Double Perovskite System Research Articles

Magnetic transition in double perovskite systems

The search for materials having complete spin polarization and high Curie temperature have received a lot of attention in view of spintronics applications, especially the ferromagnetic (F) Sr 2 FeMoO 6 , because of its fairly high Curie temperature ( T C = 450 K), half-metallic character, large magnetoresistance and potential applications. On the other hand, Sr 2 FeWO 6 is insulating and antiferromagnetic (AF) with T N = 37 K . With a double exchange type model it has been shown that F – AF transition can be driven by super-exchange interactions with increasing Fe – M ( M = Mo , W) charge transfer energy. So, the charge transfer energy is expected to be larger in FeW than in FeMo compounds. Using a tight-binding model with the renormalized perturbation expansion technique, we determine the density of states for the AF phase and the electronic energy difference for the F- and AF-phases as a function of the Fe – M charge transfer energy. The F – AF transition in the ordered system Sr 2 FeMo x W 1 − x O 6 occurs for x ∼ 0.3 , in good agreement with the experimental value. We also studied the effect of the diagonal disorder in the variation of the number of conduction electrons on Fe and M sites. Finally, the behavior of the Curie temperature as a function of the Mo/W concentration is determined.

Magnetic and magneto-transport properties of double perovskite Ba 2−xSr xFeMoO 6 system

The structural magnetic and magneto-transport properties of double perovskite system Ba 2− x Sr x FeMoO 6 (0⩽ x⩽1.0) prepared in bulk polycrystalline form are reported in this paper. X-ray diffraction analysis showed that samples are single phase and the lattice constants decreases with increase in the Sr content. The degree of Fe–Mo ordering has been found decreasing in the series with an increase in the Sr content. Parent compound Ba 2FeMoO 6 exhibits saturation magnetic moment value of 3.54 μ B/f.u. at 85 K in a magnetic field of 6000 Oe. Temperature dependence of resistivity shows metallic behavior for all the samples. The magneto-resistance (MR) of the compound with x=0.4 is higher than that of the other samples. At room temperature this system shows a saturation magnetization value of 1.73 μ B/f.u. and MR value of 7.08% (1 T). The observed variations in the structural and magnetic properties are attributed to the change of chemical pressure due to the substitution of Sr in place of Ba. The effect of antisite disorder (ASD) defects on magneto-transport properties is studied in more detail.

Magnetization and Mössbauer study of double layer perovskites La1.5Ca1.5Mn2 − x Fe x O7 prepared by sol–gel method

Magnetization and Mossbauer studies have been made for understanding magnetic behavior of three double perovskite systems La1.5Ca1.5Mn2 − x Fe x O7 corresponding to x = 0.05, 0.10 and 0.50. These have been prepared following sol–gel route. Substitution of Fe does not lead to any major change in the tetragonal cell but increased iron leads to greater distortion in octahedral site. The three samples undergo paramagnetic–ferromagnetic transition. Curie temperature (T c) for the system with 0.05 Fe is ∼150 K which is lower than (190 K) for the system without iron; with 0.50 Fe T c goes below 50 K. Iron goes as Fe3 + and replaces Mn in ab plane. With increasing Fe the valence states of Mn get re-distributed in a way that number of the Jahn–Teller ions Mn3 + increases and that of the pairs of Mn3 + –O–Mn4 + experiencing double exchange decreases.

DC and AC Electrical Properties of Sr<sub>2-x</sub>Li<sub>x</sub>FeMnO<sub>6</sub> Compound

The variations of dc conductivities (σ) of the Sr2-xLixFeMnO6 double perovskite system over a range of temperatures (100 ~ 300 K) and for several concentrations (0≤ x ≤ 0.4) of incorporated Li+ ions are well described by the equation σ = (B/T) exp(−Ea/kT) and show a decrease with decreasing temperature for a given doping. At a given temperature, σ tends to decrease with increasing x. The derived activation energy shows a gradual increase with x with a range of values that are lower than what is usually reported. The measured ac impedance is used to extract the relaxation frequencies as a function of composition and temperature.

Ferroelectric and magnetic properties of Ho 2CuTiO 6 double perovskite

Ho 2CuTiO 6 double pervoskite is synthesized by solid state reaction method and the phase formation was confirmed by X-ray diffraction. Ho 2CuTiO 6 crystallizes in space group P63 cm and are isostructural in AMnO 3 structure. We have investigated the effect of the presence of a magnetic ion (Ho 3+∼10.4μ B) in Ho 2CuTiO 6, on the magnetic properties, compared to that of an isostructural double perovskite containing Y-ion (non-magnetic), i.e., Y 2CuTiO 6. Magnetization measurements on Ho 2CuTiO 6 showed paramagnetic moment of 0.026μ B/Ho at room temperature in an applied field of 7 T although no saturation is observed at room temperature, whereas, at 5 K, the paramagnetic moment was 0.3μ B/Ho in the same field. Since this compound is non-centrosymmetric, it is of interest to look into the possibility of realizing multiferroicity (coexistence of magnetic and ferroelectric properties) in these double perovskite systems, especially the Ho-containing compound, for possible applications.

Structural, electronic, and magnetic properties of Sr2−xBaxFeMoO6(⩽x⩽2)

Here we report on the investigation of structural, transport, and magnetic properties in double perovskite system, Sr2−xBaxFeMoO6(0⩽x⩽2). X-ray Diffraction analysis shows that the compounds form in single phase across the whole solid solution range. The Rietveld refinement of the x-ray data clearly indicates a structural transition from tetragonal (I4∕m) for x⩽0.6 to cubic (Fm3m) for x⩾1.6 with an orthorhombic (Fmmm) structure in the intermediate region (0.8⩽x⩽1.4). Magnetization studies show a regular increase in saturation magnetization from 2.0μB∕f.u.(x=0) to 3.0μB∕f.u.(x=2) with increasing Ba concentration. However, magnetization as well as electron-spin resonance (ESR) measurements confirmed a systematic decrease in the Curie temperatures from 420 K (x=0) to 310 K (x=2) with increasing x. Resistivity measurements carried out in zero magnetic field and for a field of 0.8 T reveal appreciably higher magnetoresistance for intermediate compositions in comparison to those at the ends.

Sr2FeMoO6: A Prototype to Understand a New Class of Magnetic Materials

We propose a new mechanism to explain the magnetic structure of a recently discovered magnetoresistive double perovskite oxide system, Sr2FeMoO6, with the help of detailed experimental and theoretical results. This model, based on a strong antiferromagnetic coupling between the local moment and the charge carriers arising from local hopping interactions, can give rise to ferromagnetic metallic as well as ferromagnetic insulating ground states. The relevance of this mechanism in understanding the magnetism in dilute magnetic semiconductors such as Ga1−xMnxAs, is also discussed.

Effect of quenching on structure and antiferroelectric instability of La (2− x)/3 Li xTiO 3 compounds: a Raman study

A Raman study of the La (2− x)/3 Li x TiO 3 ionic conductors has been performed as a function of temperature, lithium content and synthesis cooling rate. Irrespective of thermal treatment or lithium content, the spectrum can always be explained within a tetragonal symmetry (space group P4/mmm), which suggests that, at a local scale of several unit cells, some cation ordering persists even in samples submitted to quenching conditions. The mode involving mostly in-plane titanium vibration softens on lowering temperature, though a phase transition never occurs. The effect is attributed to the tendency of these double-perovskite systems to antiferroelectric ordering, frustrated by structural disorder. Quenching the samples from high temperature increases cation disorder and suppresses the tendency of these systems toward antiferroelectric instability. A correlation is found between the degree of instability and the parameter η=La1−La2−x, where La1 and La2 stand for the site occupancy factors of lanthanum ions in sites La1 and La2, respectively.

Effective three-band model for double perovskites

We start from a six-band model describing the transition-metal t2g orbitals of half-metallic double perovskite systems, such as Sr2FeMoO6, in which only one of the transition metal ions (Fe) contains important intratomic repulsion Ufe. By eliminating the Mo orbitals using a low-energy reduction similar to that used in the cuprates, we construct a Hamiltonian which contains only effective t2g Fe orbitals. This allows to treat exactly Ufe, and most of the Fe-Mo hopping. As an application, we treat the effective Hamiltonian in the slave-boson mean-field approximation and calculate the position of the metal-insulator transition and other quantities as a function of pressure or on-site energy difference.

Antiferroelectric Instability in Double Perovskite Systems La (2−x)/3 Li x TiO 3 , La (1−x)/3 Li x TaO 3 and La (1−x)/3 Li x NbO 3

Anomalous softening of the in-plane, out-of-phase vibration of Ti, Nb or Ta cations in these double perovskite systems has been observed by Raman spectroscopy upon decreasing temperature. In the titanates softening ceases below , 60 K, while in Ta and Nb systems a broad minimum in the frequency of the soft-mode at about 140 K has been found, suggesting that a real phase transition takes place in these cases. In general, an increase in lithium content results in the gradual disappearance of the soft-mode behavior. This effect has been interpreted as due to the enhancement of the random field at the Ti site due to increasing structural disorder, a behavior that is intimately related to the cation ordering in the A site of the perovskite cell. Since the behavior in the titanates resembles that of incipient ferroelectrics such as SrTiO 3 and KTaO 3 the phenomenon in double perovskites may be termed as incipient antiferroelectricity , though the explanation for the low temperature frequency stabilization is not the same.

Raman study of antiferroelectric instability inLa(2−x)/3LixTiO3(0.1<~x<~0.5)double perovskites

A Raman study of the double-perovskite system ${\mathrm{La}}_{(2\ensuremath{-}x)/3}{\mathrm{Li}}_{x}{\mathrm{TiO}}_{3}$ is presented. The in-plane vibration of Ti cations shows a soft-mode behavior on lowering the temperature, though the softening ceases below 60 K. The freezing-in of the atomic displacements involved would result in an antiferroelectric order, but the tendency to instability is suppressed by structural disorder arising from lithium doping and incomplete cation ordering in the A site of the perovskite.

Comment on the magnetic properties of the system Sr2−xCaxFeMoO6, 0≤x≤2

The structural and magnetic properties of the n-type-metallic double-perovskite system Sr2−xCaxFeMoO6 are reported. At 5 K, the saturation magnetization remains Ms(5 K)=3.5 μB/formula unit compared to a theoretical spin-only moment of 4.0 μB/formula unit for a perfectly ordered compound. From the shape of the M–H hysteresis loops below Tc, it is suggested that ferromagnetic long-range ordered domains are coupled antiferromagnetically across antiphase boundaries; random disorder within domains may be small. An unprecedented dependence of the paramagnetic susceptibility on the applied magnetic field is reported and attributed to an overlapping Fe3+/Fe2+ and Mo6+/Mo5+ redox couple, first suggested by Sleight and Weiher [Sleight AW, Weiher JF. J. Phys Chem Solids 1972; 33: 679.], which results in localized S=5/2 spins on the iron atoms coexisting with itinerant minority-spin electrons in a narrow π* band.

ダブルペロブスカイト型Ｓｒ２Ｆｅ（Ｒｕ１−ｘＷｘ）Ｏ６の合成

We synthesized polycrystalline materials of a double perovskite system Sr2Fe(Ru1-xWx)O6 with x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0. These materials were made by the solid-state-reaction method, in three types of atmospheres: as-grown in air, annealed in O2, and annealed in Ar-H2 (Ar with 1% H2). From X-ray diffraction and the Mössbauer effect, we found that the samples of Sr2FeRuO6 became single-phase after annealing in O2. On the other hand, single-phase Sr2FeWO6 was obtained after annealing in Ar-H2(Ar with 1% H2).The lattice constants showed a monotonic increase from x = 0.0 to 0.8, and Fe had a trivalent state in this region.