Magnetism-Iron Oxide Magnetite

1.1 The history of magnetism

Magnetism goes back to early 600 B.C., but scientist were not able to fully understand how it worked or what was involved until the twentieth century, when they began developing technologies and research based on their understanding. Magnetism was first seen in a mineral form called lodestone. Iodestone consist of an iron oxide compound of both iron and oxygen. It is said that the ancient Greeks were the first known to have ever used this mineral because of it magnetic ability to attract other pieces to itself [1]. It wasn’t until in the 1800, that a Danish scientist named Hans Christian Oersted, was demonstrating to his friends that the flow of an electric current in a wire caused a nearby compass which he had to move in the direction of the current flow. This new phenomenon was studied in France by Andre-Marie Ampere, who came to the conclusion that the nature of magnetism was quite different from what everyone believed or thought it was. He explained the nature of magnetism as a force between electric currents: two parallel currents in the same direction attract, where as in opposite directions repel. Iron magnets are a very special case, which Ampere was also able to explain [2].

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Figure 1. What Oersted observed…

1.2 Types of Magnetism

The different types of magnetism are classified based on their magnetic behavior of materials in response to magnetic fields at different temperatures. These types of magnetism are: ferromagnetism, ferrimagnetism, antiferromagnetism, paramagnetism, and diamagnetism.

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Figure 2. Types of Magnetism

1.2.1 Ferromagnetism

Ferromagnetic materials are those such as iron and nickel that are able to maintain their magnetic properties even after the magnetic field is removed. The prefix, Ferro is the Latin word for iron (this is the reason behind the atomic symbol of iron- Fe), a material which displays strong magnetic properties. Electrons are said to exhibit a small magnetic field as they spin and orbit the nucleus of an atom. For most, the combinations of electrons in their orbits cancel each other out. For ferromagnetic materials, the case is not the same as the electron fields in the atoms do not cancel out, so they exhibit a long-range ordering phenomenon at the atomic level. Due to this long range ordering phenomenon, the unpaired electron spins to line up parallel with each other in a region called a domain. Ferromagnetic materials will respond mechanically to an impressed magnetic field, which changes the length slightly toward the direction of the applied field [3].

1.2.2 Anti-Ferri and Ferrimagnetism

Ferrimagnetisms like ferromagnetism occurs when the magnetic moments in a magnetic material line up spontaneously at a temperature below the so-called Curie temperature, to produce net magnetization. These magnetic moments are aligned at random at temperatures above the Curie point, and become ordered below the Curie temperature. In a ferrimagnet, the moments are unequal in magnitude and order in and are also anti-parallel their arrangement. Antiferromagnetism occurs when the moments are in equal magnitude and orders at a temperature called the Neel Temperature. When this occurs there is no net magnetization. These transitions from disorder to order represent classic examples of phase transitions [3].

1.2.3 Diamagnetism

Diamagnetism is a very weak form of magnetism and is only exhibited in the presence of an external magnetic field. Diamagnetism occurs as a result of changes in the orbital motion of electrons due to this external magnetic field. It is said that the induced magnetic moment is very small and opposite to the direction of the applied field. When diamagnetic materials are placed between the poles of a strong electromagnet, they become attracted towards regions where the magnetic field is weak. Although diamagnetism is found in all materials it can only be observed in materials that do not exhibit any other forms of magnetism. However, an exception to this weak nature of diamagnetism occurs when there is a large number of materials that become superconducting. Superconductors are said to be perfect diamagnets, and when placed in an external magnetic field expel the field lines from their interiors, which s dependent upon the field intensity and temperature. Superconductors also have zero electrical resistance due to their diamagnetism. In an attempt to escape the external magnetic field, superconducting structures have been known to tear themselves apart with astonishing force Superconducting magnets are the major component of most MRI systems, perhaps the only important application of diamagnetism [4]. Diamagnetic materials have a relative magnetic permeability that is less than 1, and a magnetic susceptibility that is less than 0. The negative magnetic susceptibility in diamagnetic materials is the result of a current induced in the electron orbits of the atoms by the applied magnetic field. The electron current that is induced by the magnetic moment is of opposite sign to that of the applied field.

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Figure 3.Curie Point refers to a characteristic property of a ferromagnetic material. In Ferromagnetic materials, the magnetic moments are aligned at random at temperatures above the Curie point, and become ordered below the Curie Point.As the temperature is increased towards the Curie point, the alignment (magnetization) within each domain decreases. Above the Curie point, the material is purely paramagnetic and there are no magnetized domains of aligned moments

1.2.4 Paramagnetism

Paramagnetism is a weak form of magnetism which displays a positive response to an applied magnetic field. This positive response is described by its magnetic susceptibility per unit volume. Magnetic susceptibility is a dimensionless quantity defined by the ratio of the magnetic moment to the magnetic field intensity. Paramagnetism is seen in in atoms and molecules with an odd number of electrons, because net magnetic moment cannot be zero[4].

1.3 Relative Permeability

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Figure 4. Simplified comparison of permeabilities for: ferromagnets (μf), paramagnets(μp), free space(μ0) and diamagnets (μd)

Relative permeability is the degree of magnetization of a material that responds linearly to an applied magnetic field. Relative permeability is defined as a material property that describes the ease with which a magnetic flux is established in a component. It is said that the wider hysteresis loop- lower permeability, higher residual magnetization and the narrower hysteresis loop- higher permeability and lower residual magnetization. Relative permeability can be calculated using the equation shown below[10].

m = B/H
Equation 1

It is said that high frequency magnetic effects has a complex permeability. At low frequencies in a linear material the magnetic field and the auxiliary magnetic field are proportional to each other through some scalar permeability. Whereas at higher frequencies they will react to each other with some lag time[11]. These fields can be written as [10]

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Equation 2

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Equation 3

where δ is the phase delay of B from H.

By Euler's formula it transforms to the equation below

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Equation 4

The ratio of the imaginary to the real part of the complex permeability is called the loss tangent.The loss tangent is a parameter of a dielectric material that quantifies its inherent dissipation of electromagnetic energy.

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Equation 5

1.4 Hysteresis Loop (B-H)

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Figure 5. Hysteresis Loop of Magnetic Materials

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Figure 6. Hysteresis Loop

Hysteresis is a retardation of the effect when the forces acting upon a body are changed (as if from viscosity or internal friction); or the lagging in the values of resulting magnetization in a magnetic material such as iron due to a changing magnetizing force [8]. The hysteresis loop can tell the magnetic property of a material as it shows the relationship the induced magnetic flux density (B) and the magnetizing force (H). An example of the loops is shown in the figures below.
The hysteresis loop is produced by measuring the magnetic flux of a ferromagnetic material while altering the magnetized force. The dashed line represents a ferromagnetic material that has never been magnetized. From the dashed line, it shows that, the greater the amount of current applied (H), the greater the magnetic field in the component (B). To understand the hysteresis loop, one must understand what is happening at each point on the loop. At point "a" almost all of the magnetic domains are aligned, and any additional increase in the magnetizing force will not change the magnetic flux by a lot as the material has reached the point of magnetic saturation. When current (H) is reduced to zero, the curve will move from point "a" to point "b." which some of the magnetic flux is remained in the material. This is referred to as the point of retentivity on the graph and indicates the remanence or level of residual magnetism in the material. When the magnetizing force is reversed, the curve moves to point "c", and the flux is zero; this is called the point of coercivity on the curve. (The reversed magnetizing force has flipped enough of the domains so that the net flux within the material is zero.) As the magnetizing force is increased in the negative direction, the material will again become magnetically saturated but in the opposite direction (point "d"). As H goes to zero brings the curve to point "e." [9]

1.5 Magnetic susceptibility

The magnetic susceptibility of a material is a unitless constant determined by the physical properties of the magnetic material. The magnetic susceptibility can either be positive or negative. When the magnetic susceptibility is positive, it implies that the induced magnetic field,I, is in the same direction as the inducing field, H. When the magnetic susceptibility is negative, it implies that the induced magnetic field is in the opposite direction as the inducing field [5]. If the magnetic susceptibility is zero it means that the material does not respond with any magnetization. So both quantities give the same information, and both are dimensionless quantities.For ordinary solids and liquids at room temperature, the relative permeability Km is typically in the range 1.00001 to 1.003. We recognize this weak magnetic character of common materials by the saying "they are not magnetic", which recognizes their great contrast to the magnetic response of ferromagnetic materials. The gases N2 and H2 are weakly diamagnetic with susceptibilities -0.0005 x 10-5 for N2 and -0.00021 x 10-5 for H2. That is in contrast to the large paramagnetic susceptibility of O2 in the table [6].

χm = Km – 1

Equation 6

where Km is called the relative permeability

1.6 Magnetic Field

A magnetic field (B) is a field that exerts magnetic force on substances that are sensitive to magnetism. Magnetic fields are produced by electric currents, and can either be macroscopic currents in wires, or microscopic currents associated with electrons in atomic orbits. Lorentz Force Law describes the magnetic field B as the force exerted on moving charge [12].

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Equation 7

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Figure 7. Magnetic Field

1.7 Applications of Magnetic Particles (Iron Oxide) in the Medical Field

Magnetic nanoparticles such as magnetite and maghemite, have drawn a lot of attention because of their unique properties and application in various fields such as biotechnology and cancer therapy just to name a few. Magnetite is a widespread material of common use as:

◦ It is conspicuous because of its magnetic properties and low toxicity.
◦ Magnetic properties of magnetite nanoparticles can be attracted by applying a strong magnetic field
◦ With the application of magnetic field, one is able to control their orientation and arrangement

These magnetic nanoparticles contain hydrophobic characteristics and are easily aggregated and can be attacked by the immune system and removed from the body to avoid high level of toxicity. In order to avoid their aggregation, they are required to be coated with a desirable material that is biocompatible, water-soluble, and nontoxic. With the coating on the surface of the nanoparticles, the surface properties of the magnetic nanoparticles change from hydrophobic to hydrophilic and the magnetic dipolar attraction between magnetic nanoparticles can be screened to avoid their aggregation. Magnetic iron oxide (IO) nanoparticles are also said to have a long blood retention time, which makes them very good target labels for both in vivo and in vitro applications. Although these nanoparticles have large surface areas, they can be engineered to provide a large number of functional groups for cross-linking to tumor-targeting ligands such as monoclonal antibodies, peptides, or small molecules for diagnostic imaging or delivery of therapeutic agents. Magnetic nanoparticles possess unique paramagnetic properties, which generate significant susceptibility effects resulting in strong T2 contrast, as well as T1effects at very low concentrations for magnetic resonance imaging (MRI), which is widely used or clinical oncology imaging [7].

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Figure 8. Schematic picture of Coated Iron Oxide

Table 1. Different polymers/molecules which can be used for nanoparticle coating to stabilize the iron oxide nanoparticles and also for other biological applications [13]

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Iron

Iron coated with one half of fluorescent material

Videos were made by Vincent Palumbo

For More information on:

Ferroelectric Materials and their properties http://electrons.wikidot.com/ferroelectrics

[[bibliography]]

[1] http://science.jrank.org/pages/4081/Magnetism-History-magnetism.html

[2] http://www.iki.rssi.ru/mirrors/stern/Education/Imagnet.html

[3] http://science.jrank.org/pages/4083/Magnetism-Types-magnetism.html

[4] http://knowledgerush.com/kr/encyclopedia/Diamagnetism/

[5]http://www.earthsci.unimelb.edu.au/ES304/MODULES/MAG/NOTES/suscept.html

[6] http://hyperphysics.phy-astr.gsu.edu/HBASE/tables/magprop.html

[7] Peng et al. Targeted magnetic iron oxide nanoparticles
for tumor imaging and therapy
Review Article

[8]http://www.lassp.cornell.edu/sethna/hysteresis/WhatIsHysteresis.html

[9]http://www.ndted.org/EducationResources/CommunityCollege/MagParticle/Physics/HysteresisLoop.htm

[10]http://en.wikipedia.org/wiki/Permeability_(electromagnetism)

[11] M. Getzlaff, Fundamentals of magnetism, Berlin: Springer-Verlag, 2008

[12] http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfie.html

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