Magnetical Behavior

7/31/2019 Magnetical Behavior

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MAGNETICAL BEHAVIOR

All materials of the world respond to the presence of a magnetic field, there is no

"non-magnetic" materials, due to the fact that everything is composed of atoms

whose electrons revolve around these, similarly in which one exhale carrier current

generates a magnetic field.

The way in which we measure the response between electrons and atoms

determines if this is very magnetic or little magnetic.

Clasification ofMagnetic

Materials

Ferromagnetic

Ferrimagnetic

Antiferromagnetic

Paramagnetic

Superparamagnetic

Diamagnetic


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We can find the magnetic behavior of the materials in their atoms.

The orbital motion of the electron around the nucleus and the spin of the electron

around its own axis cause separate magnetic moments.

The nucleus of the atoms also has a spin, however the magnetic moment is much

lower compared to the electron, so it is neglected.

The magnetic moment of an electron due to its rotation is known as the Bohr

magneton.

MAGNETICAL BEHAVIOR


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We can consider that the electrons in materials are like elementary magnets. If the

magnetic moments due to electrons could be aligned in the same direction, the Earth

would be a magnetic site. Fortunately this is not the case, since the magnetic moments

associated with the spin of the electron and its orbital motion are cancelled in most

materials, leaving only a few materials that are magnetic.

We describe several effects that make the majority of the materials 'not magnetic:

The Pauli exclusion principle: a single orbital can be occupied only by two electrons with opposite

spins (represented upward and the other downward ).

Cancellation between the orbital moments: in a layer fulfilled the spin and orbital moments are

cancelled with each other.

The cancellation of the magnetic moments for some individual atoms when these forms crystalline

structures

MAGNETICAL BEHAVIOR


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MAGNETIZATION, PERMEABILITY AND THE

MAGNETIC FIELD

When a electric current is passed by a coil it produces a magnetic field (h); the intensity of

the field is given by:

n = number of turns

l = coil length

I = current

When a magnetic field is applied in a vacuum, it induces

magnetic flux lines. The set of lines of flux are called

density flow or inductance,and is related to the magnetic

field by means of:

B = inductance

H = magnetic field

0 = magnetic permeability of vacuum


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When we place a material within the magnetic field, the magnetic-flux density is

determined by the manner in which induced and permanent magnetic dipoles interactwith the field. The flux density now is:

= permeability of the material in the field

If the magnetic moments reinforce the applied field, then > 0 a greater number oflines of flux that can accomplish work are created, and the magnetic field is magnified.

If the magnetic moments oppose the field, however,0 > .

We can describe the influence of the magnetic material by the relative permeabilityr ,

where:


MAGNETIC FIELD


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The magnetization M represents the increase in the inductance due to the core

material, so we can rewrite the equation for inductance as

The magnetic susceptibility m

, which is the ratio between magnetization and the

applied field, gives the amplification produced by the material:

Both r and m refer to the degree to which the material enhances the magnetic field

and are therefore related by

For ferromagnetic and ferrimagnetic materials, the terms r y m depends on the

applied field (H) in wich 0M >>

0H. Thus, for these materials:


MAGNETIC FIELD


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DIAMAGNETIC, PARAMAGNETIC,

FERROMAGNETIC, FERRIMAGNETIC, AND

SUPERPARAMAGNETIC MATERIALS

Diamagnetismo: A magnetic field acting on any atom induces

a magnetic dipole for the entire atom by influencing the

magnetic moment caused by the orbiting electrons. These

dipoles oppose the magnetic field, causing the magnetization

to be less than zero.

Paramagnetismo: When materials have unpaired electrons,a net magnetic moment due to electron spin is associated

with each atom. When a magnetic field is applied, the

dipoles align with the field, causing a positive magnetization.

Because the dipoles do not interact, extremely large

magnetic fields are required to align all of the dipoles. In

addition, the effect is lost as soon as the magnetic field is

removed.


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Ferromagnetismo: Ferromagnetic behavior is caused by the

unfilled energy levels in the 3dlevel of iron, nickel, and

cobalt. Similar behavior is found in a few other materials,

including gadolinium (Gd). In ferromagnetic materials, the

permanent unpaired dipoles easily line up with the imposed

magnetic field due to the exchange interaction, or mutual

reinforcement of the dipoles. Above the Curie temperature,

ferromagnetic materials behave as paramagnetic materials.

Antiferromagnetismo: In materials such as manganese,

chromium, MnO, and NiO, the magnetic moments producedin neighboring dipoles line up in opposition to one another in

the magnetic field, even though the strength of each

dipole is very high. These materials are antiferromagnetic

and have zero magnetization.





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Ferrimagnetism: In ceramic materials, different ions

have different magnetic moments. In a magnetic

field, the dipoles of cationA may line up with the

field, while dipoles of cation B oppose the field.

Because the strength or number of dipoles is not

equal, a net magnetization results.

Superparamagnetism: When the grain size of

ferromagnetic and ferrimagnetic materials falls

below a certain critical size, these materials behave

as if they are paramagnetic. The magnetic dipole

energy of each particle becomes comparable to

the thermal energy. This small magnetic moment

changes its direction randomly (as a result of the

thermal energy).





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DOMAIN STRUCTURE AND THE HISTERESIS

LOOP

Within the single crystal or polycrystalline structure of a ferromagnetic or ferrimagnetic

material, a substructure composed of magnetic domains is produced, even in theabsence of an external field. This spontaneously happens because the presence of many

domains in the material, arranged so that the net magnetization is zero, minimizes the

magnetostatic energy. Domains are regions in the material in which all of the dipoles are

aligned in a certain direction. In a material that has never been exposed to a magnetic

field, the individual domains have a random orientation. Application of a magnetic field

(poling) will coerce many of the magnetic domains to align with the magnetic fielddirection.

Boundaries, called Bloch walls, separate the individual magnetic domains. The Bloch

walls are narrow zones in which the direction of the magnetic moment gradually and

continuously.


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Movement of Domains in a Magnetic Field: When a magnetic field is imposed on the

material, domains that are nearly lined up with the field grow at the expense ofunaligned domains. In order for the domains to grow, the Bloch walls must move; the

field provides the force required for this movement.

Initially, the domains grow with difficulty, as it increases the intensity of the field,

domains with favourable orientation grow more easily, and at the same time, also

increases the permeability.

DOMAIN STRUCTURE AND THE HISTERESIS

LOOP


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When the field is removed, the resistance offered by the domain walls prevents

regrowth of the domains into random orientations.

As a result, many of the domains remain oriented near the direction of the original

field and a residual magnetization, known as the remanance (Mr) is present in the

material. In this case de material acts like a permanent magnet.

The magnetic field needed to bring the induced magnetization to zero is the coercivity

HC of the material. This is a microstructure-sensitive property.

If we apply a magnetic field in the opposite

direction, the domains will start to align in

the direction of the applied field, in which

the magnetization and the field form a

hysteresis loop.

DOMAIN STRUCTUREAND THE HISTERESIS

LOOP


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THE CURIE TEMPERATURE

When the temperature of a ferromagnetic or ferrimagnetic material is increased, the added

thermal energy increases the mobility of the domains, making it easier for them to become

aligned, but also preventing them from remaining aligned when the field is removed.

Consequently, saturation magnetization, remanance, and the coercive field are all reduced

at high temperatures.

If the temperature exceeds the Curie temperature (Tc), ferromagnetic or ferrimagnetic

behavior is no longer observed. Instead, the material behaves as a paramagnetic material.


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APPLICATION OF MAGNETIC

MATERIALS


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Magnetical Behavior