Transverse Magnetic Components Research Articles

Ultrafast dynamical path for the switching of a ferrimagnet after femtosecond heating

Ultrafast laser-induced magnetic switching in rare earth-transition metal ferrimagnetic alloys has recently been reported to occur by ultrafast heating alone. Using atomistic simulations and a ferrimagnetic Landau-Lifshitz-Bloch formalism, we demonstrate that for switching to occur it is necessary that angular momentum is transferred from the longitudinal to transverse magnetization components in the transition metal. This dynamical path leads to the transfer of the angular momentum to the rare earth metal and magnetization switching with subsequent ultrafast precession caused by the intersublattice exchange field on the atomic scale.

Analysis of TM mode light extraction efficiency enhancement for deep ultraviolet AlGaN quantum wells light-emitting diodes with III-nitride micro-domes

Analysis of transverse magnetic (TM) mode light extraction efficiency enhancement for AlGaN quantum wells (QWs) based deep ultraviolet (UV) light-emitting diodes (LEDs) with III-nitride micro-hemisphere and micro-dome structures on the p-type layer are studied and compared to that of the conventional deep-UV LEDs with flat surface. The transverse electric (TE) and TM components of the spontaneous emission of AlGaN QWs with AlN barriers were calculated by using a self-consistent 6-band k∙p method, which shows the TM component overtakes the TE component and becomes the dominant contribution of the spontaneous emission when the Al-content of the AlGaN QWs is larger than 0.66. The TM mode light extraction efficiency of the deep-UV LEDs emitting at 250 nm with AlGaN micro-domes as compared to the conventional LEDs with flat surface is calculated based on three dimensional finite difference time domain (3D-FDTD) method. The effects of the III-nitride micro-dome diameter and height as well as the p-type layer thickness on the light extraction efficiency were comprehensively studied. The results indicate optimized light extraction efficiency enhancement (>7.3 times) of the dominant TM polarized spontaneous emission for deep-UV LEDs with III-nitride micro-domes.

The influence of a current on the magnetization dynamics in a ferromagnet-antiferromagnet structure: Simulation

A ferromagnet-antiferromagnet junction in the presence of an in-plane magnetostatic field and a spin-polarized current flowing perpendicularly to the junction layers is considered. In the macrospin approximation, the system of nonlinear equations describing the dynamics of magnetization of the antiferromagnet layer in such a junction is numerically solved with allowance for the spin torque transfer and the spin equilibrium disturbance caused by spins injected by the current into the antiferromagnet. It is shown that, when the current exceeds a certain threshold, the magnetization becomes instable and that, beyond the instability region, the results are in complete agreement with the theory using linearization in small deviations from the equilibrium. It is found that the development of instability causes switching from the antiparallel configuration to the parallel one and that, in the instability region, nondecaying oscillations of the longitudinal and transverse components of the antiferromagnet magnetization are formed.

Estimating side-chain order in methyl-protonated, perdeuterated proteins via multiple-quantum relaxation violated coherence transfer NMR spectroscopy

Relaxation violated coherence transfer NMR spectroscopy (Tugarinov et al. in J Am Chem Soc 129:1743-1750, 2007) is an established experimental tool for quantitative estimation of the amplitudes of side-chain motions in methyl-protonated, highly deuterated proteins. Relaxation violated coherence transfer experiments monitor the buildup of methyl proton multiple-quantum coherences that can be created in magnetically equivalent spin-systems as long as their transverse magnetization components relax with substantially different rates. The rate of this build-up is a reporter of the methyl-bearing side-chain mobility. Although the build-up of multiple-quantum (1)H coherences is monitored in these experiments, the decay of the methyl signal during relaxation delays occurs when methyl proton magnetization is in a single-quantum state. We describe a relaxation violated coherence transfer approach where the relaxation of multiple-quantum (1)H-(13)C methyl coherences during the relaxation delay period is quantified. The NMR experiment and the associated fitting procedure that models the time-dependence of the signal build-up, are applicable to the characterization of side-chain order in [(13)CH(3)]-methyl-labeled, highly deuterated protein systems up to ~100 kDa in molecular weight. The feasibility of extracting reliable measures of side-chain order is experimentally verified on methyl-protonated, perdeuterated samples of an 8.5-kDa ubiquitin at 10°C and an 82-kDa Malate Synthase G at 37°C.

Free induction decays in entangled polymer melts

The relaxation of the transverse magnetization components caused by both dipolar interactions between the spins of different polymer chains and the dipolar coupling between CH-protons on an isolated Kuhn segment along a single polymer chain have been calculated. Explicit expressions for the transverse relaxation function are given in terms of the absolute mean squared displacement of the Kuhn segment during melt gr(t), the tangent vector dynamical correlation function 〈bn(t)b(0)〉, the segmental relaxation time τs, the Kuhn segment length b, the bond length a0, the internuclear distance d, and the spin number density ρs. It is shown that the functional dependence of the intramolecular relaxation function on 〈bn(t)b(0)〉 is fairly weak. The time-dependence of the intramolecular contribution to the transverse relaxation function is dominated by the probability density distribution function of the end-to-end vector of the Kuhn segment. The long-time decay of the intramolecular contribution to the transverse relaxation function is found to scale as t−3/2 for τs<<t<<τmax, where τmaxis the maximum relaxation time of polymer chains in melts. For times much less than the spin–spin relaxation time, T2≈10−3−10−2s, we show that the intermolecular contribution to the relaxation function is given by the following expression: exp(−λ1(b, τs, ρs)t2/gr3/2(t)). Both the numerical coefficient and the functional dependence of λ1on b, τs and ρs reproduce the expression obtained from the frequently used second cumulant approximation. For longer times (T2≤t<<τmax), the intermolecular contribution is determined by the following relation: exp(−λ2(b, τs, ρs, t)gr(t)). We show that λ2 increases logarithmically with t. The molecular mass independence of λ1and λ2 shows that, in polymer melts with molecular masses Mw far above the critical value Mc, the relevant experimental window for the decay of the intermolecular relaxation function is connected with the anomalous diffusion regime. Comparison with the experimental data suggests that the intermolecular contribution plays a significant role in the NMR relaxation process in polymer systems close to the melting point.

An Optimized Relaxation-Based Coherence Transfer NMR Experiment for the Measurement of Side-Chain Order in Methyl-Protonated, Highly Deuterated Proteins

Relaxation violated coherence transfer NMR spectroscopy has emerged as a powerful experimental tool for the quantitative measurement of amplitudes of motion of methyl containing side-chains. Typically, the experiments, performed on proteins that are highly deuterated and methyl-protonated, monitor the build-up of methyl (1)H double-quantum magnetization. Because all three protons in a methyl group are degenerate, such coherences can only result from differential relaxation of transverse magnetization components, which in turn reflect the extent and time-scale of motion of the methyl probe [Tugarinov, V., Sprangers, R.; Kay, L.E. J. Am. Chem. Soc. 2007, 129, 1743-1750]. We show here that a 50% gain in the sensitivity of the experiment can be achieved through selection of (1)H triple-quantum coherence, thereby significantly increasing the utility of the approach. A theoretical treatment rationalizes the sensitivity gain that is subsequently verified through experiment. The utility of the methodology is demonstrated on a number of proteins, including the 360 kDa α(7)α(7) "half-proteasome".

Experimental evidence for exchange bias in polycrystalline BiFeO3/Ni81Fe19 thin films

We report on experimental evidence of exchange bias between a polycrystalline antiferromagnetic BiFeO3 and a ferromagnetic Ni81Fe19 film at room temperature. The measured 17 Oe hysteresis loop shift corresponds to an exchange energy of 11×10-3 erg/cm2 per unit area of the interface coupling the two films, which is comparable with those observed for similar epitaxially-grown systems. The azimuthal behavior of the longitudinal and transverse magnetization components revealed the presence of induced unidirectional and biquadratic anisotropies. A misalignment between unidirectional and biquadratic anisotropy axes was also observed.

Magnetization reversal and domain correlation for a non-collinear and out-of-plane exchange-coupled system

We have investigated the impact of out-of-plane ferromagnetic (FM) anisotropy (which can be coincident with the direction of unidirectional anisotropy), where antiferromagnetic (AF) anisotropy is along the film plane. This provides a platform for non-collinear exchange coupling in an archetypal exchange coupled system in an unconventional way. We probe the in-plane magnetization by the depth-sensitive vector magnetometry technique. The experimental findings reveal a magnetization reversal (i) that is symmetric for both the branches of the hysteresis loop, (ii) that is characterized by vertically correlated domains associated with a strong transverse component of magnetization and (iii) that remains untrained (suppression of trained state) with field cycling. This scenario has been compared with in-plane magnetization reversal for a conventionalin-plane unidirectional anisotropic case in the same system that shows usual asymmetric reversal and training for vertically uncorrelated domains. We explain the above observations for the out-of-plane case in terms of inhomogeneous magnetic states due to competing perpendicular anisotropies that result in non-collinear FM–AF coupling. This study provides direct evidence for the vertical correlation of domains mediated by out-of-plane exchange coupling.

Energy flow characteristics of vector X-Waves

The vector form of X-Waves is obtained as a superposition of transverse electric and transverse magnetic polarized field components. It is shown that the signs of all components of the Poynting vector can be locally changed using carefully chosen complex amplitudes of the transverse electric and transverse magnetic polarization components. Negative energy flux density in the longitudinal direction can be observed in a bounded region around the centroid; in this region the local behavior of the wave field is similar to that of wave field with negative energy flow. This peculiar energy flux phenomenon is of essential importance for electromagnetic and optical traps and tweezers, where the location and momenta of micro-and nanoparticles are manipulated by changing the Poynting vector, and in detection of invisibility cloaks.

High phase retardation by waveguiding in slanted photonic nanostructures

We report a physical mechanism leading to high phase retardation in slanted photonic nanostructures. The phenomenon is based on the waveguiding of the transverse electric polarization component inside the slanted pillars, while the transverse magnetic component is not guided. Such a mechanism leads to very high phase retardation even with shallow structures that are suitable also for lithographical mass production. We present physical principle, numerical analysis of the phenomenon and designs for half-wave retarders. As an experimental result, a slanted grating producing 177 degrees retardation and 95.5% efficiency is presented.

Polarization control of electroluminescence from vertically stacked InAs/GaAs quantum dots

We have developed a technique to control the polarization dependence of quantum dot (QD)-semiconductor optical amplifiers (SOAs) using vertically stacked self-assembled InAs QDs with moderately thick intermediate layers. By increasing the number of stacking layers, the transverse magnetic polarization component of electroluminescence (EL) from the cleaved edge surface of the SOA has been enhanced dramatically. Broadband and almost isotropic EL with a polarization difference of less than 1.2 dB has been demonstrated in a 1.3 μm optical communication band for nine-layer stacked QDs in the active region of the SOA.

Imaging Electric Properties of Biological Tissues by RF Field Mapping in MRI

The electric properties (EPs) of biological tissue, i.e., the electric conductivity and permittivity, can provide important information in the diagnosis of various diseases. The EPs also play an important role in specific absorption rate calculation, a major concern in high-field MRI, as well as in nonmedical areas such as wireless telecommunications. The high-field MRI system is accompanied by significant wave propagation effects, and the RF radiation is dependent on the EPs of biological tissue. On the basis of the measurement of the active transverse magnetic component of the applied RF field (known as B(1)-mapping technique), we propose a dual-excitation algorithm, which uses two sets of measured B(1) data to noninvasively reconstruct the EPs of biological tissues. The finite-element method was utilized in 3-D modeling and B(1) field calculation. A series of computer simulations were conducted to evaluate the feasibility and performance of the proposed method on a 3-D head model within a TEM coil and a birdcage coil. Using a TEM coil, when noise free, the reconstructed EP distribution of tissues in the brain has relative errors of 12%-28% and correlated coefficients of greater than 0.91. Compared with other B(1)-mapping-based reconstruction algorithms, our approach provides superior performance without the need for iterative computations. The present simulation results suggest that good reconstruction of EPs from B1 mapping can be achieved.

Longitudinal and transverse magnetization components in thin films: A resonant magnetic reflectivity investigation using circularly polarized soft x-rays

An in-plane vectorial analysis of the magnetization of thin magnetic films is presented. Longitudinal soft x-ray resonant magnetic reflectivity curves display characteristic nodes where the longitudinal scattering component is suppressed by x-ray interference. The transverse magnetic component can be effectively retrieved at these nodal points, despite the use of circular polarization and longitudinal scattering geometry. Using a single geometric configuration, transverse and longitudinal magnetic hysteresis loops can be clearly separated. Calculations based on a Stoner–Wohlfarth model satisfactorily describe both loops. Therefore, this method presents a viable alternative to standard vectorial analysis techniques, with the additional benefit of element specificity.

Saving transverse magnetization

A magnetization storage sequence, ALT-1 (alternating longitudinal and transverse components), is reported. The ALT-1 sequence is a hybrid of two types of storage sequences, the Carr–Purcell type and store-and-restore sequences. During incremental storage periods within the ALT-1 sequence, essentially half of the initially transverse magnetization is stored along the z-axis and the other half is prolonged by an echo-generating pulse. The portions of initial magnetization that are stored as longitudinal components or transverse components are alternated by a π/2 pulse during the cycle. Both transverse components of the initial magnetization are treated the same in the ALT-1 sequence and orientational (phase) information of the initial magnetization is kept during the storage period. The ALT-1 sequence can preserve magnetization more effectively than a published class of modified Carr–Purcell type sequences, because essentially half of the magnetization during incremental storage periods is not subjected to relaxation from T 2 effects.

Anomalous magnetization reversal mechanism in unbiased Fe/FeF 2 investigated by means of the magneto-optic Kerr effect

The coercive fields of Fe/FeF 2(1 1 0) bilayers above the Néel temperature of FeF 2 have been studied by means of the magneto-optic Kerr effect (MOKE). To distinguish the anisotropies, the longitudinal as well as the transverse magnetization components were measured and simulated. The latter allow for a conclusion about the different magnetization reversal mechanisms above and below the Néel temperature of FeF 2, i.e. for unbiased and exchange-biased bilayers, leading to a modified description of the magnetization reversal processes at high temperatures with explicitly forbidden local energy minima.

Angular dependence of the relaxation times of 139La nuclei in La1 − x Sr x MnO3

Relaxation of the magnetization of 139La nuclei is considered in lanthanum manganites, which are materials with anisotropic interactions of localized electronic spins, namely, the Dzyaloshinskiĭ-Moriya interactionand the interaction with a crystal field. Expressions are derived for the relaxation times of the longitudinal and transverse components of the nuclear magnetization, and the angular dependences of these relaxation times are found for the La0.95Sr0.05MnO3 compound. In contrast to electronic relaxation, the anisotropy of nuclear relaxation contains a contribution from the shift in the electron Zeeman frequency. The theoretically calculated numerical values of the nuclear relaxation times and their ratios correspond to the range of experimental values in the compounds studied. The results can be of importance for designing devices based on these materials and for further investigation.

Theoretical study on all-optical magnetic recording using a solid immersion lens

We present a simple model for performing high-density all-optical magnetic recording using a solid immersion lens. The magnetization distribution in the magneto-optic film placed in the vicinity of the solid immersion lens is studied using the vector diffraction theory and the inverse Faraday effect. Simulation results show that although the transverse components of magnetization are nonzero, the axial component dominates. The magnetization direction of the axial component can be reversed by changing the helicity of the incident circularly polarized laser pulses. For a lower-numerical-aperture (NA) system, a larger and circular magnetization zone is obtained and the deviation angle of magnetization direction departing from the optical axis is smaller in the effective magnetization zone, which is useful to vertical magnetic recording. For a higher-NA system, a smaller magnetization zone is generated, but the three-dimensional magnetization distribution has to be considered.

Magnetic Behavior of Ni/Cu Multilayer Nanowire Arrays Studied by First-Order Reversal Curve Diagrams

Arrays of Ni/Cu multilayer nanowires were fabricated by electrodeposition into alumina templates (pore diameter = 175 nm, interpore distance = 300 nm), with an Ni disc thickness between 20 and 50 nm, and a Cu thickness between 10 and 35 nm. Vector major hysteresis loops were measured with an out-of-plane (along the nanowire axis) and inplane (perpendicular to the nanowire axis) applied field. During the magnetization reversal, we did not observe any net component of transverse magnetization (My) , perpendicular to the applied field Hx. Extrapolated first-order reversal curve (FORC) diagrams were measured, out-of-plane and inplane. While the major hysteresis loops of all arrays have similar shapes, their FORC diagrams reveal distinct patterns, suggesting different magnetic behavior. The magnetic anisotropy can be controlled by the ratio of Cu to Ni thickness, the nanowire axis becoming harder as the ratio increases. In the out-of-plane direction, the maximum interaction field is proportional to the Ni thickness. The FORCs show evidence of a large coercive field distribution, which we attribute to the smaller interaction fields in the nanodiscs along the edges of the samples. In the inplane direction, depending upon the easy axis direction, there can be a mixture of nanodiscs reversed by coherent and incoherent rotation, or by incoherent rotation only.

Control of Domain Wall Polarity by Current Pulses

Direct observation of current-induced propagation of purely transverse magnetic domain walls with spin-polarized scanning electron microscopy is reported in Fe30Ni70 nanowires. After propagation, the domain walls keep their transverse nature but switch polarity in some cases. For uniform Ni70Fe30 wires, the effect is random and illustrates domain-wall propagation above the Walker threshold. In the case of Ni{70}Fe_{30}/Fe wires, the transverse magnetization component in the wall is entirely determined by the polarity of the current pulse, an effect that is not reconciled by present theories even when taking into account the nonuniform Oersted field generated by the current.

A Tunable Lyot Birefringent Filter With Variable Channel Spacing and Wavelength Using Nonlinear Polarization Rotation in an SOA

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We experimentally demonstrate a tunable fiber-optical Lyot birefringent comb filter with independent tuning of the channel spacing and wavelength. The filter configuration is based on sandwiching a fiber-optic programmable differential group delay line (DGDL) and a semiconductor optical amplifier (SOA) between two polarizers. The channel spacing of the filter can be tuned by adjusting the programmable DGDL. Using the nonlinear polarization rotation effect in the SOA, the phase shift between the transverse electric and the transverse magnetic components of the input probe beam can be adjusted by varying the power of the pump light, and hence fast and continuously wavelength tuning of the filter can also be achieved. The wavelength tuning range is up to 50% of the channel spacing when the pump power is increased from <emphasis><formula formulatype="inline"><tex>${-}$</tex></formula></emphasis>10 to 17 dBm. </para>