Héctor Corte-León

Anisotropic Magnetoresistance State Space of Permalloy Nanowires with Domain Wall Pinning Geometry

The domain wall-related change in the anisotropic magnetoresistance in L-shaped permalloy nanowires is measured as a function of the magnitude and orientation of the applied magnetic field. The magnetoresistance curves, compiled into so-called domain wall magnetoresistance state space maps, are used to identify highly reproducible transitions between domain states. Magnetic force microscopy and micromagnetic modelling are correlated with the transport measurements of the devices in order to identify different magnetization states. Analysis allows to determine the optimal working parameters for specific devices, such as the minimal field required to switch magnetization or the most appropriate angle for maximal separation of the pinning/depinning fields. Moreover, the complete state space maps can be used to predict evolution of nanodevices in magnetic field without a need of additional electrical measurements and for repayable initialization of magnetic sensors into a well-specified state.

Magnetic bead detection using domain wall-based nanosensor

We investigate the effect of a single magnetic bead (MB) on the domain wall (DW) pinning/depinning fields of a DW trapped at the corner of an L-shaped magnetic nanodevice. DW propagation across the device is investigated using magnetoresistance measurements. DW pinning/depinning fields are characterized in as-prepared devices and after placement of a 1 μm-sized MB (Dynabeads® MyOne™) at the corner. The effect of the MB on the DW dynamics is seen as an increase in the depinning field for specific orientations of the device with respect to the external magnetic field. The shift of the depinning field, ΔBdep = 4.5–27.0 mT, is highly stable and reproducible, being significantly above the stochastic deviation which is about 0.5 mT. The shift in the deppinning field is inversely proportional to the device width and larger for small negative angles between the device and the external magnetic field. Thus, we demonstrate that DW-based devices can be successfully used for detection of single micron size MB.

Simultaneous magnetoresistance and magneto-optical measurements of domain wall properties in nanodevices

Simultaneous anisotropic magnetoresistance and magneto-optical Kerr effect measurements have been performed on L-shaped Permalloy nanowires. It is shown that through magnetoresistance measurements at the corner of the device, the switching information of both arms in a single loop can be accessed. This is a very efficient method that allows for the characterization of the pinning properties in such a device as well as the understanding of the fundamental physics behind the nucleation and propagation processes in such a system. Experimental results are in good agreement with micromagnetic simulations.

V-Shaped Domain Wall Probes for Calibrated Magnetic Force Microscopy

Magnetic force microscopy (MFM) qualitatively resolves stray magnetic fields, but its fundamental flaws include limited quantitative analysis and difficulties in measuring samples with heterogeneous magnetic areas. We propose a custom-made domain wall probe (DWP) with a

Calibration of multi-layered probes with low/high magnetic moments

V

Influence of Geometry on Domain Wall Dynamics in Permalloy Nanodevices

-shaped magnetic nanostructure on one face of a non-magnetic probe, which behaves as a low moment probe with high coercivity to reduce magnetic switching in the presence of strong stray fields. The performance of the DWP is compared against commercial standard and low moment probes with different approaches to quantify resolution from striped domain structures of a thin reference film. The three probes are calibrated by acquiring the tip-transfer function (TTF) from a Fourier transform approach. The calculated TTF is used to predict the MFM response from a permalloy nanostructure and compared to experimental results.

Tailoring of Domain Wall Devices for Sensing Applications

We present a comprehensive method for visualisation and quantification of the magnetic stray field of magnetic force microscopy (MFM) probes, applied to the particular case of custom-made multi-layered probes with controllable high/low magnetic moment states. The probes consist of two decoupled magnetic layers separated by a non-magnetic interlayer, which results in four stable magnetic states: ±ferromagnetic (FM) and ±antiferromagnetic (A-FM). Direct visualisation of the stray field surrounding the probe apex using electron holography convincingly demonstrates a striking difference in the spatial distribution and strength of the magnetic flux in FM and A-FM states. In situ MFM studies of reference samples are used to determine the probe switching fields and spatial resolution. Furthermore, quantitative values of the probe magnetic moments are obtained by determining their real space tip transfer function (RSTTF). We also map the local Hall voltage in graphene Hall nanosensors induced by the probes in different states. The measured transport properties of nanosensors and RSTTF outcomes are introduced as an input in a numerical model of Hall devices to verify the probe magnetic moments. The modelling results fully match the experimental measurements, outlining an all-inclusive method for the calibration of complex magnetic probes with a controllable low/high magnetic moment.

Detection of a magnetic bead by hybrid nanodevices using scanning gate microscopy

We perform magnetoresistance (MR) and magneto-optical Kerr effect (MOKE) measurements to experimentally track magnetic domain wall (DW) pinning/depinning process in L-shaped Permalloy nanostructures of different geometries (i.e., square/round corner and absence/presence of circular disks) with widths in the range 75-400 nm. With MR and MOKE measurements being in a good quantitative agreement, we demonstrate that the field interval between pinning and depinning events increases with reduction of the nanowire width. In addition, the DW pinning/depinning behavior is geometry dependent, where round corners break the symmetry in the device orientation with respect to the field. The interpretation of experimental results is supported by micromagnetic simulations.

Current induced domain wall motion in cylindrical nanowires

We perform magnetoresistance (MR) measurements to experimentally track magnetic domain wall pinning/depinning process in L-shaped Permalloy (Py) nanostructures with widths in the range 50-400 nm. We demonstrate that the field interval between the pinning/depinning events increases with the reduction of the nanowire width. The most reproducible measurements are obtained from the narrowest devices. MR measurements reveal that the stochastic contribution to pinning/depinning processes is higher when the applied field is oriented symmetrically with respect to both arms of the device. The interpretation of experimental results is supported by micromagnetic simulations.

Hybrid normal metal/ferromagnetic nanojunctions for domain wall tracking

Hybrid ferromagnetic(Py)/non-magnetic metal(Au) junctions with a width of 400 nm are studied by magnetotransport measurements, magnetic scanning gate microscopy (SGM) with a magnetic bead (MB) attached to the probe, and micromagnetic simulations. In the transverse geometry, the devices demonstrate a characteristic magnetoresistive behavior that depends on the direction of the in plane magnetic field, with minimum/maximum variation when the field is applied parallel/perpendicular to the Py wire. The SGM is performed with a NdFeB bead of 1.6 μm diameter attached to the scanning probe. Our results demonstrate that the hybrid junction can be used to detect this type of MB. A rough approximation of the sensing volume of the junction has the shape of elliptical cylinder with the volume of ∼1.51 μm3. Micromagnetic simulations coupled to a magnetotransport model including anisotropic magnetoresistance and planar Hall effects are in good agreement with the experimental findings, enabling the interpretation of the ...