Exchange Bias Field Research Articles

Exchange Bias and Vertical Magnetic Shift on Co−CoO Thin Films

AbstractThis work reports the results of an experimental study on Si(001)/Au(111)/Co/CoO/ZrO2 elaborated by electron beam evaporation. The chemical composition of the oxidized state of cobalt and zirconium cover layers derived from metallic deposition is confirmed by X‐ray photoelectron spectroscopy (XPS). Magnetic properties are probed by magneto‐optical Kerr effect (MOKE). The polar magneto‐optic Kerr effect (PMOKE) measurements reveal that the cooling at 77 K under an applied magnetic field induces an exchange coupling which is manifested by a positive exchange bias (EB) field accompanied by a negative vertical shift of the magnetization. In longitudinal magneto‐optical Kerr effect (LMOKE), the exchange coupling depends on the orientation of the applied measuring magnetic field. It is found that both the exchange field shift rate hshift and the vertical shift rate in magnetization mshift exhibit a peak at 45° between the applied field and the [100] in‐plane direction which may be associated with the presence of uncompensated pinned spins at the Co−CoO interface.

The exchange bias effect and Griffiths phase in La1.5Sr0.5Co1-xNixMnO6: The impact of the divalent Ni

The effects of Co-site doping of Ni2+ ions on the crystal structure, electronic structure, and magnetic properties of La1.5Sr0.5CoMnO6 are investigated. The doping causes an increase in the anti-site disorder and a change in the content of Co2+/3+ and Mn3+/4+, as well as a change in the spin state of Co ions. Ni doping introduces Ni2+-O2--Mn4+ ferromagnetic interactions to increase the ferromagnetic transition temperature. Samples display different dominant clusters, and field cooling promotes the presence of ferromagnetic clusters, allowing an increase in the Griffiths phase. Interestingly, Ni doping enables tuning the conventional and spontaneous exchange bias (EB) effects, increasing the EB field for intermediate concentrations. Moreover, the maxima of both EB fields do not occur in the same sample, which leads to a new understanding of the relationship between these two effects. The possible mechanisms related to the uncompensated spin and competition between ferromagnetic and antiferromagnetic clusters are discussed, respectively.

Electrical Control of Spin Hall Efficiency and Field-Free Magnetization Switching in W/Pt/Co/Pt Heterostructures with Competing Spin Currents.

Reversal of magnetization via current-induced spin-orbit torque (SOT) is one of the core issues in spintronics. However, an in-plane assistant field is usually required for the deterministic switching of a perpendicularly magnetized system. Additionally, the efficiency of SOT is low, which is detrimental to device applications. This study achieved a reversible and non-volatile control of the critical current for magnetization switching and spin Hall efficiency in the TaN/W/Pt/Co/Pt/TaN heterostructures by ionic liquid (IL) gating-induced hydrogen ion adsorption and desorption in the upper Pt layer. Furthermore, the thinning of the Pt and TaN capping layers activated the oxygen ion migration toward the Co layer under IL gating, resulting in an exchange bias field and allowing field-free magnetization switching and Boolean logic operation. The results of this study offer an intriguing opportunity to promote the development of SOT-based spintronic devices from the perspective of iontronics with low energy dissipation.

Roles of magnetic coupling and spin-orbit torque in the electrical manipulation of exchange bias in a Pt/Co/IrMn heterostructure

Heavy-metal/ferromagnet/antiferromagnet heterostructures have attracted significant attention in manipulating antiferromagnetic ordering electrically. We report electric control of the exchange bias field along both the out-of-plane and in-plane directions in Pt/Co/IrMn with assist magnetic fields. Experiments with different directions and amplitudes of the assist magnetic field show the role of magnetic coupling between ferromagnet and antiferromagnet spins. Furthermore, we find that the magnitude and direction of the exchange bias can be manipulated by the magnitude of the current pulse. The observed results can be explained based on a numerical macrospin model; the antiferromagnetic spins are excited and precessed from their ground state via the spin-orbit torque-driven oscillation and are reoriented, being mediated by magnetic coupling with the ferromagnet. Our results provide an approach to achieve electrical omnidirectional exchange bias manipulation and an understanding of the spin dynamics at the antiferromagnet-ferromagnet interface.

Exploration of low field magnetic states in NdCe x CrO3

We report the structural and magnetic properties of NdCe x CrO3 () single-phase samples to classify the influence of Ce substitution on the Nd-site. The electron density profile indicates the possible covalent nature of Cr–O bonds. The x-ray photoelectron spectroscopy confirms a mixed Ce valency with a constant ratio of Ce3+/Ce4+ ions in all substituted compounds and the charge neutralization through the oxygen vacancies. The magnetization measurements reveal an increase in antiferromagnetic ordering temperature () and spin-reorientation transition temperature () and unfold soft spin-reorientation attributed to diluted superexchange interactions upon Ce incorporation. The presence of mixed Ce ions induces the merging of the hysteresis loop with a significant exchange bias (EB) field. We demonstrate for the first time that the magnitude of the magnetization is different for the same applied field in positive and negative directions, indicating the existence of two different magnetic states. The difference between these two magnetic states possibly arises from the pinning of Cr3+ spins, which requires an additional Zeeman energy for it to rotate. This maximum Zeeman energy from the normalized magnetic susceptibility vs. temperature curves correlates with the maximum EB field, validating unusual EB in these compounds.

Magnetism and Exchange Bias Properties in Ba2ScRuO6

This paper presents structural, detailed magnetic, and exchange bias studies in polycrystalline Ba2ScRuO6 synthesized at ambient pressure. In contrast to its strontium analogue, this material crystallizes in a 6L hexagonal structure with space group P3¯m1. The Rietveld refinement using the room-temperature powder XRD pattern suggests a Ru-Sc disorder in the structure. The temperature variation of the DC electrical resistivity highlights a semiconducting behavior with the electron conduction corresponding to Mott’s 3D variable range hopping (VRH) model. The detailed magnetization measurements show that Ba2ScRuO6 develops antiferromagnetic ordering at TN≈ 9 K. Interestingly, below 9 K (TN), the field-cooled magnetic field variation (FC) of the magnetization curves highlights an exchange bias effect in the sample. The exchange bias field reaches a maximum value of 1.24 kOe at 2 K. The exchange bias effect below the magnetic ordering temperature can be attributed to the inhomogeneous magnetic correlations due to the disorder in the structure. Remarkably, the appearance of a large exchange bias field in Ba2ScRuO6 indicates that inhomogeneous hexagonal double perovskites are a promising class to explore new materials having potential applications in spintronics.

Exchange biased surface acoustic wave magnetic field sensors

Magnetoelastic composites which use surface acoustic waves show great potential as sensors of low frequency and very low amplitude magnetic fields. While these sensors already provide adequate frequency bandwidth for most applications, their detectability has found its limitation in the low frequency noise generated by the magnetoelastic film. Amongst other contributions, this noise is closely connected to domain wall activity evoked by the strain from the acoustic waves propagating through the film. A successful method to reduce the presence of domain walls is to couple the ferromagnetic material with an antiferromagnetic material across their interface and therefore induce an exchange bias. In this work we demonstrate the application of a top pinning exchange bias stack consisting of ferromagnetic layers of (Fe90Co10)78Si12B10 and Ni81Fe19 coupled to an antiferromagnetic Mn80Ir20 layer. Stray field closure and hence prevention of magnetic edge domain formation is achieved by an antiparallel biasing of two consecutive exchange bias stacks. The set antiparallel alignment of magnetization provides single domain states over the complete films. This results in a reduction of magnetic phase noise and therefore provides limits of detection as low as 28 pT/Hz1/2 at 10 Hz and 10 pT/Hz1/2 at 100 Hz.

Flexible Manipulation of Operating Range in Giant Magnetoresistance Unipolar Switch Based on Double-Pinned Spin Valve

Giant magnetoresistance (GMR) magnetic switches have been widely investigated due to their attractive advantages such as good stability, small size, large saturation field, and high integration. However, in currently available commercial GMR unipolar switches, the operation and release points are limited to a narrow range. In this work, a modified double pinned spin valve (SV) structure is proposed to achieve a wide operating range and easy modulation of magnetic unipolar switches. In particular, a modified SV structure of Ta/NiFe/IrMn/CoFe/Ru/CoFe/Cu/CoFe/IrMn/Ta was chosen, where the large and small exchange bias fields were obtained from the bottom synthetic antiferromagnetic (SAF) structure and the top CoFe/IrMn layers, respectively. Accordingly, the operating range can be easily modulated by adjusting the thickness of top pinned CoFe layer. It is worthwhile to note that the controllable range exceeds 100Oe, which is much larger than that exhibited by current commercialized SV structure. Furthermore, in the patterned sample having the modified double pinned SV, the operation and release points are independent of length and width. The reason is attributed to the exchange bias field, which weakens the impact of shape anisotropy. In addition, this result also facilitates the simplification of the design process. That is to say, the proposed modified double pinned SV offers a broad application region for GMR magnetic unipolar switches and reveals a new paradigm for the development of next-generation magnetic sensors.

Manipulating exchange bias in 2D magnetic heterojunction for high-performance robust memory applications

The exchange bias (EB) effect plays an undisputed role in the development of highly sensitive, robust, and high-density spintronic devices in magnetic data storage. However, the weak EB field, low blocking temperature, as well as the lack of modulation methods, seriously limit the application of EB in van der Waals (vdW) spintronic devices. Here, we utilized pressure engineering to tune the vdW spacing of the two-dimensional (2D) FePSe3/Fe3GeTe2 heterostructures. The EB field (HEB, from 29.2 mT to 111.2 mT) and blocking temperature (Tb, from 20 K to 110 K) are significantly enhanced, and a highly sensitive and robust spin valve is demonstrated. Interestingly, this enhancement of the EB effect was extended to exposed Fe3GeTe2, due to the single-domain nature of Fe3GeTe2. Our findings provide opportunities for the producing, exploring, and tuning of magnetic vdW heterostructures with strong interlayer coupling, thereby enabling customized 2D spintronic devices in the future.

Structural and magnetic characteristics of NiO/NiFe2O4/α-Fe2O3 nanocomposite

Effect of Fe concentration in forming NiO/NiFe2O4/α-Fe2O3 nanocomposite (NC) was probed through structural and magnetic characterizations. Presence of two-magnon mode in Raman spectra confirms the existence of NiFe2O4 and α-Fe2O3 phases in the background of antiferromagnetic (AFM) NiO matrix. Magnetic study further confirms the occurrence of weak ferromagnetism in the AFM background. The behaviour of temperature dependence of saturation magnetization revealed that the samples exhibit some type of inhomogeneity. The NC samples show robust exchange bias characteristics at room temperature. The occurrence of ferromagnetic like feature in the NC samples could be due to the combine effect of the formation of BMP along with the magnetic inhomogeneity. The samples show the persistence of robust exchange bias field at 300K. The amplification of anisotropic constant with higher Fe concentration in the NC sample could be ideal for device applications like magnetic recording media.

The spin polarization of palladium on magneto-electric Cr2O3

While induced spin polarization of a palladium (Pd) overlayer on antiferromagnetic and magneto-electric Cr2O3(0001) is possible because of the boundary polarization at the Cr2O3(0001), in the single domain state, the Pd thin film appears to be ferromagnetic on its own, likely as a result of strain. In the conduction band, we find the experimental evidence of ferromagnetic spin polarized in Pd thin films on a Cr2O3(0001) single crystal, especially in the thin limit, Pd thickness of around 1–4 nm. Indeed there is significant spin polarization in 10 Å thick Pd films on Cr2O3(0001) at 310 K, i.e. above the Néel temperature of bulk Cr2O3. While Cr2O3(0001) has surface moments that tend to align along the surface normal, for Pd on Cr2O3, the spin polarization contains an in-plane component. Strain in the Pd adlayer on Cr2O3(0001) appears correlated to the spin polarization measured in spin polarized inverse photoemission spectroscopy. Further evidence for magnetization of Pd on Cr2O3 is provided by measurement of the exchange bias fields in Cr2O3/Pd(buffer)/[Co/Pd] n exchange bias systems. The magnitude of the exchange bias field is, over a wide temperature range, virtually unaffected by the Pd thickness variation between 1 and 2 nm.

Acoustically assisted energy efficient field free spin orbit torque switching of out of plane nanomagnet

Deterministic spin orbit torque (SOT) magnetization switching of the perpendicular magnetic anisotropy structures requires an external magnetic field, which is unsuitable for on-chip applications. Various approaches are there to debar the external magnetic field requirement. In this work, we use static stress, which can generate a virtual magnetic field via magnetoelastic coupling. We show that this field can be used for deterministic magnetization switching. In our simulations, we use an antiferromagnetic material for generating the SOT and exchange bias field and avail the benefits of the field like torque (FLT). With the exclusion of thermal noise, this reduces the threshold current density from 114 to 36 MA/cm2, thus mitigating the energy dissipation by more than nine times. To study the effect of thermal noise, we simulate 500 trajectories and find out the switching probability. We perform simulations to study the effects of current density, stress, pulse width, and FLT on the switching probability in great details. Our simulation results show that we can get sub-ns switching with a probability of 0.99 requiring only 45.5 fJ of energy dissipation. This can be very attractive for artificial neural network kind of applications.

Giant Exchange-Bias-Like Effect at Low Cooling Fields Induced by Pinned Magnetic Domains in Y2 NiIrO6 Double Perovskite.

Exchange bias (EB) is highly desirable for widespread technologies. Generally, conventional exchange-bias heterojunctions require excessively large cooling fields for sufficient bias fields, which are generated by pinned spins at the interface of ferromagnetic and antiferromagnetic layers. It is crucial for applicability to obtain considerable exchange-bias fields with minimum cooling fields. Here, an exchange-bias-like effect is reported in a double perovskite, Y2 NiIrO6 , which shows long-range ferrimagnetic ordering below 192 K. It displays a giant bias-like field of 1.1 T with a cooling field of only 15Oe at 5 K. This robust phenomenon appears below 170 K. This fascinating bias-like effect is the secondary effect of the vertical shifts of the magnetic loops, which is attributed to the pinned magnetic domains due to the combination of strong spin-orbit coupling on Ir, and antiferromagnetically coupled Ni- and Ir-sublattices. The pinned moments in Y2 NiIrO6 are present throughout the full volume, not just at the interface as in conventional bilayer systems.

Substitutional effect of Mg2+ on structural and magnetic properties of La2MgxNi1‐xMnO6 double‐perovskite thin films

AbstractBy doping different concentrations of Mg2+ at Ni site, the (00l)‐oriented La2MgxNi1‐xMnO6 (abbreviated as LMxNMO, x = 0, 0.1, 0.2, 0.3, 0.4) double‐perovskite thin films were epitaxially grown by pulsed laser deposition. The substitutional effect of Mg2+ on the structural and magnetic properties of the films is comprehensively investigated. It is found that with the increase of Mg‐doping concentration, the in‐plane and out‐of‐plane lattice constants as well as the cell volume of the LMxNMO thin films increase, which could be ascribed to the radius difference between Mg2+ and Ni2+/Ni3+ ions, resulting in the in‐plane compressive stress in LMxNMO films. When the Mg‐doping concentration is small (x ≤ 0.1), the doped Mg2+ tends to substitute Ni3+, which restrains the intensity of antiferromagnetic interaction between Ni3+‐O‐Mn3+, resulting in the reduced the exchange bias field as well as the increased the saturation magnetization. However, when the Mg‐doping concentration increases to x ≥ 0.2, Mg2+ becomes to mainly replace Ni2+ position, which could inhibit the super‐exchange ferromagnetic interaction between Ni2+‐O‐Mn4+ magnetic paths and thus reduce the saturation magnetization. The enhanced magnetic properties can be obtained in the LM0.1NMO double‐perovskite thin film, with a large saturation magnetization of 492.12 emu/cm3 and a high Curie temperature of 262.7 K.

Giant exchange bias by tuning Co/CoO core/shell structure

In this report, the exchange-bias effect of Co/CoO core/shell nanoparticles with the amorphous CoO shell has been investigated. At low temperatures, the developed spin-glass state with net magnetic moments in the CoO shell provides a unidirectional anisotropy in the Co core via exchange coupling, leading to a giant exchange bias field up to 3.6 kOe. By improving the crystallinity of CoO shell by performing surface oxidation at an elevated temperature, the spin-glass ordering is suppressed by the antiferromagnetic ordering, resulting in a significantly lower exchange-bias field (2.2 kOe) and a prominent training effect due to the existence of multiple easy axes in the multi-crystalline CoO shell. Theoretical studies confirm that the emerging spin-glass state in the CoO shell has a significant impact on the exchange bias of the system. These new findings provide a novel approach toward manipulating and fine-tuning the exchange bias effects in nanoscale systems for spintronics applications.

Giant and Nonvolatile Control of Exchange Bias in Fe3GeTe2/Irradiated Fe3GeTe2/MgO Heterostructure Through Ultralow Voltage

AbstractThe discovery of van der Waals magnets has provided a new platform for the electrical control of magnetism. Recent experiments have demonstrated that the magnetic properties of van der Waals magnets can be tuned by various gate modulations, although most of them are volatile and require gate voltages no lower than several volts. Here, the realization of nonvolatile control of exchange bias and coercive fields in Fe3GeTe2/MgO heterostructures, and the gate voltage is as low as tens of mV which is two orders of magnitude smaller than those in previous experiments is presented. The discovery of an ionic‐irradiated phase formed in Fe3GeTe2 by MgO sputtering revealed that an exchange bias effect can be obtained in this heterostructure and tuned from ≈700 to 0 Oe through voltages ranging from 5 to 20 mV. Owing to the high stability of oxidized Fe3GeTe2, the voltage‐driven oxygen incorporated into Fe3GeTe2 from the irradiated phase induces a nonvolatile magnetism modulation that can be retained after turning off the gate voltage. These findings demonstrate a methodology to modulate the magnetism of van der Waals magnets, opening new opportunities to fabricate all‐solid, long‐retention, and low‐dissipation nano‐electronic devices using van der Waals materials.

Manipulation of the Martensitic Transformation and Exchange Bias Effect in the Ni45Co5Mn37In13 Ferromagnetic Shape Memory Alloy Films

The martensitic phase transition and exchange bias effect of the Ni-Mn-based ferromagnetic shape memory alloys (FSMAs) Ni45Co5Mn37In13 (Ni-Co-Mn-In) films are investigated in this paper. The martensitic transformation properties of the Ni-Co-Mn-In alloy target material are manipulated by the process of electric arc melting, melt-fast quenching, and high-temperature thermal pressure. The Ni-Co-Mn-In alloy films with martensite phase transition characteristics are obtained by adjusting deposition parameters on the (001) MgO substrate, which shows a significant exchange bias (EB) effect at different temperatures. With increasing sputtering power and time, the film thickness increases, resulting in a gradual relaxation of the constraints at the interface between the film and the substrate (the interfacial strain decreases as the increase of thin film thickness), which promotes the martensite phase transition. Between zero-field cooling (ZFC) and field-cooled (FC) curve obvious division zone, the decrease of exchange bias field (HEB) and coercive force field (Hc) with an increase in test temperature is due to ferromagnetic (FM) interaction begins to dominate, resulting in a reduction of antiferromagnetic (AFM) anisotropy at the interface. The maximal HEB and Hc reach ~465.7 Oe and ~306.9 Oe at 5 K, respectively. The manipulation of the martensitic transformation and EB effect of the Ni-Co-Mn-In alloy films demonstrates potential application in the field of information and spintronics.

Large exchange bias in Mn-Ni-Sn Heusler alloys: Role of cluster spin glass state

We report a combined experimental and theoretical study of the exchange bias (EB), structural and magnetic properties of manganese-rich ${\mathrm{Mn}}_{50}{\mathrm{Ni}}_{41.5+x}{\mathrm{Sn}}_{8.5\ensuremath{-}x}$ $(x\phantom{\rule{4pt}{0ex}}=0, 0.75, \text{and} 1)$ Heusler alloys. All these alloys crystallize in a tetragonal structure at room temperature. Ab initio simulation using density functional theory (DFT) on off-stoichiometric ${\mathrm{Mn}}_{50}{\mathrm{Ni}}_{42.1875}{\mathrm{Sn}}_{7.8125}$ alloy (very close to the actual composition) predicts the tetragonal structure with the ferrimagnetic arrangement as the ground state, in good agreement with the experimental results. These alloys are found to exhibit a large exchange bias at low temperatures, with a maximum EB field of 7.1 kOe at 2 K for $x\phantom{\rule{4pt}{0ex}}=1$, which is significantly larger than that reported for any Mn-Ni--based Heusler systems. Frequency dependence of spin freezing temperature obeying the scaling law confirms the presence of a cluster spin glass (CSG)--like state in these alloys. The latter is also confirmed from the memory and aging effect studies in zero-field cooling and field-cooled protocols for these alloys. The large EB is attributed to the strong exchange coupling between the CSG clusters embedded in an otherwise ferrimagnetic matrix. The exchange mechanism behind the large EB effect is discussed in the light of varying bond lengths between different magnetic pairs, and hence the magnetic coupling, due to the large tetragonal distortion. The present experimental findings are found to be in good agreement with the DFT results.

Gate-induced switching of perpendicular exchange bias with very low coercivity in Pt/Co/Ir/Cr2O3/Pt epitaxial film

We investigate magnetoelectric switching of perpendicular exchange bias with very low coercivity in a Pt/Co/Ir/Cr2O3/Pt epitaxial film. We also optimize the suitable Ir spacer thickness so that the film exhibits the perpendicular exchange bias greater than the coercivity up to the vicinity of the Néel temperature. Main impact of the Ir spacer layer is the significant reduction of coercivity less than 0.5 mT in maintaining both the perpendicular magnetic anisotropy and the perpendicular exchange bias. For the suitable structure, the perpendicular exchange bias was isothermally switched by the gate voltage in combination with the magnetic field. Analysis of the hysteresis of the exchange bias field as a function of the gate voltage suggested that the magnetoelectric coefficient was comparable to the reported value. This implies that the Ir layer does not degrade the efficiency to transfer the magnetoelectrically controlled antiferromagnetic order parameter to the ferromagnetic Pt/Co/Ir trilayer.

Modulating Exchange Bias, Anisotropic Magnetoresistance, and Planar Hall Resistance of Flexible Co/MnN Epitaxial Bilayers on Mica by Bending Strain.

The integration of ferromagnetic/antiferromagnetic bilayers with exchange bias effect on flexible substrates is crucial for flexible spintronics. Here, the epitaxial Co/MnN bilayers are deposited on mica by facing-target sputtering. A large in-plane exchange bias field (HEB) of 1800 Oe with a coercive field (HC) of 2750 Oe appears in the Co (3.8 nm)/MnN (15.0 nm) bilayer at 5 K after field cooling from 300 to 5 K. Effective interfacial exchange energy Jeff of the Co/MnN bilayer is 0.83 erg/cm2. The strain-induced maximum increase of HEB and HC reaches 18% and 21%, respectively, in the Co(3.8 nm)/MnN(15.0 nm) bilayer. Strain-modulated HEB is attributed to the change of interfacial exchange coupling between Co and MnN layers. HEB is inversely proportional to Co thickness but independent of MnN thickness. The change of HEB is less than 5% after 100 bending cycles, indicating mechanical durability. The out-of-plane exchange bias also appears since Co spins are not fully reversed due to the strong pinning effect. Anisotropic magnetoresistance (AMR) and planar Hall resistance (Rxy) show obvious hysteresis due to HEB. Exchange bias-induced phase difference of AMR and Rxy almost remains unchanged at different bending strains. The results provide the basis for understanding the bending strain tailored exchange bias.