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] V 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 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 K, HC of $100 Oe at 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 % and N\ % 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 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 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 (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: tuanna@itims.edu.vn Tel.: 08-4-38680787/ext 210 Fax: 08-4-38692963 0003-6951/2011/99(16)/162507/3/$30.00 99, 162507-1 C 2011 American Institute of Physics V Author complimentary copy Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp 162507-2 N A Tuan and N P Duong FIG (a) M-H curves measured at (n) and 300 K (h) with H parallel to the film plane (b) Coercive field HC of $100 Oe at K (c) M-H curves at room temperature with H parallel (h) and normal ($) to the film plane saturation of the n-MR was observed at high magnetic fields above 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 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 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 þ GBMRðHÞ; 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 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 Author complimentary copy Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp 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 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\ % 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 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 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 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 R A de Groot, F M Mueller, P G van Engen, and K H J Buschow, Phys Rev Lett 50, 2024 (1983) C Hordequin, J Pierre, and R Currat, J Magn Magn Mater 162, 75 (1996) M J Otto, R A M van Woerden, P J van der Valk, J Wijngaard, C F van Bruggen, and C Haas, J Phys.: Condens Matter 1, 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21 W Gil, D Goărlitz, M Horisberger, and J Koătzler, Phys Rev B 72, 134401 (2005) 22 A W Rushforth, K Vy´borny´, C S King, K W Edmonds, R P Campion, C T Foxon, J Wunderlich, A C Irvine, P Vasˇek, V Nova´k, K Olejnı´k, J Sinova, T Jungwirth, and B L Gallagher, Phys Rev Lett 99, 147207 (2007) 23 M Ziese, Phys Rev B 60, R738 (1999) Author complimentary copy Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp View publication stats ... 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... 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. .. 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