Magnetoresistance, Giant
Magnetoresistance, and You
The Future is Now
A Learning Summary
• A circular aperture of diameter d

sin   1.22 (1st minimum)
d
• Capacitors store charge, thereby storing electric field
and maintaining a potential difference
• Capacitors can be used to store binary info
• Capacitance is found in many different aspects of
integrated circuits: memory (where it’s desirable),
interconnects (where it slows stuff down), and
transistors (ditto)
Review of Magnetic Storage
• Each bit requires two domains to allow for error
identification
• If two domains are magnetized in same direction,
the bit is a 0; opposite directions makes the bit a 1
• Direction of magnetization must change at the
start of each new bit.
• Magnetic data is written by running a current
through a loop of wire near the disk
Magnetic Storage: Reading by
Induced Currents
• As magnetic data passes by coil of wire,
changing field induces currents
• Effect described by Faraday’s Law:

d B
dB
 iR  
 A
dt
dt
 B   B  dA  BA
Magnetic Forces
 Charges moving through a magnetic field
experience a force (Fact #10)
 This force is perpendicular to both the magnetic
field and the direction of motion
 If the charge is at rest, it experiences no magnetic
force
 If the charge moves parallel (or antiparallel) to
magnetic field, it experiences no magnetic force
Magnetic Forces
 Mathematically,
FB = qv x B
|FB| = |qv| |B| sin 
(  is angle between v and B)
direction given by right-hand rule
Magnetoresistance
 Electrons moving through a current-carrying wire
are moving charges
 If a magnetic field is present in the wire (not in the
direction of current flow), the conduction electrons
will experience a magnetic force perpendicular to
direction of current
 This force pushes electrons off track, increasing
resistance
Conduction
electrons
Magnetic field pointing
into page (screen)
Current-Carrying Wire
Direction of velocity v
of electrons
Direction of qv of
(negative) electrons
Direction of force on
conduction electrons
Magnetic field pointing
into page (screen)
Current-Carrying Wire
Direction of velocity v
of electrons
Direction of qv of
(negative) electrons
So where’s the application?
 The presence of a magnetic field increases the
resistance of a wire
 If a potential difference is applied to the wire,
current will flow inversely proportional to
resistance (i=V/R)
 A change in magnetic field produces a change in
current which can be measured
 This yields a sensitive indicator of change in
magnetic field
Comparison
 Magnetoresistance is a much larger effect than
induction
 Magnetoresistance detects magnetic field, not just
the change in magnetic field, so it is less sensitive
to changes in tape/disk speed and other variables
 Equipment needed to detect magnetoresistance
simpler than coils for inductance
 Magnetoresistance replaced induction in mid1990s
Magnetic Storage: Reading by
Giant Magnetoresistance
• Giant Magnetoresistance (GMR) is a completely
different effect from Magnetoresistance (MR)
– Both utilize magnetic data’s effect on resistance, but
that’s the only similarity
• MR is the regular “Lorenz” force on charges
moving in a magnetic field
• GMR exploits spin-dependent scattering and
requires very carefully-crafted devices such as
spin valves
Spins and ferromagnetism
 Ferromagnetism due to spins of electrons
 Can classify electrons as “spin-up” or “spindown”, based on the component of magnetic field
along a chosen axis
Chosen axis (z)
Electrons with intrinsic
magnetic field indicated
Up Down Up Down Up
Up Down
Spins and Scattering
 An electron moving into a magnetized region will
exhibit spin-dependent scattering
 Electrons with spins in the direction of the
magnetic field will scatter less than electrons with
spins opposite the direction of the magnetic field
Magnetization
Magnetic Superlattices
 Alternate layers of ferromagnetic material will naturally
align with opposite magnetization
 All electrons coming in will scatter since they’ll have
opposite spin from magnetization in some region
Ferromagnetic material
with magnetization in
direction of turquoise arrow
Non-ferromagnetic
material spacer
Warning: Figure
not to Scale
Magnetic Superlattice in Field
 If an external field is present, ferromagnetic layers will all
align with external field
 Only half of the electrons coming in will scatter maximally,
those with spin opposite external field
Externally applied
magnetic field
Warning: Figure
not to Scale
Giant magnetoresistance
 When magnetic field is present in magnetic
superlattice, scattering of electrons is cut
dramatically, greatly decreasing resistance
 Superlattices are hard to mass-produce, but the
effect has been seen in three-layer devices called
“spin valves”
 The origin of giant magnetoresistance is very
different from that of regular magnetoresistance!
The Future is Now
 Magnetoresistance read heads have been produced at IBM
since 1992
 Magnetoresistance read heads have been exclusively used
at IBM since 1994
 Giant magnetoresistance spin valves were used to pack
16.8 gigabytes onto a PC hard drive in 1998
 As of 2002, a density of 35.3 Gbits/in2 has been achieved
 As of 2002, IBM was working toward density of 100
Gbits/in2
What have we learned?
 A charge moving through a magnetic field
experiences a force perpendicular to the field and
the direction of motion of the charge
 The magnetic force is proportional to the charge,
the magnitude of the field, the velocity of the
charge, and the sine of the angle between v and B
 The effects of this force on charges in a currentcarrying wire lead to effect of magnetoresistance
What have we learned about GMR?
• Electrons (and other elementary “particles”) have
intrinsic magnetic fields, identified by spin
• The scattering of electrons in a ferromagnetic
material depends on the spin of the electrons
• Layers of ferromagnetic material with alternating
directions of magnetization exhibit maximum
resistance
• In presence of magnetic field, all layers align and
resistance is minimized