8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University
Enhancement technique in the measurement of ac susceptibility for studying magnetic properties of nano materials
Dr. S. Mukherjee,
Lecurer, Department of Physics, The University of Burdwan, Burdwan
Abstract :
The AC susceptibility enhancement method is the study of AC susceptibility in the presence of a second biasing field (pulsed DC or another AC field of frequency incommensurate with the measuring one). For bulk samples having a multi-domain structure, the application of a second biasing field causes an extra perturbation of the domain walls and thereby, a general loosening of the domain pinning. Thus the sample state is changed to one in which the domains are far more free to follow the applied field compared to the previous case. If from the complicated magnetization versus time curve thus obtained, only the Fourier coefficient of the measuring sine wave is computed (or measured, as with a lock-in amplifier with its reference driven by the same signal) the coefficient χ is found to increase; this is enhancement. [1]
For nano-sized ferromagnetic particles, the magnetic anisotropy energy (responsible for keeping the magnetization oriented in certain directions) is comparable to the thermal energy (kT ).When this happens, the particles become super-paramagnetic above TB ; as thermal fluctuations randomly flip the magnetization direction between parallel and anti-parallel orientations. They exhibit single-domain ferromagnetic behaviour below the blocking temperature TB and show hysteresis. In the super-paramagnetic state, he moment of each particle freely rotates, so a collection of particles acts like a paramagnet where the constituent moments are ferromagnetic particles (rather than atomic moments as in a normal paramagnet). The imaginary component of the ac susceptibility peaks at TB. Since TB depends on the measurement frequency, the peak in χ" vs. T occurs at different temperatures for different frequencies. The nature of the barrier potential can be probed with enhancement technique in the measurement of ac susceptibility.
Reference [1] S. Mukherjee, R. Ranganathan, A. Chakravarti and S. Sil; Journal of Magnetism and
magnetic materials Volume 224, Issue 3, 2001, Pages 210-220
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