Temperature Dependence of Magnetization and Induced Magnetic Anisotropy of Some Fe, Co and Ni-Based Amorphous Ribbons

Abstract

Ferromagnetic amorphous ribbons of Fe-Si-B, Ni-Fe-B and Co-Fe-B-Si series have been prepared by rapid quenching technique and the amorphousity of the samples has been confirmed by X-ray diffraction technique. The kinetics of glass formation and crystallization as affected by a slight change in composition is studied by differential thermal analysis (DTA). It is observed that the thermodynamics of the amorphous ribbons in respect of the formation of glassy state and its stability is affected by the complexity of the composition. The magnetic ordering of Fe-based, Ni-based and Co-based amorphous ribbons is studied by measuring A.C. and D.C. magnetization as functions of temperature. D.C. measurements are made by using a vibrating sample magnetometer and A.C. initial permeability is measured, using a furnace in which the heating wire is wound in accordance with the bifiller technique. Coercivity, remanence, saturation magnetization and maximum permeability have been determined as static magnetic properties for Fe-based, Ni-based and Co-based specimens from the hysteresis loops obtained by Ballistic method. The dynamic characteristic such as the real and imaginary components of the complex permeability in A.C. conditions have been measured as a function of the instantaneous value of a sine wave core current density by means of an adapted LCR bridge method. Initial permeability, frequency dependence of complex permeability and relative quality factor have been measured in the frequency range 0.5KHz to 13MHz for all the different compositions and for annealing effect of initial permeability. The measurements of saturation specific magnetization of Fe-based, Ni-based and Co-based amorphous ribbons have been measured by V.S.M. We have correlated the behaviour of magnetic moment in amorphous alloys containing 3d transition metals on the basis of rigid band model, assuming that the metalloid atoms contribute some of their S electrons and P electrons to fill the d band of the transition metal. This accounts for the reduction of magnetization of all the amorphous ribbons with increasing metalloid content. The temperature dependence of magnetization, of amorphous system decreases faster with increasing temperature as compared to crystalline materials. According to the mean field approximation, reduced magnetization versus reduced temperature graphs for different Fe-based, Ni-based and Co-based amorphous ribbons leads to fluctuation in the exchange interaction giving rise to a structural disorder. Induced magnetic anisotropy of amorphous ribbons having Fe-based, Ni-based and Co-based system have been measured for temperature and field dependence using torque magnetometer with proportional integrating differentiating compensation. The results of compositional dependence of initial permeability, coercive force and induced anisotropy are found to be mutually consistent for all the samples. The origin the uniaxial anisotropy in these amorphous materials is assumed to be the stress, induced during the preparation. The temperature dependence of uniaxial anisotropy is caused by stress, relieving and decreases with the increase of temperature. The corresponding curves of In [Ku(0)/ Ku(T)] versus In [σs(0)/ σs(T)] in amorphous ribbons with Fe-based, Ni-based and Co-based system follow straight lines arising from the disorder caused by randomly oriented easy axes. The values of the exponents are different for the different samples. All amorphous ribbons have been investigated using Mössbauer spectroscopy. The Mossbauer measurements were performed with a conventional constant acceleration spectrometer at room temperature in transmission geometry, using 57Co in a rhodium matrix. The velocity scale of a thin 57Fe sample and the isomer shifts are given relative to the centroid of the spectrum. The isomer shifts of Fe-atoms in all the Fe, Ni and Co-based samples behave as trivalent ions. The spectrum for the as prepared samples consists of six lines which are broadened as compered with the Mössbauer spectrum of a pure 57Fe sample in the form of a thin foil. The curve for the experimental spectrum is obtained by using the best fit of the distribution of hyperfine fields. The consistency between the average magnetization measured by V.S.M. and the magnetic moment of the Fe-atom as estimated from the hyperfine field distribution confirms the collinearity of the magnetic ordering of the sample.