Ferromagnetic Resonance near the Upper Limit of the Spin Wave Manifold

Abstract

Measurements of line widths of polycrystalline garnets as a function of frequency and temperature are reported. The variation of these parameters allows the uniform precessional mode to move above the k = 0 upper limit of the spin wave manifold so that this mode is no longer degenerate with the low k value spin wave states. When this occurs the line width undergoes a radical change, since these low k states can no longer take place in the relaxation process. When the uniform precession is thus “outside” the manifold, the behavior of the line width should be attributed to the theory of Clogston, Suhl, Walker, and Anderson. When the uniform precession is “inside”the manifold, the behavior is best described by the theory of Schlömann. It is believed that the measurements reported indicate convincingly the utility of the spin wave analysis of ferromagnetic relaxation.

Ferromagnetic resonance linewidth mechanisms in polycrystalline ferrites: Role of grain-to-grain and grain-boundary two-magnon scattering processes

“…The first experimental signature of two-magnon scattering as a valid FMR relaxation process came from the seminal work of Buffler in 1959. 7 Schloemann's work, 9 noted above, had pointed out a peculiar spin-wave band effect for sphere shaped samples. As one increases the applied field, there is a point at which the FMR frequency crosses the top edge of the low wave-number limit of the spin-wave band.…”

“…It occurs for spheres but not for other sample shapes. Buffler 7 was the first to observe this effect experimentally. This direct early experimental evidence for TMS processes provided the key data that pointed the way to further theory and experiments on anisotropy, porosity, and surface pit TMS processes.…”

“…The second point concerns the interplay between the actual coupling to the degenerate modes and the crossover effect that leads to the TMS signature in the linewidth vs frequency profile discovered by Buffler. 7 In this context, the key is in the shaded vertical strip of modes at the extreme low k side for each of the spin-wave band diagrams in Fig. 1.…”

Hamiltonian formalism for two magnon scattering microwave relaxation: Theory and applications

“…Two magnon scattering processes are often invoked to account for additions to the intrinsic ferromagnetic resonance ͑FMR͒ relaxation rate or resonance linewidth for bulk 3, 4 and thin film 5 ferrites as well as metallic magnetic films and multilayers. [6][7][8] The driven FMR mode is usually spatially uniform or near uniform and is taken to correspond to a k = 0 spin wave excitation.…”

Limiting long-range-ordered solids to finite sizes in condensed-matter physics

“…We believe that a large increase in grain boundaries results from decreased grain size and these act as scatterers for the two-magnon relaxation channel. A large conversion of the k = 0 modes going to the degenerate k # 0 modes has been ascribed to grain boundary scattering by Herring and Kittel (1951) and Buffler (1959). The non-appearance of such a broadening in earlier work may be due to the relatively large grain size (less scattering centres per unit volume).…”