Ferromagnetic Transition Temperature Research Articles

Non-Fermi-liquid behavior consistent with a preasymptotic disorder-tuned ferromagnetic quantum critical point in the heavy fermion system CeTi1−xVxGe3

An investigation of the thermodynamic and electrical transport properties of the isoelectronic chemical substitution series CeTi1−xVxGe3 (CTVG) single crystals is reported. As x increases, the ferromagnetic (FM) transition temperature is suppressed, reaching absolute zero at the critical concentration x=0.4, where a non-Fermi-liquid low-temperature specific heat and electrical resistivity, as well as the hyperscaling of specific heat and magnetization, are found. Our study suggests the presence of an FM quantum critical point (QCP) in CTVG. The obtained critical exponents indicate that CTVG falls in the preasymptotic region of the disorder-tuned FM QCP predicted by the Belitz-Kirkpatrick-Vojta theory. Published by the American Physical Society 2024

Integrated study of the structural and magnetic characteristics of Nd(Mn, Cr)0.5O3: exploring perovskite material

In this work, a perovskite material, Nd(Mn,Cr)0.5O3, is synthesized by a sol–gel chemical route method, and the structural and magnetic properties of the prepared samples were studied for a range of annealing temperatures. The variation of structural parameters against annealing temperatures is analyzed via Rietveld refinement, and it exhibits orthorhombic cubic geometry with non-monotonically increasing lattice parameters. lattice strain is due to canting in the Cr3+ and Mn3+ ions and the presence of strain is also confirmed by the shifting of XRD peaks towards a higher 2θ angle. The obtained Goldschmidt tolerance factor is 0.87, which confirms the stability of the perovskite structure. The magnetic properties of the prepared samples show the ferromagnetic transition temperature ranging from 86.5 K to 82.2 K due to super-exchange interaction between Cr3+ and Mn3+ ions via oxygen ions. The stability in crystal structure, lower value of strain, and ferromagnetic nature at low temperature for the samples annealed up to1000 °C may make this material applicable for magnetic data storage.

High entropy alloys (FeCoNi)0.75Cr0.25-xCux – thermal stability and physical properties

The paper reports on the phase stability of the (FeCoNi)0.75Cr0.25-xCux HEA system with equimolar ratio of Fe, Co and Ni by differential scanning calorimetry (DSC) and measurements of physicochemical properties: density, electrical resistivity, Seebeck coefficient, thermal conductivity, and magnetic behaviour in a broad temperature region as well as hardness and elastic modulus at room temperature as a function of the gradual substitution of chromium by copper in a series of (FeCoNi)0.75Cr0.25-xCux alloys with different mole fraction of Cu (x = 0, 0.05, 0.1, 0.15 and 0.2). DSC measurements showed that all alloys are thermally stable. Increasing content of Cu was found (i) to increase the formation of a fcc Cu-rich phase, (ii) to strengthen ferromagnetic interactions, resulting in rising ordered magnetic moments, as well as in growing ferromagnetic transition temperatures, and (iii) to distinctly change physical properties like electrical resistivity, thermal expansion, and mechanical properties. Experimental data regarding the phase stability are supported by CALPHAD calculations.

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.

Hysteresis-free reversible magnetocaloric effect and critical behavior of Ni39.56Fe7.27Co2.97Mn40.65Sn9.55 full heusler alloy

Utilizing the magnetocaloric effect (MCE), magnetic refrigeration technology offers an advantage over traditional gas-based refrigerators, which produce harmful gases. Herein, we study the MCE and critical magnetic behavior of Fe and Co substituted Ni39.56Fe7.27Co2.97Mn40.65Sn9.55 Heusler alloy near the paramagnetic (PM) to ferromagnetic (FM) transition temperature. With Fm3̅m space group, the present sample crystallizes in cubic L21 structure. Arrott plots and hysteresis-free continuous phase transition confirm a second-order transition from FM to PM. Again, this second-order transition is verified by collapsing scaled entropy curves for various fields into a single universal curve. The critical exponent values obtained using several analytical techniques, like the Kouvel-Fisher method, modified Arrott plot, and critical isotherm, correspond to the mean-field theory universality class having long-range ordering. The above findings suggest our sample to be a suitable magnetic refrigerant by overcoming the difficulty of obtaining a reversible and high value of magnetic entropy change along with substantial relative cooling power across the second-order phase transition.

Highly Efficient Room-Temperature Nonvolatile Magnetic Switching by Current in Fe3GaTe2 Thin Flakes.

Effectively tuning magnetic state by using current is essential for novel spintronic devices. Magnetic van der Waals (vdW) materials have shown superior properties for the applications of magnetic information storage based on the efficient spin torque effect. However, for most of known vdW ferromagnets, the ferromagnetic transition temperatures lower than room temperature strongly impede their applications and the room-temperature vdW spintronic device with low energy consumption is still a long-sought goal. Here, the highly efficient room-temperature nonvolatile magnetic switching is realized by current in a single-material device based on vdW ferromagnet Fe3GaTe2. Moreover, the switching current density and power dissipation are about 300 and 60000 times smaller than conventional spin-orbit-torque devices of magnet/heavy-metal heterostructures. These findings make an important progress on the applications of magnetic vdW materials in the fields of spintronics and magnetic informationstorage.

Crystal, ferromagnetism, and magnetoresistance with sign reversal in a EuAgP semiconductor

We synthesized the ferromagnetic EuAgP semiconductor and conducted a comprehensive study of its crystalline, magnetic, heat capacity, band gap, and magnetoresistance properties. Our investigation utilized a combination of X-ray diffraction, optical, and PPMS DynaCool measurements. EuAgP adopts a hexagonal structure with the P63/mmc space group. As the temperature decreases, it undergoes a magnetic phase transition from high-temperature paramagnetism to low-temperature ferromagnetism. We determined the ferromagnetic transition temperature to be TC = 16.45(1) K by fitting the measured magnetic susceptibility using a Curie-Weiss law. Heat capacity analysis of EuAgP considered contributions from electrons, phonons, and magnons, revealing η = 0.03 J/(mol·K2), indicative of semiconducting behavior. Additionally, we calculated a band gap of ∼1.324(4) eV based on absorption spectrum measurements. The resistivity versus temperature of EuAgP measured in the absence of an applied magnetic field shows a pronounced peak around TC, which diminishes rapidly with increasing applied magnetic fields, ranging from 1 to 14 T. An intriguing phenomenon emerges in the form of a distinct magnetoresistance transition, shifting from positive (e.g., 1.95% at 300 K and 14 T) to negative (e.g., −30.73% at 14.25 K and 14 T) as the temperature decreases. This behavior could be attributed to spin-disordered scattering.

Interlayer engineering of Fe3GeTe2: From 3D superlattice to 2D monolayer

The discoveries of ferromagnetism down to the atomically thin limit in van der Waals (vdW) crystals by mechanical exfoliation have enriched the family of magnetic thin films [C. Gong et al., Nature 546, 265-269 (2017) and B. Huang et al., Nature 546, 270-273 (2017)]. However, compared to the study of traditional magnetic thin films by physical deposition methods, the toolbox of the vdW crystals based on mechanical exfoliation and transfer suffers from low yield and ambient corrosion problem and now is facing new challenges to study magnetism. For example, the formation of magnetic superlattice is difficult in vdW crystals, which limits the study of the interlayer interaction in vdW crystals [M. Gibertini, M. Koperski, A. F. Morpurgo, K. S. Novoselov, Nat. Nanotechnol. 14, 408-419 (2019)]. Here, we report a strategy of interlayer engineering of the magnetic vdW crystal Fe3GeTe2 (FGT) by intercalating quaternary ammonium cations into the vdW spacing. Both three-dimensional (3D) vdW superlattice and two-dimensional (2D) vdW monolayer can be formed by using this method based on the amount of intercalant. On the one hand, the FGT superlattice shows a strong 3D critical behavior with a decreased coercivity and increased domain wall size, attributed to the co-engineering of the anisotropy, exchange interaction, and electron doping by intercalation. On the other hand, the 2D vdW few layers obtained by over-intercalation are capped with organic molecules from the bulk crystal, which not only enhances the ferromagnetic transition temperature (TC), but also substantially protects the thin samples from degradation, thus allowing the preparation of large-scale FGT ink in ambient environment.

Mapping the phase-separated state in a 2D magnet.

Intrinsic 2D magnets have recently been established as a playground for studies on fundamentals of magnetism, quantum phases, and spintronic applications. The inherent instability at low dimensionality often results in coexistence and/or competition of different magnetic orders. Such instability of magnetic ordering may manifest itself as phase-separated states. In 4f 2D materials, magnetic phase separation is expressed in various experiments; however, the experimental evidence is circumstantial. Here, we employ a high-sensitivity MFM technique to probe the spatial distribution of magnetic states in the paradigmatic 4f 2D ferromagnet EuGe2. Below the ferromagnetic transition temperature, we discover the phase-separated state and follow its evolution with temperature and magnetic field. The characteristic length-scale of magnetic domains amounts to hundreds of nanometers. These observations strongly shape our understanding of the magnetic states in 2D materials at the monolayer limit and contribute to engineering of ultra-compact spintronics.

Magnetic properties of microwave-processed ferromagnetic La2CoMnO6

We synthesized ferromagnetic La2CoMnO6 by irradiating a stoichiometric mixture of oxide powder with microwave (MW) of frequency 2.45 GHz and studied the impact of microwave power (P) on its structural and magnetic properties. The MW power (P) was varied from 1000 to 1600 W at a pre-set sintering temperature of 1200 °C and a dwelling time of 20 min. The ferromagnetic transition temperature and saturation magnetization depend on P. The highest saturation magnetization value of 5.54 μB/f.u. was realized in the sample irradiated with P = 1400 W which is closer to the theoretical value of 6 μB/f.u. for spin-only contributions from Co2+ (S = 3/2) and Mn4+ (S = 3/2). The observed results can be attributed to varying degrees of B-site ordering of Co2+ and Mn4+ ion with the MW power, however, caution has to be taken to ensure that the sample is maintained at a specified temperature for the desired dwelling time. Our results indicate that varying the microwave power at a fixed sintering temperature provides a different approach for manipulating saturation magnetization and Curie temperature.

Near-room temperature ferromagnetism and a tunable anomalous Hall effect in atomically thin Fe4CoGeTe2.

Itinerant ferromagnetism at room temperature is a key factor for spin transport and manipulation. Here, we report the realization of near-room temperature itinerant ferromagnetism in Co doped Fe5GeTe2 thin flakes. The ferromagnetic transition temperature TC (∼323 K-337 K) is almost unchanged when the thickness is as low as 12 nm and is still about 284 K at 2 nm (bilayer thickness). Theoretical calculations further indicate that the ferromagnetism persists in monolayer Fe4CoGeTe2. In addition to the robust ferromagnetism down to the ultrathin limit, Fe4CoGeTe2 exhibits an unusual temperature- and thickness-dependent intrinsic anomalous Hall effect. We propose that it could be ascribed to the dependence of the band structure on thickness that changes the Berry curvature near the Fermi energy level subtly. The near-room temperature ferromagnetism and tunable anomalous Hall effect in atomically thin Fe4CoGeTe2 provide opportunities to understand the exotic transport properties of two-dimensional van der Waals magnetic materials and explore their potential applications in spintronics.

Broken Inversion Symmetry in Van Der Waals Topological Ferromagnetic Metal Iron Germanium Telluride.

Inversion symmetry breaking is critical for many quantum effects and fundamental for spin-orbit torque, which is crucial for next-generation spintronics. Recently, a novel type of gigantic intrinsic spin-orbit torque is established in the topological van der Waals(vdW) magnet iron germanium telluride. However, it remains a puzzle because no clear evidence exists for interlayer inversion symmetry breaking. Here, the definitive evidence of broken inversion symmetry in iron germanium telluride directly measured by the second harmonic generation (SHG) technique is reported. The data show that the crystal symmetry reduces from centrosymmetric P63/mmc to noncentrosymmetric polar P3m1 space group, giving the threefold SHG pattern with dominant out-of-plane polarization. Additionally, the SHG response evolves from an isotropic pattern to a sharp threefold symmetry upon increasing Fe deficiency, mainly due to the transition from random defects to ordered Fe vacancies. Such SHG response is robust against temperature, ensuring unaltered crystalline symmetries above and below the ferromagnetic transition temperature. These findings add crucial new information to the understanding of this interesting vdW metal, iron germanium telluride: band topology, intrinsic spin-orbit torque, and topological vdW polar metal states.

Estimation of magnetocaloric effect in multicomponent material Gd0.2Tb0.2Dy0.2Ho0.2Er0.2Ni2: Exploring high-entropy design

Polycrystalline rare earth intermetallic compound Gd0.2Tb0.2Dy0.2Ho0.2Er0.2Ni2 has been synthesized by arc melting and it has MgCu2-type cubic crystal structure at 300 K (space group Fd-3m). Magnetization measurement on arc-melted Gd0.2Tb0.2Dy0.2Ho0.2Er0.2Ni2 reveals a paramagnetic to ferromagnetic transition at ∼32 K (TC). The TC is close to the average value of the ferromagnetic transition temperatures of the end member GdNi2, TbNi2, DyNi2, HoNi2 and ErNi2 compounds. Using the field-dependent magnetization data measured around TC, isothermal magnetic entropy change (ΔSm) has been estimated. A maximum value of ΔSmmax = −14.3 J kg−1 K−1 is obtained at 27.5 K for 5 T field change. This value is significant when compared to that in the parent RNi2 (R = Gd, Tb, Dy, Ho and Er) compounds. The multicomponent compound Gd0.2Tb0.2Dy0.2Ho0.2Er0.2Ni2 has also been prepared by melt-spinning under inert atmosphere. The melt-spun sample shows granular microstructure with an average grain size of ∼1 µm. It is found that the bulk magnetic properties are almost retained in the melt-spun sample and the magnetocaloric effect is nearly equal to that in arc-melted Gd0.2Tb0.2Dy0.2Ho0.2Er0.2Ni2. The relative cooling power is large and is about 568 J kg−1 and 531 J kg−1 for 5 T field change for the arc-melted and melt-spun Gd0.2Tb0.2Dy0.2Ho0.2Er0.2Ni2 samples respectively.

The dynamics of glassy behavior in SrRu1-xFexO3 heterostructure

We report the magnetic interactions in SrRu1-xFexO3 epitaxial thin films. All the films showed a single-phase and good step-terrace structure with minimal surface roughness. The X-ray photoelectron spectra of Fe and Ru revealed that both transition metal elements exist in the films as Fe3+ and Ru4+. With Fe doping, the itinerant ferromagnetic behavior and ferromagnetic transition temperature gradually decreased, but the coercivity field increased. The magnetic interplay between Fe3+-O-Fe3+ is coined as antiferromagnetic. There was another interaction between Ru4+-O-Fe3+ which is best described as ferromagnetic. Furthermore, the magnetic frustration in our films caused by the doped Fe resulted in dynamic glassy behavior. On the other hand, the epitaxial strain from the imminent SrTiO3 substrate along with change in oxygen vacancy might have strong evidence for the spin-glass nature in our thin films.

Impact of epitaxial strain relaxation on ferromagnetism in a freestanding La2 / 3Sr1 / 3MnO3 membrane

Manganite perovskites host emerging physical properties of strongly-correlated electrons with charge, spin, and lattice degrees of freedom. Using the epitaxial lift-off technique, we report enhancement of saturation magnetization and the ferromagnetic transition temperature of freestanding La2/3Sr1/3MnO3 membrane compared with as-grown film on SrTiO3 substrate involving lateral tensile strain. Structural analysis reveals shrinkage of unit-cell volume by tensile strain relaxation in the freestanding membrane, which causes enhancement of the ferromagnetic interaction. The impact of the microscopic lattice deformation on the ferromagnetism of La2/3Sr1/3MnO3 indicates a high potential of this material for flexible electronics applications with intriguing functionalities in strongly-correlated electron systems.

Role of structural, morphological, and electronic characteristics in investigating the low temperature transport and magnetic behavior of Sr doped Sm2NiMnO6

Oxide double perovskites of the form R2NiMnO6 (R= rare-earth) are known to possess the multifunctional behavior. Herein, we have induced structural and electronic modifications in Sm2NiMnO6 system by Sr doping to form Sm2−xSrxNiMnO6 (x = 0.0, 0.2, and 0.4) and accordingly investigated the structural, morphological, electronic, dielectric, magnetic and magnetoresistance properties. Sm2NiMnO6 has monoclinic structure with P21/n space group and doping with Sr does not change the phase of compound. Microstructure studies suggest a decrease in the grain size with increase in Sr doping while elemental overlay and analysis suggest the presence of known elements in uniformity. Chemical valency indicates the existence of mixed valence of Ni and Mn cations along with presence of oxygen vacancies in all the sample of Sm2−xSrxNiMnO6 (x = 0.0, 0.2, and 0.4). Frequency (200 Hz to 2 MHz) and low temperature (100–320 K) dependence of dielectric measurements have been carried out to observe the behavior of dielectric constant and dielectric loss. With increase in concentration of Sr, the dielectric constant decreases while the dielectric loss increases. Temperature variation of ac conductivity as well as resistivity suggests the negative temperature coefficient of resistance, depicting the semiconducting behavior of all the samples. Temperature variation of magnetization in terms of field cooled (FC) and zero field cooled (ZFC) curves shows the presence of two magnetic transitions in all the three samples. One pertains to paramagnetic to ferromagnetic transition temperature and another is due to downturn in magnetization at lower temperature. Sr doping also causes an increase in the magnetic ordering temperature. Field variation of magnetization shows the ferromagnetic nature of all the three samples, where the saturation magnetization decreases with increase in temperature as well as increase in Sr concentration. Temperature dependence of resistivity at different magnetic fields indicates the presence of magnetoresistance effect in all the three samples. These studies highlight the role of Sr doping in determining and influencing the various physical properties discussed in this paper and the importance of these double perovskite systems for future material design and various technological applications.

Concentration and temperature dependence of the structural, magnetic, and dielectric properties of La2Ni(Mn1-xRux)O6 solid solutions

The crystal structure, magnetic ordering, and dielectric characteristics of polycrystalline powder samples of La2NiMn1-xRuxO6 (x = 0.25–0.75) solid solutions have been studied. The X-ray diffraction results were analysed in detail, and reliably show that all compounds have a monoclinic (P21/n) structure with Ni and Mn/Ru ions occupying the 2c and 2d sites, respectively. Magnetic susceptibility measurements reveal a ferromagnetic order for La2NiMn0.75Ru0.25O6 and La2NiMn0.5Ru0.5O6. The ferromagnetic transition temperature decreases from 200 K for La2NiMn0.75Ru0.25O6 to 175 K for La2NiMn0.5Ru0.5O6. Dielectric measurements show large dielectric constants (∼104) at room temperature. In addition, the temperature-dependent loss-tangent Tanδ(T) curves reveal relaxation characteristics due to charge transfer between nearest cations.

The exchange bias effect and Griffiths phase in La1.5Sr0.5Co1-xNixMnO6: The impact of the divalent Ni

The effects of Co-site doping of Ni2+ ions on the crystal structure, electronic structure, and magnetic properties of La1.5Sr0.5CoMnO6 are investigated. The doping causes an increase in the anti-site disorder and a change in the content of Co2+/3+ and Mn3+/4+, as well as a change in the spin state of Co ions. Ni doping introduces Ni2+-O2--Mn4+ ferromagnetic interactions to increase the ferromagnetic transition temperature. Samples display different dominant clusters, and field cooling promotes the presence of ferromagnetic clusters, allowing an increase in the Griffiths phase. Interestingly, Ni doping enables tuning the conventional and spontaneous exchange bias (EB) effects, increasing the EB field for intermediate concentrations. Moreover, the maxima of both EB fields do not occur in the same sample, which leads to a new understanding of the relationship between these two effects. The possible mechanisms related to the uncompensated spin and competition between ferromagnetic and antiferromagnetic clusters are discussed, respectively.

Pressure-Dependent Magnetic Properties of Quasi-2D Cr2Si2Te6 and Mn3Si2Te6

Recently, pressure has been used to induce structural and magnetic phase transitions in many layered quantum materials whose layers are linked by van der Waals forces. Materials with such weakly held layers allow for relatively easy manipulation of the superexchange mechanism, which can give rise to novel magnetic behavior. Using hydrostatic pressure as a disorderless means to manipulate the interlayer coupling, we applied pressure on two quasi-2D sister compounds, namely, Cr2Si2Te6 (CST) and Mn3Si2Te6 (MST), up to ∼1 GPa. Magnetic property measurements with the application of pressure revealed that the ferromagnetic transition temperature decreases in CST, while the opposite occurs for the ferrimagnetic MST. In MST, magnetization decreases with the increase in pressure, and such a trend is not clearly observed within the pressure range studied for CST. The overall pressure effect on magnetic characteristics such as exchange couplings and magnetic anisotropy energies is also examined theoretically using density functional theory. Exchange coupling in MST is strongly frustrated, and the first nearest neighbor interaction is the most dominant of the components with the strongest pressure dependence. In CST, the exchange coupling parameters exhibit very little dependence on pressure. This combined experimental and theoretical work has the potential to expand to other relevant quantum materials.

Enhancement of spin-charge conversion efficiency for Co3Sn2S2 across transition from paramagnetic to ferromagnetic phase

Co$_{3}$Sn$_{2}$S$_{2}$ (CSS) is one of the shandite compounds and becomes a magnetic Weyl semimetal candidate below the ferromagnetic phase transition temperature ($\textit{T}_\textrm{C}$). In this paper, we investigate the temperature ($\textit{T}$) dependence of conversion between charge current and spin current for the CSS thin film by measuring the spin-torque ferromagnetic resonance (ST-FMR) for the trilayer consisting of CSS / Cu / CoFeB. Above $\textit{T}_\textrm{C}$ ~ 170 K, the CSS / Cu / CoFeB trilayer exhibits the clear ST-FMR signal coming from the spin Hall effect in the paramagnetic CSS and the anisotropic magnetoresistance (AMR) of CoFeB. Below $\textit{T}_\textrm{C}$, on the other hand, it is found that the ST-FMR signal involves the dc voltages ($\textit{V}_\textrm{dc}$) not only through the AMR but also through the giant magnetoresistance (GMR). Thus, the resistance changes coming from both AMR and GMR should be taken into account to correctly understand the characteristic field angular dependence of $\textit{V}_\textrm{dc}$. The spin Hall torque generated from the ferromagnetic CSS, which possesses the same symmetry as that for spin Hall effect, dominantly acts on the magnetization of CoFeB. A definite increase in the spin-charge conversion efficiency ($\xi$) is observed at $\textit{T}$ < $\textit{T}_\textrm{C}$, indicating that the phase transition to the ferromagnetic CSS promotes the highly efficient spin-charge conversion. In addition, our theoretical calculation shows the increase in spin Hall conductivity with the emergence of magnetic moment at $\textit{T}$ < $\textit{T}_\textrm{C}$, which is consistent with the experimental observation.