Magnetic- Hard Disk Drives

Question 1

What properties are needed for magnetic data storage?

Answer 1

Non-volatile (can store information without consuming power). Switchable (information can be deleted non-destructively). Anisotropy (easy to create digital states). Creates field (provides way to detects state of data)

Question 2

3 main functions of hard disk drives

Answer 2

Writing data, storing data, reading data. Each process requires magnets with different properties (sometimes in conflict with each other)

Question 3

Parts of hard disk drive

Answer 3

Read/write head- writes data into platter, reads data back.
Platter/recording media- thin magnetic film into which data encoded.
Slider motor/actuator- controls radial position of read/write head.
Platter/spindle motor- controls circumferential positioning of read/write head.
All set up like record player

Question 4

Coercivity of hard and soft magnets

Answer 4

Hard- high coercivity

Soft- low coercivity

Question 5

What determines whether a magnet is hard or soft?

Answer 5

The strength of their magnetocrystalline anisotropy. Higher means harder

Question 6

Slider/spindle motor

Answer 6

The slider and spindle motors are standard electrical motors. Motion produced by passing electric current through wires in presence of magnetic field. Motion produced by Lorentz force. Requires permanent magnets to provide strong and constant magnetic field to maximise torque. Magnet needs to be highly stable over long time so efficiency of motor doesn’t decay. Need hard magnet with high remenance and coercivity. NdFeB

Question 7

Describe the recording media material

Answer 7

Are granular (no exchange coupling between grains) thin films of magnetic material. Typically CoCrPt. Strong uniaxial anisotropy for encoding digital data. Non-magnetic Cr migrates to grain boundaries so neighbouring grains are decoupled and can be switched independently. Hard magnetic material so magnetisation is stable against data loss (not too hard so can write data). Needs high Ms so creates strong magnetic field for reading data. Typically fabricated by sputter deposition

Question 8

How is data stored in recording media?

Answer 8

Each bit of data on the hard disk consists of a collection of grains that are magnetised in one of two anti-parallel directions (1,0).

Question 9

Why is the magnetisation of a magnet always fluctuating?

Answer 9

Due to thermal energy. Always a finite probability that the fluctuations will reverse the magnetisation and turn 1 into 0 and cause data loss

Question 10

Arrhenius-Néel law

Answer 10

1/τ=f0exp(ΔE/kBT)
τ is characteristic timescale of the thermal activation
f0 is the attempt frequency (how often system tried to jump energy barrier).
ΔΕ is size of energy barrier
T is temperature

Question 11

Probability of magnetisation having switched due to thermal activation

Answer 11

P(t)=1-exp(-t/τ)

Question 12

Formula for energy barrier for a grain

Answer 12

For a film dominated by uniaxial anisotropy
ΔE=KuV
V is grain volume

Question 13

Ratio of energy barrier to thermal energy for hard disk media

Answer 13

KuV/kBT greater than 60
Ensures grains are stable even in presence of local demagnetising fields or slight perturbations when neighbouring grains written

Question 14

What is the write head?

Answer 14

Essentially a miniaturised electromagnet. Electric current passed through a wire wrapped around a magnetic core. Magnetises core and creates strong magnetic field in air gap

Question 15

What type of magnetic material needed for write head?

Answer 15

Requires soft (low anisotropy) magnetic materials with high saturation magnetisation (to create large field). Need low coercivity to easily change what is being written. Needs low remenance so it isn’t writing all the time. Use FeCo alloys (Ms=1909kA/m) which have highest Ms possible

Question 16

How does write head work?

Answer 16

Use perpendicularly magnetises media (magnetisation points out of plane of platter). Is like half of electromagnet. Goes along top of platter to magnetise the grains. Field sent and received through write and return poles (circuit completed through soft magnetic underlayer). Different size of write and return poles create different flux densities in recording layer so only one write position (at narrow pole).

Question 17

What does the read head do?

Answer 17

Needs to sense the magnetic field coming from the storage media. Uses phenomenon of magnetoresistance. Electrical resistance of magnet depends of its magnetic state so field from data stored in media can be turned into electrical signal (example of spintronic effect)

Question 18

Magnetisation in read head

Answer 18

Read head sensor composed based on magnetic thin film (called spin-valve). The magnetisation of the sensing layer lies orthogonal to field from bits in recording media. If field points up magnetisation rotates up and vice versa

Question 19

What type of material does free layer of read head need to be?

Answer 19

Soft ferromagnetic material because it needs to respond easily to the applied field of the data bits

Question 20

What is SNR and what does it depend on?

Answer 20

Signal to noise ratio (signal/noise). It is proportional to square root of number of grains per bit (N).

Question 21

The 3 important points of hard-disk development

Answer 21

For storing data the energy barrier can’t be too small or data will be lost (high KuV) For writing data the coercivity of media can’t be too high (low Ku). For reading data SNR from sensor can’t be too low

Question 22

What needs to happen for data density to grow?

Answer 22

Bits need to be made smaller

Question 23

The trilemma of hard-disk development starting from thermal stability

Answer 23

Start with requirement of ΔE=KuV must be high. To shrink size of data bits can use smaller grains (same grains per bit) and increase Ku to retain thermal stability. Or can use same size grains but fewer per bit and keep Ku same.

Question 24

Problem with two ways of shrinking bits starting from thermal stability

Answer 24

For smaller grains and larger Ku, root N stays same so food for SNR, but greater Ku increases write fields. For same sized grains coercivity stays same for write fields but root N decreases so lower SNR for reading

Question 25

Roadmap for hard-disk development

Answer 25

Have had PMR (perpendicular magnetic recording) and PMR+ (PMR with 2D magnetic recording). Starting HAMR (heat assisted magnetic recording). Further in future is HDMR (heated-dot magnetic recording)

Question 26

HAMR

Answer 26

Heat assisted magnetic recording. Use laser to locally heat film when wanting to write data. Data stability now different during writing and storing phases. Decrease stability during writing=low coercivity. Higher stability at other times=stable data. Can now use smaller grains (SNR) with higher anisotropy so higher data density

Question 27

Difference between standard granular media and bit patterned media

Answer 27

Standard granular media each bit is composed from multiple, weakly coupled grains. Means stability controlled by grain volume (much smaller than data bit). In bit patterned media (for HDMR) each bit is stored by an individual nanostructure which each contain multiple, strongly coupled grains

Question 28

Advantage of bit patterned media

Answer 28

Strong coupling between grains means stability controlled by the full nanostructure/bit volume. Means can reduce Ku while maintaining KuV/kT. Reduce coercivity while maintaining stability. Can use smaller bits without pushing coercivity beyond write head limits. Also uniform magnetisation within a bit so high SNR for read out

Question 29

How are solid state drives different?

Answer 29

Based on semiconductor technology. Read and write dats much quicker than conventional hard-disk drives as fewer moving parts