Shift Of Hysteresis Loop Research Articles

Current-Induced Spin-Orbit Torque and Field-Free Switching in Mo -Based Magnetic Heterostructures

Magnetic heterostructure Mo/CoFeB/MgO has strong perpendicular magnetic anisotropy and thermal stability. Through current-induced hysteresis loop shift measurements, we show that the dampinglike spin-orbit torque (SOT) efficiency of Mo/CoFeB/MgO heterostructure is $\xi_{DL}\approx -0.003\pm 0.001$ and fairly independent of the annealing temperature from 300$^\circ$C to 400$^\circ$C. Though $|\xi_{DL}|$ is small while compare to those from Ta or W-based heterostructures, reversible current-induced SOT switching of a thermally-stable Mo/CoFeB/MgO heterostruture can still be achieved. Furthermore, we observe field-free current-induced switching from a Mo/CoFeB/MgO structure with the Mo layer being wedge-deposited. Our results indicate that even for a weak spin-orbit interaction 4d transition metal such as Mo, it is still possible to generate sufficient spin current for conventional SOT switching and to realize field-free current-induced switching by structural engineering.

Morphology-induced spin frustration in granular BiFeO3 thin films: Origin of the magnetic vertical shift

Pronounced room temperature vertical shifts in the magnetic hysteresis loops of granular, highly polycrystalline and ferromagnetic-like BiFeO3 thin films are observed upon field-cooling from a temperature above the Néel temperature of bulk BiFeO3. This is ascribed to the interplay between the preferential alignment, established by the field-cooling process, of the net magnetic moment, which arises from uncompensated antiferromagnetic spins, and the pinning of a fraction of these spins at the particle boundaries. Conversely, field-cooling of an epitaxially grown BiFeO3 film results in no vertical shift, confirming the effective role played by the particle boundaries (i.e., morphology) of the granular-like BiFeO3 films in the process of spin frustration.

Magnetic coupling in Mn3O4-coated γ-MnOOH nanowires

Mn3O4-coated γ-MnOOH nanowires were synthesized by using the hydrothermal method. X-ray diffraction and transmission electron microscopy studies reveal that the nanowires have a core (γ-MnOOH)–shell (Mn3O4) structure. The magnetic transition temperature of Mn3O4 is slightly lower than the previously reported value because of the increased thermal disturbance for nanomaterials and the influence of the helical magnetism of γ-MnOOH. The hysteresis loop at 50 K keeps the same shape as was measured below the Néel temperature of Mn3O4 because of the short-range order of γ-MnOOH above the transition temperature. The short-range ordering is responsible for the deviation of the hysteresis loop at 300 K from the linear behavior of a normal paramagnetic phase. The magnetic coupling behavior between Mn3O4 and γ-MnOOH also induces an exchange bias effect in the system. The hysteresis loop shifts to the positive direction with increasing measurement temperature. A lower barrier energy is requested for the reversal of magnetic moments on the interface to generate the exchange bias behavior because of the asymmetric magnetic natures of Mn3O4 and γ-MnOOH, which should possess shorter responsive time compared with the traditional antiferromagnetic-ferromagnetic coupling induced exchange bias systems.

The Influence of Loop Geometry on the Asymmetric Pinning of Domain Walls in Films with Uniaxial Anisotropy

The asymmetric pinning of a vortex-domain wall in regions with enhanced values of saturation magnetization and the anisotropy constant (defects) has been studied by a three-dimensional micromagnetic simulation at different geometric parameters of these regions. The dependences of the geometry of shifted hysteresis loops on a spacing between two defects and their size in a 100-nm-wide uniaxial magnetic film have been obtained.

Role of Ni concentration on structural and magnetic properties of inverse spinel Ferrite

In present work, Nickel (Ni) substituted Mn-Co ferrite nanoparticles, with doping content varying from 0 to 0.1, have been synthesized. The inverse spinel structure of doped cobalt ferrite has been claimed by X-ray diffractometer with average particle size of 20 nm. The decrease in lattice parameter and increase in X-ray density has been found with increase in Ni concentration in ferrite structure. Fourier transform infrared spectroscopy analysis showed two vibrational frequency bands (υ1 and υ2) for octahedral and tetrahedral sites depicting the formation of spinel ferrite. In addition, Transmission electron microscopy analyses were carried out to investigate particle size, which indicates that TEM and XRD analyses are in agreement. Magnetic analyses depict that coercivity value increases from 635 Oe to 841 Oe in Ni doped Mn-Co ferrite. Exchange interactions between Ni and Mn0.2Co0.8-xFe2O4 result for enlargement in coercivity and shift in hysteresis loop towards positive direction along the field axis. The increment in magnetic coercive field and exchange bias has been observed as temperature decreases. Enhanced magnetic properties at low temperature are attributed to magneto-crystalline anisotropy and surface effects.

Spin Valves with the Controlled Shift of Low-Field Hysteresis Loop and High-Sensitive Sensing Elements on Their Basis

Metallic multilayer spin-valve nanostructures that comprise the exchange-coupled ferromagnet/ Ru/ferromagnet structure in the free layer have been synthesized by magnetron sputtering. For microobjects (meanders) formed from CoFe/Cu/CoFe/Ru/CoFe/FeMn/Ta spin valves with different thicknesses of ruthenium and copper layers, the dependences of the magnetoresistive sensitivity and low-field hysteresis loop shift on the meander strip width have been studied. Sensing elements characterized by high magnetoresistive sensitivity have been manufactured.

Mictomagnetic State in an EuBaCo2–xO5.5–δ Single Crystal

The magnetic properties of an EuBaCo1.9O5.36 single crystal are studied in the temperature range T = 2–300 K and the magnetic field range H ≤ 90 kOe. This binary layered cobaltite single crystal has vacancies in the cobalt and oxygen sublattices, in contrast to the stoichiometric EuBaCo2O5.5 composition. All cobalt ions in EuBaCo1.9O5.36 are in a trivalent state. The single crystal has an orthorhombic structure with space group Pmmm, and its unit cell parameters are a = 3.883 A, b = 7.833 A, and c = 7.551 A. The field and temperature dependences of the magnetization of the single crystal demonstrate that it is ferrimagnet below TC = 242 K. At T < 300 K, all three spin states of the Co3+ ions are present. The nearest-neighbor interactions give antiferromagnetic (AFM) and ferromagnetic (FM) contributions to the exchange energy. The ratio of the AFM to the FM contributions changes when temperature decreases because of a change in the spin state of the Co3+ ions. The single crystal exhibits signs of mictomagnetism at low temperatures in high magnetic fields. At T = 2 K and H = 90 kOe, the zero-field and nonzero-field magnetizations are strongly different because of a uniaxial magnetic anisotropy, which tends to set magnetization along the magnetic field applied in cooling throughout the crystal volume. As a result, a complex ferrimagnetic structure with a noncollinear direction of Co3+ spins appears. The following phenomena characteristic of mictomagnets are also observed in the EuBaCo1.9O5.36 single crystal: a shift in a magnetization hysteresis loop when temperature decreases, retained hysteretic phenomena and no magnetization saturation in high magnetic fields, and an orientation transition. The mictomagnetic state in EuBaCo1.9O5.36 is shown to be caused by the structural distortions induced by vacancies in the cobalt and oxygen sublattices and by the frustration of AFM and FM exchange interactions.

Magnetic coupling in Mn3O4-coated γ-MnOOH nanowires

Mn3O4-coated g-MnOOH nanowires were synthesized by using the hydrothermal method. X-ray diffraction and transmission electron microscopy studies reveal that the nanowires have a core (g-MnOOH)-shell (Mn3O4) structure. The magnetic transition temperature of Mn3O4 is slightly lower than the previously reported value because of the increased thermal disturbance for nanomaterials and the influence of the helical magnetism of g-MnOOH. The hysteresis loop at 50 K keeps the same shape as was measured below the Neel temperature of Mn3O4 because of the short-range order of g-MnOOH above the transition temperature. The short-range ordering is responsible for the deviation of the hysteresis loop at 300 K from the linear behavior of a normal paramagnetic phase. The magnetic coupling behavior between Mn3O4 and g-MnOOH also induces an exchange bias effect in the system. The hysteresis loop shifts to the positive direction with increasing measurement temperature. A lower barrier energy is requested for the reversal of magnetic moments on the interface to generate the exchange bias behavior because of the asymmetric magnetic natures of Mn3O4 and g-MnOOH, which should possess shorter responsive time compared with the traditional antiferromagnetic-ferromagnetic coupling induced exchange bias systems.

Spin-orbit torque magnetometry by wide-field magneto-optical Kerr effect

Magneto-optical Kerr effect (MOKE) is an efficient approach to probe surface magnetization in thin film samples. Here we present a wide-field MOKE technique that adopts a Köhler illumination scheme to characterize the current-induced damping-like spin-orbit torque (DL-SOT) in micron-sized and unpatterned magnetic heterostructures with perpendicular magnetic anisotropy. Through a current-induced hysteresis loop shift analysis, we quantify the DL-SOT efficiency of a Ta-based heterostructure with bar-shaped geometry, Hall-cross geometry, and unpatterned geometry to be |ξDL| ≈ 0.08. The proposed wide-field MOKE approach therefore provides an instant and direct characterization of DL-SOT, without the need of any further interpretation on electrical signals.

Current induced domain wall motion and tilting in Pt/Co/Ta structures with perpendicular magnetic anisotropy in the presence of the Dyzaloshinskii–Moriya interaction

Current induced domain wall motion (CIDWM) was studied in Pt/Co/Ta structures with perpendicular magnetic anisotropy and the Dyzaloshinskii–Moriya interaction (DMI) by the spin-orbit torque (SOT). We measured the strength of DMI and SOT efficiency in Pt/Co/Ta with the variation of the thickness of Ta using a current induced hysteresis loop shift method. The results indicate that the DMI stabilizes a chiral Néel-type domain wall (DW), and the DW motion can be driven by the enhanced large SOT generated from Pt and Ta with opposite signs of spin Hall angle in Pt/Co/Ta stacks. The CIDWM velocity, which is 104 times larger than the field driven DW velocity, obeys a creep law, and reaches around tens of meters per second with current density of ~106 A cm−2. We also found that the Joule heating accompanied with current also accelerates the DW motion. Meanwhile, a domain wall tilting was observed, which increases with current density increasing. These results can be explained by the spin Hall effect generated from both heavy metals Pt and Ta, inherent DMI, and the current accompanying Joule heating effect. Our results could provide some new designing prospects to move multiple DWs by SOT for achieving racetrack memories.

Comparative study on spin-orbit torque efficiencies from W/ferromagnetic and W/ferrimagnetic heterostructures

It has been shown that W in its resistive form possesses the largest spin-Hall ratio among all heavy transition metals, which makes it a good candidate for generating efficient dampinglike spin-orbit torque (DL-SOT) acting upon adjacent ferromagnetic or ferrimagnetic (FM) layer. Here we provide a systematic study on the spin transport properties of W/FM magnetic heterostructures with the FM layer being ferromagnetic Co$_{20}$Fe$_{60}$B$_{20}$ or ferrimagnetic Co$_{63}$Tb$_{37}$ with perpendicular magnetic anisotropy. The DL-SOT efficiency $|\xi_{DL}|$, which is characterized by a current-induced hysteresis loop shift method, is found to be correlated to the microstructure of W buffer layer in both W/Co$_{20}$Fe$_{60}$B$_{20}$ and W/Co$_{63}$Tb$_{37}$ systems. Maximum values of $|\xi_{DL}|\approx 0.144$ and $|\xi_{DL}|\approx 0.116$ are achieved when the W layer is partially amorphous in the W/Co$_{20}$Fe$_{60}$B$_{20}$ and W/Co$_{63}$Tb$_{37}$ heterostructures, respectively. Our results suggest that the spin Hall effect from resistive phase of W can be utilized to effectively control both ferromagnetic and ferrimagnetic layers through a DL-SOT mechanism.

Control of Low-Field Hysteresis Loop Shift of Spin Valves

Spin valves that comprise synthetic antiferromagnet as a component of pinned layer and an exchange-coupled ferromagnet/Ru/ferromagnet structure in the free layer have been prepared by magnetron sputtering. Microobjects have been formed from spin valves by optical and electron-beam lithography. It has been shown that the shift of the low-field magnetoresistance hysteresis loop decreases as the thicknes of the Ru spacer in the free layer of spin valve increases. The almost hysteresis-free odd-field dependences of the magnetoresistance were obtained for micron-sized samples; in this case, the sensitivity is 0.2%/Oe.

Enhancing Multifunctionality through Secondary Phase Inclusion by Self-Assembly of Mn3O4 Nanostructures with Superior Exchange Anisotropy and Oxygen Evolution Activity

While altering physical properties by self-assembly is a common phenomenon, controlled inclusion of a secondary phase that in turn enhances the properties of the ensemble is a rare occurrence. Herein monodisperse Mn3O4 spherical nanoparticles were self-assembled into hierarchical flakes and cubes by regulating the surfactant–metal precursor molar ratio, reaction atmosphere, and time. The secondary phase of Mn2O3 was incorporated differently, depending on the type of self-assembly as 2, 3.5, and 6.5 wt % in the flake, spherical, and cubic morphologies, respectively. The highest percentage of Mn2O3 in the cubes boosts its multifunctionality in terms of enhanced magnetic exchange coupling and oxygen evolution reaction (OER) activity. With a 2 T cooling field, the hysteresis loop shift corresponding to coupling between antiferromagnetic Mn2O3 and ferrimagnetic Mn3O4 reached 3813 ± 2 Oe for the cubes, which is a record high for any reported Mn3O4–Mn2O3 system. The presence of a eg1 electron due to a higher Mn2...

Spin-Orbit Torque from a Magnetic Heterostructure of High-Entropy Alloy

High-entropy alloy (HEA) is a family of metallic materials with nearly equal partitions of five or more metals, which might possess mechanical and transport properties that are different from conventional binary or tertiary alloys. In this work, we demonstrate current-induced spin-orbit torque (SOT) magnetization switching in a Ta-Nb-Hf-Zr-Ti HEA-based magnetic heterostructure with perpendicular magnetic anisotropy (PMA). The maximum damping-like SOT efficiency from this particular HEA-based magnetic heterostructure is further determined to be $|\zeta^{\operatorname{HEA}}_{DL}|\approx0.033$ by hysteresis loop shift measurements, while that for the Ta control sample is $|\zeta^{\operatorname{Ta}}_{DL}|\approx0.04$. Our results indicate that HEA-based magnetic heterostructures can serve as a new group of potential candidates for SOT device applications.

Reduction of in-plane field required for spin-orbit torque magnetization reversal by insertion of Au spacer in Pt/Au/Co/Ni/Co/Ta

Spin-orbit torques and current-induced switching are studied in perpendicularly magnetized Pt/Au/(Co/Ni/Co) films as a function of Au insertion layer thickness tAu. By simultaneously varying the ferromagnet layer thickness, a parametric series of samples with nearly constant anisotropy were prepared. On this series, spin orbit torques were characterized by harmonic voltage and hysteresis loop shift measurements, and current-induced switching was examined as a function of the in-plane bias field. Little variation is seen for tAu &amp;lt; 0.5 nm, whereas for tAu &amp;gt; 0.5 nm, a series of well-correlated effects appear. Both the loop shift efficiency and the Slonczewski-like spin-orbit torque effective field double, while the in-plane field required to saturate the loop shift efficiency decreases by a factor of ∼10. Correspondingly, the current and in-plane field required for spin-orbit torque switching are reduced by about 90%. These results suggest that a thin Au insertion layer reduces the Dzyaloshinskii-Moriya interaction strength and improves spin transmission at the spin Hall metal/ferromagnet interface, substantially reducing the in-plane field and currents for spin orbit torque switching.

Robust pinning of magnetic moments in pyrochlore iridates

Pyrochlore iridates ${A}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ ($A$ = rare earth elements, Y or Bi) hold great promise for realizing novel electronic and magnetic states owing to the interplay of spin-orbit coupling, electron correlation, and geometrical frustration. A prominent example is the formation of all-in/all-out (AIAO) antiferromagnetic order in the ${\mathrm{Ir}}^{4+}$ sublattice that comprises corner-sharing tetrahedra. Here we report on an unusual magnetic phenomenon, namely, a cooling-field-induced shift of magnetic hysteresis loop along the magnetization axis, and its possible origin in pyrochlore iridates with nonmagnetic Ir defects (e.g., ${\mathrm{Ir}}^{3+}$). In a simple model, we attribute the magnetic hysteresis loop to the formation of ferromagnetic (FM) droplets in the AIAO antiferromagnetic (AFM) background. The weak ferromagnetism originates from canted antiferromagnetic order of the ${\mathrm{Ir}}^{4+}$ moments surrounding each nonmagnetic Ir defect. The shift of hysteresis loop can be understood quantitatively based on an exchange-bias-like effect in which the moments at the shell of the FM droplets are pinned by the AIAO AFM background via mainly the Heisenberg ($J$) and Dzyaloshinsky-Moriya ($D$) interactions. The magnetic pinning is stable and robust against sweeping cycle and sweeping field up to 35 T, which is possibly related to the magnetic octupolar nature of the AIAO order.

Effect of doping and annealing on crystal structure and magnetic properties of La1–xAgxMnO3 magnetic nanoparticles

We study crystal structure and magnetic properties of La1–xAgxMnO3 nanopowders prepared by glycine—nitrate method. The particle size and crystal structure were modified by heat treatment. The average size of particle varies from about 26.6(4) nm for as prepared sample to 63.3(9) nm for annealed sample at 900 °C/2 h. Crystal structure changes from orthorhombic Pnma to rhombohedral R3¯c after annealing at 600 °C/2 h. The saturated magnetization μs and the Curie temperature TC increase with annealing; TC is more than doubled after annealing at 600 °C/2 h. Exchange bias phenomena were first observed on nanoparticles with orthorhombic crystal structure. The hysteresis loop shifts in horizontal and vertical direction after cooling in magnetic field μ0Hcf ≠ 0 through TC. The values of exchange bias field μ0HE, coercive field μ0Hc, remnant asymmetry μE and coercive magnetization μc increase with increasing value of cooling field μ0Hcf. In addition the training effect was observed.

Interface-induced spontaneous positive and conventional negative exchange bias effects in bilayer La0.7Sr0.3MnO3/Eu0.45Sr0.55MnO3 heterostructures

We report zero-field-cooled spontaneous-positive and field-cooled conventional-negative exchange bias effects in epitaxial bilayer composed of La0.7Sr0.3MnO3 (LSMO) with ferromagnetic (FM) and Eu0.45Sr0.55MnO3 (ESMO) with A-type antiferromagnetic (AF) heterostructures respectively. A temperature dependent magnetization study of LSMO/ESMO bilayers grown on SrTiO3 (001) manifest FM ordering (TC) of LSMO at ~320 K, charge/orbital ordering of ESMO at ~194 K and AF ordering (TN) of ESMO at ~150 K. The random field Ising model has demonstrated an interesting observation of inverse dependence of exchange bias effect on AF layer thickness due to the competition between FM-AF interface coupling and AF domain wall energy. The isothermally field induced unidirectional exchange anisotropy formed at the interface of FM-LSMO layer and the kinetically phase-arrested magnetic phase obtained from the metamagnetic AF-ESMO layer could be responsible for the spontaneous exchange bias effect. Importantly, no magnetic poling is needed, as necessary for the applications. The FM-AF interface exchange interaction has been ascribed to the AF coupling with sum {J}_{ex}vec{{S}_{{rm{FM}}}}cdot vec{{S}_{{rm{AF}}}} ({J}_{ex}approx {J}_{AF}, coupling constant between AF spins) for the spontaneous positive hysteresis loop shift, and the field-cooled conventional exchange bias has been attributed to the ferromagnetically exchanged interface with {J}_{ex}approx {J}_{F} (coupling constant between FM spins).

Surfactant-Mediated Resistance to Surface Oxidation in MnO Nanostructures.

The intrinsic physical properties of nanostructures of metals and their oxides are altered when they are prone to surface oxidation in ambient atmosphere. To overcome this limitation, novel synthesis methodologies are required. In this study, solid octahedral shapes of MnO limit the inward oxygen diffusion compared to that of the MnO-nanoparticle-assembled octahedra. In addition to morphology control, which restricts the thickness of the Mn3O4 surface layer, the binding chemistry of the surfactants plays an essential role. For example, the Mn3O4 surface layer is 0.4 nm thinner with trioctylphosphine oxide than with trioctylamine as the surfactant. The nanostructures were prepared by varying the surfactants, surfactant-to-precursor molar ratio, accelerating agent, and reaction heating rate. The surface oxidation of MnO nano-octahedra was probed by Rietveld analysis of X-ray diffraction patterns and X-ray photoelectron spectroscopy and characterized by magnetic measurements, as the presence of ferrimagnetic Mn3O4 shell on the antiferromagnetic MnO core provides an exchange coupling at the core–shell interface. Thicker the Mn3O4 shell, higher is the exchange-biased hysteresis loop shift.

Relative importance of surface anisotropy and defects on loop shifts in antiferromagnetic nanoparticles

The shifts in field-cooling hysteresis loops of antiferromagnetic nanoparticle systems have been widely observed in experiments. Here, numerical calculation is used to study the influence of the surface anisotropy and defects. It is found that surface defects alone are sufficient to yield the loop shifts (LS) at proper field angles. Neither the disordered nor the radial surface anisotropy is the indispensable ingredient to generate the LS. However, both of them play a role in enhancing the LS. The possible mechanisms are discussed.