Highly Charged Ion Modified Magnetic Tunnel Junctions
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0960-N08-02
Highly Charged Ion Modified Magnetic Tunnel Junctions H. Grube, J. M. Pomeroy, A. C. Perrella, and J. D. Gillaspy National Institute of Standards and Technology, 100 Bureau Drive, Mail Stop 8423, Gaithersburg, MD, 20899
ABSTRACT We have used highly charged ions (HCIs) such as Xe44+ to modify ultrathin aluminum oxide barriers in magnetic tunnel junctions (MTJs) in order to controllably adjust their electrical properties independently of oxide thickness. We have reduced the resistance area (RA) product of our MTJ devices by up to three orders of magnitude down to our present measurement uncertainty limit of 30 Ω·µm2 by varying the HCI dose. Preliminary experiments indicate that HCI modified Co/Al2O3/Co MTJs have a reduced magnetoresistance (MR) of ≈ 1% at room temperature as compared to ≈ 10% for undosed devices. The goal of this effort is to fabricate a magnetic field sensor in current-perpendicular-to-plane (CPP) geometry with an RA optimized for hard drive read heads. This is an improvement over presently demonstrated CPP architectures based on giant magnetoresistance or tunnel junctions, whose RAs are either too low or too high.
INTRODUCTION As ever higher data densities in magnetic recording media are achieved, the conventional current-in-plane (CIP) spin valve read head can no longer be shrunk at the necessary pace. One obstacle to continued size reduction is the need for electrical isolation layers between the spin valve sensor element and the surrounding magnetic shields that are needed to maximize spatial sensitivity of the head. In read head architectures based on current perpendicular-to-plane (CPP) geometry, isolation is not necessary allowing a closer shield spacing and therefore smaller recording bit size [1]. Due to their all metal construction, CPP spin valve sensors inherently have very low impedances, which severely limits obtainable signal amplitudes [2]. The magnetoresistance (MR) ∆R/Rmin of typical spin valve sensors is only a few percent, but antiferromagnetically coupled multilayer structures have accomplished 65% MR at room temperature [3]. Large sensor signals help maintain signal-to-noise ratio as magnetic bits shrink and shield gaps narrow; both are factors reducing the magnetic signal strength available for sensing, since the head fly height cannot be scaled down at the same rate. Magnetic tunnel junctions (MTJs) are CPP devices offering large MR in the tens to hundreds of percent, however, generally with high impedances [4]. Combined with the capacitance across the tunnel barrier, the high RC time constant limits available bandwidth, precluding high-speed data read out. The RA product of amorphous aluminum oxide and crystalline magnesium oxide tunnel barriers have recently been markedly reduced through process advances, although consistent fabrication of reliable ultrathin barriers is a challenge and the resulting devices are electrically fragile [1,5].
This work explores an entirely different path toward magnetoresistive CPP sensors with intermediate RA product. We fabric
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