Research

  • Metadynamics

Knowledge of free energy barriers separating different states is critically important for assessment of long-term stability of information stored in magnetic devices. This information, however, is not directly accessible by standard simulations of microscopic models because of the ubiquitous time-scale problem, related to the fact that the transitions among different free energy minima have character of rare events. Here we show that by employing the metadynamics algorithm based on suitably chosen collective variables, namely helicity and circulation, it is possible to reliably recover the free energy landscape. We demonstrate the effectiveness of the new approach on the example of vortex nucleation process in magnetic nanodot with lowered spatial symmetry. With the help of reconstructed free energy surfaces (FES) we show the origin of the symmetry broken vortex nucleation, where one polarity of the nucleated vortex core is preferred, even though only in-plane magnetic field is present.

TóbikJ., Martoňák, R., Cambel, V., : Free-energy landscapes in magnetic systems from metadynamics. Phys. Rev. B 96 (2017) 140413(R). (APVV 0088-12, VEGA 2/0180/14, VEGA 2/0200/14).

  • Study of surface charges in III-N heterostructures for preparation of GaN power switching devices

Heterostructure field effect transistors (HFET) based on gallium nitride (GaN) represent a new promising technology for high-power and mixed-signal electronics. Despite ongoing industrialization of GaN electronic devices, there is still limited understanding on the formation of GaN surface charges, being vital for the device’s threshold voltage adjustment. Specifically, origin of the oxide/semiconductor interface charges in HFETs with metal-oxide-semiconductor (MOS) gate is still under debate. In our works, we demonstrated several technological approaches to manipulate interface charges in GaN MOS-HFETs with gate dielectrics grown by different deposition techniques. First, we evaluated all relevant charges located at oxide/semiconductor interface that were then correlated with microstructural and chemical properties of the interface. The results show possibilities to manipulate the surface charges and indicate their origin. Such knowladge is unavoidable for development of the next-generation high-power and high-speed GaN transistors.

HRTEM micrograph of Al2O3/AlGAN/GaN structure

Ťapajna, M., StoklasR., GregušováD., GucmannF., HušekováK., HaščíkŠ., FröhlichK., Tóth, L., Pecz, B., Brunner, F., KuzmíkJ., : Investigation of ‘surface donors’ in Al2O3/AlGaN/GaN metal-oxide-semiconductor heterostructures: Correlation of electrical, structural, and chemical properties. Applied Surface Sci 426 (2017) 656-661. (CENTE II, APVV 15-0031, VEGA 2/0138/14).

Ťapajna, M., VálikL., GucmannF., GregušováD., FröhlichK., HaščíkŠ., DobročkaE., Tóth, L., Pecz, B., Kuzmík, J., : Low-temperature atomic layer deposition-grown Al2O3 gate dielectric for GaN/AlGaN/GaN MOS HEMTs: Impact of deposition conditions on interface state density. J. Vacuum Sci Technol. B 35 (2017) 01A107. (SAFEMOST, VEGA 2/0138/14, CENTE).

  • Thermo-mechanical analysis of uncooled La0.67Sr0.33MnO3 microbolometer made on circular SOI membrane

We report on thermal and mechanical analysis of uncooled antenna-coupled La0.67Sr0.33MnO3 microbolometer made on circular SOI (Silicon On Insulator) membrane with no limitation in its active area (circular membrane with diameter up to 2 500 mm). A simple method how to investigate the thermal conversion efficiency (thermal resistance value – Rth) is introduced. Thermal analysis is supported by the ANSYS modelling and simulation. It is found that Rth and thermal time constant (t) of our LSMO microbolometer (bolometer sensitivity and time response) can be tuned by the SOI membrane thickness. Rth value as high as 188 K/mW and t value as low as 0.88 ms are estimated from the thermal simulation for SOI membrane with total thickness of 300 nm (SiO2-200 nm, Si-100 nm). Genesis of the induced mechanical stress changes after main processing steps is found and evaluated to explain the mechanical stability of the LSMO based MEMS microbolometer.

LalinskýT., DzubaJ., Vanko, G., Kutiš, V., Paulech, J., Gálik, G., Držík, M., ChromikŠ., and LobotkaP.:Thermo-mechanical analysis of uncooled La0.67Sr0.33MnO3 microbolometer made on circular SOI membrane. Sensors Actuators A 265 (2017) 321–328. (APVV 14-0613, APVV 0450-10, APVV 0455-12, CENTE II).

  • Skyrmion stability in nanodots

In collaboration with Adam Mickiewicz Univeristy in Poznan, Poland and University of the Basque Country, Spain we have studied the stability of skyrmion in nanodots. Magnetic skyrmions are stable, circularly symmetric inhomogeneous magnetization configurations. These topologically protected states of nanometer size have a potential to provide useful solution for low-power, high-density data storage and processing.  Typically, ultrathin layers of ferromagnetic and heavy metal layers with various number of repeats are used to stabilize skyrmions in thin films. We have studied influence of confinement, dipole interactions[Zelent 2017] and pinning [Zeissler 2017] on the skyrmion stability. We show how these effects lead to enhancement of skyrmion stability. We have demonstrated, that due to the lateral confinement and strong magnetostatic interactions introduced by multiple repetition of the layers, the two skyrmions of different sizes can stabilize in the same nanodot  in dependence on the magnetic reversal history [Zelent2017]. Finally, we have proposed a nanodot with the bi-stable skyrmion states at zero magnetic field, with feasible switching between these two states. Bi-stability was also found experimentally in nanodot and our micromagnetic simulations incorporating disorder were used to explain the hysteresis in skyrmion size [Zeissler]. Our result can open a new route to develop an efficient skyrmion memory, where information would be coded as a skyrmion’s equilibrium size.

Zelent, M., TóbikJ., Krawczyk, M., Guslienko, K., Mruczkiewicz, M., : Bi‐stability of magnetic skyrmions in ultrathin multilayer nanodots induced by magnetostatic interaction. Physica Status Solidi RRL 11 (2017) 1700259. (SASPRO 1244/02/01).

Zeissler, K., Mruczkiewicz, M., Finizio, S., Raabe, J., Shepley, P., Sadovnikov, A., Nikitov, S., Fallon, K., McFadzean, S., McVitie, S., Moore, T., Burnell, G., Marrows, C., : Pinning and hysteresis in the field dependent diameter evolution of skyrmions in Pt/Co/Ir superlattice stacks. Sci Rep. 7 (2017) 15125.(SASPRO 1244/02/01).