Short-range Magnetic Interactions Research Articles

High spin polarization in all-3d-metallic Heusler compounds: the case of Fe2CrZ and Co2CrZ (Z = Sc,Ti,V)

Employing ab initio electronic structure calculations, we study a new class of magnetic Heusler compounds, which consist only of 3d transition metal atoms, namely, Fe2CrZ and Co2CrZ, where Z is an early 3d-metal, such as Sc, Ti or V. Although these compounds show a pseudogap and high spin polarization at the Fermi level, the so-called Slater–Pauling rule is not applicable to them. Co2CrTi and Co2CrV compounds present robust magnetic properties due to the short range magnetic exchange interactions and, thus, relatively high values of Curie temperature far above the room temperature. Moreover, these two compounds keep their magnetic properties upon B2 disorder that makes them very promising materials for spintronic and magnetoelectronic applications.

Three-dimensional Heisenberg critical behavior in amorphous Gd65Fe20Al15 and Gd71Fe3Al26 alloys

Amorphous Gd65Fe20Al15 and Gd71Fe3Al26 alloys have ever been reported to exhibit giant refrigerant capacity around their magnetic ferromagnetic-paramagnetic phase transition, which is desirable in magnetic refrigeration. In this paper, we have investigated the critical behavior of amorphous Gd65Fe20Al15 and Gd71Fe3Al26 alloys in order to better understand their magnetic phase transition. We compared the critical exponents estimated using modified Arrott plot and Kouvel-Fisher plot methods. The critical exponents obtained by Widom's law are in good agreement with those calculated from critical isotherm analysis. According to the obtained critical exponents, the magnetic properties of these two samples were re-scaled, indicating that the magnetic behavior of these alloys could be expressed by a universal and unified law. By analyzing the effective critical exponents and exchange interaction of the two alloys, we have determined that Gd65Fe20Al15 and Gd71Fe3Al26 exhibit isotropic short-range magnetic interaction and close to the 3D-Heisenberg model.

Dispersion, damping, and intensity of spin excitations in the monolayer (Bi,Pb)2(Sr,La)2CuO6+δ cuprate superconductor family

Using Cu-$L_3$ edge resonant inelastic x-ray scattering (RIXS) we measured the dispersion and damping of spin excitations (magnons and paramagnons) in the high-$T_\mathrm{c}$ superconductor (Bi,Pb)$_{2}$(Sr,La)$_{2}$CuO$_{6+\delta}$ (Bi2201), for a large doping range across the phase diagram ($0.03\lesssim p\lesssim0.21$). Selected measurements with full polarization analysis unambiguously demonstrate the spin-flip character of these excitations, even in the overdoped sample. We find that the undamped frequencies increase slightly with doping for all accessible momenta, while the damping grows rapidly, faster in the (0,0)$\rightarrow$(0.5,0.5) nodal direction than in the (0,0)$\rightarrow$(0.5,0) antinodal direction. We compare the experimental results to numerically exact determinant quantum Monte Carlo (DQMC) calculations that provide the spin dynamical structure factor $S(\textbf{Q},\omega)$ of the three-band Hubbard model. The theory reproduces well the momentum and doping dependence of the dispersions and spectral weights of magnetic excitations. These results provide compelling evidence that paramagnons, although increasingly damped, persist across the superconducting dome of the cuprate phase diagram; this implies that long range antiferromagnetic correlations are quickly washed away, while short range magnetic interactions are little affected by doping.

Tuning of magnetic structure and its effect on magnetic properties in Co(Cr1–xMnx)2O4 (x = 0–0.3)

We report here the magnetic properties of Co(Cr1–xMnx)2O4 (x = 0.00–0.30) through bulk-magnetization measurements and temperature variable neutron powder diffraction experiments. We have noticed the compensation temperature (Tcomp) and then the onset of negative magnetization at T = 60 K and 83 K only for Co(Cr0.73Mn0.27)2O4, i.e., x = 0.27, and Co(Cr0.70Mn0.30)2O4, i.e., x = 0.30 samples. Low temperature neutron diffraction experiments on these polycrystalline samples (i.e., x = 0.27 and 0.30) are performed to explore the understanding of observed negative magnetization, complex magnetic properties, and correlation between chemical and magnetic structures. A broad hump has been observed around the (111) Bragg reflection which gives the signature of diffuse scattering from short-range magnetic interactions coexisting with long range magnetic order below ordering temperature. The variation of bond lengths, bond angles, and magnetic moments with temperature is correlated, which clearly indicates magneto-elastic coupling around compensation temperature in these compounds. The presence of this magneto-elastic coupling results in the sign change of the magneto-caloric effect at compensation temperature.

Scaling study of magnetic phase transition and critical behavior in Nd0.55Sr0.45Mn0.98Ga0.02O3 manganite

Microscopic magnetic interactions and magnetism can be probed by studying the critical paramagnetic–ferromagnetic phase transition of magnetic materials. In this work, the critical behavior of the lightly Ga-doped manganites Nd0.55Sr0.45Mn0.98Ga0.02O3 (NSMGO) has been investigated. Scaling analysis reveals the critical exponents β=0.308, γ=1.197, and δ=4.916, where their self-consistency and reliability are verified by the Widom scaling law and the scaling equation. These exponents indicate that the magnetic phase transition in NSMGO belongs to the anisotropic 3D-Ising universality class. Nevertheless, the asymptotic critical exponents βeff and γeff are found to approach to the values of 3D-Heisenberg model, implying the existence of isotropic short-range magnetic interactions. We suggest that, although the Nd-based manganites possess a strong magnetocrystalline anisotropy, this effect can be partly averaged out in view of the present polycrystalline NSMGO. Therefore, in the asymptotic critical region, the phase transition reveals the crossover from the scaling of 3D-Ising toward 3D-Heisenberg universality class.

A-site cationic disorder induced significantly large magnetoresistance in polycrystalline La0.2Gd0.5Ba0.3MnO3 compound

The observation of significantly large magnetoresistance at the liquid nitrogen temperature range in the polycrystalline La0.2Gd0.5Ba0.3MnO3 (LGBMO) compound has been addressed in the present manuscript. The motivation of considering LGBMO sample is the average ‘A’ site ionic radius 〈rA〉 and tolerance factor (t), almost same as that of La0.7Sr0.3MnO3 (LSMO), which is a well studied colossal magnetoresistive material. Magnetoresistance of the LGBMO compound has been compared with the LSMO as well as parent compound La0.7Ba0.3MnO3(LBMO) to show the enhancement of magnetoresistance in LGBMO compound. This observed nature has been elucidated considering the disorder induced short range magnetic interaction due to the enhance size disorder parameter (σ2). Our study revels that, size disorder parameter plays the crucial role for enhancing the colossal magnetoresistance.

Static critical behavior of the ferromagnetic transition in La0.67Ba0.33–xZnxMnO3 manganites

ABSTRACTThe critical properties of perovskite manganite La0.67Ba0.33−xZnxMnO3 around the paramagnetic–ferromagnetic phase transition are investigated through various techniques such as modified Arrott plot, Kouvel-Fisher method and critical isotherm analysis. Through the nature of this transition was found to be in second order. The value of critical exponent, derived from the magnetic data using the Kouvel-Fisher method, yields , and 4.600.034.66 ± 0.06 with a TC of 259.98 ± 0.32 − 221.73 ± 0.05 K. The exponent values are close to those expected for three-dimensional Heisenberg ferromagnets with short-range magnetic interaction.

Cadmium oxide nanoparticles grown in situ on reduced graphene oxide for enhanced photocatalytic degradation of methylene blue dye under ultraviolet irradiation

Cadmium oxide (CdO) nanoparticles (NPs), reduced graphene oxide (rGO) and rGO-CdO nanocomposites have been synthesized using one step hydrothermal method. The structural and optical properties of CdO NPs, rGO, and rGO-CdO nanocomposites were investigated by X-ray diffraction (XRD), energy dispersive X-ray (EDX), high resolution transmission electron microscopy (HR-TEM), Raman spectroscopy (RS), ultraviolet–visible spectroscopy (UV–Vis.) and photoluminescence (PL) spectroscopy techniques. The rGO has a sharp 2D peak compared to GO. The sharp nature of 2D band may be due to the larger contribution from single layer sheet. The photocatalytic activity of the synthesized samples has been investigated under UV irradiation. The results of photocatalytic measurements revealed that ~80% of MB dye is degraded by adding the rGO-CdO nanocomposites as photocatalysts into the dye solution. The decrease in the intensity of emission peaks indicates that the photogenerated charge carriers have been transferred from CdO NPs to rGO sheets, which causes to increase the density of O2− and OH radicals in the dye solution. The CdO nanoparticles gown on the rGO sheets showed enhanced ferromagnetism (FM) at room temperature, which may be attributed to the short range magnetic interaction of magnetic moments of CdO NPs and spin units present on the rGO sheets.

Short range magnetic exchange interaction favors ferroelectricity.

Multiferroics, where two or more ferroic order parameters coexist, is one of the hottest fields in condensed matter physics and materials science. To search multiferroics, currently most researches are focused on frustrated magnets, which usually have complicated magnetic structure and low magnetic ordering temperature. Here, we argue that actually simple interatomic magnetic exchange interaction already contains a driving force for ferroelectricity, thus providing a new microscopic mechanism for the coexistence and strong coupling between ferroelectricity and magnetism. We demonstrate this mechanism by showing that even the simplest antiferromagnetic insulator like MnO, could display a magnetically induced ferroelectricity under a biaxial strain. In addition, we show that such mechanism also exists in the most important single phase multiferroics, i.e. BiFeO3, suggesting that this mechanism is ubiquitous in systems with superexchange interaction.

Critical phenomenon of the near room temperature skyrmion material FeGe.

The cubic B20 compound FeGe, which exhibits a near room temperature skyrmion phase, is of great importance not only for fundamental physics such as nonlinear magnetic ordering and solitons but also for future application of skyrmion states in spintronics. In this work, the critical behavior of the cubic FeGe is investigated by means of bulk dc-magnetization. We obtain the critical exponents (β = 0.336 ± 0.004, γ = 1.352 ± 0.003 and β = 5.276 ± 0.001), where the self-consistency and reliability are verified by the Widom scaling law and scaling equations. The magnetic exchange distance is found to decay as r−4.9, which is close to the theoretical prediction of 3D-Heisenberg model (r−5). The critical behavior of FeGe indicates a short-range magnetic interaction. Meanwhile, the critical exponents also imply an anisotropic magnetic coupling in this system.

Dominant ferromagnetic coupling over antiferromagnetic in Ni doped ZnO: First-principles calculations

The low magnetic moment (MM) in diluted magnetic semiconductors (DMS) at low impurity doping levels has triggered considerable research into condensed magnetic semiconductors (CMS).This work reports an ab-initio investigation of the electronic structures and magnetic properties of ZnO in a zinc-blende (ZB) structure doped with nickel ions. Ni-doped ZnO-based DMS and CMS exhibit a dominance of ferromagnetic coupling over antiferromagnetic. A robust increase in the magnetization has been observed as a function of Ni impurity levels. This material favors short-range magnetic interactions at the ground state, suggesting that the observed ferromagnetism is defined by the double exchange mechanism. The spin-polarized density of states (DOS) of Ni-doped ZnO characterizes it as half-metallic with a considerable energy gap for up-spin components and as metallic for-down spins. Half-metallic Ni:ZnO based magnetic semiconductors with high magnetization are expected to have potential applications in spintronics.

Cooperative magnetic behaviour in the new valence fluctuating compound Ce2Rh3Ge

In this study we report the physical properties of the new ternary compound Ce2Rh3Ge that crystallizes in the rhombohedral, triple hexagonal MgCu2-type of structure. The electronic ground state properties of Ce2Rh3Ge were characterized by magnetic susceptibility, specific heat, electrical resistivity and thermal transport measurements. The results indicate the presence of short range magnetic interaction, probably of ferromagnetic origin below TC = 4 K. The shape of χ−1(T) deviates from the Curie–Weiss behavior with a broad minimum at about = 450 K reminiscent of valence fluctuating cerium systems. At T = 10 K, the magnetic part of the resistivity ρ4 f (T) exhibits a shallow minimum followed by increase of resistivity ρ(T) −lnT, which hints at a substantial Kondo screening effect. Ce2Rh3Ge belongs to a small group of strongly correlated cerium compounds in which the two competing effects of Kondo and RKKY interactions produce long-range magnetic order from strongly hybridized and intermediate-valent 4 f spins. At sufficiently low temperatures Ce2Rh3Ge scales well with the Kadowaki–Woods ratio A/γ2 and the value of the Wilson ratio χ(T → 0)/γ found for this compound classifies it as a mixed-valence compound. The presence of valence fluctuation and magnetic order it is rare for these attributes to be found simultaneously in same compound, in same temperature range. In our opinion a novelty of presented results of Ce2Rh3Ge is that this compound adds a new member to a small but growing class of systems bearing a strongly mixed- or intermediate-valent 4 f magnetic moment, but in which the lattice of spins nevertheless end up finding it possible to order magnetically.

Critical behavior and magnetic entropy change in Nd0.7Sr0.1Ba0.1Ca0.1MnO3−δ perovskite manganite

The aim of our study was to investigate the critical exponents of Nd0.7Sr0.1Ba0.1Ca0.1MnO3−δ material around its paramagnetic to ferromagnetic phase transition. Reliable critical exponents i.e. β=0.355, γ=1.329, and δ=4.867 at the critical temperature TC=117K were refined using modified Arrott plot and Kouvel–Fisher methods. The critical exponent δ=4.760±0.129 was further separately determined from the isothermal magnetization data. Finally these critical exponents fulfill the Widom scaling relation δ=1+γ/β. Based on these analysis, the magnetization–field–temperature (M–H–T) data around TC collapses into two independent curves obeying the single scaling equation M(H,ε)=|ε|βf±(H/|ε|β+γ), with ε=(T−TC)/TC is the reduced temperature. Once again, the critical exponents are confirmed by the field dependence of the magnetic entropy change relation ∆SM|T=TC ∞Hn with n=1+(β−1)/(β+1). The obtained critical exponents for Nd0.7Sr0.1Ba0.1Ca0.1MnO3−δ manganite are in accordance with the prediction of the 3D-Heisenberg model with short-range magnetic interactions. The temperature dependence of specific heat and magnetic entropy change ΔSM further supports second order PM–FM phase transition and shed light the A-site-disordered perovskite effect on the magnetocaloric properties.

Slow magnetic fluctuations and superconductivity in fluorine-doped NdFeAsO

Among the widely studied superconducting iron-pnictide compounds belonging to the Ln1111 family (with Ln a lanthanide), a systematic investigation of the crossover region between the superconducting and the antiferromagnetic phase for the Ln = Nd case has been missing. We fill this gap by focusing on the intermediate doping regime of NdFeAsO(1-x)F(x) by means of dc-magnetometry and muon-spin spectroscopy measurements. The long-range order we detect at low fluorine doping is replaced by short-range magnetic interactions at x = 0.08, where also superconductivity appears. In this case, longitudinal-field muon-spin spectroscopy experiments show clear evidence of slow magnetic fluctuations that disappear at low temperatures. This fluctuating component is ascribed to the glassy-like character of the magnetically ordered phase of NdFeAsO at intermediate fluorine doping.

Magnetic and vibrational properties of Gd2(Mo0.9W0.1O4)3 and Gd1.8Er0.2(MoO4)3

Abstract In this work, we report on the analysis of both vibrational and magnetic properties of heavy rare-earth molybdates Gd 2 (Mo 0.9 W 0.1 O 4 ) 3 and Gd 1.8 Er 0.2 (MoO 4 ) 3 . The crystal structures, as determined by X-ray powder diffraction and the Rietveld refinement analysis, established that at room temperature both of these materials belong to orthorhombic Pba 2 space group. Magnetization measurements showed that no saturation is present up to 8 T at T =10 K, and the magnetic susceptibility curves show paramagnetic behavior down to T =10 K. The effective magnetic moments and possible short-range magnetic interactions between Gd 3+ ions are treated and discussed. Raman spectra of both of these molybdates are similar to that of the pure compound Gd 2 (MoO 4 ) 3 , indicating that changes in the Raman spectra due to the introduction of either Er or W doping are negligible at the subject dopant concentrations.

Magnetocaloric effect and critical behavior in Pr0.5Sr0.5MnO3: an analysis of the validity of the Maxwell relation and the nature of the phase transitions

The Maxwell relation, the Clausius–Clapeyron equation, and a non–iterative method to obtain the critical exponents have been used to characterize the magnetocaloric effect (MCE) and the nature of the phase transitions in Pr0.5Sr0.5MnO3, which undergoes a second-order paramagnetic to ferromagnetic (PM-FM) transition at , and a first-order ferromagnetic to antiferromagnetic (FM-AFM) transition at . We find that around the second-order PM-FM transition, the MCE (as represented by the magnetic entropy change, ΔSM) can be precisely determined from magnetization measurements using the Maxwell relation. However, around the first-order FM-AFM transition, values of ΔSM calculated with the Maxwell relation deviate significantly from those calculated by the Clausius–Clapeyron equation at the magnetic field and temperature ranges where a conversion between the AFM and FM phases occurs. A detailed analysis of the critical exponents of the second-order PM-FM transition allows us to correlate the short-range type magnetic interactions with the MCE. Using the Arrott–Noakes equation of state with the appropriate values of the critical exponents, the field- and temperature-dependent magnetization curves, and hence the curves, have been simulated and compared with experimental data. A good agreement between the experimental and simulated data has been found in the vicinity of the Curie temperature TC, but a noticeable discrepancy is present for . This discrepancy arises mainly from the coexistence of AFM and FM phases and the presence of ferromagnetic clusters in the AFM matrix.

Effect of Ni doping on magnetic and electrical properties of CuCr2Se4 single crystals

The X-ray studies showed that the new CuCr2−xNixSe4 single crystals crystallized in the spinel structure with the space group Fd3¯m. Magnetic measurements revealed that both the long and short-range magnetic interactions are comparable and do not significantly depend on the Ni-content. Electrical measurements showed that p-type metallic conduction, thermopower and power factor decrease as the Ni-content increases. These results are considered in a framework of the structural defects, double exchange mechanism and mixed valence band of the chromium ions.

Control of defect binding and magnetic interaction energies in dilute magnetic semiconductors by charge state manipulation

Dilute magnetic semiconductors exhibit a unique entanglement of magnetism and semiconductor properties. Their properties are dominated by short-range chemical and magnetic interactions among the magnetic impurities diluted therein. The microscopic structure and defect distribution are of crucial importance; and indeed, it has been shown that clustering, phase separation, and complex formation of the magnetic impurities, possibly involving other intrinsic or extrinsic defects, can dramatically alter the magnetic properties of a given sample. Detailed knowledge of the underlying short-range chemical and magnetic interactions, in turn, can be used to “design” new materials with target magnetic properties. This paper describes the Fermi-level dependence of these short-range chemical and magnetic interactions, i.e., how these interactions depend on defect charge states.

Critical behavior in Fe-doped manganites La0.8Ba0.2Mn1−xFexO3 (x=0.15 and x=0.2)

Abstract The critical properties of La 0.8 Ba 0.2 Mn 1− x Fe x O 3 polycrystalline samples near ferromagnetic–paramagnetic phase transition temperature have been investigated by analyzing isothermal magnetization data. Experimental results revealed that all samples exhibit a second-order magnetic phase transition. Dc-magnetization data near the transition temperature have been analyzed with the help of the modified Arrott plot, Kouvel–Fisher method. The value of critical exponents, derived from the magnetization data using the Kouvel–Fisher method, yield 0.370 ± 0.02 ⩽ β ⩽ 0.379 ± 0.01, 1.359 ± 0.02 ⩽ γ ⩽ 1.392 ± 0.03 and 4.40 ± 0.0 ⩽ δ ⩽ 4.67 ± 0.01with a T C of 125 ± 0.02–157.50 ± 0.02 K. The exponent values are close to those expected for three-dimensional (3D) Heisenberg ferromagnets which is short-range magnetic interaction.

Crystal Growth, Structure, Polarization, and Magnetic Properties of Cesium Vanadate, Cs2V3O8: A Structure–Property Study

Cesium vanadate, Cs2V3O8, a member of the fresnoite-type structure, was synthesized via a hydrothermal route and structurally characterized by single-crystal X-ray diffraction. Cs2V3O8 crystallizes in a noncentrosymmetric polar space group, P4bm, with crystal data of a = 8.9448(4) Å, c = 6.0032(3) Å, V = 480.31(4) Å(3), and Z = 2. The material exhibits a two-dimensional layered crystal structure consisting of corner-shared V(5+)O4 and V(4+)O5 polyhedra. The layers are separated by the cesium cations. The alignment of the individual polyhedra results in a macroscopic polarity for Cs2V3O8. Frequency-dependent polarization measurements indicate that the material is not ferroelectric. A pyroelectric coefficient of -2.0 μC m(-2) K(-1) was obtained from pyroelectric measurements taken as a function of the temperature. The magnetic susceptibility data were measured as a function of the temperature and yielded an effective magnetic moment of 1.78 μB for the V(4+) cation. Short-range magnetic ordering was observed around 7 K. The susceptibility data were fit to the Heisenberg square-lattice model supporting that the short-range magnetic interactions are antiferromagnetic and two-dimensional. IR and thermal properties were also characterized.