Transcript Magnetoresistance and Giant Magnetoresistance and Introduction to Atoms and

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.
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
Magnetic Storage: Faraday’s Law
• 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:
d B
  iR  
dt
 B   B  dA
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
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
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
Spins and ferromagnetism
• Ferromagnetism due to spins of electrons
• Can classify electrons as “spin-up” or “spindown”, 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
Up Down
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
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 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
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
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
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=4
n=3
n=2
n=1
Really eight closely spaced
energies, since no two electrons
can occupy same state
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 l = hc/E = 1020 Å
E=0 (unbound)
n=4
n=3
n=2
n=1
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
n=4
n=3
n=2
n=1
E=0 (unbound)
Really eight closely spaced
energies, since no two electrons
can occupy same state
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=4
n=3
n=2
n=1
Before the next class, . . .
• Finish Homework 14
• Do Activity 13 Evaluation by Midnight
tonight
• Read Chapters 2-3 in Turton
• Do Reading Quiz
Do Today’s Activity
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/ ni2)
• The wavelength is given by l = 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
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