Keith Gilmore

Identification of the Dominant Precession-Damping Mechanism in Fe, Co, and Ni by First-Principles Calculations

The Landau-Lifshitz equation reliably describes magnetization dynamics using a phenomenological treatment of damping. This Letter presents first-principles calculations of the damping parameters for Fe, Co, and Ni that quantitatively agree with existing ferromagnetic resonance measurements. This agreement establishes the dominant damping mechanism for these systems and takes a significant step toward predicting and tailoring the damping constants of new materials.

Spin-orbit precession damping in transition metal ferromagnets (invited)

We provide a simple explanation, based on an effective field, for the precession damping rate due to the spin-orbit interaction. Previous effective field treatments of spin-orbit damping include only variations of the state energies with respect to the magnetization direction, an effect referred to as the breathing Fermi surface. Treating the interaction of the rotating spins with the orbits as a perturbation, we include also changes in the state populations in the effective field. In order to investigate the quantitative differences between the damping rates of iron, cobalt, and nickel, we compute the dependence of the damping rate on the density of states and the spin-orbit parameter. There is a strong correlation between the density of states and the damping rate. The intraband terms of the damping rate depend on the spin-orbit parameter cubed, while the interband terms are proportional to the spin-orbit parameter squared. However, the spectrum of band spacings is also an important quantity and does no...

Magnetic properties of Co3O4 nanoparticles mineralized in Listeria innocua Dps

Temperature-dependent magnetic measurements are reported for 4.34 nm antiferromagnetic Co3O4 nanoparticles mineralized in the Listeria innocua Dps protein cage. ac measurements show a superparamagnetic blocking temperature of roughly 5.4 K and give an extracted anisotropy energy density of (7.6±0.4)×104J∕m3. The Neel temperature for the Co3O4 nanoparticles, determined with dc magnetometry, was determined to be roughly 15±2K.

Surface contribution to the anisotropy energy of spherical magnetite particles

The surface contribution to the magneto-crystalline anisotropy energy of spherical magnetite nanoparticles has been investigated. Magnetite particles of three sizes (3.5, 7, and 18nm diameter) were grown inside protein cages. Alternating current magnetic susceptibility measurements revealed the particles to be noninteracting, and allowed a determination of the average anisotropy energy for each sample. Surface atoms were found to increase the volume anisotropy energy density of the particles, and this effect increased sublinearly with particle curvature.

Nonadiabatic spin-transfer torque in real materials

The motion of simple domain walls and of more complex magnetic textures in the presence of a transport current is described by the Landau-Lifshitz-Slonczewski (LLS) equations. Predictions of the LLS equations depend sensitively on the ratio between the dimensionless material parameter β which characterizes non-adiabatic spin-transfer torques and the Gilbert damping parameter α. This ratio has been variously estimated to be close to 0, close to 1, and large compared to 1. By identifying β as the influence of a transport current on α, we derive a concise, explicit and relatively simple expression which relates β to the band structure and Bloch state lifetimes of a magnetic metal. Using this expression we demonstrate that intrinsic spin-orbit interactions lead to intraband contributions to β which are often dominant and can be (i) estimated with some confidence and (ii) interpreted using the “breathing Fermi surface” model. PACS numbers:

Modeling of the magnetic behavior of γ-Fe2O3 nanoparticles mineralized in ferritin

The temperature dependent initial magnetization of γ-Fe2O3 (maghemite) mineralized inside ferritin protein cages has been investigated with a vibrating sample magnetometer up to 8 T. The data are fit to different magnetic models to extract values of the magnetic moment of each cluster. It is found that the application of a simple Langevin model with a first and second order term in the susceptibility greatly enhances the quality of the fit to the data suggesting that the inclusion of crystalline anisotropy is important in extracting the magnetic moment of each core.

Electron magnetic resonance of iron oxide nanoparticles mineralized in protein cages

Magnetic and structural properties determined by electron magnetic resonance (EMR) spectroscopy are reported for maghemite (γ‐Fe2O3) nanoparticles formed through template-constrained mineralization within three protein cages with nominal diameters of 5, 8, and 24 nm. EMR spectra, obtained at 4.0, 9.2, 34.6, 94.9, and 130.0 GHz, and at ambient temperature, show dramatic frequency dependent effects in the line shapes, line-widths, and resonance-field shifts. Simulations of the spectra are used to obtain moment distribution parameters, which are consistent with size limitations imposed by the protein cages, but which reflect significant departures from bulk magnetization properties.

Numerical quantification of the vibronic broadening of the SrTiO3 Ti L-edge spectrum.

The 2p(5)3d(1) excited state of the Ti(4+) ion in SrTiO(3) couples to e(g) distortions of the local oxygen cage, leading to a Jahn-Teller vibronic broadening of the excited states. We quantify this contribution to the broadening of the spectral features of the Ti L edge of SrTiO(3) by solving a model Hamiltonian, taking parameters for the Hamiltonian from previous first-principles calculations. Evaluation of the model Hamiltonian indicates that vibronic coupling accounts for the majority of the broadening observed for the L(3) edge, but only a minority of the L(2)-edge spectral width. The calculations reveal that, with increasing temperature, the spectral features become increasingly asymmetric and the amount of vibronic broadening grows approximately linearly.

Evaluating the locality of intrinsic precession damping in transition metals

The Landau-Lifshitz-Gilbert damping parameter is typically assumed to be a local quantity, independent of magnetic configuration. To test the validity of this assumption we calculate the precession damping rate of small amplitude non-uniform mode magnons in iron, cobalt, and nickel. At scattering rates expected near and above room temperature, little change in the damping rate is found as the magnon wavelength is decreased from infinity to a length shorter than features probed in recent experiments. This result indicates that non-local effects due to the presence of weakly non-uniform modes, expected in real devices, should not appreciably affect the dynamic response of the element at typical operating temperatures. Conversely, at scattering rates expected in very pure samples around cryogenic temperatures, non-local effects result in an order of magnitude decrease in damping rates for magnons with wavelengths commensurate with domain wall widths. While this low temperature result is likely of little practical importance, it provides an experimentally testable prediction of the non-local contribution of the spin-orbit torque-correlation model of precession damping. None of these results exhibit strong dependence on the magnon propagation direction.

Quantitative first-principles calculations of valence and core excitation spectra of solid C 60

We present calculated valence and C 1s near-edge excitation spectra of solid C60 and experimental results measured with high-resolution electron energy-loss spectroscopy. The near-edge calculations are carried out using three different methods: solution of the Bethe-Salpeter equation (BSE) as implemented in the OCEAN suite (Obtaining Core Excitations with ab initio methods and the NIST BSE solver), the excited-electron core-hole approach (XCH), and the constrained-occupancy method using the Stockholm-Berlin core-excitation code, StoBe. The three methods give similar results and are in good agreement with experiment, though the BSE results are the most accurate. The BSE formalism is also used to carry out valence level calculations using the NIST Bethe-Salpeter Equation solver (NBSE). Theoretical results include self-energy corrections to the band gap and band widths, lifetime-damping effects, and Debye-Waller effects in the core-excitation case. A comparison of spectral features to those observed experimentally illustrates the sensitivity of certain features to computational details, such as self-energy corrections to the band structure and core-hole screening.