Unidirectional Magnetic Anisotropy Research Articles

Electrically Controllable Exchange Bias via Interface Magnetoelectric Effect

Exchange bias is a unidirectional magnetic anisotropy that often arise from interfacial interaction of a ferromagnetic and antiferromagnetic layers. In this article, we show that a metallic layer with spin-orbit coupling can induces an exchange bias via an interface magnetoelectric effect. In linear response regime, the interface magnetoelectric effect is induced by spin-orbit couplings that arises from the broken symmetry of the system. Furthermore, we demonstrate that the exchange bias can be controlled by electric field.

Locally Reconfigurable Exchange Bias for Tunable Spintronics by Pulsed Current Injection

Unidirectional magnetic anisotropy induced by exchange bias (EB) between ferromagnetic and antiferromagnetic layers is of paramount importance in various spintronic devices. Traditional setting and optimization of EB involve cooling through the Néel temperature of the antiferromagnets under biasing field in vacuum furnace which takes hours of time and cannot realize localized control of EB. Herein, an alternative process to configure EB by injection of microsecond current impulse for locally heating of the heterostructure is reported. 180‐degree exchange bias switching has been realized in both anisotropic magnetoresistance and giant magnetoresistance units, resulting in tunable magnetic field sensing functionalities. This mechanism for pinpoint control of EB provides more freedom for the design and optimization of various sensing and logic spintronics.

Fourfold magnetic anisotropy induced in CoFeB/IrMn bilayers by interfacial exchange coupling

Exchange bias (EB) occurring in ferromagnetic (FM)/antiferromagnetic (AFM) bilayers conventionally can lead to a unidirectional magnetic anisotropy () as well as an accompanied uniaxial magnetic anisotropy (). We observed an additional fourfold magnetic anisotropy () induced by interfacial exchange coupling in amorphous CoFeB/epitaxial IrMn bilayers with an EB. Because of the combined effect of the three kinds of magnetic anisotropies, one- and two-step magnetic switching processes were observed at different magnetic field orientations, which usually appear in single-crystal FM layer with an intrinsic magnetocrystalline anisotropy but not in amorphous FM layer. The angular dependent magnetic switching fields can be nicely fitted by a phenomenological model based on domain wall nucleation and propagation with the in-plane along <100>. The ferromagnetic resonance measurements indicate that the specific strength of for EB along [100] is larger than that for EB along [110]. The induced can be understood by considering two types of AFM domains caused by both monatomic steps and defects and their induced net uncompensated spins along the in-plane <100> axes. The different dependence of on the EB direction are because of the different effects of growth magnetic field on the presence of AFM domains.

Electric field control of magnetic states in ferromagnetic-multiferroic nanostructures.

Multiferroic oxides are considered as key elements of energy-consuming devices required for the development of scalable logic and information storage technologies. In this regard, understanding the mechanisms of magnetoelectric switching and finding the optimal way to switch magnetization by an electric field is of crucial importance. In this study, we develop a model for studying magnetic states in a nanoscale exchange-coupled ferromagnetic-multiferroic heterostructure subjected to the action of an electric field. Based on bias effects emerging due to the coupling between a ferromagnetic subsystem and an antiferromagnetically ordered multiferroic material, we explore the magnetic textures and the magnetization reversal processes in a ferromagnet. As the multiferroic material, we consider BiFeO3, where magnetic ordering and ferroelectric ordering are determined by the mutually perpendicular antiferromagnetic (L), weak ferromagnetic (M) and polarization (P) vectors. Application of an electric voltage removes degeneration from eight energetically equivalent positions of P|| 〈111〉, allocates the definite directions of vectors P, M, and L and as a consequence the unidirectional magnetic anisotropy axis in the reference ferromagnetic layer. Our study reveals the features of the magnetic configurations in systems of different geometries, with varying exchange and magnetic anisotropy, necessary to determine the optimal conditions for switching magnetic states in a multiferroic bi-layer by an electric field.

Tunnel magnetoresistance sensors with symmetric resistance-field response and noise properties under AC magnetic field modulation

Magnetoresistance (MR) devices with symmetric (even-function) resistance change to an external magnetic field is useful for various magnetic sensor applications, such as encoders for position and rotation detection and ultra-sensitive magnetic field sensors, when combined with a frequency modulation method. In this study, we developed tunnel magnetoresistance (TMR) sensors with even-function resistance–field (R–H) responses and investigated their noise properties. The developed TMR sensors achieved a higher MR ratio (∼120%) than those of conventional giant magnetoresistive multilayers exhibiting even-function R–H responses. The TMR sensors were realized by stabilizing the magnetization of the magnetic free layer (FL) in an anti-parallel configuration with respect to that of the reference layer. Orange-peel ferromagnetic coupling between the FL and a pinned layer through an AgSn spacer layer was used to control the strength of the unidirectional magnetic anisotropy of the FL, by which even-function R–H responses with negligible hysteresis were obtained. The sensors enabled us to study the intrinsic noise properties of TMR devices, particularly under an external AC modulation magnetic field. We found that the magnetic 1/f noise originating from thermal magnetization fluctuation was the dominant noise of the TMR sensors at low frequencies. We observed that the application of an AC modulation field to the TMR sensors increased the noise owing to the magnetic domain fluctuation and the transfer of magnetic 1/f noise to the modulation frequency.

Exchange-coupling-induced fourfold magnetic anisotropy in CoFeB/FeRh bilayer grown on SrTiO3(001)

Exchange coupling across the interface between a ferromagnetic (FM) layer and an antiferromagnetic (AFM) or another FM layer may induce a unidirectional magnetic anisotropy and/or a uniaxial magnetic anisotropy, which has been extensively studied due to the important application in magnetic materials and devices. In this work, we observed a fourfold magnetic anisotropy in amorphous CoFeB layer when exchange coupling to an adjacent FeRh layer which is epitaxially grown on an SrTiO3(001) substrate. As the temperature rises from 300 K to 400 K, FeRh film undergoes a phase transition from AFM to FM phase, the induced fourfold magnetic anisotropy in the CoFeB layer switches the orientation from the FeRh〈 110〉 to FeRh〈 100 〉 directions and the strength is obviously reduced. In addition, the effective magnetic damping as well as the two-magnon scattering of the CoFeB/FeRh bilayer also remarkably increase with the occurrence of magnetic phase transition of FeRh. No exchange bias is observed in the bilayer even when FeRh is in the nominal AFM state, which is probably because the residual FM FeRh moments located at the interface can well separate the exchange coupling between the below pinned FeRh moments and the CoFeB moments.

Magnetic anisotropy and reversal in epitaxial FeGa/IrMn bilayers with different orientations of exchange bias

Epitaxial FeGa/IrMn bilayers with exchange biases along the FeGa[100] and [110] directions are prepared on MgO(001) single crystal substrates by magnetron sputtering through controlling the orientation of the external field &lt;i&gt;in situ&lt;/i&gt; applied during growth. The effect of the exchange bias orientation on the magnetic switching process and the magnetic switching field are studied. The X-ray &lt;i&gt;φ&lt;/i&gt;-scan indicates that the FeGa layer is epitaxially grown with a 45° in-plane rotation on the MgO(001) substrate along the FeGa(001)[110] direction and the MgO(001)[100] direction. The measurements of the angular dependence of the ferromagnetic resonance field and the corresponding fitting to the Kittel equation show that the samples have a superposition of fourfold symmetric magnetocrystalline anisotropy &lt;inline-formula&gt;&lt;tex-math id="M4"&gt;\begin{document}$ {K}_{1} $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M4.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M4.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, unidirectional magnetic exchange bias anisotropy &lt;inline-formula&gt;&lt;tex-math id="M5"&gt;\begin{document}$ {K}_{\mathrm{e}\mathrm{b}} $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M5.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M5.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, and uniaxial magnetic anisotropy &lt;inline-formula&gt;&lt;tex-math id="M6"&gt;\begin{document}$ {K}_{\mathrm{u}} $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M6.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M6.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; with configuration of &lt;inline-formula&gt;&lt;tex-math id="M7"&gt;\begin{document}$ {K}_{\mathrm{e}\mathrm{b}}//\left[100\right] $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M7.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M7.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; or &lt;inline-formula&gt;&lt;tex-math id="M8"&gt;\begin{document}$ {K}_{\mathrm{e}\mathrm{b}}//\left[110\right] $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M8.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M8.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;. The combined longitudinal and transverse magneto-optical Kerr effect measurements show that sample with &lt;inline-formula&gt;&lt;tex-math id="M9"&gt;\begin{document}$ {K}_{\mathrm{e}\mathrm{b}}//\left[100\right] $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M9.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M9.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; exhibits square loops, asymmetrically shaped loops, and one-sided two-step loops in different external magnetic field directions. In contrast, the sample with &lt;inline-formula&gt;&lt;tex-math id="M10"&gt;\begin{document}$ {K}_{\mathrm{e}\mathrm{b}}//\left[110\right] $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M10.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M10.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; exhibits one-sided two-step and two-sided two-step loops as the magnetic field orientation changes. Because the &lt;inline-formula&gt;&lt;tex-math id="M11"&gt;\begin{document}$ {K}_{1} $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M11.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M11.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; is superimposed by &lt;inline-formula&gt;&lt;tex-math id="M12"&gt;\begin{document}$ {K}_{\mathrm{u}} $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M12.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M12.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; and &lt;inline-formula&gt;&lt;tex-math id="M13"&gt;\begin{document}$ {K}_{\mathrm{e}\mathrm{b}} $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M13.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M13.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, the in-plane fourfold symmetry of the magnetic anisotropy energy is broken. The local minima are no longer strictly along the in-plane &lt;inline-formula&gt;&lt;tex-math id="M14"&gt;\begin{document}$ \left\langle{100}\right\rangle $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M14.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M14.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; directions, but make a deviation angle which depends on the relative orientation and strength of magnetic anisotropy. A model based on the domain wall nucleation and propagation is proposed with considering the different orientations of &lt;inline-formula&gt;&lt;tex-math id="M15"&gt;\begin{document}$ {K}_{\mathrm{e}\mathrm{b}} $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M15.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M15.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, which can nicely explain the change of the magnetic switching route with the magnetic field orientation and fit the angular dependence of the magnetic switching fields, indicating a significant change of domain wall nucleation energy as the orientation of &lt;inline-formula&gt;&lt;tex-math id="M16"&gt;\begin{document}$ {K}_{\mathrm{e}\mathrm{b}} $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M16.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220166_M16.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; changes.

Features of unidirectional magnetic anisotropy in multilayer nanostructured systems [Co–Al[formula omitted]O[formula omitted

We study the magnetic anisotropy of multilayer nanostructures [Co–Al2O3]N with fixed Co island layer thickness (d≈1.2nm) and varying Al2O3 layer thickness. We use the Kerr effect to measure the magnetization hysteresis loops at different orientations of the magnetic field. We observed a unidirectional magnetic anisotropy in structures with small thicknesses of Al2O3 layers (d<1.4nm) when the exchange interaction between the Co layers is large. This anisotropy was oriented at 150° with respect to the easy axis of a continuous Co film. We believe that the unidirectional anisotropy can be caused by the super-vortex magnetization of the multilayer structure, while the unidirectional magnetization is caused by the projection of the super-vortex core on the sample plane.

Isotropic FMR frequency enhancement in thin Py/FeMn bilayers under strong magnetic proximity effect

Exchange biasing in ferromagnet/antiferromagnet bilayers is known to enhance the material’s ferromagnetic resonance frequency and make it strongly angle dependent due to the unidirectional anisotropy induced at the interface. We observe a ten-fold enhancement in frequency and angle-independent ferromagnetic resonance in bilayers of Py/FeMn with ultrathin FeMn, accompanied by a significantly enhanced magnetic moment. The observed isotropic frequency enhancement is consistent with rotatable rather than unidirectional magnetic anisotropy and the induced magnetic moment links this anisotropy with the ferromagnet-proximity effect. The estimated effective anisotropy field acting on the proximity-induced moment in ultrathin FeMn can be as high as 0.5 T at room temperature. Our results show the potential of the ferromagnetic proximity effect combined with the inherent exchange anisotropy in antiferromagnets for high-speed spintronic applications.

Unidirectional Magnetic Anisotropy in Dense Vertically-Standing Arrays of Passivated Nickel Nanotubes.

We report the facile and low-cost preparation as well as detailed characterization of dense arrays of passivated ferromagnetic nickel (Ni) nanotubes (NTs) vertically-supported onto solid Au-coated Si substrates. The proposed fabrication method relies on electrochemical synthesis within the nanopores of a supported anodic aluminum oxide (AAO) template and allows for fine tuning of the NTs ferromagnetic walls just by changing the cathodic reduction potential during the nanostructures’ electrochemical growth. Subsequently, the experimental platform allowed further passivation of the Ni NTs with the formation of ultra-thin antiferromagnetic layers of nickel oxide (NiO). Using adequately adapted magnetic measurements, we afterwards demonstrated that the thickness of the NT walls and of the thin antiferromagneticNiO layer, strongly influences the magnetic behavior of the dense array of exchange-coupled Ni/NiO NTs. The specific magnetic properties of these hybrid ferromagnetic/antiferromagnetic nanosystems were then correlated with the morpho-structural and geometrical parameters of the NTs, as well as ultimately strengthened by additionally-implemented micromagnetic simulations. The effect of the unidirectional anisotropy strongly amplified by the cylindrical geometry of the ferromagnetic/antiferromagnetic interfaces has been investigated with the magnetic field applied both parallel and perpendicular to the NTs axis.

Influence of interfacial magnetic ordering and field-cooling effect on perpendicular exchange bias and magnetoresistance in nanoporous IrMn/[Co/Pd] films

We have studied systematically the effect of field cooling on the magnetic properties of continuous and porous IrMn/[Co/Pd] films. It is found that the coexistence of two ferromagnetic (FM) phases in the porous film, namely, hard-magnetic and soft-magnetic ones, with significantly different magnetic properties relates to the role of pore edges and modifies its magnetic and magnetoresistive properties. It is shown that annealing of the films with their subsequent cooling in an external magnetic field applied for aligning the magnetic moments in the antiferromagnetic (AFM) IrMn layer improves effectively the uniaxial perpendicular anisotropy of the [Co/Pd] layer and induces unidirectional anisotropy in its hard-magnetic regions, blocking simultaneously the soft-magnetic parts by pinning their magnetic moments along the film plane. Magnetoresistance of both continuous and porous films is found to be determined mainly by electron–magnon scattering, whereas the complex morphology of the porous film providing different orientations of exchange coupling at the AFM/FM interface in different film regions modifies significantly the spin-dependent electron transport. The revealed asymmetry of the field dependences of magnetoresistance is attributed both to unidirectional magnetic anisotropy of the FM layer and its splitting into magnetically nonequivalent regions in the porous films. The origin of the observed phenomenon is associated with a local influence on the orientation of AFM magnetic moments by an adjacent ferromagnet.

Effect of AFM and FM exchange interaction on magnetic anisotropy properties of single domain SmFeO3 at nanoscale

Multifunctional Samarium orthoferrites (SmFeO3) nano-particles have been studying extensively due to their applications in digital memory, ultrafast switching, temperature dependent spin valves and sensor applications. Magnetic properties of SFO were investigated by varying particle size, initial magnetic state of material and applied field under Zero Field Cooling (ZFC) and Field Cooling (FC) protocol. It was found that the remnant magnetization, M−H behavior and coercive field are extremely dependent on particle size. It was also found that the magnetic behavior of single domain SFO nanoparticle is completely different to that of multi domain bulk due to uniaxial magneto crystalline anisotropy. The magnetization vs temperature (M−T) behavior of nano-particle depends on unidirectional anisotropy along with uniaxial magneto-crystalline anisotropy of SFO. The Sm3+ spin ordering in nano-SFO compensates to that of Fe3+ spin ordering at higher temperature (~13 K) compare to that of bulk (~4K). The Sm3+ spin unidirectional magnetic anisotropy is more effective in between ~13 K to ~65 K and Fe3+ spin unidirectional anisotropy is effective at higher temperature beyond ~65 K. Magnetically poled nano-particles show completely different behavior to that of non-poled nano-SFO particles at low temperature. In nutshell, the manuscript shows why one can’t predict the magnetic behavior of the SFO without knowing its particle or grain size, initial magnetic state of the particle, procedure of measurement, the magnitude of applied field and temperature etc., since all these affects the magneto crystalline anisotropy as well as unidirectional anisotropy of SFO.

Concomitant Magnetic Memory Effect in CrO2–Cr2O3 Core–Shell Nanorods: Implications for Thermal Memory Devices

The origin of the concomitant memory effect is still a controversial issue and poor evidence in the observation of nanocrystal systems. We report on a type of concomitant memory effect driven by first-field-induced unidirectional magnetic anisotropy at the interface of ferromagnetic CrO2-core and antiferromagnetic Cr2O3-shell nanorods, with the effect becoming less significant in pure CrO2 nanorods. To corroborate the results, a core–shell anisotropic energy model was used to determine the coherent rotation of magnetization and exchange coupling constant, giving direct access to the anisotropic interfacial properties of the core–shell nanoscale system without spatially resolved magnetic measurement. The outcome of this study will be useful for the future development of thermal memory devices.

Strain‐Gradient‐Induced Unidirectional Magnetic Anisotropy in Nanocrystalline Thin Permalloy Films

Herein, the effect of inhomogeneous elastic deformation on the magnetic anisotropy of Ni71.5Fe28.5 wt% nanocrystalline films is investigated. The in‐plane controlled strains are induced in the film by bending of a glass substrate which has a thickness step in the middle. Ferromagnetic resonance measurements reveal the existence of in‐plane unidirectional magnetic anisotropy. The anisotropy behavior directly correlates with the calculated strain gradients. It is shown that this correlation is well explained by the flexomagnetic effect, which establishes the relation between the magnetization and the inhomogeneous strains. The experimental value of the flexomagnetic coefficient for the thin Ni71.5Fe28.5 wt% film is 1.5 × 10−3 T m.

Zero-field-cooled exchange bias effect in phase-segregated La2−xAxCoMnO6−δ (A=Ba,Ca,Sr;x=0,0.5)

In the zero-field-cooled exchange bias (ZEB) effect the unidirectional magnetic anisotropy is set at low temperatures even when the system is cooled in the absence of external magnetic field. La$_{1.5}$Sr$_{0.5}$CoMnO$_{6}$ stands out as presenting the largest ZEB reported so far, while for La$_{1.5}$Ca$_{0.5}$CoMnO$_{6}$ the exchange bias field ($H_{EB}$) is one order of magnitude smaller. Here we show that La$_{1.5}$Ba$_{0.5}$CoMnO$_{6}$ also exhibits a pronounced shift of its magnetic hysteresis loop, with intermediate $H_{EB}$ value in respect to Ca- and Sr-doped samples. In order to figure out the microscopic mechanisms responsible for this phenomena, these compounds were investigated by means of synchrotron X-ray powder diffraction, Raman spectroscopy, muon spin rotation and relaxation, AC and DC magnetization, X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). The parent compound La$_{2}$CoMnO$_{6}$ was also studied for comparison, as a reference of a non-ZEB material. Our results show that the Ba-, Ca- and Sr-doped samples present a small amount of phase segregation, and that the ZEB effect is strongly correlated to the system's structure. We also observed that mixed valence states Co$^{2+}$/Co$^{3+}$ and Mn$^{4+}$/Mn$^{3+}$ are already present at the La$_{2}$CoMnO$_{6}$ parent compound, and that Ba$^{2+}$/Ca$^{2+}$/Sr$^{2+}$ partial substitution at La$^{3+}$ site leads to a large increase of Co average valence, with a subtle augmentation of Mn formal valence. Estimates of the Co and Mn valences from the $L$-edge XAS indicate the presence of oxygen vacancies in all samples (0.05$\leq \delta \leq$0.1). Our XMCD results show a great decrease of Co moment for the doped compounds, and indicate that the shift of the hysteresis curves for these samples is related to uncompensated antiferromagnetic coupling between Co and Mn.

Interface assisted high magnetoresistance in BiFeO3/Fe97Si3 thin film at room temperature

Bilayer thin films of multiferroic oxide BiFeO3 and metallic alloy Fe97Si3 deposited by RF-magnetron sputtering have been investigated to understand spin polarized electron transport across bilayer interface. Effect of BiFeO3 thickness variation on magnetic and magnetotransport properties of bilayer thin films has been analyzed. An induced unidirectional magnetic anisotropy in bilayer thin films has been observed by interface coupling between antiferromagnetic BiFeO3 and ferromagnetic Fe97Si3 spin magnetic moments. Increasing interface roughness with increase in BiFeO3 film thickness has been found to be a regulatory factor for inducing in-plane unidirectional anisotropy and exchange interaction at bilayer interface. A high magnetic moment ∼315 emu/cc originated at room temperature by uncompensated interface spin density has been measured in 30 nm thick BiFeO3 film. Magnetic inhomogeneities at interface give rise to a spin glass like phase responsible for spin flip scattering at bilayer interface. Enhanced low field magnetoresistance ∼30% in 60 nm thick BiFeO3 film is ascribed to intensified scattering from disordered interface states with increasing BiFeO3 thickness. Magnetodielectric measurements (applied magnetic field 1 kOe) have revealed five times increase in relaxation time of charge carriers inside grains confirming intergrain charge transport dominated by alignment of magnetic moments of nearby grains via exchange coupling. High value of magnetoresistance ∼46% in 60 nm thick film of increased grain size is due to enhanced critical length for charge transport. Observed room temperature magnetoresistive properties of BiFeO3/Fe97Si3 thin films are critical for developing multiferroic antiferromagnetic based spintronic devices.

Influence of a Strong Magnetic Field on Magnetic Order in Polycrystalline Layered Cobaltites EuBaCo2 –xO5.5 – δ

The magnetic properties of polycrystalline EuBaCo2 –xO5.5 – δ (x ≤ 0.1, δ ≤ 0.14) are studied in the temperature range 2 K < T < 300 K in magnetic fields H ≤ 90 kOe. A comparison is made of the dependencies of magnetization M on temperature T and magnetic field H for polycrystalline and single crystal EuBaCo1.90O5.36, in which only Cu(III) ions are present. It is shown that in the magnetically ordered temperature range of magnetic order, the samples are ferrimagnetic, in which antiferromagnetic interactions between the nearest Co3+ ions with the same spins compete with the ferromagnetic exchange. With a decrease in temperature, a strong magnetic field affects the magnetic order. At T = 2 K in fields H = 50–90 kOe, all samples exhibit the properties of mictomagnetism. There are a shift in the hysteresis loops M(H) and a large difference in the values of the magnetization of the sample cooled in a magnetic field M (FC) and without a field M (ZFC) due to unidirectional magnetic anisotropy. In this case, there is no saturation of magnetization. The field dependences of magnetization taken in the cooling and heating in unequal fields (CHUF) mode indicate a metastable magnetic state. Near T = 20–30 K, an orientational transition characteristic of micromagnets takes place and a complex ferrimagnetic structure is formed. The main reasons for the manifestation of mictomagnetism properties in layered cobaltites are the presence of magnetoactive Co3+ ions in the states with intermediate and high spins at low temperatures and structural defects that affect the exchange interaction between the nearest neighbors. Nonmagnetic Co3+ ions in the low-spin state can also act as defects.

Asymmetry in magnetic behavior caused by superposition of unidirectional and four-fold magnetic anisotropies in CoPt/FeMn bilayers

We investigated magnetic anisotropy and magnetization reversal process in CoPt/FeMn bilayers by four-probe measurements of anisotropic magnetoresistance (AMR) effect. Here, a single crystal-CoPt/polycrystal-FeMn interface has been established, where A1-CoPt layer with four-fold magnetic anisotropy was epitaxially grown on MgO(100) substrate, while the top FeMn layer was 111 textured showing in-plane polycrystalline feature. Due to the noncollinear 3Q AFM spin structure and existence of uncompensated spins in ultrathin FeMn, the magnetic anisotropy of the bilayer can be tuned through interfacial exchange coupling, controlled by field cooling treatment. As a result, it gives rise to asymmetric AMR curves. We show that this asymmetry strongly relies on the direction of induced unidirectional magnetic anisotropy with respect to the easy axis of CoPt layer, leading to a modulated magnetization reversal process.

Effect of Selective Thermomagnetic Treatment on Unidirectional Magnetic Anisotropy in FeNi/TbCo Films

The effect of selective thermomagnetic treatment on the exchange interaction between magnetic layers in Fe11Ni89/Tb30Co70/Ti films has been investigated. Dependencies of the exchange bias field induced in the Fe11Ni89 layer on the annealing temperature have been obtained and interpreted. We consider structural modifications of the interface and stress-relaxation processes in the Fe11Ni89 layer to be the main reasons for the observed modifications of hysteresis properties.

Preferential weakening of rotational magnetic anisotropy by keV-He ion bombardment in polycrystalline exchange bias layer systems

The consequences of keV light-ion bombardment of polycrystalline exchange bias layer systems on individual magnetic anisotropies composing the effective unidirectional magnetic anisotropy are investigated by vectorial magneto-optic Kerr effect measurements. Preferential reduction of rotatable magnetic anisotropy as compared to the other magnetic anisotropies is observable, disproving the intuitive assumption of an equable weakening of all magnetic anisotropies due to the bombardment. It is concluded that light-ion bombardment has a stronger impact on the magnetic characteristics of smaller grains in a polycrystalline antiferromagnet as compared to those of larger ones.