Magnetic Avalanches Research Articles

Interface depinning, self-organized criticality, and the Barkhausen effect.

We report measurements of magnetic avalanches in a Fe-Ni-Co alloy. The distribution of avalanche sizes is given by $P(A)\ensuremath{\propto}{A}^{\ensuremath{-}\ensuremath{\alpha}}\mathrm{exp}({\ensuremath{-}A/A}_{0})$, with $\ensuremath{\alpha}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.33\ifmmode\pm\else\textpm\fi{}0.10$. The role of demagnetizing effects in the avalanche statistics is demonstrated. We analyze a simple model of interface growth and find a power law distribution of avalanche sizes over a wide range of parameters when an infinite-range demagnetization field is included. The exponents agree with those found at the critical field in the absence of demagnetization effects.

Avalanches of Magnetic Reconnection

AbstractActive region coronal magnetic fields are expected to be in a twisted tangled state due to photospheric convective motions. These motions can drive the magnetic field to a statistically steady state where energy is released impulsively (Lu and Hamilton 1991). These relaxation events in the magnetic field can be interpreted as avalanches of many small reconnection events. We argue that the frequency distribution of these magnetic reconnection avalanches must be a power law. Furthermore, we calculate the expected distributions in a simple model of magnetic energy release events in a 3-dimensional complex magnetized plasma, and compare these to the distributions of solar flares. These distributions are found to match the observed power law distributions of solar flare energies, peak fluxes, and durations. This model implies that the energy-release process is fundamentally the same for flares of all sizes. Observational predictions of this model are discussed.

A Magnetic Sensor Utilizing an Avalanching Semiconductor Device

A new semiconductor device for sensing uniaxial magnetic fields has been realized. The device is basically a dual-collector open-base lateral bipolar transistor operating in the avalanche region, and is referred to as a Magnetic Avalanche Transistor. It exhibits high magnetic transduction sensitivity compared to traditional Hall-effect and conventional nonlinear magnetoresistive devices. Several hundred experimental devices have been designed, fabricated, and tested over the past two years. Many structural and some process parameters were varied. The magnetic sensitivity of a typical device was found to be proportional to substrate resistivity. A sensitivity of 30 volts per tesla was measured for devices which used 5-ohm-cm p-type substrates. The output signal measured between collectors is differential and responds linearly with field magnitude and polarity. A typical signal-to-noise ratio is 20,000 per tesla. The bandwidth is known to extend well beyond 5 MHz. The sensitive area is calculated to be on the order of 5 µm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . This communication describes the basic structure, fabrication, and characteristics for the magnetic avalanche transistor.