Role of interfaces in higher order angular terms of magnetic anisotropies:ultrathin film structures

The role of interface inhomogeneities on macroscopic magnetic anisotropy te rms was investigated using simple mathematical modeling. It is shown that t he intrinsic anisotropies caused by the spin-orbit interaction in the prese nce of lateral inhomogeneities can create additional magnetic anisotropies with higher order angular terms, The lateral inhomogeneous distribution of the uniaxial anisotropy can result in the presence of a fourth order angula r term. These anisotropies decrease the total energy, and their strength is affected by the exchange averaging effect. Large atomic terraces allow the local magnetic moment to deviate from the mean orientation resulting in ne gative higher order angular terms in magnetic anisotropies satisfying the s ample symmetry. The negative sign results in the magnetic easy axes being o riented 45 degrees with respect to the intrinsic uniaxial axis. It is shown that the additional higher order terms do not always have to satisfy the t ime inversion symmetry. Odd powers in directional cosines can arise when th e exchange restoring torque becomes comparable to or weaker than torques fr om the internal fields. The additional higher order angular terms in magnet ic anisotropies decrease usually more rapidly with the sample thickness tha n the intrinsic interface anisotropies. Their rapid decrease in strength wi th an increasing sample thickness allows one to distinguish them from the i ntrinsic interface anisotropies. It will be shown that the magnetic anisotr opies originating in interface inhomogeneities can play an important role i n the magnetic phase transitions of ultrathin film structures. (C) 2001 Els evier Science B.V. All rights reserved.