Reliable Propagation of Magnetic Domain Walls in Cross Structures for Advanced Multiturn Sensors

[EN] We develop and analyze an advanced concept for a domain-wall-based sensing of rotations. Moving domain walls in n closed loops with n - 1 intersecting convolutions by rotating fields, we are able to sense n rotations. By combining loops with coprime numbers of rotations, we create a sensor system allowing for the total counting of millions of turns of a rotating applied magnetic field. We analyze the operation of the sensor and identify the intersecting cross structures as the critical component for reliable operation. Specifically, depending on the orientation of the applied field angle with the magnetization in the branches of the cross, a domain wall is found to propagate in an unwanted direction, yielding failures and counting errors in the device. To overcome this limiting factor, we introduce a specially designed syphon structure to the controlled pinning of the domain wall before the cross and depinning and propagation only for a selected range of applied field angles. By adjusting the syphon and the cross geometry, we find that the optimized combination of both structures prevents failures in the full sensor structure yielding robust operation. Our modeling results show that the optimized element geometry allows for the realization of the sensor with cross-shaped intersections and an operation that is tolerant to inaccuracies of the fabrication.Comisión Europea (P7-PEOPLE-2013-ITN 608031) Gobierno de España (MAT2014-52477-C5-4-P) Junta de Castilla y Leon (SA090U16) European Research Council (MultiRev ERC-2014-PoC 665672) German Research Foundation (SFB TRR173 Spin+X

Magnetooptical response of permalloy multilayer structures on different substrate in the IR-VIS-UV spectral range

The magnetooptical (MO) response of Ru/Py/Ta thin film stacks with 4, 8, and 17 nm thick Ni81Fe19 permalloy (Py) films on a SiO2/Si and a ZnO substrate was measured by vector magnetooptical generalized ellipsometry. The MO response from VMOGE was modelled using a 4  ×  4 Mueller matrix algorithm. The wavelength-dependent, substrate-independent and thickness-independent complex MO coupling constant (Q) of Py in the Ru/Py/Ta thin film stacks was extracted by fitting Mueller matrix difference spectra in the spectral range from 300 nm to 1000 nm. Although the composition-dependent saturation magnetization of NixFe1−x alloys (x  =  0.0...1.0), e.g. of Ni81Fe19, is predictable from the two saturation magnetization end points, the MO coupling constant of NixFe1−x is not predictable from the two Q end points. However, in a small alloy range (0.0  <  x  <  0.2 and 0.8  <  x  <  1.0) the composition-dependent Q of NixFe1−x can be interpolated from a sufficiently high number of analyzed NixFe1−x alloys. The available complex MO coupling constants of six different NixFe1−x (x  =  1.0 to 0.0) alloys were used to interpolate MO response of binary NixFe1−x alloys in the range from x  =  0.0 to x  =  1.0

Mega, Giga, Terra - was der Nobelpreis für Physik 2007 mit dem immer größerem Speichervolumen der Festplatten zu tun hat

Die rasante Entwicklung des Speichervolumens der Festplatten ist untrennbar mit der Entdeckung von neuartigen Effekten verbunden, für die die Herren Prof. Grünberg und Fert im Jahre 2007 den Nobelpreis für Physik erhielten. Sie begründeten mit ihren Entdeckungen eine neue Art von Elektronik, die Magnetoelektronik, bei der der Drall der Elektronen, der so genannte Spin, eine entscheidende Rolle spielt. Dabei werden Effekte genutzt, die in metallischen Viellagenschichtsystemen auftreten, bei denen die Dicke der einzelnen Lagen nur ein 5.000stel bis ein 50.000stel des Durchmessers eines menschlichen Haares beträgt. In der Vorlesung werden grundlegende Effekte sowie Anwendungen der Magnetoelektronik in einer allgemein verständlichen Form vorgestellt

Magnetic domain wall gratings for magnetization reversal tuning and confined dynamic mode localization

High density magnetic domain wall gratings are imprinted in ferromagnetic-antiferromagnetic thin films by local ion irradiation by which alternating head-to-tail-to-head-to-tail and head-to-head-to-tail-to-tail spatially overlapping domain wall networks are formed. Unique magnetic domain processes result from the interaction of anchored domain walls. Non-linear magnetization response is introduced by the laterally distributed magnetic anisotropy phases. The locally varying magnetic charge distribution gives rise to localized and guided magnetization spin-wave modes directly constrained by the narrow domain wall cores. The exchange coupled multiphase material structure leads to unprecedented static and locally modified dynamic magnetic material properties

Towards multiple readout application of plasmonic arrays

In order to combine the advantages of fluorescence and surface-enhanced Raman spectroscopy (SERS) on the same chip platform, a nanostructured gold surface with a unique design, allowing both the sensitive detection of fluorescence light together with the specific Raman fingerprint of the fluorescent molecules, was established. This task requires the fabrication of plasmonic arrays that permit the binding of molecules of interest at different distances from the metallic surface. The most efficient SERS enhancement is achieved for molecules directly adsorbed on the metallic surface due to the strong field enhancement, but where, however, the fluorescence is quenched most efficiently. Furthermore, the fluorescence can be enhanced efficiently by careful adjustment of the optical behavior of the plasmonic arrays. In this article, the simultaneous application of SERS and fluorescence, through the use of various gold nanostructured arrays, is demonstrated by the realization of a DNA detection scheme. The results shown open the way to more flexible use of plasmonic arrays in bioanalytics

Current-induced vortex dynamics and pinning potentials probed by homodyne detection

Using a homodyne detection scheme, we show that we can determine the polarity and chirality of a magnetic vortex in an asymmetric magnetic disk as well as the resonance frequency and phase shift of the dynamic vortex gyration excited by a spin-polarized current. From systematic phase measurements, we deduce the relative contributions of the spin torque and the Oersted field, which is found to dominate the excitation. Local pinning sites in the disk lead to an increased resonance frequency and a reduced amplitude. This allows us to draw a map of the pinning sites and thus to characterize the full potential in the disk

Direct imaging of current-induced domain wall motion in CoFeB structures

By direct x-ray photoemission electron microscopy imaging, we probe current-induced domain wall motion in 20 nm thick CoFeB wires. We observe transverse walls for all wire widths up to 1500 nm as a consequence of the small saturation magnetization of the material. High critical current densities above 1x10 12 A/m2 for wall displacement due to the spin transfer torque effect are found. The critical current densities jc increase further with decreasing wire width indicating that jc is governed by extrinsic pinning due to edge defects. In addition to wall displacements, we observe wall transformations to energetically favorable wall types due to heating. Owing to the high Curie temperature though, the sample temperature stays below the Curie temperature even for the highest current densities where structural damage sets in

Advanced giant magnetoresistance technology for measurement applications

Weiss R, Mattheis R, Reiss G. Advanced giant magnetoresistance technology for measurement applications. Measurement Science And Technology. 2013;24(8): 82001.Giant magnetoresistance (GMR) sensors are considered one of the first real applications of nanotechnology. They consist of nm-thick layered structures where ferromagnetic metals are sandwiched by nonmagnetic metals. Such multilayered films produce a large change in resistance (typically 10 to 20%) when subjected to a magnetic field, compared with a maximum change of a few per cent for other types of magnetic sensors. This technology has been intensively used in read heads for hard disk drives and now increasingly finds applications due to the high sensitivity and signal-to-noise ratio. Additionally these sensors are compatible with miniaturization and thus offer a high spatial resolution combined with a frequency range up to the 100 MHz regime and simple electronic conditioning. In this review, we first discuss the basics of the underlying magnetoresistance effects in layered structures and then present three prominent examples for future applications: in the field of current sensing the new GMR sensors offer high bandwidth and good accuracy in a space-saving open loop measurement configuration. In rotating systems they can be used for multiturn angle measurements, and in biotechnology the detection of magnetic particles enables the quantitative measurement of biomolecule concentrations