Domain Structure and Thermal Dependence of the Coercive Field in Nanocrystalline FeZrBCu
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Domain structure and thermal dependence of the coercive field in nanocrystalline FeZrBCu A. Hernando and J. Arcas Instituto de Magnetismo Aplicado (UCM, RENFE) P.O. Box 155, 28230, Las Rozas (Madrid), Spain ABSTRACT Nanocrystalline Fe85Zr7B6Cu2 and Fe87.2Zr7.4B4.3Cu1.1 (at. %) samples have been obtained by annealing Melt-spun ribbons for 1hr at various temperatures in the range 593K to 953K. Structural characterization by means of X-ray diffraction and thermomagnetic analyses reveal the manifestations of Fe nanocrystals embedded in an amorphous matrix.. The coercive field has been measured by a Förster coercimeter at temperatures ranging between 50 K and 300 K, and the room temperature domain structure has been monitored by magneto-optical Kerr effect. Both, the as cast as well as the sample annealed above 813K display soft magnetic properties at room temperature, exhibiting a coercive field below 10 A/m, and wide regular domains. In contrast, the samples annealed at 750 K, corresponding to the beginning of the crystallization process, undergo a magnetic hardening, showing higher coercive fields (up to 150 A/m for Fe87.2Zr7.4B4.3Cu1.1) and a dull domain pattern. When these magnetically harder samples are cooled, their coercivity is reduced. Thus, the magnetic hardening is attributed to the exchange decoupling between crystallites, due to the proximity of the Curie temperature of the amorphous phase.
INTRODUCTION A class of magnetic materials of interest today is composed of a set of nanocrystals (phase 1), which interact through their grain boundaries or through a different intergranular phase (phase 2). Thus, softer magnetic materials of composition Fe79Zr7B14 consist of Fe rich nanocrystallites embedded in a soft amorphous matrix and display coercivities of 10-3 Oe [1]. Whereas, harder magnets, of composition Fe79Nd7B14, can achieve coercivities of 104 Oe and are formed by mixing at nanoscale highly anisotropic rare earth-transition metals crystallites with a soft magnetic matrix, which enhances both magnetization and Curie temperature [2]. This broad range (of seven order of magnitude) is obtained on substituting Nd by Zr at the same 7 at.% level in the amorphous alloy of composition Fe79B14. Both materials are nanostructured, the former consists of Fe nanocrystals, with anisotropy constant, k1=104Jm-3, embedded in an amorphous matrix of FeZrB, with k2=102 Jm-3; whereas the later is formed of Fe2Nd14B1 nanocrystals with k1=5 106 Jm-3, embedded in a matrix of Fe. In both cases, the magnetic anisotropy constant of crystallites, k1, is two orders of magnitude larger than that of the matrix, k2. One should also notice that the anisotropy constant of Fe2Nd14B1 is only two and a half orders of magnitude larger than that of Fe. Therefore, the difference in coercivity is due to the effects of the nanostructure rather than for compositional reasons. The coercive field of a ferromagnetic material is given by,
L7.4.1
HC = p
k eff µ0 MS
(1)
where keff is the effective anisotropy, MS the saturation magnetization and p
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