[Show abstract][Hide abstract] ABSTRACT: This paper reports a thorough microstructural investigation of bismuth ferrite (BFO) thin films subjected to various processing conditions and discusses their influence on the stability of the BiFeO3 perovskite phase. The formation of secondary phases in BFO thin films is studied as a function of annealing temperature and time, film thickness, Bi excess, and Ti substitution. While films annealed at 600 °C consist of the desired BiFeO3 phase, higher temperatures induce the decomposition leading to a significant amount of secondary phases, particularly the iron-rich Bi2Fe4O9 phase. A longer annealing time at 700 °C further enhances the decomposition of BiFeO3. Qualitative microstructural analysis of the films is performed by electron backscattered diffraction which provides phase analysis of individual grains. The morphology of the single-crystalline Bi2Fe4O9 grains that are embedded in the BiFeO3 matrix drastically changes as a function of the film thickness. Nucleation of these Bi2Fe4O9 grains probably occurs at the film/substrate interface, after which grain growth continues toward the surface of the film through the depletion of the BFO phase. Addition of Bi excess or the substitution of Fe with Ti in the precursor solutions significantly reduces the formation of an iron-rich secondary phase. Influence of the secondary phases as well as Ti substitution on magnetic properties of BFO films was investigated.
No preview · Article · Jul 2015 · Journal of Materials Science
[Show abstract][Hide abstract] ABSTRACT: Indium tin oxide (ITO) is broadly used as a transparent conducting material for electrodes in optoelectronic devices. Irradiation of ITO with low energy ions can result in the formation of periodic surface nanopatterns which can serve as an alternative for the polymer alignment layer in liquid crystal devices. We investigated the formation of the ion-induced surface nanopatterns on ITO with focus on the influence of the crystalline structure of the material. We find that the crystallinity plays a crucial role in the pattern formation, with no pattern developing on an amorphous ITO surface. We discuss these findings in the context of the state-of-the-art theory for ion-induced patterning. We show that the ion-induced pattern plays a critical role in the liquid crystal alignment on ITO surfaces.
No preview · Article · May 2015 · Thin Solid Films
[Show abstract][Hide abstract] ABSTRACT: The trend towards miniaturization of electronic devices has enhanced the demand for multifunctional materials, mostly in the form of thin films or nanostructures. Multifferoic materials are interesting candidates for applications in electronics because they combine ferroelectric and ferromagnetic properties resulting in new functionalities when orders are coupled [1,2]. To obtain thin films of the complex oxides, solution-gel chemistry is considered as a flexible and relatively cost-effective method.
We synthesized aqueous solutions with different compositions, as precursors for single phase or composite multiferroic materials. Thus, polycrystalline and epitaxial films of BiFeO3 were deposited on different substrates overcoming the issues concerning phase formation and stabilization, secondary phase formation and interaction between film and substrate. Furthermore, flexibility of solution processing is shown by growing self-assembled heterostructures of a two-phase system consisting of ferroelectric (BaTiO3 or BiFeO3) and magnetic (CoFe2O4) phases. A thorough structural and morphological investigation is correlated with the magnetic properties of the obtained films.
[Show abstract][Hide abstract] ABSTRACT: We present an extended X-ray absorption fine structure investigation of the local environment of Sn atoms in strained and relaxed Ge
1−xSnx layers with different compositions. We show that the preferred configuration for the incorporation of Sn atoms in these Ge
1−xSnx layers is that of a α-Sn defect, with each Sn atom covalently bonded to four Ge atoms in a classic tetrahedral configuration. Sn interstitials, Sn-split vacancy complexes, or Sn dimers, if present at all, are not expected to involve more than 2.5% of the total Sn atoms. This finding, along with a relative increase of Sn atoms in the second atomic shell around a central Sn atom in Ge
1−xSnx layers with increasing Sn concentrations, suggests that the investigated materials are homogeneous random substitutional alloys. Within the accuracy of the measurements, the degree of strain relaxation of the Ge
1−xSnx layers does not have a significant impact on the local atomic surrounding of the Sn atoms. Finally, the calculated topological rigidity parameter a** = 0.69 ± 0.29 indicates that the strain due to alloying in Ge
1−xSnx is accommodated via bond stretching and bond bending, with a slight predominance of the latter, in agreement with ab initio calculations reported in literature.
No preview · Article · Mar 2015 · Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: The coexistence of superconductivity and ferromagnetism is investigated in granular Sn–Co nanocomposites. The nanocomposites have been prepared by co-deposition of Sn atoms and Co clusters, the morphology and composition of which can be tuned by varying the deposition rate of Co clusters relative to Sn atoms. Flat isolated Sn islands are obtained at zero or low Co cluster flux, while granular nanocomposites are formed with increasing Co cluster flux, reaching Co concentrations up to 44 vol.%. Interfaces with a low electronic transparency between superconductor and ferromagnet are obtained by a combination of the granular nature of the nanocomposites and the formation of Sn–Co alloys at the Sn/Co interfaces. The structure and composition of the nanocomposites have been thoroughly characterized by atomic force microscopy, X-ray diffraction and conversion electron Mössbauer spectroscopy. Over the entire Co concentration range, the hybrids show a ferromagnetic response. The superconducting phase boundary and the Meissner response depend on the morphology and composition of the nanocomposites. In particular, the superconducting critical temperature decreases with increasing Co concentration, while the Meissner response varies from a reversible to a strongly hysteretic behaviour depending on the morphology of the samples with different Co content. The persistence of superconductivity at high Co concentrations is attributed to a suppression of the superconducting proximity effect in these nanocomposites, which is ascribed to the low interface transparency between the Sn and Co components that make up these hybrid systems.
No preview · Article · Mar 2015 · Journal of Alloys and Compounds
[Show abstract][Hide abstract] ABSTRACT: We studied surface morphology induced changes of magnetic anisotropy, magnetization reversal, and symmetry of the anisotropic magnetoresistance (AMR) in ion sputtered Ni films grown on MgO (001). Grazing-incidence ion sputtering generally develops anisotropic surface roughness of the Ni films, i.e., nanometer wide ripples parallel to the ion beam direction, giving rise to uniaxial magnetic anisotropy with the easy axis along the ion beam direction. The formed ripples act as domain wall nucleation and pinning sites during magnetization reversal, while two-jump domain wall motion dominates in the as-grown Ni films. More importantly, the azimuthal angular dependence of the AMR indicates a superposition of twofold symmetry and fourfold symmetry. By relying on grazing-incidence ion sputtering along specific crystallographic directions, we are able to tailor the relative weight of twofold and fourfold symmetry of AMR. We demonstrate that in contrast to the bulk case, the symmetry of the AMR becomes also sensitive to the surface morphology in thin films, which is in particular relevant for the design of magnetotransport based sensors.
No preview · Article · Mar 2015 · Physical Review B
[Show abstract][Hide abstract] ABSTRACT: Magnetic spin structures in epitaxial BiFeO3 single layer and an epitaxial BaTiO3/BiFeO3 multilayer thin film have been studied by means of nuclear resonant scattering of synchrotron radiation. We demonstrate a spin reorientation in the 15 × [BaTiO3/BiFeO3] multilayer compared to the single BiFeO3 thin film. Whereas in the BiFeO3 film, the net magnetic moment m → lies in the (1-10) plane, identical to the bulk, m → in the multilayer points to different polar and azimuthal directions. This spin reorientation indicates that strain and interfaces play a significant role in tuning the magnetic spin order. Furthermore, large difference in the magnetic field dependence of the magnetoelectric coefficient observed between the BiFeO3 single layer and multilayer can be associated with this magnetic spin reorientation.
No preview · Article · Feb 2015 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: The structure, magnetic properties, and lattice dynamics of ordered Fe-Pt
alloys with three stoichiometric compositions, Fe$_3$Pt, FePt and FePt$_3$,
have been investigated using the density functional theory. Additionally, the
existing experimental data have been complemented by new measurements of the Fe
projected phonon density of states performed for the Fe$_3$Pt and FePt$_3$ thin
films using the nuclear inelastic scattering technique. The calculated phonon
dispersion relations and phonon density of states have been compared with the
experimental data. The dispersion curves are very well reproduced by the
calculations, although, the softening of the transversal acoustic mode TA$_1$
leads to some discrepancy between the theory and experiment in Fe$_3$Pt. A very
goood agreement between the measured spectra and calculations performed for the
tetragonal structure derived from the soft mode may signal that the tetragonal
phase with the space group $P4/mbm$ plays an important role in the martensitic
transformation observed in Fe$_3$Pt. For FePt$_3$, the antiferromagnetic order
appearing with decreasing temperature has been also investigated. The studies
showed that the phonon density of states of FePt$_3$ very weakly depends on the
Full-text · Article · Jan 2015 · Journal of Alloys and Compounds
[Show abstract][Hide abstract] ABSTRACT: We determined the lattice location of Mn in ferromagnetic (Ga,Mn)As using the electron emission channeling technique. We show that interstitial Mn occupies the tetrahedral site with As nearest neighbors (TAs) both before and after thermal annealing at 200 °C, whereas the occupancy of the tetrahedral site with Ga nearest neighbors (TGa) is negligible. TAs is therefore the energetically favorable site for interstitial Mn in isolated form as well as when forming complexes with substitutional Mn. These results shed new light on the long standing controversy regarding TAs
versus TGa occupancy of interstitial Mn in (Ga,Mn)As.
No preview · Article · Jan 2015 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: Multiferroic (BaTiO3-BiFeO3) × 15 multilayer heterostructures show high magnetoelectric (ME) coefficients αME up to 24 V/cm·Oe at 300 K. This value is much higher than that of a single-phase BiFeO3 reference film (αME = 4.2 V/cm·Oe). We found clear correlation of ME coefficients with increasing oxygen partial pressure during growth. ME coupling is highest for lower density of oxygen vacancy-related defects. Detailed scanning transmission electron microscopy and selected area electron diffraction microstructural investigations at 300 K revealed antiphase rotations of the oxygen octahedra in the BaTiO3 single layers, which are an additional correlated defect structure of the multilayers.
No preview · Article · Jan 2015 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: We investigated the training effect and magnetization reversal in CoO/Co bilayer films grown epitaxially on MgO (001) substrates. The asymmetry of the magnetization reversal, which appears due to the exchange bias after field cooling, survives after training, in contrast to the case of polycrystalline bilayers. By applying hysteresis loops with the magnetic field perpendicular to the cooling field, we are able to modify the orientation of the average uncompensated magnetization of the antiferromagnetic CoO. Subsequently, the untrained state can be partially restored, and more importantly the magnetization reversal asymmetry can be inverted by starting the perpendicular loop with the appropriate field polarity. Consequently, we succeeded in manipulating the magnetization reversal asymmetry and even in achieving opposite reversal asymmetries in the same exchange bias system.
Full-text · Article · Dec 2014 · Physical Review B
[Show abstract][Hide abstract] ABSTRACT: We have investigated the structural and optical properties of metastable amorphous and crystalline GeSn layers on Si substrates. The as-deposited amorphous layers crystallize during annealing at 500°C. This transition leads to a significant change in the local environment of the Sn atoms and in the optical properties. The Ge-Sn bond length is decreased after crystallization. The as-deposited GeSn layers, with nominal 4.5% and 11.3% Sn content, do not show Sn segregation. For the crystallized GeSn with nominal 4.5%, the Sn appears to be substitutional, as no Sn clustering was observed. However, for the crystallized GeSn with nominal 11.3% Sn, Sn segration and the presence of β-Sn is observed by EXAFS. A method to suppress Sn segregation and increase the substitutional Sn concentration is discussed. Furthermore, we determined the optical properties of amorphous and crystalline GeSn with nominal 4.5% Sn. The bandgap energy decreases significantly from 0.89 eV (1392 nm) ± 0.05 eV for the amorphous layer to 0.52 eV (2383 nm) ± 0.05 eV for crystalline GeSn, leading to significant reduction in penetration depth.
[Show abstract][Hide abstract] ABSTRACT: The magnetic and electric properties of impurities in semiconductors are strongly dependent 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 complicated to predict, detect and identify. We have carried out extensive beta(-) emission channeling studies on the lattice location of transition metal impurities in wide-gap dilute magnetic semiconductors, namely Co and Mn in GaN and ZnO, making use of radioactive Co-61 and Mn-56 implanted at the ISOLDE facility at CERN. In addition to the majority occupation of cation (Ga, Zn) sites, we located significant fractions (of the order of 20%) of the Co and Mn impurities in anion (N, O) sites, which are virtually unaffected by thermal annealing up to 900 degrees C. Here, we present the beta(-) emission channeling experiments on Co-61-implanted GaN. We discuss these results in the context of our recent reports of minority anion substitution in Mn-implanted GaN Pereira et al. (2012)  and Mn/Co-implanted ZnO Pereira et al. (2011) , particularly in terms of the advantages of the emission channeling technique in such cases of multi-site occupancy.
No preview · Article · Aug 2014 · Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms
[Show abstract][Hide abstract] ABSTRACT: The training effect for exchange bias in field-cooled Co/CoO bilayers films is investigated. Previous experiments on the same system have shown that, starting from the ascending branch of the first hysteresis loop, coherent magnetization rotation is the dominant reversal mechanism. This is confirmed by the performed numerical simulations, which also indicate that the training is predominantly caused by changes of the rotatable anisotropy parameters of uncompensated spins at the Co/CoO interface. Moreover, in contrast with what is commonly assumed, the exchange coupling between the rotatable spins and the ferromagnetic layer is stronger than the coupling between the ferromagnet and the spins responsible for the bias. Thus, uncompensated spins strongly coupled to the ferromagnet contribute to the coercivity rather than to the bias, whatever the strength of their magnetic anisotropy.
Full-text · Article · Jun 2014 · Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: Surface treatments using multiple Ar ion irradiation processes with a maximum energy and fluence of 200 keV and 1x1016 ions/cm2, respectively, have been performed on two different metallic glasses: Zr55Cu28Al10Ni7 and Ti40Zr10Cu38Pd12. Analogous irradiation procedures have been carried out at room temperature (RT) and at T = 620 K (around 0.9 Tg, where Tg denotes the glass transition). The structure, mechanical behavior, wettability and corrosion resistance of the irradiated alloys have been compared with the properties of the as-cast and annealed (T = 620 K) non-irradiated specimens. While ion irradiation at RT does not significantly alter the amorphous structure of the alloys, ion irradiation close to Tg promotes decomposition/nanocrystallization. Consequently, the hardness (H) and reduced Young’s modulus (Er) decrease after irradiation at RT but they both increase after irradiation at 620 K. While annealing close to Tg increases the hydrophobicity of the samples, irradiation induces virtually no changes in the contact angle when comparing with the as-cast state. Concerning the corrosion resistance, although not much effect is found after irradiation at RT, an improvement is observed after irradiation at 620 K, particularly for the Ti-based alloy. These results are of practical interest in order to engineer appropriate surface treatments based on ion irradiation, aimed at specific functional applications of bulk metallic glasses.
Full-text · Article · May 2014 · Journal of Alloys and Compounds
[Show abstract][Hide abstract] ABSTRACT: The interdependence between training and magnetization reversal in granular Co-CoO exchange bias (EB) systems prepared by O ion implantation in Co thin films is demonstrated by polarized neutron reflectometry. While high-fluence O-implanted thin films show reduced relative training values and no asymmetry in magnetization reversal (all reversals take place by domain wall nucleation and motion), low-fluence O ion implantation results in an increased relative training and a magnetization reversal asymmetry between the first descending and the first ascending branches. Whereas the untrained decreasing field reversal occurs mainly by domain wall nucleation and motion, traces of a domain rotation contribution are evidenced in the increasing field reversal. This is explained by the evolution of the CoO structure and the contribution of the out-of-plane magnetization with ion implantation. The amount of incorporated O, which determines the threshold between both behaviors, is around 20 at.%. This reveals that the interdependence between training and magnetization reversal is insensitive to the morphology of the constituents (i.e., granular or layered), indicating that this is an intrinsic EB effect, which can be conveniently tailored by the interplay between the intrinsic properties of the investigated materials and ion implantation.
Full-text · Article · Apr 2014 · Physical Review B