Nature of the magnetic ordering for small mean-size and large-size mismatch of A-site cations in CMR manganites

Abstract

We have addressed the question of the exact nature of magnetic correlations/ordering in CMR manganites, R_(1−x)M_xMnO₃ (R=lanthanides and M=alkaline earths) with decreasing average radius, 〈r_R〉, and/or increasing size mismatch, σ² of the rare earth-site cations where below a certain 〈r_R〉, and/or above a certain σ², an insulating spin-glass-like ground state is widely reported. We have investigated the magnetic ordering in (Nd_1−xTb_x)_0.55Sr_0.45MnO₃ with x=0.6 (〈r_A〉 =1.2067 Å, tolerance factor, t=0.9168, σ² = 0.0093 Ų) and 0.7 (〈r_A〉 = 1.2030 Å, t=0.9154, σ² = 0.0099 Ų) using DC magnetization (M), neutron depolarization (NDP) and SANS techniques. These studies show that the low-temperature ferromagnetic ground state is suppressed drastically for these Tb-substituted compounds. M vs. T curves show a broad peak around 50 and 43 K for the compounds with x = 0.6 and 0.7, respectively. For the x = 0.6 sample, the NDP study confirms the presence of spin clusters below 150 K with an average spin cluster size of 0.034 μm at 12.6 K. For x = 0.7, no depolarization and spontaneous M have been observed. However, a SANS study confirms the presence of spin clusters of finite correlation length (∼6 Å) at all temperatures up to 300 K. M(H/T) curves indicate that interactions exist among these clusters for this (x = 0.7) compound.