Magnetic- Spintronic and Magnetoresistive Materials Flashcards
What are spintronic/magnetoresistive materials used in?
The read head sensors of a hard disk drive
How do conventional electronics operate?
By transport of electrical charge. High voltage on a terminal means charge transport and potential difference is negative. No voltage on terminal means no charge transport and potential difference is zero.
How are spintronic materials used?
The spin of the electrons is instead/additionally used to encode information. Where spin up could mean 1 and spin down mean 0. In devices the spin orientation is generally read out by changes in electrical resistance (magnetoresistance).
Simplest form of magnetoresistance
Anisotropic magnetoresistance (AMR). Observed in single layer thin films. Effect illustrated by considering domains of alternating magnetisation and applying a current passed through it either perpendicular or parallel to the length of the domains and a magnetic field applied either perpendicular or parallel to the current and observing the changes in electrical resistance.
How does resistivity depend on orientation of current, magnetisation and field in AMR?
Resistivity high when J and M (of the domains) along same axis and is low when they are perpendicular. Resistivity increases/decreases when M turns towards/away from field axis (only rotates when M and H orthogonal).
Resistivity as function of angle between current and magnetisation for AMR
ρ(θ)=ρ(para)+(ρ(para)-ρ(perp))cos^2θ
Where θ is angle between I and M
Resistivity in different directions
What is electrical resistance due to?
Conduction electrons scattering. Infrequent scattering means low resistance, frequent scattering means high resistance.
How does scattering relate to relative orientation of magnetisation and current?
Magnetism of 3d metals primarily comes from electron spin but there is also angular momentum (so moment) which means electron density asymmetric in space. Spin-orbit coupling means when M changes direction it twists the electron orbits around with it. When M and I are colinear, the electron density is in an orientation with a higher scattering cross-section for conduction electrons than when they are orthogonal. This explains AMR effect.
How strong is AMR effect?
Relatively weak effect and typically produces resistance changes of only a few %
How was giant magnetoresistance originally observed?
GMR. Super-lattices composed of alternating magnetic and non-magnetic layers. Shows oscillatory interlayer exchange coupling with non-magnetic layer thickness t. Alternate between ferromagnetic coupling between magnetic layers and antiferromagnetic coupling between magnetic layers with increasing t.
Eex=-2JexSi•Sj
Si and Sj are spins of adjacent layers. Sign of Jex oscillates with and decreases in magnitude with increasing t.
What happens if apply field to material with GMR?
For ferromagnetic coupling, spins are locked together and rotate as a whole to orientate to the field. Easy to saturate.
For anti-ferromagnetic coupling, spins gradually rotate to parallel alignment in the direction of the field. Hard to saturate.
Anti-ferromagnetic coupling case for GMR with applied field
Resistance changes by up to 50%. Resistance depends on alignment of magnetic layers’ magnetisations. High resistance when still anti-ferromagnetic alignment, low resistance once aligned with field. Fractional change in resistivity 0 when angle between M vectors of layers is 0, it is maximum when angle is 180
Things need to know to understand GMR
Electron flow consists of up and down spin electrons. Electrons don’t often flip spin so can be considered as two separate conduction channels. Scattering much more common for spin down electrons.
How are spins defined?
Relative to the magnetisation of the layer. Up means they are in same direction as M, down means in opposite direction to M
Explain basic GMR behaviour
M of magnetic layers parallel: spin up electrons pass through easily, spin down electrons scatter frequently, overall low resistance.
M of layers anti-parallel: spin up electrons are in opposite direction for alternate layers so frequent scattering, same for spin down, overall high resistance.
Problem with super lattices for GMR and what we want
Produce very high MR signals but require high fields to switch them. Want simple two magnetic layer structure where one layer is pinned and the other moves at low field. This is a spin valve. These layers are still either side of non-magnetic layer.
Exchange bias
When ferromagnetic layer grown next to anti-ferromagnetic layer, exchange bias occurs. This is where hysteresis loop of the ferromagnet is shifted left along the field axis. The shift is called the exchange bias field. The ferromagnetic layer will always be magnetised in one direction at remanence and takes a lot of field to reverse it.
Structure of spin valves
Anti-ferromagnetic layer fixes the bottom pinned ferromagnetic layer to point in one direction. Non-magnetic layer thick enough that the two ferromagnetic layers are not exchange coupled. Top free layer can rotate easily to sense a magnetic field.
Read head
In GMR read heads the free layer equilibrium position is orthogonal to both pinned layer and axis of field it needs to sense. Will now sense positive or negative resistance as an increase or decrease of resistance as the layer rotates towards or away from the pinned layer magnetisation direction.
