Magnetic Multilayers Research Articles

Magnetic skyrmions: Basic properties and potential applications

AbstractMagnetic skyrmions are particle‐like topological magnetic textures that are potential information carriers in future spintronics. An enormous body of research confirms their existence in a broad range of magnetic materials since their first discovery in 2009. To date, magnetic skyrmions can not only be found in asymmetric systems but also in centrosymmetric ones. Notably, engineered magnetic multilayers are promising structures for skyrmion‐based spintronics because they can stabilize small‐sized skyrmions at room temperature and facilitate their electric manipulation. In this overview, we introduce the topological nature, their special properties, and nucleation methods of skyrmions, and show their potential for applications. Perspectives on skyrmionic devices and developments toward other, more three‐dimensional particle‐like magnetic nanostructures, are discussed at the end.

Oscillatory exchange bias controlled by RKKY in magnetic multilayers

Ferromagnetic/antiferromagnetic bilayers are interfaced with normal metal/ferromagnetic bilayers to form F*/AF/N/F valves. The N-spacer thickness is chosen such that it mediates strong indirect exchange [Ruderman–Kittel–Kasuya–Yosida (RKKY)] between the outer ferromagnetic layers, which varies in strength/direction depending on the N thickness and changes its direction on switching F. The system exhibits a strong modulation of the F*/AF exchange bias, oscillating in strength synchronously with the oscillation in the interlayer RKKY exchange across the normal metal spacer. The effect is explained as due to a superposition taking place within the antiferromagnetic layer of the direct-exchange proximity effect from the F*/AF interface and the indirect RKKY exchange from F penetrating AF via N. The modulation, expressed via the strength of the F*/AF bias field, reaches 400% at the first RKKY peak.

Design of Multilayer Winding-Based Halbach Array PM Compulsator

Compulsator provides higher energy density and comparable power density for pulse power supply than capacitor-based sources. With the added feature of natural current, zero-crossing capabilities make compulsator the ideal choice for pulse power supply to electromagnetic launch applications. A multilayer Halbach array permanent magnet (PM) compulsator is designed to achieve the desired performance. The multilayer winding arrangement is considered to minimize the self-inductance and to maximize the short-circuit current capability of the compulsator. Thus, different winding configurations with series and parallel connections in the compulsator are presented in detail. The mathematical modeling of the designed compulsator is also presented in brief. The designed compulsator is simulated by considering temperature rise, PM demagnetization effect, and so on. The designed compulsator performance in a dead short circuit of all phases and with single-phase short-circuit condition is presented. The obtained results show that the designed compulsator achieves the target objective.

Erratum: Accurate model of the stripe domain phase of perpendicularly magnetized multilayers [Phys. Rev. B 95 , 174423 (2017)

Erratum: Accurate model of the stripe domain phase of perpendicularly magnetized multilayers [Phys. Rev. B <b>95</b> , 174423 (2017)

Tailoring of the Interfacial Dzyaloshinskii–Moriya Interaction in Perpendicularly Magnetized Epitaxial Multilayers by Crystal Engineering

The interplay between the interfacial crystalline structure and Dzyaloshinskii-Moriya interaction (DMI) was investigated by Fe insertion in epitaxial Pt/Co/Ir perpendicular magnetized multilayers. The experimental results with the support of first-principles calculation indicate that the Fe/Ir interface exhibits a positive interfacial DMI (iDMI) originating from the fcc crystalline structure inserted by 2 monolayers (ML) Fe, while a negative one from the structure with a layer shifting of 1-ML Fe insertion. The total iDMI of the multilayers increases (decreases) due to the additive enhancement (competitive counteraction) between the iDMI of Fe/Ir and Pt/Co interfaces. Comparing the iDMI of single-crystalline and textured multilayers, the iDMI of multilayers is found to be particularly sensitive to the crystallinity nearby the heterointerfaces. This work is of vital importance to reveal a deeper insight into the physical mechanism of the iDMI and provides a viable strategy for tailoring the iDMI of the multilayers by crystal engineering.

Broadband Nonreciprocal Thermal Emission

The reciprocity between thermal emission and absorption in materials that satisfy the Lorentz reciprocity places a fundamental constraint on photonic energy conversion and thermal management. For approaching the ultimate thermodynamic limits in various photonic energy conversions and achieving nonreciprocal radiative thermal management, broadband nonreciprocal thermal emission is desired. However, existing designs of nonreciprocal emitters are narrowband. Here, we introduce a gradient epsilon-near-zero magneto-optical metamaterial for achieving broadband nonreciprocal thermal emission. We start by analyzing the nonreciprocal thermal emission and absorption in a thin layer of epsilon-near-zero magneto-optical material atop a substrate. We use temporal coupled-mode theory to elucidate the mechanism of nonreciprocal emission in the thin-film emitter. We then introduce a general approach for achieving broadband nonreciprocal emission by using a gradient epsilon-near-zero magneto-optical metamaterial. We numerically demonstrate broadband nonreciprocal emission in gradient-doped semiconductor multilayer, as well as in a magnetic Weyl semimetal multilayer with gradient chemical potential. Our approach for achieving broadband nonreciprocal emitters is useful for developing broadband nonreciprocal devices for energy conversion and thermal management.

Stacking-induced magnetic frustration and spiral spin liquid

Like the twisting control in magic-angle twisted bilayer graphene, the stacking control is another mechanical approach to manipulate the fundamental properties of solids, especially the van der Waals materials. We explore the stacking-induced magnetic frustration and the spiral spin liquid on a multilayer triangular lattice antiferromagnet where the system is built from ABC stacking with competing intralayer and interlayers couplings. By combining the nematic bond theory and the self-consistent Gaussian approximation, we establish the phase diagram for this ABC-stacked multilayer magnet. It is shown that the system supports a wide regime of spiral spin liquid with multiple degenerate spiral lines in the reciprocal space, separating the low-temperature spiral order and the high-temperature featureless paramagnet. The transition to the spiral order from the spiral spin liquid regime is first order. We further show that the spiral-spin-liquid behavior persists even with small perturbations such as further neighbor intralayer exchanges. The connection to the ABC-stacked magnets, the effects of Ising or planar spin anisotropy, and the outlook on the stacking-engineered quantum magnets are discussed.

Proximity induced moment at Pt/Co interfaces and isolated skyrmion bubble stabilization at zero magnetic field

Magnetic multilayers with heavy metal/ferromagnetic interfaces play an important role in data storage and spintronics due to magnetic phenomena such as perpendicular magnetic anisotropy (PMA) and magnetic skyrmion stabilization. Properties such as saturation magnetization (Ms), PMA, and interfacial Dzyaloshinskii–Moriya interaction (iDMI) can be tuned by stacking engineering to tailor the multilayer magnetic ground state. Here, it is demonstrated that isolated skyrmion bubbles can be stabilized at zero magnetic field in nominally symmetric Pt/Co/Pt multilayers grown by magnetron sputtering. Interface disorder and asymmetry are strongly enhanced as the Co layer is thinned, leading to nonzero iDMI and Pt proximity induced moment (PIM) that affects both the multilayer Ms and PMA. The interplay between PIM and the iDMI is behind the nucleation of skyrmion bubbles, highlighting the importance of controlling the interface phenomena to develop devices for future applications. These results also manifest the importance of considering the PIM to determine the correct magnetic parameters of ultrathin films on the zero-field spin textures stability.

Boosting Spin-Orbit-Torque Efficiency in Spin-Current-Generator/Magnet/Oxide Superlattices

Efficient manipulation of magnetic materials is essential for spintronics. In spin-current generator/magnet bilayers, the efficiency of spin-orbit torques per magnetic layer thickness scales inversely with the magnetic layer thickness, leading to considerable power increase in applications with large magnetic layer thickness. Here, we develop a 3D spin-orbit material scheme in which the spin torque efficiency can be remarkably boosted up by stacking [spin-current generator/magnet/oxide]n superlattices, with the oxide layers breaking the inversion symmetry. In contrast, the spin torque diminishes in [spin-current generator/magnet]n superlattices lacking inversion symmetry breaking. These results advances the understanding of spin-orbit torques in magnetic multilayers and establishes spin-current generator/magnet/oxide superlattices as advantageous bricks for development of high energy-efficiency, high-endurance, and high-density spintronic memory and computing.

Magnetocaloric effect in multilayers studied by membrane-based calorimetry

We study magnetic multilayers, incorporating dilute ferromagnetic spacers between strongly-ferromagnetic layers exhibiting a proximity-enhanced magnetocaloric effect (MCE). Using magnetometry and direct measurements of the adiabatic temperature change based on a nanomembrane-calorimetry, we find that the MCE in the studied multilayer is indeed enhanced compared to that in the bulk spacer material. We develop a phenomenological numerical model of the studied trilayer and find that a long-range exchange interaction through the weakly-ferromagnetic spacer is required to adequately describe the magnetic and magnetocaloric properties of the system.

Key Points in the Determination of the Interfacial Dzyaloshinskii–Moriya Interaction From Asymmetric Bubble Domain Expansion

Different models have been used to evaluate the interfacial Dzyaloshinskii–Moriya interaction (DMI) from the asymmetric bubble expansion method using magneto-optics. Here, we investigate the most promising candidates over a range of different magnetic multilayers with perpendicular anisotropy. Models based on the standard creep hypothesis are not able to reproduce the domain wall (DW) velocity profile when the DW roughness is high. Our results demonstrate that the DW roughness and the interface roughness of the sample layers are correlated. Furthermore, we give guidance on how to obtain reliable results for the DMI value with this popular method. A comparison of the results with Brillouin light scattering (BLS) measurements on the same samples shows that the BLS approach often results in higher measured values of DMI.

Manufacturing and Characterization of Novel Multilayer Magnets for Electrical Machine Applications

This paper presents the potentialities of diverse kinds of permanent magnet materials in order to build and characterize multilayer magnets, as well as to investigate their prospective application in electrical machines. In particular, bonded magnet and hybrid magnet powders are used to build two-layer samples, which pave the way for constructing special electrical machines with complex magnetic structures, such as the variable flux permanent magnet machines. In this study, samples of innovative double-layer magnets are built through a single press and the following magnetization procedure. A particular electromagnetic circuit is adapted to experimentally achieve the demagnetization characteristic to estimate the air gap magnetic flux behaviour in electrical machines. The measurement system flexibility allowed the testing of two double-layer magnets: the so-called parallel and series arrangements.

Detent Force Reduction in Linear Interior Permanent Magnet Generator for Direct-Drive Wave Power Conversion

The permanent magnet linear generator is widely applied in the direct-drive wave energy converter (DD-WEC) because of its high power density. In this paper, a novel tubular permanent magnet linear generator, which consists of multilayer and interior permanent magnets (MI-TLPMGs), is presented for DD-WEC, which improves the output power and back electromotive force (back EMF) through the flux concentrating effect. However, MI-TLPMGs with multilayer embedded permanent magnets have severe problems regarding force ripples and detent force, which affect the DD-WEC’s dynamics. Therefore, a DD-WEC system with MI-TLPMGs is proposed, and the effect of the detent force on the dynamic performance of the DD-WEC is analyzed theoretically. Then, the L-type auxiliary teeth and magnetic barriers, which are optimized by the Taguchi method, are introduced to minimize the detent force of the MI-TLPMGs. After optimization using the Taguchi method, the amplitude of the detent force is reduced from the initial 21.7 N to 5.2 N, which means it has weakened by nearly 76.1%. Finally, a prototype has been manufactured and measured in the wave tank to verify the optimization results.

Modelling of Magnetic Stray Fields in Multilayer Magnetic Films with In-Plane or Perpendicular Anisotropy

The magnetic stray field is an unavoidable consequence of magnetic multilayers, which may have a significant influence on the performance of spintronic devices. Based on Maxwell’s magnetostatics theory, here we numerically calculated the distributions of magnetic stray fields and self-demagnetizing fields in a series of patterned multilayer thin-film structures with either an in-plane or a perpendicularly magnetized ferromagnetic layer. The stray field above the ferromagnetic layer is inhomogeneous, showing the dramatic changes near the sample edge, but the uniformity in the center region could be improved with the increasing sample size. The stray field strength tends to zero for large samples, increases with the increase in the hard-layer thickness, and decreases with the increase in the distance D away from the ferromagnetic layer. In the multilayer samples, the separately simulated stray field and self-demagnetizing field within the soft layer agree well with the classic magnetostatic relationship of B=μ0(Hd+M). For the in-plane magnetized trilayer sample, the magnetic-flux density within the soft ferromagnetic layer slightly decreases in the antiparallel magnetization alignment and increases in the parallel alignment state with the increase in the intermediate non-magnetic-layer thickness. In contrast, for the sample with the perpendicular magnetization, the magnetic-flux density decreases as the non-magnetic layer is thickened for both the antiparallel and parallel state. This study may provide a theoretical basis for the design of thin-film spintronic devices.

Three-dimensional skyrmionic cocoons in magnetic multilayers

Three-dimensional spin textures emerge as promising quasi-particles for encoding information in future spintronic devices. The third dimension provides more malleability regarding their properties and more flexibility for potential applications. However, the stabilization and characterization of such quasi-particles in easily implementable systems remain a work in progress. Here we observe a three-dimensional magnetic texture that sits in the interior of magnetic thin films aperiodic multilayers and possesses a characteristic ellipsoidal shape. Interestingly, these objects that we call skyrmionic cocoons can coexist with more standard tubular skyrmions going through all the multilayer as evidenced by the existence of two very different contrasts in room temperature magnetic force microscopy. The presence of these novel skyrmionic textures as well as the understanding of their layer resolved chiral and topological properties have been investigated by micromagnetic simulations. Finally, we show that the skyrmionic cocoons can be electrically detected using magneto-transport measurements.

Topological Hall effect in Pd/CoZr/MgO multilayer films

The topological Hall effect (THE) in annealed Pd/CoZr/MgO multilayers was investigated and disentangled in the temperature range of 50–300 K. The anomalous Hall curves change polarity with decreasing temperature. Moreover, crests and troughs develop in the curves. The topological Hall resistivity is obtained by subtracting the anomalous Hall and ordinary Hall resistivities from the temperature dependence of the total transverse resistivity. The interfacial Dzyaloshinskii–Moriya interaction (DMI) strength is affected by the ambient temperature, annealing temperature, and thickness of the CoZr layer, resulting in the manipulation of the topological Hall effect. The room temperature THE was obtained in this multilayer system, and the maximum THE resistivity reached 0.52 μΩ cm when the thickness of the CoZr layer was 3.25 nm at 100 K. Our study indicates the potential application of CoZr alloys in magnetic multilayers for the performance of spin textures and THE.

Theoretical foundation for designing multilayer Halbach array magnets for benchtop NMR and MRI

Multilayer Halbach array magnets support portable NMR and MRI, but optimizing their design to maximize performance and minimize the use of expensive magnet materials is challenging. This is partly because our theoretical understanding of such arrays is incomplete and computationally intensive. Here we provide a theoretical description of the magnetic field distribution and we demonstrate that inhomogeneity is greatest along the z axis in multilayer Halbach array magnets. This allows the configuration of the multilayer Halbach array magnets to be optimized in a way that takes into account homogeneity, magnet volume, and magnetic flux density. At the same time, our description simplifies the design of multilayer array magnets, while accommodating the possibility of different outer radii, lengths for each layer array, or the presence of separation between the rings. We validated the theoretical description in simulations of a three-layer Halbach array magnet, then with a prototype three-layer 1-T Halbach array magnet. After adjusting the position of magnet blocks in the neighboring rings, we achieved homogeneity of 220 ppm for a standard 5 mm NMR tube while the inner diameter of the magnet is 20 mm. Our work provides a theoretical foundation for designing multilayer Halbach array magnets to maximize homogeneity and minimize the use of magnet materials.

Optimized multi-layer radial permanent magnet bearing with an eddy current damping systems

This research work proposes the conventional and novel eddy current damping systems (CECDS & NECDS) for an optimized multi-layer radial permanent magnet bearing, which is axially magnetized. Firstly, the design and optimization of a bearing are carried out by adopting the general procedure for maximum radial force and stiffness by selecting stator outer diameter, radial air gap, and length as parameters. The maximum radial force and stiffness concerning an optimized structure are also calculated using semi-analytical equations, and force results are validated with three-dimensional (3D) finite element analysis results. Then, the analysis of both damping systems is carried out for damping forces and coefficients using 3D electromagnetic transient analysis in ANSYS. The proposed dampers provide adequate damping to the bearing structure, and conductor plate thickness significantly affects the damping characteristics. The NECDS could be used to replace the conventional type as it eliminates the effect of reduced radial air gap on the performance of the bearing.

Observation of the giant magneto-Seebeck effect in a metastable Co50Fe50/Cu multilayer

We report the observation of the giant magneto-Seebeck (GMS) effect in an epitaxially grown Co50Fe50/Cu multilayer film with metastable bcc Cu spacers under an in-plane temperature gradient. The magnetization-dependent switching ratio of the Seebeck coefficient, GMS ratio, and switching ratio of the thermoelectric power factor reach approximately −50% and 280% at room temperature, respectively, which are higher than those previously reported in magnetic multilayers with the current-in-plane geometry. By measuring the temperature dependence of both GMS and giant magnetoresistance (GMR) effects, we found that the GMS ratio remains high at high temperatures, while the GMR ratio quickly decreases with increasing temperature, where the spin-dependent electron scattering dominantly affects the large GMS effect in the Co50Fe50/Cu multilayer.

Control of transport phenomena in magnetic heterostructures by wavelength modulation

We demonstrate the tunability of the ultrafast energy flow in magnetic/nonmagnetic bilayer structures by changing the wavelength of the optical excitation. This is achieved by an advanced description of the temperature-based $\ensuremath{\mu}T$ model that explicitly considers the wavelength- and layer-dependent absorption profile within multilayer structures. For the exemplary case of a Ni/Au bilayer, our simulations predict that the energy flow from Ni to Au is reversed when changing the wavelength of the excitation from the infrared to the ultraviolet spectral range. These predictions are fully supported by characteristic signatures in the time-dependent magneto-optical Kerr traces of the Ni/Au model system. Our results will open up avenues to steer and control the energy transport in designed magnetic multilayers for ultrafast spintronic applications.