This is a minimalist webpage designed just to give some information regarding me, Matt Vannette, and my research. It is also a place where I will attempt to flesh out my understanding of my research. Therefore, I encourage anyone to comment on the clarity of my explanations. I know that not everyone who reads this is a physicist, but I hope that you feel comfortable telling me what you think is unclear.

I am currenty a graduate student in Condensed Matter Physics working with Ruslan Prozorov. The lab website can be found at http:// www.cmpgroup.ameslab.gov/supermaglab/.

The group is studying an array of physical properties and methods to measure them. My focus is on novel measurement techniques of various magnetic transitions. Particularly, I use a tunnel diode resonator (TDR) to examine the divergence of magnetic susceptibility in ferromagnets at the Curie temperature. Then I apply a weak (100's of oersteds) magnetic field to suppress that divergence. At least that's how it's supposed to work. Another interesting property we study is the difference between local moment and itinerant systems. For a brief discussion of the differences between local moment and itinerant systems go here. It appears the TDR is useful in measuring the different effects of the conduction electrons versus those localized around the lattice ions. The last aspect we are examining is the anisotropic responses of various magnets. This is interesting because we can probe in a very weak field, specifically 20 mOe. For comparison, the Earth's field is about 500 mOe. The usual way of determining which way magnets want to order is to measure the magnet's strength at some low temperature (5 K or about -440 F) in increasing background fields. The background field (called the applied field) is aligned with different directions of the sample. Then you see which direction saturates in magnetic strength first. The problem with this is that some samples never seem to saturate in any direction. Further, once you apply a field you are adding energy to the system. This is energy that can tilt the individual moments off their preferred directions. Therefore, you may not be measuring the true direction the sample wants to magnetize in.

The TDR is basically a self-driven inductor-capacitor circuit. Our main system resonates at about 14 MHz, which is in the radio-frequency band. A sample is placed in the inductor, which is a single layer, 60 turn coil of 42 AWG copper wire held up by a thin epoxy film. As the magnetic properties of the sample change with either temperature or magnetic field, the resonant frequency changes. The design of our circuit allows us to resolve changes of frequency on the order of 0.05 Hz. To help visualize that, imagine holding a cup with 1,000,000,000 (1 billion) grains of sand. The equivalent resolution of our system would allow us to know if as little as 5 grains where added or removed.

Our lab has a ³He system with a sample base temperature of ~400 mK. In Fahrenheit that corresponds to about -459 F. We also have a ⁴He system that has a sample base temperature of about 1.9 K (-457 F) and runs up to 300 K (80 F). The ⁴He system has an in house built magnet that can apply a dc bias field of 15 kOe. The field capabilities, while small by most standards, is more than sufficient for magnetic transitions. Further, tests on carbon doped MgB₂ show noticeable shifts in T_C due to the applied field. Another system we are working on is a high temperature system that will allow for measurements from room temperature up to 1000 K (1,340 F).

Unrelated to temperature scales, I'm building a capacitor version of the resonator, which will allow us to explore the electric response of materials with the same resolution. We hope this will open up a study of ferroelectrics, materials which develop a spontaneous electric polarization below a certain temperature that depends on the material.

On another unrelated note, I'm interested in other unique measurements. Our lab uses a magneto-optical setup to directly image magnetic domains in various materials. It's pretty cool, and there are some images and movies on the lab website. One technique that we don't have is resonant ultrasound spectroscopy (RUS) measurements. This is a technique that allows one to measure the elastic constants of materials. From what I can find, few groups do this. It seems to be a powerful tool for understanding why certain materials behave the way they do. I think when there is a beautiful technique that few groups do it generally comes down to a difficulty in either performing the measurement or analyzing the results. My eyes see a fantastic opportunity to do some good, unique science. I hope to gain a post-doc position with someone who can teach me RUS or some other, little done technique.

Here is a list of presentations and publications I have been directly involved with.