Spin wave stiffness and the range of exchange interactions in amorphous (Fe_1-xNi_x)₇₅B₂₅ and (Fe_1-yW_y)₈₃B₁₇ alloys

Abstract

The variation of saturation magnetisation, M_s, with temperature has been derived from FMR measurements performed on amorphous (a-)(Fe_1-xNi_x)₇₅B₂₅ and (Fe_1-yW_y)₈₃B₁₇ alloys in the temperature range 77 to 300 K. These data have been analysed to yield reliable values of the coefficients B, C of the T^3/2, T^5/2 terms in the spin wave expression for the fractional decrease in magnetisation with temperature and the mean-square range of the exchange interaction, (r²). The spin wave theory for amorphous ferromagnets due to Krey adequately describes the observed dependence of the above parameters (B, C, (r²)) on the Curie temperature, T_C (determined from low-field (<or=10 Oe) bulk magnetisation data). The plot of spin wave stiffness coefficient, D, against T_C for a-(Fe_1-xNi_x)₇₅B₂₅ alloys is a straight line, well represented by the empirical equation D=D₀+mT_C, whose slope m exactly coincides with that predicted by the nearest-neighbour (nn) Heisenberg model and intercept, D₀, on the ordinate gives the strength of the next-nearest-neighbour (nnn) exchange coupling constant which is at least one order of magnitude smaller than the nn exchange coupling constant. A plot of D against T_C for a-(Fe_1-yW_y)₈₃B₁₇ alloys is again a straight line with the same slope m but passing through the origin (i.e. D₀=0). This result is shown to imply that the competing interactions in W containing glasses confine the direct exchange to the nearest neighbours only whereas the exchange interactions in Ni containing alloys involve both the nearest as well as the next-nearest neighbours. Other important findings include (i) mixed exchange interactions cause more drastic reduction in the value of T_C, M_s(0) and D than does simple magnetic dilution and (ii) for a given value of T_C, D possesses a considerably lower value for the W substituted alloys than for the Ni substituted ones.