Crystal and magnetic structure transitions in BiMnO3 d ceramics driven by cation vacancies and temperature

The crystal structure of BiMnO3 d ceramics has been studied as a function of nominal oxygen excess and temperature using synchrotron and neutron powder diffraction, magnetometry and differential scanning calorimetry. Increase in oxygen excess leads to the structural transformations from the monoclinic structure C2 c to another monoclinic P21 c , and then to the orthorhombic Pnma structure through the two phase regions. The sequence of the structural transformations is accompanied by a modification of the orbital ordering followed by its disruption. Modification of the orbital order leads to a rearrangement of the magnetic structure of the compounds from the long range ferromagnetic to a mixed magnetic state with antiferromagnetic clusters coexistent in a ferromagnetic matrix followed by a frustration of the long range magnetic order. Temperature increase causes the structural transition to the nonpolar orthorhombic phase regardless of the structural state at room temperature; the orbital order is destroyed in compounds BiMnO3 d d lt; 0.14 at temperatures above 470

Magnetic properties and magnetoresistance in La0.5Sr0.5Co1 xMexO3 Me Cr, Ga, Ti, Fe cobaltites

Magnetic and magnetotransport properties of stoichiometric cobaltites La0.5Sr0.5Co1 amp; 8722;xMexO3 Me Cr, Ga, Ti, Fe, x amp; 8804; 0.25 have been investigated by neutron powder diffraction, magnetization and electrical measurements in fields up to 14 T. It is shown that doping with Fe up to x 0.2 slightly increases magnetization herewith the Curie point decreases. The chromium doping leads to dramatic decrease of magnetization and the Curie point, associated coherent magnetic contribution in NPD patterns quickly decreases with doping and becomes nearly negligible for x 0.2 compound. The substitution with diamagnetic Ga and Ti ions decreases the magnetization to a lesser extent than in the chromium doped compound. The Cr, Ti and Ga substitution leads to a strong increase in magnetoresistance at low temperature as compared with undoped cobaltite. The obtained results indicate that the magnetic interactions between Co and Fe are positive whereas those between Co and Cr ions are negative. Enhancement of magnetoresistance is attributed to the magnetic field induced transition from local antiferromagnetic order to ferromagnetic on

Crystal Structure and Magnetic Properties of Bi1-yBa(Sr)Fe1-yTiyO3 Solid Solutions

Abstract: Usages of various chemical substitution schemes of the initial multiferroic BiFeO3 can significantly reduce known drawbacks specific for the functional oxides based of iron ions and thus foster a creation of novel magnetoelectric compounds perspective for various technological applications. In the present study the co-doped compounds of the system Bi1-y(Ba1- xSrx)yFe1-yTiyO3 (x = 0.0 – 1.0; y ≤ 0.4) synthesized using sol-gel technique were analyzed focusing on the crystal structure stability and the correlation between the structure and magnetic properties. The concentration driven evolution of the crystal structure as well as the unit cell parameters were investigated based on the X-ray diffraction data, the correlation between the crystal structure and the magnetic properties of the compounds has been studied by magnetometry techniques. The compounds Bi1-y(Ba1- xSrx)yFe1-yTiyO3 with x = 0; y = ≤ 0.2 are characterized by single-phase rhombohedral structure, and increase in the dopant concentration to y = 0.4 leads to the stabilization of the pseudocubic phase. An increase in the Sr content leads to the phase transition in the compounds to the single phase state with the cubic structure which is accompanied by an increase in the value of the remanent magnetization

Effect of combined Ca Ti and Ca Nb substitution on the crystal and magnetic structure of BiFeO3

Herein, we report on the crystal structure, magnetic and local ferroelectric properties of the Bi1 amp; 8722;xCaxFe1 amp; 8722;xTixO3 and Bi1 amp; 8722;xCaxFe1 amp; 8722;x 2Nbx 2O3 perovskites prepared by a solid state reaction method. It has been found that the Ca2 Nb5 containing series is characterized by a narrower concentration range x amp; 8239; amp; 8804; amp; 8239;0.2 over which the acentric R3c structure specific to the pure BiFeO3 can be stabilized. The compositional variation in the critical concentration defining the polar nonpolar R3c Pnma phase boundary can be understood as related to the chemical modification induced changes in the lattice spacing diminishing the stability of the a amp; 8722;a amp; 8722;a amp; 8722; tilting in favor of the a amp; 8722;b a amp; 8722; one. Both the Ca2 Ti4 and Ca2 Nb5 substitutions ensure the suppression of a cycloidal antiferromagnetic order, thus leading to the formation of a weak ferromagnetic polar state. While this effect is proven to be associated with a composition driven reduction in polar displacements, lattice defects are supposed to contribute to the instability of the cycloidal spin arrangemen

The structural origin of composition driven magnetic transformation in BiFeO3 based multiferroics a neutron diffraction study

Having been considered as materials of exceptional technological importance, magnetoelectric multiferroics continue to attract tremendous research interest. While the steady progress achieved in this field over the past decades has made our understanding of the underlying physics substantially exhaustive and deep, some topics still remain to be debated. In particular, the reasons behind the composition driven instability of the cycloidal antiferromagnetic order in a classical room temperature multiferroic, BiFeO3, are not fully understood yet. Herein, we present experimental evidence indicating that the evolution of the magnetic state in chemically substituted bismuth ferrites is determined by the competition between the polarization and oxygen octahedra rotation related components of the Dzyaloshinskii Moriya interaction. Taking into account that the Bi1 amp; 8722;xCaxFe1 amp; 8722;xTixO3 and Bi1 amp; 8722;xBaxFe1 amp; 8722;xTixO3 perovskites exhibit totally different compositional evolution of the magnetic behavior contrary to the Ba Ti doping, the Ca Ti substitution stabilizes a weak ferromagnetic state in the polar phase , we use the neutron diffraction technique to compare the crystal structures of these materials and explore how the variation in the magnetic properties correlates with the parameters describing structural distortions. Both the Ca Ti and Ba Ti substitutions are proven to diminish the polar ionic displacements. The resulting effect of the chemical modification on the magnetic structure is shown to be largely dependent on the magnitude of the oxygen octahedra tiltin