Nanomagnetism (Part I)
Fundamentals
Nanoscale magnets have at least one dimension in the nanometre range. They exhibit size-specific magnetic properties such as superparamagnetism, remanence enhancement, exchange averaging of anisotropy and giant magnetoresistance when the small dimensions become comparable to a characteristic magnetic or electrical length scale. Thin films are the most versatile magnetic nanostructures, and interface effects such as spin-dependent scattering and exchange bias influence their magnetic properties. Thin-film stacks form the basis of modern magnetic sensors and memory elements.
Matter behaves differently down in the nanoworld, where the length scales of interest range from about 1 nm up to about 100 nm. The atomic-scale structure of matter can usually be ignored, but the mesoscopic dimensions of the magnetic nano-objects are comparable to some characteristic length scale, below which the physical properties change. We have already encountered one important nanoscale object in bulk magnetic material – the domain wall. It is extended in two directions, but not in the third; the domain wall width δw is one of the characteristic lengths that concern us here.
The number of small dimensions in a nanoscale magnet may be one, two or three. The one-small-dimension class includes magnetic thin films, which are at the heart of many modern magnetic devices. Magnetic and nonmagnetic layers can be stacked to make thin-film heterostructures, such as spin valves and tunnel junctions. The films are usually grown on a macroscopic substrate.
Contemporary needs for increasing information storage and high density magnetic recording have stimulated new research in mesoscopic magnetism. In analogy to semiconductor physics, the process of miniaturizing magnetic materials has revealed a variety of new and unexpected classical and quantum mechanical phenomena. On a small enough length scale the interactions between individual atomic spins cause their magnetic moments to be aligned in the ordered pattern of a single domain, without the complication of domain walls separating regions of varying orientation. For particle sizes at or below that of a single domain, standard theoretical models of dynamical behavior predict simple stable magnets with controllable properties. However, recent experimental studies of micron-scale ferromagnetic particles show them to be far less stable and to switch more easily than expected from traditional classical theories of thermal activation. In even smaller systems, new theoretical predictions and experimental observations suggest that at low enough temperature, quantum mechanical tunneling may ultimately control the particle’s magnetization.
Magnetic nanoparticles
Magnetic nanoparticles are nanomaterials consist of magnetic elements, such as iron, nickel, cobalt, chromium, manganese, gadolinium, and their chemical compounds. Magnetic nanoparticles are superparamagnetic because of their nanoscale size, offering great potentials in a variety of applications in their bare form or coated with a surface coating and functional groups chosen for specific uses. Especially, ferrite nanoparticles are the most explored magnetic nanoparticles, which can be greatly increased by clustering of a number of individual superparamagnetic nanoparticles into clusters to form magnetic beads.
Magnetic nanoparticles can be selective attached to a functional molecules and allow transportation to a targeted location under an external magnetic field from an electromagnet or permanent magnet. In order to prevent aggregation and minimize the interaction of the particles with the system environment, surface coating may be required. The surface of ferrite nanoparticles is often modified by surfactants, silica, silicones, or phosphoric acid derivatives to increase their stability in solution. In general, coated magnetic nanoparticles have been widely used in several medical applications, such as cell isolation, immunoassay, diagnostic testing and drug delivery.
Properties
1. Magnetic Property
The properties of magnetic nanoparticles depend on the synthesis method and chemical structure. In most cases, the magnetic nanoparticles range from 1 to 100 nm in size and can display superparamagnetism. Superparamagnetism is caused by thermal effects that the thermal fluctuations are strong enough to spontaneously demagnetize a previously saturated assembly; therefore, these particles have zero coercivity and have no hysteresis. In this state, an external magnetic field is able to magnetize the nanoparticles with much larger magnetic susceptibility. When the field is removed, magnetic nanoparticles exhibit no magnetization. This property can be useful for controlled therapy and targeted drug delivery.
2. Magnetocaloric Effect
Some magnetic materials heat up when they are placed in a magnetic field and cool down when they are removed from a magnetic field, which is defined as the magnetocaloric effect (MCE). Magnetic nanoparticles provide a promising alternative to conventional bulk materials because of their particle size-dependent superparamagnetic features. In addition, the large surface area in magnetic nanoparticles has the potential to provide better heat exchange with the surrounding environment. By careful design of core-shell structures, it would be possible to control the heat exchange between the magnetic nanoparticles and the surrounding matrix, which provide a possible way for improving therapy technologies, such as hyperthermia.
3. Superparamagnetism
Superparamagnetism is a form of magnetism which appears in small ferromagnetic or ferrimagnetic nanoparticles. In sufficiently small nanoparticles, magnetization can randomly flip direction under the influence of temperature. The typical time between two flips is called the Neel relaxation time. In the absence of an external magnetic field, when the time used to measure the magnetization of the nanoparticles is much longer than the Neel relaxation time, their average value of magnetization appears to be zero: they are said to be in the superparamagnetic state. In this state, an external magnetic field is able to magnetize the nanoparticles, similar to a paramagnet. However, their magnetic susceptibility is much larger than that of paramagnets.
Normally, any ferromagnetic or ferrimagnetic material undergoes a transition to a paramagnetic state above its Curie temperature.
Superparamagnetism is different from this standard transition since it occurs below the Curie temperature of the material.
Superparamagnetism occurs in nanoparticles which are single domain, i.e., composed of a single magnetic domain. This is possible when their diameter is below 3–50 nm, depending on the materials. In this condition, it is considered that the magnetization of the nanoparticles is a single giant magnetic moment, the sum of all the individual magnetic moments carried by the atoms of the nanoparticle.
Magnetic nanostructures are being developed for use in many aspects of our daily life, spanning areas such as data storage, sensing and biomedicine. Whereas patterned nanomagnets are traditionally two-dimensional planar structures, recent work is expanding nanomagnetism into three dimensions; a move triggered by the advance of unconventional synthesis methods and the discovery of new magnetic effects. In three-dimensional nanomagnets more complex magnetic configurations become possible, many with unprecedented properties.
Imagen Courtesy: Schematic view comparing some examples of geometries and magnetic configurations (indicated by blue arrows) for (a) 3D and (b) 2D nanomagnetism. The dependence of the magnetization M on spatial coordinates (black arrows) is indicated for both cases. New synthesis, characterization and computational methods have the potential to make the leap to 3D. The combination of more complex magnetic states and additional degrees of freedom in 3D nanomagnets leads to the emergence of new physical phenomena, which may find applications in multiple areas. (a) Examples of 3D nanomagnets, from left to right: magnetic sphere with vortex configuration. Magnetic thin film element with a skyrmion. Symmetry breaking is caused by bulk or interfacial Dzyaloshinskii-Moriya interaction. Möbius strip with perpendicular magnetization; a DW is present in the ground state due to the object’s topology. Cylindrical NW with modulated diameter, with different magnetic configurations depending on the diameter. Antiferromagnetic (AF) superlattice (interlayers not shown for clarity) with a wide soliton in the middle. (b) Examples of 2D nanomagnets, from left to right: Single-domain magnet. Magnetic multi-layered element with perpendicular anisotropy. Nanostrip with protrusions for DW trapping. Bi-layered magnet with AF coupling due to indirect exchange via an interlayer. Amalio Fernández-Pacheco (Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge, CB3 0HE, UK)
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