Onset of ferromagnetism in pseudobinary FCC (Fe_1-xNi _x)₉₂C₈ alloys. II. Magnetovolume effect

Thermal expansion, spontaneous volume magnetostriction and forced volume magnetostriction of the pseudobinary FCC (Fe_1-xNi_x) ₉₂C₈ alloys (0.1<or=x<or=0.4) fabricated by rapid quenching have been measured at temperatures from 4.2 to 800K. The spontaneous volume magnetostriction omega _s is observed in alloys with x>or=0.2, being a maximum of omega _s approximately 1.3*10^-2 at x=0.27 in the strong ferromagnet. The forced volume magnetostriction at 4.2K reaches its maximum at about x=0.225 in the mixture region of weak ferromagnetism and paramagnetism. The large positive thermal expansion due to the spin fluctuation effect is found at x<0.2 in the spin glass or Pauli-type paramagnetic alloys. The instability of local magnetic moments of Fe at about x=0.25 which is responsible for the magnetovolume effect is discussed. The composition dependence of the spontaneous volume magnetostriction is explained by the variation of the local magnetic moment with x, accompanied by the change of the number of nearest-neighbour Ni atoms.