Magnetic behavior of RMn_2+xAl_10−x (R=La,Gd) crystals

The crystallographic and magnetic properties of the solution grown RMn_2+xAl_10−x (R=Gd and La) crystals with tetragonal (P4∕nmm) structure are investigated. For these, single-crystal x-ray diffraction results have shown the preferential occupation of excess manganese on the aluminum 8i crystallographic site. Due to excess Mn, there is evidence of local-moment magnetism in LaMn_2+xAl_10−x and their random distribution is thought to be responsible for the magnetic correlations below ∼50 K and the spin-glass behavior below ∼3 K. For GdMn_2+xAl_10−x, the extra manganese influences temperature below which the sample enters into a state with a net ferromagnetic component: T_C≈16 K for GdMn_2.21(4)Al_9.79(4) and 25.5 K for GdMn_2.39(2)Al_9.61(2). Assuming a linear dependence between T_C and excess Mn concentration, the fully stoichiometric and ordered GdMn₂Al₁₀ should have T_C≈5 K.