Spin wave stiffness and the range of exchange interactions in amorphous (Fe1-xNix)75B25 and (Fe1-yWy)83B17 alloys

Kaul, S. N. ; Mohan Babu, T. V. S. M. (1989) Spin wave stiffness and the range of exchange interactions in amorphous (Fe1-xNix)75B25 and (Fe1-yWy)83B17 alloys Journal of Physics: Condensed Matter, 1 (44). p. 8509. ISSN 0953-8984

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Official URL: http://iopscience.iop.org/0953-8984/1/44/021

Related URL: http://dx.doi.org/10.1088/0953-8984/1/44/021

Abstract

The variation of saturation magnetisation, Ms, with temperature has been derived from FMR measurements performed on amorphous (a-)(Fe1-xNix)75B25 and (Fe1-yWy)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 T3/2, T5/2 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, TC (determined from low-field (<or=10 Oe) bulk magnetisation data). The plot of spin wave stiffness coefficient, D, against TC for a-(Fe1-xNix)75B25 alloys is a straight line, well represented by the empirical equation D=D0+mTC, 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 TC for a-(Fe1-yWy)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 TC, Ms(0) and D than does simple magnetic dilution and (ii) for a given value of TC, D possesses a considerably lower value for the W substituted alloys than for the Ni substituted ones.

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