L. Seetha Lakshmi

Possible magnetic phase separation in Ru-doped La0.67Ca0.33MnO3

Abstract X-ray diffraction, resistivity, AC susceptibility and magnetization studies on La0.67Ca0.33Mn1−xRuxO3 (0⩽x

Transport and thermal properties of La0.67Ca0.33Mn1−xMxO3 (M=Fe, Zr and Hf)

Mn site substitution with Fe, Zr and Hf (1 at.%) were carried out in colossal magneto-resistive material La 0.67 Ca 0 33 MnO 3 . It is found from the resistivity, ac susceptibility and specific heat (C p ) measurements that Zr and Hf substitutions suppress the transition temperature (T c ) more effectively than Fe substitution. The excess entropy under the C p curve associated with the magnetic transition also substantially lowered compared to that of undoped and Fe-doped systems. From the consideration of the excess entropy under the C p curve and the ionic radii, it is speculated that both Zr 4+ and Hf 4+ ions predominantly substitute Mn 3+ site. Larger suppression of T c is attributed to local strain produced due to larger ion substitution as well as hole filling due to replacement of trivalent ion (Mn 3+ ) with tetravalent ions (Hf 4+ and Zr 4+ ).

Dependence of the high-field grain-boundary magnetoresistance of ferromagnetic manganites on Curie temperature

We analyze the high-field magnetoresistance of polycrystalline ferromagnetic manganites of varied composition in magnetic fields up to μ0H=47T. Small to medium deviations from a linear field dependence of the conductance {as discovered for a La0.7Sr0.3MnO3 film recently, [N. Kozlova et al., J. Magn. Magn. Mater. 261, 48 (2003)]} are typical. The conductance is always well described by a quadratic polynomial G(H)=a+bH+cH2 with temperature-dependent coefficients a, b, and c. Both b and c increase with decreasing Curie temperature (TC) of the samples. The conductance slope 1∕G0 dG∕dH is related to an average susceptibility of the magnetically disturbed layer at the grain boundaries. The nonsaturated magnetic order of grain-boundary (GB) spins at 5K and 47T means the presence of strong antiferromagnetic interactions. The observation of a systematic correlation between GB magnetoresistance and bulk TC indicates that magnetic order at GBs is governed by parameters such as doping and average ionic size (electron...

Charge state modification in Mn site substituted CMR manganites: strong deleterious influence on the ferromagnetic?metallic state

The effect of charge state modification at the Mn site on the physical properties of CMR manganites is reported. With a view to avoiding additional complexity of local spin coupling effects, Mn site substitution of La0.67Ca0.33MnO3 is carried out with appropriate diamagnetic ions—Zn2+, Zr4+, Ta5+ and W6+—of different valence states. The substitution results in size changes of the unit cell and enhanced local structural distortions, which increase in the order Zn, Zr, Ta and W. The ground state is ferromagnetic–metallic below a certain critical concentration xc of the substituents, beyond which the magnetic ground state shows a glassy behaviour. The phase transition temperatures (TMI and Tc) decrease with substitution, but to different extents. The observed suppression rates of the Curie temperature, Tc, of ~39 K/at.% and ~45 K/at.% respectively for Ta5+ and W6+ substituted compounds are the highest reported in the Mn site substituted CMR manganites. Besides the modification of majority carrier concentration due to the increased (decreased) Mn3+ concentration and enhanced local structural effects, the local electrostatic potential of the substituents seems to contribute to the unusually strong reduction in the itinerant ferromagnetism and the observed glassy states.