Complete Magnetization Reversal Research Articles

Comparison of driven equilibrium and standard spin-echo sequence in MR microscopy: Analysis of signal dependence on RF pulse imperfection and diffusion

Rapid MR imaging of slowly relaxing samples is often challenging. The most commonly used solutions are found in multi spin-echo (RARE) sequences or gradient-echo (GE) sequences, which allow faster imaging of such samples with multiple acquisitions of k-space lines per excitation or imaging with very short repetition times (TRs). Another solution is the use of a spin-echo (SE) sequence superimposed with a driven equilibrium Fourier transform (DEFT) method. Such a (DE-SE) imaging sequence has two refocusing RF pulses that produce two spin-echoes. In the first echo, the signal is acquired from the k-space line, and in the second echo, a 90° RF pulse is applied, typically 180° out of phase with respect to the excitation RF pulse. This last RF pulse allows almost complete magnetization reversal back to the longitudinal orientation with minimal magnetization loss. The DE-SE sequence and its RARE variant are widely used in clinical imaging, but its use in MR microscopy has some peculiarities related to the usually less perfect RF pulse flip angles and diffusion. In this study, their effects are first theoretically analyzed and later verified by experiments on test samples performed on a 9.4 T system for MR microscopy. Experiments on a water-filled tube for TE = 3.4 ms and TR = 25–200 ms showed that the DE-SE sequence produces about 10 times more signal than the SE sequence in this TR range. Finally, the performance of the DE-SE sequence compared to the SE sequence was demonstrated on a biological sample. The presented DE-SE sequence has been shown to be effective for rapid imaging of samples with long T1 relaxation times in MR microscopy and can also be considered as a suitable method for rapid proton density weighed imaging of materials.

Angle selective piezoelectric strain-controlled magnetization switching in artificial spin ice based multiferroic system

The prospect of electrically controlled writing of ferromagnetic bits is highly desirable for developing scalable and energy-efficient spintronics devices. In this direction, various efforts have been made to achieve electrically controlled magnetization switching utilizing an artificial multiferroic system. To date, the magnetization switching has been realized in a diverse nanopatterned magnetic system. However, the demonstration of electric field-induced strain-controlled magnetization switching in artificial spin ice (ASI) coupled with a piezoelectric material is still unexplored. In the present work, we perform micromagnetic simulations to investigate the electric field-induced strain-mediated magnetization switching in an ASI based multiferroic system. Here, the piezoelectric strain-controlled magnetization switching has been studied by applying the electric-field pulse at different angles with respect to the axes of the system. Remarkably, magnetization switches by 180° only if the external electric-field pulse is applied at some specific angles, close to the anisotropy axis of the system (≈30°–60°). Our detailed analysis of the demagnetization energy variation reveals that the energy barrier becomes antisymmetric in such cases, facilitating complete magnetization reversal. Moreover, we have also proposed a possible magnetization reversal mechanism with two sequential electric-field pulses of a relatively smaller magnitude. We believe that the present work could pave the way for a future ASI-based multiferroic system for scalable magnetic field-free low power spintronics devices.

Experimental observation and micromagnetic understanding of the current-induced magnetization reversal process in Ta/Pt/[Co/Ni] n Co/Ta multilayers

We investigate a current-induced magnetization reversal process mediated by spin–orbit torque (SOT) in Ta/Pt/(Co/Ni) n Co/Ta multilayers experimentally and by micromagnetic simulation. Experimental results show that the current-induced magnetization reversal in these samples is completed by nucleation and subsequent domain wall (DW) propagation. For samples with a small number (n) of Co/Ni bi-layers, which show strong Dzyaloshinskii–Moriya interaction (DMI), when the in-plane magnetic field (Hx ) is weaker than a critical value, the tilted DWs form in the final stabilized state, corresponding to a partial magnetization reversal. Since Hx is close to the DMI effective field, the domains expand asymmetrically to semicircular areas, finally resulting in complete magnetization reversal. For the sample with large n and weak DMI, the reversed domains grow obliquely along a special orientation, which varies with the magnitude of Hx . These experimental phenomena are well reproduced by micromagnetic simulation after taking into account the SOT effect and the DMI-modulated magnetic moment distribution in the DW. The agreement betweenexperimental observation and micromagnetic simulation suggests that the current-induced magnetization reversal process in the Ta/Pt/[Co/Ni] n Co/Ta system can be well understood within the framework of SOT and the DMI scenario.

Design parameters for field-free spin–orbit torque switching of perpendicular synthetic antiferromagnets

We report micromagnetic simulations of spin–orbit torque (SOT) induced magnetization switching of a ferromagnetic layer with perpendicular anisotropy in the absence of an external magnetic field. Field-free switching is achieved by antiferromagnetic interlayer exchange coupling (IEC) between two perpendicular ferromagnetic layers. At appropriate IEC values and an SOT current density exceeding the critical value (Jc), magnetization reversal can be achieved within sub-ns. The complete magnetization reversal of the synthetic antiferromagnetic free layer occurs upon removing the current pulse. Higher damping is preferred for the proposed switching scheme, as Jc decreases with the increase of damping. Remarkably, we also found that Jc has a parabolic dependence on the nanomagnet's diameter, with the vertex occurring at 90 nm.

Electric current and field control of vortex structures in cylindrical magnetic nanowires

Magnetization dynamics in a cylindrical Permalloy nanowire under simultaneously applied electric current and field is investigated by means of micromagnetic simulations. The reversal process starts with the creation of open vortex structures with different rotation senses at the nanowire ends. Our results conclude that the current alone enlarges or reduces the size of these vortex structures according to the rotational sense of the associated Oersted field. Large current intensity creates a vortex structure which covers the whole nanowire surface. At the same time the magnetization in the nanowire core remains the same, i.e., no complete magnetization reversal is possible in the absence of external field. The simultaneous action of the current and field allows for the complete control of the vortex structures in terms of setting the polarity and vorticity. The state diagram for the minimum field and current required for the vorticity and axial magnetization switching is presented. This control is essential for future information technologies based on three-dimensional vertical structures, and the presented state diagram will become very useful for future experiments on current-induced domain wall dynamics in cylindrical magnetic nanowires.

Threshold Current Density for Perpendicular Magnetization Switching Through Spin-Orbit Torque

We theoretically investigate the threshold current density for spin-orbit-torque- (SOT) induced perpendicular magnetization switching. According to the relative sign of fieldlike torque (FLT) and dampinglike torque (DLT), we derive the corresponding analytical formulas of the threshold current density ${J}_{\mathrm{th}}$ required to switch magnetization from equilibrium states to nearly in-plane states. These formulas, which agree well with numerical results among a wide range of material parameters, indicate that SOT current density can be significantly reduced in the presence of a large FLT. The conditions are then explored to achieve complete magnetization reversal after removing the SOT currents. When the ratio of FLT to DLT \ensuremath{\eta} is negative in our sign convention, we find that SOT pulses with long enough fall time, for example, 0.2 ns in our simulations, can guarantee stable switching. In contrast, a small in-plane magnetic field or a large damping is necessary when \ensuremath{\eta} g 0. Based on the conditions, the measured threshold current densities in several reported experiments, which deviate from previous models, can be well reproduced by our derived ${J}_{\mathrm{th}}$. We further study the possible incubation delay induced by SOT, clarifying the recent contradiction on incubation time during SOT switching. Our work sheds insights on the SOT material optimization and the comprehension of magnetization switching dynamics induced by SOT.

Responses of an Isolated Anisotropic Magnetic Nanoparticle and Nanoparticle Lattice to a Magnetic Field Pulse

We have performed numerical simulations to evaluate responses of the magnetic moments of a single nanoparticle possessing uniaxial anisotropy and a planar nanoparticle lattice to excitation by a magnetic field pulse. The precession dynamics of magnetic moment for a wide range of the pulse parameters and the lattice anisotropy value has been studied. We have explored and analyzed periodic dependences of the response duration and the resulted orientation of the magnetic moments on the pulse duration and its amplitude. The effect of an additional noise signal on the lattice magnetization reversal is studied. The regimes of partial and complete magnetization reversal of dipole systems achievable only in the presence of noise have been revealed. We report on the conditions enabling the pulse magnetizations reversal most and least sensitive to the noise signal.

Vectorial observation of the spin Seebeck effect in epitaxial NiFe2O4 thin films with various magnetic anisotropy contributions

We have developed a vectorial type of measurement for the spin Seebeck effect (SSE) in epitaxial NiFe2O4 thin films which have been grown by pulsed laser deposition on MgGa2O4 with (001) and (011) orientations as well as CoGa2O4 (011), thus varying the lattice mismatch and crystal orientation. We confirm that a large lattice mismatch leads to strain anisotropy in addition to the magnetocrystalline anisotropy in the thin films using vibrating sample magnetometry and ferromagnetic resonance measurements. Moreover, we show that the existence of a magnetic strain anisotropy in NiFe2O4 thin films significantly impacts the shape and magnitude of the magnetic-field-dependent SSE voltage loops. We further demonstrate that bidirectional field-dependent SSE voltage curves can be utilized to reveal the complete magnetization reversal process, which establishes a vectorial magnetometry technique based on a spin caloric effect.

Monopole-Induced Emergent Electric Fields in Ferromagnetic Nanowires.

We predict that complete magnetization reversal in simple metallic ferromagnetic nanoparticles is directly linked to the pair creation of topological point defects in the form of hedgehog-antihedgehog pairs. These dynamical point defects move at exceptionally high speeds in excess of 1500 m/s, faster than any other known magnetic object. Their rapid motion generates unprecedented solenoidal emergent fields on the order of megavolts per meter, in analogy to the magnetic field of a moving electric charge, providing a striking example that a moving hedgehog constitutes an emergent magnetic monopole.

Voltage pulse induced repeated magnetization reversal in strain-mediated multiferroic nanomagnets: a size- and material-dependent micromagnetic study

Realization of complete magnetization reversal is a major challenge for electrical control of magnetism. This paper focuses on voltage pulse induced magnetization reversal which is mediated by localized strain generated in a two-phase magnetostrictive/piezoelectric multiferroic heterostructure. This paradigm takes advantage of the damping and precessional property of magnetization dynamics, of which the magnetization reversal needs precise control of the voltage pulse. We have performed a micromagnetic study of varied-size elliptical single-domain Ni and Terfenol-D nanomagnets at T = 300 K. By using the actual ground states of the nanomagnets at T = 300 K as the initial states, we obtained the key parameters of the voltage pulse which can induce repeated magnetization reversal. Then we uncovered the mechanism of the size-dependent incoherent magnetization switching of Terfenol-D nanomagnets by a quantitative analysis of the interaction between the exchange energy, demagnetization energy and stress anisotropy energy. These results lay the foundation for the application of strain-mediated magnetization switching technology, and also provide a set of guidelines for signal design and the choice of nanomagnets with proper material and size in the design of specific straintronic circuits.

Dynamics of magnetization reversal of double-row magnetic moment arrays

Double-row magnetic moment arrays coupled by dipole-dipole and exchange interactions have been studied. The processes of system excitation by a local external field affecting a fraction of magnetic moments have been considered; the conditions of partial and complete magnetization reversal of the system have been investigated. It has been shown that excitation of even two magnetic moments can cause magnetization reversal of the entire array. It has been revealed that the interface between oppositely oriented array regions can have both zero and nonzero total magnetic moments, depending on the exchange interaction parameter. The interdomain wall displacement due to the external magnetic field affecting the entire system has been considered.

Quasistatic and Pulsed Current-Induced Switching With Spin-Orbit Torques in Ultrathin Films With Perpendicular Magnetic Anisotropy

Spin-orbit interaction derived spin torques provide a means of reversing the magnetization of perpendicularly magnetized ultrathin films with currents that flow in the plane of the layers. A basic and critical question for applications is the speed and efficiency of switching with nanosecond current pulses. Here, we investigate and contrast the quasistatic (slowly swept current) and pulsed current-induced switching characteristics of micrometer scale Hall crosses consisting of very thin (<;1 nm) perpendicularly magnetized CoFeB layers on β-Ta. While complete magnetization reversal occurs at a threshold current density in the quasistatic case, short duration (≤10 ns) larger amplitude pulses (≏10 times the quasistatic threshold current) lead to only partial magnetization reversal and domain formation. We associate the partial reversal with the limited time for reversed domain expansion during the pulse.

Electric-field-induced magnetization reorientation in a (Ga,Mn)As/(Ga,Mn)(As,P) bilayer with out-of-plane anisotropy

Combined electric- and magnetic-field control of magnetization orientation and reversal is studied using anomalous Hall effect in an ultrathin ferromagnetic (Ga,Mn)As/(Ga,Mn)(As,P) bilayer. Its anisotropy results from the electrically tunable competition between the in-plane and out-of-plane anisotropies of both layers. The magnetic hysteresis loop shape is sensitive to the bias electric field. In the loop reversible part, an electric-field variation is found to reorient reversibly the magnetization. In this case, the magnetization direction follows the easy anisotropy direction controlled by electric field. In contrast, in the hysteretic part, an almost complete nonreversible magnetization reversal is achieved. This is interpreted as resulting from the electric-field-induced enhancement of domain nucleation and domain-wall propagation.

Complete magnetization reversal in a magnetostrictive nanomagnet with voltage-generated stress: A reliable energy-efficient non-volatile magneto-elastic memory

Rotating the magnetization of a magnetostrictive nanomagnet with electrically generated strain dissipates far less energy than any other rotation method and would have been the perfect choice for writing bits in non-volatile magnetic memory, except strain cannot ordinarily rotate the magnetization of magnet in a memory cell by more than 90° and “flip” it. Here, we describe a scheme to achieve complete 180° rotation with strain alone without having to precisely time the strain cycle. At room temperature, this writing method results in: (1) energy dissipation &amp;lt;6200 kT per bit, (2) write error probability &amp;lt;10−6, (3) write time of ∼1 ns, and (4) low read error.

Current-driven switching property of MgO-based magnetic tunnel junctions with a CoFeB free layer with in-plane magnetization

Spin-transfer torque enables magnetization switching by passing a spin-polarized current through nanostructures of spin valves or magnetic tunnel junctions. In this study, current-driven switching is investigated for magnetic tunnel junctions with a CoFeB free layer having in-plane magnetization. The critical switching current is found to depend on the feature of the switching modes and on the junction geometries. Especially, long-pulse mode switching generates equilibrium remnant magnetization states before complete magnetization reversal, which results in robust switching by reducing the local magnetization anomalies. The current-driven switching can be understood by using the combined effects of spin transfer and thermal activation.

Magnetization switching modes in nanopillar spin valve under the external field

The current-induced magnetic switching is studied in Co/Cu/Co nanopillar with an in-plane magnetization traversed under the perpendicular-to-plane external field. Magnetization switching is found to take place when the current density exceeds a threshold. By analyzing precessional trajectories, evolutions of domain walls and magnetization switching times under the perpendicular magnetic field, there are two different magnetization switching modes: nucleation and domain wall motion reversal; uniform magnetization reversal. The first mode occurs at lower current density, which is realized by the formation of the reversal nucleus and domain wall motion; while the second mode occurs through complete magnetization reversal at higher current density. Furthermore, the switching time reduces as the spin-polarized current density increases, which can also be grouped into two reversal modes.

Cumulative minor loop growth in Co/Pt and Co/Pd multilayers

The behavior of minor hysteresis loops in perpendicular anisotropy [Co/Pt] and [Co/Pd] multilayers has been investigated. Upon applying a succession of identical magnetic field cycles, we observe a very substantial cumulative growth of the minor loop area. For the [Co/Pt] multilayers this effect only saturates near complete magnetization reversal while the behavior is slightly more limited for [Co/Pd] multilayers. We also find this cumulative growth to occur even if the minor loop field cycles are made asymmetric by means of a positive bias field. The cumulative behavior persists up to a sample-dependent threshold value above which this effect disappears. In all samples, the cumulative minor loop growth is correlated with a small reduction in the maximum magnetization value in each cycle. Magneto-optical Kerr microscopy studies correlate the minor loop growth with the memory and cumulative expansion of lateral domain cycling. All experimental observations can be consistently explained as an accumulation of small nucleation domains that aid subsequent reversals and facilitate the cumulative minor loop growth.

AC-polarized neutron reflectometry: Application to domain dynamics in thin Fe film

We report on the first application of polarized neutron reflectometry (PNR) to probe the re-magnetization kinetics in thin films subjected to periodic magnetic fields. Specular PNR and off- specular scattering were recorded from a 100 nm thick MBE grown Fe film under the application of AC magnetic fields with amplitudes up to 60 Oe and frequencies in the range of 0.3 − 1.8 MHz. It was found that up to 0.4 MHz a complete magnetization reversal of the Fe film takes place during each AC field cycle. Higher frequencies cause only a partial re-magnetization and above 0.8 MHz the AC field no longer alters the sample magnetization. High frequencies suppress magnetization fluctuations on a micrometer scale which are otherwise seen via off-specular scattering.

Magnetization reversal via internal spin waves in magnetic nanoparticles

By numerically solving the equations of motion for atomic spins we show that internal spin-wave processes in large enough magnetic particles, initially in unstable states, lead to complete magnetization reversal and thermalization. The particle's magnetization $m$ strongly decreases in the middle of reversal and then recovers. The closer is the initial orientation of $\mathbf{m}$ to the energy minimum, the slower is the relaxation toward it and the smaller is the decrease in $m$ in the course of relaxation. We identify two main scenarios, exponential and linear spin-wave instabilities. For the latter, the longitudinal and transverse relaxation rates have been obtained analytically. Orientation dependence of these rates leads to a nonexponential relaxation of the particle's magnetization at long times.

RE-TM antiferromagnetic coupling and laser induced ultrafast magnetization reversal dynamics in GdFeCo magneto-optical films

The RE-TM antiferromagnetic coupling behavior and magnetization reversal dynamics in passing through the fermimagnetic compensation temperature (Tm) due to laser heating of GeFeCo films are investigated using femtosecond time-resolved pump-probe Kerr spectroscopy. Owing to sample temperature passing through Tm and hence the exchange of net magnetic moment carriers, an abnormal hysteresis loop is observed. The saturation magnetization difference between magnetization states above and below coercive field is found under a same direction of magnetic field, which reveals the incomplete rigid coupling between RE and TM sub-lattices in GdFeCo. A complete ultrafast magnetization reversal and recovery dynamic process is experimentally observed at the saturation magnetic bias on the sample with Al heat sink layer. Besides, by comparing magnetization reversal at saturation magnetic bias with magnetization recovery at remanence, it was found that the magnetic field applied externally does not affect the rate of domain nucleation and growth, but only their magnetization direction.