Unusual Magnetic Hysteresis and Transition between Vortex and Double Pole States Arising from Interlayer Coupling in Diamond-Shaped Nanostructures.

Controlling the magnetic ground states at the nanoscale is a long-standing basic research problem and an important issue in magnetic storage technologies. Here, we designed a nanostructured material that exhibits very unusual hysteresis loops due to a transition between vortex and double pole states. Arrays of 700 nm diamond-shaped nanodots consisting of Py(30 nm)/Ru(tRu)/Py(30 nm) (Py, permalloy (Ni80Fe20)) trilayers were fabricated by interference lithography and e-beam evaporation. We show that varying the Ru interlayer spacer thickness (tRu) governs the interaction between the Py layers. We found this interaction mainly mediated by two mechanisms: magnetostatic interaction that favors antiparallel (antiferromagnetic, AFM) alignment of the Py layers and exchange interaction that oscillates between ferromagnetic (FM) and AFM couplings. For a certain range of Ru thicknesses, FM coupling dominates and forms magnetic vortices in the upper and lower Py layers. For Ru thicknesses at which AFM coupling dominates, the magnetic state in remanence is a double pole structure. Our results showed that the interlayer exchange coupling interaction remains finite even at 4 nm Ru thickness. The magnetic states in remanence, observed by magnetic force microscopy (MFM), are in good agreement with corresponding hysteresis loops obtained by the magneto-optic Kerr effect (MOKE) and micromagnetic simulations.

Realization of macroscopic ratchet effect based on nonperiodic and uneven potentials.

Ratchet devices allow turning an ac input signal into a dc output signal. A ratchet device is set by moving particles driven by zero averages forces on asymmetric potentials. Hybrid nanostructures combining artificially fabricated spin ice nanomagnet arrays with superconducting films have been identified as a good choice to develop ratchet nanodevices. In the current device, the asymmetric potentials are provided by charged Néel walls located in the vertices of spin ice magnetic honeycomb array, whereas the role of moving particles is played by superconducting vortices. We have experimentally obtained ratchet effect for different spin ice I configurations and for vortex lattice moving parallel or perpendicular to magnetic easy axes. Remarkably, the ratchet magnitudes are similar in all the experimental runs; i. e. different spin ice I configurations and in both relevant directions of the vortex lattice motion. We have simulated the interplay between vortex motion directions and a single asymmetric potential. It turns out vortices interact with uneven asymmetric potentials, since they move with trajectories crossing charged Néel walls with different orientations. Moreover, we have found out the asymmetric pair potentials which generate the local ratchet effect. In this rocking ratchet the particles (vortices) on the move are interacting each other (vortex lattice); therefore, the ratchet local effect turns into a global macroscopic effect. In summary, this ratchet device benefits from interacting particles moving in robust and topological protected type I spin ice landscapes.

Little-Parks effect governed by magnetic nanostructures with out-of-plane magnetization.

Little-Parks effect names the oscillations in the superconducting critical temperature as a function of the magnetic field. This effect is related to the geometry of the sample. In this work, we show that this effect can be enhanced and manipulated by the inclusion of magnetic nanostructures with perpendicular magnetization. These magnetic nanodots generate stray fields with enough strength to produce superconducting vortex-antivortex pairs. So that, the L-P effect deviation from the usual geometrical constrictions is due to the interplay between local magnetic stray fields and superconducting vortices. Moreover, we compare our results with a low-stray field sample (i.e. with the dots in magnetic vortex state) showing how the enhancement of the L-P effect can be explained by an increment of the effective size of the nanodots.

Topologically protected superconducting ratchet effect generated by spin-ice nanomagnets.

We have designed, fabricated and tested a robust superconducting ratchet device based on topologically frustrated spin ice nanomagnets. The device is made of a magnetic Co honeycomb array embedded in a superconducting Nb film. This device is based on three simple mechanisms: (i) the topology of the Co honeycomb array frustrates in-plane magnetic configurations in the array yielding a distribution of magnetic charges which can be ordered or disordered with in-plane magnetic fields, following spin ice rules; (ii) the local vertex magnetization, which consists of a magnetic half vortex with two charged magnetic Néel walls; (iii) the interaction between superconducting vortices and the asymmetric potentials provided by the Néel walls. The combination of these elements leads to a superconducting ratchet effect. Thus, superconducting vortices driven by alternating forces and moving on magnetic half vortices generate a unidirectional net vortex flow. This ratchet effect is independent of the distribution of magnetic charges in the array.

Magnetic order and disorder in nanomagnets probed by superconducting vortices.

We have studied two nanomagnet systems with strong (Co/Pd multilayers) and weak (NdCo alloy films) stray magnetic fields by probing the out-of-plane magnetic states with superconducting vortices. The hybrid samples are made of array of nanomagnets embedded in superconducting Nb thin films. The vortex motion detects relevant magnetic state features, since superconducting vortices are able to discriminate between different magnetic stray field strengths and directions. The usual matching effect between the superconducting vortex lattice and the periodic pinning array can be quenched by means of disorder magnetic potentials with strong stray fields at random. Ordered stray fields retrieve the matching effect and yield asymmetry and shift in the vortex dissipation signal. Furthermore vortices can discriminate the sizes of the nanomagnet magnetic domains, detecting magnetic domain sizes as small as 70 nm. In addition, we observe that the vortex cores play the crucial role instead of the supercurrents around the vortex.

Superconducting/magnetic Three-state Nanodevice for Memory and Reading Applications.

We present a simple nanodevice that can operate in two modes: i) non-volatile three-state memory and ii) reading device. The nanodevice can retain three well defined states -1, 0 and +1 and can operate in a second mode as a sensor for external magnetic fields. The nanodevice is fabricated with an array of ordered triangular-shaped nanomagnets embedded in a superconducting thin film gown on Si substrates. The device runs based on the combination of superconducting vortex ratchet effect (superconducting film) with the out of plane magnetization (nanomagnets). The input signals are ac currents and the output signal are dc voltages. The memory mode is realized without applying a magnetic field and the nanomagnet stray magnetic fields govern the effect. In the sensor mode an external magnetic field is applied. The main characteristic of this mode is that the output signal is null for a precise value of the external magnetic field that only depends on the fabrication characteristics of the nanodevice.

Quantitative decoding of interactions in tunable nanomagnet arrays using first order reversal curves.

To develop a full understanding of interactions in nanomagnet arrays is a persistent challenge, critically impacting their technological acceptance. This paper reports the experimental, numerical and analytical investigation of interactions in arrays of Co nanoellipses using the first-order reversal curve (FORC) technique. A mean-field analysis has revealed the physical mechanisms giving rise to all of the observed features: a shift of the non-interacting FORC-ridge at the low-HC end off the local coercivity HC axis; a stretch of the FORC-ridge at the high-HC end without shifting it off the HC axis; and a formation of a tilted edge connected to the ridge at the low-HC end. Changing from flat to Gaussian coercivity distribution produces a negative feature, bends the ridge, and broadens the edge. Finally, nearest neighbor interactions segment the FORC-ridge. These results demonstrate that the FORC approach provides a comprehensive framework to qualitatively and quantitatively decode interactions in nanomagnet arrays.

Rocking ratchets in nanostructured superconducting-magnetic hybrids.

Two rectification mechanisms in vortex lattice dynamics in Nb films have been studied. These two effects are based on ratchet effects, that is, an ac driving force induces a net dc vortex flow. In our case, an input ac current applied to the Nb films, grown on top of arrays of Ni nanotriangles, yields an output dc voltage. These two rectification effects occur when the vortex lattice moves in periodic asymmetric potentials. These pinning potentials are induced by the array of Ni triangles. In one configuration (longitudinal effect) the driven force is applied perpendicular to the triangle reflection symmetry axis; in the second one (transverse effect) the input current is injected parallel to the triangle reflection symmetry axis. In the framework of the rocking ratchet mechanism, the appropriate Langevin equation allows us to model the experimental data, taking into account the vortex-vortex interaction.

Field induced vortex dynamics in magnetic Ni nanotriangles.

The magnetization states in Ni triangular dots under an applied magnetic field have been studied using variable-field magnetic force microscopy (VF-MFM) imaging. In order to understand their dynamics we performed micromagnetic simulations which are in remarkable agreement with the experimental MFM results. The nanostructures present magnetic vortices as ground states which move under an external magnetic field. The combination of micromagnetic simulations and MFM imaging allows us to identify correctly the vortex chiralities and polarizations. The triangular geometry produces an improved contrast of the vortex core. Additionally, the vortices of different chiralities present clearly different MFM images under an applied field.

A superconducting reversible rectifier that controls the motion of magnetic flux quanta.

We fabricated a device that controls the motion of flux quanta in a niobium superconducting film grown on an array of nanoscale triangular pinning potentials. The controllable rectification of the vortex motion is due to the asymmetry of the fabricated magnetic pinning centers. The reversal in the direction of the vortex flow is explained by the interaction between the vortices trapped on the magnetic nanostructures and the interstitial vortices. The applied magnetic field and input current strength can tune both the polarity and magnitude of the rectified vortex flow. Our ratchet system is explained and modeled theoretically, taking the interactions between particles into consideration.

Scaling of the interface roughness in Fe-Cr superlattices: self-affine versus non-self-affine.

We have analyzed kinetic roughening in Fe-Cr superlattices by energy-filtered transmission electron microscopy. The direct access to individual interfaces provides both static and dynamic roughness exponents. We find an anomalous non-self-affine scaling of the interface roughness with a time dependent local roughness at short length scales. While the deposition conditions affect strongly the long-range dynamics, the anomalous short-range exponent remains unchanged. The different short- and long-range dynamics outline the importance of long-range interactions in kinetic roughening.