Doping of inorganic materials in microreactors – preparation of Zn doped Fe₃O₄ nanoparticles

Microreactor systems are now used more and more for the continuous production of metal nanoparticles and metal oxide nanoparticles owing to the controllability of the particle size, an important property in many applications. Here, for the first time, we used microreactors to prepare metal oxide nanoparticles with controlled and varying metal stoichiometry. We prepared and characterised Zn-substituted Fe₃O₄ nanoparticles with linear increase of Zn content (ZnxFe₃−xO₄ with 0 ≤ x ≤ 0.48), which causes linear increases in properties such as the saturation magnetization, relative to pure Fe₃O₄. The methodology is simple and low cost and has great potential to be adapted to the targeted doping of a vast array of other inorganic materials, allowing greater control on the chemical stoichiometry for nanoparticles prepared in microreactors

Ferrimagnetic large single domain iron oxide nanoparticles for hyperthermia applications

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 the green rust synthesis and characterized regarding their structural and magnetic properties. For increasing preparation temperatures (5 to 85 °C), an increasing particle size in the range of 30 to 60 nm is observed. Magnetic measurements confirm a single domain ferrimagnetic behavior with a mean saturation magnetization of ca. 90 Am2/kg and a size-dependent coercivity in the range of 6 to 15 kA/m. The samples show a specific absorption rate (SAR) of up to 600 W/g, which is promising for magnetic hyperthermia application. For particle preparation temperatures above 45 °C, a non-magnetic impurity phase occurs besides the magnetic iron oxides that results in a reduced net saturation magnetization

Structural Insights into Hysteretic Spin‐Crossover in a Set of Iron(II)‐2,6‐bis(1 H ‐Pyrazol‐1‐yl)Pyridine) Complexes

Bistable spin-crossover (SCO) complexes that undergo abrupt and hysteretic (ΔT$_{1/2}$) 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)$_{2}$⋅CH$_{3}$NO$_{2}$ (X=ClO$_{4}$, 1; X=BF$_{4}$, 2). Stable spin-state switching – T$_{1/2}$=288 K; ΔT$_{1/2}$=62 K – is observed for 1, whereas 2 undergoes above-room-temperature lattice-solvent content-dependent SCO – T$_{1/2}$=331 K; ΔT$_{1/2}$=43 K. Variable-temperature single-crystal X-ray diffraction studies of the complexes revealed pronounced molecular reorganizations – from the Jahn-Teller-distorted HS state to the less distorted LS state – and conformation switching of the ethyl group of the COOEt substituent upon SCO. Consequently, we propose that the large structural reorganizations rendered SCO hysteretic in 1 and 2. Such insights shedding light on the molecular origin of thermal hysteresis might enable the design of technologically relevant molecule-based switching and memory elements

Reversed ageing of Fe3_3O4_4 nanoparticles by hydrogen plasma

Magnetite (Fe3O4) nanoparticles suffer from severe ageing effects when exposed to air even when they are dispersed in a solvent limiting their applications. In this work, we show that this ageing can be fully reversed by a hydrogen plasma treatment. By x-ray absorption spectroscopy and its associated magnetic circular dichroism, the electronic structure and magnetic properties were studied before and after the plasma treatment and compared to results of freshly prepared magnetite nanoparticles. While aged magnetite nanoparticles exhibit a more γ-Fe2O3 like behaviour, the hydrogen plasma yields pure Fe3O4 nanoparticles. Monitoring the temperature dependence of the intra-atomic spin dipole contribution to the dichroic spectra gives evidence that the structural, electronic and magnetic properties of plasma treated magnetite nanoparticles can outperform the ones of the freshly prepared batch

Impact of lattice dynamics on the phase stability of metamagnetic FeRh: Bulk and thin films

We present phonon dispersions, element-resolved vibrational density of states (VDOS) and corresponding thermodynamic properties obtained by a combination of density functional theory (DFT) and nuclear resonant inelastic X-ray scattering (NRIXS) across the metamagnetic transition of B2 FeRh in the bulk material and thin epitaxial films. We see distinct differences in the VDOS of the antiferromagnetic (AF) and ferromagnetic (FM) phase which provide a microscopic proof of strong spin-phonon coupling in FeRh. The FM VDOS exhibits a particular sensitivity to the slight tetragonal distortions present in epitaxial films, which is not encountered in the AF phase. This results in a notable change in lattice entropy, which is important for the comparison between thin film and bulk results. Our calculations confirm the recently reported lattice instability in the AF phase. The imaginary frequencies at the $X$-point depend critically on the Fe magnetic moment and atomic volume. Analyzing these non vibrational modes leads to the discovery of a stable monoclinic ground state structure which is robustly predicted from DFT but not verified in our thin film experiments. Specific heat, entropy and free energy calculated within the quasiharmonic approximation suggest that the new phase is possibly suppressed because of its relatively smaller lattice entropy. In the bulk phase, lattice degrees of freedom contribute with the same sign and in similar magnitude to the isostructural AF-FM phase transition as the electronic and magnetic subsystems and therefore needs to be included in thermodynamic modeling.Comment: 15 pages, 12 figure

Magnetic measurements on single-phase and composite multiferroics

This thesis focuses on the detailed characterization of multiferroic phenomena in single-phase and composite materials, which have become a key aspect of research interest due to their numerous possible uses for application purposes. The main topic deals with the interplay of electric and magnetic material properties, linked either intrinsically in single-phase materials or via a mediator such as strain coupling in purposefully engineered biphasic composites. The influence of different material compositions, synthesis techniques and resulting morphologies was analyzed in order to obtain a detailed overview of the physical phenomena that govern the strength of magnetoelectric (ME) coupling. The main methods used for this purpose include Mössbauer spectroscopy, a non-destructive nuclear physical method utilizing resonant absorption of radiation, as well as a magnetoelectric measurement setup. Latter is based on a modified SQUID AC susceptometer, making it possible to record the magnetic response of ceramic samples exposed to electric fields with high precision. One section of this thesis is devoted to the single-phase material bismuth ferrite (BiFeO3), which is antiferromagnetic and ferroelectric. This multiferroic property was never utilized for applications, as its antiferromagnetic nature leads to zero net magnetic moment. It was discovered that BiFeO3 contains a long range modulation of magnetic moments (spin cycloid), which is incommensurable with the crystal lattice. Furthermore, weak ferromagnetism was observed in nanoparticles, which was the main motivation for our measurements. Using Mössbauer spectroscopy, we examined the behavior of this spin cycloid in great detail while recording spectra in a large temperature range (4K- 800 K), for different particle sizes (54nm- 1 m). This allowed us to assess the temperature dependence of the spin cycloid, which was distorted towards an anharmonic state at lower temperatures, while no significant influence of the particle size was witnessed. This work represents the first instance of the anharmonic cycloid being characterized in detail for nanoparticle samples in a wide temperature range. The second section deals with a number of composite ceramics that combine magnetostrictive and piezoelectric materials with different constituents and connectivity schemes. These were characterized with our converse ME measurement setup at different temperatures (4K- 350 K) and applied magnetic fields (±5 T), in order to ascertain their behavior, and to optimize the measurement parameters. This was followed by the recording of the electric field dependence of sample magnetization, providing direct access to the converse ME coupling coefficient, which provides a measure for the strength of ME coupling in our samples. Through the course of our work, we were able to significantly increase the coupling coefficient by modifications of the sample composition, preparation and constituents. Our studies also provide information about the underlying physical driving forces that affect the strength of the ME coupling, while also uncovering possible pitfalls of the measurement setup. The knowledge of these factors made it possible to effectively optimize our samples, allowing us to increase coupling coefficients nearly fourfold relative to our initial work.Diese Arbeit beschäftigt sich mit der detaillierten Charakterisierung multiferroischer Phänomene in einphasigen Materialien und Verbundmaterialien, die wegen ihrer möglichen Anwendungen in den Fokus der Wissenschaft gerückt sind. Das Hauptaugenmerk liegt auf dem Zusammenspiel elektrischer und magnetischer Eigenschaften, welche entweder intrinsisch oder über Vermittler, wie die mechanische Spannung, miteinander gekoppelt sind. Der Einfluss verschiedener Materialzusammensetzungen, Synthesemethoden und daraus resultierender Morphologien wurde analysiert, um eine Übersicht der physikalischen Phänomene zu erhalten, welche die Stärke der magnetoelektrischen (ME) Kopplung bestimmen. Bei den hauptsächlich verwendeten Methoden handelt es sich um Mössbauerspektroskopie, einer zerstörungsfreien kernphysikalischen Messmethode, die auf der resonanten Absorption von Strahlung beruht, und um einen Aufbau für ME Messungen. Letzterer basiert auf einem modifizierten SQUID AC Suszeptometer, was die Messung der magnetischen Reaktion auf elektrische Felder erlaubt. Ein Abschnitt der Arbeit ist dem einphasigen Material Bismutferrit (BiFeO3) gewidmet, welches gleichzeitig antiferromagnetisch und ferroelektrisch ist. Diese multiferroische Eigenschaft wurde nie genutzt, da das antiferromagnetische Material keine Nettomagnetisierung besitzt. Es ist bekannt, dass BiFeO3 eine langreichweitige, mit dem Kristallgitter inkommensurable Modulation der magnetischen Momente aufweist (Spin-Zykloide). Weiterhin wurde schwach ferromagnetisches Verhalten in BiFeO3-Nanopartikeln beobachtet, was die Hauptmotivation für unsere Messungen war. Mittels Mössbauerspektroskopie konnten wir das Verhalten dieser Spinzykloide im Detail beobachten, während Spektren in einem sehr weiten Temperaturbereich (4K- 800 K) für verschieden große (54nm- 1 m) Partikel aufgenommen wurden. Dies erlaubte uns die Observierung der Temperaturabhängigkeit der Spinzykloide, welche bei tiefen Temperaturen zunehmend verzerrt und anharmonisch wurde, während kein nennenswerter Einfluss der Partikelgröße zu sehen war. Im Zuge dieser Arbeit wurde erstmalig die anharmonische Zykloide in Nanopartikeln im Detail temperaturabhängig charakterisiert. Im zweiten Abschnitt beschäftigen wir uns mit Kompositen, welche aus einer magnetostriktiven und piezoelektrischen Phase mit verschiedenen Bestandteilen und Konnektivitäten bestehen. Diese wurden bei verschiedenen Temperaturen (4K- 350 K) und angelegten Magnetfeldern (±5 T) charakterisiert, sodass ihr Verhalten beobachtet und die Messparameter optimiert werden konnten. Es folgte die Aufnahme des magnetischen Signals in Abhängigkeit von angelegten elektrischen Feldern, was die Berechnung des konversen ME Koeffizienten erlaubt, der ein Maß für die Stärke der ME Kopplung darstellt. Somit konnten wir den Kopplungskoeffizienten durch Modifikation der Probenbestandteile, Präparation und Zusammensetzung stark erhöhen. Wir erhielten so Informationen über die zugrundeliegenden physikalischen Phänomene des ME Effekts, während auch potentielle Probleme des Messaufbaus erkannt wurden. Das Wissen über diese Faktoren ermöglichte die Optimierung der Probeneigenschaften, sodass der Kopplungskoeffizient verglichen mit unseren ersten Arbeiten um etwa den Faktor 4 erhöht werden konnte

Bistable spin-state switching characteristic of a charge-neutral iron(II) complex

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 SCO complexes to fabricate clean and impurity-free thin films. Charge-neutral iron(II) complexes are a class of SCO complexes that are reported to undergo sublimation, and their spin-state switching characteristics in the thin film state have been studied. However, hysteretic SCO in the thin film state is a scarcely observed phenomenon, requiring the development of iron(II) charge-neutral complexes that can undergo bistable spin-state switching in the bulk and thin film states. Herein, we report a new iron(II) charge-neutral complex—[Fe(H2Bpz2)24,4\u27-Br2-bpy] (H2Bpz2 = di¬hydro¬bis¬(pyrazol-1-yl)borate; 4,4\u27-Br2-bpy = 4,4\u27-dibromo-2,2\u27-bipyridine)—that undergoes abrupt and hysteretic spin-state switching in the bulk-state with T1/2 = 113 K and ΔT1/2 = 13 K at a scan rate of 0.25 K/min. The HS-to-LS switching of the complex is scan-rate-dependent, whereas the LS-to-HS switching is scan-rate-independent. Moreover, a reverse-SCO phenomenon was observed upon heating the sample in the 78 K-105 K temperature range at a scan rate of 3 K/minute. However, the reverse SCO was not observed when the complex was studied at scan rates of 1 and 0.5 K/min. Such observations indicate the presence of a kinetically trapped HS-fraction (frozen-in effect) during the HS-to-LS switching, when the sample was studied at the scan rate of 3 K/min. Crucially, the complex can be sublimed; efforts are on to elucidate the nature of SCO in the thin film state. Overall, a simple and easy to prepare sublimable complex—[Fe(H2Bpz2)24,4’-Br2-bpy]—shows bistable spin-state switching characteristics that can be leveraged to fabricate spin-state switchable thin film architectures