Heiko Wende

• Dr. rer. nat.
• Scientific Director CENIDE at University of Duisburg-Essen

To realize molecular spintronic devices, it is important to externally control the magnetization of a molecular magnet. One class of materials particularly promising as building blocks for molecular electronic devices is the paramagnetic porphyrin molecule in contact with a metallic substrate. Here, we study the structural orientation and the magne...

Magnetic nanoparticles are of immense current interest because of their possible use in biomedical and technological applications. Here we demonstrate that the large magnetic anisotropy of FePt nanoparticles can be significantly modified by surface design. We employ X-ray absorption spectroscopy offering an element-specific approach to magnetocryst...

Ferrimagnetic CoFe2O4 nanopillars embedded in a ferroelectric BaTiO3 matrix are an example for a two-phase magnetoelectrically coupled system. They operate at room temperature and are free of any resource-critical rare-earth element, which makes them interesting for potential applications. Prior studies succeeded in showing strain-mediated coupling...

Future applications of molecular units in quantum information technologies require a fine control at the single molecule level. This includes the choice of each functional element, the intramolecular interaction and the robustness of molecules when dispersed on a substrate. Keeping these goals in mind, we designed and synthesized a heterometallic p...

Hysteresis is more than just an interesting oddity that occurs in materials with a first-order transition. It is a real obstacle on the path from existing laboratory-scale prototypes of magnetic refrigerators towards commercialization of this potentially disruptive cooling technology. Indeed, the reversibility of the magnetocaloric effect, being es...

High entropy alloys (HEAs) have a wide range of applications across various fields, including structural engineering, biomed-ical science, catalysis, magnetism, and nuclear technology. Nanoscale HEA particles show promising catalytic properties. Nevertheless, attaining versatile composition control in nano-particles poses a persistent challenge. Th...

Metal-doped spinel MxCo3−xO4 nanoparticles are promising catalysts for the electrochemical oxygen evolution reaction (OER). We here report a robust and scalable route for the synthesis of metal-doped spinel MxCo3−xO4 nanoparticles (M = Al, V, Cr, Mn, Fe, Ni; x = 0.05, 0.1, 0.2) by thermal decomposition of cobalt(ii) acetylacetonate and the respecti...

Spin-state switching in iron(II) complexes composed of ligands featuring moderate ligand-field strength—for example, 2,6-bi(1H-pyrazol-1-yl)pyridine (BPP)—is dependent on many factors—examples include lattice solvent, counter anion, and substituent. Herein, we show that spin-state switching in isomeric iron(II) complexes (1·CH3CN and 2) composed of...

The rise of Fe magnetic moment, changes in Al electronic structure and a variation of Al magnetic polarization in thin films of transition metal aluminide Fe 60 Al 40 have been probed through the order-disorder phase transition by soft X-ray absorption spectroscopy and X-ray resonant magnetic reflectivity in the extreme ultraviolet regime. In a cou...

The photoinduced switching of Fe(II)-based spin-crossover complexes from singlet to quintet takes place at ultrafast time scales. This a priori spin-forbidden transition triggered numerous time-resolved experiments of solvated samples to elucidate the mechanism at play. The involved intermediate states remain uncertain. We apply ultrafast x-ray spe...

The reaction of UO2 with the respective lanthanide metal and purified graphite in an arc-melting furnace led to the formation of solid solutions of the composition LnxU1−xC2, with Ln = Tb, Dy, Ho, Tm, and Lu. They all crystallize in the tetragonal CaC2 type structure (I4/mmm, Z = 2). Elemental analyses of selected samples (EDX) confirm that the com...

The sticking properties of dust in early phases of planet formation depend on the thermal history and ambient atmosphere. Therefore, dust will change its ability to build larger aggregates in collisions, depending on its location in protoplanetary disks. We aim at quantifying the change in sticking properties as chondritic dust is heated under vari...

Free-electron lasers provide bright, ultrashort, and monochromatic x-ray pulses, enabling novel spectroscopic measurements not only with femtosecond temporal resolution: The high fluence of their x-ray pulses can also easily enter the regime of the non-linear x-ray–matter interaction. Entering this regime necessitates a rigorous analysis and reliab...

This work investigates the effect of copper substitution on the magnetic properties of SmCo5 thin films synthesized by molecular beam epitaxy. A series of thin films with varying concentrations of Cu were grown under otherwise identical conditions to disentangle structural and compositional effects on the magnetic behavior. The combined experimenta...

Alloys with a first-order magnetic transition are central to solid-state refrigeration technology, sensors and actuators, or spintronic devices. The discontinuous nature of the transition in these materials is a consequence of the coupling between the magnetic, electronic, and structural subsystems, and such transition can, in principle, cross seve...

Nb 3 Sn is a promising thin film material for superconducting radio frequency (SRF) applications. Although surface resistivity, critical temperature , and critical field are advantageous in comparison to pure Nb, currently the performance of Nb 3 Sn is lacking behind due to its complex defect structure and phase inhomogeneities. In this work, the i...

The shell-ferromagnetic effect originates from the segregation process in off-stoichiometric Ni-Mn-based Heusler alloys. In this work, we investigate the precipitation process of L21-ordered Ni2MnSn and L10-ordered NiMn in off-stoichiometric Ni50Mn45Sn5 during temper annealing, by X-ray diffraction (XRD) and 119Sn Mössbauer spectroscopy. While XRD...

The novel vacuum-evaporable complex [Fe(pypypyr)2] (pypypyr = bipyridyl pyrrolide) was synthesised and analysed as bulk material and as a thin film. In both cases, the compound is in its low-spin state up to temperatures of at least 510 K. Thus, it is conventionally considered a pure low-spin compound. According to the inverse energy gap law, the h...

Using magnetic nanoparticles for extracorporeal magnetic heating applications in bio-medical technology allows higher external field amplitudes and thereby the utilization of particles with higher coercivities (HC). In this study, we report the synthesis and characterization of high coercivity cobalt ferrite nanoparticles following a wet co-precipi...

Pronounced magnetocaloric effects are typically observed in materials that often contain expensive and rare elements and are therefore costly to mass produce. However, they can rather be exploited on a small scale for miniaturized devices such as magnetic micro coolers, thermal sensors, and magnetic micropumps. Herein, a method is developed to gene...

Photo-doped states in strongly correlated charge transfer insulators are characterized by $d$-$d$ and $d$-$p$ interactions and the resulting intertwined dynamics of charge excitations and local multiplets. Here we use femtosecond x-ray absorption spectroscopy in combination with dynamical mean-field theory to disentangle these contributions in NiO....

The complex electronic structure of metallic ferromagnets is determined by a balance between exchange interaction, electron hopping leading to band formation, and local Coulomb repulsion. By combining high energy and temporal resolution in femtosecond time-resolved X-ray absorption spectroscopy with ab initio time-dependent density functional theor...

The interaction of 2-propanol with Co 3 O 4 (001) was studied by vibrational sum frequency spectroscopy (vSFS) and by ab initio molecular dynamics (AIMD) simulations of 2-propanol dissolved in a water film to gain insight at the molecular level into the pathways of catalytic oxidation. The experimental study has been performed under near ambient co...

This work investigates the effect of copper substitution on the magnetic properties of SmCo$_{5}$ thin films synthesized by molecular beam epitaxy. A series of thin films with varying concentrations of Cu were grown under otherwise identical conditions to disentangle structural and compositional effects on the magnetic behavior. The combined experi...

Nanoscaled magnetic particle ensembles are promising building blocks for realising magnon based binary logic. Element-specific real-space monitoring of magnetic resonance modes with sampling rates in the GHz regime is imperative for the experimental verification of future complex magnonic devices. Here we present the observation of different phasic...

Magnetic cooling has the potential to replace conventional gas compression refrigeration. Materials such as La(Fe,Si)$_{13}$ exhibit a sizeable first-order magnetocaloric effect, and it is possible to tailor the phase transition towards room temperature by Mn-H-doping, resulting in a large temperature range for operation. Within this work, we discu...

The Shell-ferromagnetic effect originates from the segregation process in off-stochiometric Ni-Mn-based Heusler. In this work, we investigate the precipitation process of L2$_1$-ordered Ni$_2$MnSn and L1$_0$-ordered NiMn in off-stochiometric Ni$_{50}$Mn$_{45}$Sn$_{5}$ during temper annealing, by X-ray diffraction (XRD) and $^{119}$Sn M\"ossbauer sp...

We present a statistically motivated method to extract magnonic contrast from time-resolved scanning transmission x-ray microscopy (TR-STXM) measurements. TR-STXM is an element-specific method for resolving spin-dynamics in space and time. It offers nanometer spatial resolution and picosecond temporal resolution. The presented method makes it possi...

X-ray absorption spectroscopy (XAS) is a well-established method for in-depth characterization of electronic structure. In practice hundreds of energy-points should be sampled during the measurements, and most of them are redundant. Additionally, it is also tedious to estimate reasonable parameters in the atomic Hamiltonians for mechanistic underst...

Alloys with a first-order magnetic transition are central to solid-state refrigeration technology, sensors and actuators, or spintronic devices. The discontinuous nature of the transition in these materials is a consequence of the coupling between the magnetic, electronic and structural subsystems, but in a real experiment, it is difficult to obser...

The product properties of mixed oxide nanoparticles generated via spray-flame synthesis depend on an intricate interplay of solvent and precursor chemistries in the processed solution. The effect of two different sets of metal precursors, acetates and nitrates, dissolved in a mixture of ethanol (35 Vol.%) and 2-ethylhexanoic acid (2-EHA, 65 Vol.%)...

Femtosecond transient soft X-ray absorption spectroscopy (XAS) is a very promising technique that can be employed at X-ray free-electron lasers (FELs) to investigate out-of-equilibrium dynamics for material and energy research. Here, a dedicated setup for soft X-rays available at the Spectroscopy and Coherent Scattering (SCS) instrument at the Euro...

In laboratory experiments, we heated chondritic material up to 1400K in a hydrogen atmosphere. Moessbauer spectroscopy and magnetometry reveal that, at high temperatures, metallic iron forms from silicates. The transition temperature is about 1200K after 1 h of tempering, likely decreasing to about 1000K for longer tempering. This implies that in a...

The synthesis of bimetallic iron-nickel nanoparticles with control over the synthesized phases, particle size, surface chemistry, and oxidation level remains a challenge that limits the application of these nanoparticles. Pulsed laser ablation in liquid allows the properties tuning of the generated nanoparticles by changing the ablation solvent. Or...

Multilayered phosphate bonded CoFe2O4–BaTiO3–CoFe2O4 (CBC) and BaTiO3–CoFe2O4 – BaTiO3 (BCB) multiferroic structures were formed by means of uniaxial pressing. The dielectric properties were studied in 20 Hz – 1 GHz frequency and 120–500 K temperature ranges. The complex dielectric permittivity is 15–0.17i for CBC and 22–0.04i for BCB, it is temper...

We have performed a combined experimental and theoretical investigation of the vibrational properties of Ni2MnSn Heusler alloys. Sn-partial vibrational density of states (VDOS) of Sn119 was measured by nuclear resonant inelastic x-ray scattering at temperatures of 15 and 300 K, while magnetism and local environment of Sn was resolved by Sn119 Mössb...

The pulse intensity from X-ray free-electron lasers (FELs) can create extreme excitation densities in solids, entering the regime of non-linear X-ray-matter interactions. We show L3-edge absorption spectra of metallic nickel thin films with fluences entering a regime where several X-ray photons are incident per absorption cross-section. Main featur...

The main text contains 5 pages and 4 figures, the total length including supplement is 14 pages, 7 figures and one table. See also the simultaneously submitted paper about the rate model

In laboratory experiments, we heated chondritic material up to 1400 K in a hydrogen atmosphere. Mössbauer spectroscopy and magnetometry reveal that, at high temperatures, metallic iron forms from silicates. The transition temperature is about 1200 K after 1 h of tempering, likely decreasing to about 1000 K for longer tempering. This implies that in...

Femtosecond transient soft X-ray Absorption Spectroscopy (XAS) is a very promising technique that can be employed at X-ray Free Electron Lasers (FELs) to investigate out-of-equilibrium dynamics for material and energy research. Here we present a dedicated setup for soft X-rays available at the Spectroscopy & Coherent Scattering (SCS) instrument at...

We present the element-specific and time resolved visualization of uniform ferromagnetic resonance excitations of a Permalloy (Py) disk–Cobalt (Co) stripe bilayer microstructure. The transverse high frequency component of the resonantly excited magnetization is sampled in the ps regime by a combination of ferromagnetic resonance (FMR) and scanning...

The complex electronic structure of metallic ferromagnets is determined by a balance between exchange interaction, electron hopping leading to band formation, and local Coulomb repulsion. The interplay between the respective terms of the Hamiltonian is of fundamental interest, since it produces most, if not all, of the exotic phenomena observed in...

Ionic engineering is exploited to substitute Bi cations in BiFe0.95Mn0.05O3 NPs (BFM) with rare-earth (RE) elements (Nd, Gd, and Dy). The sol-gel synthesized RE-NPs are tested for their magnetic hyperthermia potential. RE-dopants alter the morphology of BFM NPs from elliptical to rectangular to irregular hexagonal for Nd, Gd, and Dy doping, respect...

The results show that the formation of magnetic phases during temperature dependent magnetic measurements are challenging to deal with in order to study the magnetic behavior of pristine BFO or BFM NPs at high temperatures. However, we could estimate the Néel temperature of BFM and RE-doped BFM NPs using derivative of M-T curves.

Controlling the surface composition of colloidal nanoparticles is still a challenging yet mandatory prerequisite in catalytic studies to investigate composition-activity trends, active sites, and reaction mechanisms without superposition of particle size or morphology effects. Laser post-processing of colloidal nanoparticles has been employed previ...

Magnetic materials on the micro-and nanoscale are addressing a wide spectrum of contemporary challenges ranging from cancer research towards spintronic and magnonic solutions for modern information technology. The synchrotron-based technique of Scanning Transmission X-ray Microscopy (STXM) allows the element-specific and spatially resolved chemical...

An iron(II) complex (1·CH3CN) composed of a tridentate all nitrogen coordinating ligand—ethyl 2,6-bis(1H-pyrazol-1-yl)isonicotinate (BPP-COOEt)—shows bistable spin-state switching characteristic with thermal hysteresis width (ΔT1/2) = 44 K and switching temperature (T1/2) = 298 K in the first cycle. Crystal structures of the LS and HS forms of the...

The chirality-induced spin selectivity (CISS) effect facilitates a paradigm shift for controlling the outcome and efficiency of spin-dependent chemical reactions, for example, photoinduced water splitting. While the phenomenon is established in organic chiral molecules, its emergence in chiral but inorganic, nonmolecular materials is not yet unders...

Spin-crossover (SCO) complexes that show abrupt and hysteretic spin-state switching characteristics—termed as bistable spin-state switching—are proposed suitable to realize molecule-based switching and memory elements. For realistic applications, spin-state switching needs to be demonstrated in the thin film state, requiring vacuum sublimation of S...

The organometallic on-surface synthesis of the eight-membered sp2 carbon-based ring cyclooctatetraene (C8H8, Cot) with the neighboring rare-earth elements ytterbium and thulium yields fundamentally different products for the two lanthanides, when conducted on graphene (Gr) close to the charge neutrality point. Sandwich-molecular YbCot wires of more...

Pulsed laser ablation in liquids was utilized to prepare NiFe2O4 (NFO) and CoFe2O4 (CFO) nanoparticles from ceramic targets. The morphology, crystallinity, composition, and particle size distribution of the colloids were investigated. We were able to identify decomposition products formed during the laser ablation process in water. Attempts to frac...

From macroscopic heavy-duty permanent magnets to nanodevices, the precise control of the magnetic properties in rare-earth metals is crucial for many applications used in our daily life. Therefore, a detailed understanding and manipulation of the 4f-metals' magnetic properties are key to further boosting the functionalization and efficiency of futu...

The Cover Feature illustrates the pulsed laser defect engineering in liquid (PuDEL) of CoFe2O4‐nanoparticles. In their Research Article, S. Zerebecki et al. show the pulsed laser induces rapid particle heating which resulted in the generation or healing of defects (e.g. stacking faults) depending on the starting material. The method was used to tun...

Bulk BaTiO3-xCoFe2O4, x = 0.1 – 0.6 magnetoelectric composites were prepared using the phosphate bonded ceramics approach. XRD analysis proved the purity of both phases. The dielectric properties are governed by a series of composition-dependent Maxwell-Wagner relaxations and conductivity at lower frequencies and a phase transition-related anomaly...

A single-crystal sheet of graphene is synthesized on the low-symmetry substrate Ir(110) by thermal decomposition of C2H4 at 1500 K. Using scanning tunneling microscopy, low-energy electron diffraction, angle-resolved photoemission spectroscopy, and ab initio density functional theory, the structure and electronic properties of the adsorbed graphene...

Due to the variability of the cation occupancy of octahedral and tetrahedral sites, spinel ferrites and cobaltites are particularly interesting to investigate activity trends in oxidation catalysis. Yet, the preparation of the respective catalyst series remains challenging. We employed pulsed laser defect engineering of CoFe2O4 nanoparticles in wat...

The impact of reduction post-treatment and phase segregation of cobalt iron oxide nanowires on their electrochemical oxygen evolution reaction (OER) activity is investigated. A series of cobalt iron oxide spinel nanowires are prepared via the nanocasting route using ordered mesoporous silica as a hard template. The replicated oxides are selectively...

Materials with strong magneto-structural coupling have complex energy landscapes featuring multiple local ground states, thus making it possible to switch among distinct magnetic-electronic properties. However, these energy minima are rarely accessible by a mere application of an external stimuli to the system in equilibrium state. Here, we report...

The Co‐based complex [Co(H2B(pz)(pypz))2] (py=pyridine, pz=pyrazole) deposited on Ag(111) was investigated with scanning tunneling microscopy at ≈5 K. Due to a bis(tridentate) coordination sphere the molecules aggregate mainly into tetramers. Individual complexes in these tetramers undergo reversible transitions between two states with characterist...

Bistable spin‐state switching near room temperature: Two mononuclear iron(II) complexes showing technologically relevant abrupt and wide hysteretic, bistable spin‐state switching characteristics in the vicinity of room temperature are reported. The large structural reorganizations from the Jahn–Teller‐distorted high‐spin state to the less distorted...

This paper describes the preparation and obtained magnetic properties of large single domain iron oxide nanoparticles. Such ferrimagnetic particles are particularly interesting for diagnostic and therapeutic applications in medicine or (bio)technology. The particles were prepared by a modified oxidation method of non-magnetic precursors following t...

Bistable spin‐crossover (SCO) complexes that undergo abrupt and hysteretic (ΔT1/2) spin‐state switching are desirable for molecule‐based switching and memory applications. In this study, we report on structural facets governing hysteretic SCO in a set of iron(II)‐2,6‐bis(1H‐pyrazol‐1‐yl)pyridine) (bpp) complexes – [Fe(bpp−COOEt)2](X)2⋅CH3NO2 (X=ClO...

The paper addresses coupling of magnetic nanoparticles (MNPs) with the polymer matrix of temperature-sensitive microgels and their response to magnetic fields. Therefore, CoFe2O4@CA (CA = citric acid) NPs are embedded within N-isopropylacrylamid (NIPAM) based microgels. The volume phase transition (VPT) of the magnetic microgels and the respective...

The Co-based complex [Co(H 2 B(pz)(pypz)) 2 ] (py = pyridine, pz = pyrazole) deposited on Ag(111) was investigated with scanning tunneling microscopy at ≈ 5 K. Due to a bis(tridentate) coordination sphere the molecules aggregate mainly into tetramers. Individual complexes in these tetramers undergo reversible transitions between two states with cha...

Controlling the surface composition of colloidal nanoparticles is still a challenging yet mandatory prerequisite in catalytic studies to investigate composition-activity trends, active sites, and reaction mechanisms without superposition of particle size- or morphology-effects. Laser post-processing of colloidal nanoparticles has been employed prev...

TSol-gel chemistry, while being extremely established, is to this day not fully understood, and much of the underlaying chemistry and mechanisms are yet to be unraveled. Here, we elaborate on...

We present a new instrument for advanced magnetic studies based on the high field X-ray magnetic circular dichroism (XMCD) end-station developed at the beamline ID12 of the European Synchrotron Radiation Facility (ESRF, Grenoble, France). It offers a unique possibility to measure simultaneously element-specific and macroscopic properties related to...

Correction for ‘Towards a mechanistic understanding of the sol–gel syntheses of ternary carbides’ by Jan P. Siebert et al. , Inorg. Chem. Front. , 2022, DOI: 10.1039/d2qi00053a.

Nanoparticulate CoxFe3-xO4 (0 ≤ x ≤ 3) catalysts were prepared by spray-flame synthesis and applied in liquid phase cyclohexene oxidation with O2 as oxidant. The catalysts were characterized in detail...

The electrochemical behavior of iron-doped nickel hydroxide is remarkably consistent across the diverse fabrication protocols employed throughout the literature, which contrasts with variations reported in composition-dependent changes in structure and...

High entropy oxides (HEOs) are a rapidly emerging class of functional materials consisting of multiple principal cations. The original paradigm of HEOs assumes cationic occupations with the highest possible configurational entropy allowed by the composition and crystallographic structure. However, the fundamental question remains on the actual degr...

In the standard model of charge density wave (CDW) transitions, the displacement along a single phonon mode lowers the total electronic energy by creating a gap at the Fermi level, making the CDW a metal–insulator transition. Here, using scanning tunneling microscopy and spectroscopy and ab initio calculations, we show that VS 2 realizes a CDW whic...

Liquid crystal (LC) based magnetic materials consisting of LC hosts doped with functional magnetic nanoparticles enable optical switching of the mesogens at moderate magnetic field strengths and thereby open the pathway for the design of novel smart devices. A promising route for the fabrication of stable ferronematic phases is the attachment of a...

Perovskites are interesting oxidation catalysts due to their chemical flexibility enabling the tuning of several properties. In this work, we synthesized LaFe1−xCoxO3 catalysts by co‐precipitation and thermal decomposition, characterized them thoroughly and studied their 2‐propanol oxidation activity under dry and wet conditions to bridge the knowl...