We demonstrate a room-temperature spin dynamo where the precession of electron spins in ferromagnets converts energy from microwaves to a bipolar current of electricity. The current/power ratio is at least 3 orders of magnitude larger than that found previously for spin-driven currents in semiconductors. The observed bipolar nature and intriguing symmetry are fully explained by the spin rectification effect via which the nonlinear combination of spin and charge dynamics creates dc currents.

We investigate the dc electric response induced by ferromagnetic resonance in ferromagnetic Permalloy (Ni80Fe20) microstrips. The resulting magnetization precession alters the angle of the magnetization with respect to both dc and rf current. Consequently the time averaged anisotropic magnetoresistance (AMR) changes (photoresistance). At the same time the time-dependent AMR oscillation rectifies a part of the rf current and induces a dc voltage (photovoltage). A phenomenological approach to magnetoresistance is used to describe the distinct characteristics of the photoresistance and photovoltage with a consistent formalism, which is found in excellent agreement with experiments performed on in-plane magnetized ferromagnetic microstrips. Application of the microwave photovoltage effect for rf magnetic field sensing is discussed.

Quantized spin excitations in a single ferromagnetic microstrip have been measured using the microwave photovoltage technique. Several kinds of spin wave modes due to different contributions of the dipole-dipole and the exchange interactions are observed. Among them are a series of distinct dipole-exchange spin wave modes, which allow us to determine precisely the subtle spin boundary condition. A comprehensive picture for quantized spin excitations in a ferromagnet with finite size is thereby established. The dispersions of the quantized spin wave modes have two different branches separated by the saturation magnetization.

An approach to measure precisely nonlinear ferromagnetic damping is demonstrated by using spin dynamos in combination with sensitive electrical probing techniques. The directly measured intrinsic foldover effect unravels a 50-year-old mystery of ferromagnetic metals. Pivotal importance of nonlinear ferromagnetic damping is uncovered via its distinct dependence on the frequency, amplitude, and initial conditions. The experimental results are in excellent agreement with a nonlinear oscillator model, which revises the pioneer work of Anderson and Suhl for nonlinear magnetization dynamics.Nonlinear dynamics differ distinctly from linear response. For example, introducing a nonlinear restoring force into Galileo's pendulum 1 breaks the isochronism of harmonic oscillators and causes effects such as amplitude-dependent resonance frequency, foldover and bistability. 2 These nonlinear fingerprints are ubiquitous in nature, as found in mechanical 2 and magnetic 3,4 systems. Demanding technological issues such as microwave-assisted switching 5 and spin-torque nano-oscillators 6 require a clear grasp of nonlinear magnetization dynamics. Yet, the long-standing significant challenge in making quantitative measurements of nonlinear ferromagnetic dissipation has hampered understanding.For over half a century, the foundation of ferromagnetic dissipation 7 has been built on the Gilbert damping model. 8 Based on the linearization of the Landau-Lifshitz equation, the intrinsic Gilbert damping constant ␣ is commonly determined from the linewidth ⌬H 0 of ferromagnetic resonance ͑FMR͒, 7 via the well-known relationwhere ⌬H i describes the nonintrinsic magnetic damping induced by inhomogeneities, 9 is the microwave frequency and ␥ is the gyromagnetic ratio. Equation ͑1͒ has been experimentally verified in the linear regime at sufficiently small magnetization precession angles . A recent theoretical breakthrough has elucidated the intrinsic nonlinear dissipation in ferromagnetic metals. 10 It leads to the open question of whether the standard Gilbert model is adequate in the nonlinear regime, 11 and it motivates us to develop an experimental approach to measure precisely nonlinear ferromagnetic dissipation in metals by utilizing spin dynamos. 12 Our primary finding is that for above a few degrees, Eq. ͑1͒ is replaced bywhich is dominated by the third term caused by nonlinear damping. Here M 0 is the saturation magnetization, ␤ is a dimensionless but frequency-dependent damping constant describing nonlinear ferromagnetic dissipation. Equation ͑2͒is established by studying FMR of Py microstrips. It fundamentally changes the picture of ferromagnetic dissipation in its frequency and amplitude dependences, which reveals the reason why foldover and bistability effects were not observed in any ferromagnetic metals previously, despite the pioneer work of Anderson and Suhl 3 which predicted such nonlinear fingerprints more than 50 years ago. 13 To highlight the general features of nonlinear dynamics, we begin with a simple model of a classical os...