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Mar 1, 2016 0    
The wonderful world of complex oxides
Let There Be Light! (And a Little Bit of Magnetism)

Of all the materials we know, which are the most studied, and why? My guess is that a poll of scientists might put two materials—silicon and carbon—at the top. Silicon enables the huge array of electronic devices that we know and love and use every day, while carbon gives us diamonds, pencil “lead,” and even single-atom-thick sheets called graphene. A good third choice for most-studied material might be a compound known as strontium titanate, or SrTiO3.

First made and studied over 50 years ago, few materials have held the attention of scientists for so long. One reason is that SrTiO3 (or, more familiarly, “STO”) is the poster child for a whole family of materials called complex oxides. These materials have tremendous potential across many fields of science. They’re used in electronics, in cell phones, and in computer memories, and they could even be used in fuel cells to make electricity with water as the only waste product.

Understanding these materials is hard work, however. One major reason is that complex oxides are . . . well . . . complex. This is where STO comes in—it is, in principle, one of the simplest complex oxides to work with. In essence, STO is to many physicists what the fruit fly is to biologists: the simplest example imaginable for experiments. But even STO has mysteries. Examples include its strange superconductivity (see Jeremy Levy’s article on Swing Dancing Electron Pairs) and its weird behavior at low temperatures.  Recent research from the University of Minnesota and Los Alamos National Lab has discovered another mystery of STO, and at the same time shows that it may be able to perform a very useful trick.

STO is a non-magnetic material. The new research shows, however, that this can be changed simply by shining a light on it. The light is of a special type, called circularly polarized. In most light beams, the oscillations that make up the waves are oriented randomly, but in polarized light they have specific orientations. In circularly polarized light these orientations rotate in a specific way, and it is this particular variety of light that is found to induce magnetism in this non-magnetic material. The most surprising part, however, is that the magnetism stays even after the light is shut off. It is persistent. So persistent, in fact, that we can write magnetic patterns in STO, wait, and then come back and read the patterns, as in the image above. The purple background is normal, non-magnetic STO, while the pink and green correspond to oppositely directed magnetic regions, written into the STO with light. This persistent magnetism is the vital element in computer memories, which is one reason this discovery is so exciting.

There is a catch (or two), however. First, the magnetism is only persistent at very low temperatures. Second, while we have some ideas about what might cause it–in particular, certain defects in STO crystals–we don’t understand the details. If we can better understand how this effect is produced, it may be possible to raise the temperature at which it occurs.  If we can figure out how to make STO exhibit persistent magnetism at room temperature, we can potentially create a completely new type of memory technology.

STO may be an old dog, but it still has new tricks!

Strontium titanate crystals used as artificial diamonds. (geology.com)
Fig. 1 (Click to enlarge) Strontium titanate crystals used as artificial diamonds. (geology.com)
The crystal structure of STO. Each titanium atom is bonded to six oxygen atoms, forming an octahedral unit cell that repeats throughout the crystal. (Michael O'Keeffe,
Fig. 2 (Click to enlarge) The crystal structure of STO. Each titanium atom is bonded to six oxygen atoms, forming an octahedral unit cell that repeats throughout the crystal. (Michael O'Keeffe, "Materials Science: Edge Effects," Nature 419, 28-29)
The blue and green waves are oriented at right angles to one another, combining to form the circular pattern shown in red. When light waves are organized in this way, the light is called circularly polarized. (Wikimedia Commons)
Fig. 3 (Click to enlarge) The blue and green waves are oriented at right angles to one another, combining to form the circular pattern shown in red. When light waves are organized in this way, the light is called circularly polarized. (Wikimedia Commons)
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