Reduced Curie Temperature Research Articles

Valence-state mixing and reduced magnetic moment in Fe3−δGeTe2 single crystals with varying Fe content probed by x-ray spectroscopy

We present a spectroscopic study of the magnetic properties of Fe3−δGeTe2 single crystals with varying Fe content, achieved by tuning the stoichiometry of the crystals. We carried out x-ray absorption spectroscopy and analyzed the x-ray circular magnetic dichroism spectra using the sum rules, to determine the orbital and spin magnetic moments of the materials. We find a clear reduction of the spin and orbital magnetic moment with increasing Fe deficiency. Magnetic susceptibility measurements show that the reduction in magnetization is accompanied by a reduced Curie temperature. Multiplet calculations reveal that the Fe2+ state increasingly mixes with a higher valence state when the Fe deficiency is increased. This effect is correlated with the weakening of the magnetic moment. As single crystals are the base material for exfoliation processes, our results are relevant for the assembly of 2D magnetic heterostructures.

Impact of dimensionality on the magnetocaloric effect in two-dimensional magnets

Magnetocaloric materials, which exploit reversible temperature changes induced by magnetic field variations, are promising for advancing energy-efficient cooling technologies. The potential integration of two-dimensional materials into magnetocaloric systems represents an emerging opportunity to enhance the magnetocaloric cooling efficiency. In this study, we use atomistic spin dynamics simulations based on first-principles parameters to systematically evaluate how magnetocaloric properties transition from three-dimensional (3D) to two-dimensional (2D) ferromagnetic materials. We find that 2D features such as reduced Curie temperature, sharper magnetic transition, and higher magnetic susceptibility are beneficial for magnetocaloric applications, while the relatively higher lattice heat capacity in 2D can compromise achievable adiabatic temperature changes. We further propose GdSi2 as a promising 2D magnetocaloric material. Our calculation predicts that GdSi2 exhibits an isothermal entropy change ΔSM of 22.5 J kg−1 K−1 and an adiabatic temperature change ΔTad of 6.2 K, near the hydrogen liquefaction temperature (TC≈25 K). Our analysis offers valuable theoretical insights into the magnetocaloric effect in 2D ferromagnets and demonstrates that 2D ferromagnets hold promise for cooling and thermal management applications in compact and miniaturized nanodevices.

Uncovering the universality of critical phenomenon in a ferromagnet Gd5Si4 prepared under high pressure

Manipulation of phase transition is of great interest not only because it can enhance the application ability of accompanying physical effects, but also because it allows us to uncover the physical connotations near the critical temperature point. In this work, we systematically examine the influence of high-pressure annealing (HPA) on critical phenomena in Gd5Si4 ferromagnet. A second-order magnetic phase transition is determined in the annealed sample under 3 GPa with a reduced Curie temperature. Various techniques including modified Arrott plot, Kouvel-Fisher method, and critical isotherm analysis are adopted to calculate critical exponents β, γ, and δ of HPA compound. Using the obtained parameters, we employ the renormalization group theory to calculate the spin exchange range, which satisfies J(r) ∼ r−4.454, exhibiting a longer-range-ordered ferromagnetic interaction. Our work indicates that HPA can manipulate the degree of order inside matter with a novel universality class, thus acquiring desired functionality through reasonable HPA treatment.

Aluminium substituted yttrium iron garnet thin films with reduced Curie temperature

Aluminium substituted yttrium iron garnet thin films with reduced Curie temperature

Simultaneously Achieving Large Piezoelectricity and Low Dielectric Loss in High-Temperature BiScO3-PbTiO3-Based Ceramics.

High-temperature piezoelectric materials are pivotal to technology applications fields including defense, aerospace, nuclear energy, and oil well logging. However, the acquisition of excellent piezoelectric properties is usually at the cost of temperature stability (reduced Curie temperature and increased high-temperature dielectric loss), which hinders the application of piezoelectric ceramics in harsh environments. In this study, we investigated the effect of Nb5+ donor and Mn2+/3+ acceptor doping on the dielectric and piezoelectric properties of BiScO3-PbTiO3 (BS-PT)-based ceramics. In contrast to the acceptor doping, it was found that the donor doping not only enhances the piezoelectric properties but also effectively suppresses the dielectric loss at a high temperature by reducing the oxygen vacancy concentration. Eventually, we simultaneously attained an excellent piezoelectric performance (d33 is 553 pC/N at room temperature and 1528 pC/N at 400 °C, respectively) and a low dielectric loss (less than 2% in the temperature range of 150-300 °C) but still with a high Curie temperature (TC ∼ 445 °C) in Nb5+-doped BS-PT ceramics. Furthermore, different in situ measurements were used to demonstrate the remarkable temperature stability up to a high depolarization temperature of ∼400 °C. This work represents significant progress in high-temperature piezoelectric materials and provides a guideline for future efforts on enhancing the piezoelectricity and suppressing the dielectric loss at high temperature.

Close-to-ideal spin polarization in zinc-doped Fe–Mo double perovskites at the nanoscale

A high degree of spin polarization in half-metallic double perovskites is a prerequisite for several applications in spintronics, which depends crucially on the cationic order of the systems. This paper reports a study on tailoring the structure and morphology of nano-sized Sr2Fe1-xZnxMoO6 (x = 0.05, 0.1, 0.15) materials to improve their spin polarization. The combined analysis of synchrotron X-ray diffraction and magnetization data shows that Zn replaces Fe in the B sites. Although the majority of particles have lateral dimensions in the range 30–60 nm as observed by scanning electron microscope, the samples with x = 0.1 and 0.15 show finite-size effects with superparamagnetism below room temperature and a reduced Curie temperature (from 410 K for x = 0.05–390 K for x = 0.15). The results are due to the formation of networks of insulating Mo–O–Zn–O–Mo linkages and anti-phase boundaries, which divide the particles into smaller domains with a mean diameter of ∼11 nm as determined via a Langevin fit. The almost perfectly ordered structure in the nanodomains is responsible for a high magnetoresistance ratio. A value of -42% at 5 K in 50 kOe is recorded for the sample x = 0.15. Via fitting the magnetoresistance curve using the Inoue-Mekagawa theory, the spin polarization of 99% is determined.

Effect of Fe substitution on temperature coefficient of resistance and magnetoresistance of La0.67Ca0.33MnO3 polycrystalline ceramics

La0.67Ca0.33MnO3 perovskite manganate exhibits high temperature coefficient of resistance (TCR) and large magnetoresistance (MR) effect, these enable novel multifunctionalities. Mn site substitution can change magnetic order of the manganates, thereby tailoring both electrical and magnetic transport. Herein, La0.67Ca0.33Mn1-xFexO3(0 ≤ x ≤ 0.06) polycrystalline ceramics were prepared by sol-gel route. The effect of Fe substitution on TCR and MR of La0.67Ca0.33MnO3 was studied. Fe replacing Mn ions, would weaken double exchange, and significantly reduced Curie temperature and ferromagnetism. Additionally, Fe doping promoted the development of grain. TCR increased first with Fe content x and then decreased, and reached 45.2%·K-1 at x = 0.01, which is the highest value reported. Notably, with Fe doping, MR gradually increased and reached 81.1% (1 T) at x = 0.06. Fe doping can significantly enhance TCR and MR, which generates promising potential in (uncooled) thermistor/infrared detecting and magnetic sensors.

ON THE TRANSITION BETWEEN FERROMAGNETIC AND PARAMAGNETIC STATES IN MESOPOROUS MATERIALS WITH FRACTAL MORPHOLOGY

In this paper, we have shown how the presence of pores and pore morphology influence on magnetic phase transition temperatures in mesoporous ferromagnetic materials. Model calculations have demonstrated the possibility to obtain macroscopic mesoporous samples with notably reduced Curie temperatures which is also further depressed in the case the pore morphology is more complicated. The results have been obtained on the basis of the experimentally verified correlation between the Curie temperature and cohesive energy of the material and illustrated using the examples of pure mesoporous iron, nickel and cobalt while pore morphology has been determined by the methods of fractal geometry. Several practical applications of mesoporous materials with tuned values of the Curie temperature have also been discussed in the final section.

Composition driven (Ba,Ca)(Zr,Ti)O3 lead‐free ceramics with large quality factor and energy harvesting characteristics

AbstractA comprehensive study on energy harvesting characteristics as well as electro‐mechanical properties of lead‐free (1−x)(BaZr0.2Ti0.8)O3—x(Ba0.7Ca0.3Ti)O3 ceramics were systematically carried out. Raman and Rietveld analyses show a formation of rhombohedral‐orthorhombic‐tetragonal (R‐O‐T) phase boundary region between 4/6 BZT/BCT and 6/4 BZT/BCT compositional range. Raman modes shift toward lower frequencies with increased Zr/Ca stoichiometric ratio attributed to asymmetric Ti‐O phonon vibrations, which caused local disorder, widening of energy band and reduced Curie temperature. The large mechanical quality factor Qm = 556 is related to the hardening effect and significantly high energy conversion efficiency η = 96% was discovered for 3/7 BZT/BCT composition. Largely, the noblest electro‐mechanical properties were retrieved for 5/5 BZT/BCT ceramics, in which d33 = 500 pC/N (from quasi‐static d33 meter), d33 = 540 pm/V (from field‐dependent d33 curves) indicating that the both methods are analogous with a deviation of 8%. The outstanding energy harvesting characteristics such as voltage constant g33 = 27 × 10−3 Vm/N, transduction coefficient d33 × g33 = 13 301 × 10−15 m2/N, figure of merit under off‐resonance conditions FOMoff = 12.1 × 10−10 m2/N and fairly large η of 94% were attained again for 5/5 BZT/BCT ceramics. These outstanding characteristics were ascribed to the R‐O‐T phase boundary region that comprises a low energy barrier, consequently facilitated easy polarization rotation and triggered an increased electro‐mechanical conversion. These characteristics outperform other lead‐free and even most commercially available lead‐based ceramics, and thus suitable for sensors, actuators, resonators, and energy harvesting applications.

Изменение температуры Кюри в пористом материале

In this paper, the dependence of magnetic transitions temperatures of nanoporous materials on pore geometric characteristics (volume and shape) have been studied in the framework of the cohesive energy-based model in the case of pure nanoporous nickel and cobalt. Pore geometric characteristics have been taken into account using two parameters: its effective diameter and shape coefficient of a pore which characterizes the degree of deviation of its shape from the spherical one. The estimates allow expecting the possibility to obtain nanoporous macroscopic objects of ferromagnetic materials with a reduced Curie temperature which value also decreases with “complicating” the pore shape.

Monte Carlo simulations of the magnetocaloric effect in PrSi-like compound

Magnetic properties and magnetocaloric effect of the praseodymium–silicon (PrSi)-like compound are studied using Monte Carlo simulation. Thermal magnetization, dM/dT, normalized magnetic entropy and the normalized specific heat of PrSi are given. The temperature dependence of the normalized magnetic entropy and the normalized adiabatic temperature change are obtained. The Curie temperature of PrSi is obtained. The field dependence of normalized relative cooling power of PrSi has been determined. The magnetic hysteresis cycle of PrSi is obtained. Large magnetic entropy change, ΔSm, the relative cooling power and characteristics of giant magnetocaloric effect are achieved near the reduced Curie temperature.

Local spin moments, valency, and long-range magnetic order in monocrystalline and ultrathin films of Y3Fe5O12 garnet

We investigate and compare the electronic structure of a bulk single crystal of ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ garnet [YIG, a high-${T}_{C}$ (= 560 K) ferrimagnet] with that of an epitaxial ultrathin (3.3 nm) film of YIG with a reduced ferrimagnetic temperature ${T}_{C}=380\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, using bulk-sensitive hard x-ray photoelectron spectroscopy (HAXPES), x-ray absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD). The Fe $2p$ HAXPES spectrum of the bulk single crystal exhibits a purely trivalent ${\mathrm{Fe}}^{3+}$ state for octahedral and tetrahedral sites. The Fe $3s$ spectrum shows a clear splitting which allows us to estimate the on-site Fe $3s\text{\ensuremath{-}}3d$ exchange interaction energy. The valence band HAXPES spectrum shows Fe $3d$, O $2p$, and Fe $4s$ derived features and a band gap of $\ensuremath{\sim}2.3\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ in the occupied density of states, consistent with the known optical band gap of $\ensuremath{\sim}2.7\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. Fe $L$-edge XAS identifies the octahedral ${\mathrm{Fe}}^{3+}$ and tetrahedral ${\mathrm{Fe}}^{3+}$ site features. XMCD spectra at the Fe ${L}_{2,3}$ edges show that bulk single-crystal YIG exhibits antiferromagnetic coupling between the octahedral- and tetrahedral-site spins. The calculated Fe $2p$ HAXPES, Fe $L$-edge XAS, and XMCD spectra using full multiplet cluster calculations match well with the experimental results and confirm the full local spin moments. In contrast, HAXPES, XAS, and XMCD of the Pt/YIG (3.3 nm) ultrathin epitaxial film grown by a pulsed laser deposition method show a finite ${\mathrm{Fe}}^{2+}$ contribution and a reduced ${\mathrm{Fe}}^{3+}$ local spin moment. The ${\mathrm{Fe}}^{2+}$ state is attributed to a combination of oxygen deficiency and charge transfer effects from the Pt capping layer to the ultrathin film. However, the conserved XMCD spectral shape for the ultrathin film indicates that the 3.3-nm epitaxial film is genuinely ferrimagnetic, in contrast to recent studies on films grown by radio-frequency magnetron sputtering which have shown a magnetic dead layer of $\ensuremath{\sim}6$ nm. The presence of ${\mathrm{Fe}}^{2+}$ and the reduced local spin moment in the epitaxial ultrathin film lead to a reduced Curie temperature, quantitatively consistent with well-known mean-field theory. The results establish a coupling of the local Fe spin moments, valency, and long-range magnetic ordering temperature in bulk single crystal and epitaxial ultrathin-film YIG.

Pressure-enhanced magnetocaloric effects in Mn2Sb1-xSnx system with uniaxial magnetocrystalline anisotropy

The magnetocaloric effects in Sn-substituted Mn2Sb1-xSnx system (x = 0.1, 0.2, 0.3) have been studied. With more Sn content, metamagnetic phase transition temperature (Tt) is increased with gradually reduced Curie temperature, thermal hysteresis and the maximum of magnetic entropy change (ΔSmax). By hydrostatic pressure of 2.5 kbar, Tt is lowered but ΔSmax is enhanced from 4.2 JKg−1K−1 to ∼5.5 Jkg−1K−1 in Mn2Sb1-xSnx (x = 0.1, 0.2) ingots. The uniaxial magnetocrystalline anisotropy is found in all Mn2Sb1-xSnx ingots and abnormally increased with elevated temperature, leading to the abnormal coercivity-temperature dependence in the corresponding ribbons. The microstructure of the ribbons is sensitively dependent on Sn composition. Our work demonstrates that high ΔSmax tuned by hydrostatic pressure and high magnetocrystalline anisotropy can be realized together in Mn2Sb1-xSnx system, which may help the development of such bifunctional alloy exhibiting refrigerating capability as well as the hard magnetic properties simultaneously.

Site-Resolved Contributions to the Magnetic-Anisotropy Energy and Complex Spin Structure of Fe/MgO Sandwiches

Fe/MgO-based Magnetic Tunnel Junctions (MTJs) are among the most promising candidates for spintronic devices due to their high thermal stability and high tunneling magnetoresistance. Despite its apparent simplicity, the nature of the interactions between the Fe and MgO layers leads to complex finite size effects and temperature dependent magnetic properties which must be carefully controlled for practical applications. In this letter, we investigate the electronic, structural and magnetic properties of MgO/Fe/MgO sandwiches using first principles calculations and atomistic spin modeling based on a fully parameterized spin Hamiltonian. We find a large contribution to the effective interfacial magnetic anisotropy from the two-ion exchange energy. Minimization of the total energy using atomistic simulations shows a surprising spin spiral ground state structure at the interface owing to frustrated ferromagnetic and antiferromagnetic interactions, leading to a reduced Curie temperature and strong layer-wise temperature dependence of the magnetization. The different temperature dependences of the interface and bulk-like layers results in an unexpected non-monotonic temperature variation of the effective magnetic anisotropy energy and temperature-induced spin-reorientation transition to an in-plane magnetization at low temperatures. Our results demonstrate the intrinsic physical complexity of the pure Fe/MgO interface and the role of elevated temperatures providing new insight when interpreting experimental data of nanoscale MTJs.

Effect of Fe Substitution on the Physical Properties of La0.62Er0.05Ba0.33Mn1\u2212xFexO3(x =\u20090.00, 0.05 and 0.15)

Morphology, magnetic and magnetocaloric properties of La0.62Er0.05Ba0.33Mn1−xFexO3(x = 0.00, 0.05 and 0.15) were experimentally investigated. Solid-state reaction method was used in the preparation of the samples. The microstructure of the samples was determined by scanning electron microscopy (SEM). From the XRD study, it has been found that all samples are single-phased and crystallized in the rhombohedral structure with the R3-c space group. For x = 0.05 and 0.15, a steep drop of zero field-cooled (ZFC) magnetization at low temperature signifies the formation of cluster- or spin-glass state. This is caused by the competition between the ferromagnetic and antiferromagnetic interaction. A sensitive response to substituting Fe for Mn is observed in the magnetic and magnetocaloric properties. We found that Fe doping is powerful enough to reduce Curie temperature TC and it brings about cluster glass behaviours. The magnetocaloric effect is calculated from the measurement of initial isothermal magnetization versus magnetic field at various temperatures. The maximum entropy change left | {{Delta } S_{M}^{max } } right | reaches the highest values of 3.31, 3.12, and 2.57 J/kg K in magnetic field. However, the relative cooling power decreases with Fe content from x = 227.44 to 188.68 J/kg for x = 0.00 to x = 0.15 compositions, respectively.

Decrement of magnetic spin effect in films of polymer composites with rubrene: Interaction of excited states with magnetic nanoparticles

For films of poly(alkane etherimide) composites containing rubrene microcrystals, the effect of Cu–Ni magnetic nanoparticles (MNP) with a reduced Curie temperature T C (40–60°C) on luminescence and photoconductivity, as well as on the magnetic spin effect, has been observed and studied. It has been shown that this effect depends on the excitation intensity and weak heating (up to T < T C°C). The assumption has been made that the excited states of the microcrystals (or charge carriers) interact with the MNP surface, leading to a change in their magnetic characteristics.

Surface morphology driven non-uniform magnetism in epitaxial BiMnO3

Thin films of BiMnO3 were grown on (001) SrTiO3 substrates by fine tuning the growth conditions. The films are epitaxial and impurity-free, but show island growth mode. From detailed magnetization measurements and surface analysis, we found evidence of morphology driven non-uniform magnetism. The non-uniform magnetism resulted in reduced saturation magnetization, reduced Curie temperature (TC), and nonlinear Arrott plots. In spite of non-uniform magnetism, the temperature dependence of magnetization showed conventional mean field behavior near TC. Our data suggest that non-uniform strain leads to coexistence of separate ferromagnetic and ferroelectric regions in BiMnO3 films which is not observed in bulk samples.

Composition-dependent magnetic properties of melt-spun La or/and Ce substituted nanocomposite NdFeB alloys

Aiming at high-performance low-cost NdFeB magnets, the magnetic properties and microstructure for melt spun nanocrystalline (Nd1−xMx)10Fe84B6 (M=La, Ce, or La0.5Ce0.5; x=0–0.7) alloys were investigated. Relatively, LaCe-substituted alloys show high values of the remanent magnetization Mr, the maximum energy product (BH)max and the coercivity Hc, up to 114emu/g (1.07T), 147kJ/m3 and 471kA/m, respectively, at x=0.1. The unusual increase in coercivity for the alloys with 10% La or 10% La0.5Ce0.5 substitution is possibly attributed to the phase segregation in alloys with certain La or LaCe contents. The reduced Curie temperature and spin-reorientation temperature were obtained for La, Ce or LaCe substituted alloys. Transmission electron microscope analysis has revealed that a fine and uniform distributed grain structure leads to remanence enhancement for La0.5Ce0.5 substituted alloys. The present results indicate that partially substituting Nd by La or/and Ce cannot only effectively reduce the cost of nanocrystalline NdFeB based magnetic powders but also can maintain a relatively good combination of magnetic properties.

Microstructural and magnetic characterization of ion-beam bombarded [Ni80Fe20-Cr]50 thin films

Ion-beam bombardment during thin film deposition can effectively change the microstructure and thus influence the magnetic properties of the thin film system. In this paper, we studied plain (un-bombarded) and argon ion-beam bombarded multilayered [Ni80Fe20-Cr]50 thin films fabricated using a dual ion-beam deposition technique. The atomic force microscopy images of the bombarded thin films showed a much rougher surface and an increased average grain size. No noticeable exchange bias phenomenon was observed in the thin films deposited with or without bombardment, indicating that the interfacial uncompensated spins were prevented from providing the necessary unidirectional anisotropy due to interfacial intermixing. This significant intermixing between Ni80Fe20 and Cr layers, and the etching process caused by the bombardment, led to the formation of an nano-composite [Ni80Fe20-Cr]50 thin film with a reduced Curie temperature. An enhancement of the coercivity attributed to the stronger NiFe–Cr coupling was also found, which was the result of additional pinning sites created by the bombardment. The bombarded thin film also exhibited a higher ZFC/FC divergence temperature compared to the un-bombarded film, which indicated a much stronger NiFe–Cr coupling and more coordinated alignment of the magnetization of the film's larger crystallites.

Magneto-transport properties of La0.7Sr0.3MnO3–manganese oxide composite films on YSZ substrates prepared using aerosol deposition

We report the magneto-transport properties of pure La0.7Sr0.3MnO3 (LSMO) and 40mol% LSMO–Mn3O4 (LSMM) composite films on YSZ (yttrium stabilized zirconia) substrates prepared using aerosol deposition (AD). As-deposited films via AD were post-annealed at 1100°C and 1200°C for 2h in air. While the as-deposited films commonly exhibited a poor low-field magnetoresistance (LFMR) property with reduced Curie temperatures (Tc), post-annealed films exhibited a very good LFMR property with high Tc values ranging from 355 to 370K. The highest LFMR value of 1.2% in 0.5kOe and an unprecedentedly high (dMR/dH)max value of 158.8%kOe−1 were obtainable at 300K from the LSMM composite film annealed at 1200°C. Compared with a pure LSMO film, the LSMM composite films possess higher LFMR values with higher (dMR/dH)max values, which is primarily attributed to a greatly reduced magnetically disordered region near the LSMO grain boundary because of the chemically compatible Mn3O4 phase.