Ferromagnetic resonance in nanostructures, rediscovering its roots in paramagnetic resonance

Abstract

Both techniques went different routes: The EPR explored an enormous variety of paramagnets in solids, liquids, and gas phase. The focus was to determine orbital- and spin-magnetic moments (g-tensor), hyperfine interactions, and from the linewidth the spin dynamics (T1, T2 relaxation). In FMR most of the experiments and theory assumed the total value M to be constant in the equation of motion and used only one effective damping parameter (Gilbert). This is an enormous, unnecessary limitation for today's analysis of magnetism in nanostructures and ultrathin films. To assume M = const ignores spin wave excitations, scattering between longitudinal and transverse components of M. Moreover, in the framework of itinerant ferromagnetism, the magnetic moment/atom μ was assumed to be isotropic with g ≈ 2! That ignores the anisotropy of μ in nanostructures and the importance of the orbital magnetic moments with μL/μS = (g − 2)/2. Without finite μL we would have no magnetic anisotropy energy (MAE), no hard magnets, no magnetic storage media. Only recently the "language" of EPR was adapted to FMR in ultrathin films. A g-tensor is discussed and its interrelation with the MAE is pointed out. Also recent theory points out, that" there is no reason to assume a fixed magnetization length for nanoelements". This allows a detailed discussion of magnon-magnon scattering, spin-spin, and spin-lattice relaxation - useful, for example, for fs spin dynamics. Recent FMR experiments using frequencies from 1 GHz up to several hundred GHz, will allow measuring the proper g-factor components and μL,μS. From the frequency dependent linewidth magnon-magnon scattering can be separated from dissipative spin-lattice damping.