Exchange Bias Direction Research Articles

Reversal asymmetry and anomalous magnetic viscosity in exchange-bias systems

Employing vector magnetometry, interaction plots and model simulations, we probe the magnetization reversal in Co/Cu/IrMn exchange-biased trilayers. In the films with thin or no Cu spacer, the magnetization reversal is governed by domain-wall motion along one branch of the easy-axis hysteresis loop and by coherent rotation along the other. For thicker Cu spacers, both magnetization paths are dominated by coherent rotation. The latter also appears to be responsible for the reversal when the magnetic field is applied away from the exchange-bias direction for all films. We reveal a delayed domain-wall reversal along the descending-field path as compared to that predicted for coherent rotation. By exploiting in-field interaction plots, we evidenced a correlation between the asymmetric reversal and the exchange-bias coupling. Anomalous viscosity along recoil curves is detected and attributed to strong ferromagnetic coupling into the ferromagnet. The anomaly tends to fade away with the increase in the bias.

Asymmetric magnetization reversal of the Heusler alloy Co2FeSi as free layer in an CoFeB/MgO/Co2FeSi magnetic tunnel junction

We report the in-plane magnetization reversal behavior of the Co2FeSi Heusler alloy free layer in a bottom-pinned magnetic tunnel junction film with 149% TMR. Using magneto-optic indicator film imaging, we visualize the magnetic domain dynamics of this buried layer integrated within a full magnetic tunnel junction stack. The magnetic domain dynamics reveal anisotropic magnetization reversal within Co2FeSi under applied in-plane magnetic fields. While the reversal behavior under in-plane fields applied perpendicular to the in-plane exchange bias direction indicates smooth, coherent rotation away from the easy axis, asymmetric magnetic reversal behavior is observed along the easy axis. Reversed domains propagate from the interior of the film laterally outward toward the edges as the free layer magnetization switches into parallel alignment with the pinned synthetic antiferromagnetic reference layer (IrMn/CoFe/Ru/CoFeB). On the other hand, domains propagate from the film edges inward for the free layer transition into antiparallel alignment with the reference layer. These results have important implications for Heusler magnetic tunnel junction device performance.

Assessing interface coupling in exchange-biased systems via in-field interaction plots

An in-field interaction plot, δMR, has been recently introduced, presenting important advantages over the classical remanence plots. Here a general δMR is proposed, allowing to assess interactions even in systems with shifted and asymmetric major loops. To construct such a plot, a recoil loop (which incorporates a first-order reversal curve, FORC) and the position of the center of the major loop are only needed. Applying the method on exchange-biased Co/IrMn bilayer gives two types of δMR obtained for measuring field either parallel or antiparallel to the exchange-bias direction. This provides valuable information on the reversal mechanism and allows distinguishing between effects coming from coupling into the ferromagnet (Co) and those stemming from interactions in its interface with the antiferromagnet (IrMn). The essentially nonzero general δMR plot obtained from the major loop revealed to be a precise measure of the reversal asymmetry. The technique can readily be adjusted for use in other scientific fields where hysteresis is observed. We provide free software which generates such δMR plot(s).

Independence of spin-orbit torques from the exchange bias direction in Ni81Fe19/IrMn bilayers

We investigated a possible correlation between spin Hall angles and exchange bias in Ni$_{81}$Fe$_{19}$/IrMn samples by performing spin torque ferromagnetic resonance measurements. This correlation is probed by patterning of Ni$_{81}$Fe$_{19}$/IrMn bilayers in different relative orientations with respect to the exchange bias direction. The measured voltage spectra allow a quantitative determination of spin Hall angles, which are independent of the orientation around 2.8\pm0.3%.

Inverse magnetostrictive stress sensors based on crossed pinned CoFeB/MgO/CoFeB tunnel junctions

The usage of double pinned magnetic CoFeB/MgO/CoFeB tunnel junctions with noncollinear magnetizations as sensors of mechanical stress is shown. For this purpose, tunnel magnetoresistive (TMR) stacks with MnIr based pinning of both electrodes have been prepared. By using different MnIr thicknesses, we were able to realize noncollinear magnetizations. A series of annealing steps with a field perpendicular to the initial exchange bias direction have been performed in order to find the optimum thermal treatment for the best performance of the sensors. On these systems, bending experiments were completed which, show the suitability of our double-pinned TMR elements as stress sensors and their performance in differentiating tensile and compressive stress.

Magnetic anisotropies and its optical manipulation in epitaxial bismuth ferrite/ferromagnet heterostructure

Exchange-coupled BiFeO3/ferromagnet thin film heterostructures are promising for a fast and power efficient control of magnetization of the ferromagnetic layer. In the paper, the results of the study of magnetic properties of the epitaxial Fe2B/BiFeO3 heterostructure on (001)-SrTiO3 substrate with ferromagnetic resonance (FMR) spectroscopy are presented. The hierarchy of magnetic anisotropies that determine the angular variation of the resonance field in-plane and in-/out-of the plane of the system include the tetragonal four-fold and uniaxial terms for the Fe2B layer, uniaxial term for antiferromagnetic BiFeO3 layer and the exchange coupling at the interface. We find out that the exchange bias direction can be switched by a strong enough applied magnetic field. Both the exchange bias and an in-plane FMR resonance field of the heterostructure are strongly affected by the illumination with λ = 405 nm light. An optical tunability of the exchange bias and ferromagnetic resonance fields of the heterostructure has been demonstrated.

Study of angular dependence of exchange bias and misalignment in uniaxial and unidirectional anisotropy in NiFe(111)/FeMn(111)/CoFeB(amorphous) stack

We report the investigation of the in-plane azimuthal angular dependence of the magnetization reversal in the ion beam sputtered exchanged biased NiFe(111)/FeMn(111)/CoFeB(amorphous) stack. Compared to the as-deposited case, the magnetic annealing resulted in 3 fold enhancement in exchange bias but decrease in coercivity. The observed cosine dependence of exchange biased CoFeB layer on the in-plane azimuthal angle of applied field is corroborated with Meiklejohn and Bean model. The training effect associated with the exchange bias showed unconventional increase in coercivity after first cycle of hysteresis loop, while the exchange bias decreases sharply, and for subsequent cycles the exchange bias follows the empirical relation based on the energy dissipation in the AF layer. The ferromagnetic resonance (FMR) measurements also exhibited the in-plane azimuthal angle dependence of the magnetic resonance field indicating that the uniaxial and unidirectional anisotropies are not collinear, although they lie in the same plane. However, no misalignment between the unidirectional anisotropy and the exchange bias direction is observed. The misalignment angle between the uniaxial and unidirectional anisotropy, as measured by FMR, is found to be 10° and 14° for CoFeB and NiFe, respectively. This misalignment is attributed to the interface roughness as revealed by x-ray reflectance measurements.

Concentric 360° domain wall nesting in magnetic tunnel junction films: a Lorentz TEM study

We describe the formation of an unusual concentric magnetic domain wall pattern in the free layer of a bottom pinned magnetic tunnel junction. Lorentz microscopy reveals that repeated switching of the free layer with a magnetic field applied perpendicular to the exchange bias direction can produce a series of concentric 360° domain wall loops, a phenomenon we refer to as domain wall nesting. We propose two necessary ingredients for the behaviour: (i) inhomogeneities in the grain-by-grain magnetic dispersion that break local symmetry to produce a preferential sense of magnetic rotation upon field switching; and (ii) structural defects that act to pin 360° domain walls. Further control of this behaviour may provide new functionality for future device applications.

Optimum Laser Exposure for Setting Exchange Bias in Spin Valve Sensors

The specific local alignment of the magnetization of a reference layer in spin valves has a rapidly growing significance in the implementation of monolithically integrated sensor devices. The spatial sensitivity of such sensors is defined by the exchange bias effect arising in strongly coupled ferromagnetic (FM) and antiferromagnetic (AFM) heterostructures. The (re-)setting of the orientation of the exchange bias direction can be achieved through local annealing by means of a focused laser beam and subsequent cooling in the presence of a magnetic field. The parameters of such a laser exposure will have a significant influence to the FM/AFM system as well as the remaining layers, playing therefore an important role on the sensing characteristics. In order to analyse this effect, bottom-pinned IrMn / CoFe / Cu / CoFe/ NiFe spin valves are patterned into a meander shape and further annealed using a focused pulsed laser beam. Different processing peak intensities are tested. The analysis of the magnetotransport properties after this exposure shows how strongly the electrical conductivity is affected for higher intensities, as well as the effect of the laser peak intensity on the magnetic coupling between the ferromagnetic electrodes. These changes are discussed as a result of the structural modifications induced by the laser exposure on the spin valve structures.

Controllable exchange bias in Fe/metamagnetic FeRh bilayers

We report the studies of tuning the exchange bias at ferromagnetic Fe/metamagnetic FeRh bilayer interfaces. Fe/FeRh(111) bilayers show exchange bias in the antiferromagnetic state of FeRh while no exchange bias occurs at Fe/FeRh(001) interface. The contrasting results are attributed to the spin configurations of FeRh at the interface, i.e., the uncompensated ferromagnetic spin configuration of FeRh appears exclusively for (111) orientation. The exchange bias disappears as the bilayers are warmed above the antiferromagnetic-ferromagnetic transition temperature. The direction of the exchange bias for Fe/FeRh(111) is also found to be perpendicular to the cooling-field direction, in contrast to the commonly observed direction of exchange bias for ferromagnetic/antiferromagnetic interfaces. In view of these results, the exchange bias in Fe/FeRh bilayers with the (111) crystallographic orientation should be useful for the design of rapid writing technology for magnetic information devices.

Sign change of exchange bias in [Pt/Co]3/IrMn multilayer

The properties of exchange bias in a multilayer of [Pt(1.0 nm)/Co(1.0 nm)]2/Pt(tPt nm)/Co(1.0 nm)/ IrMn(12.0 nm) were investigated with a variation of Pt layer thickness, tPt. For tPt ≤ 1.6 nm, it was typically observed that Co layers were ferromagnetically coupled while IrMn layer exhibited negative exchange bias. With increasing Pt thickness, antiferromagnetic (AF) interlayer coupling strength increased and caused AF spin configuration between the Co layers. With further increasing of Pt thickness (tPt = 2.5 nm), the exchange bias between Co and IrMn layers was changed from negative to positive. Therefore, a large enhancement of AF interlayer coupling induced the sign change of exchange bias from negative to positive and resulted in a drastic change of switching behavior in a magnetization reversal. Both extraordinary Hall-effect and magnetoresistance were measured to verify the exchange bias direction and spin configurations upon magnetization reversal.

Tuning the direction of exchange bias in ferromagnetic/antiferromagnetic bilayer by angular-dependent spin-polarized current

The angular dependence of an external magnetic field applied with an in-plane alternating pulse that induces a new direction of exchange bias (EB) is observed in an NiFe/IrMn bilayer. Depending strongly on the direction of the external field, EB field gradually orients along the external field with the increase in current pulse, and the new direction of EB is finally established. Furthermore, the new direction of EB can also be induced along the external field at once when the applied pulse is larger than the critical current. Because the strength and direction of the EB is highly correlated with the micromagnetic state distribution of the antiferromagnet, the observations are explained by the spin torque exerted on the antiferromagnetic moments and provide evidence for the prediction of spin transfer and current-induced switching in antiferromagnets.

Current-induced switching of exchange bias in nano-scaled magnetic tunnel junctions with a synthetic antiferromagnetic pinned layer

This report investigates the current-induced switching of exchange bias without an external magnetic field in nano-scaled magnetic tunnel junction (MTJ) cells. An MTJ stack film was patterned into an ellipse with dimensions of 120 nm × 270 nm by using standard electron beam lithography in combination with ion beam etching. A spin-polarized current pulse with a duration of 100 ns was used to switch the exchange bias direction of the synthetic antiferromagnetic (SAF) pinned layer. It is worth noting that the MTJ cell was initialized in a high resistance state before applying the current pulse. For the application of both positive and negative current pulses, the resistance can be switched from the high (antiparallel) state to the low (parallel) one at 2.95 and −2.80 mA, respectively. After the current-induced switching, it was found that the magnetoresistance curve is reversed relative to the one before the current-induced switching. Predominantly, this behavior is independent of the polarity of the current pulse. As a result, it is shown that the exchange bias in the SAF pinned layer changes its direction with a sufficient supply of current pulse.

Thermal Decay and Reversal of Exchange Bias Field of CoFe/PtMn Bilayer after Ga+ Irradiation

An applied field is used to perform Ga+ ion irradiation on a CoFe/PtMn bilayer. Effects of the applied field and energy transfer between Ga+ ions and antiferromagnetic (AFM) atoms on the exchange bias field Hex are investigated. A partially reversed Hex is found in CoFe/PtMn specimens irradiated at a dose of 1 × 1014 ions/cm2 with an applied field anti-parallel to the original exchange bias direction. We believe that the rapid energy transfer and local temperature increase originating from the interaction between Ga+ ions and AFM atoms result in spin reversal and the formation of reversed AFM domains when specimens are irradiated with anti-parallel fields. The decrease in Hex when annealing the film in a negative saturation field indicates a thermal decay process. The AFM moments are reversed by thermal activation over an energy barrier distribution, which may change in some way as the temperature increases.

Angular dependence of the exchange bias direction and giant magnetoresistance on different cooling-field strengths

We studied the effect of different cooling-field strengths on the exchange bias by measuring the angular-dependent sheet resistance and the giant magnetoresistance of exchange-biased spin valves using a PtMn antiferromagnetic and a synthetic antiferromagnetic layer. When we annealed the spin valve at a cooling-field of 100 Oe, the exchange bias was antiparallel to the cooling-field. As we increased the cooling-field to 4000 Oe, the exchange bias direction gradually rotated and it ended up parallel to the cooling-field direction. The giant magnetoresistance also changed with the cooling-field strength. In the cooling-field range between 100 and 4000 Oe, the magnetoresistance ratios measured along the cooling-field direction were significantly reduced. However, the magnetoresistance ratios measured along the exchange bias direction increased, although still remaining smaller than those of the spin valve annealed at 100 or 4000 Oe. On the other hand, the exchange bias strength did not change significantly with the cooling-field strength.

Tuning the exchange bias in large area Co/CoO nanowire arrays

The exchange bias effect in Co (25 nm)/Co (5 nm)/Cu (2 nm) nanowire arrays has been systematically investigated. The exchange bias field HE is strongly dependent on the orientation of field cooling relative to the easy axis of the nanowires. We have exploited the competition between field cooling induced unidirectional anisotropy and shape anisotropy of the nanowires to tune both the direction and magnitude of exchange bias. A strong interplay between thermal activation effects and CoO domain size confinement in the nanowires also results in HE for the nanowire arrays being either smaller or larger than the continuous film.

Competing magnetic interactions in exchange-bias-modulated films

The magnetization reversal in stripelike exchange-bias-patterned ${\text{Ni}}_{81}{\text{Fe}}_{19}/\text{IrMn}$ thin films was investigated by complementary inductive and high-resolution magneto-optical magnetometry, magneto-optical Kerr microscopy, and polarized neutron reflectometry to clarify the effects of competing interfacial exchange-bias and lateral interface contributions. Structures of varying ferromagnetic layer thickness and stripe period were analyzed systematically at the frozen-in domain state of oppositely aligned stripe magnetization. For all samples the mean magnetization of the magnetic hybrid structures was found to be aligned nearly orthogonally with respect to the stripe axis and the set exchange-bias direction. Due to the interaction of interfacial coupling, exchange, and magnetostatic energy contributions, the opening angle of neighboring stripe magnetizations increases with decreasing ferromagnetic layer thickness and increasing stripe period. The experimental observations are in agreement with an earlier proposed model for designing micropatterned exchange-bias films.

Local exchange anisotropy of microstructured CoFe / MnIr (001) bilayers

Molecular beam epitaxy (MBE) grown CoFe/MnIr (001) epitaxial bilayers were microfabricated by electron beam (EB) lithography and Ar+ ion etching, and their local exchange biasing was studied by using a magnetic force microscopy. The circular patterned CoFe/MnIr annealed applying a magnetic field Ha parallel to [100] direction was found to show significant distribution of the direction of exchange bias from [100] direction: the mean direction of the exchange bias. On the other hand, when the CoFe/MnIr was annealed applying a field Ha // [110], the distribution of the exchange bias direction was found to be quite narrow, and almost all the circular patterns exhibit the exchange bias parallel to [110] direction. These results suggest that the local exchange anisotropy is controlled by the applied field direction during the annealing relative to the crystallographic axis of antiferromagnetic MnIr.

Spin torque-driven switching of exchange bias in a spin valve

We show that the strength and direction of the exchange bias in a spin valve can be changed and recovered by applying a spin-polarized current pulse. In other words, once the exchange bias has been changed by a spin-polarized current pulse with the external magnetic field antiparallel to the exchange bias direction, it can be returned to the initial state by a current pulse of the same magnitude with reversal of the external field. Furthermore, the exchange bias field reverts to the same magnitude, irrespective of whether one or multiple current pulses are applied. Based on a model for spin-momentum transfer, the experimental observations can be rationalized in terms of changing micromagnetic distributions at the ferromagnet/antiferromagnet interface.

Reorientation of exchange anisotropy in epitaxial (002) IrMn/CoFe system

We investigated the reorientation of exchange anisotropy in the epitaxial (002) IrMn/CoFe bilayers. Transitions of exchange-bias field from a negative field to a positive field with increasing time scale were observed at room temperature. The negatively magnetized state combining with the strong magnetocrystalline anisotropy of CoFe may raise the probability that the interfacial spins of thin IrMn layers were thermally relaxed to the direction of CoFe magnetization; therefore, after a period of time, the exchange-bias direction was reversed. This process can be further enhanced when samples were placed under the setting field. The double-shifted magnetization curve observed in this system was used as a good indicator for the relaxation of interfacial spins of IrMn.