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DFA > Magnetism > Flashcards

Magnetism Flashcards

1
Q

Rank the 3 main forms of magnetism and explain their properties

A

Dia magnetism
Para magnetism
Ferro magnetism

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2
Q

What is a magnetic dipole?

A

Electron spin generates magnetic dipoles, and all electrons spin (up or down)
If electrons are paired their spins appose = dipoles appose and cancel - completely filled orbitals have no magnetic moments

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3
Q

Describe Hunds rule

A

When electrons add to a shell they do so maximises these things:
- spin
- orbital
Ie iron has 3d6 (6 electrons in 3d shell, has 5 orbitals), each electron has a spin of +/- 0.5, so will set up one spin pair and 5 single spins to maximise spins (more pairs = less spin as cancel)

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4
Q

What happens to electrons spin in a magnetic field?

A

Coupling between magnetic field and electron spin to minimise energy (causing some electrons to rotate) creating a magnetic moment
Withdrawing a magnetic field will cause the electron to rotate back to its original state

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5
Q

Describe dia magnetism

A

Material apposeS magnetic field meaning no magnetic moment is induced in the material

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6
Q

Describe para magnetism

A

Ions are fixed in lattice position waning only electrons orientate to magnetic field - resulting in a low magnetic moment

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7
Q

Describe ferromagnetism

A

Ions and electrons align with magnetic field meaning there is a large net magnetic moment

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8
Q

Draw the Bethe-slater curve and annotate

A

Exchange Energy vs atomic radius
Start negative then curved up to peak and reduced with asymptote as 0
Fe, Co and then Ni

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9
Q

What is the curie temperature?

A

Temperature at which all dipole alignment is lost = material is completely demagnetised

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10
Q

State the curie temps of the ferromagnetic materials and how this affects magnetic moment

A

Fe = 800
Co = 1100
Ni = 350
Curie temperature doesn’t affect magnetic moment

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11
Q

Explain which ferromagnetic material has a higher magnetic moment

A

Fe is 3d6 - 4 unpaired e-
Co is 3d7
Ni is 3d8 - 2 unpaired
More spin in Fe = higher dipole and moment

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12
Q

Why isn’t chromium ferromagnetic?

A

Although it has 3d5 shell (5 unpaired e-) desperation between atoms is too big to have a strong magnetic reaction

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13
Q

How does tension/compression affect the bethe-slater curve?

A

Compression moves atom to left (atoms closer together)

Tension moves atoms to right (further apart)

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14
Q

How do you make a paramagnetic material ferromagnetic?

A

By changing the atomic spacing between atoms you change their position on the bethe-slater curve
By alloying you changed the deportation distance which means you can change the magnetism of a material
Deformation/tension&compression can also change it

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15
Q

What is meant by magnetic easy direction?

A

As magnetism is directional, a material behaves differently depending on the orientation
All directions will reach the same saturation but some take much more applied field to reach this point

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16
Q

How does domain shape affect magnetisation?

A

Long thin (needle) shaped domains are easy to magnetise across short side but hard along long side

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17
Q

Define a magnetic domain

A

Regions where magnetic dipoles are aligned

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18
Q

Where are domains found and how are they positioned?

A

Usually Positioned parallel to easy magnetic direction and can be found within grains but also cross grain boundaries

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19
Q

When can ferromagnets show 0 net magnetisation?

A

When multiple domains are present and closure domains form with 90° block walls

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20
Q

Draw the stages of ferromagnetic magnetisation

A

1 - demagnetised closure domains state
2- domains in easy magnetisation direction and with applied field grow (others shrink) -> due to ion rotation
3 - only easy domains remain
4 - domain rotates to applied field to achieve full saturation

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21
Q

Explain the demagnetisation sequence of ferromagnetic materials

A
  • When applies field removed, domains rotate to easy direction (remnance)
  • Reverse domain is nucleated in reverse easy direction when reverse field applied
  • Reverse domain grows until covers entire material, domain rotates to applied direction = saturation achieved
22
Q

What is the nucleation point of demagnetisation?

A

Likely to be heterogeneous point (requires less energy) such as domain wall or inclusion

23
Q

Why does nucleation occur instead of complete domain rotation in ferromagnetic demagnetisation?

A

Nucleation and domain growth requires less energy than the complete domain flipping

24
Q

Explain domain growth in terms of energy

A

When domains grow they release energy, however there is an energy penalty because a domain wall forms - means that domains only grow when energy is above a minimum value = when energy released from increasing volume > energy cost of domain area

25
Q

Describe the curie temperature in terms of energy

A

ΔG = ΔH - TΔS
G = Gibbs energy, H = enthalpy, T = temp,
S= entropy (disorder)
Curie temp is when ΔG < 0, means that disorder is favourable = no domain alignment = demagnetisation

26
Q

Describe and explain the material requirements for transformer cores

A

1- high permeability (easy magnetisation parallel to applied field)
2- high saturation to hold more magnetisation
3 - low coercivity to have a narrow BH loop = low energy losses during cycles
- likely iron core in <100> orientation

27
Q

How does the processing of transformer cores affect the structure?

A
  • Fe is alloyed with Si = lower saturation but increases resistivity = increased efficiency as eddy currents reduced
  • alloying increases impingement but point defects have limited effect, material annealed, decarbonised (remove unwanted inclusions) and heat treated (increase grain size = reduces boundaries)
28
Q

What are the possible requirements of recording media and thus the materials?

A
  • Easily written = low coercivity
  • Easily re-written = medium coercivity
  • not easily erased = high coercivity
  • long term storage = high Tc, non brittle and corrosion resistant
  • high capacity = high saturation (small grains)
  • cheap - non exotic materials
29
Q

Why is iron not used for recording media? What replaces it and why?

A
  • Oxidises too quickly = no long term storage

- Fe oxide used instead as corrosion resistant

30
Q

Describe the iron oxide used for recording media

A

y Fe2O3 used as it has a cubic spinel structure (FCC oxide arrangement, Fe in interstitials) which means it’s ferrimagnetic of bethe-slater curve (due to u equal distribution of Fe)

31
Q

How can Cr be used in recording media?

Compare It’s usage with Fe2O3

A

Cr antiferromagnetic on its own, oxidised to CrO2 = higher remnance and coercivity than Fe2O3
Cr - more stable and longer storage
Fe - more storage per M2 and higher resolution

32
Q

Why are oxides used for data storage?

A
  • Single domain = below minimum size for domain wall to form = only two states (magnetised or not)
  • Narrow BH loop = energetically favourable
  • ceramic particles that only bond with polymer carrier (not eachother) = particles move separately and stress doesn’t build up = higher life span
33
Q

How are oxides processed?

A
  • Fe oxides start as rust and prepared into needle shaped particles
  • dehydrated to form spinnel structure (reduces O so then reoxidised)
  • particles then combined with solvent in hopper, put onto substrate and aligned
  • solvent vaporised off, rolled to remove excess binders
  • needles saturated (data recorded via demagnetisation)
34
Q

What are the problems with videotape storage?

A

Video - running the tape deforms polymer substrate, stress causes rape to stretch or wrinkle = changed separation = changed magnetism = lost data

35
Q

What are the data storage differences between swipe cards and credit cards?

A

Swipe - 3 Fe oxide strips = limited data storage, cheap as short life time, must be rewritable (medium coercivity)
Credit - small data capacity needed (low saturation), can’t lose data (high coercivity = small particle size = reverse domain growth difficult)

36
Q

Describe Hdd magnetic requirements

A 
High density storage - high saturation 
Long term storage - low corrosion 
Rewritable - medium coercivity 
Portable - lightweight 
Accurate - fine magnetic grain size
37
Q

What are a hard drives requirements?

A
  • Small grain size (texturing material)
  • High magnetisation density
  • Low inertia (travel at high speed)
  • Stiff (flexing doesn’t occur during reading = inaccurate readings)
  • High specific stiffness (surface defects don’t corrupt data)
38
Q

What is Moore’s law and how is it achieved?

A

Moore’s law requires doubling of data density very 2 years
Comes from: greater packing density of particles with higher saturation (more data per area), finer write heads to allow smaller regions to be accurately recorded

39
Q

What are Fe & Co magnetic easy direction?

A

Iron - <100>

Co - <0001> -> HCP structure

40
Q

Describe magnetic sputtering

A
  • Sealed chamber with substrate and sputtering target
  • Ionised Argon has put into chamber and settle on -Ve plate (target)
  • Target releases particles which move to substrate & build up layers
  • Slow process so gas can be extracted before it reacts, uniform layers build & small grain size as high Xc
41
Q

Describe the HDD structure

A
  • Polished AlMg plate (to remove surface defects)
  • sputtered NiP glue layer
  • Cr sputtered BCC later (control orientation of Co)
  • CoPtCrB sputtered layer (soft magnet)
  • sputtered C layer for corrosion resistance
42
Q

Why is CoPtCrB used instead of pure Co in HDD structures?

A
  • Pt stabilises HCP structure
  • Cr segregates to boundaries making them para magnetic (while needles stay ferro)
  • B segregates to boundaries to widen them & give uniform grain size distribution (less interference = greater recording accuracy)
43
Q

What are the two types of ceramics?

A
  • Chemically made ceramics (Powder which reconsolidate into shape), shape and size dependant on reaction, spherical flow better but have lower strength, processing depends on green strength of powder
  • Naturally occurring clays (add water to make them plastic solid or liquid)
44
Q

What are slip clays and how are they cast?

A
  • Naturally occurring clays that form a liquid when water is added
  • Pour slip clay into a porous mould, water escapes through the mould, slip clay poured from mould = hollow shape (wall thickness depends on time in mould), low temp so low hydrolysis
45
Q

How are ferrites useful in magnetic applications and why can’t they be conventionally processed?

A

Higher resistivity = lower energy losses

- high Tm (no casting) and brittle so can’t be machines

46
Q

How do you increase the strength of clays?

A
  • By firing them = decreasing moisture = increased bond energy between particles
  • Firing above 1000° = structure changes = silica forming which fills interstitials and creates surface layer or silica (forms glass on cooling and shrinks up to 1/3 = closer bonding of clay)
47
Q

What is green strength?

A

Strength of a powder when it’s compressed

High porosity = low green strength and lower energy surface bonds

48
Q

Describe power processing

A

Powder fills die, upper and lower pinch compact (so no density gradient)

  • high surface activity powders increase production rates
  • lubricants added to increase flow ability of powder
49
Q

Describe powder heating and sintering

A

Powder heating - heated to low temp to remove solvent and binder, high temp for sintering to occur, cool at slow rate to avoid thermal shock
Sintering - very slow diffusion (hours) of material to porous area and moves particles closer together to increase contact area = bond strength

50
Q

What is isotactic pressing

A

Uses liquid to 3D press powder
Cold - like powder pressing
Hot - reduces porosity but increases cost
Both types then sintered

DFA (5 decks)

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