Alignment and analyses of MnBiBi nanostructures

Article abstract of DOI:10.1063/1.2008368

A Mn5 Bi95 alloy was rapidly solidified into a mixture of nanocrystalline Bi and metastable Bi(Mn). Heating the ribbons to temperature T=525 K in a dc magnetic field causes formation and c -axis alignment of low-temperature phase (LTP) MnBi nanorods along the applied field direction. Nanorod alignment increases with increased magnetic field, with a calculated alignment half-angle of 47° for a sample heated to 520 K at 50 kOe. In situ magnetization changes suggest that nanorod alignment is achieved by rotation of MnBi particles. Particle alignment enables the measurement of the MnBi nanorod spin reorientation temperature of 100 K, the same as its bulk counterpart.

Full text of DOI:10.1063/1.2008368

Alignment and analyses of MnBi/Bi nanostructures
K. Kang, L. H. Lewis, and A. R. Moodenbaugh
Citation: Appl. Phys. Lett. 87, 062505 (2005); doi: 10.1063/1.2008368
View online: http://dx.doi.org/10.1063/1.2008368
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APPLIED PHYSICS LETTERS 87, 062505 ͑2005͒
Alignment and analyses of MnBi/Bi nanostructures
a͒
K. Kang, L. H. Lewis, and A. R. Moodenbaugh
Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973-5000
͑
Received 21 February 2005; accepted 20 June 2005; published online 3 August 2005͒
A Mn Bi alloy was rapidly solidified into a mixture of nanocrystalline Bi and metastable Bi͑Mn͒.
5
95
Heating the ribbons to temperature T=525 K in a dc magnetic field causes formation and c-axis
alignment of low-temperature phase ͑LTP͒ MnBi nanorods along the applied field direction.
Nanorod alignment increases with increased magnetic field, with a calculated alignment half-angle
of 47° for a sample heated to 520 K at 50 kOe. In situ magnetization changes suggest that nanorod
alignment is achieved by rotation of MnBi particles. Particle alignment enables the measurement of
the MnBi nanorod spin reorientation temperature of 100 K, the same as its bulk counterpart. © 2005
American Institute of Physics. ͓DOI: 10.1063/1.2008368͔
The crystallographic alignment of uniaxial magnetic
nanocrystals with respect to their supporting substrate or ma-
trix is an important technological challenge, often with in-
completely understood governing forces and conditions.
Crystallographic alignment of the easy axis of magnetization
is essential to the realization of a high maximum energy
relative to the bulk LTP MnBi Curie temperature 628 K. The
high coercivity of the MnBi nanoparticles reported in the
current study may attract the interest of the permanent mag-
net community; work of similar intent was performed by
1
6
Yelon et al. In this letter we report on the easy-axis align-
ment of the nanoparticles resulting from in situ application of
a magnetic field during low-temperature annealing. We re-
port on the phenomenon and conditions of field-induced
crystallographic texturing of the MnBi nanorods, presenting
a method to tailor the alignment of MnBi nanostructures.
Mn-Bi ribbons ͑4 mm wideϫ40 ␮m thick, with lengths
less than 5 mm͒ were fabricated from a charge of nominal
composition Mn5-Bi95 ͑at %͒ by melt-spinning. The sample
was induction melted then ejected using 525 Torr com-
pressed Ar gas onto a water-cooled rotating copper wheel
1
product ͑BH͒max in advanced permanent magnets, underlies
the design of future high-density perpendicular magnetic and
2
magneto-optic recording media, and facilitates the optimiza-
3
tion of ultrasoft magnetic materials.
Because of its high uniaxial magnetic anisotropy ͑ϳ2
7
3
ϫ10 erg/cm at 500 K͒ along its c-axis and record
4
magneto-optical Kerr rotation of 1.25° at room temperature,
the low-temperature phase ͑LTP͒ of equiatomic MnBi with
the hexagonal NiAs structure has received much attention
since the 1950s due to potential applications in high-
͑
28 m/s͒ for rapid solidification. X-ray diffraction and plan-
5
6
view transmission electron microscopy ͑TEM͒ reveal that the
as-quenched ribbons consist only of polycrystalline Bi. Evi-
dence of LTP MnBi is obtained only after annealing to
ϳ400 K, suggesting that rapid solidification causes the Mn
to be metastably retained in the Bi matrix as either a substi-
tutional or an interstitial impurity. The as-quenched ribbons
were sealed in silica tubes in a vacuum better than3
ϫ10− Torr and data was collected in a furnace insert to the
superconducting quantum interference device ͑SQUID͒ mag-
netometer cryostat upon heating and cooling in the range
temperature permanent magnets and in data storage.
Among other factors, however, the implementation of MnBi
7
in these advanced applications has been limited by typical
synthesis methods resulting in rather large coupled grains
8
that cause high media-noise readout and low energy
9
products. Its technological potential notwithstanding, moti-
vation exists to investigate the properties of nanoscaled
MnBi as it is anticipated that its magnetic and structural
attributes will differ from those observed in its larger-grained
forms. Improved control and understanding of the structure-
magnetic connections in MnBi may allow its implementation
into multifunctional sensors that simultaneously detect ther-
mal and magnetic environment changes, an emerging tech-
nology. Recent work done by the present authors builds upon
6
3
00 KഛTഛ525 K in the presence of a selection of dc mag-
netic fields Hഛ50 kOe. Additionally, ac and dc susceptibil-
ity measurements were made on the aligned samples in the
range 5 KഛTഛ300 K. The temperature dependence of the
ac magnetic susceptibility ␹ac was measured with a zero dc
bias field and ac field of 2.0 Oe at a frequency of 300 Hz.
The low-temperature dc magnetic moment was investigated
with fields ഛ10 Oe. No corrections to the data were made
for demagnetization effects. Ribbon pieces were mounted
with their flat surfaces perpendicular to the direction of the
applied field in the silica tube and care was taken to maintain
this alignment for subsequent x-ray diffraction studies. The
heating and cooling rates were controlled to 2.5 K/min.
Typical demagnetization curves measured at 300 K for
ribbons heated to 520 K in applied fields of 0 kOe, 10 kOe,
and 50 kOe are presented in Fig. 1. The temperature of
1
0–12
and extends previous research
that exploits the low-
temperature eutectic reaction in the binary Mn-Bi phase dia-
gram at approximately 5%Mn-95%Bi to synthesize a regular
two-phase microstructure of MnBi and Bi. The synthesis of
isolated LTP MnBi nanorods with average diameter 10 nm
and average length 30 nm embedded in a Bi matrix was
achieved by rapid solidification of the eutectic composition
1
3
followed by a moderate annealing treatment. The nanorods
are sparsely ͑ϳ7 vol %͒ but isotropically distributed and, in
1
4,15
agreement with related studies,
possess coercivities on
the order of 17 kOe at room temperature and a Curie tem-
perature T =540 K on heating that is significantly depressed
C
5
20 K was chosen because it is below the Bi melting tem-
perature of 545 K but is well above the nucleation tempera-
ture ͑ϳ400 K͒ of the LTP MnBi nanoparticles in the as-
a͒Electronic mail: kkang@bnl.gov
0003-6951/2005/87͑6͒/062505/3/$22.50
87, 062505-1
© 2005 American Institute of Physics
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062505-2
Kang, Lewis, and Moodenbaugh
Appl. Phys. Lett. 87, 062505 ͑2005͒
FIG. 1. 300 K demagnetization curves for samples heated to 520 K in 10
and 50 kOe fields.
quenched ribbons. Because previous studies indicate that
the uniaxial MnBi nanorods are well-separated and
1
3
single-domain, it is a simple exercise to use the remanence
to calculate the degree of particle alignment. Ribbons heated
in the zero-field condition reveal a remanence ratio M /M of
r
s
0
.5, where M is the remanence and M is the saturation
r
s
magnetization. This result is in agreement with that expected
from a ferromagnetic material composed of randomly-
1
7
oriented uniaxial single-domain particles. Determination of
the orientation of the particle ensemble is obtained by a cal-
culation of the distribution cone angle corresponding to a
measured remanence as follows. Let ␣ be the angle of largest
deviation of the easy magnetization axis from the direction
of the previously applied magnetic field direction. At rema-
nence the particle magnetization lies within a cone of half-
angle ␣ in the positive direction and the distribution covers a
portion of a corresponding positive hemisphere. The rema-
nence is given by
FIG. 2. ͑a͒ 300 K demagnetization curves for samples as-spun and heated to
5
10, 520, and 525 K in 10 kOe; ͑b͒ M-T curve in 10 kOe measured upon
heating and cooling. The inset is an enlarged M-T curve in range 500 K
ഛTഛ530 K.
corded upon heating and cooling in a 10 kOe magnetic field,
Fig. 2͑b͒. In comparison with samples with Tmax=510 and
5
20 K, the sample heated to 525 K shows a more complex
␣
␣
−1
change in the magnetization trends: the magnetization exhib-
its an abrupt drop at 525 K ͓inset Fig. 2͑b͔͒ followed by a
significant increase in the moment upon cooling, greater than
that produced by excursions to the lower temperatures. The
decreased magnetization above 525 K is attributed to the
first-order phase transformation from LTP MnBi to the high-
temperature-phase ͑HTP͒ form of MnBi. In this phase the
bipyramidal interstices of the NiAs structure are stuffed with
Mn cations to a nominal composition of Mn1.08Bi with the
M =
͵
Ms
͵
sin ␣ d␣
sin ␣ cos ␣ d␣.
͑1͒
r
ͩ ͪ
0
0
An isotropic particle distribution is represented by ␣=90°,
with M /M =0.5. Ribbons heated to 520 K in an applied
r
s
field of 10 kOe yield M /M =0.67, indicating partial align-
r
s
ment of particles’ c-axes along the field direction with a cone
half-angle ␣Ϸ70°. When the same heating process is applied
to an as-quenched sample under a higher field of 50 kOe, a
remanence ratio M /M of 0.84 was observed, consistent
distorted Ni In structure, creating a paramagnetic or an anti-
r
s
2
1
9,20
with an alignment cone half-angle of ␣Ϸ47°. Simple ex-
trapolation of the trend of applied field versus remanence
ratio M /M suggests that a field of ϳ80 kOe applied at a
ferromagnetic state.
A room-temperature hysteresis curve
measurement of the 525 K-heated sample yields a rema-
nence ratio of unity, Fig. 2͑a͒.
r
s
maximum temperature of 520 K should result in close to
00% nanoparticle alignment, M /M =1 or ␣=0°.
The nanorod alignment is confirmed by room-
1
temperature Cu K x-ray diffraction spectra measured on rib-
r
s
␣
Demagnetization curves measured at 300 K for samples
heated to the maximum temperatures Tmax of 510, 520 or
25 K are presented in Fig. 2͑a͒. The curves for the samples
with the Tmax=510 and 520 K overlap with M /M =0.67. In
bons in their as-spun state and annealed in zero-field or
10 kOe, Fig. 3. The zero-field-annealed sample’s Bragg peak
intensities are consistent with those of bulk MnBi, and thus
no appreciable preferred orientation is produced. However,
the sample heated in a 10 kOe field shows a stronger LTP
MnBi ͑002͒ peak, consistent with c-axis preferred orientation
of particles aligned perpendicular to the ribbon surface and
parallel to the applied field.
5
r
s
contrast, the sample with Tmax=525 K exhibits a remanence
ratio M /M ϳ1.0, consistent with nominally complete align-
r
s
ment of the MnBi nanorods along the direction of the applied
field. These results underscore the importance of heating
temperature for tailoring the alignment degree of MnBi
nanocomposites, in agreement with results obtained on
Other researchers have observed alignment of MnBi in
the presence of a magnetic field and have proposed two
mechanisms for MnBi alignment: recrystallization and crys-
1
8
coarse-grained MnBi composites by Ren et al. To clarify
2
1
the alignment mechanism, the magnetization signal was re-
tallite rotation. Chen and Stutius report that heating a MnBi
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062505-3
Kang, Lewis, and Moodenbaugh
Appl. Phys. Lett. 87, 062505 ͑2005͒
FIG. 3. X-ray spectra for samples ͑a͒ in as-spun state and ͑b͒ heated in
zero-field, and ͑c͒ 10 kOe field conditions.
FIG. 4. dc M-T curve measured at 10 Oe and the real component ␹Ј of the
ac-susceptibility ␹ac for a sample with aligned MnBi particles oriented along
the applied field direction.
single crystal in a magnetic field ͑15–20 kOe͒ oriented per-
pendicular to the easy magnetization c-axis of the crystal
causes the crystal to subdivide and recrystallize with an ori-
entation parallel to the field. It should be noted that a rela-
tively high heating temperature of 603 K and a very long
annealing time ͑2500 min͒ are required in that study for re-
crystallization. In contrast, in the present study complete par-
ticle alignment was achieved in the present study within 60 s
at the lower temperature of 525 K. On the other hand, Ren et
This manuscript has been authored by Brookhaven Sci-
ence Associates, LLC under Contract No. DE-AC02-98CH1-
8
86 with the U.S. Department of Energy. The United States
Government retains, and the publisher, by accepting the ar-
ticle for publication, acknowledges, a world-wide license to
publish or reproduce the published form of this manuscript,
or allow others to do so, for the United States Government
purposes.
1
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