Effect of heat treatment on the secondary magnetic effects in iron and cobalt based amorphous magnetic materials

Abstract

Co-based amorphous magnetic materials with composition C08o_xFexBIOSilO (x = 0, 2, 4 and 6) have been prepared by melt spinning technique as a possible soft magnetic material. These specimens have been checked for their amorphousity by X-ray diffraction. Magnetostriction as a secondary effect, which plays one of the most important roles in determining the magnetic characteristics like coercivity, initial permeability, remanance etc. have been studied by straingauge technique using a Wheatstone Bridge in out of balance condition. The effect of composition and temperature on magnetostriction have been measured with a sensitivity of 10-6 as fractional strain. Since magnetostriction arises from spin orbit coupling such that a ferromagnetic material spontaneously develop lattice distortion in order to reduce magnetic anisotropy energy, the value of magnetostriction provide information regarding the magnetoelastic interaction of a magnetic material. Its importance is not only technological because low magnetostriction is related to low constrain in domain wall movement and hence higher permeability, but also of theoretical interest. Amorphous material in its perfect isotropic form should not have magnetic anisotropy and also magnetostriction but due to the mechanism used in the production of amorphous ribbons there is a strain induced anisotropy and an associated magnetostriction. The strain arises due to the tension along the length of the ribbon which the specimen is subjected to during the preparation of the ribbons. In this alloy system, the magnetic elements Iron and cobalt have positive and negative magnetostriction respectively their pure crystalline forms. It is observed that with the substitution of iron for cobalt magnetostriction has decreased gradually, which shows that iron atom has cancelled out part of the negative magnetostriction due to cobalt atoms. This result although quiet interesting in showing that one can tailor amorphous materials in respect of their magnetostrictions and hence magnetic softness by adjusting the proportions of elements having opposite magnetostrictions, can not be deduced theoretically. Because magnetoelastic effect is a property of the structure of the material and not of its chemical composition only. Our experimental finding is therefore very important in establishing a relation and a guideline for preparing magnetically soft . amorphous material. Only one sample was without iron which gave maximum negative magnetostriction value. For other samples there is monotonous decrease of magnetostriction with increasing iron substitution, maximum value is -21.519 x 1O~for composition COso_xFexBJOSiJO (x = 2) and minimum value is -14.308 x 1O~ for composition C08o-xFexBIOSilO (x = 6). Maximum value for magnetostriction at liquid nitrogen temperature is -26.302 x 1O~ for composition C08o_xFexBIOSiJO (x = 2) for minimum value is -16.174 x 1O~ for composition C08o_xFexBJOSiJO (x = 6). Thermal expansion measurements have also been carried out by straingauge technique which shows that amorphous state give rise to lower thermal expansion coefficient because of lower anharmonicity of the atomic vibrations in this system.