
--~ --~ ~
as-cast 335"C, 3min 350°C, lOmin
i 2
2;
---c--- as-cast 335°C, 3rain + 330°C, 30min 330°C, 60min 350°C. lOmin
tl
s . . . . . . ive i treatments
O0 00
05
10 '
15 i
20
H (Oe)
H (Oe)
Fig. 1. Hysteresis loops of CosoB20 and CovsB25 in the as-cast state and after successive annealings.
Fig. 2. Switching curves of C08oB2o and C075B25 in the as-cast state and after successive annealings.
The hysteresis loops and switching curves are plotted in Figs. 1 and 2, respectively, for both compositions in the as-cast and annealed states. As can be seen in Fig. 1, the as-cast CosoB2o loop presents a higher squareness character than the as-cast C07sB2s loop, which is more round. This difference is better reflected in the switching curves, see Fig. 2. Thus, in the switching curves of CoaoB2o it is possible to define a field H~ that separates the magnetization process in two parts. For fields higher than H~ almost the whole material is magnetized to saturation. Whereas for fields lower than Hs the magnetization falls down very quickly, meaning that only a very small part of the magnetization can follow the alternating field. The value of this field increases with the annealing temperature or time, i.e., with the number of cobalt clusters. On the other hand, the switching curves of C0~5B25 decrease continuously towards zero, and after a first relaxation of the amorphous phase, they do not change upon further annealings. The magnetization process in C08oB2o is characterized by the presence of defects (cobalt nanoclusters that are amorphous in a sense discussed above) which act as
pinning centres to the wall displacements [3,4]. Whereas the walls can move uniformly through the material in the case of C075B25, the amorphous matrix of which exhibits a homogeneous structure. It is possible to compare the contribution of the defects to the wall pinning in both compositions, which should be described by the product of coercivity times the saturation magnetization. We obtain that for the as-cast samples the contribution in C080Bzo is twice as much that of Co75B2s. Besides, this product increases with the annealing treatments in the CosoB2o sample: while the coercive field of Co75B25 remains constant (He ~ 0.20e), it grows continuously for Co8oB2o (from 0.3 to 0.60e).
References
[t] R. Hasegawa, R. Ray, J. Appl. Phys. 50 (1979) 1586. [2] A. Zern, J. Appl. Phys. (1999) in press. [3] A. Hernando, M. Vazquez, T. Kulik, C. Prados, Phys. Rev. B 51 (1995) 3581. [4] S. Chikazumi. Physics of Magnetism, Krieger, New York, 1964, p. 285.