Magnetic phase transition and critical point behaviour of some iron and based amorphous materials

Abstract

In the present work the detailed study of magnetic phase transition and critical point behavior, magneto stricti on and temperature dependence of resistivity of the amorphous alloys Coso_xFexBloSilO and Ni sO-xFe xB20with x= 2,4,6,8 and x= 20,30,40,50 has been carried out respectively. Metallic glass ribbons of Co-Fe-B-Si and Ni-Fe-B alloys are prepared by melt spinning technique. A review of theory of magnetic phase transition and critical point behavior of some iron - nickel based amorphous materials is presented. The magnetic phase transition and critical phenomena of COso-xFexBIOSiIOand Niso_xFexB2oalloys are investigated by using the experimental values of magnetization as a function of magnetic field and temperature. Magnetization measurements on the Coso-xFexBIOSiiOand Niso_xFexB2oalloy samples were carried out in the temperature range 301 to 716 K and in magnetic fields up to 4 Koe_ The critical exponents are deduced from magnetization measurements. The ferromagnetic transition temperature (Te), and critical exponents (~, y and 8) have been determined by using the magnetic state equations in the critical region_ The values of~, y and 8 are discussed and compare~ with the results of various theoretical models. The data are fitted to a magnetic equation of state characteristics of a second - order phase transition over a rather wide range of temperature both above and below Te. Magnetostriction of the above samples prepared by melt spinning technique have been measured by resistance strain gauge technique at room temperature as a function of field and from liquid nitrogen temperature to room temperature at constant malsnetic field. The linear saturation magnetostriction of the amorphous Coso-xFexBIOSiiOwith x= 2,4,6,8 ribbons have been calculated by rotating the magnetization vector from perpendicular position to parallel position of the magnetization direction with respect to the direction of strain measurement. It is considered that the direction of the field and the direction of magnetization practically coincide at maximum field. The d•irection of the strain measurement is taken on the direction of the bonded strain gauge. The saturation magnetostriction decreases as Co concentration increases and Fe-concentration decreases. Here the observed magnetostriction has been calculated in terms of the theory explained in chapter 5.The results are described in terms of domain motion and domain rotation that are spontaneously strained.