Nonmagnetic Layer Research Articles

Modulating the electronic and optical properties of CrI3/In2Se3 van der Waals heterostructures by external fields

The van der Waals heterostructures (vdWH) composed of two-dimensional (2D) magnetic materials and non-magnetic atomic layers provide promising candidates for innovative device design. Here, we construct the CrI3/α-In2Se3 vdWH and regulate its band structure, magnetism and optical absorption through external fields. The intrinsic vdWH possesses type-Ⅱ band alignment with 0.38 eV band gap. As a result of proximity effects and superexchange interaction, magnetism is induced in the α-In2Se3 layer. The Curie temperature of the vdWH with FM coupling has increased to 99.3 K. Interestingly, with the increasing of the applied electric field, the band alignment exhibits a transition from type-Ⅱ to type-Ⅰ and then back to type-Ⅱ. When the electric field is −4.31 V/nm, the vdWH convert from semiconductor to half-metal. Moreover, the applying biaxial and vertical strain can influence the energy level of hybridization, and transferred charges. For example, the increased magnetic moment of the CrI3 monolayer increased by four times when the distance gap is decreased by 0.5 Å. These properties can offer novel strategies for engineering band structure and regulating magnetic properties, thereby enabling researchers to design tunable multiferroic vdWH devices.

Observation of topological hall effect and skyrmions in Pt/Co/Ir/Co/Pt system

Abstract The interlayer exchange coupling (IEC) between two ferromagnetic (FM) layers separated by a non-magnetic (NM) spacer layer gives rise to different types of coupling with the variation of spacer layer thickness. When the NM is metallic, the IEC is attributed to the well-known Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction which shows an oscillatory decaying nature with increasing thickness. Due to this, it is possible to tune the coupling between the two FM to be either ferromagnetic or antiferromagnetic. In this work we have studied a Pt/Co/Ir/Co/Pt system where the Co thickness has been taken in the strong perpendicular magnetic anisotropy regime which is much less than the spin reorientation transition thickness. By tuning the Ir thickness to 2.0 nm, a canted state of magnetization reversal in the system is observed which gives rise to a possibility of nucleating topologically non-trivial spin textures like skyrmions. Further, with the combination of transport and magnetic force microscopy (MFM) measurements, we have confirmed the presence of skyrmions in our system. These findings may be useful for potential applications in emerging spintronic and data storage technologies using skyrmions.

Thickness-Dependent Gilbert Damping and Soft Magnetism in Metal/Co-Fe-B/Metal Sandwich Structure.

The achievement of the low Gilbert damping parameter in spin dynamic modulation is attractive for spintronic devices with low energy consumption and high speed. Metallic ferromagnetic alloy Co-Fe-B is a possible candidate due to its high compatibility with spintronic technologies. Here, we report thickness-dependent damping and soft magnetism in Co-Fe-B films sandwiched between two non-magnetic layers with Co-Fe-B films up to 50 nm thick. A non-monotonic variation of Co-Fe-B film damping with thickness is observed, which is in contrast to previously reported monotonic trends. The minimum damping and the corresponding Co-Fe-B thickness vary significantly among the different non-magnetic layer series, indicating that the structure selection significantly alters the relative contributions of various damping mechanisms. Thus, we developed a quantitative method to distinguish intrinsic from extrinsic damping via ferromagnetic resonance measurements of thickness-dependent damping rather than the traditional numerical calculation method. By separating extrinsic and intrinsic damping, each mechanism affecting the total damping of Co-Fe-B films in sandwich structures is analyzed in detail. Our findings have revealed that the thickness-dependent damping measurement is an effective tool for quantitatively investigating different damping mechanisms. This investigation provides an understanding of underlying mechanisms and opens up avenues for achieving low damping in Co-Fe-B alloy film, which is beneficial for the applications in spintronic devices design and optimization.

Effects of Ti doping on the magnetic and microstructure properties of the lower magnetic anisotropy CoCrPtRu-oxides layer in a perpendicular recording media

Areal density capacity is constrained by linear and track density, and the recent advancements in magnetic layer structures have led to the development of segmented media. This configuration incorporates a graded magnetic anisotropy layer introduced through a non-magnetic exchange control layer. Optimizing this structural design yielded improvements in the signal-to-noise ratio (SNR) and an elevation in areal density capacity. To comprehensively investigate the magnetic properties and microstructure of the lower magnetocrystalline anisotropy constant (Ku) CoCrPtRu-oxides magnetic capping layer individually, we deposited the (0002) textured magnetic capping layer CoCrPtRu-oxides on (0002) Ru/NiW/NiTa underlayers. Additionally, metallic Ti was introduced into CoCrPtRu targets, replacing half of Ru, resulting in the deposition of the (0002) textured CoCrPtRuTi-oxides film was also deposited and compared to CoCrPtRu-oxides film. A comparative analysis was conducted between the CoCrPtRu-oxides and CoCrPtRuTi-oxides films. Microstructural image mapping and chemical composition analysis revealed that the introduced Ti in core grains diffused to the grain boundaries, enhancing grain separation. This diffusion was facilitated by the more negative formation energy of TiO2 (-3.512 eV/atom) compared with other oxides in the film. Consequently, the CoCrPtRuTi-oxides film exhibited slightly larger grain boundary areas (52%) and smaller grain sizes. Moreover, perpendicular coercivity (243.50 kA/m) and anisotropy were higher. This insight suggests a potential strategy for improving segregation by doping metallic elements, such as Ti, into the core grains, emphasizing the importance of not merely increasing the oxide content (e.g., TiO2).

Study of the Long-Range Exchange Coupling in Nd-Fe-B/Ti/Fe Multilayered Structure

The exchange coupling between two ferromagnetic thin films, one with magnetically hard and the other with soft phases, separated by a thin non-magnetic layer, is studied. Nd-Fe-B/Ti/Fe thin film heterostructures were fabricated using DC magnetron sputtering on Si substrates, which were heated in situ at 650 °C using a house-built vacuum-compatible heater. The effect of the thickness of the Ti buffer layer and the annealing temperature on the formation of various phases of Nd-Fe-B was investigated. The effect of the thickness of the non-magnetic Ti spacer layer on the exchange coupling strength between the hard phase Nd-Fe-B ferromagnetic thin layer and the soft phase transition metal Fe layer was experimentally investigated. Hysteresis loops of multilayer thin films indicate an antiferromagnetic coupling was observed when the thickness of the spacer layer was 2 nm. This is within the range of an antiferromagnetic coupling calculation based on RKKY theory predictions.

Spatially inhomogeneous inverse Faraday effect provides tunable nonthermal excitation of exchange dominated spin waves

Abstract We demonstrate optical nonthermal excitation of exchange dominated spin waves of different orders in a magnetophotonic crystal. The magnetophotonic structure consists of a thin magnetic film and a Bragg stack of nonmagnetic layers to provide a proper nonuniform interference pattern of the inverse Faraday effect induced by light in the magnetic layer. We found a phenomenon of the pronounced phase slippage of the inverse Faraday effect distribution when the pump wavelength is within the photonic band gap of the structure. It allows to tune the interference pattern by a slight variation of light wavelength which results in the modification of excitation efficiency of the different order spin waves. The approach can be applied for different magnetic dielectrics expanding their application horizons for spin-wave based devices.

Strong perpendicular magnetic anisotropy and interlayer coupling in CoRh/Rh/Fe multilayers tailored by Rh spacer layer thickness

Magnetic interlayer coupling is a critical study topic for spintronic device technology because it is an essential component in designing magnetic multilayers with functionality that can be expanded into application areas. Particularly, the interlayer coupling between two ferromagnetic (FM) layers with different magnetic properties in the magnetic field is the specific situation to develop spintronic applications, such as increasing the sensitivity of magnetic field sensors. Therefore, the film structure was chosen as two layers in this study; the first ferromagnetic layer (FM1-CoRh) has a perpendicular and the second ferromagnetic layer (FM2-Fe) has an in-plane (IP) magnetic easy axis in the presence of a magnetic field. Here, a non-magnetic (NM) Rh spacer layer separated the two FM layers. The magnetic properties dependent on spacer layer thickness (t) in CoRh/Rh/Fe multilayers were investigated in detail. It is found that the strength of interlayer coupling alters depending on the t.

Surface acoustic wave-spin wave coupling and magneto-acoustic nonreciprocal devices

Surface acoustic wave (SAW) is a new means of exciting and controlling spin wave (SW), which has not only high excitation efficiency, but also long transmission length up to millimeter order. Based on the SAW-SW coupling (phonon-magnon coupling), a wide variety of new devices and applications such as high-sensitivity weak magnetic field sensors, energy-efficient spintronic devices, solid-state acoustic isolators, and nonreciprocal phase shifters, have been realized. Therefore, it is of great value to study the physical mechanism of magneto-acoustic coupling, develop new magneto-acoustic coupling effects, and improve the efficiency of magneto-acoustic coupling. In this work, different types of physical mechanisms of magneto-acoustic coupling are reviewed. The effective driven magnetic fields of magnetoelastic coupling, spin-vorticity coupling (including injection of alternating spin current from a non-magnetic layer and Barnett effect inside magnetic material itself), and magneto-rotation coupling under different modes of SAW excitation are compared. The angular dependence of these driven fields and the frequency dependence of the corresponding power absorption are discussed, which provides theoretical support for distinguishing and utilizing various magneto-acoustic coupling in practical applications. In addition, we also introduce two methods to realize nonreciprocal SAW transmission by magneto-acoustic coupling, including the helicity mismatch effect and nonreciprocal spin-wave dispersion magnetic structures, and discuss their physical mechanisms as well as advantages and disadvantages. For such magneto-acoustic nonreciprocal devices, the properties of higher isolation, lower insertion loss and wider bandwidth are always desired. In order to improve the properties of the devices, it is important to find magnetic structures with stronger SW nonreciprocity, reduce the insertion loss introduced by magnetic structure, and fully consider the effective driven field characteristics of different modes of SAW. We hope that this review can serve as a guide for future design and development of solid acoustic isolators and circulators in the RF and microwave frequency bands.

Terahertz emission characterization of silicon based ferromagnetic heterostructures

Terahertz spectroscopy and imaging have many applications, so the generation of broadband terahertz radiation is very important, but now it faces some challenges. Opto-spintronic terahertz emitters, composed of nanometer-thin magnetic multilayer, can produce high-quality broad-band terahertz pulses. Integration of opto-spintronic terahertz emitters onto the silicon wafers is the first step towards their usage in modern photonic devices. In this work, Ta/CoFeB/Ir heterostructures are deposited on thermally oxidized silicon wafers by dc magnetron sputtering. Under the illumination of a femtosecond laser pulse on the Ta/CoFeB/Ir trilayer heterostructure grown on silicon substrate, a spin current can be generated in the ferromagnetic layer due to the ultrafast demagnetization. The spin current is transported and injected into the neighboring non-magnetic metal layers of Ta and Ir. Consequently, the spin current can be converted into the charge current due to the strong spin-orbit coupling. The sub-picosecond transient charge current gives rise to the terahertz radiation that enters into the free space. The terahertz electric field is fully inverted when the magnetization is reversed, which indicates a strong connection between THz radiation and spin order of the heterostructure. The THz radiation from Ta/CoFeB/Ir heterostructure covers the 0.1–2.5 THz frequency range with a maximum value of about 0.64 THz. We also investigate the dependence of THz peak-to-peak value on the pump fluence. The THz emission is found to be saturated at a pump fluence of ~0.73 mJ/cm<sup>2</sup>. Our results demonstrate the existence of the strong spin-orbit coupling in the heavy metal Ir. Furthermore, we optimize the THz emission from the Ta/CoFeB/Ir heterostructure by changing the thickness of Ir layer. According to the thickness dependence of THz emission from the heterostructure, the propagation length of the spin current at THz frequencies is extracted to be about 0.59±0.12 nm, which is shorter than the GHz experimental measurement (~1.34 nm). Our experimental observation is consistent with that in the antiferromagnet IrMn layer, which may be attributed to different transport regimes. Theoretically, the optimized thickness values for CoFeB and Ir layers are 2.4 nm and 1.1 nm, respectively.

Interlayer coupling of the Kittel mode and the perpendicular standing spin wave in magnetic multilayers

Magnetic/nonmagnetic multilayers are complex and fascinating systems with immense potential for various technological applications. To meet the requirements of spintronic devices, it is essential to comprehend their magneto-dynamic behaviour and interlayer coupling mechanisms. Here, we systematically study the static and dynamic coupling of different spin precession modes, including the Kittel mode and the perpendicular standing spin wave (PSSW) mode, between permalloy (Py) layers with variable thicknesses. Our main focus centers around three distinct types of magnetic multilayers featuring different spacers (Ta, Pt, Ru). According to the ferromagnetic resonance results, we find that the nonmagnetic layers inside these structures display distinct magnetic static and dynamic characteristics over varying ranges of magnetic layer thickness. When the thickness of the Py layer is 10 nm, it demonstrates a ferromagnetic exchange coupling between the Py layers via the Ta and Pt layers. Conversely, when the spacer layer is composed of Ru, a clear antiferromagnetic coupling is detected. All of the multilayers, however, exhibit increased ferromagnetic exchange coupling as the thickness of the Py layer increases. Furthermore, it is found that the Pt spacer layer still plays a crucial role in dynamic interlayer coupling in the uniform precession mode, while the Py/Ru multilayer exhibits a stronger coupling in the non-uniform precession mode.

Tuning magnon spectra via interlayer coupling in pseudo-3D nanostructured artificial spin ice arrays

We have investigated the static and dynamic behavior of “pseudo-3D” trilayer square artificial spin ice structures. The trilayer stack comprises of two ferromagnetic Ni81Fe19 (Permalloy, Py) layers with 30 and 70 nm thickness, separated by a nonmagnetic copper layer of varying thickness from 2 to 40 nm. We show that the copper thickness enables interlayer coupling between layers to be finely controlled, leading to bespoke magnetization states and resonance spectra tuning. Our results demonstrate a further route to control the interaction in artificial spin ices beyond planar structures, enabling tunable magnetization dynamics, a potentially programmable degree of freedom for magnonic and microwave devices.

Observation of higher-order contribution to anisotropic magnetoresistance of thin Pt/[Co/Pt] multilayered films

We studied the magnetoresistance mechanisms in a Pt/[Co/Pt]x5 film consisting of a ferromagnetic [Co/Pt]x5 layer with strong perpendicular magnetic anisotropy and a nonmagnetic Pt layer with strong spin–orbit coupling. We revealed two competing contributions of the sin2θ and cos4θ types to its angular and magnetic field dependences of electrical resistance at T = 10–250 K corresponding to the out-of-plane rotation of the magnetization M(θ) perpendicularly to the electric current. They were attributed to different magnetoresistance mechanisms. The higher-order cos4θ contribution, which emerges and increases with decreasing temperature, is attributed to the anisotropic magnetoresistance of the ferromagnetic layer, while the sin2θ contribution, which prevails at room temperature and then decreases, is mainly associated with the spin Hall magnetoresistance originating from the Pt layer. The analysis of the corresponding angular dependences of the Hall voltage revealed non-trivial periodic oscillations in the second harmonic. Their appearance is found to be consistent with the manifestation of higher-order angle-dependent contributions to the field-like spin–orbit torque. The revealed strong influence of the electric current on the magnetization of the film studied, which ensures the higher-order effects manifestation, is of high relevance for magnetic memory design technologies.

Ultrafast magnetization enhancement and spin current injection in magnetic multilayers by exciting the nonmagnetic metal

A systematic investigation of spin injection behavior in Au/FM (FM = Fe and Ni) multilayers is performed using the superdiffusive spin transport theory. By exciting the nonmagnetic layer, the laser-induced hot electrons may transfer spin angular momentum into the adjacent ferromagnetic (FM) metals resulting in ultrafast demagnetization or enhancement. We find that these experimental phenomena sensitively depend on the particular interface reflectivity of hot electrons and may reconcile the different observations in the experiment. Stimulated by the ultrafast spin currents carried by the hot electrons, we propose the multilayer structures to generate highly spin-polarized currents for the development of future ultrafast spintronics devices. The spin polarization of the electric currents carried by the hot electrons can be significantly enhanced by the joint effects of bulk and interfacial spin filtering. Meanwhile, the intensity of the generated spin current can be optimized by varying the number of repeated stacking units and the thickness of each metallic layer.

Monte Carlo study of magnetocaloric effect and magnetic properties of the nano- borophene bilayer with RKKY exchange interactions of a mixed spin 2 and 3/2

In this study, we have studied the magnetic properties and magnetocaloric effect of the nano-borophene bilayer with RKKY (Ruderman-Kittel-Kasuya-Yosida) exchange interactions of mixed spin 2 and 3/2 using Monte Carlo calculations. In the first part of this investigation, the ground state phase diagrams for zero temperature are reported and discussed. We also determined the critical temperature for varying the number of non-magnetic layers (NML) and the interacting couplings Jss. In the second part, we investigated the influence of the parameter NML, the exchange couplings Jss and Jσσ, and the crystal field on the total magnetizations and the magnetic hysteresis loop. The specific heat and magnetic entropy changes versus of the temperature under the effect of different external magnetic fields and the parameter Jss. Finally, we analyzed the relative cooling power (RCP) for distinct external magnetic field and Jss.

Interlayer Interaction and Coercivity of Three-Layer Films Оbtained bу Chemical Deposition

Abstract—The results of experimental and theoretical studies of the coercivity and the displacement field of the hysteresis loop on the thickness of the nonmagnetic interlayer in magnetic films obtained by electroless plating are presented. With the help of model calculations based on the Landau–Ginzburg equations, the exchange interactions between magnetic layers with the participation of atoms of the nonmagnetic layer are studied. The resulting expression for the displacement field describes well the exponential changes in the displacement field as a function of the interlayer thickness in structures with both magnetically soft layers and layers with significantly different values of the coercivity.

Investigation of the effects of Cu layer thickness and annealing temperature under air atmosphere on the properties of Co/Cu multilayer films

In this study, the suitability of Co/Cu multilayer films with different physical conditions (varying thickness values and systematic annealing processes) in magnetic technology applications is discussed. Co/Cu films were deposited by the sputtering technique. The effects of different thicknesses of non-magnetic (Cu) layers and annealing temperature on magneto-structural properties were investigated. Different thicknesses of Cu layers were determined as 34 nm, 10 nm, 4 nm. Additionally, two annealed situations were considered to investigate the annealing effect. While the first one is exposing the films to 80 °C and 130 °C for 240 s, second one is annealing the films at 180 °C for different exposure times (50 s and 150 s). All films that have different thicknesses of Cu layers crystallized in (111) plane of the face centered cubic (fcc) structure. The intensity of this peak increased with increasing Cu layers thickness. Variation in the thickness of Cu layers has an important effect on the film surface. Saturation magnetization (Ms), coercivity (Hc) and squareness (Mr/Ms, Mr: remanent magnetization) were considerably affected by variation of the Cu content and film surface caused by the change in the thickness of the Cu layers. The film with 4 nm Cu layer thickness has the highest Ms, lowest Hc values and high Mr/Ms ratio. This indicates magnetically high efficient compared to the other films in the same series. The fcc structure continued to exist for the films annealed at 80 °C and 130 °C for 240 s. It was found that the annealing procedure transfigured the film surface and the differences in Ms and Hc values can be mostly attributed to this transfiguration because of the same film content revealed. An increase in Ms value, and a slight decrease in Hc and Mr/Ms values were detected for the annealed film at 130 °C, compared to the film annealed at 80 °C. It was also seen that the film structure was damaged at 180 °C because of excessive heat transfer.

Chemical Approach Towards Broadband Spintronics on Nanoscale Pyrene Films.

The injection of pure spin current into the non-magnetic layer plays a crucial role in transmitting, processing, and storing data information in the realm of spintronics. To understand broadband molecular spintronics, pyrene oligomer film (~20 nm thickness) was prepared using an electrochemical method forming indium tin oxide (ITO) electrode/pyrene covalent interfaces. Permalloy (Ni80Fe20) films with different nanoscale thicknesses were used as top contact over ITO/pyrene layers to estimate the spin pumping efficiency across the interfaces using broadband ferromagnetic resonance spectra. The spintronic devices are composed of permalloy/pyrene/ITO orthogonal configuration, showing remarkable spin pumping from permalloy to pyrene film. The large spin pumping is evident from the linewidth broadening of 5.4 mT at 9 GHz, which is direct proof of spin angular momentum transfer across the interface. A striking observation is made with the high spin-mixing conductance of ~1.02 1018 m-2, a value comparable to the conventional heavy metals. Large spin angular moment transfer was observed at the permalloy-pyrene interfaces, especially at the lower thickness of permalloy, indicating a strong spinterface effect. Pure spin current injection from ferromagnetic into electrochemically grown pyrene films ensures efficient broadband spin transport, which opens a new area in molecular broadband spintronics.

Effects of structure, size and non-magnetic Cu layer thickness on magnetization switching behavior in Ni/Cu/M (M [formula omitted] Ni, Co) cylindrical nanowires

We have investigated the dependence of the magnetization switching behavior on the structure, size and nonmagnetic Cu layer thickness (tCu) in Ni/Cu/M (M = Ni, Co) cylindrical nanowires fabricated by electrodeposition using porous polycarbonate templates. The crystal structure of the Ni nanowires was (111)- or (200)-oriented polycrystalline fcc, while that of Co nanowires was (100)- or (101)-oriented hcp and/or (111)-oriented polycrystalline fcc. When the wire diameter (d) was 50 nm, the squareness ratio (Mr/Ms) for M = Ni, as estimated from the magnetization curve, is almost constant at Mr/Ms∼0.79 (Mr/Ms∼0.64 for M = Co) when tCu increases from 0 to 500 nm. When d=100nm, Mr/Ms decreases by 41% from 0.71 to 0.42 with increasing tCu from 0 to 700 nm for M = Ni, while Mr/Ms decreases by 25% from 0.53 to 0.40 with varying tCu from 0 to 500 nm for M = Co. The threshold Cu layer thickness (tCuth) beyond which Mr/Ms becomes constant at d=100nm is ∼500 nm for M = Ni and ∼300 nm for M = Co. The results are qualitatively consistent with those obtained from our micromagnetic simulation and analysis. The observed difference in the tCuth dependence of Mr/Ms between M = Ni and Co is attributed to the change in the magnetic domain structure of the nanowires as it sensitively varies with the magnetocrystalline anisotropy. On the other hand, the coercive force of the nanowires shows a small tCu dependence for both d=50 and 100 nm. These results indicate that the layered structure in the nanowires governs the magnetic domain structure, though the effect on the magnetization switching behavior is indeed relatively small.

Ferromagnetism emerged from non-ferromagnetic atomic crystals

The recently emerged ferromagnetic two-dimensional (2D) materials provide unique platforms for compact spintronic devices down to the atomic-thin regime; however, the prospect is hindered by the limited number of ferromagnetic 2D materials discovered with limited choices of magnetic properties. If 2D antiferromagnetism could be converted to 2D ferromagnetism, the range of 2D magnets and their potential applications would be significantly broadened. Here, we discovered emergent ferromagnetism by interfacing non-magnetic WS2 layers with the antiferromagnetic FePS3. The WS2 exhibits an order of magnitude enhanced Zeeman effect with a saturated interfacial exchange field ~38 Tesla. Given the pristine FePS3 is an intralayer antiferromagnet, the prominent interfacial exchange field suggests the formation of ferromagnetic FePS3 at interface. Furthermore, the enhanced Zeeman effect in WS2 is found to exhibit a strong WS2-thickness dependence, highlighting the layer-tailorable interfacial exchange coupling in WS2-FePS3 heterostructures, which is potentially attributed to the thickness-dependent interfacial hybridization.

Magnetic characteristics of the ferrimagnetic bilayer MXene-like nanostructure with interlayer exchange interactions: Monte Carlo study

Monte Carlo simulations are used to investigate the magnetic properties of the nanostructured spin-7/2 and spin-1 ferrimagnetic bilayers within interlayer exchange interactions. The effects of nonmagnetic layers thickness, magnetic coupling and the crystal field on the blocking temperature, magnetization and hysteresis loops are studied. It is found that the blocking temperature decreases when increasing the magnetic coupling between layers and/or increasing the thickness of the magnetic layer. Besides, the model exhibits multiple hysteresis loops for which the area decreases when increasing the thickness of the magnetic layer, the crystal field and/or the temperature. Moreover, the effect of temperature on the behavior of the partial and the total magnetization and the total susceptibility are also as investigated.