Benchtop time-resolved magneto-optical Kerr magnetometer

Magneto-Electronic Hydrogen Gas Sensors: A Critical Review

Devices enabling early detection of low concentrations of leaking hydrogen and precision measurements in a wide range of hydrogen concentrations in hydrogen storage systems are essential for the mass-production of fuel-cell vehicles and, more broadly, for the transition to the hydrogen economy. Whereas several competing sensor technologies are potentially suitable for this role, ultra-low fire-hazard, contactless and technically simple magneto-electronic sensors stand apart because they have been able to detect the presence of hydrogen gas in a range of hydrogen concentrations from 0.06% to 100% at atmospheric pressure with the response time approaching the industry gold standard of one second. This new kind of hydrogen sensors is the subject of this review article, where we inform academic physics, chemistry, material science and engineering communities as well as industry researchers about the recent developments in the field of magneto-electronic hydrogen sensors, including those based on magneto-optical Kerr effect, anomalous Hall effect and Ferromagnetic Resonance with a special focus on Ferromagnetic Resonance (FMR)-based devices. In particular, we present the physical foundations of magneto-electronic hydrogen sensors and we critically overview their advantages and disadvantages for applications in the vital areas of the safety of hydrogen-powered cars and hydrogen fuelling stations as well as hydrogen concentration meters, including those operating directly inside hydrogen-fuelled fuel cells. We believe that this review will be of interest to a broad readership, also facilitating the translation of research results into policy and practice.

Sagnac interferometer for time-resolved magneto-optical measurements

We introduce a time-resolved magneto-optical measurement technique based on a zero-area Sagnac interferometer. By replacing a continuous wave light source to a pulsed one, temporal resolution of hundreds of picoseconds is achieved. Because two lights passing through a Sagnac loop always travel the same optical path length, the interference from the phase modulation and Kerr rotation occurs in a pulse mode. For illustration of the apparatus, we present ferromagnetic resonance of a Permalloy film caused by a magnetic field pump. The instrument still possesses the favorable properties of a Sagnac interferometer, such as rejection of all the reciprocal effects, and shows 1μrad/Hz sensitivity at a 3 µW optical power in the pulse mode.

Applications of nanomagnets as dynamical systems: II

In Part I of this topical review, we discussed dynamical phenomena in nanomagnets, focusing primarily on magnetization reversal with an eye to digital applications. In this part, we address mostly wave-like phenomena in nanomagnets, with emphasis on spin waves in myriad nanomagnetic systems and methods of controlling magnetization dynamics in nanomagnet arrays which may have analog applications. We conclude with a discussion of some interesting spintronic phenomena that undergird the rich physics exhibited by nanomagnet assemblies.

Direct mapping of spin wave modes of individual Ni80Fe20 nanorings

Ferromagnetic nanorings exhibit tunable magnetic states with unique magnetization reversal processes and dynamic behavior that can be exploited in data storage and magnonic devices. Traditionally, probing the magnetization dynamics of individual ferromagnetic nanorings and mapping the resonance modes has proved challenging. In this study, micro-focused Brillouin light scattering spectroscopy is used to directly map the spin wave modes and their intensities in nanorings as a function of ring width and applied magnetic field. Micromagnetic simulations provide further insights into the experimental observations and are in good agreement with the experimental results. These results can help in improving the understanding of spin wave confinement in single elements for magnonic devices and waveguides.

Three-dimensional frequency- and phase-multiplexed magneto-optical microscopy

We describe a new approach to scanning magneto-optical Kerr effect (MOKE) microscopy in which two opto-mechanical choppers modulate the spatial profile of a probe laser beam to separately encode all three magnetization components at different frequencies and phases in one signal. We demonstrate this multiplexed technique in two representative regimes: the equilibrium and non-equilibrium response of a magnetic vortex to a changing magnetic field. We observe the translation of the vortex state in equilibrium and the spiraling gyrotropic trajectory of the vortex position out of equilibrium. We compare the results to a traditional MOKE measurement and to micromagnetic simulations. We find that the multiplexed method presented here provides better agreement with simulation than previous methods and equal or better signal-to-noise ratio.

Spatial mapping of focused surface acoustic waves in the investigation of high frequency strain induced changes

The field of straintronics, in which strain is used to drive phase transitions, ordering and structural changes, has conventionally been limited to dc or low frequency strain. High frequency large strains, which have the potential to serve as a high frequency trigger of strain sensitive physical phenomena, can be generated using focused surface acoustic waves, which produce two dimensional standing strain waves with very high strain at the elliptical focus. Here, the strain standing wave pattern generated by a focused surface acoustic wave is mapped and quantified as a function of voltage and frequency with high spatial resolution. A knife-edge optical reflection method is used to map the strain standing wave pattern generated by a 87.95 MHz annular interdigital transducer on 128° Y-Cut LiNbO3. Subsequent to strain mapping, ferromagnetic Co/Pt multilayers nanostructures are lithographically patterned within the high strain region for preliminary measurements of magnetization changes arising from high frequency fast strain. The knife edge technique is simple, results in excellent spatial resolution and is fully compatible with other optical measurements, such as focused magneto-optic Kerr measurements, while maintaining spatial information. This ability to accurately and reproducibly determine the position of maximum strain and to lock onto a specific strain region is an important step in the investigation of the effects of high frequency strain on thin film materials, which range from magnetic reorientations to strain induced phase transitions.

Development of an in situ magnetoelastic magneto-optical Kerr effect magnetometer

Reported here is the development and implementation of an integrated in situ magnetoelastic measurement setup with a MOKE magnetometer, repositionable electromagnet, and sample transfer/straining device. The former were used within a molecular beam epitaxial vacuum growth chamber. Consequently the magnetostriction constants for both Cr capped and uncapped Fe/GaAs(100) films were acquired without film oxidization occurring. Samples were bent in a four point bending geometry to produce a quantifiable tensile mechanical strain on the films during magnetoelastic measurements. In addition, a laser measurement system was developed to confirm the induced strain in the samples.

Scanning magneto-optical Kerr microscope with auto-balanced detection scheme

We have developed a scanning magneto-optical Kerr microscope dedicated to localization and measurement of the in-plane magnetization of ultra-thin layered magnetic nanostructures with high sensitivity and signal-to-noise ratio. The novel light detection scheme is based on a differential photodetector with automatic common mode noise rejection system with a high noise suppression up to 50 dB. The sensitivity of the developed detection scheme was tested by measurement of a single Co layer and a giant magnetoresistance (GMR) multilayer stack. The spatial resolution of the Kerr microscope was demonstrated by mapping an isolated 5×5 μm spin-valve pillar.

Note: A time-resolved Kerr rotation system with a rotatable in-plane magnetic field

A time-resolved Kerr rotation system with a rotatable in-plane magnetic field has been constructed to study anisotropic spin relaxation of electrons in semiconductors. A permanent magnet magic ring is placed on top of a motor-driven rotation stage (RS) to create the rotatable in-plane magnetic field. The RS is placed on a second translation stage to vary the local magnetic field around a sample. The in-plane magnetic field in such a system varies from 0.05 to 0.95 T, with full-round 360° rotatablity, thus offering a convenient and low-cost way to study the anisotropy of spin dynamics in semiconductors. Its performance was demonstrated via measurement of the anisotropy of the spin dephasing time (SDT) of electrons in a two-dimensional electron system embedded in a GaAs/Al(0.35)Ga(0.65)As heterostructure. The SDT with B∥[110] was observed to be 10% larger than that with B∥[110], consistent with the results of others, which was measured via rotating sample.