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Magnetoresistance and Giant Magnetoresistance and Introduction to Atoms and Energies

Magnetoresistance and Giant Magnetoresistance and Introduction to Atoms and Energies

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Magnetoresistance and Giant Magnetoresistance and Introduction to Atoms and Energies. Magnetic Storage. The smallest region with uniform magnetism is called a “domain” Each bit requires two domains to allow for error identification - PowerPoint PPT Presentation

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Page 1: Magnetoresistance and Giant  Magnetoresistance  and  Introduction to Atoms and Energies

Magnetoresistance and Giant Magnetoresistance

and Introduction to Atoms and

Energies

Page 2: Magnetoresistance and Giant  Magnetoresistance  and  Introduction to Atoms and Energies

Magnetic Storage

• The smallest region with uniform magnetism is called a “domain”

• Each bit requires two domains to allow for error identification

• If two domains are magnetized in same direction, the bit is a 0

• If two domains are magnetized in opposite directions, the bit is a 1

• Direction of magnetization must change at the start of each new bit.

Page 3: Magnetoresistance and Giant  Magnetoresistance  and  Introduction to Atoms and Energies

Magnetic Storage: Writing• Magnetic fields have two sources:

– Currents (electromagnetism)– Alignment of intrinsic “spin” of particles

(ferromagnetism)• Magnetic data is written by running a current

through a loop of wire near the disk– resulting magnetic field aligns spins in region

of disk and produces magnetic domain– switching current produces magnetic domain

with magnetism in opposite direction

Page 4: Magnetoresistance and Giant  Magnetoresistance  and  Introduction to Atoms and Energies

• A changing magnetic field induces a current in a coil of wire proportional to the derivative (rate of change) of the field with respect to time.

• The emf, and current also depend upon the field, area A of the loop, and the number of turns in a coil.

• This is summarized in Faraday’s Law:

Magnetic Storage: Faraday’s Law

dtdiR B

dABB

Page 5: Magnetoresistance and Giant  Magnetoresistance  and  Introduction to Atoms and Energies

Magnetic Storage: Reading by Induced Currents

• As magnetic data passes by coil of wire, changing field induces currents– increase in field (more positive or less negative)

induces current in opposite direction of that induced by a decrease in field (more negative or less positive)

– Number of changes in a bit indicates whether bit is 0 or 1

Page 6: Magnetoresistance and Giant  Magnetoresistance  and  Introduction to Atoms and Energies

Magnetic Storage: Reading by Magnetoresistance

• Charges traveling through magnetic field experience magnetic force (provided velocity and field are not aligned):

FB = qv x B• Force is perpendicular to velocity (and to field), so charges

are pushed “off track”, resulting in more frequent collisions and thus an increased resistance

• Current through a loop of wire near magnetic data will vary as magnetic field does, giving a very sensitive indication of magnetic data

Page 7: Magnetoresistance and Giant  Magnetoresistance  and  Introduction to Atoms and Energies

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 “Lorentz” force on charges

moving in a magnetic field• GMR exploits spin-dependent scattering and

requires very carefully-crafted devices such as spin valves

Page 8: Magnetoresistance and Giant  Magnetoresistance  and  Introduction to Atoms and Energies

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

Page 9: Magnetoresistance and Giant  Magnetoresistance  and  Introduction to Atoms and Energies

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

Page 10: Magnetoresistance and Giant  Magnetoresistance  and  Introduction to Atoms and Energies

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

Page 11: Magnetoresistance and Giant  Magnetoresistance  and  Introduction to Atoms and Energies

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

Page 12: Magnetoresistance and Giant  Magnetoresistance  and  Introduction to Atoms and Energies

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!

Page 13: Magnetoresistance and Giant  Magnetoresistance  and  Introduction to Atoms and Energies

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 have been used to pack 16.8 gigabytes onto a PC hard drive in 1998

• Currently a density of 35.3 Gbits/in2 has been achieved

• IBM is working toward density of 100 Gbits/in2

Page 14: Magnetoresistance and Giant  Magnetoresistance  and  Introduction to Atoms and Energies

Stuff to remember 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

Page 15: Magnetoresistance and Giant  Magnetoresistance  and  Introduction to Atoms and Energies

On To Atoms

• Around the turn of the century, Bohr proposed that electrons in atoms can only occupy certain, quantized energy “states”

• When an electron moves from one allowed state to another, it needs to absorb or emit a particular amount of energy– Often that energy takes the form of light– Only specific energies (and therefore wavelengths) of light will be

emitted by a particular element– The collection of energies emitted or absorbed by an element is

called the atomic spectrum of that element

Page 16: Magnetoresistance and Giant  Magnetoresistance  and  Introduction to Atoms and Energies

Our Model of the Atom• If the atom is in the “ground state” of lowest energy, electrons fill the

states in the lowest available energy levels. The first shell has two possible states, and the second shell has eight possible states. Higher shells have more states, but we’ll represent them with the eight states in the first two sub-shells.

• Electrons in the outermost shell are called “valence” electrons. We’ll make them green to distinguish from e- in filled shells

E=0 (unbound)

n=1

n=2n=3n=4

Really eight closely spaced energies, since no two electrons can occupy same state

Page 17: Magnetoresistance and Giant  Magnetoresistance  and  Introduction to Atoms and Energies

The Hydrogen Atom• Has one electron, normally in the ground state n=1• This electron can absorb energy and go to a higher state, like n=3• The atom will eventually return to its ground state, and the electron

will emit the extra energy in the form of light.• This light will have energy E = (13.6 ev)(1/1 – 1/32) = 12.1 eV• The corresponding wavelength is = hc/E = 1020 Å

E=0 (unbound)

n=1

n=2n=3n=4

Page 18: Magnetoresistance and Giant  Magnetoresistance  and  Introduction to Atoms and Energies

Other Atoms• Electrons can absorb energy and move to a higher level

– White light (all colors combined) passing through a gas will come out missing certain wavelengths (absorption spectrum)

• Electrons can emit light and move to a lower level• Calculating the allowed energies extremely complicated for

anything with more than one electron• But can deduce allowed energies from light that is emitted

E=0 (unbound)

n=1

n=2n=3n=4

Really eight closely spaced energies, since no two electrons can occupy same state

Page 19: Magnetoresistance and Giant  Magnetoresistance  and  Introduction to Atoms and Energies

Atomic Bonding• Electrons in an unfilled valence shell are loosely bound• Atoms will form bonds to fill valence shells, either by

sharing valence electrons, borrowing them, or loaning them

• When atoms bond in solids, sharing electrons, each atom’s energy levels get slightly shifted

E=0 (unbound)

n=1

n=2n=3n=4

Page 20: Magnetoresistance and Giant  Magnetoresistance  and  Introduction to Atoms and Energies

Before the next class, . . .

• Finish Homework 14• Do Activity 13 Evaluation by Midnight

tonight• Read Chapters 2-3 in Turton• Do Reading Quiz

Page 21: Magnetoresistance and Giant  Magnetoresistance  and  Introduction to Atoms and Energies

Do Today’s Activity

Page 22: Magnetoresistance and Giant  Magnetoresistance  and  Introduction to Atoms and Energies

What Have We Learned About Atoms?

• ENERGY IS QUANTIZED• Electrons can absorb energy and move to a higher level;

they can emit light and move to a lower level• In hydrogen the emitted light will have energy

E = (13.6 ev)(1/nf2 – 1/ ni

2) • The wavelength is given by = hc/E = 1240(nm eV)/E• Energy levels of nearby atoms are slightly shifted from

each other, producing bands of allowed energies• Electrons move from the locality of one atom to the next

only if an energy state is available within the same band

Page 23: Magnetoresistance and Giant  Magnetoresistance  and  Introduction to Atoms and Energies

What Have We Learned About Spectra?

• ENERGY IS QUANTIZED• Different elements have different allowed energies (since

different numbers of protons and electrons provide different structure of attraction)

• Light emitted when electrons move from a high energy level to a lower energy level in an atom will have only certain, QUANTIZED, allowed energies and wavelengths.

• Those wavelengths depend solely on the element emitting the light and compose the characteristic emission spectrum for that element