Chapter 2 - Magnetism of Interfaces and Nanostructures

Question 1

Where can we find ferromagnetic bulk materials?

Answer 1

In some 3d transitions metals (Fe, Ni, Co) and some 4f rare earth metals. This is due to unpaired spin in valence band.

Question 2

What is the magnetic susceptibilities for a (i) diamagnet, (ii) paramagnet and (iii) ferromagnet?

Answer 2

i) chi 0 (chi 0 (chi&raquo_space; 1)

Question 3

Draw the magnetic ordering for a paramagnet and for a ferromagnet.

Answer 3

You know this..

Question 4

In one simple sentence, explain ferromagnetism in 3d transition metals.

Answer 4

We can get exchange splitting of d-bands, which, if this splitting reduces the overall energy (Stoner criterion), will give rise to ferromagnetism.

Question 5

What three types of exchange do we have?

Answer 5

Direct exchange, can give rise to either antiparallel or parallel alignment depending on the spin distance.

Indirect exchange: magnet species polarize conduction electrons, then these conduction electrons polarize the next magnetic species.

Superexchange: magnet ions couple via non-magnetic neighbours, such as oxide (through their p-orbitals).

Question 6

Name two examples of where we find nanomagnetism in nature.

Answer 6

In the in-built compasses of certain bacteria and migratory birds.

Question 7

What is magnetocrystalline anisotropy?

Answer 7

The fact that the orbital momentum follows preferred crystal lattice directions. This means that there are axes where it is harder to magnetize (along direction of highest energy) and easier to magnetize (along preferred direction).

Question 8

What is the magnetic anisotropy energy?

Answer 8

The energy required to rotate magnetic moment from easy to hard axis.

Question 9

What is the Faraday effect?

Answer 9

It is the rotation of light induced by a magnetic field as it passes through a Faraday-active crystal

Question 10

What is the Kerr effect?

Answer 10

The change of light polarization upon reflection at magnetic surface.

Question 11

In ferromagnets, what is the magnetic ordering?

Answer 11

We get Weiss domains that have the same cooperative spin orientation. In the material there are many such domains, which is randomly orientated until influenced by magnetic field.

Question 12

What is a scanning electron microscopy with polarization analysis (SEMPA)?

Answer 12

A regular SEM, but with an electron detector that detects the spin polarization of the incoming electrons. This allows us to look at the magnetic domains.

Question 13

What is the origin of Giant Magnetoresistance?

Answer 13

In a ferromagnetic material, the two different spin densities will have different resistance (due to their energy diagrams being different). When we sandwich a non-magnetic material (Cr) between two layers of ferromagnetic materials (Fe), the magnetic layers are without an external field oppositely polarized. When a field is applied, they will be parallel. In the first situation, the current of both spin densities will be the same (the spin up will experience one resistance in one of the layers, and the one in the other. The spin down will experience the same, but in opposite succession). When the field is applied, one of the spin densities will have a much lower resistance and one will have a higher.

Question 14

How can half-metals be used to exhibit the giant magnetoresistance effect?

Answer 14

In half-metals, the spin down band is metallic and spin up is insulating. In this case the spin down will flow uninhibited when the polarization of both domains are parallel. When they are anti-parallel, there will be no current, since both layers will totally block one of the spin densities.

Question 15

How does tunneling magneto resistance differ from giant magnetoresistance?

Answer 15

Instead of a non-magnetic layer as the sandwiched layer, we use an insulating material. This means there is only a tunneling current through this thin barrier. The effect of TMR is higher than GMR.

Question 16

How does spin-polarized STM work?

Answer 16

We have a tip covered with a ferromagnetic thin film which is polarized in a certain direction. When scanning over the surface, the current will be different depending on the polarization of the domains being scanned. This will give a contrast of the different magnetic domains.

Question 17

How can we prepare a tip for spin-polarized STM?

Answer 17

We deposit a thing magnetic film on an atched and annealed W-tip. The magnetization direction is given by the surface and interface, and not by the shape anisotropy. This means we can make tips polarized both in-plane and out of plane, depending on the material and the number of monolayers deposited.

Question 18

What is spin-exciation spectroscopy? What are the requirements?

Answer 18

Spectroscopy based on the Zeeman-splitting of energy levels in a magnetic field.

Requirements: low temperatures and high magnetic fields.

Question 19

What are skyrmions?

Answer 19

Quasi-particles on the surface of certain materials, spiral spins. Whirling configuration with a long modulation period.

Question 20

How can skyrmions act as information bits?

Answer 20

Depending on their circulation direction (clockwise or counterclockwise) they can represent 1 and -1 states. If we can manipulate and read these in a reliable manner, they can be used to store information.

Question 21

What is the areal density of magnetic storage these days?

Answer 21

In the 100Gbit/in^2 regime.

Question 22

Draw a schematic of a longitudinal recording system.

Answer 22

See slide 4 of second magnetic powerpoint.

Question 23

How big are the CoPt-particles used in magnetic storage, and what limits this size?

Answer 23

They are about 10nm in size. The superparamagnetic limit is the fundamental limitation where the nanoparticles become superparamagnetic. Under this we’re under the threshold for permanent magnetism, as the thermal energy flips the magnetic orientation of the whole particle.

Question 24

How is anisotropy of magnetic materials depending on shape, crystallinity and surface?

Answer 24

Shape: for needle-shaped particles, it is parallel to the axis. For thin-film it is preferably in-plane.

Crystalline: tends to align with a specific crystallographic direction.

Surface: on surfaces and interfaces, it is often perpendicular to the interface.

Question 25

What is hard and soft magnet? What are they used for?

Answer 25

Hard magnet: large anisotropy, hard to switch.
Used as a permanent magnet and as magnetic storage.

Soft magnet: small anisotropy, easy to switch.
Transofmer core and sensor.

Question 26

How can we attain higher storage densities?

Answer 26

We need nano-patterened media with linewidths down to 12.5 nm. It is a challenge to find a low cost way to do this.

Question 27

What is the advantage of perpendicular recording over longitudinal?

Answer 27

Higher bit densities.

Question 28

What can be said about head-media spacing in magnetic recording?

Answer 28

This is the spacing between the read/write head and the media. This needs to be low, and it can be very challenging to make this reliably. For Tbpsi recording, one need a spacing of about 5 nm.