A detailed understanding of noise characteristics is essential for the design of a high signal-to-noise ratio (SNR) reader sensor. We intend to correlate the microstructure to the source of magnetic noises for improving the magnetic stability of the recording heads. A dynamic magnetic sensitivity mapping (MSM) system is designed to image the magnetic noise sources in sub-micrometer sized recording heads. A nanometer sized magnetic force microscopy (MFM) tip was used to apply a well-defined, localized magnetic field on the air bearing surface (ABS) of the head. For a certain area position of the free layer with incoherent rotation of the magnetic moment, this localized magnetic field will cause magnetic instability in the head, and this instability will show up as electrical noise on the output signal. Because most of the noise related to magnetic domain fluctuation is dominated at the low frequency region, our study concentrates on the spatial characterization of the noise source in a frequency range of 20 kHz to 60 kHz. Recording the average amplitude of the noise spectrum due to the excitation in the measured frequency range as a function of the tip's position, the location of the magnetic noise source can be identified. Magnetic noise images have been obtained by our system for some recording giant magnetoresistance (GMR) and tunnel magnetoresistance (TMR) recording heads. Noise MSM images of some unstable recording heads clearly show the spatially uneven noise.

A spin-polarized dc current can induce steady-state, microwave frequency magnetization dynamics in a nanoscale ferromagnet. The torque that drives these dynamics originates from the exchange of spin angular momentum between conduction electrons and the magnetization. We present measurements of current perpendicular to the plane (CPP) giant magnetoresistance (GMR) nanopillar devices in which this phenomenon occurs. We focus on devices that contain one reference ferromagnetic layer that has a fixed magnetization and one free ferromagnetic layer with a magnetization that responds to spin torque. The resulting spin transfer induced magnetization dynamics combined with GMR lead to resistance noise, which we measure in both the frequency- and time-domain. The appearance of these dynamical states is consistent with spin transfer in that dynamics are observed only for those combinations of current direction and magnetic configuration in which spin torque opposes the FL configuration set by the magnetic field. Furthermore, the amplitude of the resultant resistance noise increases rapidly with increasing current until saturating at a value that is a large fraction of the magnetoresistance between parallel and antiparallel states. This behaviour is contrasted with similar measurements of a current-in-plane (CIP) GMR device in which the magnetic resistance noise is thermally activated.

Barkhausen noise (BN) is a prototypical example of a complex system where the probability distributions for most relevant quantities follow a power-law behavior. In our experiment we investigated the role of temperature in determining the statistical properties of BN in thin Fe films. In the temperature range between 10 K and 295 K the probability distribution for the amplitude of the magnetization avalanches is always a power-law. The critical exponent a, however, undergoes a strong variation since its value changes from α = 1 at 295 K to α = 1.8 at 10 K. At our knowledge this is the first experimental evidence of the role of temperature in BN and, more generally, in complex systems. The experimental results are discussed in terms of a generalized version of the energy equipartition principle. Within this ansatz, the energy released during the dynamical evolution of the system is "shared" between all the available avalanches, depending on their size. Avalanches of a given size constitute a "mode" of evolution of the system: the energy globally released during the magnetization process is equally shared between all the available modes. In our experiment this behavior is actually observed at room temperature. At low temperature a "freezing" of the system prevents the occurrence of large size events and therefore energy is mostly released through jumps with small amplitude.

We analyze the current experimental and theoretical research about the magnetization (Barkhausen) noise in magnetic thin films. We observe that, in respect to three-dimensional systems, the situation is much more complicated, and many details still have to be analyzed and understood. In particular, the critical exponents must be correctly analyzed when compared with the theoretical predictions. We observe, for instance, that the easured magnetization steps in optical measurements do not actually correspond to the avalanche size, as claimed, but to a different quantity having a different critical exponent. We also observe as the lack of a significative demagnetizing field does not assure to have a stationary Barkhausen signal, which again implies a different set of exponents. For comparison, we report in detail all the possible theoretical values of the critical exponents calculated for a single DW model.

We report our direct observation of the Barkhausen avalanche in ferromagnetic thin film systems, where a collective spin behavior produces nontrivial fluctuations in magnetization change under an external magnetic field. For this study, we develop and use two direct full-field magnetic imaging techniques: magneto-optical microscope magnetometer (MOMM) and magnetic transmission X-ray microscopy (MTXM). From a direct visualization and a statistical analysis of the fluctuating domain images for Co thin films, we investigate the scaling behavior of the Barkhausen avalanche both on spatial and temporal scales using MOMM. We also investigate the reproducibility of the Barkhausen avalanche process. Interestingly, the partially stochastic nucleation behavior is observed for CoCrPt alloy films by means of MTXM on a nanometer scale comparable to the fundamental length scales such as the Barkhausen volume and the grain size of the polycrystalline films. Via these direct full-field observation techniques, dynamic details of Barkhausen avalanche are revealed.

During magnetization reversal of ferromagnets random flux changes occur, which are responsible for the Bark-hausen noise. Using magnetic force microscopy, this noise can be visualized in real space so that individual Barkhausen events can be distinguished and analyzed with respect to size and location. Reducing the external field from saturation to zero in a granular ferromagnetic La_0.7Sr_0.3MnO₃ thin film, the observed power law size distribution of growth events is discussed with respect to self-organized criticality and domain wall motion through a disordered medium.

Random spin fluctuations in an equilibrium ensemble of paramagnetic spins are shown to contain valuable information about the system itself. We use off-resonant Faraday rotation to passively and sensitively "listen" to the random magnetization fluctuations (spin noise) in atomic alkali vapors. These random fluctuations generate spontaneous spin coherences which precess and decay with the same characteristic energy and time scales as the macroscopic magnetization of an intentionally polarized or driven ensemble. Correlation spectra of the measured spin noise reveals g-factors, nuclear spin, isotope abundance ratios, hyperfine splittings, nuclear moments, and spin coherence lifetimes -- without having to excite, optically pump, or otherwise drive the spin system away from thermal equilibrium. These noise signatures scale inversely with interaction volume, suggesting routes towards non-perturbative, sourceless magnetic resonance of small solid state spin systems.

Mesoscopic shot noise is not only probabilistic: it has features which reflect quantum mechanical entanglement. We discuss recent proposals of orbital entanglement generation and detection in mesoscopic coherent conductors. Orbital entanglement avoids the difficulty of detecting spin and leads to simpler structures. Orbital entanglement schemes invoke two two-particle sources. The index of the source plays the role of a pseudo-spin. The rotation of qubits can be implemented with beam-splitters or even just quantum point contacts. Entanglement is detected via violation of a Bell inequality. The necessary correlations can be extracted from shot noise measurements. Possible two particle sources are Cooper pairs emitted from superconductors or even simpler electron-hole pairs generated at tunnel contacts or generated dynamically with the help of oscillating potentials.

We consider the non-equilibrium noise properties of bistable systems. The stochastic path integral formalism is derived and used to investigate the dynamics and distribution of transmitted charge. Microscopic fluctuations induce transitions between the two stable states, with rates found from an instanton calculation. On a long time scale, the system exhibits a random telegraph signal between the currents produced by the two stable states. We predict a universal ellipse law for the log-distribution of transmitted charge in the bistable current range, which applies to any type of bistable system, regardless of its origin.

We show how the semiclassical Langevin method can be extended to calculations of higher-than-second cumulants of noise. These cumulants are affected by indirect correlations between the fluctuations, which may be considered as "noise of noise." We formulate simple diagrammatic rules for calculating the higher cumulants and apply them to mesoscopic diffusive contacts and chaotic cavities. As one of the application of the method, we analyze the frequency dependence of the third cumulant of current in these systems and show that it contains additional peculiarities as compared to the second cumulant. The effects of environmental feedback in measurements of the third cumulant are also discussed in terms of this method.

Measurements of the electronic current fluctuations of free-standing hydrogenated amorphous silicon nanoparticles are described. The nanoparticles are synthesized by high-density plasma chemical vapor deposition and are deposited onto conducting substrates. An insulating matrix, either silicon oxide or silicon nitride is then grown in order to electrically isolate the particles. Electronic measurements are performed in this transverse geometry, and underneath a top electrode of area 1mm x 1mm are typically 10,000 nanoparticles with an average diameter of 150 nm in parallel. The spectral density of the current fluctuations in the a-Si:H nanoparticles is well described by a 1/f frequency dependence for frequency f, as in the case of bulk a-Si:H films. The variation of the correlation coefficients with frequency octave separation of the noise power fluctuations in bulk a-Si:H films indicates serial interactions between fluctuators. In contrast, the octave separation dependence of the correlation coefficients for the nanoparticles are very well described by an ensemble of fluctuators whose amplitudes are independently modulated in parallel.

The current noise of a voltage biased interacting quantum wire adiabatically connected to metallic leads is computed in presence of an impurity in the wire. We find that in the weak backscattering limit the Fano factor characterizing the ratio between shot noise and backscattering current crucially depends on the noise frequency relative to the ballistic frequency v_F/gL, where v_F is the Fermi velocity, g the Luttinger liquid interaction parameter, and L the length of the wire. In contrast to chiral Luttinger liquids, the noise is not only due to the Poissonian backscattering of fractionally charged quasiparticles at the impurity, but also depends on Andreev-type reflections of plasmons at the contacts, so that the frequency dependence of the noise needs to be analyzed to extract the fractional charge of the bulk excitations. We show that the frequencies needed to see interaction effects in the Fano factor are within experimental reach.

We present time-domain electron counting studies on charge fluctuations in a mesoscopic system. In this measurement, a radio-frequency single-electron transistor (RF-SET) acting as a fast electrometer is capacitively coupled to a quantum dot (QD) electrostatically defined in a GaAs/AlGaAs heterostructure. Random telegraph signals (RTSs) were observed on the RF-SET and were interpreted as resulting from individual electrons tunneling into/out of the QD hence switching the QD charge state between N and N+1 electrons near the charge degeneracy point. Periodic behavior of the switching events is observed as the number of average electrons in the electron box is decreased one by one. The occupation time of the excess electron is directly measured and changes from a few microseconds to a few milliseconds as the tunneling resistance of the QD is increased. Histogram of the occupation time obtained from the time-domain electron counting measurements agrees well with the frequency-domain power spectrum analysis, both suggesting a Poissonian process at the charge degenerate point, in agreement with a model consisting of two charge states. Real time electron counting techniques can play a powerful role in studies on temporal electronic correlations, as well as quantum information processing schemes. Techniques that will improve the charge sensitivity of the RF-SET will also be discussed.

We address anomalous transport phenomena in arrays of semiconductor nanocrystals (quantum dots): Transient power-law decay of current as a response to a step in large bias voltage applied across the array, as well as memory effects observed after successive applications of the bias voltage. A novel phenomenological model of transport in such systems is proposed, capable of rationalizing both anomalous transport and memory. The model describes electron transport by a stationary Levy process of transmission events and therefore requires no time dependence of system properties. The long tail in the waiting time distribution gives rise to a nonstationary response in the presence of a voltage pulse. Noise measurements agree well with the predicted non-Poissonian fluctuations in current. We briefly discuss possible microscopic mechanisms that could cause the anomalous statistics in transmission.

Nanowires with high aspect ratio can become unstable due to Rayleigh-Plateau instability. The instability sets in below a certain minimum diameter when the force due to surface tension exceeds the limit that can lead to plastic flow as determined by the yield stress of the material of the wire. This minimum diameter is given d_m ≈ 2σ_S/σ_Y , where σ_S is the surface tension and σ_Y is the Yield force. For Ag and Cu we estimate that d_m ≈ 15nm. The Rayleigh instability (a classical mechanism) is severely modified by electronic shell effect contributions. It has been predicted recently that quantum-size effects arising from the electron confinement within the cross section of the wire can become an important factor as the wire is scaled down to atomic dimensions, in fact the Rayleigh instability could be completely suppressed for certain values of k_F r₀. Even for the stable wires, there are pockets of temperature where the wires are unstable. Low-frequency resistance fluctuation (noise) measurement is a very sensitive probe of such instabilities, which often may not be seen through other measurements. We have studied the low-frequency resistance fluctuations in the temperature range 77K to 400K in Ag and Cu nanowires of average diameter ≈ 15nm to 200nm. We identify a threshold temperature T* for the nanowires, below which the power spectral density S_V(f) ~1/f. As the temperature is raised beyond T* there is onset of a new contribution to the power spectra. We link this observation to onset of Rayleigh instability expected in such long nanowires. T* ~ 220K for the 15nm Ag wire and ~ 260K for the 15nm Cu wire. We compare the results with a simple estimation of the fluctuation based on Rayleigh instability and find good agreement.