Magnetocaloric and magnetic properties of La_2NiMnO_6 double perovskite

The magnetic effect and the magnetocaloric effect in La_2NiMnO_6(LNMO) double perovskite are studied using the Monte Carlo simulations.The magnetizations,specific heat values,and magnetic entropies are obtained for different exchange interactions and external magnetic fields.The adiabatic temperature is obtained.The transition temperature is deduced.The relative cooling power is established with a fixed value of exchange interaction.According to the master curve behaviors for the temperature dependence of △S_m~(max) predicted for different maximum fields,in this work it is confirmed that the paramagnetic-ferromagnetic phase transition observed for our sample is of a second order.The near room-temperature interaction and the superexchange interaction between Ni and Mn are shown to be due to the ferromagnetism of LNMO.     

Magnetocaloric effect in Sr2TiMoO6 double perovskite

Environmental friendly double perovskite Sr₂TiMoO₆ has been studied by using the density functional theory with generalized gradient approximation (GGA+U) approaches, and the Monte Carlo simulation in the framework of Ising model. The parameter of the unit cell has been optimized. The value of the crystal field and the exchange coupling have been estimated from the partial electronic density of states and the energy calculations, respectively. The magnetic properties and the magnetocaloric effect of the compound have been also studied. The Relative cooling power has been performed. It is found that Sr₂TiMoO₆ is an interesting material for the magnetic refrigeration applications.     

Structural,electronic and magnetic properties of the double perovskite Pb2FeReO6

The structural, electronic and magnetic properties of the double perovskite Pb₂FeReO₆ have been studied by using the first-principles projector augmented wave (PAW) potential within the generalized gradient approximation (GGA) as well as taking into account the on-site Coulomb repulsive and exchange coupling interactions (GGA+U). The optimized crystal structure of the Pb₂FeReO₆ is a body-centered tetragonal (BCT) with a space group of I4/m and the lattice constants of a=b=5.59 Å and c=7.93 Å, consistent with the experimental results. The two axial transition metal and oxygen (TM–O) distances are slightly larger than the four equatorial TM–O distances and shows the existence of the Jahn–Teller structural distortion in FeO₆ and ReO₆ octahedra. The Fe³⁺ and Re⁵⁺ ions are in the states (3d⁵, S=5/2) and (5d², S=1) with magnetic moments 3.929 and −0.831μ_B respectively and thus antiferromagnetic (AFM) coupling via oxygen between them. The half-metallic (HM) ferromagnetic (FM) nature implies a potential application of this new compound in magnetoelectronic and spintronics devices.     

Raman scattering in the magnetically frustrated double perovskite Sr2YRuO6

The spin correlations and excitations of the Sr₂YRuO₆ double perovskite are investigated by means of Raman scattering, complemented by synchrotron X‐ray diffraction measurements. Anomalous softening of a breathing mode of the oxygen octahedra is observed below ~200 K, much above the long‐range antiferromagnetic ordering temperature, T_N1 = 32 K, due to a spin‐phonon coupling mechanism in the presence of magnetic correlations. A diffusive Raman signal is also observed, possibly associated with spin excitations within magnetically correlated regions. Our results point to a characteristic energy and temperature scale of ~25 meV/200 K below which unusual behavior associated with magnetic correlations is observed in this material. Copyright © 2013 John Wiley & Sons, Ltd.     

Magnetic phase transitions and iron valence in the double perovskite Sr 2 FeOsO 6

The insulating and antiferromagnetic double perovskite Sr₂FeOsO₆ has been studied by ⁵⁷Fe Mössbauer spectroscopy between 5 and 295 K. The iron atoms are essentially in the Fe^3?+? high spin $( {t_{2\mathrm{g}}{^3} e_\mathrm{g}{^2} } )$ and thus the osmium atoms in the Os $^{5+} ( {t_{2\mathrm{g}}{^3} } )$ state. Two magnetic phase transitions, which according to neutron diffraction studies occur below T _N?= 140 K and T ₂?= 67 K, give rise to magnetic hyperfine patterns, which differ considerably in the hyperfine fields and thus, in the corresponding ordered magnetic moments. The evolution of hyperfine field distributions, average values of the hyperfine fields, and magnetic moments with temperature suggests that the magnetic state formed below T _N is strongly frustrated. The frustration is released by a magneto-structural transition which below T ₂ leads to a different spin sequence along the c-direction of the tetragonal crystal structure.     

Structural, magnetic and electrical transport properties of Co doping in LaKFeMoO6 double perovskite

The structural, magnetic and transport properties of La_1+xK_1−xFe_1−yCo_yMoO₆ (0.0≤x≤0.1 and 0.1≤y≤0.2) series are studied. At room temperature, the crystal structure is a monoclinic system with space group P2₁/n. The antisite defect lowers with Co doping in LaKFe_1−yCo_yMoO₆ series. However, it increases with the substitution of K by La. Magnetizations increase with the increase in Co content (x=0) and with the La substitution for K, respectively. All compounds demonstrate semiconducting behavior. Their electrical resistivities increase with Co content for LaKFe_1−yCo_yMoO₆ and also increase with La for La_1+xK_1−xFe_1−yCo_yMoO₆. For the LaKFe_1−yCo_yMoO₆ the electrical transport behavior can be described by Mott variable range hopping model in the studied temperature range, whereas for the La_1+xK_1−xFe_1−yCo_yMoO₆ (x≠0 and y≠0) the electrical transport behavior follows the Mott and ES variable range hopping model in high and low temperature ranges, respectively. Each sample exhibits a large magnetoresistance effect.     

Magnetic properties of the double perovskite compound Sr_2YRuO_6

We study the magnetic properties of the double perovskite ruthenate compound Sr_2YRuO_6 using Monte Carlo simulations(MCS).We elaborate the ground state phase diagrams for all possible and stable configurations.The magnetizations and the susceptibilities as a function of temperature for the studied system are also reported.The effects of the exchange coupling interactions and the crystal field are examined and discussed.On the other hand, since the compound Sr_2YRuO_6 exhibits an antiferromagnetic behavior, we find its Néel temperature, T_N≈ 31 K, which is in good agreement with the experimental results in the literature.To complete this study, the hysteresis loops and the coercive field as a function of the external magnetic field are also obtained for fixed values of the physical parameters.     

Critical behavior and effect of Sr substitution in double perovskite Ca_2CrSbO_6

The double perovskite Ca2CrSbO6 exhibits a ferromagnetic long-range order below Tc=13 K and a saturation magnetization of 2.35μB at 2 K.In this study,the polycrystalline Ca2CrSbO6 is synthesized under high pressure and high temperature,and the critical behavior of the ferromagnetic material as well as the effects of the magnetic behavior due to the isovalent substitution of Sr2+for Ca2+is investigated.Also studied are the ferromagnetic criticality of the double perovskite Ca2CrSbO6 at the ferromagnetic transition temperature Tc≈12.6 K from the isotherms of magnetization M(H)via an iteration process and the Kouvel–Fisher method.The critical exponents associated with the transition are determined as follows:β=0.322,γ=1.241,andδ=4.84.The magnetization data in the vicinity of Tc can be scaled into two universal curves in the plot of M/|ε|βversus H/|ε|β+γ,whereε=T/Tc?1.The obtainedβandγvalues are consistent with the predicted values from a three-dimensional Ising model.The effects of Sr substitution on the double perovskite Ca2CrSbO6 are taken into consideration.As the Sr content increases,the(Ca2?xSrx)CrSbO6 polycrystal shows a continuous switch from ferromagnetic to antiferromagnetic behavior.     

Orbital-decomposed electronic and magnetic properties of the double perovskite Sr2FeReO6

The detailed orbital-decomposed electronic structures and magnetic properties of the double perovskite Sr₂FeReO₆ have been studied using the first-principles projector augmented wave (PAW) potential within the generalized gradient approximation (GGA). Both occupied and unoccupied s and three p states of Fe³⁺ ion are located far away from the Fermi level, while all up-spin states and most down-spin states are completely filled for the s and three p states of Re⁵⁺ ion. The octahedral crystal field of the oxygen atoms around transition-metal (TM) sites splits the five-fold degenerate d states of the free TM atoms into triply degenerate t_2g states with smaller bonding-antibonding splitting and doubly degenerate e_g states with larger bonding-antibonding splitting. The Fe³⁺ and Re⁵⁺ ions are in the states (3d⁵, S=5/2) and (5d², S=1) with magnetic moments 3.70 and −0.86μ_B, respectively and thus antiferromagnetic coupling via oxygen between them. There are no direct interactions between two nearest Fe-Fe or Re-Re pairs, whereas along each Fe-O-Re-O-Fe or Re-O-Fe-O-Re chains, the hybridizations between Fe 3d and 4s, O 2s and 2p, as well as Re 5p, 5d and 6s orbitals are fairly significant.     

Hole doping effects in (Sr2-xNax)FeMoO6 double perovskite

Hole-doped double perovskite compounds (Sr,Na)₂FeMoO₆ with the Na content of x=0,0.02,0.03,0.07 and 0.17 have been synthesized by sol–gel method. Effects of hole doping on the crystal structure and magnetic properties of Sr₂FeMoO₆ have been investigated by means of X-ray powder diffraction (XRD) and magnetic measurements. The XRD pattern indicates that all the samples are of single phase and belong to the space group I4/m. Due to the smaller ionic radius of Na⁺ than that of Sr²⁺ ions, the lattice constants and unit cell volume of the compound decrease slightly with x. The degree of cation ordering in the Na-doped Sr₂FeMoO₆ compounds shows a non-monotonic variation with the doping level, increasing from x=0 to x=0.03 and decreasing slightly with further increase of the doping. In contrast to the composition dependence of the degree of ordering, the Curie temperature of the compound decreases at low doping level and increases at high doping level. The saturation magnetization of the compound increases with x for x<0.17. Similar to the electron-doped Sr₂FeMoO₆, provided that the doped hole enters selectively the spin-down band, the variation of the saturation magnetization can be explained in light of the ferrimagnetic model (FIM). PACS 61.72.Ww; 75.50.-y; 75.30.Cr; 75.50.Gg     

Effect of cation disorder on structural, magnetic and dielectric properties of La2MnCoO6 double perovskite

The origin of dielectric anomalies and magnetodielectric response of La(2)MnCoO(6) has been investigated by means of ultra-high resolution synchrotron x-ray powder diffraction, neutron powder diffraction, resistivity, magnetization and dielectric measurements. The study has been performed on two different bulk samples presenting different degrees of Mn/Co order: 95 and 74%. Beside the well-known influence on magnetic properties, our results show that the main effect of disorder lies on the electrical resistivity. Bond distances clearly show Mn(4+)/Co(2+) valence states in the well-ordered sample, while for the disordered one this picture still holds. AC resistivity data show dielectric anomalies and a small magnetodielectric effect, but impedance complex plane analyses prove that these phenomena appear at the frequency-temperature region where extrinsic effects dominate the dielectric response.     

Preparation and dielectric properties of the KBiScNbO6 double perovskite

Ceramic KBiScNbO₆ compound was synthesized for the first time. X‐ray diffraction analysis revealed that at room temperature the samples consist of a mixture of the perovskite and pyrochlore phases. The dominant phase is the perovskite one. This phase can be described as distorted pseudocubic Pm3m structure with unit‐cell parameter a_c = 4.069 Å. Rhombohedral distortion leading the polar R3m phase is assumed to be one of possible structural distortions. Dielectric measurements of the compound under study demonstrated anomalies associated with relaxor ferroelectric behavior (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Monte Carlo study of the critical and the compensation behaviors of the double perovskite Sr2CrIrO6

The phase diagrams and magnetic properties of double perovskite $S{r}_{2}CrIr{O}_6$ have been studied by using Monte Carlo simulation based on the heat bath algorithm. The ground-state diagrams of the compound $S{r}_{2}CrIr{O}_6$ have been calculated for different combinations of system parameters. The diagrams obtained are very rich and they give an idea of all the most stable configurations. The effects of the exchange interactions and the crystal field on the phase diagrams and magnetic properties of the system have been examined. A number of interesting phenomena have been observed such as the compensation temperature, the first and second order phase transitions, the critical triple point and the terminal critical point.     

Synthesis, structure and magnetic properties of the new double perovskite Ca2CrSbO6

The title double perovskite has been synthesized by solid-state reaction in air. The crystal structure has been studied from powder X-ray diffraction data. Rietveld fits to the pattern show that this compound has a monoclinic symmetry [a=5.4932(3) Å, b=5.4081(3) Å, c=7.6901(5) Å, β=90.0022(1)°, at 300 K] defined in the space group P2₁/n, where the Cr and Sb cations are almost completely ordered in the B-sublattice of the perovskite structure. Magnetic susceptibility and magnetization measurements show that this compound behaves as a Curie-Weiss paramagnet at high temperatures with μ_eff=3.53(1) μ_B and θ_P=8 K, and exhibits a robust ferromagnetic component below the ordering temperature of T_C=13 K, with a saturation magnetization of 2.36 μ_B/f.u. at 5 K. To our knowledge, this is the first example of a ferromagnetic double perovskite containing a non-magnetic element, such as Sb, occupying one half of the B positions of the perovskite structure.