Effect of size reduction on the ferromagnetism of the manganite La1-xCaxMnO3 (x=0.33)

In this paper, we report an investigation of the ferromagnetic state and the nature of ferromagnetic transition of nanoparticles of La0.67Ca0.33MnO3 using magnetic measurements and neutron diffraction. The investigation was performed on nanoparticles with crystal size down to 15 nm. The neutron data show that even down to a size of 15 nm the nanoparticles show finite spontaneous magnetization (M-S) although the value is much reduced compared to the bulk sample. We observed a non-monotonic variation of the ferromagnetic-to-paramagnetic transition temperature T-C with size d and found that T-C initially enhances upon size reduction, but for d < 50 nm it decreases again. The initial enhancement in T-C was related to an increase in the bandwidth that occurred due to a compaction of the Mn-O bond length and a straightening of the Mn-O-Mn bond angle, as determined from the neutron data. The size reduction also changes the nature of the ferromagnetic-to-paramagnetic transition from first order to second order with critical exponents approaching mean field values. This was explained as arising from a truncation of the coherence length by the finite sample size.