This material is based upon work supported by the National Science Foundation under Grant No. 0355171. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author-(s) and do not necessarily reflect the views of the National Science Foundation.
Major research and education activities
The major research and education activities pursued under this award were geared towards a collaborative study between Europe and USA for the study of magnetic binary alloys, particularly FePd and FePt and to the fabrication of thin films and nano-structures using these materials. Highly ordered L1o thin films of these alloys exhibit very large magnetic anisotropy, and can be deposited with the anisotropy axis perpendicular to the film plane, making them suitable for perpendicular media. In this case, the system has fcc structure where the Fe and Pt (Pd, or Ir, similar to Pt) atoms occupy alternate plane along (001) directions. This leads to the formation of a super-lattice with 2 atomic layers period and strong tetragonal distortion along the (001) direction. The chemical order alters the electronic structure of the system inducing strong magnetic anisotropy as well a magneto-optical activity.
The UHV sputtering/laser-ablation deposition system used in this research. The
chamber houses 6 sputtering guns.
Detail of the sputtering/laser-ablation system showing various sputtering guns (total of six) as well as the sample motion arm.
A series of FePd thin film samples were grown to establish the optimum conditions for long-range L1o order as well as perpendicular magnetic anisotropy. The structural properties of the samples were characterized ex-situ with XRD. The surface morphology was studied with ex-situ atomic force microscopy (AFM), and the images were correlated with those obtained with magnetic force microscopy (MFM). Additional magnetic characterization was performed using the magneto-optical Kerr effect (MOKE).
A series of samples with varying thickness (15-300 Angstroms) were grown afterwards at the optimal temperature for long range order growth (450oC). The samples were characterized using ex-situ XRD, AFM/MFM and polar MOKE. These samples were brought to my laboratory where longitudinal MOKE measurements where performed. Additional structural characterization within my group using cross-sectional TEM and XRD was carried out. The subsequent series of samples grown at IMM consisted of FePt epitaxial films deposited on (001) MgO buffer layer. Again the substrate temperature and deposition conditions were varied to optimized long range L1o order. The samples were characterized ex-situ with XRD and polar MOKE. These samples were also brought to my laboratory where longitudinal MOKE measurements as well as additional structural characterization using cross sectional TEM were carried out. Additional XRD studies on post-growth annealing of partially ordered FePt epitaxial films have also been carried out within my research group.
Follow this link to read about our Major Findings (2004-2008).
A series of Pt epitaxial thin films were deposited on (001) MgO substrates to exploit Fe ion-implantation using the heavy ions accelerator at the University of Toledo. The films were characterized with ex-situ XRD before and after implantation and MOKE after implantation.
Simulations to estimate the optimum “dose” to achieve shallow ion penetration with very high concentration of dopants have been carried out using SRIM. The goal is to constrain the range of penetration to the topmost 50 Angstroms with high concentration of Fe ions aiming at nano-cluster formation near the surface. The first trial sample has already been fabricated. This sample consisted of Fe implanted at low energy (30 KeV) on Silicon to establish the feasibility of the experiment. The magnetic properties have been characterized using longitudinal MOKE. A series of Pt thin film samples are currently being ion-implanted with Fe to continue these studies. Ultra-fast MOKE studies are also in progress at the College of William and Mary, VA.
A thin film deposition system in my laboratory was upgraded to include two sputtering guns so that we could carry out the growth of the alloy thin films at the University of Toledo. A new system with 5 guns, in-situ RHEED and RGA was recently installed in my new laboratory at the College of William and Mary to continue some of these studies. Two graduate students as well as two undergraduate students participated in this effort.
A proposal for beam-time at the advanced photon source was approved (Beam time was allocated for Runs 1 and 2 in 2005) to perform real time X-ray rapid thermal annealing (XRTA) studies of disordered/ordered phase changes in stoichometric epitaxial but chemically disordered FePt and FePd thin films, as well as Fe ion-implanted Pt films. This is an innovative application of undulator radiation to simultaneously perform RTA and to probe structural changes that occur during annealing on FePt and FePd films.This is made possible by the high power-density of undulator beams (~100 W/mm2), combined with their excellent properties as a probe of crystal structure (especially collimation and high-brightness).XRTA is similar to laser annealing, but there are important differences and unique advantages: first, the penetrating power of x-rays ensures a more uniform and controllable delivery of thermal energy; second, the x-ray energy can be tuned to enhance the coupling into the absorption edge of a particular species, thereby permitting selective annealing of buried layers and nanostructures.In the proposed studies we aim to address these specific questions:
- does the segregation into the ordered phase occur by means of island nucleation and growth?
- s the layering initiated at the substrate (MgO)- film interface and how does it propagate through the film?
- what role does epitaxy play in the layering mechanism?
- how does the formation of layering order scale with time and temperature?
- what is the correlation between the emergence of perpendicular magnetization and the nanostructure of the annealed samples?
- is the evolution of long range order different in-plane and out-of-plane, and if so, can this be used to manipulate the magnetic anisotropy associated with the self-assembled layers?
Additional studies regarding the modification of the magnetic anisotropy with different cap layers were carried out using XMCD and resonant XRR at the Advanced Photon Source, Argonne National Laboratory
We are also interested in the study of induced ferromagnetic properties. To this end V/Fe/V tri-layered samples have been deposited on (001) MgO to perform exploratory search for induced ferromagnetic phases. Beam-time was allocated at the Lujan neutron facility, Los Alamos National Laboratory and at the National Synchrotron Light Source at Brookhaven National Laboratory to carry out complementary studies on the magnetic profile of V-Fe-V trilayered samples.