Johan Van Lierop

University of Manitoba | UMN · Department of Physics and Astronomy

Graphene‐based magnetic materials exhibit novel properties and promising applications in the development of next‐generation spintronic devices. Modern synthesis techniques have paved the way to design precisely the local environments of metal atoms anchored onto a nitrogen‐doped graphene matrix. Herein, it is demonstrated that grafting cobalt (Co)...

Employing the elemental sensitivity of x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD), we study the valence and magnetic order in the heavy fermion superconductor CeCoIn$_5$. We probe spin population of the f-electrons in Ce and d-electrons in Co as a function of temperature (down to 0.1 K) and magnetic field (up t...

The preparation of radicals with intense and redox‐switchable absorption beyond 1000 nm is a long‐standing challenge in the chemistry of functional dyes. We now report a series of unprecedented stable neutral nickel(II) and copper(II) complexes of “Manitoba‐dipyrromethenes” (MB‐DIPYs) in which the organic chromophore is present in the anion–radical...

The preparation of radicals with intense and redox-switchable absorption beyond 1000 nm is a long-standing challenge in the chemistry of functional dyes. Here we report the preparation of a series of unprecedented stable neutral nickel(II) and copper(II) complexes of "Manitoba-dipyrromethenes" (MB-DIPYs) in which the organic chromophore is present...

Multi-metal oxide (MMO) materials have significant potential to facilitate various demanding reactions by providing additional degrees of freedom in catalyst design. However, fundamental understanding of the (electro)catalytic activity of MMOs is limited because of the intrinsic complexity of multi-element nature. Additional complexities arise when...

Multi‐Metal Oxide (Electro)catalysts Multi‐metal oxide (MMO) nanomaterials have significant potential to facilitate various demanding (electro)catalytic reactions, but their intrinsic complexity hinders in‐depth understanding of the origin of the catalytic activity. In article number 2107868, Kug‐Seung Lee, Seoin Back, Yung‐Eun Sung, Taeghwan Hyeon...

Morphological, structural and magnetic properties of Co nanowire/IrMn film heterostructures are studied by atomic force microscopy, non-coplanar X-ray diffraction and magnetization measurements, the latter performed with magnetic field parallel and perpendicular to the nanowire length direction and using remnant (as-grown) and field-cooling protoco...

Although strain underpins the behavior of many transition‐oxide‐based magnetic nanomaterials, it is elusive to quantify. Since the formation of orbital molecules is sensitive to strain, a metal–insulator transition should be a window into nanocrystallite strain. Using three sizes of differently strained Fe3O4 polycrystalline nanorods, the impact of...

Density Functional Theory (DFT) calculations coupled with several exchange-correlation functionals were used for the prediction of Mössbauer hyperfine parameters of 36 bis-axially coordinated iron(II) phthalocyanine complexes with the general formulas PcFeL2, PcFeL'L″, and [PcFeX2]2-, including four new compounds. Both gas-phase and PCM calculation...

A series of Fe and Ru coordination complexes supported by bidentate P^N ligands bearing π-extended phenanthridine (3,4-benzoquinoline; PhenLR,R') or quinoline (QuinLH,Ph) N-heterocycles have been synthesized and fully characterized. Tetrahedral monomeric (P^N)FeBr2 and trigonal bipyramidal dimeric ((P^N)FeBr)2(μ-Br)2 complexes were found to be high...

In Cu-doped Co3O4 nanoparticles (CuxCo3−xO4;0≤x≤0.5) Cu occupies both octahedral and tetrahedral sites with a 2+ oxidation state. As the Cu doping increases, we observe changes in the crystal structure corresponding to a Jahn-Teller distortion of the Cu2+ sites. To mediate charge balance with Cu2+ entering the octahedral sites, a hole forms in the...

The synthesis, structures and electronic characterization of three strongly coloured, pseudo-octahedral Ni(II) complexes supported by redox-active diarylamido ligands featuring benzannulated N-heterocyclic donor arms are reported. The S = 1 paramagnets each present two singly occupied molecular orbitals (SOMOs) identified as metal-based by density...

The applications of low energy argon or oxygen ion-beam assisted deposition and high energy oxygen ion implantation for thin film fabrication and modification of the microstructural, compositional and magnetic properties are demonstrated. Five representative model systems are discussed with the state-of-the art characterization tools such as atomic...

We have synthesized three different shapes of Co3O4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox{Co}_{3}\hbox{O}_{4}$$\end{document} nanoparticles to investigat...

The photo-spin-voltaic effect is revealed by the presence of a spin voltage generated by photons when a non-magnetic metal (e.g., Pt) is in close proximity to a ferrimagnetic insulator (e.g., Y3Fe5O12 (YIG)). This is attributed to the excited electrons and holes diffusing from the proximized layer near the interface to the metallic surface. By usin...

Magnetic nanodot structures have significant potential in spintronic applications. The magnetism of nanodots are predominately influenced by external conditions such as temperature. While the magnetization reversal modes of sub-100 nm Fe nanodots has been reported, currently there is a lack of information about the temperature evolution of the magn...

A Cu-substituted, Co-based spinel catalyst (CuxCo3-xO4) is introduced for direct NO decomposition to N2 and O2. A series of CuxCo3-xO4 catalysts with varying Cu content (0 ≤ x ≤ 1) were synthesized via a co-precipitation method. Reactivity for direct NO decomposition was measured at 450 °C, with the maximum activity of 2.8 × 10⁻² [(µmol NO to N2) g...

A modified magnetron sputtering setup was used to fabricate planar nanowire (NW) arrays of Co. The structural characterization of these polycrystalline Co NW arrays confirms the coexistence of both hcp and fcc phases, in which their relative fractions depend on the Co deposition time as well as the presence (or absence) of a bcc nanowire seed layer...

Biofilms that form on reusable medical devices are a cause of hospital acquired infections; however, sanitization of biofilms is a challenge due to their dense extracellular matrix. This work presents an innovative strategy using biocide-loaded iron oxide nanoparticles transported within the matrix via a magnetic field to eradicate biofilms. Result...

Exchange-coupled ferromagnetic/antiferromagnetic (FM/AF) bilayers are widely used in spin valves, magnetic tunnel junctions and spin-orbit torque devices. While most studies focused on AF layers exchange coupled with single FM layer, the exchange interaction with FM multilayer was not well understood. In this work, [Co/Ni]n/PtMn multilayers with di...

The electronic structures of a set of PcFe(azole) 2 complexes (azole = imidazole, [Formula: see text]-methylimidazole, pyrazole, isoxazole, thiazole, 1,2,4-triazole, 3-amino-1,2,4,-triazole, and 5-amino-1,2,3,4-tetrazole) were examined by Mössbauer spectroscopy and Density Functional Theory (DFT) calculations. In addition, the geometric distortions...

Replacing current benchmark rare-element photosensitizers with ones based on abundant and low-cost metals such as iron would help facilitate the large-scale implementation of solar energy conversion. To do so, the ability to extend the lifetimes of photogenerated excited states of iron complexes is critical. Here, we present a sensitizer design in...

Fe and Fe–O dispersed on nanoscale CeO supports are considered promising catalysts for oxidation and reduction catalysis and regarded as one of the better alternatives to precious metal catalysts for NO reduction. To understand the role of Fe ions, promoters, and factors that control the NOx reduction, we probed the local environments of Fe, Ce, an...

The strength of exchange bias can be influenced by interface roughness and antiferromagnetic morphology. Here, we studied the interface profile of an exchange biased, nanocrystalline Ni80Fe20/α-Fe2O3 bilayer. Magnetometry determined the bilayer’s exchange bias is observed below a blocking temperature of 75 K. Polarized neutron reflectometry measure...

8-nm ε−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 tetrahedra...

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 tetra...

A study of spherical maghemite nanoparticles on a two-dimensional triangular array was carried out using a stochastic Landau-Lifshitz-Gilbert approach. The simulation method was first validated with a triangular array of simple dipoles, where results show the expected phase transition to a ferromagnetic state at a finite temperature. The ground sta...

We show that the magnetism of Fe/Cu nanostructured thin films can be controlled by adjusting an interfacial FeCu alloy formed during in situ ion-beam bombardment of the Fe atoms, promoting intermixing between the Fe and Cu, forming an FeCu alloy. This intermixing results in prominent changes to the inter- and intra-crystallite interactions which go...

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 modeling, we show that the nanostructure, defects and disorder, and nonstoichiometry create magnetically polarized Ce 4f and O 2p hybridized states captured by the vacancy...

We report a synthesis strategy to simplify often cumbersome post-synthesis ligand exchange protocols and use that approach to synthesize EDTA-Na3 (N -(trimethoxysilylpropyl)ethylenediaminetriacetate, trisodium salt) functionalized hydrophilic and biocompatible Fe3O4 nanoparticles. The grafting density of EDTA-Na3 has been controlled from 0.07-0.37...

A study of spherical maghemite nanoparticles on a two dimensional triangular array was carried out using a stochastic Landau-Lifshitz-Gilbert (sLLG) approach. The simulation method was first validated with a triangular array of simple dipoles, where results show the expected phase transition to a ferromagnetic state at a finite temperature. The gro...

A main feature of biofilms is the self-produced extracellular polymeric substances (EPSs) that act as a protective shield, preventing biocide penetration. We use magnetic iron oxide nanoparticles (MNPs) in combination with magnetic fields to damage the biofilm matrix and cause detachment. A Methicillin-resistant Staphylococcus aureus (MRSA) biofilm...

In this study, the AC magnetic hyperthermia responses of spinel CoFe2O4, MnFe2O4 and NiFe2O4 nanoparticles of comparable sizes (∼20 nm) was investigated to evaluate their feasibility of use in magnetic hyperthermia. The heating ability of EDT-coated nanoparticles which were dispersed in the two different carrier media, DI H2O and EG, at concentrati...

In this study, the ac magnetic hyperthermia responses of spinel CoFe2O4, MnFe2O4, and NiFe2O4 nanoparticles of comparable sizes (∼20 nm) were investigated to evaluate their feasibility of use in magnetic hyperthermia. The heating ability of EDT-coated nanoparticles which were dispersed in two different carrier media, deionized water and ethylene gl...

MnBi is a promising ferromagnetic material for applications at elevated temperatures due to its high Curie temperature, sizable magnetocrystalline anisotropy, and increasing coercivity with temperature. However, fabrication of MnBi thin films with ferromagnetic low-temperature phase (LTP) is conventionally difficult due to the peritectic Mn-Bi reac...

The magnetic and magnetotransport properties of Ni80Fe20/La0.7Sr0.3MnO3 (NiFe/LSMO) bilayers were investigated after bombarding the LSMO surface with low-energy Ar+ or O2+/Ar+ ion beams before the growth of the top NiFe layer. A variety of magnetic properties are revealed, including an asymmetric two-stepped hysteresis loop with an exchange bias lo...

A rapid zeolite synthesis procedure was investigated using pulverized waste glass cullet via microwave radiation and compared with zeolites synthesized using convection heating. It was observed that constant temperature microwave synthesis method made zeolites more efficiently under superheating conditions with a maximum relative crystallinity of 6...

Widespread reports of spontaneous ferromagnetism in nanoscale wide-band gap semiconducting oxides have challenged theorists - a phenomenon that seemly breaks the conventional rubric of magnetism requiring unpaired d-electrons. The prototypical system that presents this d-zero ferromagnetism is nanoscale CeO2 (nanoceria) that has attracted significa...

Exchange-biased bilayers are widely used in the pinned layers of spintronic devices. While magnetic field annealing (MFA) was routinely engaged during the fabrication of these devices, the annealing effect of NiO/CoFe bilayers is not yet reported. In this paper, the transition from NiO/Co90Fe10 bilayer to nanocomposite single layer was observed thr...

We investigated the structural, compositional, and magnetic properties as well as the AC magnetic hyperthermic response of CoFe2-xLaxO4 (x=0.0,0.2,0.5) nanoparticles. We found that the La³⁺ ions substituted into the Fe³⁺ ion sites, and resulted in an increased magnetocrystalline anisotropy with x, and altered the time-dependent magnetism. To provid...

Magnetic nanoparticles (MNPs) have become increasingly important in biomedical applications like magnetic imaging and hyperthermia based cancer treatment. Understanding their magnetic spin configurations is important for optimizing these applications. The measured magnetization of MNPs can be significantly lower than bulk counterparts, often due to...

A facile one-pot method of synthesizing amine-functionalized non-spherical Fe3O4 nanoparticles in gram-scale quantities is presented using just a single source of iron (iron (II) chloride) and an amine (triethylamine). The amine not only reduces iron salt to Fe3O4, but also directs the morphology of the nanoparticles along with the temperature of t...

The ferromagnetic (FM)/antiferromagnetic (AF) bilayer structures have drawn intensive attention because of their wide applications in modern spintronic devices. While abundant published works have been reported on the interface effects of the FM/AF bilayers caused by the magnetic field annealing (MFA) process, the volume effects caused by the MFA t...

The exchange bias effect in ferromagnetic (FM)/antiferromagnetic (AF) bilayer structures has been widely investigated because its underlying principle is critical for spintronic applications. In this work, the effect of Ar+ beam bombardment on the microstructural and magnetic properties of the Mn/NiFe thin films was investigated. The in-situ Ar+ bo...

The applications of low energy argon or oxygen ion-beam assisted deposition and high energy oxygen ion implantation for thin film fabrication and modification of the microstructural, compositional and magnetic properties are demonstrated. Five representative model systems are discussed with the state-of-the art characterization tools such as atomic...

The microstructure, composition and magnetism of CoFe2O4, MnFe2O4 and NiFe2O4 nanoparticles of comparable sizes (∼20 nm) and interparticle spacings (∼20 nm) have been characterized from 10 to 400 K. The cation distributions of the tetrahedral and octahedral sites of the particles, that have cubic spinel structures, have a high degree of inversion,...

Our work introduces a novel technique based on the magnetic response of Ce(3+) and molecular oxygen adsorbed on the surface of nanoceria and ceria-based catalysts that quantifies the number and type of defects, and demonstrates that this information is the missing link that finally enables predictive design of NOx catalysis in ceria-based systems....

We discuss the characterization of magnetic core–shell nanoparticles by describing typical experimental techniques applied to nanoparticle characterization, in addition to more specialized atomic-scale and element-specific characterization techniques which provide in-depth insight to the origin and nature of the magnetism of core–shell nanoparticle...

BiFeO3 in the bulk is antiferromagnetically ordered—it lacks a magnetic handle that can be readily changed with magnetic fields. In article number 1700790, Er-Jia Guo, Michael R. Fitzsimmons and co-workers report a high-temperature large interfacial net magnetization in thin BiFeO3 layers in close proximity to La0.7Sr0.3MnO3. Simultaneously, ultrat...

γ−Fe2O3 particles, surface modified with NiO crystallites, form a unique nanocomposite that points to how to tune strong interfacial exchange coupling. We find that Ni2+ migrates into the octahedral sites of the γ−Fe2O3 nanoparticle surface, and this NiFe2O4-like layer permits effective magnetic coupling of Ni and Fe sites that strengthens the inte...

In exchange-biased systems, antiferromagnetic Cr2O3 has drawn intensive attention because of its highly anisotropic structure. However, other Cr-oxide phases can be formed during fabrication. The influence of different stoichiometric Cr-oxides on the magnetism of the NiFe/Cr-oxide bilayers was studied. We find that paramagnetic CrO3 grains could pr...

An unprecedented stable neutral radical nickel(II) complex of 3,5-bis(dimedonyl)azadiisoindomethene (1) was prepared by the direct reaction between 1,3-diiminoisoindoline and dimedone. A new radical complex 1 has an intense and narrow absorption at 1008 nm and can be reduced to a less stable anionic [1](-) with a typical aza(dibenzo)boron dipyrrome...

In this work, we obtained a quantitative depth dependent magnetization profile across the planar interfaces in BiFeO3/La0.7Sr0.3MnO3 (BFO/LSMO) superlattices using polarized neutron reflectometry (PNR). We observed an enhanced magnetization of 1.83 ± 0.16 μB/Fe in BFO layers when they are interleaved between two manganite layers. The enhanced magne...

In this work, we present a procedure to modify the exchange bias (EB) properties of antiferromagnetic Mn/ferromagnetic NiFe bilayers by in-situ low energy Ar⁺ bombardment of the Mn layer during sample deposition. We present structural and magnetic results for unassisted and Ar⁺ assisted Mn/NiFe bilayers. X-ray diffraction, transmission electron mic...

A series of atomistic finite temperature simulations on a model of an FCC lattice of maghemite nanoparticles using the stochastic Landau-Lifshitz-Gilbert (sLLG) equation are presented. The model exhibits a ferromagnetic transition that is in good agreement with theoretical expectations. The simulations also reveal an orientational disorder in the o...

While the exchange bias in ferromagnetic/antiferromagnetic (FM/AF) bilayer and FM1/AF/FM2 trilayer configurations have been widely investigated, the role of an AF2 layer in FM/AF1/AF2 trilayer configurations is still not well understood. In this work, the magnetic properties of NiFe/CoO, NiFe/α-Fe2O3 bilayers, and NiFe/CoO/α-Fe2O3 trilayer were stu...

Exchange bias and interlayer exchange coupling are interface driven phenomena. Since an ideal interface is very challenging to achieve, a clear understanding of the chemical and magnetic natures of interfaces is pivotal to identify their influence on the magnetism. We have chosen Ni80Fe20/CoO(t CoO)/Co trilayers as a model system, and identified no...

Nanoparticles targeting endothelial cells to treat diseases such as cancer, oxidative stress, and inflammation have traditionally relied on ligand-receptor based delivery. The present studies examined the influence of nanoparticle shape in regulating preferential uptake of nanoparticles in endothelial cells. Spherical and brick shaped iron oxide na...

FeCo nanoparticles (4 ± 1 nm), encapsulated by SiO2, were synthesized with and without a 2% (atomic ratio) vanadium doping. The impact from the presence of vanadium, an additive often used in the bulk to alter both physical and mechanical properties, on the nanomagnetism was probed by element-specific X-ray spectroscopy and magnetometry techniques....

Effective treatment of brain disorders requires a focus on improving drug permeability across the blood-brain barrier (BBB). Herein, we examined the pharmacokinetic properties of negatively charged iron oxide nanoparticles (IONPs) and the capability of using lysophosphatidic acid (LPA) to transiently disrupt the tight junctions and allow IONPs to e...

Dynamical freezing of Fe nanocrystallites in a Cu matrix with magnetic ordering of an FeCu interfacial phase provides a unique window into the magnetocaloric effect. The FeCu alloy altered the Fe nanocrystallite surface atoms, and with a magnetic ordering temperature comparable to the dynamical freezing temperature of the nanocrystallites enabled F...

Results from finite temperature sLLG simulations of an atomistic core-shell model of γ-Fe2O3 nanoparticles are presented. Radial surface anisotropy gives rise to a surface magnetization with a Néel-like domain wall separating the magnetic poles. It is shown that the pinning of the domain wall by the oxygen anion sites plays an important role in low...

The source of the unusual ferromagnetism of nanowires (NWs) such as CoSi-SiO2 has been studied by first-principles calculations. While previous experiments on ferromagnetic NWs presumed that their magnetism was the result of metal ions at the interface suffering reduced coordination, first-principles calculations of such a configuration revealed th...

MnTiO3 has been studied for many decades, but it was only in the last few years that its 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 R3¯ symmetry. We also present the temperature and field dependence of the...

We propose a new and highly effective tool to add to the ever shrinking toolbox for combating infections caused by antibiotic resistant bacteria; N-chloramine and quaternized N-chloramine were coated onto iron-oxide magnetic nanoparticles to generate antibacterial MNPs. Two differently-sized primary iron-oxide nanoparticles (3 nm and 10 nm) were sy...

We discuss how understanding the physics of magnetism is used to direct the “pushing” of atoms and their electrons to make and enable novel applications.

The detailed characterization of core/shell iron-oxide/silica nanoparticles reveals how these superparamagnetic systems are actually composed of a Fe3O4 inner core, γ-Fe2O3, outer core, iron orthosilicate interphase layer, and exterior silica shell. The performance of a superparamagnetic inductor device is then reported for the nanocomposite formed...

We examine the role of Fe(3+) and vacancies ([Formula: see text]) on the magnetism of Fe-doped CeO2 nanoparticles. Magnetic nanoparticles of Ce100-xFexO2 (x = 0, 0.26, 1.82, 2.64, 5.26, 6.91, and 7.22) were prepared by a co-precipitation method, and their structural, compositional and magnetic properties were investigated. The CeO2 nanoparticles ha...

Si/Fe and SiO2/Fe thin-film heterostructures are commonly seen in magnetic multilayer devices, whose magnetic properties are strongly influenced by intermixing at the interfaces. In this paper, Si-oxide/Fe bilayers were formed by depositing Si on Fe with in situ O2/Ar ion-beam bombardment during the Si deposition. The surface oxidation conditions w...