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Register a new userAC Measurement of the Nernst effect of thin films at low temperatures
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We describe an alternating current method to measure the Nernst effect in superconducting thin films at low temperatures. The Nernst effect is an important tool in the understanding superconducting fluctuations and, in particular, vortex motion near critical points. However, in most materials, the Nernst signal in a typical experimental setup rarely exceeds a few $mu$V, in some cases being as low as a few nV. DC measurements of such small signals require extensive signal processing and protection against stray pickups and offsets, limiting the sensitivity of such measurements to $>$5nV. Here we describe a method utilizing a one-heater-two-thermometer setup with the heating element and thermometers fabricated on-chip with the sample, which helped to reduce thermal load and temperature lag between the substrate and thermometer. Using AC heating power and 2$omega$ measurement, we are able to achieve sub-nanovolt sensitivity in 20-30nm MoGe thin films on glass substrate, compared to a sensitivity of $sim$9nV using DC techniques on the same setup.
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We have studied the influence of a square array of pinning centers on the dynamics of vortex avalanches in Pb thin films by means of ac- and dc- magnetization measurements. Close to the superconducting transition Tc the commensurability between the vortex lattice and the pinning array leads to the well known local increments of the critical current. As temperature T decreases, matching features progressively fade out and eventually disappear. Further down in temperature vortex avalanches develop and dominate the magnetic response. These avalanches manifest themselves as jumps in the dc-magnetization and produce a lower ac-shielding giving rise to a paramagnetic reentrance in the ac-screening x(T). Within the flux jump regime two subregimes can be identified. Close to the boundary where vortex avalanches develope, the field separation between consecutive jumps follow the periodicity of the pinning array and a field and temperature dependent screening is observed. In this regime, the response also depends on frequency f in agreement with theoretical models for magnetothermal instabilities. At low enough temperatures and fields, the screening saturates to a constant value independent of T, H, and f, where jumps are randomly distributed. We have also found that vortex instabilities occupy a larger portion of the H-T diagram in patterned samples than in films without nanoengineered pinning sites. Finally, we discuss the possible origin of the vortex avalanches and compare our results with previous experimental and theoretical studies.
We report Nernst effect measurements for some crystalline films of Ca and Zn-doped yttrium barium copper oxide grown by pulsed laser deposition. We argue that our results and most of the published data for LSCO are consistent with the theory of Gaussian superconducting fluctuations.
The magneto-thermoelectric properties of Heusler compound thin films are very diverse. Here, we discuss the anomalous Nernst response of Co$_2$MnGa thin films. We systematically study the anomalous Nernst coefficient as a function of temperature, and we show that unlike the anomalous Hall effect, the anomalous Nernst effect in Co$_2$MnGa strongly varies with temperature. We exploit the on-chip thermometry technique to quantify the thermal gradient, which enables us to directly evaluate the anomalous Nernst coefficient. We compare these results to a reference CoFeB thin film. We show that the 50-nm-thick Co$_2$MnGa films exhibit a large anomalous Nernst effect of -2$mu$V/K at 300 K, whereas the 10-nm-thick Co$_2$MnGa film exhibits a significantly smaller anomalous Nernst coefficient despite having similar volume magnetizations. These findings suggest that the microscopic origin of the anomalous Nernst effect in Co$_2$MnGa is complex and may contain contributions from skew-scattering, side-jump or intrinsic Berry phase. In any case, the large anomalous Nernst coefficent of Co$_2$MnGa thin films at room temperature makes this material system a very promising candidate for efficient spin-caloritronic devices.
A commercially available calorimeter has been used to investigate the specific heat of a high-quality kn single crystal. The addenda heat capacity of the calorimeter is determined in the temperature range $0.02 , mathrm{K} leq T leq 0.54 , mathrm{K}$. The data of the kn crystal imply the presence of a large $T^2$ contribution to the specific heat which gives evidence of $d$-wave order parameter symmetry in the superconducting state. To improve the measurements, a novel design for a calorimeter with a paramagnetic temperature sensor is presented. It promises a temperature resolution of $Delta T approx 0.1 , mathrm{mu K}$ and an addenda heat capacity less than $200 , mathrm{pJ/K}$ at $ T < 100 , mathrm{mK}$.
We report on the first experimental visualization of domain structure in films of weakly ferromagnetic Cu$_{0,47}$Ni$_{0,53}$ alloy with different thickness at liquid helium temperatures. Improved high-resolution Bitter decoration technique was used to map the magnetic contrast on the top of the films well below the Curie temperature T$_{Curie}$ ($sim$ 60 K). In contrast to magnetic force microscopy, this technique allowed visualization of the domain structure without its disturbance while the larger areas of the sample were probed. Maze-like domain patterns, typical for perpendicular magnetic anisotropy, were observed. The average domain width was found to be about 100 nm.
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