Metallic Magnetic Surfaces

Thin cobalt (Co) films are currently of interest because they exhibit interesting properties, such as a phase transformation from hcp to fcc, uniaxial magnetic anisotropy, magnetization reversal, etc.

This project studies how thermal annealing affects the surface morphology and crystallinity of Co films in the thickness range of 3 to 30 nm, using ex-situ atomic force microscopy (AFM) and x-ray diffraction (XRD) at room temperature. Samples were grown on (11-20) oriented sapphire substrates in a DC magnetron sputtering system at 315 ^oC, annealed in vacuum right after that at different temperatures T_A (400 ^oC to 700 ^oC ), followed by quenching by liquid nitrogen in order to preserve the metastable high temperature surface structure. AFM reveals that the surface becomes rough when T_A is less than 500 ^oC. When T_A is approximately 550 ^oC, the surface becomes very smooth, becoming rough once again when T_A is even higher. Correspondingly, XRD shows that while there is a hcp to fcc transition when T_A is less than 500 ^oC, samples reoriented themselves from fcc(111) to fcc(100) when T_A is around 550 ^oC, followed by a transition from fcc(100) back to fcc(111) when T_A is even higher. Reflection high energy electron diffraction (RHEED) is also being employed to demonstrate such a transition.

The mechanism causing this reentrant surface roughening transition is being investigated. Possible applications are envisioned in the fields of nanolithography and magnetic storage.

AFM image of 6.3 nm Co film annealed at 550 ^oC

GMR Superlattices with Magnetic Anisotropy

So why study material systems that exhibit the GMR effect? GMR systems can be used as magnetic sensors, like the ones in your hard disk drive. Companies like Seagate and IBM are starting to use this technology to increase the capacity of current hard drives.

What is the GMR effect? GMR is seen when you measure the resistance of antiferromagnetically coupled multilayers as a function of applied magnetic field. A typical features of a GMR measurement is the resistance at low fields is greater (sometimes as much as 50%) than that at high fields. The current accepted explanation for this phenomenon deals with how spin up and spin down electrons scatter differently from magnetic field. Simply put at low fields, where the magnetization in adjacent magnetic layers are aligned antiparallel, both the spin up and down electrons are scattered at the interface. While at high fields the magnetization of adjacent magnetic layers are aligned parallel. Now one of the electron species is scattered less than before effectively
decreasing the resistance.

What we do. We study multilayers composed of cobalt and rhenium bilayers grown by dc magnetron sputtering. We then learn about their structural characteristics like interface roughness and layer thicknesses through x-ray diffraction. Magnetic properties are discovered by SQUID magnetometry, MOKE measurements, and soon a Vibrating Sample Magnetometer.

To get an even better idea of the magnetic characteristics of our samples we collaborate with Argonne National Lab and use their neutron diffraction experiment. The neutron experiments are ideal for us since they measure the antiferromagnetic moment directly as a function of magnetic field. We also collaborate with Michael Pechan's group at Miami University in Oxford, Ohio. Through ferromagnetic resonance experiments they extract information about the strength and direction of the anisotropy axis as well as the effective magnetization.

In our samples the magnetoresistance (MR) have some odd structure. From all of these experiments we can now simulate the MR curves. It turns out that the MR curves are a superposition of GMR and an anisotropic magnetoresistance (AMR). This AMR can actually boost the MR when the current is applied perpendicular to the anisotropy axis and perpendicular to the applied field. It maybe possible to exploit this in GMR based magnetic sensors like those in hard drives.

MR data and simulation

Antiferromagnetic Thin Films and Superlattices

Antiferromagnetic films and superlattices can provide a wealth of information regarding basic magnetic interactions in reduced dimensions. We are particularly interested in studying the interactions between ferromagnets and antiferromagnets resulting in a unidirectional exchange anisotropy. One of the areas of interest is the effect of dilute antiferromagnets (e.g. Fe_xZn_1-xF₂) on the exchange anisotropy, and the effect of the ferromagnet in raising the dilute system's critical (Neel) temperature beyond what would be expected in the bulk material.

We are also interested in the collective magnetic excitations in antiferromagnetic superlattices. We are able to grow such epitaxial superlattices (FeF₂/CoF₂) via molecular beam epitaxy (MBE). Theoretically, one expects the effective Brillouin zone to be reduced in the direction perpendicular to the surface due to the artificial superlattice periodicity, causing new standing wave modes to occur at k = 0. We are currently attempting to study this using optical spectroscopy techniques.