Temperature dependent structural studies on the spin correlated system A2FeCoO6 (A= Sm, Eu, Dy and Ho) using synchrotron radiation

Spin reorientation and disordered rare earth magnetism in Ho2FeCoO6.

Abstract: We report the experimental observation of spin reorientation in the double perovskite Ho2FeCoO6. The magnetic phase transitions in this compound are characterized and studied through magnetization and specific heat, and the magnetic structures are elucidated through neutron powder diffraction. Two magnetic phase transitions are observed in this compound-one at K, from paramagnetic to antiferromagnetic, and the other at K, from a phase with mixed magnetic structures to a single phase through a spin reorientation process. The magnetic structure in the temperature range 200–45 K is a mixed phase of the irreducible representations and , both of which are antiferromagnetic. The phase with mixed magnetic structures that exists in Ho2FeCoO6 gives rise to a large thermal hysteresis in magnetization that extends from 200 K down to the spin reorientation temperature. At T N2, the magnetic structure transforms to . Though long-range magnetic order is established in the transition metal lattice, it is seen that only short-range magnetic order prevails in the Ho3+ lattice. Our results should motivate further detailed studies on single crystals in order to explore the spin reorientation process, spin switching and the possibility of anisotropic magnetic interactions giving rise to electric polarization in Ho2FeCoO6.

Spin reorientation and disordered rare earth magnetism in Ho$_2$FeCoO$_6$

Abstract: We report the experimental observation of spin reorientation in the double perovskite Ho$_2$FeCoO$_6$. The magnetic phase transitions in this compound are characterized and studied through magnetization and specific heat, and the magnetic structures are elucidated by neutron powder diffraction. Two magnetic phase transitions are observed in this compound - one at $T_\mathrm{N1} \approx$ 250~K, from paramagnetic to antiferromagnetic, and the other at $T_\mathrm{N2} \approx$ 45~K, from a phase with mixed magnetic structures to a single phase through a spin reorientation process. The magnetic structure in the temperature range 200~K - 45~K is a mixed phase of the irreducible representations $\Gamma_1$ and $\Gamma_3$, both of which are antiferromagnetic. The phase with mixed magnetic structures that exists in Ho$_2$FeCoO$_6$ gives rise to a large thermal hysteresis in magnetization that extends from 200~K down to the spin reorientation temperature. At $T_\mathrm{N2}$, the magnetic structure transforms to $\Gamma_1$. Though long-range magnetic order is established in the transition metal lattice, it is seen that only short-range magnetic order prevails in Ho$^{3+}$ - lattice. Our results should motivate further detailed studies on single crystals in order to explore spin reorientation process, spin switching and the possibility of anisotropic magnetic interactions giving rise to electric polarization in Ho$_2$FeCoO$_6$.

Magnetocaloric effect and spin-phonon correlations in RFe0.5Cr0.5O3 (R = Er and Yb) compounds

Abstract: We report the results of our investigation of the physical properties of mixed metal oxides RFe0.5Cr0.5O3 (R = Er and Yb). ErFe0.5Cr0.5O3 undergoes an antiferromagnetic ordering around 270 K followed by spin reorientation (SR) transitions around 150 and 8 K respectively. In contrast, in YbFe0.5Cr0.5O3 a single SR transition is noted at 36 K, below the AFM ordering temperature of 280 K. In ErFe0.5Cr0.5O3, a significant value of magnetic entropy change ( Δ S M ) ∼ -12.4 J/kg-K is noted near the 2nd SR transition, however, this value is suppressed in YbFe0.5Cr0.5O3. Temperature dependent dielectric permittivity of ErFe0.5Cr0.5O3 and YbFe0.5Cr0.5O3 at different frequencies, reveal the presence of Debye-like relaxation behaviour in both compounds, which can be due to the effect of charge carrier hopping between localized states of Fe and Cr ions. Temperature dependent Raman scattering studies divulge that spin-phonon coupling plays a crucial role in defining the physical properties of these compounds.

The order of magnetic phase transitions in disordered double perovskite oxides Sm2FeCoO6 and Dy2FeCoO6

Abstract: We present the results on the order of phase transition around the spin re-orientation region using isothermal magnetization data performed on the sol-gel synthesized disordered double perovskite oxides Sm2FeCoO6 and Dy2FeCoO6 by Arrott plot method. The temperature variation of the DC magnetization data of both the samples show that there are two magnetic phase transitions; one at high temperature regime, the conventional paramagnetic (PM) to ferro or ferrimagnetic (FM or FIM) and the other at low temperatures, ferro or ferrimagnetic (FM or FIM) to antiferromagnetic (AFM). From the Arrott plot method, it is inferred that spin re-orientation transitions of both the compounds has a first order nature corresponding to the FM (FIM) to AFM transition.

Dielectric response of the magnetic perovskite oxide Eu2FeCoO6

Abstract: The rare earth based transition metal perovskites are well known for their tunable physical properties. Here we present the dielectric response of the magnetic perovskite Eu2FeCoO6 from impedance spectroscopy measurements. The compound having a weak ferromagnetic ground state, is subjected to impedance spectroscopy measurements through the frequency range 100 Hz to 10 MHz for the temperature range 123 K-473 K. The dielectric permittivity shows huge values at high temperatures with low frequency. The two step relaxation process is inferred from Arrhenius fit.The rare earth based transition metal perovskites are well known for their tunable physical properties. Here we present the dielectric response of the magnetic perovskite Eu2FeCoO6 from impedance spectroscopy measurements. The compound having a weak ferromagnetic ground state, is subjected to impedance spectroscopy measurements through the frequency range 100 Hz to 10 MHz for the temperature range 123 K-473 K. The dielectric permittivity shows huge values at high temperatures with low frequency. The two step relaxation process is inferred from Arrhenius fit.

Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides

Abstract: The effective ionic radii of Shannon & Prewitt [Acta Cryst. (1969), B25, 925-945] are revised to include more unusual oxidation states and coordinations. Revisions are based on new structural data, empirical bond strength-bond length relationships, and plots of (1) radii vs volume, (2) radii vs coordination number, and (3) radii vs oxidation state. Factors which affect radii additivity are polyhedral distortion, partial occupancy of cation sites, covalence, and metallic character. Mean Nb5+-O and Mo6+-O octahedral distances are linearly dependent on distortion. A decrease in cation occupancy increases mean Li+-O, Na+-O, and Ag+-O distances in a predictable manner. Covalence strongly shortens Fe2+-X, Co2+-X, Ni2+-X, Mn2+-X, Cu+-X, Ag+-X, and M-H- bonds as the electronegativity of X or M decreases. Smaller effects are seen for Zn2+-X, Cd2+-X, In2+-X, pb2+-X, and TI+-X. Bonds with delocalized electrons and therefore metallic character, e.g. Sm-S, V-S, and Re-O, are significantly shorter than similar bonds with localized electrons.

Recent advances in magnetic structure determination by neutron powder diffraction

Abstract: In spite of intrinsic limitations, neutron powder diffraction is, and will still be in the future, the primary and most straightforward technique for magnetic structure determination. In this paper some recent improvements in the analysis of magnetic neutron powder diffraction data are discussed. After an introduction to the subject, the main formulas governing the analysis of the Bragg magnetic scattering are summarized and shortly discussed. Next, we discuss the method of profile fitting without a structural model to get precise integrated intensities and refine the propagation vector(s) of the magnetic structure. The simulated annealing approach for magnetic structure determination is briefly discussed and, finally, some features of the program FullProf concerning the magnetic structure refinement are presented and discussed. The different themes are illustrated with simple examples.

A First-Order Phase Transition Induced by a Magnetic Field

Abstract: An electronic (metal-to-insulator) phase transition of the first order, which can be caused by an external magnetic field, was discovered in Nd 1/2 Sr 1/2 MnO 3 . A clear hysteresis was observed during the increase and decrease of an external magnetic field at a fixed temperature. The hysteretic field region was observed to depend critically on temperature and to drastically expand with a decrease of temperature, perhaps as a result of suppression of the effect of thermal fluctuations on the first-order phase transition. Although it has seldom been observed, this is thought to be a generic feature of the first-order phase transition at low temperatures near 0 kelvin.

Raising the Curie temperature in Sr 2 FeMoO 6 double perovskites by electron doping

Abstract: Electron doping in the ferromagnetic and metallic double perovskites ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6}$ is achieved via the partial substitution of ${\mathrm{Sr}}^{2+}$ by ${\mathrm{La}}^{3+}.$ We show that this doping promotes an extraordinary rising of the Curie temperature of about 70 K above that of the pristine compound. This finding reveals that the ferromagnetic coupling is mediated by itinerant carriers, thus providing solid experimental support to the double-exchange picture for the magnetic interactions. The observation that ${T}_{C}$ can be substantially enhanced may be of relevance for technological applications of these materials in advanced spin devices.

Hydrothermal synthesis and magnetic properties of multiferroic rare-earth orthoferrites

Abstract: Rare-earth orthoferrites RFeO3 are gifted with fantastic magnetic properties. They have become a research focus for developing multiferroics in recent years. However, it is difficult to obtain pure-phase RFeO3 from the high-temperature synthesis methods. In this work, we report hydrothermal synthesis of well-crystallized pure-phase orthoferrites RFeO3 (R = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) in low temperatures. Structures, morphologies and magnetic properties of the products were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), magnetic property measurement system (MPMS), vibrating sample magnetometer (VSM), etc. The expected products were determined to be orthorhombic crystals of perovskite structure. The influences of alkalinity, reaction temperature and time on the formation and growth of RFeO3 crystals were investigated in details, and a dissolution–precipitation mechanism was proposed for describing the formation and growth. Optimum hydrothermal conditions for synthesizing orthoferrites RFeO3 were concluded. The magnetic properties were systematically characterized and studied, including antiferromagnetic transition of Fe (TN1), spin reorientation (TSR), compensation effect (Tcomp), ordering of R (TN2), etc. The relation between the magnetic properties of RFeO3 and the ionic radii of R was investigated and discussed. The experimental results in this work may provide fundamental support to the research and development of multiferroic materials.