TEMPERATURE-DEPENDENCE OF THE EXCHANGE COUPLING IN THE FE(001) WHISKER 11ML CR/20ML FE STRUCTURE/

The exchange coupling between iron layers separated by 11 monolayers ( ML) of Cr(001) has been investigated using a structure in which the Cr (001) was grown on a bulk iron whisker Fe(001) surface at a temperatur e of approximately 300-degrees-C. This temperature was selected to pro duce near optimum smoothness of the Cr layer. The Cr(001) deposition w as followed by the deposition of 20 ML of Fe(001) at room temperature, and by the deposition of a 20 ML Au(00 1) protective layer. The frequ encies corresponding to the magnetic excitations in this structure wer e measured by means of Brillouin light scattering (BLS). One of the ob served frequencies corresponds to a surface mode in the bulk iron whis ker. Another observed frequency corresponds to the lowest lying preces sional mode of the magnetization in the 20 ML thick Fe(001) thin film. Typically, the thin film frequency exhibits a dependence on applied m agnetic field that displays two cusps. The positions of the cusps are dependent on the exchange coupling between the 20 ML Fe film and the b ulk iron substrate. The surface mode frequency increases monotonically with increasing field over most of the field range investigated. Howe ver, at the field corresponding to the low field cusp in the thin film frequency, the surface mode frequency undergoes an abrupt jump in mag nitude. We have used the position of the cusps in the thin film data t o deduce values for the bilinear, J1, and biquadratic, J2, coupling te rms, where the coupling energy is written in the form E(AB) = -J1 Cos( DELTAphi) + J2 CoS2(DELTAphi); DELTAphi is the angle between the thin film and bulk iron magnetizations. Measurements of J1 and J2 have been carried out at six temperatures that span the range 100-350 K. Both J 1 and J2 are found to depend strongly on temperature. The data are wel l described by the quadratic expression J2 = -0.54 + 1.46 X\J1\ - 0.52 X j1(2), where J1 and J2 are expressed in erg/cm2. The large nonzero intercept and the linear term probably imply a significant intrinsic c ontribution to the biquadratic exchange.