Relaxing-precessional magnetization switching

A new way of magnetization switching employing both the spin-transfer torque and the torque by a magnetic field is proposed. The solution of the Landau–Lifshitz–Gilbert equation shows that the dynamics of the magnetization in the initial stage of the switching is similar to that in the precessional switching, while that in the final stage is rather similar to the relaxing switching. We call the present method the relaxing-precessional switching. It offers a faster and lower-power-consuming way of switching than the relaxing switching and a more controllable way than the precessional switching.     

Improving exchange-coupling field in the same thickness of pinned magnetic layer

A conventional Ta/NiFe/Cu/NiFe/FeMn spin valve was prepared to investigate the exchange bias properties with the variations of deposition field. By enhancing the deposition magnetic fields from 50 to 650 Oe, increase of exchange bias fields at a given thickness of the pinned NiFe layer has been found in the spin valves. In this paper, we show that this increase is due to the change of magnetic moment distribution at the ferromagnetic and antiferromagnetic interface by comparison of measured results with the interfacial uncompensated model. Therefore, by enhancing deposition magnetic fields, a large exchange-coupling field can be achieved in relatively thicker magnetic films for application.     

Micromagnetic simulations of the magnetization dynamics in nanostructures with special applications to spin injection

We present several micromagnetic simulation examples of magnetization dynamics driven by the spin injection. First, we address the validity of the macrospin approximation often used to interpret experimental data. Next, we discuss the interpretation of experimental results obtained on columnar multilayer structures and show that a sophisticated micromagnetic model which takes into account a polycrystalline structure of a nanoelement can explain qualitatively the most important features of the magnetization oscillation spectra observed experimentally. A quantitative agreement with experimental results, however, could not be achieved in the region of reasonable parameter values. The third part of our contribution deals with simulations of the point-contact experiments. Here, we find an important qualitative disagreement between the experiment and simulations. The latter predict the existence of two current regions of a steady-state precession of the point-contact area (before and after spin-polarized current-driven magnetization switching), whereas experimentally only one such region is observed. In conclusion, we discuss some explanations of the above-mentioned discrepancies.     

Anatomy of field effects on magnetization dynamics and spin transfer noise

Spin transfer-related phenomena in nanomagnets have attracted extensive studies. In this paper we shall focus on analysis of individual and combined effects of the external, anisotropy, and demagnetization fields on magnetization dynamics and spin transfer noise. It is found that individual roles of the external, anisotropy, and demagnetization fields, as well as the combined roles of external plus anisotropy fields and anisotropy plus demagnetization fields, do not change the behavior of current induced magnetization switching. Such magnetization reversal procedures are of low noise. Our dynamics and power spectral density calculations show that it is the demagnetization field that plays a major role in inducing spin transfer noise: the demagnetization field itself or in combination with the anisotropy field will result in wave-like switching; moreover, the demagnetization field, together with the external field (not too small), will lead to precession and hence the system would be in noisy states. Our modeling work for an elliptical Py alloy is qualitatively consistent with Cornell's experiment and simulation [Science 307 (2005) 228].     

Accessing fast magnetization dynamics by XPEEM: Status and perspectives

Being already well established as a versatile technique for high-resolution static magnetic domain imaging, X-ray photoemission electron microscopy (XPEEM) is now also capturing the field of time-resolved magnetic investigations. Using appropriate operation modes at synchrotron radiation sources, a time resolution of 10 ps and less can be achieved in recent magnetodynamics studies, giving access even to phenomena involving precessional processes.     

On the range of interaction from finite size scaling in magnetic Ni nanowires

A recent experiment reporting a thickness dependence of magnetic properties of Ni nanowires has been re-interpreted in terms of spin–spin interactions eventually ranging beyond nearest neighbors. An analysis following the Ginzburg argument indicated that the system is in the crossover regime from 3D Ising to classical critical behavior as a function of the normalized range of interactions R$R$. The range of the interactions covering three to four coordination spheres of an Ni atom has been found by the fit to literature experimental data on Ni nanowires. Consistency with recent first-principles calculations are discussed.     

Current-induced domain wall motion

The present understanding of domain wall motion induced by spin-polarized electric current is assessed by considering a subset of experiments, analytical models, and numerical simulations based on an important model system: soft magnetic nanowires. Examination of this work demonstrates notable progress in characterizing the experimental manifestations of the “spin-torque” interaction, and in describing that interaction at a phenomenological level. At the same time, an experimentally verified microscopic understanding of the basic mechanisms will require substantial future efforts, both experimental and theoretical.     

Electron correlations, spontaneous magnetization and momentum density in quantum dots

The magnetization of quantum dots (QDs) is discussed in terms of a relatively simple but exactly solvable model Hamiltonian. The model predicts oscillations in spin polarization as a function of dot radius for a fixed electron density. These oscillations in magnetization are shown to yield distinct signature in the momentum density of the electron gas, suggesting the usefulness of momentum resolved spectroscopies for investigating the magnetization of dot systems. We also present variational quantum Monte Carlo calculations on a square dot containing 12 electrons in order to gain insight into correlation effects on the interactions between like and unlike spins in a QD.     

The effect of additional Si and SiGe layers on the confinement potential of GeMn diluted ferromagnetic semiconductor

In this work we analyze the spin-polarized charge density distribution in the GeMn diluted ferromagnetic semiconductors (DFS). The calculations are performed within a self-consistent k·p method, in which the exchange correlation effects in the local density approximation, as well as the strain effects due to the lattice mismatch, are taken into account. Our findings show that the extra confinement potential provided by the barriers and the variation of the Mn content in the DFS are responsible for a separation between the different spin charge densities, giving rise to higher mobility spin-polarized currents or high ferromagnetism transition temperatures systems.     

Magnetic and magnetization properties of electrodeposited fcc CoPt nanowire arrays

Magnetic and magnetization properties of fcc Co_1−xPt_x (x?0.3) alloy nanowires fabricated by electrodeposition into self-synthesized anodic alumina templates are investigated. Magnetization curves, measured for varying wire geometries, show a crossover of easy axis of magnetization from parallel to perpendicular to the nanowire axis as a function of the diameter and length. The measured values of coercivity (H_c) and remanent squareness (SQ) of CoPt nanowire arrays, as a function of angle (θ) between the field and wire axis, support the crossover of easy axis of magnetization. The curling mode of the magnetization reversal process is observed for CoPt nanowire arrays. At low temperatures, the easy axis for magnetization of the nanowires is observed to deviate from the room-temperature orientation.     

Structural,magnetic and Mössbauer spectral studies of nanosize aluminum substituted nickel zinc ferrites

Nanosize aluminum substituted nickel zinc ferrites were prepared through aerosol route and characterized using TEM, XRD, magnetic measurements and Mössbauer spectroscopy. The particle size of as obtained samples was found to be ∼10 nm which increases up to ∼85 nm upon annealing at 1200 °C. The unit cell parameter ‘a’ decreases linearly with concentration of aluminum due to the small ionic radius of aluminum. The saturation magnetization for all the samples after annealing at 1200 °C lies in the range 12.9–72.6 emu/g and decreases linearly with concentration of aluminum. Room temperature Mössbauer spectra of all as obtained samples of ferrite compositions exhibited a broad doublet suggesting super paramagnetic nature. This doublet is further resolved into two doublets and assigned to the surface region and internal region atoms of the particles. The samples annealed at 1200 °C show broad sextets, which were fitted with five sextets, indicating different local environment of both tetrahedrally and octahedrally coordinated Fe cation.     

Rotational power loss of magnetic steel sheets in a circular rotational magnetic field in CCW/CW directions

This paper presents rotational power loss properties of magnetic steel sheets under high flux density conditions using two-dimensional vector magnetic properties measurement. Recently it was reported by some research groups that the magnetic power loss measured in a counter clockwise (CCW) rotating field differed from that in a clockwise (CW) rotating field. This phenomenon was only observed in case of higher magnetic flux density excitation condition. We call this the CCW/CW problem. To clarify the reasons why the disagreement exists in the CCW/CW direction, we have examined angle errors of H- and B-coils by using geometrical, optical and magnetic methods. Then we compensated the measured vector components including different signals due to the angle errors. In the components of irremovable small angle error, we have also used the measured field strength waveforms in CCW/CW conditions. We have applied the compensation method to measurement of a grain-oriented electrical steel sheet.     

Simulations of magnetic microstructure in thin film elements used for programmable motion of magnetic particles

The results of two-dimensional micromagnetic modeling of magnetization patterns in Permalloy ellipses under the influence of rotating constant-amplitude magnetic fields are discussed. Ellipses of two different lateral sizes have been studied, 0.5 μm×1.5 μm and 1 μm×3 μm. The amplitude of the rotating magnetic field was varied between simulations with the condition that it must be large enough to saturate or nearly saturate the ellipse with the field applied along the long axis of the ellipse. For the smaller ellipse size it is found that the magnetization pattern forms an S state and the direction of the net magnetization lags behind the direction of the applied field. At a critical angle of the rotating magnetic field the direction of the magnetization switches by a large angle to a new S state. Both the critical angle and the angle interval of the switch depend on field amplitude. For this new state, it is instead the applied field direction that lags behind the magnetization direction. The transient magnetization patterns correspond to multi-domain patterns including two vortices, but this state never exists for the equilibrated magnetization patterns. The behavior of the larger ellipse in rotating field is different. With the field applied along the long-axis of the ellipse, the magnetization of the ellipse is nearly saturated with a vortex close to each apex of the ellipse. As the field is rotated, this magnetization pattern remains and the net-magnetization direction lags behind the direction of the field until for a certain angle of the applied field an equilibrium multi-domain state is created. Comparisons are made with corresponding experimental results obtained by performing in-field magnetic force microscopy on Permalloy ellipses.     

Back to the future: Polarised positron beams

Plans for the development of a polarised mono-energetic positron beam at the University of Bath are described. Pilot measurements with the existing beam, modified only by increasing the source-moderator separation, have demonstrated that the helicity of the present beam is sufficient to show sensitivity to ferromagnetism in bulk iron. However, an increase in flux and helicity is required if the beam is to be successfully used to probe magnetic structures of current technological interest, and strategies to achieve both are described. Potential applications in magnetic semiconductor structures and magnetic nanoparticles are discussed.     

Theoretical simulations of magnetic nanotubes using Monte Carlo method

We present the results of the Monte Carlo simulations of magnetic nanotubes, which are based on the plane structures with the square unit cell at low temperatures. The spin configurations, thermal equilibrium magnetization, magnetic susceptibility and the specific heat are investigated for the nanotubes of different diameters, using armchair or zigzag edges. The dipolar interaction, Heisenberg model interaction and also their combination are considered for both ferromagnetic and anti-ferromagnetic cases. It turns out that the magnetic properties of the nanotubes strongly depend on the form of the rolling up (armchair or zigzag). The effect of dipolar interaction component strongly manifests itself for the small radius nanotubes, while for the larger radius nanotubes the Heisenberg interaction is always dominating. In the thermodynamic part, we have found that the specific heat is always smaller for the nanotubes with smaller radii.     

Exact soliton solution of spin chain with an external magnetic field in linear wave background

Employing a simple, straightforward Darboux transformation we construct exact N-soliton solution for anisotropic spin chain driven by an external magnetic field in linear wave background. As a special case the explicit one- and two-soliton solution dressed by the linear wave corresponding to magnon in quantum theory is obtained analytically and its property is discussed in detail. The dispersion law, effective soliton mass, and the energy of each soliton are investigated as well. Our result show that the stability criterion of soliton is related with anisotropic parameter and the amplitude of the linear wave.     

Micromagnetic simulations of absoption spectra

Further development of a previously introduced method for numerically simulating magnetic spin waves is presented. Together with significant improvements in speed, the method now allows one to calculate the energy absorbed by the various modes excited by a position- and time-dependent H₁ field in a ferromagnetic body of arbitrary shape in the presence of a (uniform or non-uniform) static H₀ field as well as the internal exchange and anisotropy fields. The method is applied to the case of the single vortex state in a thin disc, a ring, and various square slabs, for which the absorption spectra are calculated and the most strongly excited resonance modes are identified.     

The effect of randomly oriented anisotropy on the zero-field-cooled magnetization of a non-interacting magnetic nanoparticle assembly

A model based on localized partition function and master equation was set up to calculate the zero-field-cooled (ZFC) and field-cooled (FC) curves of a non-interacting magnetic nanoparticle assembly with randomly oriented anisotropy. The peak temperature of the ZFC curve corresponds to the highest energy barrier that acts against the unblocking process, and could be described well by an equation covering the heating rate effect. The predicted H^2/3 field dependence of the peak temperature is in good agreement with published results.     

Giant magnetic susceptibility enhancement in field-structured nanocomposites

We demonstrate through experiment and simulation that when mono-domain Fe nanoparticles are formed into chains by the application of a magnetic field, the susceptibility of the resulting structure is greatly enhanced (11.4-fold) parallel to the particle chains and is much larger than transverse to the chains. Simulations show that this significant enhancement is expected when the susceptibility of the individual particles approaches 5 in MKS units, and is due to the spontaneous magnetization of individual particle chains, which occurs because of the strong dipolar interactions. This large enhancement is only possible with nanoparticles, because demagnetization fields limit the susceptibility of a spherical multi-domain particle to 3 (MKS). Experimental confirmation of the large susceptibility enhancement is presented, and both the enhancement and the susceptibility anisotropy are found to agree with simulation. The specific susceptibility of the nanocomposite is 54 (MKS), which exceeds the highest value we have obtained for field-structured composites of multi-domain particles by a factor of four.