A Racetrack Memory Based on Exchange Bias

This thesis describes preliminary studies for a new type of computer memory, racetrack memory. Racetrack memory was initially proposed by scientists at IBM. Data in racetrack memory is stored in domains within ferromagnetic nanowires which are separated by domain walls. The data is moved in the wires by moving the domain walls. Control over the movement of domain walls was initially attempted via use of notches cut into the wires, but these were not only expensive and difficult to fabricate but also proved to be unreliable. The method for pinning domain walls described in this thesis uses antiferromagnetic wires grown perpendicular to ferromagnetic wires so that exchange bias is induced at the crossing points. Exchange bias occurs when an antiferromagnet is in contact with a ferromagnet. When the structure is cooled in an applied field from near the Néel temperature of the antiferromagnet, the hysteresis loop shifts along the field axis resulting in pinning of the ferromagnetic layer. Multiple ferromagnetic materials were considered for the ferromagnetic layer. Initially unpinned ferromagnetic films were grown and characterised. Exchange biased films were then grown in configurations where the antiferromagnetic layer was either under or above the ferromagnetic layer but showed no major differences in the exchange bias. Ferromagnetic wires were patterned on Si substrates using e-beam and photolithography. Coercivity of the wires was measured along the length of the wires. Exchange biased wires in both top and bottom pin configurations were fabricated afterwards using the same methods and characterised using the same technique as the unbiased wires. The comparison between the biased and unbiased wires showed that domain walls can be pinned in nanowires using exchange bias. The top bias configuration showed a maximum value for pinning of about 55 Oe which is comparable to that initially reported in notched systems.