Increase Of Mn4 Research Articles

Thermal hysteresis of permeability and transport properties of Mn substituted Mg–Cu–Zn ferrites

Mn substituted Mg–Cu–Zn ferrites of composition Mg0.35Cu0.20Zn0.45O(Fe2−xMnx O3)0.97 have been prepared by the standard double sintering ceramic technique. X-ray diffraction patterns of the samples showed single phase cubic spinel structure without any detectable impurity phases. The lattice constant is found to increase linearly with increase in Mn3+ ion concentration obeying Vegard's law. The initial permeability (μi) of the Mg–Cu–Zn ferrites exhibits thermal hysteresis when the temperature is cycled from above the Curie temperature TC to below. The sharp decrease of μi at T = TC indicates that the samples have high homogeneity according to Globus et al. The Curie temperature TC of the studied ferrite system was determined from the μi–T curves where the Hopkinson type of effect at the TC has been observed with the manifestation of a sharp fall in permeability. The Curie temperature TC is found to increase with increasing Mn content. Dc electrical resistivity increases significantly with the increase in Mn content. The ac resistivity (ρac) and dielectric constant (ε′) of the samples are found to decrease with increase in frequency, exhibiting normal ferrimagnetic behaviour. Dielectric relaxation peaks were observed for the frequency dependence of dielectric loss tangent curves. ε′ increases as the temperature increases, which is the normal dielectric behaviour of the magnetic semiconductor ferrite. The observed variation of electrical and dielectric properties are explained on the basis of Fe2+/Fe3+ ionic concentration as well as the electronic hopping frequency between Fe2+ and Fe3+ ions in the present samples.

Structure and electrochemical properties of LiMn2O4−xF x

LiMn2O4−xFx prepared by the sol-gel method has a perfect crystal formation. The crystal particle size of the material was medium and distributed uniformly. The substitution of F for O increased the specific capacity of the material at the cost of the cycleability. The explanation of this results is that the F decreases the valence of Mn, that is, more Mn3+ and less Mn4+ exist in the material. The increase of Mn3+ will improve the initial specific capacity and Mn3+ is the original reason for Jahn-Teller effect that caused the poor cycleability of the cathode material by the micro-distortion of the crystal structure. In addition, the expanded measurement of the crystal lattice is also the reason for the poor cycleability. Therefore, the results of F-substitution and cation-substitution are opposite. If the two methods are combined, they can compensate the inability each other and the satisfactory results may be obtained.

Magnetic interaction in Mg, Ti, Nb doped manganites

An effect of Mn substitution with Me=Mg2+, Ti4+, Nb5+ in manganites has been investigated by preparing La0.7Sr0.3(Mn1-xMex)O3 and La1-xSrx(Mn1 - x/2Nbx/2)O3 series. It was established that substitution of manganese with magnesium up to x = 0.16 leads to a collapse of a long-range ferromagnetic order whereas La0.7Sr0.3(Mn 3 +0.85Nb 5 +0.15)O3 is ferromagnet with TC = 123 K and exhibits a large magnetoresistance below Curie point despite an absence of four-valent manganese. Hypothetical magnetic phase diagrams are constructed for La0.7Sr0.3(Mn1-xMex)O3 and La1-xSrx(Mn1 - x/2Nbx/2)O3. Our results show that Mn3+-O-Mn3+ exchange interaction is ferromagnetic in the orbitally disordered manganites as well as an increase of Mn4+ content above 50% from a total amount of manganese ions leads to formation of a spin glass state due to a competition between antiferromagnetic Mn4+-O-Mn4+ and ferromagnetic Mn3+-O-Mn4+(Mn3+) superexchange interactions.

X-ray, IR and bulk magnetic properties of Cu1 + xMnxFe2–2xO4 ferrite system

The structural, IR and magnetic properties of the mixed spinel Cu1 + xMnxFe2-2xO4 (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) system have been investigated by means of X-ray diffraction, Infrared spectroscopy, magnetization and a.c. susceptibility measurements. X-ray results confirm single-phase spinel structure for all the concentrations. The structure of CuFe2O4 is tetragonal and changes to cubic for x = 0.1 to 0.4. For x = 0.5 and 0.6 the structure become tetragonal. The lattice parameter and X-ray density are deduced and their variation with Mn4+ concentration is studied. The cation distribution derived from the X-ray diffractometry data was found to agree very well with the cation distribution obtained from magnetization measurements. X-ray intensity calculations indicate that Mn4+ occupy only octahedral [B] sites and Cu2+ and Fe3+ ions occupy both octahedral [B] and tetrahedral (A) sites. The infrared spectra obtained at room temperature in the range 200–800 cm−1 showed two absorption bands. The force constants have been obtained from IR data. The variation of the saturation magnetization per formula unit as a function of Mn4+ content x has been satisfactorily explained on the basis of Neel's collinear spin ordering model for all values of x. The Curie temperature TC determined from a.c. susceptibility data decreases with increase of Mn4+ concentration x, suggesting decrease in ferrimagnetic behaviour.

Effect of valence fluctuations in A sites on the transport properties of La1-xRxMnO3 (R = Ce, Pr)

La1-xRxMnO3 (R = Ce or Pr) exhibit valence fluctuations (R3+ and R4+) in the A sublattice of the ABO3 perovskite structure. They also display a metal-insulator (M-I) transition and magnetoresistance effect even without the substitution of divalent cations. In La1-xCexMnO3, as Ce concentration increases (x = 0.05 to 0.6), the magnitude of resistivity decreases and the magnetic transition temperature, Tc, shifts towards higher temperature. The change in transport properties is attributed to the increase in Mn3+-Mn4+ pairs. The electronic and magnetic phase diagram is obtained for La1-xCexMnO3 from the resistivity and dc susceptibility measurements over a broad range of temperature and concentrations. The thermopower and magnetoresistance are also measured for La1-xCexMnO3 for two x values (x = 0.3 and 0.6). The high temperature thermopower data fit to an adiabatic small polaron hopping mechanism.

Giant magnetoresistance in bulk samples of LaMnO3 with varying Mn4+ content

Magnetoresistance (MR) in bulk samples of LaMnO3 has been investigated by varying the Mn4+ content from 10 to 33 per cent by chemical means, without aliovalent doping. With the increase in Mn4+ content, the structure of LaMnO3 changes first from orthorhombic to rhombohedral and then to cubic and the material becomes increasingly ferromagnetic, exhibiting a resistivity maximum akin to an insulator-metal transition at T-Peak, just below the ferromagnetic T-c. The magnitude of MR is highest in the cubic sample (with 33% Mn4+) around the T-Peak, and negligible in the non-magnetic orthorhombic sample (12% Mn4+).