46 research outputs found
Switching of +/-360deg domain wall states in a nanoring by an azimuthal Oersted field
We demonstrate magnetic switching between two domain wall vortex
states in cobalt nanorings, which are candidate magnetic states for robust and
low power MRAM devices. These 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
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
Evidence for phonon skew scattering in the spin Hall effect of platinum
\u3cp\u3eWe measure and analyze the effective spin Hall angle of platinum in the low-residual resistivity regime by second-harmonic measurements of the spin-orbit torques for a multilayer of Pt|Co|AlOx. An angular-dependent study of the torques allows us to extract the effective spin Hall angle responsible for the damping-like torque in the system. We observe a strikingly nonmonotonic and reproducible temperature dependence of the torques. This behavior is compatible with recent theoretical predictions which include both intrinsic and extrinsic (impurities and phonons) contributions to the spin Hall effect at finite temperatures.\u3c/p\u3
Disorder-independent control of magnetic monopole defect population in artificial spin-ice honeycombs
Breakdown of the ice rule in artificial spin-ice nanostructures results in magnetic monopole defects with zero magnetic moment. Such defects exist during the magnetic switching process in some nanostructures and yet are absent in other apparently similar arrays having the same geometry and made from the same material components. One explanation proposed for this discrepancy is that it is due to the variation of disorder across samples, with monopole defect formation occuring only in highly disordered samples. Although disorder can indeed play a role in the determination of monopole density, in this paper we show, by experiment and simulation, that in samples of similar, low disorder, the factor controlling the nature of magnetic switching is whether the domain walls are in the transverse wall regime or in the vortex wall regime. This work illustrates that monopole formation can be controlled by intrinsic micro-magnetic behaviour as well as by extrinsic quenched disorder
On the aromagnetism and anapole moment of anthracene nanocrystals
We report the results of the study of orientation of small anthracene crystals (suspended in liquid) by static and oscillating magnetic fields. They contradict the previous claim that magnetic response of such crystals displays aromagnetism associated with the static toroidal (anapole) moments of the aromatic molecule. The existence of strong aromagnetism in anthracene would be a fundamental result of high importance for the understanding of interactions on the molecular and bio-molecular levels. Instead, our observations are fully consistent with conventional diamagnetism of anthracene molecules
Self-Similar Nested Flux Closure Structures in a Tetragonal Ferroelectric
In specific solid-state materials, under the right conditions, collections of magnetic dipoles are known to spontaneously form into a variety of rather complex geometrical patterns, exemplified by vortex and skyrmion structures. While theoretically, similar patterns should be expected to form from electrical dipoles, they have not been clearly observed to date: the need for continued experimental exploration is therefore clear. In this Letter we report the discovery of a rather complex domain arrangement that has spontaneously formed along the edges of a thin single crystal ferroelectric sheet, due to surface-related depolarizing fields. Polarization patterns are such that nanoscale "flux-closure" loops are nested within a larger mesoscale flux closure object Despite the orders of magnitude differences in size, the geometric forms of the dual-scale flux closure entities are rather similar.</p