Probabilistic aspects of magnetization relaxation in single-domain nanomagnets.

A single-domain nanomagnet is a basic example of a system where relaxation from high to low energy is probabilistic in nature even when thermal fluctuations are neglected. The reason is the presence of multiple stable states combined with extreme sensitivity to initial conditions. It is demonstrated that for this system the probability of relaxing from high energies to one of the stable magnetization orientations can be tuned to any desired value between 0 and 1 by applying a small transverse magnetic field of appropriate amplitude. In particular, exact analytical predictions are derived for the conditions under which the probability of reaching one of the stable states becomes exactly 0 or 1. Under these conditions, magnetization relaxation is totally insensitive to initial conditions, and the final state can be predicted with certainty, a feature that could be exploited to devise novel magnetization switching strategies or novel methods for the measurement of the magnetization damping constant.

Magnetization switching and microwave oscillations in nanomagnets driven by spin-polarized currents.

A novel theoretical approach to magnetization dynamics driven by spin-polarized currents is presented. Complete stability diagrams are obtained for the case where spin torques and external magnetic fields are simultaneously present. Quantitative predictions are made for the critical currents and fields inducing magnetization switching, for the amplitude and frequency of magnetization self-oscillations, and for the conditions leading to hysteretic transitions between self-oscillations and stationary states.

Spin-wave instabilities in large-scale nonlinear magnetization dynamics.

The stability of large magnetization motions in systems with uniaxial symmetry subject to a circularly polarized radio-frequency field is analytically studied. Instability conditions valid for arbitrary values of the amplitude and frequency of the driving field are derived. In the limit of small motions, these conditions yield Suhl's theory of spin-wave instabilities for the case of ferromagnetic resonance. It is shown that the input powers capable of inducing spin-wave instabilities are bounded from both below and above, so that large enough motions are always stable. In addition, it is demonstrated that stability of uniform motions depends on their preparation history.

Nonlinear magnetization dynamics under circularly polarized field.

Exact analytical results are presented for the nonlinear large motion of the magnetization vector in a body with uniaxial symmetry subject to a circularly polarized field. The absence of chaos, the existence of pure time-harmonic magnetization modes with no generation of higher-order harmonics, and the existence of quasiperiodic magnetization modes with spontaneous breaking of the rotational symmetry are proven. Application to ferromagnetic resonance and connection with the Stoner-Wohlfarth model are discussed.

Limitations of the computerized laryngeal analyzer (CLA) in the assessment of the swallowing mechanism.

This study was devised to assess the repeatability of measurements obtained from the Computerized Laryngeal Analyzer (CLA), a commercial system which monitors global laryngeal activity through a piezoelectric transducer positioned at the level of the thyroid cartilage. Twenty-two healthy subjects (37+/-7 yr) were asked to repeat three consecutive times the deglutition of three types of bolus: dry (saliva only), liquid (10 ml water) and solid (1 cm(3) biscuit). The whole recording session was repeated 4+/-3 days apart. Two parameters were considered: amplitude and duration of the deglutition. Both intra- and inter-sessions repeatability were assessed by the standard error of measurement. The amplitude measurements were often found to be clipped at 100 mV, making duration the only parameter suitable for analysis. The duration was homogenous for the three types of bolus and no trend effect was found either intra- or inter-sessions. Inter-sessions repeatability was significantly lower than the intra-session one (P<0.05). As a practical implication, a change in duration >/=1 s (=65% of the observed mean value) between two separate measurements is required in order to be 95% confident that it is not due to chance alone. We conclude that the observed limitations in the measurements provided by the CLA system, in conjunction with its intrinsic characteristic of providing only an uncalibrated measurement of global laryngeal activity, seriously question the possibility of using the CLA system either as a diagnostic tool or as a substitute of the gold standard method (videofluorography).

Functional integration approach to hysteresis.

A general formulation of scalar hysteresis is proposed. This formulation is based on two steps. First, a generating function g(x) is associated with an individual system, and a hysteresis evolution operator is defined by an appropriate envelope construction applied to g(x), inspired by the overdamped dynamics of systems evolving in multistable free-energy landscapes. Second, the average hysteresis response of an ensemble of such systems is expressed as a functional integral over the space G of all admissible generating functions, under the assumption that an appropriate measure mu has been introduced in G. The consequences of the formulation are analyzed in detail in the case where the measure mu is generated by a continuous, Markovian stochastic process. The calculation of the hysteresis properties of the ensemble is reduced to the solution of the level-crossing problem for the stochastic process. In particular, it is shown that, when the process is translationally invariant (homogeneous), the ensuing hysteresis properties can be exactly described by the Preisach model of hysteresis, and the associated Preisach distribution is expressed in closed analytic form in terms of the drift and diffusion parameters of the Markovian process. Possible applications of the formulation are suggested, concerning the interpretation of magnetic hysteresis due to domain wall motion in quenched-in disorder and the interpretation of critical state models of superconducting hysteresis.