Temperature Dependence Of Exchange Bias Research Articles

Effect of thickness and temperature on interfacial interaction between Cr and Ni nanolayers: An exchange bias study

The phenomenon of negative exchange bias is typically observed in ferromagnetic and antiferromagnetic materials following a field cooling protocol. However, the occurrence of positive exchange bias is hardly found. The majority of models show positive exchange bias after undergoing field cooling or zero-field-cooling process. In this work, we achieved a novel positive exchange bias effect for Cr/Ni bilayer after gradually increasing the temperature from low temperature (5 K) to high temperature (up to RT) in the absence of magnetic field. The M−H loops at different temperatures reveal not only temperature dependent exchange bias but also thickness dependent exchange bias for Cr/Ni layer. A systematic investigation was conducted to analyse the influence of the exchange bias field on the thickness of the ferromagnetic layer. Significantly, our experimental observations also reveal a notable room temperature exchange bias field measuring 320 Oe for the optimum bilayer. Moreover, to obtain a detailed understanding of coercivity in the Cr/Ni bilayer, Kerr hysteresis was also recorded at room temperature in longitudinal geometry (L-MOKE). Hence, the discovery of our work not only suggests a novel methodology to achieve positive exchange bias in ferromagnetic and antiferromagnetic bilayers but also the oscillatory behavior of HEB with respect to increase in temperature is observed for polycrystalline Cr (AFM) layer.

Controlling the Magneto-Optical Response in Ultrathin Films of EuO1-x via Interface Engineering with Ferroelectric BaTi2O5.

Utilizing pulsed laser deposition, a film of EuO1-x was deposited onto a Si(001) substrate with MgO buffer and compared to the same heterostructure with an additional BaTi2O5 thin film on top of the EuO1-x surface. X-ray diffraction (XRD) indicates the films crystallize into a preferred EuO(111) orientation; it also reveals the clear presence of EuSi2, which suggests Si or Eu diffuses across the MgO buffer layer. EuO1-x films exhibit a ferromagnetic (FM) signature and temperature-dependent exchange bias, indicated by MOKE measurements, suggesting the presence of a magnetic order well above the EuO Curie temperature with possible origins in charge carrier density near the interface. In comparison, an antiferromagnetic character persists well above the EuO Curie temperature of 69 K and the enhanced Curie temperature of 150 K for BaTi2O5 films grown on the EuO1-x films. The antiferromagnetic behavior is not seen in thicker EuO1-x thin films when integrated with other ferroelectric (FE) phases of the BaO-TiO2 system, suggesting an origin in the perturbed charge population at the BaTi2O5/EuO1-x interface.

Hyperthermia of Magnetically Soft-Soft Core-Shell Ferrite Nanoparticles

Magnetically soft-soft MnFe2O4-Fe3O4 core-shell nanoparticles were synthesized through a seed-mediated method using the organometallic decomposition of metal acetyl acetonates. Two sets of core-shell nanoparticles (S1 and S2) of similar core sizes of 5.0 nm and different shell thicknesses (4.1 nm for S1 and 5.7 nm for S2) were obtained by changing the number of nucleating sites. Magnetic measurements were conducted on the nanoparticles at low and room temperatures to study the shell thickness and temperature dependence of the magnetic properties. Interestingly, both core-shell nanoparticles showed similar saturation magnetization, revealing the ineffective role of the shell thickness. In addition, the coercivity in both samples displayed similar temperature dependencies and magnitudes. Signatures of spin glass (SG) like behavior were observed from the field-cooled temperature-dependent magnetization measurements. It was suggested to be due to interface spin freezing. We observed a slight and non-monotonic temperature-dependent exchange bias in both samples with slightly higher values for S2. The effective magnetic anisotropy constant was calculated to be slightly larger in S2 than that in S1. The magnetothermal efficiency of the chitosan-coated nanoparticles was determined by measuring the specific absorption rate (SAR) under an alternating magnetic field (AMF) at 200-350 G field strengths and frequencies (495.25-167.30 kHz). The S2 nanoparticles displayed larger SAR values than the S1 nanoparticles at all field parameters. A maximum SAR value of 356.5 W/g was obtained for S2 at 495.25 kHz and 350 G for the 1 mg/mL nanoparticle concentration of ferrogel. We attributed this behavior to the larger interface SG regions in S2, which mediated the interaction between the core and shell and thus provided indirect exchange coupling between the core and shell phases. The SAR values of the core-shell nanoparticles roughly agreed with the predictions of the linear response theory. The concentration of the nanoparticles was found to affect heat conversion to a great extent. The in vitro treatment of the MDA-MB-231 human breast cancer cell line and HT-29 human colorectal cancer cell was conducted at selected frequencies and field strengths to evaluate the efficiency of the nanoparticles in killing cancer cells. The cellular cytotoxicity was estimated using flow cytometry and an MTT assay at 0 and 24 h after treatment with the AMF. The cells subjected to a 45 min treatment of the AMF (384.50 kHz and 350 G) showed a remarkable decrease in cell viability. The enhanced SAR values of the core-shell nanoparticles compared to the seeds with the most enhancement in S2 is an indication of the potential for tailoring nanoparticle structures and hence their magnetic properties for effective heat generation.

Temperature-Independent Coercivity in Compositionally Graded Ferromagnetic Multilayers

We devise nanoscale-designed ferromagnetic thin films with graded exchange-coupling (J) profiles that allow us to obtain constant coercivities over a very extended temperature range. The magnetic structure designed here is composed of two ferromagnetic layers, which are separated by a spacer layer with a linear exchange-graded profile. Our detailed magnetometry analysis on single-crystal $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Ru}$ alloy thin films reveals a coercivity plateau with a relative change of less than 2% over a temperature range of 75 K, whereas conventional films exhibit changes larger than 20%. This remarkable behavior is achieved by means of a strongly temperature-dependent exchange bias between adjacent ferromagnetic portions of our film structures, which are designed to fully compensate for the conventional temperature dependence of the coercive field. This strongly temperature-dependent bias field is mediated by a paramagnetic interlayer in our film design and can be adjusted by a suitable choice of both the spacer-layer composition profile and thickness. Our particular design can be applied to a large variety of magnetic materials and adapted to relevant temperature ranges that are needed to maintain stable coercive field values over a wide operation range in real applications.

A Study on the Temperature Dependence of Exchange Bias on the Planar Hall Effect of NiFe/IrMn Bilayer

A Study on the Temperature Dependence of Exchange Bias on the Planar Hall Effect of NiFe/IrMn Bilayer

Angular Deviation of the Exchange Bias in Bilayer CoFe/IrMn Under Rotating Magnetic Field

A model of exchange bias in a polycrystalline bilayer based on thermal fluctuations was extended to account for an arbitrary magnetization angle of the ferromagnetic (FM) layer. It allows for a more accurate description of the thermally activated reversal of the Néel vector in antiferromagnetic (AFM) grains since the extended model takes into account the variation of barrier height resulting from the magnetization angle of the FM layer. Field-annealing (setting of the exchange bias) and further relaxation of the FM/AFM bilayer was simulated using a sequence of iterations based on this model. An array of AFM grains is generated based on a lognormal distribution of their volume. Chosen parameters of distribution are based on the fit to the experimentally measured temperature dependence of exchange bias in sputter-deposited CoFe/IrMn films. The angular deviation of the exchange bias and the degradation of its amplitude in an applied field, with relative angle varied from 0° to 180°, are calculated. Hysteresis of the exchange bias with field exposure is observed. A temperature–time ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\tau$ </tex-math></inline-formula> ) diagram of the deviation angle, predicting the stability of the exchange bias under specific temperature and time, is presented.

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.

Temperature dependence of exchange bias in NiFe2O4/BiFeO3 bilayers

In BiFeO3-based ferromagnet/anitferromagnet (FM/AFM) bilayers, field cooling is required through the Néel temperature to achieve significant exchange bias (EB) at room temperature (RT). In this work, NiFe2O4/BiFeO3 (NFO/BFO) all-oxide bilayer films were selected and deposited on SrTiO3 substrates by pulsed laser deposition to avoid the possible interfacial oxidization during the field cooling process. In combination with X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS) and transmission electron microscopy (TEM) characterizations, the NFO/BFO bilayers are shown to have good crystalline layered structure. After field cooling at various temperatures, the samples show sizable EB effect at low temperature region, which is attributed to spin-glass-pinned effect. More importantly, significant EB can be obtained at RT after the sample was field cooled from temperatures higher than 400 K. The maximal values of the exchange bias field (HE) and the ratio between HE and coercivity (i.e. HE/HC) are 48 Oe and 0.6 at RT, respectively. The EB effect at RT is considered to result from the pinned and uncompensated spins near the NFO-BFO interface.

Exchange bias effect in epitaxial La0.35Sr0.65MnO3/La0.7Sr0.3MnO3 bilayers: Impact of antiferromagnet growth conditions

Antiferromagnets have shown great potential for replacing ferromagnets in spintronics applications in recent years. In this work, antiferromagnetic La0.35Sr0.65MnO3 (AFM-LSMO) thin films were grown on ultrathin ferromagnetic La0.7Sr0.3MnO3 (FM-LSMO) layer over a wide temperature range. The different AFM-LSMO growth temperatures introduced variation of strain states in the AFM-LSMO layer, changing the AFM-LSMO magnetic properties. Considering the thermally activated switching model of antiferromagnetic grains, the behaviour of AFM-LSMO growth temperature-dependent exchange bias effect are explained. This work demonstrates the modulation of exchange bias effect through the control of AFM-LSMO growth conditions, suggesting possibility to control and probe oxide antiferromagnets via exchange bias coupling.

Structural phase transition and spontaneous exchange bias in NiCr1.9Fe0.1O4 nanoparticles: XRD, EXAFS and magnetic measurements

Abstract Here, we report structural and magnetic phase transitions in NiCr1.9Fe0.1O4 nanoparticles of size ∼60 nm synthesized through cost-effective co-precipitation route. X-ray diffraction pattern shows a stable cubic phase at RT in contrast to tetragonal phase in bulk NiCr2O4. Decreasing the temperature, a complete tetragonal symmetry is observed at 50 K, while a coexistence of cubic and tetragonal phases in the intermediate temperature range. Interestingly, no tetragonal to orthorhombic phase transformation has been observed down to 12 K as observed in NiCr2O4 at 70 K. The local structure examined from temperature dependent EXAFS demonstrates that although no change in distribution of cations among tetrahedral (A) and octahedral (B) sites down to 10 K has been observed, Fe3+ ions occupy A site replacing equivalent amount of Ni2+ towards B site which is contradicting to the bulk NiCr2O4 where Ni2+ occupies only A site. Magnetic studies demonstrate a two-fold increase in para to long-range ferrimagnetic transition, TC, due to increase in A-B exchange interaction and no change in spiral ordering temperature, TS indicates strong B-B interaction. In addition, these nanoparticles exhibit unusually high spontaneous exchange bias of ∼1.265 kOe at 60 K. We discuss the temperature dependent exchange bias on the basis of competition between uncompensated B site moments and ferromagnetically ordered spins at A site. The tunable exchange bias without field highlight the potential applications of these materials in spintronics devices.

Exchange Bias Enhancement and Magnetic Proximity Effect in FeVO4-Fe3O4 Nanoparticles

We study the behavior of the exchange bias (EB) and the blocking temperature in an antiferromagnetic FeVO4–ferrimagnetic Fe3O4 nanocomposite system upon annealing in Ar atmosphere. Surprisingly, the blocking temperature of post-annealed samples increased to ∼ 50 K, more than two-fold compared the Neel temperature (TN = 22 K) of individual FeVO4 nanoparticles. This significant enhancement of the blocking temperature was accompanied by the corresponding increase of EB, from ∼ 50 Oe in as-prepared samples to ∼ 110 Oe in post-annealed samples. The temperature dependence of EB can be described by two approximately linear regions with different slopes, with an inflection point at T ∼ 21 K coinciding with the Neel temperature of FeVO4 nanoparticles. The region above the inflection point with non-zero EB is characterized by a weaker temperature dependence and is expanded well beyond TN. The x-ray photoemission spectroscopy measurements indicate that the surface of post-annealed Fe3O4 particles becomes oxygen deficient, which leads to a modification of the electronic, magnetic and morphological properties of the FeVO4/Fe3O4 interface. We associate this unusual behavior with a magnetic proximity effect, in which the ordering temperature of the antiferromagnetic FeVO4 nanoparticles and the corresponding exchange bias are strongly affected by the adjacent ferrimagnetic Fe3O4 layer.

Choking Nonlocal Magnetic Damping in Exchange-Biased Ferromagnets

We investigated the temperature dependence of the magnetic damping in the exchange biased Pt/ Fe50Mn50 /Fe20Ni80 /SiOx multilayers. In samples having a strong exchange bias, we observed a drastic decrease of the magnetic damping of the FeNi with increasing temperature up to the blocking temperature. The results essentially indicate that the non-local enhancement of the magnetic damping can be choked by the adjacent antiferromagnet and its temperature dependent exchange bias. We also pointed out that such a strong temperature dependent damping may be very beneficial for spintronic applications.

Design of a multifunctional sample probe for transport measurements

We describe a multifunctional sample probe for transport measurements, equipped with a two-axis goniometer providing computer controlled 360${^\circ}$ out of plane and manual 360${^\circ}$ in-plane rotation. The multifunctional sample probe has been successfully implemented to a host dewar and electromagnet system. The developed probe is capable of performing transport measurements and device characterizations with high flexibility of controlling magnetic and electrical fields, angle, temperature and current/voltage parameters in a wide range. The design of the multifunctional sample probe allows easy connection of other external devices and easy installation into other dewar and magnet systems. A software interface based on NI Labview visual programming language has been developed to control the system. The setup has been tested in the hysteresis loop measurements of perpendicularly magnetized Co/Pt/CoO ultra-thin films with anomalous Hall effect (AHE) method. We have also performed temperature dependent exchange bias measurements of the samples. The developed probe revealed much better resolution in the case of perpendicularly magnetized ultra-thin Co/Pt/CoO films compared to a standard vibrating sample magnetometry system.

Temperature dependence of exchange bias in (NiFe/IrMn)n multilayer films studied through static and dynamic techniques

The in-plane temperature dependence of exchange bias was studied through both dc magnetometry and ferromagnetic resonance spectroscopy in a series of [NiFe/IrMn]n multilayer films, where n is the number of layer repetitions. Major hysteresis loops were recorded in the temperature range of 300 K to 2 K to reveal the effect of temperature on the exchange bias in the static regime while temperature-dependent continuous-wave ferromagnetic resonance for frequencies from 3 to 16 GHz was used to determine the exchange bias dynamically. Strong divergence between the values of exchange bias determined using the two different types of measurements as well as a peak in temperature dependence of the resonance linewidth were observed. These results are explained in terms of the slow-relaxer mechanism.

Thickness dependent interfacial magnetic coupling in [La2NiMnO6/LaMnO3] multilayers

In the present work, interfacial magnetic exchange coupling at FM/AFM interface has been studied by varying the thickness of AFM layer (LaMnO3) in (La2NiMnO6/LaMnO3)15 multilayer thin film based system. In multilayer thin films, the thickness of LMO was varied from 30 to 50Å, while the thickness of LNMO was kept constant at 100Å. Thin films of LNMO, LMO and LNMO/LMO multilayers have been deposited by pulsed laser deposition technique on (001) LaAlO3 substrate. The thin films have been studied for their structural and magnetic properties. XRD analysis reveals the c-axis epitaxial growth of LNMO, LMO and their multilayer thin films. Exchange bias (EB) effect has been observed in the field cooled hysteresis loops of multilayer thin films and the interaction between FM and AFM spins at the interface is responsible for the observed effect. EB measurements reveal that the thickness variation influences the interfacial interaction between two layers. The EB increases with increasing AFM film thickness in multilayer thin film samples and maximum EB (∼740Oe) is observed for sample with 50Å thickness of AFM layer. Temperature dependence of EB and training effect measurements have also been performed to confirm the EB in the sample.

Effect of shell thickness on the exchange bias blocking temperature and coercivity in Co-CoO core-shell nanoparticles

The exchange bias blocking temperature distribution of naturally oxidized Co-CoO core-shell nanoparticles exhibits two distinct signatures. These are associated with the existence of two magnetic entities which are responsible for the temperature dependence of an exchange bias field. One is from the CoO grains which undergo thermally activated magnetization reversal. The other is from the disordered spins at the Co-CoO interface which exhibits spin-glass-like behavior. We investigated the oxide shell thickness dependence of the exchange bias effect. For particles with a 3 nm thick CoO shell, the predominant contribution to the temperature dependence of exchange bias is the interfacial spin-glass layer. On increasing the shell thickness to 4 nm, the contribution from the spin-glass layer decreases, while upholding the antiferromagnetic grain contribution. For samples with a 4 nm CoO shell, the exchange bias training was minimal. On the other hand, 3 nm samples exhibited both the training effect and a peak in coercivity at an intermediate set temperature Ta. This is explained using a magnetic core-shell model including disordered spins at the interface.

Antisite disorder induced spin glass and exchange bias effect in Nd2NiMnO6 epitaxial thin film

We report the observation of the exchange bias effect and spin glass behaviour at low temperature in a ferromagnetic Nd2NiMnO6 epitaxial thin film. Along with the ferromagnetic transition at ∼194 K, an additional transition is observed at lower temperature (∼55 K) as seen from M-T curves of the sample. A shift in the ac susceptibility peak with frequency has been observed at low temperature, which is a signature of a glassy phase within the sample. The detailed investigation of the memory effect and time dependent magnetic relaxation measurements reveals the presence of a spin glass phase in the Nd2NiMnO6 thin film. The exchange bias effect observed at low temperature in the sample has been associated with an antisite disorder induced spin glass phase, which results in a ferromagnetic/spin glass interface at low temperature. The exchange bias behaviour has been further confirmed by performing cooling field and temperature dependence of exchange bias along with training effect measurements.

Enhanced exchange bias fields for CoO/Co bilayers: influence of antiferromagnetic grains and mechanisms

The emergence and optimization of devices that can be applied to spintronics have attracted considerable interest, and both experimental and theoretical approaches have been used in studies of exchange bias phenomena. A survey of the literature indicates that great efforts have been devoted to improving exchange bias fields, while only limited attempts have been made to controll the temperature dependence of exchange bias. In this study, the influence of antiferromagnetic grains on exchange bias phenomena in CoO/Co bilayers on a semiconductor surface was investigated. Based on an antiferromagnetic grain model, a correlation between grain size, grain density, blocking temperature, and the exchange bias field was established. For crystallites with a smaller median diameter, the dependence of the thickness of the CoO layer on blocking temperature showed a less pronounced variation. This is due to the larger thermal agitation of the atomic spin moments in the grain, which causes a weaker exchange coupling between atomic spin moments. The enhanced density of antiferromagnetic/ferromagnetic pinning sites resulting from an increased grain density is responsible for the enhancement in the exchange bias fields. The results reported herein provide insights into our knowledge related to controlling the temperature dependence of exchange bias and related mechanisms.

Modeling temperature dependent exchange bias in systems with magnetoelectric chromia

Exchange bias in a magnetoelectric Cr2O3/ferromagnet system at finite temperature, based on the formation of a domain wall in Cr2O3, has been investigated using Monte Carlo simulation. It has been shown that the calculation of the exchange bias based on domain wall formation yields a more realistic value than that calculated using interfacial exchange coupling between Cr2O3 and the adjacent ferromagnet. Possible shortcoming of the magnetoelectric effect in setting the switchable exchange bias in the low temperature regime has also been demonstrated based on an energy threshold requirement. Specifically, it has been found that the magnetoelectric effect becomes intrinsically less effective in switching the exchange bias at low temperature, thus making the applicability of the system limited to only a certain temperature range.

Coexistence of perpendicular and in-plane exchange bias using a single ferromagnetic layer in Pt/Co/Cr/CoO thin film

We studied the temperature dependence of magnetization and exchange bias in a Pt/Co/Cr/CoO multilayer thin film. These magnetic multilayers are of particular interest since the easy axis of ultra-thin Co is strongly affected by the interfacial anisotropies of neighbouring Pt and Cr layers. The room temperature measurements show that the sample has a magnetic easy axis only in the film plane. However, upon cooling the sample, the easy axis of the magnetization departs from its initial orientation and typical easy-axis hysteresis loops are obtained for both in-plane and perpendicular directions. In accordance with this change in the magnetization direction at lower temperatures, the sample shows an unexpected coexistence of perpendicular and in-plane exchange bias below the antiferromagnetic transition of CoO. The temperature dependence of the exchange bias field for both directions is also significantly different. Along the film plane, the exchange bias field monotonically decreases and disappears at 220 K with increasing temperature. For the perpendicular direction, however, the exchange bias field increases and reaches a maximum value at 80 K. Then it decreases and disappears at 150 K with further increasing temperature. The mechanisms behind this anomalous temperature dependence of the exchange bias as well as the step-like behaviour in the hysteresis curves are discussed.