File Name: permanent magnet materials and their application .zip
The use of non-invasive alternating magnetic field AMF on biocompatible, small sized iron oxide nanoparticles can be used for heat generation in magnetic hyperthermia or for contrast enhancement of biological tissue. However, the behavior of magnetic nanoparticle in the presence of magnetic field is restricted by their size, shape, surface defects, and coatings.
Hence, it becomes imperative to closely monitor the magnetic properties of the nanoparticles as current novel formulations of nanoparticles being developed for tissue targeting involves conjugating a magnetic nanoparticle with a site specific ligand or a peptide. Thus, in this review article we have reviewed the effect of size, shape, doping, and surface coating on the magnetic properties of the nanoparticle.
Finally we have concluded with the clinical status of magnetic nanoparticle in the field of magnetic fluid hyperthermia. Nanotechnology, a science which deals with design and applications of nanomaterials, can provide valuable information to differentiate abnormalities in various body structures and organs to determine the extent of disease, and evaluate the effectiveness of treatment rendered. Hence, controlling nanomaterials in vivo is desired for optimum diagnostic or therapeutic outcome.
Among nanomaterials, magnetic nanoparticles MNPs have received significant attention in the field of biomedical engineering.
This is due to the fact that the intrinsic properties of MNPs provide a non-invasive means to control their fate for a wide variety of applications such as biosensors, magnetic fluid hyperthermia MFH , magnetic resonance imaging MRI , magnetic drug and gene delivery, and magnetic separation 1—4.
The latter has already been FDA approved for its application as an MRI contrast agent because of its inoffensive toxicity profile and biocompatibility. Besides, along with their tunable magnetic properties, MNPs also provide reactive surface that can be readily modified with biocompatible coatings for targeting tissues. Thus aided aggregation of magnetic nanoparticles in tumor tissues can be achieved by conjugating nanoparticles with tumor targeting peptides or antibodies 5, 6.
However, these functionalization and modification of the MNPs, lead to change in the size, shape which greatly affect the magnetic properties such as coercivity Hc of these nanosphere which is of utmost importance for their application in the field of magnetic fluid hyperthermia MFH.
For example, the MFH requires a particle which behaves as a soft magnet so that it will retain some magnetization after the removal of the magnetic field. Of course, care has to be taken to retain its inertness while synthesizing a soft magnet with the highest possible saturation. These intrinsic properties are mainly affected by particles shape, size, surface defects, surface ligands, and temperature to name few 7— Hence, it is imperative to mention that the design of novel MNPs for biomedical application requires careful evaluation of surface modification, size, and shape, on its magnetic properties.
A thorough consideration of each design parameter must be evaluated to produce MNPs that can overcome biological barriers and carry out its function. Even though it is impossible to consider all these effects in details for this review article, in the next section we have tried to outline the striking effects of the above mentioned factors on the magnetic properties of the MNPs.
Furthermore, we have also reviewed the basic of magnetism such as ferromagnetism soft and hard magnets , and superparamagnetism along with various MNPs for their biological application followed by their current clinical status in the field of magnetic fluid hyperthermia.
Magnetic materials encompass a variety of materials which are used in a diverse range of applications. They can be classified in terms of their magnetic behavior. Most common types of magnetic behaviors are diagmagnetism and paramagnetism which account for the magnetic properties of elements at room temperature.
Consequently, most elements in the periodic table are usually referred as non-magnetic, whereas, those which are cited as magnetic are classified as ferromagnetic 7.
Most of magnetic materials of industrial interests are ferromagenetic materials. In general, the magnetic effects are caused by movements of particles that have both mass and electric charge. These particles can be electrons, holes, protons and positive and negative ions. We are aware that a spinning electric charged particle creates a magnetic dipole, called magneton which are associated in groups in a ferromagnetic material 7.
Furthermore, the bulk of ferromagnetic material consists of a number of small regions of magnetons which are called domains as shown in Figure 1. The boundaries between domains are called domain walls. These domain walls are not thin surfaces but should be visualized as zones of transition of finite thickness in which the magnetization gradually changes the direction from one side to another Figure 1 7, Magnetic moment in both ferromagnetic and superparamagnetic materials.
On application of the magnetic field the domain walls in ferromagnetic materials are washed away and aligned to the direction of the magnetic field. Whereas, in superparamagnetic materials which are usually defined as single domain structures have no domain walls, but the magnetic moments align to the direction of the applied external magnetic field. The domain structure of the magnetic materials has been drawn for simplicity.
Thus, a magnetic domain in a ferromagnetic material refers to the volume of the material in which all magnetons are aligned in the same direction by the exchange forces. This concept of domains distinguishes ferromagnetism from paramagnetism. The ferromagnetic materials in a demagnetized state does not show any magnetization as the total magnetization is cancelled because of the random orientation of the magnetizations in magnetic domains.
However, on the application of an external magnetic field, the magnetic domain walls are washed away and magnetic moments become aligned to the direction of the magnetic field and saturate the magnetization Figure 1. This magnetization is called saturation magnetization Ms Figure 2. On removal of the applied magnetic field, instead of retracing its original path, ferromagnets retain some memory of the applied field called as remanence Point A in the curve, Figure 2.
To reduce the magnetization of that material to zero, a coercive force Pont B in the Curve, Figure 2 must be applied to a ferromagnetic material so as to close the loop. Thus coercivity measures the resistance of a ferromagnetic material to become demagnetized.
This behavior of the ferromagnetic material is known as the hysteresis and the path which it follows is known as the hysteresis loop Figure 2. The hysteresis depicts the behavior of ferromagnets under the influence of the magnetic field and differentiates them from paramagnets.
A typical hysteresis loop such as that obtained from superparamagnetic and ferromagnetic soft and hard materials. Due to its varied dependence on the magnetic field the ferromagnetic materials can be categorized into soft and hard magnetic materials Figure 2 11, Soft magnetic materials are those which can be demagnetized at low magnetic fields and hence the coercivity Hc is low.
On the other hand, soft magnetic materials can be easily magnetized and hence the permeability is high. Consequently, for a ferromagnetic material to be soft, their magnetocrystalline anisotropy must be low which can be responsible for the easy migration of the magnetic domains 7, 11, However, when the domain wall is difficult to migrate a higher magnetic field is required for the alignment of the magnetic moments of the ferromagnetic material 7, 11, These types of ferromagnetic materials are referred to as hard magnets 7, 11, In other words, these types of ferromagnetic materials are difficult to magnetize, but once magnetized, they are difficult to demagnetize.
Hence, it is an obvious fact that in contrast to the soft magnetic materials the hard magnetic materials have high magnetocrystalline anisotropy and coercivity Hc Normally, hard ferromagnetic materials have memory because they remain magnetized after the external magnetic field has been removed. Whereas, soft ferromagnetic materials such as iron or silicon steel have very narrow magnetic hysteresis loops resulting in very small amounts of residual magnetism making them ideal for a variety of biological applications.
Additionally, since a coercive force must be applied to overcome this residual magnetism, work must be done in closing the hysteresis loop. This magnetic hysteresis results in the dissipation of energy in the form of heat with the energy wasted being in proportion to the area of the magnetic hysteresis loop.
To our advantage, the resulting heat can be used for the in-situ heating of the tumor cells. Since the heat loss is determined by the hysteresis loop which is in turn determined by the magnetic material, the type of magnetic material plays an important role in its biological application. Thus the coercivity Hc of the material usually defines the application of a particular magnet.
In a broader sense, the magnetic properties of any ferromagnets depend on various factors such as particle size, shape, defects, surface effects, and temperature to name but a few. In the following section we have tried to discuss the dependence of coercivity on the particle size, shape, and surface coatings of the magnetic materials as these are the controlling factors in the engineering of nanoparticles for its biomedical applications.
The domain structure of a ferromagnetic material determines the size dependence of its magnetic behavior In fact, the two most studied finite-size effects in nanoparticles are the single domain limit and the superparamagnetic limit As discussed earlier, a large magnetic particle is well known for its multidomain structure with regions of uniform magnetization which are separated by domain walls Figure 1.
This formation of the domain walls is energetically favorable if the energy consumption for the formation of the domain walls is lower than the difference between the magnetostatic energy of the single domain and the multidomain states 7. As the dimensions of the particles are reduced, the relative contribution of the various energy terms to the total energy of the ferromagnetic material is changed. Thus, the surface energy of the domain walls becomes more important than the magnetostatic energy.
Below the critical diameter it costs more energy to create a domain wall than to support the magnetostatic energy of the single-domain state Hence, when the size of a ferromagnetic material is reduced below the critical diameter, it becomes a single domain.
As stated previously, the effect of ferromagnetic materials to an applied field is well described by the hysteresis loop, which is characterized by two main parameters viz remanence and coercivity Dealing with fine particles, the coercivity is the single property of most interest and it is strongly size-dependent.
It has been found that as the particle size is reduced, the coercivity increases to a maximum, and then decreases toward zero as shown in Figure 3. With a further decrease in particle size below the critical diameter, the coercivity becomes zero and such particles become superparamagnetic Figure 3. Schematic illustration of the coercivity-size relations of small particles. Copyrighted from reference When considering only the dipolar interactions between magnetic particles, for smaller particles the simple magnetization reversal energy becomes equal to the energy at room temperature 7, 10, Therefore, superparamagnetic nanoparticles become magnetic in the presence of an external magnet, but revert to a non-magnetic state when the external magnet is removed.
This behavior of superparamagnetic materials has led to its potential advantages for bio applications. Recently, Kim et al. Similarly, Krishnan and colleagues demonstrated that the 16 nm magnetic particles responded better to the alternating magnetic field as compared to the particles of larger diameter However, Osaka and colleagues, recently, reported that the larger sized particle 44 nm had better heating efficiency in vitro as compared to the smaller sized particle 13 nm In the same study, the authors reported that the magnetic properties of the particles synthesized with a 13 nm diameter were superparamagnetic in nature, whereas, those with a 44 nm diameter showed a ferromagnetic behavior Figure 5.
Finally, the heat treatment of these cells lines MCF-7 internalized with these two nanoparticles 13 and 44 nm revealed that the magnetic nanoparticles with a diameter of 44 nm demonstrated a higher increase in temperature in vitro as compared to the nanoparticles with a diameter of 13 nm Figure 6.
This difference in temperature rise can be attributed to the difference in the coercivity of both particles and hence their magnetic nature. Magnetic behavior of the Fe 3 O 4 nanocubes measured at K: A M—H curves for nm- red , nm- blue , and nm- black sized nanocubes; B size-dependent coercivity error bar: size distribution. Shows the magnetization curves of 44 black line and 13 nm MNPs red line.
There are a number of crystalline materials of various sizes that exhibit ferromagnetism. Since ferrite oxide-magnetite Fe 3 O 4 is the most magnetic of all the naturally occurring minerals on the earth it is widely used in the form of superparamagnetic nanoparticles for all sorts of biological applications such as magnetic field hyperthermia MFH and magnetic resonance imaging MRI magnetic separation and magnetic drug and gene delivery.
However, for its use in tissue specific applications such as tumors, a tumor specific ligand is usually conjugated onto the surface of these nanoparticles. This leads to the change in the surface of the magnetic nanoparticles, leading to the change in magnetic properties of these nanoparticles via the changes in surface anisotropy.
Thus in addition to the size, the magnetic properties of the nanoparticles depend to a great extent on its surface and shape which are discussed in the next section. Another source for the change in the coercivity of magnetic nanoparticles is the shape anisotropy.
The departure from sphericity for magnetic nanoparticles has significant influence on the coercivity as is shown in Table 1 which list the particles with different aspect ratios As the aspect ratio changes, the shape of the particle changes and hence the coercivity changes.
Moreover, the shape of nanoparticles can influence its magnetic properties in different ways. For example, classical electrodynamics teaches us that the homogenous magnetization is achievable only for ellipsoidal bodies. Hence, an ideal single-domain particle has to be ellipsoidal.
2 Permanent magnet processes. Introduction. Magnetic domains. Ceramic ferrite magnets. Alnico magnets. Samarium-cobalt magnets.
The use of non-invasive alternating magnetic field AMF on biocompatible, small sized iron oxide nanoparticles can be used for heat generation in magnetic hyperthermia or for contrast enhancement of biological tissue. However, the behavior of magnetic nanoparticle in the presence of magnetic field is restricted by their size, shape, surface defects, and coatings. Hence, it becomes imperative to closely monitor the magnetic properties of the nanoparticles as current novel formulations of nanoparticles being developed for tissue targeting involves conjugating a magnetic nanoparticle with a site specific ligand or a peptide. Thus, in this review article we have reviewed the effect of size, shape, doping, and surface coating on the magnetic properties of the nanoparticle. Finally we have concluded with the clinical status of magnetic nanoparticle in the field of magnetic fluid hyperthermia.
Skip to Main Content. A not-for-profit organization, IEEE is the world's largest technical professional organization dedicated to advancing technology for the benefit of humanity. Use of this web site signifies your agreement to the terms and conditions. High-coercive-force permanent-magnet materials and their application Abstract: The characteristics of high-coercive-force permanent-magnet alloys are reviewed and some of the opportunities and problems involved in their application are discussed.
Magnetic Materials and their Applications discusses the principles and concepts behind magnetic materials and explains their applications in the fields of physics and engineering. The book covers topics such as the principal concepts and definitions related to magnetism; types of magnetic materials and their electrical and mechanical properties; and the different factors influencing magnetic behavior.
Skip to search form Skip to main content You are currently offline. Some features of the site may not work correctly. DOI: Fundamentals of magnetism 2. Permanent magnet processes 3.
Отпусти меня! - попросил Хейл. - Я ничего не сделал. - Ничего не сделал? - вскричала Сьюзан, думая, почему Стратмор так долго не возвращается. - Вы вместе с Танкадо взяли АНБ в заложники, после чего ты и его обвел вокруг пальца. Скажи, Танкадо действительно умер от сердечного приступа или же его ликвидировал кто-то из ваших людей. - Ты совсем ослепла. Как ты не понимаешь, что я ко всему этому непричастен.
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Тогда всему придет конец. Директор нахмурился и повернулся к экрану. - Мистер Беккер, я был не прав.
Новый стандарт шифрования приказал долго жить. Никому не показалось удивительным, что два дня спустя АНБ приняло Грега Хейла на работу. Стратмор решил, что лучше взять его к себе и заставить трудиться на благо АНБ, чем позволить противодействовать агентству извне.
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