Magnetism and Its ApplicationsBy Nadia Ramlagan
Perhaps tantamount to the attention, growth, and interest the APS Topical Group on Magnetism and its Applications (GMAG) has experienced over the past decade, last year’s 2007 Nobel Prize in Physics went to Albert Fert of France and Peter Grunberg of Germany, for their (independent) discovery of giant magnetoresistance (GMR). “The March meeting was a lot of fun, we sponsored a special reception and lecture to honor the Nobel Laureates and had a huge audience. Both Grunberg and Fert were able to attend. They gave a successful lecture to GMAG people,” says GMAG chair William Butler.
Giant magnetoresistance is the change in the electrical resistance of a metallic magnetic multilayer that occurs when an external magnetic field aligns the moments in different layers. The discovery has radically transformed methods of retrieving data from hard disks, improved magnetic sensors, and spawned the development of a whole new field of electronics called “spintronics.”
“Things seem to be coming along every few years”, notes Butler. In 1995 there was the discovery of about 10% tunneling magnetoresistance (TMR) at room temperature by Terunobu Miyazaki at Tokyo University and Jagadeesh Moodera at MIT. The following year in 1996, John Slonczewski and Luc Berger independently predicted the phenomena of spin torque, a sort of converse to GMR. “In GMR the relative orientation of the magnetic moment in two layers will influence the current that goes through it. In spin torque, the spin-polarized current can influence the relative orientation of the moment, and you can actually apply a current that will cause the orientation of the moments to switch,” explains Butler.
In 2001, very high TMR was predicted for magnesium oxide and cobalt iron. This was observed experimentally in 2004, and researchers were able to demonstrate about 1,000% TMR, a huge advancement from the 10% in 1995. Even more recently, there has been a huge investment in these new materials to make spin torque- switched magnetic random access memory. “The largest application of spintronics so far has been to the read sensor for hard drives. There was a lot of publicity talk about GMR and the IPod connected with the 2007 Nobel Prize because the first important application of GMR was for read sensors in hard drives. Nowadays hard drive read sensors use TMR, (tunneling magnetoresistance), using these new materials and most IPODS actually use flash memory,” says Butler.
Currently, magnetic oxide research has been receiving a lot of attention. “In fact, the largest number of papers we get at APS are on magnetic oxides,” Butler notes. Magnetic oxides (and to a lesser extent sulfides) generate lots of exciting phenomena. “It’s interesting for the theorists because they are ionic materials, so the atoms all know where they want to be (in contrast to metals where you can have almost amorphous or fine microcrystalline samples). In the oxides you have the positive and negative atoms wanting to arrange themselves in a particular structure, and that makes things a little simpler to think about from a geometric point of view,” says Butler.
Magnetic oxide materials allow researchers to generate what is called a spin filter effect, in which tunneling occurs through two magnetic layers that are insulators. The relative orientation of these layers can have a huge effect on the tunneling current. “These materials are also of extreme interest because they are very difficult to understand. The ordinary tools of band structure that we use are not always reliable when you apply them to these materials, because of the strong electron correlation”, Butler says.
GMAG continues to play an important role at the APS March Meeting. This year, magnetism-related papers accounted for 14% (839 papers) of the March Meeting total, similar to the previous year. The group also sponsors student dissertation awards, a $500 prize and an invited talk at the March Meeting. “We also give awards for outreach. This year we funded a graduate student who has developed a program that’s been going out and engaging students at local middle schools and high schools in science and magnetism activities,” says Butler. As part of its public outreach effort, GMAG partially funds an Outreach Program (MINT-SOUP), which purchases kits for magnetism demos designed for high school and middle school students.
GMAG was founded in 1997 by David Giles, the first chair. “David was really active in working to get the group recognized. All of the past chairs have done a good job, and this has been reflected in our growing membership,” says Butler.