Spontaneous Exchange Bias Research Articles

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.

Observation of the multiple magnetic phases in double perovskite Pr1.8La0.2CoFeO6

The structural and magnetic properties of the double perovskite Pr1.8La0.2CoFeO6 have been investigated. The X-ray diffraction study shows that the system acquires room-temperature orthorhombic phase with the Pnma space group. The X-ray photoemission spectroscopy (XPS) measurement confirmed that the B-site ions are present in mixed valence states i.e. Co has been found in Co2+ and Co3+ valance states whereas Fe has been found in Fe3+/Fe4+. Magnetic measurements of the system confirm the existence of several fascinating magnetic behaviors, such as long-range canted antiferromagnetism withTN ∼ 271 K, giant exchange bias, and re-entrant cluster glass phase (TG ∼ 33 K). Field-dependent magnetization shows a large spontaneous exchange bias; HSEB∼ 1.8 kOe and giant conventional exchange bias, HCEB∼ 2.4 kOe at 5 K. Antiferromagnetic materials with large exchange bias can be utilized in high-density spintronic devices. Temperature-dependent Raman study demonstrates that the observed phonon mode exhibits anomalous behavior close to the magnetic transition temperature. Analysis of anomalous softening of phonon mode below TN, clarifies the presence of significant spin-phonon coupling while the magnetostriction effect does not play any significant role in the observed phonon anomaly.

Competition between Cubic and Tetragonal Phase Induced Unusual Vertical Magnetization Shift and Exchange Bias in CoxMn3−xO4 (1 ≤ x ≤ 2) Spinel Nanoparticles

CoxMn3−xO4 (1 ≤ x ≤ 2) spinel nanoparticles synthesized through conventional coprecipitation technique exhibit the coexistence of tetragonal and cubic phases in the range of 1 ≤ x ≤ 1.5, while a pure cubic phase is observed for 1.75 ≤ x ≤ 2 at room temperature. Reduction in tetragonal phase fraction from 92 % for x = 1 to 47% for x = 1.5 is attributed to diminution of Jahn–Teller (J–T) active Mn3+ ions occupying the octahedral site of spinel lattice. X‐ray photoelectron spectroscopy confirms both +2 and +3 oxidation states for Co and Mn. Surprisingly, cubic and tetragonal phases exhibit magnetic transition, Tc1 and Tc2, corresponding to a paramagnetic to a high and low temperature ferrimagnetic state, respectively. Tetragonal phase induces high spontaneous (HSEB) and conventional (HCEB) exchange bias with unusually high vertical magnetization shift (VMS) than that of the pure cubic phase, shows maximum HCEB of 4.062 kOe for x = 1.5 and a VMS of 2.5 emu g−1 for x = 1. Such dependence of VMS and exchange bias on tetragonal to cubic phase ratio in CoxMn3−xO4 is demonstrated for the first time and interpreted based on the interaction between different arrangement of spins in tetrahedral and octahedral sublattice.

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.

Structural phase transition and tunable exchange bias effect in La1·5Sr0·5CoMnO6 nanoparticles with particle size

The effect of particle size on the crystal structure and magnetic behaviors of the double perovskite oxide La1·5Sr0·5CoMnO6 was systematically investigated. As the particle size increases from 45 nm to 308 nm, the crystal structure undergoes a gradual transformation from a cubic phase with the Pm-3m space group to the rhombohedral phase (R-3c). The magnetic data reveals that the Griffiths phase (GP) can be induced and enhanced by an increase in particle size. Importantly, the modulation of the conventional and spontaneous exchange bias (EB) of the system was effectively achieved by crystal growth, i.e., a monotonically increased EB field with an increase of particle size. This phenomenon can be attributed to the enhancement in the antiferromagnetic (AFM) interfacial anisotropy based on the Meiklejohn-Bean model. Our results provide evidence that crystal growth plays a crucial role in modulating the EB effect in double perovskite.

Low field-cooled induced large exchange bias effect and DFT calculations in ferromagnetic Sm2CoMnO6

In the present report, we study the large exchange bias effect in Sm2CoMnO6 (SCMO) polycrystalline samples synthesized with the presence of two crystallographic phases: ordered-phase (monoclinic; P21/n) and disordered-phase (orthorhombic; Pnma). X-ray photoelectron spectroscopy study revealed the presence of mixed valence states for Co (2+ & 3+) and Mn (4+ & 3+). M(T) data exhibits an inhomogeneous magnetic state with the presence of ferromagnetic ordering at TC ∼128 K due to the super-exchange interactions of Co2+-O2--Mn4+ and antiferromagnetic-like spin correlations for T < 50 K, attributed to Co3+-O2--Co3+, and Mn3+-O2--Mn3+ interactions. M(H) loop shift with a conventional exchange bias (EB) effect of 10 kOe for a field-cooled (HFC) of 10 kOe at 2 K was observed. Such a large value of the EB effect for low HFC in SCMO is comparable to that of large EB compounds, such as La1·5Sr0·5CoMnO6 and NiFe2O4/CoO nanocomposites. The zero-field cooled asymmetry in M(H) loop termed as spontaneous exchange bias effect (SEB) observed for T < 20 K. The systematic study of EB effects like HEB and MEB with T(K) and HFC was explained qualitatively by the presence of unidirectional anisotropy formed at the interface of inhomogeneous magnetic phases. Further, density functional theory (DFT) calculations validate the ferromagnetic ground state of SCMO with Co and Mn networks. Moreover, the semiconductor characteristics of SCMO are established with a band gap of 1.3 eV.

Large room-temperature spontaneous exchange bias effect in LaFeO3 micro-polyhedrons

Single-phase materials with exchange bias (EB) effect at room temperature (RT) are very important for future applications in spintronic devices, magnetic storage, and sensors. To explore materials showing the EB effect at RT is an interesting and important issue. In this paper, LaFeO3 micro-polyhedrons composed of an average particle size of 66 nm nanoparticles have been synthesized by annealing LaFe(CN)6 precursors in air at 630 °C for 8 h. Magnetic hysteresis loops show that LaFeO3 micro-polyhedrons present a large spontaneous EB (SEB) effect (∼ 1.5 kOe) at RT after zero-field cooling (ZFC) and a gigant conventional EB (CEB) effect (∼ 5.6 kOe) after field cooling (FC) of 70 kOe. To the best of our knowledge, the current compound shows the largest negative EB field after ZFC at RT among SEB materials discovered so far. The successful fitting of training effect (TE) data with Binek’s model shows that the origin of SEB lies in the interface exchange coupling between different magnetic phases. The interface with the “pinning” effect of antiferromagnetic (AFM) matrix relative to ferromagnetic (FM) clusters exhibits unidirectional anisotropy, which is formed during the initial magnetization process. This interesting finding of the SEB effect at RT in single-phase perovskite LaFeO3 system may open a new direction for possible technological applications.

Field-Free Spin-Orbit Torque-Induced Magnetization Switching in the IrMn/CoTb Bilayers with a Spontaneous In-Plane Exchange Bias.

Spin-orbit torque (SOT)-induced magnetization switching in ferrimagnetic materials is promising for application in a new generation of information storage devices. Here, we demonstrate SOT-induced field-free magnetization switching of the perpendicularly magnetized CoTb ferrimagnet layer in the IrMn/CoTb bilayer, in which the in-plane magnetic inversion symmetry is broken by a spontaneous in-plane exchange bias (IEB) established by isothermal crystallization of the IrMn layer. We obtain a significant SOT effective field acting on the CoTb layer and a large effective spin Hall angle in this system, derived by the second harmonic voltage measurement method. Moreover, the IrMn/CoTb bilayer demonstrates multistate magnetic switching behavior with different amplitudes of current pulses at zero field, which can mimic the synaptic weight updates in the neuromorphic network. These findings make the IrMn/CoTb bilayer with spontaneous IEB a promising candidate for potential applications in multilevel storage and neuromorphic computing.

Spontaneous exchange bias and magnetic characteristics in Ni50Mn35Sn10V5 Heusler alloy

The magnetic state transformations and exchange bias effect of Ni50Mn35Sn10V5 shape memory alloy have been studied systematically. Curie transformation and martensitic transformation of Ni50Mn35Sn10V5 alloy were observed close to room temperature. Static and dynamic magnetic measurements were performed to reveal two magnetic states of the martensitic phase, with superparamagnetic and superspin glass. In addition, two exchange bias effects were observed at low temperatures, which can be attributed to the interaction of antiferromagnetism with both ferromagnetism and superferromagnetism.

Coexistence of tetragonal and cubic phase induced complex magnetic behaviour in CoMn2O4 nanoparticles

CoMn2O4, known for its extensive range of applications, has been subject to limited investigations regarding its structure dependent magnetic properties. Here, we have examined the structure dependent magnetic properties of CoMn2O4 nanoparticles synthesized through a facile coprecipitation technique and are characterized using x-ray diffractometer, x-ray photoelectron spectroscopy (XPS), RAMAN spectroscopy, transmission electron microscopy and magnetic measurements. Rietveld refinement of the x-ray diffraction pattern reveals the coexistence of 91.84% of tetragonal and 8.16% of cubic phase. The cation distribution for tetragonal and cubic phases are (Co0.94Mn0.06)[Co0.06Mn1.94]O4 and (Co0.04Mn0.96)[Co0.96Mn1.04]O4, respectively. While Raman spectra and selected area electron diffraction pattern confirm the spinel structure, both +2 and +3 oxidation states for Co and Mn confirmed by XPS further corroborate the cation distribution. Magnetic measurement shows two magnetic transitions, Tc1 at 165 K and Tc2 at 93 K corresponding to paramagnetic to a lower magnetically ordered ferrimagnetic state followed by a higher magnetically ordered ferrimagnetic state, respectively. While Tc1 is attributed to the cubic phase having inverse spinel structure, Tc2 corresponds to the tetragonal phase with normal spinel. In contrast to general temperature dependent H C observed in ferrimagnetic material, an unusual temperature dependent H C with high spontaneous exchange bias of 2.971 kOe and conventional exchange bias of 3.316 kOe at 50 K are observed. Interestingly, a high vertical magnetization shift (VMS) of 2.5 emu g−1 is observed at 5 K, attributed to the Yafet–Kittel spin structure of Mn3+ in the octahedral site. Such unusual results are discussed on the basis of competition between the non-collinear triangular spin canting configuration of Mn3+ cations of octahedral sites and collinear spins of tetrahedral site. The observed VMS has the potential to revolutionize the future of ultrahigh density magnetic recording technology.

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.

Structural transitions and spontaneous exchange bias in La2−xBaxCoMnO6 series

The structural, electronic, and magnetic properties of La2−xBaxCoMnO6(0.25 ≤ x ​≤ ​1.0) compounds were investigated employing x-ray powder diffraction and magnetometry. The crystal structure evolves from orthorhombic Pnma to rhombohedral R3̄c and then to hexagonal P63/mmc space group as the Ba concentration increases. The magnetization as a function of temperature measurements revealed a ferromagnetic-like behavior for all samples, but antiferromagnetic interactions seem to also be present throughout the whole series. The competition between magnetic phases leads to the phenomenon of spontaneous exchange bias effect, observed in the zero field cooled magnetization as a function of applied magnetic field curves. The evolution of this effect along the series is discussed in terms of changes in the crystal structure.

Sign reversal of both spontaneous and conventional exchange bias in nanoparticles of Y2FeCrO6 double perovskite

Single-phase nanoparticles sample of Y2FeCrO6 double perovskite with an average particle size of 67 nm was prepared and its structural and magnetic behavior is reported. Magnetization vs temperature (M–T) study revealed that the Neel temperature, TN is 327 K, which is greater than the previous finding [Maiti et al., J. Phys. D: Appl. Phys. 46, 415303 (2013)]. Super-exchange interactions in Cr3+–O–Cr3+ and Fe3+–O–Fe3+ networks are responsible for AFM transition. Magnetization reversal (MR) with a high compensation temperature Tcomp = 192 K and sign reversal in both spontaneous and conventional exchange bias fields are observed. In addition, a spin reorientation is observed at TSR ∼ 30 K. At 5 K, a zero-field cooled spontaneous exchange bias (SEB) field of 265 Oe is observed. At the same temperature, after field cooling the sample at 0.5 T, the maximum value of the exchange bias field (CEB) is found to be 2.1 kOe. The SEB shows a sign reversal across TSR, and the CEB reverses its sign across Tcomp. The observed thermal hysteresis of magnetization indicates a first-order phase transition. The loop width of magnetic hysteresis as a function of magnetization exhibits a double peak confirming the multiple magnetic phase behavior. Bipolar switching of magnetization has been demonstrated at 50 K. The competition between single-ion anisotropy and Dzyaloshinsky–Moriya interaction is responsible for the above behaviors.

Large Exchange Bias Triggered by Transition Zone of Spin Glass

AbstractExchange bias has increasingly practical significance in magnetoresistive and spintronic devices. However, the underlying mechanism of exchange bias in bulk compounds with the structural single‐phase and inhomogeneous magnetic phases is still elusive. Herein, based on experimental and simulation results, two important parameters are studied, i.e., the antiferromagnetic (AFM) volume fraction and the ferromagnetic (FM)/AFM interface area, which essentially determine the (spontaneous) exchange bias of Mn‐rich Ni44Co6Mn44‐xSn6+x (x = 0 ∼ 6) magnetic shape memory alloys. The substitution Sn for Mn changes magnetic ground state following the sequence of superparamagnetic/AFM → dilute spin glass/AFM → transition zone → cluster spin glass/AFM → FM, accompanying the growth of FM cluster and the weakening of AFM interactions. The results reveal that the magnetic ground state for exchange bias is optimized at transition zone between dilute spin glass/strong AFM and cluster spin glass/weak AFM, in which the optimal AFM volume fraction and FM/AFM interface area are achieved by tuning magnetic fields. A giant exchange bias field of 702.7 mT and a spontaneous exchange bias field of 318.7 mT are demonstrated. The work contributes to in‐depth understanding of (spontaneous) exchange bias in magnetically inhomogeneous compounds.

Spontaneous exchange bias and large dielectric constant in Bi0.8Tb0.2Fe0.8Mn0.2O3 multiferroic

The structural, dielectric, and magnetic properties of Bi0.8Tb0.2Fe0.8Mn0.2O3 have been studied in detail. A structural shift from rhombohedral (space group R3c) to orthorhombic (space group Pn21a + Pnma) phase is observed with doping. The substitution of Mn and Tb gives a very large value of the dielectric constant. It has been demonstrated that the observed spontaneous exchange bias is larger than the conventional exchange bias at room temperature. Moreover, doping reduces the Neel temperature from 643 to 521 K and magnetization increases. Thermoremanent magnetization studies at room temperature show that the system is composed of an interfacial layer of an antiferromagnetic core and a two-dimensional diluted antiferromagnet shell with a net magnetization under the field. Furthermore, the system's reasonable HEB and HC values at room temperature make it intriguing and appealing for a variety of multifunctional devices.

Effects of Crystalline Disorder on Interfacial and Magnetic Properties of Sputtered Topological Insulator/Ferromagnet Heterostructures

Thin films of Topological insulators (TIs) coupled with ferromagnets (FMs) are excellent candidates for energy-efficient spintronics devices. Here, the effect of crystalline structural disorder of TI on interfacial and magnetic properties of sputter-deposited TI/FM, Bi2Te3/Ni80Fe20, heterostructures is reported. Ni and a smaller amount of Fe from Py was found to diffuse across the interface and react with Bi2Te3. For highly crystalline c-axis oriented Bi2Te3 films, a giant enhancement in Gilbert damping is observed, accompanied by an effective out-of-plane magnetic anisotropy and enhanced damping-like spin-orbit torque (DL-SOT), possibly due to the topological surface states (TSS) of Bi2Te3. Furthermore, a spontaneous exchange bias is observed in hysteresis loop measurements at low temperatures. This is because of an antiferromagnetic topological interfacial layer formed by reaction of the diffused Ni with Bi2Te3 which couples with the FM, Ni80Fe20. For increasing disorder of Bi2Te3, a significant weakening of exchange interaction in the AFM interfacial layer is found. These experimental results Abstract length is one paragraph.

The spontaneous exchange bias effect in La2−xCaxCoMnO6 series

Structural, electronic and magnetic properties of polycrystalline La2-xCaxCoMnO6 (0 $\leq$ x $\leq$ 0.75) compounds are investigated by x-ray diffraction and magnetometry. All the samples have an orthorhombic structure and show a slight decrease in the unit cell with Ca-doping. Temperature-dependent magnetization measurements reveal a complex magnetic behavior with two ferromagnetic transitions. These transitions are ascribed to Co2+--Mn4+ and Co3+--Mn3+ couplings and suggest the presence of additional antiferromagnetic couplings in these disordered compounds. Field-dependent magnetization curves, measured after cooling the samples in a zero external magnetic field, reveal the spontaneous exchange bias (SEB) effect for the Ca-doped samples. The strengthening of the uncompensated magnetic coupling at the interfaces, caused by the increased antiferromagnetic phase, explains the increase of SEB with increasing the Ca-content.

Double glassy states and large spontaneous and conventional exchange bias in La1.5Ca0.5CoFeO6 ferrimagnetic double perovskite

The structural and magnetic properties of hole doped double perovskite La1.5Ca0.5CoFeO6 have been investigated by measuring x-ray photoemission spectroscopy, neutron powder diffraction and magnetization. A ferrimagnetic transition is observed at T C∼ 167 K. The presence of anti-site disorder (ASD) in La1.5Ca0.5CoFeO6 has also been demonstrated. Double re-entrant cluster glass transitions (T 1∼ 11 K and TS ∼ 35 K) were observed which has been attributed to the ASD effect. The presence of both large spontaneous exchange bias H SEB ∼ 2.106 kOe and giant conventional exchange bias H CEB ∼ 1.56 T at 5 K has also been observed which can be attributed to the coexistence of long range magnetic ordering and glassy state. The experimental observations were explained with the results obtained by the density functional theory calculation. The presence of double glassy states, large exchange-bias effect and different magnetic phases make this system a potential candidate for spintronic applications.

Epitaxial growth of high-entropy alloy thin film with spontaneous exchange bias

High-entropy alloys (HEAs) have attracted extensive research interest for their outstanding mechanical, electrochemical, and magnetic properties due to the highly random occupation of near equimolar multi-principal elements. The functionalities are severely influenced by the highly disordered lattice structure at grain boundaries and random orientation of grains in polycrystalline samples. Here, epitaxial CrMnFeCoNi HEA thin films of (001) orientation are prepared by laser-assisted molecular beam epitaxy on MgO substrates using a Cu buffer layer. A spontaneous exchange bias (SEB) effect is observed in a 98 nm thick CrMnFeCoNi HEA thin film. The SEB effect is related to a newly formed ferromagnetic (FM)/spin glass (SG) interface generated by field-induced irreversible growth of FM regions. The interfacial spins of SG domains are gradually consumed during the initial magnetizing process, leading to the consequent reservation of unidirectional anisotropy. The observation of SEB in HEA films expands their functions, which may be applied in novel magnetic devices.