Magnetic-dipole mechanism for biquadratic interlayer coupling

A mechanism resulting in biquadratic interlayer coupling is proposed and analyzed theoretically. This mechanism is connected with the magnetic-dipole field, created by magnetic layers with roughness. This field decays exponentially with the distance from the layer, but it shows oscillating behavior in the lateral direction. The scale of both exponential and oscillating dependencies corresponds to the scale of the interface roughness. The oscillating variation of the field makes 90° alignment of the magnetization energetically favorable in analogy to the Slonczewski mechanism. Computer simulations and estimates show that this mechanism can provide a coupling strength of the order of 0.01 erg/${\mathrm{cm}}^2$ for Fe films with 1 nm interlayer thickness.