Effect of oxygen non-stoichiometry on the antiferromagnet-ferromagnet transition in Nd0.5Ca0.5MnO3−δ(δ≤0.12)

A decrease in the oxygen content in Nd_0.5Ca_0.5MnO_3?δ down to γ≤0.12 is shown to bring about a strong decrease in the magnetic field inducing a transition from the antiferromagnetic charge-ordered to the ferromagnetic charge-disordered state. The ferromagnetic phase in a Nd_0.5Ca_0.5MnO_2.92 sample is stable in the absence of an external magnetic field. A further increase in the content of oxygen vacancies stabilizes the antiferromagnetic charge-disordered state.     

Crystal Structure and Ferromagnetic Component in Layered Perovskite Sr0.8Y0.2CoO2.65

Crystallography Reports - The crystal structure and magnetic state of Sr0.8Y0.2CoO2.65 layered perovskite have been studied using neutron diffraction and synchrotron radiation diffraction and...     

Magnetic structure and properties of the Nd0.6Ca0.4Mn0.5Cr0.5O3 manganite

The effect of high chromium concentrations on the charge and orbital ordering in manganites is studied using neutron diffraction and magnetic measurements. It is found that the Nd_0.6Ca_0.4Mn_0.5Cr_0.5O₃ manganite exhibits a CE-type antiferromagnetic ordering with a weak ferromagnetic component below 160 K. In the Nd_0.6Ca_0.4Mn_0.5Cr_0.5O₃ manganite, the Mn and Cr ions form antiferromagnetic zigzag chains that are likewise antiferromagnetically coupled with one another in the basal plane and are arranged along the b axis of the orthorhombic structure with oppositely directed spins. An applied magnetic field of 5 T does not change the magnetic structure.     

The influence of oxygen deficiency on the magnetic and electric properties of La<Subscript>0.70</Subscript>Ba<Subscript>0.30</Subscript>MnO<Subscript>3 −γ</Subscript> (0≤γ≤0.30) manganite with a perovskite structure

The crystal structure and the magnetic and electric properties of La_0.70Ba_0.30MnO_{3 ? γ} manganite (0≤γ≤0.30) with a perovskite structure were studied experimentally depending on the concentration of oxygen vacancies. The stoichiometric La_0.70Ba_0.30MnO₃ compound (γ = 0) had cubic unit cell symmetry, which did not change as oxygen deficiency increased up to γ=0.30. A decrease in the content of oxygen in the compound under study caused the occurrence of several sequential magnetic phase transitions in the ground state, from the ferromagnetic state at γ=0 through the cluster spin glass state (γ=0.15) to the antiferromagnetic state (γ=0.30) with the presence of a small ferromagnetic component. The specific electric resistance grew to become activation in character at γ=0.11, and the metal-semiconductor transition disappeared as oxygen deficiency increased. The magnetoresistance of anion-deficient compositions included (1) magnetoresistance close to the temperature of the transition to the magnetically ordered state and (2) low-temperature magnetoresistance. The magnetoresistance peak at T_C disappeared as γ increased (γ=0.11), whereas the low-temperature magnetoresistance component first increased to attain a maximum of about 34% at γ=0.15 and then decreased. The results of experimental studies were used to construct a magnetic phase diagram. These results could be interpreted within the framework of superexchange magnetic ordering theory. The suggestion was made that Mn³⁺-O-Mn³⁺ indirect exchange interactions were positive in the orbitally disordered phase only when manganese was in octahedral coordination, whereas these interactions became negative if at least one of the Mn³⁺ ions was five-coordinate or had a smaller coordination number.     

Morphotropic phase boundary, weak ferromagnetism, and strong piezoelectric effect in Bi1 ? xCaxFeO3 ? x/2 compounds

The crystal structure and magnetic and piezoelectric properties of the Bi_{1 − x} Ca _x FeO_{3 − x/2} system (x ≤ 0.2) are studied. The crystal-structure transformations occur in the following sequence: rhombohedral (R3c) polar phase (x ≤ 0.06) → modulated polar phase (0.07 ≤ x ≤ 0.1) → modulated antipolar phase (0.11 ≤ x ≤ 0.14). The modulated polar and antipolar phases are weakly ferromagnetic with a spontaneous magnetization of 0.25 G cm³/g (x = 0.09). In the polar weak ferromagnet with x = 0.09, a uniform piezoelec- tric response 2.5 times stronger than in the initial BiFeO₃ compound is detected by the piezoelectric force microscopy.     

The nonlinear magnetic properties of the pseudocubic Nd0.77Ba0.23MnO3 single crystal in the critical paramagnetic region and phase separation

The second magnetization harmonic was studied for a moderately doped Nd_0.77Ba_0.23MnO₃ neodymium manganite single crystal in parallel constant and harmonic magnetic fields in the critical paramagnetic region. According to the neutron and X-ray diffraction data, the crystal was crystallographically single-phase and had a pseudocubic structure both at room temperature and below the Curie point T _C=124.1 K. Although the specific resistance of this compound had a singularity near T _C and exhibited giant magnetoresistance, it remained an insulator in the ferromagnetic state. Nonlinear response measurements in the T _C<T<T _*≈146.7 K paramagnetic region were indicative of the existence of two magnetic phases. Above T _*, the crystal was magnetically single-phase, and its critical behavior was well described by dynamical similarity theory for isotropic 3D ferromagnets. The unexpected appearance of a new magnetic phase in the structurally homogeneous crystal was discussed based on phase separation ideas; such a phase separation could occur in moderately doped cubic manganites experiencing orbital ordering.     

High resolution diffraction and small angle scattering neutron investigations of LaCo<Subscript>0.5</Subscript>Mn<Subscript>0.5</Subscript>O<Subscript>3+</Subscript><Subscript>δ</Subscript>: effect of oxygen content

We have investigated LaCo_0.5Mn_0.5O_{3+ δ} compounds with different oxygen content by means of magnetization, high resolution and small-angle neutron diffraction measurements. Oxygen content decrease down to stoichiometric composition leads to an essential increase of T_C and magnetic moment while Co/Mn ionic ordering degree is kept almost constant. It is assumed that upon oxygen reduction Co³⁺ ions change their valence state down to 2+ one that leads to dominating of Co²⁺-Mn⁴⁺ ferromagnetic interactions as well as T_C increase. Magnetic properties can be explained in terms of coexistence of long-range ferromagnetic order and short-range clusters with antiferromagnetic interactions prevailing. Size distribution of the mentioned short-range magnetic inhomogeneities is rather mild within the samples but it is strongly temperature dependent.     

Investigation of a Spin Transition in a LaCoO<Subscript>3</Subscript> Single Crystal by the Method of X-Ray Magnetic Circular Dichroism at the Cobalt <Emphasis Type="Italic">K</Emphasis>- and <Emphasis Type="Italic">L</Emphasis><Subscript>2,3</Subscript>-Edges

Spin transitions of cobalt ions in LaCoO₃ single crystals have been studied by the method of X-ray magnetic circular dichroism (XMCD) at the K- and L_2,3-edges of Co³⁺ ions. The orbital momentum of cobalt ions obtained for the K-edge at the 3d level in the region of the spin transition in the temperature range from 25 to 120 K increases by a factor of approximately 1.6, whereas the slope of the magnetization curve value in the same temperature range and magnetic field increases by a factor of more than 10. XMCD experiments at the cobalt L_2,3-edges demonstrate gradual growth of the ratio of the orbital momentum to the spin one L/S from 0.48 to 0.53 in the temperature range from 60 K to 120 K.     

Magnetic and crystal structures of iron-ion-doped TbBaCo<Subscript>2</Subscript>O<Subscript>5 + γ</Subscript>

Neutron diffraction and magnetic studies of a TbBaCo₂O_{5 + γ} polycrystalline sample with a perovskite-like layered structure have revealed that spontaneous magnetization appears at 305 K and decreases sharply below 200 K. It has been shown that the crystal structure is described by space group Pmmm with a 2a _p · a _tp · 2a _p unit cell. The magnetic structure in the high-temperature magnetic phase consists of the G-type antiferromagnetic and ferromagnetic components, whereas the magnetic structure in the low-temperature phase is a G-type antiferromagnetic. The ferromagnetic component appears due to the noncollinearity of the magnetic moments of Co³⁺ ions, which are in the high-spin state in pyramids and in the low-spin state in octahedrons. The antiferromagnetic component is present in the octahedral sublattice, because oxygen vacancies are partially ordered.     

Crystal structure and the magnetic state of Pr0.5Sr0.5Co0.5Fe0.5O3

The magnetic properties and crystal structure of the Pr_0.5Sr_0.5Co_0.5Fe_0.5O₃ compound are studied by neutron and x-ray diffractions using synchrotron radiation. These measurements show that this compound is a dielectric spin glass with a magnetic moment freezing temperature of about 70 K. As temperature decreases in the range 30–95 K, a structure phase transition of the first order occurs with an increase in the symmetry from orthorhombic (space group Imma) to tetragonal (space group I4/mcm). It is assumed that the transition is caused by a change in the 4f electron configuration of the Pr³⁺ ions.