Large magnetoresistance in MnBi point contacts

Article abstract of DOI:10.1016/j.jmmm.2003.12.777

An MnBi alloy was produced by arc melting under helium followed by annealing. Point contacts were then created in a piezosystem using small crystals taken from the bulk sample. The I:V curves were then measured with and without an applied field. Large magnetoresistances (ΔR/R ₀>70%) in a field of 10mT were observed in some cases.

Full text of DOI:10.1016/j.jmmm.2003.12.777

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 1614–1615
Large magnetoresistance in MnBi point contacts
E. Clifford*, M. Venkatesan, J.M.D. Coey
Physics Department, Trinity College, Dublin 2, Ireland
Abstract
An MnBi alloy was produced by arc melting under helium followed by annealing.Point contacts were then created in
a piezosystem using small crystals taken from the bulk sample.The I:V curves were then measured with and without an
applied field.Large magnetoresistances ( DR=R0 > 70%) in a field of 10 mT were observed in some cases.
r 2003 Published by Elsevier B.V.
PACS: 81.07. Lk
Keywords: MnBi; Point contact; Magnetoresistance; Half metal
1. Introduction
helium atmosphere.The samples were then heat treated
at 570 K in a vacuum, maintaining slow heating and
cooling steps.X-ray diffraction analysis reveals the
formation of a-phase MnBi along with some unreacted
Mn and Bi (Fig.1 ).A composition estimation using
EDAX measurements gives a concentration of 75–80%
MnBi, with the rest mostly being unreacted bismuth.
This compares favourably with previous efforts using
this method [4].
MnBi has attracted a lot of interest in recent years due
to its high uniaxial magnetic anisotropy [1] and its
possible applications in permanent magnets and magne-
to-optical memory devices.Recent band structure
calculations suggest that the ferromagnetic low-tem-
perature phase of MnBi behaves like a half metal [2].We
have, therefore, carried out an investigation into MnBi
point contacts in the light of earlier results with
magnetite, a half-metallic oxide which shows remarkable
magnetoresistive effects in point contacts [3].
Point contacts of MnBi were created using small
crystals (1–2 mm) taken from the bulk sample.The size
of the contacts could be varied and controlled using a
piezo system.The magnetoresistance of these contacts
was then measured by taking I:V curves across the
contact with and without an applied field of 10 mT.
I:V curves obtained have a nonlinear contribution and
are fitted according to
The low-temperature a-phase of MnBi has a NiAs-
type structure.Upon heating to 628 K, it transforms to a
high-temperature phase Mn1.08Bi with a zincblende
structure.The preparation of high-quality MnBi sam-
ples is quite problematic, and at present the authors are
unaware of a method that can prepare single-phase
MnBi in large crystals.
I ¼ GV þ CV3
The magnetoresistance is then calculated using
2. Experimental methods
MR ¼ ½ðGðHÞ ꢀ Gð0ÞÞ=GðHÞꢁ;
MnBi was prepared by arc melting high-purity
(99.98%) manganese and bismuth (99.999%) under
where G(H) and G(0) are the conductances with and
without an applied field.(Note that with this definition,
a positive value of magnetoresistance means that the
resistance reduces when a field is applied.)
*Corresponding author.Tel:. 4353-1-6082171; fax: +353-1-
6711759.
E-mail address: cliffoea@tcd.ie (E. Clifford).
0304-8853/$ - see front matter r 2003 Published by Elsevier B.V.
doi:10.1016/j.jmmm.2003.12.777

ARTICLE IN PRESS
E. Clifford et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1614–1615
1615
100
80
60
40
20
0
-20
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Conductance (G0)
20
40
60
2θ (deg)
Fig 2.Plot of magnetoresistance as
conductance across the contact for MnBi point contacts.
a
function of the
Fig.1.X-ray diffraction analysis of MnBi sample.Peaks
indicate the presence of unreacted bismuth and manganese.
6.0x10-7
B = 10mT
4.0x10-7
3. Results and discussion
2.0x10-7
0.0
Large values of magnetoresistance (up to 70%) were
observed even in relatively high conductive contacts.
However, some contacts showed no magnetoresis-
tance at all, even at very low conductances (Fig.2 ).
Except for one contact, all observed magnetoresistances
were positive.
-2.0x10-7
no field
-4.0x10-7
-6.0x10-7
The I:V curves obtained also show some nonlinearity
(Fig.3 ), however this nonlinearity is not as apparent as
in point contacts of other materials such as magnetite
and nickel [3,5].
-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4
V
(V)
Bias
Fig 3. I:V curves obtained across a point contact of MnBi.The
conductance of this contact is 0.003G0 in zero field.A
magnetoresistance of 72% is seen here.
The observed magnetoresistance is more than likely
due to the removal of a domain wall by alignment of the
magnetic domains either side of the nanocontact.It has
been predicted that smaller contacts have narrower
domain walls [6].It has been shown that such narrow
domain walls have a high resistance associated with
behaviour.However, to obtain more conclusive results,
higher purity samples need to be obtained.
them, and thus their removal leads to
a larger
Acknowledgements
magnetoresistance value [7].This idea has been dis-
cussed in more detail elsewhere [8].
This work was partly supported by Enterprise Ireland.
Magnetostriction is not likely to be an issue in these
MnBi contacts because the field dependence is as B2, and
the effect is negligible at 10 mT.Many of the contacts
show little or no magnetoresistance.This is probably
due to the high level of impurities present.Taking the
percentage of MnBi in the sample as 75%, the
probability of both sides of a contact consisting of
MnBi becomes B56%.Even if both contacts are MnBi,
we are not assured that a domain wall will become
trapped at the constriction.
References
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(2001) 022601.
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Phys.Lett.79 (2001) 1846.
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(1999) 2923.
4. Conclusion
[6] P.Bruno, Phys.Rev.Lett.83 (1999) 2425.
[7] J.L. Prieto, M.G. Blamire, J.E. Evans, Phys. Rev. Lett. 90
(2003) 027201.
Large magnetoresistance was observed in MnBi point
contacts, which may be due to polarised spin transport
across the contact caused by the materials half-metallic
[8] J.M.D Coey, J.J. Versluijs, M. Venkatesan, J. Phys. D.:
Appl Phys.35 (2002) 2457.