Mon., Feb. 04 & Wed., Feb. 06, 2013

A few more instructive slides related to GMR

and GMR sensors

Oscillating sign of Interlayer Exchange Coupling between

two FM films separated by Ruthenium spacers of

thickness varying from 0.3 nm to 3.3 nm (measured data).

Ruthenium – an exotic metal from the Platinum group, with Z = 44. It had no

major technological applications until it was discovered that it is particularly

efficient in conveying interlayer exchange coupling between Cobalt-rich

ferromagnetic films.

A schematic of a simplest GMR sensor. The thickness

of the non-magnetic spacer is such that the coupling

between the two FM films is antiferromagnetic.

However, both FM layers are “free”, i.e., their magnetization

vectors M1 and M2 are not “anchored” to anything. Hence,

their mutual orientation can be changed by an external field

exerted in any direction. Consequently, such devices are

are sensitive only to the external field magnitude.

A schematic of a “spin valve” GMR structure. The top FM layer is

a “free” layer – its magnetization direction can be changed by app-

lying an external magnetic field B. The other FM layer is exchange-

coupled, or “pinned” to a thick antiferromagnetic substrate, and

therefore its magnetization does not react to B.

A spin valve does not react to fields exerted in certain directions:

B

This field does not change

the magnetization direction

in the top layer – no change

in the resistance!

B

But field in this direction will

change the magnetization di-

rection in the top layer,

and thus the sensor re-

sistance will decrease.

“Directional sensitivity” is often needed in technological applications!

However, such a design is still not perfect!

The “pinned” layer is a source a field that produces an

“offset” in the R vs. B characteristic…

Fortunately, the “offset problem” can be solved by a more

sophisticated design, in which a single “pinned” FM layer

is replaced by two FM layers separated by a thin Ru spacer

that introduces a strong AFM coupling between them.

Such a “trilayer” is usually referred to as an “artificial antiferromagnet”.

The B fields produced by the two FM components cancel out one

another.

In such a spin valve design, the

“pinning” AFM layer may even

not be needed….

Spintronics

The emergence of GMR devices marked the beginning

of the Spintronics Era.

What is spintronics? It is a novel branch of electronics.

Conventional electronics is based on controlling the

magnitude of electric currents. In contrast, in spintronics

it is the spin state of the current (or its spin polarization,

if you prefer) that is controlled.

What advantages may controlling of the

current’s spin-state offer compared to

conventional current’s magnitude

controlling?

Let’s take a short “brainstorming session”!

I want to know your opinions…

GMR sensors

are not

EXACTLY

spintronics

devices…

There are “spintronics-

like elements” in GMR

sensor operation, but

the signal produced

by the sensor is still

a current signal.

So, GMR sensors are still “spintronics and

Conventional electronics HYBRIDES”…

But there is nothing wrong with it, too radical

Revolutions are not always good…

There are no “100% spintronics devices” yet, but things are

certainly evolving in this direction….

A device that is “more spintronics” than a GMR sensor, is a

Tunnel Magnetoresistance (TMR) junction.

The design is similar to that of GMR sensors, except that

instead of a metallic non-magnetic spacer there is an ultra-

thin insulating layer. It acts as a barrier the electrons can

pass through due to the quantum effect of tunneling.

The probability of tunneling is different for the “parallel” and the

“anti-parallel” configuration of the FM layers.

TMR sensors are even more efficient that the GMR sensors. They

are now widely used in computer hard-drive reading heads.

Another application of TMR sensors that seems to be “right

Around the corner” is in Magnetic Random Access Memory (MRAM).

Each TMR junction can store one bit of info.

In magnetic random access memory

(MRAM) the magnetic moment of a

magnetic material is used to store data.

In this case, a magnetic moment

pointing left can represent a "0", while

a magnetic moment pointing right can

represent a "1". (b) Data can be written

to the material by sending an electric

current down conductors that pass

nearby. In this case, the magnetic field

produced by current x puts the magne-

tization into an intermediate state, and

current y then triggers the magnetic

moment to move to a particular

orientation.

Next item: another class of magnetic materials that are highly

Interesting from the viewpoint of spintronics are the so-called

“half-metals”.

In a half-metal, for

one electron spin

orientation (↑) the

structure of the

electronic bands

is like that in a

metal…

But for the other

spin orientation (↓)

it is like in a typical

semiconductor,

with a distinct

“energy gap”.

One interesting application of half-metals is in “spin-filters”

that can be used for obtaining nearly 100% spin-polarized

currents:

An ordinary electron current is a mixture of 50% spin-up

electrons, and 50% spin-down ones – therefore, the net

angular momentum it carries is exactly ZERO.

In contrast, a spin-polarized current does carry angular

momentum.

MOREOVER, the angular momentum carried by the

electrons can be transferred to other object.

Adding angular momentum to an object may haeve the

same effect as EXERTING TORQUE on the object!

The torque-transfer effect, discovered independently

by J. Slonczewski and L. Berger, can be used for

changing the state of a TMR “memory cell”.