Comparing Magnetism in Isostructural Oxides A_0.8La_1.2MnO_4.1: Anisotropic Spin Glass (A = Ba) versus Long-Range Order (A = Sr)

This study presents the strikingly distinct magnetic properties of two isostructural compounds, Ba_0.8La_1.2MnO_4.1 and Sr_0.8La_1.2MnO_4.1 (K₂NiF₄type, I4/mmm). Spectroscopic studies have shown that Mn is in a +3.0(1) oxidation state only, in both compounds; therefore, the charge is balanced by accommodating extra oxygen at interstitial sites, as confirmed by neutron powder diffraction. We found that the Ba compound exhibits an exceedingly rare anisotropic spin glass behavior, T_g = 26.4(4) K, with the moment freezing along the c-axis only while the in-plane spin components remain dynamic well below T_g. Experimental results including neutron diffraction, heat capacity, and magnetic (dc and ac) measurements performed on an oriented single crystal support this conclusion. This is a remarkable result, the only other known example of an anisotropic spin glass being Fe₂TiO₅. The spin glass state in Ba_0.8La_1.2MnO_4.1 is argued to arise due to competing antiferromagnetic and ferromagnetic 180° Mn³⁺–O–Mn³⁺ superexchange interactions. In contrast, the Sr analogue shows 2D antiferromagnetic correlations and long-range antiferromagnetic order below 95(1) K with a remarkably reduced ordered moment of 1.449(49) μ_B/Mn³⁺ instead of the ∼4 μ_B expected for an S = 2 ion.