# Presentation: 100493

## SrTiO3

- Chris Leighton, University of Minnesota, **DMR 0509666**

- Perovskite oxides are a class of materials that display
- some of the most exciting and technologically useful
- properties known. High temperature superconductivity, colossal magnetoresistance and multiferroic behavior are just a few examples of their spectacular properties, many of which involve magnetism. In our recent work at the U of M we have discovered that a special class of perovskite oxides (the cobalt oxides) can, when grown in the form of very thin films, *spontaneously assemble* a magnetic device known as an “exchange spring”.

- As shown in the figure above, this is achieved via a
- strain relaxation mechanism, leading to formation of a
- magnetic hard layer and a magnetic soft layer. The spin
- structure is similar to that of a wound spring (hence the
- term “exchange spring”). Such structures typically
- require the demanding epitaxy of dissimilar materials
- with different magnetic properties. Exchange springs
- have a characteristic magnetic signature (see the fig to the
- left), have found use as hard permanent magnets, and are
- considered strong candidates for future recording media
- in hard disks. Our work opens up the possibility of using
- “strain engineering” to simply fabricate such devices.

**SrTiO****3**

**HARD**

**SOFT**

**LaSrCoO****3**

**Notes:**
Our work involves the synthesis and measurement of an extraordinarily versatile class of materials known as the “Perovskite Oxides”. These materials, which form in a very flexible crystal structure called “Perovskite” can be “grown” as ultra thin films with thicknesses down to just a few nanometers. In this research we have discovered that a certain class of these materials (cobalt based perovskite oxides) possess some remarkable and rather unique magnetic properties in thin film form. In essence one single film of these oxides behaves magnetically as though it were composed of two separate magnetic films with very different magnetic properties. This comes about because the atomic lattice of the “substrate” crystal that the films are grown on does not match exactly that of the film. The fraction of the film closer to the substrate is forced to “match” the underlying lattice leading to mechanical strain which alters the properties significantly. On the other hand, the top part of the film “relaxes” and achieves similar properties to the bulk materials. The film therefore “self-assembles” itself into two different magnetic films. It so happens in this case that the strained layer has the properties of a soft magnet (like Fe), while the relaxed layer has the properties of a hard magnet. The result is a hard/soft bilayer which is the active component in a device called an “exchange spring”. This device is so called because the hard layer resists the application of magnetic fields while the soft layer responds easily. The coupling together of the two layers means that a “spring” forms through the depth of the film (as in the figure). It is unique that we are able to achieve this behavior in a single magnetic film. Moreover, these springs are useful magnetic structures. They are already used in the engineering of hard permanent magnets and are being considered as an excellent candidate for future extremely high density recording media. Our work shows for the first time that such a device can be generated by strain engineering a single film as opposed to the difficult task of growing dissimilar materials on top of one another in a controlled fashion. The perfection of the interface between the materials is expected to be considerably higher in our case.         

## Magneto-electronic Properties of Perovskite Heterostructures

- Chris Leighton, University of Minnesota, **DMR 0509666**

_**Broader impact: **_In addition to the technological relevance of our research (i.e. to the
- magnetic recording and microelectronic industries that greatly impact our society), and the vital
- educational opportunities for graduate students and postdocs, we have also undertaken a number
- of outreach activities. As an example, the PI organized and presented (in collaboration with
- C.D. Frisbie at UMN) a presentation on “*Electronic and Magnetic Materials: Science and *
*Technology*” at the 2007 annual meeting of the Davidson Institute in St. Paul, MN. This is an
- organization (www.Davidson-Institute.org) that strives to provide a stimulating learning
- environment for exceptionally gifted young children. Our two 1 1⁄2 hour presentations were based
- on hands-on demonstrations on topics including plastics, silicon, conducting polymers, solar cells, magnetism, and superconductivity, and impacted almost 80 children. Photographs of the PI, his students, and postdocs presenting at the event are shown below.