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WS05 I Advance materials in the information technology: Fundamentals and applications

Magnetism and magnetic materials

Why spins?

Non volatile magnetic storage information exists.

Low consumption of power.

Interesting electron spin correlation

with light polarization.

In magneto-electronics it is important to know:

Spin-dependent phenomena (transport, tunnelling, scattering,interference, exchange interactions, spin lifetime, excitations,),

which lead to:Spin dependent effects (interlayer exchange coupling, giantmagnetoresistance, tunnel magnetoresistance, ),

used in devices (spin valves, magnetic memories, spin injectors,).

In microelectronic devices information is carried by charge.

Magnetoelectronics (or spintronics) information is carried by the electronic spin.

Memory

Logic

read

write

Transport

Transport

inject

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Phenomena in magnetoelectronics

Exchange interactions

Anisotropy

Spin dependent scattering

Spin dependent tunnelling

Spin injection

Interlayer exchange coupling

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Exchange interaction.

Indirect exchange interaction.A magnetic ion induces a spin polarisation inthe conduction electrons in its neighbourhood.

This spin polarisation in the itinerant electronsis felt by the moments of other magnetic ionswithin range leading to an indirect coupling.

Direct exchange interaction between atoms

N

S

N

S

N

S N

S

Superexchange.A magnetic ions induces a spin polarisation inother magnetic ion which is coupled by theircommon non-magnetic neighbour.

Electron motion and local ion magnetic momentsare coupled.The magnetic property of the material is thusimportant for the transport of spins

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Magnetic ferromagnetic dommains.

A ferromagnet possesses spontaneousmagnetization below the Curie temperature.

The stray field tends to demagnetise theferromagnet.

Domains are formed.

The size of domains can be unbalanced with anexternal field

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Special anisotropies at surfaces and interfaces, and .

Anisotropy:Anisotropy: existence of an easy axes ofmagnetization: To rotate the magnetization, energymust be applied, i.e. the magnetic anisotropyenergy.

Heating the system, the spin alignment along theeasy axis is lost and a paramagnetic phase isreached.

Quantum anisotropyQuantum anisotropy..-The allowed values of mJ are quantized.

Magnetocrystalline anisotropy.

-Bulk anisotropy typicallyB~105 J/m3d.

-Orbital momentum follows defined orientations inthe crystal lattice, due to the atomic bonding orbitalarrangements. Since the spin is coupled to the orbit,the lattice define easy axis for spin orientation.

-Introducing stress in the crystal lattice further

increases the anisotropy, and thus the magneticmoment of the solid: Magnetoelastic anisotropy.

Surface anisotropy.

-At the surface the atomic structure from the bulk isterminated, creating an additional anisotropy.

-Purely surface anisotropy, i.e., it does not dependon thickness.

-This anisotropy can also be perpendicular to thesurface, and therefore, fight against the tendency ofa shape anisotropy.

-S=KS ~mJ/m2

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thickness dependence of magnetic ordering.

Spontaneous re-orientation of magnetic moments at thesurface depending on the balance between shape, bulk andsurface anisotropies.

Since surface anisotropy does not depend on the film

thickness, and the shape does, for a thin film withperpendicular surface anisotropy, it will be a critical ticknessabove which the spins re-orient.

Typical systems where this occurs: Fe on Ag and Au, Co onPt and Pd,.

Shape anisotropy:

-Spins tend to align parallel to theinterface.

Shape~106 J/m3d.

Thickness dependence of

Curie Temperature.

The size alsomaters inmagnetism.

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Exchange anisotropy.

The magnetic alignment at an interfacebetween two magnetic layers will bedominated by the exchange interaction.

The effect of the exchange coupling in an interface isthe asymmetry of the hysteresis loop by an amount

defined as the exchange bias, and the

increase of the coercitivity.

Defects, steps,may also help to pinmagnetic moments at the interface.

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Spin dependent reflectivity at materials interface

The transport of electrons through a ferromagneticmaterial interface is a process that depend on therelative orientation of the local polarization and thespin.

The reflection of an conduction electron at theinterface also depends on the relative orientation

of their magnetic moments.

When conduction electrons are injected from anormal material, both spin components impinge atthe interface. In this case, the transmission thoughthe ferromagnetic material filters only one spin

component.

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The giant magnetoresistance effect (GMR)

When electrons are injected into two ferromagneticlayers separated by a normal material, thetransmision (i.e. the resistance) depends on therelative orientation of the two magnetic layers.

The GMR effect is the large increase in

resistance when the relative orientation of themagnetizations in neighboring ferromagneticlayers is switched from paralel to antiparallelby applying a magnetic field.

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There are two types of configuration giving rise togiant magnetoresistance:

The giant magnetoresistance effect (GMR)

Current perpendicularto plane (CPP) Current in plane (CIP)

P

PAP

R

RR

R

R =

To control the resistance it is necessary change byexternal means the relative orientation of the twoferromagnetic layers. This is usually done by anexternal magnetic field, having one of the layerspinned by an antiferromagnetic layer (spin valve).

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Interlayer exchange coupling

Similarly to the indirect exchangeinteraction between atoms,ferromagnetic layers can couplethrough non-ferromagnetic media.

Two ferromagnetic layers can be couple effectivelyby electrons in an antiferromagnet (AF). The typeof coupling does depend on the thickness of theAF layer.

Two ferromagnetic layers can also coupleeffectively by electrons in an non-magnetic media.

The type of coupling (AF or F) at zeromagnetic field depends on the thickness ofthe non-magnetic spacer!!!!.

Co

CuCoCuCo

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Origin of the oscillations in the magnetic coupling between ferromagnetic layers

a) Spina) Spin--dependent reflectivity.dependent reflectivity.

b) Quantum well states in the spacer layer.b) Quantum well states in the spacer layer.

c) Energy minimizationc) Energy minimization

Higher DOS at EFmeans higher energy.

To minimize the energy, anAF coupling is preferred.

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Spin injection mechanisms

1) Optical injectionOptical injection: If either left or right circularly polarized light is used to opticallygenerate non-equilibrium carriers, the excitation selection rules require S = +1 andrespectively S = 1, therefore one spin direction will be generated preferentially.

2) Tunnelling of carriers from a ferromagnetic material:Tunnelling of carriers from a ferromagnetic material:The tunnelling conductance isproportional to the product of densities of states at the Fermi level on both sides of the

barrier. Due to the different densities of states in the spin sub-bands in the ferromagnet, thetunnelling rates from the ferromagnet to the normal region are different for the two spin sub-bands (i.e the tunnelling current is spin polarized). A non-equilibrium spin population is thusinjected in the normal region. This effect is purely an interface effect.

3) Injection via clean interfaces, by passing current from a ferromInjection via clean interfaces, by passing current from a ferromagnetic regionagnetic region

into a noninto a non--magnetic onemagnetic one. This mechanism is different from tunnelling, as the spinaccumulation is due to the bulk properties of the materials. However, one has to keep inmind that the realization of good tunnelling barriers on semiconductors is rather challengingfrom technological point of view. Thus the fact that spin injection via clean interfaces dependonly on bulk properties makes this an attractive route towards realizing spin injection.

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