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Monday, January 31, 2011
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 applying an external magnetic field B. The other FM layer is exchangecoupled, 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 direction in the top layer,
and thus the sensor resistance 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 “spintronicslike 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 ultrathin 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 magnetization 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”.