Spintronic Materials and Devices

The integration of magnetic materials and impurities into nanoelectronic devices enables the use of the electron spin, as well as its charge, for carrying information.  This new paradigm in information processing devices has been called “spintronics” in analogy to electronics.  Achieving functional spintronic devices involves development of new materials and integration of diverse materials with atomic-level control.

Magnetic tunnel junctions (MTJs) are prototypical spintronic devices . They consist of three layers: a ferromagnetic metal, an insulator, and another ferromagnetic metal.  The insulator is only a few nanometers thick, which is thin enough to allow tunneling of electrons from one metallic electrode to the other.  When the magnetizations of the ferromagnetic layers are aligned, the tunneling current is large and the device resistance is low.  When the magnetizations of the ferromagnetic layers are anti-aligned, the tunneling current is small and the device resistance is large.  If the magnetization of one electrode is fixed, for example by exchange coupling to a neighboring antiferromagnet, and the other layer can switch depending on an applied magnetic field, the MTJ exhibit magnetoresistance, in which the resistance state of the device depends on the sign of the applied field.  MTJs are already used as sensors in the read heads of magnetic hard disk drives.

(Clockwise from top left): Fe3O4 / Fe interface, Fe3-xTixO4 at the Fe / TiN interface, EELS of the Fe3-xTixO4 layer, EELS of the Fe3O4 layer

We have studied integration of two types of new materials into MTJ devices.  The first type is materials with negative spin polarization, so that the spin of the conduction electrons are anti-aligned to the magnetization of the material.  A MTJ with one negative spin polarization electrode and one positive spin polarization electrode exhibits negative magnetoresistance, which could be used with a normal MTJ to create a three state device or a combined high gain sensors.  Fe3O4 (magnetite) and Fe4N are two materials with negative spin polarization.  We have studied thin films of both materials deposited onto Si with a TiN buffer film on Si.  The Fe3O4 films were deposited by a novel selective oxidation procedure, which resulted in exceptional smooth, low defect density films.  At higher oxidation temperature, O diffused through the residual Fe to the interface with the TiN and created an undesireable paramagnetic Fe3-xTixO4 reaction layer.

A MTJ consisting of CoFe / MgO / Co2MnSi layers. The termination of the MgO/Co2MnSi interface with Co, Mn, or Si has a large influence on the properties of the device.

The second material type are Huesler alloys, which are intermetallic phases in the L21 structure.  These materials are half metals: only one spin band crosses the Fermi level, so the current is in principle 100% spin polarized, which has the potential to enable devices with extremely high magnetoresistance ratios.  The particular Huesler alloy we have studied is Co2MnSi, in MTJs with a MgO tunnel barrier.  The device performance of a Co2MnSi depends on the interface termination with the MgO.  Only the MnMn/O termination preserves 100% spin polarization in the tunneling current.  A CoCo/O interface has 50% spin polarized tunneling, and the MnSi/O loses the spin polarization of the tunneling current completely.  Based on Z-contrast STEM images, we have demonstrated devices with MnMn/O termination for the first time.  Learning to control that interface through processing will be critical to future devices.

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