Magnetoresistance, Giant Magnetoresistance, and You

1. Magnetoresistance, Giant Magnetoresistance, and You The Future is Now

2. 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 • Capacitance is found in many different aspects of integrated circuits: memory (where it’s desirable), interconnects (where it slows stuff down), and transistors (ditto)

3. 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

4. Magnetic Storage: Reading by Induced Currents • As magnetic data passes by coil of wire, changing field induces currents • Effect described by Faraday’s Law:

5. 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

6. Magnetic Forces • Mathematically, FB = qv x B |FB| = |qv| |B| sin q ( q is angle between v and B) direction given by right-hand rule

7. 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

8. Conduction electrons Magnetic field pointing into page (screen) Direction of velocity v of electrons Direction of qv of (negative) electrons Current-Carrying Wire

9. Direction of force on conduction electrons Magnetic field pointing into page (screen) Direction of velocity v of electrons Direction of qv of (negative) electrons Current-Carrying Wire

10. 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

11. 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

12. 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

13. Chosen axis (z) Electrons with intrinsic magnetic field indicated 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 Up Up Up Up Down Down Down

14. 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

15. 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 Non-ferromagnetic material spacer Ferromagnetic material with magnetization in direction of turquoise arrow Warning: Figure not to Scale

16. 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

17. Giantmagnetoresistance • 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!

18. 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

19. 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

20. 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