Hard Axis Magnetic Field Research Articles

Universality of stripe domain width change by an in-plane magnetic field

We establish a model for a stripe domain pattern width change under hard-axis magnetic fields in a perpendicularly magnetized system with Dzyaloshinskii–Moriya interaction. With the presence of hard-axis magnetic fields, tilted magnetizations in domains modulate an exchange interaction, a perpendicular magnetic anisotropy and a dipole–dipole interaction into specific forms, which are the main competing interactions to determine the specific form of the stripe domain. In addition, we found that the stripe domain pattern change under a hard-axis perturbation can be analyzed by only considering the amount of magnetization tilting from the initial easy-axis. The proposed model was used to draw a single universal curve for stripe width change as a function of the hard-axis magnetic fields.

Magnetoelectric coupling in self-assembled BiFeO3–CoFe2O4 nanocomposites on (110)-LaAlO3 substrates

We describe the magnetic domain structures and magnetoelectric coupling in self-assembled BiFeO3–CoFe2O4 (BFO–CFO) nanocomposite thin films grown on (110)-LaAlO3 substrates, consisting of CFO slab-shaped crystals (nanofins) embedded in a BFO matrix. The nanofins exhibit magnetic single-domain states with in-plane magnetization along their length due to shape and magnetoelastic anisotropies. The piezoresponse of the BFO matrix is affected by applying an in-plane hard axis magnetic field, indicating magnetoelectric coupling within the multiferroic composite. Conversely, an applied electric field leads to the magnetic reversal of a fraction of the nanofins as the strain transferred from the BFO alters the magnetic anisotropy, enabling switching driven by magnetostatic interactions.

Paramagnetic Effects of j-electron Superconductivity and Application to UTe2

The putative spin-triplet superconductor UTe2 has an Ising-like magnetic anisotropy, indicating the Cooper pair spins parallel to the easy-axis. However, the upper critical field for the hard-axis direction is much larger than that for the easy-axis, implying the strong suppression of the Pauli depairing effect even for the hard-axis direction. This observation is consistent with the recent NMR measurements which show the tiny decrease of the Knight shift below the transition temperature Tc for hard-axis magnetic fields. We clarify that these behaviors are naturally understood as a result of multi-orbital f -electron bands with total angular momentum j=5/2. Taking into account spin-orbit couplings for an orthorhombic structure, we demonstrate that for hard-axis magnetic fields, the decrease of the spin susceptibility below Tc and the Pauli depairing effect are strongly suppressed.

Bi-directional high speed domain wall motion in perpendicular magnetic anisotropy Co/Pt double stack structures

We report bi-directional domain wall (DW) motion along and against current flow direction in Co/Pt double stack wires with Ta capping. The bi-directionality is achieved by application of hard-axis magnetic field favoring and opposing the Dzyloshinskii-Moriya interaction (DMI), respectively. The speed obtained is enhanced when the hard-axis field favors the DMI and is along the current flow direction. Co/Pt double stack is a modification proposed for the high spin-orbit torque strength Pt/Co/Ta stack, to improve its thermal stability and perpendicular magnetic anisotropy (PMA). The velocity obtained reduces with increase in Pt spacer thickness due to reduction in DMI and enhances on increasing the Ta capping thickness due to higher SOT strength. The velocity obtained is as high as 530 m/s at a reasonable current density of 1 × 1012 A/m2 for device applications. The low anisotropy of the device coupled with the application of hard-axis field aids the velocity enhancement by preventing Walker breakdown.

Detection of Electrically Modulated Inverse Spin Hall Effect in anFe/GaAsMicrodevice

We report the detection of the inverse spin Hall effect (ISHE) in n-gallium arsenide (n-GaAs) combined with electrical injection and modulation of the spin current. We use epitaxial ultrathin-Fe/GaAs injection contacts with strong in-plane magnetic anisotropy. This allows us to simultaneously perform Hanle spin-precession measurements on an Fe detection electrode and ISHE measurements in an applied in-plane hard-axis magnetic field. In this geometry, we can experimentally separate the ordinary from the spin-Hall signals. Electrical spin injection and detection are combined in our microdevice with an applied electrical drift current to modulate the spin distribution and spin current in the channel. The magnitudes and external field dependencies of the signals are quantitatively modeled by solving drift-diffusion and Hall-cross response equations.

Single-Shot Time-Domain Studies of Spin-Torque-Driven Switching in Magnetic Tunnel Junctions

We report single-shot measurements of resistance versus time for thermally assisted spin-torque switching in magnetic tunnel junctions. We achieve the sensitivity to resolve the magnetic dynamics prior to as well as during switching, yielding detailed views of switching modes and variations between events. Analyses of individual traces allow measurements of coherence times, nonequilibrium excitation spectra, and variations in magnetization precession amplitude. We find that with a small in-plane hard-axis magnetic field the switching dynamics are more spatially coherent than for a zero field.

Bistable control of ferromagnetic resonance frequencies in ferromagnetic trilayered dots

The ferromagnetic resonance (FMR) properties of submicron-scale Ni81Fe19/Cu/Ni81Fe19 trilayered dots were investigated for application in programable microwave absorbers for broadband filter devices. The trilayered dot shows binary FMR frequencies, depending on the relative orientation of magnetization in the top and the bottom magnetic layers. A parallel configuration of magnetization, which exhibits a lower FMR frequency than that for an antiparallel configuration, can be metastable as the lateral aspect ratio of the dot is larger than the threshold value governed by the Cu layer thickness. The difference in the FMR frequencies between the parallel and antiparallel configurations is gradually decreased with elongation of the dot. This is associated with the suppression of the magnetostatic coupling energy that stabilizes the antiparallel configuration. It was also found that the magnetic configurations can be programed by the selective application of easy- and hard-axis magnetic fields used to saturate the magnetization. This is an appropriate manner to realize the antiparallel configuration of magnetization in the case of a dot with a plural stack of magnetic layers with the same coercive field.

Hard axis magnetic field dependence on current-induced magnetization switching in MgO-based magnetic tunnel junctions

We conducted a detailed study of hard axis magnetic field (Hhard) dependence on current-induced magnetization switching (CIMS) in MgO-based magnetic tunnel junctions (MTJs) with various junction sizes and various uniaxial anisotropy fields. The decreases in critical current density (Jc) and the intrinsic critical current density (Jc0) estimated from the pulse duration dependence on Jc in CIMS are observed when applying Hhard for all MTJs. The decrease in energy barrier of CIMS is also observed except for the largest sample. These results indicate that the reduction of Jc is attributable to both the increase of spin-transfer efficiency and the decrease in energy barrier in the case of applying Hhard. The Jc0 decreases with increase in the mutual angle between the direction of magnetization and the easy axis (θf), which is consistent with the theoretical prediction proposed by Slonczewski. The degree of the reduction of Jc0 for the same value of Hhard decreases with decreasing size of MTJs. This behavior is considered to be related to not only decrease in θf due to the increase in anisotropy field in MTJs, but also to the increase in the variance of the initial angle of magnetization due to the thermally activated magnon excitation. The stable switching endurance related to CIMS was observed in a wide range of MTJ sizes when applying Hhard. Moreover, we proposed a new architecture and a new switching method considering write disturbance. These results would be useful for application to spin memory and other spin-electronic devices.

Micromagnetic simulation of spin transfer torque switching combined with precessional motion from a hard axis magnetic field

The authors have performed micromagnetic simulations of spin transfer torque (STT) switching, combined with precessional motion induced by a pulsed or static hard axis field. They have found a significant reduction in the current required for STT switching in the presence of precessional motion. In particular, the switching current can be reduced below the zero-temperature threshold current at dc, when the hard axis field exceeds a fourth of the free layer coercivity. The simulations indicate that the critical current reduction arises from the strong STT present when the magnetization nearly passes through the hard axis during the initial few precessional turns.

Magnetization switching by spin torque using subnanosecond current pulses assisted by hard axis magnetic fields

We have induced magnetization switching by spin transfer in pillar-shaped spin valves using nanosecond-scale current pulses combined with hard axis fields. Switching events requiring 100ps pulses were obtained. In comparison with the switching induced by a single current pulse, biasing the device with a hard axis field decreases the pulse duration needed for error-free switching. The reduction of the cost in pulse amplitude scales with the magnitude of the applied field, and can be quantitatively understood from micromagnetic simulations and macrospin arguments taking into account the thermal fluctuations.

Precessional switching of a 3×1 micrometer elliptical element in a crossed-wire geometry

Switching dynamics of a thin-film 3×1 micrometer elliptical permalloy element in a crossed-wire geometry was investigated for different pulsed field amplitudes using time resolved scanning Kerr microscopy. We observed coherent switching of magnetization by fast-risetime hard axis magnetic field pulses. The combined action of easy- and hard-axis fields resulted in reliable half-select switching of the sample magnetization. Varying the delay between the pulses one can control the postswitching oscillations of magnetization or eliminate them altogether achieving an ultrafast half-select switching of risetime of just 180 ps. Our results agree quantitatively with Landau–Lifshitz–Gilbert-based micromagnetic simulations.

Precessional magnetization reversal in microscopic spin valve cells

We demonstrate stable and reversible precessional switching of the magnetization in microscopic spin valve cells. Magnetization reversal is achieved by applying sub-ns-hard axis magnetic field pulses under compensation of the easy axis offset field. Multiple application of a given hard axis field pulse leads to a stable and reversible switching back and forth of the cell's magnetization. Applications of this precessional switching process in ultra fast low power magnetic random access memories can be foreseen.

Amplitude and asymmetry correlation study between quasistatic and dynamic electrical testing of MR sensors

The peak amplitudes of a shielded MR sensor are shown to be well correlated over a wide range of hard-axis magnetic fields and bias currents between quasi-static and dynamic electrical testing. The peak asymmetry correlation appears to be head design specific. Write disturbances significantly increase the standard deviation of the amplitude and asymmetry repeatability measurements. The presence of a small dc easy-axis magnetic field has little effect on MR responses to uniform fields.

Motion of a magnetic domain wall traversed by fast-rising current pulses

A Bloch wall is predicted to undergo finite displacements when traversed by a current pulse with short rise time ≤20 ns and long fall time, in thin films of metallic ferromagnets. In Ni–Fe films of thickness 85–150 nm, pulses with peak current density ≂1×107 A/cm2 are expected to induce wall displacements of order 0.1–1 μm. This effect originates from the s-d exchange interaction. It is phenomenologically similar to the well-known ‘‘wall streaming’’ motion of Bloch walls subjected to fast-rising pulses of hard-axis magnetic field. The effect is related to the existence of a novel, current-induced, term in the expression for the momentum of a magnetic domain wall.

Observation of galvanomagnetic voltages at a magnetic domain wall in Ni-Fe films

A Ni81Fe19 film of 110 nm thickness is traversed by a dc current density normal to the easy axis. Two sharp tungsten wires serve as potential probes, distant by 210 μm along the easy axis. A charged wall, oriented at an angle to the easy axis, is made to creep slowly across the sample by applying dc easy axis and 60-Hz hard axis magnetic fields. The dc voltage between the probes is found to vary whenever the wall passes by the probes. The variation has the form of a voltage peak of ≂40 μV typical height. These peaks are caused by the planar Hall effect, in combination with a canting of the domain magnetization near the charged wall. In the case of a current parallel to the easy axis and normal to the line joining the probes, a steplike voltage variation ≤150 μV is observed for an uncharged wall in the presence of a dc hard axis field. Planar Hall effect and (field induced) domain canting are again responsible.

Prediction of Shapiro steps in the dc voltage across a magnetic domain wall traversed by a dc current and exposed to high-frequency magnetic fields

In metallic ferromagnets, a current of electrons traversing a domain wall exerts a torque on the wall spins through the s-d exchange interaction. This torque may induce a precession of the wall spins around the easy axis. In turn, the wall precession at a rate ω0 generates a dc voltage δV across the wall, given by the formula eδV = ℏω0 similar to the Josephson equation for superconducting junctions. In the present theoretical work, a high-frequency (hf), in-plane, hard-axis magnetic field is also applied. For a certain range of values of the dc current density, the rate of wall precession is synchronous with that of the hf drive field. In other ranges, precession is asynchronous, or there is no precession. As a result, the voltage δV has a stepwise dependence on the current density. This is the analog of the well-known Shapiro steps for the superconducting Josephson voltage.

Unusual magnetic recording applications utilizing highly anisotropic recording materials

The extreme anisotropy of certain magnetic recording tapes results in unusual recording properties. Oriented, low coercive force barium ferrite recording tapes are exceptionally resistant to erasure by hard-axis magnetic fields. This property makes it possible to anhysteretically contact duplicate digital signals from a barium ferrite recording tape with H\min_{c}\max_{e} (easy-axis) of 715 Oe to cobalt-doped γ-Fe 2 O 3 tapes with H\min_{c}\max_{e} of 875-1130 Oe, using hard-axis bias fields up to 2000 Oe. The large uniaxial magnetocrystalline anisotropy of oriented barium ferrite tapes also makes them unresponsive to recording-head fields with components along hard-axes-of-magnetization. Consequently, dual-layer barium ferrite recording tapes with in-plane easy-axes-of-magnetization oriented at +45° and -45°, respectively, to the longitudinal-tape-axis can be independently recorded, read, and erased using heads with gap-length parallel to the easy-axis of a selected layer. Signal amplitude losses on one layer, resulting from recording or erasing the other layer, are about 11% (1dB) for 27 flux transition/cm digital signals.

Influence of imperfections in magnetic thin films on the relaxation of the uniaxial anisotropy

Skew aging and its dependence on stabilization treatments have been investigated in cylindrical, electroplated permalloy films by annealing in hard axis magnetic fields. The skew fieldH ± was measured with the Belson test method. Plots ofH ± versus a logarithmic time scale show nearly straight lines in a range of four decades for temperatures between ambient temperature and 200 °C. This behaviour is supposed to be explained with a distribution of the relaxation processes in activation energy between 0·8 and 1·6 eV independent of the stabilization treatment. The frequency factor lying between 109 and 1010 sec is found to be a function of the stabilization temperature. The values can be interpreted by first-order kinetic processes, while this is not possible by assuming discrete activation energies. The reason for the distribution is the highly defective structure of thin films, especially in grain boundaries, dislocations and around impurities. By stabilization treatments the total volume of these regions and the vacancy concentration decrease. Only the Fe-Fe pair ordering mechanism is necessary to interpret the annealing data, if one assumes that also pairs in locally defective regions contribute to the anisotropy.

Influence of a Continuous Activation Energy Spectrum on the Aging Behavior of Magnetic Thin Films

Skew aging and its dependence on stabilization treatments have been investigated in cylindrical electroplated Permalloy films by annealing in hard-axis magnetic fields. The skew field Hβ was measured using the Belson test method. Plots of Hβ vs a logarithmic time scale show nearly straight lines over a range of four decades for temperatures between the ambient temperature and 200°C. This behavior may be explained by assuming that the activation energies of the relaxation processes have a continuous distribution between 0.8 and 1.6 eV, independent of the stabilization treatment. The frequency factor is found to be a function of the stabilization temperature, lying between 109 and 1010 sec−1. These values can be interpreted as being due to first-order kinetic processes, which is not possible if discrete activation energies are assumed. The reason for the distribution is the highly defected structure of thin films, especially at grain boundaries and dislocations and around impurities. Stabilization treatments reduce the total volume of these regions and the vacancy concentration. Only the Fe–Fe pair ordering mechanism is necessary to interpret the annealing data if one assumes that pairs in local defected regions also contribute to the anisotropy.

Magnetization spreading in a thin film memory

The hard axis magnetic field distribution within the uniaxial magnetic film element of a thin film memory device is calculated. In the memory device considered, the driving stripline and magnetic film are located between a magnetic keeper and a conducting ground plane. The difficulty encountered in the keeper-ground plane problem arises from the requirement of satisfying boundary conditions at the keeper and ground plane surfaces simultaneously, or in other words, from considering an infinite set of film and stripline images. A method of solution using Fourier transforms proves satisfactory. The dependence of the magnetic field distribution on the relative position of the elements and on variations in material properties is assessed.