Anca Meffre

A Simple Chemical Route toward Monodisperse Iron Carbide Nanoparticles Displaying Tunable Magnetic and Unprecedented Hyperthermia Properties

We report a tunable organometallic synthesis of monodisperse iron carbide and core/shell iron/iron carbide nanoparticles displaying a high magnetization and good air-stability. This process based on the decomposition of Fe(CO)(5) on Fe(0) seeds allows the control of the amount of carbon diffused and therefore the tuning of nanoparticles magnetic anisotropy. This results in unprecedented hyperthermia properties at moderate magnetic fields, in the range of medical treatments.

Increase of magnetic hyperthermia efficiency due to dipolar interactions in low-anisotropy magnetic nanoparticles: Theoretical and experimental results

When magnetic nanoparticles (MNPs) are single-domain and magnetically independent, their magnetic properties and the conditions to optimize their efficiency in magnetic hyperthermia applications are now well-understood. However, the influence of magnetic interactions on magnetic hyperthermia properties is still unclear. Here, we report hyperthermia and high-frequency hysteresis loop measurements on a model system consisting of MNPs with the same size but a varying anisotropy, which is an interesting way to tune the relative strength of magnetic interactions. A clear correlation between the MNP anisotropy and the squareness of their hysteresis loop in colloidal solution is observed : the larger the anisotropy, the smaller the squareness. Since low anisotropy MNPs display a squareness higher than the one of magnetically independent nanoparticles, magnetic interactions enhance their heating power in this case. Hysteresis loop calculations of independent and coupled MNPs are compared to experimental results. It is shown that the observed features are a natural consequence of the formation of chains and columns of MNPs during hyperthermia experiments: in these structures, when the MNP magnetocristalline anisotropy is small enough to be dominated by magnetic interactions, the hysteresis loop shape tends to be rectangular, which enhance their efficiency. On the contrary, when MNPs do not form chains and columns, magnetic interactions reduces the hysteresis loop squareness and the efficiency of MNPs compared to independent ones. The present work should improve the understanding and interpretation of magnetic hyperthermia experiments.

Large specific absorption rates in the magnetic hyperthermia properties of metallic iron nanocubes

We report on the magnetic hyperthermia properties of chemically synthesized ferromagnetic 11 and 16 nm Fe(0) nanoparticles of cubic shape displaying the saturation magnetization of bulk iron. The specific absorption rate measured on 16 nm nanocubes is 1690±160 W/g at 300 kHz and 66 mT. This corresponds to specific losses-per-cycle of 5.6 mJ/g, largely exceeding the ones reported in other systems. A way to quantify the degree of optimization of any system with respect to hyperthermia applications is proposed. Applied here, this method shows that our nanoparticles are not fully optimized, probably due to the strong influence of magnetic interactions on their magnetic response. Once protected from oxidation and further optimized, such nano-objects could constitute efficient magnetic cores for biomedical applications requiring very large heating power.

Use of long chain amine as a reducing agent for the synthesis of high quality monodisperse iron(0) nanoparticles

This article reports the synthesis of iron(0) nanoparticles at moderate temperature—from 120 °C to 150 °C—using the reduction of the organometallic iron(II) precursor {Fe[N(SiMe3)2]2}2 by hexadecylamine (HDA) in the absence of dihydrogen (H2). The nanoparticles are monodisperse in size and self-assemble into 2D super-lattices suitable for transport measurements. The nanoparticles are stabilized in mesitylene by a mixture of HDA and hexadecylammonium chloride (HDA·HCl). The resulting truncated single-crystalline nanocubes have a narrow size distribution and a high magnetization close to the bulk value. The products are characterized by transmission electronic microscopy (TEM and HRTEM), SQUID measurements, Mossbauer and Infra-Red spectroscopies. Fe(II) reduction is accompanied by oxidation of amines into imines which was detected as a by-product. This reduction occurs at 120 °C and above. The temperature, in conjunction with the reaction time, allows for a fine control of the nano-objects final size. The latter can also be tuned with the HDA·HCl concentration. Finally, this one-pot synthesis produces high-quality magnetic nanoparticles with mean sizes in the range 6 to 10 nm depending on the conditions.

Complex Nano-objects Displaying Both Magnetic and Catalytic Properties: A Proof of Concept for Magnetically Induced Heterogeneous Catalysis

Addition of Co2(Co)9 and Ru3(CO)12 on preformed monodisperse iron(0) nanoparticles (Fe(0) NPs) at 150 °C under H2 leads to monodisperse core-shell Fe@FeCo NPs and to a thin discontinuous Ru(0) layer supported on the initial Fe(0) NPs. The new complex NPs were studied by state-of-the-art transmission electron microscopy techniques as well as X-ray diffraction, Mössbauer spectroscopy, and magnetic measurements. These particles display large heating powers (SAR) when placed in an alternating magnetic field. The combination of magnetic and surface catalytic properties of these novel objects were used to demonstrate a new concept: the possibility of performing Fischer-Tropsch syntheses by heating the catalytic nanoparticles with an external alternating magnetic field.

Room-Temperature Tunnel Magnetoresistance in Self-Assembled Chemically Synthesized Metallic Iron Nanoparticles

We report on room temperature magnetoresistance in networks of chemically synthesized metallic Fe nanoparticles surrounded by two types of organic barriers. Electrical properties, featuring Coulomb blockade, and magnetotransport measurements show that this magnetoresistance arises from spin-dependent tunnelling, so the organic ligands stabilizing the nanoparticles are efficient spin-conservative tunnel barrier. These results demonstrate the feasibility of an all-chemistry approach for room temperature spintronics.

Magnetic anisotropy determination and magnetic hyperthermia properties of small Fe nanoparticles in the superparamagnetic regime

We report on the magnetic and hyperthermia properties of 5.5 nm in diameter iron nanoparticles synthesized by organometallic chemistry, which display the bulk magnetization. Quantitative analysis of alternative susceptibility measurements allows the determination of the effective anisotropy Keff=1.3×105 J m−3. Hyperthermia measurements are performed at a magnetic field up to 66 mT and at frequencies in the range of 5–300 kHz. Maximum measured specific absorption rate (SAR) is 280 W/g. SAR displays a square dependence with the magnetic field below 30 mT but deviates from this power law at higher value. SAR is linear with the applied frequency for μ0H=19 mT. These results are discussed in the light of linear response theory.

How to Modulate Catalytic Properties in Nanosystems: The Case of Iron–Ruthenium Nanoparticles

Ultrasmall FeRu bimetallic nanoparticles were prepared by co‐decomposition of two organometallic precursors, {Fe[N(Si(CH3)3)2]2}2 and (η4‐1,5‐cyclooctadiene)(η6‐1,3,5‐cyclooctatriene)ruthenium(0) (Ru(COD)(COT)), under dihydrogen at 150 °C in mesitylene. A series of FeRu nanoparticles of sizes of approximately 1.8 nm and incorporating different ratios of iron to ruthenium were synthesized by varying the quantity of the ruthenium complex introduced (Fe/Ru=1:1, 1:0.5, 1:0.2, and 1:0.1). FeRu nanoparticles were characterized by TEM, high‐resolution TEM, and wide‐angle X‐ray scattering analyses. Their surface was studied by hydride titration and IR spectroscopy after CO adsorption and their magnetic properties were analyzed by using a superconducting quantum interference device (SQUID). The FeRu nanoparticles were used as catalysts in the hydrogenation of styrene and 2‐butanone. The results indicate that the selectivity of the nanoparticle catalysts can be modulated according to their composition and therefore represent a case study on fine‐tuning the reactivity of nanocatalysts and adjusting their selectivity in a given reaction.

Off-Axis Electron Holography for the Quantitative Study of Magnetic Properties of Nanostructures: From the Single Nanomagnet to the Complex Device

Electromagnetic properties are one of the keys for understanding and mastering nano systems used in many applications, as in medical treatment, optics, microelectronic or data storage. Various methods exist to map magnetic fields. Some are based on near field microscopy, like magnetic force microscopy, other on X-ray set-ups, like photoemission electron microscopy. Electron holography (EH), a powerful transmission electron microscopy method, is another appropriate tool which combines high sensitivity with a high spatial resolution. EH allow the quantitative measurement of both internal and external fields in individual nano-objects instead of assemblies of nanoobjects. This interferometric method can also be used for performing in situ/in operando experiments. We thus developed and applied EH on very different systems, from the single nanoparticles to the thin layer and the complex magnetic device, for studying their magnetic properties. In this presentation, we will present our investigations on a single Fe nanocube and an FeRh thin layer. - Nanomagnets recently attracted considerable interest due to their possible application as building blocks for hard drive disks and permanent magnets or as nanobiological vectors for drug delivery and hyperthermia. Despite theoretical studies, the size-dependence of spin arrangements in single nanomagnets has not yet been evidenced experimentally due to sensitivity limitations of the investigation tools. The single domain limit, corresponding to the critical nanomagnet size separating vortex/single domain configurations, has never been observed although it will dictate the optimized size for applications. In such small nano-objects, micromagnetic simulations show that the magnetic internal structure changes from single domain (SD) to vortex states as the cube size increases (Fig. 1). Some years ago, we reported symmetrical vortices, i.e. vortex of <001> axis, in isolated 30 nm single crystal Fe cubes with a 14 nm vortex core size [2]. Next, we showed that vortices can also be stabilized in the presence of dipolar interactions thanks to holes in the cubes inducing a pinning of the vortex core [3]. Here we will present the spin configuration phase diagram in size-controlled single Fe nanocubes combining EH experiments and micromagnetic simulations [4]. High sensitivity imaging explicitly reveals how three different spin arrangements can be stabilized within a 3 nm window, evidencing the key importance of nanometric size control of magnetic nanoparticles. Moreover, it gives a deeper understanding of the single domain limit, which is more complex than expected with the appearance of a previously unreported <111> vortex state. Such a measurement opens the door to fine magnetic control of nano-objects. - In situ heating/cooling EH has been used to quantitatively map the magnetization of a cross-sectional FeRh thin film through its magnetic transition [5]. This alloy presents a remarkable and unusual magnetic transition from a low temperature antiferromagnetic state (AFM) to a high temperature ferromagnetic state (FM) close to 370K accompanied by a 1% volume expansion. [6- 8]The transition is obtained for a narrow composition range 0.48<x<0.56 in the B2-ordered a’ crystal phase of Fe1-xRhx. These remarkable features make FeRh particularly well suited for advanced magnetic devices for instance in heatassisted or even electrically –assisted magnetic recording [9- 10]and magnetic random access memories based on AFM spintronics [11], or for magnetocaloric materials [12]. Further developments of reliable devices including thin FeRh films or others materials presenting a magnetic transition require a full control of the magnetic state within the film, a perfect knowledge of the mechanisms involved in the transition and a deep understanding of the influence of interfaces and low dimensionality. However, the mechanisms involved in the transition are still under debate. EH experiments provide a direct observation of an inhomogeneous spatial distribution of the transition temperature along the growth direction. Most interestingly, a regular spacing of the ferromagnetic domains nucleated upon monitoring of the transition is also evidenced. Beyond these findings on the fundamental transition mechanisms, EH also brings insights for in operando analysis of magnetic devices and a very promising approach to investigate the mechanisms of phase transitions in various magnetic systems (as MnAs [13]) at all pertinent scales.