Low Curie Temperature Research Articles

Magnetic proximity effect in the two-dimensional ε-Fe2O3/NbSe2 heterojunction

Two-dimensional (2D) magnet/superconductor heterostructures can promote the design of artificial materials for exploring 2D physics and device applications by exotic proximity effects. However, plagued by the low Curie temperature and instability in air, it is hard to realize practical applications for the reported layered magnetic materials at present. In this paper, we developed a space-confined chemical vapor deposition method to synthesize ultrathin air-stable ε-Fe2O3 nanosheets with Curie temperature above 350 K. The ε-Fe2O3/NbSe2 heterojunction was constructed to study the magnetic proximity effect on the superconductivity of the NbSe2 multilayer. The electrical transport results show that the subtle proximity effect can modulate the interfacial spin–orbit interaction while undegrading the superconducting critical parameters. Our work paves the way to construct 2D heterojunctions with ultrathin nonlayered materials and layered van der Waals (vdW) materials for exploring new physical phenomena.

Enhanced ferromagnetism, magnetic anisotropy, and spin polarization in Janus CrSeTe via strain and doping

Recently, experimental 2D magnetic CrSe2 and CrTe2 inspired the study on their Janus CrSeTe, which breaks the mirror symmetry of Cr–Se(Te)–Cr. However, the CrSeTe monolayer possesses a lower Curie temperature and the in-plane magnetic anisotropy, and few works concerned both its zigzag- and stripy-antiferromagnetic states, which are crucial to determine the magnetic ground state and magnetic transition temperature. In this Letter, using first-principles calculations and Monte Carlo simulations, we systematically explore the strain, carrier-doping, and electric field effects on magnetic and electronic properties of monolayer CrSeTe. We demonstrate that monolayer CrSeTe preserves ferromagnetism in a large range of whether strain or doping. The estimated Curie temperature can be increased from 167 K to the room-temperature by the combination of 4% tensile strain and 0.1 e doping due to the enhanced ferromagnetic Cr–Se(Te)–Cr super-exchange interaction. Interestingly, either a small tensile strain (1%) or a low hole doping (0.025 h) can induce the transition of the easy axis from in-plane to out-of-plane due to the increased out-of-plane magnetic anisotropy from hybridized pz and py orbitals of Te atoms. In addition, the spin polarization is improved by over 60% by the tensile strain. Our results suggest that small strain and low carrier-doping induced room-temperature ferromagnetism, large out-of-plane magnetic anisotropy, and high spin polarization make CrSeTe useful in 2D spintronic applications and will stimulate broad studies on the regulation of magnetic and electronic properties of this class of magnetic Janus monolayers.

Controllable high Curie temperature through 5d transition metal atom doping in CrI3

Two-dimensional (2D) CrI3 is a ferromagnetic semiconductor with potential for applications in spintronics. However, its low Curie temperature (T c) hinders realistic applications of CrI3. Based on first-principles calculations, 5d transition metal (TM) atom doping of CrI3 (TM@CrI3) is a universally effective way to increase T c, which stems from the increased magnetic moment induced by doping with TM atoms. T c of W@CrI3 reaches 254 K, nearly six times higher than that of the host CrI3. When the doping concentration of W atoms is increased to above 5.9%, W@CrI3 shows room-temperature ferromagnetism. Intriguingly, the large magnetic anisotropy energy of W@CrI3 can stabilize the long-range ferromagnetic order. Moreover, TM@CrI3 has a strong ferromagnetic stability. All TM@CrI3 change from a semiconductor to a half-metal, except doping with Au atom. These results provide information relevant to potential applications of CrI3 monolayers in spintronics.

Atomic order, magnetic and transport phenomena in half-Heusler CoMnSb0.9Z0.1 alloys (Z = Si, Al, Sn, and Bi)

In this study, we investigate the effects of doping with ∼10 at% of Si, Al, Sn, and Bi on the atomic order, microstructure, and magnetic properties of CoMnSb samples prepared using the arc-melting method. X-ray diffraction, scanning electron microscopy, and magnetometry techniques were employed to derive the structural and magnetic parameters. The samples demonstrate a superstructure with an Fm-3m space group. They exhibit a saturation moment of 3.71–4.71 μB per formula unit at 5 K and a Curie temperature ranging from 474 to 507 K. Rietveld refinement performed on the XRD data revealed a possible occupation of magnetic atoms in the vacant 32f (x’, x’, x’) sites and atomic swapping, both of which may affect the magnitudes of the magnetic moment and Curie temperature. The electrical measurement results show that the grain boundaries and voids govern the conductivity of the samples. The spin polarization of the Si-doped sample was determined based on the grain-boundary magnetoresistance effect, which showed that this sample has a high degree of atomic order, consistent with the low magnetic moment, Curie temperature, and lattice constant.

Magnetothermal spider silk-based scaffolds for cartilage regeneration

Developing biocompatible, magnetically controlled polymers is a multifunctional solution to many surgical complications. By combining nanoparticle technology with the latest advancements in polymer materials science, we created a multicomponent hybrid system comprised of a robust native spider silk-based matrix; a Mn0.9Zn0.1Fe2O4 nanoparticles coating to provide a controlled thermal trigger for drug release; and liposomes, which act as drug carriers. Fluorescent microscope images show that the dye loaded into the liposomes is released when the system is exposed to an alternating magnetic field due to heating of ferromagnetic nanoparticles, which had a low Curie temperature (40–46°С). The silk matrix also demonstrated outstanding biocompatibility, creating a favorable environment for human postnatal fibroblast cell adhesion, and paving the way for their directed growth. This paper describes a complex approach to cartilage regeneration by developing a spider silk-based scaffold with anatomical mechanical properties for controlled drug delivery in a multifunctional autologous matrix-induced chondrogenesis.

Magnetocrystalline anisotropy of interstitially and substitutionally Sn-doped MnBi for high temperature permanent magnet applications

First-principles calculations were performed to calculate the electronic structures of low temperature phase (LTP) MnBi (Mn50Bi50) and substitutionally and interstitially Sn-doped MnBi [Mn50Bi25Sn25, (Mn0.5Bi0.5)66.7Sn33.3]. Brillouin function predicts the temperature dependence of saturation magnetization M(T). Sn substitution for Bi in MnBi (Mn50Bi25Sn25) changes the magnetocrystalline anisotropy constant (Ku) from −0.202 MJ/m3 (the in-plane magnetization) for LTP MnBi to 1.711 MJ/m3 (the out-of-plane magnetization). In comparison, the Ku remains negative but slightly decreases to −0.043 MJ/m3 when Sn is interstitially doped in MnBi [(Mn0.5Bi0.5)66.7Sn33.3]. The Curie temperature (TC) decreases from 716 K for LTP Mn50Bi50 to 445 K for Mn50Bi25Sn25 and 285 K for (Mn0.5Bi0.5)66.7Sn33.3. Mn50Bi25Sn25 has a lower magnetic moment of 5.034 μB/f.u. but a higher saturation magnetization of 64.2 emu/g than (Mn0.5Bi0.5)66.7Sn33.3 with a magnetic moment of 6.609 μB/f.u. and a saturation magnetization of 48.2 emu/g because the weight and volume of the substitutionally Sn-doped MnBi are smaller than the interstitially Sn-doped MnBi. The low Curie temperature and magnetization for Sn-doped MnBi are attributed to the high concentration of Sn. Thus, future study needs to focus on low Sn-concentrated MnBi.

Te doping effects on the ferromagnetic performance of the MnGe/Si quantum dots grown by ion beam sputtering deposition

Dilute magnetic MnGe quantum dots (QDs) have been a popular research topic due to their great potential application on spintronics and their natural compatibility with today's highly developed silicon-based complementary metal oxide semiconductor (CMOS) technology. However, the relatively low Curie temperature impedes their ability to maintain the ferromagnetic phase and creates difficulty for their practical application in spintronics at room temperature. Here, we report the first preparation of the Te-doped dilute-magnetic Mn0.064Ge0.895Te0.041/Si QDs by using the ion beam composite target sputtering technique. Growth temperature and deposition thickness dependent QDs density and magnetic properties were investigated systematically. The experimental results indicate that the QD density of Mn0.064Ge0.895Te0.041/Si sample increases first and then reduces with the deposited thickness, while the QDs' diameters increase monotonously from 10.18 ± 0.21–33.6 ± 0.34 nm. Curie temperature of 369 K observed in Mn0.064Ge0.895Te0.041 QDs is higher than that of 295 K observed in Mn0.061Ge0.939 counterparts grown at the same conditions. Correspondingly, the saturation and residual magnetization in the former are much higher than in the latter. The ferromagnetic enhancement can be attributed to that the nonpolar Mn0.064Ge0.936 QDs transform into Rashba polar Mn0.064Ge0.895Te0.041 QDs with the doping of Te ions, which obviously promotes the exchange coupling between holes and Mn atoms. Our work provides a practical approach for improving the ferromagnetism and Curie temperature of silicon-based diluted magnetic semiconductor QDs.

Nanocomposite Didymium-Fe-B/Alnico magnetic alloys with core shell structure and improved magnetic properties

NdFeB-based permanent magnets have the highest maximum energy products, but their low Curie temperatures limit their applications. Alnico permanent magnets have relatively modest magnetic energy products, but have very high Curie and working temperatures. In this manuscript, we report new NdPrFeB/Alnico nanocomposite magnetic materials with enhanced Curie temperatures and maximum magnetic energy products. A special microstructure with the Alnico phase dispersed within the NdPrFeB phase results in excellent magnetic properties. The Curie temperature of the composite alloys is enhanced from 570 K to 643 K by adding 25% low-grade Alnico. The enhanced Curie temperature which results from the cobalt and nickel replacement for iron in the 2:14:1 phase, and the molecular field penetration improves the temperature stability of the magnets. The beneficial heterogeneous distribution of the constituent phases and elements was confirmed by transmission electron microscopy and electron diffraction spectroscopy analyses. The maximum energy product is enhanced from 110 kJ/m3 to 133 kJ/m3. Magnetostatic interaction and exchange interaction in the alloys were studied by means of Henkel plots and minor hysteresis loop measurements. Despite the high content of the Alnico phase with relatively large special dimensions, there were no kinks in the demagnetization curves either at 300 K or 400 K. Furthermore, no kink was noted at low and high applied fields in the demagnetization-magnetization process which shows the effective magnetic interactions. This work would elucidate the development of nanocomposite magnetic material with high energy product without the use of heavy rare earths and with the economical use of Nd and Pr.

Interface-Tuning of Ferroelectricity and Quadruple-Well State in CuInP2S6 via Ferroelectric Oxide.

Ferroelectric van der Waals CuInP2S6 possesses intriguing quadruple-well states and negative piezoelectricity. Its technological implementation has been impeded by the relatively low Curie temperature (bulk TC ∼ 42 °C) and the lack of precise domain control. Here we show that CuInP2S6 can be immune to the finite size effect and exhibits enhanced ferroelectricity, piezoelectricity, and polar alignment in the ultrathin limit when it is interfaced with ferroelectric oxide PbZr0.2Ti0.8O3 films. Piezoresponse force microscopy studies reveal that the polar domains in thin CuInP2S6 fully conform to those of the underlying PbZr0.2Ti0.8O3, where the piezoelectric coefficient changes sign and increases sharply with reducing thickness. High temperature in situ domain imaging points to a significantly enhanced TC of >200 °C for 13 nm CuInP2S6 on PbZr0.2Ti0.8O3. Density functional theory modeling and Monte Carlo simulations show that the enhanced polar alignment and TC can be attributed to interface-mediated structure distortion in CuInP2S6. Our study provides an effective material strategy to engineer the polar properties of CuInP2S6 for flexible nanoelectronic, optoelectronic, and mechanical applications.

Large perpendicular magnetic anisotropy, high curie temperature, and half-metallicity in monolayer CrSI induced by substitution doping

The experimental discovery of two-dimensional (2D) intrinsic ferromagnets have sparked intense interest due to their potential applications in spintronics. However, the small magnetic anisotropy and low Curie temperature (TC) hinder their applications. Herein, through comprehensive first-principles calculations and Monte Carlo simulations, we investigate how Mn substitution doping affects the structure, electronic structure, magnetic properties, and TC of monolayer CrSI. Our calculations show that in contrast to ferromagnetic semiconductor with a TC of 175 K of monolayer CrSI, Mn doped monolayer CrMnS2I2 is a half-metallic ferromagnet with a TC of 312 K. Different from in-plane magnetic anisotropy of monolayer CrSI, the easy axis of monolayer CrMnS2I2 is out-of-plane, and the magnetic anisotropy of monolayer CrMnS2I2 can reach to 1.434 meV/atom, which is much larger than 0.001 meV/atom of monolayer CrSI. Additionally, the phonon dispersion, ab initio molecular dynamics simulations, formation energy, and elastic constants calculations indicate that monolayer CrMnS2I2 possesses excellent stability. Our findings provide a promising route to realize 2D intrinsic ferromagnetic half-metallic materials with a high TC and a large perpendicular magnetic anisotropy.

Electronic phase transition, perpendicular magnetic anisotropy and high Curie temperature in Janus FeClF

How to enhance the spin polarization, the Curie temperature and the perpendicular magnetic anisotropy (PMA) is crucial for the applications of 2D magnets in spintronic devices. In this work, based on the experimental FeCl2 flakes and the predicted in-plane magnetic anisotropy (IMA) and lower Curie temperature of FeCl2 monolayer, we use first-principles and Monte Carlo simulation to explore the strain and carrier-doping effects on the electronic and magnetic properties of Janus FeClF monolayer. The structure is stable within −10% to 2% biaxial strain. Janus FeClF monolayer can experience transitions from a half-semiconductor to a spin gapless semiconductor (SGS) around the −6% compressive strain, and from the IMA to the PMA at the −7% compressive strain. The super-exchange Fe–F/Cl–Fe interaction induces the ferromagnetic coupling, and the Curie temperature can be considerably enhanced from 56 K to 281 K at the −10% compressive strain. The half-metallicity can be achieved whether under electron doping or hole doping. The Fe-d orbitals and the spin–orbit coupling interaction between occupied and unoccupied intraorbital states are responsible for the electronic phase transition and the magnetic anisotropy, respectively. Remarkably, the compressive −10% strain and the 0.02 e doping collectively increase the Curie temperature to near room temperature (286 K). The high spin polarization (exhibiting SGS and half-metal), the PMA and the near-room-temperature ferromagnetism induced by strain and doping make Janus FeClF a promising candidate for 2D spintronic applications, which will stimulate experimental and theoretical broad studies on this class of Janus monolayers FeXY (X,Y = F, Cl, Br, and X ≠ Y).

Modulation of the Transient Magnetization of an EuO/Co Bilayer Tuned by Optical Excitation

AbstractThe magnetic proximity effect provides a promising way to increase the low Curie temperature (TC) of europium monoxide (EuO) toward or even above room temperature, while keeping its stoichiometry and insulating properties. This work studies EuO/Co bilayers using static and time‐resolved magneto‐optical Kerr effect measurements, and explores the influence of magnetic proximity on TC and on the spin dynamics in EuO. Excitation above the EuO bandgap results in an ultrafast enhancement of the EuO magnetization followed by a demagnetization within nanoseconds. This behaviour is also visible upon selectively photoexciting Co in the EuO/Co bilayer placed in an out‐of‐plane magnetic field, which is attributed to propagation of a superdiffusive spin current from Co into EuO. As the spin dynamics of Co shows a transient thermal demagnetization, the bilayer provides a system where the transient magneto‐optical signal can be tuned in amplitude and sign by varying external parameters such as the sample temperature or pump fluence. Moreover, in a strong excitation regime it is possible to measure the magnetic hysteresis of the underlying EuO, which is present up to room temperature – giving experimental evidence for the presence of a tuneable magnetic proximity coupling between Co and EuO.

Na3VAs2 monolayer: A two-dimensional intrinsic room-temperature ferromagnetic half-metal with large desired perpendicular magnetic anisotropy

Recent experiments have proved that the intrinsic two-dimensional (2D) magnetism has aroused strong interest in the application of advanced spintronics. However, low Curie temperature and low magnetic anisotropy energy (MAE) greatly limit their application range. By density functional theory calculation, we predict a stable 2D Na3VAs2 monolayer, which shows intrinsic ferromagnetic (FM) order and a large MAE (570 μeV per V atom). The Na3VAs2 monolayer has a local magnetic moment about 2 μB/V. Monte Carlo simulation shows that the Curie temperature (TC) of Na3VAs2 monolayer is up to 305 K based on anisotropic Heisenberg mode. More interestingly, the 2D Na3VAs2 monolayer shows an ideal half-metallic abundance, and it still maintains half-metallic characteristics under various strains. In addition, this monolayer has good dynamic, thermal, and mechanical stability. The excellent properties of the Na3VAs2 monolayer makes it great potential to be used in spintronic devices.

Electromagnetic Performance of NiMgCuZn Ferrite for Hyperthermia Application in Cancer Treatment.

Ferrite with high permeability, high magnetic loss, and a low Curie temperature is a highly promising material for hyperthermia applications in cancer treatment. In this work, Ni(0.29-x)MgxCu0.14Zn0.60Fe1.94O3.94 (NiMgCuZn) ferrites were prepared at different Mg ion concentrations. The dependence of the Mg ion contents on the crystal structure, magnetic, and permeability properties of NiMgCuZn ferrite was investigated. Structural characterization showed a high crystallinity of the materials and an increase in the lattice constant with the Mg ion content. The magnetic hysteresis loops revealed that the saturation magnetization decreased with an increasing Mg content. Furthermore, the permeability measurements showed an increase in permeability and magnetic loss with the doping of Mg. More importantly, the Curie temperature of the NiMgCuZn ferrite decreases with the Mg ion content and reaches 42.37 °C for x = 0.26 as well as a real permeability of ∼797.88@1 MHz and a magnetic loss of 0.316@1 MHz, which meet the requirements of tumor hyperthermia.

Revealing Site Occupancy in a Complex Oxide: Terbium Iron Garnet.

Complex oxide films stabilized by epitaxial growth can exhibit large populations of point defects which have important effects on their properties. The site occupancy of pulsed laser-deposited epitaxial terbium iron garnet (TbIG) films with excess terbium (Tb) is analyzed, in which the terbium:iron (Tb:Fe)ratio is 0.86 compared to the stoichiometric value of 0.6. The magnetic properties of the TbIG are sensitive to site occupancy, exhibiting a higher compensation temperature (by 90 K) and a lower Curie temperature (by 40 K) than the bulk Tb3 Fe5 O12 garnet. Data derived from X-ray core-level spectroscopy, magnetometry, and molecular field coefficient modeling are consistent with occupancy of the dodecahedral sites by Tb3+ , the octahedral sites by Fe3+ , Tb3+ and vacancies, and the tetrahedral sites by Fe3+ and vacancies. Energy dispersive X-ray spectroscopy in a scanning transmission electron microscope provides direct evidence of TbFe antisites. A small fraction of Fe2+ is present, and oxygen vacancies are inferred to be present to maintain charge neutrality. Variation of the site occupancies provides a path to considerable manipulation of the magnetic properties of epitaxial iron garnet films and other complex oxides, which readily accommodate stoichiometries not found in their bulk counterparts.

Single domain Co2FeGa nanoparticles with high crystalline order synthesized by template-less chemical method

Co2FeGa nanoparticles with stable L21 type full Heusler alloy structure have been synthesized with average crystallite sizes of 57 nm, 47 nm and 39 nm using a simple template-less chemical route. All these nanoparticles in single domain regime exhibit soft ferromagnetic nature with high saturation magnetization (>93 % of theoretical estimate), low coercive field (<10 Oe) and high Curie temperature (>1100 K). Near zero coercive field and remanence magnetization values of these nanoparticles indicate their superparamagnetic nature. Ab initio calculations were performed to estimate the magnetic moment and the half metallic nature of the Heusler alloy particles from the density of states of the spin-up and spin-down bands.

Development of Intrinsic Room-Temperature 2D Ferromagnetic Crystals for 2D Spintronics

Two-dimensional (2D) ferromagnetic crystals with fascinating optical and electrical properties are crucial for nanotechnology and have a wide variety of applications in spintronics. However, low Curie temperatures of most 2D ferromagnetic crystals seriously hinder their practical applications, thus searching for intrinsic room-temperature 2D ferromagnetic crystals is of great importance for development of information technology. Fortunately, progresses have been achieved in the last few years. Here we review recent advances in the field of intrinsic room-temperature 2D ferromagnetic crystals and introduce their applications in spintronic devices based on van der Waals heterostructures. Finally, the remaining challenge and future perspective on the development direction of intrinsic room-temperature 2D ferromagnetic crystals for 2D spintronics and van der Waals spintronics are briefly summarized.

Optimization of the electrical properties on lead-free barium titanate-based piezoelectric by combining grain growth with orthorhombic-tetragonal phase boundary

Barium titanate-based lead-free piezoceramics are considered as a key material for the fabrication of dielectric capacitors due to their large relative dielectric constant (εr) and are also identified as a remarkable electrocaloric material on account of their low Curie temperature (TC) and high spontaneous polarization (Ps). At this work, to optimize the electrical performances of BaTiO3-based Pb-free piezoceramics, chemical doping was used to tailor their composition. The aim is to boost Curie temperature while sustaining decent electrical properties. Besides, to enlighten the underlying physical mechanisms, first-principles calculations based on atomic coupling, orbital hybridization as well as electron localization were carried out. The experimental results show that the dielectric, ferroelectric, and piezoelectric properties are mainly determined by the orthorhombic-tetragonal (O-T) two-phase coexistence and grain size. Notably, a boosted dielectricity, ferroelectricity, and piezoelectricity accompanied by a raised TC is achieved at x = 0.10 mol%. Moreover, the highest TC∼125.40 ℃ is obtained at x = 0.40 mol% along with a degradation in dielectricity, ferroelectricity, and piezoelectricity.

Photoinduced Room-Temperature Magnetism of Low Curie Temperature Cu–Ni Nanoparticles in Polymer Composites with Rubrene Microcrystals

A relation has been established between the decrement of the low-field magnetic spin effect (measured by the luminescence of polymer films of composites with rubrene microcrystals), caused by the addition of magnetic Cu–Ni nanoparticles with a low Curie temperature (TC = 40–60°C), and the photoinduced particle magnetic moment exceeding the dark moment. Based on the study of the temperature dependence of the decrement and its comparison with the thermal demagnetization of the dark magnetic moment, a conclusion was made about a possible mechanism for the photothermal magnetization of nanoparticles

Ferrimagnetic coupling between cobalt and light rare-earth samarium induced by dense hydrogenation of SmCo5 permanent magnet under high pressures

X-ray magnetic circular dichroism, x-ray powder diffraction and first-principles calculations revealed that dense hydrogenation of a Gd-doped ${\mathrm{SmCo}}_{5}{\mathrm{H}}_{x}$ compound $({\mathrm{Sm}}_{0.92}{\mathrm{Gd}}_{0.08}{\mathrm{Co}}_{5}{\mathrm{H}}_{x})$, ranging up to $x\ensuremath{\approx}13.5$, flips the direction of Sm $4f$ magnetic moments. Consequently, the magnetic coupling of Sm $4f$ with Co $3d$ moments changes to ferrimagnetic from the conventional ferromagnetic coupling. The ferrimagnetic structure possessed significantly lower Curie temperatures of ${T}_{\text{C}}\ensuremath{\le}131$ K, which is in contrast to the extremely high ${T}_{\text{C}}\ensuremath{\sim}1300$ K of a ${\mathrm{SmCo}}_{5}$ permanent magnet. Structural parameters of two hydrogenation-induced crystal structures were determined carefully by x-ray powder diffraction. Large volume expansion exceeding $V/{V}_{0}=1.3$ indicates that hydrogen occupation in the Sm-3Co tetrahedron occurred above 1 GPa. The hydrogen in the tetrahedron dominates the ferromagnetic coupling between the interatomic Co 3$d$--Sm 5$d$ electron orbitals, which plays an important role in the induction of the ferrimagnetic structure of ${\mathrm{SmCo}}_{5}{\mathrm{H}}_{x}$.