Anisotropic nanocrystalline MnBi with high coercivity at high temperature

Article abstract of DOI:10.1063/1.3630001

Magnetic hard nanocrystalline MnBi has been prepared by melt spinning and subsequent low temperature annealing. A coercivity of 2.5 T can be achieved at 540 K for MnBi with an average grain size of about 20-30 nm. The coercivity iH_c, mainly controlled by the coherent magnetization rotation, shows a strong dependence on the time of grinding and exhibits a positive temperature coefficient from 100 up to 540 K. The unique temperature dependent behavior of the coercivity (magnetocrystalline anisotropy) has a relationship with the variations in the crystal lattice ratio of c/a with temperatures. In addition, discontinuity can not be found in the lattice parameters of a, c, and c/a ratio at the magnetostructural transition temperature. The nanocrystalline MnBi powder fixed in an epoxy resin and under an applied magnetic field of 24 kOe shows a maximum energy product of 7.1 MGOe at room temperature and shows anisotropic characteristics with high Mr/Ms ratio up to 560 K.

Full text of DOI:10.1063/1.3630001

Anisotropic nanocrystalline MnBi with high coercivity at high temperature
J. B. Yang, Y. B. Yang, X. G. Chen, X. B. Ma, J. Z. Han et al.
Citation: Appl. Phys. Lett. 99, 082505 (2011); doi: 10.1063/1.3630001
View online: http://dx.doi.org/10.1063/1.3630001
View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v99/i8
Published by the American Institute of Physics.
Related Articles
Mechanism of reversing the Neel domain walls in the Co nanostripes with transverse magnetic anisotropy
Appl. Phys. Lett. 101, 252412 (2012)
Structural and magnetic properties of biaxially textured NiFe2O4 thin films grown on c-plane sapphire
J. Appl. Phys. 112, 123910 (2012)
Thick CoFeB with perpendicular magnetic anisotropy in CoFeB-MgO based magnetic tunnel junction
AIP Advances 2, 042182 (2012)
Ferromagnetic enhancement and magnetic anisotropy in nonpolar-oriented (Mn, Na)-codoped ZnO thin films
Appl. Phys. Lett. 101, 242401 (2012)
The nature of Cr center in GaN: Magnetic anisotropy of GaN:Cr single crystals
J. Appl. Phys. 112, 113914 (2012)
Additional information on Appl. Phys. Lett.
Journal Homepage: http://apl.aip.org/
Journal Information: http://apl.aip.org/about/about_the_journal
Top downloads: http://apl.aip.org/features/most_downloaded
Information for Authors: http://apl.aip.org/authors
Downloaded 05 Jan 2013 to 152.14.136.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

APPLIED PHYSICS LETTERS 99, 082505 (2011)
Anisotropic nanocrystalline MnBi with high coercivity at high temperature
J. B. Yang,1,a) Y. B. Yang,1 X. G. Chen,1 X. B. Ma,1 J. Z. Han,1 Y. C. Yang,1 S. Guo,2
A. R. Yan,2 Q. Z. Huang,3 M. M. Wu,4 and D. F. Chen4
1State Key Laboratory for Mesoscopic Physics, School of Physics, Peking University, Beijing 100871,
People’s Republic of China
2Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201,
People’s Republic of China
3National Institute of Standards and Technology, Gaithersburg, Maryland 20878-9957, USA
4China Institute of Atomic Energy, P. O. Box-275-30, Beijing 102413, People’s Republic of China
(Received 13 June 2011; accepted 8 August 2011; published online 25 August 2011)
Magnetic hard nanocrystalline MnBi has been prepared by melt spinning and subsequent low
temperature annealing. A coercivity of 2.5 T can be achieved at 540 K for MnBi with an average
grain size of about 20-30 nm. The coercivity iHc, mainly controlled by the coherent magnetization
rotation, shows a strong dependence on the time of grinding and exhibits a positive temperature
coefficient from 100 up to 540 K. The unique temperature dependent behavior of the coercivity
(magnetocrystalline anisotropy) has a relationship with the variations in the crystal lattice ratio of
c/a with temperatures. In addition, discontinuity can not be found in the lattice parameters of a, c,
and c/a ratio at the magnetostructural transition temperature. The nanocrystalline MnBi powder
fixed in an epoxy resin and under an applied magnetic field of 24 kOe shows a maximum energy
product of 7.1 MGOe at room temperature and shows anisotropic characteristics with high Mr/Ms
C
V
ratio up to 560 K. 2011 American Institute of Physics. [doi:10.1063/1.3630001]
The intermetallic ferromagnetic compound MnBi has
attracted much attention because of its high uniaxial mag-
netic anisotropy, high magneto-optical Kerr effect, and high
spin polarization at room temperature.1–13 For instance, the
low temperature (LTP) phase with NiAs-type hexagonal
crystal structure has a magnetic anisotropy energy of 2 ꢀ 107
erg/cm3 at 500 K.1 Upon heating to 628 K, a first-order mag-
netostructural transition takes place, corresponding to a
phase decomposition of MnBi into Mn1.08Bi and free Bi with
a 4% decrease in the hexagonal c/a ratio.2 In addition, the
magnetic hard LTP MnBi exhibits a positive temperature
coefficient for coercivity, which is unique in magnetic mate-
rials.1,3,4 High coercivity of 17 kOe at room temperature
(RT) was found in nanostructured MnBi/Bi composites,5,6
Its high temperature phase exhibits a high polar Kerr rotation
of 1.25ꢁ.7–9 Recently, a spin polarization of 63% has also
been observed for MnBi with a large magnetoresistance.10,11
Despite these improvements, the application of LTP MnBi,
however, has been limited by conventional synthesis meth-
ods such as arc-melting and sintering that failed to get MnBi
in a single phase. High purity or increased coervicity of LTB
MnBi ribbons could be obtained by melt spinning.3,12 But
MnBi with both high saturation and coercivity has not been
achieved by this approach. Since the magnetic properties of
magnets are associated with phases, compositions, and
microstructures of compounds, a detailed study of the struc-
tures and magnetic properties is required to improve the
magnetic properties of MnBi. In this letter, we prepared high
quality MnBi ribbons with nanocrystaline grains by melt
spinning. The coercivity of nanocrystalline MnBi was found
to be mainly controlled by the coherent magnetization rota-
tion. A maximum energy product (BH)max of 7.1 MGOe was
achieved at RT. The neutron diffraction was used to study
the structure and phase transition of MnBi at various temper-
atures. It was found that the unique changes of anisotropy
and coercivity are related to differences in crystal lattice
parameters varied with temperatures.
MnBi ingots were prepared by arc-melting with high pu-
rity manganese (99.99%) and bismuth (99.99%) in suitable
atomic ratios. The ribbons of MnBi were obtained by eject-
ing the melt of ingot from a quartz tube onto the surface of a
rotating copper wheel under the argon atmosphere. The
tangential speed of the wheel was in the range from 10 to
70 m/s. The ribbons were annealed at various temperatures
in order to prepare MnBi with optimal magnetic properties.
The obtained samples were grinded into powders, mixed
with epoxy resin, and aligned in a magnetic field of 1.5 T.
The volumetric and gravimetric ratios of the magnetic pow-
ders to the epoxy resin are about 3/50 and 1/3, respectively,
which ensure a good alignment of the powders under the
magnetic field. The density of the specimen was estimated to
be 7.8 g/cm3 using the lattice parameters and the molecular
weight of MnBi. The structure of MnBi was studied by x-ray
diffraction (XRD), neutron diffraction (ND), and transmis-
sion electron microscopy (TEM). Magnetic properties were
measured using vibrating sample magnetometer and super-
conductivity quantum interference device.
In order to obtain high purity of LTP MnBi, MnxBi with
various nominal compositions (x ¼ 0.9–2.33) were synthe-
sized and investigated. Fig. 1 shows the typical XRD pat-
terns of MnxBi (x ¼ 1.22). The amorphous ribbons were
achieved using this rotation speed (Fig. 1(a)). Thus, Mn and
Bi atoms can be homogeneously mixed, making them easier
to form MnBi after the annealing. High purity of MnBi phase
was found with x ¼ 1.20–1.22 (Fig. 1(b)). Large amount of
a)Author to whom correspondence should be addressed. Electronic mail:
jbyang@pku.edu.cn.
C
V
0003-6951/2011/99(8)/082505/3/$30.00
99, 082505-1
2011 American Institute of Physics
Downloaded 05 Jan 2013 to 152.14.136.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

082505-2
Yang et al.
Appl. Phys. Lett. 99, 082505 (2011)
FIG. 1. XRD patterns of the MnBi after melt spinning at a speed of 65 m/s
(a) and subsequently annealed at 573 K for 3 h (b).
Bi phase appears if Mn content is less than 1.1, while Mn
and MnO2 phases appear if Mn content is too high in the
composition as shown in the XRD pattern of Mn2.33Bi. After
refinement of the neutron diffraction data of the high purity
MnBi, the room temperature lattice parameters of a ¼ 4.2854,
FIG. 3. (Color online) The magnetic properties of MnBi. The hysteresis
loop of MnBi at RT (a), the temperature dependence of coercivity for nano-
crystalline MnBi (b).
˚
c ¼ 6.1229 A with hexagonal symmetry were obtained. The
observed content of MnBi is higher than 95%, and Mn shows
a magnetic moment of 3.6lB per atom. The measured mag-
netization of MnBi sample is about 75 emu/g at 7 T, very
close to its theoretical saturation magnetization value of 80
emu/g at room temperature, confirming that high purity of the
sample can be achieved with a quenching speed of 65 m/s.
The TEM images of the annealed MnBi with various
quenching speeds are shown in Fig. 2. Well shaped hexagonal
MnBi single crystal grains with an average size of 200 nm are
observed in the samples with a speed of 20 m/s (Fig. 2(a)).
Those single crystal-like grains are crystallographically oriented
along the c-axis of the MnBi according to the electron diffrac-
tion patterns, which shows typical hexagonal spot pattern along
the [001] direction (see Fig. 2(b)). This orientation is critical to
form anisotropic magnetic powders. However, the average grain
size decreases to about 20–30 nm when a quenching speed of
65 m/s is applied (see high resolution lattice image Fig. 2(c)).
In order to improve the magnetic properties, the
annealed ribbons were grinded for various lengths of time. It
is found that the coercivity increases with the length of
grinding time and reaches a value of 1.1 T at RT. The decou-
pling between hard magnetic grains may account for the
coercivity change. Interestingly, the increase of the coercive
field is accompanied with slight decrease of the saturation
magnetization and remanence. In order to avoid the oxidiza-
tion of the powders, MnBi was grinded in a mortar protected
by petroleum ether. The oxygen content in the MnBi pow-
ders was monitored. The oxygen content is about 0.15 wt. %
after grinding of 5 min and is about 1.1 wt. % after grinding
of 7 h. The maximum energy product at RT is about 7.1
MGOe after 7-h grinding. The corresponding hysteresis
loop of MnBi is plotted in Figure 3(a) and a very high Mr/
Ms > 90% is obtained. Since most nanocrystalline
compounds prepared by melt spinning is isotropic and has a
Mr/Ms ratio of normally less than 60%, such a high Mr/Ms
ratio of nanocrystalline MnBi is unique and the crystallo-
graphically orientation of the single crystal grains is likely to
be the cause. In addition, the grain size of the above MnBi
magnet is about 20–30 nm, much less than that of a single
domain size of 250 nm. Thus, the magnetization reversal
may be associated with the magnetization rotation controlled
FIG. 2. TEM images of the annealed
MnBi with quenching speed, 20 m/s (a),
the electron diffraction pattern of MnBi
grains (b), and the TEM images of
annealed MnBi with quenching speed of
65 m/s (c).
Downloaded 05 Jan 2013 to 152.14.136.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

082505-3
Yang et al.
Appl. Phys. Lett. 99, 082505 (2011)
from ferromagnetic to paramagnetic occurs (See Fig. 4(a)).
At the magnetostructural transition, the c/a ratio of the nano-
structured MnBi remains unchanged whereas that of the bulk
material decreases abruptly to 1.37.2 Similar to MnBi nano-
rods in the Bi matrix,16 no evidence of discontinuity is
noticed for the lattice a- and c-parameters at 625 K for high
purity nanocrystalline MnBi, while the discontinuity is easily
seen in the lattice parameters of the bulk at this temperature.2
The magnetic moment of Mn atom decreases slowly with
increase of the temperature (Fig. 4(b)) and shows no discon-
tinuity around 100 K corresponding to the spin reorientation
of the Mn atom. A sharp drop of the magnetic moment was
observed at 540–550 K due to the magnetic phase transition,
and it reaches a value of 1.4 lB at 600 K.
In conclusion, we report that over 95 wt. % nanocrystal-
line LTP MnBi can be prepared by melt spinning. The aver-
age size of MnBi grains is 20–30 nm, much smaller than the
single domain size of 250 nm. Coercivity Hc can be greatly
improved by grinding and a positive temperature coefficient
is observed. The maximum energy product (BH)max of the
magnet is about 7.1 MGOe at room temperature. The mag-
netic anisotropy changes from planar to uniaxial with the
changes of the c/a ratio, which may contribute to the unique
temperature coefficient of the coercivity of MnBi.
FIG. 4. (Color online) The lattice parameters a, c and c/a ratio of MnBi at
different temperatures (a), and the temperature dependence of the Mn mag-
netic moment in MnBi (b).
by magnetocrystalline anisotropy.14 The domain wall pinning
is probably not the dominant mechanism since no domain wall
nucleates can be produced during the coherent magnetization re-
versal. In order to identify the magnetization reversal mecha-
nism for MnBi nanocrystalline materials, the angular
dependence of coercivity is obtained from the hysteresis loops
measured at various angles. The coercivity decreases with the
increasing angle between the easy axis and the applied field
direction, indicating that a coherent rotation of magnetization
occurs in MnBi according to the classical Stoner–Wohlfarth
model.15 The field dependence of the coercivity further supports
this finding. The temperature dependence of the coercivity is
shown in Fig. 3(b), which exhibits a positive temperature coeffi-
cient. The coercivity of MnBi increases with the temperature
from 100 to 540 K, and reaches a maximum of 2.5 T at 540 K.
The positive temperature coefficient of the coercivity may have
some relationship with the magnetocrystalline anisotropy field
of MnBi. From 150 K to 530 K, the anisotropy field increases
with temperature and reaches the maximum at 530 K.3
This work has been supported by the National Natural
Science Foundation of China (Grant No. 50701003), the
National 973 Project (No. 2010CB833104, MOST of China),
and the program for New Century Excellent Talents (NCET-
10-1097) and the Scientific Research Foundation for Returned
Overseas Chinese Scholars, State Education Ministry.
1T. Chen and W. E. Stutius, IEEE Trans. Magn. MAG-10, 581 (1974).
2R. R. Heikes, Phys. Rev. 99, 446 (1955).
3X. Guo, X. Chen, Z. Altounian, and J. O. Strom-Olsen, Phy. Rev. B 46,
14578 (1992).
4J. B. Yang, K. Kamaraju, W. B. Yelon, W. J. James, Q. Cai, and A. Bol-
lero, Appl. Phys. Lett. 79, 1846 (2001).
5K. Kang, L. H. Lewis, and A. R. Moodenbaugh, Appl. Phys. Lett. 87,
062505 (2005).
6K. Kang, A. R. Moodenbaugh, and L. H. Lewis, Appl. Phys. Lett. 90,
153112 (2007).
7G. Q. Di and S. Uchiyama, Phys. Rev. B 53, 3327 (1996).
8D. J. Sellmyer, R. D. Kirby, J. Chen, K. W. Wierman, J. X. Shen, Y. Liu,
B. W. Robertson, and S. S. Jaswal, J. Phys. Chem. Solids 56, 1549 (1995),
and references therein.
9Y. Liu, J. Zhang, S. Cao, X. Zhang, G. Jia, Z. Ren, X. Li, C. Jing, and K.
Deng, Phys. Rev. B 72, 214410 (2005).
In order to have a better understanding of the mecha-
nism of this phenomenon, ND was performed to study the
phase and composition evolution of MnBi with various tem-
peratures. Fig. 4 is the temperature dependence of lattice pa-
rameters and magnetic moments of MnBi obtained from
refined ND data. A kink is observed around 100 K in both a
and c axes, suggesting a spin reorientation of the Mn
atoms.3,16,17 The relaxation of the lattice is found to be asso-
ciated with changes in the magnetocrystalline anisotropy
from planar to c-axis. When the c/a ratio is less than 1.425,
the LTP MnBi shows planar anisotropy; as c/a > 1.425, the
uniaxial anisotropy appears and the anisotropic field
increases with the increase of c/a up to 1.433. When the c/a
ratio reaches 1.433, a magnetostructural phase transition
10E. Clifford, M. Venkatesan, and J. M. D. Coey, J. Magn. Magn. Mater.
272, 614 (2004).
11P. Kharel, P. Thapa, P. Lukashev, R. F. Sabirianov, E. Y. Tsymbal, D. J.
Sellmyer, and B. Nadgorny, Phys. Rev. B 83, 024415 (2011).
12S. Saha, R. T. Obermyer, B. J. Zande, V. K. Chandhok, S. Simizu, S. G.
Sankar, and J. A. Horton, J. Appl. Phys. 91, 8525 (2002).
13R. Coehoorn and R. A. Degroot, J. Phys. F: Met. Phys. 15, 2135 (1985).
14T. Schrefl, V. D. Tsiantos, D. Suess, W. Scholz, H. Forster, and J. Fidler,
“Micromagnetic simulations and applications,” in Scattering and Biomedi-
cal Engineering: Modeling and Applications, edited by D. Fotiadis and C.
Massalas (World Scientific, 2001), pp. 141–158.
15H. Kronmu¨ller and M. Fa¨hnle, Micromagnetism and the Microstructure of Fer-
romagnetic Solids (Cambridge University Press, Cambridge, 2003), pp. 90–95.
16K. Kang, W. Yoon, S. Park, A. R. Moodenbaugh, and L. H. Lewis, Adv.
Funct. Mater. 19, 1100 (2009).
17H. Yoshida, T. Shima, T. Takahashi, H. Fujimori, S. Abe, T. Kaneko,
T. Kanomata, and T. Suzuki, J. Alloys Compd. 317, 297 (2001).
Downloaded 05 Jan 2013 to 152.14.136.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions