Increase In Tetragonal Distortion Research Articles

Structural transformation and magnetic properties of copper ferrite nanoparticles prepared by sol–gel method

Copper ferrite nanoparticles were synthesized by sol–gel method by varying metal nitrate to citric acid (M:C) ratio as 1:1, 1:2, 1:3 and 2:1 and were subsequently annealed at different temperatures ranging from 400 to 900 °C in air for 3 h. These nanoparticle samples were characterized by X-ray diffraction, Fourier transformed infrared spectroscopy, Raman spectroscopy, field emission gun scanning electron microscopy and vibrating sample magnetometer. X-ray diffraction studies showed the as-prepared nanoparticles prepared with M:C ratio 1:1, 1:2, and 2:1 are mostly cubic Cu-ferrite. After annealing phase transformation occurred from cubic to tetragonal Cu-ferrite with increase in c/a ratio. Magnetization value decreased and coercivity increased with the increase in annealing temperature and c/a ratio for the samples prepared with M:C = 1:1 and 1:2. For the samples prepared with M:C = 1:3, the magnetization increased with the increase in annealing temperature whereas for the samples prepared with M:C = 2:1, a minimum was observed in the magnetization values for the sample annealed at 600 °C. The highest magnetization values of 45.6 and 49.5 emu/g at 300 and 60 K, respectively were observed in the present study for the as prepared sample prepared with M:C = 2:1. The highest coercivity of 1530 and 1690 Oe at 300 and 60 K, respectively were observed for the sample prepared with M:C = 1:3 and annealed at 800 °C. The observed magnetic properties can be understood on the basis of phase transformation, increase in tetragonal distortion and c/a ratio, and grain growth in these nanoparticles.

Tuning magnetic coercivity with external pressure in iron-rhenium based ferrimagnetic double perovskites

We studied the effect of physical pressure on the electronic and magnetic properties of ferrimagnetic double perovskites ${A}_{2}{\mathrm{FeReO}}_{6}$ ($A=\mathrm{Ca}$, Ba) using Re ${L}_{2,3}$ edge x-ray absorption spectroscopy and powder diffraction measurements. Volume compression is shown to dramatically increase the magnetic coercivity (${H}_{c}$) in polycrystalline samples of both compounds with $\mathrm{\ensuremath{\Delta}}{H}_{c}/\mathrm{\ensuremath{\Delta}}V\ensuremath{\sim}150$--200 $\mathrm{Oe}/{\AA{}}^{3}$. A nearly eight-fold increase in ${H}_{c}$, from 0.2 to 1.55 T, is obtained in ${\mathrm{Ba}}_{2}{\mathrm{FeReO}}_{6}$ at $P=29$ GPa. While no signs of structural phase transitions are seen in either sample to $\ensuremath{\sim}30$ GPa, the structural data points to a pressure-driven increase in tetragonal distortion of ${\mathrm{ReO}}_{6}$ octahedra. A sizable but pressure-independent Re orbital-to-spin magnetic moment ratio is observed, pointing to the critical role of spin-orbit interactions at Re sites. We present a ${j}_{\mathrm{eff}}$ description of the electronic structure that combines effects of crystal field and spin-orbit coupling on the Re $5{d}^{2}$ orbitals and use this description to provide insight into the pressure-induced enhancement of magnetic anisotropy.

Effect of decreasing Mg2+ and Zr4+ substitution on the structural, ferroelectric and magnetic properties of BF-PT solid solutions

Mg2+ and Zr4+ co-substitution in perovskite materials is known to have a pronounced effect on their tetragonal distortion and other physical properties. This study reports the variation in structural, ferroelectric and magnetic properties of (BiMg0.5(1−x)Zr0.5(1−x)FexO3)0.45− (PbTiO3)0.55 solid solutions with x varying from 0.7 to 1.0. The XRD analysis shows an increase in tetragonal distortion from 1.07 to 1.12 with increase in x. The increasing tetragonal distortion has been observed to be directly correlated with the decrease in BO bond length from 1.55 Å to 1.20 Å. The AO bond length remains unchanged. The grain size was observed to decrease slightly with increasing x (up to x = 0.9). The ferroelectric measurements show that remnant polarisation (Pr) decreases from 10.19 μC/cm2 to 0.64 μC/cm2. The magnetic measurements revealed that all the samples exhibit antiferromagnetic behaviour with a maximum Mr value of 11.205* 10−4emu/g for x = 0.9.

Mechanism of spin crossover in LaCoO3 resolved by shape magnetostriction in pulsed magnetic fields.

In the scientific description of unconventional transport properties of oxides (spin-dependent transport, superconductivity etc.), the spin-state degree of freedom plays a fundamental role. Because of this, temperature- or magnetic field-induced spin-state transitions are in the focus of solid-state physics. Cobaltites, e.g. LaCoO3, are prominent examples showing these spin transitions. However, the microscopic nature of the spontaneous spin crossover in LaCoO3 is still controversial. Here we report magnetostriction measurements on LaCoO3 in magnetic fields up to 70 T to study the sharp, field-induced transition at Hc ≈ 60 T. Measurements of both longitudinal and transversal magnetostriction allow us to separate magnetovolume and magnetodistortive changes. We find a large increase in volume, but only a very small increase in tetragonal distortion at Hc. The results, supported by electronic energy calculations by the configuration interaction cluster method, provide compelling evidence that above Hc LaCoO3 adopts a correlated low spin/high spin state.

Transformation behaviour of freestanding epitaxial Ni–Mn–Ga films

► The complex martensite microstructure of free-standing epitaxial Ni–Mn–Ga films. ► A two-stage transformation in the temperature range between 40 °C and 160 °C. ► Temperature dependent mechanical properties of free-standing Ni–Mn–Ga films. ► With increasing temperature, the twinning stress decreases due to thermal activation of twin boundaries. ► Large superplastic strain increases from about 10% at 110 °C to 14% at RT. We analyze the transformation behaviour of a 2 μm thick epitaxial Ni–Mn–Ga film by combining temperature dependent measurements of magnetization, electrical resistance, X-ray diffraction (XRD) and tensile stress–strain characteristics. While the magnetization measurements hint for a simple austenite–martensite transformation below the Curie temperature at about 90 °C, resistivity measurements reveal a two-stage transformation in the temperature regimes (I) of 40–80 °C and (II) of 140–160 °C. XRD and pseudoplastic behaviour prove the presence of martensite well above the Curie point. The combination of four independent methods suggests that the transformation at (II) may originate from a weakly first order transformation followed by an intermartensitic transformation at (I). This interpretation is in line with the large superplastic strain observed for the tensile direction parallel to the [1 0 0] direction of the Ni–Mn–Ga unit cell. The strain increases from about 10% at 110 °C to 14% at room temperature suggesting an increase in tetragonal distortion.

X-ray study of the thermal expansion anisotropy in the quarternary semiconductor CuGaSn□Se 4

Accurate lattice parameters a and c of the tetragonal chalcopyrite quaternary semiconductor CuGaSn□Se 4 have been determined as a function of temperature by the X-ray powder diffraction method in the temperature range 300 K to about 900 K. The data have been used to evaluate the axial expansion coefficients α a and α c at various temperatures. The thermal expansion studies revealed the anisotropy between the axial expansion coefficients having a larger coefficient of expansion along the a-axis than that along the c-axis (α a > α c ). The mean values α a and α c , in the temperature range 300–900 K, are found to be 14.02 × 10 -6K -1 and 5.02 × 10 -6K -1 respectively, and the axial ratio, c/a, changes with a coefficient of -8.96 × 10 -6K -1. This result indicates an increase in the tetragonal distortion, δ = 2 - c/a with temperature. An attempt is made to explain the increase in tetragonal distortion with temperature and the anisotropic thermal expansion of CuGaSn□Se 4 in terms of the thermal expansion of the A>−;Se (where A is Cu and Ga randomly distributed) and B>−;Se (where B is Sn and vacancy randomly distributed) bonds. The results are also discussed in terms of the principal Grüneisen parameters of chalcopyrite structure compounds.

E.p.r. and spectroscopic studies of configurational changes in 1,4-diazacycloheptane copper(II) complexes

E.p.r. and spectroscopic studies of bis(1,4-diazacycloheptane)copper(II) perchlorate, bromobis(1,4-diazacycloheptane)copper(II) perchlorate and dibromo-bis(1,4-diazacycloheptane)copper(II), were carried out in MeNO2, DMSO and H2O solutions as well as in the polycrystalline state. The ligation of bromide ion to bis(1,4-diazacycloheptane)copper(II) along thez-axis was investigated. The e.p.r. spectra are explained in terms of energy changes involving the electronic ground state and the lowest excited state as a function of ligation of nucleophiles to the metal atom along thez-axis. In the polycrystalline state, the weaker the apical interaction (square-planar species) the lower theg-value and the higher the copper(II) hyperfine coupling constant. In the liquid state, theg-values observed in non-coordinating solvents such as MeNO2 are similar to those for the polycrystalline material suggesting structural similarity in the two states. The electronic absorption maxima for the three complexes in the liquid state shift to higher energy with an increase in tetragonal distortion around copper(II). In strongly coordinating solvents the e.p.r. spectra and absorption maxima are independent of the anion present. The increasing strength of solvation results in an increase in the wavelength of the optical transition and theg-values, and a decrease in the copper(II) hyperfine coupling constant. It appears that an increase in the basicity of the solvent has the same effect on thed-orbital energy levels and on the unpaired electron density on copper(II)_as does increasing the electronegativity of the substituents on the ligand.

Thermal expansion of CuGa0.5In0.5Se2

The tetragonal unit cell dimensions a and c of a ternary chalcopyrite semiconductor CuGa0.5In0.5Se2 have been measured over the temperature range 28 to 442 degrees C by the X-ray diffraction method. In this compound the parameter a increased nonlinearly and c linearly with temperature. The linear coefficient of thermal expansion in the a direction ( alpha perpendicular to ) was greater than that in the c direction ( alpha /sub ///). This result indicates an increase in the tetragonal distortion (2-c/a) of this compound with temperature. The anisotropic thermal expansion ( alpha perpendicular to > alpha /sub ///) and the increase in tetragonal distortion of this compound are interpreted in terms of the thermal expansion of the I-VI and III-VI bonds, taking into account both the covalent and the ionic forces.