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.

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.

Domain walls can be driven by current at very high speeds in nanowires formed from ultra-thin, perpendicularly magnetized cobalt layers and cobalt/nickel multilayers deposited on platinum underlayers due to a chiral spin torque. An important feature of this torque is a magnetic chiral exchange field that each domain wall senses and that can be measured by the applied magnetic field amplitude along the nanowire where the domain walls stop moving irrespective of the magnitude of the current. Here we show that this torque is manifested when the magnetic layer is interfaced with metals that display a large proximity-induced magnetization, including iridium, palladium and platinum but not gold. A correlation between the strength of the chiral spin torque and the proximity-induced magnetic moment is demonstrated by interface engineering using atomically thin dusting layers. High domain velocities are found where there are large proximity-induced magnetizations in the interfaced metal layers.

A ferromagnetic material shows a sequence of discrete and jerky domain jumps, known as the Barkhausen avalanche 1,2 , in the presence of an external magnetic field. Studies of Barkhausen avalanches reveal power-law scaling behaviour that suggests an underlying criticality 3-8 , as observed in a wide variety of systems such as superconductor vortices 9 , microfractures 10 , earthquakes 11 , lung inflations 12 , mass extinctions 13 , financial markets 14 and charge-density waves 15 . The most interesting unsolved fundamental question is whether the universality in the scaling exponent holds regardless of the material and its detailed microstructure. Here we show that the scaling behaviour of Barkhausen criticality in a given ferromagnetic film is experimentally tunable by varying the temperature (not dimensionality). We observe for the first time that the scaling behaviour in the Barkhausen criticality of a given system crosses over between two universality classes when the relative contributions from the dipolar interaction and domain-wall energies are altered by an experimental parameter.All theoretical works so far predict that the universality of the scaling exponent depends only on the dimensionality of a system, even though the value of the scaling exponent varies according to the theory [16][17][18][19][20][21][22] . However, the measured scaling exponents reported in the literature span a relatively wide range of values despite the same dimensionality 3-8 . Thus, the universality has been questioned, and our understanding is far from complete. To test the validity of the universality of Barkhausen criticality, a desirable approach is to make systematic measurements of the scaling exponent under well-controlled experimental conditions with reliable statistics in a given system while maintaining the same dimensionality. A ferromagnetic (FM) MnAs film on GaAs(001) substrate is considered an ideal system for such purposes: it reveals a systematic variation of domain-evolution patterns with temperature during a Barkhausen avalanche, a variation that results from the decrease in the saturation magnetization M S with temperature 23 . Thus, scaling behaviour in different domain-evolution patterns in a given system can be investigated in an experimentally controllable manner. Figure 1 shows representative domain-evolution patterns of the 50 nm MnAs film observed three consecutive times by means of a magneto-optical microscope magnetometer (MOMM) at each designated temperature in a range of 20−35 • C. In Fig. 1, we can clearly see that the domain evolution patterns at each temperature show a sequence of discrete and jerky domain jumps during the magnetization reversal. Also, we found that the domain jumps proceed with randomness of interval, size and location for the repeated experiments, as clearly seen from the three representative domain images at each temperature. From this evidence, it Figure 1 Representative domain-evolution patterns at several temperatures in the temperature range of 20−35 • C. These pattern...

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.

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.

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We demonstrate a highly efficient and simple scheme for injecting domain walls into magnetic nanowires. The spin transfer torque from nanosecond long, unipolar, current pulses that cross a 90° magnetization boundary together with the fringing magnetic fields inherently prevalent at the boundary, allow for the injection of single or a continual stream of domain walls. Remarkably, the currents needed for this "in-line" domain wall injection scheme are at least one hundred times smaller than conventional methods.