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Center for Nonlinear Studies |
Universality has successfully brought together disparate areas of science from astronomy[1] to biology[2] and to plate tectonics[3].  A well-studied[4] example is the Barkhausen effect[5], in which slowly increasing magnetic fields drive magnetic domain walls through a landscape of pinning centers giving noisy magnetic behavior, which displays spatial and temporal scale invariance over many orders of magnitude. Similar noise has also been observed in the absence of driving fields[6, 7], but has not yet been studied in as much detail. Here we study an ultrathin cobalt film prepared near a spin reorientation transition[8] that exhibits these zero field fluctuations. Near this transition, domain wall pinning is greatly reduced, and thermal excitation causes domain walls to fluctuate over hundreds of nanometers. These fluctuations are easily observable at ambient conditions with standard magneto-optical techniques. We present data showing that our sample has a varied magnetic anisotropy landscape, and that trends between local magnetic properties and the fluctuations are correlated. Then, we measure spatial correlations between fluctuations, finding they are uncorrelated over short time scales but over long time scales act to minimize magnetostatic energy and keep the sample demagnetized. Finally, we perform a scaling analysis on the fluctuation areas, determining a critical exponent t. This exponent remarkably matches theoretical predications[9] and observations[10] for the field driven Barkhausen effect in a number of magnetic systems.
References:
1. M. W. Choptuik, Physical Review Letters 70 (1), 9 (1993).
2. M. Kleiber, Physiol. Rev 27 (4), 511-541 (1947).
3. K. Aki, Journal of Geophysical Research 72 (4), 1217-1231 (1967).
4. S. Zapperi, P. Cizeau, G. Durin and H. E. Stanley, Physical Review B 58 (10), 6353-6366 (1998).
5. E. P. T. Tyndall, Physical Review 24 (4), 439-451 (1924).
6. O. Portmann, A. Vaterlaus and D. Pescia, Physical review letters 96 (4), 047212 (2006).
7. N. Bergeard, J. Jamet, A. Mougin, J. Ferré, J. Gierak, E. Bourhis and R. Stamps, Physical Review B 86 (9), 094431 (2012).
8. J.-W. Lee, J.-R. Jeong, S.-C. Shin, J. Kim and S.-K. Kim, Physical Review B 66 (17), 172409 (2002).
9. A. Benassi and S. Zapperi, Physical Review B 84 (21), 214441 (2011).
10.S.-C. Shin, K.-S. Ryu, D.-H. Kim, S.-B. Choe and H. Akinaga, Journal of Magnetism and Magnetic Materials 310 (2), 2599-2603 (2007).
Host: Nikolai Sinitsyn