magnetoresistance, giant magnetoresistance, and you
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Magnetoresistance, Giant Magnetoresistance, and You. The Future is Now. A Learning Summary. A circular aperture of diameter d Capacitors store charge, thereby storing electric field and maintaining a potential difference Capacitors can be used to store binary info - PowerPoint PPT PresentationTRANSCRIPT
Magnetoresistance, Giant Magnetoresistance, and You
The Future is Now
• A circular aperture of diameter 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)
A Learning Summary
minimum)(1st 22.1sind
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:
dtdBA
dtdiR B
BAdABB
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
Magnetic field pointing into page (screen)
Current-Carrying Wire
Conduction electrons
Direction of velocity v of electrons
Direction of qv of (negative) 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
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 mid-1990s
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 “spin-
down”, based on the component of magnetic field along a chosen axis
Chosen axis (z) Electrons with intrinsic magnetic field indicated
Up DownUp UpDownDown Up
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
Warning: Figure not to Scale
Externally applied magnetic field
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 current-carrying 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