Showing papers on "Ferrimagnetism published in 2020"

Single-shot all-optical switching of magnetization in Tb/Co multilayer-based electrodes.

Abstract: Ever since the first observation of all-optical switching of magnetization in the ferrimagnetic alloy GdFeCo using femtosecond laser pulses, there has been significant interest in exploiting this process for data-recording applications. In particular, the ultrafast speed of the magnetic reversal can enable the writing speeds associated with magnetic memory devices to be potentially pushed towards THz frequencies. This work reports the development of perpendicular magnetic tunnel junctions incorporating a stack of Tb/Co nanolayers whose magnetization can be all-optically controlled via helicity-independent single-shot switching. Toggling of the magnetization of the Tb/Co electrode was achieved using either 60 femtosecond-long or 5 picosecond-long laser pulses, with incident fluences down to 3.5 mJ/cm2, for Co-rich compositions of the stack either in isolation or coupled to a CoFeB-electrode/MgO-barrier tunnel-junction stack. Successful switching of the CoFeB-[Tb/Co] electrodes was obtained even after annealing at 250 °C. After integration of the [Tb/Co]-based electrodes within perpendicular magnetic tunnel junctions yielded a maximum tunneling magnetoresistance signal of 41% and RxA value of 150 Ωμm2 with current-in-plane measurements and ratios between 28% and 38% in nanopatterned pillars. These results represent a breakthrough for the development of perpendicular magnetic tunnel junctions controllable using single laser pulses, and offer a technologically-viable path towards the realization of hybrid spintronic-photonic systems featuring THz switching speeds.

First-principles calculations to investigate magnetic and thermodynamic properties of new multifunctional full-Heusler alloy Co2TaGa

Abstract: The structural, elastic, electronic, magnetic and thermodynamic properties of Co2TaGa full-Heusler alloy are investigated using density functional theory-based full-potential linearized augmented plane waves method. Our results present Co2TaGa full-Heusler in CuHg2Ti-type structure FM phase that is mechanically and dynamically stable at pressure. The negative formation energy of Co2TaGa is −1.516 eV that can be synthesized experimentally. The electronic properties of 3d transition metal-based full-Heusler compound Co2TaGa are calculated within Perdew–Burke–Ernzerhof generalized gradient approximation. Co2TaGa is predicted to be half-metallic ferrimagnet with an indirect band gap and 100% spin polarization. The calculated total magnetic moment is 2 μB, which is mainly determined by Co partial moment, and total spin magnetic moment is in conformity with Slater–Pauling rule Mt that gives a simple function of valence electrons number, Zt, formulated as Mt = Zt − 18.

Ultrafast and energy-efficient spin–orbit torque switching in compensated ferrimagnets

Abstract: Spin–orbit torque can be used to manipulate magnetization in spintronic devices. However, conventional ferromagnetic spin–orbit torque systems have intrinsic limitations in terms of operation speed due to their inherent magnetization dynamics. Antiferromagnets and ferrimagnets with antiparallel exchange coupling exhibit faster spin dynamics and could potentially overcome these limitations. Here, we report ultrafast spin–orbit torque-induced magnetization switching in ferrimagnetic cobalt-gadolinium (CoGd) alloy devices. Using a stroboscopic pump–probe technique to perform time-resolved measurements, we show that the switching time in the ferrimagnets can be reduced to the subnanosecond regime and a domain wall velocity of 5.7 km s‒1 can be achieved, which is in agreement with analytical modelling and atomistic spin simulations. We also find that the switching energy efficiency in the ferrimagnets is one to two orders of magnitude higher than that of ferromagnets. Time-resolved measurements show that current-induced magnetization switching in ferrimagnetic devices is faster and more energy-efficient than in ferromagnet devices.

Controlled Growth and Thickness‐Dependent Conduction‐Type Transition of 2D Ferrimagnetic Cr 2 S 3 Semiconductors

Abstract: 2D magnetic materials have attracted intense attention as ideal platforms for constructing multifunctional electronic and spintronic devices. However, most of the reported 2D magnetic materials are mainly achieved by the mechanical exfoliation route. The direct synthesis of such materials is still rarely reported, especially toward thickness-controlled synthesis down to the 2D limit. Herein, the thickness-tunable synthesis of nanothick rhombohedral Cr2 S3 flakes (from ≈1.9 nm to tens of nanometers) on a chemically inert mica substrate via a facile chemical vapor deposition route is demonstrated. This is accomplished by an accurate control of the feeding rate of the Cr precursor and the growth temperature. Furthermore, it is revealed that the conduction behavior of the nanothick Cr2 S3 is variable with increasing thickness (from 2.6 to 4.8 nm and >7 nm) from p-type to ambipolar and then to n-type. Hereby, this work can shed light on the scalable synthesis, transport, and magnetic properties explorations of 2D magnetic materials.

Cation distribution, magnetic and hyperfine interaction studies of Ni–Zn spinel ferrites: role of Jahn Teller ion (Cu2+) substitution

Abstract: This study reports cation distribution, magnetic, and hyperfine interaction studies of Cu2+-substituted mixed Ni–Zn nano-spinel ferrites prepared by combustion technique. X-ray diffraction and electron microscopy were used to study the structural and morphological aspects of all the samples. Rietveld refined diffraction patterns exhibited a cubic-spinel lattice structure with the Fd3m space group for all the samples. Morphological investigations revealed the spherical morphology of particles with some agglomeration. The magnetic properties investigated at 300 K and 5 K implied a soft ferromagnetic character of all the samples. The magnetization at 5 K progressively enhanced due to surface effects. Field-cooled and zero-field-cooled measurements indicated net irreversibility for all the samples. Hyperfine interaction studies revealed the ferrimagnetic nature of Cu2+-substituted mixed Ni–Zn spinel nano-ferrites. All the obtained results show that the prepared nanoparticles are useful for magnetic fluid hyperthermia and other bio-applications.

Single pulse all-optical toggle switching of magnetization without gadolinium in the ferrimagnet Mn2RuxGa.

Abstract: Energy-efficient control of magnetization without the help of a magnetic field is a key goal of spintronics. Purely heat-induced single-pulse all-optical toggle switching has been demonstrated, but so far only in Gd-based amorphous ferrimagnet films. In this work, we demonstrate toggle switching in films of the half-metallic ferrimagnetic Heusler alloys Mn2RuxGa, which have two crystallographically-inequivalent Mn sublattices. Moreover, we observe the switching at room temperature in samples that are immune to external magnetic fields in excess of 1 T, provided they exhibit a compensation point above room temperature. Observation of the effect in compensated ferrimagnets without Gd challenges our understanding of all-optical switching. The dynamic behavior indicates that Mn2RuxGa switches in 2 ps or less. Our findings widen the basis for fast optical switching of magnetization and break new ground for engineered materials that can be used for nonvolatile ultrafast switches using ultrashort pulses of light. Femtosecond laser pulses allow for extremely fast switching of magnetization in ferromagnetic films, but all examples so far contained gadolinium. Here the authors demonstrate room temperature all-optical toggle switching in a ferrimagnetic manganese-based half-metal without gadolinium.

Analysis of Magnetic Properties of Nano-Particles Due to a Magnetic Dipole in Micropolar Fluid Flow over a Stretching Sheet

Abstract: This article explores the impact of a magnetic dipole on the heat transfer phenomena of different nano-particles Fe (ferromagnetic) and Fe3O4 (Ferrimagnetic) dispersed in a base fluid ( 60 % water + 40 % ethylene glycol) on micro-polar fluid flow over a stretching sheet. A magnetic dipole in the presence of the ferrities of nano-particles plays an important role in controlling the thermal and momentum boundary layers. The use of magnetic nano-particles is to control the flow and heat transfer process through an external magnetic field. The governing system of partial differential equations is transformed into a system of coupled nonlinear ordinary differential equations by using appropriate similarity variables, and the transformed equations are then solved numerically by using a variational finite element method. The impact of different physical parameters on the velocity, the temperature, the Nusselt number, and the skin friction coefficient is shown. The velocity profile decreases in the order Fe (ferromagnetic fluid) and Fe3O4 (ferrimagnetic fluid). Furthermore, it was observed that the Nusselt number is decreasing with the increasing values of boundary parameter ( δ ) , while there is controversy with respect to the increasing values of radiation parameter ( N ) . Additionally, it was observed that the ferromagnetic case gained maximum thermal conductivity, as compared to ferrimagnetic case. In the end, the convergence of the finite element solution was observed; the calculations were found by reducing the mesh size.

Observation of Magnetic Antiskyrmions in the Low Magnetization Ferrimagnet Mn2Rh0.95Ir0.05Sn.

Abstract: Recently, magnetic antiskyrmions were discovered in Mn1.4Pt0.9Pd0.1Sn, an inverse tetragonal Heusler compound that is nominally a ferrimagnet, but which can only be formed with substantial Mn vacan...

Magnetic and structural properties of single-phase Gd3+-substituted Co–Mg ferrite nanoparticles

Abstract: Nanocrystalline Gd3+-doped Co–Mg ferrite nanoparticles with the chemical formula Co0.7Mg0.3Fe(2−x)GdxO4 (x = 0.02) were prepared by coprecipitation for the first time. The properties of the nanoparticles were investigated by X-ray diffraction, confirming a single-phase, highly crystalline cubic spinel structure in the space group Fdm and an average crystallite size of 54 nm. The Fourier-transform infrared spectrum showed two fundamental absorption bands in the wavenumber range of 437–748 cm−1 attributed to the stretching vibration of tetrahedral and octahedral sites in the spinel structure. Scanning electron microscopy analysis showed that the nanoparticles are different in shape and slightly agglomerated. Energy-dispersive X-ray spectroscopy demonstrated the purity of the nano-ferrite powder. Magnetic measurements revealed ferrimagnetic behavior at room and low temperatures with high coercivity and a high saturation magnetization of 95.68 emu g−1, larger than that of pure bulk cobalt ferrite (80.8 emu g−1). Only ferrite cobalt synthesized sonochemically has been reported to have a higher saturation magnetization (92.5 emu g−1).

Comparative study on the physical properties of rare-earth-substituted nano-sized CoFe 2 O 4

Abstract: Nanotechnology manufacturing is rapidly developing and promises that the essential changes will have significant commercial and scientific impacts be applicable in an extensive range of areas. In this area, cobalt ferrite nanoparticles have been considered as one of the competitive candidates. The present study is based on the investigation of the effect of rare-earth (RE) incorporation on the physical properties of CoFe2O4. Rare-earth ions doped cobalt ferrites with composition CoRE0.025Fe1.975O4 where RE are Ce, Er and Sm have been synthesized by citrate auto combustion technique. Characterization is achieved using X-Ray diffraction (XRD) technique for structural analysis. The obtained data show that the samples exhibit a single-phase spinel structure. RE is successfully substituted into the spinel lattice without any distortion and it acts as inhibiting agent for grain growth. Room temperature M–H curves exhibit ferrimagnetism behavior with a decrease in saturation magnetization and coercivity indicating these materials can be applicable for magnetic data storage and magneto-recording devices. The electrical conductivity is studied as a function of frequency in the temperature range of 300–700 K. The conduction mechanism is attributed to the hopping mechanism. The Seebeck coefficient S is found to be positive for Ce indicating that Co/Ce ferrite behaves as a p-type semiconductor. While it is fluctuated between positive and negative for Er/Sm-doped samples throughout the studied temperature range. The cobalt doped with Er3+ and Sm3+ exhibits degenerated semiconductor trends at higher temperatures. Such data offer a new opportunity for optimizing and improving the performance of cobalt ferrite where the physical properties are decisive.

Polarizing an antiferromagnet by optical engineering of the crystal field

Abstract: Strain engineering is widely used to manipulate the electronic and magnetic properties of complex materials. An attractive route to control magnetism with strain is provided by the piezomagnetic effect, whereby the staggered spin structure of an antiferromagnet is decompensated by breaking the crystal field symmetry, which induces a ferrimagnetic polarization. Piezomagnetism is especially attractive because unlike magnetostriction it couples strain and magnetization at linear order, and allows for bi-directional control suitable for memory and spintronics applications. However, its use in functional devices has so far been hindered by the slow speed and large uniaxial strains required. Here, we show that the essential features of piezomagnetism can be reproduced with optical phonons alone, which can be driven by light to large amplitudes without changing the volume and hence beyond the elastic limits of the material. We exploit nonlinear, three-phonon mixing to induce the desired crystal field distortions in the antiferromagnet CoF$_2$. Through this effect, we generate a ferrimagnetic moment of 0.2 $\mu_B$ per unit cell, nearly three orders of magnitude larger than achieved with mechanical strain.

Half-metallicity in new Heusler alloys Mn2ScZ (Z = Si, Ge, Sn)

Abstract: Study of half-metallicity has been performed in a new series of Mn2ScZ (Z = Si, Ge and Sn) full Heusler alloys using density functional theory with the calculation and implementation of a Hubbard correction term (U). Volume optimization in magnetic and non-magnetic phases for both the Cu2MnAl and Hg2CuTi type structures was done to predict the stable ground state configuration. The stability was determined by calculating their formation energy as well as from elastic constants under ambient conditions. A half-metal is predicted for Mn2ScSi and Mn2ScGe with a narrow band gap in the minority spin whereas Mn2ScSn shows a metallic nature. The magnetic moments of Mn and Sc are coupled in opposite directions with different strengths indicating that the ferrimagnetic order and the total magnetic moment per formula unit for half-metals follows the Slater Pauling rule. And a strong effect was shown by the size of the Z element in the electronic and magnetic properties.

Stacking-dependent interlayer magnetic coupling in 2D CrI3/CrGeTe3 nanostructures for spintronics

Abstract: The recent emergence of two-dimensional (2D) materials with intrinsic long-range magnetic order opens the avenue of fundamental physics studies and the spintronics application; however, the mechanism of interlayer magnetic coupling and the feasible way to control magnetic states are yet to be fully investigated. In the present study, from first-principle calculations, we studied the interlayer magnetic coupling of 2D CrI3/CrGeTe3 heterostructures and revealed the stacking-dependent magnetic states. It is found that AB and AB1 stacking are prefer ferromagnetic interlayer coupling, while the other two stacked configurations are in the ferrimagnetic state. The underlying mechanism has contributed to the competition between nearest-neighbor (NN) and second-nearest-neighbor (SNN) Cr-Cr atoms interaction between layers. Meanwhile, it is also found that the electronic properties are stacking dependent, while the band edge states are separated to the different layers. The magnetic and electronic states can be effectively tuned by the external strain. Based on these findings, the magnetic domain devices are proposed in the twisted magnetic heterostructures with the domain size and interlayer coupling being controlled by the rotation angle. Our study thus provides an approach to achieve the controllable magnetic/electronic properties which is not only important for fundamental research but also useful for the practical applications in spintronics.

Magnetic properties of an Ising ladder-like graphene nanoribbon by using Monte Carlo method

Abstract: In this paper, we proposed a ferrimagnetic mixed-spin (1, 3/2) Ising model to describe the ladder-like graphene nanoribbon. Using the Monte Carlo simulation, the effects of the anisotropies, exchange couplings, longitudinal magnetic field and temperature on the magnetic and thermodynamic properties were investigated. Typical temperature dependences of thermodynamics quantities such as magnetization, susceptibility and internal energy were presented under the influence of various physical parameters. The phase diagrams of blocking temperature were obtained in the different parameter planes. In particular, we also found some interesting double hysteresis loops behavior for certain physical parameters. Our results are consistent with those obtained by some theoretical and experimental researches.

Magnetization and susceptibility behaviors in a bi-layer graphyne structure: A Monte Carlo study

Abstract: In this paper, the magnetic properties of a bi-layer graphyne structure have been studied using Monte Carlo simulations. The considered system is composed of two planes of graphyne with mixed spins: σ = 7/2 and S = 1. Firstly, the ground state phase diagrams for zero temperature are reported and discussed. Secondly, the magnetic properties for the studied system are examined for non-zero temperature. The effects of exchange coupling interaction and temperature on magnetization, susceptibility and transitional temperature. Furthermore, the effects of the crystal field on total magnetization of the system have been exhibited. Finally, the effect of the ferrimagnetic parameter, temperature and crystal field on the hysteresis cycles have been determined. The obtained results have been compared with theoretical and experimental researches.

Impact of Tm3+ and Tb3+ Rare Earth Cations Substitution on the Structure and Magnetic Parameters of Co-Ni Nanospinel Ferrite.

Abstract: Tm-Tb co-substituted Co-Ni nanospinel ferrites (NSFs) as (Co0.5Ni0.5) [TmxTbxFe2−2x]O4 (x = 0.00–0.05) NSFs were attained via the ultrasound irradiation technique. The phase identification and morphologies of the NSFs were explored using X-rays diffraction (XRD), selected area electron diffraction (SAED), and transmission and scanning electronic microscopes (TEM and SEM). The magnetization measurements against the applied magnetic field (M-H) were made at 300 and 10 K with a vibrating sample magnetometer (VSM). The various prepared nanoparticles revealed a ferrimagnetic character at both 300 and 10 K. The saturation magnetization (Ms), the remanence (Mr), and magneton number (nB) were found to decrease upon the Tb-Tm substitution effect. On the other hand, the coercivity (Hc) was found to diminish with increasing x up to 0.03 and then begins to increase with further rising Tb-Tm content. The Hc values are in the range of 346.7–441.7 Oe at 300 K to 4044.4–5378.7 Oe at 10 K. The variations in magnetic parameters were described based on redistribution of cations, crystallites and/or grains size, canting effects, surface spins effects, super-exchange interaction strength, etc. The observed magnetic results indicated that the synthesized (Co0.5Ni0.5)[TmxTbxFe2−x]O4 NSFs could be considered as promising candidates to be used for room temperature magnetic applications and magnetic recording media.

Magnetic and thermodynamic characteristics of a rectangle Ising nanoribbon

Abstract: Using Monte Carlo simulation, we investigated magnetic and thermodynamic characteristics of the rectangle core–shell ferrimagnetic mixed-spin (3/2, 2) Ising nanoribbon under the effects of sublattice crystal field, exchange coupling and temperature. Typical temperature dependence of the total magnetization, the core and shell magnetization, the susceptibility and the internal energy for several fixed values of Hamiltonian parameters were discussed in detail. The phase diagrams of the critical and compensation temperatures were presented in various Hamiltonian parameter planes. In addition, particular emphasis was given to the interesting multi-cycle hysteresis behaviors of the total system and sublattices. Our results show reasonable consistency compared with many previous theoretical and experimental studies.

Polarizing an antiferromagnet by optical engineering of the crystal field

Abstract: Strain engineering is widely used to manipulate the electronic and magnetic properties of complex materials. For example, the piezomagnetic effect provides an attractive route to control magnetism with strain. In this effect, the staggered spin structure of an antiferromagnet is decompensated by breaking the crystal field symmetry, which induces a ferrimagnetic polarization. Piezomagnetism is especially appealing because, unlike magnetostriction, it couples strain and magnetization at linear order, and allows for bi-directional control suitable for memory and spintronics applications. However, its use in functional devices has so far been hindered by the slow speed and large uniaxial strains required. Here we show that the essential features of piezomagnetism can be reproduced with optical phonons alone, which can be driven by light to large amplitudes without changing the volume and hence beyond the elastic limits of the material. We exploit nonlinear, three-phonon mixing to induce the desired crystal field distortions in the antiferromagnet CoF2. Through this effect, we generate a ferrimagnetic moment of 0.2 μB per unit cell, nearly three orders of magnitude larger than achieved with mechanical strain. This paper shows how lattice distortions induced by a laser pulse can create a ferrimagnetic moment in an antiferromagnet. This mechanism gives a magnetic response that is orders of magnitude larger than using mechanical strain.

Exploring the Structural, Dielectric and Magnetic Properties of 5 Mol% Bi 3+ -Substituted CoCr 2 O 4 Nanoparticles

Abstract: In the present work for the first time, we report in-depth structural, electrical, optical and magnetic properties of a family of cobalt chromate nanoparticles with 5 mol% Bi3+ substitution of the average crystallite size of 15 nm, fabricated by a solution combustion method using urea and glucose as a fuel. Co0.95Bi0.05Cr2O4 shows a single phase with spinel cubic structure with a space group of Fd3m with a lattice parameter of 8.334 A. The morphology of the family of Bi3+-doped CoCr2O4 shows a highly porous nature. Transmission electron microscopy (TEM) shows samples are in nano size, i.e. 22 nm with well crystalline nature. The energy gap was estimated by using UV spectrum and found in the range of 3.86 eV. Temperature-dependent dielectric constant (e′), dielectric loss (e″) and loss tangent (tan δ) are explained by using Maxwell–Wagner and Koop’s phenomenological theory. The evolution of magnetic behaviour was studied as a function of temperature and magnetic field to study the magnetic transitions such as paramagnetic to long-range collinear ferrimagnetism transitions, and it was found at 98 K and non-collinear ferrimagnetism at 26 K. M−H loop at 300 K nearly shows a paramagnetic phase at 98 K, and it clearly suggests that samples exhibit superparamagnetic nature.

Monolayer Ti2C MXene: manipulating magnetic properties and electronic structures by an electric field

Abstract: Two-dimensional (2D) layered Ti2C MXene has been synthesized experimentally, and the magnetism of monolayer Ti2C MXene has been predicted theoretically. In this study, based on first-principles calculations, five magnetic configurations of monolayer Ti2C were constructed to predict the magnetic ground state. We have found that the antiferromagnetic (AFM) state has the lowest energy. By applying an external electric field, monolayer Ti2C changes from an AFM semiconductor to a ferrimagnetic (FIM) semiconductor, half-metal, magnetic metal, non-magnetic (NM) metal, and NM semiconductor. When the electric field increases beyond a certain value, the magnetic moments of Ti atoms sharply decrease. With the increase in the electric field, the effective masses decrease significantly, carrier mobility increases and conductivity increases. The magnetic anisotropy energies were calculated and the results showed that the out-of-plane direction was the magnetic easy axis. Using the mean-field approximation method, the Neel temperature of monolayer Ti2C is estimated to be 50 K. By applying an electric field, the Neel temperature significantly decreases, which shows that the electric field can effectively reduce the high Neel temperature. Therefore, our research suggests that the magnetic and electronic properties of 2D materials can be manipulated by an external electric field, which provides a feasible direction for the tuning of nanomagnetic devices.