Strong biquadratic coupling and antiferromagnetic-ferromagnetic crossover in NiFe/Cu multilayers

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Abstract
We report clear manifestations of biquadratic exchange in the magnetoresistance (MR) of (110) single-crystal NiFe/Cu multilayers. Magnetoresistance curves show a low-field MR minimum which results from an asymmetric canting of the moments away from the applied magnetic field. The stability of these magnetic configurations indicates that the biquadratic coupling can be significantly stronger than the bilinear coupling. In samples with ferromagnetic (F) bilinear coupling the strength of both the bilinear and biquadratic coupling decays in the temperature range from 200 to 400 K. In samples that are coupled antiferromagnetically (AF) at room temperature, we observe a crossover from AF to F bilinear coupling as the temperature is lowered below 200 K. We attribute the strong biquadratic coupling and temperature dependence of the bilinear coupling to pinholes in the Cu spacer layers.