Structure and magnetic properties of directionally solidified Bi-MnBi eutectic alloys

An alloy containing about 2 at% Mn and 98 at% Bi should solidify at 230°C to give a eutectic consisting of about 3 vol % ferromagnetic MnBi in a matrix of diamagnetic Bi. Directional solidification of this alloy produces a structure of elongated parallel MnBi rods with diameters from 0.1 to 1.5 μm, decreasing with increasing growth rate. Samples solidified slowly (growth rate less than 5 cm/h) show magnetic properties at room temperature and down to 4.2 K that are consistent with the known magnetic properties of bulk equilibrium MnBi. Samples solidified more rapidly (20-80 cm/h) show considerably more complicated magnetic behavior. At room temperature they have less than half the moment expected for the equilibrium MnBi composition, and a second magnetic phase appears when the temperature is lowered below about 240 K. The coercive field of this phase rises rapidly with decreasing temperature, reaching the limit of our experimental apparatus (about 120 kOe) near 100 K. Magnetic measurements with the field applied perpendicular to the growth axis show the effects of anisotropy, due to particle alignment, although samples prepared at very rapid freezing rates (300 cm/h) appear to be poorly aligned. Samples prepared by arc melting on a water-cooled hearth are similar to high-growth-rate samples. Annealing at 200°C was necessary in order to obtain reproducible magnetic data. The results are generally consistent with the presence in the samples of a mixture of the normal equilibrium MnBi phase and a variant of the known high-temperature phase (HTP) with a substantially depressed Curie temperature. Structural studies using electron diffraction from thinned sections indicate the presence of both normal (LTP) and high-temperature (HTP) MnBi.