Interface Exchange Interaction Research Articles

Modeling of the effect of the thickness of the antiferromagnetic layer on the energy of interfacial exchange interaction in Ferromagnetic/Antiferromagnet films

By means of thermodynamic and configuration averaging of the energy of the exchange interaction between the magnetic spin moments adjacent through the ferromagnetic/antiferromagnetic interphase boundary, the expression for the energy of the interphase exchange interaction Eex is obtained. The dependence of the energy Eex on the temperature and the characteristic dimensions of the ferromagnetic/antiferromagnetic two-phase film is modeled. The experimentally observed regularity of the growth of the energy of the interfacial exchange interaction due to the increase of the thickness of the antiferromagnetic layer is confirmed. A method for the estimation of the constants of the interfacial exchange interaction Jint and the exchange interaction in antiferromagnetics JAFM is proposed based on the agreement of the experimental and theoretical curves of the temperature dependence of the exchange bias field HebT.

Static and dynamic magnetic behavior of YBCO/Co/IrMn heterostructures

The effect of the YBCO superconducting (SC) state on the magnetic properties of as-grown YBCO/Co/IrMn heterostructures has been systematically studied using magnetometry and ferromagnetic resonance. The obtained data showed that the superconductivity of the YBCO substrate strongly affects the ferromagnetic properties of the deposited Co layer deeper (up to 50 nm) than the coherence length of the YBCO (≃4nm) by an exchange interaction between the Co magnetic moments and the superconducting pairs at the YBCO/Co interface. The interfacial exchange interaction, switched on while the YBCO enters the SC state, pins Co spins and yields an enhancement of the Co magnetic properties.

Switching of magnetic bilayer nanotube ring with antiferromagnetically coupled layers in an annular field

The magnetization switching behavior of magnetic nanotube rings with core–shell structure is investigated by solving the Landau-Lifshitz-Gilbert (LLG) equation, focusing on the effects of the magnetic structure, interfacial exchange interactions, anisotropy constants, and transverse field on the coercivity of the system. The reduction of the coercivity of the magnetic nanorings can increase the magnetization switching rate and reduces the energy consumption when the magnetic nanorings are used as magnetic storage units. In the study, it is found that the antiferromagnetic exchange interaction between the spins in the core can greatly reduce the coercivity of the system, and the coercivity decreases with the increase of the interface exchange interaction. When the anisotropy constant of the antiferromagnetic core gradually increases, the coercivity of the system varies nonlinearly and a point of minima occurs. Finally, the effect of the transverse magnetic field on the hysteresis behavior of the system is also discussed, and it is found that the coercivity of the system decreases gradually with the increase of the transverse magnetic field, but the hysteresis loop of the system is distorted when the critical value is exceeded.

Modeling of the Influence of Oxidation on the Energy of Interfacial Exchange Interaction in Co/CoO Films

The interphase exchange interaction energy of ferromagnetic/antiferromagnetic films is determined through the mean spin theory. The thickness of the oxidized layer affects the exchange field, as shown by the Co/CoO film.

Deterministic Magnetization Reversal in Synthetic Antiferromagnets using Natural Light.

Traditional current-driven spintronics is limited by localized heating issues and large energy consumption, restricting their data storage density and operation speed. Meanwhile, voltage-driven spintronics with much lower energy dissipation also suffers from charge-induced interfacial corrosion. Thereby finding a novel way of tuning ferromagnetism is crucial for spintronics with energy-saving and good reliability. Here, a visible light tuning of interfacial exchange interaction via photoelectron doping into synthetic antiferromagnetic heterostructure of CoFeB/Cu/CoFeB/PN Si substrate is demonstrated. Then, a complete, reversible magnetism switching between antiferromagnetic (AFM) and ferromagnetic (FM) states with visible light on and off is realized. Moreover, a visible light control of 180° deterministic magnetization switching with a tiny magnetic bias field is achieved. The magnetic optical Kerr effect results further reveal the magnetic domain switching pathway between AFM and FM domains. The first-principle calculations conclude that the photoelectrons fill in the unoccupied band and raise the Fermi energy, which increases the exchange interaction. Lastly, a prototype device with visible light control of two states switching with a 0.35% giant magnetoresistance ratio change (maximal 0.4%), paving the way toward fast, compact, and energy-efficient solar-driven memories is fabricated.

Self‐Reversed Magnetization in Sediments Caused by Greigite Alteration

AbstractMultipolarity remanence in greigite‐bearing sediments has long been recognized, but the cause of this anomalous remanence behavior is not well understood. Here, we use electron microscopic and magnetic analyses to investigate the origin of such multipolarity in Miocene greigite‐bearing sediments from the Pannonian Basin (Hungary). We find a magnetic softening and partial transformation of iron sulfides to magnetite and pyrrhotite from “single‐polarity” to “multi‐polarity” samples. The inward alteration of sulfide grains is topotactic and is size‐dependent with higher alteration in smaller grains. We propose a multi‐phase self‐reversal chemical remanent magnetization (CRM) mechanism in altered greigite: the neoformed magnetite/pyrrhotite shell acquires a CRM coupled in the opposite direction to the primary CRM of the greigite core, likely through magnetostatic interactions or interfacial exchange interactions between the closely contacting core and shell. This new greigite self‐reversal model can explain the commonly observed antiparallel polarities and has broad geochronological, tectonic and paleoenvironmental implications.

Renormalization of antiferromagnetic magnons by superconducting condensate and quasiparticles

The ability to modify and tune the spin-wave dispersion is one of the most important requirements for engineering of magnonic networks. In this study we demonstrate the promise of synthetic thin-film hybrids composed of an antiferromagnetic insulator (AF) and a normal (N) or superconducting (S) metal for tuning and modifying the spin-wave dispersion in antiferromagnetic insulators. The key ingredient is the uncompensated magnetic moment at the AF/S(N) interface, which induces an effective exchange field in the adjacent metal via the interface exchange interaction. The exchange field spin polarizes quasiparticles in the metal and induces spinful triplet Cooper pairs screening the magnon. The quasiparticle and Cooper pair polarization renormalizes the magnon dispersion. The renormalization results in the splitting of the otherwise degenerate AF magnon modes with no need to apply a magnetic field. It is also proposed that measurements of the renormalized dispersion relations can provide the amplitude of the effective exchange field induced by the AF in the adjacent metal.

Ultrafast Control of Interfacial Exchange Coupling in Ferromagnetic Bilayer

AbstractFast spin manipulation in magnetic heterostructures, where magnetic interactions between different materials often define the functionality of devices, is a key issue in the development of ultrafast spintronics. Although recently developed optical approaches such as ultrafast spin‐transfer and spin–orbit torques open new pathways to fast spin manipulation, these processes do not fully utilize the unique possibilities offered by interfacial magnetic coupling effects in ferromagnetic multilayer systems. Here, ultrafast optically controlled interfacial exchange interactions in the ferromagnetic Co2FeAl/(Ga,Mn)As system at low laser fluence levels are experimentally demonstrated. The excitation efficiency of Co2FeAl with the (Ga,Mn)As layer is 30–40 times higher than the case with the GaAs layer at 5 K due to the modification of exchange coupling interaction via photoexcited charge transfer between the two ferromagnetic layers. In addition, the coherent spin precessions persist to room temperature, excluding the drive of pump‐modulated magnetization in the (Ga,Mn)As layer and indicating a proximity‐effect‐related optical excitation mechanism. The results highlight the importance of interfacial exchange interactions in ferromagnetic heterostructures and how these magnetic coupling effects can be utilized for ultrafast, low‐power spin manipulation.

The Effect of the Interfacial Exchange Interaction on the Metastability of Magnetic States of Core/Shell Nanoparticles

The Effect of the Interfacial Exchange Interaction on the Metastability of Magnetic States of Core/Shell Nanoparticles

The Effect of the Interface Width on the Exchange Interaction 
 Constant between Ferro- and Antiferromagnets

Within the model of two-phase (core/shell) ellipsoidal nanoparticles, the effects of the interfacial exchange interaction, size, elongation, and orientation of the major axes of the core and shell on the meta-stability of magnetic states have been simulated. Using the example of Fe2.44Ti0.56O4 nanoparticles,Fe3O4 it is shown that the metastability of magnetic states is realized in a limited range of the constant of interfacial exchange interaction between the core and shell, Ain , and geometric parameters. With an increase in the absolute value of the interfacial exchange interaction constant, , from Ain = 0 to some finite value, theAin metastability of magnetic states decreases monotonically from maximum to zero, regardless of the angle between the major axes of the core and the shell.

The Effect of the Interface Width on the Exchange Interaction 
 Constant between Ferro- and Antiferromagnets

In the approximation of the average spin method, a system of equations for determining the average magnetic moments of atoms at the interface between a ferromagnet and an antiferromagnet is formulated. It is possible to simulate the dependences of interfacial exchange interaction constant Ain on the temperature, antiferromagnetic layer thickness, and interface width by solving the system of equations for an ultrathin Ni/NiO film. It is found that the interfacial exchange interaction constant in a film with a fixed interface width at low temperatures increases with an increase in the thickness of the antiferromagnetic layer. With anincrease in the interface width, the Ain value decreases by a factor of 1.3 and reaches its minimum level.

Controlling Antiferromagnetic Magnon Polarization by Interfacial Exchange Interaction

We demonstrate highly efficient control of antiferromagnetic (AFM) magnon spins by the interfacial exchange interaction in heterostructures of ferrimagnetic yttrium iron garnet (YIG) and AFM ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$. At low temperatures, ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ is antiferromagnetically ordered. The interfacial exchange interaction exerted by YIG lifts the degeneracy between the AFM magnon modes in ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$, resulting in a net spin polarization and a spin current dominated by left-handed magnons, even at zero magnetic field, which is detected by the spin Seebeck effect with a 5-nm-thick $\mathrm{Pt}$ film. In the AFM magnon-dominated region, even if the magnetic field is not sufficiently strong to induce the spin-flop transition in ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$, the total spin Seebeck signal polarity flips when the YIG magnetization switches. It clearly indicates that the ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ magnon polarization is controlled by YIG magnetization through the interfacial exchange interaction. The demonstration of the efficient control of AFM magnon polarization opens a pathway to manipulate AFM magnon quantum states using FM-AFM heterostructures.

Proximity-magnetized quantum spin Hall insulator: monolayer 1 T’ WTe2/Cr2Ge2Te6

Van der Waals heterostructures offer great versatility to tailor unique interactions at the atomically flat interfaces between dissimilar layered materials and induce novel physical phenomena. By bringing monolayer 1 T’ WTe2, a two-dimensional quantum spin Hall insulator, and few-layer Cr2Ge2Te6, an insulating ferromagnet, into close proximity in an heterostructure, we introduce a ferromagnetic order in the former via the interfacial exchange interaction. The ferromagnetism in WTe2 manifests in the anomalous Nernst effect, anomalous Hall effect as well as anisotropic magnetoresistance effect. Using local electrodes, we identify separate transport contributions from the metallic edge and insulating bulk. When driven by an AC current, the second harmonic voltage responses closely resemble the anomalous Nernst responses to AC temperature gradient generated by nonlocal heater, which appear as nonreciprocal signals with respect to the induced magnetization orientation. Our results from different electrodes reveal spin-polarized edge states in the magnetized quantum spin Hall insulator.

In-situ investigation of exchange bias property in FePt (L10)/FeCo bilayer with increasing FeCo thickness

Magnetization reversal of soft ferromagnetic (FM) FeCo layer coupled to hard FM FePt (L10) layer in FePt (L10)/FeCo) bilayer has been studied in-situ with increasing thickness of FeCo layer. During FePt(L10) phase formation, an in-plane magnetic saturation field (HMAX) is applied to keep the hard FePt(L10) layer in its remanent state. Magneto-optical Kerr effect measurements reveal the absence of hysteresis loops initially up to 4.8 nm thickness of FeCo layer while loops with significant shift, i.e. exchange bias (EB) opposite to the HMAX direction, were observed on further deposition. The EB is found to decrease with the increasing thickness of the FeCo layer. It is due to the combined effect of both magnetostatic coupling and interfacial exchange interaction, where the contribution of magnetostatic interaction to EB is almost constant, and interfacial exchange coupling exhibits an inverse relation. Furthermore, different nucleation mechanisms during magnetization reversal in the descending and ascending branches of hysteresis loops lead to asymmetry in magnetization reversal at the lower thickness of the FeCo layer. This asymmetry decreases with the decrease in the interfacial coupling as the thickness increases. Azimuthal angle dependence of exchange shift with respect to the direction of HMAX further revealed the unidirectional character of the EB.

Magnon-exciton proximity coupling at a van der Waals heterointerface

We report on an optically probed ferromagnetic resonance experiment which elucidates the magnon-exciton coupling at the interface between a magnetic thin film and an atomically thin semiconductor. Our approach allies the long-lived magnons hosted in a film of yttrium iron garnet (YIG) to strongly bound excitons in a flake of a transition metal dichalcogenide, ${\mathrm{MoSe}}_{2}$. The magnons induce on the excitons a dynamical valley Zeeman effect ruled by interfacial exchange interactions. This nascent class of hybrid system suggests new opportunities for information transduction between microwave and optical regions.

Domain wall depinning from FM/AFM interface defects by spin-polarized current

We report a theoretical investigation of the impact of spin-transfer torque on magnetic head-to-head domain walls of Fe and Ni80Fe20 (Permalloy PyTM) nanowires exchange coupled to a two-sublattice uniaxial antiferromagnetic substrate. Our results indicate that provided the interface exchange interaction is large enough, the domain walls pin to interface defects consisting of steps perpendicular to the easy antiferromagnetic axis, separating terraces with opposite effective interface exchange fields. We also found that the dipolar and interface energies lead to narrow V-shaped domain walls and that the spin-transfer torque effects are restricted to the domain wall. Depinning walls from the step defect at the interface require polarized spin current densities of the order of 107 A/cm2 for both materials.

Modelling the influence of interphase and magnetostatic interaction on the magnetic characteristics of core/shell nanoparticles

In this work, we present the results of a theoretical study of the dependence of the hysteresis characteristics of core/shell nanoparticles on their size, interphase exchange and interparticle magnetic interactions. By modelling the influence of the size effect on the saturation magnetisation Ms, the remanent magnetisation Mrs and the coercive field Hc of nanoparticles (Fe/Fe3O4andCo/Au), in which there is a difference in the magnetostatic interaction between the core and shell of the systems, we show that an increase in the size of the nanoparticles leads to an increase in Ms and Mrs,and non-monotonic (Co/Au) and monotonic (Fe/Fe3O4) changes in Hc. The increased hysteresis characteristics of the nanoparticles are due to both a decrease in the proportion in the superparamagnetic state and the peculiarities of the reversal of magnetisation. The interphase exchange interaction, regardless of its sign, leads to an increase in the coercive force and remanent saturation magnetisation of Fe/Fe3O4 nanoparticles, and has only very weak effects on the hysteresis characteristics of Co/Au nanoparticles. It was found that an increase in the interparticle magnetic interaction (volume concentration) of core/shell nanoparticles, and consequently their chaotisation, leads to a decrease in the coercive force and remanent saturation magnetisation. Moreover, an increase in the size of the nanoparticles leads to a higher rate of decrease in the hysteresis characteristics. A calculation of the blocking temperature TB for the core/shell nanoparticles shows that it is determined not only by their volume but also by the size dependence of the critical fields of the phase magnetisation reversal. With an increase in the interfacial exchange interaction constant Ain, the blocking temperature of the Fe/Fe3O4 nanoparticles quickly reaches saturation, while for Co/Au nanoparticles, TB is practically independent of Ain. The magnetic interaction between nanoparticles leads to chaos in the distribution of their magnetic moments, and hence to an increase in the blocking temperature. The results obtained here are in good agreement with experimental data.

Epitaxial growth and magnetic characterization of EuSe thin films with various crystalline orientations

We report different growth modes and corresponding magnetic properties of thin EuSe films grown by molecular beam epitaxy on BaF2, Pb1−xEuxSe, GaAs, and Bi2Se3 substrates. We show that EuSe grows predominantly in the (001) orientation on GaAs(111) and Bi2Se3, but along the (111) crystallographic direction on BaF2 (111) and Pb1−xEuxSe (111). High resolution transmission electron microscopy measurements reveal a sharp and highly crystalline interface for both (001) and (111) EuSe films. In agreement with previous studies, ordered magnetic phases include antiferromagnetic, ferrimagnetic, and ferromagnetic phases. In contrast to previous studies, we found a strong hysteresis of the antiferromagnetic–ferrimagnetic transition. The ability to grow epitaxial films of EuSe on Bi2Se3 and of Bi2Se3 on EuSe enables further investigation of interfacial exchange interactions between various phases of an insulating metamagnetic material and a topological insulator.

Interaction of magnons with plasmons in a system of antiferromagnetic insulators coupled to a superconductor microwave cavity through interfacial exchange interaction

A superconductor can interact with magnets through the magnetic field which is induced by the supercurrent. However, this interaction is rather weak and requires relatively large magnets to reach a sizable hybridization of microwave electromagnetic modes with magnons. The size of the device can be drastically reduced by means of the large interfacial exchange interaction between spins in a magnet and a superconductor. In the spin-orbit coupled superconductor this interaction can be transmitted to the s-wave condensate through a mixing of triplet and singlet Cooper pairs. In this paper we theoretically consider thin antiferromagnetic films, which make a spin compensated contact to a superconducting wire, which forms a cavity for Mooij-Sch\"{o}n plasma waves. The effective Lagrangian is derived where the interaction of these waves with magnons is taken into account. Frequencies of hybrid magnon-plasmon modes are calculated. A system of two micromagnets placed far apart are also considered and their mutual coupling through the cavity mode is calculated.

Epitaxial growth of Ho<sub>3</sub>Fe<sub>5</sub>O<sub>12</sub> films with perpendicular magnetic anisotropy and spin transport properties in Ho<sub>3</sub>Fe<sub>5</sub>O<sub>12</sub>/Pt heterostructures

Rare-earth iron garnet films with perpendicular magnetic anisotropy could open new perspectives for spintronics. Holmium iron garnet (Ho<sub>3</sub>Fe<sub>5</sub>O<sub>12</sub>, HoIG) films with thickness ranging from 2 to 100 nm are epitaxially grown on (111) orientated gadolinium gallium garnet single crystal substrate doped with yttrium and scandium (Gd<sub>0.63</sub>Y<sub>2.37</sub>Sc<sub>2</sub>Ga<sub>3</sub>O<sub>12</sub>, GYSGG) by ultra-high vacuum magnetron sputtering. A 3-nm Pt film is further deposited on each of the HoIG films. The magnetic anisotropy and magneto-transport properties of heterostructures at room temperature are investigated. It is shown that the HoIG film as thin as 2 nm (less than two unit cells in thickness) exhibits the ferromagnetic properties at room temperature, and perpendicular magnetic anisotropy is achieved in the 2-60 nm thick films, and a maximum effective perpendicular anisotropy field reaches 350 mT due to the strain induced magnetoelastic anisotropy. The HoIG/Pt heterostructure shows significant anomalous Hall effect (AHE) and appreciable spin-Hall magnetoresistance (SMR) and/or anisotropic magnetoresistance (AMR). Remarkably, the AHE starts to decline gradually when the HoIG thickness is less than 4 nm, but the magnetoresistance decreases rapidly with the HoIG layer becoming less than 7 nm in thickness. The fact that the AHE in the heterostructure is less sensitive to the HoIG thickness suggests that the interface effect is more dominant in the AHE mechanism, whereas the bulk magnetic properties of the HoIG plays a more important role for the observed magnetoresistance. In addition, the spin Seebeck effect decreases exponentially with the decrease of HoIG thickness till the ultrathin limit, which was previously validated in the micrometer-thick YIG/Pt stacks in the frame of thermally excited magnon accumulation and propagation. The present results show that the nanometer HoIG/Pt heterostructure with tunable perpendicular magnetic anisotropy and efficient interfacial spin exchange interaction could be a promising candidate for insulating magnet based spintronic devices.