Whenever
a current carrying wire is placed in a magnetic field, the electrical
resistance increases very slightly. This effect is known as
"magnetoresistance" (MR) and arises becauses the electrons tend to be
curved in their motion because they are charged particles moving in a
magnetic field. The vector cross-product rule indicates that they
tend to deflect to the side of the wire. The effect is common to all
current carrying materials but it is very small in magnitude. In the
mid-1980's, thin film technology was used to form multilayer Fe/Cr
structures. In 1988, Baibich et al. demonstrated resistance changes
in the electrical flow through these structures of several percent.
Since then, numerous systems involving multilayers of ferromagnetic
and non-magnetic metals have been fabricated. The large change in
resistance has been attributed to the different scattering
cross-sections of opposite spin electrons when passing through these
ferromagnetic arrays. This dramatic increase in magnetoresistance
spawned the name "giant magnetoresistance" (GMR) and these structures
have quickly been spun off into a new technology. The latest
generation of read-heads for computer disk drives are all using MR
technology to achieve smaller bit dimensions and therefore greater
packing density. The standard "starter" hard drive is now about 4 GB
and this is due to the development of these new heads.
In more recent years,a new family of materials, based on LaMnO3. have appeared. These materials also demonstrate magnetoresistance but now the change can approach 100%. This effect is known as Collosal Magnetoresistance (CMR). The mechanism has been linked to the notion of double-exchange of electrons between neighboring Mn3+ and Mn4+ ions with a strong on-site Hund's coupling; in essence, the neighboring spin states must be aligned for an electron to hop between ions. The mechanisms of MR, GMR, and CMR are each completely different and CMR is by far the least mature of the three topics. However, the revolution in data storage that has commenced with GMR read-heads promises to be repeated at the next level using these new materials.
We have just started a new collaboration with two research groups in the Ukraine. Prof. Anatoly Belous of the Institute of General and Inorganic Chemistry and Prof. Anatoly Pogorily of the Institute of Magnetism are heading up teams to develop and test these materials in an attempt to find the right material whose magnetoresistance transition is large enough and stable enough at room temperature to be amenable to detecting magnetic fields on hard disk drive platters. One challenge is to determine the source of this effect. We will be studying the surface of these materials with Scanning Tunneling Spectroscopy and Magnetic Force Microscopy in an attempt to answer that question. We have recently characterized our first sample and have found a 30% MR change in a field of 0.88 T at a temperature of about 175 K. This is shown in the graph here. Note how the sample's resistance is measured under field-free and H = 0.88 T conditions. The percentage increase because of the presence in a magnetic field is the MR coefficient. Though this material has a large MR effect, it is not sufficiently large at room temperature to be viable in a consumer product. Other dopants will need to be investigated. The accompanying graph shows the material resistance as a function of temperature at field strengths of both 0 T and 0.88 T. The percentage difference in resistance is shown as the MR coefficient.
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