Thermoelectric performance of spin Seebeck effect in Fe3O4/Pt-based thin film heterostructures

All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license.-- et al.We report a systematic study on the thermoelectric performance of spin Seebeck devices based on Fe3O4/Pt junction systems. We explore two types of device geometries: a spin Hall thermopile and spin Seebeck multilayer structures. The spin Hall thermopile increases the sensitivity of the spin Seebeck effect, while the increase in the sample internal resistance has a detrimental effect on the output power. We found that the spin Seebeck multilayers can overcome this limitation since the multilayers exhibit the enhancement of the thermoelectric voltage and the reduction of the internal resistance simultaneously, therefore resulting in significant power enhancement. This result demonstrates that the multilayer structures are useful for improving the thermoelectric performance of the spin Seebeck effect.This work was supported by the Spanish Ministry of Science (through Project Nos. PRI-PIBJP-2011-0794 and MAT2011-27553-C02, including FEDER funding), the Aragón Regional Government (Project No. E26), Thermo-spintronic Marie-Curie CIG (Grant Agreement No. 304043), JST-PRESTO “Phase Interfaces for Highly Ecient Energy Utilization” from JST, Japan, Grant in-Aid for Scientific Research on Innovative Areas “Nano-Spin Conversion Science” (Grant No. 26103005), Grant-in-Aid for Challenging Exploratory Research (Grant No. 26600067), Grant-inAid for Scientific Research (A) (Grant No. 15H02012) from MEXT, Japan, NEC Corporation, and The Noguchi Institute.Peer Reviewe

Contact-free mapping of electronic transport phenomena of polar domains in SrMnO3 films

Under the terms of the Creative Commons Attribution license.-- et al.High-resolution mapping of electronic transport phenomena plays an increasingly important role for the characterization of ferroic domains and their functionality. At present, spatially resolved electronic transport data are commonly gained from local two-point measurements, collected in line-by-line scans with a conducting nanosized probe. Here, we introduce an innovative experimental approach based on low-energy electron microscopy. As a model case, we study polar domains of varying conductance in strained SrMnO3. By a direct comparison with conductive atomic force and electrostatic force microscopy, we reveal that the applied low-energy electron-microscopy experiment can be considered as an inverse I(V) measurement, providing access to the local electronic conductance with nanoscale resolution and short data-acquisition times in the order of 10-102 ms. Low-energy electrons thus hold yet unexplored application opportunities as a minimal-invasive probe for local electronic transport phenomena, opening a promising route towards spatially resolved, high-throughput sampling at the nanoscale.We thank HZB for the allocation of synchrotron beam time and we thankfully acknowledge financial support by HZB. Research at the ETH was financed in part by the SNF (Proposal No. 200021_149192). L. M., E. L., P. A. A., and J. A. P. acknowledge financial support from Ministerio de Economía y Competitividad under Project No. MAT2014-51982-C2 and Gobierno de Aragón under Project No. E26.Peer Reviewe

Relaxation Mechanisms and Strain-Controlled Oxygen Vacancies in Epitaxial SrMnO3 Films

: SrMnO3 has a rich epitaxial strain-dependent ferroic phase diagram, in which a variety of magnetic orderings, even ferroelectricity, and thus multiferroicity, are accessible by gradually modifying the strain. Different relaxation processes, though, including the presence of strain-induced oxygen vacancies, can severely curtail the possibility of stabilizing these ferroic phases. Here, we report on a thorough investigation of the strain relaxation mechanisms in SrMnO3 films grown on several substrates imposing varying degrees of strain from slightly compressive (−0.39%) to largely tensile ≈+3.8%. First, we determine the strain dependency of the critical thickness (tc) below which pseudomorphic growth is obtained. Second, the mechanisms of stress relaxation are elucidated, revealing that misfit dislocations and stacking faults accommodate the strain above tc. Yet, even for films thicker than tc, the atomic monolayers below tc are proved to remain fully coherent. Therefore, multiferroicity may also emerge even in films that appear to be partially relaxed. Last, we demonstrate that fully coherent films with the same thickness present a lower oxygen content for increasing tensile mismatch with the substrate. This behavior proves the coupling between the formation of oxygen vacancies and epitaxial strain, in agreement with first-principles calculations, enabling the strain control of the Mn3+/Mn4+ ratio, which strongly affects the magnetic and electrical properties. However, the presence of oxygen vacancies/Mn3+ cations reduces the effective epitaxial strain in the SrMnO3 films and, thus, the accessibility to the strain-induced multiferroic phase

Strain-induced coupling of electrical polarization and structural defects in SrMnO3 films

Letter.-- et al.Local perturbations in complex oxides, such as domain walls, strain and defects, are of interest because they can modify the conduction or the dielectric and magnetic response, and can even promote phase transitions. Here, we show that the interaction between different types of local perturbations in oxide thin films is an additional source of functionality. Taking SrMnO 3 as a model system, we use nonlinear optics to verify the theoretical prediction that strain induces a polar phase, and apply density functional theory to show that strain simultaneously increases the concentration of oxygen vacancies. These vacancies couple to the polar domain walls, where they establish an electrostatic barrier to electron migration. The result is a state with locally structured room-temperature conductivity consisting of conducting nanosized polar domains encased by insulating domain boundaries, which we resolve using scanning probe microscopy. Our 'nanocapacitor' domains can be individually charged, suggesting stable capacitance nanobits with a potential for information storage technology.M.F., D.M. and M.L. acknowledge funding from grant ‘ETH-06 12-2’ and from SNF proposal no. 200021-149192. The authors acknowledge funding through the SNF R’equip Program (no. 206021-144988). Financial support from Spanish Ministerio de Economía y Competitividad through projects MAT2011-27553-C02, MAT2012-38213-C02-01, MAT2014-51982-C2 and from Regional Gobierno de Aragón through project E26 is acknowledged.Peer Reviewe

Characteristic length scale of the magnon accumulation in Fe3O4/Pt bilayer structures by incoherent thermal excitation

The dependence of Spin Seebeck effect (SSE) with the thickness of the magnetic materials is studied by means of incoherent thermal excitation. The SSE voltage signal in Fe3O4/Pt bilayer structure increases with the magnetic material thickness up to 100 nm, approximately, showing signs of saturation for larger thickness. This dependence is well described in terms of a spin current pumped in the platinum film by the magnon accumulation in the magnetic material. The spin current is generated by a gradient of temperature in the system and detected by the Pt top contact by means of inverse spin Hall effect. Calculations in the frame of the linear response theory adjust with a high degree of accuracy the experimental data, giving a thermal length scale of the magnon accumulation (Λ) of 17 ± 3 nm at 300 K and Λ = 40 ± 10 nm at 70 K.This work was supported by the Spanish Ministry of Science (through Project No. MAT2014-51982-C2-R, including European social fund), the Aragon Regional government (Project No. E26), and Thermo-Spintronic Marie Curie CIG Project (Grant Agreement No. 304043).Peer Reviewe

Polar-Graded Multiferroic SrMnO3 Thin Films

Engineering defects and strains in oxides provides a promising route for the quest of thin film materials with coexisting ferroic orders, multiferroics, with efficient magnetoelectric coupling at room temperature. Precise control of the strain gradient would enable custom tailoring of the multiferroic properties but presently remains challenging. Here we explore the existence of a polar-graded state in epitaxially strained antiferromagnetic SrMnO3 thin films, whose polar nature was predicted theoretically and recently demonstrated experimentally. By means of aberration-corrected scanning transmission electron microscopy we map the polar rotation of the ferroelectric polarization with atomic resolution, both far from and near the domain walls, and find flexoelectricity resulting from vertical strain gradients. The origin of this particular strain state is a gradual distribution of oxygen vacancies across the film thickness, according to electron energy loss spectroscopy. Herein we present a chemistry-mediated route to induce polar rotations in oxygen-deficient multiferroic films, resulting in flexoelectric polar rotations and with potentially enhanced piezoelectricity

Growth and structural characterization of strained epitaxial Hf0.5Zr0.5O2 thin films

Ferroelectricity was recently reported in thin films with several compositions in the HfO2-ZrO2 system with orthorhombic crystal structure. In the present paper we study the growth by pulsed laser deposition and the structural characterization of strained epitaxial Hf0.5 Zr0.5 O2 films on (001)-oriented yttria-stabilized zirconia (YSZ) substrates. We have determined the conditions for the coherent growth and correlated the deposition parameters with the films structure and microstructure studied through a combination of x-ray diffraction, electron backscatter diffraction, and scanning transmission electron microscopy. In the range of experimental parameters explored, all the films show monoclinic structure with distorted lattice parameters relative to bulk

Nature of antiferromagnetic order in epitaxially strained multiferroic SrMnO3 thin films

et al.Epitaxial films of SrMnO3 and bilayers of SrMnO3/La0.67Sr0.33MnO3 have been deposited by pulsed laser deposition on different substrates, namely, LaAlO3 (001), (LaAlO3)0.3(Sr2AlTaO6)0.7 (001), and SrTiO3 (001), allowing us to perform an exhaustive study of the dependence of antiferromagnetic order and exchange bias field on epitaxial strain. The Néel temperatures (TN) of the SrMnO3 films have been determined by low-energy muon spin spectroscopy. In agreement with theoretical predictions, TN is reduced as the epitaxial strain increases. From the comparison with first-principles calculations, a crossover from G-type to C-type antiferromagnetic orders is proposed at a critical tensile strain of around 1.6±0.1%. The exchange bias (coercive) field, obtained for the bilayers, increases (decreases) by increasing the epitaxial strain in the SrMnO3 layer, following an exponential dependence with temperature. Our experimental results can be explained by the existence of a spin-glass (SG) state at the interface between the SrMnO3 and La0.67Sr0.33MnO3 films. This SG state is due to the competition between the different exchange interactions present in the bilayer and favored by increasing the strain in the SrMnO3 layer.This work was supported by the Spanish Ministerio de Economía y Competitividad through Project Nos. MAT2011-28532-C03-02, MAT2011-27553-C02, MAT2012- 38213-C02-01, and MAT2014-51982-C2 including FEDER funding, by the Aragon Regional Government through projects E26 and CTPP4/11 and by the European Union under the Seventh Framework Programme under a contract for an Integrated Infrastructure Initiative Reference 312483-ESTEEM2. N. Marcano acknowledges the support of the Centro Universitario de la Defensa en Zaragoza (through Project 2013-03).Peer Reviewe

Origin of the giant magnetic moment in epitaxial Fe3O4 thin films

7 páginas, 7 figuras.-- PACS number(s): 75.70.Rf, 75.70.Ak, 75.50.Bb.-- et al.We study the enhanced magnetic moment observed in epitaxial magnetite (Fe3O4) ultrathin films (t<15 nm) grown on MgO (001) substrates by means of pulsed laser deposition. The Fe3O4 (001) thin films exhibit high crystallinity, low roughness, and sharp interfaces with the substrate, and the existence of the Verwey transition at thicknesses down to 4 nm. The evolution of the Verwey transition temperature with film thickness shows a dependence with the antiphase boundaries density. Superconducting quantum interference device (SQUID) and vibrating sample magnetometry measurements in ultrathin films show a magnetic moment much higher than the bulk magnetite value. In order to study the origin of this anomalous magnetic moment, polarized neutron reflectivity (PNR), and x-ray magnetic circular dichroism (XMCD) experiments have been performed, indicating a decrease in the magnetization with decreasing sample thickness. X-ray photoemission spectroscopy measurements show no metallic Fe clusters present in the magnetite thin films. Through inductively coupled plasma mass spectroscopy and SQUID magnetometry measurements performed in commercial MgO (001) substrates, the presence of Fe impurities embedded within the substrates has been observed. Once the substrate contribution has been corrected, a decrease in the magnetic moment of magnetite thin films with decreasing thickness is found, in good agreement with the PNR and XMCD measurements. Our experiments suggest that the origin of the enhanced magnetic moment is not intrinsic to magnetite but due to the presence of Fe impurities in the MgO substrates.This work has been financially supported by Spanish Ministry of Science through Projects No. MAT2008-06567-C02 and No. MAT2008-01077, and the Regional Government of Aragón (E-26, E-69, and E-34).Peer reviewe

Electron scattering processes in Ho5(SixGe1−x)4 compounds: Electrical resistivity studies

8 páginas, 7 figuras, 1 tabla.-- PACS number(s): 75.30.Sg, 71.70.Ch, 75.30.Kz, 75.10.DgWe present a detailed study of the temperature dependence of the electrical resistivity [ρ(T)] in the range 13–300 K for the Ho5(SixGe1-x)4 system. Three distinct ρ(T) behaviors are observed, associated with different magnetic and crystallographic structures along the series. In the samples with an antiferromagnetic phase (AFM) one observes a shoulder near the Néel temperature (TN) attributed to the formation of a gap on the Fermi surface. This gap is analyzed using a phenomenological two-band model for an AFM with distinct atomic and magnetic periodicities, and its effect seems to extend well above TN. We also found the presence of short-range magnetic clusters in the paramagnetic (PM) phase. On the ferromagnetic (FM) materials, the distinct ρ(T) scattering contributions (phonon, magnetic, and residual terms) are extracted from the measurements, with ρ(T) mainly dominated by electron spin scattering. An additional contribution is also observed, arising from the strong crystal field effect in these materials. The effect is mainly observed in the PM phase, leading to a curvature on ρ(T) in this phase. Using a two-level crystal field model the corresponding gap was estimated for the different Si/Ge ratio samples, revealing that the crystal field splitting increases linearly with Si content.Work partially supported by the projects POCI/CTM/61284/2004, PTDC/CTM/NAN/115125/2009, and FEDER/POCTI n0155/94 from Fundaçâo para a Ciência ex Tecnologia (FCT), Portugal. A.M.P. thanks FCT for Grant No. SFRH/BPD/63150/2009. C.M. acknowledges the support of the Fundación ARAID. The financial support of the Spanish MEC (MAT2008-06567-C02) and DGA (Grant No. E26) is acknowledged.Peer reviewe