Search

Motivated by the importance of magnetization dynamics in nanomagnets for the development and optimization of magnetic devices and sensors, we measured and modeled spin wave spectra in patterned elliptical nanomagnets. Ferromagnetic resonance spectra for multiple nanomagnets of Ni80Fe20, fabricated by electron-beam lithography to have nominal short-axes of 200 nm or 100 nm, were measured by use of heterodyne magneto-optical microwave microscopy. Scanning electron microscope images taken of the same nanomagnets were used to define element shapes for micromagnetic simulations. The measured spectra show significant differences between nominally identical nanomagnets, which could be only partially attributed to uncontrolled shape variations in the patterning process, as evidenced by the limited agreement between the measured and simulated spectra. Agreement between measurements and simulations was improved by including a zone of reduced magnetization and exchange at the edges of the nanomagnets in the simulations. Our results show that the reduction of shape variations between individual magnetic random-access memory elements can potentially improve their performance. However, unambiguous determination of materials parameters in nanomagnets based on analysis and modeling of spin wave spectra remains problematic. [ABSTRACT FROM AUTHOR]

We present a quantitative, systematic study of the effect of magnetostriction on the dynamical properties in NiFe alloys. Both the ferromagnetic resonance frequency and the damping times are correlated to the magnetostriction. In addition, we find that the Gilbert damping parameter varies by more than 100% over the range of Ni percentage tested (61.9%–86.7%). [ABSTRACT FROM AUTHOR]

We derive equations for the phase noise spectrum of a spin torque oscillator in the macrospin approximation for the highly symmetric geometry where the equilibrium magnetization, applied field, anisotropy, and spin accumulation are all collinear. This particular problem is one that can be solved by analytical methods, but nevertheless illustrates several important general principles for phase noise in spin torque oscillators. In the limit, where the restoring torque is linearly proportional to the deviation of the precession amplitude from steady-state, the problem reduces to a sum of the Wiener-Lévy (W-L) and Ornstein-Uhlenbeck (O-U) processes familiar from the physics of random walks and Brownian motion. For typical device parameters, the O-U process dominates the phase noise and results in a phase noise spectrum that is nontrivial, with 1/ω² dependence at low Fourier frequencies, and 1/ω4 dependence at high Fourier frequencies. The contribution to oscillator linewidth due to the O-U process in the low temperature limit is independent of magnetic anisotropy field and scales inversely with the damping parameter, whereas in the high temperature limit the oscillator linewidth is independent of the damping parameter and scales as √|Hk|. Numerical integration of the fully nonlinear stochastic differential equations is used to determine the temperature and precession amplitude ranges over which our equations for phase noise and linewidth are valid. We then expand the theory to include effects of spin torque asymmetry. Given the lack of experimental data for nanopillars in the geometry considered here, we make a rough extrapolation to the case of nanocontacts, with reasonable agreement with published data. The theory does not yield any obvious means to reduce phase noise to levels required for practical applications in the geometry considered here. [ABSTRACT FROM AUTHOR]

We present a method for simultaneously measuring the two normalized in-plane magnetization components in magnetic thin films that allows for the reconstruction of the magnetization angle and the normalized value of the magnetization with subnanosecond temporal resolution. The calibrated method relies on the large contrast associated with magnetization-induced optical second-harmonic generation, permitting measurements with a dynamic range in excess of 25 dB. Calibration is achieved by the determination of the relative magnitude and phase of the essential nonlinear optical tensor components, including those of a magneto-optic origin. Ellipsometric determination of the polarization of the second-harmonic signal, when the magnetization is saturated along four in-plane Cartesian directions, is sufficient to determine the second-order nonlinear tensor components. These four directions represent the two saturated states along the longitudinal and transverse directions of the magnetization, respectively. The tensor components are subsequently used to uniquely determine the in-plane magnetization amplitude and direction. [ABSTRACT FROM AUTHOR]

Investigates the high frequency behavior and damping of the iron-cobalt-nitrogen-based films by performing magnetization dynamics measurements at a subnanosecond time scale. Critical building blocks in numerous electromagnetic devices; Typical time domain signals with different longitudinal bias fields; Limitations of the Kittel equation.

We used a wide-field Kerr microscope to measure magnetization reversal in high coercivity thin film media that were subjected to nanosecond field pulses. Coplanar waveguides were used as a field source. Two different samples of CoCr10Ta4 were measured. Sample A had a coercivity of 83 kA/m and sample B had a coercivity of 167 kA/m. For sample A, we find that after a step change in H, the magnetization initially relaxes exponentially with a time constant of 5 ns, and then relaxes logarithmically. We interpret this result as indicating a transition from dynamic reversal to thermal relaxation. In higher fields, the exponential relaxation time decreases according to τ=Sw/(H-H0), where Sw=29.7 μs·A·m-1 (373 ns·Oe). For sample B, only logarithmic relaxation is observed, implying that the dynamic magnetization response time is subnanosecond. We observe correlated regions of reversed magnetization in our Kerr images of sample A with a typical correlation length of 1 μm along the applied field direction. We propose a microscopic model of nucleation and growth of reversed regions by analogy to viscous domain wall motion. [ABSTRACT FROM PUBLISHER]

Presents information on a study which determined the magnetization response times in thin-film Permalloy made by both electrical and optical methods. Methodology used; Results and discussion.

We studied the microwave assisted magnetic reversal of 65×71 nm2 elliptical Ni80Fe20 nanomagnets. Hysteresis curves were measured by magneto-optical Kerr effect for a range of microwave frequencies and amplitudes. The coercive field Hc was reduced by 26% for an rf field of 0.08Hc when the microwave frequency coincided with the minimum of the experimentally determined ferromagnetic resonance frequency with varying dc field. The experimental results for the fractional reduction in Hc with rf field amplitude are in good agreement with numerical simulations for an array of interacting macrospins with a physically realistic shape anisotropy distribution. [ABSTRACT FROM AUTHOR]

We studied current-induced magnetization reversal in elliptical spin valves with CoFeB free layers. The data obtained from high-speed pulsed switching experiments showed that the intrinsic switching current densities were size dependent and 50%–100% higher than predicted by a single-domain model. Micromagnetic simulations reveal a complex behavior of magnetization switching in which end-mode oscillations are important, and indicate that the switching current density depends on the device dimensions. Experimental values for the intrinsic switching current density agree with those predicted by micromagnetic simulations. [ABSTRACT FROM AUTHOR]

The effect of finite coplanar waveguide (CPW) width on the measurement of the resonance frequency in thin ferromagnetic films has been characterized for pulsed inductive microwave magnetometry. A shift in resonant frequency is a linear function of the ratio of sample thickness to CPW width. The proportionality constant is experimentally determined to be 0.74±0.1 times the saturation magnetization of the film. The frequency shift may be modeled as arising from an effective magnetic-anisotropy field. [ABSTRACT FROM AUTHOR]

We have measured the Lande g factor, the effective magnetization M[SUBeff], the uniaxial anisotropy H[SUBk], and the Gilbert damping parameter α, as a function of Permalloy film thickness from 2.5 to 50 nm. We used a pulsed inductive microwave magnetometer capable of generating dc bias fields of 35.2 kA/m (440 Oe). A significant decrease in g is observed with decreasing thickness below 10 nm. Also, M[SUBeff] decreases with decreasing thickness consistent with a surface anisotropy constant of 0.196±0.025 mJ/m[SUP2]. The decrease in g can arise from the orbital motion of the electrons at the interface not being quenched by the crystal field. We also compare our data to a model of an effective g factor suggesting that the decrease in g factor might also stem from the Ni-Fe interface with a Ta underlayer. [ABSTRACT FROM AUTHOR]

Studies the in-plane magnetization dynamics of Ni[sub 81]Fe[sub 19] thin films using the surface and interface-sensitive second-harmonic magneto-optic Kerr effect. Measurement of dynamical magnetization as a function of an in-plane magnetic field applied parallel to the anisotropy axis; Dynamics of impulse and stop excitation.

We demonstrate a method of eliminating overshoot and ringing in magnetization dynamics when the system bandwidth includes the intrinsic ferromagnetic resonance (FMR). The method employs staggered step excitation for the cancellation of FMR oscillations while maximizing the risetime of the magnetization response. The second-harmonic magneto-optic Kerr effect is used to measure the magnetic response at a localized spot on the sample. The measured response is adequately modeled with the Landau-Lifshitz-Gilbert differential equation. We explain the observed behavior in terms of destructive interference. [ABSTRACT FROM AUTHOR]