The formation of single-phase equiatomic MnBi by rapid solidification
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I. INTRODUCTION
The equiatomic intermetallic compound MnBi has long been of interest because of the unusually large magnetic anisotropy (k = 107 ergs cm"3) of the low temperature phase (LTP)1 and the favorable magnetooptical properties of the quenched high temperature phase (QHTP).2'3 However, the formation of pure single-phase MnBi is difficult by conventional means, because at the peritectic temperature 719 K (see the phase diagram,4-5 Fig. 1), Mn tends to segregate from the Mn-Bi liquid. On the other hand, if the temperature is lowered to below that of the eutectic, the reaction becomes sluggish.6 A complicating feature is that the diffusion of Mn through MnBi is extremely slow,6"8 so that formation of MnBi at the interface actually impedes any further reaction. A further problem is that MnBi exists in several closely related forms (see Table I); along with LTP and QHTP is a high temperature phase (HTP)1-9 and a so-called new phase NP.11"13 Chen and Stutius1 pulled LTP and QHTP crystals from a molten solution at 628 K and 700 K, respectively, by adding excess Bi. A small volume percent of MnBi embedded in Bi was also obtained by the Czochralski and Bridgman techniques.11'12 Andresen et al.6 made (inter alia) some LTP by sintering Mn and Bi powders at 708 K for up to 30 days. At present, however, there is no known technique which can prepare single phase MnBi in large amounts. In the present paper we report on an alternative approach to MnBi formation: rapid quenching from the melt to room temperature followed by thermal annealing. By melt spinning (which may achieve quench rates in excess of 106 Ks"1) Mn segregation may be reduced or even eliminated, LTP being produced by subsequent heat treatment. A similar technique was recently reported by Xu et al.,15 although they succeeded only in producing a sample with a small fraction of LTP. Here 2646
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J. Mater. Res., Vol. 5, No. 11, Nov 1990
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we show that it is in fact possible, by suitable processing parameters, to obtain almost complete formation of LTP. II. EXPERIMENTAL METHODS
Mn-Bi ingots for melt-spinning were prepared by induction melting appropriate amounts of Mn and Bi (99-99% purity) on a water-cooled copper boat under titanium-gettered argon at a pressure of 33 kPa. Melt spinning was carried out by ejecting the melt from a quartz crucible onto a copper wheel with a tangential speed of 50 ± 5 ms"1 under argon at 33 kPa. To enhance the quenching rate, the nozzle diameter of the quartz crucible was never more than 0.25 mm. The resulting ribbons were brittle, about 1 mm wide and 10 /i thick, and formed in lengths of about 1 cm. Subsequent
T(K) / /
\
Ll + L2
\ \ 1C17 \
1
\l416 A 1352
14UU
mnn
' 983
719
HTP \ 628
613
600
535
LTP1
Mn
\ 544.5 97.8
I
1
MnBi
Bi
FIG. 1. Mn-Bi equilibrium phase diagram taken from Refs. 4 and 5. 1990 Materials Research Society
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X. Guo et a/.: The formation of single-phase equiatomic MnBi by rapid solidification
TABLE I. Crystalli
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