Magnetic Characterization Techniques for Nanomaterials

Rotational anisotropy nonlinear harmonic generation (RA-NHG) is an all-optical technique by which crystallographic, magnetic, and electronic symmetries of crystalline materials’ bulk surface and interfaces may be examined. It also allows characterization of nanostructures and biological tissue as well as imaging applications. In this chapter, we describe the principles behind RA-NHG, discuss current experimental approaches, and review key experimental findings since 2009.

In situ characterization of minute amounts of complex fluids is a challenge. Magnetic rotational spectroscopy (MRS) with submicron probes offers flexibility and accuracy providing desired spatial and temporal resolution in characterization of nanoliter droplets and thin films when other methods fall short. MRS analyzes distinct features of the in-plane rotation of a magnetic probe, when its magnetic moment makes full revolution following an external rotating magnetic field. The probe demonstrates a distinguishable movement which changes from rotation to tumbling to trembling as the frequency of rotation of the driving magnetic field changes. In practice, MRS has been used in analysis of gelation of thin polymer films, ceramic precursors, and nanoliter droplets of insect biofluids. MRS is a young field, but it has many potential applications requiring rheological characterization of scarcely available, chemically reacting complex fluids.

Iron oxide nanoparticles are one of the most important materials for magnetic resonance imaging. The possibility of multifunctionalization, lack of toxicity, and variety of compositions make them ideal for many applications. Furthermore, the new generation of nanoparticles for “positive” contrast will increase even more their utility, particularly in the clinic.

Magnetic nanoparticles are found in computer memory and in futuristic biomedical applications. General models for the particle dynamics are essential to understanding and predicting dynamical behaviors in diverse conditions. Many approaches have been used to varying degrees of success. Here we present the most general methods for modeling: nonlinear, nonequilibrium models that typically require computational solving. We maintain rigor throughout so that the expressions arise from as close to first principles calculations as possible. We present the intuitively simpler, conceptually satisfying models too but are clear on their ranges of validity. We are also explicit in the computational implementation where necessary. At the end, we summarize the state of the art and the interesting problems that remain unsolved.

Magnetic force microscopy (MFM) is one of the operational modes of atomic force microscopy (AFM). In this mode, a magnetic probe is brought close to the sample surface and interacts with the magnetic stray fields emanating from the sample. The strength of the local magnetostatic interaction determines the vertical motion of the tip as it scans across the sample. Since early 1990s, it has been widely used in fundamental research on magnetic materials, as well as in the development of magnetic recording components. It has the capacity to map the local stray fields emanating from individual magnetic nanostructures of the sample, hence providing insight into its magnetic behavior.

Gold nanoparticles have been discovered to possess peculiar properties not found in the bulk metal, such as optical, catalytic, or magnetic properties. Whereas some of the properties are well understood, such as the optical ones, the magnetic properties are much more mysterious. This chapter is devoted to the magnetic characterization of nanoparticulate gold samples, using a variety of techniques.

Magnetic force microscopy (MFM) is scanning probe technique which enables the analysis of magnetic properties of the samples at the nanoscale using a microfabricated tip coated with a magnetic layer. In this chapter, we describe MFM and give an overview of its applications, ranging from well established to advanced new applications.

Certain magnetic nanoparticles are able to generate heat through magnetic moment reversal processes under the action of an adequate alternating magnetic field. This ability, together with biocompatibility and nanosize of the particles, makes them promising materials for biomedical applications. Among the potential applications is magnetic hyperthermia, an oncological therapy expected to battle malignant tumors with minimal side effects by using localized heating. The success of the therapy requires, among others, accurate quantification of the released heat leading to the prediction of the temperature increase in and around the treatment area. This chapter is devoted to the recent advances in the determination of this heating ability.

Quantitative Lorentz microscopy and electron holography are applied to probe the local magnetic properties of ferromagnetic nanostructures. We show here the possibilities of these techniques for the mapping of the magnetization states of nanoscale ferromagnets grown by focused electron beam induced deposition (FEBID) and for the analysis of the magnetization processes by the in situ application of magnetic fields.

Neutron reflectivity is a powerful nondestructive technique to characterize thin films and nanostructured materials. This technique works equally well for various types of systems like organic, inorganic, and biological materials both in solid and liquid forms. Neutron reflectivity measurements provide information regarding the thickness and density of a thin film as a function of depth and also about the roughness of the top surface and buried interfaces. In comparison with x-ray reflectivity study, the neutron reflectivity measurements provide much improved contrast for elements with close values of atomic numbers, even for isotopes of same element. Furthermore, the detail of the in-plane spin arrangement can be obtained from polarization analysis. Neutron reflectivity is more than a complementary technique to x-ray reflectivity measurement for structural and morphological studies and is essential for the study of magnetic ordering due to its capability to measure the average magnetic moment in absolute units simultaneously with the structural information. In this chapter we discuss the theory of neutron reflectivity technique and illustrate the merit of this technique with some recent examples. We explain also the analysis techniques of neutron reflectivity data in detail. Although the nature of interaction of neutron with matter is different from that of x-ray, the basic formalism for reflectivity presented here utilizing the wave nature of thermal/cold neutrons remain valid for both measurements.

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 nanoparticles. To demonstrate how a clear understanding of the total magnetism of the core–shell nanoparticle is obtained through the characterization techniques presented, we discuss how the magnetism of core–shell nanoparticles made of maghemite (γ-Fe₂O₃) cores and transition metal and metal oxide shells and identify how the overall nanoparticle magnetism is altered substantially by the interface, an extremely difficult region to characterize within the core–shell nanoparticle, which is critically important to the magnetism.

Bimetallic ferromagnetic nanoparticles can be characterized using various techniques such as neutron scattering, magnetometry, electron microscopy, etc. The results from most of those techniques are from the average sample, not from individual nanoparticle. The property of nanoparticle is affected by its shape, size, chemical order, and composition. The atomic level characterization of each nanoparticle is essential and can be done by employing scanning/transmission electron microscopy (S/TEM). The use of Z-contrast imaging in STEM analysis of material permits distinguishing the atomic columns of the constituents, and the spectroscopic techniques (electron energy loss and energy-dispersive X-ray spectroscopy) allow mapping the positions of different metals and the chemical order can be seen. In addition, the magnetic property can be investigated using electron holography and Lorentz microscopy, where the change in phase information is recorded, which is directly related with the local variation in magnetic induction and the electrostatic potential.

The nanodentritic noble metals structure 3D spatial porous nanoclusters with high specific area. The magnetism of this kind of nanoparticles depends on fabrication techniques greatly.

Magneto-optical (MO) techniques are sensitive and versatile tools for the study of magnetic nanomaterials. Interaction of polarized light with a magnetized medium brings information on the magnetic properties of the sample, thus making MO techniques a valid alternative to standard magnetometric techniques. On the other hand, spectroscopic degrees of freedom arising from the tuneability of the incoming photon energy give access to an additional set of information, inaccessible to other investigation methods.

Magnetic resonance imaging (MRI) has developed at an exponential rate over the last decades, and the development of contrast agents to enhance the visualization of organs has followed the same trend. Meanwhile, magnetic nanoparticles that generate either “positive” or “negative” contrast in MRI have become one of the most important biomedical applications of nanotechnology. Indeed, superparamagnetic iron oxide nanoparticles, as negative contrast agents for T ₂/T ₂ ^* -weighted imaging, have found numerous applications in preclinical and clinical MRI (cell labeling, vascular contrast, lymph node imaging, liver contrast). In addition to this, paramagnetic and antiferromagnetic nanoparticles based on the elements Gd³⁺ and Mn²⁺ have mainly been exploited in vascular procedures and targeted imaging, for their capacity to enhance the MR signal of blood and of molecular signatures of endovascular disease. They are commonly referred to as “positive” contrast agents for T ₁-weighted imaging.

The present chapter is an introduction to the fundamental principles of nanoparticle-based MRI contrast agents. It addresses the main considerations guiding the relaxometric characterization of aqueous suspensions of magnetic nanoparticles, based on the elements iron, manganese, and gadolinium (Fe, Mn, Gd). The relaxivity of MRI contrast agents depends on their nanoparticulate structure, on their magnetic properties, on the distance between water molecules and their surface, and on the kinetics and rotational rate of the compound in biological fluids and in tissues. Among the main parameters guiding the relaxation time of water protons in the vicinity of contrast agents, figure the number of water molecules bound to the contrast agent, the size of the nanocrystals, the total hydrodynamic diameter of nanoparticles, their rotational correlation time, and the exchange rate between the water and the nanoparticle surface.

The general magnetic and relaxometric characteristics of the major classes of nanoparticles used as MRI contrast agents will be reviewed. Examples of nuclear magnetic relaxation dispersion profiles (NMRD), revealing the relaxometric potential of magnetic particles at increasing magnetic field strengths, are also presented and discussed.