University of Minnesota
School of Physics & Astronomy

Spotlight

Building the right sandwich: novel magnetic tunnel junctions

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Barry Costanzi is a graduate student in Professor Dan Dahlberg’s Magnetic Microscopy and Transport group. Costanzi’s research involves a novel method of creating magnetic tunnel junctions by using Electron Beam Induced Deposition (EBID). His work is in collaboration with the Royal Institute of Technology in Stockholm, and requires him to travel to Sweden every couple of months.

Costanzi explains that within a magnetic tunnel junction you have a thin sandwich of a ferromagnet, an insulator and another ferromagnet. Classically, the insulator should not allow current to flow through it, but if the insulator is thin enough, electrons can quantum mechanically tunnel from one ferromagnet to the other. You can keep adding to the sandwich to get some interesting physics. It is relatively straight forward to get the electrical contact to the top and bottom, bread of the sandwich, but getting electrical contact to the middle piece of bread is hard. "The folks in Sweden are using a deposition process the size of a 3D printer to make really big bread, if you will." They use long magnets, in order to get access to the center. The advantage to having this electrical contact in the center, is that you can play around with energy regimes and higher resolution on the tunneling process. Costanzi says there are a lot of potential applications in the magnetic recording industry once the physics of the electrons tunneling through the layers is better understood. There are already read/write recorders which used the phenomenon, but a more complete understanding would allow new and better devices to be built.

Physicists use ferromagnets for magnetic tunnel junctions because they have an excess of one type of spin, whereas with a normal metal, you pick one direction or the other and have the same number of electrons up or down. With the ferromagnet, you have more in one direction or the other, it is a bar magnet on a macro scale. You can start to play games such as flipping the direction of the magnet to test their behavior.

A lot of the work that Costanzi has done so far is proof of concept that this technique, Electron Beam Induced Deposition, works to construct devices. Costanzi says there have been real problems with materials quality in the past. "You can shape them how you want but they don’t always work how you’d like electronically. So a lot of our work has been refining this and proving that you can even make workable devices this way."

Costanzi carries his sandwich shop metaphor one step further in explaining his role in the project: "before we make the club sandwich that we care about, we have to make a lot of single sandwiches to prove it works. What I’ve done on campus is make the contact pads that we build the devices on later." The devices were built in the clean room at the U of MN using photolithography. He designed the contact pads in close conjunction with the folks in Sweden, working with the person running the EBID system to design and manufacture the tunnel junctions they want. After the junctions are completed in Sweden, Costanzi brings them back do the transport measurements at Minnesota. After that, the research is fine-tuned based on what he learned from measurements.

Costanzi says that they have now proven that EBID could make a single sandwich that works. He is currently working on creating a double sandwich to prove that can be done.