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Fluctuation Dissipation Theorem Violation in a Superspin Glass

Katsuyoshi Komatsu


The fluctuation-dissipation theorem (FDT) is a cornerstone of statistical physics. It led to important results in many domains, such as conduction theory, magnetic resonance, neutron scattering, etc. However, the FDT concerns only systems at equilibrium, although the most of the objects in nature are in out-of-equilibrium states. Glasses are typical examples of out-of-equilibrium systems, thus violations of the FDT are expected in them. Recently the extension of the FDT to out-of-equilibrium systems ; i.e. the generalized FDT, has been extensively investigated.

In the present work, we have examined the limits of FDT applicability via the magnetic noise of a concentrated frozen ferrofluid. A ferrofluid consists of nanomagnets suspended in a liquid matrix (in our case, glycerol), with a typical individual nanomagnet possessing a large magnetic moment of the order of 104 µB. Nanomagnets are sufficiently small to be considered magnetically single domained and thus called ‘superspins’. When ‘superspins’ are sufficiently concentrated, long-range dipole-dipole interactions among them produce spin-glass like behavior (aging, memory, etc.) at low temperatures. The resulting out-of-equilibrium state is called a ‘superspin glass’.

Magnetic noise was measured using a high resolution 2-dimensional electron gas (2DEG) Hall sensor (QWHS) 2 µm x 2 µm in size with the spinning current technique. The technique suppresses simultaneously the low frequency noise and the Hall voltage offset, which effectively improves the resolution of the sensor. With a resulting field resolution of 2mG/√Hz around 0.1 Hz, we succeeded in measuring the magnetic noise of a frozen ferrofluid sample that indicates the FDT violation below the glass transition temperature.

 

References :

K. Komatsu, D. L’Hôte, S. Nakamae, V. Mosser, A. Kerlain, M. Konczykowski, E. Dubois, V. Dupuis and R. Perzynski, J. Appl. Phys. 107, 09E140 (2010).

 

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