Eliezer Adar

Wire Machine Technologies Ltd., Israel

Spin Seebeck Effect in Magnetic Nanowires

Possibilities of obtaining filiform magnetic structures which exhibit spin effects are explored. The structures are fabricated on the base of cast microwire bundles embedded in a magnetic glass isolation with cores made of amorphous and nanostructured alloys based on iron, manganese, cobalt and copper. To enhance the degree of amorphousness, the alloys comprise silicon, boron, as well as some other elements. The filiform structures are made of densely packed bundles of the aforementioned wires thinned down to a diameter of 40…200 nm. The magnetic glass isolation is made of borosilicate glass comprising up to 20% of iron, manganese and cobalt oxides. Both magnetically hard and soft structures with coercivities from 200 to 1600 A/m were produced.
The spin transfer effect is implemented over a sample length of 0.8…1.5 mm. The measured thermopower at room temperature amounts to 0.9 mV/ºC. This value is by an order of magnitude higher than the thermopower of metals. This witnesses to the spin origin of the observed effect which is found in magnetic fields above 0.5 T.
Though the observed thermopower is well below the values reported by the authors of Ref. [1] (K. Uchida et al.: “Observation of the spin Seebeck effect”) at low temperatures, the decay length of the spin coherence is here substantially longer. No measurements were done yet at low temperatures and in stronger magnetic fields.
The possible existence of micro-interruptions in the wires does not impedes the spin transfer and the appearance of a potential difference between the ends of the samples with contacts made of indium-gallium eutectics.

Vítor Amaral

Universidade de Aveiro, Portugal

Oxides: magnetism and beyond

Oxide materials attract lot of interest due to its flexibility and compatibility for many design applications, with a broad range of functional responses, which include magnetism, ferroelectricity, superconductivity and other highly correlated electron behavior. The coupling between several degrees of freedom (charge, spin, orbital) paves the way to new multifunctional materials and challenging situations occur at heterointerfaces between oxides and other materials. Orbital control of magnetism, bias induced behavior and interfacial control of magnetic moment will be addressed, in perovskite-based materials.

Mikalai Kalanda

Scientific and Practical Materials Research Centre of the NAS of Belarus, Belarus

Structure and valence state of iron in strontium ferromolybdate Sr2FeMoO6‒d under conditions of Fe/Mo cations ordering and oxygen nonstoichiometry

Sr2FeMoO6‒d single-phase powders with different degrees of superstructural ordering (P) of Fe/Mo cations and oxygen indices (6‒d) have been obtained from the SrFeO2.52 and SrMoO4 initial reagents by the single-phase technique. It has been determined by the x-ray and neutron diffraction that the Fe–O and Mo–O bond lengths in the crystal lattice increase with decreasing oxygen content. At the same time, the Fe–O and Mo–O bond lengths decrease due to the rise of the P value. According to the Mössbauer spectroscopy data, the values of the hyperfine field Bhf and the isomer shift δIS correspond to a valence of the iron cations being intermediate between Fe3+ and Fe2+. The presence of the mixed valence state of the iron cations may be explained by the occurrence of point defects in the magnetic structure, leading to the presence of a delocalized Mo electron near a Fe atom. It was revealed that with increasing concentration VO•• of oxygen vacancies, the interaction between them takes place accompanied by the formation of (VOVO)•• associates, which leads to a decrease of the coordination number. According to the values of the isomer shift of the Mössbauer spectra, a fraction of iron ions take positions in the tetrahedral surrounding, or a similar one.
A decrease of the unit cell volume takes place with rising P value at a constant oxygen index. This fact promotes an increase of the bonds’ covalency degree, and stimulates a redistribution of the electronic density and an increase of the concentration of spin-polarized charge carriers which are placed on the Mo(t2g) electron orbitals in the conduction band. This leads to a rise of the density of states on the Fermi level, strengthening of the exchange interactions and increase of the Curie temperature, which indicates the dominating role of spin-polriased charge carriers on the Fermi level in the exchange interactions.

Vladislav Kataev

Leibniz-Institut für Festkörper- und Werkstoffforschung – IFW Dresden, Germany

Inversion of the Orbital States in the Intrinsically Stacked Oxide Sr2IrO4

A promising route to tailoring the electronic properties of quantum materials and devices rests on the idea of orbital engineering in multilayered oxide heterostructures. In this talk we will show that the interplay of interlayer charge imbalance and ligand distortions provides a knob for tuning the sequence of electronic levels even in intrinsically stacked oxides. We resolve in this regard the d-level structure of layered Sr2IrO4 by electron spin resonance (ESR) and ab initio quantum chemistry methods. Sr2IrO4 is a prototypical spin-orbital Mott insulator which has attracted considerable interest in the recent past. It has a rather simple crystalline structure displaying stacked, quasi two-dimensional IrO2 and double SrO layers.
Using ESR measurements at sub-THz frequencies in strong magnetic fields we were able to untangle the 5d-shell electronic structure of Sr2IrO4, in particular, the exact order of the Ir t2g levels. To do that, we have experimentally determined the spectroscopic g-tensor which appears inverted as compared to predictions of canonical ligand-field theory. The inversion of the g-tensor implies an inversion of the ordering of the t2g orbital states. This has been confirmed on the quantitative level by ab initio quantum chemistry methods which yielded experimentally observed g-factors. We show that in the layered structure of Sr2IrO4 a specific distribution of ionic charges between the IrO2 and SrO layers modifies the sequence of energy levels within the t2g and eg manifolds and consequently very fundamental physical properties such as the magnetic g-factors, which determine the relation between the magnetic moment and quantum number of a magnetic particle.
Given the nonpolar character of the metal-oxygen layers, our findings highlight the tetravalent transition-metal 214 oxides as ideal platforms to explore d-orbital reconstruction in the context of oxide electronics.

Dong-Hyun Kim

Chungbuk National University, Korea

Magnetization dynamics of ferromagnetic thin films over wide time scales

Magnetization dynamics of ferromagnetic thin films on a slower (~ sec) to a faster (~ fs) timescale will be presented. The ferromagnetic multilayer films such as Co/Pt and CoFeB/Pd as well as simple Co, Ni, Fe films will be explored by means of magneto-optical Kerr effect. On a slower time scale, minor hysteresis behavior will be presented, where corresponding microscopic magnetic domain patterns and domain dynamics are discussed together. On the other hand, photoinduced demagnetization/remagnetization behavior are examined on a faster time scale by means of time-resolved magneto-optical Kerr effect based on a femtosecond laser setup. Ultrafast signal of Kerr effect and reflectivity are fitted based on a 3-temperature model to analyze the temperatures of spin-, electron-, and lattice-subsystems. Fluence- and field-dependent ultrafast magnetization dynamics is experimentally investigated, where a separation of spin sub-system on a sub-ps time scale is found to lead to a giant magnetic cooling effect. Several other interesting topics will be discussed as well.

Andrii Korostil

Institute of Magnetism of the NAS of Ukraine and the MES of Ukraine, Ukraine

On controlled remagnetization in nanoheterostructures

Controlling magnetization in magnetic nanoheterostructures at ultrafast time scales and ultrasmall space scales and ultra low power consumption is the top problem of the physics of magnetism, which has both fundamental scientific and applied significance with huge potential applications in novel magnetic devices. Utilizing the electronic spin degree of freedom creates new functionalities that lead to applications in information technologies such as sensors and memories. The magnetization control can be realized by different external fields possessing by corresponding mechanisms of the magnetization manipulation.
For the electromagnetic field, it is possible the mechanism of the effective bias field of the magneto-optic inverse Faraday effect and for magnetic with antiferromagnetic exchange interaction, the laser heating-induced mechanism associated with the effect of ferromagnetic-like transitional state. The electric field control can is realized via mechanism of the spin transfer torque exerting on localized magnetization states in the magnetic nanoheterostructures. Herewith, it is possible the voltage-controlled magnetic anisotropy, where the perpendicular magnetic anisotropy is controlled by the external gate voltage applied across a dielectric layer. In this case, the microscopic origin of the anisotropy change is attributed to voltage control by an effective interfacial Rashba spin-orbit coupling. In the conventional case, when the applied voltage involves current flow, the mechanism of the spin transfer torque is related to the electron spin polarization and the spin flow generated by the spin Hall effect.
Special attention is paid to features of the current spin-orbit induced magnetic dynamics in multilayer nanostructures with nonmagnetic heavy metal layers possessing by a strong spin-orbit interaction. These structures include ferromagnetic (antiferromagnetic AF)/normal metal nanostructures based on both conductive and insulating magnetics and heavy normal metals (e. g., FeCoB/Ta, YIG/Pt, Nio/Pt).The spin Hall effect of the conversion of an incoming charge current into a transverse (with respect to the charge current) spin current induces a spin-transfer torque and magnetic dynamics including a magnetic precession and switching. The magneto-dynamic effect of a spin current pumping generation together with the inverse spin Hall effect of conversion of the spin current into the incoming charge current provide the influence of the magnetic dynamics on the incoming charge current. These feedforward and feedback between the incoming charge current and the magnetic dynamics can be the basis for the spin-orbit driven self-sustained auto-oscillations of a magnetic order in the nanostructures. It is shown that these magnetic nanostructures possess by properties of controlled microwave radiation attaining tens THz in the antiferromagnetic case.

Ilya V. Kubasov

National University of Science and Technology “MISiS”, Russia

Bidomain lithium niobate crystals: technology, investigation, applications

Lithium niobate (LiNbO3) single crystals have been attracting the attention of researchers and engineers for over five decades due to their nonlinear optical, acoustic, and ferroelectric properties. The possibility of engineering a ferroelectric domain structure allows creating devices with unique properties. For a long time the main interest to lithium niobate was associated with optics and acoustics, while usage of this material in static or low-frequency piezoelectric devices was disregarded. In this presentation we want to show perspectives to utilize lithium niobate single crystals for some new applications. There are sensors and actuators based on single crystals with a so-called ferroelectric bidomain structure. Such crystals contain only two domains having antiparallel spontaneous polarization vectors and a charged domain wall between them. The presence of only 180º domains as well as a very high coercive field allow the existence of the bidomain structure in a very wide temperature range without degradation or depolarization. A bidomain crystal is an analog to a classic piezoelectric bimorph, so that it can convert mechanical deformations into voltage and vice versa with a high efficiency due to its single-crystalline nature and absence of any intergrain boundaries or epoxy layers. Moreover, a high Curie temperature and chemical stability make lithium niobate a promising material for high-temperature applications.

Peter K. Petrov

Imperial College London, United Kingdom

Strain Engineered Properties of Complex Oxide Thin Films

Heteroepitaxial strain in complex oxide thin films is known to have a significant impact on both their low and high frequency dielectric properties. For example, SrRuO3 (SRO) has become the most popular epitaxial electrode for complex oxide heterostructures owing to its favourable fatigue characteristics, chemical and thermal stabilities and also its nearly isotropic conductivity. Conductive oxide thin films can be engineered through the deposition conditions. The three major influences on their structural, electrical, magnetic and optical properties are strain, stoichiometry, and thickness.
Here we will discuss the properties of several complex oxide thin films, namely barium-strontium titanite (BaxSr1-xTiO3), strontium ruthenate (SrRuO3), strontium molybdate (SrMoO3), strontium iron molybdate (SFMO) and strontium niobate (SrNbO3).
Thin films and multilayer structures based on these materials, were deposited by pulsed laser deposition and were comprehensively characterised by means of spectroscopic ellipsometry, XRD, AFM, SEM, and SIMS, along with DC resistivity and AC Hall effect measurements. The results of these experiments will be presented and the corresponding microstructure-properties relationship will be revealed.
New applications and devices based on these materials will be discussed.

Anatoli I. Popov

Institute of Solid State Physics, University of Latvia, Latvia

Thermal annealing of radiation damage in binary and complex oxides

The binary and complex oxide (MgO, ZnO, BeO, Al2O3, MgAl2O4, Y3Al5O12 etc) insulators as well as some other wide-band gap insulator and semiconductor compounds (AlN, BN, PLZT) are very important for many applications, including those when they are either exposed to radiation beams or function in radiation environment. This includes a huge number of applications, such as optical materials, storage phosphors, laser active elements, dosimeter, imaging plates, materials for fusion technology etc. The resulting radiation defects determine and influence the functional characteristics of the corresponding instruments and/or their specifics parts or elements.
Thus, it is very important to predict/simulate not only the kinetics of diffusion-controlled defect accumulation under irradiation, but also a long-time defect structure evolution including the thermal annealing of radiation-induced defects.
After introducing some basics on the radiation point defects in halides, binary oxides and oxide perovskites as well as the mechanisms of point defect formation under particle irradiation (neutron, ion, proton, electron), the current understanding of the point defect thermal annealing processes will be briefly reviewed.
We will present recent results and provide current understanding of the kinetics of the F-type center (F and F+) annealing in above-mentioned compounds after electron, heavy ions or neutron irradiation, which were treated as the bimolecular process with equal concentrations of the complementary F and Oi interstitial defects. The process is controlled by the interstitial oxygen ion mobility, which is much higher than that of the F centers. The appropriate migration energies were obtained for available in the literature experimental annealing kinetics for electron, neutron and ion irradiated MgO, Al2O3, MgAl2O4, Y3Al5O12, BeO, ZnO, YSZ, PLZT etc. The results obtained are used for the evaluation of interstitial oxygen migration parameters and are compared with the available ab initio calculations.

Carlos Rosário

Universidade de Aveiro, Portugal

Origin of the temperature and magnetic field dependence of the electrical transport in substoichiometric TaOx thin films

Memristive devices built upon redox-based resistive switching show good prospects for the implementation in Storage Class Memory and neuromorphic computing architectures. Ta2O5-based structures are some of the most commonly used for the realization of memristive devices. These are based on the nonvolatile change of the resistance via the modulation of the oxygen content in conductive filaments. However, the filaments’ structure and exact composition are still a matter of debate. In a previous work we showed a clear correlation between the electrical transport mechanisms of conductive filaments in Ta2O5-based memristive devices and of substoichiometric TaOx thin films with x ~ 1. In this work we tried to deepen our understanding of the origin of the electrical transport in the substoichiometric TaOx thin films. We sputtered TaOx films with different oxygen concentrations, as well as pure Ta films. We then characterized the films’ structure by grazing incidence X-ray diffraction (GIXRD) and measured the temperature and magnetic-field dependence of the in-plane electrical transport in these films at temperatures from 300 K down to 1.8 K.
The GIXRD patterns of the TaOx films show a mixture of two phases: a disordered β-Ta phase containing oxygen interstitials and Ta2O5. The resistivity of the TaOx thin films with x ~ 1 shares a negative temperature coefficient of resistance (TCR) and the same transport mechanisms with the Ta films. Hall measurements show a very high carrier concentration of 2.6×1022 cm–3 in the TaOx films with x = 1.0 – 1.3. These films also exhibit a positive magnetoresistance associated with weak antilocalization at temperatures below approximately 30 K. The same magnetotransport characteristics are observed for the Ta film. Therefore, we conclude that the transport in the substoichiometric TaOx films with x ~ 1 is dominated by the Ta phase. The conductivity in the Ta is determined by disorder-induced effects that lead to a power-law temperature dependence of the conductivity.

Gunnar Suchaneck

Technische Universität Dresden, Germany

Multi-target reactive sputter deposition of strontium ferromolybdate – Challenges and approaches

The deposition of Sr2FeMoO6 (SFMO) thin films with appropriate properties on silicon wafers is extremely difficult. This is caused by the formation of antisite defects of FeMo and MoFe even in stoichiometric films, the appearance of reduced phases, e.g. Fe, at lower and the SFMO decomposition into SrMoO4 and SrFeO3-x at too high oxygen partial pressure, respectively. The formation of oxygen vacancies narrows the available technology window additionally. On the other hand, the relatively high synthesis temperature of SFMO (ca. 1150 K) is not compatible with Si wafers already covered by oxide and electrode films.
Conventional RF sputtering using oxide targets comprising several cations suffer from different sputter yield of the constituents leading to the preferential sputtering phenomenon. Moreover, single phase SFMO ceramic targets exhibiting a low porosity and a homogeneous density are difficult to fabricate, particularly beyond diameters of 100 mm. They often crack during high-power sputtering due to local heating.
Multi-target reactive sputtering (MTRS) has the advantage to control the film stoichiometry, especially the Fe/Mo ratio of deposited thin films by process parameters like target power and substrate temperature. MTRS was carried out in a LS730S sputtering system (VON ARDENNE GmbH, Dresden, Germany). The computer- controlled system is equipped with a process chamber and a load-lock, both evacuated by turbo-molecular pumps. The process chamber consists of up to four 200 mm metal targets, two stationary radiation heaters and a wafer carousel holding four 150 mm silicon wafers. The base of the Fe target was directly connected with the RF (13.56MHz) generator. A unit for pulsed DC and arc suppression was connected between power supply and Mo target. Also, high-power-pulse sputtering was performed by connecting a switching unit between the DC power supply and either the Sr- or the Mo-target in order to increase the effective surface temperature by an additional energy flux from the low-pressure plasma. Base argon flow into the chamber was kept constant while an additional oxygen flow was carried directly to all three metal targets. The oxygen gas flow of one channel was controlled by a close-loop feedback circuit of a plasma emission monitor.
Film phase composition, texture, crystallite size and antisite disorder were characterized by XRD, film composition by EDX and XPS, and film microstructure by SEM and AFM. Electrical resistivity was measured by a standard four-point method carrying out switching the electric and magnetic fields directions. Magnetic and magnetotransport were obtained in the range of 4.2–300 K in constant magnetic fields of up to 8 T.
The results demonstrate that direct synthesis of SFMO by reactive multitarget sputtering is a promising technology for the fabrication of magnetoresistive devices operating at room temperature. However, the processing temperature should be adapted to the limits of silicon wafer technology. On the other hand, the narrow window of pure SFMO phase deposition requires a careful in-situ process control.

Sigitas Tamulevicius

Kauno Technologijos Universitetas, Lithuania

Technologies for 2-D nanostructures

Technologies of production of two types of ordered nanoparticle systems (linearly arranged nanoparticles in 1- D periodic structures either 2-D nanometer structures of nanoparticles) as potential building blocks for optical sensors, catalytic and other applications (potentially including magnetic materials) will be presented, exploring both top-down and bottom-up approaches. In the top – down approach, reactive magnetron sputtering (high-power impulse magnetron sputtering mode-HIPIMS) of metalic target was used to deposit thin films of diamond like carbon (DLC) nanocomposites including silver nanoparticles (modelling material). Structuring features of the nanocomposites using reactive ion sputtering combined with nanoimprint lithography or femtosecond laser irradiation was used to produce 1-D or 2-D submicron structures of DLC:Ag. Influence of plasma parameters applied during reactive ion etching as well as role of laser beam parameters like fluence, polarization were studied systematically paying attention to the evolution of 2-D system of nanoparticles, including size distribution of nanoparticles.
Alternatively (bottom– up approach), 2-D submicron system of silver nanoparticles was produced applying capillary assisted particle assembly (CAPA) method. Response of colloidal solution of silver nanocubes with edge dimensions ranging from few to tenth nanometers were studied applying pump-probe spectroscopy and UV-Vis spectroscopy. Applying different types of templates produced by holographic lithography either e-beam lithography, 2-D systems of silver nanocubes was produced employing an original CAPA setup. Role of different solvents, humidity and substrate temperature were studied to produce highly ordered 2-D nanometer structures.
Both top-down and bottom-up approaches in production of 2D nanostructures of magnetic nanoparticles are in the implementation stage.

Maxim Varenik

Weizmann Institute of Science, Israel

Van Vleck paramagnetism and sign-reversal of magnetostriction in non-magnetic materials with cubic symmetry

The magnetic properties of bulk CeO2 are not fully understood yet. It is slightly paramagnetic at room temperature, despite not having any magnetic ions. Weak, temperature-independent paramagnetism in non-metals is usually attributed to a second order, Van Vleck-type magnetization. This type of paramagnetism can stem from a break in local symmetry. By doping ceria with a non-magnetic Lu3+ ion we were able to discern that this paramagnetic contribution comes solely from the Ce4+ ions. Ceria, despite having an average cubic structure, has been shown to have non-cubic Ce-O bond arrangement. We have proven experimentally the reduction in local symmetry by measuring magnetostriction in ceria. Magnetostriction can only be observed in materials that have a non-cubic magnetic unit cell. We also observe magnetostriction in SrTiO3, which also dynamically deviates from its average cubic structure. The magnetostriction curves in these two materials follows a similar shape – contraction at low fields and expansion at high fields. Although the nature of the magnetic moment and magnetostriction mechanism is not yet clear, we observe that magnetostriction is governed by local symmetry reduction and magnetic susceptibility. In addition, we show that magnetostriction measurements is a powerful tool to detect local reduction in symmetry.