Room temperature ferromagnetism in InSb-Mn nanowires | Nano Letters

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Room temperature ferromagnetism in InSb-Mn nanowires Katarzyna El#bieta Hnida, Antoni #ywczak, Marcin Sikora, Marianna Marciszko, and Marek Przybylski Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b02690 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Room temperature ferromagnetism in InSb-Mn nanowires Katarzyna E. Hnida1,*, Antoni Żywczak1, Marcin Sikora1, Marianna Marciszko1, Marek Przybylski1,2 1AGH

University of Science and Technology, Academic Centre for Materials and Nanotechnology, A. Mickiewicza 30, 30-059 Krakow, Poland

2AGH

University of Science and Technology, Faculty of Physics and Applied Computer Science, A. Mickiewicza 30, 30-059 Krakow, Poland

ABSTRACT Successful synthesis of one dimensional nanostructures of narrow band gap semiconductor exhibiting ferromagnetic response at room temperature is reported. High-quality nanowires of InSb-Mn have been produced by template-assisted pulse electrodeposition. Detailed structural and spectroscopic characterization reveal good crystallinity, narrow size distribution of the nanostructures and ability to control of the Mn doping level. The dominating magnetic response at cryogenic temperature evolves with increasing Mn concentration from paramagnetic through antiferromagnetic to ferromagnetic. Robust ferromagnetic response of InSb nanowires doped with 2.5% at. of Mn is retained up to Curie temperature of nearly 500K.

KEYWORDS InSb, magnetism in semiconductors, manganese dopant, electrodeposition

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Development of new spintronic devices combining useful properties of semiconductors and ferromagnets1 triggered a renaissance in studies seeking for reproducible and scalable production routes of nanostructures of magnetic semiconductors. Indium antimonide (semiconductor with large Bohr radius, excellent electron mobility – up to 7.8 m2V-1s-1 and small band gap 0.17 eV) doped with manganese atoms is considered a promising platform to realize room temperature ferromagnetism/ferrimagnetism in narrow gap semiconducting systems.2,3 Due to very low equilibrium solubility of Mn in III-V semiconductors it is difficult to incorporate these magnetic atoms into the host material at high concentrations. In most cases the doping is realized by dislocation in the semiconductor lattice, which deteriorate the mechanical,4 magnetic5,6 and electrical7,8 properties of crystals. However, for Curie temperature (TC) to be increased to room temperature, appreciably high concentration of the 3d element is required, which often results in the formation of clusters. The resulting clusters may exhibit net magnetic moments, which can render unique properties to the InSb-Mn system. Depending on dopant concentration and its influence on semiconductor structure three mechanisms of incorporation of ferromagnetism/ferrimagnetism in InSb-Mn can be designated. When the concentration of dopant in InSb structure is very low, Mn atoms substitute In sites in crystal lattice leading to a carrier-mediated ferromagnetism at temperatures below 20K.9 At higher concentrations dopants form clusters of atomic-scale Mn embedded in the InSb host what usually leads to increasing TC. Upon further increase of concentration of dopant a formation of separate crystal phases, namely MnSb (TC tetragonal

bulk

= 587 K)10 and/or Mn2Sb (TC

bulk, hexagonal

= 363 K, TC

bulk,

= 546-550 K)11, can occur.12 Those additional phases are suspected of causing magnetic

response of InSb-Mn system at and above room temperature.13 In the most cases reported in the literature, at least two of the described magnetic phases (i. e. Mn clusters and MnSb inclusions on


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grain boundaries) are liable for magnetic response of InSb-Mn system.12,14,15 Applicability of these materials is determined by the fact that their magnetic properties can be tuned by varying the size and concentration of magnetic clusters. Controlled doping of indium antimonide with Mn has been confirmed in bulk monocrystalline13 or polycrystalline12 samples as well as thin films.16 Until now, InSb-Mn in form of nanowires (NWs) was obtained only by Zhang et al.17 However, a synthesis of InSb-Mn nanowires exhibiting room temperature ferromagnetism has not been reported so far. In this communication the successful production of InSb-Mn nanowires of controlled morphology and robust ferromagnetism at room temperature is described. Presented study shows in detail how (i) to generate the room temperature magnetism in InSb nanowires via Mn doping and (ii) determine the effect of the dopant content on magnetic properties of InSb-Mn NWs obtained by pulse electrodeposition.


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Template-assisted pulse electrodeposition was used due to its plural advantages i.e. good control


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of chemical composition and geometrical parameters of nanowires, possibility of large scale


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production, high level of growth control and high synthesis rate. In comparison with traditional


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electrodeposition techniques, pulse electrodeposition gives more compact structure, with low porosity, finer grain and more homogenous distribution of atoms of deposited material.18 This

technique has been already employed for synthesis of high-quality NWs of InSb19 and tetradymite semiconductors20. As reported previously Eon and Eoff strongly influence the stoichiometry of InSb nanowires.19 Therefore, the main challenge of the presented study was to optimize the pulses of potential in order to maximize the possibility of manganese deposition while maintaining the correct semiconductor stoichiometry. For all experimental details regarding nanowires preparation please refer to Supporting Information.


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Figure 1. A - Schematic representation of pulse electrodeposition synthesis of InSb-Mn nanowires. B – Dependence of the amount of Mn embedded into InSb NWs as a function of concentration of the manganese precursor in the electrolyte. C - SEM image of NWs with 1.4% at. Mn. D – Manganese L-edge and indium M-edge XAFS spectra of NWs with 2.5% at. Mn. E XRD pattern of NWs with 2.5% at. Mn. Schematic representation of nanowires preparation using pulse electrodeposition method is presented in Figure 1A. As shown in Figure 1B a measurable amount of Mn was successfully introduced to the InSb nanowires. When 0.003M Mn2+ was added to the electrochemical bath a relatively small amount of manganese dopants of approx. 0.5% at. was incorporated in the nanostructures. Increasing the amount of Mn2+ in the bath led to higher dopant concentration. The dependence is monotonic and reveal an asymptotic character. The highest concentration of 2.5% at. Mn was observed when the concentration of manganese ions in the electrolyte was increased to 0.3 M. Further increase of the precursor concentration has not led to enhanced incorporation of Mn. Scanning electron microscopy (SEM; FEI Versa 3D) observations (Figure 1C) reveals a typical picture of as prepared nanowires after AAO removal. Any noticeable change in morphology between doped and pristine InSb NW19 is observed, including the high aspect ratio. What is more, no manganese precipitations were observed on the lateral surface of nanowires. Chemical state and local structure of Mn embedded within few nm from the NW surface was examined using Xray absorption fine structure (XAFS; XAFS spectra of free-standing InSb-Mn nanowire arrays were collected at PEEM/XAS beamline of Solaris National Synchrotron Radiation Centre, Poland) spectroscopy (Figure 1D). Characteristic shape of Mn L2,3 edge spectra is similar to that observed in epitaxial In0.965Mn0.035Sb film.21 It is attributed to the localized Mn2+ ions with a 3d5 ground


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state configuration.22 In the case of nanowires it may be plausibly ascribed to oxidized surface.23,24 However, a ferromagnetic response in the studied nanowires implies that other atomic environments containing Mn must be present too. Lack of substantial line broadening rules out the scenarios of Mn-Sb precipitates or individual Mn diluted in semiconducting host, but it is in line with the possible existence of Mn2+ and Mn3+ clusters.25 Such scenario is consistent with X-ray diffraction (XRD; Panalytical Empyrean diffractometer (Cu Kα = 1.5406 Å)) analysis. As shown in diffraction pattern (Figure 1E), studied nanowires are predominantly formed from high quality InSb crystallites. In addition, a residue of Mn2O3 and In2O3 phases formed on the NWs surface are present. But there is no evidence for the formation of crystallites of manganese antimony salts. Thus, it can be concluded that nanowires obtained via pulse electrodeposition absorbs dopants in the form of small Mn clusters embedded into InSb host structure. Magnetic characteristics of the InSb-Mn nanowires (LakeShore 7407 magnetometer) are summarized in Figure 2. Results were scaled to the volume of each sample in order to track the effective magnetization evolution vs. Mn content. The dependence of magnetization (M) vs external magnetic flux density (B) probed at temperature (T) of 100K (Figure 2A) reveals substantial variation depending on Mn concentration. At low concentration it is a linear function characterized by weak, but positive magnetic susceptibility typical for paramagnetic (PM) materials. In view of diamagnetic character of pristine InSb, it is a clear evidence of Mn incorporation into the structure of NWs. The three characteristic ranges can be defined. Up to 1.1% at. Mn, the M(B) dependence shows the linear response with susceptibility roughly proportional to concentration of dopants. In the middle concentration range, up to 2.0% at., the M(B) dependence changes significantly. Susceptibility gets smaller with increasing Mn content and a weak indication of nonlinearity below 0.1T is present. Such behaviour was already observed in thin films of InSb-


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Mn.14 It is due to mixture of a few different magnetic phases: paramagnetic Mn atoms diluted in InSb matrix and antiferromagnetic (AF) surface oxide and Mn clusters. The magnetic response of the sample with 2.5% at. Mn is predominantly ferromagnetic. It is well documented by open hysteresis loop with Bc = 14 mT and 7 mT at 100K and 300K (see Figure S1 in Supplementary Information), respectively, and clear tendency towards magnetic saturation. Antiferromagnetic (paramagnetic) contribution is also present, as indicated by linear M(B) at high filed (Figure 2A) that decreases with increasing temperature (Figure S1). Depending on the number of manganese atoms in the cluster, few stable isomers of Mn can be distinguish.26 Mn2 dimers form two stable magnetic states – ferromagnetic or antiferromagnetic – depending on interatomic distance, while Mn3 trimmers response is either ferromagnetic or ferrimagnetic (FM). According to the DFT calculations in larger Mn clusters antiferromagnetic arrangement of spins is predominant.26 Figure 2B shows the concentration dependence of net magnetization probed at 500 mT along with the expectation values given by the probability of finding diluted PM ions, AF Mn2 dimers and FM Mn3 trimers given by the binomial distribution taking into account nearest-neighbour environment (for more details please see Supporting Information). This model is nicely followed at low concentration of dopants and at the highest concentration achieved, namely 2.5% at. Mn. The drop of net magnetization visible in the middle concentration range in Figure 2B can be rationalized assuming a strong tendency of clustering, i.e. any Mn atom doped at concentration higher than approx. 1.1% at. localises in the close vicinity of lone PM Mn ion forming AF dimer. It seems that at higher concentrations the tendency towards cluster formation switches suddenly to trimmers as proven by the ferromagnetic response of nanowires containing 2.5% at. Mn discussed above (Figure 2A).


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As shown in the approximation presented in Figure 2C, both AF and FM contributions shape the unusual M(T) dependence of NWs with 2.5% at. Mn. Considering the different models proposed to explain the M(T) dependence in ferromagnetic semiconductors,27,28 we see that critical behaviour following the 3D Heisenberg model fits best. However, within the accuracy achieved it is not possible to judge, which magnetic coupling scheme is predominant. Nevertheless, the critical temperature given by all of the models is similar.

Figure 2. A - The dependence of magnetization as a function of external magnetic flux for the NWs samples with the distinct doping levels. B – Comparison of net magnetization at B = 500 mT


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as a function of doping level. The three characteristic ranges described in text (PM, PM+AF, FM+AF) are highlighted. Dashed lines represent two models of cluster formation - black one follows classical binomial distribution, while the red one include strong tendency to AF cluster formation. C – Temperature dependence of net magnetization at B = 500mT for NWs with 2.5% at. Mn. Solid lines present the results of dual phase (AF+FM) approximation using the different FM models. Dashed line shows antiferromagnetic component. See text and Supporting Information for details. Estimated value of TC in nanowires containing 2.5% at. Mn equal to 445 K, 460 K, 470 K for 3D Heisenberg (3DHM), polaronic (PM), and static mean field (SMFM) magnetic models, respectively (for more details please see Supporting Information). All these values are close to the theoretical value predicted in DFT calculations of atomic-scale Mn clusters.29 This corroborates presented assumptions of the cluster origin of robust room temperature ferromagnetism of the produced nanowires. Presented study demonstrated that nanowires of narrow gap ferromagnetic semiconductor can be produced using pulse electrodeposition technique. It is a straightforward way of preparing large amounts of high-quality Mn-doped InSb nanowires. The amount of Mn dopants build in the host structure can be easily tuned by varying the concentration of precursor in electrochemical bath. Maximum amount of Mn introduced to the nanowire structures was 2.5% at. Magnetic response of InSb-Mn NWs can be controlled by the concentration of dopants, which reveal a tendency to form small clusters. Through precise selection of electrodeposition conditions, it is possible to obtain well-defined doped semiconductor nanowires that reveal ferromagnetic response at room temperature and above.


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ASSOCIATED CONTENT Supporting Information content: Details of nanowires preparation; Magnetic response of InSbMn nanowires containing 2.5% at. Mn; Binomial distribution of the doping dependence of magnetization; Temperature dependence of magnetization of InSb-Mn nanowires containing 2.5% at. Mn;

AUTHOR INFORMATION Corresponding Author *K.E. Hnida, e-mail: [email protected], [email protected] AGH University of Science and Technology, Academic Centre for Materials and Nanotechnology, av. A. Mickiewicza 30, 30-059 Krakow, Poland; Tel: + 48 12 617 53 09 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Part of these research took place at National Synchrotron Radiation Centre SOLARIS at the PEEM/XAS beamline. The experiment was performed thanks to collaboration of SOLARIS Staff. The research was partially carried out with the materials and equipment purchased thanks to the financial

support

of

the

National

Science

Centre,

Poland,

(grant

no.

UMO-

2015/17/D/ST5/021332).


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A - Schematic representation of pulse electrodeposition synthesis of InSb-Mn nanowires. B – Dependence of the amount of Mn embedded into InSb NWs as a function of concentration of the manganese precursor in the electrolyte. C - SEM image of NWs with 1.4% at. Mn. D – Manganese L-edge and indium M-edge XAFS spectra of NWs with 2.5% at. Mn. E - XRD pattern of NWs with 2.5% at. Mn.


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A - The dependence of magnetization as a function of external magnetic flux for the NWs samples with the distinct doping levels. B – Comparison of net magnetization at B = 500 mT as a function of doping level. The three characteristic ranges described in text (PM, PM+AF, FM+AF) are highlighted. Dashed lines represent two models of cluster formation - black one follows classical binomial distribution, while the red one include strong tendency to AF cluster formation. C – Temperature dependence of net magnetization at B = 500mT for NWs with 2.5% at. Mn. Solid lines present the results of dual phase (AF+FM) approximation using the different FM models. Dashed line shows antiferromagnetic component. See text and Supporting Information for details. 160x299mm (300 x 300 DPI)


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Mn-doped InSb nanowires that exhibit room-temperature ferromagnetism have been synthesized for the first time using electrochemical methods.


Room temperature ferromagnetism in InSb-Mn nanowires | Nano Letters

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