Magneto-Optical Imaging of Magnetic-Domain Pinning Induced by Chiral Molecules

Chiral molecules have the potential for creating new magnetic devices by locally manipulating the magnetic properties of metallic surfaces. When chiral polypeptides chemisorb onto ferromagnets they can induce magnetization locally by spin exchange interactions. However, direct imaging of surface magnetization changes induced by chiral molecules was not previously realized. Here, we use Magneto-optical Kerr microscopy to image domains in thin films and show that chiral polypeptides strongly pin domains, increasing the coercive field locally. In our study, we also observe a rotation of the easy magnetic axis towards the out-of-plane, depending on the sample's domain size and the adsorption area. These findings show the potential of chiral molecules to control and manipulate magnetization and open new avenues for future research on the relationship between chirality and magnetization.Comment: 11 pages, 4 figure

Effects of interdot dipole coupling in mesoscopic epitaxial Fe(100) dot arrays

The domain structure and the coercivity of epitaxial Fe(100) circular dot arrays of different diameters and separations have been studied using magnetic force microscopy (MFM) and focused magneto-optical Kerr effect (MOKE). The MFM images of the 1 µm diameter single domain dot arrays show direct evidence of strong interdot dipole coupling when the separation is reduced down to 0.1 µm. The coercivity of the dots is also found to be dependent on the separation, indicating the effect of the interdot dipole coupling on the magnetization reversal process

Current-induced domain wall motion including thermal effects based on Landau-Lifshitz-Bloch equation

We employ the Landau-Lifshitz-Bloch (LLB) equation to investigate current-induced domain wall motion at finite temperatures by numerical micromagnetic simulations. We extend the LLB equation with spin torque terms that account for the effect of spin-polarized currents and we find that the velocities depend strongly on the interplay between adiabatic and non-adiabatic spin torque terms. As a function of temperature, we find non-monotonous behavior, which might be useful to determine the relative strengths of the spin torque terms experimentally.Comment: 20 page, 8 figure

Switching of +/-360deg domain wall states in a nanoring by an azimuthal Oersted field

We demonstrate magnetic switching between two $360^\circ$ domain wall vortex states in cobalt nanorings, which are candidate magnetic states for robust and low power MRAM devices. These $360^\circ$ domain wall (DW) or "twisted onion" states can have clockwise or counterclockwise circulation, the two states for data storage. Reliable switching between the states is necessary for any realistic device. We accomplish this switching by applying a circular Oersted field created by passing current through a metal atomic force microscope tip placed at the center of the ring. After initializing in an onion state, we rotate the DWs to one side of the ring by passing a current through the center, and can switch between the two twisted states by reversing the current, causing the DWs to split and meet again on the opposite side of the ring. A larger current will annihilate the DWs and create a perfect vortex state in the rings.Comment: 5 pages, 5 figure

Magnetization Vorticity and Exchange Bias Phenomena in Arrays of Small Asymmetric Magnetic Rings

Arrays of nanoscopic magnetic asymmetric rings, 150 nm in outer diameter, are fabricated using the techniques of electron-beam lithography, angular deposition and ion-beam etching. Magnetic measurements for cobalt asymmetric rings at room temperature verifies previous reports of vortex magnetic state formation of a desired circulation direction for the application of external magnetic field along the asymmetry axis of the rings. However, the main theme of this article is the observation of exchange bias phenomena when the ring samples are cooled down to low temperature in the presence of a positive magnetic field. Very interestingly, the observed exchange bias effect is negative for along and perpendicular orientations of ring's asymmetry axis with respect to the in-plane external magnetic field. This is in good quantitative agreement with the random interface model proposed by Malozemoff et al. For the application of inplane external magnetic field at 45 degree with respect to the asymmetry axis, the exchange bias effect is positive. Unlike the exchange bias effects in thin films, this is a very unusual observation indicating that exchange bias phenomena of opposite natures can be manipulated by appropriate combination of geometrical constraint and external magnetic field direction, in addition to the interfacial interactions between ferromagnetic (FM) and antiferromagnetic (AFM) layer.Comment: Asymmetric magnetic rings arrays; Exchange bias phenomen

Tailoring the magnetic properties of Fe asymmetric nanodots

Asymmetric dots as a function of their geometry have been investigated using three-dimensional (3D) object oriented micromagnetic framework (OOMMF) code. The effect of shape asymmetry of the disk on coercivity and remanence is studied. Angular dependence of the remanence and coercivity is also addressed. Asymmetric dots are found to reverse their magnetization by nucleation and propagation of a vortex, when the field is applied parallel to the direction of asymmetry. However, complex reversal modes appear when the angle at which the external field is applied is varied, leading to a non monotonic behavior of the coercivity and remanence.Comment: 5 pages, 7 figure

Perspective on unconventional computing using magnetic skyrmions

Learning and pattern recognition inevitably requires memory of previous events, a feature that conventional CMOS hardware needs to artificially simulate. Dynamical systems naturally provide the memory, complexity, and nonlinearity needed for a plethora of different unconventional computing approaches. In this perspective article, we focus on the unconventional computing concept of reservoir computing and provide an overview of key physical reservoir works reported. We focus on the promising platform of magnetic structures and, in particular, skyrmions, which potentially allow for low-power applications. Moreover, we discuss skyrmion-based implementations of Brownian computing, which has recently been combined with reservoir computing. This computing paradigm leverages the thermal fluctuations present in many skyrmion systems. Finally, we provide an outlook on the most important challenges in this field.Comment: 19 pages and 3 figure

Poly[μ3-aqua-μ2-2,4-dinitro­phenolato-rubidium(I)]

The asymmetric unit of the title compound, [Rb(C6H3N2O5)(H2O)]n, comprises a rubidium cation, a 2,4-dinitro­phenoxide anion and a water mol­ecule. The Rb+ cation is 11-coordinated by O atoms from 2,4-dinitro­phenolate anions and water mol­ecules. The metal centre is firstly coordinated by two μ3-H2O to form a one-dimensional ladder-shaped unit, [Rb2(μ3-H2O)2], which is further linked by 2,4-dinitro­phenolate to give the three-dimensional framework of the title compound. The crystal structure involves O—H⋯O hydrogen bonds

Hysteresis mediated by a domain wall motion

The position of an interface (domain wall) in a medium with random pinning defects is not determined unambiguously by a current value of the driving force even in average. Based on general theory of the interface motion in a random medium we study this hysteresis, different possible shapes of domain walls and dynamical phase transitions between them. Several principal characteristics of the hysteresis, including the coercive force and the curves of dynamical phase transitions obey scaling laws and display a critical behavior in a vicinity of the mobility threshold. At finite temperature the threshold is smeared and a new range of thermally activated hysteresis appears. At a finite frequency of the driving force there exists a range of the non-adiabatic regime, in which not only the position, but also the average velocity of the domain wall displays hysteresis

Vertical current induced domain wall motion in MgO-based magnetic tunnel junction with low current densities

Shifting electrically a magnetic domain wall (DW) by the spin transfer mechanism is one of the future ways foreseen for the switching of spintronic memories or registers. The classical geometries where the current is injected in the plane of the magnetic layers suffer from a poor efficiency of the intrinsic torques acting on the DWs. A way to circumvent this problem is to use vertical current injection. In that case, theoretical calculations attribute the microscopic origin of DW displacements to the out-of-plane (field-like) spin transfer torque. Here we report experiments in which we controllably displace a DW in the planar electrode of a magnetic tunnel junction by vertical current injection. Our measurements confirm the major role of the out-of-plane spin torque for DW motion, and allow to quantify this term precisely. The involved current densities are about 100 times smaller than the one commonly observed with in-plane currents. Step by step resistance switching of the magnetic tunnel junction opens a new way for the realization of spintronic memristive devices