Transport and thermal properties of La0.67Ca0.33Mn1-XMXO3 (M=Fe, Zr and Hf)

Mn site substitution with Fe, Zr and Hf (approximate to1 at.%) were carried out in colossal magneto -resistive material La0.67Ca0.33MnO3. It is found from the resistivity, ac susceptibility and specific heat (CP) measurements that Zr and Hf substitutions suppress the transition temperature (T-c) mor e effectively than Fe substitution. The excess entropy under the CP curve a ssociated with the magnetic transition also substantially lowered compared to that of undoped and Fe-doped systems. From the consideration of the exce ss entropy under the CP curve and the ionic radii, it is speculated that bo th Zr4+ and Hf4+ ions predominantly substitute Mn3+ site. Larger suppressio n of T-c is attributed to local strain produced due to larger ion substitut ion as well as hole filling due to replacement of trivalent ion (Mn3+) with tetravalent ions (Hf4+ and Zr4+). (C) 2001 Elsevier Science BY All rights reserved.