Two-step Magnetization Reversal Research Articles

Significant role of thermal effects in current-induced exchange bias field switching at antiferromagnet/ferromagnet interface

The current-induced switching of in-plane exchange bias field (<i>H</i><sub>eb</sub>) has many advantages, such as switching without assistance of external magnetic field, excellent immunity to magnetic field, and robust magnetic anisotropy. However, the blocking temperature of the nanoscale antiferromagnet/ferromagnet (AFM/FM) heterostructure is relatively low and susceptible to thermal effects. Therefore, the Joule heating theoretically plays a substantial role in the switching of <i>H</i><sub>eb</sub> driven by current, but its underlying mechanism requires further investigation and verification. We prepare a series of Pt/IrMn/Py heterostructures with varying antiferromagnet IrMn thicknesses and systematically investigate the role of thermal effects in current-driven <i>H</i><sub>eb</sub> switching. These results demonstrate that under millisecond-level current pulses, Joule heating heats the device above the blocking temperature, leading to the decoupling of exchange coupling at AFM/FM interface. Simultaneously, the Oersted field and spin-orbit torque field generated by the current switch the ferromagnetic moments, and then a new <i>H</i><sub>eb</sub> will be induced along the direction of the ferromagnetic moments in the cooling process. Furthermore,in the switching process of <i>H</i><sub>eb</sub>, the anisotropic magnetoresistance curve of the AFM/FM heterostructure exhibits a temperature-dependent two-step magnetization reversal phenomenon. Theoretical analysis indicates that this phenomenon arises from the competitive relationship between exchange bias coupling at AFM/FM interface and direct exchange coupling between the ferromagnetic moments. The findings of this study elucidate the crucial role of thermal effects in the current-driven switching of <i>H</i><sub>eb</sub>, thereby contributing to the advancement of spintronic devices based on electrically controlled <i>H</i><sub>eb</sub>.

Interfacial antiferromagnetic phase induced two-step magnetization reversal in PbZr0.52Ti0.48O3/La0.67Sr0.33MnO3 superlattices

Artificial multiferroic heterostructures have recently attracted much interests due to the demonstrated magnetoelectric coupling (MEC) and unique functionalities, promising a tantalizing perspective of novel applications in next-generation electronic, memory, sensor, and energy harvesting technologies. Herein, we report a two-step magnetization reversal in PbZr0.52Ti0.48O3/La0.67Sr0.33MnO3 (PZT/LSMO) superlattices, which originates from the strongly entangled strain-, ferroelectric (FE)-polarization-, and exchange-dependent effects. Specifically, the preferential occupancy of the in-plane Mn dx2−y2 orbitals is triggered via the collective effects of the large tensile strain and FE polarization, giving rise to an interfacial antiferromagnetic (AFM) layer with strong AFM anisotropy. The strong spin exchange coupling between the AFM layer and the adjacent ferromagnetic (FM) layer facilitates the magnetic stratification of the FM layer, leading to two coercivities, i.e., two-step magnetization reversal. Meanwhile, a sizeable exchange bias (EB) field is induced. The emerged two-step magnetization reversal concomitant with the pronounced EB phenomenon should be a signature of an enhanced MEC in PZT/LSMO superlattices. Our results will stimulate further interests in multiferroic superlattices in applications of multiferroic-based devices.

Magnetization Reversal Process and Magnetostatic Interactions in Fe56Co44/SiO2/Fe3O4 Core/Shell Ferromagnetic Nanowires with Non-Magnetic Interlayer.

Nowadays, numerous works regarding nanowires or nanotubes are being published, studying different combinations of materials or geometries with single or multiple layers. However, works, where both nanotube and nanowires are forming complex structures, are scarcer due to the underlying difficulties that their fabrication and characterization entail. Among the specific applications for these nanostructures that can be used in sensing or high-density magnetic data storage devices, there are the fields of photonics or spintronics. To achieve further improvements in these research fields, a complete understanding of the magnetic properties exhibited by these nanostructures is needed, including their magnetization reversal processes and control of the magnetic domain walls. In order to gain a deeper insight into this topic, complex systems are being fabricated by altering their dimensions or composition. In this work, a successful process flow for the additive fabrication of core/shell nanowires arrays is developed. The core/shell nanostructures fabricated here consist of a magnetic nanowire nucleus (Fe56Co44), grown by electrodeposition and coated by a non-magnetic SiO2 layer coaxially surrounded by a magnetic Fe3O4 nanotubular coating both fabricated by means of the Atomic Layer Deposition (ALD) technique. Moreover, the magnetization reversal processes of these coaxial nanostructures and the magnetostatic interactions between the two magnetic components are investigated by means of standard magnetometry and First Order Reversal Curve methodology. From this study, a two-step magnetization reversal of the core/shell bimagnetic nanostructure is inferred, which is also corroborated by the hysteresis loops of individual core/shell nanostructures measured by Kerr effect-based magnetometer.

Effect of stripe domains on magnetization reversal and domain wall motion-like boundary expansion of the stripe domain region

We investigated the magnetization reversal and domain wall motion-like boundary expansion of the stripe domain region in ultrathin FeCo/Pt multilayers with perpendicular magnetic anisotropy. We found that magnetization reversal is realized in two steps, and the proportion of these two steps changes with the repeat number and the thickness of the magnetic layer. The stripe domain structure appears during the magnetization reversal and is responsible for the two-step magnetization reversal process. The repeat number and magnetic layer thickness have an obvious effect on the size of the stripe domain. The boundary of the stripe domain region expands just like a domain wall when it is driven by a magnetic field, and the velocity follows the creep law which is consistent with typical domain walls.

Stepwise magnetization reversal of geometrically tuned in diameter Ni and FeCo bi-segmented nanowire arrays

Magnetization reversal processes of hexagonal dense arrays of bi-segmented Ni and Fe50Co50 nanowires consisting of two well defined diameters (45 and 80 nm) have been studied. The nanowires were grown inside of tailored pores of anodic alumina templates by combined anodization, atomic layer deposition (ALD) and electrodeposition techniques. The experiments have allowed to identify their two-step magnetization reversal process ascribed to the respective segments of different diameter. This is concluded from the differential susceptibility observed in the hysteresis loops, contrary to those for nanowires with homogeneous diameter. These results are also confirmed by the first-order reversal curve (FORC) distribution diagrams, where an elongation parallel to the interaction axis around two coercive field values is obtained, which is correlated to the difference in diameter of the two segments. This well-defined two-step magnetization reversal process through the nanowire diameter design is thought to be very useful for the advanced control of the remagnetization in arrays of magnetic multidomain systems.

Investigation of magnetic properties of Sr3(Co, Zn)2Fe24O41 Z-type hexaferrites

The magnetic properties of Sr3Co2−xZnxFe24O41 (0 ≤ x ≤ 2) hexaferrite powders were investigated at different temperatures. The Results indicate that increasing Zn2+/Co2+ ratio has not led to significant changes in both lattice constant and grain structure. The hysteresis loops show that with an increase of Zn2+/Co2+ ratio, the two-step magnetization reversal process turns into three-step reversal process at certain Zn2+/Co2+ ratio. Moreover, with increasing temperature, the three-step magnetization reversal process turns into four-step and finally returns to three-step process when temperature reaches 350 K. In general, the saturation magnetization of the hexaferrite increases with higher percentage of Zn2+, but decreases when temperature rises. The coercivity is lowest for Sr3Co2Fe24O41 and Sr3Zn2Fe24O41. A partial substitution of Co2+ with Zn2+ leads to an increase in coercivity.

Two-Step Magnetization Reversal FORC Fingerprint of Coupled Bi-Segmented Ni/Co Magnetic Nanowire Arrays.

First Order Reversal Curve (FORC) analysis has been established as an appropriate method to investigate the magnetic interactions among complex ferromagnetic nanostructures. In this work, the magnetization reversal mechanism of bi-segmented nanowires composed by long Co and Ni segments contacted at one side was investigated, as a model system to identify and understand the FORC fingerprint of a two-step magnetization reversal process. The resulting hysteresis loop of the bi-segmented nanowire array exhibits a completely different magnetic behavior than the one expected for the magnetization reversal process corresponding to each respective Co and Ni nanowire arrays, individually. Based on the FORC analysis, two possible magnetization reversal processes can be distinguished as a consequence of the ferromagnetic coupling at the interface between the Ni and Co segments. Depending on the relative difference between the magnetization switching fields of each segment, the softer magnetic phase induces the switching of the harder one through the injection and propagation of a magnetic domain wall when both switching fields are comparable. On the other hand, if the switching fields values differ enough, the antiparallel magnetic configuration of nanowires is also possible but energetically unfavorable, thus resulting in an unstable magnetic configuration. Making use of the different temperature dependence of the magnetic properties for each nanowire segment with different composition, one of the two types of magnetization reversal is favored, as demonstrated by FORC analyses.

Laser MBE-grown CoFeB epitaxial layers on MgO: Surface morphology, crystal structure, and magnetic properties

Epitaxial layers of CoFeB were grown on MgO by means of laser molecular beam epitaxy using $\mathrm{C}{\mathrm{o}}_{40}\mathrm{F}{\mathrm{e}}_{40}{\mathrm{B}}_{20}$ target. The growth was combined with in situ structural characterization by three-dimensional reciprocal space mapping obtained from reflection high energy electron diffraction (RHEED) data. High-temperature single stage growth regime was adopted to fabricate CoFeB layers. As confirmed by the atomic force microscopy, the surface of CoFeB layers consists of closely spaced nanometer sized islands with dimensions dependent on the growth temperature. As shown by RHEED and XRD analysis, the CoFeB layers grown at high-temperature on MgO(001) possess body centered cubic (bcc) crystal structure with the lattice constant $a=2.87\phantom{\rule{0.16em}{0ex}}\AA{}$ close to that of the $\mathrm{C}{\mathrm{o}}_{75}\mathrm{F}{\mathrm{e}}_{25}$ alloy. It was further shown that following the same high-temperature growth technique the MgO/CoFeB/MgO(001) heterostructures can be fabricated with top and bottom MgO layers of the same crystallographic orientation. The CoFeB layers were also grown on the GaN(0001) substrates using MgO(111) as a buffer layer. In this case, the CoFeB layers crystallize in bcc crystal structure with the (111) axis perpendicular to the substrate surface. The magnetic properties of the CoFeB/MgO (001) heterostructures have been investigated by measuring magnetization curves with a vibrating sample magnetometer as well as by performing magneto-optical Kerr effect (MOKE) and ferromagnetic resonance (FMR) studies. FMR spectra were obtained for the variety of the magnetic field directions and typically consisted of a single relatively narrow resonance line. The magnetization orientations and the resonance conditions were calculated in the framework of a standard magnetic energy minimization procedure involving a single ${K}_{1\mathrm{c}}$ cubic term for the magnetocrystalline anisotropy. This allows a fairly accurate description of the angular dependences of the resonance fields---both in-plane and out-of-plane. It was shown that CoFeB layers exhibit in-plane fourth-order magnetic anisotropy. A two-step magnetization reversal model has been adopted for the CoFeB layers based on the VSM measurement analysis. Magnetization reversal studies performed by polar MOKE indicate that the magnetization lies in-plane in absence of magnetic field. Observed magnetic field dependences of reflected light ellipticity in geometry of longitudinal Kerr effect give convincing evidence for contribution of quadratic in magnetization terms in the dielectric tensor and clearly show the in-plane magnetization rotation.

Angular-dependent magnetization dynamics of kagome artificial spin ice incorporating topological defects

We report angular-dependent spin-wave spectroscopy on kagome artificial spin ice made of large arrays of interconnected ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ nanobars. Spectra taken in saturated and disordered states exhibit a series of resonances with characteristic in-plane angular dependencies. Micromagnetic simulations allow us to interpret characteristic resonances of a two-step magnetization reversal of the nanomagnets. The dynamic properties are consistent with topological defects that are provoked via a magnetic field applied at specific angles. Simulations that we performed on previously investigated kagome artificial spin ice consisting of isolated nanobars show characteristic discrepancies in the spin-wave modes which we explain by the absence of vertices.

Signature of exchange bias and magneto-electric coupling in BiFeO3/SrRuO3 heterostructure

The magnetic interaction between BiFeO3 and SrRuO3 layers in a heterostructure grown on (001) oriented SrTiO3 substrate is investigated. A two-step magnetization reversal was observed in M−H hysteresis loop measurement of the heterostructure at 10K. The first step in the hysteresis loop is associated with the switching of the free SrRuO3 moments whereas the second step arises from the switching of the pinned moments. The total amount of the pinned SrRuO3 moment was observed to decrease with increasing thickness of the BiFeO3 layer. The presence of exchange bias effect in the heterostructure was confirmed by the field cooled M−H measurements where only the second step of hysteresis loop was observed to shift along the field axis. The coercivity of the second step of hysteresis loop decreases with increasing temperature and merges with that of the first step above 100K which infers the desertion of the pinned magnetic moments. Temperature dependent capacitance measurement shows a kink at the ferromagnetic transition temperature of SrRuO3 and a pronounced dip was observed in the second derivative of capacitance with respect to temperature, indicating a strong magneto-electric coupling between the two layers.

Exchange bias effect and large coercivity enhancement in SrRuO3/NiO multilayer

Magnetic multilayers consisting of itinerant ferromagnetic SrRuO3 (SRO) and antiferromagnetic NiO were pulse-laser deposited on (0 0 1) SrTiO3 substrates. After small-field cooling, enhancements from both the unconventional exchange bias (EB) and the large coercive field were observed in the multilayers. Coercivity values for the in-plane hysteresis loop increased by almost 50 times in comparison with a pure SRO film. The maximum value of the bias moment in the minor loop and the EB field can be achieved in different cooling fields. Moreover, hysteresis indicates a two-step magnetization reversal. We speculate that a pinned layer exists at the SRO/NiO interface producing magnetic regions that pin the ferromagnetic SRO layer.

Super spin-glass state and exchange bias in Fe/CoO hybrid nanostructures

Fe/CoO heterostructures were realized by depositing Fe thin films on CoO nanoparticle arrays. Magnetization measurements revealed that 1 nm Fe exhibits a superparamagnetic behavior at 300 K and a super spin-glass state at temperatures below 80 K. The superparamagnetic as well as super spin-glass state vanishes for higher Fe film thicknesses once Fe starts to form a continuous layer across the CoO nanoparticle arrays. Furthermore, all samples exhibit an exchange bias effect at 6 K after field cooling, with a maximum exchange bias field of about 60 Oe for a Fe thickness of 2 nm. M–H loops of thicker Fe samples show a two-step magnetization reversal where Fe in the area in between CoO nanoparticles reverses at low fields, while, in proximity to the CoO nanoparticles, Fe switches at substantially higher fields. Both reversals are exchange biased.

Magnetization reversal in Fe48Pt34B18 magnetic foils

Exchange-spring magnet L1-FePt/(Fe2B+α-Fe) is fabricated by flash annealing a melt-spun Fe48Pt34B18 foil. A coercivity of 8500 Oe (1 Oe = 79.5775 A/m), squareness (Mr/Ms) of 0.70, saturation magnetization of 10.2 kGs (1 Gs = 10−4 T) and an effective anisotropy Keff = 2.0 × 107 ergs/cm3 are obtained. A two-step magnetization reversal feature is characterized in this paper. An exchange bias phenomenon is also observed in a low saturation field.

Three-dimensional magnetization profile and multiaxes exchange bias in Co antidot arrays

Magnetic properties of Cu/Co/Cu trilayers deposited on nanoporous alumina membranes are investigated using a superconducting quantum interference device. Hysteresis loops of the resulting Co antidot arrays show two-step magnetization reversal. Due to oxidation when samples are exposed to air, CoO forms and exchange bias is observed, whether the cooling field is applied within or perpendicular to the surface plane. In the former case, the exchange bias changes sign close to the blocking temperature of Co/CoO. We attribute these effects to the local, crescent shape of the Co films induced by the surface morphology of the membranes, which leads to a three-dimensional magnetization distribution.

Coercivity variation in exchange-coupled Fe/FePt bilayer with perpendicular magnetization

The soft/hard Fe/FePt film with perpendicular magnetization has been deposited on a glass substrate. The (001) oriented L1 0 FePt film was obtained when annealed by rapid thermal process at 800 °C and a Fe layer was deposited at room temperature with thicknesses of 2 nm to 20 nm. Controlling the Fe layer thickness allowed modification of the hysteresis loops from out-of-plane rigid magnet to in-plane exchange-spring like magnet due to the nanometer scale interface coupling. When the Fe layer thickness increased to 2 nm, the out-of-plane coercivity is reduced to 5.9 kOe but the remanence ratio (0.98) is still high. The Fe (2 nm)/FePt film shows perpendicular magnetization with linear in-plane hysteresis loop. The remanence ratio is reduced to 0.85 when the Fe layer thickness increased to 5 nm. When the Fe layer thickness was varied up to 10–20 nm, the in-plane hysteresis loop shows exchange-spring like behavior with two-step magnetization reversal processes. The films with perpendicular coercivity were moderated by the thickness of soft magnetic layer.

Influence of Different Onion States on Magnetization Reversal Processes in Permalloy Rings

Simulation and experiment are performed to investigate the influence of onion states with different-helicity pair vortices on the magnetization reversal process. It is found that for the onion state that has pair vortices with the same helicity, flux-closure state occurs in the reversal process; for the onion state that has pair vortices with opposite helicity, flux-closure state does not occur in the reversal process, and the onion state transforms directly to the reverse onion state. In addition, notches at the same side and diagonal notches with respect to the field direction are designed on the permalloy rings to induce onion states with same-helicity pair vortices and opposite-helicity pair vortices, respectively, for the control of the three-step and two-step magnetization reversal processes.

Tunneling Anisotropic Magnetoresistance in Co/AlOx/Au Tunnel Junctions

We observe spin-valve-like effects in nanoscaled thermally evaporated Co/AlOx/Au tunnel junctions. The tunneling magnetoresistance is anisotropic and depends on the relative orientation of the magnetization direction of the Co electrode with respect to the current direction. We attribute this effect to a two-step magnetization reversal and an anisotropic density of states resulting from spin-orbit interaction. The results of this study points to future applications of novel spintronics devices involving only one ferromagnetic layer.

In-plane anisotropy of coercive field in permalloy square ring arrays

Magnetic ring arrays are promising candidates for application in magnetic random access memory devices. The magnetic reversal processes and anisotropy of the coercivity in arrays of square-shaped nanorings with different spacings were investigated by vector magneto-optical Kerr effect magnetometry, magnetic force microscopy, and micromagnetic simulations. Two-step magnetization reversal demonstrates fourfold symmetry in the film plane resulting from the shape anisotropy in rings. Our numerical simulations show good agreement with the experiment.

Tunneling anisotropic magnetoresistance: Creating a spin-valve-like signal using a single ferromagnetic semiconductor layer

This article reports on a spintronics device based on the ferromagnetic semiconductor (Ga,Mn)As. Our transport measurements on a Au∕AlOx∕(Ga,Mn)As tunnel junction yield the surprising result that it is possible to get a spin-valve-like signal using only one magnetic layer. The strong spin-orbit coupling in (Ga,Mn)As creates significant anisotropies in the density of states with respect to the magnetization orientation. This, together with a two-step magnetization reversal creates a bistable magnetoresistive device with properties unattainable in current metal based spin-valves.

Tunneling anisotropic magnetoresistance: a spin-valve-like tunnel magnetoresistance using a single magnetic layer.

We introduce a new class of spintronic devices in which a spin-valve-like effect results from strong spin-orbit coupling in a single ferromagnetic layer rather than from injection and detection of a spin-polarized current by two coupled ferromagnets. The effect is observed in a normal-metal-insulator-ferromagnetic-semiconductor tunneling device. This behavior is caused by the interplay of the anisotropic density of states in (Ga,Mn)As with respect to the magnetization direction and the two-step magnetization reversal process in this material.