Crystal field effect and geometric frustration in Er2Ti2O7 —an XY antiferromagnetic pyrochlore

Magnetic susceptibility of powder Er₂Ti₂O₇ (ErT) is measured between 300 K and 80 K. shows a Curie-Weiss (CW) type behaviour with   ErTiO_3.5 and . A crystal field (CF) analysis of our experimental data, g-values (g_∥=0.27 and g_⊥=7.8) and the positions of two CF levels (reported earlier from an inelastic neutron scattering study) provide CF parameters and CF levels of the ground ⁴I_15/2 and excited multiplets of ErT. The theoretical follows a CW-type behaviour, with . Single-ion magnetic anisotropy (χ_⊥−χ_∥) is 9500×10⁻⁶ emu/mol ErTiO_3.5 at 300 K, which increases by ∼54 times at 10 K and ErT resembles an XY planar system. It can be inferred from CF analysis that the earlier observed change of from −13 K to −22 K below 50 K is not due to the CF effect. Nuclear hyperfine (HF) levels of ¹⁶⁷ErT and ¹⁶⁶ErT are calculated and the theoretical curve of vs. T (K) for T<T_N matches the observed results. Mössbauer lines expected for ¹⁶⁶ErT are also predicted.     

Magnetic states and exchange interaction in hexagonal MnFeAs: A first principles study

The electronic structure and magnetic states for hexagonal-MnFeAs have been studied by a first-principles density functional theory (DFT) calculation. The ground state is ferromagnetic and the calculated magnetic moments for Fe and Mn are 1.1 and 3.1μ_B, respectively, leading to a total magnetization of 4.1μ_B per formula unit due to the small negative moments of As atoms. The exchange interaction between Fe and Mn layers () is positive and tends to form the ferromagnetic ordering. On the other hand, the exchange interaction at the Fe-As₁ layer () is negative while that at the Mn-As₂ layer () is positive. The field induced first order magnetic transition at T_C is related to the competed exchange interaction in the compound.     

Optical phonon modes and structure of ZnGa2Se4 and ZnGa2S4

We present the infrared and Raman study of the optical phonon modes of the defective compounds ZnGa₂Se₄ and ZnGa₂S₄. Most of the compounds have been found to crystallize in the thiogallate structure (defect chalcopyrite) with space group where all cations and vacancies are ordered. For some Zinc compounds a partially disordered cationic sublattice with various degrees of cation and vacancy statistical distribution, which lead to the higher symmetry (defect stannite), has been reported. For ZnGa₂Se₄ we have found three modes of A symmetry, showing Raman activity only. In addition, we have observed each five modes of B and E symmetry, showing infrared as well as Raman activity. The number of modes and their symmetry assignment, based on polarized measurements, clearly indicate space group for the investigated crystals of ZnGa₂Se₄.Regarding ZnGa₂S₄ we have found three modes exclusively showing Raman activity (2A⊕1B₁), and only eight modes showing infrared as well as Raman activity (3B₂⊕5E). The assignment of the modes has been derived by analyzing the spectral positions of the vibrational modes in comparison to a number of compounds. From the number and symmetry assignment of the optical phonon modes we confirm that ZnGa₂S₄ most likely crystallizes in space group .     

Single crystal EPR studies of Mn(II) doped zinc sodium sulphate hexahydrate: a case of interstitial substitution

Single crystal Electron Paramagnetic Resonance studies on Mn(II) doped zinc sodium sulphate hexahydrate are carried out at room temperature. The angular variation spectra of the crystal in the three orthogonal planes show that the paramagnetic impurity Mn(II) has entered the host lattice interstitially. The spin Hamiltonian parameters calculated are: g_xx=1.899, g_yy=1.944, g_zz=2.024; and The probable location for the interstitial site has been identified from the position of various atoms in the lattice. The variable temperature study for polycrystalline sample has been carried out, which indicates no phase transition. The percentage covalency of Mn-O bond has been estimated to be 8.5%.     

Compression of CdCu3Ti4O12 perovskite to 55 GPa

Synchrotron x-ray diffraction measurements of CdCu₃Ti₄O₁₂ (CDCTO) were performed up to 55.5 GPa. There is no structural phase transformation in this pressure range. The irregular curvature shifts of the P-V curve are attributed to the grain surface effect. Analysis indicates that the grain surface of CDCTO is stiffer than the grain interior at higher pressures. We point out that the atoms on grain surfaces must be either densely packed or have a strong correlation with the gain interior in order to have a high dielectric constant, as in CaCu₃Ti₄O₁₂. The derived bulk modulus of CDCTO is approximately 235±7 GPa with K^′=5.1±0.4.     

Charge and orbital ordering in Na0.5CoO2

The ground state of Na_0.5CoO₂ has been calculated using the full potential local orbital method and local density approximation plus Hubbard U (); the results demonstrate that charge and orbital ordering evidently exist in the present system in association with the antiferromagnetic state. Notable structural features observed between 300 and 30 K have been carefully examined using in situ TEM investigations, a superstructure with a wave vector of Q1=a^∗/2, becoming commonly visible below , can be interpreted as the charge/orbital ordering on the Co1 and Co2 sites. Moreover, we have also observed another notable superstructure with Q2=a^∗/4 below the phase transition of , which suggests a more complicated orbital ordered state existing at lower temperatures.     

The magnetocaloric effect and low temperature specific heat of SmNi

The magnetocaloric effect (MCE) in a magnetic SmNi sample was evaluated from magnetization and heat capacity measurements. The MCE phenomena in the vicinity of magnetic phase transitions in terms of magnetic entropy change, , and adiabatic temperature change, , are reported. Isothermal magnetization measurements at several temperatures around the transition were carried out and used for versusT calculations. A similar dependence of the magnetic entropy change was evaluated from heat capacity C_p(T) measurements under zero field and 5 T. The SmNi system provides magnetic refrigerants that induce an adiabatic cooling of about during the magnetization process with a field of 5 T in the temperature range of 35-45 K. The temperature dependence of C_p(T) is analyzed in terms of the magnetic and the lattice contributions.     

Approach of single-molecule magnets to thermal equilibrium

We study the spin-lattice relaxation of single-molecule-magnets (SMM) using time-dependent specific heat C_m measurements. These molecular clusters, intermediate between paramagnetic atoms and ferromagnetic nanoparticles, are ideal systems to investigate if quantum phenomena contribute to relaxation at the mesoscopic scale. Experiments show indeed that relaxation to equilibrium proceeds by quantum tunnelling through the magnetic anisotropy energy barrier. For sufficiently high temperatures tunnelling takes place between excited magnetic states. Tunnelling via lower lying states can be promoted by applying a magnetic field B_⊥ perpendicular to the anisotropy axis. For sufficiently large B_⊥, the lowest energy states become quantum coherent superpositions. The equilibrium C_m is dominated, for T<1 K, by dipolar interactions between the molecular spins. A nearly isotropic Mn₆ cluster compound shows a transition to a ferromagnetic phase at For Ising-like SMM's, such as Mn₄, relaxation takes place by incoherent tunnelling between the lowest lying ±S states, assisted by interactions with phonons and nuclear spins. Tunnelling can then be promoted by lowering the symmetry of the molecule. In this case too, the molecular spins order if tunnelling remains sufficiently fast down to      

Magnetic study of Mg0.95Mn0.05Fe2O4 ferrite nanoparticles

The magnetic properties of Mg_0.95Mn_0.05Fe₂O₄ ferrite samples with an average particle size of ∼6.0±0.6 nm have been studied using X-ray diffraction, Mössbauer spectroscopy, dc magnetization and frequency dependent real χ^′(T) and imaginary χ^″(T) parts of ac susceptibility measurements. A magnetic transition to an ordered state is observed at about 195 K from Mössbauer measurements. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization have been recorded at low field and show the typical behavior of a small particle system. The ZFC curve displays a broad maximum at , a temperature which depends upon the distribution of particle volumes in the sample. The FC curve was nearly flat below , as compared with monotonically increasing characteristics of non-interacting superparamagnetic systems indicating the existence of strong interactions among the nanoparticles. A frequency-dependent peak observed in χ^′(T) is well described by Vogel-Fulcher law, yielding a relaxation time and an interaction parameter . Such values show the strong interactions and rule out the possibility of spin-glass (SG) features among the nanoparticle system. On the other hand fitting with the Néel-Brown model and the power law yields an unphysical large value of τ₀ (∼6×10⁻⁶⁹ and 1.2×10⁻²² s respectively).     

Quantum critical behaviour in doped Y bB12 studied by ab initio calculations

The Kondo insulator Y bB₁₂ is known to undergo a transition to the metallic state with doping or under an external magnetic field. Within the virtual crystal approximation (VCA), we calculated the occupation of the Yb 4f and 5d shells, and , as a function of doping of Y bB₁₂ with the rare earths Tm and Lu. We found that exhibits an anomalous change at the critical concentration of the dopant, in agreement with experiment ( for Y b_1−xLu_xB₁₂ and for Y b_1−xTm_xB₁₂). We suggest that the critical behaviour seems to be strictly connected with the change of and in consequence the change of the Yb valency.     

Effect of Al doping on the martensitic transition and magnetic entropy change in Ni-Mn-Sn alloys

Effect of Al doping on the martensitic transition and magnetic entropy change in Mn₅₀Ni₄₀Sn_10−xAl_x was investigated. The experimental results show that the martensitic transition temperatures increase with the increase of Al content due to cell contraction, while the martensitic transition temperature range decreases rapidly. Mn₅₀Ni₄₀Sn₈Al₂ alloy has the largest value of  (3.14 J/kg K) for the magnetic field changing from 0 to 10 kOe, which is nearly twice as large as that of Mn₅₀Ni₄₀Sn₁₀ alloy. It is demonstrated that a larger can be obtained due to the sharper magnetization change around martensitic transition.     

Radiation and thermally induced effects in YAlO3:Mn crystals

We have studied effects induced by γ-radiation and temperature in Mn-doped YAlO₃ crystals. The studies have been performed by means of optical spectroscopy that include measuring of optical absorption changes induced by γ-radiation and elevated temperature as well as thermally stimulated luminescence (TSL). It has been shown that under γ-irradiation of YAlO₃:Mn crystals, along with the ionization of Mn_Al⁴⁺ ions (Mn_Al⁴⁺→Mn_Al⁵⁺+e⁻), some additional coloration processes take place. This additional coloration is characterized by a wide intense band centered at 26,000- that is ascribed to color centers intrinsic to YAlO₃ lattice. This coloration is removed by the way of crystal warming at , while the coloration caused by Mn_Al⁵⁺ ions is removed at higher temperature . The observed TSL glow of irradiated crystals reveals three peaks near 360, 400 and that correspond to three types of traps. Parameters of the traps have been determined. The TSL emission corresponds to intra-center luminescence of Mn_Al⁴⁺ and Mn_Y²⁺ ions. The possible ionization and trapping mechanisms in YAlO₃:Mn crystals are discussed.     

Pressure cycle of superconducting Cs0.8Fe2Se2 : A transport study

We report measurements of the temperature and pressure dependence of the electrical resistivity (ρ) of single-crystalline iron-based chalcogenide Cs_0.8Fe₂Se₂. In this material, superconductivity with a transition temperature develops from a normal state with extremely large resistivity. At ambient pressure, a large “hump” in the resistivity is observed around 200 K. Under pressure, the resistivity decreases by two orders of magnitude, concomitant with a sudden T_c suppression around . Even at 9 GPa a metallic resistivity state is not recovered, and the ρ(T) “hump” is still detected. A comparison of the data measured upon increasing and decreasing the external pressure leads us to suggest that the superconductivity is not related to this hump.     

Equation of state of cubic hafnium(IV) nitride having Th3P4 -type structure

Pressure dependence of the specific volume, V(P), of the recently discovered high-pressure compound Hf₃N₄ having cubic Th₃P₄-type structure (c- Hf₃N₄) has been measured at room temperature up to 43.9 GPa in a diamond anvil cell using energy-dispersive X-ray powder diffraction combined with synchrotron radiation. A least-square fit of the Birch-Murnaghan equation of state to the experimental V(P)-data yielded for c- Hf₃N₄ the bulk modulus of and its first pressure derivative of . For fixed at 4 the bulk modulus of c- Hf₃N₄ was determined to be . The obtained B₀-value is only insignificantly below that estimated in preliminary measurements. Existing theoretical predictions for B₀ scatter around the present experimental data. The observation of a high bulk modulus of c- Hf₃N₄ supports the suggestion that this compound could have high hardness.     

Magnetocaloric properties of DyCo2−xGax alloys

The influence of the substitution of Ga atoms for Co atoms in DyCo₂ compounds on magnetocaloric properties has been investigated. A series of DyCo_2−xGa_x alloys with x=0, 0.03, 0.06, 0.1, 0.15, and 0.2 was prepared by the arc-melting method for this investigation. Experimental results revealed that the Ga substitution for Co in DyCo₂ can form a single phase with the cubic Laves phase structure up to x=0.2. As the Ga content x increases, the lattice parameter and the Curie temperature T_c increases from 143 to 196 K linearly. The maximum magnetic entropy changes in a low field change of 0-1.5 T, increasing from 8.24 to 10.61 J/K kg when the Ga content x increases from 0 to 0.03, but decreasing gradually to 3.51 J/K kg as the Ga content further increases to x=0.2. All the samples show a relatively large magnetic entropy change with very small hysteresis loss.     

Synthesis of new La nitrides at high pressure and temperature

We have succeeded in synthesizing two new lanthanum nitrides in a supercritical nitrogen fluid at high pressure (about 30 GPa) and high temperature (about 2000 K), using a diamond anvil cell and a YAG laser heating system. These nitrides were found to be stable down to 5 GPa and ∼300 K in a nitrogen atmosphere. One of the new lanthanum nitrides is a cubic P lattice-type phase, which is a main phase synthesized nitride. The calculated lattice parameter is at 5 GPa, 300 K. The other nitride is of a trigonal P lattice-type. The calculated lattice parameters are and at 5 GPa, 300 K. The most likely phase of the former new La nitride is , the structure of which may be similar to the   Mn₂O₃-type (Ia80). The phase of the latter nitride is , the structure of which is the same as the   La₂O₃-type (hP5).     

Hydrogen bonds mediated magnetism in Cu(bmen)2Pd(CN)4

Results of synthesis, X-ray structure analysis, electron spin resonance, susceptibility, magnetization and specific heat measurements of powdered Cu(bmen)₂Pd(CN)₄ (,N^′-dimethyl-1,2-diaminoethane) are reported. Its structure is formed of quasi-linear chains of the [-Cu(bmen)₂-NC-Ni(CN)₂-CN-]_n composition; these are interlinked by hydrogen bonds (HBs) leading to two-dimensional patterns. Upon magnetic, spectral and thermodynamic measurements the compound was identified as an S=1/2 Heisenberg antiferromagnet on a square lattice with due to the dominant role of HBs in creating a square network. The long-range ordering observed at 0.24 K is proposed to be of a Néel type. The possibility of tuning the exchange interactions in various directions is considered.     

Experimental and theoretical studies on X-ray induced secondary electron yields in Ti and TiO2

Generation of X-ray induced secondary electrons in Ti and TiO₂ was studied from both experimental and theoretical approaches, using X-ray photoelectron spectroscopy (XPS) attached to a synchrotron radiation facility and Monte Carlo simulation, respectively.The experiment revealed that the yields of secondary electrons induced by X-rays (electrons/photon) at photon energies to 4950 and 5000 eV for Ti and TiO₂ are δ_Ti(4950 eV) = 0.002 and δ_Ti(5000 eV) = 0.014 while those for TiO₂ are δTiO₂(4950 eV)=0.003 and δTiO₂(5000 eV)=0.018.A novel approach to obtain the escape depth of secondary electrons has been proposed and applied to Ti and TiO₂. The approach agreed very well with the experimental data reported so far. The Monte Carlo simulation predicted; and while and .An experimental examination on the contribution of X-ray induced secondary electrons to photocatalysis in TiO₂ has also been proposed.