Recent Developments in Ferromagnetic Resonance at High Power Levels

Abstract

The influence of inhomogeneities on the saturation of the ferromagnetic resonance is investigated. In the region of moderate power levels, the susceptibility at resonance χ′′ varies linearly with the square of the rf field h. The magnitude of the slope ∂χ′′/∂h2 depends on the nature of the dominant scattering mechanism. If the uniform mode scatters primarily to spin waves of very large wavelength, the slope should be negative. Scattering to spin waves of short wavelength gives a positive contribution to the slope and can lead to a reversal of the sign. The theoretical predictions agree with measurements at X band on various polycrystalline garnets and ferrites. At very high power levels the opening angle of the precessing magnetization vector approaches a limiting value, which is related to the “line width” ΔHk of z directed spin waves having the same frequency as the uniform mode. Experiments on single crystals and polycrystals of rare earth substituted garnets show that ΔHkincreases approximately linearly with the rare earth content. The materials investigated contain Gd, Yb, Er, Sm, Dy, Ho, or Tb and for a given ratio of substitution ΔHk increases in that order. The line width ΔHk of z directed spin waves is found to be approximately proportional to the line width ΔH of the uniform mode as measured in single crystals. Experimental results on cobalt and zinc substituted nickel ferrite are reported. ΔHk increases linearly with the cobalt content. For the nickel-zinc ferrites with a large magnetic moment the saturation curve (χ′′ vs h2) measured at X band shows a maximum well below the initial onset of nonlinearity. A theoretical explanation for this extraordinary behavior is given. A new nonlinear effect arising from spin wave instability in a microwave magnetic field applied parallel to the dc field has been observed. Spin waves which propagate in directions perpendicular to the dc field are most susceptible to this instability. The observed variation of the critical rf field strength agrees well with the theoretical predictions. It indicates that the spin-wave line width increases with increasing wave number and decreasing angle between propagation direction and dc magnetic field.

Parametric excitation of eigenmodes in microscopic magnetic dots

“…In this configuration, the microwave field cannot linearly excite magnetization dynamics, since the corresponding component of the dynamic magnetic susceptibility tensor is equal to zero 16 . Instead, the dynamics can be excited by a higherorder parametric excitation process 35 . In the quasiparticle picture, this process can be understood as splitting of a microwave photon with frequency f P and wave vector k P ≈0…”

Spectral shape deformation in inverse spin Hall voltage in Y3Fe5O12|Pt bilayers at high microwave power levels

“…YIG also offers a playground for exploring non-linear magnetization phenomena at high microwave power levels, such as spin wave instabilities, foldover and bistable behaviour. 18,19 In recent years, there has been significant interest in observing non-linear spin dynamics in YIG via ISHE measurements and harnessing these effects in spintronics a) Electronic mail: lustikova@imr.tohoku.ac.jp devices. [20][21][22][23] The simplest example of a non-linear regime in spin pumping is the effect of decreasing static component of magnetization.…”

Thickness dependence study of current-driven ferromagnetic resonance in Y3Fe5O12/heavy metal bilayers

“…Within the family of ferromagnetic insulators, yttrium iron garnet (YIG) holds a special place owing to several favourable properties, including ultra-low damping, high Curie temperature and chemical stability [1][2][3]. By growing an overlayer of heavy metal (HM), such as platinum or tantalum, several important spintronic phenomena have been explored in the YIG/HM bilayer system, including the magnetic proximity effect [4,5], spin pumping [6,7], spin Hall magnetoresistance (SMR) [8,9], spin Seebeck effect [10,11] and so on.…”