Oleg Petracic

Forschungszentrum Jülich

Electric field-assisted storage of information using magnetoelectric materials permits a novel class of memory devices. Magnetic nanoparticles can be considered as building blocks for artificial super-structures for magnetoelectric composites. In this work, monodisperse ferrimagnetic nanoparticles (NPs) embedded into ferroelectric barium titanate (...

Iron oxide nanoparticles find a wide variety of applications, including targeted drug delivery and hyperthermia in advanced cancer treatment methods. An important property of these particles is their maximum net magnetization, which has been repeatedly reported to be drastically lower than the bulk reference value. Previous studies have shown that...

Oxygen diffusivity and surface exchange kinetics underpin the ionic, electronic, and catalytic functionalities of complex multivalent oxides. Towards understanding and controlling the kinetics of oxygen transport in emerging technologies, it is highly desirable to reveal the underlying lattice dynamics and ionic activities related to oxygen variati...

Complex oxides provide rich physics related to ionic defects. For the proper tuning of functionalities in oxide heterostructures, it is highly desired to develop fast, effective and low temperature routes for the dynamic modification of defect concentration and distribution. In this work, we report on the use of helium-implantation to efficiently c...

Iron oxide nanoparticles are presently considered as main work horses for various applications including targeted drug delivery and magnetic hyperthermia. Several questions remain unsolved regarding the effect of size onto their overall magnetic behavior. One aspect is the reduction of magnetization compared to bulk samples. A detailed understandin...

Magneto-ionic control of magnetism is a promising route towards the realization of non-volatile memory and memristive devices. Magneto-ionic oxides are particularly interesting for the purpose, exhibiting magnetic switching coupled to resistive switching, with the latter emerging as a perturbation of the oxygen vacancy concentration. Here, we repor...

We report on the observation of strain- and magneto-electric coupling in a system consisting of a thin film of ferromagnetic La (1− x ) Sr x MnO 3 (LSMO, x = 0.5 and 0.3) on a ferroelectric BaTiO 3 (BTO) substrate. Pronounced magnetization steps occur at the BTO structural phase transitions. We associate these steps with a strain induced change of...

We report on the influence of the Al content (from 3.5 to 54 at.%) on phase formation and magnetic properties in FeMnCoCrAl high entropy alloy thin film libraries. Al additions to FeMnCoCr crystallizing in the alpha-Mn structure cause the formation of the body centered cubic (BCC) structure. This is consistent with density functional theory predict...

The manipulation of magnetic states in nanoparticle supercrystals promises new pathways to design nanocrystalline magnetic materials and devices. Trench-patterned silicon substrates were used as templates to guide the self-assembly of iron oxide nanoparticles. Grazing incidence small angle x-ray scattering shows that the nanoparticles form a long-r...

The vacancy distribution of oxygen and its dynamics directly affect the functional response of complex oxides and their potential applications. Dynamic control of the oxygen composition may provide the possibility to deterministically tune the physical properties and establish a comprehensive understanding of the structure–property relationship in...

Ferrimagnetic iron oxide nanoparticle monolayers on top of ferroelectric BaTiO3 substrates were prepared and a magnetoelectric coupling effect was observed. We employed hereby a magnetoelectric AC susceptibility setup as modification of a commercial superconducting quantum interference device magnetometer. The magnetoelectric coefficient shows two...

X-ray photoemission electron microscopy (XPEEM) and magneto-optic Kerr effect (MOKE) microscopy have been performed on a metal-insulator multilayer of [Co80Fe20 (t = 1.8 nm)/Al2O3 (3nm)]9 to image 360◦ domain walls (DWs) along easy and hard axes, respectively. Their creation and annihilation can be directly visualized under application of a magnetic...

The manipulation of magnetism of self-assembled iron oxide nanoparticle (NP) monolayers on top of BaTiO3 (BTO) single crystals is reported. We observe strain induced magnetoelectric coupling (MEC) as shown by measurements of both the magnetization and magneto-electric AC susceptibility (MEACS). The magnetization, coercivity, remanent magnetization...

We report about the magnetic properties of antiferromagnetic (AF) MnO nanoparticles (NPs) with different sizes (6-19nm). Using a combination of polarized neutron scattering and magnetometry we were able to resolve previously observed peculiarities. Magnetometry, on the one hand, reveals a peak in the zero field cooled (ZFC) magnetization curves at...

Zero field cooled (ZFC)/Field Cooled (FC) magnetization curves of a bulk MnO single crystal show a peculiar peak at low temperatures (~40K) similar to the low temperature peak observed in MnO nanoparticles. In order to investigate the origin of this peak, the spin structure of a MnO single crystal has been studied and compared with a single phase p...

In the present work we demonstrate, how the self-assembly of nanoparticles provides a simple and straightforward way to fabricate ordered nanocomposites on length scales of up to 300…1000 µm. We realize this by employing a centrifugation assisted sedimentation technique. We start with magnetic iron oxide nanoparticles (NPs) with an average diameter...

We prepared monolayers of iron oxide nanoparticles via self-assembly on a bare silicon wafer and on a vanadium film sputter deposited onto a plane sapphire substrate. The magnetic configuration of nanoparticles in such a dense assembly was investigated by polarized neutron reflectivity. A theoretical model fit shows that the magnetic moments of nan...

In this review we focus onto the magnetic properties of NP assemblies at variant concentrations. Hence, we consider the effects of interparticle interactions of different strengths giving rise to a variety of magnetic states such as the superparamagnetic (SPM), the superspin glass (SSG) and the superferromagnetic (SFM) ones, altogether forming the...

We report about the fabrication and characterization of iron oxide nanoparticle thin film superlattices. The formation into different film morphologies is controlled by tuning the particle plus solvent-to-substrate interaction. It turns out that the wetting vs. dewetting properties of the solvent before the self-assembly process during solvent evap...

We have studied the effect of Cr capping on the magnetic properties of Co nanoparticles (NPs). The NPs have an average diameter of 2.2 nm. The blocking temperature T-B of the bare Co particles is 13.2 K. By capping with a thin Cr layer up to a thickness of t(Cr) = 0.52 nm, we first observe a decrease of T-B up to t(Cr) = 0.14 nm, followed by an inc...

We have studied the magnetic interaction of circular magnetic islands with a dipole character on a square lattice. The square pattern consists of lithographically prepared polycrystalline PdFe islands, 150 nm in diameter and a periodicity of 300 nm. Below the Curie temperature at 260 K, the islands are in a single domain state with isotropic in-pla...

Nanoparticles (NPs) of cobalt are synthesized in shallow layers of polyimide using 40 keV implantation of Co+ ions with a few different fluences at various ion current densities. Nucleation of individual NPs at low fluencies and their percolation at high fluencies are crucial processes governing the electrical and magnetic properties of the metal/p...

We studied the magnetic and transport properties of Co nanoparticles (NPs) being capped with varying amounts of Pt. Beside field and temperature dependent magnetization measurements we performed delta-M measurements to study the magnetic interactions between the Co NPs. We observe a transition from demagnetizing towards magnetizing interactions bet...

The editors gratefully thank the authors for their contributions to this special issue and the reviewers for their constructive comments and dedication.

Single domain magnetic nanoparticles (MNPs) have been a vivid subject of intense research for the last fifty years. Preparation of magnetic nanoparticles and nanostructures has been achieved by both bottom-up and top-down approaches. Single domain MNPs show Néel-Brown-like relaxation. The Stoner-Wohlfarth model describes the angular dependence of t...

We have investigated the structure and magnetism of self-assembled, 20 nm diameter iron oxide nanoparticles covered by an oleic acid shell for scrutinizing their structural and magnetic correlations. The nanoparticles were spin-coated on an Si substrate as a single monolayer and as a stack of 5 ML forming a multilayer. X-ray scattering (reflectivit...

We have prepared thin films of the ferromagnetic Heusler alloy Co${}_{2}$MnGe on $A$-plane Al${}_{2}$O${}_{3}$ substrates exhibiting growth-induced, superimposed fourfold and uniaxial magnetic anisotropies, the symmetry being determined by the single-crystalline Al${}_{2}$O${}_{3}$ substrate. The magnitude of the uniaxial anisotropy compared to the...

The deliberate control over the spatial arrangement of nanostructures is the desired goal for many applications such as, for example, in data storage, plasmonics or sensor arrays. Here we present a novel method to assist the self-assembly process of magnetic nanoparticles. The method makes use of nanostructured aluminum templates obtained after ano...

We show that by capping Co nanoparticles with small amounts of Pt strong changes of the magnetic properties can be induced. The Co nanoparticles have a mean diameter of 2.7 nm. From magnetometry measurements we find that for zero and for small amounts of Pt (nominal thickness t(Pt) < 0.7 nm) the nanoparticles behave superparamagnetic like. With inc...

We report about a combined structural and magnetometric characterization of self-assembled magnetic nanoparticle arrays. Monodisperse iron oxide nanoparticles with a diameter of 20 nm were synthesized by thermal decomposition. The nanoparticle suspension was spin-coated on Si substrates to achieve self-organized arrays of particles and subsequently...

Discontinuous metal-insulator multilayers (DMIMs) of [CoFe(t(n))/Al2O3](m) containing soft ferromagnetic (FM) Co80Fe20 nanoparticles embedded discontinuously in a diamagnetic insulating Al2O3 matrix are ideal systems to study interparticle interaction effects. Here the CoFe nanoparticles are treated as superspins with random size, position and anis...

Magnetic γ-Fe(2)O(3) nanoparticles with a mean diameter of 20 nm and size distribution of 7% were chemically synthesized and spin-coated on top of a Si-substrate. As a result, the particles self-assembled into a monolayer with hexagonal close-packed order. Subsequently, the nanoparticle array was coated with a Co layer of 20 nm thickness. The magne...

Magnetic single-domain nanoparticles constitute an important model system in magnetism. In particular ensembles of superparamagnetic nanoparticles can exhibit a rich variety of different behaviors depending on the inter-particle interactions. Starting from isolated single-domain ferro- or ferrimagnetic nanoparticles the magnetization behavior of bo...

We report on self-assembled iron oxide nanoparticle films on silicon substrates. In addition to homogeneously assembled layers, we fabricated patterned trenches of 40-1000 nm width using electron beam lithography for the investigation of assisted self-assembly. The nanoparticles with a diameter of 20 nm +/- 7% were synthesized by thermal decomposit...

Antiferromagnetic (AF) nanostructures from Co3O4, CoO and Cr2O3 were prepared by the nanocasting method and were characterized magnetometrically. The field and temperature dependent magnetization data suggests that the nanostructures consist of a core-shell structure. The core behaves as a regular antiferromagnet and the shell as a two-dimensional...

The spin structure and magnetization reversal in Co / Al ₂ O ₃/ Py triple layer nanodots have been investigated both via micromagnetic simulations and experimentally by magneto-optical Kerr effect measurements. Depending on the size, isolated Py dots show either a vortex state or single domain state during magnetization revers...

The domain wall nucleation and motion processes in Permalloy nanowires with a thickness gradient along the nanowire axis have been studied. Nanowires with widths, w = 250 nm to 3 um and a base thickness of t = 10 nm were fabricated by electron-beam lithography. The magnetization hysteresis loops measured on individual nanowires are compared to corr...

The magnetic behavior of antiferromagnetic nanostructures is usually dominated by a core-shell structure. Here we report on a magnetometry study on antiferromagnetic Co3 O4 nanostructures showing a magnetic decoupling of the core and its surface contribution below a critical structure size. Systems with sizes larger than 8 nm yield a common coupled...

We investigated the magnetization reversal of arrays of exchange biased NiO/Ni squares with superconducting quantum interference device magnetometry and micromagnetic simulations. The edges of the squares were 0.5, 1.5, and 3.0 μ m long. The NiO/Ni structures exhibit vortexlike hysteresis loops, and micromagnetic calculations show that this feature...

A coexistence of lateral and in-depth domain walls in antiferromagnet/ferromagnet (AF/FM) thin films exhibiting double hysteresis loops (DHLs) is demonstrated. Comparison of single and DHLs together with local and global measurements confirms the formation of two oppositely oriented domains in the AF that imprint a lateral domain structure into the...

Exchange biased Fe/CoO bilayers with a Cr spacer layer between Fe and CoO layers have been studied using magnetometry. The Cr "dusting" layer thickness was varied between 0 and 10 A. Independent of the growth method we find that the coercive field H(c) drops much faster than the exchange bias field H(eb) as a function of increasing Cr thickness. We...

The magnetic properties and electrical resistance of nanoparticle systems [Co80Fe20(tn)/Al2O3(3 nm)]m are investigated. The temperature dependence of the coercive field for different samples with different tn shows a sudden jump at the nominal thickness 1.4 nm, showing a clear transition from superferromagnetism between nanoparticles in the non-per...

Employing magnetometry measurements, we have studied Co3O4 nanowires focusing on the core-shell behavior. We find two magnetic contributions, i.e., a regular antiferromagnetic and an additional irreversible one. The first contribution can be attributed to the antiferromagnetically ordered wire cores. The nature of the second one can be identified u...

We demonstrate electrical spin injection from ferromagnetic Fe/Tb multilayer structures with remanent perpendicular magnetization into GaAs-based light-emitting diodes at room temperature. Using a reverse-biased Schottky contact and a MgO tunnel contact, respectively, we achieve spin injection at remanence. The maximum degree of circular polarizati...

We investigated the magnetization reversal of arrays of exchange biased NiO/Ni squares with superconducting quantum interference device magnetometry and micromagnetic simulations. The edges of the squares were 0.5, 1.5, and 3.0 $\mu$m long. The NiO/Ni structures exhibit vortexlike hysteresis loops and micromagnetic calculations show that this featu...

We demonstrate a photodetector with ferromagnetic contacts which can electrically detect the polarization degree of incoming light using spin filtering of photoinduced spin-polarized electron currents. Our structure is a pin diode with a single GaAs quantum well as active region and a Fe/Tb multilayer on top of a MgO tunnel barrier as n-contact whe...

Magnetization reversal in a periodic magnetic field is studied on an ultrathin, ultrasoft ferromagnetic Pt=Co�0:5 nm�=Pt trilayer exhibiting weak random domain wall (DW) pinning. The DW motion is imaged by polar magneto-optic Kerr effect microscopy and monitored by superconducting quantum interference device susceptometry. In close agreement with m...

Polarized neutron reflectivity (PNR) studies have been performed on granular multilayers [Co80Fe20(tn)/Al2O3(3nm)]m. For a non-percolating superferromagnetic sample (tn = 1.3 nm) no spin-flip scattering was observed, measured at various points of the hysteresis, confirming that the magnetization reversal is achieved merely by domain nucleation and...

The magnetization reversal of Permalloy (Ni(81)Fe(19)) nanowires has been investigated by magneto-optic Kerr effect (MOKE) magnetometry, where one end of the wire exhibits a slope in the thickness. Straight nanowires with a thickness of 7.5 nm, widths of 150 nm, and length of 100 mu m were prepared by electron-beam lithography. The sloped ends were...

When a ferromagnet is in proximity to an antiferromagnet, lateral length scales such as the respective magnetic domain sizes drastically affect the exchange bias. Bilayers of FeF2 and either Ni, Co or Fe are studied using SQUID and spatially resolved MOKE. When the antiferromagnetic domains are larger than or comparable to the ferromagnetic domains...

The depth dependence of the magnetization has been studied in antiferromagnet/ferromagnet (AF/FM) exchange coupled systems. Results from vector magnetometry and magneto-optical Kerr effect probing both the AF/FM and FM/air interfaces demonstrate the existence of a magnetization depth profile in FeF2/FM (FM=Fe, Ni, and Py) bilayers, contrary to the...

Asymmetric magnetization reversal is an unusual phenomenon in antiferromagnet/ferromagnet (AF/FM) exchange biased bilayers. We investigated this phenomenon in a simple model system experimentally and by simulation assuming inhomogeneously distributed interfacial AF moments. The results suggest that the observed asymmetry originates from the intrins...

We discuss specular reflectivity and off-specular scattering of neutrons and X-rays from magnetic films. Both these techniques are capable of providing information about the morphology of the chemical and magnetic roughness and the magnetic domain structure. The use of neutrons with polarization analysis enables the spatial distribution of differen...

Polarized neutron reflectivity (PNR) and magnetometry studies have been performed on the metal-insulator multilayer ${[{\mathrm{Co}}_{80}{\mathrm{Fe}}_{20}(1.6\phantom{\rule{0.3em}{0ex}}\mathrm{nm})∕{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(3\phantom{\rule{0.3em}{0ex}}\mathrm{nm})]}_{9}$ which exhibits dominant dipolar coupling between the ferromagnetic la...

The magnetization of diluted nanoparticle systems ${[{\mathrm{Co}}_{80}{\mathrm{Fe}}_{20}({t}_{n})∕{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(3\phantom{\rule{0.3em}{0ex}}\mathrm{nm})]}_{10}$ with ${t}_{n}=0.7$ and $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ reveals strong paramagnetic contributions at low temperatures. They are due to atomically small magne...

Independently exchange biased subsystems can coexist in ferromagnet/antiferromagnet bilayers after various field-cooling protocols. We find well separated double hysteresis loops in FeF2/Ni bilayers for intermediate cooling fields, while for small or large cooling fields a negatively or positively shifted single loop, respectively, are encountered....