Exchange Bias Interaction Research Articles

Ion irradiation modulated magnetic properties of materials and its applications

Ion irradiation, a technology in which ion beams are used to irradiate materials, has high manipulation precision, short processing time, and many applications in the fields of material modification, chip manufacturing, biomedicine, energy and chemicals. Especially in magnetic material modification, customized modifications of magnetic materials can be achieved by precisely controlling the energy, dose, and direction of the ion beam. To further enhance the performances of magnetic materials and explore new magnetic devices, this study focuses on how ion irradiation precisely modulates various magnetic interactions and the analysis of its influence on the spin Hall effect and magnetic structural dynamics. Firstly, the latest research achievements are emphasized of ion irradiation regulated magnetic characteristics such as perpendicular magnetic anisotropy, exchange bias, and RKKY interaction. These regulation methods are crucial for understanding and optimizing the microstructure and properties of magnetic materials. Secondly, the significant role played by ion irradiation in regulating spin-orbit torque devices is discussed in detail. These applications demonstrate the potential of ion irradiation technology in designing high-performance magnetic storage and processing devices. Finally, the future applications of ion irradiation technology in the preparation of multifunctional magnetic sensors and magnetic media for information storage are discussed, highlighting its great enormous innovation and application potential in the field of magnetic materials.

Magnetic properties of Fe-doped NiO nanoparticles

Undoped and Fe-doped NiO nanoparticles were successfully synthesized using a lyophilization method and systematically characterized through magnetization techniques over a wide temperature range, with varying intensity and frequency of the applied magnetic fields. The Ni1-xFexO nanoparticles can be described by a core-shell model, which reveals that Fe doping enhances exchange interactions in correlation with nanoparticle size reduction. The nanoparticles exhibit a superparamagnetic blocking transition, primarily attributed to their cores, at temperatures ranging from above room temperature to low temperatures, depending on the Fe-doping level and sample synthesis temperature. The nanoparticle shells also exhibit a transition at low temperatures, in this case to a cluster-glass-like state, caused by the dipolar magnetic interactions between the net magnetic moments of the clusters. Their freezing temperature shifts to higher temperatures as the Fe-doping level increases. The existence of an exchange bias interaction was observed, thus validating the core-shell model proposed.

Structural engineering of porous biochar loaded with ferromagnetic/anti-ferromagnetic NiCo2O4/CoO for excellent electromagnetic dissipation with flexible and self-cleaning properties

Design of multi-functional microwave absorption materials with strong dissipation ability is a practical approach to address the current issue of electromagnetic radiation pollution. Herein, based on the exchange bias interaction between ferromagnetic and anti-ferromagnetic interfaces, a series of absorbers composed of the porous biochar loaded with ferromagnetic/anti-ferromagnetic NiCo2O4/CoO were successfully prepared via a fairly simple process of one-step calcination. By regulating the calcination temperature, the pore size and porosity of porous carbon, morphology of loaded NPs as well as the electromagnetic response property and impedance matching characteristic in such composites can be modulated. The porous biochar/NiCo2O4/CoO composite prepared under 350 °C possesses a remarkable electromagnetic absorption reflection loss (RL) of –48.41 dB at 9.12 GHz with 2.5 mm thickness, and the effective absorption bandwidth (EAB) of 4.32 GHz with 2.2 mm thickness is located at X band, this is owing to the strong coupling effect of multiple dielectric polarizations, magnetic resonances, and eddy current consumption under matched impedance. In addition, the microwave absorbing patch prepared with this composite exhibits a well-hydrophobic property for self-cleaning function, and the large extensibility with substantial breaking strength endows its practical usage as a flexible absorber.

Tunable interplay between exchange coupling and uniaxial magnetic anisotropy in epitaxial CoO/Au/Fe trilayers

We show that the interaction between ferromagnetic Fe(110) and antiferromagnetic CoO(111) sublayers can be mediated and precisely tuned by a nonmagnetic Au spacer. Our results prove that the thickness of the Fe and Au layers can be chosen to modify the effective anisotropy of the Fe layer and the strength of the exchange bias interaction between Fe and CoO sublayers. Well-defined and tailorable magnetic anisotropy of the ferromagnet above Néel temperature of the antiferromagnet is a determining factor that governs exchange bias and interfacial CoO spins orientation at low temperatures. In particular, depending on the room temperature magnetic state of Fe, the low-temperature exchange bias in a zero-field cooled system can be turned “off” or “on”. The other way around, we show that exchange bias can be the dominating magnetic anisotropy source for the ferromagnet and it is feasible to induce a 90-degree rotation of the easy axis as compared to the initial, exchange bias-free easy axis orientation.

Transition from AFM Spin Canting to Spin Glass-AFM Exchange as Particle Size Decreases in LaFeO3.

In this work, we have studied structural and magnetic properties of LaFeO3 as a function of the particle size d, from bulk (d >> 1 µm) to nanoscale (d ≈ 30 nm). A large number of twins were observed for large particles that disappear for small particle sizes. This could be related to the softening of the FeO6 distortion as particle size decreases. It was observed that the bulk sample showed spin canting that disappeared for d ~ 125 nm and can be associated with the smoothening of the orthorhombic distortion. On the other hand, for d < 60 nm, the surface/volume ratio became high and, despite the high crystallinity of the nanoparticle, a notable exchange effect bias appeared, originated by two magnetic interactions: spin glass and antiferromagnetism. This exchange bias interaction was originated by the formation of a "magnetic core-shell": the broken bonds at the surface atoms give place to a spin glass behavior, whereas the inner atoms maintain the antiferromagnetic G-type order. The LaFeO3 bulk material was synthesized by the ceramic method, whereas the LaFeO3 nanoparticles were synthesized by the sol-gel method; the particle size was varied by annealing the samples at different temperatures. The physical properties of the materials have been investigated by XRD, HRTEM, TGA, and AC and DC magnetometry.

Nanocomposite Obtained in the Plasma of a Pulsed High Voltage Discharge Using Nickel Electrodes and PTFE

In the plasma of pulsed high-voltage discharge, initiated between nickel electrodes in air, when the fluoroplastic is placed in the discharge gap, powder nanocomposite material has been synthesized. The nanocomposite contains NiF2 nanoparticles less than 5 nm in size, dispersed in a matrix consisting of carbon and fluorocarbon substances. The carbonaceous substance contains nanoscale disordered graphite-like regions. The fluorocarbon component of the composite contains fragments of PTFE molecules and fluorocarbon molecular fragments that differ in structure from PTFE molecule’s structure. After annealing the composite in air at 773 K, the initial nanocomposite is transformed into a nanocomposite containing nanosized PTFE and nanoparticles of NiF2 less than 5 nm in size, scattered in a matrix composed of nanographite and low-layer nanosized graphene, after aneling at 1173 K into a material containing NiO nanoparticles less than 10 nm in size. After annealing of the initial nanocomposite in argon atmosphere at 1073 K, the obtained nanocomposite contains Ni nanoparticles with sizes less than 5 nm and carbon and fluorocarbon components. The magnetic susceptibility of the unannealed nanocomposite is investigated. A transition to the antiferromagnetic phase at 73 K was detected. At T = 4K, exchange bias interaction of the AFM / FM type takes place in the composite. There is divergence of the FC and ZFC curves, which can be explained by the presence of a superparamagnetic phase or a spin glass phase in the sample. The field dependences of the magnetic susceptibility measured at T = 300 K show sharp changes that occur at certain values of the magnetic field. Elucidation of the nature of these changes requires additional research.

Structural studies and exchange bias interaction in Sr+2 doped LaFeO3 synthesised via PVA based sol-gel method

Modification in the crystal structure often exhibits different functionalities and characteristics. In the present study it is demonstrated that Sr2+ modified lanthanum ferrite La(1-x)SrxFeO3 (x = 0, 0.02, 0.04 and 0.06) prepared using PVA based sol gel method shows unique structure and magnetic properties. Structural analysis and phase formation study was performed using X-ray diffraction data and various structural parameters were calculated using Rietveld refinement. It is observed that with the increase in Sr2+ content the structural parameters of LaFeO3 unit cell change. The FESEM and TEM studies were used to study the size and morphology of La(1-x)SrxFeO3 nanoparticles. Magnetic hysteresis measurement reveals the appearance of zero field cooled exchange bias in La(1-x)SrxFeO3, originating due to the interfacial exchange coupling between AFM/FM. In addition, the magnetization increases while exchange bias vanishes at higher strontium concentration. These results help in understanding the in-depth interfacial coupling interaction mechanism.

Effect of non-$$180^{\circ }$$ polarization invariants on the exchange bias of tetragonal $$\langle 001\rangle $$ and rhombohedral $$\langle 111\rangle $$ orientations of bismuth ferrite epitaxial thin films

\(\hbox {BiFeO}_{3}\) (BFO) is the only room temperature multiferroic material that has been extensively studied due to its multifunctional properties. BFO with a canted G-type antiferromagnetic (AFM) ordering exhibits strong exchange bias characteristics with NiFe which offers the potential to design and utilize devices working based on multiferroic features. In the past, it was known that the presence of \(180^{\circ }\) domain walls of BFO hinders a plausible exchange bias interaction. To understand the role of the strain-induced effects on such \(180^{\circ }\) domain walls and its effect on the exchange bias, NiFe, a soft ferromagnetic layer, was grown on the epitaxial BFO AFM layers. An approximately 80 nm BFO layer was grown epitaxially in both the tetragonal (001) phase on the \(\hbox {LaAlO}_{3}\) (001) substrate and the rhombohedral (111) phase on the \(\hbox {SrTiO}_{3}\) (111) substrate, with a thin (10 nm) layer of \(\hbox {Ni}_{80}\hbox {Fe}_{20}\) on top of it. An exchange bias of 510 Oe was observed in the tetragonal phase of BFO with a c / a ratio of 1.22, which is comparable with the exchange bias shown by the (111) oriented rhombohedral phase (360 Oe). Both the tetragonal (001) and rhombohedral (111) layers possess ferroelectric polarization normal to the sample surface and so the domain walls are mostly \(180^{\circ }\) oriented which is expected to have a minimum effect on the exchange bias. However, the weak strain-induced structural variants in the (111) oriented rhombohedral BFO and the monoclinic distortion present in the tetragonal BFO introduce non-\(180^{\circ }\) domain walls in the system. These variants arising due to the structural distortion are expected to play a key role in defining the ferroelectric domain wall nature, thereby exhibiting exchange bias characteristics.

Manipulating exchange bias using all-optical helicity-dependent switching

Deterministic all-optical control of magnetization without an applied magnetic field has been reported for different materials such as ferrimagnetic and ferromagnetic thin films and granular recording media. These findings have challenged the understanding of all-optical helicity-dependent switching of magnetization and opened many potential applications for future magnetic information, memory and storage technologies. Here we demonstrate optical control of an antiferromagnetic layer through the exchange bias interaction using the helicity of a femtosecond pulsed laser on IrMn/[Co/Pt]xN antiferromagnetic/ ferromagnetic heterostructures. We show controlled switching of the sign of the exchange bias field without any applied field, only by changing the helicity of the light, and quantify the influence of the laser fluence and the number of light pulses on the exchange bias control. We also present the combined effect of laser pulses and applied magnetic field. This study opens applications in spintronic devices where the exchange bias phenomenon is routinely used to fix the magnetization orientation of a magnetic layer in one direction.

Asymmetric magnetoimpedance effect in CoFeSiB amorphous ribbons by combination of field and current annealing for sensor applications

The roles of applied magnetic field during the current annealing of Co68.15Fe4.35Si12.5B15 soft magnetic amorphous ribbons are studied. Samples heat treated by Joule heating effect in open air and simultaneously in the present of longitudinal external magnetic field showed asymmetric magnetoimpedance (AMI) behavior. The AMI profile can be related to the exchange bias interaction between the soft magnetic amorphous material and a harder magnetic crystalline phase formed on the surface of the ribbon. This effect stems from thermal effect, the transverse Oe field generated from the annealing current which is thickness dependent and the longitudinal external field. The single peak AMI with the field sensitivity of 101%/Oe for DC annealing current is achieved. Our results could address a simple way to achieve the AMI response toward developing high sensitive magnetic field sensors.

One-pot facile synthesis of cobalt oxide nanocubes and their magnetic properties

Co3O4 nanocubes with perfect shape were synthesized by a simple hydrothermal route in which Co3O4 was directly prepared in the one-pot process with aging temperature set at 160 °C without a subsequent calcination. In this study, as the whole process is sintering free, this can successfully avoid the agglomeration of nanoparticles and protect the integrity of perfect crystal form to the maximum extent. Cubic Co3O4 nanoparticles were characterized by field emission scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray diffraction. Important factors influencing the nanoparticles’ formation were discussed, which include temperature, molar ratio of Co(Ac)2 to NaOH, reactant concentration, and the use of surfactant. Optical property was investigated by Raman spectroscopy and UV–Vis spectroscopy. Magnetic property measurement indicated that the sample exhibited a low Neel temperature (33 K) and bulk antiferromagnetic coupling due to geometric confinement of antiferromagnetic order within the nanoparticles. Below TN, the noteworthy exchange bias interaction between ferromagnetically coupled surface spins and underlying antiferromagnet-like coupled surface spins has also been observed.

Controlling the magnetic characters co-existing at the novel nanometric multiferroic PrAlxFe1−xO3; 0.0 ⩽ x ⩽ 0.6 via an exchange bias interaction to open a new era of applications

For the first time, the nanometric multiferroic perovskites PrAlxFe1−xO3; 0.0⩽x⩽0.6, were synthesized and extensively studied. XRD results and tolerance factor calculations revealed the distorted orthorhombic single-phase structure for all synthesized samples. HRTEM micrographs for PrFeO3 (x=0.0) indicated homogeneous orthorhombic nanoparticles with high local crystallinity and highly ordered lattice planes. Interplanar d-spacing calculated from HRTEM accorded well with the d-values estimated from XRD. Introducing the undersized Al3+ cations to the B-site on the expenses of Fe3+ cations affected the crystallites’ size negatively and resulted in weak lattice inflation due to the BO6 octahedral tilting. As a result, PrAl0.1Fe0.9O3 (x=0.1) showed polycrystalline nature with deteriorated lattice arrangements, lower ordering and fair homogeneity. Magnetic hysteresis and DC molar magnetic susceptibility verified that this novel and unique series of nanometric multiferroics carries the antiferromagnetic (AFM) character as a secondary magnetic component inside all of its compositions. The existence of superparamagnetism (SPM) and ferromagnetism (FM) among selected x-values were also verified. PrFeO3 showed the superparamagnetic character, after which SPM to FM phase transition was established at x=0.1 with soft magnetic features and improved Ms, Mr and Hc values. Interesting magnetic hardening appeared at x=0.2 up to x=0.4 with further improvement of the magnetic constants. FM to AFM phase transition was reached at x=0.6 with the coexistence of the former two phases with different extents at 0.1⩽x-values⩽0.6. Competition between ferromagnetic and antiferromagnetic sub-lattices lead to a ferrimagnetic-like state and induced a phenomenon of exchange bias interaction which resulted in hysteresis necking at x=0.2 and 0.5. Our results are signatures of a complex magnetic structure considered as a magnetic core–shell-like system or a magnetic multilayer-like system with an exchange interaction between the FM/AFM heterosystems at their interfaces. Recorded Ms and Mr values exceed those reported in literature for RFeO3 multiferroics with weak ferromagnetic moment. Each sample in this series showed its own magnetic behavior and can be applied to a specific tailored application. New era of applications are expected for this new multiferroic series of perovskites as exchange bias materials.

Controlled growth of Ni/NiO core–shell nanoparticles: Structure, morphology and tuning of magnetic properties

We performed a detailed study of Ni/NiO core–shell nanoparticles (NP) obtained with a gas aggregation source. The NP oxide shells were produced by oxidizing the NP with different procedures: deposition in oxygen atmosphere, post-annealing in air, sequential deposition of (a) first NiO layer, (b) Ni NP and (c) third NiO Layer. X-ray photoelectron spectroscopy from Ni 2p core-level gave information about the chemical state of Ni in the core and in the oxide shell, while scanning electron microscopy was used for investigation of the NP morphology. High quality scanning transmission electron microscopy in high angle annular dark field mode data demonstrated core–shell structure also for NiO/Ni NP/NiO samples. Field-cooled/zero-field-cooled magnetization curves and field-cooled isothermal hysteresis cycles at T = 5 K were recorded by a SQUID magnetometer. In this way, the relation between magnetic properties and oxide shell structure was assessed, showing the role played by the control of the formation of oxide on the exchange bias and interparticle magnetic interaction.

Magnetic proximity effect in Pr0.5Ca0.5MnO3/La0.7Sr0.3MnO3 bilayered films

Magnetotransport properties of the Pr0.5Ca0.5MnO3/La0.7Sr0.3MnO3 bilayer and the Pr0.5Ca0.5MnO3and La0.7Sr0.3MnO3 films deposited on LaAlO3 (001) substrate by pulse laser deposition were investigated. X-ray diffraction and high-resolution electron-microscopy analysis reveal that lattice parameters for the constituent sublayers in the bilayer are very close to those for the individual films. It was found that a ferromagnetic transition in the La0.7Sr0.3MnO3 sublayer significantly modifies magnetotransport properties of the Pr0.5Ca0.5MnO3 constituent sublayer due to the magnetic proximity effect. Evidence of this effect includes the appearance of exchange bias interaction between the constituent sublayers, a localized-to-itinerant crossover in the system of polarized electrons, which results in formation of the Griffiths-like ferromagnetic state, and an unusual polar transport of carriers. Experimental results were analyzed within the framework of the modern theoretical approach.

Exchange biasing in SFMO/SFWO double perovskite multilayer thin films

In the present study, we have studied the exchange bias interaction in ferromagnetic Sr 2FeMoO 6 (SFMO)/antiferromagnetic Sr 2FeWO 6 (SFWO) multilayer thin films deposited on single crystal LaAlO 3 substrates using KrF pulsed laser deposition technique. XRD pattern revealed that SFMO, SFWO and their multilayer thin films were highly oriented along the c-axis. The microstructure studied by atomic force microscopy was found to be uniform, fine, dense and homogenous in nature. The observed magnetization-temperature curves showed Neel temperature T N ∼ 37 K for SFWO and Curie temperature T C > 320 K for SFMO thin films. For multilayer, the field cooled magnetization–field curve was shifted horizontally and the direction of the horizontal shift is opposite to that of H FC, indicating an exchange bias effect. Exchange bias field H E was found to decrease with increase in temperature and approached to zero at blocking temperature.

Nanoscale Control of Exchange Bias with BiFeO3 Thin Films

We demonstrate a direct correlation between the domain structure of multiferroic BiFeO3 thin films and exchange bias of Co 0.9Fe 0.1/BiFeO3 heterostructures. Two distinct types of interactions - an enhancement of the coercive field ( exchange enhancement) and an enhancement of the coercive field combined with large shifts of the hysteresis loop ( exchange bias) - have been observed in these heterostructures, which depend directly on the type and crystallography of the nanoscale ( approximately 2 nm) domain walls in the BiFeO3 film. We show that the magnitude of the exchange bias interaction scales with the length of 109 degrees ferroelectric domain walls in the BiFeO 3 thin films which have been probed via piezoresponse force microscopy and X-ray magnetic circular dichroism.

Ultrafast laser excitation of spin waves and the permanent modification of the exchange bias interaction in IrMn∕Co

Single mode coherent magnetization oscillations are observed in exchange biased IrMn∕Co thin films by the pump-probe time-resolved magneto-optical Kerr effect, using moderate intensity pump pulses. Unlike past experiments on exchange biased systems, the oscillations produced can be excited when the Co magnetization is saturated along the pinning axis. The oscillation frequencies correspond to ferromagnetic resonance measurements and can be described using a FMR equation. It is also demonstrated that ultrafast laser pulses can induce pinning along the direction of the applied magnetic field.

Exchange bias in Co-Cr2O3 nanocomposites

The possibility of using exchange bias in a ferromagnetic-antiferromagnetic system to overcome the effect of superparamagnetism in small ferromagnetic nanoparticles is explored. We have prepared Co-Cr2O3 nanocomposite powders using a chemical method and shown that the effect of superparamagnetism in cobalt nanoparticles could be overcome using exchange bias between Co and Cr2O3. The superparamagnetic blocking temperature of 3 nm cobalt particles has been increased to above room temperature. The choice of Cr2O3 is vital as its TN is higher compared to other antiferromagnetic materials used for this purpose such as CoO. The field cooled and zero field cooled hysteresis measurements of the samples confirm the existence of exchange bias interaction in this system.

Dynamic superparamagnetic state in nano-clustered Nd 0.5Sr 0.5MnO 3 films

The structural and the magnetic properties of Nd 0.5Sr 0.5MnO 3 films with column-like nano-clustered microstructure have been investigated. The high-resolution electron microscopy, X-ray diffraction and electron diffraction reveal that the average cluster diameter was about 10 nm, and that the charge-ordered state, typical for this composition, was not observed down to 87 K. The films manifest a superparamagnetic behavior with increasing temperature, and an additional antiferromagnetic transition at T N⩽50 K. The analysis of the temperature-dependent magnetization shows that an increase in the applied magnetic field from 0.01 to 1 T leads to a drastic decrease from 3000 to 150 μ B in the effective magnetic moment of superparamagnetic particles, which is simply described by μ eff∼1/ H. Therefore, the superparamagnetic particles in these films are not rigidly bound with the crystalline clusters and the size is controlled by applied magnetic field. The greatly asymmetric hysteresis loops at low temperatures are explained by the exchange-bias interaction between the ferro- and the antiferro-magnetic clusters.

Thermal stability of magnetic nanostructures in ion-bombardment-modified exchange-bias systems

In magnetic bilayer systems consisting of a ferromagnet and an antiferromagnet the strength and direction of the exchange bias coupling can be set by ion bombardment in an external magnetic field. Magnetic nanostructures with a laterally varying exchange bias direction can be produced by local ion bombardment (ion bombardment induced magnetic patterning). We have investigated the thermal stability of these magnetic nanostructures by in situ x-ray photoemission electron microscopy while heating the samples above their blocking temperature. The investigations have been done at a $10.4\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}\ifmmode\times\else\texttimes\fi{}10.4\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ large checkered pattern with a minimum size of the magnetic patterns of $800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}\ifmmode\times\else\texttimes\fi{}800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ on a field cooled $\mathrm{Mn}\mathrm{Ir}∕\mathrm{Co}\mathrm{Fe}$ stack and a pattern with $1.6\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ wide lines with a periodicity of $5\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ on an as-prepared $\mathrm{Mn}\mathrm{Ir}∕\mathrm{Co}$ stack. The temperature dependence of the magnetization pattern can be explained by the temperature dependence of the exchange bias interaction, the exchange interaction energy, and the stray field energy. No substantial change of the thermal stability of magnetic patterns in remanence by the ion bombardment was found.