# Presentation: 033263

## Noise Studies of Disordered Condensed Matter – Colossal Magnetoresistive MaterialsM. B. Weissman, UIUC, DMR-0240644

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- Colossal Magnetoresistive (CMR) materials have inspired interest for their potential as sensors of magnetic fields, most evident in a regime where a magnetic electrically conducting phase and an insulating phase coexist. Understanding the nature of the phases and why they coexist is important for optimizing the materials. We have recently used an experiment on how the noise, random conductance fluctuations, depend on magnetic field history to show that a special type of insulating phase, called antiferromagnetic, plays a key role in an unexpected way. The ferromagnetic metal sits in a background of antiferromagnetism, not simple non-magnetic material.

- The noise power at zero field depends on whether the last applied field greater than 100 G was positive or negative, showing a fixed symmetry-breaking magnetic background. The large field scale required to change the background shows it is antiferromagnetic.

**Notes:**
The sensitivity of most CMR materials to magnetic fields comes mainly from the mixture of electrically conducting and electrically insulating regions. Applying a magnetic field makes the conducting regions, which are magnetic, grow. Most simple theories, however, do not predict that such mixed-phase states would be stable. We have shown elsewhere that the simple disorder in where the different atoms sit is not enough to stabilize the mixture. Here we show that actually a distinct third phase is involved. It is magnetic, as we have shown conclusively by a symmetry argument, but it is not the familiar sort of magnetism (ferromagnetism) that is strongly sensitive to applied fields. Rather, it is antiferromagnetism, with alternating directions of the internal magnetic alignment. Since the volumes of each of the three phases are different, that opens up interesting possibilities for how their coexistence can be maintained by the mechanical strain due to the misfit. That mechanical strain can itself be manipulated by depositing the material on different crystal substrates, as we have shown elsewhere. One practical result is that relatively noise-free sensors can be built by tuning the material-substrate combination

## Noise Studies of Disordered Condensed MatterM. B. Weissman, UIUC, DMR-0240644

- The work described in this nugget was done by recent student Aki Palanisami (now a post-doc in Japan) working with N. Mathur’s group from Cambridge University and with J. Eckstein’s group at Illinois. This grant currently supports work by a graduate student (Lambert Chao), an undergraduate (Kara Lamb)  and a Visiting Research Professor (Eugene Colla). Recent graduates from this group have gone on to work for several hard-drive developers, as well as for other hardware developers, national labs, and universities.

**Outreach:**
- The PI has prepared and edited hundreds of answers for a question-and-answer _Web site_ run by the Physics Department _http://van.__hep__.__uiuc__.__edu__/van/__qa__/__qaform__.__htm_
- This site draws questions on all sorts of science topics from students of all ages from around the world. Here’s the PI’s favorite:
- http://van.hep.uiuc.edu/van/qa/section/stuff_about_space/the_earth_and_the_moon/20020821210810.htm.
- Here’s another, with more serious substance: http://van.hep.uiuc.edu/van/qa/section/states_of_matter_and_energy/boiling_evaporating_and_condensing/20030626151340.htm.

- Noise Studies of Disordered Condensed MatterM. B. Weissman, UIUC, DMR-0240644

**Notes:**
This project is classic ‘small science’, in which participants get to do more than work at narrow roles within a big group. Students and post-docs working on it have a chance to build their own electronics, to make apparatus, to use the sorts of modern lithographic techniques used to make chips, to write computer programs to take and analyze data, and to do some of their own calculations to connect results with theory. The graduating students thus have many options to contribute to industry, national labs, and academics. Since the topics are close to some important electronics materials applications, the students have been particularly well positioned to go into computer hardware development. The student count is currently low because two students just finished up.