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Materials module 1 > Magnetic properties > Flashcards

Magnetic properties Flashcards

1
Q

How is an applied magnetic field created?

A

created by a current through a coil

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2
Q

What happens due to the applied magnetic field? (2)

A

magnetic induction results in the material, the material changes the magnetic field
If there is nothing in the coil B=H

Magnetic susceptibility

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3
Q

What are the different conditions due to magnetic susceptibility? (Graph that is B (magnetic induction inside the material) against H.)

A

x, dimensionless
x>0 is steeper than the rest (there is a positive susceptibility, a larger field B) , vacuum x =0, x<0 (negativity susceptibility gives a smaller field B)

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4
Q

Why is there a magnetic moment?

A

The sum of moments from all electrons in a conductor an effect due to the fact the electrons can move due to these points in the magnetic field.

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5
Q

What is the net magnetic moment?

A

sum of moments from all electrons

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6
Q

What is the origin of magnetic moments?

A

electrons produce magnetic moments, there is an orbital and spin component.

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7
Q

What the magnetic susceptibility measure?

A

the response of electrons to a magnetic field

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8
Q

What are the three types of material response?

A

diamagnetic, paramagnetic and ferromagnetic

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9
Q

What happens to the magnetic induction and strength of the applied magnetic field when the material size changes?

A

The larger the material, the higher the magnetic induction (B) but the smaller the strength of applied magnetic field (H)

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10
Q

Rank the sizes of the three types of magnetism, ferromagnetic, paramagnetic, diamagnetic.

A

ferromagnetic (X as large as 10^6) > paramagentic (X ~ 10^-4) > diamagnetic (X ~ -10^-5

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11
Q

What happens in diamagnetic moments? before and after applying a magnetic field

A

No dipoles, no magnetic dipoles or bohr magnetrons before applied

Dipoles are opposed to the applied magnetic field

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12
Q

What happens in paramagnetic moments? before and after applying a magnetic field

A

Dipoles are random and present before applied

The dipoles begin to align with the applied magnetic field

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13
Q

What happens in ferromagnetic, ferrimagnetic moments? before and after applying a magnetic field

A

Dipoles are already aligned

They are aligned already with the applied magnetic field

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14
Q

What are diamagnetic materials?

A

Xm < 0
Diamagnetic materials don’t have magnetic dipoles until a magnetic field is applied – the induced magnetic dipoles align in the opposite direction (anti-align) to the applied magnetic field

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15
Q

Why are strong diamagnetic materials diamagnetic?

A

because they are good electrical conductors – any change in the magnetic field induces current in a metal that opposes the change in the magnetic field -so called Landau diamagnetism.

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16
Q

What are strongly diamagnetic things?

A

Superconductors which are excellent electrical conductors

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17
Q

Is water strongly diamagnetic or not?

A

Water is weakly diamagnetic and with a very strong B field (>10T)- it can be used to levitate objects

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18
Q

What are paramagnetic materials

A

Paramagnetic materials already have magnetic dipoles in the material and
they line up with the applied magnetic field

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19
Q

What are paramagnetic materials

A

Xm>0
Paramagnetic materials already have magnetic dipoles in the material and
they line up with the applied magnetic field

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20
Q

What is Hunds rule?

A

For a given electron configuration, the term with maximum multiplicity has the lowest energy.

maximum total spin has the lowest energy

21
Q

What is Hunds rule?

A

For a given electron configuration, the term with maximum multiplicity has the lowest energy.

maximum total spin has the lowest energy

if ↑↓ then S = 1/2 -1/2 = 0

22
Q

When can electrons share the same energy level?

A

when they have opposite spin

23
Q

What is multiplicity equal to?

A

2S + 1, where S is the total spin angular momentum for all electrons, therefore the term with lowest energy is also the term with maximum S.

24
Q

Why can electrons share an energy state?

A

Electrons are anti-social, they prefer to have their own energy state but if necessary they will share an energy state but only with an electron of the opposite spin

25
Q

What are ferromagnetic materials?

A

Xm»0
Ferromagnetism is like paramagnetism – that is the magnetic dipole moments align with the applied magnetic field, H, but the dipoles form domains that get stuck with a given alignment.

26
Q

Why can’t ferromagnetic materials not be gases?

A

The ferromagnetic domains can be the same as the crystallites in polycrystalline material – or the crystallites can align to form bigger domains. So usually gases cannot be ferromagnetic because they don’t have crystallites but they can be paramagnetic.

A material can only be ferromagnetic when the temperature (thermal energy) is below the Currie temperature so that the domain structure can form – Currie Temperature is different for different ferromagnetic materials

27
Q

When can a material lose its magnetisation?

A

Even when the H field is removed there is a permanent magnetization left behind. Only if the material is heated to a point where the thermal energy disrupts the domain structure can the material lose its magnetisation.

28
Q

Describe a ferromagnetic and hysteresis curve, (B, H, R, C, S)

A

The B left when the H field returns to zero is called the remanence, R, and means there’s a permanent magnetization left – thus ferroelectric materials are used in permanent magnets and data storage devices such hard drive disks in computers.

Also from this curve a property called the coercivity, C, can be measured which is a measure of how large a field has to applied to cause magnetisation

S is point of saturation

29
Q

Describe what occurs in a ferromagnetic and hysteresis curve.

A

The curve increases with initial magnetisation.

as the field is removed it doesn’t go back down the blue curve instead continues onto the red curve, as it remembers that a magnetic field has been applied in the past it does not return to 0 and so retains some of the magnetisation

30
Q

How can you get back zero in a ferromagnetic and hysteresis curve.

A

To get back to zero you must apply magnetic field in the opposite direction

31
Q

What happens to ferro and ferri magnetic materials as a applied field (H) increases?

A

As the applied field (H) increases… –the magnetic moment aligns with H.

“Domains” with aligned magnetic moment grow at expense of poorly aligned ones!

32
Q

What happens to ferro and ferri magnetic materials as a applied field (H) increases? (3)

A

As the applied field (H) increases… –the magnetic moment aligns with H.

“Domains” with aligned magnetic moment grow at expense of poorly aligned ones!

Arrows are different lengths and begin to grow as applied magnetic field and magnetic induction increase
All domains align to form one large domain

33
Q

What is an anti-ferromagnet?

A

If the moments of the aligned and anti-aligned ions balance completely so as to have zero net magnetization, despite the magnetic ordering

34
Q

What happens if the temperature gets above curie temeprature for ferri and ferro magnetic materials?

A

If you have a hot material it can misalign the domains, if it gets above the curie temperature then the material will no longer be ferromagnetic. Because you then have enough thermal energy that domains can misalign randomly since they jiggle around due to the thermal energy

35
Q

What are permanent magnets? (ferri and ferro magnets)

A

As your ferromagnetic material is heated it up and becomes non ferromagnetic, apply a field as it is cooling down and it will line up all the magnetic domains

36
Q

What is the process of permanent magnets? (ferri and ferro magnets)

A
  1. initial (unmagnetized state)
  2. apply H, cause
    alignment
  3. remove H, alignment stays! => permanent magnet!
  4. Coercivity, HC
    Negative H needed to demagnitize!
37
Q

How do you create a hard magnet? (ferri and ferro magnets)

A

large coercivity
–good for perm magnets –add particles/voids to
make domain walls hard to move (e.g., tungsten steel:
Hc = 5900 amp-turn/m

38
Q

How do you create a soft magnet? (ferri and ferro magnets)

A

mall coercivity–good for elec. motors (e.g., commercial iron 99.95 Fe)

39
Q

What are limits of superconductivity?

A
  • 26metals + 100’sofalloys & compounds
    • Unfortunately,not this simple:
    Jc (critical density) = critical current density if J > Jc not superconducting Hc (magnetic field) = critical magnetic field if H > Hc not superconducting
40
Q

Is electrical resisitivity higher in a super conductor or a normal metal?

A

It is higher in a super conductor, but it has to be at a temperature below which material is superconductivity

41
Q

What are two advances in superconductivity?

A 
•  1987- new results published for Tc > 30 K
–  ceramics of form Ba1-x Kx BiO3-y
–  Started enormous race.
-- Y Ba2Cu3O7-x Tc = 90 K
--Tl2Ba2Ca2Cu3Ox Tc = 122 K

•Values now stabilized at ca. 120K–important to get above 77K the temperature of liquid nitrogen because liquid nitrogen can be used as a much less expensive coolant than liquid He (4K).

42
Q

What is the meissner effect?

A

Super conductors expel magnetic fields
normal superconductor
This is why a super conductor will float above a magnet

43
Q

What are the two types of current flow in super conductors?

A

current only in outer skin
- so amount of current limited

current flows within wire

44
Q

What is magnetostrsiction?

A

When a magnetic materials with a magnetic domain structure – ferromagnets - have a magnetic field applied the domains re-order and change their dimension –this behaviour is called Magnetostriction and it causes the humming noise that comes from electrical transformers

45
Q

What happens when you increase the temperature in invar materials?

A

When the temperature is increased in Invar materials the thermal expansion is opposed by the magnetostriction because the material is less ferromagnetic as it approached its Currie temperature.

46
Q

Where is the saturation flux density in a B verses H curve?

A

Where the two lines of the graph meet in the postive x and y axis

47
Q

What is the saturation magnetisation in a B verses H curve?

A

The saturation flux density / 1.26 x 10^5 (permeability in a vacuum)

48
Q

What is the remanence in a B verses H curve?

A

when the graph hits the positive y axis

49
Q

What is the coercivity in a B verses H curve?

A

when the graph hits the negative x axis

Materials module 1 (12 decks)

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