New Trends in Nanoparticle Magnetism

In this chapter we discuss some intrinsic features of nano-scaled magnetic systems, such as finite-size, boundary, shape and surface effects. We mainly review in a succinct manner the main results of previous works. We first present the basics of theoretical models and computational techniques and their applications to individual nanomagnets. Results of both simulations and analytic calculations for specific materials, compositions and shapes are given based on these models.

In this chapter, we discuss the interparticle interaction effects in assemblies of magnetic nanoparticles. For our study, we have developed a mesoscopic scale model that takes into account: (a) the morphology of the assemblies and (b) the interplay between the interparticle and intra-particle characteristics of the nanoparticles. The hysteresis loops, the virgin magnetization curves and the temperature-dependent (Field Cooled (FC)/zero-field cooled (ZFC)) magnetization curves have been calculated with our model. Results are presented for three case studies of different nanoparticles’ morphologies assemblies and they show that our mesoscopic model reproduces well the experimentally studied systems and reveals the origin of the observed magnetic behavior.

The mechanisms responsible for magnetic interaction between nanoparticles are described and modelled in the previous chapter of this book. Here, the collective superspin glass state resulting from such interaction is discussed, using a collection of experimental results. Superspin glasses display qualitatively similar dynamical magnetic properties as canonical spin glasses, including ageing, memory and rejuvenation phenomena. In the Introduction, the dynamical properties of spin and superspin glasses are illustrated and contrasted. These properties are discussed in more detail in Case studies, taking into account the nanoparticle concentration, size and size distribution, using results from studies of ferrofluids and compacts of γ-Fe₂O₃ particles. The Outlook section illustrates recent findings suggesting that the temperature dependence of the low-field isothermal remanent magnetization (IRM) and magnetization as a function of magnetic field (hysteresis or M-H) curves of superspin glasses include information on the superspin dimensionality and magnetic anisotropy. The possibility to engineer nanocomposites with tailored magnetic interaction and anisotropy is also discussed.

The advances in the physical and chemical fabrication methods have enabled the possibility to produce artificial nanostructures whose properties are different from that of their constituent materials. The presence of interfaces in core/shell bimagnetic nanoparticles introduces additional interactions that could radically modify the static and dynamic magnetic behavior of the systems. The number of parameters that governs the magnetic behavior grows enormously and the opportunity to manipulate, control, and understand the role played by each one of them, opens a wide range of possibilities to design novel materials with suited properties. The magnetic response changes depend on the magnetic ordering and anisotropy of the phases, the core size and shell thickness, the quality of the interface, and the strength of the interface exchange coupling. In this chapter, we discuss the new properties found in core/shell bimagnetic nanoparticles and analyze the main characteristics that have to be taken into account to design a system with a particular response.

Magnetoplasmonics nanoparticles encompass in a single nano-entity all the rich science and promising applications of the plasmonics and magnetic nanoworlds. The difficult liaison and a certain incompatibility between plasmonics and magnetic phenomena, due to the different chemical-physical origins and supporting materials, are overcome thanks to the design and synthesis of novel nanostructures. The variations of properties, interactions and synergies of both phenomena and materials demonstrate how rich and surprising the matter is at nanoscale and the promising applications. In fact, we show how not only light and magnetism can interplay but also other phenomena like forces, heat, electric field and chemical interactions, between others, can show synergism. Magnetoplasmonic systems are excellent benchmark materials to develop and investigate multi-responsive multifunctional nanosystems that now are required in an increasing number of technologies, such as biomedicine, pharmacy, catalysis, optoelectronics and data storage.

Hollow magnetic nanoparticles present a characteristic morphology that gives rise to interesting magnetic behaviors and novel applications. In this chapter, we describe the synthesis methods utilizing the Kirkendall effect and the magnetic properties of these nanoparticles, with a focus on the analysis of their enhanced surface anisotropy, spin disorder, and exchange bias effect. The experimental studies are complemented by atomistic Monte Carlo simulations. Finally, we review a variety of applications of these nanoparticles, especially in biomedicine, batteries, sensing, and data storage, and also discuss some of the limitations that need to be overcome for their implementation.

Magnetotactic bacteria are aquatic microorganisms that have the ability to align in the geomagnetic field lines, using a chain of magnetic nanoparticles biomineralized internally (called magnetosomes) as a compass needle. Here we describe the biogenesis of magnetosomes, focusing in the formation of the mineral core. We then discuss the magnetic properties of the magnetosomes and the chain of magnetosomes, a natural paradigm of a magnetic 1D nanostructure. Finally, we review the use of magnetosomes and magnetotactic bacteria in biomedical and biotechnological applications, with special mention to the application in magnetic hyperthermia treatments.

In this chapter, we show that thanks to the use of micellar and organometallic approaches, one can favor the growth of uniform spherical Co NPs with controlled surface passivation (dodecanoic acid or oleylamine), tunable size (from around 4 to 9 nm) and tunable nanocrystallinity (from fcc to hcp structure). As a result of the balance between van der Waals attractions between the metallic NPs, magnetic interactions between the magnetic NPs and solvent-mediated interactions between ligands, these uniform colloidal NPs can be used as building units to form a full set of assemblies which morphology depends on the deposition strategy, involving solvent evaporation. In the case of spontaneous self-assembling of magnetic NPs, compact hexagonal 2D arrays and 3D superlattices called supercrystals can form. In the latter case, either face-centered cubic supercrystalline films or single colloidal crystals can be obtained. Mesostructures of hexagonally ordered columns, labyrinths and void structures can result from assisted self-assembling, induced by the application of an external magnetic field. In highly ordered superlattices, individual NPs act as “artificial atoms” and occupy the lattice sites to form repetitive, periodic “artificial planes". From a fundamental point of view, these artificial solids constitute good models for investigating crystallization behavior. Resulting from collective interactions between neighboring NPs, they exhibit novel magnetic properties. The magnitude of these interactions, and then, the magnetic properties, can be tuned by various parameters including (1) the (crystallographic) nature of the magnetic NP, (2) the NP size, (3) the nature of the coating agent, (4) the nature of the solvent, (5) the evaporation rate and (6) if appropriate, the application of an external field during the solvent evaporation. On the one hand, simulations based on a flory-type solvation theory using Hansen solubility colloidal parameters allow to predict the cobalt NP size. On the other hand, Monte Carlo simulations and free energy theories are able to predict the size and type of patterns appearing during the evaporation of a solution of magnetic NPs under a magnetic field.

Magnetic nanoparticles are of great interest for applications in fields ranging from biomedicine to spintronics. However, despite considerable work, their size-dependent magnetic properties are still poorly understood. In this chapter, we will introduce x-ray photoemission electron microscopy (XPEEM) as a spectromicroscopy technique ideally suited for the investigation of the magnetic properties of large numbers of individual nanoparticles in extended ensembles. Moreover, XPEEM can be combined with other microscopy techniques to achieve a direct correlation between magnetism, size, shape, and structure of the very same nanoparticles. This approach has led to the discovery of novel magnetic states in 3d transition metal nanoparticles characterized by strongly enhanced magnetic energy barriers, attributed to the strong impact of structural defects rather than to surface or interface contributions to the total magnetic anisotropy.

The chapter describes the development of X-ray magnetic circular dichroism (XMCD) using circularly polarised soft X-ray photons from synchrotron sources. Following the derivation of X-ray absorption sum rules for magnetic materials, the technique became a powerful probe of magnetism able to separately measure the atomic and spin orbital magnetic moments independently for each magnetic element in the sample. The majority of the experiments have focused on the L-absorption edges of transition metals and the method has been particularly useful in identifying the source of enhanced magnetic moments in nanostructures. The chapter illustrates the power of the method with a specific example, that of Fe@Cr core–shell nanoparticles with different Cr shell thicknesses. Here, it was shown that at least two Cr atomic layers are required to see the onset of the exchange bias effect at the ferromagnetic–antiferromagnetic interface. The future perspectives of the technique are described including spatially resolved XMCD and time-resolved XMCD measurements.

The recent advances in TEM instrumentation with faster and more sensitive detectors, and the ever-increasing number of advanced algorithms capable of achieving quality 3D reconstructions with fewer acquired projections, are transforming electron tomography in one of the most versatile tools for a materials scientist, as the possible field of application for this technique is open to virtually any nanoscaled material. The complete three-dimensional characterization of magnetic nanoparticles is not an exception. Not only the 3D morphology is resolved, but also the elemental composition in 3D, by combining the available reconstruction algorithms and TEM spectral characterization techniques, seeking to retrieve the so-called spectrum volume. Among them, electron energy loss spectroscopy (EELS) stands out, given its unchallenged lateral resolution and its unique ability to resolve variations of the oxidation states, and even atomic coordination, through the analysis of the fine structure of the elemental edges in the acquired spectra. This chapter is a revision of electron tomography strategies applied to magnetic nanomaterials, beginning in a chronologically ordered description of some of the commonly used algorithms and their underlying mathematical principles: from the historical Radon transform and the WBP, to the iterative ART and SIRT algorithms, the later DART and recently added compressed sensing-based algorithms with superior performance. Regarding the spectral reconstruction, dimensionality reduction techniques such as PCA and ICA are also presented here as a viable way to reduce the problem complexity, by applying the reconstruction algorithms to the weighted mappings of the physically meaningful resolved components. In this sense, the recent addition of clustering algorithms to the possible spectral unmixing tools is also described, as a proof of concept of its potentiality as part of an analytical electron tomography routine. Throughout the text, a series of published experiments are described, in which electron tomography and advanced EELS data treatment techniques are used in conjunction to retrieve the spectrum volume of several magnetic nanomaterials, revealing details of the NPs under study such as the 3D distribution of oxidation states.

Among the various techniques for the characterization of magnetic NPs, magnetic force microscopy (MFM) represent one of the most widespread and versatile methods due to its lateral resolution, sensitivity, imaging capability, the need for a relatively simple and widespread experimental setup, minimal/no specific requirements about sample preparations, capability to operate in air at room conditions as well as in vacuum or liquid environment. Indeed, MFM enables the quantitative characterization of magnetic properties of single magnetic NPs, can be used to detect single magnetic NPs in nonmagnetic (e.g., polymeric or biological) matrices, as well as to perform mechanical or magnetic nanomanipulation of single NPs. In this chapter, applications of MFM in the study of magnetic NPs are briefly reviewed and intriguing perspectives are depicted, focusing on current limitations to overcome and challenges to take up.

The ongoing research on the applications of magnetic nanoparticles in Bio-medicine and the results obtained up to now open a wide range of possibilities for their use in Life Science disciplines, for example in general plant research and agronomy. The work presented here focuses on the interaction of two types of magnetic core-shell nanoparticles with plants and microorganisms. The research carried out with carbon coated iron nanoparticles aims to investigate their penetration and translocation in whole living plants and into plant cells, as response of the nanoparticles to magnetic field gradients. This study is essential to evaluate the suitability of any nanoparticles as magnetic responsive carriers for the localized delivery of phytosanitary or pest control treatments. The study carried out with silica coated nanoparticles focuses on their interaction with fungal cells, taking a soil borne plant pathogen as in vitro model. Our research paves the way to use magnetic nanoparticles for detection, selective control and eventual elimination of pathogenic fungi.

The increased ability in manipulating matter at the nanoscale has paved the way towards the creation of a plethora of novel systems endowed with extremely appealing properties exploitable in a wide number of clinical applications, the two most prominent being, undoubtedly, Magnetic Resonance Imaging (MRI) and Magnetic Fluid Hyperthermia (MFH). In this Chapter, we review a few recent examples to convey to the reader a picture of the promising role magnetic nanoparticles (MNPs) may play in the medicine of the next future. After a general overview of the two techniques, we summarize the physical principles at their base to provide the reader the necessary tools to understand limits and advantages of employing MNPs as contrast agents in MRI and heat mediators in MFH. Among the countless examples of MNP-based materials proposed in the recent years for these applications, we select and report in detail some of the most representative and promising ones to underline the challenges that this branch of the material science must address to interplay with the complexity of the human body. Finally, we try to photograph the state of the art of the clinical applications, which, mainly concerning MFH, is in continuous evolution.

This chapter provides an overview of the various types of magnetic micro- and nanoparticle systems used in biomedical applications. We broadly divide particle types into colloidally synthesized and lithographically defined on silicon wafers. The applications relevant to each particle type are highlighted followed by research case studies. Each case study highlights a novel approach to the engineering of magnetic particles for a specific application. Finally, future perspectives for the field are described with an emphasis on the challenges remaining to be solved for all the main application areas of magnetic particles.

The unique properties of magnetic nanoparticles (MNP) and their interactions with their environment have given rise to innovative R&D possibilities outside the field of conventional magnetism. One such example is in the field of energy science, and in particular, the thermal engineering. In this respect, research on refrigeration technology based on the magnetoconvection property of ferrofluids (FF) has attracted great attention in the past decades. On the other hand, the thermoelectric energy conversion (or more commonly known as “thermopower”) in ferrofluids has so far remained underexplored. This subchapter describes this very new research path in the field of magnetic nanoparticle science, from its theoretical background and motivation, a few existing example of experimental investigations and the future perspectives. The unique properties of magnetic nanoparticles (MNP) and their interactions with their environment have given rise to innovative R&D possibilities outside the field of conventional magnetism. One such example is in the field of energy science, and in particular, the thermal engineering. In this respect, research on refrigeration technology based on the magnetoconvection property of ferrofluids (FF) has attracted great attention in the past decades. On the other hand, the thermoelectric energy conversion (or more commonly known as “thermopower”) in ferrofluids has so far remained underexplored. This subchapter describes this very new research path in the field of magnetic nanoparticle science, from its theoretical background and motivation, a few existing example of experimental investigations and the future perspectives.

Permanent magnets are exploited in a variety of devices (e.g. motors, generators, sensors, actuators) used in various fields of applications including transportation (e.g. (hybrid)electric vehicles), energy management (wind turbines…) and information technology (e.g. hard disc drives). Permanent magnet research today is concerned with improving the performance of magnets based on various hard magnetic phases while reducing dependence on any critical materials used. Nanocomposite magnets which combine a high coercivity hard magnetic phase with a high magnetisation soft magnetic phase hold great potential to rise to this challenge. In this chapter, we briefly outline the history of permanent magnets and explain the basic physical concepts behind nanocomposite permanent magnets. We recall the metallurgical and physical vapour deposition synthesis routes used to fabricate bulk and thin film nanocomposites, respectively. We then focus on chemical synthesis methods which offer the possibility to produce hard and soft magnetic nanoparticles or core-shell nanoparticles that can be used as building blocks to fabricate bulk hard-soft nanocomposites. We present three case studies concerning the fabrication and structural and magnetic characterisation of FePt-Fe₃Pt, FePd-Fe and SmCo₅-Fe nanocomposites. We wrap up the chapter with an outline of the challenges faced in producing hard-soft nanocomposite magnets using chemically synthesised nanoparticles, and an overview of the advanced magnetic characterisation tools being used to study the complex magnetisation reversal processes at play in hard-soft nanocomposites.