Exchange Bias Field Research Articles

Polarity-Reversal of Exchange Bias in van der Waals FePS3/Fe3GaTe2 Heterostructures.

Exchange bias (EB) in antiferromagnetic (AFM)/ferromagnetic heterostructures is crucial for the advancement of spintronic devices and has attracted significant attention. The common EB effect in van der Waals heterostructures features a low blocking temperature (Tb) and a single polarity. In this work, a significant EB effect with a Tb up to 150K is observed in FePS3/Fe3GaTe2 heterostructures, and in particular, the EB exhibits an unusual temperature-dependent polarity-reversal behavior. Under a high positive field-cooling condition (e.g., μ0H ≥0.5T), a negative EB field (HEB) is observed at low temperatures, and with increasing temperature, the HEB crosses zero at ≈20K, subsequently becomes positive and later approaches zero again at Tb. A model composed of a top FePS3/interfacial FePS3/Fe3GaTe2 sandwich structure is proposed. The charge transfer from Fe3GaTe2 to FePS3 at the interface induces net magnetic moments (∆M) in FePS3. The interface favors AFM coupling, and thus the reversal of ∆M of the interfacial FePS3 leads to the polarity-reversal of EB. Moreover, the EB can be extended to the bare Fe3GaTe2 region of the Fe3GaTe2 flake partially covered by FePS3. This work provides opportunities for a deeper understanding of the EB effect and opens a new route toward constructing novel spintronic devices.

In situ observation of the structural transformations behind the emergence and disappearance of exchange bias in polycrystalline Ni-Mn/Fe20Ni80 films

The antiferromagnetic L10θ-NiMn compound has a Néel temperature of up to 800 ºC and is capable of providing the exchange bias effect in multilayered structures with adjacent ferromagnetic layers, such as Fe20Ni80, up to 350 ºC. The value of the exchange bias field can be non-decreasing up to 150 ºC, making Ni-Mn a compelling choice for applied devices based on the exchange bias effect. In this work we study the structural transformations occurring in magnetron-sputtered Ni-Mn/Fe20Ni80 films that first lead to the emergence of exchange bias in these structures, and then to its disappearance. Employing in situ X-ray diffractometry and vibrating-sample magnetometry we observe how under annealing the phases of Ni-Mn follow Ostwald’s rule of stages on their way to form the equilibrium θ-NiMn. Our findings suggest that at temperatures of around 350 ºC diffusion of iron from the ferromagnetic layer causes decomposition of θ-NiMn, ultimately resulting in the irreversible disappearance of exchange bias. Our results reveal the role of adjacent Fe20Ni80 layers in the transformations, and how crystal texture in the Ni-Mn layer affects exchange bias. The results of our work not only clarify the fundamental mechanisms but also provide valuable insights for engineering and utilizing of applied devices with Ni-Mn, making it of interest to material scientists and engineers of different specializations.

Surface weak ferromagnet coupling induced giant room-temperature spontaneous exchange bias in antiferromagnet Fe3BO6 polycrystals.

The spontaneous exchange bias effect (SEB) has wide application prospects in information storage technologies. In this study, nanoscale raw materials were used to fabricate antiferromagnetic Fe3BO6 polycrystals. The obtained Fe3BO6 exhibited a large SEB effect, where the value of the spontaneous exchange bias field at room temperature was as large as ∼4234 Oe. The room-temperature training effect, temperature-dependence, and maximum field-dependence of the HSEB were investigated. We propose that this giant SEB originates from the exchange-coupling interactions between the weak ferromagnetic surface state and the bulk antiferromagnetic state. The theoretical analysis results were further verified by comparing the magnetic properties of the Fe3BO6 with relatively low crystallinity. The results of this investigation will help find promising candidate materials for devices based on the SEB effect.

Non-exchange bias hysteresis loop shifts in dense composites of soft-hard magnetic nanoparticles: New possibilities for simple reference layers in magnetic devices

Exchange bias has been extensively studied in both exchange-coupled thin films and nanoparticle composite systems. However, the role of non-exchange mechanisms in the overall hysteresis loop bias is far from being understood. Here, dense soft-hard binary nanoparticle composites are used not only as a novel tool to unravel the effect of dipolar interactions on the hysteresis loop shift but also as a new strategy to enhance the bias of any magnet exhibiting an asymmetric magnetization reversal. Mixtures of equally sized, 6.8 nm, soft maghemite (γ-Fe2O3) nanoparticles (no bias—symmetric reversal) and hard cobalt doped γ-Fe2O3 nanoparticles (large exchange bias—asymmetric reversal) reveal that, for certain fractions of soft particles, the loop shift of the composite can be significantly larger than the exchange-bias field of the hard particles in the mixture. Simple calculations indicate how this emerging phenomenon can be further enhanced by optimizing the parameters of the hard particles (coercivity and loop asymmetry). In addition, the existence of a dipolar-induced loop shift (“dipolar bias”) is demonstrated both experimentally and theoretically, where, for example, a bias is induced in the initially unbiased γ-Fe2O3 nanoparticles due to the dipolar interaction with the exchange-biased hard nanoparticles. These results open a new paradigm in the large field of hysteresis bias and pave the way for novel approaches to tune loop shifts in magnetic hybrid systems beyond interface exchange coupling.

Robust Electric-Field Control of Colossal Exchange Bias in CrI3 Homotrilayer.

Controlling exchange bias (EB) by electric fields is crucial for next-generation magnetic random access memories and spintronics with ultralow energy consumption and ultrahigh speed. Multiferroic heterostructures have been traditionally used to electrically control EB and interfacial ferromagnetism through weak/indirect coupling between ferromagnetic and ferroelectric films. However, three major bottlenecks (lattice mismatch, interface defects, and weak/indirect coupling in multiferroic heterostructures) remain, resulting in only a few tens of milli-tesla EB field. Here, this study reports a robust electric-field control recipe to dynamically tailor the EB effect in a pure CrI3 homotrilayer on a ferroelectric Y-doped HfO2 (HYO) substrate, and demonstrate a colossal and tunable EB field (HE) from -0.15 to +0.33 T, giving rise to an EB modulation of 0.48 T. The charge doping due to ferroelectric HYO film divides a homo-configuration of CrI3 homotrilayer into one antiferromagnetic (AFM) bilayer CrI3 and one ferromagnetic (FM) monolayer CrI3, favoring direct exchange coupling. The synergies of charge doping and electric field induce a transition of magnetic orders from AFM to FM phase in bilayer CrI3, which is also supported by first-principles calculations, leading to the robust electric control of colossal EB effect. The results therefore open numerous opportunities for exploring 2D spintronics, memories, and braininspired in-memory computing.

Role of spin-glass-like interfaces in exchange-biased MnN/Fe thin films grown on W buffer layers

This study reports the growth and investigation of (001)-oriented MnN/Fe thin films on the W buffer layer, focusing on detailed exchange bias (EB) studies, including thermal evolution (300–10 K) and the training effect. At room temperature, the magnetically annealed α-W/MnN/Fe/Ta stack exhibits an EB field (HEB) of 118 Oe. With decreasing temperature, particularly below 100 K, both HEB and coercive field (HC) show substantial increases, with HEB displaying a more pronounced enhancement. Analysis of the temperature-dependent HEB and HC data reveals an exponential trend, indicative of a spin-glass-like interface in the MnN/Fe system. At 10 K, the pronounced EB is accompanied by an asymmetric “kinked” magnetization reversal, suggesting a transition from uniaxial to biaxial anisotropy below 50 K due to spin-glass-like magnetic frustration at the interdiffused MnN/Fe interface. Training effect measurements further support the spin-glass-like MnN/Fe interface, with two distinct training mechanisms observed at 10 K: “athermal” and “thermal.” Finally, the spin-glass model demonstrates an excellent fit for the training effect data, validating the presence of spin-glass-like disorder at the MnN/Fe interface.

Manipulation of Exchange Bias in Two-Dimensional van der Waals Ferromagnet Near Room Temperature.

As a host for exchange bias (EB), van der Waals (vdW) magnetic materials have exhibited intriguing and distinct functionalities from conventional magnetic materials. The EB in most vdW systems is far below room temperature, which poses a challenge for practical applications. Here, by using Kerr microscopy, we demonstrate a record-high blocking temperature that approaches room temperature and a huge positive EB field that nears 2 kOe at 100 K in naturally oxidized two-dimensional (2D) vdW ferromagnetic Fe3GaTe2 nanoflakes. Moreover, we realized a reversible manipulation of both the presence/absence and positive/negative signs of EB via a training magnetic field without multiple field cooling processes. Thus, our study clearly reveals the robust, sizable, and sign-tunable EB in vdW magnetic materials up to near room temperature, thereby establishing Fe3GaTe2 as an emerging room-temperature-operating vdW material and paving the way for designing practical 2D spintronic devices.

Flexible Exchange‐Biased Films with Superior Strain Stability

AbstractThe exchange bias (EB) effect plays an undisputed role in the state‐of‐art spintronic devices, both rigid and flexible. However, the poor stability of EB under mechanical strain is detrimental to the construction of reliable flexible spintronic devices, which poses a great challenge for potential wearable applications. Here, it is revealed that the strain‐induced variation of EB field can be greatly suppressed by engineering flexible IrMn/[Co/Pt]3 systems featured with perpendicular magnetic anisotropy (PMA). Particularly, in the multi‐stacks with strong PMA, poorly crystallized IrMn structure, and parallel alignment of the ferromagnetic (FM) moments with the biasing magnetic field applied during deposition, an exceptional good strain stability of EB among reported flexible systems is realized, which results from the stable perpendicular FM moments, the low strain transfer efficiency, and the eliminated strain‐driven training effect. These results provide an effective approach to develop flexible spintronic devices with superior strain stability, rendering them strong contenders in the realms of sensor and storage applications.

Tunable magnetic switching behavior in La0.7Sr0.3MnO3/PbTiO3 multilayers

Tuning magnetic switching behavior and related magnetic configuration of transition metal oxides (TMOs) is critical for realizing multifunctional materials and devices. In this study, we manipulated the magnetic switching behavior through interface engineering by introducing ferroelectric PbTiO3 (PTO) to form La0.7Sr0.3MnO3 (LSMO)/PTO multilayers. Both the normal hysteresis loop (NHL) and the inverted hysteresis loop (IHL) could be tuned by changing the PTO thickness. All multilayers displayed a transition from NHL to IHL with increasing temperature, and the transition temperature increased with the insertion of the PTO layer or increasing thickness of the PTO. In addition, it was found that the increase in the Curie temperature (Tc) and saturation magnetization (Ms), as well as the decrease in the coercive field (Hc) and exchange bias field (Heb) with a reduction in the thickness of the PTO, is possibly related to the interface phase. This study illustrates the influence of the ferroelectric PTO thickness on the magnetic switching behavior and related magnetic configuration in LSMO, which is possibly driven by the unscreened depolarization charge and oxygen octahedral tilting. This finding may benefit the application of TMOs in multifunctional memory devices.

Medium dynamic field range linear bipolar spin valve sensor through soft pinning the sensing layer

Magnetic sensor with spin valve-GMR technology with medium dynamic range is designed for a diversity of applications, including linear and rotary position measurements, proximity switches, and current sensors. For this, the sensing layer (SL) of the spin valve stack was modified by a soft pinning layer (SPL) through an exchange bias field created by an antiferromagnetic layer which has a lower blocking temperature than the one that is kept adjacent to the pinned layer. Numerical simulation was carried out to control the bias field by keeping a non-magnetic Ru spacer layer between the SPL and SL layers and the results were experimentally verified. The magnetic sensor was fabricated with linear operating field range of the order ±100 Oe having a sensitivity of the order of 0.1 m V V−1 Oe−1 near zero field. The thermal performance confirms that the device can be operated in the temperature range of −40 ∘C to 125 ∘C and it has a thermal coefficient of voltage around 15 µV V−1∘C−1.

The martensitic structures, exchange bias effect and training effect in Ni-Mn-Ga ribbons

The exchange bias effect is the physical basics of spin valves and magnetic recording devices. A stable exchange bias effect would be suitable for practical application. In this work, Ni50Mn50-xGax (11≤ x ≤16) ribbons were obtained by melt spinning. The grain size and microstructure have been tuned through melt spinning, which changes the exchange interaction in ribbons and thus gets a giant and stable exchange bias effect. As the increase of x, the structure of Ni50Mn50-xGax (11≤ x ≤16) ribbons undergoes a transition from non-modulated martensite to non-modulated martensite &10 layers modulated martensite to 10 layers modulated martensite. The average grain size of all ribbons is less than 10 μm, and the martensitic plate widths are smaller than 200 nm. With the increasing x or the cooling fields, the magnetization of the ribbons has been enhanced. As a result, the competition between ferromagnetic and antiferromagnetic interactions has been changed and thus influencing spin glass behavior. Furthermore, a low degree of attenuation for the exchange bias effect after continuous cycles has been achieved which arises from the stable spin configuration with a larger contribution of frozen spin which has a slow relaxation rate. For example, the exchange bias field of 8.78 kOe in Ni50Mn38Ga12 ribbons can be achieved under the 40 kOe field cooling at 10 K, and the exchange bias field after infinite cycles can be kept at 8.58 kOe, the degree of attenuation is only 3.4 %. This work enriches the research on the exchange bias effect of Heusler alloys and provides the experimental basis and theoretical support for designing a stable exchange bias effect.

Asymmetric Manipulation of Perpendicular Exchange Bias and Programmable Spin Logical Cells by Spin-Orbit Torque in a Ferromagnet/Antiferromagnet System.

Antiferromagnets are competitive candidates for the next generation of spintronic devices owing to their superiority in small-scale and low-power-consumption devices. The electrical manipulation of the magnetization and exchange bias (EB) driven by spin-orbit torque (SOT) in ferromagnetic (FM)/antiferromagnetic (AFM) systems has become focused in spintronics. Here, the realization of a large perpendicular EB field in Co/IrMn and the effective manipulation of the magnetic moments of the magnetic Co layer and EB field by SOT in Pt/Co/IrMn system is reported. During the SOT-driven switching process, an asymmetrically manipulated state is observed. Current pulses with the same amplitude but opposite directions induce different magnetization states. Magneto-optical Kerr measurements reveal that this is due to the coexistence of stable and metastable antiferromagnetic domains in the AFM. Exploiting the asymmetric properties of these FM/AFM structures, five spin logic gates, namely AND, OR, NOR, NAND, and NOT, are realized in a single cell via SOT. This study provides an insight into the special ability of SOT on AFMs and also paves an avenue to construct the logic-in-memory and neuromorphic computing cells based on the AFM spintronic system.

Strain-Induced Robust Exchange Bias Effect in Epitaxial La0.7Sr0.3MnO3/LaFeO3 Bilayers.

The ground state of correlated electrons in complex oxide films can be controlled by applying epitaxial strain, offering the potential to produce unexpected phenomena applicable to modern spintronic devices. In this study, we demonstrate that substrate-induced strain strongly affects the coupling mode of interfacial magnetic moments in a ferromagnetic (FM)/antiferromagnetic (AFM) system. In an epitaxial bilayer comprising AFM LaFeO3 (LFO) and FM La0.7Sr0.3MnO3 (LSMO), samples grown on a LaAlO3 (LAO) substrate exhibit a larger exchange bias field than those grown on a SrTiO3 substrate. Our results indicate a transition in the alignment of magnetic moments from perpendicular to collinear due to the large compressive strain exerted by the LAO substrate. Collinear magnetic moments at the LSMO/LFO interface generate strong exchange coupling, leading to a considerable exchange bias effect. Thus, our findings provide a method for tailoring and manipulating the orientations of magnetic moments at the FM/AFM heterogeneous interface using strain engineering, thereby augmenting methods for exchange bias generation.

Exchange bias in heterostructures combining magnetic topological insulator MnBi2Te4 and metallic ferromagnet Fe3GeTe2

Introducing magnetism into topological insulators enables exotic phenomena such as quantum anomalous Hall effect. By fabricating van der Waals (vdW) heterostructures using layered magnetic materials, we can not only induce a gap in the non-magnetic topological surface states through magnetic proximity but also further manipulate the magnetic properties of magnetic topological insulators. However, the scarcity of 2D ferromagnetic insulator materials limits the fabrication of such heterostructures. Here, we demonstrate the vdW heterostructure devices comprising metal ferromagnetic Fe3GeTe2 nanoflakes and few-layer antiferromagnetic topological insulator MnBi2Te4 separated by an insulating hexagonal-boron nitride spacer. These devices exhibit significant exchange bias with the exchange bias field of over 100 mT under certain conditions. Our results prove that besides magnetic insulators, metallic magnets can also effectively adjust the magnetic properties of topological insulators, thereby inspiring diverse configurations of the heterostructures between topological insulators and magnetic materials.

Insights into a Defective Potassium Sulfido Cobaltate: Giant Magnetic Exchange Bias, Ionic Conductivity, and Electrical Permittivity

AbstractThe novel potassium sulfido cobaltate, K2[Co3S4] is introduced, with 25% vacancies of the cobalt positions within a layered anionic sublattice. The impedance and dielectric investigations indicate a remarkable ionic conductivity of 21.4 mS cm−1 at room temperature, which is in the range of highest ever reported values for potassium‐ions, as well as a high electrical permittivity of 2650 at 1 kHz, respectively. Magnetometry results indicate an antiferromagnetic structure with giant intrinsic exchange bias fields of 0.432 and 0.161 T at 3 and 20 K respectively, potentially induced by a combination of the interfacial effect of combined magnetic anionic and nonmagnetic cationic sublattices, as well as partial spin canting. The stability of the exchange bias behavior is confirmed by a training effect of less than 18% upon 10 hysteresis cycles. The semiconductivity of the material is determined, both experimentally and theoretically, with a bandgap energy of 1.68 eV. The findings render this material as a promising candidate for both, active electrode material in potassium‐ion batteries, and for spintronic applications.

Different exchange bias behavior caused by tuning glassy state in Ni50Mn37Ga13 ribbons

Exchange bias plays a significant role in the application of spin valves and magnetic sensors. A spontaneous exchange bias field of 0.75 kOe can be observed in the annealed Ni50Mn37Ga13 ribbons, while a giant exchange bias field of 6.14 kOe occurs in the quenched samples under field cooling. This is due to the different lattice parameters and microscopic morphology making the annealed and quenched samples, respectively, form superspin glass and spin glass embedded in the antiferromagnetic matrix. The conditions for superspin glass and spin glass to overcome the energy barrier of spin reconfiguration are different, resulting in the difference in exchange bias behavior. This work combines the magnetic properties with the crystal structure and microscopic morphology, providing a method of controlling the glassy state by heat treatments to regulate the exchange bias of Heusler alloys.

Dependence of Magnetic Properties of As-Prepared Nanocrystalline Ni2MnGa Glass-Coated Microwires on the Geometrical Aspect Ratio.

We have prepared NiMnGa glass-coated microwires with different geometrical aspect ratios, ρ = dmetal/Dtotal (dmetal-diameter of metallic nucleus, and Dtotal-total diameter). The structure and magnetic properties are investigated in a wide range of temperatures and magnetic fields. The XRD analysis illustrates stable microstructure in the range of ρ from 0.25 to 0.60. The estimations of average grain size and crystalline phase content evidence a remarkable variation as the ρ-ratio sweeps from 0.25 to 0.60. Thus, the microwires with the lowest aspect ratio, i.e., ρ = 0.25, show the smallest average grain size and the highest crystalline phase content. This change in the microstructural properties correlates with dramatic changes in the magnetic properties. Hence, the sample with the lowest ρ-ratio exhibits an extremely high value of the coercivity, Hc, compared to the value for the sample with the largest ρ-ratio (2989 Oe and 10 Oe, respectively, i.e., almost 300 times higher). In addition, a similar trend is observed for the spontaneous exchange bias phenomena, with an exchange bias field, Hex, of 120 Oe for the sample with ρ = 0.25 compared to a Hex = 12.5 Oe for the sample with ρ = 0.60. However, the thermomagnetic curves (field-cooled-FC and field-heating-FH) show similar magnetic behavior for all the samples. Meanwhile, FC and FH curves measured at low magnetic fields show negative values for ρ = 0.25, whereas positive values are found for the other samples. The obtained results illustrate the substantial effect of the internal stresses on microstructure and magnetic properties, which leads to magnetic hardening of samples with low aspect ratio.

Modeling of the effect of the thickness of the antiferromagnetic layer on the energy of interfacial exchange interaction in Ferromagnetic/Antiferromagnet films

By means of thermodynamic and configuration averaging of the energy of the exchange interaction between the magnetic spin moments adjacent through the ferromagnetic/antiferromagnetic interphase boundary, the expression for the energy of the interphase exchange interaction Eex is obtained. The dependence of the energy Eex on the temperature and the characteristic dimensions of the ferromagnetic/antiferromagnetic two-phase film is modeled. The experimentally observed regularity of the growth of the energy of the interfacial exchange interaction due to the increase of the thickness of the antiferromagnetic layer is confirmed. A method for the estimation of the constants of the interfacial exchange interaction Jint and the exchange interaction in antiferromagnetics JAFM is proposed based on the agreement of the experimental and theoretical curves of the temperature dependence of the exchange bias field HebT.

Enhancement of Magnetic Stability in Antiferromagnetic CoO Films by Adsorption of Organic Molecules.

Antiferromagnets are a class of magnetic materials of great interest in spintronic devices because of their stability and ultrafast dynamics. When interfaced with an organic molecular layer, antiferromagnetic (AF) films are expected to form a spinterface that can allow fine control of specific AF properties. In this paper, we investigate spinterface effects on CoO, an AF oxide. To access the magnetic state of the antiferromagnet, we couple it to a ferromagnetic Co film via an exchange bias (EB) effect. In this way, the formation of a spinterface is detected through changes induced on the CoO/Co EB system. We demonstrate that C60 and Gaq3 adsorption on CoO shifts its blocking temperature; in turn, an increase in both the EB fields and the coercivities is observed on the EB-coupled Co layer. Ab initio calculations for the CoO/C60 interface indicate that the molecular adsorption is responsible for a charge redistribution on the CoO layer that alters the occupation of the d orbitals of Co atoms and, to a smaller extent, the p orbitals of oxygen. As a result, the AF coupling between Co atoms in the CoO is enhanced. Considering the granular nature of CoO, a larger AF stability upon molecular adsorption is then associated with a larger number of AF grains that are stable upon reversal of the Co layer.

Programmable Racetrack for Magnetic Domain Wall Motion via Local Tuning of Exchange Biased Field

AbstractTo overcome memory bottleneck issues in memory‐centric chip technologies, in‐memory computing has been considered an alternative route involving simultaneous data storage and computing in a memory network. In the context of spintronics, a memory‐in‐logic device based on spin‐transfer‐torque or spin‐orbit‐torque magnetoresistive random‐access memory, and a magnetic domain wall (DW) racetrack has been studied. To expand the functionalities of a conventional magnetic DW racetrack, the study devises a reprogrammable exchange‐biased DW racetrack with local engineering of the exchange bias field (HE) in continuous magnetic films without requiring a lithography process for specific patterning of the films. Furthermore, current‐driven and field‐driven DW motion along the exchange‐biased racetrack is demonstrated. Additionally, within the route of the locally different exchange‐biased racetrack, a gate function can be performed to guide or stop DW motion by locally tuning HE. The complex maze racetrack is rewritable, and multiple input channels can be controlled.