Biquadratic exchange coupling in Fe/Cr/Fe(100) and Fe/Ag/Fe(100) thin film sandwiches (abstract)

Abstract

We have investigated the magnetic exchange coupling between an Fe film and an Fe(100) single crystal whisker substrate separated by varying thickness Cr or Ag spacer layers. The magnetization of the Fe was measured using scanning electron microscopy with polarization analysis. Reflection high-energy electron diffraction and scanning tunneling microscopy (STM) were used to analyze the film structure. In addition to the oscillatory thickness-dependent bilinear exchange coupling, biquadratic exchange coupling was observed in both films. At spacer thicknesses that correspond to the transitions between ferromagnetic and antiferromagnetic coupling the Fe film magnetization was in-plane and orthogonal to the ferromagnetic alignment. Our observations generally agree with the model proposed by Slonczewski1 in which the biquadratic coupling originates from the bilinear coupling through spatial fluctuations of the spacer thickness. If these fluctuations occur over small enough length scales and the bilinear coupling varies between ferromagnetic and antiferromagnetic with monolayer thickness changes, then the Fe film intralayer exchange stiffness forces biquadratic coupling of the film to the substrate. We have observed the necessary short-period coupling in both Fe/Cr/Fe and Fe/Ag/Fe and we have measured the length scale of the thickness variations on a similar sample using a STM. One unexpected result in Fe/Ag/Fe was that the amount of biquadratic coupling increased with increasing top Fe layer thickness. No similar dependence was observed for Fe/Cr/Fe.