Magnetic Quantum-Mechanical Tunneling and its Applications in Ultrasensitive Magnetic Sensors
Magnetic tunnel junctions (MTJs) are a new class of thin film device which was first successfully fabricated in the mid-1990s. In its simplest form, the MTJ is a tri-layer “sandwich”consisting of two layers of magnetic material separated by an ultra-thin insulating film. If a voltage is applied to the top and bottom of this structure, classical physics does not allow a current to flow; however, if the insulating layer (also referred to as the “barrier layer”) is sufficiently thin, electrons can flow by quantum mechanical tunneling through the barrier layer.
The reason for the relative newness of this technology is that, for devices with a reasonable resistance values, the thickness of the insulatingbarrier layer must be extremely low (0.7 -1.6 nanometers, or 4-10 atomic monolayers). For tunneling between two magnetized materials, the tunneling current is maximum if the magnetization directions of the two electrodes are parallel and minimum then they are aligned anti-parallel. Therefore,the tunneling current, and therefore the resistance of the device, will change as external magnetic fields alter the magnetic orientation of these twoelectrodes.
Figure 1 shows a schematic of a recent MTJ layer structure. The two electrodes are fabricated from CoFeB, whilethe insulating barrier is composed of MgO, which, as discussed later,greatly enhances the properties of the device. The remaining layers in the structure are chosen to enhance the material and magnetic characteristics of the device. Typically, in order to achieve a linear, bipolar operation, one of the two magnetic electrodes (the “pinned layer”) in each sensor has its magnetization fixed by the exchange biasing phenomenon, while the remaining electrode (the “free layer”) is left free to respond to external fields. In this case, the pinned layer is fixed by the adjacent IrMn layer, which is termed the “pinning layer”.
The resulting structure has an electrical resistance which varies linearly as a function of the magnetic field strength over a substantial field range. Like the older and better-known anisotropic (AMR) and giant magnetoresistive (GMR) technologies, the magnetic tunnel junction is a magnetoresistive device. Such devices are often compared using magnetoresistance (MR) as the figure of merit. This quantity is defined as total change in resistance between the two saturated states, divided by the low (parallel-state) value. For the purposes of comparison, AMR sensors generally have a magnetoresistance of 2-3% and current GMR devices have MR values of 20-25%. By comparison, MTJ sensors feature magnetoresistance ratios of 100-200%.
These devices can be fabricated using conventional semiconductor methods, making them potentially very cheap in large quantities. In addition, because of their two-terminal nature, they are customizable and easy to use. Since they were discovered, there has been a great deal of effort towards introducing magnetic tunnel junction technology into two potential billion-dollar markets: as read/write heads for the disk drive industry, and as the cornerstone of a new non-volatile magnetic memory technology for the semiconductor industry.
AlOx-based MTJ sensors
From their realization in 1995 until two years ago, the vast majority of MTJ devices were grown using Al2O3 as the insulating barrier material. AlOx barriers are relatively simple to fabricate and allow for moderate magnetoresistance values of up to 35% with high yields. AlOx tunnel barriers are typically grown by depositing elemental aluminum and then using either a natural or plasma oxidation process to create the oxide.
MgO-based MTJ devices
In 2001, theoretical work predicted the possibility of extremely large magnetoresistance values in certain new tri-layer structures which used MgO as the barrier material. Due to a coherent tunneling effect, it was calculated that a properly prepared MTJ structure using an MgO barrier might exhibit up to 5000% magnetoresistance. In 2004, two groups independently reported record magnetoresistance ratios of over 200% in MgO-based MTJ devices. In the case of AlOx-based junctions, the only conditions required for the barrier were that it be sufficiently non-conductive. In contrast, MgO-based junctions rely on the quantum mechanical properties of the MgO layer for the realization of ultra-high magnetoresistance. Because of this, creating successful tunnel junctions using MgO as the barrier layer is significantly more complicated. One major reason for this is that the coherent tunneling processes required for ultra-high magnetoresistance can only occur if the MgO layer has a certain crystal orientation. Achieving this crystal orientation requires tight control of deposition and annealing parameters, and only a handful of groups worldwide have reported success in fabricating MgO-based tunnel junction devices.
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Professor Gang Xiao