Curie Temperature Increase Research Articles

Two-dimensional Cr2Cl3S3 Janus magnetic semiconductor with large magnetic exchange interaction and high-TC

Abstract The two-dimensional (2D) Janus monolayers are promising in spintronic device application due to their enhanced magnetic couplings and Curie temperatures. Van der Waals CrCl3 monolayer has been experimentally proved to have an in-plane magnetic easy axis and a low Curie temperature of 17 K, which will limit its application in spintronic devices. In this work, we propose a new Janus monolayer Cr2Cl3S3 based on the first principles calculations. The phonon dispersion and elastic constants confirm that Janus monolayer Cr2Cl3S3 is dynamically and mechanically stable. Our Monte Carlo simulation results based on magnetic exchange constants reveal that Janus monolayer Cr2Cl3S3 is an intrinsic ferromagnetic semiconductor with T C of 180 K, which is much higher than that of CrCl3 due to the enhanced ferromagnetic coupling caused by S substitution. Moreover, the magnetic easy axis of Janus Cr2Cl3S3 can be tuned to the perpendicular direction with a large magnetic anisotropy energy (MAE) of 142 μeV/Cr. Furthermore, the effect of biaxial strain on the magnetic property of Janus monolayer Cr2Cl3S3 is evaluated. It is found that the Curie temperature is more robust under tensile strain. This work indicates that the Janus monolayer Cr2Cl3S3 presents increased Curie temperature and out-of-plane magnetic easy axis, suggesting greater application potential in 2D spintronic devices.

Τhe effect of Ni on the magnetic properties of power or high permeability MnZn ferrites

In this article an extensive study is presented on the Ni addition in three subcategories of MnZn ferrites under conditions of constant Mn/Zn ratio and Fe excess, namely i) low frequency (∼100 kHz) power ferrites, ii) medium frequency (∼500 kHz) power ferrites and iii) high permeability (μi ∼ 8000, 25 °C) ferrites. As found, Ni is a useful additive for optimizing the high temperature performance of MnZn ferrites since it increases Curie temperature, saturation flux density, magnetic permeability and decreases power losses at temperatures above 100 °C. In addition, the addition of Ni increases the resonance frequency and the frequency stability of the initial permeability and partly counteracts the ageing effects caused by Co, when it is present in medium frequency ferrites. The magnetic results are explained on the basis of Ni-substitutions on octahedral sites associated with a negative contribution to the total magnetocrystalline anisotropy, thereby shifting the temperature at which the magnetocrystalline anisotropy constant K1 changes sign, to higher temperatures. The morphological results are explained assuming that the presence of Ni is associated with decreased cation vacancy and increased anion vacancy concentrations.

High‐Cerium‐Content Fe–Ce–Nd–B Sintered Magnets with High Coercivity

Ce substitution of Nd in FeNdB sintered magnets is very interesting for reasons of resource efficiency, sustainability and costs. However, the magnetic properties of high Ce‐content magnets are poor. This is mainly due to the lower values of intrinsic magnetic properties of Fe14Ce2B compared to Fe14Nd2B as well as the Laves phase Fe2Ce, which is formed in the grain boundaries and forms flat aggregates for higher Ce‐content. In this article, sintered magnets with 75 at% degree of substitution of the composition Fe70.9–[(Ce1−xLax)0.75Nd0.25]18.8–B5.8–M4.5 (M = Co, Ti, Al, Ga, Cu, 0 ≤ x ≤ 0.3) are produced and analyzed. It is shown that a new rare earth rich grain boundary phase is formed adding La and that the Laves phase fraction can be reduced by up to 85%. With increasing La content, the remanence and Curie temperature increase and the temperature coefficient of Hc improves. A coercivity, remanence, and maximum energy density of up to μ0Hc = 0.74 T, Jr = 0.98 T, and (BH)max = 139.9 kJ m−3 have been achieved.

Near room-temperature ferromagnetism in BixCr1−xTe2 epitaxial thin films grown on GaAs(111)B by molecular beam epitaxy

Two-dimensional (2D) ferromagnetic materials (FMs) are potentially the material foundation for future spintronics devices. However, at present, the Curie temperature (TC) of most 2D FM is relatively low and cannot meet the need for practical applications. Nowadays, CrTe2 thin films grown by molecular beam epitaxy (MBE) are reported to be room-temperature ferromagnetic only on graphene substrate instead of 3D substrates. In this work, we report high-quality Bi-doped CrTe2 (BixCr1−xTe2) thin films grown on conventional substrates of GaAs(111)B by MBE. Magnetotransport measurements reveal strong ferromagnetism of all the films, with out-of-plane magnetic anisotropy. More importantly, as more Bi atoms are doped into the film, the Curie temperature increases and reaches 305 K at x = 0.1. This improvement is a step forward for its application in spintronics and other fields.

Tuning Ferroelectric Properties of Barium Titanate by Lateral Strain: A Molecular Dynamics Simulation Study

BaTiO3 (BT), a lead‐free ferroelectric material, is employed in energy‐conversion devices, including actuators and capacitors. However, its ferroelectric properties, such as the piezoelectric coefficient and Curie temperature, fall short of those of harmful lead‐based ferroelectrics, such as Pb(Zrx, Ti1−x)O3. In this study, a BT crystal is examined under biaxial strain (i.e., strain in the a‐ and b‐directions of the unit cell) to investigate the alterations in its ferroelectric properties. The piezoelectric coefficient, Curie temperature, polarization magnitude, and coercive electric field are calculated under varying biaxial strains using molecular dynamics simulations. The simulations utilize a core–shell‐type interatomic potential developed from first‐principles electronic structure calculations. In the findings, it is revealed that 1) the Curie temperature, polarization magnitude, and coercive electric field increase, whereas the piezoelectric coefficient in the c‐direction decreases under biaxial compression; 2) modifications in the ferroelectric properties depend on the area of ab‐plane and not on its aspect ratio. Comprehensive analyses of the Ti‐displacement distribution offer microscopic insights into strain‐induced shifts in ferroelectric properties.

Understanding Crystal Spatial Symmetry of Sm/Mg Heterovalent‐Doped BST–BNT Ceramics: An Effect Mechanism for Electrical Properties

The effect mechanism of lattice distortion caused by heterovalent rare‐earth and alkaline‐earth ions co‐doping in the electrical properties of piezo‐ceramics is generally concerning. Herein, the Sm2O3/MgO co‐doped 0.65(Ba0.96Sr0.04TiO3)–0.35(Bi0.50Na0.50TiO3) (BST–BNT: x(Sm, Mg)) (0 ≤ x ≤ 0.25 wt%) ceramics are prepared by Pechini sol–gel method. The reversibility transition from the tetragonal P4/mmm relaxation paraelectric phase to the tetragonal P4mm relaxation ferroelectric phase confirms the doping‐modulated piezoelectric constants’ response to the spatial symmetry. Excessive Sm3+ and Mg2+ in the interstitial lattice inhibit the Jahn–Teller effect of distortion in the crystal spatial symmetry. The restored P4/mmm spatial symmetry of BST–BNT: x(Sm, Mg) (x = 0.15 wt%) ceramics contributes to the large electric‐induced strain, increased Curie temperature, and the stabilized frequency dependence of the Curie temperature (TC [1 MHz] = 167 °C, εr [1 MHz] = 2151.92, d33 = 43 pC N−1, Smax/Emax = 535.22 pm V−1). The samples with large electric‐induced strain and good temperature stability of electrical properties can be applied in the heat dissipation actuators.

Effects of heat treatment on the structural properties and magnetic hyperfine parameters of Fe72Cr8Si8B12 amorphous ribbons

Fe-based amorphous alloys are extensively used in power electronics and other industries due to their high magnetic permeability, low coercivity and exceptional soft magnetic properties, which are closely related to their microstructure. In this study, the microstructure and hyperfine parameters of Fe72Cr8Si8B12 amorphous ribbons annealed at different temperatures were investigated using X-ray diffraction, scanning electron microscopy, Mössbauer spectrometry, and vibrating sample magnetometer. The results show that the crystallization process is due to the formation of α-Fe(Si), Fe2B and Fe23B6 phases. The measured 57Fe Mössbauer spectra were calculated by the superposition of several crystalline magnetic components for each annealing temperature (800, 825, 850 and 875 K), and the calculated values of the hyperfine fields for each component reflect a crystallization of the sample. Furthermore, the average nanograin size as well as the Curie temperature (TC) increase as annealing temperature increases. The simulation of the M(T) curve agrees well with the experimental data.

Gadolinium manganites substituted with barium Gd1−XBaXMnO3 (x = 0.0; 0.15; 0.18; 0.25): Structural transitions, microstructure, magnetic and electrical properties

The effect of gadolinium substitution by barium on the crystal structure, microstructure, magnetic and electrical properties of Gd1−XBaXMnO3 (x = 0.0; 0.15; 0.18; 0.25) prepared by the solid-state reaction method has been studied. An increase in the concentration of barium to x = 0.18 leads to a critical decrease in the concentration of Mn3+ ions involved in the distortion mechanism and changes in the structural order (Jahn-Teller transition). The porosity of the samples increases and average the grain size and decreases as the concentration of barium increases. The electrical conductivity of Gd1−xBaxMnO3 increases up to 1.5 times compared to the initial material when the concentration of barium increases to x = 0.25. All manganites have TN = 42 K. The Curie temperature increases and changes the sign from negative to positive as the substitution level increases. For the first time, a phase diagram of the evolution of structural, magnetic and electrical properties when replacing Gd with Ba is presented. The evolution of magnetic and electrical properties of manganites is associated with structural distortions of the crystal lattice caused by the substitution of gadolinium with barium.

Enhancement of coercivity and thermal stability of Pr85Al15 doped NdFeB sintered magnets followed with grain boundary diffusion

Magnetic-property modification and thermal stability improvement for high-performance NdFeB magnets by dual-alloy (DA) sintering with Pr85Al15 (PrAl) together with grain boundary diffusion (GBD) with Tb55Pr20Cu25 (TbPrCu) is demonstrated. The coercivity is increased from 14.9 to 16.4–18.0 kOe for the magnet DA sintering with 2–4 % PrAl. Pr and Al at grain boundary strengthens magnetic decoupling effect. In addition, a thin layer of (Nd, Pr)2Fe14B with higher magnetocrystalline anisotropy field (HA) is formed at grain surface. Both effects contribute to the coercivity enhancement for the magnets prepared by DA sintering with PrAl. Furthermore, remarkable coercivity enhancement, from 14.9 to 18.0 kOe to 22.7–27.1 kOe, with improved thermal stability is attained for the above magnets after GBD with Tb55Pr20Cu25 alloy. Besides, the increased Curie temperature of 2:14:1 phase indicates the entrance of Tb into 2:14:1 phase and helps to improve thermal stability. Most importantly, various magnetic levels of high-performance NdFeB magnets, including 48–52 M, 45H, 50SH and 40-45UH, can be easily attained by modifying the amount of PrAl for DA sintered magnets and/or GBD with TbPrCu alloys.

Calculated magnetic exchange interactions in the van der Waals layered magnet CrSBr

Intrinsic van der Waals materials layered magnets have attracted much attention, especially the air-stable semiconductor CrSBr. Herein, we carry out a comprehensive investigation of both bulk and monolayer CrSBr using the first-principles linear-response method. Through the calculation of the magnetic exchange interactions, it is confirmed that the ground state of bulk CrSBr is A-type antiferromagnetic, while there are five sizable large intralayer exchange interactions with small magnetic frustration, which results in a relatively high magnetic transition temperature of both bulk and monolayer CrSBr. Moreover, the significant electron doping effect and strain effect are demonstrated, with further increased Curie temperature for monolayer CrSBr, as well as an antiferromagnetic to ferromagnetic phase transition for bulk CrSBr. We also calculate the magnon spectra using linear spin-wave theory. These features of CrSBr can be helpful to clarify the microscopic magnetic mechanism and promote the application in spintronics.

Increased Curie temperature and magnetoresistive response by modifying Fe/Mo ratio in Sr[formula omitted]FeMoO[formula omitted] thin films

We investigated the effect of deposition distance on a set of otherwise identically grown Sr2FeMoO6 (SFMO) thin films grown by pulsed laser deposition. Based on the detailed magnetic and transport measurements, we found that the optimal properties can be realized at longer deposition distances than earlier expected. The achieved onset Curie temperature of the order of 400K with the middle transition value of 372K, which are clearly the highest presented in the literature for the SFMO thin films. In addition, the increased metallicity and magnetoresistive response is observed in films deposited at longer distances. The improvements are widely discussed in the light of discovered stoichiometric imbalance between the cations Fe and Mo, which modify the magnetic interactions and thus magnetic and electric properties. Therefore, this study shows a new approach in the deposition process to provide the SFMO thin films and multilayers of high quality for future spin valve devices working at room temperature.

On Curie temperature of B20-MnSi films

B20-type MnSi is the prototype magnetic skyrmion material. Thin films of MnSi show a higher Curie temperature than their bulk counterpart. However, it is not yet clear what mechanism leads to the increase of the Curie temperature. In this work, we grow MnSi films on Si(100) and Si(111) substrates with a broad variation in their structures. By controlling the Mn thickness and annealing parameters, the pure MnSi phase of polycrystalline and textured nature as well as the mixed phase of MnSi and MnSi1.7 are obtained. Surprisingly, all these MnSi films show an increased Curie temperature of up to around 43 K. The Curie temperature is likely independent of the structural parameters within our accessibility including the film thickness above a threshold, strain, cell volume and the mixture with MnSi1.7. However, a pronounced phonon softening is observed for all samples, which can tentatively be attributed to slight Mn excess from stoichiometry, leading to the increased Curie temperature.

Unexpected wide tuning of ferroelectric properties by varying the Er concentration in La2-xErx(MoO4)3 (x = 0.75, 1, 1.25) solid solutions

Powder samples of the series La2-xErx(MoO4)3 (x ​= ​0.75, 1, 1.25) were prepared by solid-state reaction from the La and Er trimolybdates. The correlation of the ferroelectric properties with the crystal structure, as a function of the x concentration, was studied by X-ray diffraction (XRD), dielectric and optical spectroscopy, and the ferroelectric hysteresis loop measurements. The β′-Gd2(MoO4)3 structure type was identified by a non-conventional Rietveld method, including a symmetry analysis. Contrary to the expectations, the ferroelectric properties were intensified by the Er3+ concentration increase: the maximum polarization for x ​= ​1.25 reaches one order of magnitude larger than that of x ​= ​0.75, and the Curie temperature increase was around 200 ​K. This unexpected behavior is opposite of that observed for the improper ferroelectrics RE2(MoO4)3, where the ferroelectric properties are enhanced with the RE ionic radius increase. Hence, the series La2-xErx(MoO4)3 show an “wider tuning range” for ferroelectric properties opposite to the rare earth trimolybdate family.

Effect of pulsed laser annealing on the properties of (Ga,Mn)As layers

• Ferromagnetic (Ga,Mn)As layers were obtained by pulsed laser deposition and annealing. • Results of X-ray diffraction, TEM and the magnetization study are presented. • Decomposition of MnAs inclusions is observed after laser annealing of thin (Ga,Mn)As layer. • The Curie temperature increased to 105–110 K in the annealed thin (Ga,Mn)As layer. We have investigated the layers of the (Ga,Mn)As diluted magnetic semiconductor with the thickness in the range of 150 – 400 nm, which have been formed by the combination of the methods of a pulsed laser deposition and a post-growth pulsed laser annealing. It has been found that the laser annealing has a significant effect on the structural, magnetic, and electrical properties of the thinnest (∼150 nm) (Ga,Mn)As layer. For this layer the additional activation of the Mn impurity, the Curie temperature increase and the dissolution of the second phase MnAs clusters have been observed. The results of model calculations of heat spread in (Ga,Mn)As layer have shown that in thicker layers a MnAs melting point temperature is not achievable (for the chosen conditions of pulsed laser annealing) in the entire layer.

Investigation of Phase Transitions in Ferromagnetic Nanofilms on a Non-Magnetic Substrate by Computer Simulation.

Magnetic properties of ferromagnetic nanofilms on non-magnetic substrate are examined by computer simulation. The substrate influence is modeled using the two-dimensional Frenkel-Kontorova potential. The film has a cubic crystal lattice. Cases of different ratio for substrate period and ferromagnetic film period are considered. The difference in film and substrate periods results in film deformations. These deformations result in a change in the magnetic properties of the film. The Ising model and the Metropolis algorithm are used for the study of magnetic properties. The dependence of Curie temperature on film thickness and substrate potential parameters is calculated. Cases of different values for the coverage factor are considered. The deformation of the film layers is reduced away from the substrate when it is compressed or stretched. The Curie temperature increases when the substrate is compressed and decreases when the substrate is stretched. This pattern is performed for films with different thicknesses. If the coating coefficient for the film is different from one, periodic structures with an increased or reduced concentration of atoms are formed in the film first layer. These structures are absent in higher layers.

Microstructure and Magnetocaloric Effect by Doping C in La-Fe-Si Ribbons.

The melt-spun ribbons of LaFe11.5Si1.5Cx (x = 0, 0.1, 0.2, 0.3) compounds are prepared by the melt fast-quenching method. The doping of C is beneficial to the nucleation and precipitation of the La (Fe, Si)13 phase, which is indicated by the microstructure observation and the elemental analysis. Subsequently, the ribbons of LaFe11.5Si1.5C0.2 are annealed at different times, and the phase composition, the microstructures, and the magnetic properties are investigated. The LaFe11.5Si1.5C0.2 ribbons annealed at 1273 K for 2 h achieved the best magnetic properties, and the maximum isothermal magnetic entropy change with a value of 9.45 J/(kg·K) upon an applied field of 1.5 T at an increased Curie temperature 255 K.

Influence of substitutions and hydrogenation on the structural and magnetic properties of (R’R’’)2Fe17 (R’, R’’ = Sm, Er, Ho): Compositions with promising fundamental characteristics

The structural and magnetic properties of the (R,R’)2Fe17-type (R and R’ are heavy (Ho, Er) and light (Sm) rare earth metals, respectively) compounds and the (R,R’)2Fe17Hy hydrides are reported. The hydrides with a high hydrogen concentration (y ≥ 4) are obtained by direct hydrogen absorption by the intermetallics. The rhombohedral Th2Zn17-type of structure is inherent in the Sm containing parent compounds and hydrides. The unit cell increase and increased Curie temperature are characteristic of the hydrides. Magnetic properties of the samples are investigated in pulsed magnetic fields up to 60 T. Strong magnetic fields induce phase transitions in the compounds with a high content of heavy rare earth elements. The parameter of the intersublattice exchange interaction is estimated.

Crystallographic, electronic and magnetic properties of Sr2FeW1-xMoxO6 double perovskite oxides

The structural, electronic and magnetic properties of Sr2FeW1-xMoxO6 with 0 ≤ x ≤ 0.3 are studied. Powder samples have been elaborated using the conventional solid - state reaction at 1300 °C. At room temperature our systems are single phase and crystallized in the tetragonal structure with the P4 space group. The Sr2FeWO6 possesses the metallicity ground state and antiferromagnetic ordering. Sr2FeW0.5Mo0.5O6 possesses the half metallicity ground state and ferromagnetic ordering. The difference energy ΔE between the magnetic configurations EFM and EAFM was calculated for x = 0, x = 0.25 and x = 0.5. The Mo doping in W atom leads to the half metallicity and ferromagnetic behavior. This result is confirmed by experimental. Magnetization measurements and calculations exhibit a paramagnetic-ferromagnetic transition. Sr2FeWO6 is antiferromagnetic at low temperature. The Curie temperature and spontaneous magnetization increase with increasing the value of x. The experiments results obtained are comparable with those obtained by simulations.

Intrinsic Magnetic Properties of a Highly Anisotropic Rare‐Earth‐Free Fe2P‐Based Magnet

AbstractPermanent magnets are applied in many large‐scale and emerging applications and are crucial components in numerous established and newly evolving technologies. Rare‐earth magnets exhibit excellent hard magnetic properties; however, their applications are limited by the price and supply risk of the strategic rare‐earth elements. Therefore, there is an increasing demand for inexpensive magnets without strategic elements. Here, the authors report the intrinsic highly‐anisotropic magnetic properties of Co and Si co‐doped single crystals (Fe1−yCoy)2P1−xSix (y ≈ 0.09). Co increases Curie temperature TC; Si doping decreases magnetocrystalline anisotropy K1 and also increases TC significantly because of the enhanced interlayer interaction. The maximum room temperature magnetocrystalline anisotropy K1 = 1.09 MJ m−3 is achieved for x = 0.22, with saturation magnetization µ0Ms = 0.96 T and TC = 506 K. The theoretical maximum energy product is one of the largest for any magnet without a rare earth or Pt. Besides its promising intrinsic magnetic properties and absence of any strategic elements, other advantages are phase stability at high temperatures and excellent corrosion resistance, which make this material most promising for permanent magnetic development that will have a positive influence in industry and daily life.

Effect of Mn doping and point vacancy on stability and magnetism of ZnO

Understanding the source of magnetism in Mn doped ZnO (ZnO:Mn) is an active area of research, made difficult experimentally by the challenge of controlling point vacancies in ZnO:Mn. Therefore, the stability and magnetic properties of ZnO:Mn with point vacancies were studied by first-principles calculations based on density functional theory (DFT). Formation energy and magnetic moment calculation results show that Mn prefers to cluster in ZnO:Mn. This clustering can become uniformly distributed through modifications by Zn vacancies (V Zn ) or O vacancies (Vo). Magnetic analysis shows that Mn dopant in Zn site (ZnO:Mn Zn ) coupled with V Zn or interstitial Mn (Mn i ) coupled with V O makes ZnO having long-range ordered ferromagnetism (FM) and an increased Curie temperature ( Tc ) above room-temperature. The ZnO:Mn Zn with V Zn exhibits obvious half-metallic characteristics that are of essential benefit to the design and preparation of a new-type of spin-electron injection source dilute magnetic semiconductor. The magnetic sources of ZnO:Mn Zn with V Zn and Mn i -doped ZnO (ZnO:Mn i ) with V O arise from carrier-mediated magnetic interaction and the double exchange of Mn3d, O2p and Zn4s orbitals, consistent with the theories of average field approximation and the double exchange mechanism.