Magnetic Transition Temperature Research Articles

In Situ Local Imaging of Ferromagnetism and Superconductivity in RbEuFe4As4.

The coexistence of superconductivity and ferromagnetism is an intrinsically interesting research focus in condensed matter physics, but the study is limited by low superconducting (Tc) and magnetic (Tm) transition temperatures in related materials. Here, we used a scanning superconducting quantum interference device to image the in situ diamagnetic and ferromagnetic responses of RbEuFe4As4 with high Tc and Tm. We observed significant suppression of the superfluid density in the vicinity of the magnetic phase transition, signifying fluctuation-enhanced magnetic scatterings between Eu spins and Fe 3d conduction electrons. Intriguingly, we observed multiple ferromagnetic domains that should be absent in an ideal magnetic helical phase. The formation of these domains demonstrates a weak c-axis ferromagnetic component probably arising from the Eu spin-canting effect, indicative of possible superconductivity-driven domain Meissner and domain vortex-antivortex phases, as revealed in EuFe2(As0.79P0.21)2. Our observations highlight that RbEuFe4As4 is a unique system that includes multiple interplay channels between superconductivity and ferromagnetism.

Effects of Fe substitution for Mn on structural, magnetic and magnetocaloric properties of TbMn2-xFexSi2

The structural and magnetic properties of TbMn2-xFexSi2 compounds (x = 0.0–0.4) have been investigated in details by high-resolution synchrotron x-ray and neutron powder diffraction, specific heat, and dc magnetization measurements. The replacement of Fe for Mn in TbMn2-xFexSi2 does not change the crystal structure but leads to a significant contraction of the unit cell (dV/dx ∼ - 6 Å3) and modification of three magnetic phase transition temperatures – the Curie temperatures Tc1, Tc2 and the Néel temperature TN. For example, the Tc1, Tc2 and TN values of TbMn2Si2 decrease from Tc1 = 50 K, Tc2 = 64 K and TN = 500 K to Tc1 = 27 K, Tc2 = 37 K and TN = 440 K for TbMn1.6Fe0.4Si2. Detailed neutron diffraction investigations have allowed us to determine the magnetic structures and construct the magnetic phase diagram as well as confirm the presence of magnetoelastic coupling effects. The significantly different responses of Tc1 and Tc2 to applied magnetic fields (e.g. dTc1/dB = 0.52 K/T and dTc2/dB = 4.7 K/T for TbMn1.6Fe0.4Si2), leads to expansion of the temperature region (ΔT = Tc2- Tc1) for the canted ferromagnetic state Fmc-ii between Tc1 and Tc2. In the case of TbMn1.6Fe0.4Si2, ΔT increases from ΔT = 9 K for applied magnetic field B = 0.2 T to ΔT = 36 K for B = 6 T, resulting in plateau-like behaviour of magnetic entropy change and enhances the relative cooling power (RCP) in this system. Under a change of magnetic field of 0 T – 5 T, the maximum value of the magnetic entropy changes -ΔSmax and adiabatic temperature change, ΔTad are derived to be -ΔSmax = 13.1 J/kg K and ΔTad = 8.4 K with the RCP = 411 J/kg for x=0.4 sample. The plateau-like magnetocaloric effect and relatively large RCP make these materials potentially suitable for application in cryogenic magnetic refrigeration.

Enhanced magnetic transition temperature through ferromagnetic and antiferromagnetic interaction in cobalt-substituted Fe5GeTe2

Two-dimensional (2D) magnetism is an incredibly intriguing phenomenon in condensed matter physics. The exploration of 2D magnets holds great promise for various applications, even though they often exhibit low magnetic transition temperature. Among these materials, Fe5GeTe2 has emerged as a compelling candidate for room-temperature spintronics due to its intrinsic ferromagnetism and high Curie temperature. In this study, we investigate the impact of Co substitution at the Fe sites in Fe5GeTe2, which induces a transition of the magnetic ground state to the antiferromagnetic state when the substitution level exceeds 0.36. Additionally, we observe the coexistence of ferromagnetic (FM) and antiferromagnetic states in the magnetic transition region of (Fe1−xCox)5GeTe2 crystals. Notably, the interaction between the two magnetic phases results in Néel temperature (TN) up to 374 K, establishing a record among known van der Waals antiferromagnets. Our findings present a strategy for enhancing the magnetic temperature of 2D magnets, paving the way for potential advancements in spintronics applications.

Ubiquitous Order-Disorder Transition in the Mn Antisite Sublattice of the (MnBi2Te4)(Bi2Te3)n Magnetic Topological Insulators.

Magnetic topological insulators (TIs) herald a wealth of applications in spin-based technologies, relying on the novel quantum phenomena provided by their topological properties. Particularly promising is the (MnBi2Te4)(Bi2Te3)n layered family of established intrinsic magnetic TIs that can flexibly realize various magnetic orders and topological states. High tunability of this material platform is enabled by manganese-pnictogen intermixing, whose amounts and distribution patterns are controlled by synthetic conditions. Here, nuclear magnetic resonance and muon spin spectroscopy, sensitive local probe techniques, are employed to scrutinize the impact of the intermixing on the magnetic properties of (MnBi2Te4)(Bi2Te3)nand MnSb2Te4. The measurements not only confirm the opposite alignment between the Mn magnetic moments on native sites and antisites in the ground state of MnSb2Te4, but for the first time directly show the same alignment in (MnBi2Te4)(Bi2Te3)n with n = 0, 1 and 2. Moreover, for all compounds, the static magnetic moment of the Mn antisite sublattice is found to disappear well below the intrinsic magnetic transition temperature, leaving a homogeneous magnetic structure undisturbed by the intermixing. The findings provide a microscopic understanding of the crucial role played by Mn-Bi intermixing in (MnBi2Te4)(Bi2Te3)n and offer pathways to optimizing the magnetic gap in its surfacestates.

High-temperature magnetic anomaly via suppression of antisite disorder through synthesis route modification in a Kitaev candidate Cu2IrO3

By incorporating inert KCl into the Na2IrO3 + 2CuCl → Cu2IrO3 + 2NaCl topochemical reaction, we significantly reduced the synthesis temperature of Cu2IrO3 from the 350 °C reported in previous studies to 170 °C. This adjustment decreased the Cu/Ir antisite disorder concentration in Cu2IrO3 from ∼19% to ∼5%. Furthermore, magnetic susceptibility measurements of the present Cu2IrO3 sample revealed a weak ferromagnetic-like anomaly with hysteresis at a magnetic transition temperature of ∼70 K. Our research indicates that the spin-disordered ground state reported in chemically disordered Cu2IrO3 is an extrinsic phenomenon, rather than an intrinsic one, underscoring the pivotal role of synthetic chemistry in understanding the application of Kitaev model to realistic materials.

Prediction of carbon-based metal-free compounds with antiferromagnetism and superhardness

Magnetic superhard materials are still scarce, although they are very important to magnetic devices in extreme environment. Here, we predict a series of antiferromagnetic carbon-based compounds through stacking diamane and 2D C4N3. The result exhibits that all the ground states of these compounds display antiferromagnetic ordering and superhard nature. The antiferromagnetism originates from the unpaired electrons of the low-coordinated C atoms at the interface. Interestingly, their hardness, magnetic moment, dynamic and mechanical stabilities are independent of the stacking sequence of the two precursors. The Vickers hardness is ∼70 GPa given by Chen model, the magnetic moment is ∼0.44μB. Notably, the magnetic moment is insensitive to the external pressure, whereas the magnetic transition temperature would increase with increasing the external pressure. On the contrary, the electronic property is strongly dependent on the stacking sequence, resulting in energy gaps ranging from 1.00 to 1.30 eV, despite all behaving as semiconductors. This work opens a potential way to design magnetic carbon-based materials with superhard feature.

Enhanced magnetic property and magnetodielectric coupling effect in multiferroic Gd2BaCuO5 via Zn2+ doping

The multiferroics with remarkable magnetoelectric (ME) coupling and technological application have drawn significant attention. Here, we report a chemical doping tunable effect on the magnetic, ferroelectric, and dielectric properties in type-II multiferroic Gd2BaCuO5. The magnetic measurement results show that a long-range antiferromagnetic ordering forms at the Néel temperature (TN1) of ∼12.1 K, and a second magnetization anomaly occurs at a lock-in transition temperature (TN2) of ∼6.4 K. Sharp peaks in the pyroelectric and dielectric curves are observed at the magnetic transition temperatures, indicating intrinsic coupling between magnetic and ferroelectric orderings. The doping of Zn2+ results in a gradual enhancement of magnetization due to the increase of uncompensated spins, and large magnetodielectric (MD) coupling parameter up to ∼7.65 % is obtained in Gd2BaCu0.7Zn0.3O5 because of strong spin-phonon coupling. The ferroelectric property is weakened in Zn2+ doped samples, suggesting the critical role of the 4f-3d magnetic interactions in determining the ME coupling effect. Moreover, an effective manipulation of electric field on magnetization is observed in Gd2BaCuO5. This work helps understand the importance of 4f-3d magnetic interactions in inducing ferroelectricity and explore the ME and MD coupling effects with multiple magnetic orderings.

Structural and magnetic transformations in the system YBaCo4O7+X (x = 0, 0.1, 0.2)

The modification of magnetic and elastic properties of YBaCo4O7+x (x = 0, 0.1) cobaltite series at a small controlled deviation x = 0, 0.1, 0.2 from stoichiometry has been investigated. The non-stoichiometric cobaltites with x > 0.1 are found to be separated into two structural phases with different values of x under certain synthesis and heat-treatment conditions of the YBaCo4O7+x system (x = 0, 0.1, 0.2). A non-trivial magnetic behavior is observed in stoichiometric YBaCo4O7. Magnetic moment ΔM = MFC–MZFC induced by an external magnetic field (an analog of thermo-residual magnetization) exhibits anomalies at magnetic phase transition temperatures TN1 and TN2 that coincide with Young’s modulus anomalies. Conversely, in the magnetic susceptibility curves taken in the FC and ZFC modes, phase transitions are not discerned. When x deviates from stoichiometry, the ratio between two magnetic ions Co3+/Co2+ changes, the induced moment ΔM increases, and a residual ferromagnetic moment of ∼ 10-2μB appears in the magnetization curves at low temperatures. At a slight deviation from the oxygen stoichiometry in the samples, the structural distortion disappears, and the anomalies in the elastic characteristics near the TN region rapidly diffuse and vanish. It has been found that when YBaCo4O7+x cobaltites deviate from stoichiometry, the evolution of their magnetic properties is similar to that observed at the transition from Y-based to Ca-based cobaltite. Two scenarios of the cobalt subsystem magnetic behavior with an increasing fraction of cobalt Co3+ ions have been discussed.

Structure, magnetism and magnetocaloric properties in performance GdClWO4 compound

Rare earth elements possessing a diverse range of optical, electrical, and magnetic properties has attracted more attention. In the present work, GdClWO4 compound with monoclinic structure was prepared, the structure, magnetism and magnetocaloric effect (MCE) were investigated. The magnetic phase transition temperature (TN), magnetic ground state and maximum magnetic entropy change (−ΔSMmax) were derived from magnetization measurements. Both first principles calculation and experimental results suggest that the compound displays antiferromagnetic (AFM) interactions, with a magnetic transition occurring at 0.6 K, shifting from a paramagnetic (PM) state to an AFM state. The −ΔSMmax is up to 35.2 J kg−1 K−1 (205.9 mJ cm−3 K−1) for GdClWO4 at T = 1.3 K under a magnetic field change of 0–50 kOe. In particular, the volumetric magnetic entropy change still remains 83.1 mJ cm−3 K−1 at 0.5 K and μ0ΔH = 20 kOe, which is almost twice as much as that of commercial refrigerant Gd3Ga5O12 (GGG) under μ0ΔH = 20 kOe, establishing GdClWO4 compound as a potential candidate for cryogenic refrigeration applications.

Electrostatic doping tunable magnetic transition and half-metallicity in the monolayer CrCTe3

The electrical manipulation of the magnetic transition and spin-polarized states has attracted extensive attention in the field of spintronics. In this work, we perform a detailed study on the electronic and magnetic properties of the carrier-doped monolayer CrCTe3 by using first-principles calculation. It is found that, the magnetic transition from Néel-antiferomagnetic (nAFM) to ferromagnetic (FM) is observed in the case of the electron doping, while for hole doping a magnetic transition sequence of nAFM → zigzag-AFM → FM is observed in the monolayer CrCTe3. Interestingly, the carrier doping induced FM ground state always exhibits half-metallicity with full spin polarization. Moreover, the spin polarity of the half-metallic electronic states is opposite for electron and hole doping, meaning that the spin polarization direction can be tuned by manipulating a gate voltage. The Monte Carlo calculations show that the magnetic transition temperature of the doped FM CrCTe3 is rapidly increased with the increasing doping concentration and is extremely expected to achieve room temperature at a suitable doping concentration. These findings demonstrate that the monolayer AFM system possesses a potential application in spintronic devices with electrically tunable spin polarization.

Effect of Nd-M (M = Al/Co) alloys addition on microstructures and magnetic properties of hot deformed CeFeB magnets

In this study, the magnetic properties, thermal stabilities, elemental distributions and phase transition temperatures of hot deformed CeFeB magnets were investigated by adding 20 wt% of Nd85Al15 and Nd64Co36 alloys, respectively. The results revealed that the addition of low melting point alloys efficiently enhanced the comprehensive magnetic performance and thermal stability of the CeFeB magnets. The optimal magnetic properties were obtained with the addition of 20 wt% Nd85Al15. Compared to the original sample, the coercivity increased from 104 kA/m to 604 kA/m, and the maximum magnetic energy product increased from 8 kJ/m3 to 34 kJ/m3. The incorporation of Nd-Al/Co alloys effectively reduced the area fraction of the coarse grain region and facilitated a more uniform distribution of the RE-rich phase. TEM observations demonstrated that Nd partially replaced Ce, leading to the formation of (Nd, Ce)2Fe14B hard magnetic phase. Additionally, the analysis of the M–T curves indicated the presence of two Curie temperature points for Nd85Al15 and Nd64Co36 added CeFeB hot deformed magnets. The addition of Nd85Al15 improved the fluidity of the RE-rich phase, thereby enhancing the coercivity. Conversely, the incorporation of Nd64Co36 resulted in the entry of Co elements into the main phase, contributing to improved thermal stability.

Large anomalous Hall conductivity induced by spin chirality fluctuation in an ultraclean frustrated antiferromagnet PdCrO2

Magnetic frustration, realized in the special geometrical arrangement of localized spins, often promotes topologically nontrivial spin textures in the real space and induces significantly large unconventional Hall responses. This spin Berry curvature effect in itinerant frustrated magnets mainly works with a static spin order, limiting the effective temperature range below the magnetic transition temperature and yielding the typical anomalous Hall conductivity below ~ 103 Ω−1cm−1. Here we show that an ultraclean triangular-lattice antiferromagnet PdCrO2 exhibits a large anomalous Hall conductivity up to ~ 106 Ω−1cm−1 in the paramagnetic state, which is maintained far above the Neel temperature (TN) up to ~ 4TN. The reported enhancement of anomalous Hall response above TN is attributed to the skew scattering of highly mobile Pd electrons to fluctuating but locally-correlated Cr spins with a finite spin chirality. Our findings point at an alternative route to realizing high-temperature giant anomalous Hall responses, exploiting magnetic frustration in the ultraclean regime.

Synthesis, Crystal Structure, and Optical and Magnetic Properties of the New Quaternary Erbium Telluride EuErCuTe3: Experiment and Calculation.

This paper reports for the first time on a new layered magnetic heterometallic erbium telluride EuErCuTe3. Single crystals of the compound were obtained from the elements at 1120 K using CsI as a flux. The crystal structure of EuErCuTe3 was solved in the space group Cmcm (a = 4.3086(3) Å, b = 14.3093(9) Å, and c = 11.1957(7) Å) with the KZrCuS3 structure type. In the orthorhombic structure of erbium telluride, distorted octahedra ([ErTe6]9-) form two-dimensional layers (Er(Te1)2/2e(Te2)4/2k-)∞2, while distorted tetrahedra ([CuTe4]7-) form one-dimensionally connected substructures (Cu(Te1)2/2e(Te2)2/1t5-∞1) along the [100] direction. The distorted octahedra and tetrahedra form parallel two-dimensional layers (CuErTe32-∞2) between which Eu2+ ions are located in a trigonal-prismatic coordination environment (EuTe610-). The trigonal prisms are connected by faces, forming chains (Eu(Te1)2/2(Te2)4/22-∞1) along the [100] direction. Regularities in the variations in structural parameters were established in the series of erbium chalcogenides (EuErCuCh3 with Ch = S, Se, and Te) and tellurides (EuLnCuTe3 with Ln = Gd, Er, and Lu). Ab-initio calculations of the crystal structure, phonon spectrum, and elastic properties of the compound EuErCuTe3 were performed. The types and wavenumbers of fundamental modes were determined, and the involvement of ions in the IR and Raman modes was assessed. The experimental Raman spectra were interpreted. The telluride EuErCuTe3 at temperatures below 4.2 K was ferrimagnetic, as were the sulfide and selenide derivatives (EuErCuCh3 with Ch = S and Se). Its experimental magnetic characteristics were close to the calculated ones. The decrease in the magnetic phase transition temperature in the series of the erbium chalcogenides was discovered.

The arising of ferromagnetism in Al-doped Mn2(Ga1−xAlx)C MAX phases

The magnetic properties of ordered MAX phases Mn2(AlxGa1−x)C (x = 0.125, 0.25, 0.5, 0.75 and 0.875) have been studied within the DFT-GGA. We have found that increase of Al atom at A-site leads to the formation of the ferromagnetic phase with large magnetization of about 3.6 μB/f.u. The investigation of the phase stability is performed by comparing the total energy of the MAX phases with that of a set of competitive phases for calculation of the phase formation enthalpy. Up to a concentration of Al atoms x = 0.7 the compound remains thermodynamically stable. The exchange constants analysis shows the crucial role of exchange interactions between manganese atoms along the c-axis in forming of ferromagnetism. The magnetic transition temperature of Mn2(AlxGa1−x)C alloys increases with increase of the aluminum concentration.

Quantum Anomalous Hall Effect with Tunable Chern Numbers in High-Temperature 1T-PrN2 Monolayer

Quantum anomalous Hall (QAH) insulators have highly potential applications in spintronic device. However, available candidates with tunable Chern numbers and high working temperature are quite rare. Here, we predict a 1T-PrN2 monolayer as a stable QAH insulator with high magnetic transition temperature of above 600 K and tunable high Chern numbers of C = ±3 from first-principles calculations. Without spin-orbit coupling (SOC), the 1T-PrN2 monolayer is predicted to be a p-state Dirac half metal with high Fermi velocity. Rich topological phases depending on magnetization directions can be found when the SOC is considered. The QAH effect with periodical changes of Chern number (±1) can be produced when the magnetic moment breaks all twofold rotational symmetries in the xy plane. The critical state can be identified as Weyl half semimetals. When the magnetization direction is parallel to the z-axis, the system exhibits high Chern number QAH effect with C = ±3. Our work provides a new material for exploring novel QAH effect and developing high-performance topological devices.

Mutual control of electric and magnetic orders near room temperature in Al doped Y-type hexaferrite single crystals

Realizing robust magnetoelectric (ME) coupling effect near room temperature is still a long-standing challenge for the application of multiferroic materials in next-generation low-power spintronic and memory devices. Here we report a systematic study on the magnetic, dielectric, and ME coupling properties of Y-type hexaferrite Ba0.5Sr1.5Co2Fe12–xAlxO22 (x = 0.0, 0.5, 1.0) single crystals. The Al doping can induce the shifting of the alternating longitudinal conical (ALC)-proper screw (PS) magnetic phase transition temperature from 200 K for x = 0–365 K for x = 1.0. The most interesting feature is that the Ba0.5Sr1.5Co2Fe11AlO22 single crystal displays a direct and converse ME coupling coefficient with αH ∼3,100 ps/m and αE ∼3,900 ps/m at 250 K, respectively, due to the Al-doped enhanced stability of ALC phase. Moreover, the exchange bias also verifies the strong coupling of electric and magnetic orders. These results provide a valuable insight on the modulation of ALC structure and the mechanism of ME effect in Y-type hexaferrites.

The varying nature of the martensitic phase transition and associated entropy changes in a Co and Cr doped pseudo-ternary Heusler alloy system

We have investigated the magnetostructural phase transitions and the associated entropy changes in a system of three Heusler alloys, namely: Ni2Mn0.55Cr0.45Ga, Ni2Mn0.55Co0.10Cr0.35Ga, and Ni2.15Mn0.75Cr0.1Ga. All three alloys exhibited first-order martensitic phase transition. An inter-martensitic phase transition raised the magnetic and structural phase transition temperatures, and partially decoupled the said transitions. Furthermore, field magnetization measurements demonstrated that a significant reduction in magnetic hysteresis is achieved in the Ni2Mn0.55Co0.10Cr0.35Ga compound, resulting in improved effective refrigerant capacity values. The largest magnetic entropy change of −23.5 J/kg K and refrigeration capacity of 97.7 J/kg were observed in Ni2.15Mn0.75Cr0.1Ga. This observation of mitigated deficiencies in materials experiencing a first-order phase transition is relevant towards their application in magnetic refrigeration.

Inverse Magnetocaloric Effect in Altermagnetic 2D Non‐van der Waals FeX (X = S and Se) Semiconductors

AbstractThe concept of altermagnet (AM), which is neither ferromagnet nor antiferromagnet, has recently been proposed as compensated collinear magnet. Among 2D non‐van der Waals crystals, one of the thinnest altermagnets is identified and their interesting physical phenomenon at 2D limit is investigated. According to symmetry analysis and first‐principles calculations, 2D FeX (X = S, Se) sheets with one‐unit‐cell thickness are intrinsic altermagnetic semiconductors with observable magnitude of spin splitting (103−193 meV), high magnetic transition temperature (TAM = 400 and 190 K), large out‐of‐plane magnetic anisotropy energy (0.24 and 0.74 meV per atom) and considerable metal‐like anomalous Hall conductivity (143 and 249 Ω−1 cm−1). Moreover, the spin lattices of these 2D FeX ultrathin films are formed by parallel diamond‐like chains with spin frustrated interaction through spin exchange along the b direction. Consequently, an inverse magnetocaloric effect with positive ∆Sm values of 1.7–8.2 mJ kg−1 K−1 near TAM is observed. These results enrich the database of altermagnets and provide deep insights into the spin frustration effect in 2D magnets, which paves the avenue for the next‐generation spintronics.

Nonlinear Hall effect and scaling law in Sb-doped topological insulator MnBi4Te7

The nonlinear Hall effect (NLHE), as a new member of Hall effect family, has been realized in many materials, attracting a great deal of attention. Here, we report the observation of NLHE in magnetic topological insulator Sb-doped MnBi4Te7 flakes. The NLHE generation efficiency can reach up to 0.06 V−1, which is comparable to that observed in MnBi2Te4. Differently, the NLHE can survive up to 200 K, much larger than the magnetic transition temperature. We further study the scaling behavior of the NLHE with longitudinal conductivity. The linear relationship with opposite slope when temperature is below and above the magnetic transition temperature is uncovered. It reveals that the NLHE originates from skew scattering. Our work provides a platform to search NLHE with larger generation efficiency at higher temperatures.

Investigation of Magnetocaloric Effect and Critical Field Analysis of Nd0.7−xLaxSr0.3MnO3 (x = 0.0−0.3) Manganites

This research explored the temperature dependence magnetization, magnetocaloric effect, and critical field analysis of Nd0.7−xLaxSr0.3MnO3 (x = 0.0, 0.1, 0.2 and 0.3) manganites samples synthesized through solid state reaction route. The substitution of La3+ at Nd3+ site enhances the A-site average ionic radius ( rA ) from 1.321 Å (for x = 0.0) to 1.348 Å (for x = 0.3). An increase in rA reduced the internal chemical pressure inside the lattice such that the Mn–O–Mn bond angle approached 180°. In addition, an increase in Mn–O–Mn bond angle supresses the spin-lattice interaction, which consequently reduces the change in maximum magnetic entropy (ΔSMmax) at the magnetic transition temperature. This implies that variation of A-site average ionic radius influences the change in maximum magnetic entropy. The critical behavior of the manganite samples was addressed based on the data of magnetization measurements around the transition temperature TC. Various techniques such as modified Arrott plot, a Kouvel–Fisher method, and critical isotherm analysis were used to measure the values of the ferromagnetic transition temperature TC, as well as the critical exponents of β, γ, and δ. The magnetic order parameter n is very well obeyed with scaling relation with change in magnetic entropy.