arXiv subjects

Johan van Lierop

Publications and source records attributed to Johan van Lierop.

6 recordsLinked to original sources

arXiv2026

Positive Temperature Coefficient of Anisotropy due to $d^6L$ Groundstate in $ε$-Fe$_2$O$_3$

Positive anisotropy temperature coefficients ($dK/dT>0$) usually arise from hybridization between magnetic 3d states and strongly spin-orbit-coupled (SOC) subsystems. Yet $ε$-Fe$_2$O$_3$ shows $dK/dT>0$ (125-200~K) without an obvious SOC partner. We study pure and Cr-doped (with weakened Fe-O hybridization) $ε$-Fe$_2$O$_3$ across the transition from low temperature incommensurate to high-anisotropy magnetic phases. We identify a $d^6L$ groundstate in $ε$-Fe$_2$O$_3$ while reduced hybridization yields $d^6 + d^6L^2$ in the Cr-doped system, highlighting metal-ligand hybridization as a route to tune magnetic, electronic and orbitronic properties.

cond-mat.str-el↗

arXiv2025

Monopole current control in artificial spin ice via localized fields

Artificial spin ice systems are metamaterials composed of interacting nanomagnets arranged on a lattice, exhibiting geometrical frustration and emergent phenomena such as monopole excitations. We explore magnetization dynamics and monopole current control in square artificial spin ice with added vertical control elements. Using Monte Carlo simulations, we examine how localized magnetic fields from these elements influence vertex configurations and domain propagation, enabling directional and polarity control of monopole currents. The control elements suppress monopole nucleation along one edge, steering monopole flow across the lattice, sometimes even against the applied field direction. These elements also reshape the system's energy landscape, producing tailored hysteresis and guided state transitions. Our results offer a strategy for manipulating collective behaviours in artificial spin ice using localized fields. This has implications for magnetic memory, physical reservoir computing, enabling reconfigurable magnetic logic and spin-based information processing, and device architectures requiring directional magnetic charge transport.

cond-mat.mes-hall↗

arXiv2024

Active Inference Demonstrated with Artificial Spin Ice

A numerical model of interacting nanomagnetic elements is used to demonstrate active inference with a three dimensional Artificial Spin Ice structure. It is shown that thermal fluctuations can drive this magnetic spin system to evolve under dynamic constraints imposed through interactions with an external environment as predicted by the neurological free energy principle and active inference. The structure is defined by two layers of magnetic nanoelements where one layer is a square Artificial Spin Ice geometry. The other magnetic layer functions as a sensory filter that mediates interaction between the external environment and the hidden Artificial Spin Ice layer. Spin dynamics displayed by the bilayer structure are shown to be well described using a continuous form of a neurological free energy principle that has been previously proposed as a high level description of certain biological neural processes. Numerical simulations demonstrate that this proposed bilayer geometry is able to reproduce theoretical results derived previously for examples of active inference in neurological contexts.

cond-mat.mes-hall↗

arXiv2019

Temperature and field evolution of site-dependent magnetism in $ε$-Fe$_2$O$_3$ nanoparticles

8~nm epsilon-Fe2O3 nanoparticles exhibit a spin reorientation transition that begins at 150 K which is a hallmark of this unique iron-oxide polymorph. We find that the change from the high to low temperature magnetic structures has been suppressed by ~50 K. At the spin reorientation temperature, a change of the field-dependent response of the tetrahedral sites in intermediate field strengths (0.25 - 1.5 T) indicates that a collective tetrahedral distortion occurs to which the octahedral sites adjust, altering the magnetic anisotropy. An abrupt step in the hyperfine parameters' temperature dependencies, especially at 125 K for the hyperfine field associated with the Fe4 tetrahedral sites, suggests strongly a change in the superexchange pathways are responsible for the spin reorientation.

cond-mat.mes-hall↗

arXiv2019

Role of Ce 4f hybridization in the origin of magnetism in nanoceria

Nanoscale CeO2 (nanoceria) is a prototypical system that presents d0 ferromagnetism. Using a combination of x-ray absorption spectroscopy, x-ray magnetic circular dichroism and modelling, we show that nanostructure, defects and disorder, and non-stoichiometry create magnetically polarized Ce 4f and O 2p hybridized states captured by the vacancy orbitals (Vorb) that are vital to ferromagnetism. Further, we demonstrate that foreign ions (Fe and Co) enhance the moment at Ce 4f sites while the number of Vorb is unchanged, pointing clearly to the mechanism of orbital hybridization being key missing ingredient to understanding the unexpected ferromagnetism in many nanoscale dilute magnetic oxides and semiconductors.

cond-mat.mtrl-sci↗

arXiv2016

Incommensurate crystal supercell and polarization-flop observed in the magnetoelectric ilmenite, MnTiO3

MnTiO3 has been studied for many decades, but it was only in the last few years that its strong magnetoelectric behavior had been observed. Here, we use neutron scattering on two separately grown single crystals and two powder samples to show the presence of a supercell that breaks R-3 symmetry. We also present the temperature and field dependence of the dielectric constant and pyroelectric current, and show evidence of non-zero off-diagonal magnetoelectric tensor elements (forbidden by R-3 symmetry) followed by a polarization flop accompanying the spin flop transition at μ0HSF = 6.5 T. Mössbauer spectroscopy on MnTiO3 gently doped with 57Fe was used to help shed light on the impact of the supercell on the observed behavior. Although the full supercell structure could not be solved at this time due to a lack of visible reflections, the full-scope of the results presented here suggest that the role of local spin-lattice coupling in the magnetoelectric properties of MnTiO3 is likely more important than previously thought.

cond-mat.mtrl-sci↗