Trinity College Dublin

Conventional elcetronics ignore the spin of the electron. The new science of spin electronics will deliver smaller, faster devices with novel properties. Our focus is on spin polarised electron transport in nanoconstrictions and then film stacks

The great advances in thin-fillm fabrication technology and lithographic patterning techniques over the years has made it possible to produce extremely small-scale magnetic structures and more directly access properties that depend on the electron spin. A new paradigm of electronics will be based on the spin degree of freedom. The CINSE Research program addresses this new horizon.

Spin Polarisation

Ferromagnetic metals are the obvious first choice as a source of spin-polarised electrons Since the ferromagnetic moment reflects an imbalance of the up and down electron spins in the conduction band Spin polarization P is defined in terms of the density of states of spin up and spin down electrons at the Fermi Energy: P = ( N↑ - N↓)/( N↑ + N↓ ) The Spin Polarization P for Fe, Co or Ni is approximately 0.45; for half-metals it should be 1

Diagrams of the spin-resolved electronic density of states (x-axis) as a function of energy (y-axis). (A) Normal metal, (B) typical ferromagnet, and (C) half-magnetic ferromagnet

Magnetoresistive Devices

Giant Magnetoresistance (GMR) was discovered by Albert Fert's group in Co/Cr multilayers in 1988. It is the basis of the spin valve read head which is in common use in hard drives since its introduction by IBM in 1998.

Magnetic, non-volatile random access memory (MRAM) based on magnetic tunnel junctions (MTJ) is in active development by major semiconductor companies in the US, EU, Japan and Korea.

These magnetoresistive devices are based on ferromagnetic metal/spacer (metal or insulator/ferromagnetic metal sandwiches.

The resistance depends on the relative magnetic orientation of the layers which can be changed from parallel to antiparallel by an applied magnetic field, as shown for a MTJ in the picture:

The Magnetoresistance (MR) of a MTJ is described by Julliere's formula which depends on the spin polarization P1 and P2 of the two magnetic layers: MR=2P1P2 / 1 - P1P2 The response is maximised by using half-metals with P-1.

 

Powder Magnetoresistance

Contacts between crystallites of ferromagnetic metals can themselves be either metallic of insulating tunnel junctions. In a pressed powder, the resistance is dominated by the most resistve contacts along a critical path. A large change of resistance in a low magnetic field (PMR) is associated with the magnetization process of contiguous ferromagnetic grains.

 

Beyond Magnetoresistance

Most work has been done on two-terminal transistor magnetoelectric devices (magnetoresistor, spin valves, tunnel junctions) which are operated by an external magnetic field. However, there are good prospects for developing three terminal devices (spin transistors) and for using spin polarized current instead of an external field to do the switching.