Laser-induced demagnetization is theoretically studied by explicitly taking into account interactions among electrons, spins and lattice. Assuming that the demagnetization processes take place during the thermalization of the sub-systems, the temperature dynamics is given by the energy transfer between the thermalized interacting baths. These energy transfers are accounted for explicitly through electron-magnons and electron-phonons interaction, which govern the demagnetization time scale. By properly treating the spin system in a self-consistent random phase approximation, we derive magnetization dynamic equations for a broad range of temperature. The dependence of demagnetization on the temperature and pumping laser intensity is calculated in detail. In particular, we show several salient features for understanding magnetization dynamics near the Curie temperature. While the critical slowdown in dynamics occurs, we find that an external magnetic field can restore the fast dynamics. We discuss the implication of the fast dynamics in the application of heat assisted magnetic recording.

Ultrasound current source density imaging (UCSDI) potentially transforms conventional electrical mapping of excitable organs, such as the brain and heart. For this study, we demonstrate volume imaging of a time-varying current field by scanning a focused ultrasound beam and detecting the acoustoelectric (AE) interaction signal. A pair of electrodes produced an alternating current distribution in a special imaging chamber filled with a 0.9% NaCl solution. A pulsed 1 MHz ultrasound beam was scanned near the source and sink, while the AE signal was detected on remote recording electrodes, resulting in time-lapsed volume movies of the alternating current distribution. Electrical mapping helps identify abnormal electric pathways in the heart (i.e., arrhythmias) and brain (i.e., epileptic seizures) during treatment.1,2 This invasive procedure requires assumptions for reconstructing the current distribution, has limited spatial resolution, and is sensitive to registration errors.3 Ultrasound current source density imaging (UCSDI) has been proposed as a complement and possible alternative to traditional mapping of electrophysiological signals.4-11 UCSDI exploits the acoustoelectric effect (AE), an interaction between pressure and current, to map electric field distributions while scanning a focused ultrasound beam. A previous study in the rabbit heart demonstrated that UCSDI has sufficient sensitivity to map the cardiac activation wave. 4 Potential advantages of UCSDI include 1) remote detection of current with a spatial resolution determined by the size of the ultrasound focus (<5 mm 3 ); 2) volume images of current flow and biopotentials with as few as one electrode and ground without major assumptions regarding conductivity; 3) automatic coregistration of UCSDI with pulse echo (PE) ultrasound for simultaneously portraying current flow with anatomy. This study demonstrates volume imaging of a timevarying current field produced by scanning an ultrasound beam and detecting the generated AE signal.UCSDI is based on reciprocal theory. 4,6,11 In an electric field produced from a distributed current source J I ¼ J I (x, y, z; t s ) changing in physiologic time t s , the voltage V i measured by lead i at coordinate x 0 , y 0 , z 0 can be expressed in four dimensions under the assumption of far field detection of the AE signal. If an ultrasound beam is centered at C(x 0 , y 0 , z 0 ), then any point (x, y, z) in the pressure field can be represented in the electric field as (x þ x 0 , y þ y 0 , z þ z 0 ). The induced AE voltage (V AE ) detected by two distant electrodes can be represented bywith pressure pulse amplitude P 0 , interaction constant K I , resistivity q 0 , current distribution of detector J i L (x, y, z), ultrasound beam pattern b(x, y, z) defined with the transducer at the origin, speed of sound c, ultrasound pulse waveform a(t-z/c), and fast ultrasound time t. The instantaneous local field potential is reconstructed by demodulating (or basebanding) the AE signal. The current density J I of the dipole is modele...

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