Magnetic Exchange Bias Effect Research Articles

In-situ electric field-tailored exchange bias in the manganite/ferroelectric multiferroic heterostructures

The application of electric field-induced magnetic structure changes has significantly advanced nonvolatile data storage with ultralow energy consumption, as well as spintronics and quantum computing processes. In artificial perovskite oxide multiferroic structures composed of ferromagnetic and ferroelectric layers, the evolution of magnetic phases and ferroelectric domains often contribute to the magnetoelectric coupling. To fully understand the mechanisms behind electric field-induced changes in magnetic structures and enable the miniaturization of magnetoelectric devices, it is crucial to achieve local ferroelectric domain-triggered ferromagnetic evolution. In this study, we have fabricated La0.7Ca0.3MnO3/Pb(Zr0.52Ti0.48)O3 (LCMO/PZT) heterostructures, where the magnetic phase separation of LCMO and the ferroelectric domain of PZT can be modulated by substrates and piezoelectric force microscopy, respectively. Our experiments demonstrate that by switching the polarization states of PZT, we can observe electric field control of the magnetic exchange bias effect through changes in the phase separation of the LCMO layer. These results contribute to the development of magnetoelectric devices with enhanced properties and offer valuable insights for future design strategies.

Effects of intrinsic and extrinsic multi-phased lattice structure and chemical heterogeneity on the control of ferrimagnetic properties in Co rich spinel ferrite

The present work reports the structural and magnetic properties in spinel ferrite composition around Co2.25Fe0.75O4 as the function of self-composite structure (ratio of two spinel phase components), coexisting impurity phase of hematite, and non-stoichiometric chemical composition. The neutron diffraction patterns have been used to determine lattice structure and magnetic moment. The neutron depolarization experiments have indicated spin reorientation at lower temperature for the single phased sample. The magnetization measurements confirmed the blocking of magnetic domains, possible magnetic spin compensation and magnetic exchange bias effect. The sample coexisting with small hematite phase has shown the maximum coercivity (∼14.682 kOe), remanent magnetization (∼1.4657 μB) and saturation magnetization (∼2.027 μB) with small exchange bias shift (- 71 Oe) at 5 K. The single phased sample exhibited the lowest magnetic moment (∼0.3775 μB) with maximum exchange bias shift (- 543 Oe) and the highest cooling-field induced enhancement of coercivity (3691 Oe). The experiments confirmed a large scope of tuning the ferrimagnetic parameters by controlling the phase fraction and interfacial phases in Co rich spinel ferrite.

Interface induced exchange bias effect in La0.67Sr0.33MnO3/SrIrO3 multilayer

Epitaxial superlattices of ferromagnetic/paramagnetic La0.67Sr0.33MnO3/SrIrO3 materials have been prepared on SrTiO3 (100) substrate using pulse laser deposition technique. An unexpected onset of interface magnetic interaction has been observed around 40 K. Interestingly, magnetic exchange bias effect has been observed in both field cooled and zero field cooled magnetization loops, however, the shifting of loop is opposite in both measurements. Exchange bias field vanishes as temperature increases to interface magnetic ordering temperature. Moreover, exchange bias field is found to decrease with increasing cooling field. We believe that tuning of magnetic exchange at interface during field cooling induces this evolution in nature of exchange bias field.

Evidence for exchange bias coupling at the perovskite/brownmillerite interface in spontaneously stabilized SrCoO3−δ/SrCoO2.5 bilayers

Interface effect in complex oxide thin film heterostructures lies at the vanguard of current research to design technologically relevant functionality and explore emergent physical phenomena. While most of the previous works focus on the perovskite/perovskite heterostructures, the study on perovskite/brownmillerite interfaces remain at its infancy. Here, we investigate spontaneously stabilized perovskite-ferromagnet (SrCoO3-{\delta})/brownmillertite-antiferromagnet (SrCoO2.5) bi-layer with TN > TC and discover an unconventional interfacial magnetic exchange bias effect. From magnetometry investigations, it is rationalized that the observed effect stems from the interfacial ferromagnet/antiferromagnet coupling. The possibility for coupled ferromagnet/spinglass interface engendering such effect is ruled out. Strikingly, a finite coercive field persists in the paramagnetic state of SrCoO3-{\delta} whereas the exchange bias field vanishes at TC. We conjecture the observed effect to be due to the effective external quenched staggered field provided by the antiferromagtic layer for the ferromagnetic spins at the interface. Our results not only unveil a new paradigm to tailor the interfacial magnetic properties in oxide heterostructures without altering the cations at the interface, but also provide a purview to delve into the fundamental aspects of exchange bias in such unusual systems paving a big step forward in thin film magnetism.

Antiferromagnetic Order in Epitaxial FeSe Films on SrTiO_{3}.

Single monolayer FeSe film grown on a Nb-doped SrTiO_{3}(001) substrate shows the highest superconducting transition temperature (T_{C}∼100 K) among the iron-based superconductors (iron pnictides), while the T_{C} value of bulk FeSe is only ∼8 K. Although bulk FeSe does not show antiferromagnetic order, calculations suggest that the parent FeSe/SrTiO_{3} films are antiferromagnetic. Experimentally, because of a lack of a direct probe, the magnetic state of FeSe/SrTiO_{3} films remains mysterious. Here, we report direct evidence of antiferromagnetic order in the parent FeSe/SrTiO_{3} films by the magnetic exchange bias effect measurements. The magnetic blocking temperature is ∼140 K for a single monolayer film. The antiferromagnetic order disappears after electron doping.

The exchange bias behavior of BiFeO3 nanoparticles with natural core-shell structure

The surface and interface effects of small antiferromagnetic nanostructures are important on the modulation of their magnetic properties. In this report, temperature and particle size dependent magnetic exchange bias effect was investigated in BiFeO3 (BFO) nanoparticles that possess natural core-shell structure. Nonmonotonic variation of exchange bias field, interesting surface spin-glass state and improved exchange bias training effect are only obtained in 18 nm BFO particles. Based on comparative experiments on particles with different sizes, we found that the surface spins and the interaction among them show great effect on the interfacial exchange coupling of the core-shell structure, and thus are responsible for the peculiar exchange bias behavior in small BFO nanoparticles. Our work provides the effect of surface spin state on the magnetic characteristics of nanomaterials and will favor their applications on spintronic devices.

The regulation of magnetoelectric coupling and magnetic exchange bias effects in La doped Bi5Ti3FeO15 multiferroic films

La doped Bi5Ti3FeO15 (BTFO) multiferroic films with various doping concentration were synthesized by chemical solution deposition method. The structure, ferroelectric, magnetic properties, and magnetoelectric coupling effect were systemically investigated. Room temperature magnetoelectric effect was obtained. And the magnetoelectric coupling and magnetic exchange bias effects are regulated by the doping concentration of La. Such regulation originates from the evolution of magnetic structure derived from doping rare earth La. The coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) orders can be controlled by the doping concentration of La. Thus, the superexchange interaction between FM and AFM orders was enhanced strongly, which contributes to the strong magnetoelectric coupling effect. This work provides an avenue on regulating the magnetic structure and magnetoelectric coupling effects.

Novel highly ordered core–shell nanoparticles

Core–shell nanoparticles have potential for a wide range of applications due to the tunability of their magnetic, catalytic, electronic, optical, and other physicochemical properties. A frequent drawback in the design of core–shell nanoparticles and nanocrystals is the lack of control over an extensive, disordered, and compositionally distinct interface that occurs due to the dissimilarity of structural and compositional phases of the core and shell. In this work, we demonstrate a new hydrothermal nanophase epitaxy (HNE) technique to synthesize highly structurally ordered α-Cr2O3@α-Co0.38Cr1.62O2.92 inverted core–shell nanoparticles (CSNs) with evidence for the nanoscale growth of corundum structure beginning from the core and extending completely into the shell of the CSNs with minimal defects at the interface. The high-resolution TEM results show a sharp interface exhibiting epitaxial atomic registry of shell atoms over highly ordered core atoms. The XPS and Co K-edge XANES analyses indicate the +2 oxidation state of cobalt is incorporated in the shell of the CSNs. Our XPS and EXAFS results are consistent with oxygen vacancy formation in order to maintain charge neutrality upon substitution of the Co2+ ion for the Cr3+ ion in the α-Co0.38Cr1.62O2.92 shell. Furthermore, the CSNs exhibit the magnetic exchange bias effect, which is attributed to the exchange anisotropy at the interface made possible by the nanophase epitaxial growth of the α-Co0.38Cr1.62O2.92 shell on the α-Cr2O3 core of the nanoparticles. The combination of a well-structured, sharp interface and novel nanophase characteristics is highly desirable for nanostructures having enhanced magnetic properties.

The ferromagnetic–antiferromagnetic properties of Ni–Cr2O3composite hollow spheres prepared by an in situ reduction method

Considerable efforts have been exerted on the facile synthesis of magnetic composite materials because of their unique properties and potential applications. Especially for ferromagnetic–antiferromagnetic systems, the magnetic exchange bias effect is essential for the development of magneto-electronic switching devices and magnetic storage media. In this research, a facile ethylene glycol in situ reduction strategy has been successfully employed in the preparation of Ni–Cr2O3 composite hollow spheres. X-Ray powder diffraction was used to determine the phase composition. Scanning electron microscopy and transmission electron microscopy was employed to characterize the morphologies of the as-prepared samples. Experiments proved that the volume ratio of ethylene glycol to water played a determinative role in the final morphology of the products. The magnetization vs. temperature results revealed a spin-glass-like behavior with blocking temperature of about 150 K for the as-prepared Ni–Cr2O3 composites. Induced by the coupling between ferromagnetic Ni and antiferromagnetic Cr2O3, a small exchange bias effect could be observed in the magnetic hysteresis loops. At lower temperature, a larger exchange bias field and coercivity are obtained. A high surface area of 145.1 m2 g−1 was obtained for the prepared porous hollow spheres.

Exchange bias effect in nickel zinc ferrite–mesoporous silica nanocomposites

Nickel zinc ferrite–mesoporous silica nanocomposite (NZF–MS) was synthesized using impregnation method. The microstructure was investigated by transmission electron microscopy. A magnetic exchange bias effect was exhibited by the nanocomposites. This was ascribed to the presence of a ferromagnetic core and antiferromagnetic shell structure. Electron microscopic studies confirmed the presence of a core–shell structure with NZF forming the core. The zero-field cooled magnetization data as a function of temperature indicated the presence of an antiferromagnetic phase which is believed to be formed by the diffusion of Fe3+ or Ni2+ ions into the silica glass.

Phase separation and exchange biasing in the ferromagnetic IV-VI semiconductor Ge1−xMnxTe

Ferromagnetic Ge1−xMnxTe grown by molecular beam epitaxy with Mn content of xMn≈0.5 is shown to exhibit a strong tendency for phase separation. At higher growth temperatures apart from the cubic Ge0.5Mn0.5Te, a hexagonal MnTe and a rhombohedral distorted Ge0.83Mn0.17Te phase is formed. This coexistence of antiferromagnetic MnTe and ferromagnetic Ge0.5Mn0.5Te results in magnetic exchange-bias effects.