Exchange Bias Field Research Articles

Observation of large exchange bias field in Mn rich NiMnAl Heusler alloy thin film

In the present work exchange bias in Mn rich Ni-Mn-Al film has been studied. The film has been deposited by cosputtering from the targets of Ni, Mn and Al. It exhibits an antiferromagnetic B2 phase at room temperature. At low temperature, magnetization versus temperature (M-T) measurement shows a bifurcation among ZFC and FC curves. Hysteresis (M-H) loops measurements reveal the occurrence of exchange bias phenomenon at low temperature. Large HEB of 2.2 kOe for 20 kOe cooling field has been observed. There occurs a field induced ordering at temperature ~50 K reflected as coinciding ZFC (zero-field cooled) and FC (field-cooled) curves at large magnetic field. AC susceptibility measurement indicates the occurrence of cluster glass behaviour at low temperature. The exchange constant obtained from the best fit to the experimental data was found to be Ji ≈ −2 meV. The negative interface exchange constant indicates AFM coupling occurs among the coexisting phases. Exchange bias phenomenon may be associated to the coexistence of antiferromagnetic and ferromagnetic like ordering at low temperature. Cooling field dependence of HEB and coercive field HC indicate that an increase in anisotropy and growth of ferromagnetic clusters occurs at high magnetic field.

Core Size and Interface Impact on the Exchange Bias of Cobalt/Cobalt Oxide Nanostructures

Two series of Co/Co-oxide nanostructures have been synthesized by the co-precipitation method followed by different reduction and oxidation processes in an attempt to optimize their exchange bias (EB) properties. The samples are characterized by X-ray diffraction, scanning and transmission electron microscopy, and SQUID (superconducting quantum interference device) magnetometry. The two series differ with respect to their average Co core grain sizes: in one (the l-series), the size is ≈100 nm, and in the other (the s-series, obtained using lower synthesis temperatures than the l-series), it is ≈10 nm. In the l-series, progressive oxidation yields an increase in the EB field together with a reduction in Co core size. In contrast, progressive oxidation in the s-series results in growth of the Co-oxide fraction at the expense of the Co core upon oxidation, which is accompanied by a decrease in the EB effect that is attributed to an ordering of the ferromagnetic–antiferromagnetic interface and therefore a reduction of uncompensated spins density. These results illustrate how the interface details become relevant only for small enough ferromagnetic cores.

Dependence of exchange bias and coercive field on Cu spacer thickness in oblique deposited Co/Cu/CoO multilayers

Effect of growth-induced uniaxial anisotropy and thickness of non-magnetic Cu spacer between the ferromagnetic/antiferromagnetic Co/CoO multilayers on the exchange bias, coercive field, and magnetic anisotropy have been studied in 10–300 K temperature range. Co(8 nm)/Cu(tCu)/CoO(4 nm) (tCu = 0.6–2.6 nm) multilayers on Si (100) substrates were prepared using the oblique-angle magnetron sputtering deposition to induce growth induced magnetic anisotropy. The top Co layer was allowed to be naturally oxidized to obtain antiferromagnetic CoO. The films show a strong uniaxial in-plane anisotropy determined by ferromagnetic resonance measurements. Magnetization measurements for both easy and hard magnetization directions revealed that the exchange bias and coercive fields decrease by increasing Cu thickness. The existence of strong exchange bias across the thickest Cu layer shows the long-range character of exchange and dipolar interactions at 10 K. Dependence of Cu spacer thickness shows an oscillatory behavior that can be ascribed to the competitions between Ruderman–Kittel–Kasuya–Yosida (RKKY) and dipolar interactions.

Influence of temperature on thermal relaxation of exchange bias field in CoFe/Cu/CoFe/IrMn spin valve* *Project supported by the Yunnan Provincial Ten Thousand Talents Plan Young Talents Training Fund, China (Grant No. KKRD201952029), the Applied Basic Research Program of Yunnan Province, China (Grant No. 2011FB037), and the School Talent Cultivation Foundation, China (Grant No. KKSY201252017).

A multilayered spin valve film with a structure of Ta (5 nm)/Co75Fe25 (5 nm)/Cu(2.5 nm)/Co75Fe25 (5 nm)/Ir20Mn80 (12 nm)/Ta(8 nm) is prepared by the high-vacuum direct current (DC) magnetron sputtering. The effect of temperature on the spin valve structure and the magnetic properties are studied by x-ray diffraction (XRD), atomic force microscopy (AFM), and vibrating sample magnetometry. The effect of temperature on the exchange bias field thermomagnetic properties of multilayered spin valve is studied by the residence time of samples in a reverse saturation field. The results show that as the temperature increases, the IrMn (111) texture weakens, surface/interface roughness increases, and the exchange bias field decreases. Below 200 °C, the exchange bias field decreases with the residence time increasing, and at the beginning of the negative saturation field, the exchange bias field H ex decreases first quickly and then slowly gradually. When the temperature is greater than 200 °C, the exchange bias field is unchanged with the residence time increasing.

Strong Interfacial Perpendicular Magnetic Anisotropy in Exchange-Biased NiO/Co/Au and NiO/Co/NiO Layered Systems.

The ability to induce and control the perpendicular magnetic anisotropy (PMA) of ferromagnetic layers has been widely investigated, especially those that offer additional functionalities (e.g., skyrmion stabilization, voltage-based magnetization switching, rapid propagation of domain walls). Out-of-plane magnetized ferromagnetic layers in direct contact with an oxide belong to this class. Nowadays, investigation of this type of system includes antiferromagnetic oxides (AFOs) because of their potential for new approaches to applied spintronics that exploit the exchange bias (EB) coupling between the ferromagnetic and the AFO layer. Here, we investigate PMA and EB effect in NiO/Co/Au and NiO/Co/NiO layered systems. We show that the coercive and EB fields increase significantly when the Co layer is coupled with two NiO layers, instead of one. Surrounding the Co layer only with NiO layers induces a strong PMA resulting in an out-of-plane magnetized system can be obtained without a heavy metal/ferromagnetic interface. The PMA arises from a significant surface contribution (0.74 mJ/m2) that can be enhanced up to 0.99 mJ/m2 by annealing at moderate temperatures (~450 K). Using field cooling processes for both systems, we demonstrate a wide-ranging control of the exchange bias field without perturbing other magnetic properties of importance.

Size dependence of magnetic and dielectric properties in YCr0.5Fe0.5O3 ceramics

The YCr0.5Fe0.5O3 ceramics with different sintering temperatures (TS) have been synthesized by a sol-gel method, and the structural, magnetic and dielectric properties are studied. With the decrease of TS, continuous decrease of grain size is evidenced via the micromorphology analysis. Due to the competition between the single ion magnetic anisotropy and the antisymmetric Dzyaloshinsky-Moriya interaction, the total magnetization becomes zero at the compensation temperature (TComp) in the presence of a lower magnetic field, and the TComp exhibits size dependent behavior. With the decrease of grain size, the TComp becomes lower due to the increase of uncompensated spins. Meanwhile, the TComp is adjustable by the magnetic field, and a larger external field drives the TComp to lower temperature. After cooling the samples in magnetic field, the magnetic anisotropy is established, which results in a positive exchange bias (EB) effect below the TComp. The decrease of TS would increase the ferromagnetic component, which leads to the decrease of EB field (HEB). The dielectric measurements suggest that the samples of TS = 900 and 800 °C possess large permittivity accompanied with dielectric relaxation near room temperature, and the dielectric relaxation which arises from the single-ionized oxygen vacancies moves to higher temperature with the decrease of TS.

Effect of hydrostatic pressure on martensitic transformation and low-temperature magnetic properties in Ni45Cu5Mn35In15 Heusler alloy

The effect of hydrostatic pressure on martensitic transformation (MT), superparamagnetic (SPM) and exchange bias (EB) behaviors in polycrystalline Ni45Cu5Mn35In15 has been intensively investigated by various experimental methods. The experimental results show that the studied alloy experiences a MT from the austenitic state with a L21-type cubic structure to the martensitic state with a seven-layer (7 M) modulated orthorhombic structure with decreasing temperature, accompanied by a significant change in magnetization. It is found that applying hydrostatic pressure can induce the MT to occur in the higher temperature, while the Curie transition almost remains unchanged due to the suppression of ferromagnetic (FM) coupling. In addition, the applied hydrostatic pressure can suppress the EB field and coercivity of the studied alloy due to the reduction of the competition between FM and AFM interactions. This phenomenon can be attributed to the fact that the application of hydrostatic pressure reduces the size of the SPM cluster and its average magnetic moment.

Magnon valve effect and resonant transmission in a one-dimensional magnonic crystal

We theoretically investigate the transmission of exchange-dominated spin waves in a one-dimensional magnonic crystal (MC) with a periodic exchange bias field. By recasting the Landau-Lifshitz-Gilbert equation into an effective Schr\"odinger equation and establishing spin-wave functions, it is found that MCs with upward (up) and downward (down) magnetization, respectively, correspond to the rectangular $N$-fold barriers and wells for magnons. The broadband transmission spectra in up and down states are systematically investigated. We show the phenomena of the magnon resonant transmission in both states and calculate the resonant transmission wave functions, which are related to the magnon density. Our results also show a transmission spectra shift effect (TSSE) between up and down states, which is found to be general in this system. Furthermore, the TSSE is useful to design a type of magnon valve, the magnonic-crystal-based magnon valve (MCMV), which has a large on/off ratio and bandwidth. By high-throughput screening, 125 000 groups of parameters of the MC are calculated, and 1948 parameter groups of high-performance MCMVs are screened out. Our work clarifies the physical details of the exchange-dominated spin-wave transmission in rectangular $N$-fold barriers and wells and also provides a promising route for designing novel magnonic devices.

Evidence for linear dependence of exchange bias on pinned uncompensated spins in an Fe/FeO bilayer

We use a magnetic field to manipulate the pinned spins in an Fe/FeO bilayer at room temperature, observing the behavior of the pinned magnetic moment via magnetometry. Our experiment reveals a linear relationship between the exchange bias field and the pinned magnetic moment. Based on the pinned uncompensated spin density at the interface as a function of the pinned moment, we provide a quantitative description of the exchange bias effect via exchange interaction of ferromagnetic and antiferromagnetic spins.

Spin induced exchange bias and lattice modulation in Nd1−x Eu x CrO3

We report the evolution of coupled phonons and exchange bias (EB) in perovskite-type Nd1−x Eu x CrO3 (x = 0.0, 0.05, and 0.10) samples by means of temperature-dependent Raman spectroscopy and dc magnetization measurements. We observed a non-monotonic behavior of the EB field around the temperature T *, which lies between the antiferromagnetic ordering temperature (T N) and spin-reorientation transition temperature (T SR). The temperature dependence of phonon modes related to antistretching and bending of CrO6 octahedra and Nd3+/Eu3+ ion vibration below T N confirms the strong spin–phonon coupling. The T * found from the non-monotonicity of the EB is imprinted with the additional anomaly observed in the low-temperature spin–phonon behavior. The phonon modes and phonon anomaly are also verified using the density functional theory-based calculations.

Exchange bias in BiFeO3 and Bi0.9La0.1FeO3 nanoparticles

The exchange bias (EB) effects of BiFeO3 (BFO) and Bi0.9La0.1FeO3 nanopowders are investigated. An EB field of 152 Oe and 902 Oe at 60 K are observed for BFO and Bi0.9La0.1FeO3, respectively, when cooled in 20 kOe magnetic field. The enhancement of EB values in the case of Bi0.9La0.1FeO3 is explained based on the combination of Malozemoff’s and domain state models. Training effect measurements data fitted with Binek’s model suggested that the origin of EB lies in the interaction between the antiferromagnetic (AFM) core and the soft magnetic shell. Memory effect measurements, thermoremanent and isoremanent magnetization studies, and time decay of thermoremanent magnetization studies are done to understand the nature of the shell of both the samples. Time decay of thermoremanent magnetization of BFO is fitted with a stretched exponential based on Kohlrausch–Williams–Watt model, the obtained shape parameter value lies in the range of Heisenberg type spin-glasses. From thermoremanent and isoremanent magnetization plots, a 2D-diluted AFM shell is observed for Bi0.9La0.1FeO3. The field cooling and the temperature dependence of EB are investigated and the results are explained based on the available models of EB.

Current-Induced Magnetization Switching of Exchange-Biased NiO Heterostructures Characterized by Spin-Orbit Torque

In this work, we study magnetization switching induced by spin-orbit torque in W(Pt)/Co/NiO heterostructures with variable thickness of heavy-metal layers W and Pt, perpendicularly magnetized Co layer and an antiferromagnetic NiO layer. Using current-driven switching, magnetoresistance and anomalous Hall effect measurements, perpendicular and in-plane exchange bias field were determined. Several Hall-bar devices possessing in-plane exchange bias from both systems were selected and analyzed in relation to our analytical switching model of critical current density as a function of Pt and W thickness, resulting in estimation of effective spin Hall angle and perpendicular effective magnetic anisotropy. We demonstrate in both the Pt/Co/NiO and the W/Co/NiO systems the deterministic Co magnetization switching without external magnetic field which was replaced by in-plane exchange bias field. Moreover, we show that due to a higher effective spin Hall angle in W than in Pt-systems the relative difference between the resistance states in the magnetization current switching to difference between the resistance states in magnetic field switching determined by anomalous Hall effect ($\Delta R/\Delta R_{\text{AHE}}$) is about twice higher in W than Pt, while critical switching current density in W is one order lower than in Pt-devices. The current switching stability and training process is discussed in detail.

Configurable Artificial Spin Ice with Site-Specific Local Magnetic Fields.

We demonstrate ground state tunability for a hybrid artificial spin ice composed of Fe nanomagnets which are subject to site-specific exchange-bias fields, applied in integer multiples of the lattice along one sublattice of the classic square artificial spin ice. By varying this period, three distinct magnetic textures are identified: a striped ferromagnetic phase; an antiferromagnetic phase attainable through an external field protocol alone; and an unconventional ground state with magnetically charged pairs embedded in an antiferromagnetic matrix. MonteCarlo simulations support the results of field protocols and demonstrate that the pinning tunes relaxation timescales and their critical behavior.

Изучение поля обменного смещения в ферромагнитной пленке на антиферромагнитной подложке

Изучение поля обменного смещения в ферромагнитной пленке на антиферромагнитной подложке

Magnetic properties of FeCo-iron oxide core–shell nanoparticles investigated through first order reversal studies

Core–shell magnetic nanoparticles exhibit exchange bias, whereas the magnitude of the exchange bias field is influenced by various factors that are still under investigation. We present a detailed analysis of the magnetic behavior in Fe- and Co-rich FeCo particles with a ferrimagnetic oxide shell. The as-synthesized alloys, with an average particle size above 150 nm, show high saturation magnetization and exhibit multidomain nature. The magnetic behavior has been analyzed through first order reversal studies at room temperature, and low temperature (15 K) for the as-synthesized and size-reduced alloys. The size-reduced alloys exhibit exchange bias that enhances with decreasing average particle size and found to be maximum for the Co-rich FeCo. First order reversal curve (FORC) studies could resolve the reversible, irreversible magnetization components, and magnetostatic interactions. The low-temperature field cooled measurements expose random field bias acting at the interfaces. The studies reveal that FORC could be utilized to obtain information about particle size and its distribution dependent magnetic behavior in core–shell nanostructures.

Effect of spin glass frustration on exchange bias in NiMn/CoFeB bilayers.

Exchange bias in ferromagnetic/antiferromagnetic systems can be explained in terms of various interfacial phenomena. Among these, spin glass frustration can affect the magnetic properties in exchange bias systems. Here we have studied a NiMn/CoFeB exchange bias system in which spin glass frustration seems to play a crucial role. In order to account for the effect of spin glass frustration on magnetic properties, we have investigated the temperature and cooling field dependence of exchange bias. We have observed the decrease of exchange bias field (μ0HEB) with cooling field (μ0HFC) whereas there is a negligible effect on coercive field (μ0HC). Exponential decay of μ0HEB and μ0HC is found in this exchange bias system. Furthermore, training effect measurements have been performed to study the spin relaxation mechanism. We have fitted the training effect data with a frozen and rotatable spin relaxation model. We have determined the ratio of relaxation rates of interfacial rotatable and frozen spins in this study. The training effect data are also fitted with various other models. Furthermore, we have observed the shifting of the peak temperature towards higher temperature with frequency from the ac susceptibility data. The peak temperature vs. frequency data can be described by the Vogel-Fulcher law, which indicates the spin glass like state in the bilayer system.

Emergence of large exchange anisotropy in Pr doped nanocrystalline spinel ferrites

This paper reports the influence of rare earth Pr3+ doping on microstructural and magnetic properties in soft nickel nanoferrites. All the samples were prepared by soft chemical co-precipitation route with the generic formula NiPrxFe2-xO4 (x = 0.00, 0.04, 0.08 and 0.12). The prepared nanoparticles were single-phase as verified by x-ray diffractograms. Mean crystallite sizes of all the samples were found in the range of 4 nm–15 nm from the Williamson-Hall plot. Raman spectra signalled the lowering of crystal symmetry of octahedral co-ordinated sites due to the doping of large size Pr3+ ions. Field cooled M(H) loops registered at 5 K detected the existence of significant exchange bias field and large coercivity in the samples. The superparamagnetic behavior of prepared nanoferrites at room temperature became more prominent for higher Pr concentration as confirmed by both hysteresis loops and M(T) curves.

Exchange bias in flexible freestanding La0.7Sr0.3MnO3/BiFeO3 membranes

Exchange bias lies in the core of cutting-edge technologies, for instance, high-density data storage, spin valves, and high-frequency magnetic devices. As the devices with flexibility have become vital in the forefront of technology, the maintenance of exchange bias under high flexion is highly desired. This paper reports the strong exchange bias in flexible freestanding La0.7Sr0.3MnO3/BiFeO3 membranes with decent mechanical durability. The La0.7Sr0.3MnO3 layer is biased strongly by the BiFeO3 layer even in the freestanding state. At the same time, the strain is demonstrated to affect the strength of the exchange bias of the La0.7Sr0.3MnO3/BiFeO3 bilayer. Furthermore, the saturation magnetization and exchange bias field of the freestanding La0.7Sr0.3MnO3/BiFeO3 membranes are preserved well after 1000 bending cycles. Our work on freestanding membranes paves the way for manufacturing advanced flexible spintronic devices.

Vertical Strain Engineering of Epitaxial La2/3Sr1/3MnO3 Thin Films by Spontaneously Embedding ZrO2 Nanopillar Arrays

AbstractRational control of local strain distributions and thus the functional properties of epitaxial thin films has been a long‐standing goal in the development of new physics and novel devices based on strain‐sensitive materials. Here, the fabrication of La2/3Sr1/3MnO3 (LSMO) films with strain fields arising from vertical epitaxial embedding of ultra‐small ZrO2 nanopillars, diameter 4.0 ± 0.6 nm, is reported. High quality films are obtained with average distance between adjacent nanopillars of 9.0 ± 0.3 nm for x = 0.2 in (LSMO)1−x:(ZrO2)x. The strain distribution of the vertical interface is analyzed in detail and the dominant state of the interfacial strain is verified. Remarkably, with increasing x, the Curie temperature TC and metal–insulator (MI) transition temperature TMI show a surprisingly large depression, revealing the significant tuning capability of the vertical tensile stress originating from the small‐size ZrO2 pillars. A systematic tunability of the low field magnetoresistance is also found. The field dependence of the magnetization exhibits both horizontal and vertical shifts. The exchange bias field HE increases with increasing x, while magnetization shift Mshift is unchanged. The results suggest the possibility of strain tuning through epitaxial nanostructures for multifunctional applications across many fields with appropriate selection of matrix and nanopillar materials.

Evidence of two dimensional dilute antiferromagnet mediated exchange bias field and vertical shift in LaFeO3-NiO nanocomposite

Low dimensional antiferromagnetic systems, such as nanoparticles, are known to exhibit weak ferromagnetic behavior due to the uncompensated surface spins. In this report, we present a detailed magnetic study of LaFeO3/NiO nanocomposite, synthesized by propylene glycol gel method. The X-ray diffraction (XRD) based analyses, in combination with transmission electron microscopy (TEM), confirms 50:50 ratio composite with ~35–40 nm size particles. Owing to the high Néel temperatures (TN) of LaFeO3 and NiO, significant exchange bias (EB) and vertical magnetization shift (VMS) have been observed in the nanocomposite at room temperature. These features are attributed to the exchange interaction at the interface between the core and shell of the nanoparticles that are homogeneously dispersed in the nanocomposite. In our studies, the EB field and the VMS attains a value as high as 2050 Oe & 1.8 × 10-1 emu/g at 5 K and 75 Oe & 1.7 × 10-2 emu/g at 300 K, respectively. The thermoremanent magnetization (TRM) and isothermoremanent magnetization (IRM) analyses are done to identify the two-dimensional (2D) diluted antiferromagnet (DAFF) behavior of the surface shell of nanoparticles. We propose a phenomenological antiferromagnetic-(2D-DAFF)-antiferromagnetic magnetic structure model, to explain all the rich features and the observed magnetic properties of this nanocomposite.