Uncompensated Interface Research Articles

Zero-field magnetic skyrmions in exchange-biased ferromagnetic–antiferromagnetic bilayers

We report on the stabilization of ferromagnetic skyrmions in zero external magnetic fields, in exchange-biased systems composed of ferromagnetic–antiferromagnetic (FM-AFM) bilayers. By performing atomistic spin dynamics simulations, we study cases of compensated, uncompensated, and partly uncompensated FM-AFM interfaces, and investigate the impact of important parameters such as temperature, inter-plane exchange interaction, Dzyaloshinskii–Moriya interaction, and magnetic anisotropy on the skyrmions appearance and stability. The model with an uncompensated FM-AFM interface leads to the stabilization of individual skyrmions and skyrmion lattices in the FM layer, caused by the effective field from the AFM instead of an external magnetic field. Similarly, in the case of a fully compensated FM-AFM interface, we show that FM skyrmions can be stabilized. We also demonstrate that accounting for interface roughness leads to stabilization of skyrmions both in compensated and uncompensated interface. Moreover, in bilayers with a rough interface, skyrmions in the FM layer are observed for a wide range of exchange interaction values through the FM-AFM interface, and the chirality of the skyrmions depends critically on the exchange interaction.

Mechanisms of linear control of exchange bias in Fe/FeO bilayer

The microscopic nature of pinning uncompensated moment interface, how it is pinned by an antiferromagnet, and how the mechanism with this coupling drives exchange bias remain outstanding challenges. In this work, the Fe/FeO ferromagnet/antiferromagnet bilayer with an ultrathin FeO layer is modeled and the correlation between exchange bias and microscopically uncompensated antiferromagnetic magnetization is studied, based on first-principles calculations and modified Monte-Carlo simulations, where spin energies are exactly calculated and energy barriers are detectable to avoid the occurrence of unphysical tunneling during spin reversals. The linear dependence of exchange bias on uncompensated antiferromagnetic magnetization in Fe/FeO bilayer experimentally is reproduced and interpreted by exploring the monolayer-resolved magnetization behavior in the FeO layer, which depends on antiferromagnetic anisotropy, cooling field, fraction of Fe atoms permeated in the FeO layer, and magnetic frustration effect. It is evidenced that this linear dependence requires plenty of controllable pinning uncompensated antiferromagnetic spins located in the layer depths close to but not in contact with the ferromagnetic layer.

Theory of propagation of nonlinear spin wave through an antiferromagnetic magnonic crystal with four-sublattice interfaces

The purpose of the research is constructing an analytical model of a nonlinear spin wave propagating through a single antiferromagnet/antiferromagnet boundary as well as in one-dimensional antiferromagnetic magnonic crystal comprised of two sorts of antiferromagnets (AFM). Both AFMs that comprise the magnonic crystal are assumed to be two-sublattice uniaxial ones. The Landau-Lifshitz equations have been used in the sigma model with account for the exchange bias between magnetic sublattices of both AFMs, the magnetic anisotropy, the magnetic dipole–dipole interaction and the Dzyaloshynskyi-Moriya interaction. As a result, the allowed discrete sets of frequencies and velocities for the considered spin wave are obtained. In the process of investigation, the boundary conditions for the Néel vector on the interface between two AFMs are derived with the exchange bias between magnetic sublattices of both AFMs taken into account. These boundary conditions are, in particular, applied for both fully uncompensated interface and fully compensated one for the illustration purposes. Analysis of the results show that the considered wave is reflectionless (thus, energy losses on reflection on the interfaces are absent), phase-coherent and possess a number of parameters that can be considered as degrees of freedom for encoding information. These findings open up new possibilities of digital data processing utilizing nonlinear spin waves propagating through antiferromagnetic magnonic crystal.

Magnon drag in a metal–insulating antiferromagnet bilayer

We study a bilayer structure consisting of an antiferromagnetic insulator and a normal metal. An electron current is driven in the normal metal with direction parallel to the interface between the materials. Due to interfacial exchange coupling between the localized spins in the antiferromagnet and the itinerant electrons in the normal metal, a magnon current can then be induced in the antiferromagnet. Using an uncompensated antiferromagnetic interface, creating an asymmetry in the interfacial coupling to the two degenerate magnon modes, we find that it is possible to generate a magnon spin current. The magnon spin current can be enhanced by increasing the temperature or by spin-splitting the magnon modes.

Spontaneous magnetization reversal and positive exchange bias effect in CoFeSiB/FeCrO3 heterostructures

The exchange bias (EB) phenomenon is an interface-driven effect; it depends on the interface spin configuration, which in turn is immensely related to antiferromagnetic spin configuration. Here, we chose an antiferromagnet, FeCrO3, with a corundum structure with randomly occupied cations. We report EB in the bilayer heterostructures that are composed of an amorphous CoFeSiB ferromagnet interfaced with an epitaxial FeCrO3 layer fabricated in-situ by pulsed laser deposition. The bilayers exhibit crossover from conventional negative EB to positive EB from room temperature down to 5 K. The crossover occurs for a cooling field that exceeds the interfacial coupling strength at the interface. Systems that show positive EB effects sometimes also display spontaneous magnetization reversal; the present bilayers showed spontaneous EB below 50 K. Temperature and field-dependent magnetisation studies reveal that the prepared heterostructure consists of uncompensated interface spins due to the disorder in the FeCrO3 layer that gets frozen at low temperatures, pins the spins in the ferromagnetic layer and causes the spontaneous EB effect.

Lamellar magnetism and exchange bias in billion-year-old metamorphic titanohematite with nanoscale ilmenite exsolution lamellae – III. Atomic-magnetic basis for experimental results

SUMMARY Lamellar magnetism is a source of remanent magnetization in natural rocks different from common bulk magnetic moments in ferrimagnetic minerals. It has been found to be a source for a wide class of magnetic anomalies with extremely high Koenigsberger ratio. Its physical origin are uncompensated interface moments in contact layers of nanoscale ilmenite lamellae inside an hematite host, which also generate unusual low-temperature (low-T) magnetic properties, such as shifted low-T hysteresis loops due to exchange bias. The atomic-magnetic basis for the exchange bias discovered in the hematite-ilmenite system is explored in a series of papers. In this third article of the series, simple models are developed for lamellae interactions of different structures when samples are either cooled in zero-field, or field-cooled in 5 T to temperatures below the ordering temperature of ilmenite. These models are built on the low-temperature measurements described earlier in Paper II. The important observations include: (i) the effects of lamellar shapes on magnetic coupling, (ii) the high-T acquisition of lamellar magnetism and low-T acquisition of magnetization of ilmenite lamellae, (iii) the intensity of lamellar magnetism and the consequent ilmenite magnetism in populations of randomly oriented crystals, (iv) lattice-preferred orientation of the titanohematite host crystal populations and (v) the effects of magnetic domain walls in the host on hysteresis properties. Based on exemplary growth models of lamellae with different geometries and surface couplings we here provide simple models to assess and explain the different observations listed above. Already the simplified models show that the shapes of the edges of ilmenite lamellae against their hematite hosts can control the degree of low-T coupling between ilmenite, and the lamellar magnetic moments. The models also explain certain features of the low-T exchange bias in the natural samples and emphasize the role of lattice-preferred orientation upon the intensity of remanent magnetization. The inverse link between ilmenite remanence and exchange-bias shift in bimodal low-T ilmenite lamellae is related to different densities of hematite domain walls induced by the clusters of ilmenite lamellae.

Thermally induced spin torque and domain-wall motion in superconductor/antiferromagnetic-insulator bilayers

We theoretically investigate domain wall motion in an antiferromagnetic insulator layer caused by thermally generated spin currents in an adjacent spin-split superconductor layer. An uncompensated antiferromagnet interface enables the two crucial ingredients underlying the mechanism - spin splitting in the superconductor and absorption of spin currents by the antiferromagnet. Treating the superconductor using the quasiclassical theory and the antiferromagnet via Landau-Lifshitz-Gilbert description, we find domain wall propagation along the thermal gradient with relatively large velocities $\sim 100$ m/s. Our proposal exploits the giant thermal response of spin-split superconductors in achieving large spin torques towards driving domain wall and other spin textures in antiferromagnets.

Schwinger boson study of superconductivity mediated by antiferromagnetic spin fluctuations

We study superconductivity in a normal metal, arising from effective electron-electron interactions mediated by spin-fluctuations in a neighboring antiferromagnetic insulator. Introducing a frustrating next-nearest neighbor interaction in a N\'eel antiferromagnet with an uncompensated interface, the superconducting critical temperature is found to be enhanced as the frustration is increased. Further, for sufficiently large next-nearest neighbor interaction, the antiferromagnet is driven into a stripe phase, which can also give rise to attractive electron-electron interactions. For the stripe phase, as previously reported for the N\'eel phase, the superconducting critical temperature is found to be amplified for an uncompensated interface where the normal metal conduction electrons are coupled to only one of the two sublattices of the magnet. The superconducting critical temperature arising from fluctuations in the stripe phase antiferromagnet can be further enhanced by approaching the transition back to the N\'eel phase.

Charge-magnon conversion at the topological insulator/antiferromagnetic insulator interface

Efficient generation of magnons is highly desirable for magnonics. Here we theoretically investigate the charge-magnon conversion at the interface of topological insulator/antiferromagnetic insulator (AFMI), where the Dirac electrons and the antiferromagnetic magnons are coupled by the fluctuation of s-d exchange field. Our model reveals that the charge-magnon conversion efficiency for uncompensated interface is 2--3 orders higher than that for compensated interface. The conversion is more efficient with smaller magnon gap and larger s-d exchange coupling. We find a significant efficiency enhancement when the thickness of the AFMI is in the two-dimensional regime, where the magnons experience a quantum confinement effect. $\mathrm{B}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{3}/\ensuremath{\alpha}\ensuremath{-}\mathrm{MnTe}$ bilayers are then proposed to be a promising system to realize strong charge-magnon conversion. Our work provides the principles of manipulating charge-magnon conversion and designing efficient charge-magnon conversion structure, which would help to improve the performance of antiferromagnetic magnonics, such as magnon transistor and magnon torque memories.

Magnon-mediated superconductivity on the surface of a topological insulator

We study superconductivity on the surface of a topological insulator, mediated by magnetic fluctuations in an adjacent ferromagnetic or antiferromagnetic insulator. Superconductivity can arise from effective interactions between helical fermions induced by interfacial fermion-magnon interactions. For both ferromagnetic and antiferromagnetic insulators, these fermion-fermion interactions have the correct structure to facilitate pairing between particles located on the same side of the Fermi surface, also known as Amperean pairing. In antiferromagnets, the strength of the induced interactions can be enhanced by coupling the topological insulator asymmetrically to the two sublattices of the antiferromagnet. This effect is further amplified by next nearest neighbor frustration in the antiferromagnetic insulator. The enhancement makes the induced interactions significantly stronger in the antiferromagnetic case, as compared to the ferromagnetic case. These results indicate that an uncompensated antiferromagnetic interface might be a better candidate than a ferromagnetic interface for proximity-induced magnon-mediated superconductivity on the surface of a topological insulator.

Enhancement of superconductivity mediated by antiferromagnetic squeezed magnons

We investigate theoretically magnon-mediated superconductivity in a heterostructure consisting of a normal metal and a two-sublattice antiferromagnetic insulator. The attractive electron-electron pairing interaction is caused by an interfacial exchange coupling with magnons residing in the antiferromagnet, resulting in p-wave, spin-triplet superconductivity in the normal metal. Our main finding is that one may significantly enhance the superconducting critical temperature by coupling the normal metal to only one of the two antiferromagnetic sublattices employing, for example, an uncompensated interface. Employing realistic material parameters, the critical temperature increases from vanishingly small values to values significantly larger than 1 K as the interfacial coupling becomes strongly sublattice-asymmetric. We provide a general physical picture of this enhancement mechanism based on the notion of squeezed bosonic eigenmodes.

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.

Spin Splitting Induced in a Superconductor by an Antiferromagnetic Insulator.

Inspired by recent feats in exchange coupling antiferromagnets to an adjacent material, we demonstrate the possibility of employing them for inducing spin splitting in a superconductor, thereby avoiding the detrimental, parasitic effects of ferromagnets employed to this end. We derive the Gor'kov equation for the matrix Green's function in the superconducting layer, considering a microscopic model for its disordered interface with a two-sublattice magnetic insulator. We find that an antiferromagnetic insulator with effectively uncompensated interface induces a large, disorder-resistant spin splitting in the adjacent superconductor. In addition, we find contributions to the self-energy stemming from the interfacial disorder. Within our model, these mimic impurity and spin-flip scattering, while another breaks the symmetries in particle-hole and spin spaces. The latter contribution, however, drops out in the quasiclassical approximation and thus, does not significantly affect the superconducting state.

Monte Carlo simulation of exchange bias in spin valve systems

We have proposed an atomistic model for the investigation of exchange bias effect in spin valve systems. Using Monte Carlo method, we have studied the field cooled thermal and magnetic properties such as the magnetization, magnetic susceptibility, and specific heat of the system belonging to different regions. Using the cooled spin configuration, we have evaluated the hysteresis loops, and discussed the validity of several theories regarding the physical origin of exchange bias. From our simulations, we have found that, in contrast to spherical particle models, the existence of uncompensated interface spins may not be a sufficient condition for origination of nonzero loop shifts in magnetic multilayers. In other words, a spin valve system with a purely flat interface may not exhibit exchange bias behavior. We have also discussed the influence of interface structure on the aforementioned effect due to a “uniaxial anisotropy” term acting only on the interface spin moments.

Monte Carlo study of the exchange bias effect in Co/CoO core–shell nanowires

We study the magnetic properties of cylindrical ferromagnetic core—antiferromagnetic shell nanowires using Monte Carlo simulations and a classical Heisenberg Hamiltonian in order to elucidate the impact of the oxidized shell on the magnetic properties and the magnetization reversal mechanism. We find that the coupling to the antiferromagnetic shell leads to suppression of the coercivity and emergence of a weak exchange bias effect. Comparison of the magnetization reversal mechanism in the bare and the surface-oxidized nanowire reveals that the domain wall propagation and annihilation remains the dominant reversal mechanism in surface oxidized nanowires as in their ferromagnetic counterparts. However, the interface exchange coupling introduces a secondary reversal mechanism activated in the central part of the wire with characteristics of coherent rotation, which acts in synergy to wall propagation leading to enhancement of the wall mobility. This effect is more pronounced in nanowires with large exchange bias values and is attributed to the uncompensated interface moments that act as nucleation centers for magnetization reversal. Our results are in good agreement with recent measurements in Co and Co/CoO nanowires.

Evidence of spin-glass like ordering and exchange bias effect in antisite-disordered nanometric La1.5Ca0.5CoMnO6 double perovskite

Single-phase polycrystalline La1.5Ca0.5CoMnO6 double perovskite nanoparticles (∼25nm) have been synthesized by chemical sol-gel method. We report here the structural, magnetic and transport properties using X-ray diffraction, dc magnetization, ac susceptibility, exchange bias and dc resistivity measurements. The Rietveld refinement of X-ray diffraction pattern reveals that the La1.5Ca0.5CoMnO6 (LCCMO) system crystallizes in orthorhombic structure with pbnm space group. Mn and Co ions are not completely ordered on the B sites due to the presence of about 30% antisite-disorder in the system. The ordering of Co2+ and Mn4+ gives rise to the ferromagnetism below 145K. A spin glass like ground state has also been observed near 37.6(4)K, arising mainly due to the presence of competing magnetic interactions and antisite-disorder in the LCCMO nanoparticles. The frequency dependence peak shift of the Ac-susceptibility peak in the glassy state follows the critical slowing down model. The observed memory effect in ac susceptibility data reveals the existence of interacting clusters in a competing magnetic interactions state. The presence of noticeable exchange bias effect can be best explained on the basis of uncompensated interface (ferromagnetic/spin-glass) spins of antisite-disordered LCCMO system. This anti-site disordered nanocompound exhibits semiconducting behavior with variable range hopping kind of electronic conduction mechanism in the temperature range of 200–300K. We have also observed large negative magnetoresistance (−30% at 100K and 60kOe) mainly due to the spin-polarized transport across the grain boundaries.

Antisite-disorder driven large exchange bias effect in phase separated La1.5Ca0.5CoMnO6 double perovskite

Investigations of structural and magnetic properties of polycrystalline hole doped double perovskite La1.5Ca0.5CoMnO6 has clearly revealed the existence of structural antisite-disorder (either, Co–O–Co or Mn–O–Mn) in the system. The ordering of Co2+ and Mn4+ gives rise to a ferromagnetic transition around 157K. A spin-canted antiferromagnetic transition is found in this material at TCAFM ~9K. The effect of antisite-disorder in the double perovskite structure is most likely the prime reason for antiferromagnetic interaction. The temperature dependent inverse susceptibility exhibits Curie-Weiss like behaviour and it yields an effective paramagnetic moment of 6.49 μB. At very low temperature (T<TCAFM), it shows a strong hysteresis loop shift with large exchange bias (EB) field of HEB ~5.5kOe and can be tuned by the cooling field. The presence of zero-field cooled spontaneous EB effect (P-type and N-type) is confirmed to be not an experimental artefact - an inherent property of this double perovskite material. A phenomenological model has been proposed to explain the exchange coupling between the ferromagnetic and canted-antiferromagnetic interfaces of antisite-disordered La1.5Ca0.5CoMnO6 mainly on the basis of uncompensated interface spins.

Exchange bias in (La,Ca)MnO3 bilayers: influence of cooling process

The exchange bias (EB) phenomenon in La2/3Ca1/3MnO3/La1/3Ca2/3MnO3 bilayers was studied using Monte Carlo simulations combined with the Heisenberg model and the Metropolis algorithm. These simulations were carried out using the model proposed by Kiwi for an uncompensated interface. The Hamiltonian considered several terms corresponding to the nearest neighbor interaction, magnetocrystalline anisotropy and Zeeman effect. Several interactions in the ferromagnetic (FM), antiferromagnetic (AFM) and FM/AFM interface were considered, depending on the type of interacting ion (Mn3+eg, Mn3+eg′ or Mn4+d3). The influence of field cooling and cooling temperature on the EB was analyzed and discussed. Regarding the field cooling, it caused an increase in the EB until a certain critical value was reached. After that, its effect was almost negligible. On the other hand, at low values of cooling temperature, not only the EB but also the coercive field were enhanced.

Frustrated multilayer ferromagnet-antiferromagnet structures: Beyond the scope of the exchange approximation (a review)

The frustrations of exchange interaction between ferromagnetic and antiferromagnetic layers, which arise at the uncompensated interface between the layers due to the interface roughness, have been described. The distribution of magnetic order parameters in the vicinity of the interface between the layers has been investigated, and the “layer thickness-roughness” magnetic phase diagram has been obtained in the case of the two-layer ferromagnet-antiferromagnet system and the ferromagnet-antiferromagnet-ferromagnet spin-valve system. An analysis has been performed taking into account the single-ion anisotropy energy, i.e., beyond the scope of the exchange approximation. It has been demonstrated that the number of easy axes in the layer plane, in many respects, determines the existence of an exchange shift of the hysteresis loop of the ferromagnet due to its interaction with the antiferromagnetic substrate.

Effect of Interface Roughness on Exchange Bias of an Uncompensated Interface: Monte Carlo Simulation

By means of Monte Carlo simulation, we investigate the effects of interface roughness and temperature on the exchange bias and coercivity in ferromagnetic (FM)/antiferromagnetic (AFM) bilayers. Both exchange bias and coercivity are strongly dependent on interface roughness. For a perfect uncompensated interface a domain wall is formed in the AFM system during FM reversal, which results in a very small exchange bias. However, a finite interface roughness leads to a finite value of the exchange bias due to the existence of pinned spins at the AFM surface adjacent to the mixed interface. It is observed that the exchange bias decreases with increasing temperature, consistent with the experimental results. It is also observed that a bump in coercivity occurs around the blocking temperature.