arXiv subjects

Robert Laskowski

Publications and source records attributed to Robert Laskowski.

7 recordsLinked to original sources

arXiv2022

Oxygen Octahedral Tilt Controlled Topological Hall Effect in Epitaxial and Freestanding SrRuO3/SrIrO3 Heterostructures

The fabrication technique of freestanding oxide flakes by epitaxial lift-off has made significant contributions to the multifunctional oxide thin film research. Several highly impactful work have recently demonstrated the robustness of freestanding oxide flakes retaining their desirable properties after detachment from the substrate by dissolving away the water-soluble Sr3Al2O6 buffer layer. Firstly, in epitaxial SrRuO3/SrIrO3 systems showing Hall-humps reminiescence of Topological Hall Effect, first-principle calculations revealed that octahedral tilts can modify the sign of Dzyaloshinskii-Moriya interaction and avoid cancellation in a symmetric trilayer structure. Secondly, freestanding flakes of the trilayer is also able to retain the Hall-humps across a wide temperature range. The behaviour of humps' peak field does not vary with field direction rotation away from surface normal, consistent to a micromagnetic simulation result of N\'eel-type magnetic Skyrmions. The layer-resolved octahedral tilts in the freestanding heterostructures also crucially controls the occurrence of Hall-humps, consistent to the insight from the epitaxial ones. This work offers a new perspective to understanding Hall-humps in perovskite oxides, as well as demonstrates the fabrication of oxide heterostructure membranes with high interfacial quality.

cond-mat.mtrl-sci↗

arXiv2021

Quantifying symmetric exchange in ultrathin ferromagnetic films with chirality

The symmetric (Heisenberg) exchange interaction is fundamental to magnetism and assumes critical importance in designing magnetic materials for novel emergent phenomena and device applications. However, quantifying exchange is extremely challenging for ultrathin ($\sim$ 1 nm) magnetic films, as techniques and approximations reliably used for bulk materials are largely inapplicable in the two-dimensional (2D) limit. Here we present and contrast the measurement of exchange stiffness, $A$, by several methods on a series of five Co/Pt-based ultrathin ($1-2$ nm) films. We compare results from (a) spin-wave spectroscopy by Brillouin light scattering (BLS), (b) three analytical models describing the temperature dependence of magnetization obtained by magnetometry, (c) microscopic domain periodicity measurements and simulations, and (d) ab initio density functional theory (DFT) calculations. While different methods present some qualitatively consistent trends across samples, we note, for any given sample, considerable differences (up to $5\times$) in the absolute values of $A$ across the techniques, consistent with discrepancies of $A$ reported in literature for nominally similar samples. We analyze possible sources of the discrepancies across various methods, notably including their relationship to the spin-wave dispersion, and the wave-vector ranges probed. We compare the strengths and limitations of the techniques, and outline directions for their future use in characterizing exchange interactions in ultrathin films.

cond-mat.mtrl-sci↗

arXiv2021

Equilibrium Energy and Entropy of Vortex Filaments on a Cubic Lattice: A Localized Transformations Algorithm

In this work we propose a new algorithm for the computation of statistical equilibrium quantities on a cubic lattice when both an energy and a statistical temperature are involved. We demonstrate that the pivot algorithm used in situations such as protein folding works well for a small range of temperatures near the polymeric case, but it fails in other situations. The new algorithm, using localized transformations, seems to perform well for all possible temperature values. Having reliably approximated the values of equilibrium energy, we also propose an efficient way to compute equilibrium entropy for all temperature values. We apply the algorithms in the context of suction or supercritical vortices in a tornadic flow, which are approximated by vortex filaments on a cubic lattice. We confirm that supercritical (smooth, "straight") vortices have the highest energy and correspond to negative temperatures in this model. The lowest-energy configurations are folded up and "balled up" to a great extent. The results support A. Chorin's findings that, in the context of supercritical vortices in a tornadic flow, when such high-energy vortices stretch, they need to fold.

cond-mat.stat-mech↗

arXiv2021

Tuning Dzyaloshinskii-Moriya Interaction in Ferrimagnetic GdCo: A First Principles Approach

We present a systematic analysis of our ability to tune chiral Dzyaloshinskii-Moriya Interactions (DMI) in compensated ferrimagnetic Pt/GdCo/Pt1-xWx trilayers by cap layer composition. Using first principles calculations, we show that the DMI increases rapidly for only ~ 10% W and saturates thereafter, in agreement with experiments. The calculated DMI shows a spread in values around the experimental mean, depending on the atomic configuration of the cap layer interface. The saturation is attributed to the vanishing of spin orbit coupling energy at the cap layer and the simultaneous constancy at the bottom interface. Additionally, we predict the DMI in Pt/GdCo/X (X=Ta, W, Ir) and find that W in the cap layer favors a higher DMI than Ta and Ir that can be attributed to the difference in d-band alignment around the Fermi level. Our results open up exciting combinatorial possibilities for controlling the DMI in ferrimagnets towards nucleating and manipulating ultrasmall high-speed skyrmions.

cond-mat.mtrl-sci↗

arXiv2013

Oxide Heterostructures for Efficient Solar Cells

We propose an unexplored class of absorbing materials for high-efficiency solar cells: heterostructures of transition-metal oxides. In particular, LaVO_3 grown on SrTiO_3 has a direct band gap ~1.1 eV in the optimal range as well as an internal potential gradient, which can greatly help to separate the photo-generated electron-hole pairs. Furthermore, oxide heterostructures afford the flexibility to combine LaVO_3 with other materials such as LaFeO_3 in order to achieve even higher efficiencies with band-gap graded solar cells. We use density-functional theory to demonstrate these features.

cond-mat.mtrl-sci↗

arXiv2012

First Principles Calculation of Helical Spin Order in Iron Perovskite SrFeO3 and BaFeO3

Motivated by recent discovery of ferromagnetism in cubic perovskite BaFeO3 under small magnetic field, we investigate spin order in BaFeO3 and isostructual SrFeO3 by the first principles calculation. The on-site Coulomb and exchange interactions are necessary for the helical spin order consistent with experiments. SrFeO3 exhibits stable G-type helical order, while A- and G-type helical orders in BaFeO3 are almost degenerate at short propagating vector with tiny energetic barrier with respect to ferromagnetic spin order, explaining ferromagnetism under small field. The results are consistent with model calculation where negative charge-transfer energy is explicitly taken into account.

cond-mat.str-el↗

arXiv2004

The role of intra-atomic non-collinear magnetization density in weak ferromagnetism

We investigate the mechanism behind the breakdown of the compensation of large magnetic moments leading to weak ferromagnetism. For this we use first-principles calculations within density functional theory and we focus on the weak ferromagnetic compound Mn3Sn. Our new implementation allows for an exact treatment of the spin-density matrix and non-collinearity. In order to gain some insight, our results are compared to the ones obtained by using the atomic moment approximation (AMA) and its role is discussed. We find that the appearance of the weak ferromagnetic moment in this compound originates not so much as an effect of spin-orbit coupling as suggested previously from AMA calculations, as from the non-collinearity of the Sn atom magnetization density. This is confirmed by non-collinear calculations in which the SOC effects are neglected.

cond-mat.other↗