Structural, magnetic, and magnetotransport properties of nimnsb thin films deposited by flash evaporation

APPLIED PHYSICS LETTERS 99, 162507 (2011)

Structural, magnetic, and magnetotransport properties of NiMnSb thin films
deposited by flash evaporation
Nguyen Anh Tuan,a) and Nguyen Phuc Duong
International Training Institute for Materials Science (ITIMS), Hanoi University of Technology (HUT),
01 Dai Co Viet Rd., Hai Ba Trung Dist., Hanoi 10000, Vietnam

(Received 12 August 2011; accepted 22 September 2011; published online 18 October 2011)
To date, the use of flash evaporation (FE) as a deposition technique for NiMnSb thin films has not
yet been reported. In this letter, we report on NiMnSb thin films deposited on heated Si (111)
substrates at 300 C via FE. Investigations of the structural characteristics and magnetic and
magnetotransport properties of these thin films show typical features of a half-metallic
ferromagnetic semi-Heusler alloy. The origin of the film’s extraordinary magnetotransport behavior is
examined under the perspective of spin-order levels attached to a grain-grain boundary-type structure.
C 2011 American Institute of Physics. [doi:10.1063/1.3651337]
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NiMnSb thin films, which have half-metallic ferromagnetism (HMF) with 100% spin polarization at the Fermi
level,1 a magnetic moment of 4.0 lB/f.u.,2 and a Curie temperature of $730 K,3 have attracted considerable attention
because of their applications in a state-of-the-art generation of
spintronic devices. Many physical deposition techniques have
been used to obtain NiMnSb thin films.2,4–6 To date, however,
no work has yet been published on the use of flash evaporation
(FE) to prepare NiMnSb thin films. The FE technique was
used in 1973 to deposit ternary compounds.7 Since then, FE
has been widely used for the deposition of multi-component
thin films.8 In this letter, we report our findings on NiMnSb
thin films deposited via the FE technique. We also present our
opinions on their extraordinary magnetotransport behavior.
The source NiMnSb alloys were prepared by arcmelting under an Ar atmosphere, annealed at 1000 K for two
days, quenched in water, and then crushed to a mean diameter size of $50 lm in a protected environment. During evaporation, a hopper funnel-like feeder filled with the fine

NiMnSb powder was regularly shaken to continuously sprinkle with regular amounts of the powder onto a heated W
boat. NiMnSb films with an average thickness of 140 nm
were deposited onto Si(111) substrates heated to 300  C.
Energy dispersive x-ray and x-ray diffraction (XRD) measurements confirmed an approximate ratio of Ni:Mn:Sb stoichiometry and an FCC polycrystalline-type structure with
non-preferred orientation for both the bulk source alloy and
the NiMnSb thin films. The XRD data also showed that only
the NiMnSb single phase was formed. A similarity was
observed between the crystallographic phases of the bulk
alloy and thin film, as presented in Fig. 1(a). By analyzing
and comparing the XRD data of NiMnSb thin films prepared
using various techniques,4–6,9,10 the NiMnSb thin films fabricated through FE were judged to have the C1b type structure
(F
43m space group) of semi-Heusler crystals. The topography of the FE NiMnSb thin films was observed using fieldemission scanning microscopy. A fine-grained structure with
average grain sizes of 40 nm and an average roughness of
˚ were observed (Fig. 1(b)). Each of the grains perhaps
6À8 A

contained a few crystalline particles with mean sizes of
25 nm as estimated by the Scherrer method from XRD data.
Fig. 2(a) shows the M-H curves measured at 5 and
300 K using a quantum design physical parameter measuring
system (PPMS). The results showed evident ferromagnetic
characteristics, with MS of about 538 emu/cm3 at 5 K, HC of
$100 Oe at 5 K (Fig. 2(b)), $2 Oe at 300 K due to the ultrafine grain structure and low surface roughness, and in-plane
anisotropy with an anisotropy field HA of $0.5 T (Fig. 2(c)).
These properties were all in good agreement with the findings on other NiMnSb thin films.6,10 Since the demagnetizing factors Njj ¼ NT % 0 and N\ % 1 for perpendicular fields,
the demagnetizing field also plays the role of the anisotropy
field such that HD % HA $ 0.5 T.
The magnetoresistance (MR) and anomalous Hall effect
(AHE) of NiMnSb films fabricated using various methods

have been previously studied.3,11–13 In this study, a fourterminal probe system was used. A constant current of 5 mA
was placed in the sample plane, and a magnetic field in the
range of 61.35 T was applied to the plane in three configurations: parallel (longitudinal, jj) to the current, crosswise
(transverse, >) to the current, and perpendicular (\) to the
plane. The MR ratio is defined as MR ¼ [R(H) À R(0)]/R(0).
Fig. 3 presents the MR data of the FE NiMnSb thin films at
room temperature. The negative MR behavior (n-MR) for all
three configurations, with a ratio of $0.4% for the longitudinal configuration and $0.2% for the two others in the maximum applied fields (1.35 T), were notable. No sign of

a)

FIG. 1. (a) XRD spectra for arc-melted NiMnSb bulk (source alloy) and FE
NiMnSb thin film. (b) FE-SEM surface image of FE NiMnSb thin film.

Author to whom correspondence should be addressed. Electronic mail:
Tel.: 08-4-38680787/ext. 210. Fax: 08-4-38692963.

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99, 162507-1

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162507-2

N. A. Tuan and N. P. Duong

FIG. 2. (a) M-H curves measured at 5 (n) and 300 K (h) with H parallel to
the film plane. (b) Coercive field HC of $100 Oe at 5 K. (c) M-H curves at
room temperature with H parallel (h) and normal ($) to the film plane.

saturation of the n-MR was observed at high magnetic
fields above 1 T for all three configurations. A shapeanisotropic magnetoresistance (SAMR) was manifested by
jMRjjj >jMR\j % jMR>j.
The origin of the n-MR in low fields has been attributed to
some mechanisms, such as inelastic s-d scattering, disorderedspin, or weak localization spin scattering, and nonsaturation
behavior in high fields is due to forced magnetization for those
spins.12 However, we believe that these mechanisms must be
assigned to grain (G)-grain boundary (GB)-type structures.
Based on the GB models,14 the NiMnSb grains were regarded
as a stoichiometric main phase with a low zero-field resistivity
due to high-order levels of spins. The GBs were regarded as a
nonstoichiometric second phase, where impurities and dislocations are located, resulting in high disorder levels of spins and
in turn in high zero-field resistivity. Thus, GBs may be paramagnetic, antiferromagnetic, or somewhat similar to diluted
magnetic semiconductors (DMS) and doped or narrow-gap
semiconductors.12 As a result, the n-MR behavior in low magnetic fields can be controlled mainly by spin-dependent scattering (SDS) between neighboring grain pairs separated by GBs
similar to the mechanism that generates the giant magnetoresistive (GMR) effect in granular ferromagnetic systems.15 Consequently, the low-field n-MR component is described as the
granular giant magnetoresistance (GGMR), upon which the
SDS mechanism depends on the arrangement of the total spin
moments in each grain, called “giant spin.” The alignment by
the external field of the “giant spins” reduces the SDS and leads
to increases in the MR ratio. A small part of the n-MR comes
from the reordering of disordered spins in the GBs by forced
magnetization. This process quickly reaches the technical saturation state in high fields (starting at $0.5 T) because of the
“soft” character of super-fine ferromagnetic grains. The low
MR magnitude observed in Fig. 3 can be assigned to the
decrease of the GGMR component because of the existence of

a noncollinear surface layer outside the grains,16 leading to a
reduction in the “giant spin” moment. The n-MR and nonsaturation phenomena in high-field regions, which are displayed by
a “tail” extended as far as high fields of the MR curve, are due
to SDS and forced magnetization of disordered or weakly local-

Appl. Phys. Lett. 99, 162507 (2011)

FIG. 3. MR data at room temperature for longitudinal (), perpendicular
(h), and transverse (the right inset, $) configurations. Left inset (n): SAMR
as a function of H with the eye-guiding lines expressing a parabolic or linear
form below or above $0.5 T, respectively.

ization spins, which have been mentioned in various DMS and
HMF systems,13,14 presented just in GBs. The n-MR components at high fields created by the GB factors are commonly
called grain boundary magnetoresistance (GBMR). Thus,
the total MR in this case can be presented as MRHị
ẳ GGMRHị ỵ GBMRHị; where the first term is most important component to contribute to the n-MR in low and moderate magnetic field regions, and the second term dominates in
high magnetic fields.
From Fig. 3, MRjj(H) > MR\(H) and MR>(H) $ MR\(H)
(inset in Fig. 3), which displays considerable anisotropy in the
magnetotransport of FE NiMnSb thin films. This is called the
SAMR effect because it reflects the in-plane magnetic anisotropy consistent with observations from the magnetization
measurements (Fig. 2(c)). The amplitude of the SAMR
effect is defined as SAMRHị ẳ jMRjj j jMR? j ẳ ẵqjj Hị
q? Hị=q0 ; where q0 denotes the resistivity at H ¼ 0 and is
a constant for a given sample. The highest SAMR ratio which
relates to maximum applied magnetic field was determined to
be SAMR(1.35 T) $ 0.2%. The variation of SAMR as a function of the magnetic field, SAMR(H) vs. H is drawn based on
MRjj and MR\ data and is calculated for SAMR(H > 0) (inset
in Fig. 3). A quadratic-like increase with H, which implies that

SAMR(H) $ ÀH2 below 0.5 T, and an almost linear behavior,
SAMR(H) $ H, above 0.5 T of the SAMR curve are showed.
Information on the anisotropic magnetoresistance (AMR)
of NiMnSb thin films is scarce, except for a report on a very
small AMR effect in nonstoichiometric NiMnSb films17 and
in bulk NiMnSb alloys.18 The arguments on the AMR mechanism used for different ferromagnetic systems19–23 can be
applied to the SAMR effect, but they have to be associated
with the G-GB-type structure in FE NiMnSb thin films. For
example, the crystalline and noncrystalline components contributing to the AMR22 should be attached to the Gs and GBs,
respectively. The magnetic factors contributed to SAMR are
different for the Gs and GBs. For example, for GBs, DR/R0
! Mgb (Ref. 2) and the high-field slope d(DR/R0)
/dH ! vgbMgb,23 while for Gs, it must be introduced Mg and
vg to these relations. The parabolic behavior at low fields of

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162507-3

N. A. Tuan and N. P. Duong

SAMR may reflect the synchronic rotation of the “giant
spins” of NiMnSb Gs from in-plane to out-of-plane, which is
controlled by the spin-orbit interaction (SOI).21 The linear
behavior at high fields is unlikely to be governed by SOI
because spontaneous polarization leads to complete suppression of the spin-orbital scattering due to lack of spin-mixed
states generated by spin-flip scattering.19 However, the linear
behavior at high fields may be the result of the so-called highfield forced effect for the disordered or weak localization
spins20 in GBs. Therefore, when the GB is put in a strong
magnetic field, the restriction for an in-plane SDS occurs

faster and is more powerful than that for an out-of-plane SDS.
In other words, when the magnetic field is directed out-ofplane, qjj decreases faster than q\, which means that the
SAMR ratio increases with the magnetic field.
Fig. 4 shows the total Hall resistance of the FE NiMnSb
thin film as a function of the perpendicular magnetic field,
which includes ordinary Hall effect (OHE) and AHE:
Ryx ẳ R0Bz ỵ RSl0Mz, where R0 and RS are the ordinary and
anomalous Hall coefficients, respectively, Bz is the magnetic
induction along the z-axis (perpendicular to the film plane), and
l0 is the magnetic permeability of the vacuum. As a result,
Bz ¼ l0 H due to the demagnetizing factor N\ % 1. The range
of actions for each effect is determined by a knee at the anisotropic field HA. The separate contributions of the OHE and
AHE effects were divided as demonstrated in Fig. 4. R0 is
taken as the slope of the Ryx versus H curve above 0.5 T, in
which the positive slope indicates dominant conduction of the
holes.3 The RS value can be evaluated by extrapolating the
high-field curve to H ¼ 0, that is, Ryx(0) ¼ l0RSMz, which hides
components of the skew scattering and side-jump processes.3
The inset in Fig. 4 shows a very good match between the AHE
and M-H curves as evidence of SDS and proves that the asymmetric scattering at high fields is mainly caused by disordered
spins, but not by orbital scattering.3 Analyzing the obtained
Hall data and comparing them with other works,3,11,12 the FE
NiMnSb thin films appeared to have high spin polarization, as
indicated through RS > R0 by a factor of over five (RS % 5.3R0).

FIG. 4. Hall resistance at room temperature as a function of H applied perpendicular to the film plane. The inset shows the form of the AHE curve
(*), following closely the M-H curve measured in the perpendicular magnetic field (~).

Appl. Phys. Lett. 99, 162507 (2011)

Spin asymmetric scattering takes place in the stoichiometric
Gs, in which the spin-up majority holes are dominant carriers
and the spin-down minority states at EF are absent.
In summary, we showed the successful deposition of
NiMnSb thin films using the FE technique. The FE NiMnSb
thin films exhibited a semi-Heusler structure with HMF
properties. The extraordinary behavior of FE NiMnSb thin
films, including n-MR and nonsaturation at high fields,
SAMR, and AHE effects, was observed. Such behavior may
be attributed to SDS and spin reversal in the grains and grain
boundaries, where the spin-order or spin-localization levels
are the key factors. FE appears to be a suitable technique for
preparing multi-component magnetic thin films.
This work was supported by the National Foundation for
Science and Technology Development (NAFOSTED) of
Vietnam under Project Code No. 103.02.50.09.
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