Cooperative emission study in ytterbium doped NdVO4

The infrared and visible cooperative emissions of ytterbium ions are studied in Yb-doped NdVO₄ single crystals. The absorption of optical phonons allows the emissions at room temperature when a Nd:YAG laser is used. Low temperature emissions are observed due to the Nd³⁺→Yb³⁺ energy transfer following an argon ion laser excitation of the Nd³⁺ ions. Analysis of the cooperative emission at low doping concentration (1%) indicates that it is generated by distant pair forming Yb³⁺ ions while at high doping concentration (≥ 5%) close ions magnetically coupled and superexchange mechanisms prevail in the emitting process.     

Preparation and investigation of magnetic properties of MnNiTi-substituted strontium hexaferrite nanoparticles

A series of M-type strontium hexaferrite powders with substitution of Mn²⁺, Ni²⁺ and Ti⁴⁺ ions for Fe³⁺ ions according to the formula SrFe₉(Mn_0.5−xNi_xTi_0.5)₃O₁₉, where x ranges from 0 to 0.5 with a step of 0.1, has been prepared via the conventional ceramic method. In order to get nanoparticles, the obtained powders were milled in a high energy SPEX mill for 1 h. XRD investigations of the unmilled and milled powders show that the prepared samples are all single phase hexaferrite. Lattice parameters and mean crystallite sizes of the powders were determined from the XRD data and Scherrer’s formula. Transmission electron microscope (TEM) was used to analyze their structures. Room temperature magnetizations and coercivities of the samples in a magnetic field of 15 kOe have been determined from the hysteresis loops. It was found that magnetizations of the milled samples were smaller than the magnetization of the unmilled samples. This decrease, based on core-shell model, has been attributed to the presence of a magnetically dead layer on the particles’ surface of the milled powders. In addition, the magnetizations of the milled samples decrease with the increase in x value. This decrease has been discussed according to site occupation of the substituted cations on the sublattices. The discussion also supports the increase of lattice parameters and the decrease of Curie temperature as x increases.     

Symmetry and Intervalent Charge Transfer in Copper-Oxide Superconducting Precursors

A vibrationally coupled intervalent charge transfer theory is proposed for the copper oxide-superconductors. The proposed B_1g-vibrational distortion exactly interchanges the chemical environment of the neighboring copper atoms. A quadruple cell is constructed to accommodate the non-stoichiometry and a double copper-oxide-conducting chain with distorted symmetry is postulated. The distorted structure, prone to switching of the copper sites by vibration, is postulated to be the superconducting precursor. The left over-oxygen, due to oxygen deficiency, is allowed to serve as a bridge between the chains and 10 oscillate with the vibration. Symmetry of the system, before and after distortion, is used to explain the spin-angular momentum change, needed to arrive at Cooper-paired electron/holes. The O^? and O⁼ atoms are postulated to be the bridge for superexchange and charge flow between the Cu⁺¹/Cu⁺³ and Cu⁺¹/Cu⁺² pairs. The same theory is used to interpret the mechanisms for the following three types of copper oxide-superconductors: YBa₂Cu₃O_T-x; La_2-xSr_xCuO₄ and the newly discovered electron superconductor Nd_2-xCe_xCuO_4-y. The bridge oxygen oscillation may be chosen not to come from the breathing mode but from the bending deformation mode that involves the relative motion of the Cu atoms (to the oxygen) giving rise to smaller oxygen isotope effect.     

The study of superexchange interaction of ordered Li 0.5 Fe 1.0 Rh 1.5 O 4

Li_0.5Fe_1.0Rh_1.5O₄ has been studied by X-ray diffraction, Mössbauer spectroscopy. The crystal structure is characterized by the additional reflection (200) that is described by 1:1 ordered structure of Li, Fe at tetrahedral (A) site and can be assigned to the space group F $\bar{4}Li_0.5Fe_1.0Rh_1.5O₄ has been studied by X-ray diffraction, M?ssbauer spectroscopy. The crystal structure is characterized by the additional reflection (200) that is described by 1:1 ordered structure of Li, Fe at tetrahedral (A) site and can be assigned to the space group F3m. The lattice constant (a ₀) is 8.4348 ?. The temperature dependence of the magnetic hyperfine field is analyzed by the Néel theory of ferrimagnetism. The inter-sublattice superexchange interaction is found to be antiferromagnetic with a strength of J _A–B = –3.78 k _B while the intra-sublattice superexchange interactions are ferromagnetic with strengths of J _A–A = 5.40 k _B and J _B−B = 7.39 k _B . The Debye temperatures of the tetrahedral and octahedral sites are determined to be 388 and 464 ± 3 K, respectively, and the Néel temperature has been found to be 260 ± 3 K.     

Hydrothermal synthesis, characterization and composition-dependent magnetic properties of LaFe1−xCrxO3 system (0≤x≤1)

Hydrothermal synthesis, characterization and magnetic properties of a series of ABO₃-perovskites LaFe_1−xCr_xO₃ (0≤x≤1) are reported. The alkalinity in initial reaction mixtures plays a critical role in controlling the designed stoichiometry of the final compositions. Their magnetic properties are strongly dependent on the compositions and a maximum magnetic moment is found for the sample at x=0.5. Weak ferromagnetic interaction observed for the samples from x=0 to 0.9 arises from the presence of Fe-O-Fe antisymmetric exchange and Fe-O-Cr superexchange interaction. The weak ferromagnetism as well as the linear variation of the lattice parameters implies the possible random distribution of Fe and Cr ions in B sites of the perovskites. The evolution of magnetic ordering transition temperatures has a close relationship with substituent ratios, for the competition of antiferromagnetism and ferromagnetism. The saturated magnetic moment shows a great improvement compared with that for the samples synthesized by solid state method.     

First-principle calculation on nearly half-metallic antiferromagnetic behavior of double perovskites La2VReO6

While it was recently found that La₂VTcO₆ and La₂VCuO₆ are promising candidates for half-metallic antiferromagnets (HM-AFM), the search continues for other potential candidates of HM-AFM in the double perovskites structure La₂BB′O₆ (B, B′=transition metal). La₂VReO₆ is found to be a nearly HM-AFM. Furthermore, considering correlation and spin-orbital coupling (SOC) effects in transition metals, it is still nearly a HM-AFM after generalized gradient approximation with correction of on-site Coulomb interaction and SOC calculations, as reported herein.     

Microscopic model for superexchange interactions and photomagnetism in binuclear transition metal complexes

Recent experiments show that the superexchange interaction in molecular clusters containing transition metal ions A?=?Ni^II and B?=?W^V, Nb^IV or Mo^V in some cases is antiferromagnetic, contrary to the conventional superexchange rules. To understand this anomaly, we develop a quantum many-body model Hamiltonian and solve it exactly using a valence bond (VB) approach. We identify the various model parameters which control the ground state spin in different clusters of the A-B system. We present quantum phase diagrams that delineate the high and low-spin ground states in the parameter space. We fit the spin gap to a spin Hamiltonian and extract the effective exchange constant within the experimentally observed range, for reasonable parameter values. We also find a region of intermediate spin ground state in the parameter space, in clusters of larger size. The spin spectrum of the microscopic model cannot be reproduced by a simple Heisenberg exchange Hamiltonian. The above microscopic model is generic and can also be employed to explain photomagnetism in the MoCu₆ system. We solve the model for MoCu₆ and find that ground state is degenerate and is spanned by the S?=?0,?1,?2 and 3 manifolds with doubly occupied Mo site corresponding to Mo(IV) and singly occupied Cu sites corresponding to Cu(II) configurations. In each of these spin spaces, we observe that there exist charge-transfer (CT) states at ≈3?eV above the ground state which are dipole coupled to the ground state. The transition dipole in the S?=?3 manifold is the largest for the CT excitations. Coupled with the fact that the density of states of the S?=?3 manifold is sparse, compared to other spin manifolds, we expect that the S?=?3 CT excited state to be long-lived, thereby explaining the experimentally observed photomagnetism in the MoCu₆ system.     

High-pressure synthesis,structure and magnetic properties of LaCrO3 ceramics

Single-phase LaCrO₃ ceramics was synthesized successfully by the solid state sintering method under high pressure (5 GPa). X-ray diffraction measurements suggest that the high-pressure synthesized sample is well-crystallized, but the cell volume is larger than that of the sample synthesized under ambient pressure, which can be ascribed to the occurrence of Cr²⁺ converted form part of Cr³⁺. Such mixed valent state with coexistence of Cr²⁺ and Cr³⁺ could be confirmed by X-ray photoelectron spectroscopy measurements. Magnetic measurement results indicate that in addition to antiferromagnetic superexchange interaction of Cr³⁺-O-Cr³⁺ network accompanied by weak ferromagnetism at high temperature, another kind of ferromagnetic behavior can be observed at low temperature (below 30 K), which could be attributed to double exchange interaction of Cr²⁺-O-Cr³⁺ networks.     

Structural and Magnetic Properties of Pr2BaCuO5 Oxide

The Pr₂BaCuO₅ oxide has been prepared by using a precursor method. X‐ray and neutron diffraction data reveal that this mixed oxide crystallizes with the so‐called Nd₂BaPtO₅‐type structure, showing tetragonal symmetry and space group P4/mbm. Magnetic susceptibility, magnetization and specific heat measurements reveal that Pr₂BaCuO₅ oxide behaves as antiferromagnet, where the Pr³⁺ and Cu²⁺ sublattices become simultaneously ordered at 15 K. Preliminary neutron diffraction studies confirm this proposed antiferromagnetic ordering and it can be described by the wave vector k = [0,0,0].