Magnetic- Thin Films

Question 1

What uses thin films in a magnetic hard disk drive?

Answer 1

The storage media

Question 2

What are bulk materials?

Answer 2

Magnets with dimensions in all directions greater than 100s of μm. We can increase the scale of the object without causing any significant change in its magnetic behaviour

Question 3

Relative size of domains in bulk material

Answer 3

The characteristic size of domains in a given magnet (width) is much smaller than the magnet dimensions.

Question 4

What does the relative size of domains in bulk materials mean?

Answer 4

Domain wall energy insignificant compared to magnetostatic energy. Very large number of domains. Complex domain configurations controlled by defects

Question 5

How to visualise thin films

Answer 5

Reduce one dimension of a bulk magnet until it reaches nanoscopic proportions. For magnetostatics, this film resembles an infinite sheet

Question 6

Demagnetising factors and easy and hard axes in thin films

Answer 6

In-plane demagnetising factors
Nx=Ny=0.
Out-of-plane demagnetising factor
Nz=1
Means very unfavourable for magnetisation to point out of plane so magnetisation of system effectively becomes 2D (unless...). Out of plane direction becomes hard axis

Question 7

In and out-of-plane hysteresis loops for thin films

Answer 7

In-plane: very open square shape with high coercivity and remanence.
Out-of-plane: basically no enclosed space and is like y=x until saturation

Question 8

What is an exchange length?

Answer 8

Distance over which magnetisation can change direction

Question 9

What is true for film thickness less than a few exchange lengths?

Answer 9

Magnetisation not only lies in-plane but will be in the same direction at the top and bottom surface of the film. In-plane the dimensions are still large compared to characteristic domain width. Means complex defect dominated domain structure

Question 10

Why are thin films not just 2D versions of bulk materials?

Answer 10

They are grown layer by layer on substrates, typically atomically flat semiconductor wafers. Surface area to volume ratio much higher so stronger influence of surfaces and interfaces than in bulk materials

Question 11

How to make single crystal films

Answer 11

Growth performed very slowly (under nm/s) and under ultra-high vacuum (<10^-9mbar) we can get epitaxial films by molecular beam epitaxy providing the lattice parameters of the source and substrate are well matched. Means atomic positions in substrate and deposited layer are correlated.

Question 12

What happens if the lattice parameters are not as well matched for single crystal thin films?

Answer 12

The film is strained and the properties will be affected by magnetostatic anisotropy

Question 13

Which type of energy dominates switching behaviour of single crystal films?

Answer 13

Magnetostatic anisotropy

Question 14

How to make single crystal films

Answer 14

Under nm

Question 15

Describe general textured films

Answer 15

They are polycrystalline where there is random orientation of grains in in-plane directions. Well-defined orientation of grains in out-of-plane directions. Generally due to growth of columnar grains from a substrate

Question 16

What 2 phases can FePt form?

Answer 16

Disordered FCC (different elements could be in any position). Magnetism here is isotropic.
L1 subscript 0 ordered FCT where there is a plane of 4 Pt atoms on face centres with the rest Fe atoms. Strong uniaxial magnetocrystalline anisotropy présent along c-axis.

Question 17

How to make textured films

Answer 17

Optimise process conditions for L1 sub 0 phase of FePt with c-axis pointing perpendicular to substrate. Control substrate temperature during growth, post growth annealing, add buffer layers of non-magnetic material under film (RuCr). This promoted texturing. Anisotropy induced in texturing strong enough to overcome shape anisotropy and force film’s magnetisation out-of-plane.

Question 18

What changes at the interfaces/surfaces?

Answer 18

Change in the electronic environment. Lower dimensionality of bonding at surface than atoms inside where bonding is 3D. Means changes in atomic spacing at surface. Electronic structure also changes at interface. Causes changes in spin-orbit coupling and therefore magnetocrystalline anisotropy. Also changes magnetic moment per atom. Effects combine to creat surface anisotropy

Question 19

Effective anisotropy constant formula

Answer 19

Keff=Kbulk+Ksurface/t
t is film thickness
K surface units J/m^2

Question 20

What does it mean when effective anisotropy constant is positive or negative?

Answer 20

When positive, out-of-plane is easy axis. When negative it is the hard axis

Question 21

Why are textured films used for magnetic hard disk drives?

Answer 21

Can be made much thicker than interface anisotropy materials and more easily fabricated

Question 22

General process for fabricating thin films

Answer 22

Usually use physical vapour deposition. Atoms removed from source material into a vapour. This condenses onto an atomically flat substrate (like Si wafer)

Question 23

Two ways of creating the vapour for physical vapour deposition

Answer 23

Evaporation

Sputtering

Question 24

Common feature of all techniques to create vapour for PVD

Answer 24

Take place in high vacuum. Reduces number of impurity atoms incorporated into the magnetic films and prevents oxidation of magnetic materials. For same reason also use high purity materials as source, e.g 99.99%

Question 25

How does evaporation work for PVD?

Answer 25

A substance is rated until it melts and begins to evaporate forming a vapour in the chamber. Atoms travel to a substrate and are deposited

Question 26

Thermal evaporation

Answer 26

Material heated by passing current through a resistive element. As the whole crucible (and surrounding parts of chamber) get hot this can be relatively dirty process and may cause impurities in deposited films.

Question 27

Electron beam evaporation

Answer 27

Material heated by incident electron beam. Only source material heated so is cleaner process. If done slowly in very high vacuum this is suitable for single crystal growth

Question 28

General process for sputtering

Answer 28

Atoms removed from a target by impact from ions of an inert gas plasma. Chamber filled with low pressure of inert gas (Ar). Voltage applied across gas creating a plasm. Positive Ar ions accelerated towards cathode where target of the material to be deposited is placed. These sputter atoms from the deposition source which are condensed onto a substrate

Question 29

Difference with modern sputter chambers

Answer 29

Cathode and anode integrated into a single sputter gun with the substrate placed some distance away. Plasma localised close to gun gather than in larger area

Question 30

Types of voltage used in sputtering

Answer 30

For Gallic materials can use DC voltage between anode and cathode. For insulating material use alternating voltage to prevent charge build up on target (RF sputtering)

Question 31

Why can we sputter practically anything?

Answer 31

Don’t have to melt the target so can use materials with very high melting points

Question 32

Effect of using Ar in sputtering

Answer 32

Doesn’t dramatically affect film quality because base pressure (before Ar added) usually very low so very few other impurities, Ar pressure use is quite low (10^-3mbar), Ar is inert. Does make big difference to stress in film as it controls how much energy deposited atoms have as they arrive at substrate. Sputtered atoms get gradually thermalised by collisions with Ar atoms reducing their KE.