Magnetoelectronic Devices Research Articles

High frequency dynamics and magnetic anisotropy of bcc Co films grown on Si (0 0 1) substrate

Dynamic magnetization and magnetic anisotropy of Cu/bcc-Co/CoSi2 /Si(001) system were investigated by Magneto-Optical Kerr Effect (MOKE) analysis and vector network analyzer ferromagnetic resonance (VNA-FMR). Typical fourfold magnetocrystalline anisotropy of bcc Co film and superposed uniaxial magnetic anisotropy were observed. The uniaxial magnetic anisotropy originated from shape anisotropy as confirmed by micromagnetic simulation. The large magnetocrystalline anisotropy results in high resonance frequency even in the zero applied field. The Gilbert damping parameter α obtained by analysis of the dynamic process is close to the theoretically estimated value of bcc phase Co. The Gilbert damping parameter of the bcc-Co film is larger than those of fcc-Co and hcp-Co films. This work indicates the Co damping parameter can be enhanced by using bcc structure, which will help create high-speed magnetoelectronic devices.

Electric field control of magnetism in FePt/PMN-PT heterostructures

Laminar heterostructures composed by materials with different ferroic properties have been proposed for the control of magnetism with an applied voltage. We present here a study of the structural and magnetic properties of chemically disordered FePt ferromagnetic films of different thicknesses that have been sputter-deposited on ferroelectric PMN-PT (011) single crystals.By means of static and dynamic magnetic measurements we have found that it is possible to switch the magnetization easy axis by 90° if an electric field is applied in a direction perpendicular to the film plane. Using ferromagnetic resonance techniques we have estimated the strain induced magnetoelectric coupling of the FePt/PMN-PT heterostructure, obtaining ME=477(15) Oe m/MV for FePt thicknesses greater than 26 nm, which corresponds to an average magnetoelastic constant B=−13.2(1.1) MJ/m3. The associated saturation magnetostriction was estimated in λs∼90 ppm. Films thinner than 26 nm present corresponding values that are smaller by approximately 20%. A possible explanation for this difference is the well known transition from planar to stripe-like magnetic domains that occurs in FePt around this film thickness. However, this thickness dependence of ME is relatively smaller than that found in other ferromagnets such as Fe or FeGa, which makes FePt a potential candidate for thin film applications in magnetoelectronic devices.

Air-Stable Chiral Single-Molecule Magnets with Record Anisotropy Barrier Exceeding 1800 K.

Design and synthesis of air-stable and easily tailored high-performance single-molecule magnets (SMMs) are of great significance toward the implementation of SMMs in molecular-based magneto-electronic devices. Here, by introducing electron-withdrawing fluorinated substituents on equatorial ligand, two chiral Dy(III) macrocyclic complexes, RRRR-Dy-D6hF12 (1) and SSSS-Dy-D6hF12 (2), with a record anisotropy barrier exceeding 1800 K and the longest relaxation time approaching 2500 s at 2.0 K for all known air-stable SMMs, were obtained. The nearly perfect axiality of the ground Kramers doublet (KD) enables the open hysteresis loops up to 20 K in the magnetically diluted sample. It is notable that they are structurally rigid with high thermal stability and the apical ligand can be tailored to carry proper surface-binding groups. This finding not only improves the magnetic properties for air-stable SMMs but also provides a new avenue for deposition of SMMs on surfaces.

Experimental and theoretical investigations of the structural, magnetic and electronic characteristics of copper ferrite nanostructures as potential spin-filtering candidates

Employing both experimental and first-principle techniques, the structural, dielectric, magnetic, and electronic characteristics of spinel-structured copper ferrite (CuFe2O4) nanoparticles (NPs) are examined. The CuFe2O4 NPs are synthesized by a co-precipitation technique (CPT) and characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM), and X-ray photoelectron spectroscopy (XPS). The SEM results display that the CuFe2O4 NPs possess mixed spinel phases. The composition of NP elements is determined via Energy-dispersive X-ray (EDX) spectroscopy. The magnetization and coercivity are evaluated by VSM measurements. The principal peaks of the O 1s, Cu 2p, and Fe 2p states are appeared at peculiar binding energies in XPS spectra. The magnetic features and electronic structures of the CuFe2O4 NPs are examined via density functional theory (DFT) together with on-site correction for Coulomb interaction using the generalized gradient approach (GGA + U) technique for exchange and correlation term. The CuFe2O4 NPs derived from the bulk spinel structure possess a magnetic semiconducting character. The results of density of states (DOSs) reveal an insulating character having an energy band gap of ~1.75 eV for both spin channels. The local magnetic moments display a ferrimagnetic spin arrangement in the CuFe2O4 nanostructure, manifesting through the hybridized d states of Fe/Cu and the p orbitals of O atoms. The K-edge X-ray absorption spectra (XAS) of Cu, Fe, and O ions in the CuFe2O4 nanostructure are also modeled with GGA + U technique and compared with the XPS results. The structural features of the local environment of Cu2+, Fe3+, and O2- ions are scrutinized theoretically to discern the ion sites centered at octahedral or tetrahedral lattice voids. The alternating current (AC) conductivity diminishes with the augmentation in frequency, yielding a semiconductive nature. The characterization of CuFe2O4 NPs can provide insights into their preferential magnetic and dielectric behaviors applicable in miniature magnetoelectronic and energy-storage devices.

Van der Waals epitaxial growth of air-stable CrSe2 nanosheets with thickness-tunable magnetic order.

The discovery of intrinsic ferromagnetism in ultrathin two-dimensional van der Waals crystals opens up exciting prospects for exploring magnetism in the ultimate two-dimensional limit. Here, we show that environmentally stable CrSe2 nanosheets can be readily grown on a dangling-bond-free WSe2 substrate with systematically tunable thickness down to the monolayer limit. These CrSe2/WSe2 heterostructures display high-quality van der Waals interfaces with well-resolved moiré superlattices and ferromagnetic behaviour. We find no apparent change in surface roughness or magnetic properties after months of exposure in air. Our calculations suggest that charge transfer from the WSe2 substrate and interlayer coupling within CrSe2 play a critical role in the magnetic order in few-layer CrSe2 nanosheets. The highly controllable growth of environmentally stable CrSe2 nanosheets with tunable thickness defines a robust two-dimensional magnet for fundamental studies and potential applications in magnetoelectronic and spintronic devices.

Influence of nickel dopant concentration on structural, optical, magnetic and electrochemical properties of TiO2 nanoparticle

In this work, we report enhanced photocatalytic and magnetisation effects of nickel (Ni) ion doped titanium dioxide (TiO2) nano particles. The nano crystalline TiO2 doped with different concentrations of Ni (0.05-0.2%) were prepared by sol-gel method and characterised by various analytical techniques such as X-ray diffraction (XRD), UV-Vis spectroscopy, scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM). The Ni metal ions incorporation modified the microstructure of TiO2 nano particles considerably which are evident from XRD and SEM analyses. The band gap reduction made by the Ni impurity level extends the TiO2 optical absorption edge to visible region. The photocatalytic activity of TiO2 and Ni-TiO2 was studied by photo degradation of methyl orange dye as a function of irradiation time under visible light. The results signify that TiO2 nanoparticles doped with the higher of concentration of 0.2% Ni improves the reaction rate of hydroxyl radical production. The electrochemical behaviour of the prepared samples was analysed by cyclic voltammetry study. The existence of hard ferromagnetic behaviour with the enhanced magnetisation in Ni doped TiO2 samples make this as an appropriate material for possible applications in spintronic and magneto-electronic devices.

Room-temperature negative magnetoresistance of helium-ion-irradiated defective graphene in the strong Anderson localization regime

Anderson localization (AL), a major topic in condensed matter physics, has been extensively studied. Since the discovery of graphene, its unique AL in Dirac materials, particularly weak localization (WL), has been studied intensively. Strong localization (SL) has also been investigated in graphene with intentionally introduced defects. The precise control of spacing/strength of defects using conventional methods is challenging; thus, He-ion-irradiation is a promising technology. However, magnetotransport, a sensitive tool to probe AL, has not yet been studied for He-ion-irradiated graphene. Herein, we systematically investigate the magnetotransport of He-ion-irradiated graphene. We observe negative magnetoresistance (MR) due to SL-dominated hopping transport. By systematically tuning the device parameters, negative MR at room temperature is first revealed for graphene field-effect transistor devices. Our study reveals carrier localization via searching in the multidimensional parameter space of Dirac materials, contributing to the development of fundamental AL physics and magnetoelectronic device applications.

Integer quantum Hall effect of the α−T3 model with a broken flat band

Despite the intense amount of attention and huge potential of two-dimensional (2D) magnets for applications in novel magnetic, magneto-optical, magneto-thermal and magneto-electronic devices, there has yet to be a robust strategy developed to systematically understand magnon-magnon (MMI) interactions at finite temperature. In this paper, we present a first-principles theoretical method to introduce the finite temperature magnon-magnon interaction into Heisenberg Hamiltonian through a nonlinear correction energy. The Wick theorem is used to decouple the four-magnon operators to two-magnon order. We demonstrate the capabilities of this method by studying the strength of MMI in Cr2Ge2Te6 (CGT) monolayer. The spin wave spectrum at finite temperature and the time-dependent spin autocorrelation function are explored. It is found that the magnon relaxation time due to magnon-magnon scattering increases with temperature because of the reduction in magnon energy, while decreases with wavevector and external magnetic field. Our results provide a new insight to understand the magnon damping and energy dissipation in two-dimensional ferromagnetic materials.

Interface-induced ferromagnetism in μ-Fe2O3/β-Ga2O3 superlattices

Superlattices of antiferromagnetic μ-Fe2O3 and diamagnetic β-Ga2O3 are grown by plasma-assisted molecular beam epitaxy on (010) oriented β-Ga2O3 substrates in which ferromagnetism emerges above room temperature. To investigate the suspected interface origin of the ferromagnetic phase, identical superlattice structures are grown at various substrate temperatures and beam fluxes. Atomic-resolution scanning transmission electron microscopy images confirm the registry of μ-Fe2O3 to the β-Ga2O3 layers in these superlattices. Atomic force microscopy and high-resolution x-ray diffraction are used to examine the growth morphology and characterize the superlattice interface roughness. The saturation magnetization of the ferromagnetic phase is observed to increase strongly with the interface roughness. Conversely, smoother superlattices exhibit a weaker ferromagnetic response and a higher density of paramagnetic moments along with evidence of superparamagnetic clusters. These findings are consistent with the interface origin for the ferromagnetic response in these superlattices. The demonstration of an interface magnetic phase in nearly lattice-matched monoclinic Fe2O3/Ga2O3 opens the door to ultrawide bandgap heterostructure-engineered magnetoelectronic devices, where ferromagnetic switching of the interface phase can be incorporated into high-field devices.

Large magnetodielectric effect based on spin-dependent charge transfer in metal–insulator type Co-(BaF2) nanogranular films

Cobalt-metal-fluoride nanogranular films exhibit high electrical resistivity (ρ) and high permittivity (ɛ′) as well as high magnetic moments, making them potentially suitable for applications in low power magnetoelectronic devices at a high-frequency range. Here, we present the DC/AC magnetoelectric effect of high-ρ Cox-(BaF2)1 − x nanogranular films that are constituted of Co nanogranules of about 2 nm in diameter in an insulating BaF2 crystal. Under the application of magnetic field (H), we observed the ρ variations (Δρ/ρ0) of 2.0%–5.9% for x ranging 0.19–0.54 and the ɛ′ variations (Δɛ/ε0′) of 0.7%–6.0% for x ranging 0.19–0.42, respectively. These magnetically controllable ρ and ɛ′ are realized based on the spin-dependent quantum-mechanical charge transfer. The Cox-(BaF2)1 − x film for x = 0.42 possesses high ρ, while remaining in a notably high Δɛ/ε0′ of 6% at a megahertz frequency range. This study demonstrates a candidate of high-resistive magnetoelectric nanogranular films, which may bring prosperous applications in high-frequency magnetoelectric devices with low power consumption.

Magnetically controllable and flexible phototransistor for artificial intelligent skin with additional perception

With rational design, emerging artificial intelligent systems enable to possess capabilities far beyond human beings. Here, we report a novel magnetically controllable and flexible phototransistor using a tandem structure composed of an organic solar cell (OSC) of ITO/ZnO/P3HT:PC61BM/MoO3/Ag and a magnetic micropyramid array film of FeNi/PDMS composite covered with silver nanowires (AgNWs) (FeNi/PDMS/AgNWs) separated by an air gap. The FeNi/PDMS/AgNWs micropyramid structure takes the advantage of the Ag electrode of the bottom OSC forming a magnetoelectronic device. This novel seamless integrated device can interact with external stimuli, including pressure, light and magnetic field, forming an intriguing type of transistor. The output current of the developed transistor can be modulated by both optical excitation and magnetic field simultaneously, making it possess new functionalities. Additionally, the phototransistor carries several outstanding features, including fast photo/magnetic-response time, non-contact magnetic/optical control, and mechanical flexibility. These promising properties diversify the applications of the proposed phototransistor for the implementation in wearable electronics, touchless technology, and optical communication.

Spin caloritronics in a CrBr3-based magnetic van der Waals heterostructure

The recently reported magnetic ordering in insulating two-dimensional (2D) materials, such as chromium triiodide (CrI$_3$) and chromium tribromide (CrBr$_3$), opens new possibilities for the fabrication of magneto-electronic devices based on 2D systems. Inevitably, the magnetization and spin dynamics in 2D magnets are strongly linked to Joule heating. Therefore, understanding the coupling between spin, charge and heat, i.e. spin caloritronic effects, is crucial. However, spin caloritronics in 2D ferromagnets remains mostly unexplored, due to their instability in air. Here we develop a fabrication method that integrates spin-active contacts with 2D magnets through hBN encapsulation, allowing us to explore the spin caloritronic effects in these materials. The angular dependence of the thermal spin signal of the CrBr$_3$/Pt system is studied, for different conditions of magnetic field and heating current. We highlight the presence of a significant magnetic proximity effect from CrBr$_3$ on Pt revealed by an anomalous Nernst effect in Pt, and suggest the contribution of the spin Seebeck effect from CrBr$_3$. These results pave the way for future magnonic devices using air-sensitive 2D magnetic insulators.

Multiple Transitions in Permalloy Half-Ring Wires with Finite-Size Effect.

Six permalloy (Py) half-rings with finite-size from 120 nm to 360 nm were connected in series on five corners. The magnetization reversal processes were investigated by the measurement of anisotropic magnetoresistance (AMR). The number of switching jumps in the AMR loops, from zero to five, varied with the longitudinal applied field. These discrete jumps resulted from domain wall (DW) nucleating and depinning on the corners. The larger external field had a fewer number of jumps in the magnetoresistance (MR) curve. This reproducible and particular response of the domain wall device in the half-ring wires pattern might be one of the new promising magnetoelectronic devices.

Detecting current-induced quantum magnetization fluctuations with a spin-torque nano-oscillator

Interactions between conduction electrons and quantum fluctuations of ferromagnetic order have seldom been observed in magnetoelectronic devices. We show that current-induced quantum magnetization fluctuations can be detected using a spin-torque nano-oscillator by measuring its linewidth at different temperatures. The relative linewidth in a special dynamic region of the device can distinguish quantum magnetization fluctuations from their thermal counterparts, which is important in understanding magnetization dynamics beyond the mean-field level in magnetoelectronic devices.

Spin-polarized exciton formation in Co-doped GaN nanowires

Diluted magnetic semiconductors often show a spin-dependent interaction and induced ferromagnetism with high Curie temperature which is a desirable feature to be utilized for magneto-electronic devices. However, the origin of ferromagnetism and optical emission dynamics has not yet been well-understood due to their complicated microstructural and compositional characters. Herein, un-doped and Co-doped GaN nanowires have been synthesized by chemical vapor deposition and the effect of Co dopant on structural, magnetic and optical properties has been investigated. Electron-phonon couple caused by Co dopant, plays a dominant role for enhancement and shift of LO- phonon mode toward higher wavenumber. The incorporation of Co (II) ion in GaN lattice is attributed to the presence of room temperature ferromagnetism which influences their optical properties through exciton-spin interactions. It happens when the Co atom prefers to substitute on Ga sites and induces coupling with neighboring Co atoms to align ferromagnetically, leading to the formation of independent exciton magnetic polarons, as a new elementary excitation that is one of the origins of the bosonic lasing. These findings can pave new way in the future solid-state optoelectronic quantum information technology.

Facile magnetoresistance adjustment of graphene foam for magnetic sensor applications through microstructure tailoring

Abstract Graphene foam is becoming a material of choice for magnetoelectronic devices due to its large, linear and unsaturated room temperature magnetoresistance. However, the magnetoresistance of graphene foam is not as large as that of monolayer graphene. Herein, we describe how magnetoresistance ~ 100% was detected at room temperature under a magnetic field of 5 T that is comparable to the magnetoresistance in monolayer graphene; the highest magnetoresistance of ~158% was detected at 5 K under a magnetic field of 5 T. Unlike monolayer graphene, graphene foam is far more comfortable with producing in gram scale and utilizing in magnetoelectronic devices.

Understanding the role of electrons in the magnetism of a colossal permittivity dielectric material

Evidence for magnetic order at room temperature in a colossal permittivity dielectric indicates a potential for the development of magneto-electronic devices.

Unusual magnetotransport properties in graphene fibers.

Graphene, purely sp2-hybridized, has already been extensively studied for magnetoelectronics, however, the magnetotransport properties of graphene fibers (GrFib) have not been explored very well to date. Herein, unique magnetotransport properties of graphene fibers are detected. All the GrFib-samples show the highest positive magnetoresistance (MR ∼ 60%) at room temperature (300 K) that gradually decreases (MR ∼ 37%) at low temperature (5 K), indicating quite different behavior for a graphene derivative. The MR of three different morphologies are compared: single graphene sheet (60-100% at 300 K and 100-110% at 5 K under an applied magnetic field of 5 T), graphene foam (GF-100% at 300 K and 158% at 5 K under an applied magnetic field of 5 T), and graphene fiber (60% at 300 K and 37% at 300 K under an applied magnetic field of 5 T), and found that each morphology has a different magnitude of MR under similar magnitude of magnetic field and temperature. Unlike graphene and GF, GrFib shows a decreasing trend of MR at low temperatures, violating commonly used weak anti-localization phenomena in graphene. Technologically, each morphology of graphene has a unique set of magnetotransport properties that can be considered for particular magnetoelectronic devices depending upon the mechanical, electrical, and magnetotransport properties.

Voltage-controlled ON switching and manipulation of magnetization via the redox transformation of β-FeOOH nanoplatelets

Redox-based metal/metal oxide transformations achieved via electrolytic gating recently emerged as a novel, magneto-ionic route for voltage control of magnetism. So far, mainly metal or oxide thin films and nanoporous metal alloy structures are used as starting materials. The present study demonstrates a magneto-ionic transformation starting from a stable electrodeposited FeOOH nanoplatelet structure. The application of a low voltage in a Li-based electrolyte results in the reduction of the virtually non-magnetic FeOOH into ferromagnetic Fe, yielding an ON switching of magnetization. The magnetization can be tuned in a large range by the time of voltage application and remains stable after voltage-switch off. A reversible magneto-ionic change of magnetization of up to 15% is achieved in the resulting iron films with a thickness of about 30 nm. This large magneto-ionic effect is attributed to the enhanced roughness of the iron films obtained from the nanoplatelet structure. The robust, voltage-controlled, and non-volatile ON switching of magnetism starting from a stable oxide structure is promising for the development of energy-efficient magnetic switches, magnetic actuation and may offer new avenues in magnetoelectronic devices.

Topological Magnetic-Spin Textures in Two-Dimensional van der Waals Cr2Ge2Te6.

Two-dimensional (2D) van der Waals (vdW) materials show a range of profound physical properties that can be tailored through their incorporation in heterostructures and manipulated with external forces. The recent discovery of long-range ferromagnetic order down to atomic layers provides an additional degree of freedom in engineering 2D materials and their heterostructure devices for spintronics, valleytronics, and magnetic tunnel junction switches. Here, using direct imaging by cryo-Lorentz transmission electron microscopy we show that topologically nontrivial magnetic-spin states, skyrmionic bubbles, can be realized in exfoliated insulating 2D vdW Cr2Ge2Te6. Due to the competition between dipolar interactions and uniaxial magnetic anisotropy, hexagonally packed nanoscale bubble lattices emerge by field cooling with magnetic field applied along the out-of-plane direction. Despite a range of topological spin textures in stripe domains arising due to pair formation and annihilation of Bloch lines, bubble lattices with single chirality are prevalent. Our observation of topologically nontrivial homochiral skyrmionic bubbles in exfoliated vdW materials provides a new avenue for novel quantum states in atomically thin insulators for magneto-electronic and quantum devices.