Competing magnetic correlations and uniaxial anisotropy in (Fe_1-xMn_x)_2AlB_2 single crystals

We have succeeded for the first time in synthesizing single crystals of nanolaminated borides (Fe_1-xMn_x)_2AlB_2 in the entire Fe-Mn composition range using the Al self-flux method, and have established T-x, H-T and three-dimensional H-T-x magnetic phase diagrams from the results of magnetization measurements. The ferromagnetic correlation of Fe_2AlB_2 is weakened with the Mn substitution, whereas the antiferromagnetic correlation of Mn_2AlB_2 is enhanced with the Fe up to x=0.65. The spin direction in the magnetic ordered states changes from the a to the b axis with increasing Mn concentration and temperature. At x = 0.31-0.46, there are three magnetic phases; ferromagnetic, antiferromganetic, and intermediate phases in between. At x = 0.65 and 0.74, a spin-flop-like metamagnetic transition was observed at a finite field parallel to the spin direction. These observations indicate that in (Fe_1-xMn_x)_2AlB_2 the ferromagnetic and antiferromagnetic correlations coexist and the uniaxial magnetic anisotropy competes between the a and b axes.