Ferromagnetic Phase Transition Temperature Research Articles

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.

57Fe Mössbauer spectroscopy and high-pressure structural analysis for the mechanism of pressure-induced unique magnetic behaviour in (cation)[FeIIFeIII(dto)3] (cation = Ph4P and nPrPh3P; dto = 1,2-dithiooxalato).

A mixed-valence iron(II,III) coordination polymer, (Ph4P)[FeIIFeIII(dto)3] (2; Ph4P = tetraphenylphosphonium, dto = 1,2-dithiooxalato), exhibits a thermal hysteresis loop and a low temperature shift of the ferromagnetic phase transition temperature, with increasing pressure. The latter magnetic behaviour can also be observed in a novel compound (nPrPh3P)[FeIIFeIII(dto)3] (3; nPrPh3P = n-propyltriphenylphosphonium). To understand the structural information under pressure, we performed high-pressure powder X-ray diffraction, and the result suggests that there was no structural phase transition for either compound. Considering the 57Fe Mössbauer spectroscopy studies, both 2 and 3 may have a high transition entropy, and this finding is caused by pressure-induced unique magnetic behaviours.

Observation of Thickness-Dependent Exchange Interaction in EuO Ultrathin Films

The electronic structure of single-crystalline EuO (100) ultrathin films, together with their exchange splitting energy (ΔEEX) and ferromagnetic phase transition temperature (TC), was investigated by temperature- and thickness-dependent angle-resolved photoemission spectroscopy. Both ΔEEX and TC decreased monotonically with decreasing film thickness. The band shift showed an opposite thickness dependence at the Γ and X points, reflecting the balance of the hybridization between the Eu 4f and O 2p states (super-exchange interaction) and between the Eu 4f, O 2p, and Eu 5d states (indirect-exchange interaction). The observed transition from an indirect energy gap in the bulk to a direct gap in the ultrathin films of the ferromagnetic semiconductor EuO could be potential in future spintronic devices.

Correlation between magnetic and crystal structural sublattices in palladium-doped FeRh alloys: Analysis of the metamagnetic phase transition driving forces

FeRh alloys doped with the third element exhibit a change in the lattice and magnetic subsystems, which are manifested in antiferromagnetic- ferromagnetic (AFM-FM) first-order phase transition temperature, the shrinkage of the temperate hysteresis under transition, and the reduction of the saturation magnetization. All aforementioned parameters are crucial for practical applications. To control them it is quite important to determine the driving forces of the metamagnetic transition and its origins. In this manuscript ab initio calculations and experimental studies results are presented, which demonstrate the correlation between the structural and magnetic properties of the Fe50Rh50−xPdx alloys. The qualitative analysis of the metamagnetic phase transition driving forces in palladium-doped FeRh alloys was performed to determine their contribution to the evolution of magnetic and lattice subsystems. In addition, the impact of the impurities phases together with its magnetic behavior on the AFM-FM phase transition was considered.

Experimental observation of ferroelectricity in ferrimagnet MnCr2S4

Ferrimagnetic spinel compounds AB2X4 (A and B are the magnetic transition elements) are considered to be promising candidates for multiferroics with large magnetization and polarization. In this work, we synthesize polycrystalline spinel MnCr2S4 and characterize the magnetic and ferroelectric properties. Two well-defined ferroelectric phase transitions are demonstrated. The first one occurs at the Cr3+ ferromagnetic phase transition temperature of TC = 65 K, and the other takes place at the Yafet–Kittel (YK) magnetic phase transition temperature of TYK ≈ 5 K. It is suggested that ferroelectricity in the YK phase is driven by the noncollinear triangular YK spin orders and can be greatly tuned by an external magnetic field. Between TYK and TC ranges, another opposite electric polarization sublattice appears, which is enhanced by the external magnetic field just near TC, revealing that this opposite electric polarization is likely related to magnetostriction and the magnetic field can enhance the lattice distortions near TC. Thus, this work paves the way for exploiting ferrimagnetic multiferroicity although more studies are needed to clarify the ferroelectricity mechanism in the Cr3+ ferromagnetic phase.

Ferromagnetic frustration in ternary nitride ZnFe3N.

As a new antiperovskite nitride, ZnFe3N was synthesized and characterized by almost completely substituting iron atoms at corner positions of γ'-Fe4N. The magnetic interactions of the system with the space group Pm3[combining macron]m are fully investigated. The critical behavior was investigated based on the measured magnetic data around the ferromagnetic phase transition temperature. In this work, the values of critical exponents (β, γ and δ) were obtained systematically using the Kouvel-Fisher method in the critical region. The Widom scaling law (δ = 1 + γβ-1) and the scaling equation (m = f±(h)) were used to reveal the reliability of these values. The values of the critical exponents (β = 0.325, γ = 1.228, and δ = 4.778) are different from those predicted by the three-dimensional (3D) Heisenberg model and mean-field model, and are very close to those of the 3D-Ising model. Combined with ESR analysis, the spin clusters induced by changes in chemical bonds are considered to be the cause for the existence of an anisotropic short-range ordered state in this ferromagnetic system.

A modified piezoelectric ultrasonic composite oscillator technique for simultaneous measurement of elastic moduli and internal frictions at varied temperature.

In this work, a modified piezoelectric ultrasonic composite oscillator technique (M-PUCOT) is developed for measuring a material's elastic moduli and internal frictions, as a function of temperature. Different from the traditional PUCOT that employs two quartz bars as the drive and gauge, here, a single small piezoelectric transducer (PZT) ring is used to drive and sense longitudinal or torsional vibration in a cylinder specimen. Because of the strong piezoelectric effect and relatively large bandwidth of the PZTs compared to their quartz counterpart, the frequency match condition between the transducer and the specimen is not required in this M-PUCOT. For high temperature measurement, a fused quartz spacer, whose resonance frequency matches the specimen's, is bonded between the transducer and the specimen to provide thermal insulation. First, the united equivalent circuit of the transducer- (spacer) -specimen composite system was derived. Then, Young's modulus, longitudinal friction, shear modulus, and torsional friction were explicitly obtained by measuring the resonance frequency and antiresonance frequency of the 2- or 3-component system's electrical susceptance curve using an impedance analyzer. The accuracy of this method was validated both by measuring the system's amplitude-frequency curves using a laser vibrometer and through finite element simulations. The repeatability error of the M-PUCOT is only ∼0.2% for moduli measurement and ∼2.5% for internal friction measurement, which is very promising for studying the moduli and internal friction variations during damage, fatigue, and phase transitions. Finally, the M-PUCOT was employed to measure the variations in moduli and internal frictions of an Fe64Ni36 Invar alloy from room temperature to 500 °C. Results show that above the ferromagnetic phase transition temperature Tc, both moduli reach their maxima, and both internal frictions reach their minima. The proposed M-PUCOT is expected to be widely used in the near future for its quick measurement, high repeatability, and low cost.

Exotic magnetic behaviour and evidence of cluster glass and Griffiths like phase in Heusler alloys Fe2-xMnxCrAl (0\u2009\u2264\u2009x\u2009\u2264\u20091)

We present a detailed study of structural, magnetic and thermodynamic properties of a series of Heusler alloys Fe2-xMnxCrAl (x = 0, 0.25, 0.5, 0.75 and 1). Structural investigation of this series is carried out using high resolution synchrotron X-ray diffraction. Results suggest that with increasing Mn concentration, the L21 structure of Fe2CrAl is destabilized. The DC magnetization results show a decrement in paramagnetic (PM) to ferromagnetic (FM) phase transition temperature (TC) with increasing Mn concentration. From the systematic analysis of magnetic memory effect, heat capacity, time dependent magnetization, and DC field dependent AC susceptibility studies it is observed that, Fe2CrAl exhibits cluster glass(CG)-like transition approximately at 3.9 K (Tf2). The alloys, Fe1.75Mn0.25CrAl and Fe1.5Mn0.5CrAl exhibit double CG-like transitions near Tf1 ~ 22 K, Tf2 ~ 4.2 K and Tf1 ~ 30.4 K, Tf2 ~ 9.5 K respectively, however, in Fe1.25Mn0.75CrAl, a single CG-like transition is noted at Tf2 ~ 11.5 K below TC. Interestingly, FeMnCrAl shows the absence of long ranged magnetic ordering and this alloy undergoes three CG-like transitions at ~22 K (Tf*), 16.6 K (Tf1) and 11 K (Tf2). At high temperatures, a detailed analysis of temperature response of inverse DC susceptibility clearly reveals the observation of Griffiths phase (GP) above 300 K (T*) in Fe2CrAl and this phase persists with Mn concentration with a decrement in T*.

Enhanced Ferromagnetism in Cylindrically Confined MnAs Nanocrystals Embedded in Wurtzite GaAs Nanowire Shells.

Nearly a 30% increase in the ferromagnetic phase transition temperature has been achieved in strained MnAs nanocrystals embedded in a wurtzite GaAs matrix. Wurtzite GaAs exerts tensile stress on hexagonal MnAs nanocrystals, preventing a hexagonal to orthorhombic structural phase transition, which in bulk MnAs is combined with the magnetic one. This effect results in a remarkable shift of the magneto-structural phase transition temperature from 313 K in the bulk MnAs to above 400 K in the tensely strained MnAs nanocrystals. This finding is corroborated by the state of the art transmission electron microscopy, sensitive magnetometry, and the first-principles calculations. The effect relies on defining a nanotube geometry of molecular beam epitaxy grown core-multishell wurtzite (Ga,In)As/(Ga,Al)As/(Ga,Mn)As/GaAs nanowires, where the MnAs nanocrystals are formed during the thermal-treatment-induced phase separation of wurtzite (Ga,Mn)As into the GaAs-MnAs granular system. Such a unique combination of two types of hexagonal lattices provides a possibility of attaining quasi-hydrostatic tensile strain in MnAs (impossible otherwise), leading to the substantial ferromagnetic phase transition temperature increase in this compound.

Glassy phase of the frustrated Bethe lattice models

As a short range frustrated spin model, we study the ferromagnetic spin models on the Bethe lattice with next nearest neighbor (n.n.n.) interactions with negative sign. The iteration equations which are similar to belief propagation are derived and studied by population dynamics. At low temperatures, we find the ferromagnetic phase for zero or small n.n.n. interactions. As the strength of them increases, ferromagnetic phase transition temperature becomes zero and non-trivial distributions of the local fields arise, implying that there are glassy phases in these models.

Structural, Magnetic, Magnetocaloric and Mössbauer Spectrometry Study of $${\hbox {Gd}}_2{\hbox {Fe}}_{17-x}{\hbox {Cu}}_x$$ Gd 2 Fe 17 - x Cu x ( $$x=$$ x = 0, 0.5, 1 and 1.5) Compounds

The structure, magnetic and magnetocaloric properties of arc-melted $${\hbox {Gd}}_2{\hbox {Fe}}_{17-x}{\hbox {Cu}}_x$$ ( $$x =$$ 0, 0.5, 1 and 1.5) solid solution have been studied. The Rietveld refinement shows that these compounds crystallize in the rhombohedral $${\hbox {Th}}_2{\hbox {Zn}}_{17}$$ -type structure with the $$R{\bar{3}}m$$ space group, and the substitution of iron by copper leads to a decrease in the unit cell volume. The Curie temperature ( $$T_{\mathrm{C}}$$ ) of the prepared samples depends on the copper content. The reduction of the ferromagnetic phase transition temperature from 475 K (for $$x = 0$$ ) to 460 K (for $$x = 1.5$$ ) is due to the substitution of the magnetic element (Fe) by non-magnetic atoms (Cu). The magnetocaloric effect was determined in the vicinity of the Curie temperature $$T_{\mathrm{C}}$$ . By increasing the Cu content, an increase in the values of magnetic entropy ( $$\Delta S_{\mathrm{M}}$$ ) in a low applied field is observed. The preferred inequivalent crystallographic site of Cu atoms was determined by Mossbauer spectrometry analysis based on the isomer shift parameter correlation with the Wigner–Seitz cell volumes.

Possible magnetic correlation above the ferromagnetic phase transition temperature in Cr2Ge2Te6.

Cr2Ge2Te6 has recently emerged as a new two-dimensional ferromagnetic semiconductor (2DFMS) that is promising for spintronic applications. The origin of the ferromagnetism is a debatable point. In this study, ac/dc susceptibility and electronic spin resonance (ESR) measurements are performed to explore the origin of the ferromagnetism in Cr2Ge2Te6. Through the ac susceptibility scaling, the critical temperature TC = 62.84 K and δ = 5.24 from the critical isotherm, γ + β = 1.78 from the temperature dependence of the crossover line and γ = 1.43 from the temperature dependence of the susceptibility along the same line. Unlike Cr2Si2Te6 whose magnetism can be well described by the 2D-Ising model, Cr2Ge2Te6 cannot be simply described by a single theory model. Meanwhile, the origin of the abnormal critical behavior has been explored and it may be related to the presence of the possible magnetic correlation around the high temperature T* ∼ 160 K, which is confirmed by different probing measurements. The magnetic correlation at high temperature accompanied by the strong magnetic-crystalline anisotropy at low temperature plays an important role in the origin of the abnormal ferromagnetism in Cr2Ge2Te6. Our results may supply a typical reference to investigate the abnormal ferromagnetism of 2DFMSs.

Anomalous Hall effect of the quasi-two-dimensional weak itinerant ferromagnet Cr4.14Te8

The anomalous Hall effect (AHE), a manifestation of the Hall effect driven by the Berry curvature, has numerous applications in spintronics and valleytronics. However, its realization in quasi-two-dimensional (quasi-2D) chromium tellurides remains puzzling. In this paper, we synthesize the Cr4.14Te8 single crystal and find that it shows weak itinerant ferromagnetic (FM) metallic behavior with a large magnetocrystalline anisotropy. At the same time, Cr4.14Te8 exhibits the AHE below the FM phase-transition temperature . By taking into account the scaling behavior between the anomalous Hall resistivity and the longitudinal resistivity , the origin of the AHE in this system is suggested to stem from the skew-scattering mechanism. Moreover, the possible magnetic ground state in Cr4.14Te8 has also been discussed to reveal the origin of AHE. Our results may be helpful for exploring the potential applications of these kinds of quasi-2D FM metals.

Structural, magnetic, magnetocaloric and critical behavior studies in the vicinity of the paramagnetic to ferromagnetic phase transition temperature in [formula omitted] compound

This article comprises of the structural, magnetic, magneto-caloric and critical behavior study of single phase perovskite manganite LaMnO3+δ sample synthesized by traditional Solid State route. X-ray powder diffraction (XRD) at room temperature indicates the single phase trigonal crystalline structure of LaMnO3+δ with hexagonal setting R3‾c. Magnetic, magnetocaloric and critical behavior are studied using temperature dependent magnetization (M−T) and applied magnetic field dependent magnetization (M−H) data probed by Physical Property Measurement System (PPMS). The second order ferromagnetic to paramagnetic (FM-PM) phase transition is observe at Curie temperature (TC ∼ 180 K). This phase transition is responsible for maximum entropy change (ΔSMmax) of 0.451(1) J/kg K at applied magnetic field of 6 Tesla with relative cooling power (RCP) of 54.293(5) J/kg. At TC, the order of magnetic phase transition is examined by employing Modified Arrott plots (MAP), Kouvel Fisher (KF) and simple isotherm analysis technique. The obtained values of critical exponent’s β ≈ 0.365 ± 0.06, γ ≈ 1.14 ± 0.04 and δ ≈ 4.16 ± 0.10 elaborate that the magnetic phase transition coordinated well with the 3D-Heisenberg model at TC ≈ 180 K and are confirmed by scaling equations of state. Similarly the change in magnetic entropy (ΔSM) versus applied magnetic field recorded complies with the power law ΔS = (H)n with n = 0.64 at TC = 180 K.

Influence of grain size and sintering temperature grain size on the critical behavior near the paramagnetic to ferromagnetic phase transition temperature in La0.67Sr0.33MnO3 nanoparticles

We have undertaken a systematic study of critical behavior in La0.67Sr0.33MnO3 nanoparticles, sintered at different temperatures (L6, L8, L10 and L12 sintered at 600 °C, 800 °C, 1000 °C, 1200 °C respectively), by magnetization measurements. The critical exponents are estimated by various techniques such as the Modified Arrott plot, Kouvel–Fisher plot and critical isotherm technique. Compared to standard models, the critical exponents are close to those expected by the Mean-field model (with β = 0.5 γ = 1, and δ = 3) for (L6, L8, and L10) samples and by the (3D) Heisenberg model (β = 0.365, γ = 1.336 and δ = 4.80) for L12 sample. We conclude that the reduction of grain size strongly influences the universality class.

Effect of sodium deficiency on the critical behavior near the paramagnetic to ferromagnetic phase transition temperature in La0.8Na0.2−x□xMnO3 oxides

The critical behavior associated with the magnetic phase transition has been investigated by in La0.8Na0.2−x□xMnO3 (0.00≤x≤0.15) (□ is the sodium deficiency). The critical exponents are estimated by various techniques such as the modified Arrott plot, Kouvel–Fisher plot and critical isotherm technique. Compared to standard models, the critical exponent values determined in our work are close to those expected by the tricritical model (β=0.25, γ=1, and δ=5) for x=0.00 and 0.05 samples and by mean field theory (β=0.5, γ=1, and δ=3) for x=0.10 and 0.15 samples. We conclude that the Na deficiency rate affect the critical exponents.

The effect of post-annealing on the structure and magnetotransport properties of Pr0.5Sr0.5MnO3 thin film

Pr0.5Sr0.5MnO3 (PSMO) thin film is epitaxially grown on (001)-oriented LaAlO3 single-crystal substrate using pulsed laser deposition (PLD). It is found that the as-grown PSMO film shows compressive strain in plane and tensile strain out of the plane. Upon annealing at 900°C in the air, the strain is significantly relaxed. The paramagnetic to ferromagnetic phase transition temperature TC shifts from 200 K to 220 K, and the antiferromagnetic insulating phase is suppressed in the phase separated state at low temperature. In addition, the magnetoresistance (MR) is found to increase around the ferromagnetic transition temperature, whereas it decreases from 99% to 60% at low temperature of 20 K.

Critical behavior near the paramagnetic to ferromagnetic phase transition temperature in La0.6Sr0.4MnO3 ceramic: A comparison between sol-gel and solid state process

Critical behavior near the paramagnetic (PM) to ferromagnetic (FM) phase transition in La0.6Sr0.4MnO3 ceramics prepared by sol-gel and solid state reaction methods has been systematically investigated. The XRD result coupled with the structural Rietveld refinement method exhibited that all samples crystallized in a rhombohedral structure with a space group of R-3C, which indicated the formation of the perovskite structure of La0.6Sr0.4MnO3. The magnetic data obtained from magnetization measurements indicated a second-order phase transition from PM-FM phase around Curie temperature (TC). Critical exponents, β, γ, and δ values are estimated by various theoretical models such as the modified Arrott plot (MAP), the Kouvel-Fisher plot (KF) and the critical isotherm (CI) techniques. Grain size dependence of the critical behavior of La0.6Sr0.4MnO3 samples with different sizes from nano-scale to mic-scale was detected by fitting the experimental data to the theoretical models.

Magnetic and Magnetodielectric Properties of Epitaxial Iron Vanadate Thin Films

The magnetic and magnetodielectric behavior of epitaxial (001) transition metal oxide spinel iron vanadate (Fe1+xV2−xO4, FVO) thin films grown on (001) SrTiO3 substrates by pulsed laser deposition is reported. X‐ray absorption spectroscopy studies confirm the trivalent and nominally divalent nature of V and Fe ions, respectively, while an excess of Fe3+ ions is also found. Aberration‐corrected cross‐sectional scanning transmission electron microscopy analysis suggests that the excess Fe3+ may be accommodated through Fe3O4 stacking faults along the {111} planes. Temperature‐dependent X‐ray magnetic circular dichroism spectra reveal an anti‐parallel alignment of spins between the Fe2+ and V3+ cations. M–H loops acquired at 10 K, well below the noncollinear ferromagnetic phase transition temperature, show magnetic anisotropy and point to the existence of two distinct phases with different coercive fields. Visual evidence for magnetic domains and their switching is given by magnetic force microscopy, which finds a switching field of ≈3 T in agreement with the M–H loops. Magnetodielectric measurements for a Pd/FVO/Nb:STO heterostructure reveal a weak hysteresis in the magneto‐capacitance loops at 10 K, thus confirming magneto‐dielectric coupling. Detailed temperature and frequency‐dependent dielectric studies find that Maxwell‐Wagner‐type relaxation dominates capacitance behavior above ≈40 K due to a FVO/Nb:SrTiO3 Schottky interface.

Origin of the low critical observing temperature of the quantum anomalous Hall effect in V-doped (Bi, Sb)2Te3 film.

The experimental realization of the quantum anomalous Hall (QAH) effect in magnetically-doped (Bi, Sb)2Te3 films stands out as a landmark of modern condensed matter physics. However, ultra-low temperatures down to few tens of mK are needed to reach the quantization of Hall resistance, which is two orders of magnitude lower than the ferromagnetic phase transition temperature of the films. Here, we systematically study the band structure of V-doped (Bi, Sb)2Te3 thin films by angle-resolved photoemission spectroscopy (ARPES) and show unambiguously that the bulk valence band (BVB) maximum lies higher in energy than the surface state Dirac point. Our results demonstrate clear evidence that localization of BVB carriers plays an active role and can account for the temperature discrepancy.