Effect of size reduction on the ferromagnetism of the manganite La_1−xCa_xMnO₃ (x=0.33)

Abstract

In this paper, we report an investigation of the ferromagnetic state and the nature of ferromagnetic transition of nanoparticles of La_0.67Ca_0.33MnO₃ 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.