Magnetoresitance and Giant Magnetoresistance

Despite the increasing prevalence of CD-ROMs and the use of electronic storage in RAM, most data is still stored magnetically.  This reading assignment introduces two different effects which have been recently utilized to read magnetic data.
 

To write magnetic data, current is sent through the coil in proportion to the desired signal.  This current produces a magnetic field proportional to the current.  The magnetic field aligns the spins in the ferromagnetic material.  As the material moves away from the coil, the magnetic field decreases, and the spins remain aligned until they enter another magnetic field (when they are erased).

Unlike electric storage, magnetic storage can be either analog or digital.  The amount of spin alignment depends on the strength of magnetic field, so analog data can be recorded with a continually varying current producing a continually varying magnetic field.  Digital data can be recorded by alternating the direction of the current.  To minimize data loss or errors, binary data is not determined solely by the direction of magnitization in a domain.  Instead, it is represented by the change in magnetic orientation between two domains.  If one bit of magnetic field has the same direction as the one before it, that represents a 0 (no change).  If one bit of magnetic field has the opposite direction as the one before it, that represents a 1 (change).  So a 1 is written by changing the direction of current between the two domains comprising a bit, and a 0 is written by keeping the direction the same.  Each bit starts with a change of orientation.  This convention for recording data identifies errors, since one would never have three domains of the same orientation in a row.  In addition, the orientation should change with every other domain.  If the computer thinks a bit is complete but the orientation does not change, it knows that some error has occurred.  Some examples of domains, bits, and strings are shown below.

To read magnetic data, the ferromagnetic material is moved past the coil of wire.  The changing magnetic field caused by the material's motion induces a current in the coil of wire proportional to the change in field.  If a 0 is represented, the magnetic field does not change between the two domains of a bit, so no current is induced as the magnetic material passes the coil.  For a 1, the magnetic field changes from one direction to the other; this change induces a current in the coil.

Inductive reading of magnetic data is subject to many limitations.  Since the change in magnetic field will be greater if the ferromagnetic material is moved faster, the induced current is dependent on the speed of the material.  Thus the sensitivity of inductive read heads is limited by the precision of the material speed.  The other limiting factor on inductive heads is the strength of the magnetic field.  As efforts to increase storage density continue, the size of a data element shrinks.  Since fewer electrons are now contained in the region of one bit, the associated magnetic field is smaller.  This smaller magnetic field produces less change and thus less induced current, requiring more loops to produce a measurable current.  As mentioned above, more loops means more resistance which means more heat.  Because of these limitations, new magnetic storage devices use the phenomenon of magnetoresistance to read magnetic data.
 
 

Magnetic Force Revisited

where the bold face on F, v, and B indicates that these quantities are vectors:  they have both direction and magnitude.  The x is a cross-product, which means that the magnitude (size) of the force is found from

and the direction of the force is found from a right-hand rule:  put your thumb in the direction of (qv), your second finger in the direction of B, and your middle finger (or palm) will point in the direction of the force.  q represents the angle between B and v.  Be careful when applying this rule to a negatively-charged particle like the electron:  qv points in the opposite direction of v, since q is negative.

You should understand the following consequences of this force:
1. A charge at rest experiences no magnetic force, since v is zero.
2. A charge moving parallel to the magnetic field experiences no magnetic force: since the angle q between B and v is zero, sinq is zero.
3. A charge moving un-parallel to the magnetic field experiences a magnetic force perpendicular to both the charge's motion and the magnetic field.
4. A negatively charged particle experiences a magnetic force opposite the direction of the magnetic force on a positive charge.
 

Let us apply this magnetic force to a charge moving through a wire, as shown to the right.  The teal conduction electrons are moving to the left (red arrow) through a wire.  Since the charge of the moving charges is negative, qv points opposite the direction of v, or to the right (brown arrow). A uniform magnetic field is present throughout the wire, pointing into the page (lavender x's).  Using the right-hand rule, with the thumb to the right representing qv, the second finger into the page representing B, we find that the palm representing F points toward the top of the page.  So the conduction electrons feel a force toward the top of the wire.  This magnetic force disrupts the normal flow of electrons, causing more collisions with atoms and other electrons, which increases the resistance of the wire.

The Effect of Magnetic Fields on Resistance

Applying a magnetic field can also increase the resistance of a material, since the magnetic force on the moving charges will tend to increase the number of collisions between charges.  This dependence of resistance on magnetic field is called magnetoresistance.  Magnetoresistance is proportional to the strength of the magnetic field, with a larger field producing a higher resistance.  This property is used in computers to read magnetic data.  A potential difference is applied to a wire that is placed close to the magnetic material on disk or tape.  As the magnetic fields representing data on the material pass by the wire, the resistance of the wire changes with the magnetic field of the data.  This change in resistance changes the current through the wire.  Monitoring this current provides a reading of the magnetic field on the tape or disk.

Magnetoresistance can provide more accuracy than induction.  In addition, magnetoresistance depends on the field, not the change of field, so its use is less dependent on precise speed of magnetic material.  Finally, the circuitry needed to measure magnetoresistance (1 loop of wire with a potential difference applied, connected to an ammeter) is much simpler than the circuitry needed for induction (multiple loops of wire, arranged to maximize induced current, hooked up to an ammeter).  The size of the effect is typically measured by dividing the change in resistance (or change in current) by the magnetic field of the storage medium.  Inductive heads can give about a 1% effect, while magnetoresistance heads give about a 4% effect.

IBM started using magnetoresistance in its read heads in 1992.  By 1994, all read heads produced by IBM were using magnetoresistance.  Coils of wire are still used to write magnetic data, since a change in resistance does not cause a magnetic field.  But only one coil is needed to write data, so the combination of inductive writing with magnetoresistive reading is still a simpler arrangement than the prior inductive read/write combination.
 

In your previous reading, you learned that electrons have intrinsic magnetic fields, described by the property of spin.  We can only measure magnetic field along one axis at a time (thank the uncertainty principle again).  If a material is not magnetized, half of the electrons will have spin with a positive component along the chosen axis, and half will have a negative component.  Those with a positive component are called "spin-up"; those with a negative component are called "spin-down."

If a ferromagnetic material is magnetized along the chosen axis in the positive direction, a spin-up electron will travel through the material more easily than a spin-down electron would.  We won't do the quantum mechanical calculation here, but hopefully the conclusion is believable:  electrons with spins in the same direction as the magnetic field experience fewer collisions than electrons with spins in the opposite direction of the magnetic field.  The origin of this effect involves band structures of ferromagnetic materials, which are more complicated than the band structures already studied in this course.

Although (hopefully) believable, this property of electron collisions is not particularly useful in a homogeneous piece of magnetized material.  Since conduction electrons move into the material from non-magnetized connecting wires, etc., the conduction electrons are not magnetized.  As in our figure below, approximately half of the electrons will be spin-up, and half will be spin-down.  So no matter which way the material is magnetized, half of the conduction electrons will experience more collisions than the other half, keeping the resistance of the material constant.    In the figure below, the solid spin-up electrons have a horizontal component of spin directed to the right, and the shaded spin-down electrons have a horizontal component of spin directed to the left.  The solid spin-up electrons will experience increased mobility, while the shaded spin-down electrons will experience decreased mobility.  Since the numbers of each are even, the effects cancel.

If, however, our material is layered, the resistance can change dramatically, leading to the phenomenon of giant magnetoresistance.  Layers of certain ferromagnetic materials separated by non-ferromagnetic material will naturally have magnetizations which alternate directions (see picture).  Such constructions usually consist of many layers and are called magnetic superlattices.

When the superlattice is placed in a magnetic field, however, the magnetization of all layers will align with the external field, creating the situation depicted below.  Now only conduction electrons with spins toward the left of the page will experience the higher scattering rate.  Thus the resistance of the material decreases in a magnetic field.

Giant magnetoresistance did not get its name by being a small effect; dividing the change in resistance (or current) by the applied magnetic field can give ratios of over 100%, which should be compared to the 4% effect of regular magnetoresistance and the 1% effect of inductance.  Just like ordinary magnetoresistance, giant magnetoresistance can be used to read magnetic data by monitoring the change in current as magnetic data passes by the superlattice.

Giant magnetoresistance shows great promise for the next generation of magnetic reading devices.  IBM produced the first GMR hard drive in 1997, and the use of the new technology is gradually increasing.  Current research efforts focus on producing sensitive lattices cheaply.  The biggest effects are seen at low temperatures, so efforts are also being made to obtain large giant magnetoresistance effects at room temperatures.  Some groups have seen giant magnetoresistance in lattices with only three layers: two ferromagnetic layers separated by one spacer.  These devices are called spin valves and are one of the primary candidates for incorporation into computers.

Now go to the IBM website on GMR and read about their use of giant magnetoresistance.
 
 

Facts About the Force
(From Driving Force:  The Natural Magic of Magnets, by James D. Livingston, (Havard University Press: Cambridge), 1996)

These 10 facts about the force from Driving Force by Livingston summarize most of the information contained in this, and the previous, readings.  Of particular interest to the workings of computers are steps 4, 6, and 8.  9 and 10 are also important concepts to remember.  This reading assignment has only touched on the applications of magnets in information systems and other commonly-used technologies.  If you are interested in learning more, the book by Livingston is an excellent place to start.
 

1.	If free to rotate, permanent magnets point approximately north-south.
2.	Like poles repel, unlike poles attract.
3.	Permanent magnets attract some things (like iron and steel) but not others (like wood or glass).
4.	Magnetic forces act at a distance, and they can act through nonmagnetic barriers (if not too thick).
5.	Things attracted to a permanent magnet become temporary magnets themselves.
6.	A coil of wire with an electric current flowing through it becomes a magnet.
7.	Putting iron inside a current-carrying coil greatly increases the strength of the electromagnet.
8.	Changing magnetic fields induce electric currents in copper and other conductors.
9.	A charged particle experiences no magnetic force when moving parallel to a magnetic field, but when it is moving perpendicular to the field it experiences a force perpendicular to both the field and the direction of motion.
10.	A current-carrying wire in a perpendicular magnetic field experiences a force in a direction perpendicular to both the wire and the field.

Memos About Magnetoresistance
 

11.	The force on a current-carrying wire in a perpendicular magnetic field is due to the force on the charge carriers moving through the wire.
12.	This force on the charge carriers produces an increased resistance as the charge carriers collide more with atoms in the wire.  This phenomenon is called magnetoresistance.
13.	As charge carriers (electrons) collide with atoms in a metal, the deflection of each charge (and thus the resistance in the metal) depends on the spin of the electron.
14.	The spin-dependent scattering (and thus the resistance) will increase if adjacent magnetic domains are aligned in opposite directions.  This occurs in the absence of an external magnetic field.
15.	The spin-dependent scattering (and thus the resistance) is smallest if the magnetic field through which the charges pass all points in the same direction, which is accomplished by the external magnetic field of a bit.
16.	This spin-dependent effect is much stronger than the effect of regular magnetoresistance, and so is called giant magnetoresistance.  It is, however, a completely different effect.

Driving Force:  The Natural Magic of Magnets, by James D. Livingston, (Havard University Press: Cambridge), 1996.  An extremely good book about magnetism and their applications in our everyday activities.  It's cheap, too (about $12+shipping from Amazon, VarsityBooks, BigWords, or barnesandnoble.com).

Magnetoresistance and giant magnetoresistance are topics on the cutting edge of technology.  They have not yet been incorporated in many introductory texts or layman's books.  The following web sites, however, contain a lot of useful information.

Look at the IBM web site www.research.ibm.com/research/gmr.html(required)

IBM has more information throughout their research site.  One good page is http://www.storage.ibm.com/hardsoft/diskdrdl/technolo/gmr/gmr.htm

Also see ssdweb01.storage.ibm.com/oem/tech/eraheads.htm (this one is a little more technical)