Spin reorientation in MnBi

Article abstract of DOI:10.1016/S0925-8388(00)01352-9

High purity MnBi was prepared by arc-melting under He gas. The prepared samples have the property of being hard to oxidize in air and progress of oxidation was not observed by X-ray diffraction 1 year later. It is easy to pulverize a good quality sample into powder even in air. We confirmed that coercive force at room temperature depends on the powder size and takes a value of 8 kOe for less than 400 mesh. The permeability has a hysteresis in a temperature range between 77 and 300 K and exhibits a maximum at 100 K corresponding to the spin reorientation temperature T_t. The pressure (P) dependence of T_t was examined by measuring the permeability at various pressures. The value of dT_t/dP was obtained to be 9.6 K/kbar. The results of thermal expansion revealed that both the lattice parameters a and c increase abruptly near 250 K.

Full text of DOI:10.1016/S0925-8388(00)01352-9

Journal of Alloys and Compounds 317–318 (2001) 297–301
L
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Spin reorientation in MnBi
H. Yoshidaa , , T. Shima , T. Takahashi , H. Fujimori , S. Abe , T. Kaneko , T. Kanomata ,
a
a
a
a
a
b
*
T. Suzukib
aInstitute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-Ku, Sendai 980-8577, Japan
bFaculty of Engineering, Tohoku Gakuinn University, Tagajo 985-8537, Japan
Abstract
High purity MnBi was prepared by arc-melting under He gas. The prepared samples have the property of being hard to oxidize in air
and progress of oxidation was not observed by X-ray diffraction 1 year later. It is easy to pulverize a good quality sample into powder
even in air. We confirmed that coercive force at room temperature depends on the powder size and takes a value of 8 kOe for less than 400
mesh. The permeability has a hysteresis in a temperature range between 77 and 300 K and exhibits a maximum at 100 K corresponding to
the spin reorientation temperature Tt. The pressure (P) dependence of Tt was examined by measuring the permeability at various
pressures. The value of dTt /dP was obtained to be 9.6 K/kbar. The results of thermal expansion revealed that both the lattice parameters
a and c increase abruptly near 250 K.
2001 Elsevier Science B.V. All rights reserved.
Keywords: MnBi; Ferromagnet; Spin reorientation; Pressure effect; Thermal expansion; Permeability
1. Introduction
into the basal plane [6]. This spin reorientation has been
also investigated by NMR [7]. Robert et al. [4] carried out
the neutron diffraction measurements on the MnBi sample
at 77.3 K and at zero magnetic field. Their experimental
result indicates that a part of the moments rotate away
from the c-axis. After their work, Andresen [8] reexamined
it and concluded that the spin turning is practically
complete at 78 K. The pressure effect on the magnetic
transition from FLTP to PHTP was investigated by Samara
et al. [9]. The pressure derivative of the ferromagnetic
transition temperature is 20.75 K/kbar. Adams et al. [10]
attempted to prepare a permanent magnet of MnBi,
because of its high magnetic crystal anisotropy. Their
magnet has a large coercive force of 3.1 kOe and an
energy product (BH)MAX of 4.3 MGOe.
How to prepare a high quality MnBi sample still
remains an important problem for basic science and
application. In this paper the preparation of highly pure
MnBi by He-arc melting, the pressure effect on the spin
reorientation temperature and the thermal expansion of
MnBi are presented.
It is well known that the intermetallic compound MnBi
has a NiAs-type crystal structure and exhibits unusual
ferromagnetic properties with the ferromagnetic transition
temperature of 633 K on heating. At this temperature a
ferromagnetic low temperature phase (FLTP) with a NiAs-
type structure transforms to a paramagnetic high tempera-
ture phase (PHTP) with a distorted Ni2In-type structure
[1]. Therefore, temperature dependence of magnetization
for MnBi exhibits a hysteresis and the magnetic transition
temperature on cooling is 600 K [2]. Chen reported that the
HTP of MnBi was a separate compound with chemical
formula Mn1.08Bi [3]. By a neutron diffraction study
Roberts confirmed that MnBi is in the paramagnetic state
in the temperature range from 623 (cooling)/633 K
(heating) to 718 K and a Mn atom in FLTP has a magnetic
moment of 3.960.5 mB [4]. At room temperature MnBi
has a very high magnetocrystalline anisotropy energy [5].
The magnetizing process for MnBi oriented in the c-axis
direction exhibits strong magnetic anisotropy, where the
magnetic easy direction is the c-axis [2]. The anisotropy
constant decreases rapidly with decreasing temperature and
changes its sign at 84 K, where the magnetic moments
rotate from the c-axis of the NiAs-type crystal structure
2. Sample preparation
When the high purity manganese metals Mn (5N)
obtained commercially were melted by arc melting, col-
umns of black smoke shot up into the vessel chamber. This
*Corresponding author. Fax: 181-22-215-2231.
E-mail address: hyoshida@imr.tohoku.ac.jp (H. Yoshida).
0925-8388/01/$ – see front matter
PII: S0925-8388(00)01352-9
2001 Elsevier Science B.V. All rights reserved.

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H. Yoshida et al. / Journal of Alloys and Compounds 317–318 (2001) 297–301
black smoke is caused not only by the vaporization of Mn
but also by included gas. In order to produce high quality
samples, it is important to make Mn metals degas before
sampling. Then, the commercial Mn metal was at first arc
melted at a half of atmospheric pressure of He gas in order
to degas. The commercial Mn metals are porous and with
dull lustre. However, the degassed ones are hard and with
a metallic brightness. The strongly oriented MnBi samples
were prepared with such pretreated Mn and Bi by arc
melting in He gas. Magnetic measurements were carried
out to examine the transformation between high tempera-
ture phase (HTP) and low temperature phase (LTP).
Magnetization in HTP was measured by VSM in the
temperature range from 77 up to 750 K to examine the
phase transition. Fig. 1 shows a thermomagnetic curve for
the quenched MnBi alloy. The starting temperature of the
measurement is room temperature and the maximum
temperature is 773 K. When temperature increases, the
magnetization decreases initially and has a shallow mini-
mum around 433 K and then increases gradually. The
abrupt increase was observed near 544 K (5Tm) which is
a melting point of Bi. At 636 K, the value of the
magnetization drops abruptly to near zero. According to
the phase diagram of MnBi by Chu et al. [3], the molar
formula of MnBi in HTP, which has a distorted Ni2In-type
crystal structure, is Mn1.08Bi and it has been reported that
its Curie temperature (TC) is 433 K [3]. The above
minimum arises at the Curie temperature of HTP accom-
panying a transformation to LTP. The increase of mag-
netization above 433 K is due to the growth of LTP. The
abrupt decrease of magnetization at 636 K is due to the
transformation from FLTP to PHTP. With further increase
of temperature up to 773 K, there arises a small decrease
of the magnetization at 730 K as shown with an arrow in
the inset. When temperature decreases again after heating
up beyond 773 K, the magnetization keeps a very small
value down to 440 K and increases gradually with further
decrease of temperature. This means that MnBi in HTP
decomposes into a-Mn and Bi above 730 K and a small
amount of MnBi compound is again synthesized during
cooling down to room temperature. When the temperature
variation of magnetization is restricted under 643 K, the
magnetization decreases abruptly at 633 K on heating and
increases abruptly at 603 K on subsequent cooling (refer to
Fig. 2 in Ref. [2]). The magnetic transition shows a
hysteresis and is of the first order accompanying a crystal
phase transition. The magnetization continues increasing
although the temperature decreases through Tm.
An appearance of a slight wave near 433 K on decreas-
ing temperature is due to quenching of a small amount of
MnBi in HTP. These results give us a condition to
transformation from a quenched sample to MnBi in LTP;
the suitable temperature range for transformation is be-
tween 544 and 603 K, and the reaction time is desirably
more than 1 h. We carried out X-ray diffraction to examine
the orientation of the bulk sample. The (00l) line in the
NiAs-type crystal structure was observed for the powder
sample, but not observed for the bulk one. This shows that
the c-plane of the present bulk sample grew up in the
direction perpendicular to the bottom face of the Cu hearth.
The obtained lattice parameters a and c are 4.289 and
6.103 nm. It has been reported that MnBi was unstable to
oxidation in air. Even though the sample prepared above is
exposed to air for more than 1 year, there was no observed
variation on magnetic properties and X-ray diffraction
pattern except the appearance of pure Bi metal on the
surface. The SEM photograph and X-ray diffraction spec-
trum for an MnBi sample made with regular Mn metals
and Bi by Ar-arc melting were also taken and examined.
We observed four kinds of area in the photograph as
shown in Fig. 2. According to mapping for contents of
oxygen, a weird dark and large area contains MnBi and
oxygen, and especially small dark spots in the SEM
Fig. 1. Thermomagnetic curve for MnBi alloys prepared by He-arc
melting. The inset shows the decomposition of MnBi in HTP at 730 K.
Fig. 2. Compositional image of SEM photograph of an MnBi sample
made with regular Mn metals and Bi by Ar-arc melting.

H. Yoshida et al. / Journal of Alloys and Compounds 317–318 (2001) 297–301
299
photograph show high oxygen concentration. In the X-ray
pattern, there are observed peaks of MnO and a-Mn. The
intensity of peaks for a-Mn becomes stronger compared
with that for the present powder sample prepared by
He-arc melting. It means that MnBi is decomposed into
active a-Mn and Bi together with oxygen and MnO and
the condensed Mn exposed to the air will react with
oxygen. It was found that arc-melting under He gas is the
most suitable processing for the preparation of such good
quality MnBi.
3. Experimental results and discussion
3.1. Magnetization
The magnetization process was measured with a SQUID
magnetometer at temperatures from 4.2 up to 300 K and a
VSM one at room temperature. The results are shown in
Fig. 3. A spin easy direction is that of the c-axis and the
value of the coercive force Hc is small being |102 Oe.
There is observed a tendency of magnetic saturation near
10 kOe and the magnetic moment per Mn atom takes a
value of 3.3 mB (70 emu/g) at 10 kOe, which is consistent
with the previous results: 3.3 at 10 kOe and 3.8 mB at 140
kOe [2]. The temperature dependence of the magnetization
at 10 and 55 kOe for bulk samples is shown in Fig. 4
where the value of magnetization was taken from the
magnetization curve at a constant temperature. The curve
at H 5 10 kOe has a maximum near 100 K which gives the
magnetic moment of 4.2 mB /formula. The appearance of
the peak is due to the reorientation of spins with tempera-
Fig. 4. Temperature dependence of the magnetization at 10 and 55 kOe
for the oriented bulk MnBi.
ture. It was confirmed that the remanence is almost zero
below room temperature within the measurement accuracy,
though the temperature dependence of remanence for the
bulk sample is not presented here. The magnetization vs.
temperature curves at 50 kOe for the powdered sample are
shown in Fig. 5a. The magnetization decreases slightly
with temperature except at 4.2 K. The remanence as shown
Fig. 3. Hysteresis loops of an oriented MnBi sample measured at room
temperature.
Fig. 5. Thermomagnetic curve of powder MnBi samples at 50 kOe (a)
and at 0 kOe (5remanence) (b), respectively.

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H. Yoshida et al. / Journal of Alloys and Compounds 317–318 (2001) 297–301
in Fig. 5b starts to have a finite value from 150 K with
increasing temperature. It is quite different from that of the
bulk sample mentioned above. The magnetization at 4.2 K
for both bulk and powdered sample has a smaller value
comparing with those in the higher temperature range. It
suggests that there may exist a certain antiferromagnetic
interaction at 4.2 K which yields the forces to tilt the spins
in the ferromagnet. The good quality sample has a
preference of an orientation along the c-axis. Since it has a
large anisotropy at room temperature, the magnetic loop
has a larger coercive force. In Fig. 6 the hysteresis curve
for two kinds of powder size samples are shown. These
show that the value of coercive force strongly depends on
the grain size. These results are consistent with those by
Shur et al. [11,12]. They have examined the magnetic
domain of manganese–bismuth alloy in particles of various
sizes and observed that as a function of particle size. It was
found that the value of Hc decreases with the domain wall
number in a systematic manner. One origin of the coercive
force for the powdered samples is due to the crystalline
anisotropy of the single domains. The number of the
domain and the decrease of the magnetic anisotropy makes
Hc smaller. The remanence of powder samples at tempera-
tures lower than 150 K shown in Fig. 5b has an almost
zero value because of diminishing the single domain
structure with decreasing anisotropy energy. There is a
critical diameter size of particle (Dc) to realize the single
domain. This size is proportional to the root of the
magnetic anisotropy constant (Kc). The diminishment of
the residual magnetization at temperatures lower than 150
K is ascribed to the decrease of Kc [11,12].
3.2. Permeability and pressure effect
It was reported that the intermetallic compound MnBi
has a large magnetic anisotropy at room temperature and
its temperature dependence is particular. The spin easy
direction at room temperature is along the c-axis and the
strength of the anisotropy is reduced with decreasing
temperature down to |90 K. At lower temperatures than
|90 K, the Mn spin rotates into the c-plane. In general, the
permeability mac is sensitive to the variation of mag-
netization or magnetic anisotropy. The pressure effect on
Tt was examined by measuring the temperature variation
of mac at various pressures. The details of measurements
are given in Ref. [13]. Fig. 7 shows the temperature
dependence of the permeability at various pressures with
increasing temperature. The mac vs. T curve at normal
pressure has a maximum at 100 K. Above the maximum
temperature, mac decreases simply. When temperature is
decreased again, the curve shows a hysteresis, where the
maximum temperature appears at 98 K. The peak corre-
sponds to that observed in the magnetization vs. tempera-
ture curve and the peak temperature was defined as a
transition temperature Tt of spin reorientation. The hyster-
esis in the temperature variation of mac shows that the
transition at Tt is of the first order, which is consistent with
the discontinuous change of spin direction at Tt observed
in NMR experiment by Hihara [7]. Tt increases linearly
with pressure and its pressure derivative is dTt /dP 5 9.6
K/kbar. It means that the volume for the state with
magnetic easy direction along the c-axis is larger than for
the one in the c-plane.
Fig. 7. The temperature dependence of permeability (mac ) at 0, 3.4, 4.9,
6.8, 8.1 and 9.7 kbar. The curve at 0 kbar shows a large hysteresis in the
temperature range of 90 to about 180 K.
Fig. 6. Hysteresis loops of powder MnBi samples passed through a sieve
with 200 mesh and 400 mesh, respectively.

H. Yoshida et al. / Journal of Alloys and Compounds 317–318 (2001) 297–301
301
3.3. Thermal expansion
and c are examined from 100 to 450 K. The temperature
dependence of a and c in the NiAs-type crystal structure
and the volume of unit cell are shown in Fig. 8a–c. As
seen in the figure, the curves demonstrate unexpected
variations at 250 K. The volume varies abruptly by about
0.5%. The origin of such a peculiar volume variation
without the break of crystal symmetry is not clear at
present. The results of dTt /dP . 0 mentioned above
suggest that there is the anomalous volume increase
through Tt. To examine an anomaly of thermal expansion
at Tt, it is desired to carry out the X-ray diffraction
measurement at temperatures lower than 100 K.
It is well known that Mn2Sb with a tetragonal lattice
exhibits a spin reorientation at 250 K. The anisotropy
energy was explained well by the sum of the magnetic
dipole interaction and the single ion spin energy [14]. The
temperature dependence of the anisotropy varies in sign at
250 K. Its origin comes from two kinds of manganese
atoms present in the unit cell. Different to Mn2Sb, there
exists only one kind of manganese atom in the lattice of
MnBi. The temperature variation of lattice parameters a
Acknowledgements
The authors would like to thank Dr. H. Kimura and Prof.
K. Asami for useful suggestions and Mr. Murakami for
helping with SEM observation and X-ray analysis. This
work was performed at the Laboratory for Developmental
Research of Advanced Materials, Institute for Materials
Research, Tohoku University.
References
[1] C. Guillaud, J. Phys. Radium 12 (1951) 223.
[2] H. Yoshida, T. Shima, T. Takahashi, H. Fujimori, Mater. Trans. JIM
40 (1999) 455.
[3] T. Chen, J. Appl. Phys. 45 (1974) 2358.
[4] B.W. Roberts, Phys. Rev. 104 (1956) 607.
[5] C. Guillaud, J. Phys. Radium 12 (1951) 143.
[6] P.A. Albert, W.J. Carr Jr., J. Appl. Phys. 32 (1961) 201S.
[7] T. Hihara, Y. Koi, J. Phys. Soc. Japan 29 (1970) 343.
[8] A.F. Andresen, W. Halg, P. Fischer, E. Stol, Acta Chem. Scand. 21
(1967) 1543.
[9] G.A. Samara, A.A. Giardini, Bull. Am. Phys. Soc. 9 (1964) 635.
[10] E. Adams, W.M. Hubbard, A.M. Syeles, J. Appl. Phys. 23 (1952)
1207.
[11] Ya.S. Shur, E.V. Shtol’TS,V.I. Margolina, Sov. Phis. JETP 11 (1960)
33.
[12] I.S. Shur, Ye.V. Shtol’TS, G.S. Kandaurova, L.V. Rebneva, Fiz.
Metal. Metalloved. 8 (1959) 38.
[13] H. Yoshida, J. Chiba, T. Kaneko, H. Fujimori, S. Abe, Physica B
237–238 (1997) 525.
[14] T. Tonegawa, J. Phys. Soc. Japan B 17 (1962) 1398.
Fig. 8. Variations of lattice parameters a and c, and the volume of MnBi
as a function of temperature. The calculated curves were determined to fit
the high temperature part of the data.