Antiferromagnetic Layer Research Articles

Room temperature electric field control of magnetic properties for the α-Fe2O3/Fe3O4 composite structure

Abstract The raw nano α-Fe2O3 powders consisting of nanorods with 50–100 nm in length and 8–14 nm in width exhibit superparamagnetism. The vacuum annealing of the raw nano α-Fe2O3 at 400 °C for 3 h leads to ferromagnetic-like behaviors at room temperature (RT), due to the appearance of ferrimagnetic Fe3O4. The α-Fe2O3/Fe3O4 composite structure has an antiferromagnetic α-Fe2O3 layer and a ferrimagnetic Fe3O4 layer contains a certain amount of Fe2+ and oxygen vacancies. In addition, nano α-Fe2O3 annealed in H2 at 300 °C for 3 h completely become Fe3O4. Some magnetic properties of the α-Fe2O3/Fe3O4, such as magnetization, magnetic remanence (Mr), coercivity (Hc) and exchange bias (EB), can be modulated almost reversibly and non-volatilely by applied external RT electric field without loop current. However, for both the raw nano α-Fe2O3 and the Fe3O4, these magnetic properties cannot be modulated by applied external RT electric field. The electric field control of magnetic properties for the α-Fe2O3/Fe3O4 should be due to the mechanism of phase transition induced by migration and redistribution of oxygen vacancies within the interface of α-Fe2O3/Fe3O4.

Effect of non-$$180^{\circ }$$ polarization invariants on the exchange bias of tetragonal $$\langle 001\rangle $$ and rhombohedral $$\langle 111\rangle $$ orientations of bismuth ferrite epitaxial thin films

\(\hbox {BiFeO}_{3}\) (BFO) is the only room temperature multiferroic material that has been extensively studied due to its multifunctional properties. BFO with a canted G-type antiferromagnetic (AFM) ordering exhibits strong exchange bias characteristics with NiFe which offers the potential to design and utilize devices working based on multiferroic features. In the past, it was known that the presence of \(180^{\circ }\) domain walls of BFO hinders a plausible exchange bias interaction. To understand the role of the strain-induced effects on such \(180^{\circ }\) domain walls and its effect on the exchange bias, NiFe, a soft ferromagnetic layer, was grown on the epitaxial BFO AFM layers. An approximately 80 nm BFO layer was grown epitaxially in both the tetragonal (001) phase on the \(\hbox {LaAlO}_{3}\) (001) substrate and the rhombohedral (111) phase on the \(\hbox {SrTiO}_{3}\) (111) substrate, with a thin (10 nm) layer of \(\hbox {Ni}_{80}\hbox {Fe}_{20}\) on top of it. An exchange bias of 510 Oe was observed in the tetragonal phase of BFO with a c / a ratio of 1.22, which is comparable with the exchange bias shown by the (111) oriented rhombohedral phase (360 Oe). Both the tetragonal (001) and rhombohedral (111) layers possess ferroelectric polarization normal to the sample surface and so the domain walls are mostly \(180^{\circ }\) oriented which is expected to have a minimum effect on the exchange bias. However, the weak strain-induced structural variants in the (111) oriented rhombohedral BFO and the monoclinic distortion present in the tetragonal BFO introduce non-\(180^{\circ }\) domain walls in the system. These variants arising due to the structural distortion are expected to play a key role in defining the ferroelectric domain wall nature, thereby exhibiting exchange bias characteristics.

Charge transfer induced ferromagnetism and anomalous temperature increment of coercivity in ultrathin α-Fe2O3 decorated graphene 2D nanostructures

To overcome the detrimental effect of charge transfer from a transition metal to 2D substrates like graphene, we have grown ultrathin antiferromagnetic α-Fe2O3 layers on both sides of the graphene surface. Anomalous magnetic behavior, viz., coercivity and exchange bias, increases with increasing temperature with strong ferromagnetic ordering. The highest values of coercivity and large exchange bias are obtained as 3335 Oe and 2361 Oe, respectively. Large enhancement (646%) in exchange bias is observed with an increase in temperature from 2 K to 70 K. Interlayer exchange coupling between the ferromagnetic layers becomes strongest at 300 K to achieve an ultralow coercivity of 22 Oe by growing an α-Fe2O3 phase on both sides of the graphene surface. A 32% negative magnetoresistance is observed as a result of exchange bias which changes with temperature. All these results are explained on the basis of the charge transfer effect at the interface of the graphene/α-Fe2O3 nanostructure at the low temperature region and the spin canting effect of surface states at the higher temperature region. Theoretical Density Functional Theory calculation is also done to understand the interface interaction, quantitative evaluation of charge transfer, and density of states.

Indirect Magnetic Coupling through the Nonmagnetic Au Layer in the Fe/Au/CoO Trilayer Investigated by Resonant X-ray Magnetic Scattering at the Co K Absorption Edge

Indirect magnetic coupling between a ferromagnetic Fe layer and an antiferromagnetic CoO layer through a nonmagnetic Au layer was investigated with resonant X-ray magnetic scattering (RXMS) measure...

Voltage Control of Perpendicular Exchange Bias in Multiferroic Heterostructures

AbstractPerpendicular exchange bias (EB), which combines perpendicular magnetic anisotropy and ferromagnetic (FM)–anti‐ferromagnetic (AFM) exchange coupling, is extremely important in high‐density AFM spintronics. However, the effective modulation of EB remains challenging, since the alternant spins at the AFM/FM interface are strongly pinned by the AFM layer. Voltage tuning of EB through magnetoelectric coupling provides a potential way to achieve rapid magnetization switching in an energy‐efficient manner. Nevertheless, the interfacial strain mediation of perpendicular EB induced by E‐field remains unexplored. In this work, perpendicular EB nanostructure by room‐temperature fabrication process is obtained, and the voltage tunable perpendicular EB in Pt/IrMn/(Co/Pt)2/Ta/(011) Pb(Mg1/3Nb2/3)O3‐PbTiO3 multiferroic heterostructure is demonstrated. To enhance the voltage control effect on perpendicular EB, both strain‐mediated magnetoelectric coupling and ionic liquid gating method are further investigated in the thinned EB system with the structure of Pt/IrMn/Co/Pt/Ta. As a result, the voltage induced lattice distortion effectively transmits to the AFM/FM interface, while the charge accumulation and ion migrations in gating method generate a relatively large hysteresis loop offset that has not been observed before at room temperature. The voltage manipulation of perpendicular EB at room temperature provides new possibilities toward novel AFM devices and memories with great energy‐efficiency and ultrahigh density.

Linear Anisotropic Magnetoresistive Sensor Without Barber-Pole Electrodes

Traditional anisotropic magnetoresistive (AMR) sensors used barber-pole electrodes to rotate the current direction by 45° in order to obtain a linear electrical response. However, the adoption of barber-pole electrodes reduced the working area of the sensor. Moreover, the traditional AMR sensors need large-flipping stripes or permanent magnets for stabilization of the magnetization. Here, we report the AMR sensor using an antiferromagnetic layer to regulate the magnetization-current angle. The proposed sensor, without consuming extra area and power, could fully use the magnetoresistive area, and the stabilized magnetization could help in suppressing magnetic noise, enhancing stability, and improving linearity. The presented sensor has a stable response with a disturbing field of −1.5 T, while the commercial sensors need to be reset after being exposed to a small disturbing field of 5 Oe.

Noncollinear effective anisotropies in exchange coupled NiFe/IrMn bilayers

Abstract The influence of noncollinearity between unidirectional and uniaxial anisotropies on asymmetric magnetization reversal of NiFe/IrMn bilayers has been investigated. The experimental findings show that a noncollinear effective anisotropies originates from the competition among the applied magnetic field, magnetic anisotropy and exchange coupling, and the presence of magnetic frustration at NiFe/IrMn interfaces. Azimuthal measurements of the exchange bias suggest that the degree of effective interfacial anisotropies, which can be quantified by the angle ( γ ) between the uniaxial and unidirectional anisotropies, plays a crucial role in determining magnetization reversal. For a variable thickness of antiferromagnet layers, the experimental findings suggest a non-monotonic correlation between the magnitude of γ and the degree of interfacial roughness. Numerical analysis has been performed by considering the magnetic frustration present at the ferromagnet/antiferromagnet interfaces, suggesting a good agreement with experimental findings.

Effect of Dzyaloshinskii–Moriya Interaction Energy Confinement on Current‐Driven Dynamics of Skyrmions

Magnetic skyrmions, which are topological spin configuration, have gained interest in the past few years. However, skyrmions suffer from the skyrmion Hall effect, a phenomenon where skyrmions deflect from the path of the electron flow and annihilate at the edge of the track. There are attempts to overcome this effect using synthetic antiferromagnetic layer structures and ferrimagnetic layer structures. Herein, a new approach based on introducing Dzyaloshinskii–Moriya interaction (DMI) energy wells is reported to direct the motion of skyrmions in a nanowire. It is shown through simulations that by creating DMI energy wells with a critical DMI value, a skyrmion moves within the boundaries set by the energy wells. It is also shown that the diameter of a skyrmion can be reduced by decreasing the space between the DMI energy wells. The proposed DMI energy well enables packing of skyrmion tracks at a high density. In this view, the effect of magnetostatic interactions between multiple skyrmions on their motion in parallel tracks is also investigated. Furthermore, the effect of changing the distance between skyrmions on magnetostatic interactions is studied. These results offer a new path toward maneuvering the skyrmion motion for racetrack memory or logic devices.

Néel vector reorientation in ferromagnetic/antiferromagnetic complex oxide nanostructures

In this study, we report on a temperature-driven antiferromagnetic (AF) spin reorientation transition in micro- and nanostructures of AF/ferromagnetic (FM) LaFeO3/La0.7Sr0.3MnO3 thin film bilayers. Using a combination of x-ray photoemission electron microscopy and x-ray absorption spectroscopy, the Néel vector is shown to reorient 90° as a result of the competition between a shape-imposed anisotropy in the AF layer and interface coupling to the adjacent FM layer. We demonstrate how a temperature dependence of the AF/FM spin configuration in line-shaped nanomagnets can be tuned by variation of their linewidth. This work provides insight into the AF/FM interface exchange coupling in complex oxide heterostructures and the possibilities of spin control by nanostructuring in thin film spintronics.

Thickness effect on the easy axis distribution in exchange biased Co/IrMn bilayers

It is commonly assumed that in uncompensated FM/AF interfaces, the easy axes of antiferromagnetic grains are aligned toward the magnetic field direction applied during the film growth. However, due to several intrinsic and extrinsic factors, these easy axes can be aligned away from the unidirectional axis. In this context, the aim of this work is to study the relationship between the easy axes distribution (EAD) and thickness of both the ferromagnetic (FM) and antiferromagnetic (AF) layers in Co/IrMn bilayer systems. A phenomenological model for the free energy of the system that takes into account a domain wall formation in the AF layer and a Gaussian-type distribution of their easy axes is used. Thus, we simulate and fit the magnetization curves and get several magnetic parameters like rotatable anisotropy field, exchange bias field, coercivity and magnetic anisotropy. Our results show that when the AF thickness is increased, the width of the easy-axis distribution also increases following a power-law relationship. However, no correlation is observed when the FM thickness is varied. Furthermore, considering an easy-axis distribution we observe a significant reduction in coercivity as well as a slight decrease of the exchange bias field.

Temperature-Controlled Assembly/Reassembly of Two Dicarboxylate-Based Three-Dimensional Co(II) Coordination Polymers with an Antiferromagnetic Metallic Layer and a Ferromagnetic Metallic Chain.

Two new dicarboxylate-based three-dimensional cobalt coordination polymers, [Co(Me2mal)(bpe)0.5(H2O)]n (1) and [Co(Me2mal)(bpe)0.5]n (2), were synthesized from dimethylmalonic acid (H2-Me2mal) in temperature-controlled solvothermal reactions. Lower temperatures (60–80 °C) favored the formation of 1, while higher temperatures (120 °C) favored the production of 2. Compound 1 is comprised of Co(II) corrugated layers linked by syn–anti carboxylate bridges from the Me2mal2− ligands and pillared through bis-monodentate bpe groups. Compound 2 is comprised of a three-dimensional network involving one-dimensional Co–carboxylate chains bonded by antisymmetric µ4-Me2mal2− ligands and aligned parallel to the [001] direction. The solvothermal retreatment of crystalline samples of 1 in a DMF/H2O solvent at 120 °C allowed the structural reassembly, with complete conversion within 2 over 48 h. Magnetic analyses revealed that compound 1 exhibits both spin-orbital coupling and antiferromagnetic interactions through a syn–anti carboxylate (Me2mal2−) bridge exchange pathway [Co–Co separation of 5.478 Å] and compound 2 showed a ferromagnetic interaction resulting from the short Co–Co separation (3.150 Å) and the small Co–O–Co bridging angles (98.5° and 95.3°) exchange pathway which was provided by µ4-Me2mal2− bridging ligand.

Probing the Transfer of the Exchange Bias Effect by Polarized Neutron Reflectometry

The magnetic reversal behavior of a ferromagnet (FM) coupled through an FeMn antiferromagnet (AF) to a pinned ferromagnet has been investigated by polarized neutron reflectivity measurements. With FeMn as the AF layer it is found that there exists 90° interlayer coupling through this layer and that this plays a key role in the transfer of the exchange bias (EB) effect from the FM/AF interface to the AF/pinned-FM interface. Combined with Monte Carlo simulations, we demonstrate that the competition between the interlayer coupling and the anisotropy of the AF layer results in a control of the EB effect which has potential for device applications.

Observation of Interfacial Antiferromagnetic Coupling between Magnetic Topological Insulator and Antiferromagnetic Insulator.

Inducing magnetic orders in a topological insulator (TI) to break its time reversal symmetry has been predicted to reveal many exotic topological quantum phenomena. The manipulation of magnetic orders in a TI layer can play a key role in harnessing these quantum phenomena toward technological applications. Here we fabricated a thin magnetic TI film on an antiferromagnetic (AFM) insulator Cr2O3 layer and found that the magnetic moments of the magnetic TI layer and the surface spins of the Cr2O3 layers favor interfacial AFM coupling. Field cooling studies show a crossover from negative to positive exchange bias clarifying the competition between the interfacial AFM coupling energy and the Zeeman energy in the AFM insulator layer. The interfacial exchange coupling also enhances the Curie temperature of the magnetic TI layer. The unique interfacial AFM alignment in magnetic TI on AFM insulator heterostructures opens a new route toward manipulating the interplay between topological states and magnetic orders in spin-engineered heterostructures, facilitating the exploration of proof-of-concept TI-based spintronic and electronic devices with multifunctionality and low power consumption.

Low-Temperature Magnetization Switching of Bilayer FeNi/FeMn Films

A change in the kinetics of magnetization reversal of a bilayer ferromagnet–antiferromagnet film during a decrease in the temperature is experimentally detected. The motion of a zigzag domain wall is typical of high-temperature conditions, and the system undergoes magnetization reversal by irregular rotation at low temperatures. This phenomenon is found to occur only in a coupled system of layers with exchange anisotropy: it is absent when the antiferromagnetic layer is removed. The change in the character of magnetization reversal at low temperatures is related to the appearance of specific pinning of domain walls, which is caused by interface regions with a varying chirality of an interlayer exchange spring and a perpendicular magnetic anisotropy at interface steps.

Exchange Spring in a Heterostructure with Pd0.96Fe0.04 Low-Temperature Ferromagnet

The exchange bias of the magnetic hysteresis loop and the exchange-spring effect have been observed in thin-film heterostructures comprising layers of Pd0.96Fe0.04 low-temperature ferromagnet, metallic cobalt, and cobalt monoxide (CoO) grown on single-crystalline sapphire by methods of magnetron sputtering and molecular beam epitaxy in an ultrahigh-vacuum system. It is established that the exchange anisotropy of Pd0.96Fe0.04 layer in Pd0.96Fe0.04/CoO/Co/Al2O3 thin-film heterostructure is absent and the exchange-spring effect in this system is not manifested. In the case of a Pd0.96Fe0.04/Co/CoO/Al2O3 structure, it is shown that, according to the results of magnetometric measurements, the inversion of the order of antiferromagnetic and ferromagnetic layers leads to pinning of the magnetic moment of Pd0.96Fe0.04 layer at the interface with cobalt and to realization of the exchange-spring effect. Additional evidence in favor of this interpretation was obtained using the micromagnetic modeling of magnetic hysteresis loops.

Simulation of Strain-Assisted Switching in Synthetic Antiferromagnetic Free Layer-Based Magnetic Tunnel Junction

We perform macrospin simulations on a magnetic tunnel junction structure with perpendicular magnetic anisotropy, in which the conventional free layer is substituted with synthetic antiferromagnetic free layers to improve the thermal stability. In order to reduce the switching current Pb(Zr0.2Ti0.8)O3 material with (001) surface orientation is adopted, which is adjacent to the second free layer so that it applies biaxial stress to the interface under external voltage. We solve the Landau–Lifshitz–Gilbert–Slonczewski equation incorporating thermal and strain fields to describe the transient dynamics of magnetizations in both the free layers. The simulation results show a substantial reduction in switching current and significantly enhanced switching probability by modulating current and strain pulses.

A flexible exchange-biased spin valve for sensing strain direction

We have demonstrated that the strain direction can be detected sensitively using an exchange-biased spin valve (SV) formed on a flexible substrate. The flexible giant magnetoresistive SV used here contains a strain-sensitive Co free layer and a strain-insensitive exchange-biased NiFe pinned layer. The exchange bias is imparted from an antiferromagnetic FeMn layer stacked on the NiFe layer. When the strain is applied in a direction deviating from the exchange bias field, the free layer magnetization rotates along this directly only, resulting in a resistance change. Compared to the previously reported flexible pseudo SV, the present device allows strain direction sensing at a much smaller strain magnitude because no external magnetic field assistance is needed to fix the pinned layer magnetization, which hampers the rotation of the free layer magnetization. The robustness of the exchange bias to the strain in the simple NiFe/FeMn bilayer structure is also discussed.

Nonlinear Dynamics of an Antiferromagnetic Spintronic Oscillator

We study nonlinear dynamics of the spintronic nanosized antiferromagnetic terahertz oscillator consisting of an antiferromagnetic layer with easy-plane anisotropy (hematite) and a normal-metal (platinum) layer. Normal oscillation frequencies, namely, ferromagnetic and antiferromagnetic (terahertz) ones, are found. Their dependence on the value of a static magnetic field parallel to the sample plane is obtained. An approximate mathematical model in the form of the equations for the Neel-vector rotation angle in the azimuthal plane is developed for describing the oscillator dynamics. The adjustment characteristic, i.e., the dependence of the antiferromagnetic-mode frequency on the value of the direct current flowing in the platinum layer is obtained.

Role of anti-ferromagnetic layer thickness in exchange bias: case study of Co/CoO with Kerr microscopy

Exchange bias (HEB) and the magnetic training effect behavior of Co/CoO bilayer system with the different anti-ferromagnetic (AFM) layer thickness (tCoO = 5, 8.8 and 12.3 nm) is investigated by Kerr microscopy and it is found that the magnitude of HEB is more or less constant, but the training effect varies considerably with the AFM layer thickness. Moreover with increasing AFM layer thickness, HEB decreases more gradually with the consecutive cycling indicating that the AFM interface relax more slowly with the cycling suggesting that the increasing AFM layer thickness offer more stability to the interface coupling in conventional exchange bias systems. Kerr microscopy results reveals that there seems to be a deviation of AFM interface magnetization or pinning direction from its initial field cooling configuration for the lower AFM layer thickness, whereas for higher AFM thickness only change in domain state of AFM layer is evident with consecutive cycling.

Composition dependence of exchange anisotropy in PtxMn1−x/Co70Fe30 films

We systematically investigated the exchange anisotropy for ferromagnetic Co70Fe30 and antiferromagnetic Pt-Mn bilayer films. We focused on the relevance between the exchange bias and the composition of the PtxMn1−x (14 < x < 22 and 45 < x < 56 at%) films, and we successfully optimized the composition. The crystal structure of the PtxMn1−x films was FCC for 14 < x < 22 at% and FCT for 45 < x < 56 at% after annealing at 370 °C for 6 h. The unidirectional anisotropy constant (Jk) for fcc-Pt15Mn85 (20 nm) and fct-Pt48Mn52 (20 nm) prepared under optimum conditions in composition were 0.16 and 0.20 erg cm−2, respectively. Both Pt15Mn85 and Pt48Mn52 films showed a larger unidirectional anisotropy constant (Jk) than in other reports for the bilayer system that uses Pt-Mn antiferromagnetic layer. They also showed a flatter surface than that of other antiferromagnetic/ferromagnetic materials, which is suitable to avoid unexpected interlayer coupling in spin-valve structure. The obtained Pt-Mn films with a large exchange anisotropy and slight roughness are useful as an antiferromagnetic layer in spintronic applications.