Magnetic Materials Flashcards
What is the symbol and definition for a magnetic dipole moment?
> Symbol: μm
A magnetic dipole moment is a vector for the direction and magnitude of magnetism produced by a very small current loop
What is the equation for a magnetic dipole moment? What are all the symbols?
μm = I × A × μn > μm = Magnetic dipole moment > I = Current > A = Area > μn = Unit vector perpendicular to loop
What will happen do the magnetic dipole moment when placed in a magnetic field?
The magnetic dipole moment will align with the magnetic field
How can the small current loop inside a material be described as?
A small bar magnet
What is a good example of a small current loop?
An atoms electrons and their spin
What happens in an atom that causes a NET magnetic moment
> Electrons spinning cause a magnetic moment to be produced.
Because electrons are usually paired and spin in opposite directions this causes the magnetic moment s to cancel out.
A net magnetic moment is produced when there is an unpaired electron around an atom
What is the equation for the magnetic moment caused by an electron?
μ[el] = μ[spin] + μ[orb]
What is the equation and the symbols for the magnetic field strength in the solenoid?
B0 = μ0 × n × I B0 = Magnetic field strength μ0 = Permeability of free space n = Number of turns per unit length I = Current
What happens to the magnetic field strength of a solenoid when a material is placed inside the loop?
The magnetic field strength changes from B0 to B
What is the vector M?
> The total magnetic dipole moment per unit of volume.
> Also known as the magnetisation vector
What is the equation for the magnetisation vector?
M = n[at] × μ[av]
How is the magnetisation vector similar to polarisation?
M = n[at] × μ[av] P = n[at] × p[av]
What is the equation for the total magnetic moment?
μ[tot] = M × Az M = Magnetic moment per unit volume A = Area z = Length
What is the other equation for the total magnetic moment?
μ[tot] = I[m] × n × z × A I[m] = Current on the surface n = Number of turns per unit length A = Area z = Length
What is the equation for the magnetisation vector?
M = n × I[m]
What is the equation for the magnetic field inside a solenoid when a material has been added? What does this show?
> B = B0 + μ0M
> The material in the field contributes to the magnitude of the magnetic field strength
What is an equation for the magnetic field inside a solenoid when there is a material added with regards to the currents for these materials?
B = μ0 × n × ( I + I[m] )
What is magnetic field intensity?
This is the contribution of the solenoid to the overall magnetic field divided by the permeability of free space
What are the equations for magnetic field intensity? (3)
H = nI
H = B0/μ0
(B - μ0×M) / μ0 = H
How does it all work with the solenoid and magnetic material inside?
The solenoid induces a magnetic field inside the material which causes all the magnetic dipole moments to align which further increases the magnetic field
How can the relationship with the magnet field intensity be related to the magnetic dipole moment per unit volume?
M = χm × H M = Magnetisation vector H = Magnetic field intensity χm = Magnetic suceptability
What is the magnetic susceptibility?
This is how well the material accepts the magnetic field
What is the exceptions for magnetic susceptibility?
Ferromagnetic materials
How is the relative permeability related to the magnetic susceptibility?
μr = 1 + χm
What is a diamagnetic material?
> This is a material with a small and Negative value of magnetic susceptibility (χm)
Reduces the magnetic field within the solenoid because it is expelling it
4-5 Marks on a diamagnetic material
> Small and Negative value of magnetic susceptibility (χm)
Try to reduce the change of the magnetic field
The magnetisation vector is in the opposite direction to H; the magnetising field
Material trying to expel the magnetic field
When placed in a non-uniform magnetic field, it will tend to move in to an area of lower magnetic field strength.
Material examples
> Atoms with closed shells
> Organic polymers
> Covalently bonded solids
> Copper, Gold etc
No permanent magnetic dipole moment
Its properties are not affected by temperature. Except superconductors
What is a paramagnetic material?
> This is a material with a small and Positive value of magnetic susceptibility (χm)
Increases the magnetic field within the solenoid
Atoms have a magnetic moment
4-5 Marks on a para magnetic material
> Atoms have a net magnetic dipole moment due to an unpaired electron
Each magnetic moment is independent on it neighbours
The directions of these magnetic dipole moments will be scattered but when placed in a magnetic field they will align with it
This increases the magnetic field of solenoid.
Increasing temperature of the material will make it worse at increasing the magnetic field because the increased energy will scatter the magnetic dipole moments because the atoms will move more so less will align and more will cancel each other out
This follows the curie temperature equation:
χm=C(T-T_c)
Small positive magnetic susceptibility (χ_m)
It will move towards the regions of higher magnetic field
Attracted
Material examples
> Oxygen (Liquid or gas)
> Ferromagnetic materials
> Antiferromagnetic materials
> Ferrimagnetic materials
What is the curie temperature equation?
χm = C ( T - Tc ) C = Curie coefficient Tc = Curie temperature
What is a ferromagnetic material?
> A special type of paramagnetic material
> Large magnetic moment in the absence of a magnetic field
4-5 Marks on a ferromagnetic material
> A special type of paramagnetic material
Large magnetic moment in the absence of a magnetic field
Magnetic susceptibility (χm) is positive and VERY large and depends on the applied magnetic field intensity
The relationship between M and H is very non-linear with magnetisation saturating at high fields
All atoms and their neighbours align with each other. Due to quantum mechanics. They are not independent of their neighbors. Even at low temperatures they will align.
At very high temperatures it will become paramagnetic because this alignment will be disrupted. T> T_c. They are therefore temperature dependent
Maximum magnetization occurs when all the possible magnetic moments have been aligned
Maximum magnetization is called saturation magnetization
As the temperature increases, more lattice vibrations occur and disrupt the alignment of spins so magnetization falls
At a high temperature (curie temperature), the thermal energy is larger than the exchange interaction energy (which aligns the spins) and this will cause it to act paramagnetically
What is domain behaviour?
This is the way that a ferromagnetic material may have no net magnetic field and also how the magnetic field of a ferromagnet can be changed
Describe how domain behaviour works?
> The ferromagnetic material is split into different sections called domains.
In each domain the direction of the magnetic dipole moments of all the atoms in that domain may be different from neighbouring domains.
This can cause domains to cancel out and so no magnetic field can be seen.
How can applying an increasing magnetic field to a ferromagnetic material change its domain? (4 steps)
- Low magnetic field - The domain moves slightly (This is reversable)
- Medium Low magnetic field - The domain moves suddenly and irreversibly changing the domains shapes (jerk)
- Medium High magnetic field - The domain will align in the easy direction
- High magnetic field - The materials magnetic dipoles will allign with the magnetic field
What will happen to the ferromagnetic material after a High magnetic field has changed the domain to align with it and is then removed?
The domain will align with the easy direction and stay there
How will the ferromagnetic material be returned to its original domains?
A high magnetic field in the reverse direction is required
How can a high magnetic field in the reverse direction alter the domain of a ferromagnetic material?
> It can return the material to its original domain or it can go so far as to align the domain in the opposite direction
What is the graph called for changing the domain of a ferromagnetic material?
Hysteresis loop
What are the properties of a soft ferromagnetic material?
> Easy to magnetize and demagnetize > Require little energy to change them > Used when magnetization needs to change quickely > They need to be highly resistive > Amorphous magnetics are good choices
What are the properties of a hard ferromagnetic material?
> Hard to magnetize and demagnetize > Generally used in permanent magnets > Often made by compacting powders together > E.g. ceramic magnets > Used in magnetic storage
