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

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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:

dt

dBA

dt

diR 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