arXiv subjects

Bohdan Kundys

Publications and source records attributed to Bohdan Kundys.

8 recordsLinked to original sources

arXiv2026

Wavelength-Resolved Control of Photovoltaic Screening and Defect-Mediated Doping in Photo-Ferroelectric/Graphene Devices

Ferroelectrics enable large charge doping of two-dimensional overlayers, but the coexistence of switching and nonswitching charge dynamics complicate electro-optical analysis. Here, we investigate the optoelectronic response of a ferroelectric/graphene device under 365 and 530 nm illumination, disentangling effects on ferroelectric dipole alignment from extrinsic current pathways. Graphene acts as a high-gain sensor, amplifying subtle polarization dynamics into a pronounced resistance difference. By resolving switching and nonswitching channels in dark and illuminated conditions, we reveal a competition between photovoltaic charge screening and defect-assisted excitation that governs device electrostatics. Above-band gap illumination generates free carriers that induce leaky ferroelectric hysteresis and suppress the graphene resistance ratio between opposite remanent polarization states from 290% to 15% due to dynamic photovoltaic charge screening. In contrast, 530 nm illumination primarily induces charge redistribution in ferroelectrics via defect-state excitation, leading to a significantly weaker suppression of the resistance variation of graphene. These results establish practical guidelines for selecting photon energy and intensity to either preserve remanent polarization while tuning channel doping or deliberately reconfigure polarization through optical programming.

cond-mat.other↗

arXiv2026

Phase equilibria in MnSb2Te4-GeSb2Te4 system and magnetic properties of Mn1-xGexSb2Te4 solid solutions

As a sister compound of the antiferromagnetic topological insulator MnBi2Te4, MnSb2Te4 is also a candidate for exotic magnetic topological phases. On the other hand, the structurally analogous but nonmagnetic phase-change material GeSb2Te4 is also known to exhibit nontrivial band topology. Motivated by their shared crystal structure, the similar ionic radii of Mn2+ and Ge2+, and the opportunity to explore the interplay between magnetism and topology, here we investigate the effects of Ge substitution at Mn sites in MnSb2Te4. The Mn1-xGexSb2Te4 solid solutions were synthesized via high-temperature solid-state reaction and characterized for composition, structure, phase behavior, and magnetism using SEM-EDS, PXRD, DTA, and SQUID. Ge substitution was successful across the full composition range, producing homogeneous, single-phase samples that melt via peritectic reactions, as confirmed by the MnSb2Te4GeS-b2Te4 phase diagram. Ge substitution strengthens the sample's paramagnetism, but with ferrimagnetic ordering up to x = 0.75, with both effective moment and saturation magnetization decreasing with increasing Ge content. Two distinct magnetic transitions - high-temperature paramagnetic to ferrimagnetic and low-temperature ferrimagnetic to ferromagnetic - were identified, with a dome-like shape dependence of the low-temperature magnetic transition on Ge substitution. A negative magnetization was observed in the pristine MnSb2Te4 and x = 0.12 substituted samples, while two distinct spin-flop transitions appeared in the samples with x = 0.32 and x = 0.55 Ge substitutions as a result of competing magnetic orderings. These findings facilitate future selective single-crystal growth of homogeneous, magnetic phases, paving the way for magneto-transport and topological surface states investigations.

cond-mat.mtrl-sci↗

arXiv2023

A strain-controlled magnetostrictive pseudo spin valve

Electric-field control of magnetism via inverse magnetostrictive effect is an efficient path towards improving energy-efficient storage and sensing devices based on giant magnetoresistance effect. In this letter, we report on lateral electric-field driven strain-mediated modulation of magnetic properties in Co$/$Cu$/$Py pseudo spin valve grown on ferroelectric PMN-PT substrate. We show a decrease of the giant magnetoresistance ratio of the pseudo spin valve with increasing electric field, which is attributed to the deviation of the Co layer magnetization from the initial direction due to strain-induced magnetoelastic anisotropy contribution. Additionally, we demonstrate that strain-induced magnetic anisotropy effectively shifts the switching field of the magnetostrictive Co layer, while keeping the switching field of the nearly zero-magnetostrictive Py layer unaffected due to its negligible magnetostriction constant. We argue that magnetostrictively optimized magnetic films in properly engineered multilayered structures can offer a path to enhancing the selective magnetic switching in spintronic devices.

cond-mat.mtrl-sci↗

arXiv2020

Helium Ion Microscopy for Reduced Spin Orbit Torque Switching Currents

Spin orbit torque driven switching is a favourable way to manipulate nanoscale magnetic objects for both memory and wireless communication devices. The critical current required to switch from one magnetic state to another depends on the geometry and the intrinsic properties of the materials used, which are difficult to control locally. Here we demonstrate how focused helium ion beam irradiation can modulate the local magnetic anisotropy of a Co thin film at the microscopic scale. Real-time in-situ characterisation using the anomalous Hall effect showed up to an order of magnitude reduction of the magnetic anisotropy under irradiation, and using this, multi-level switching is demonstrated. The result is that spin-switching current densities, down to 800 kA cm$^{-2}$, can be achieved on predetermined areas of the film, without the need for lithography. The ability to vary critical currents spatially has implications not only for storage elements, but also neuromorphic and probabilistic computing.

cond-mat.mes-hall↗

arXiv2020

Reconfigurable single photon sources based on functional materials

The future of quantum photonic technology depends on the realization of efficient sources of single photons, the ideal carriers of quantum information. Parametric downconversion (PDC) is a promising route to create highly coherent, spectrally pure single photons for quantum photonics using versatile group-velocity matching (GVM) and tailored nonlinearities. However, the functionality to actively control the poling period of nonlinear crystals used in PDC is currently missing, yet would enable to dynamically modify the wavelength of single photons produced in the PDC process. Here a detailed GVM study is presented for functional PMN-0.38PT material which can be dynamically repolled at ambient conditions with fields as low as 0.4 kV/mm. Our study reveals phase-matching conditions for spectrally pure single photon creation at 5-6 microns. Further, a practical approach is proposed for on-flight wavelength switching of the created single photons. The reported reconfigurable functionality benefits a wide range of emerging quantum-enhanced applications in the mid-IR spectral region where the choice of single photon sources is currently limited.

physics.optics↗

arXiv2009

Magnetoelectric coupling in polycrystalline FeVO4

We report coupling between magnetic and electric orders for antiferromagnetic polycrystalline FeVO4 in which magnetism-induced polarization has been recently found in noncollinear antiferromagnetic state below the second antiferromagnetic phase transition at TN2=15.7K. In this low symmetry phase space group P-1, the magnetic field dependence of electric polarization evidences a clear magnetoelectric coupling in the noncollinear spin-configured antiferromagnetic phase. The discontinuity of magnetodielectric effect observed at the vicinity of the polar to nonpolar transition evidences competition between different magnetodielectric couplings in the two different antiferromagnetic states. The existence of thermal expansion anomaly near TN2 and magnetostriction effect support magnetoelastically mediated scenario of the observed magnetoelectric effect.

cond-mat.mtrl-sci↗

arXiv2008

Effect of magnetic field and temperature on the ferroelectric loop in MnWO4

The ferroelectric properties of MnWO4 single crystal have been investigated. Despite a relatively low remanent polarization, we show that the sample is ferroelectric. The shape of the ferroelectric loop of MnWO4 strongly depends on magnetic field and temperature. While its dependence does not directly correlate with the magnetocapacitance effect before the paraelectric transition, the effect of magnetic field on the ferroelectric polarization loop supports magnetoelectric coupling.

cond-mat.mtrl-sci↗

arXiv2008

Magnetic field induced ferroelectric loop in Bi0.75Sr0.25FeO3

Magnetic field induced ferroelectric hysteresis loop observed in Bi0.75Sr0.25FeO3-delta is of prime importance. The coexistence of antiferromagnetism and weak ferromagnetism is responsible for the original magnetoelastic and magnetoferroelectric properties. Upon external magnetic field application, the existence of a magnetostrictive effect supports a structural transition towards a homogeneous antiferromagnetic and ferroelectric phase. The magnetic field induced polarization is among the highest reported for BiFeO3 based systems in either thin film or bulk forms (Pr=96 microC/cm2 at 10T) while the ferroelectric coercive field is among the lowest reported (Hc=661(V/cm) at 10T). These properties make this material very attractive for technical applications.

cond-mat.mtrl-sci↗