MECHANICAL MILLING EFFECT ON CRYSTAL-STRUCTURES AND MAGNETIC-PROPERTIES OF MNAS AND MN0.8TI0.2AS COMPOUNDS

The crystal structures and some magnetic properties have been investig ated for MnAs and Mn0.8Ti0.2As compounds during mechanical milling. Th e lattice parameters and magnetization results measured for Mn0.8Ti0.2 As compound, which has no structural phase transformation at T(c)point , indicate that Ti substitution of some Mn can stabilize the B8(1) str ucture of MnAs compound, when subjected to mechanical milling. The cha nges of T-c in milling for Mn0.8Ti0.2As compound seem to result unique ly from the disordering effects, no relevent to the lattice volume cha nges as affect in the case of MnAs. Based on the thermodynamic model o f Bean and Rodbell between T-c and lattice parameters, the milling cau sed disordering degrees in the two compounds were approximated by an a tomic regularity parameter, which is shown to be monotonically decreas ing with milling period. The minimum crystal size of the milled powder samples is about 20 nm and the pseudo-coalescence phenomena occur in the vicinity of 1.8 x 10(4) s similarly for both compounds. The transi tions in the structural ordering and thermal stability during the whol e milling process have been discussed according to a step mode for bot h compounds.