Immanuel Kant Baltic Federal University

Materials for non-spintronic memories, sensors and actuators

Memories, sensors and actuators represent very important fields of application of magnetic materials. To realize such devices different types of nanomaterials (films, wires, nanoparticles) are used exploiting different types of effect (e.g interface exchange coupling, domain walls dynamics).

The enhancement of the performance of the above devices requires tuning and optimization of the the magnetic anisotropy trough manipulation of components at nanoscale, depending on the specific application.

4a) Development of the functional bio-similar memory element based on doped silicon crystal nanowire. 

(Team leader: Alexander Goickhman)

The aim of this project is to develop a technology for creating nanosized memristor clusters based on self-assembled Si:Me(Au,Ag) nanowhiskers. The main result of the project should become a neuromorphic prototype with CMOS neurons and memristor clusters based on self-assembled Si:Me(Au,Ag) nanowhiskers synapses in a crossbar configuration.

Throughout the world, in recent years there has been a strong development of memristors – passive

electrical components with a dynamic resistance that changes depending on the charge flowing through them, able to mimic a synapse, and adequate to the task of parallel processing, including learning and decision-making.

The possibility of creating 3D memristor structures based on Ag/Au-doped nanowhiskers (2-5 nm) vertically grown on a conducting Si substrate and isolated with a conformal Al2O3 layer (see figures in the Appendix 1 for part 4) is very interesting from the perspective of its potential use in electronics and rather promising due to its compatibility with the silicon technology.

Today, all known approaches to creating memristor prototypes rely on the standard planar technology of developing feedstock and further prototyping using lithography (2D approaches). A distinctive feature of the proposed approach is the formation of vertical memristor structures in Ag-doped Si:Au nanowhiskers, i.e. the creation of memristors in each nanowhisker (a 3D approach). The ultimate objective of this project is the development and testing of a methodology for ordering the developed nanowhisker memristor structure on the substrate according to a template for a simpler integration of the approach into the series technologies of electronic component production.

4b) Experimental Investigation of domain wall dynamics in amorphous ferromagnetic microwires for application in novel magnetic memory and devices.

(Ksenia Chichay; Team Leader: Valeria Rodionova)

The investigation of magnetization dynamics in nano- and micro- objects is of a great interest due to its prospects in development of novel magnetic memory and logic devices Such devices can be implemented on the fast domain wall motion. The highest domain wall velocity, up to 8000 m/s, has been obtained in bistable glass coated amorphous ferromagnetic microwires. Besides the amorphous state, the distinguishing feature of these microwires is the presence of internal stresses, which together with magnetostriction result in a significant effect on the magnetoelastic energy and thereby define the micromagnetic structure and reversal magnetization process. Moreover there are many ways of manipulating the magnetic properties and the domain wall dynamics such as annealing, applying a mechanical stresses, changing of the ρ-ratio of the metallic nucleus diameter, d, to the diameter of the glass coated microwire, D and etc.

The experimental investigation of the magnetic properties and the domain wall dynamics is topic of the joint research with the group of Prof. Arkady Zhukov (University of the Basque Country, San Sebastian, Spain), the group of Prof. Nikolai Perov (Lomonosov MSU, Faculty of Physics, Magnetism department, Moscow, Russia) and the group of Prof. Manuel Vazquez (ICMM-CSIC, Madrid, Spain).

Recently, the simulation of the reversal magnetization process has become available. The calculation and the simulation of the domain wall movement will lead to deeper understanding of the remagnetization process and will allow to predict the magnetic behavior depending on the parameters of the wire. For the simulation we collaborate with the group of Prof. Michael Farle (University of Duisburg-Essen, Duisburg, Germany). To theoretically analyze the domain wall movement and the changes in domain wall dynamics during this movement we are collaborating with the group of Prof. Sergey Leble (IKBFU, University, Kaliningrad).

4c) Theoretical investigation of domain wall propagation in thin amorphous magnetic microwires for application in novel magnetic memory and devices.

(Vereshchgin Mikhail Dmitrievich; Team leader: Sergey Leble)

Over the last few years there has been a sustained growth and increasing interest on domain wall (DW) propagation in thin amorphous magnetic microwires related to proposals of their application for logic and memory devices.

The most suitable microwires for achievement the fast and controllable results seem planar nanowires and circular cross-section amorphous microwires. However, the overwhelming majorities of investigations in this area are mainly experimental. There are only a few papers in which quite a modest theoretical analysis is presented.

At present the theory does not give answers to the most important questions in DW dynamics: What is a shape of DW? What is a velocity of DW propagation? How do shape of DW and its propagation velocity depend on external magnetic field and material properties ? The aim of the project is to fill in the above listed gaps by providing a theoretical framework for understanding the behavior of DW in amorphous bistable magnetic microwires. This is closely connected to the experimental research done by the Laboratory of Novel Magnetic Materials in IKBFU. The theory is expected to point out the possible directions of experiments, to help choosing the appropriate materials and to provide possibility for better control over experiments. Besides that, the significance of building the lacking to the moment theory is obvious by itself. One of the most important features of researched bistable ferromagnetic microwires is that the anisotropy of the material changes significantly from easy axis case in the bulk to easy plane at the surface due to internal stress and magnetostriction effects. Proper understanding of the magnetic structure of the intermediate area between the stable outer layer and the bistable bulk will be the key to describe the remagnetisation process along with dynamical properties of domain walls. For one, it will allow us to answer the question, whether the movement of the domain wall is driven by forces in the bulk or is it rather initiated by a domain wall emerging in the intermediate area moving toward the centre of the wire. Answer to that question will lead directly to determination of the shape of a moving domain wall as well as correlation of velocity with external magnetic field.

Unlike majority of researchers who mainly investigated the DW dynamics from theoretical point of view instead of Lamdau-Lifshitz-Gilbert (LLG) equation we start from Heisenberg spin chain (HSC) equation. There are several advantages of such approach. Firstly, HSG equation describes DW dynamics in either micro and nanowires, whereas LLG can be applied to microwires exclusively. Secondly, HSC admits two important particular cases, namely an easy plan and an easy axis, for which the equation descends to φ4 and Gordon sine equations, respectively. They both can be investigated by means well developed theory of integrable systems, such as the Darboux transformations, the Lax pair method, Toda chain approach etc. Continuum HSC equation is nonlinear and non-integrable. But a discrete HSC equation admits very promising complete integrable solution. The latter one is the third reason to start from HSC equation and it is crucial of our research plan. We intend to apply this solution to describe the shape of DV and its dynamics, taking benefits from easy approximation and wide numbers of methods of theory of integrable systems.

4d) Magnetic actuator based on partially-covered biphase microwires

(Irina Baraban; Team leader: Valeria Rodionova)

The controlled manipulation of miniaturized objects is essential for the development of novel advanced technologies at the nano and microscale. Nowadays, advanced devices for such manipulation make use of different instruments including optical and magnetic tweezers, piezoelectrics, computer disk drive heads or atomic force microscopy (AFM). Each of these systems provides the unique opportunities for control and study of nano- and micro- objects. Magnetic tweezers have several advantages comparing with other types of tweezers: no restrictions on transparency and the particle size (as in optical tweezers), and they allow the operation in various environments (gas, liquid, vacuum).

Alternative actuators make use of cylindrical symmetry and profit of the uniaxial directional anisotropy. Current magnetic tweezers based on non-magnetic microwires system allow to capture nano- and micro- objects by the stray fields at the microwire end and control the position of the manipulated objects by changing the value of magnetic stray fields.

A novel family of bimagnetic multilayer microwires with additional properties arising from the presence of the two magnetic phases was recently introduced. They contain an internal magnetic nucleus and and external magnetic shell. The proposed invention is a microactuator based on magnetically biphase microwires for the final manipulation of nano- and micro- objects.

Their main characteristics involve: i) Cylindrical symmetry, ii) Biphase character, iii) It profits of the different response of each phase to given agent, iv) The partial coating of the external shell confers to the microactuator specific different characteristic. The main working element of the proposed microactuator is thus partially covered by magnetically biphase microwire. The ferromagnetic materials of the metallic core and external shell have the magnetostriction coefficient of different signs: positive or nearly zero for metallic core and nearly zero or negative for external shell, correspondingly. So at applied magnetic field the internal ferromagnetic core is tending to elongate while the external shell – to shrink. This leads to bending of the microwire that is to shift of the microwire end.

The actuator based on partially covered biphase microwires has the following original aspects:

  1. the absence of the residual field

  2. two operating modes: noncontact and contact