Size Effects in Nanostructured MnBi

In addition to variations in temperature, pressure and magnetic field, nanostructuring can tailor the magnetostructural transition; in particular, transition temperatures and anisotropies may change as the material expresses large surface:volume ratios. Compositions near the Mn-Bi eutectic (Mn5Bi95, Mn10Bi90) were rapidly solidified to produce the ferromagnetic low-temperature phase (LTP) NiAs-type MnBi embedded in a Bi matrix. High-resolution TEM reveals that the two compositions have different microstructures: the Mn5Bi95 composition consists of isolated nanorods (10 nm $x$ 30 nm) self-assembled along the major hexagonal symmetry directions of the Bi matrix. In contrast, the Mn10Bi90 composition exhibits regions of equiaxed clustered MnBi precipitates (50 – 100 nm) in addition to regions of isolated nanorods. SQUID magnetometry shows that the Mn5Bi95 composition has an abrupt magnetization decrease for T $>$ 520 K associated with a first- order hysteretic magnetostructural transformation from LTP MnBi to high-temperature phase (HTP) MnBi. This transition temperature is 100 degrees lower than that of the Mn5Bi95 composition, which exhibits the bulk MnBi transition temperature of 633 K with second-order character.