arXiv subjects

Lesley F Cohen

Publications and source records attributed to Lesley F Cohen.

5 recordsLinked to original sources

arXiv2022

Anomalous Nernst effect in Mn$_3$NiN thin films

The observation of a sizable anomalous Hall effect in magnetic materials with vanishing magnetization has renewed interest in understanding and engineering this phenomenon. Antiferromagnetic antiperovskites are one of emerging material classes that exhibit a variety of interesting properties owing to a complex electronic band structure and magnetic ordering. Reports on the anomalous Nernst effect and its magnitude in this class of materials are, however, very limited. This scarcity may be partly due to the experimental difficulty of reliably quantifying the anomalous Nernst coefficient. Here, we report experiments on the anomalous Nernst effect in antiferromagnetic antiperovskite Mn$_3$NiN thin films. Measurement of both the anomalous Hall and Nernst effects using the same sample and measurement geometry makes it possible to directly compare these two effects and quantify the anomalous Nernst coefficient and conductivity in Mn$_3$NiN. We carefully evaluate the spatial distribution of the thermal gradient in the sample and use finite element modeling to corroborate our experimental results.

cond-mat.mtrl-sci↗

arXiv2019

Doping Dependence of the Second Magnetization Peak, Critical Current Density and Pinning Mechanism in BaFe$_{2-x}$Ni$_x$As$_2$ Pnictide Superconductors

A series of high quality BaFe$_{2-x}$Ni$_x$As$_2$ pnictide superconductors were studied using magnetic relaxation and isothermal magnetic measurements in order to study the second magnetization peak (SMP) and critical current behaviour in Ni-doped 122 family. The temperature dependence of the magnetic relaxation rate suggests a pinning crossover, whereas, it's magnetic field dependence hints a vortex-lattice structural phase-transition. The activation energy ($U$) estimated using the magnetic relaxation data was analyzed in detail for slightly-underdoped, slightly-overdoped and an overdoped samples, using Maley's method and collective creep theory. Our results confirm that the SMP in these samples is due to the collective (elastic) to plastic creep crossover as has been observed for the other members of 122-family. In addition, we also investigated the doping dependence of the critical current density ($J_c$) and the vortex-pinning behaviour in these compounds. The observed $J_c$ is higher than the threshold limit (10$^5$ A/cm$^2$) considered for the technological potential and even greater than 1 MA/cm$^2$ for slightly underdoped Ni-content, x = 0.092 sample. The pinning characteristics were analyzed in terms of the models developed by Dew-Hughes and Griessen $et$ $al$, which suggest the dominant role of $δl$-type pinning.

cond-mat.supr-con↗

arXiv2015

Andreev Reflection Spectroscopy in Transition Metal Oxides

Here we review the literature concerning measurement of the Andreev reflection between a superconductor (S) and ferromagnet (F), with particular attention to the case where the ferromagnet is a transition metal oxide. We discuss the practicality of utilisation of the current models for determination of the transport current spin polarisation and examine the evidence for Andreev bound states.

cond-mat.mtrl-sci↗

arXiv2009

Spin polarized transport current in n-type co-doped ZnO thin films measured by Andreev spectroscopy

We use point contact Andreev reflection measurements to determine the spin polarization of the transport current in pulse laser deposited thin films of ZnO with 1% Al and with and without 2%Mn. Only films with Mn are ferromagnetic and show spin polarization of the transport current of up to 55 $\pm$ 0.5% at 4.2 K, in sharp contrast to measurements of the nonmagnetic films without Mn where the polarization is consistent with zero. Our results imply strongly that ferromagnetism in these Al doped ZnO films requires the presence of Mn.

cond-mat.mtrl-sci↗

arXiv2009

Interactions between Josephson and pancake vortices revealed by second harmonic Hall measurements

The dynamic interactions between Josephson and pancake vortices in highly anisotropic superconductors are probed by shaking the former vortices with an ac magnetic field and measuring the second harmonic Hall response of the latter vortices with the sensor positioned above the sample. We demonstrate the efficiency of the proposed measurement method that aims to probe the coupling between the two vortex species. New experimental data obtained on Bi2Sr2CaCu2O8 single crystals reveal interesting features and the possibility to determine the phase diagram of vortex matter in such superconductors.

cond-mat.supr-con↗