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We have investigated critical scaling behavior of Barkhausen avalanches of two-dimensional Co and MnAs films using time-resolved magneto-optical microscopy, enabling to image Barkhausen avalanches at criticality. A stochastic analysis of the fluctuating size of Barkhausen jumps from numerous repetitive experiments shows a power-law scaling behavior in both systems. Strikingly, it is found that the variation of scaling exponent in the power-law distribution is closely correlated with the variation of the domain-evolution patterns. This result is understood by the fact that both the scaling exponent and the domain-evolution patterns strongly depend on the relative contribution between the dipolar interaction and domain wall energies. [ABSTRACT FROM AUTHOR]

The field dependence of the activation volumes of nucleation and wall-motion processes in magnetization reversal of Co/Pd multilayers is presented, as observed using a magneto-optical Kerr-effect microscope capable of real-time domain imaging in a wide time range of 10-5-103 s. The analytic forms of the activation volumes are derived from a theoretical consideration of equilibrium conditions of the two typical domain evolution processes in a ferromagnetic film with uniaxial perpendicular anisotropy; the theory is shown to be in good agreement with the experimental results. It was found that the field dependence of the activation volumes can be characterized by the difference between the domain wall energy and the dipolar energy. [ABSTRACT FROM AUTHOR]

We report the pinning role of the nonferromagnetic β phase in the domain wall motion of ferromagnetic MnAs film on GaAs(001) in the temperature range of 20–35 °C. The dependence of the coercivity on the applied field direction at all temperatures was found to follow an inverse cosine law, as expected from a theoretical model considering the domain wall motion in ferromagnetic films. From this analysis, it was found that the domain wall pinning energy density increases as the temperature increases. This is ascribed to the increase in the volume fraction of the nonferromagnetic β phase, resulting in an increase of the domain wall pinning effect during the domain wall motion. [ABSTRACT FROM AUTHOR]

We investigate the thermally activated magnetization reversal of Co/Pd multilayers with uniaxial perpendicular anisotropy using a magneto-optical Kerr effect microscope capable of direct domain observation in a wide time range of microsecond to second. It is found that the energy barrier in the thermally activated process is nonlinearly dependent on the applied field, irrespective of the difference in the domain evolution processes of wall motion, dendrite growth, and nucleation. These results are well explained by a theoretical model considering a ferromagnetic thin film with uniaxial perpendicular anisotropy. [ABSTRACT FROM AUTHOR]

We report the current-induced domain-wall (CIDW) motion along the electron flow in the perpendicularly magnetized Co/Ni/Co structure sandwiched between the top and bottom Pt layers, observed by means of the Kerr microscope. As the lower Co thickness increases, the DW velocity for the electron flow is significantly increased, and also its motion becomes more stable irrespective of various current pulses. From the longitudinal field dependence of the DW velocity, it is found that the increase of the lower Co thickness increases a Dyzaloshinskii–Moriya interaction in the bottom Co/Pt interface, which results in the increase of the chiral spin torque, responsible for the increase of the DW velocity along the electron flow. [ABSTRACT FROM PUBLISHER]

The operation of racetrack memories is based on the motion of domain walls in atomically thin, perpendicularly magnetized nanowires, which are interfaced with adjacent metal layers with high spin-orbit coupling. Such domain walls have a chiral Néel structure and can be moved efficiently by electrical currents. High-capacity racetrack memory requires closely packed domain walls, but their density is limited by dipolar coupling from their fringing magnetic fields. These fields can be eliminated using a synthetic antiferromagnetic structure composed of two magnetic sub-layers, exchange-coupled via an ultrathin antiferromagnetic-coupling spacer layer. Here, we show that nanosecond-long current pulses can move domain walls in synthetic antiferromagnetic racetracks that have almost zero net magnetization. The domain walls can be moved even more efficiently and at much higher speeds (up to ∼750 m s−1) compared with similar racetracks in which the sub-layers are coupled ferromagnetically. This is due to a stabilization of the Néel domain wall structure, and an exchange coupling torque that is directly proportional to the strength of the antiferromagnetic exchange coupling between the two sub-layers. Moreover, the dependence of the wall velocity on the magnetic field applied along the nanowire is distinct from that of the single-layer racetrack due to the exchange coupling torque. The high domain wall velocities in racetracks that have no net magnetization allow for densely packed yet highly efficient domain-wall-based spintronics. [ABSTRACT FROM AUTHOR]

Spin-polarized currents provide a powerful means of manipulating the magnetization of nanodevices, and give rise to spin transfer torques that can drive magnetic domain walls along nanowires. In ultrathin magnetic wires, domain walls are found to move in the opposite direction to that expected from bulk spin transfer torques, and also at much higher speeds. Here we show that this is due to two intertwined phenomena, both derived from spin-orbit interactions. By measuring the influence of magnetic fields on current-driven domain-wall motion in perpendicularly magnetized Co/Ni/Co trilayers, we find an internal effective magnetic field acting on each domain wall, the direction of which alternates between successive domain walls. This chiral effective field arises from a Dzyaloshinskii-Moriya interaction at the Co/Pt interfaces and, in concert with spin Hall currents, drives the domain walls in lock-step along the nanowire. Elucidating the mechanism for the manipulation of domain walls in ultrathin magnetic films will enable the development of new families of spintronic devices. [ABSTRACT FROM AUTHOR]

Understanding the field-driven microscopic magnetization reversal in a disordered ferromagnetic (FM) film is a fundamentally intriguing issue for the application of a disorder-induced magnetic memory. We present the field dependence of the correlation between local coercivity and switching time in the FM films showing two typical reversal processes: wall-motion and nucleation dominant ones. As the applied field strength increases, the correlation coefficient in the wall-motion sample decreases, whereas the one in the nucleation sample holds nearly zero value. The change in the wall-motion sample is well explained by the transition of the magnetization reversal mechanism from a thermal activation reversal to a viscous motion. This paper shows that the magnetization reversal mechanism depending on the time scale as well as the materials has a strong influence on the correlation. [ABSTRACT FROM PUBLISHER]

Low light-level ultraviolet and optical imaging with a photon counting image intensifier coupled to a charge coupled device camera generally results in varying levels of fixed pattern noise in the image. Here, we demonstrate that this can be minimized by the appropriate choice of photon event centroiding algorithm. We compare the fixed pattern noise generated by a center of gravity centroiding algorithm, a Gaussian centroiding algorithm, and a hybrid centroiding algorithm which uses center of gravity centroiding when one wing is zero, and Gaussian centroiding otherwise. This approach yields the best image quality with a lower fixed pattern noise parameter (9.99%) than the sole use of Gaussian centroiding (16.4%), and there is no need for a look-up table correction. In addition, the hybrid algorithm also yields maximum detective quantum efficiency by overcoming small pulse centroiding failure associated with Gaussian centroiding. The digitization error when recording the events is modeled with a Monte Carlo simulation and discussed. It is found that a center of gravity algorithm produces not only significant fixed pattern noise, but also pulse height dependent &xmacr; positions. For a Gaussian centroiding algorithm the &xmacr; positions are independent of the pulse height, the fixed pattern noise is low and the digitization error only yields a small increase of the fixed pattern noise parameter. This shows that while there is a limit to centroiding accuracy due to the digitization error, the appropriate choice of centroiding algorithm is a much more important factor to minimize fixed pattern noise. [ABSTRACT FROM AUTHOR]

We report a large converse magnetoelectric (CME) effect at room temperature in a multiferroic heterostructure formed from thin layers of perpendicularly magnetized CoxPd1-x alloys deposited on a piezoelectric single-crystal of lead magnesium niobate-lead titanate PMN-PT(001). The CME results from a strain-induced reorientation of the CoPd magnetization. By varying the composition and thickness of the CoxPd1-x film, a large converse magnetoelectric coupling constant, α=8×10-7 s/m, at room temperature was found for 10 nm Co0.25Pd0.75. This large CME effect results from combining a highly magnetostrictive CoPd alloy with highly piezoelectric PMN-PT. [ABSTRACT FROM AUTHOR]

The authors report the change of the magnetic domain structure, dependent on the film thickness of MnAs films epitaxially grown on GaAs(001), investigated by magnetic force microscopy. Interestingly, as the film thickness decreases, the domain structure within the ferromagnetic α-MnAs stripes changes from a head-on domain structure to a simple 180° one around a thickness of 250 nm. This result is understood by the change in the demagnetizing factor of the ferromagnetic stripes with the film thickness. [ABSTRACT FROM AUTHOR]

In this study, we investigated the stochastic nature of domain reversal dynamics in exchange-biased IrMn/CoFe film using a time-resolved Kerr microscope. Interestingly, the statistical distributions of the magnetization half-reversal times for both forward and backward applied fields show that the magnetization reversal dynamics is much more stochastic for the backward branch, where an applied field is parallel to the exchange-bias field direction. The enhanced stochasticity is ascribed to the large degree of magnetic disorder during backward reversal, which induces discrete and random Barkhausen jumps, whereas the forward branch reversal is dominated by a thermally activated depinning process caused by a single potential barrier. This result can be explained by the asymmetry of the magnetic disorder between both branches of a hysteresis loop. [ABSTRACT FROM AUTHOR]

We have investigated the Barkhausen critical scaling behavior of NixFe1-x (x = 0-0.5) films using a magneto-optical microscope, capable of time-resolved domain observation. Real-time direct observations of the domain evolutions in these films revealed that magnetization reversal occurred with a sequence of random Barkhausen jumps. From more than 1000 repeated experiments with each sample, it was found that the distribution of the Barkhausen jump size followed a power-law distribution. The scaling exponent of the power-law distribution was found to have the same value of 1.1, independent of the film composition, revealing a universal critical scaling behavior in these alloy films. [ABSTRACT FROM AUTHOR]

Kerr microscopy is used to investigate domain wall motion in response to nanosecond-long current pulses in perpendicularly magnetized micron-sized Co/Ni/Co racetracks. Domain wall velocities greater than 300 m/s are observed. The velocity is independent of the pulse length for a wide range of current densities. However, the domain wall dynamics depends on the pulse length just above the threshold current for motion, where slow creep motion occurs, and at very high current densities, where domain nucleation takes place. We also observe a tilting of the domain wall that cannot be accounted for by the Oersted field from the driving current. [ABSTRACT FROM AUTHOR]