Daniel Richardson and Dr. Richard Vanfleet, Physics and Astronomy
Iron platinum is an exciting material being studied for its magnetic properties for possible use as the material in magnetic memory storage (e.g. hard drive). Why use iron platinum? In today’s technological world of ever decreasing dimensions, the hunt is on for mechanisms and materials that will allow electronic devices to become smaller and smaller. Using iron platinum, the physical space needed to store one bit of information is much smaller than space needed if using the current material. Imagine being able to store 100 bits of information per square centimeter as compared to only 10 bits.
However, for iron platinum to be used in this application, it must exhibit sufficient magnetic properties. There is an essential connection between how the atoms are arranged in the iron platinum and its magnetic properties. When there are alternating layers of iron and platinum in a tetragonal crystal structure (see figure 1a.), the material is a good magnet. The material is said to be ordered if the atoms are in this structure and it is called the L10 phase. A disordered material has the iron and platinum atoms randomly mixed together in a face centered cubic array. (See figure 1b). The purpose of this research was to develop a method for detecting the degree of order for thin films of iron platinum using electron diffraction.
To the novice eye, a diffraction pattern looks like a picture of a starry night. (See figure 2). However, the spacing and intensities of the different spots in a diffraction pattern reveal information about planes of atoms in a material. While the diffraction patterns of the ordered and disordered iron platinum share many of the same spots, there are some spots that only appear if the material is ordered. By comparing the intensities of the spots that only show up when the material is ordered, to the spots that exist with or without order, the degree of order can be estimated. These results will be compared to estimates obtained by X-ray diffraction.
Thin films of iron platinum were acquired from Dr. Kevin Coffey’s research team at the University of Central Florida. Equal amounts of iron and platinum were sputtered onto magnesium oxide substrates and then annealed to produce the desired L10 phase. These same samples were analyzed by x-ray diffraction. Samples for the transmission electron microscope (TEM) were prepared by tripod polishing. A small piece of the material is carefully ground down to a wedge shape, and the thin edge of the wedge becomes electron transparent. Once the sample is thin enough, it can be put in the microscope and analyzed. Producing one sample may require many tedious hours. The thin edge of the sample is fragile, brittle, and so small that the slightest jolt can shatter it. I prepared all the samples used in this project.
The TEM is a powerful and unique tool. It uses electrons instead of visible light to create images of a sample. However, the TEM is as complicated as it is powerful. Aligning the lenses and apertures, finding an instructive image, and taking a good picture are challenging tasks. Shown below are diffraction patterns that I took from one of the iron platinum samples.
This equation comes from x-ray diffraction and compares the intensity of the 001 spot (I001), which only appears if the sample is ordered, to the 002 spot, which is always present. F is the structure factor, P is the polarization factor, L is the Lorentz factor and M is the Debye-Waller factor. These factors depend on the wavelength of the electron, the scattering angles, the material and composition. Using this equation, it was found that this sample has a degree of ordering equal to 0.87. A value of one would mean that the sample is perfectly ordered, and a value of zero would mean that the sample is completely disordered. This value is significantly higher than the results obtained from x-ray diffraction for this same sample (0.60). This discrepancy is due to complex electron scattering which does not happen in x-ray scattering. The equation does not account for this extra complexity. While this method can show the existence of the L10 phase, it does not give a suitable estimate of the order parameter. In the future, image simulations done by computer will be compared to the experimental results.