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

Structural, magnetic, and magnetotransport properties of NiMnSb thin films deposited by flash evaporation Nguyen Anh Tuan,aand Nguyen Phuc Duong International Training Institute for Mate

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

V C 2011 American Institute of Physics [doi:10.1063/1.3651337]

NiMnSb thin films, which have half-metallic

ferromag-netism (HMF) with 100% spin polarization at the Fermi

level,1a magnetic moment of 4.0 lB/f.u.,2and a Curie

temper-ature 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.2,46To 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.7Since then, FE

has been widely used for the deposition of multi-component

thin films.8In 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

arc-melting under an Ar atmosphere, annealed at 1000 K for two

days, quenched in water, and then crushed to a mean

diame-ter size of50 lm in a protected environment During

evap-oration, a hopper funnel-like feeder filled with the fine

NiMnSb powder was regularly shaken to continuously

sprin-kle 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 300C

Energy dispersive x-ray and x-ray diffraction (XRD)

meas-urements confirmed an approximate ratio of Ni:Mn:Sb

stoi-chiometry 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 6,9,10the NiMnSb thin films

fabri-cated through FE were judged to have theC1btype structure

(F43m space group) of semi-Heusler crystals The

topogra-phy of the FE NiMnSb thin films was observed using

field-emission scanning microscopy A fine-grained structure with

average grain sizes of 40 nm and an average roughness of

68 A˚ were observed (Fig.1(b)) Each of the grains perhaps

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 MSof about 538 emu/cm3at 5 K,HCof

100 Oe at 5 K (Fig.2(b)),2 Oe at 300 K due to the ultra-fine grain structure and low surface roughness, and in-plane anisotropy with an anisotropy fieldHAof0.5 T (Fig.2(c)) These properties were all in good agreement with the find-ings on other NiMnSb thin films.6,10Since the demagnetiz-ing factorsNjj¼ NT 0 and N\ 1 for perpendicular fields, the demagnetizing field also plays the role of the anisotropy field such thatHD 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 four-terminal 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 configura-tions: parallel (longitudinal, jj) to the current, crosswise (transverse, >) to the current, and perpendicular (\) to the plane The MR ratio is defined as MR¼ [R(H)  R(0)]/R(0) Fig.3presents 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 of0.4% for the longitudi-nal configuration and0.2% for the two others in the maxi-mum applied fields (1.35 T), were notable No sign of

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.

a) Author to whom correspondence should be addressed Electronic mail:

tuanna@itims.edu.vn Tel.: 08-4-38680787/ext 210 Fax: 08-4-38692963.

Author complimentary copy Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

saturation of the n-MR was observed at high magnetic

fields above 1 T for all three configurations A

shape-anisotropic 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 inelastics-d scattering,

disordered-spin, 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,14the 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

disloca-tions are located, resulting in high disorder levels of spins and

in turn in high zero-field resistivity Thus, GBs may be

para-magnetic, antiferropara-magnetic, or somewhat similar to diluted

magnetic semiconductors (DMS) and doped or narrow-gap

semiconductors.12As a result, the n-MR behavior in low

mag-netic fields can be controlled mainly by spin-dependent

scatter-ing (SDS) between neighborscatter-ing grain pairs separated by GBs

similar to the mechanism that generates the giant

magnetoresis-tive (GMR) effect in granular ferromagnetic systems.15

Conse-quently, 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

satu-ration 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

nonsatu-ration 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-ization spins, which have been mentioned in various DMS and HMF systems,13,14presented just in GBs The n-MR compo-nents 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 MRðHÞ

¼ GGMRðHÞ þ GBMRðHÞ; where the first term is most im-portant component to contribute to the n-MR in low and moder-ate magnetic field regions, and the second term dominmoder-ates 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 anisot-ropy consistent with observations from the magnetization measurements (Fig 2(c)) The amplitude of the SAMR effect is defined as SAMRðHÞ ¼ jMRjjj  jMR?j ¼ ½qjjðHÞ

ðq?HÞ=q0; where q0denotes the resistivity atH¼ 0 and is

a constant for a given sample The highest SAMR ratio which relates to maximum applied magnetic field was determined to

beSAMR(1.35 T) 0.2% The variation of SAMR as a func-tion of the magnetic field,SAMR(H) vs H is drawn based on

MRjjandMR\data and is calculated forSAMR(H> 0) (inset

in Fig.3) A quadratic-like increase withH, which implies that SAMR(H) H2below 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.18The arguments on the AMR mecha-nism 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 con-tributing to the AMR22should 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,23while for Gs, it must be introducedMgand

vgto these relations The parabolic behavior at low fields of

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.

FIG 2 (a) M-H curves measured at 5 (n) and 300 K (h) with H parallel to

the film plane (b) Coercive field H C of 100 Oe at 5 K (c) M-H curves at

room temperature with H parallel (h) and normal ($) to the film plane.

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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

suppres-sion of the spin-orbital scattering due to lack of spin-mixed

states generated by spin-flip scattering.19However, the linear

behavior at high fields may be the result of the so-called

high-field 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-of-plane, qjj decreases faster than q\, which means that the

SAMR ratio increases with the magnetic field

Fig 4shows 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:

Ryx¼ R0Bzþ RSl0Mz, where R0 and RS are the ordinary and

anomalous Hall coefficients, respectively, Bz is the magnetic

induction along thez-axis (perpendicular to the film plane), and

l0 is the magnetic permeability of the vacuum As a result,

Bz¼ l0H due to the demagnetizing factor N\ 1 The range

of actions for each effect is determined by a knee at the

aniso-tropic 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 theRyxversus 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 toH¼ 0, that is, Ryx(0)¼ l0RSMz, which hides

components of the skew scattering and side-jump processes.3

The inset in Fig.4shows a very good match between the AHE

and M-H curves as evidence of SDS and proves that the

asym-metric 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,3,11,12the FE

NiMnSb thin films appeared to have high spin polarization, as

indicated throughRS> R0by a factor of over five (RS 5.3R0)

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 atEFare 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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FIG 4 Hall resistance at room temperature as a function of H applied

per-pendicular to the film plane The inset shows the form of the AHE curve

(*), following closely the M-H curve measured in the perpendicular

mag-netic field (~).

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