Neel Temperature Research Articles

Synthesis, structural, impedance, optical, Mössbauer characterizations and magnetic studies of Pb-containing germanate compounds

The orthorhombic antiferromagnetic germanate minerals Pb2Fe2Ge2O9 and Pb2Mn2Ge2O9 have been prepared via solid state reaction procedure. An extensive study of structural, dielectric, optical and magnetic properties of both the germanates have been investigated by employing different measurements such as X-ray diffraction, FTIR, SEM, impedance, UV–Vis–NIR & Mössbauer spectroscopic and magnetometary. Rietveld refinement of room temperature XRD patterns revealed that both the compounds crystallize in the orthorhombic space group Pbcn. FTIR spectrum exhibits the presence of M−O & Ge–O bonds which confirms strong chemical bonding. Morphological analysis through SEM showed good crystallinity of both the samples. The variation of dielectric constant, loss tangent and impedance with frequency have been studied and it has been established that both the germanates are semiconducting at 300K (RT) which may be attributed to the stacking of ions at grain boundaries. The optical band gaps estimated by UV–Vis–NIR spectroscopic measurements further confirms the semiconducting nature of the prepared germanates. 57Fe Mössbauer spectroscopic measurement on synthesized Pb2Fe2Ge2O9 sample was carried out at RT. The obtained values of IS and QS showed that iron is in Fe3+ high spin state (t2g3eg2,s=5/2) and occupy two different sites, Fe1 and Fe2. The field dependent magnetization measurements show that both the compounds display dominant paramagnetic nature at 300K. The temperature dependent magnetization data reveal very sharp first order type AFM to PM transition at TN ∼50K and TN ∼8K for Pb2Fe2Ge2O9 and Pb2Mn2Ge2O9, respectively, and weak FM nature below Neel temperature. The field dependent properties of both the compounds have been analyzed by drawing the M versus H plots at low and room temperatures. The values of calculated effective magnetic moment 6.07μB and 3.95μB for Fe3+ and Mn3+ ions are in the high spin states (t2g3eg2,s=5/2 and )t2g3eg1,s=2.

Structural and magnetic properties of nanoparticles of La3Ni2NbO9 double perovskite

The nanoparticles of La3Ni2NbO9 (LNNO) have been produced via the glycine-nitrate combustion technique followed by a process of calcination with an average crystallite size of ∼ 25 nm. The powder X-ray diffraction (XRD) pattern was analyzed for structure analysis by Rietveld structure refinement. Study on the XRD pattern reveals that LNNO crystallizes in the single phase of monoclinic crystal structure with P21/n space group. Analysis of temperature-dependent magnetization reveals that nanoparticles of LNNO undergo paramagnetic to antiferromagnetic phase transition at Neel temperature TN ∼ 43 K and antiferromagnetic to spin glass transition at Tg ∼ 31.45 K. Analysis of magnetic ac-susceptibility reveals that the LNNO nanoparticles follow Vogel-Fulcher and dynamic scaling power laws in the low-temperature region.

Dielectric response in the antiferromagnetic phase of Fe1.10Te

Fe1.10Te was studied by scanning tunneling microscopy (STM) and dielectric response to elucidate the charge fluctuation. The cleaved a-b surface was clearly imaged by UHV-STM at T = 8.3 K. From the line profiles, each Fe site was thought to be equivalent suggesting no static charge order in the antiferromagnetic phase. The temperature dependence of the dielectric permittivity was obtained by measuring the complex impedance. The dielectric permittivity increased with decreasing temperature below the Neel temperature TN. The temperature dependence followed the Curie law, indicating the paraelectric state. It suggests charges fluctuate dynamically in the antiferromagnetic state. We conclude that the dynamical charge fluctuation cooperates with the antiferromagnetic order in Fe1.10Te.

Evaluation of Neel Temperatures from Fully Self-Consistent Broken-Symmetry GW and High-Temperature Expansion: Application to Cubic Transition-Metal Oxides

Using fully self-consistent thermal broken-symmetry GW, we construct effective magnetic Heisenberg Hamiltonians for a series of transition metal oxides (NiO, CoO, FeO, and MnO), capturing a rigorous but condensed description of the magnetic states. Then applying high-temperature expansion, we find the decomposition coefficients for spin susceptibility and specific heat. The radius of convergence of the found series determines the Neel temperature. The NiO, CoO, and FeO contain a small ferromagnetic interaction between the nearest neighbors (NNs) and the dominant antiferromagnetic interaction between the next-nearest neighbors (NNNs). For them, the derived Neel temperatures are in good agreement with experiment. The case of MnO is different because both NN and NNN couplings are antiferromagnetic and comparable in magnitude, for which the error in the estimated Neel temperature is larger, which is a signature of additional effects not captured by electronic structure calculations.

Tetragonal Kondo Insulator EuCd2Sb2 Discovered via High Pressure High Temperature Synthesis

AbstractMagnetic and electronic properties of quantum materials heavily rely on the crystal structure even in the same chemical compositions. In this study, it is demonstrated that a layered tetragonal EuCd2Sb2 structure can be obtained by treating bulk trigonal EuCd2Sb2 under high pressure (6 GPa) and high temperature (600 °C). Magnetization measurements of the newly formed layered tetragonal EuCd2Sb2 confirm an antiferromagnetic ordering with Neel temperature (TN) around 16 K, which is significantly higher than that (TN ≈ 7 K) of trigonal EuCd2Sb2, consistent with heat capacity measurements. Moreover, bad metal behavior is observed in the temperature dependence of the electrical resistivity and the resistivity shows a dramatic increase around the Neel temperature. Electronic structure calculations with local density approximation dynamic mean–field theory (LDA+DMFT) show that this material is strongly correlated with well‐formed large magnetic moments, due to Hund's coupling, which is known to dramatically suppress the Kondo scale.

Magnetoelectric Coupling Effects in Tb-Doped BiFeO3 Nanoparticles

The magnetic, electric, and optical properties in Tb-doped BiFeO3 nanoparticles as functions of size and doping concentrations were investigated using a microscopic model, taking into account both linear and quadratic magnetoelectric (ME) coupling. We observed improved multiferroic properties and band-gap tuning. The magnetization and polarization increased with the decreased nanoparticle size and increased Tb-doping substitution x. The Neel temperature remained nearly unchanged whereas the Curie temperature was reduced with the increased x. There was doping-induced ME coupling. The dielectric constant is discussed as a function of the size, doping, and the magnetic field. The band gap decreased with the decreased size or increased Tb dopants due to competing effects of the compressive strain, oxygen defects on the surface, and Coulomb interactions. Increasing the Tb dopants and decreasing the nanoparticle size improved the ME effect.

Anisotropic magnetoresistance and the field-induced anomalous valley inversion in half-Heusler TbPtBi

We have performed detailed magnetic and anisotropic magnetoresistance (AMR) measurements on a magnetic-field-induced topological Weyl semimetal, TbPtBi, exhibiting a weak metamagnetic transition in the antiferromagnetic state. The magnetic torque and temperature-dependent AMR measurements indicate that the spin order dominates the symmetry of AMR curves at temperature below Neel temperature (TN) and low magnetic fields. Remarkably, the AMR with a twofold symmetry gradually dominates the curves as the magnetic fields increase. Moreover, sign change in the AMR effect is confirmed for both T < TN and T > TN at high magnetic fields, which may be related to the magnetic field-induced splitting of the topological band in TbPtBi.

Growth, microstructure and magnetic properties of bixbyite structure ScFeO3: A comparative study between bulk and thin film

The growth, microstructure and magnetic property of both bulk and thin film types of ScFeO3 are studied. Results show that the mono-phase bixbyite structure bulk ScFeO3 can be obtained with sintering temperature above 1400 °C. The bulk ScFeO3 shows dominant antiferromagnetic property with an antiferromagnetic Neel temperature TN about -26 K. The epitaxial type of ScFeO3 thin film with bixbyite structure can be well grown on the SrTiO3(111) substrate. The magnetic property of ScFeO3 thin film is significantly different from that of bulk material which shows notable room temperature ferromagnetism with a saturation field below 0.4 T. The saturation magnetization MS, remanent magnetization Mr, coercivity Hc and of ScFeO3 film increase with decreasing temperature with maximum values of MS = 5.1 emu/cc, Mr = 1.5 emu/cc, and Hc = 29 mT at 5 K. The ferromagnetism of ScFeO3 thin film should be caused by the interface mismatch strain at film/substrate interface. This further proves that ferromagnetism of ScFeO3 can be stabilized with microstructure engineering including strain, which enable multifunctional properties of this material.

Hole density dependent magnetic structure and anisotropy in Fe-pnictide superconductor

The competition between different magnetic structures in hole-doped Fe-pnicitides is explored based on an extended five-orbital Hubbard model including long-range Coulomb interactions. Our results show that the stabilized magnetic structure evolves with increasing hole doping level. Namely, the stripe antiferromagnetic phase dominates at zero doping, while magnetic structures with more antiferromagnetic linking numbers such as the staggered tetramer, staggered trimer, and staggered dimer phases become energetically favorable as the hole density increases. At a certain doping level, energy degeneracy of different magnetic structures appears, indicating strong magnetic frustration and magnetic fluctuations in the system. We suggest that the magnetic competition induced by the hole doping may explain the fast decrease of the Neel temperature T N and the moderately suppressed magnetic moment in the hole doped Fe-pnicitides. Moreover, our results show a sign reversal of the kinetic energy anisotropy as the magnetic ground state evolves, which may be the mechanism behind the puzzling sign reversal of the in-plane resistivity anisotropy in hole-doped Fe-pnicitides.

Unconventional surface state pairs in a high-symmetry lattice with anti-ferromagnetic band-folding

Many complex magnetic structures in a high-symmetry lattice can arise from a superposition of well-defined magnetic wave vectors. These “multi-q” structures have garnered much attention because of interesting real-space spin textures such as skyrmions. However, the role multi-q structures play in the topology of electronic bands in momentum space has remained rather elusive. Here we show that the type-I anti-ferromagnetic 1q, 2q and 3q structures in an face-centered cubic sublattice with band inversion, such as NdBi, can induce unconventional surface state pairs inside the band-folding hybridization bulk gap. Our density functional theory calculations match well with the recent experimental observation of unconventional surface states with hole Fermi arc-like features and electron pockets below the Neel temperature. We further show that these multi-q structures have Dirac and Weyl nodes. Our work reveals the special role that band-folding from anti-ferromagnetism and multi-q structures can play in developing new types of surface states.

Magnetoelastic coupling with inverse magnetocaloric effect observed in Sr2YRuO6

Sr2YRuO6, a 4d transition metal, Ru ion based double perovskite crystallizes in monoclinic structure with Y and Ru octahedra ordering alternately. The relevance of lattice distortion to complex magnetic ground state is investigated from structural, vibrational, magnetic, transport and thermal properties. Raman modes due to monoclinic distortions are observed. From the temperature dependent magnetization studies, a magnetic transition is observed at 25 K and below that the presence of weak ferromagnetic contribution is noticed. A magnetic frustration parameter of six is evaluated from the magnetic results that suggest the presence of spin frustration on the Ru octahedra leading to geometric frustration in SYRO. Isothermal magnetization measurements are carried out for several temperatures from 8 K to 70 K in small temperature intervals to determine the magnetocaloric effect of Sr2YRuO6. The change in magnetic entropy around the Neel temperature, 25 K and Inverse magnetocaloric effect is observed in the system around this temperature. The change in isothermal magnetic entropy results indicates a canted AFM structure as the ground state. Neutron diffraction data refined using propagation vector, k = (0.5,0.5,0) reveal antiferromagnetic ordering at temperatures below 30 K down to 2 K. Variation of the lattice parameters with temperature from the neutron powder diffraction result signifies the presence of magnetoelastic coupling around 25 K. From the temperature dependent resistivity measurements, a small polaron hopping like conductivity is observed in the system. Activation energy reduces with the increase in temperature. Temperature dependent specific heat data shows no ordering, the absence of peak around magnetic transition along with magnetic contribution well above transition temperature is consistent with magnetic frustration in this system.

Microstructures and physical properties of sol–gel derived Ba2FeMnO6 double perovskite

AbstractWe report microstructures and physical (dielectric, magnetic, and optical) properties of sol–gel derived Ba2FeMnO6 (BFMO) double perovskite powders. The BFMO powders belong to hexagonal crystal structure with P‐6m2 space group. SEM images reveal the powders are nearly homogeneous distribution with spherical morphology and average particle size of 300 nm. Energy‐dispersive spectra gave out the molar ratio of the Ba:Fe:Mn elements equal to 2.16:1.00:1.00. FTIR spectrum verified the [FeO6] and [MnO6] octahedra present in the powders. X‐ray photoelectron spectroscopy spectra identified the chemical valence states of the constituent elements. The BFMO ceramics displayed a strong frequency dispersion dielectric behavior. A relaxor‐like dielectric behavior appeared around ∼475 K due to the contributions of oxygen vacancies of and the defect dipoles. A ferrimagnetic behavior was observed in the powders at 5 K with MS = 0.14 μB/f.u. and HC = 1.51 kOe. The magnetic Curie temperature (TC) was 381 K, whereas the Neel temperature (TN) was 31 K. The ferrimagnetic behavior is governed by the Mn3+–Fe4+ double‐exchange interaction via the mediated oxygen between them. The BFMO powders have a direct optical bandgap of 1.65 eV, which originates from the electron transferring from O 2p to Mn 3d (and/or Fe 3d) levels. The unique combination of high temperature ferrimagnetism and semiconductivity in the BFMO powders makes them particularly appealing for magnetic spintronics and photovoltaics.

Crystal Structure, Magnetic and Photocatalytic Properties of Solid SolutionsBi2‐xLaxFe4O9 (x=0.05, 0.1)

AbstractSolid solutions Bi2–xLaxFe4O9 at small x‐values (x=0.05, 0.1) were synthesized by the solid‐state reaction method. Here larger Bi3+ ions were partially substituted by smaller La3+ ions. It is shown that the above solid solutions had an orthorhombic mullite‐type crystal structures. Crystal lattice parameters of solid solutions were very close to the respective parameters for pure Bi2Fe4O9 ferrite. It was determined that the substitution of Bi3+ ions by La3+ ions in Bi2Fe4O9 leads to the increase of specific magnetizations of the samples. All studied samples were antiferromagnets with Neel temperatures TN of 258 K, 246 K, ∼243 K for x=0, 0.05, 0.1 respectively. Hence at the substitution of Bi3+ ions by La3+ ions, TN decreases. It is shown that solid solution Bi1.9La0.1Fe4O9 had good photocatalytic activity for the oxidation of Acid Telon Blue dye in the aqueous solution under visible light irradiation.

Raman scattering signatures of strong spin-phonon coupling in the bulk magnetic van der Waals material CrSBr

Magnetic excitations in layered magnetic materials that can be thinned down the two-dimensional (2D) monolayer limit are of high interest from a fundamental point of view and for applications perspectives. Raman scattering has played a crucial role in exploring the properties of magnetic layered materials and, even-though it is essentially a probe of lattice vibrations, it can reflect magnetic ordering in solids through the spin-phonon interaction or through the observation of magnon excitations. In bulk CrSBr, a layered A type antiferromagnet (AF), we show that the magnetic ordering can be directly observed in the temperature dependence of the Raman scattering response i) through the variations of the scattered intensities, ii) through the activation of new phonon lines reflecting the change of symmetry with the appearance of the additional magnetic periodicity, and iii) through the observation, below the Neel temperature (TN) of second order Raman scattering processes. We additionally show that the three different magnetic phases encountered in CrSBr, including the recently identified low temperature phase, have a particular Raman scattering signature. This work demonstrates that magnetic ordering can be observed directly in the Raman scattering response of bulk CrSBr with in-plane magnetization, and that it can provide a unique insight into the magnetic phases encountered in magnetic layered materials.

Crystal structure, dielectric and magnetic studies of pure and Sr substituted LaFeO3 single crystal grown by optical floating zone technique

The pure and strontium (Sr) substituted LaFeO3 polycrystalline samples were prepared through solid state synthesis process and good quality single crystals were successfully grown by four mirror optical floating zone technique for dielectric and magnetic measurements. The feed and seed rods were prepared by employing an external pressure of about 70 MPa and sintered at 1350 ⁰C on 12 h. The orthorhombic structure of the pure and Sr substituted materials were confirmed by powder X-ray diffraction analysis. The nonappearance of shoulder peaks in the Sr substituted LaFeO3 sample reveals single-phase crystal system without any sub-grain boundaries. The composition of the pure and substituted crystals was confirmed by EDS (energy dispersive X-ray spectroscopy) analysis. The dielectric studies were carried out on both samples in the temperature range from 300 to 900 K for the frequency range from 2.5 kHz to 125 kHz. The grown crystals show their Neel temperature at 742 K and the dielectric constant value increased for substituted LaFeO3 crystal. A colossal dielectric behavior and electrical conductivities were analyzed through dielectric studies. The activation energies of pure and Sr substituted LaFeO3 crystals were estimated to be 0.19 and 0.16 eV respectively. Magnetic properties of the developed crystals were tested by M-H loop. The M-T plot of Sr substituted LaFeO3 crystal was discussed with Zero field cooled (ZFC) and Field cooled (FC) modes for its potential magnetic application.

Phase transitions in multiferroic Bi1−xEuxFeO3 synthesized by sol-gel method

Bi1-xEuxFeO3 micropowders were synthesized by sol-gel method. X-ray diffraction shows that the crystal structure changes from rhombohedral to cubic symmetry at Dielectric properties and loss factor were investigated as a function of temperature. Neel temperature of the compounds was measured by differential scanning calorimetry and found to decrease from 668 K to 646 K for and bounce to 667 K for The Curie temperature TC which corresponds to the transition from ferroelectric to paraelectric phases was checked out using differential thermal analysis measurements. The TC value increases from 1130 K to 1135 K for while no transition was detected for in accordance with the structural transition from R3c to centrosymmetric Pm3m phase.

New Rare Earth-Doped Bilayered Perovskite Oxide Photocatalysts Sr2La0.5R0.5FeMnO7 (R = La, Nd, Sm, Gd, Dy) for the Degradation of Highly Toxic Methylene Blue Dye in Wastewater under Visible Light: Structural, Optical, and Magnetic Properties.

This paper presents the rare earth doping effect on the structural, optical, and magnetic properties of bilayered Ruddlesden-Popper oxides Sr2La0.5R0.5FeMnO7 (R = La, Nd, Sm, Gd, Dy). Moreover, we are reporting for the first time a new rare earth-doped bilayered perovskite oxide series for the highly toxic methylene blue dye degradation in wastewater under visible light. Structural analysis of the PXRD data using the Rietveld refinements confirms the formation of the phases in tetragonal symmetry with the I4/mmm space group. The unit cell lattice parameters (a & c) and the cell volume (V) decrease monotonically from La- to Dy-doped samples owing to the decrease in the lanthanide ionic radii. The X-ray photoelectron spectroscopy analysis indicates the existence of the Mn ions in the mixed valence state. The DRS study shows that the energy band gap value decreases on moving from La to Gd substitution; however, it further increases for the Dy-doped sample. The magnetic measurements reveal that all the phases exhibit dominant anti-ferromagnetic interactions with Neel temperature (T N) observed at 150, 147, 138, 113, and 117 K for La-, Nd-, Sm-, Gd-, and Dy-substituted phases, respectively. However, the presence of an unsaturated hysteresis loop observed in the isothermal magnetic field (H) vs magnetization (M) plot also indicates the existence of weak ferromagnetic interactions. The investigation of the photocatalytic activity of the synthesized samples was done by carrying out photo-oxidative degradation of methylene blue (MB) dye pollutants. The results show that the photodegradation enhances by doping with heavier rare earth ions with the exception of the Dy-doped sample. The Gd-doped catalyst shows the maximum degradation efficiency of 99.03% in 50 min under visible light irradiation. The scavenging experiments confirmed that the·OH was the main/dominant oxidizing agent involved in the degradation of the MB dye.

Nonlinearity and harmonics of the magnetic susceptibility of a Co-=SUP=-2+-=/SUP=- single-ion magnet in the paramagnetic region above the magnetic ordering temperature

In single-ion complexes based on Co2+ ions, the second and third harmonics of the magnetic susceptibility were found at temperatures of 2-4 K, which exceed the Neel temperature. The maxima of the second and third harmonics of the magnetic susceptibility are observed at a frequency of ~1 Hz, at which the maximum of the first harmonic is observed in a field of 3.2 kOe. An analysis of the dependences of the second and third harmonics of the magnetic susceptibility on the field and temperature showed that the nonlinearity arises as a result of the formation of a spin glass state at temperatures slightly higher than the Neel temperature. In this state, there is no long-range spin order, but there are spin clusters in the spin glass state. The spin-glass state in a compound with a Co2+ ion with a high magnetic anisotropy is unusual in that the exchange interaction is much smaller than the single-ion anisotropy energy. Keywords: molecular magnets, nonlinear magnetic susceptibility, nanostructures, spin dynamics, spin glass.

Синтез, структурные и магнитные свойства NaZnFe-=SUB=-2-=/SUB=-(VO-=SUB=-4-=/SUB=-)-=SUB=-3-=/SUB=-

A new magnetic compound NaZnFe2(VO4)3 obtained by solid-phase synthesis has been studied using X-ray diffraction, Mössbauer spectroscopy, electron paramagnetic resonance, measurement of the temperature dependence of the dielectric permeability, and magnetometry. The crystalline structure of NaZnFe2(VO4)3 is described by a triclinic spatial group of symmetry P1 with the parameters of an elementary crystalline chain: a = 6.74318 (7) Å, b = 8.1729 (1) Å, c = 9.8421 (1) Å, α = 106.2611 (9)º, β = 104.55 (1)º, γ = 102.337 (1)º, V = 479.88 (1) Å3, Z = 2. Magnetic Fe3+ cations in the cell occupy six positions populated together with diamagnetic Zn2+ cations, which leads to states of magnetic inhomogeneity and local violation of charge neutrality. Data from resonance and magnetic studies of NaZnFe2(VO4)3 confirm the main role of high-spin Fe3+ iron cations in the formation of magnetism with competing exchange magnetic interactions and a high value of the frustration index. It is shown that the magnetic subsystem of a sample with a negative asymptotic Neel temperature undergoes a magnetic transition from the paramagnetic state to the magnetic state of the spin glass when the temperature decreases. T.V. Drokina 1, M.S. Molokeev M.C. 1, 2, D.A. Velikanov 1, O.A. Bayukov 1, A.M. Vorotynov 1, A.L. Freidman 1, G.A. Petrakovskii 1 1 L.V. Kirensky Institute of Physics, Federal Research Center KSC Siberian Branch of Russian Academy of Science, Krasnoyarsk, 660036, Russia 2 Siberian Federal University, Krasnoyarsk, 660074, Russia

Evolution of the phase composition, crystal structure and magnetic properties of core@shell nanoparticles obtained during conversion of ferrocene at high pressure and high temperature

This work describes structural and magnetic properties of nanoparticles obtained during conversion of ferrocene Fe(C5H5)2 under a high pressure 8 GPa and a high temperature 900 °C (HP-HT treatment) for 10–10000 s, and subsequent self-oxidation in air. The magnetic, structural, and electronic properties of the nanocomposites were studied by XRD, TEM, HAADF-STEM, ED, EDXS, Mössbauer spectroscopy and magnetization measurements. Conversion of ferrocene leads to the formation of “pure” and carbon-encapsulated iron carbide (Fe7C3@C) nanoparticles embedded in carbon matrices with varying degrees of structural ordering. Depending on the size and structure of these nanoparticles different products can be obtained as a result of the self-oxidation of iron carbides. Along with solid iron oxide nanoparticles, hollow iron oxide particles were found in the oxidation products. The formation of hollow nanoparticles can be explained by the Kirkendall effect. It is known that magnetite Fe3O4 and maghemite γ-Fe2O3 are ferrimagnets with a high Neel point TN, while wüstite FeO is an antiferromagnet with TN about 198 K. By varying the content of these components in nanoparticles, it is possible to obtain materials with desired magnetic properties, which is of great importance for technological and biomedical applications of such nanostructures.