Family of anisotropic spin glasses Ba1-xLa1+xMnO4+δ

The synthesis, structural, and magnetic characterization of the series Ba1-xLa1+xMnO4+delta (0 <= x <= 0.5) with the K2NiF4-type structure are reported. We previously found that the x = 0.2 member exhibits the very rare anisotropic spin-glass behavior with only the c-axis spin component freezing below T-g. Here we show that each member of the Ba1-xLa1+xMnO4+delta (0 <= x <= 0.4) series exhibits the same spin-glass behavior. Moreover, T-g varies with x, reaching a maximum of 26.4(4) K for x = 0.20 compared with 19.2(2) K for x = 0. The spin-glass behavior was confirmed by both dc and ac magnetic susceptibility measurements. No long-range magnetic order was found down to 2 K. All series members adopt the I4/mmm space group and subtle structural transformations occur with increasing La content. The unit cell volume contracts for 0.0 < x < 0.3 and expands for 0.3 < x <= 0.5. Similar behavior is seen for the equatorial Mn-O bonds in the Mn-O octahedron while the axial Mn-O distances increase to x similar to 0.3 but remain unchanged for higher x. X-ray absorption near-edge structure analysis revealed that the oxidation state of Mn in the Ba1-xLa1+xMnO4+delta samples varies with x: For x <= 0.2 Mn is in the +3.0(1) oxidation state only, while for x > 0.2 a mixed +2/+3 oxidation state was found. Therefore, for the charge balance, samples with x <= 0.2 contain excess oxygen as an interstitial species, while for x > 0.2, the structure cannot sustain any more interstitial oxygen and the La excess is accommodated through the reduction of some Mn3+ to Mn2+. Based on the oxidation state of Mn, the possible origins of the spin-glass magnetism in the Ba1-xLa1+xMnO4+delta series are discussed.

Automated summary

The synthesis, structural, and magnetic characterization of the Ba1-xLa1+xMnO4+delta series with the K2NiF4-type structure is reported, showing that each member exhibits the same spin-glass behavior with subtle structural changes and variations in oxidation states. The spin-glass behavior was confirmed by magnetic susceptibility measurements, and different oxidation states of Mn were observed depending on the La content in the samples.