Temperature dependent magnetization dynamics of magnetic nanoparticles

Recent experimental and theoretical studies show that the switching behavior of magnetic nanoparticles can be well controlled by external time-dependent magnetic fields. In this work, we inspect theoretically the influence of the temperature and the magnetic anisotropy on the spin-dynamics and the switching properties of single domain magnetic nanoparticles (Stoner-particles). Our theoretical tools are the Landau-Lifshitz-Gilbert equation extended as to deal with finite temperatures within a Langevine framework. Physical quantities of interest are the minimum field amplitudes required for switching and the corresponding reversal times of the nanoparticle's magnetic moment. In particular, we contrast the cases of static and time-dependent external fields and analyze the influence of damping for a uniaxial and a cubic anisotropy.Comment: accepted by Journal of Physics: Condensed Matte

Magnetic domain-wall motion by propagating spin waves

We found by micromagnetic simulations that the motion of a transverse wall (TW) type domain wall in magnetic thin-film nanostripes can be manipulated via interaction with spin waves (SWs) propagating through the TW. The velocity of the TW motion can be controlled by changes of the frequency and amplitude of the propagating SWs. Moreover, the TW motion is efficiently driven by specific SW frequencies that coincide with the resonant frequencies of the local modes existing inside the TW structure. The use of propagating SWs, whose frequencies are tuned to those of the intrinsic TW modes, is an alternative approach for controlling TW motion in nanostripes

Phase Coherent Precessional Magnetization Reversal in Micro-scopic Spin Valve Elements

We study the precessional switching of the magnetization in microscopic spin valve cells induced by ultra short in-plane hard axis magnetic field pulses. Stable and highly efficient switching is monitored following pulses as short as 140 ps with energies down to 15 pJ. Multiple application of identical pulses reversibly toggles the cell's magnetization be-tween the two easy directions. Variations of pulse duration and amplitude reveal alter-nating regimes of switching and non-switching corresponding to transitions from in-phase to out-of-phase excitations of the magnetic precession by the field pulse. In the low field limit damping becomes predominant and a relaxational reversal is found allowing switching by hard axis fields below the in-plane anisotropy field threshold.Comment: 17 pages, 4 figure

Direct observation of domain wall structures in curved permalloy wires containing an antinotch

The formation and field response of head-to-head domain walls in curved permalloy wires, fabricated to contain a single antinotch, have been investigated using Lorentz microscopy. High spatial resolution maps of the vector induction distribution in domain walls close to the antinotch have been derived and compared with micromagnetic simulations. In wires of 10 nm thickness the walls are typically of a modified asymmetric transverse wall type. Their response to applied fields tangential to the wire at the antinotch location was studied. The way the wall structure changes depends on whether the field moves the wall away from or further into the notch. Higher fields are needed and much more distorted wall structures are observed in the latter case, indicating that the antinotch acts as an energy barrier for the domain wal

Probing the interface magnetism in the FeMn/NiFe exchange bias system using magnetic second harmonic generation

Second harmonic generation magneto-optic Kerr effect (SHMOKE) experiments, sensitive to buried interfaces, were performed on a polycrystalline NiFe/FeMn bilayer in which areas with different exchange bias fields were prepared using 5 KeV He ion irradiation. Both reversible and irreversible uncompensated spins are found in the antiferromagnetic layer close to the interface with the ferromagnetic layer. The SHMOKE hysteresis loop shows the same exchange bias field as obtained from standard magnetometry. We demonstrate that the exchange bias effect is controlled by pinned uncompensated spins in the antiferromagnetic layer.Comment: submitted to Phys. Rev. Let

Spin-wave propagation in a microstructured magnonic crystal

Transmission of microwave spin waves through a microstructured magnonic crystal in the form of a permalloy waveguide of a periodically varying width was studied experimentally and theoretically. The spin wave characteristics were measured by spatially-resolved Brillouin light scattering microscopy. A rejection frequency band was clearly observed. The band gap frequency was controlled by the applied magnetic field. The measured spin-wave intensity as a function of frequency and propagation distance is in good agreement with a model calculation.Comment: 4 pages, 3 figure

Optical detection of spin transport in non-magnetic metals

We determine the dynamic magnetization induced in non-magnetic metal wedges composed of silver, copper and platinum by means of Brillouin light scattering (BLS) microscopy. The magnetization is transferred from a ferromagnetic Ni80Fe20 layer to the metal wedge via the spin pumping effect. The spin pumping efficiency can be controlled by adding an insulating but transparent interlayer between the magnetic and non-magnetic layer. By comparing the experimental results to a dynamical macroscopic spin-transport model we determine the transverse relaxation time of the pumped spin current which is much smaller than the longitudinal relaxation time

Direct current control of three magnon scattering processes in spin-valve nanocontacts

We have investigated the generation of spin waves in the free layer of an extended spin-valve structure with a nano-scaled point contact driven by both microwave and direct electric current using Brillouin light scattering microscopy. Simultaneously with the directly excited spin waves, strong nonlinear effects are observed, namely the generation of eigenmodes with integer multiple frequencies (2 \emph{f}, 3 \emph{f}, 4 \emph{f}) and modes with non-integer factors (0.5 \emph{f}, 1.5 \emph{f}) with respect to the excitation frequency \emph{f}. The origin of these nonlinear modes is traced back to three magnon scattering processes. The direct current influence on the generation of the fundamental mode at frequency \emph{f} can be related to the spin-transfer torque, while the efficiency of three-magnon-scattering processes is controlled by the Oersted field as an additional effect of the direct current

Perivascular adipose tissue-derived nitric oxide compensates endothelial dysfunction in aged pre-atherosclerotic apolipoprotein E-deficient rats

BACKGROUND AND AIMS: Atherosclerosis is a major contributor to global mortality and is accompanied by vascular inflammation and endothelial dysfunction. Perivascular adipose tissue (PVAT) is an established regulator of vascular function with emerging implications in atherosclerosis. We investigated the modulation of aortic relaxation by PVAT in aged rats with apolipoprotein E deficiency (ApoE-/-) fed a high-fat diet as a model of early atherosclerosis. METHODS AND RESULTS: ApoE-/- rats (N = 7) and wild-type Sprague-Dawley controls (ApoE+/+, N = 8) received high-fat diet for 51 weeks. Hyperlipidemia was confirmed in ApoE-/- rats by elevated plasma cholesterol (p < 0.001) and triglyceride (p = 0.025) levels. Early atherosclerosis was supported by increased intima/media thickness ratio (p < 0.01) and ED1-positive macrophage influx in ApoE-/- aortic intima (p < 0.001). Inflammation in ApoE-/- PVAT was characteristic by an increased [18F]FDG uptake (p < 0.01), ED1-positive macrophage influx (p = 0.0003), mRNA expression levels of CD68 (p < 0.001) and IL-1β (p < 0.01), and upregulated iNOS protein (p = 0.011). The mRNAs of MCP-1, IL-6 and adiponectin remained unchanged in PVAT. Aortic PVAT volume measured with micro-PET/CT was increased in ApoE-/- rats (p < 0.01). Maximal endothelium-dependent relaxation (EDR) to acetylcholine in ApoE-/- aortic rings without PVAT was severely impaired (p = 0.012) compared with controls, while ApoE-/- aortic rings with PVAT showed higher EDR than controls. All EDR responses were blocked by L-NMMA and the expression of eNOS mRNA was increased in ApoE-/- PVAT (p = 0.035). CONCLUSION: Using a rat ApoE-/- model of early atherosclerosis, we capture a novel mechanism by which inflammatory PVAT compensates severe endothelial dysfunction by contributing NO upon cholinergic stimulation