Evolution and growth of the LTP MnBi Phase in
Mn-Bi system
V.V.Ramakrishna, S.Kavita*, D.Siva Prahasam, D. Prabhu, Ravi Gautam and R.Gopalan
International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Chennai600113, India
*
Corresponding author’s e-mail: skavita@arci.res.in, Tel.: +91-44-66632816; Fax: +91-66632802
Abstract
The paper reports the effect of ball milling on the
evolution of the LTP MnBi phase content in Mn52Bi4 alloy.
Coercivity was observed to increase with milling and Hc of
~1T was achieved in the sample milled for 5 hours.
Keywords: Intermetallics, Mn-Bi alloy, coercivity, ball
milling, magnetic properties.
Introduction
MnBi has evoked substantial interest in the
search for rare earth free permanenet magnet owing to
its good intrinsic properties. The coercivity of LTP
MnBi phase increases with temperature and is much
higher than the coercivity of Nd-Fe-B at higher
temperatures [1].Therefore; MnBi has high potential as
a permanent magnet at high temperature. Numerous
attempts have been made to obtain single-phase MnBi,
but the segregation of Mn and Bi phase could not be
avoided [2]. Here we report the effect of milling on the
evolution of LTP MnBi phase and its magnetic
properties in arc melted Mn52Bi48 alloy
the relative intensity of MnBi peak ( 2θ = 28.13 o)
compared to Bi peak (2θ = 27.26 o). The increase in the
relative intensity of the MnBi phase suggests that the
energy derived from ball milling acts a driving force for
the alloying of unreacted elemental Mn and Bi to form
LTP MnBi. On further milling the intensity of Bi phase
increases suggesting decomposition of the already
formed LTP MnBi phase. SEM micrographs shows
(fig1 (b)) a clear reduction in particle size with milling.
Fig. 1(c) shows the increase in coercivity with milling It
is found that the coercivity increases to 1T after milling
for 5hrs. The increase in coercivity could be due to the
decrease in particle size as Hc is inversely proportional
to the particle size above the single domain size which
is 500 nm for MnBi. The magnetization in MnBi system
strongly depends on atomic arrangement of Mn and Bi,
in the lattice. The decrease in magnetization (fig. 1 (d))
inspite of increasing LTP MnBi phase suggests the
rearrangement of Mn and Bi atoms within the MnBi
lattice during milling.
Experimental
An alloy ingot with a nominal composition of
Mn52Bi 48 was prepared by arc melting the constituent
elements in Ar atmosphere. The melted ingot was
vacuum sealed and subjected to annealing for 24hrs at
3000C to obtain LTP MnBi phase. Fritisch Ball mill
with a ball to powder ratio of 1:10 and rotation speed of
60rpm. Stainless steel vials and balls (10mm) were used
for milling. The structural characterization at each stage
of milling has been done using with CuKα
(=1.54059Å) radiation in a PANalytical (X’pert PRO).
The magnetic properties at each stage of milling were
done by using Microsense Easy VSM Model EV9.
Particle size variation with milling was observed using a
scanning electron microscopy (SEM) (Quanta 3D FEG).
Results and Discussion
Figure 1(a) shows the XRD pattern of the alloy
annealed at 3000C for 24hrs and subsequently ball
milled for 5hr. The annealed alloy ingot shows
elemental Bi as a major peak and LTP MnBi peaks as
minor phase. With the increase in milling time, one can
clearly observe from the XRD pattern an enhancement
in the volume fraction of the LTP MnBi phase at the
expense of Bi phase which is reflected in the increase in
Fig. 1: (a) XRD of Mn-Bi with milling (b) SEM image of as-crushed
and 5 hr milled (c) variation of coercivity(d) magnetization with
milling.
References
[1] X .Guo, X.Chen, Z.Altounian and J.O .Str¨omOlsen Phys. Rev. B 46, 14 578 (1992)
[2] H.Yoshida, T.Shima, T.Takahashi and
H.Fujimori Mater. Trans. JIM 40, 455(1999)