Magnetic Interlayer Research Articles

Mechanical-electro-magnetic coupling in strained bilayer CrI3

The discovery of intrinsic 2D ferromagnets provides exciting possibilities for spintronics applications. A particularly attractive example is CrI3, whose monolayer is ferromagnetic while bilayer shows antiferromagnetic coupling. Because of weak interlayer coupling, the magnetism of bilayer CrI3 can be easily modulated by external perturbations, such as gating or pressure. Here, we constructed a magnetic phase diagram of bilayer CrI3 under arbitrary biaxial strain (within ±4%) from compression to stretch, and found that compressive strain can effectively convert the antiferromagnetic coupling of bilayer CrI3 to ferromagnetic. Detailed analyses on electronic structure were then performed to unravel the underlying mechanism of the magnetic phase transition. It was shown that both band gap and orbital composition at conduction band minimum play important roles in determining magnetic ground states of strained bilayer CrI3. These results strengthen our understanding of the interlayer magnetism of 2D magnets and provide a feasible way to modulate the magnetism in 2D layered materials.

Organic/metal interface–modulated magnetism in [Fe/C60]3 multilayers and Fe-C60 composites

Because of the expected long spin-transport length of organic materials, the magnetic metal/organic interface is crucial to the application of organic spintronics. In this study, [Fe/C60]3 multilayers were fabricated for the investigation of C60-mediated magnetic interlayer coupling. [Fe/C60]3 thin films were characterized using the magneto-optical Kerr effect, transmission electron microscopy, Raman spectroscopy, and x-ray photoelectron spectroscopy (XPS). The thin films revealed in-plane magnetic anisotropy, and the magnetic coercivity (Hc) drastically decreased from 6–8 mT to 0.5 mT with the increase of C60 thickness from 0.1 nm to 5 nm. The insertion of the C60 layer considerably reduced Hc because a thickness greater than 1 nm of the C60 layer is sufficient for blocking magnetic exchange coupling between Fe layers. In addition, post-annealing increased Hc because of Fe inter-diffusion, which promotes magnetic exchange coupling and further Fe-C bonding, as confirmed by a comparative study of XPS C-spectra. The thermally triggered inter-diffusion between Fe and C60 layers turned the multilayers into a mixed composite film and thus caused magnetic variation. Annealing time and temperature can be used as control parameters for the tuning of magnetism in Fe-C60 composites.

Условия возникновения спонтанной осциллирующей магнитной релаксации в синтетических ферримагнетиках Pt/Co/Ir/Co/Pt

We had established that the non-monotonic magnetic relaxation appeared, when two or three magnetic fields, switching between stable magnetization states, coincide with each other, in Pt/Co/Ir/Co/Pt synthetic ferrimagnets with perpendicular magnetic anisotropy. Switching fields between stable magnetization states depend on sum of exchange interlayer energy, Zeeman energy and magnetization reversal energy barrier of each layer. This total energy depends on layers thickness and temperature. We had shown that with choose of temperature and free layer thickness we can achieve double or triple points on energy states diagram, which is reason of nonmonotonic magnetization relaxation.

Pressure-controlled interlayer magnetism in atomically thin CrI3.

Stacking order can influence the physical properties of two-dimensional van der Waals materials1,2. Here we applied hydrostatic pressure up to 2 GPa to modify the stacking order in the van der Waals magnetic insulator CrI3. We observed an irreversible interlayer antiferromagnetic-to-ferromagnetic transition in atomically thin CrI3 by magnetic circular dichroism and electron tunnelling measurements. The effect was accompanied by a monoclinic-to-rhombohedral stacking-order change characterized by polarized Raman spectroscopy. Before the structural change, the interlayer antiferromagnetic coupling energy can be tuned up by nearly 100% with pressure. Our experiment reveals the interlayer ferromagnetic ground state, which is established in bulk CrI3 but not observed in native exfoliated thin films. The observed correlation between the magnetic ground state and the stacking order is in good agreement with first principles calculations3-8 and suggests a route towards nanoscale magnetic textures by moiré engineering3,9.

Antiferromagnetic domain wall control via surface spin flop in fully tunable synthetic antiferromagnets with perpendicular magnetic anisotropy

Antiferromagnetic (AF) domain walls have recently attracted revived attention, not only in the emerging field of AF spintronics, but also more specifically for offering fast domain wall velocities and dynamic excitations up to the terahertz frequency regime. Here we introduce an approach to nucleate and stabilize an AF domain wall in a synthetic antiferromagnet (SAF). We present experimental and micromagnetic studies of the magnetization reversal in [(Co/Pt)$_{X-1}$/Co/Ir]$_{N-1}$(Co/Pt)$_X$ SAFs, where interface induced perpendicular magnetic anisotropy (PMA) and AF interlayer exchange coupling (IEC) are completely controlled via the individual layer thicknesses within the multilayer stack. By combining strong PMA with even stronger AF IEC, the SAF reveals a collective response to an external magnetic field applied normal to the surface, and we stabilize the characteristic surface spin flop (SSF) state for an even number N of AF-coupled (Co/Pt)$_{X-1}$/Co multilayer blocks. In the SSF state our system provides a well-controlled and fully tunable vertical AF domain wall, easy to integrate as no single crystal substrates are required and with uniform 2D-magnetization in the film plane for further functionalization options, such as for example lateral patterning via lithography.

Interplay of magnetic field and interlayer coupling in the quasi-two-dimensional quantum magnet Cu(en)Cl2 : Realization of the spin-1/2 rectangular/zigzag square Heisenberg lattice

The interplay of the magnetic field and interlayer correlations was experimentally investigated in a quasi-two-dimensional quantum magnet $\mathrm{Cu}(en){\mathrm{Cl}}_{2}$. For this purpose, extensive quantum Monte Carlo (QMC) studies of the finite-temperature properties of the spin-1/2 Heisenberg antiferromagnet (HAF) on the rectangular and the spatially anisotropic zig-zag square lattices were performed. The QMC studies revealed the equivalency of both models for any values of magnetic fields and spatial anisotropies $R={J}_{\mathrm{interchain}}/{J}_{\mathrm{intrachain}}$. The analysis based on the decomposition of both lattices into local Hamiltonians confirmed the equivalence. Despite the large influence of the interlayer coupling and rather complicated distribution of the intralayer exchange pathways in $\mathrm{Cu}(en){\mathrm{Cl}}_{2}$, considering several constraints in the data analysis enabled the extraction of the main features of the low-dimensional magnetic subsystem from the specific heat and magnetization. It was found that $\mathrm{Cu}(en){\mathrm{Cl}}_{2}$ can be treated as the realization of the spin-1/2 HAF on the rectangular/zig-zag square lattice with the intralayer spatial anisotropy $R\ensuremath{\approx}0.2$, the intrachain exchange coupling 2.20 \ifmmode\pm\else\textpm\fi{} 0.15 K, the saturation field 3.7 \ifmmode\pm\else\textpm\fi{} 0.1 T, the spin-flop field about 0.1 T, and the spin anisotropy of the orthorhombic symmetry. The possibility to investigate magnon instabilities in the strong-field regime of $\mathrm{Cu}(en){\mathrm{Cl}}_{2}$ is discussed.

Oscillatory dynamics of the magnetic moment of a Pt/Co/Ir/Co/Pt synthetic antiferromagnet

In this paper, we present a detailed study of the oscillating magnetic relaxation in the synthetic antiferromagnet (SAF) with two ferromagnetic Co layers of different thicknesses separated by an Ir spacer. The four stable magnetic states of the SAF are determined by the mutual alignment of magnetic moments in the layers and are controlled by both the magnetic interlayer exchange interaction and the Zeeman energy. The specific variations in the thicknesses of the layers and/or temperature allow the existence of a ``triple point,'' which corresponds to a coincidence of the critical switching fields for two or three interstate transitions. In this case, two or even three different types of magnetization reversals occur simultaneously and competitively. A nonmonotonic dependence of the domain-wall speed ${v}_{\mathrm{DW}}$ on magnetic field $H$ and an oscillating time dependence of magnetic moment $M$ in a constant magnetic field were observed in a Pt/Co/Ir/Co/Pt synthetic antiferromagnet with perpendicular anisotropy due to interplay between the magnetic nuclei produced by Dzyaloshinskii-Moriya interaction. The proximity of two or three (triple-point) critical fields of SAF switching is the necessary condition for both a nonmonotonic magnetic relaxation and the oscillating time variations of the magnetic moment. The dynamical model describing the interaction and subsequent evolution of the magnetic nuclei demonstrates that this nontrivial magnetic relaxation obeys a simple Schr\odinger equation.

A General Recipe for Nondispersive Optical Activity in Bilayer Chiral Metamaterials

AbstractBy mimicking broken mirror symmetry of naturally existing chiral materials, and exploiting even stronger magnetoelectric coupling, large optical activity has been realized in chiral metamaterials over the past decade. Nevertheless, most of chiral metamaterials demonstrated so far exhibit highly dispersive optical activity due to their resonant response, which inevitably leads to a narrow operating bandwidth in terms of the optical rotatory power as well as the transmission. Herein, conditions for possessing nondispersive optical activity are derived based on a Lagrangian model for general bilayer chiral metamaterials, and the validity is experimentally confirmed in a microwave regime. It is shown that large nondispersive optical activity can be realized in bilayer chiral metamaterials where metallic apertures are arranged in each layer with high‐rotational symmetry and the off‐diagonal terms of the magnetic interlayer coupling tensor are nonzero while the electric counterparts are eliminated. In stark contrast to most previous chiral metamaterials, a nonzero constant polarization rotation angle is observed at the quasi‐static limit, which manifests the unique nonresonant characteristic of the proposed magnetically coupled bilayer chiral metamaterials. In addition to the geometrical controllability, the amount of polarization rotation and transmission bandwidth can be further increased by stacking magnetically coupled bilayer chiral metamaterials.

Evolution of interlayer and intralayer magnetism in three atomically thin chromium trihalides

We conduct a comprehensive study of three different magnetic semiconductors, CrI3, CrBr3, and CrCl3, by incorporating both few-layer and bilayer samples in van der Waals tunnel junctions. We find that the interlayer magnetic ordering, exchange gap, magnetic anisotropy, and magnon excitations evolve systematically with changing halogen atom. By fitting to a spin wave theory that accounts for nearest-neighbor exchange interactions, we are able to further determine a simple spin Hamiltonian describing all three systems. These results extend the 2D magnetism platform to Ising, Heisenberg, and XY spin classes in a single material family. Using magneto-optical measurements, we additionally demonstrate that ferromagnetism can be stabilized down to monolayer in more isotropic CrBr3, with transition temperature still close to that of the bulk.

Study on the Preparation Technology of a Ceramic Panel with a Magnetic Interlayer for an Induction Cooker

In order to expand the range of pot materials for induction cookers, a kind of sandwich structural composite ceramic panel that consists of an Al2O3 ceramic substrate, magnetic heating interlayer, and ZrO2 ceramic substrate was developed by combining the tape casting process and the screen printing process. In this research, the slurry composition of the functional phase, glass powder, and organic carrier was optimized for preparing the heating interlayer with excellent electromagnetic properties. The influences of the glass powder content and the magnetic layer structure on the thermal shock resistance of the composite ceramic panel were studied. The finite element model of the composite ceramic panel under thermal load was established through ANSYS software. In the range of 0.1–0.3 mm thickness of a magnetic heating interlayer, the temperature field and the macroscopic stress field of the composite ceramic panel were simulated, and the influence of the magnetic layer structure on the thermal stress distribution of the composite ceramic panel was analyzed. The experimental results showed that the magnetic layer had the best quality when the amount of glass powder added was 9 wt%. The ANSYS simulation revealed that the gradient structure of the magnetic layer could reduce the stress between the alumina layer and the magnetic layer from 308 to 192 MPa, which significantly improved the thermal shock resistance of the composite ceramic panel. Therefore, the gradient structure of the magnetic layer could ensure the stability of the composite ceramic panel after five cycles of electromagnetic heating.

The annealing effect on memory state stability and interlayer coupling in perpendicular magnetic tunnel junctions with ultrathin MgO barrier

Abstract Perpendicular magnetic tunnel junctions (p-MTJs) attract great interest because of their excellent performance in spin-transfer-torque magnetic random access memories (STT-MRAMs). The annealing process is critical to achieve the required structural and magnetic properties, therefore obtaining high perpendicular tunneling magnetoresistance (p-TMR) and lower switching current. In this work, the CoFeB/MgO/CoFeB-based p-TMR stack was prepared and a high p-TMR value of 155.9% was obtained from CIPT. The p-MTJs were patterned with pillar sizes ranging from 50 nm to 1000 nm and were annealed upon different annealing temperatures (TA) of 300 and 340 °C. The resistance at parallel (RP) and antiparallel (RAP) state in p-MTJs was obtained upon the application of a ±30 mT perpendicular magnetic field, where the angle of magnetization between storage and reference layer at memory state, θP and θAP, were derived from the P and AP resistance ratios with and without magnetic field. The condition θP = θAP of about 63.5° was found in as-deposited samples, indicating a ferromagnetic interlayer coupling between storage and reference layers, where θP = 0° and θAP = 180° at TA = 300 °C and 340 °C demonstrate a stable memory state. The stability of the resistance state was modeled by considering the evolution of effective perpendicular magnetic anisotropy (Keff) and interlayer coupling constant (J), showing a method to evaluate the ratio of Keff/J via the memory state resistance characterization, where the Keff/J of about −1.26 was obtained in our as-deposited samples. Our study assesses the effect of TA on the θP and θAP and supplies the method to enhance the stability through the optimization of perpendicular magnetic anisotropy and interlayer coupling, so to better control the performance of p-MTJ-based STT-MRAMs.

Interlayer coupling-driven magnetic ordering and magnetization processes in ultrathin Au/Co/Mo/Co/Au film

Abstract Interlayer magnetic coupling in an Au/Co/Mo/Co/Au system fabricated by molecular beam epitaxy was studied as a function of the thicknesses of the cobalt and molybdenum spacer layers (denoted as dCo and dMo, respectively). Magnetization processes and magnetic domain structures were investigated using magneto-optical techniques. In the investigated thickness ranges, various magnetization configurations of the Co layers were observed: (A) both perpendicular, (B) perpendicular and in-plane, and (C) both in-plane. The interlayer exchange coupling field HIEC was determined as a function of dMo. With increasing dMo, this coupling changed from antiferromagnetic with a huge coupling field of ≈0.27 T to ferromagnetic and again to antiferromagnetic. A coupling oscillation period of around 1.4 nm was deduced by comparing the experimental data with the Ruderman–Kittel–Kasuya–Yosida model. For configurations (B) and (C), oscillations of the saturation magnetic field HSAT(dMo) perpendicular to the sample plane were found. Simplified and precise analytical expressions for HSAT were proposed by considering HIEC as well as the uniaxial magnetic anisotropy of one and both magnetic layers, respectively. Magnetic ordering was numerically modeled taking into account the dependences of layer anisotropy and exchange coupling on dCo and dMo, respectively. This modeling well described the remanent magnetization map (dCo, dMo) determined using the polar Kerr effect. It is also shown that HIEC(dMo) induced magnetization reorientation from in-plane to out-of-plane or from out-of-plane to in-plane magnetization configurations and canted magnetization of the Co layers.

Magnetic coupling of ferromagnetic SrRuO3 epitaxial layers separated by ultrathin non-magnetic SrZrO3/SrIrO3

Ferromagnetic multilayers with asymmetric interfaces and perpendicular magnetic anisotropy can stabilize non-trivial magnetic structures due to interfacial Dzyaloshinskii-Moriya interactions. Magnetic interlayer coupling between ferromagnetic layers separated by non-magnetic insulating spacers is an additional important ingredient for the properties of multilayers. We addressed the magnetic coupling between ferromagnetic SrRuO3 epitaxial layers separated by ultrathin spacers of SrIrO3/SrZrO3, so that inversion symmetry is broken at the top and bottom interfaces. Major and minor magnetization loops allowed us to assess the type and strength of the magnetic coupling. We inferred that the magnetic coupling of SrRuO3 layers through non-magnetic insulating interlayers (≤1.6 nm thick) is weakly ferromagnetic.

Reversible 90-Degree Rotation of Fe Magnetic Moment Using Hydrogen

[Pd/Fe]2 multilayers were deposited on a flat MgO(001) to study the effect of hydrogen on magnetic interlayer coupling. Complex magnetic hysteresis behavior, including single, double, and triple loops, were measured as a function of the azimuthal angle in a longitudinal and transverse direction. With a combination of a 2-fold magnetic anisotropy energy (MAE) in the bottom-Fe and a 4-fold MAE in the top-Fe, the complex magnetic hysteresis behavior could be clearly explained. Two well-split hysteresis loops with almost zero Kerr remanence were measured by choosing a suitable Pd thickness and applying the magnetic field perpendicular to the easy axis of the bottom-Fe. The split double loops originated from the 90°-rotation of the top-Fe moment. On exposure to a hydrogen gas atmosphere, the separation of the two minor loops increased, indicating that Pd-hydride formation enhanced the ferromagnetic coupling between the two Fe layers. Based on these observations, we proposed that, by applying a suitable constant magnetic field, the top-Fe moment could undergo reversible 90°-rotation following hydrogen exposure. The results suggest that the Pd space layer used for mediating the magnetic interlayer coupling is sensitive to hydrogen, and therefore, the multilayer system can function as a giant magnetoresistance-type sensor suitable for hydrogen gas.

Magnetic Properties of Coupled Co/Mo/Co Structures Tailored by Ion Irradiation

Magnonic crystals (metamaterials to control magnetism) are attractive for manipulating spin waves in digital logic and storage applications, but in the end one needs to be able to actually produce them---an interesting challenge in nanotechnology. This work shows how interlayer coupling and magnetization strength can be engineered in Co/Mo/Co ultrathin-film structures by ion irradiation. Modifications of magnetic properties are correlated to and explained by structural evolution, as revealed by numerical simulations. The authors propose numerous types of magnonic crystals that can be fabricated according to these insights.

Spin Hall driven domain wall motion in magnetic bilayers coupled by a magnetic oxide interlayer

mCell, previously proposed by our group, is a four-terminal magnetoresistive device with isolated write- and read-paths for all-spin logic and memory applications. A mCell requires an electric-insulating magnetic layer to couple the spin Hall driven write-path to the magnetic free layer of the read-path. Both paths are magnetic layers with perpendicular anisotropy and their perpendicularly oriented magnetization needs to be maintained with this insertion layer. We have developed a magnetic oxide (FeOx) insertion layer to serve for these purposes. We show that the FeOx insertion layer provides sufficient magnetic coupling between adjacent perpendicular magnetic layers. Resistance measurement shows that this magnetic oxide layer can act as an electric-insulating layer. In addition, spin Hall driven domain wall motion in magnetic bi-layers coupled by the FeOx insertion layer is significantly enhanced compared to that in magnetic single layer; it also requires low voltage threshold that poses possibility for power-efficient device applications.

Orthogonal interfacial exchange coupling in GaMnAsP/GaMnAs bilayers

We carried out a systematic study of magnetic ordering and magnetic interlayer coupling in Ga1-xMnxAs1-yPy/Ga1-xMnxAs bilayers using superconducting quantum interference device magnetometry and ferromagnetic resonance. Such bilayers are interesting, because the easy axis of the constituent materials are orthogonal. Our results show that the bilayers are strongly exchange-coupled at the interface, that manifests itself in the form of horizontal exchange-bias-like shifts of the hysteresis loops of the Ga1-xMnxAs layer, as observed in field-cooled magnetic measurements.

Investigation of Magnetic Properties of Multilayer Thin Films with Spin-Wave Resonance

A theory has been developed to simulate spin-wave resonance (SWR) modes in the multilayer systems consisting of alternate magnetic and nonmagnetic layers. An equation of motion of magnetization with Gilbert-type damping parameter for simulating SWR modes was used. It has been realized that the theory developed for the magnetic multilayer films is suitable to study the spin dynamics and extract various magnetic parameters. It has been shown that SWR modes strongly depend on an effective magnetic anisotropy constant (K eff), interlayer exchange coupling constant (A 12) and effective magnetization (M eff). The nature of the effective magnetic anisotropy and interlayer exchange coupling constants has been investigated by using the developed SWR theory in detail. The separation between optic and acoustic modes strongly depends on the magnitude of the interlayer exchange coupling constant, whereas the relative position of the acoustic and optic modes depends on the sign of $A_{12}$ . With increasing the interlayer exchange coupling constant, the resonance field of the optic mode decreases (increases) for ferromagnetic (antiferromagnetic) coupling. When the effective magnetic anisotropy constant increases, the resonance field of the acoustic and optic modes increases for both the ferromagnetic and antiferromagnetic couplings. The increasing of the effective magnetization results in decreasing of the resonance field of SWR mode at parallel geometry, whereas that of SWR mode increases at the perpendicular geometry. The results are compatible to the other theories and experimental results.

Tailoring the magnetization states in 2D arrays of multiresponse ferromagnetic nanomagnets

We have fabricated nanomagnet arrays as a function of several geometrical parameters like the spacing between nanostructures, the aspect ratio and the layers thicknesses. The nanomagnets consist in two magnetic layers, separated by a non magnetic interlayer, that interact through magnetostatic coupling. They present a multiresponse hysteresis loops with two different switching fields. We have performed micromagnetic simulations to discern the role play by the different interactions. The spacing in the array strongly modifies the saturating field along the short axis and the magnetization reversal mechanisms from coherent rotation to domain wall nucleation. A small asymmetry between the two magnetic layers favors a magnetization reversal mechanism along the long axis with two different switching fields. These fields can be tailored through the thickness of the layers or the inter-element spacing in the array. In trilayers with the same magnetic layer thicknesses, the asymmetry can be induced by growing the two magnetic layers with a different anisotropy. The well-defined reversal fields make these nanomagnets potentially useful for magnetic tagging.

Voltage-controlled interlayer coupling in perpendicularly magnetized magnetic tunnel junctions

Magnetic interlayer coupling is one of the central phenomena in spintronics. It has been predicted that the sign of interlayer coupling can be manipulated by electric fields, instead of electric currents, thereby offering a promising low energy magnetization switching mechanism. Here we present the experimental demonstration of voltage-controlled interlayer coupling in a new perpendicular magnetic tunnel junction system with a GdOx tunnel barrier, where a large perpendicular magnetic anisotropy and a sizable tunnelling magnetoresistance have been achieved at room temperature. Owing to the interfacial nature of the magnetism, the ability to move oxygen vacancies within the barrier, and a large proximity-induced magnetization of GdOx, both the magnitude and the sign of the interlayer coupling in these junctions can be directly controlled by voltage. These results pave a new path towards achieving energy-efficient magnetization switching by controlling interlayer coupling.