Magnetic Transition Temperature Research Articles

Influence of non-magnetic Cu on structural, magnetic and magnetocaloric properties on NiMnIn bulk Heusler alloys

The structural, magnetic, and magnetocaloric characteristics of bulk Cu-doped Ni47Mn40-sCusIn13 Heusler alloys (s = 0 and 1) were investigated. Temperature-dependent magnetizations for Ni47Mn40-sCusIn13(s = 0 and 1) were performed under a magnetic field (H) 0.05 T. The substitution of Copper (Cu) for Manganese (Mn) reduces the structural as well as magnetic phase transition temperatures. At room temperature (RT), PXRD pattern of Ni47Mn40-sCusIn13 (s = 0 and 1) bulk Heusler alloys shows that both samples have a cubic structure. The copper substitution in the Mn-site shifts the martensitic transformation temperature to the low temperature side as valence electron concentrations (e/a) decrease [MS = 288 K for s = 0 and MS = 251 K for s = 1]. The isothermal magnetization [M(H)] curve in the vicinity of the martensitic transition for s = 1 alloy exhibits the typical metamagnetic behavior that is not present for s = 0 Heusler alloys. The alloys reveal positive magnetic entropy changes (ΔSM) for s = 0 and 1 respectively. A maximal ΔSM of 10 J.kg-1K−1 at a temperature of 248 K has been obtained in the s = 1 compare than ΔSM of 1.27 J.kg-1K−1 (s = 0) alloys for a field change of 0–5 T.

High-Performance Thermomagnetic Gd-Si-Ge Alloys.

Exploring low-grade waste heat energy harvesting is crucial to address increasing environmental concerns. Thermomagnetic materials are magnetic phase change materials that enable energy harvesting from low-temperature gradients. To achieve a high thermomagnetic conversion efficiency, there are three main material requirements: (i) magnetic phase transition near room temperature, (ii) substantial change in magnetization with temperature, and (iii) high thermal conductivity. Here, we demonstrate a high-performance Gd5Si2.4Ge1.6 thermomagnetic alloy that meets these three requirements. The magnetic phase transition temperature was successfully shifted to 306 K by introducing Ge doping in Gd5Si4, and a sharper and more symmetric magnetization behavior with saturation magnetization of Mmax = 70 emu/g at a 2 T magnetic field was achieved in the ferromagnetic state. The addition of SeS2, as a low-temperature sintering aid, to the Gd-Si-Ge alloy improved the material's density and thermal conductivity by ∼45 and ∼275%, respectively. Our results confirm that the (Gd5Si2.4Ge1.6)0.9(SeS2)0.1 alloy is a suitable composite material for low-grade waste heat recovery in thermomagnetic applications.

Interplay between Optical Emission and Magnetism in the van der Waals Magnetic Semiconductor CrSBr in the Two-Dimensional Limit.

The van der Waals semiconductor metamagnet CrSBr offers an ideal platform for studying the interplay between optical and magnetic properties in the two-dimensional limit. Here, we carried out an exhaustive optical characterization of this material by means of temperature- and magnetic-field-dependent photoluminescence (PL) on flakes of different thicknesses down to the monolayer. We found a characteristic emission peak that is quenched upon switching the ferromagnetic layers from an antiparallel to a parallel configuration and exhibits a temperature dependence different from that of the peaks commonly ascribed to excitons. The contribution of this peak to the PL is boosted around 30-40 K, coinciding with the hidden order magnetic transition temperature. Our findings reveal the connection between the optical and magnetic properties via the ionization of magnetic donor vacancies. This behavior enables a useful tool for the optical reading of the magnetic states in atomically thin layers of CrSBr and shows the potential of the design of 2D heterostructures with magnetic and excitonic properties.

Structural and physical properties of Sr‐based 3d–5d double perovskites of Sr2Fe0.5Hf1.5O6−δ oxides

AbstractSr‐based 3d–5d double perovskite Sr2Fe0.5Hf1.5O6−δ (SFHO) oxide powders were synthesized via the solid‐state reaction method, and their structural, dielectric, magnetic, optical, and transport properties were investigated. Structural investigations revealed the SFHO powders crystallized in an orthorhombic crystal structure with Pnma space group. Scanning electron microscopy images demonstrated the relatively homogeneous distributions of spherical SFHO powders, and x‐ray energy dispersive spectra gave out the molar ratio of Sr:Fe:Hf equal to 1.94:0.5:1.87, close to the nominal value. XPS spectra confirmed the existence of Sr2+, Fe3+, Hf4+ ions in the SFHO powders, and oxygen appeared in the forms of lattice oxygen and adsorbed oxygen species, respectively. The SFHO ceramics exhibited a relaxor‐like dielectric behavior. Two dielectric abnormities appeared around 518 and 257 K, which were ascribed to the dielectric response of the oxygen vacancies ( activation energy Ea = 0.93 eV and thermal excitation of the electrons with Ea = ∼ 0.02 eV bound weakly to the, respectively. Ferromagnetic behavior was observed in the SFHO powders at 300 and 2 K, and the saturated magnetization at 2 K was 0.67 μB/f.u. Magnetic transition temperature, TC was determined to be 638 K from the temperature dependence of the magnetization under the zero‐field cooling mode. A spin glass‐like behavior was observed around 390 K, which was confirmed by the frequency‐dependent alternating current (ac) susceptibility measurements. The SFHO powders exhibit a butterfly‐like magnetoresistance (MR)–H plot at 2 K, and the MR (2 K, 7 T) is found to be −2.73% owing to the intergranular magneto‐tunneling effect. Temperature dependence of the resistivity of the SFHO ceramics displayed a semiconducting behavior, and the electrical transport properties of the SFHO ceramics were investigated by Mott's variable‐range‐hopping model, thermally activated semiconductor conductivity model, and the adiabatic small polarons hopping model, respectively. UV–vis diffuse‐reflectance spectra of the SFHO powders demonstrated a direct optical bandgap of Eg = 2.62 eV, which was contributed from the electron transfer from the O(2p) orbital to Fe(3d)/Hf(5d) orbitals. A combination of high‐temperature ferrimagnetism and semiconducting nature of the SHFO powders makes them attractive for spintronics and photovoltaics.

Three-dimensional critical behavior and magnetocaloric effect in Yb14MnSb11

Systematic investigations were performed on the nature of the magnetic properties of Yb14MnSb11, which is the first ferromagnetic Kondo lattice compound in the underscreened limit and one of the highest thermoelectrically efficient materials with p-type conduction. The Rhode-Wohlfarth ratio (RWR) is estimated to be 1.01, which demonstrates the localized nature of the ferromagnetism in Yb14MnSb11, consistent with the predicted Ruderman–Kittel–Kasuya–Yosida (RKKY) like exchange interaction. Detailed analyses on critical behavior indicate a second-order ferromagnetic phase transition with critical exponents β = 0.307(2), γ = 0.923(9), and δ = 4.003(4), demonstrating a three-dimensional critical behavior with long-range interaction. Furthermore, the magnetocaloric effect and relative cooling power are estimated to be 0.772 J kg−1 K−1 and 16.43 J kg−1 at 5 T, respectively. The magnetic transition temperature of TC = 51.9(3) K is strictly estimated by evaluating the magnetic change of specific heat [ΔCp(T,H)]. The rare d-electron ferromagnetic Kondo lattice, small magnetocaloric effect, and high-temperature large thermoelectric efficiency making Yb14MnSb11 to be a system of considerable interest.

Smectic pair-density-wave order in EuRbFe4As4.

The pair density wave (PDW) is a superconducting state in which Cooper pairs carry centre-of-mass momentum in equilibrium, leading to the breaking of translational symmetry1-4. Experimental evidence for such a state exists in high magnetic field5-8 and in some materials that feature density-wave orders that explicitly break translational symmetry9-13. However, evidence for a zero-field PDW state that exists independent of other spatially ordered states has so far been elusive. Here we show that such a state exists in the iron pnictide superconductor EuRbFe4As4, a material that features co-existing superconductivity (superconducting transition temperature (Tc) ≈ 37 kelvin) and magnetism (magnetic transition temperature (Tm) ≈ 15 kelvin)14,15. Using spectroscopic imaging scanning tunnelling microscopy (SI-STM) measurements, we show that the superconducting gap at low temperature has long-range, unidirectional spatial modulations with an incommensurate period of about eight unit cells. Upon increasing the temperature above Tm, the modulated superconductor disappears, but a uniform superconducting gap survives to Tc. When an external magnetic field is applied, gap modulations disappear inside the vortex halo. The SI-STM and bulk measurements show the absence of other density-wave orders, indicating that the PDW state is a primary, zero-field superconducting state in this compound. Both four-fold rotational symmetry and translation symmetry are recovered above Tm, indicating that the PDW is a smectic order.

Giant dielectric constant, magnetocaloric effect and spin-phonon coupling in EuTbCoMnO6 semiconductor

Transport, dielectric, magnetic and Raman studies of polycrystalline double perovskite EuTbCoMnO6 have been performed. Temperature-dependent resistivity follows the variable range and small polaron hopping mechanism. The temperature-dependent dielectric study shows the usual frequency-dependent step-like behavior. It also shows thermally activated relaxor peak in loss curves with giant dielectric constant near room temperature. The temperature-dependent magnetization demonstrates a second-order phase transition around 113 K. The critical behavior near the magnetic transition has been systematically investigated. The Widom scaling relation and magnetic entropy change calculation have verified the reliability of critical parameters. Maximum magnetic entropy change around the magnetic transition temperature shows the magnetocaloric effect. The estimated values of critical parameters are close to the value of the theoretical mean-field model. Temperature-dependent Raman study shows spin-phonon coupling in the system. The appearance of the magnetocaloric effect and colossal dielectric constant shows that these materials are interesting for real-life applications.

Absence of magnetostriction and transition from rapid positive thermal expansion to negative thermal expansion at high temperatures in Sm0.5Y0.5Fe0.58Mn0.42O3 – A synchrotron X-ray diffraction study

X-ray photoemission spectroscopic (XPS) studies and temperature dependent synchrotron X-ray diffraction (SXRD) investigation have been conducted on polycrystalline samples of Sm0.5Y0.5Fe0.58Mn0.42O3 [SYFM (58-42)]. Room temperature XPS measurements revealed the mixed valance of the Fe and Mn ions along with the traces of oxygen vacancies in the material. The room temperature synchrotron X-ray diffraction (SXRD) of specimen shows the monophasic nature of the prepared composition, without the presence of any magnetic or non-magnetic impurity phases. SXRD measurements at variable temperatures between 298 and 873 K, indicates the absence of magnetostriction in the specimen at the magnetic transition temperatures TSR1 ≈ 348 K, TSR2 ≈ 300 K and TN ≈ 361 K. At much higher temperatures above TN, an anisotropic rapid positive thermal expansion (RPTE) to negative thermal expansion (NTE) of the a lattice parameter of the unit cell is further revealed for T > 573 K. The anomalous variation of phonons, are suggested to give rise to anomalous thermal variation in a lattice parameter of the specimen.

Multi-principal rare-earth Gd-Tb-Dy-Ho-Er alloys with high magnetocaloric performance near room temperature

Magnetocaloric materials with enhanced magnetocaloric effect and high magnetic transition temperature are crucial for magnetic refrigeration, a novel cooling technology featuring high energy efficiency, low noise and superb environmental friendliness. In this study, a series of Gdx(TbDyHoEr)1-x (x=0.2–0.9) rare-earth alloys with a single-phase HCP (hexgonal-close-packed) structure, were developed utilizing a multi-principal element alloying strategy. It was found that the Gd0.8(TbDyHoEr)0.2 alloy exhibits a high magnetic transition temperature (i.e., ∼268 K), and concurrently enhanced magnetocaloric properties with a maximum magnetic entropy change value of 10.53 J kg−1 K−1 under a field change of 5 T and a refrigerant capacity value of 751.8 J kg−1. Notably, for these multi-principal element alloys, the maximum magnetic entropy change is essentially linearly correlated with the configuration entropy of mixing while the magnetic transition temperature scales with the de Gennes factor, which can be interpreted as a manifestation of ideal solid solutions. This work provides important implications for designing high-performance magnetocaloric materials for magnetic refrigeration.

Ab initio studies on structural and thermodynamic properties of magnetic Fe

The present work systematically investigates the total energy, phonon spectra, and thermodynamic properties of different polymorphs of pure Fe, i.e., FCC, BCC, and HCP, with the ab initio approach, considering various magnetic configurations. In general, the calculated energy vs. volume curves and phonon spectra agree well with previous calculations and the experimental data. In addition, their thermodynamic properties are estimated by the quasiharmonic approximation (QHA). Specifically, a superposition approach based on the latest Zentropy theory was utilized to predict magnetic transition temperatures and thermodynamic properties of pure Fe. With the ensemble of the partition function considering the multiplicity of each magnetic microstate, the current work successfully reproduced the Curie/Néel temperature and the Schottky anomaly of heat capacity in FCC, BCC, and HCP Fe purely based on the ab initio input, which exhibits good agreement with the experimental data and CALPHAD modeling.

Stacking and layer dependence of magnetic properties in Ti2C and Fe2C

Magnetic MXenes are becoming an important family of materials for exploring 2D magnetism. However, investigations into the interdependence of layer thickness, stacking patterns and magnetism in these materials, from a microscopic point of view, are still lacking. In this work, we have used density functional theory based calculations to understand the effects of layer thickness and stacking on the magnetic properties in two magnetic MXenes, Ti2C and Fe2C, in their monolayer and bilayer forms. The ground-state magnetic structures, magnetic moments, magnetic exchange interactions, magnetic transition temperatures and magnetic anisotropy energies are calculated and analysed using their electronic structures and standardised models. We find that in both systems an increase in layer thickness (monolayer to bilayer) affects the ground-state magnetic configuration, which is driven by the changes in the magnetic exchange interactions. While the effects of the stacking pattern are rather weak in Ti2C, they are substantial, both qualitatively and quantitatively, in Fe2C. The computed results are analysed from their electronic structures. The results suggest that fascinating physical effects can be obtained in Fe2C by tuning the layer thickness and stacking patterns, making it more suitable for device applications.

Magneto transport behavior of La0.67Pb0.33MnO3manganite system with various additives

The nanostructured manganites show low field magnetoresistance, where grain boundary effects influence the phenomenon. The current investigation aims to comprehend the impact of the grain boundaries on the magnetic and magneto transport characteristics of La0.67Pb0.33MnO3 manganite with 1% of additives ZrO2, Bi2O3, and Li2CO3. Two step synthesis processes was adopted to prepare the pure and additive samples. The samples were characterized with XRD, XPS and FESEM to establish the structure, phase purity, oxidation states, elemental analysis and morphology respectively. The prepared samples were formed in single-phase rhombohedral structure with the enhanced crystallite and grain size with additives when compared to the pure sample. The observed magnetic transition temperature, are well above room temperature (TC = 353 K) and a soft ferromagnetic nature with 70% of ferromagnetic fraction is noticed at T = 300 K for all the studied samples. The metal insulator transition temperature (TP) remains invariant for ZrO2, Bi2O3 additives, while it reduces for Li2CO3 additive. It is observed that 1% of additives are acting as conduction barriers at the grain boundaries as a consequence there is an increase in resistivity. The contribution to magnetoresistance is seen as intrinsic and extrinsic in pure and additive materials exhibiting high percentage both at low and room temperature. All the sample shows an appreciable temperature coefficient of resistance percentage at their respective transition temperatures which can be made useful for bolometric applications. Conduction mechanisms in the ferromagnetic and paramagnetic phases were examined with the electron-magnon scattering and small polaron hopping model respectively.

Spin-induced valley polarization in heterobilayer Janus transition-metal dichalcogenides

Inspired by potential application prospects of spintronics and valleytronics, a novel heterobilayer Janus structure is designed by replacing the chalcogenide atomic layers in the original bilayer MoS2. Based on first-principles calculations, it is found that the SMoS/SeMoS structure exhibits a direct band-gap semiconductor and a typical type-II band alignment with longer carrier lifetime. The transition metal (TM) atom represented by V/Cr/Mn can be stably adsorbed on the heterobilayer Janus SMoS/SeMoS sheet and effectively introduce magnetic moments (m). The calculation results demonstrate that the most stable adsorption site of the TM atom is CX(A), and the TM (V/Cr/Mn) adatom modified SMoS/SeMoS system is converted into metal (V-) or half-metal (Cr/Mn-), respectively. Under the coupling of different indirect exchange interactions, the structure exhibits stable intrinsic anti-ferromagnetic interactions for V-SMoS/SeMoS and ferromagnetic ground state for Cr/Mn-SMoS/SeMoS, respectively, and the magnetic transition temperature (T c) reaches a high temperature or even room temperature. Moreover, the robust out-of-plane magnetocrystalline anisotropy energy ensures stable long-range magnetic order. Specifically, the combination of spin injection and strong spin–orbit coupling interaction effectively breaks the time-reversal symmetry, which leads to valley polarization of the system. Based on this, the biaxial strain can effectively regulate the electronic structure, magnetic properties and valley polarization of TM-SMoS/SeMoS nanosheets with double breaking of spatial-inversion and time-reversal symmetry.

Additive manufacturing of Ni-Mn-Sn shape memory Heusler alloy – Microstructure and magnetic properties from powder to printed parts

Ni-Mn-based Heusler alloys such as Ni-Mn-Sn show both elastocaloric and magnetocaloric effects during the magneto-structural phase transition, making these materials interesting for solid-state cooling applications. Material processing by additive manufacturing can overcome difficulties related to the machinability of the alloys, caused by their intrinsic brittleness. Since the magnetic properties and transition temperature are highly sensitive to the chemical composition, it is essential to understand and monitor these properties over the entire processing chain. In the present work, the microstructural and magnetic properties from gas-atomized powder to post-processed Ni-Mn-Sn alloy are investigated. Directed energy deposition was used for processing, promoting the evolution of a polycrystalline microstructure being characterized by elongated grains along the building direction. A complete and sharp martensitic transformation can be achieved after applying a subsequent heat treatment at 1173 K for 24 h. The Mn-evaporation of 1.3 at.% and the formation of Mn-oxide during DED-processing lead to an increase of the transition temperature of 21 K and a decrease of magnetization, clearly pointing at the necessity of controlling the composition, oxygen partial pressure and magnetic properties over the entire processing chain.

The Study of Interplay of Singlet Superconductivity and Ferromagnetism in Superconducting HoMo6Se8

This article reports the effect of magnetic ordering and superconducting order parameter on superconducting and magnetic transition temperatures of interacting singlet superconductivity and ferromagnetism in HoMo6Se8 material. The basic superconducting parameters were calculated starting with the BCS type model Hamiltonian and using the double time temperature dependent Green function formalisms. Results showed that the superconducting critical temperature decreases with enhancement of superconducting order parameter and vice versa. This is perhaps due to the weakening of the binding energy as the temperature approaches its critical value. On the other hand, the superconducting critical temperature demonstrates attenuation with increasing magnetic order parameters . It was also observed that the enhancement of magnetic transition temperature with the increase of the magnetic order parameter demonstrates variation with the alteration of the superconducting order parameter. Moreover, the calculations revealed that there is a temperature region where both superconductivity and magnetic order coexist. Furthermore, the present theoretical analysis is broadly in agreement with existing experimental findings.

Magnetic and magnetocaloric properties of R55Co30Al10Si5 (R = Ho, Dy) metallic glass ribbons

Metallic glass alloys are suitable systems for the development of magnetocaloric refrigerated materials due to the stable physical properties and the adjustable magnetic phase transition temperature. Here, the glass formation ability (GFA), magnetic properties and magnetocaloric effects of R55Co30Al10Si5 (R = Ho, Dy) amorphous ribbons prepared by the melt-spinning technique were investigated. The X-ray diffraction and DSC measurement indicated that the metallic glass alloys have a completely amorphous structure and excellent glass formation ability. The amorphous ribbons were demonstrated to undergo a second-order magnetic phase transition (ferromagnetic to paramagnetic) around Tc ∼ 18 K for Ho55Co30Al10Si5, ∼34 K for Dy55Co30Al10Si5, respectively. The maximum magnetic entropy change (-ΔSMmax) and the corresponding refrigerant capacity (RC) values are 12.7 J/kg K, 343 J/kg for Ho55Co30Al10Si5, 9.1 J/kg K, 334 J/kg for Dy55Co30Al10Si5 under a field change ΔH= 5 T. The large -ΔSMmax and high RC suggest that R55Co30Al10Si5 (R = Ho, Dy) amorphous ribbons have potential applications in low temperature magnetic refrigeration.

BaMn2P2 : Highest magnetic ordering temperature 122-pnictide compound

We report the growth of high-quality single crystals of ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type tetragonal ${\mathrm{BaMn}}_{2}{\mathrm{P}}_{2}$ and investigation of its structural, electrical transport, thermal, and magnetic properties. Our results of basal plane electrical resistivity and heat capacity measurements show that the compound has an insulating ground state with a small band gap. Anisotropic susceptibility ${\ensuremath{\chi}}_{ab,c}(T)$ data infer a collinear local-moment N\'eel-type antiferromagnetic (AFM) ground state below the ordering temperature ${T}_{\mathrm{N}}=795(15)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, which is highest among all the ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$- and ${\mathrm{CaAl}}_{2}{\mathrm{Si}}_{2}$-type 122-pnictide compounds reported so far suggesting that the strength of magnetic exchange interactions is strongest in this material. The magnetic transition temperatures of ${\mathrm{BaMn}}_{2}P{n}_{2}$ ($Pn=\mathrm{P}$, As, Sb, Bi) compounds exhibit a monotonic decrease with the increase of tetragonal unit cell parameters $a$ and $c$, suggesting a strong dependence of the strength of the decisive magnetic exchange interactions on the separation between the localized spins residing on the Mn ions. The observed monotonic increase of both ${\ensuremath{\chi}}_{ab}$ and ${\ensuremath{\chi}}_{c}$ for $T>{T}_{\mathrm{N}}$ suggests that short-range dynamic quasi-two-dimensional AFM correlations persist above the ${T}_{\mathrm{N}}$ up to the highest temperature of the measurements. The large ${T}_{\mathrm{N}}$ of ${\mathrm{BaMn}}_{2}{\mathrm{P}}_{2}$ demands for systematic hole-doping studies on this material as similar investigations on related ${\mathrm{BaMn}}_{2}{\mathrm{As}}_{2}$ with ${T}_{\mathrm{N}}=618\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ have led to the discovery of an outstanding ground state where AFM of localized Mn spins and itinerant half-metallic ferromagnetism with ${T}_{\mathrm{c}}\ensuremath{\approx}100\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ originating from the doped holes coexist together.

Superconductivity and magnetic and transport properties of single-crystalline CaK(Fe1−xCrx)4As4

Members of the CaK(Fe$_{1-x}$Cr$_{x}$)$_{4}$As$_{4}$ series have been synthesized by high-temperature solution growth in single crystalline form and characterized by X-ray diffraction, elemental analysis, magnetic and transport measurements. The effects of Cr substitution on the superconducting and magnetic ground states of CaKFe$_4$As$_4$ ($T_c$ = 35 K) have been studied. These measurements show that the superconducting transition temperature decreases monotonically and is finally suppressed below 1.8 K as $x$ is increased from 0 to 0.038. The magnetic transition temperature increases in a roughly linear manner as Cr substitution increases. A temperature-composition (\textit{T}-\textit{x}) phase diagram is constructed, revealing a half-dome of superconductivity with the magnetic transition temperature, $T^*$, appearing near 22~K for $x$ $\sim$ 0.017 and rising slowly up to 60~K for $x$ $\sim$ 0.077. The $T$-$x$ phase diagrams for CaK(Fe$_{1-x}$$T$$_{x}$)$_4$As$_4$ for $T$ = Cr and Mn are essentially the same despite the nominally different band filling; this is in marked contrast to $T$ = Co and Ni series for which the $T$-$x$ diagrams scale by a factor of two, consistent with the different changes in band filling Co and Ni would produce when replacing Fe. Superconductivity of CaK(Fe$_{1-x}$Cr$_{x}$)$_{4}$As$_{4}$ is also studied as a function of magnetic field. A clear change in $H^\prime_{c2}$($T$)/$T_c$, where $H^\prime_{c2}$($T$) is d$H_{c2}$($T$)/d$T$, at $x$ $\sim$ 0.012 is observed and probably is related to change of the Fermi surface due to magnetic order. Coherence length and the London penetration depths are also calculated based on $H_{c1}$ and $H_{c2}$ data. Coherence lengths as the function of $x$ also shows changes near $x$ = 0.012, again consistent with Fermi surfaces changes associated with the magnetic ordering seen for higher $x$-values.

Efficient discovery of room temperature magnetic transition metal monolayers assisted by artificial neural network

The low-dimensional materials have opened a new window to new emerging science and technological applications. Recently the discovery of new two-dimensional (2D) monolayers with long-range magnetic order by theoretical method gets more attention. The key question is how to effectively explore different possible 2D magnetic configurations. However, the computational cost of optimizing and electronically analyzing a a large number of compounds on the ab initio level is very high. A practical method is the combination of density functional theory and machine learning techniques to explore a large number of 2D monolayers. As a proof of principle, the method is implemented for looking at new high transition point 2D magnetic monolayers among transition metals dichalcogenides, dihalides, and chalcogenides/halides TMXY. Our study is intended to provide physical insights into the magnetic properties in two dimensions and introduce new room-temperature magnetic monolayers. For instance, our results show that the effective electronic screening and magnetic transition temperature in 2D monolayers are strongly dependent on the type of non-magnetic ligand atoms. Three different Neural Network (NN), Random Forest (RF), and Supporting Vector Regression (SVR) algorithms are successfully employed for quick and accurate prediction of Hubbard U, cell parameters, and the magnetic exchange parameter for a large number of TMXY (TM: V, Cr, Fe, Co, Mn, Ni, Nb and X, Y: O, S, Se, Te, Cl, Br, I) monolayers. Moreover, the binary classification is employed to determine the magnetic and atomic structure ground state. Finally, the transition temperature of TMXY monolayers is estimated from the predicted value of effective magnetic exchange parameter. The magnetic monolayers with higher transition temperatures are suggested for further theoretical and experimental investigations.

Strain effects on the electronic and magnetic properties of Cr2TaC2 and Cr2TaC2O2 monolayers

Double transition metal MXene materials, such as Cr2TiAlC2, were successfully synthesized recently in addition to the synthesized mono transition metal MXenes. The physical and chemical properties of the MXenes can be tuned by varying the middle metal layer composition by other metal atoms. Thus, the desired mechanical, electronic, thermal, or magnetic properties can be revealed in that MXene material. The other method to tune material properties is by applying external strain on the structure. In this study, we systematically investigate the physical properties, such as dynamical, thermal, mechanical stability, electronic, and magnetic properties of the Cr2TaC2 and the Cr2TaC2O2 MXene monolayers for equilibrium and under biaxial strain. We find that both of MXene monolayers are energetically, thermally, and dynamically stable for certain compressive and tensile strain values. The Cr2TaC2 and the Cr2TaC2O2 MXene monolayers have a ferromagnetic ground state and show a metallic character in an equilibrium state and preserve it up to high tensile strain (ε = 0.10) value. The robust ferromagnetism above room temperature is demonstrated for the Cr2TaC2 with a Curie temperature of 714 K for the strain-free case. This value reduces to 234 K for the Cr2TaC2O2 monolayer. Moreover, our detailed Monte Carlo simulations reveal that the magnetic transition temperature of the system and also hysteresis properties can be controlled by moving from compressive to tensile strain ratios.