Structural, Magnetic, Magnetocaloric, and Magnetostrictive Properties of Pb_1-xSr_xMnBO₄ (x = 0, 0.5, and 1.0)

The solid solution Pb1-xSrxMnBO4 is reported with an orthorhombic, Pnma, structure throughout; here studies on compounds with x = 0, 0.5 and 1 are described. The structure contains chains of MnO6 octahedra that exhibit intra-chain ferromagnetic (FM) order at low temperatures. Neutron powder diffraction (NPD) reveals dominant FM order in PbMnBO4 (Tc = 30 K), whereas SrMnBO4 is primarily antiferromagnetic (AFM) with TN = 16 K; the difference is related to the link between the chains that involves the BO3 groups. PbMnBO4 has its moment along a but also has a previously unreported AFM contribution along c (magnetic space group Pnm'a'), whereas SrMnBO4 has its moment along a but also a FM canting along c (magnetic space group Pn'm'a). The end members show distinct magnetostriction at Tc/TN which correlates with the different magnetic exchange in these compounds. NPD in variable applied magnetic field shows that SrMnBO4 is converted to fully FM at 8 T. The behavior above the magnetic ordering temperature is consistent with short-range FM correlations within the chains, which is particularly apparent in PbMnBO4. The magnetocaloric effect (MCE) has been measured and compared with those previously reported for the mineral gaudefroyite. PbMnBO4 has excellent MCE behavior, especially near Tc, 30 K. The strong FM exchange within the chains and FM correlations above Tc are vital for the MC properties.