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O. Heinonen

Publications and source records attributed to O. Heinonen.

17 recordsLinked to original sources
arXiv2022

Collective ferromagnetism of artificial square spin ice

We have studied the temperature and magnetic field dependence of the total magnetic moment of large-area permalloy artificial square spin ice arrays. The temperature dependence and hysteresis behavior are consistent with the coherent magnetization reversal expected in the Stoner-Wohlfarth model, with clear deviations due to inter-island interactions at small lattice spacing. Through micromagnetic simulations, we explore this behavior and demonstrate that the deviations result from increasingly complex magnetization reversal at small lattice spacing, induced by inter-island interactions, and depending critically on details of the island shapes. These results establish new means to tune the physical properties of artificial spin ice structures and other interacting nanomagnet systems, such as patterned magnetic media.

cond-mat.mes-hall↗
arXiv2022

Topological response of the anomalous Hall effect in MnBi2Te4 due to magnetic canting

Three-dimensional (3D) compensated MnBi2Te4 is antiferromagnetic, but undergoes a spin-flop transition at intermediate fields, resulting in a canted phase before saturation. In this work, we experimentally show that the anomalous Hall effect (AHE) in MnBi2Te4 originates from a topological response that is sensitive to the perpendicular magnetic moment and to its canting angle. Synthesis by molecular beam epitaxy allows us to obtain a large-area quasi-3D 24-layer MnBi2Te4 with near-perfect compensation that hosts the phase diagram observed in bulk which we utilize to probe the AHE. This AHE is seen to exhibit an antiferromagnetic response at low magnetic fields, and a clear evolution at intermediate fields through surface and bulk spin-flop transitions into saturation. Throughout this evolution, the AHE is super-linear versus magnetization rather than the expected linear relationship. We reveal that this discrepancy is related to the canting angle, consistent with the symmetry of the crystal. Our findings suggests that novel topological responses may be found in non-collinear ferromagnetic, and antiferromagnetic phases.

cond-mat.mes-hall↗
arXiv2022

Magnetic ground states of a model for (TM)Nb$_3$S$_6$, TM=Co, Fe, Ni

The transition-metal intercalated dichalcogenide CoNb$_3$S$_6$ is a triangular antiferromagnet that has recently been shown to exhibit a large anomalous Hall effect (AHE) below the Néel temperature, even though the response to an external field is very small. This suggests that there is an interesting magnetic structure that interacts with the electronic structure to yield the AHE, as collinear antiferromagnets cannot exhibit a nonzero AHE. We propose a model for magnetic transition-metal intercalated dichalcogenides and examine its ground state as function of interaction parameters. The model exhibits transitions between planar spin spirals, non-planar spin spirals, and a particular non-coplanar so-called $3q$ state. This latter state must exhibit a nonzero AHE, while the spin spirals do not.

cond-mat.mtrl-sci↗
arXiv2021

Metamagnetism of few layer topological antiferromagnets

MnBi$_2$Te$_4$ (MBT) is a promising antiferromagnetic topological insulator whose films provide access to novel and technologically important topological phases, including quantum anomalous Hall states and axion insulators. MBT device behavior is expected to be sensitive to the various collinear and non-collinear magnetic phases that are accessible in applied magnetic fields. Here, we use classical Monte Carlo simulations and electronic structure models to calculate the ground state magnetic phase diagram as well as topological and optical properties for few layer films with thicknesses up to six septuple layers. Using magnetic interaction parameters appropriate for MBT, we find that it is possible to prepare a variety of different magnetic stacking sequences, some of which have sufficient symmetry to disallow non-reciprocal optical response and Hall transport coefficients. Other stacking arrangements do yield large Faraday and Kerr signals, even when the ground state Chern number vanishes.

cond-mat.str-el↗
arXiv2020

Controllable skyrmion chirality in ferroelectrics

Chirality, an intrinsic handedness, is one of the most intriguing fundamental phenomena in nature. Materials composed of chiral molecules find broad applications in areas ranging from nonlinear optics and spintronics to biology and pharmaceuticals. However, chirality is usually an invariable inherent property of a given material that cannot be easily changed at will. Here, we demonstrate that ferroelectric nanodots support skyrmions the chirality of which can be controlled and switched. We devise protocols for realizing control and efficient manipulations of the different types of skyrmions. Our findings open the route for controlled chirality with potential applications in ferroelectric-based information technologies.

cond-mat.mes-hall↗
arXiv2016

The effect of Ta oxygen scavenger layer on HfO$_2$-based resistive switching behavior: thermodynamic stability, electronic structure, and low-bias transport

Reversible resistive switching between high-resistance and low-resistance states in metal-oxide-metal heterostructures makes them very interesting for applications in random access memories. While recent experimental work has shown that inserting a metallic "oxygen scavenger layer" between the positive electrode and oxide improves device performance, the fundamental understanding of how the scavenger layer modifies heterostructure properties is lacking. We use density functional theory to calculate thermodynamic properties and conductance of TiN/HfO$_2$/TiN heterostructures with and without Ta scavenger layer. First, we show that Ta insertion lowers the formation energy of low-resistance states. Second, while the Ta scavenger layer reduces the Schottky barrier height in the high-resistance state by modifying the interface charge at the oxide-electrode interface, the heterostructure maintains a high resistance ratio between high- and low-resistance states. Finally, we show that the low-bias conductance of device on-states becomes much less sensitive to the spatial distribution of oxygen removed from the HfO$_2$ in the presence of the Ta layer. By providing fundamental understanding of the observed improvements with scavenger layers, we open a path to engineer interfaces with oxygen scavenger layers to control and enhance device performance. In turn, this may enable the realization of a non-volatile low-power memory technology with concomitant reduction in energy consumption by consumer electronics and significant benefits to society.

cond-mat.mes-hall↗
arXiv2016

Dynamic response of an artificial square spin ice

Magnetization dynamics in an artificial square spin-ice lattice made of Ni80Fe20 with magnetic field applied in the lattice plane is investigated by broadband ferromagnetic resonance spectroscopy. The experimentally observed dispersion shows a rich spectrum of modes corresponding to different magnetization states. These magnetization states are determined by exchange and dipolar interaction between individual islands, as is confirmed by a semianalytical model. In the low field regime below 400 Oe a hysteretic behavior in the mode spectrum is found. Micromagnetic simulations reveal that the origin of the observed spectra is due to the initialization of different magnetization states of individual nanomagnets. Our results indicate that it might be possible to determine the spin-ice state by resonance experiments and are a first step towards the understanding of artificial geometrically frustrated magnetic systems in the high-frequency regime.

cond-mat.mes-hall↗
arXiv1997

Ensemble Density Functional Approach to Charge-Spin Textures in Inhomogeneous Quantum Hall Systems

We extend our ensemble density functional approach to quantum Hall systems to include non-collinear spins to study charge-spin textures in inhomogeneous quantum Hall systems. We have studied the edge reconstruction in quantum dots at unit bulk filling factor and at 1/3 bulk filling factor as a function of the stiffness of an external confining potential. For soft enough edges, these systems reconstruct to a state in which the electron spins rotate gently as the edge is approached, with a non-trivial spin-charge texture at the edge of the system.

cond-mat.mes-hall↗
arXiv1997

Spin Ensemble Density Functional Theory for Inhomogeneous Quantum Hall Systems

We have developed an ensemble density functional theory which includes spin degrees of freedom for nonuniform quantum Hall systems. We have applied this theory using a local-spin-density approximation to study the edge reconstruction of parabolically confined quantum dots. For a Zeeman splitting below a certain critical value, the edge of completely polarized maximum density droplet reconstructs into a spin-unpolarized structure. For larger Zeeman splittings, the edge remains polarized and develops an exchange hole.

cond-mat.mes-hall↗
arXiv1996

Ensemble Density Functional Theory for Inhomogeneous Fractional Quantum Hall Systems

The fractional quantum Hall effect (FQHE) occurs at certain magnetic field strengths B*(n) in a two-dimensional electron gas of density n at strong magnetic fields perpendicular to the plane of the electron gas. At these magnetic fields strengths, the system is incompressible, i.e., there is a finite cost in energy for creating charge density fluctuations in the bulk, while the boundary of the electron gas has gapless modes of density waves. The bulk energy gap arises because of the strong electron-electron interactions. While there are very good models for infinite homogeneous systems and for the gapless excitations of the boundary of the electron gas, computational methods to accurately model finite, inhomogeneous systems with more then about ten electrons have not been available until very recently. We will here review an ensemble density functional approach to studying the ground state of large inhomogeneous spin polarized FQHE systems.

cond-mat↗
arXiv1995

Electron-phonon interactions on a single-branch quantum Hall edge

We consider the effect of electron-phonon interactions on edge states in quantum Hall systems with a single edge branch. The presence of electron-phonon interactions modifies the single-particle propagator for general quantum Hall edges, and, in particular, destroys the Fermi liquid even at integer filling. The effect of the electron-phonon interactions may be detected experimentally in the AC conductance or in the tunneling conductance between integer quantum Hall edges.

cond-mat↗
arXiv1995

Integer quantum Hall effect for hard-core bosons and a failure of bosonic Chern-Simons mean-field theories for electrons at half-filled Landau level

Field-theoretical methods have been shown to be useful in constructing simple effective theories for two-dimensional (2D) systems. These effective theories are usually studied by perturbing around a mean-field approximation, so the question whether such an approximation is meaningful arises immediately. We here study 2D interacting electrons in a half-filled Landau level mapped onto interacting hard-core bosons in a magnetic field. We argue that an interacting hard-core boson system in a uniform external field such that there is one flux quantum per particle (unit filling) exhibits an integer quantum Hall effect. As a consequence, the mean-field approximation for mapping electrons at half-filling to a boson system at integer filling fails.

cond-mat↗
arXiv1995

Ensemble density functional theory of the fractional quantum Hall effect

We develop an ensemble density functional theory for the fractional quantum Hall effect using a local density approximation. Model calculations for edge reconstructions of a spin-polarized quantum dot give results in good agreement with semiclassical and Hartree-Fock calculations, and with small system numerical diagonalizations. This establishes the usefulness of density functional theory to study the fractional quantum Hall effect, which opens up the possibility of studying inhomegeneous systems with many more electrons than has heretofore been possible.

cond-mat↗
arXiv1994

Nonlinear Steady-State Mesoscopic Transport, I. Formalism

We present an approach to steady-state mesoscopic transport based on the maximum entropy principle formulation of nonequilibrium statistical mechanics. This approach is valid in the nonlinear regime of high current, and yields the quantization observed in the integer quantum Hall effect at large currents. A key ingredient of this approach, and its success in explaining high-precision Hall measurements, is that the occupancy of single-electron states depends on their current as well as their energy. This suggests that the reservoir picture commonly used in mesoscopic transport is unsatisfactory outside of linear response.

cond-mat↗
arXiv1993

Density matrix for a mesoscopic current ribbon

We consider an ideal mesoscopic ribbon in which a steady azimuthal current is generated. We show that the closed interacting electron system in the presence of the current is described by a density matrix which is that of an equilibrium system without current but with a constrained Hamiltonian.

cond-mat↗
arXiv1993

Mesoscopic transport beyond linear response

We present an approach to steady-state mesoscopic transport based on the maximum entropy principle formulation of nonequilibrium statistical mechanics. Our approach is not limited to the linear response regime. We show that this approach yields the quantization observed in the integer quantum Hall effect at large currents, which until now has been unexplained. We also predict new behaviors of non-local resistances at large currents in the presence of dirty contacts.

cond-mat↗