\SCWO is a tetragonal ($I4/m$) double perovskite with diamagnetic W$^{6+}$ (5d$^0$) and (Jahn-Teller-active) Cu$^{2+}$ (3$d^9$) in a rocksalt-ordered arrangement on the B sites, modelling an anisotropic fcc-lattice antiferromagnet with $S=1/2$. We have studied the magnetic structure, dynamics and thermodynamic properties of this material above and below the \Neel{}-ordering transition at $T_{\textrm{N}} = 24$~K. Muon spin relaxation spectroscopy, neutron diffraction and neutron spectroscopy experiments show that the transition at $T_{\textrm{N}}$ on heating is from static 3D type-2 antiferromagnetism to 2D dynamic correlations that remain detectable up to 100~K. Above $T_{\textrm{N}}$ the muon relaxation is described by a compressed exponential decay, revealing a strong correlation between the spin fluctuation rate and the 2D correlation length. The low-temperature muon data and the entropy release around $T_{\textrm{N}}$ as obtained from the heat capacity data point to small magnetic domains, probably of the order of 60~Å. We note that the material behaves more like a quasi-2D square lattice antiferromagnet than could be expected considering its structure only. A possible explanation might lie in the unusual strong spin-orbit-coupled magnetism of the W 5$d$ levels, as the dominant exchange interaction is via the Cu-O-W-O-Cu pathway.
The synthesis and physical properties of the double perovskite (DP) compound Ba2CdReO6 with the 5d1 electronic configuration are reported. Three successive phases originating from a spin-orbit-entangled Jeff = 3/2 state, confirmed by the reduced effective magnetic moment of 0.72 μB, were observed upon cooling. X-ray diffraction measurements revealed a structural transition from a high-temperature cubic structure to a low-temperature tetragonal structure at Ts = 170 K, below which the Jeff = 3/2 state was preserved. Magnetization, heat capacity, and thermal expansion measurements showed two mor…
Sr$_2$CuWO$_6$ is a double perovskite proposed to be at the border between two and three dimensional magnetism, with a square lattice of $S=\frac{1}{2}$ Cu$^{2+}$ ions. We have used inelastic neutron scattering to investigate the spin wave excitations of the system, to find out how they evolve as a function of temperature, as well as to obtain information about the magnetic exchange interactions. We observed well defined dispersive spin wave modes at $6$~K, which partially survive above the magnetic ordering temperature, $T_N=24$~K. Linear spin wave theory is used to determine the exchange int…
We study magnetism on the fcc lattice with tetragonal distortions, with general exotic directional magnetic interactions allowed by symmetry. We consider two models, corresponding to a uniform tetragonal distortion, or a two-sublattice model with a tetragonal screw axis. We establish their low temperature phase diagrams in the semi-classical limit using classical optimization and consideration of fluctuations both analytically and by Monte Carlo simulation. Both order by disorder and exchange anisotropy mechanisms favor a $\langle 110\rangle$ easy axis for magnetization. We also show that spin…
Normally $4d$ or $5d$ transition metals are in a low-spin state. Here using first-principles calculations, we report on a rare case of a high-spin $S$=1 magnetic state for the Ag$^{3+}$ ion in the double perovskite Cs$_{2}$KAgF$_{6}$. We also explored a possibility of a conventional low-spin $S$=0 ground state and find an associated tetragonal distortion to be 0.29 Å. However, the lattice elastic energy cost and the Hund exchange loss exceed the e$_{g}$ crystal-field energy gain, thus making the low-spin tetragonal structure less favorable than the high-spin cubic structure. We conclude that t…
Structural, magnetic and electric properties of a metal-ordered perovskite YBaMn2O6 have been studied by means of powder X-ray diffraction, DSC, magnetic susceptibility, and electric resistivity. It is found that this material undergoes a 1st order structural phase transition at Tc1=520 K, from a pseudo orthorhombic (monoclinic) phase to a pseudo tetragonal (monoclinic) one. Accompanied by this transition, the susceptibility exhibits a sharp drop, where ferromagnetic exchange interaction becomes antiferromagnetic. Furthermore, two more transitions, a metal-insulator transition at Tc2=480 K and…
The resonant X-ray scattering (magnetic elastic, RXMS, and inelastic, RIXS) of Ir$^{4+}$ at the L$_{2,3}$ edges relevant to spin-orbit Mott insulators A$_{n+1}$Ir$_{n}$O$_{3n+1}$ (A=Sr, Ba, etc.) are calculated using a single-ion model which treats the spin-orbit and tetragonal crystal-field terms on an equal footing. Both RXMS and RIXS in the spin-flip channel are found to display a non-trivial dependence on the direction of the magnetic moment, $\boldsymbolμ$. Crucially, we show that for $\boldsymbolμ$ in the \emph{ab}-plane, RXMS at the L$_2$ edge is zero \emph{irrespective} of the tetragon…
Quasi two dimensional magnetic structure of the triclinic double perovskite Ca$_2$CuWO$_6$
University College London（伦敦大学学院）；Durham University（杜伦大学）；STFC Rutherford Appleton Laboratory（STFC卢瑟福阿普尔顿实验室）；National Institute for Materials Science (NIMS)（国立材料研究所）；Hokkaido University（北海道大学）
High purity polycrystalline Na0.5La0.5RuO3 was synthesized by a solid state method, and its properties were studied by magnetic susceptibility, heat capacity and resistivity measurements. We find it to be a tetragonal perovskite, in contrast to an earlier report, with random La/Na mixing. With a Curie-Weiss temperature of -231 K and effective moment of 2.74 uB/mol-Ru, there is no magnetic ordering down to 1.8 K. A broad hump at 1.4 K in the heat capacity, however, indicates the presence of a glassy magnetic transition, which we attribute to the influence of the random distribution of Na and La…
Low-temperature spin dynamics of the double-layered perovskite La_2-2x_Sr_1+2x_Mn_2_O_7 (LSMO327) was systematically studied in a wide hole concentration range (0.3 <= x < 0.5). The spin-wave dispersion, which is almost perfectly 2D, has two branches due to a coupling between layers within a double-layer. Each branch exhibits a characteristic intensity oscillation along the out-of-plane direction. We found that the in-plane spin stiffness constant and the gap between the two branches strongly depend on x. By fitting to calculated dispersion relations and cross sections assuming Heisenberg mode…
The semiconductor Sr2FeOsO6, depending on temperature, adopts two types of spin structures that differ in the spin sequence of ferrimagnetic iron - osmium layers along the tetragonal c-axis. Neutron powder diffraction experiments, 57Fe Mössbauer spectra, and density-functional theory calculations suggest that this behavior arises because a lattice instability resulting in alternating iron-osmium distances fine-tunes the balance of competing exchange interactions. Thus, Sr2FeOsO6 is an example for a double perovskite, in which the electronic phases are controlled by the interplay of spin, orbit…
We calculate the influence of transverse fluctuations on the longitudinal dynamics in the striped phase of cuprates by using the bosonization technique. We find that a charge density wave instability can arise if the stripe is quarter filled and the underlying lattice potential has a zigzag symmetry. Our results explain why static stripes are experimentally observed in underdoped La_{2-x-y}Nd_ySr_xCuO_4 exactly at the onset of the low-temperature-tetragonal transition.
The effect of local structure on the magnetic and transport properties of the layered perovskites has been investigated. The samples PrBaCo$_{2}$O$_{5+δ}$, ($δ$ = 0.80 and 0.67) though crystallize in the same 112 type tetragonal structure have different magnetic ground states. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy has been employed to explain the changes in magnetic interactions based on the rotation and tilting of CoO polyhedra in these oxygen rich double perovskites.
We present an analysis of the electronic structure of perovskite-related iridates, 5d electron compounds where a subtle interplay between spin-orbit coupling, tetragonal distortions and electron correlations determines the electronic structure properties. We suggest via electronic structure calculations that a non-collinear calculation is required to obtain solutions close to the usually quoted $j_{eff}$ = 1/2 state to describe the $t_{2g}$ hole in the $Ir^{4+} :d^5$ cation, while a collinear calculation yields a different solution, the hole is in a simpler xz/yz complex combination with a sma…
Monte Carlo simulations are performed on the three-dimensional (3D) Ising model with the 2-1-4 layered perovskite structure as a minimal model for checkerboard charge ordering phenomena in layered perovskite oxides. Due to the interlayer frustration, only 2D long-range order emerges with a finite correlation length along the c axis. Critical exponents of the transition change continuously as a function of the interlayer coupling constant. The interlayer long-range Coulomb interaction decays exponentially and is negligible even between the second-neighbor layers. Instead, monoclinic distortion …
Structural and magnetic transitions in a double perovskite hosting 5d1 Re ions are discussed on the basis of recently published high-resolution x-ray diffraction patterns [D. Hirai, et al., Phys. Rev. Res. 2, 022063(R) (2020)]. A reported structural transition below room temperature, from cubic to tetragonal symmetry, appears not to be driven by T2g-type quadrupoles, as suggested. A magnetic motif at lower temperature is shown to be composed of two order parameters, associated with propagation vectors k = (0, 0, 1) and k = (0, 0, 0). Findings from our studies, for structural and magnetic prope…
We consider a model of the double perovskite Ba2FeReO6, a room temperature ferrimagnet with correlated and spin-orbit coupled Re t2g electrons moving in the background of Fe moments stabilized by Hund's coupling. We show that for such 3d/5d double perovskites, strong correlations on the 5d-element (Re) are essential in driving a half-metallic ground state. Incorporating both strong spin-orbit coupling and the Hubbard repulsion on Re leads to a band structure consistent with ab initio calculations. Using our model, we find a large spin polarization at the Fermi level, and obtain a semi-quantita…
Pressure induced phase transitions of MIAgIIF3 perovskites where M = K, Rb, Cs, have been predicted theoretically for the first time for pressures up to 100 GPa. The sequence of phase transitions for M = K and Rb consists of an orthorhombic to monoclinic and back to orthorhombic transition, associated with progressive bending of infinite chains of corner sharing [AgF6]4 minus octahedra and their mutual approaching via secondary Ag...F contacts. In stark contrast, only a single phase transition i.e. tetragonal to triclinic, is predicted for CsAgF3, associated with substantial deformation of the…
We present a study of the lattice response to the compressive and tensile biaxial stress in La0.67Sr0.33MnO3 (LSMO) and SrRuO3 (SRO) thin films grown on a variety of single crystal substrates: SrTiO3, DyScO3, NdGaO3 and (La,Sr)(Al,Ta)O3. The results show, that in thin films under misfit strain, both SRO and LSMO lattices, which in bulk form have orthorhombic (SRO) and rhombohedral (LSMO) structures, assume unit cells that are monoclinic under compressive stress and tetragonal under tensile stress. The applied stress effectively modifies the BO6 octahedra rotations, which degree and direction c…
It is well known that if a crystal structure has no inversion symmetry, it may allow for Dzyaloshinskii-Moriya magnetic interactions, operating between different crystallographic unit cells, which in turn should lead to the formation of long-periodic spin-spiral structures. Such a behavior is anticipated for two simple perovskites PbVO3 and BiCoO3, crystallizing in the noncentrosymmetric tetragonal P4mm structure. Nevertheless, we argue that in reality PbVO3 and BiCoO3 should behave very differently. Due to the fundamental Kramers degeneracy for the odd-electron systems, PbVO3 has no single-io…
Using density-functional theory calculations, we investigate the magnetic as well as the dynamical properties of tetragonal SrRuO3 (SRO) under the influence of epitaxial strain. It is found that both the tensile and compressive strain in the xy-plane could induce the abrupt change in the magnetic moment of Ru atom. In particular, under the in-plane ~4% compressive strain, a ferromagnetic to nonmagnetic transition is induced. Whereas for the tensile strain larger than 3%, the Ru magnetic moment drops gradually with the increase of the strain, exhibiting a weak ferromagnetic state. We find that …
The model developed for LaMnO$_3$ addresses the spin-orbital order by superexchange and Jahn-Teller orbital interactions in the cubic (perovskite) symmetry up to now whereas real crystal structure is strongly deformed. We identify and explain three \textit{a priori} important physical effects arising from tetragonal deformation: (i) the splitting of $e_g$ orbitals $\propto E_z$, (ii) the directional renormalization of $d-p$ hybridization $t_{pd}$, and (iii) the directional renormalization of charge excitation energies. Using the example of LaMnO$_3$ crystal we evaluate their magnitude. It is f…
Multiferroicity and magnetodielectric coupling in metal-organic-frameworks (MOFs) is rare and so far restricted mainly to formate-based systems with perovskite structure. In the course of this work we designed a tetragonal framework [Co(C16H15N5O2)], exhibiting spin-chains of Co2+ ions, which are bridged by an organic linker containing a dipolar nitrobenzene moiety. This compound shows relaxor-like ferroelectricity at 100 K, which is followed by the onset of complex magnetic order at 15 K, indicative of weak ferromagnetism. The clear anomaly of the dielectric constant at the magnetic ordering …
We use temperature dependent neutron powder diffraction (NPD) to investigate the structural changes and magnetic interactions in double perovskite Sr$_{2-x}$La$_x$CoNbO$_6$ ($x=$ 0.4 and 0.6). A structural phase transition from tetragonal ($I4/m$) to monoclinic ($P2_1/n$) is observed between $x=$ 0.4 and 0.6 samples. Interestingly, temperature evolution of the unit cell parameters follow the Grüneisen approximation, and the analysis suggest the isotropic thermal expansion in case of the $x=$ 0.4 sample, whereas the $x=$ 0.6 sample shows the anisotropy where the thermal expansion along the $c$-…
Compositionally tunable vanadium oxyhydrides Sr2VO4-xHx (x = 0 - 1) without considerable anion vacancy were synthesized by high-pressure solid state reaction. The crystal structures and their properties were characterized by powder neutron diffraction, synchrotron X-ray diffraction, thermal desorption spectroscopy, and first-principles density functional theory (DFT) calculations. The hydrogen anions selectively replaced equatorial oxygen sites in the VO6 layers via statistical substitution of hydrogen in the low x region (x < 0.2). A new orthorhombic phase (Immm) with an almost entirely hydro…
Measurements of the static magnetization, susceptibility and ESR of Gd spin probes have been performed to study the properties of antiferromagnetically ordered La_{2-x-y}Eu_ySr_xCuO_4 (x less or equal 0.02) with the low temperature tetragonal structure. According to the static magnetic measurements the CuO_2 planes are magnetically decoupled in this structural phase. The ESR study reveals strong magnetic fluctuations at the ESR frequency which are not present in the orthorhombic phase. It is argued that this drastic enhancement of the spin fluctuations is due to a considerable weakening of the…
We combine the results of magnetic and transport measurements with neutron diffraction data to construct the structural and magnetic phase diagram of the entire family of SrMn$_{1-x}$Ru$_{x}$O$_3$ ($0 \leqslant x \leqslant 1$) perovskites. We have found antiferromagnetic ordering of the C type for lightly Ru-substituted materials ($0.06 \leqslant x \leqslant 0.5$) in a similar manner to $R_{y}$Sr$_{1-y}$MnO$_3$ ($R$=La, Pr), due to the generation of Mn$^{3+}$ in both families of manganite perovskites by either $B$-site substitution of Ru$^{5+}$ for Mn$^{4+}$ or $A$-site substitution of $R^{3+}…
Inelastic neutron scattering measurements of CaFe2As2 under applied hydrostatic pressure show that the antiferromagnetic spin fluctuations observed in the ambient pressure, paramagnetic, tetragonal (T) phase are strongly suppressed, if not absent, in the collapsed tetragonal (cT) phase. These results are consistent with a quenched Fe moment in the cT phase and the strong decrease in resistivity observed upon crossing the boundary from the T to cT phase. The suppression or absence of static antiferromagnetic order and dynamic spin fluctuations in the non-superconducting cT phase supports the no…
SrTiO$_3$ is a model perovskite compound with unique properties and technological relevance. At 105 K it undergoes a transition from a cubic to a tetragonal phase with characteristic antiferrodistortive rotations of the TiO$_6$ octahedra. Here we study systematically the effect of different exchange correlation functionals on the structural, electronic and optical properties of cubic and tetragonal STO by comparing the recently implemented strongly constrained and appropriately normed (SCAN) meta-GGA functional with the generalized gradient approximation (PBE96 and PBEsol) and the hybrid funct…
