Temperature-dependent Neutron Diffraction Research Articles

Structural-modulation-driven spin canting and reentrant glassy magnetic phase in ferromagneticLu2MnNiO6

Unusual behavior of reentrant spin-glass (RSG) compound Lu2MnNiO6 has been investigated by magnetometry and neutron diffraction. The system possesses a ferromagnetic (FM) ordering below 40 K and undergoes a RSG transition at 20 K. Additionally, Lu2MnNiO6 retains memory effect above the glassy transition till spins sustain ordering. A novel critical behavior with unusual critical exponents (beta =similar to 0.241 and gamma similar to 1.142) is observed that indicates a canting in the spin structure below the ferromagnetic transition (T-C). A comprehensive analysis of temperature-dependent neutron diffraction data and first-principles calculations divulge that a structural distortion induced by an octahedral tilting results in a canted spin structure below T-C.

Neutron diffraction studies of magnetic ordering in Ni-doped LaCoO3

Research in rare earth cobaltite has recently been intensified due to its fascinating magnetic properties. LaCoO3, an important cobaltite, exhibits two prominent susceptibility features at ~90K and 500K in low field measurement. The magnetic behavior below 100K is predominantly antiferromagnetic (AFM), but absence of pure AFM ordering and emergence of ferromagnetic coupling on further decreasing temperature made situation more intricate. The present work of studying the effect of Ni substitution at Co site in polycrystalline LaCo1−xNixO3 (0≤x≤0.3) is motivated by the interesting changes in magnetic and electronic properties. For lucid understanding, temperature dependent neutron diffraction (ND) study was carried out. ND patterns fitted with rhombohedral structure in perovskite form with R-3c space group, elucidated information on phase purity. Further temperature dependent cell parameter, Co–O bond-length and Co–O–Co bond angle were calculated for the series of Ni doped LaCoO3. The results are explained in terms of decrease in the crystal field energy which led to the transition of cobalt from low Spin (LS) state to intermediate spin state (IS).

Structural and magnetic properties of Nd2NiGe3

Structural analysis of the room temperature diffraction data of Nd2NiGe3 shows that it exhibits AlB2 type structure with space group P6/mmm. The crystal structure of Nd2NiGe3 consists of two dimensional Ni/Ge hexagonal units sandwiching the neodymium atoms between them. Temperature dependent magnetic susceptibility data follows Curie–Weiss law in the temperature range 20–300K yielding an effective magnetic moment of 3.83μB/Nd and paramagnetic Curie temperature −2.1K indicating trivalent Nd and weak antiferromagnetic interactions, respectively. Low field magnetic susceptibility measurement show a weak antiferromagnetic like ordering around 3K. Temperature dependent neutron diffraction measurements rules out long range antiferromagnetic ordering in this compound. The electrical resistivity measurement shows metallic nature of Nd2NiGe3, which exhibits sudden drop in resistivity below 3K.

Kinetics of the Thermally-Induced Structural Rearrangement of γ-MnO2

This work presents a temperature-dependent and time-resolved X-ray and neutron diffraction study of the thermally induced structural rearrangement of γ-MnO2. Here, we study electrochemically prepar...

Atomic Motion in Crystals from Neutron Diffraction Experiments

Static or dynamic disorder in crystals causes a decrease in the Bragg peak intensity, and given sufficient number of Bragg peaks over an extended Q-range, atomic displacement parameters (ADPs) can be refined that quantify the intensity reduction due to the mean square displacements of atoms about their average positions. It is becoming increasingly apparent that ADPs constitute equally important structural information as atom positions and site occupancies. Unusual, yet accurately determined, ADPs can provide telltale clues to interesting physical phenomena, e.g., approach of phase transitions, glass-like thermal conductivity, pathways for high ionic conductivity, and a variety positional disorders. Consequently, the demand for high quality ADPs is increasing, owing in part to our desire to understand and tune physical properties of technological materials. Temperature dependent neutron diffraction using single-crystals is perhaps the best possible method to determine precise individual ADPs, yet the number of these studies is surprisingly limited owing to the paucity of neutron sources and dedicated single-crystal neutron diffractometers. ADPs exhibit various temperature dependent behaviors, and can range from harmonic to anharmonic. Examples from work completed and ongoing at Oak Ridge National Laboratory (stephanite, triphylite, amblygonite, petalite, brucite, filled-skutterudites, gas clathrate hydrates, etc.) as well as previously published work will be reviewed with the aim to generalize insights and recommendations. Research conducted at ORNL's High Flux Isotope Reactor and Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

Disorder-driven spin-reorientation in multiferroic h-YMn1−xFexO3

Magnetic structure evolution of multiferroic hexagonal YMn1−xFexO3 (x = 0, 0.05, and 0.1) has been studied by carrying out detailed temperature-dependent neutron diffraction at zero and 5 T fields. Thermodynamic data confirm antiferromagnetic ordering at TN in all the compositions. Our sub-TN neutron diffraction results assign the magnetic structure of pure YMnO3 to Γ1 irreducible representation. Over the perturbative-doping range, magnetic structure changes via Γ1 + Γ2 for YMn0.95Fe0.05O3 on to Γ2 for YMn0.9Fe0.1O3, as the maiden compositional analogue of spin-reorientation; its occurrence in temperature-domain already reported for several manganites. Moreover, while the large thermal isostructural changes observed above TN are subdued in the ordered state, small alterations by the applied 5 T-field are relatively uniform across, confirming strong magneto-elastic nature of the system. Decrease of the ordered magnetic moment (μord) and planar magnetic frustration noted with Fe-doping is enhanced by the applied field, apparently through canting.

Anisotropic lattice thermal expansion of PbFeBO4: A study by X-ray and neutron diffraction, Raman spectroscopy and DFT calculations

The lattice thermal expansion of mullite-type PbFeBO4 is presented in this study. The thermal expansion coefficients of the metric parameters were obtained from composite data collected from temperature-dependent neutron and X-ray powder diffraction between 10K and 700K. The volume thermal expansion was modeled using extended Grüneisen first-order approximation to the zero-pressure equation of state. The additive frame of the model includes harmonic, quasi-harmonic and intrinsic anharmonic potentials to describe the change of the internal energy as a function of temperature. The unit-cell volume at zero-pressure and 0K was optimized during the DFT simulations. Harmonic frequencies of the optical Raman modes at the Γ-point of the Brillouin zone at 0K were also calculated by DFT, which help to assign and crosscheck the experimental frequencies. The low-temperature Raman spectra showed significant anomaly in the antiferromagnetic regions, leading to softening or hardening of some phonons. Selected modes were analyzed using a modified Klemens model. The shift of the frequencies and the broadening of the line-widths helped to understand the anharmonic vibrational behaviors of the PbO4, FeO6 and BO3 polyhedra as a function of temperature.

Low temperature magnetic studies on PbFe0.5Nb0.5O3 multiferroic

PbFe0.5Nb0.5O3 (PFN), a well-known A(B′1/2B″1/2)O3 type multiferroic, was successfully synthesized in single phase by a single step solid state reaction method. The single phase PFN was characterized through XRD, microstructure through SEM, and magnetic studies were carried out through a temperature dependent vibrating sample magnetometer (VSM) and neutron diffraction (ND) measurements. PFN exhibits a cusp at around 150K in the temperature dependent magnetic susceptibility corresponding to the Néel temperature (TN1) and another peak around 10K (TN2) corresponding to spin-glass like transition. In the temperature dependent ND studies, a magnetic Bragg peak appears at Q=1.35Å−1 (where Q=4πsinθ/λ, is called the scattering vector) below TN (150K) implying antiferromagnetic (AFM) ordering in the system. On the basis of Rietveld analysis of the ND data at T=2K, the magnetic structure of PFN could be explained by a G-type antiferromagnetic structure.

Complex magnetic behavior of the sawtooth Fe chains in Rb2Fe2O(AsO4)2

Results of magnetic field and temperature-dependent neutron-diffraction and magnetization measurements on oxy-arsenate Rb${}_{2}$Fe${}_{2}$O(AsO${}_{4}$)${}_{2}$ are reported. The crystal structure of this compound contains pseudo-one-dimensional [Fe${}_{2}$O${}_{6}$]${}^{\ensuremath{\infty}}$ sawtoothlike chains, formed by corner-sharing isosceles triangles of Fe${}^{3+}$ ions occupying two nonequivalent crystallographic sites. The chains extend infinitely along the crystallographic $b$ axis and are structurally confined from one another via diamagnetic (AsO${}_{4}$)${}^{3\ensuremath{-}}$ units along the $a$ axis and Rb${}^{+}$ cations along the $c$-axis direction. Neutron-diffraction measurements indicate the onset of a long-range antiferromagnetic order below approximately 25 K. The magnetic structure consists of ferrimagnetic chains which are antiferromagnetically coupled with each other. Within each chain, one of the two Fe sites carries a moment which lies along the $b$ axis, while the second site bears a canted moment in the opposite direction. Externally applied magnetic field induces a transition to a ferrimagnetic state, in which the coupling between the sawtooth chains becomes ferromagnetic. Magnetization measurements performed on optically aligned single crystals reveal evidence for an uncompensated magnetization at low magnetic fields that could emerge from a phase-segregated state with ferrimagnetic inclusions or from antiferromagnetic domain walls. The observed magnetic states and the competition between them are expected to arise from strongly frustrated interactions within the sawtooth chains and relatively weak coupling between them.

Long ranged structural modulation in the pre-morphotropic phase boundary cubic-like state of the lead-free piezoelectric Na1/2Bi1/2TiO3-BaTiO3

The nature of the pre-morphotropic phase boundary (MPB) cubic-like state in the lead-free piezoelectric ceramics (1−x)Na1/2Bi1/2TiO3-(x)BaTiO3 at x ∼ 0.06 has been examined in detail by electric field and temperature dependent neutron diffraction, x-ray diffraction, dielectric and ferroelectric characterization. The superlattice reflections in the neutron diffraction patterns cannot be explained with the tetragonal P4bm and the rhombohedral (R3c) phase coexistence model. The cubic like state is rather a result of long ranged modulated complex octahedral tilt. This modulated structure exhibits anomalously large dielectric dispersion. The modulated structure transforms to a MPB state on poling. The field-stabilized MPB state is destroyed and the modulated structure is restored on heating the poled specimen above the Vogel-Fulcher freezing temperature. The results show the predominant role of competing octahedral tilts in determining the nature of structural and polar states in Na1/2Bi1/2TiO3-based ferroelectrics.

Phase stability of multiferroic GaFeO3 up to 1368 K from in situ neutron diffraction

We report a detailed high-temperature powder neutron diffraction investigation of the structural behavior of the multiferroic GaFeO3 between 296 and 1368 K. Temperature dependent neutron diffraction patterns do not show any appreciable change either in intensity or appearance/disappearance of the observed peaks up to 1368 K, ruling out any structural transition in the entire temperature range. Evolution of the distortion of the oxygen polyhedra around Ga1, Ga2, Fe1, and Fe2 cations sites suggest that the Ga1-O tetrahedron is least distorted and Fe1-O is most distorted. Structural features regarding the distortion of polyhedral units would be crucial to understand the temperature dependence of the microscopic origin of polarizations. The electric polarization has been estimated using a simple ionic model and its value is found to decrease with increasing temperature.

Magnetic neutron diffraction study of Ba(Fe1−xCox)2As2critical exponents through the tricritical doping

We present temperature dependent magnetic neutron diffraction measurements on Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ for $x$ = 0.039, 0.022, and 0.021 as-grown single crystals. Our investigations probe the behavior near the magnetic tricritical point in the ($x$,$T$) plane, $x_{tr} \approx $0.022, as well as systematically exploring the character of the magnetic phase transition across a range of doping values. All samples show long range antiferromagnetic order that may be described near the transition by simple power laws, with $\beta$ =~0.306$\pm$0.060 for $x$ =~0.039, $\beta$ =~0.208$\pm$0.005 for $x$ =~0.022, and $\beta$ =~0.198$\pm$0.009 for $x$ =~0.021. For the $x$ =~0.039 sample, the data are reasonably well described by the order parameter exponent $\beta$ =~0.326 expected for a 3D Ising model while the $x$ =~0.022 and $x$ =~0.021 samples are near the $\beta$ =~0.25 value for a tricritical system in the mean-field approximation. These results are discussed in the context of existing experimental work and theoretical predictions.

Freezing of the octahedral tilt near ferromagnetic transition and appearance of a glassy phase at low temperature driven by the tilt instabilities in SrRuO3

The emergence of low temperature glassy phase in widely known itinerant ferromagnet SrRuO3 is remotely understood. In order to understand this aspect, we have undertaken a detailed temperature dependent (5–250 K) neutron diffraction study. We observe a freezing of the octahedral tilt near the ferromagnetic transition and an unusual deviation in the octahedral tilt near the onset of low temperature spin glass like phase. A reduction of the ordered magnetic moment and a decline in the total integrated magnetic intensity is observed around the temperature where the glassy behaviour starts to appear. The magnetotransport study also reveals the possibility for an additional magnetic ordering by demonstrating a peak in magnetoresistance at the low temperature side as well. The neutron diffraction study presented here provides useful information to understand the observed unusual low temperature magnetic phenomena in SrRuO3.

Antiferromagnetic Spin Correlations Between Corner-Shared [FeO5]7– and [FeO6]9– Units, in the Novel Iron-Based Compound: BaYFeO4

A novel quaternary compound in the Ba-Y-Fe-O phase diagram was synthesized by solid-state reaction and its crystal structure was characterized using powder X-ray diffraction. The crystal structure of BaYFeO4 consists of a unique arrangement of Fe(3+) magnetic ions, which is based on alternate corner-shared units of [FeO5](7-) square pyramids and [FeO6](9-) octahedra. This results in the formation of stairwise channels of FeO polyhedra along the b crystallographic axis. The structure is described in an orthorhombic crystal system in the space group Pnma with lattice parameters a = 13.14455(1) Å, b = 5.694960(5) Å, and c = 10.247630(9) Å. The temperature-dependent magnetic susceptibility data reveal two antiferromagnetic (AFM) transitions at 33 and 48 K. An upturn in the magnetic susceptibility data above these transitions is observed, which does not reach its maximum even at 390 K. The field-dependent magnetization data at both 2 and 300 K show a nearly linear dependence and do not exhibit significant hysteresis. Heat capacity measurements between 2 and 200 K reveal only a broad anomaly without any indication of long-range ordering. The latter data set is not in good agreement with the magnetic susceptibility data, which makes it difficult to exactly determine the magnetic ground state of BaYFeO4. Accordingly, a temperature-dependent neutron diffraction study is in order, which will enable resolving this issue. The theoretical study of the relative strengths of magnetic exchange interactions along various possible pathways, using extended Hückel spin dimer analysis, shows that only interactions between square pyramidal and octahedral centers are significant, and among them, the intrachannel correlations are stronger than interchannel interactions. This is the first physical property study in such a magnetic ion substructure.

Temperature dependent neutron diffraction and Mössbauer studies in zinc ferrite nanoparticles

The micro-level magnetic alignments of zinc ferrite nanoparticles synthesized by sol–gel method were investigated using magnetization study, neutron diffraction and Mössbauer spectroscopy. Magnetization behavior of the sample clearly indicates the presence of superparamagnetism. The neutron diffraction was performed at seven different temperatures between 6K and 300K. The cation distribution is found to have a variation from the from normal spinel structure. Both A and B site magnetic moment values found to vary with temperature which is found to correlates with variations in bond lengths and lattice constants. The Mössbauer spectrum at room temperature shows typical doublet while the spectrum 5K shows well defined sextet. The two sub-spectra in the sextet resolves well by the application of 5T magnetic field and the noncollinear nature of magnetic alignment with in the tetrahedral and octahedral sites of zinc ferrite are deduced.

Low temperature magnetic ground state in bulk Co0.3Zn0.7Fe2O4 spinel ferrite system: Neutron diffraction, magnetization and ac-susceptibility studies

The system under study is a bulk Co0.3Zn0.7Fe2O4 ferrite, synthesized by wet chemical route technique and having magnetic in-homogeneity at the microscopic scale, due to the concentration of magnetic ion at a tetrahedral site below the site percolation threshold for the ferrimagnetic ordering. To unravel the magnetic ground state of this system, low temperature neutron diffraction, magnetization and ac-susceptibility measurements were carried out. In the temperature-dependent neutron diffraction analysis, a diffused scattering signal appears at the low Q region below (111) magnetic Bragg peak at all temperature, indicating the presence of a finite magnetic cluster with infinite magnetic network. The diffused scattering signal intensity decreases with increases in magnetic field at T=10K. The ac-susceptibility measurement exhibits three peak behavior in χ′ and χ″ indicating the presence of finite magnetic clusters and cluster–cluster interaction in the system. The absence of magnetic (200) peak in neutron diffraction at 2K and bifurcation of zero field and field cooled magnetization indicate the phase transition from uniaxial random ferromagnetic (URF) phase to semi spin glass or canted random ferromagnetic (CRF) phase in the system with temperature.

Determination of the magnetic structure of SmFe3(BO3)4 by neutron diffraction: comparison with other RFe3(BO3)4 iron borates

Temperature dependent neutron diffraction studies were performed on SmFe3(BO3)4. The crystallographic structure was determined to stay as R32 over the whole studied temperature range of 2 K < T < 300 K. A magnetic phase transition characterized by the magnetic propagation vector κ = [0 0 3/2] takes place at TN = 34 K. The magnetic structure sees an easy-plane arrangement within the trigonal basal a–b-plane of ferromagnetic layers of iron and samarium having a canting angle of about 70° relative to each other. Neighbouring layers in the c-direction are antiferromagnetically coupled; at 2 K the magnetic moment values amount to μFe = 4.2(1) μB and μSm = 0.8(2) μB. The non-Brillouin type increase of the iron magnetic moment below TN points to a strong Fe–Sm exchange and to the simultaneous appearance of long range magnetic order on both sublattices.

Pressure and Temperature Dependent Structure Of Zircon Type ThGeO4

Herein we report the results of high pressure diffraction studies of zircon type ThGeO4. ThGeO4 exhibits anisotropic compressibility with the average compressibility along a-axis (20.8 × 10−4/GPa) larger than that along c-axis (9.98 × 10−4/GPa). Fitting the pressure dependence unit cell volume to 3rd order Birch-Murnaghan equation of states, the zero pressure bulk modulus (Ko) and volume (Vo) of 166(5) GPa and 341.6(3) Å3, respectively have been obtained. Preliminary studies on temperature dependent neutron and x-ray diffraction studies on ThGeO4 revealed anisotropic expansion behaviour with larger expansion coefficient along c-axis compared to a-axis. No structural transition under temperature or pressure is observed in between ambient pressure to 10 GPa and in the temperature range of 25 to 1273K.

On the mechanism of proton conductivity in H3OSbTeO6

Pyrochlore-type H3OSbTeO6 is a remarkable proton conductor exhibiting an outstanding electrical conductivity even at ambient temperature. It consists of a three-dimensional interconnected (Sb,Te)O6 framework, built from randomly distributed corner-shared SbO6 and TeO6 octahedra, forming large cages in which the H3O+ ions are located. The dynamics of the caged species has been investigated by temperature-dependent neutron diffraction, quasielastic neutron scattering, and NMR experiments. Three types of motion may be discerned, namely, stochastic rotations of the H3O group around its 3-fold axis, jumps between four equivalent positions within the cage, and long-range inter-cage translational diffusion. The onset of ionic conductivity is clearly reflected by structural changes. Details of the complex diffusion mechanism are given.

First order paramagnetic–ferromagnetic phase transition in Tb 3+ doped La 0.5Ca 0.5MnO 3 manganite

Effect of A-site disorder in half-doped La 0.5Ca 0.5MnO 3 created by the substitution of Tb at La site is studied through temperature dependent neutron diffraction, resistivity, and magnetization on La 0.375Tb 0.125Ca 0.5MnO 3 (LTC) to identify the evolution of phases as a function of temperature and magnetic field. Contrary to the parent system, the paramagnetic (PM) to ferromagnetic (FM) transition is found to be of first order but becomes observable only in fields above 1 T. Absence of this transition in zero and low fields along with the observation of a very intriguing and irreversible M– H curve at low temperature raises the question about the ground state of the system. It is identified from the specially designed measurement protocol that the ground state of the system becomes FM, contrary to the parent LCMO. However, this remains masked because of the hindrance to the transformation kinetics of the PM–FM first order transition in low fields. Significantly, though the thermal hysteresis of the first-order transition decreases with the measurement field, collapse of such a behavior to a critical point is not observed in LTC, unlike earlier observations in some other manganites.