Magnetic instability regions in patterned structures: influence of element shape on magnetization reversal dynamics.

We report a time-resolved imaging study of the influence of shape on magnetic instabilities in patterned magnetic structures. We find that in rectangular structures magnetization reversal initiates at the ends and interior simultaneously, while in structures with tapered ends the reversal begins in the middle of the structures and spreads out to the ends. The degree of tapering is important for both the switching field and the time required for full reversal. A model based on the concept of local instability regions yields good agreement with the observed location of the reversal onsets.

Driven dynamic mode splitting of the magnetic vortex translational resonance.

A magnetic vortex in a restricted geometry possesses a nondegenerate translational excitation that corresponds to circular motion of its core at a characteristic frequency. For 40-nm thick, micron-sized permalloy elements, we find that the translational-mode microwave absorption peak splits into two peaks that differ in frequency by up to 25% as the driving field is increased. An analysis of micromagnetic equations shows that for large driving fields two stable solutions emerge.

Dynamic origin of stripe domains.

We investigate stripe domain formation in nanometer sized Co bars. The magnetic equilibrium states and the magnetic spin wave frequencies are obtained from micromagnetic-like simulations. We find that the lowest frequency standing-wave mode has the same spatial structure as the stripe domains at remanence and it goes soft at the field where the stripe domains emerge. We show, therefore, that the final domain structure at remanence, which is not the configuration with lowest energy, is predicted from a high-field analysis of the frequencies of the standing spin waves.

Coherent inelastic light scattering from a microwave-excited array of magnetic particles.

Inelastic light scattering from an array of Permalloy particles driven by a microwave magnetic field is shown to be a coherent phenomenon in which the scattered radiation is observed only at diffraction angles corresponding to the reciprocal lattice of the array. The results are explained in terms of the phase coherence of the inelastically scattered light by each of the particles.

Magnetization reversal in submicron disks: exchange biased vortices.

Submicron, circular, ferromagnetic-antiferromagnetic dots exhibit different magnetization reversal mechanisms depending on the direction of the magnetic applied field. Shifted, constricted hysteresis loops, typical for vortex formation, are observed for fields along the exchange bias direction. However, for fields applied close to perpendicular to the exchange bias direction, magnetization reversal occurs via coherent rotation. Magnetic force microscopy imaging together with micromagnetic simulations are used to further clarify the different magnetic switching behaviors.

Effect of dipolar interaction on the antiferromagnetic resonance spectra of NiO.

We have investigated the antiferromagnetic (AF) resonance modes (AFMR) of NiO, theoretically using a model that includes the effects of exchange, dipolar coupling, and a small cubic anisotropy, and experimentally using Brillouin scattering. Using only superexchange between next nearest Ni atoms the model accounts for the observed AF structure with a [112] spin orientation. The model predicts that there are four, weakly coupled, AF lattices that should therefore exhibit eight AFMR modes. Because of degeneracies, only five distinct frequencies are predicted by the model. Three of these frequencies are consistent with the doublet observed by Raman scattering and the central peak reported in Brillouin experiments. Using Brillouin scattering we report the observation of the two missing modes.

Glass formation at the limit of insufficient network formers.

Inorganic glasses normally exhibit a network of interconnected, covalent-bonded, structural elements that has no long-range order. In silicate glasses, the network formers are based on SiO4 tetrahedra interconnected through oxygen atoms at the corners. Conventional wisdom implies that alkaline and alkaline-earth orthosilicate materials cannot be vitrified, because they do not contain sufficient network-forming SiO2 to establish the needed interconnectivity. We studied a bulk magnesium orthosilicate glass obtained by containerless melting and cooling. We found that the role of network former was largely taken on by corner and edge sharing of highly distorted, ionic Mg-O species that adopt 4-, 5-, and 6-coordination with oxygen. The results suggest that similar glassy phases may be found in the containerless environment of interstellar space.

Exchange-induced anisotropies at ferromagnetic-antiferromagnetic interfaces above and below the Néel temperature.

The exchange bias and magnetic anisotropies in a Co layer on a single-crystalline FeF2 film have been determined between 30 and 300 K. By postulating that the coupling between the ferromagnet and the antiferromagnet persists above the Néel temperature (T(N)) we develop a model that quantitatively describes the exchange bias and the anisotropies over the whole temperature range, both above and below T(N). Using only the measured low temperature exchange bias and a distribution of blocking temperatures we explain (i) the temperature dependence of the bias, (ii) the magnitude of the anisotropies, (iii) the opposite sign of the first and second order anisotropies, (iv) the observed 1/T and 1/T(3) temperature dependencies of the first and second order uniaxial anisotropies above T(N), and (v) the decrease of the anisotropies below T(N).