Unusual effects of magnetic dilution in the ferrimagnetic columnar ordered Sm₂MnMnMn_4–xTi_xO₁₂ perovskites

Powder neutron diffraction experiments have been employed to establish the effects of site-selective magnetic dilution in the Sm₂MnMnMn_4–xTi_xO₁₂ A-site columnar ordered quadruple perovskite manganites (x = 1, x = 2 and x = 3). We show that in all three compositions the Mn ions adopt a collinear ferrimagnetic structure below 27 K, 62 K and 34 K, respectively. An unexpected increase in the ordering temperature was observed between the x = 1 and x = 2 samples, which indicates a considerable departure from mean field behaviour. This result is corroborated by large reductions in the theoretical ground state magnetic moments observed across the series, which indicate the presence of spin fluctuations and or disorder. We show that long range magnetic order in the x = 3 sample, which occurs below the percolation threshold for B-B exchange, can only be understood to arise if magnetic order in Sm₂MnMnMn_4–xTi_xO₁₂ is mediated via both A-B and B-B exchange, hence confirming the importance of A-B exchange interactions in these materials. Finally we show that site-selective magnetic dilution enables the tuning of a ferrimagnetic compensation point and the introduction of temperature-induced magnetization reversal.