Magnetoelasticity of Fe₆₅(Ni_1-xMn_x)₃₅invar alloys: I. Temperature and field dependences of the elastic constants

Abstract

Temperature and field dependences of the elastic constants of Fe65(Ni1-xMnx)35 invar alloys have been measured by an ultrasonic pulse-echo technique from 4.2 up to 750 K. All of the elastic moduli, CL, C44 and C', show softening due to magnetoelastic coupling. The softening of shear moduli, Delta C44 and Delta C', is proportional to the square of the magnetisation. The softening of the bulk modulus is mainly ascribed to an inverse effect of the volume magnetostriction, Delta Bomega . The authors have shown that magnetic ordering gives rise to an increase of the bulk modulus, Delta Bm. The large negative field dependence of shear moduli in the para-process is explained as a result of the increase of magnetisation caused by an external field. The positive field dependence of the bulk modulus for x<0.1 and the negative one for x>0.1 are also explained by taking into account the magnetic contributions to the bulk modulus, Delta Bomega and Delta Bm.