Origins of giant biquadratic coupling in CoFe/Mn/CoFe sandwich structures (abstract)

Abstract

Recently Filipkowski et al. reported extremely strong, near 90 degree coupling of 2.5 erg/cm2 for epitaxial sandwiches of CoFe/Mn/CoFe, where the CoFe composition was chosen to be a good lattice match to Mn. Both CoFe and Mn have the bcc structure, but Mn is antiferromagnetic while CoFe is ferromagnetic. It was found that the data were very well described by a simple model due to Slonczewski, in which the interlayer coupling is given by Fc=C+(φ1−φ2)2+C−(φ1−φ2−π)2. While this model describes the data much better than the usual biquadratic form, it still does not connect directly to the microscopic origins of the effect. In the present work we seek to explain the results in terms of normal bilinear exchange and magnetocrystalline anisotropy, together with reasonable assumptions about the structure of the interfaces. We obtain excellent agreement with both the experimental results and the Slonczewski model under the assumptions that at least one of the two CoFe/Mn interfaces is smooth (i.e., atomically flat) on a length scale comparable to or greater than the thickness of the Mn layer and at least one interface is rough on a scale less than approximately a domain wall thickness.