Magnetic Sensors and Devices

Jack Bass, Norman Birge, Martin Crimp and William Pratt
Giant magnetoresistance in ferromagnetic/non-magnetic multilayers with current flow parallel to the layers is now being used in read heads, position sensors and magnetic memories. For higher-density magnetic applications, the current flow will need to be perpendicular to the layer planes (CPP). We initiated this field of CPP studies, for which the theory is particularly simple and allows straightforward determinations of important spin-polarized transport parameters and reliable predictions of behaviors for novel multilayer structures.

We are now probing the behavior of magnetic multilayers of 'nanometer' scale where the size of the ferromagnetic layers and the CPP current flow region is restricted. For the latter, we have confirmed theoretical predictions that a large enough CPP current density could induce magnetic excitations. The crucial experiment involved a nanometer-size point contact to a magnetic multilayer inside a microwave cavity. With the microwaves off, high current-density injection generated quantized spin-waves ('magnons') as shown in the figure. In a microwave field, the point contact acts as a non-linear mixer, producing additional magnon excitations. The magnons are partially phase coherent and the effect may possibly be used for spin-wave amplification by stimulated emission of radiation. In collaboration with Albert Fert (Orsay), and scientists at Seagate Technology, we are also studying current-driven magnetization reversal in multilayers using electron-beam lithographically produced nanowires and prototype CPP-MR sensors. Reversing layer magnetizations via current holds promise for superior control of magnetic memory states.

References:
Current-Perpendicular (CPP) Magnetoresistance in Magnetic Metallic Multilayers, J. Bass and W.P. Pratt, Jr., J. Magn. Magn. Mat. 200, 274 (1999).

Measurement of Resistance and Spin-Memory Loss (Spin Relaxation) at Interfaces Using Sputtered Current Perpendicular-to-Plane Exchange-Biased Spin Valves, W. Park et al., Phys. Rev. B 62, 1178 (2000).

Magnetoresistance of a Planar Spin Valve with Single-Domain Ferromagnetic Probes, J.A. Caballero et al., IEEE Trans. Mag. 37, 2111 (2001).