Immanuel Kant Baltic Federal University

Materials for spintronic memories and sensors

Spintronics is an emerging research field based on the manipulation of spin-polarized electrons that has been significantly developed in the last three decades, during which different outstanding spintronics phenomena and devices based on them have been proposed and developed. Giant and tunnelling magnetoresistance (GMR and TMR, respectively) heterostructures consisting, in the simplest case, of two ferromagnetic layers separated by a non-magnetic (metallic – GMR – or insulating  – TMR –) spacer are examples of spintronic systems where the flow of spin-polarized electrons and then the resistance state are manipulated by controlling the relative orientation of magnetic moments in the two ferromagnetic layers. Such a systems are already exploited in current read-head elements in hard disk drives being also at the basis of novel and advanced technologies, including magnetic random access memories and high sensitive magnetic sensors for biomolecule detections or mechanical deformation sensing.

Further improving the device performance is being tackled from various viewpoints, including i) the search for novel materials (e.g. Heusler alloys, metal oxides, etc.) with higher spin-polarization, larger anisotropy or lower Gilbert dumping, ii)  the development of novel concepts (e.g. antiferromagnetic spintronics) and  iii) the fabrication of advanced hybrid heterostructures including for example multiferroics materials with electric-field tuneable functions."

3a) Quantum heterostructures for spintronics applications

(Alexandr Vinichenko)

The proposed research goal in this application is the comprehensive study and development on the quantum heterostructures for spintronics application with high g-factor realized by enhanced In composition in quantum wells InGaAs|InAlAs. These materials have perfect crystalline structure and can be engineered for specific electronic and magnetic properties by means of layer design and doping. Field effect by voltage application can adjust electron concentration, quantum well potential profile, electron state coupling etc., thus also changing their magnetic properties. Epitaxial structures will be grown and comprehensively studied, including conductivity and Hall effect measurements, including quantum effects in high magnetic fields at low temperatures, extracting quantum scattering and single particle relaxation times, electron g-factor and quantum subbands occupation depending on the both layout structure and applied voltage. Further research will involve quantum low temperature measurements.

3b) Magnetic Skyrmions

(Ksenia Chichay; Team leader: Valeria Rodionova)

Magnetic Skyrmions are one of the fascinating and promising objects for memories and sensors because of their small size and stability to perturbations such as electric currents and magnetic fields. The major mechanism to stabilize small Skyrmions in ferromagnet/heavy metal bilayers is the presence of Dzyaloshinskii-Moriya interaction (DMI). In thin films, the DMI arises at the interface of ferromagnetic material and heavy metal due to the presence of spin-orbit interaction and broken inversion symmetry.

We’ll investigate the stability and internal structure of an isolated Skyrmion in bilayer (ferromagnet/heavy metal) and trilayer (heavy metal 1/ferromagnet/heavy metal 2) nanodisks. We plan to study the static properties of the Skyrmions and to obtain the phase diagram of the Skyrmion existence depending on the thickness of the ferromagnetic layer and the DMI strength. Our preliminary results show that fully including the dipolar interaction in the calculation is important for thicker nanodots because together with Dzyaloshinskii-Moria interaction it has the stabilizing effect and definest he skyrmion configuration.

This work is being carrying out together with Dr. Oleg Tretiakov and Dr. Joseph Barker (Institute for Material Research, Tohoku University, Sendai, Japan)

3c) Investigation of exchange bias and spin valves structures, fundamental components of spintronic devices

(Cristina Gritsenko: Team leader: Valeria Rodionova)

The Exchange Bias (EB) phenomenon is still a hot topic of investigation, since it is the basis of spin-valves structures having a wide range of applications (spintronic, MRAM, magnetic field sensors, and memory devices). The main problem is to find ways of enhancing the exchange field value. We are working on thin films based on NiFe, IrMn and FeMn, prepared in cooperation with Dr. Nikolay Chechenin from Lomonosov Moscow State University. It is well established that exchange bias effect and the coercive field depend on the antiferromagnetic layer thickness, as found in in bilayered and trilayered thin films (Ni40Fe60/Ir72Mn25/Ni40Fe60), (Ni45Fe55/Ir72Mn25/Ni45Fe55), (Ni45Fe55/Ir72Mn25), (Ir72Mn25/Ni45Fe55), Ni81Fe19/Fe50Mn50. It was also investigated the influence of the layers deposition sequence and of the composition of ferromagnetic component on the exchange field value. In addition, it was determined the influence of temperature conditions on the exchange bias in above-mentioned structures.

In collaboration with the National University of Science and Technology MISiS we started the investigation of morphology and layers surfaces structure in order to find the correlations between the exchange bias and surfaces roughness and grains sizes in the layers of the above systems. We obtained preliminary results of experiments by Atomic Force Microscope (AFM) and Transmission Electron Microscopy (TEM) (in collaboration with Dr. Mikhail Gorshenkov from The National University of Science and Technology MISiS). Moreover, by computing modeling we are studying the exchange bias dependence on the configuration and value of the external magnetic field during the preparation of thin films structures.

Experimental and theoretical investigatons are also planned in collaboration with Dr. Sara Laureti from Istituto di Struttura della Materia of the National Research Council in Rome.

3d) New multiferroics structures based on polymer piezoelectric and high   magnetostrictive wires and ribbons

(Irina Baraban; Team Leader: Valeria Rodionova)

There are multiferroic structures actively used in sensors and autonomous power sources. Amplification of the magnetoelectric effect in multiferroic structures is the main objectives for increasing the sensitivity and efficiency of the developed instruments. There is a lot of opportunities in changing settings of multiferroic structures (the using of different materials functional layers, a different arrangement relative to each other functional layers, their sizes, shapes, etc.), which change the interaction between the magnetostrictive and piezoelectric layers.

Presently, our focus is in using materials not only of different compositions but also different geometry as magnetostrictive layer: amorphous ferromagnetic glass covered microwires and rapidly quenched ribbons. On the Fe-based microwires the magnetostriction constants by small angle of rotation of the magnetization vector were measured. The magnetostriction values are in the range ~ 1-2 * 10-5 and this is enough to consider the microwires as a promising magnetostrictive material to create multifrroics based on them. These microwires will be placed in a piezoelectric polymer PVDF, which is the most promising at present. Also we will investigate the magnetostriction coefficient of the ribbons and will measure ME effect in these structures.

3e) Magnetoelectric properties of nanostructures

(Galina Kupriyanova)

The project is aimed at studying the connection between the electric and magnetic properties in magnetic nanostructures (films). In some of them it is possible to control the magnetic properties by the application of an electric field and to control the electrical properties by applying a magnetic field. Interesting class of materials is that exhibits magnetoelectric properties, such as a change in the dielectric constant at the magnetic ordering temperature. The magnetodielectric effect is often accompanied by a distortion of the unit cell of a lattice.

We plan to investigate the tetrahedrites materials (Cu12Sb4S13 and Cu10Cr2Sb4S13, Cu12Sb3.8As0.2S13 Cu11.8Au0.2Sb4S13, Cu2SeO4), focussing on establishing the relationship between the topology of the magnetic structure, the structure and the anisotropy of the electrical and magnetic properties. For this purpose, we’ll perform a comprehensive study of the structures by NMR and NQR on nuclei isotopes 35Cu, 37Cu, 77Se, Sb. Structural and anisotropic properties, magnetic and electrical properties of the samples will be studied by NMR and NQR, SQUID and FMR methods. It should be noted that the NQR frequency is directly proportional to the electric field gradient at the nucleus (EFG). NMR method is the indispensable method for the study of the structure and the establishment of phase transition when the temperature changes.