Nanoscale phase separations in spintronic materials, superconductors, and other systems

Resume : In the field of emerging nanoscale materials with switchable properties chiral structures like helices or twisted ribbons are of great interest because of their intrinsic optical and mechanical properties. In this contribution, we present a study about the mechanical properties of SiO2 and SiO2@MxOy nanotubes and helical nanosprings synthesized by an original and simple technique from organic nanotubes through inorganic transcription. These nano-objects have potential applications in NEMS, ranging from physical sensing and signal processing to ultra-low power radio frequency signal generation, thanks to their striking features. NEMS have been generally based on 1D nano-objects, such as carbon nanotubes or silicon nanowires. However, the use of 3D nanostructures such as nanohelices would allow a significant improvement the electromechanical performances of functional nanodevices, due to their specific properties. The originality of our synthesis method consists in the possibility to obtain 3D nanostructures with specific morphology and properties. Hybrid nano-helices are synthesized using amphiphilic organic chiral self-assemblies forming very well defined helix or ribbons structures and exploits them as templates for inorganic nanomaterial formation[1]. Their bio-inspired mineralization creates silica nano-helices with very well controlled morphologies usable in functional nanodevices such as sensors, actuators and resonators. To the best of our knowledge, nothing has been

Resume : The magnetic and electric properties of dopants in semiconductors strongly depend on the lattice sites which they occupy. While the majority site can often be predicted based on chemical similarities with the host elements and is usually simple to confirm experimentally, minority sites are far more difficult to predict, detect and identify. Dilute magnetic semiconductors (DMS) are perfect examples of such complex dopant lattice location underlying complex magnetic and electric phenomena. We overview recent emission channeling studies (at ISOLDE-CERN) on the lattice location of transition metals in DMS (Mn in GaAs [1]; Mn/Fe/Co/Ni in GaN and ZnO [2]) from the highly dilute (< 0.05%) to the typical DMS (several %) regime. We discuss in particular (1) the competition between substitutional and interstitial occupancy in Mn-doped GaAs, and (2) the puzzling anion substitution observed for some transition metals in ZnO and GaN. We also present ongoing work beyond DMS, in the wider context of magnetic doping of insulators (e.g. topological insulators and artificial multiferroics), involving channeling techniques and others (e.g. synchrotron-based). [1] Appl. Phys. Lett. 106, 012406 (2015); Physical Review B 86, 125206 (2012); Appl. Phys. Lett. 98, 201905 (2011). [2] Nucl. Instr. Meth. Phys. Res. 332, 143 (2014); J. Phys.: Cond. Matter 25, 416001 (2013); Appl. Phys. Lett. 103, 091905 (2013); Phys. Rev. B 86, 195202 (2012); Phys. Rev. B 84, 125204 (2011).

Resume : Decomposition of (Ga,Mn)As - a canonical dilute ferromagnetic semiconductor, has been studied since the very beginning of the research activity devoted to this material [1]. (Ga,Mn)As is a metastable compound, which can only be obtained by highly nonequilibrium growth methods like molecular beam epitaxy, hence it is fairly easy to convert it to a phase segregated material with MnAs nanocrystals embedded in GaAs matrix, applying high temperature post-growth annealing. This hybrid system is interesting because of room-temperature ferromagnetic properties of MnAs; thus it is suitable for fabrication of prototypical spintronic devices taking advantage of integration of metallic, ferromagnetic MnAs nano-magnets with the semiconducting GaAs host. In contrast to previous reports devoted to MnAs nano-inclusions embedded in GaAs layers, here we investigate quasi-one-dimensional case of MnAs nanocrystals incorporated into thin shells of GaAs nanowires. We provide evidences that the preferred location for MnAs segregation is correlated with the stacking fault (SF) defects of GaAs NW cores. This enables an adjustment of the distribution of MnAs nanocrystals along the NW axes; since the distribution of SF defects can be controlled by the NW growth conditions [2]. This work has been supported by the project 2014/13/B/ST3/04489 financed through the Polish National Science Centre. [1] J. De Boeck, et. al., Appl. Phys. Lett. 68, 2744 (1996). [2] T. Burgess, et. al. ACS Nano 7, 8105 (2013).

Resume : In the search of novel magnetic semiconductors, an attempt to incorporate magnetic element in a high content often results in a phase separation into regions with high and low contents of magnetic elements. There are two types of phase separation; the aggregation of magnetic elements with keeping crystal structure coherent to the host semiconductor (chemical phase separation) and the precipitation of nanocrystals of an extrinsic phase (structural phase separation)[1]. In this presentation, we will show our recent experiments on the phase separation and its correlation with magnetic properties in II-VI magnetic semiconductors (Zn,Cr)Te and (Zn,Fe)Te, which were grown by molecular beam epitaxy. In (Zn,Cr)Te, it is found that the co-doping of iodine as a donor impurity enhances the phase separation[2]. With the increase of Cr content or the growth temperature, the type of phase separation changes from chemical to structural phase separation[3]. In (Zn,Fe)Te, the growth with a surplus supply of Zn flux over Te flux induces the formation of Fe-rich columnar regions, which appear to be structurally coherent to the host crystal[4]. Details will be presented at the conference. [1] T. Dietl et al., Rev. Mod. Phys., in press. [2] S. Kuroda et al., Nat. Mater. 6, 440 (2007). [3] H. Kobayashi et al., Physica B 407, 2947 (2012). [4] S. Ishitsuka, Phys. Stat. Sol. (c) 11, 1312 (2014).

Resume : Cr-doped CdTe crystals were theoretically predicted to be DMS with high Curie temperature due to superexchange spin interaction between nearest-neighbor Cr2 ions substituted cadmium atoms in the host lattice [1]. However, it was also reported that room-temperature ferromagnetism in Cr-based AIIBVI compounds could possibly be associated with precipitation of dopant-related secondary phases [2] caused by poore chromium solubility in the host matrix. In the present work we investigated CdTe:Cr single crystals grown by physical vapour transport method from pre-synthesized (Cd,Cr)Te alloys with 2.5 and 5 at.% of chromium nominal content. Arranged sets of extrinsic phase precipitates were revealed by means of SEM patterning of the (110)- and (111)-oriented surfaces of the crystals, both in the form of nm-sized near-isometric particles and platelets with sub-μm thickness. Using EDX profiling, HAADF elemental mapping and HRTEM measurements coupled with FFT images analysis, it was determined that the observed defects are composed of Cr-Te intermetallic compounds coherent with {111} and {100} planes of the zinc-blende structure of CdTe environment. Magnetic properties of the grown crystals were studied by means of EPR data obtained using X-band Bruker spectrometer (9.43 GHz). The angular dependence of spectrum showes two sets of fine components originated from substitution Cr2 ions in the CdTe host matrix and from precipitated CrTe-related phases. [1] J. Blinowski, P. Kacman, J.A. Majewski, J. Cryst. Growth. 159 (1996) 972. [2] Sreenivasan, M.G., Teo, K.L.; Cheng, X.Z.; Jalil, M.B.A.; Liew, T.; Chong, T.C.; Du, A.Y.; Chan, T.K.; Osipowicz, T.,Structural, magnetic, and transport investigations of CrTe clustering effect in (Zn,Cr)Te system, Journal of Applied Physics, v 102, n 5, 2007, p 053702

Resume : Dilute magnetic semiconductors (DMSs) attracted great interests in the last several decades because of their potential for spintronic device [1]. III-V compounds especially GaAs based DMS has recently emerged as the most popular material for this new technology. However, that the low mobility of holes in p-type DMS limits the potential application in semiconductor spintronic devices. Therefore, the searching for n-type DMS is of interest. The doping of Fe in InAs is attracting research attentions due to the possibility to fabricate n-type diluted magnetic semiconductors [2, 3]. However, the low solubility of Fe in InAs is the most difficulty to achieve InFeAs DMS. In this work, we obtain Fe doped InAs layers by ion implantation and pulsed laser annealing. This approach has shown success for preparing other III-V based DMSs [4, 5]. The formed InFeAs layers are proved to be epitaxial-like on InAs substrates. The prepared InFeAs layers reveal similar magnetic properties independent of their conductivity types. While the samples are lacking of charactersistics of DMS, they appear to be superparamagnetic behavior, revealing such as time-dependent magnetiszation measurements reveal aging and memory effects. 1. T. Dietl et al., Science 287, 1019-1022 (2000) 2. M. Kobayashi et al., Appl. Phys. Lett., 105, 032403(2014) 3. P. Nam Hai et al., Appl. Phys. Lett., 101, 182403 (2012) 4. D. B?rger et al., Phys. Rev. B, 81, 115202 (2010) 5. M. Khalid et al., Phys. Rev. B, 89, 121301(R

Resume : Nowadays, the energy and environmental issues have become increasingly important. To convert sustainable energy into chemical fuels, the photocatalytic hydrogen production by semiconductor materials have received a great deal of attention. On the other hand, in order to develop efficient photocatalysts, two important factors must be taken into consideration: shifting the light absorption range to visible light and increasing the photogenerated charge carriers. Recent studies have shown that by introducing plasmonic metal, the photocatalytic efficiency of semiconductor can be improved via plasmon-enhanced light absorption. In the present work, we have successfully synthesized hexagonal close-packed core-shell Au/TiO2 hybrid nanocrystal arrays by a facile process, which includes arrangement of hexagonal close-packed metal nanocrystal arrays and deposition of TiO2 by atomic layer deposition (ALD). The localized surface plasmon resonance (LSPR) properties of the hybrid nanocrystal arrays have been investigated, which is consistent with the simulation based on Mie theory. Through tuning the thickness of TiO2 shell or changing the size of Au core, the LSPR wavelength can be systematically controlled. The hybrid Au/TiO2 nanocrystal arrays were then employed as photocatalysts in aqueous methanol solution. In comparison with bare TiO2 thin film, the hybrid Au/TiO2 nanocrystal arrays indeed exhibited notable increases in the amount of hydrogen from methanol solution splitting under both ultra-violet and visible light, which is attributed to the plasmon-enhanced light absorption. Based on finite difference time domain (FDTD) simulation, the electric field in TiO2 has been enhanced by LSPR, hence increased the generation of electron and hole pairs.

Resume : Here we describe processes to transform archetypical commercial organic pigments into colloidal micro and nanocrystals suitable for solution processing of high-quality transistors, biosensors, and photosensors. Our methodology relies on ligand-mediated syntheses, transforming industrial colored pigment powders into stable colloidal solutions of semiconductor nanocrystals, highly suitable for electronic device developments. Tuning of process conditions can yield nanocrystals with zero, one-, two- and three-dimensional shapes, exhibiting a wide range of optical absorption and photoluminescence over spectral regions from the visible to the near infrared. The utility of such colloidal nanocrystals is demonstrated in photodetectors with responsivities up to 1 A/W, and humidity sensors operating over a dynamic range of 7 orders of magnitude and ~0.1s response. Both devices are fabricated by paint brushing or drop casting of these nanocrystals on paper substrates and outperform devices prepared from the same starting materials by vacuum deposition by several orders of magnitude. Properties can be further contolled through the use of biofunctional ligands such as riboflavin, flavin mononucleotide, and phosphonic acids. We demonstrate dedicated functionality leading to selective bioresponse. Importantly, these crystals are found to exhibit excellent operational stability in both air and various aqueous ionic environments. The semiconducting nanocrystals described here offer a cheap and nontoxic alternative to inorganic nanocrystals, as well as a new paradigm for obtaining organic semiconductor materials from low-cost and nontoxic commercial colorants.

Resume : Hydrothermal synthesis is one of liquid-phase synthesis methods with water or an aqueous solution under high pressure and high temperature. This paper reports the growth of magnetic Fe3O4 particles from iron powder (spherical, <10 micron) through an alkaline hydrothermal process with different KOH Molar concentration at the same hydrothermal time. In addition, the phase and shape variations with the addition of Potassium hydroxide (KOH), Ethylene diamine (EDA), or KOH and EDA were investigated and compared. Morphologies of the synthesized magnetic particles were characterized by X-ray diffraction(XRD, 2θ= 20°~80°) with Cu-kα radiation and field emission scanning electron microscopy(FE-SEM). Structure characterization showed that the phase structure of the particles evolved from Iron powder to Fe3O4 with increasing concentration of KOH and EDA. We observed an interesting role of KOH on the formation of magnetite octahedron. Finally, the magnetite properties of Iron oxide were studied by using a vibrating sample magnetometer (VSM), showing that Fe3O4 can be simply fabricated by hydrothermal methods.

Resume : Low-dimensional composition structures, having the spin-polarized electric charge transfer through the dielectric boundaries were obtained on the base of Sr2FeMoO6-d (SFMO) compounds films in Al2O3 porous templates. The advantage of SFMO is caused by its high Curie temperature (~ 400 К) and almost 100% degree of the spin polarization. After the deposition SFMO films had a single-phase composition, but low values of their magnetization have shown the absence of the superstructural ordering of Fe3+ and Mo5+ cations, due to the presence of the antistructural defects of FeMo and MoFe type. This is caused by the appearance of the antiferromagnetic domains, because of the destruction of - Fe3+ - Mo5+- Fe3+- chains. An optimization of the structurally-perfect SFMO films synthesis in the Al2O3 template has been achieved by the ion assistance method, with a bombardment of the film surfaces by Аr+ ions. Investigation of these films microstructure has confirmed their uniformity and homogeneity, due to the formation of active centers in the form of radiation defects on the surface of the samples surface as a result of the ion assistance. This leads to the fine-grained structure of the films. Results of the measurements of the field dependence of magnetization show a decrease of the SFMO films coercitive force after the ion assistance and an increase of the electrons spin polarization.

Resume : Spin-wave resonance (SWR) is a newly emerged method for studying surface spinwave modes (SSWMs) in (Ga,Mn)As thin films. The existence of SSWMs in (Ga,Mn)As thin films was recently reported by Liu et al. [1], which observed SSWMs in the in-plane configuration (with variable azimuth angle ϕ between the in-plane magnetization of the film and the surface [100] crystal axis) – SSWMs are observed in the azimuth angle range between two in-plane critical angles ϕc1 and ϕc2. On the basis of our recent theoretical SWR studies [2, 3] we show here that the cubic surface anisotropy is an essential factor determining the existence conditions of SSWMs. The following conclusion is drawn from our considerations: in a (Ga,Mn)As thin film conditions favorable for the occurrence of surface spin-wave modes in the in-plane configuration are fulfilled first of all for those azimuth orientations of the magnetization of the sample that lie around the hard axes of cubic magnetic anisotropy. This implies that a hard cubic anisotropy axis plays the role of an easy axis for the spin pinning; we can refer to it as an easy surface pinning axis (an axis from which spins diverge easily). This study is a part of a project financed by Narodowe Centrum Nauki, Grant no. DEC-2013/08/M/ST3/00967. References [1] X. Liu, Y.-Y. Zhou, and J. K. Furdyna, Phys. Rev. B 75, 195220 (2007). [2] H. Puszkarski and P. Tomczak, Sci. Rep. 4, 6135 (2014). [3] H. Puszkarski and P. Tomczak, Phys. Rev. B 91, 195437 (2015).

Resume : We report on the fabrication of a proof-of-concept device by means of a novel direct-writing, X-ray nanolithography approach, which represents a natural extension of our recent studies on the influence of synchrotron radiation on the oxygen content of the Bi2Sr2CaCu2O8+δ (Bi-2212) superconducting oxide [1]. Selected areas of Bi-2212 microcrystals have been exposed to a 17.7 keV beam with a spot size of about 57x45 nm2 and a flux of 1-5x1011 ph/s. The irradiated regions have resulted in trenches able to force the current along the c-axis, therefore patterning a stack of intrinsic Josephson junctions. Indeed, the I-V curves clearly show the typical Josephson pattern, confirming that the delivered radiation dose was enough to locally turn the material into a non-superconducting state. Morphological analysis shows that no material has been removed from the trenched regions, but some local volume expansion can be observed. X-ray nanodiffraction frames collected at the irradiated areas show the presence of Bi-2212 peaks along with a Bi2O3 polycrystalline phase that does not appear in the non-irradiated regions. Our results represent a novel patterning method that in principle could be extended to several oxide materials. Possible advantages of this method can be represented by improved mechanical stability, absence of chemical contamination and of vacuum/oxide interfaces, and in potential higher steepness of the patterned structures. [1] A.Pagliero et al. Nano Lett. 2014, 14, 1583

Resume : Charge-ordered (CO) state and antiferromagnetism are very stable in bulk samples of half-doped and electron-doped perovskite manganites but they become significantly suppressed and accompanied by an enhanced ferromagnetism upon reduction of the grain size. Magnetic studies of nanoparticles (NPs) with reduced grain size are exciting since properties of the particle core differ significantly from those of the shell and core-shell interaction leads to unexpected effects. With decreasing particle size from 60 to 15 nm for Sm0.43Ca0.57MnO3 and from 80 to 20 nm for Sm0.27Ca0.73MnO3, the relative volume of the ferromagnetic (FM) phase increases monotonously, while the CO phase progressively weakens and disappears completely in Sm0.43Ca0.57MnO3 NPs of average 15 nm particle size. Field cooled magnetization hysteresis loops of Sm0.1Ca0.9MnO3 NPs exhibit horizontal and vertical shifts, relatively small in 60 nm and much larger in 25 nm particles, due to size-dependent exchange bias effect. The exchange bias field and the coercive field depend in a non-monotonic way on cooling magnetic field. We have ascribed the magnetic behavior of the nanoparticles to a core-shell scenario with phase separated core containing FM clusters embedded in an antiferromagnetic (AFM) matrix and partially disordered AFM or paramagnetic shell. The suppression of the FM phase in the core with decreasing particle size may account for the enhancement of the exchange bias effect seen in smaller particles.

Resume : Data encryption is the cornerstone of today's trust in the telecommunications arena. At present, complex algorithms are implemented in adjacent integrated circuits, which translate between actual and garbled information. Beyond requiring dedicated circuitry, this paradigm allows replicating the translating machine / algorithms. In this present, we present a magnetic media which spontaneously encrypts its contents at a mixed-phase boundary between ferromagnetic and antiferromagnetic phases. We show how data retrieval and editing is performed at the ferromagnetic phase, following the inspiration by Heat-Assisted Magnetic Memories, which are already coping its own niche in the non-volatile memory shelf. A detailed study allows us to conclude that the presence of coexisting phases at room temperature is an essential requirement for the observed memory effects. The ample availability of materials with mixed-phase phenomena close to room temperature anticipates that the findings reported here are not, by any theoretical perspective, limited to the specific material employed here, namely FeRh.

Resume : Group IV elements (C, Si, Ge, Sn), their binary alloys, and the alloys doped with group III and V elements can be stabilized in the form of honeycomb two-dimensional lattices. We employ first principles calculations in the framework of the density functional theory augmented by Monte Carlo calculations (within NVT ensemble and with valence force field Tersoff like potentials) to study the cohesive and electronic properties of ordered and disordered binary alloys of the whole plethora of the honeycomb monoatomic systems consisting of group IV atoms. For the ordered alloys, we determine phase diagrams, morphology of the structures leading to local total energy minima and possibility of transitions between structures under the external stress. For disordered AxB1-x alloys with x up to 50%, we determine the equilibrium structure and quantify the degree of the short- and long-range order in the alloys with the Warren-Cowley and Brag-Williams parameters, respectively. The ordered Si-C, Ge-C, and Sn-C alloys acquire highly energetically preferable low buckled equilibrium phase and have fairly large energy gap (roughly 2 eV). The disordered alloys with carbon are not random but exhibit rather large degree of short-range order. Further, we perform comparative study of the n and p-type doped systems on the basis of graphene and silicene, i.e., BC, NC, BNC, AlSi, PSi, AlPSi, to find out that these structures exhibit typically large degree of the short order.

Resume : In this talk the results of the interplay between magnetism and superconductivity, obtained for the cuprate system La2-xBaxCuO4 (x = 1/8) and for the binary helimagnet CrAs, will be presented. Magnetism and superconductivity was studied in the static stripe phase of LBCO-1/8 by means of magnetization and muon-spin rotation (µSR) experiments as a function of pressure up to p = 2.2 GPa [1]. With increasing pressure the spin order temperature and the size of the ordered moment are not changing significantly. However, application of hydrostatic pressure leads to a remarkable decrease of the magnetic volume fraction Vm(0). Simultaneously, an increase of the SC volume fraction Vsc(0) occurs. Furthermore, it was found that Vm(0) and Vsc(0) at all p are linearly correlated: Vm(0) + Vsc(0) = 1. This is an important new result, indicating that the magnetic fraction in the sample is directly converted to the SC fraction with increasing pressure. The present results provide evidence that static stripe order and bulk superconductivity occur in mutually exclusive spatial regions. This conclusion is also supported by our recent oxygen isotope effect experiments in LBCO-1/8 [2]. Another interesting issue we studied with µSR is the suppression of magnetism and the occurrence of superconductivity in CrAs as a function of pressure [3]. The magnetism remains bulk up to p ≃ 3.5 kbar while its volume fraction gradually decreases with increasing pressure until it vanishes at p ≃ 7 kbar. At 3.5 kbar superconductivity abruptly appears with its maximum Tc ≃ 1.2 K which decreases upon increasing the pressure. In the intermediate pressure region (3.5 ≤ p ≤ 7 kbar) the superconducting and the magnetic volume fractions are spatially phase separated and compete for phase volume. A scaling of the superfluid density with Tc3.2 as well as the phase separation between magnetism and superconductivity point to a conventional mechanism of the Cooper-pairing in CrAs. [1] Z. Guguchia et al., New Journal of Physics 15, 093005 (2013). [2] Z. Guguchia et al., Phys. Rev. Lett. 113, 057002 (2014). [3] R. Khasanov, Z. Guguchia et. al., arXiv:1502.07573v1 (2015).

Resume : Competition in chemically homogeneous strongly correlated transition-metal oxides leads to electronic-phase inhomogeneities on the nanoscale and amongst others to fascinating magnetoresisitve and superconducting states. In this respect, the frustrated spatially-anisotropic triangular-lattice antiferromagnet NaMnO2 is challenging as it features unexpected coexistence of long- and short- range magnetic correlations below TN = 45 K. Our complementary high-resolution synchrotron XRD, local-probe 23Na NMR and muon-spin relaxation (μ+SR) studies, corroborate that the layered NaMnO2 adopts a remarkable magnetostructurally inhomogeneous ground state in the absence of active charge degrees of freedom [1]. We show that two major opposing effects (elastic vs. magnetic exchange) of similar magnitude, lead to nearly equivalent, competing structural phases, which enable infinitesimal quenched disorder to locally lift the inherent frustration of the parent monoclinic phase. To provide an insight to this puzzling phase separation we have extended our study to the isomorphous CuMnO2 (TN = 65 K) compound, where local probes suggest that the interlayer cation (Cu) mediates the transition to a less inhomogeneous ground state [2]. NaMnO2 provides a paradigm of a rarely observed nanoscale inhomogeneity in an insulating spin system, an intriguing complexity of competition due to geometrical frustration. [1] A. Zorko et al., Nat. Commun. 5, 3222 (2014) [2] A. Zorko et al., Sci. Rep. 5, 9272 (2015)

Resume : Electron crystal - phonon glass concept of modern thermoelectrics was technologically and experimentally verified in semiconductor bulk two-phase crystalline nanocomposites (Pb,Cd)Te-CdTe and in multilayer quantum-dot PbTe-CdTe nanostructures. Very low mutual solubility of lattice-matched rock-salt PbTe and zinc-blende CdTe crystals makes this system a model thermoelectric nanocomposite of zinc-blende CdTe nano-dots epitaxially embedded in a rock-salt PbTe thermoelectric crystalline matrix. We observed these nano structures forming spontaneously in properly annealed bulk crystals as well as developed the technological procedure of growing the PbTe-CdTe multilayer quantum-dot nanostructures by molecular beam epitaxy method controlling the size (from 5 to 30 nm) and distribution of CdTe dots. The n-type PbTe-CdTe multilayer with the smallest CdTe dots revealed at room temperature an increase of thermoelectric power as compared to reference bulk n-PbTe crystals. In PbTe-CdTe bulk nanostructures doped with Bi or Na we found an increased thermoelectric power and a reduced thermal conductivity resulting in very good ZT thermoelectric figure of merit parameter up to 0.9 at T=700 K for both n- and p-type materials. M. Szot et al., Functional Materials Lett. 7, 1440007 (2014). Work supported by the European Regional Development Fund through the Innovative Economy grant (POIG.01.01.02-00-108/09).