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

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:

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, F B = qv x B |F B | = |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 Down Up Down 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/in 2 has been achieved  As of 2002, IBM was working toward density of 100 Gbits/in 2

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