arXiv subjects

N. A. Porter

Publications and source records attributed to N. A. Porter.

5 recordsLinked to original sources

arXiv2014

Scattering mechanisms in textured FeGe thin films: magnetoresistance and the anomalous Hall effect

A textured thin film of FeGe was grown by magnetron sputtering with a helimagnetic ordering temperature of TN = 276 +/- 2 K. From 5 K to room temperature a variety of scattering processes contribute towards the overall longitudinal and Hall resistivities. These were studied by combining magnetometry and magnetotransport measurements. The high-field magnetoresistance (MR) displays three clear temperature regimes: Lorentz force MR dominates at low temperatures, above T ~ 80 K scattering from spin-waves predominates, whilst finally for T > 200 K scattering from fluctuating local moments describes the MR. At low fields, where the magnetisation is no longer technically saturated, we find a scaling of magnetoresistance with the square of the magnetisation, indicating that the MR due to the unwinding of spins in the conical phase arises from a similar mechanism to that in magnetic domain walls. This MR is only visible up to a temperature of about 200 K. No features can be found in the temperature or field dependence of the longitudinal resistivity that belie the presence of the underlying magnetic phase transition at TN: the marked changes in behavior are at much lower temperatures. The anomalous Hall effect has a dramatic temperature dependence in which the anomalous Hall resistivity scales quadratically with the longitudinal resistivity: comparison with anomalous Hall scaling theory shows that our system is in the intrinsic 'moderately dirty' regime. Lastly, we find evidence of a topological Hall effect of size 100 ~Ohm cm.

cond-mat.str-el↗

arXiv2014

DMI meter: Measuring the Dzyaloshinskii-Moriya interaction inversion in Pt/Co/Ir/Pt multilayers

We describe a field-driven domain wall creep-based method for the quantification of interfacial Dzyaloshinskii-Moriya interactions (DMI) in perpendicularly magnetized thin films. The use of only magnetic fields to drive wall motion removes the possibility of mixing with current-related effects such as spin Hall effect or Rashba field, as well as the complexity arising from lithographic patterning. We demonstrate this method on sputtered Pt/Co/Ir/Pt multilayers with a variable Ir layer thickness. By inserting an ultrathin layer of Ir at the Co/Pt interface we can reverse the sign of the effective DMI acting on the sandwiched Co layer, and therefore continuously change the domain wall (DW) structure from right- to the left-handed Néel wall. We also show that the DMI shows exquisite sensitivity to the exact details of the atomic structure at the film interfaces by comparison with a symmetric epitaxial Pt/Co/Pt multilayer.

cond-mat.mtrl-sci↗

arXiv2013

Strain-induced effects on the magnetic and electronic properties of epitaxial Fe$_{1-x}$Co$_{x}$Si thin films

We have investigated the Co-doping dependence of the structural, transport, and magnetic properties of ε-FeCoSi epilayers grown by molecular beam epitaxy on silicon (111) substrates. Low energy electron diffraction, atomic force microscopy, X-ray diffraction, and high resolution transmission electron microscopy studies have confirmed the growth of phase-pure, defect-free ε-FeCoSi epitaxial films with a surface roughness of ~1 nm. These epilayers are strained due to lattice mismatch with the substrate, deforming the cubic B20 lattice so that it becomes rhombohedral. The temperature dependence of the resistivity changes as the Co concentration is increased, being semiconducting-like for low $x$ and metallic-like for x \gtrsim 0.3. The films exhibit the positive linear magnetoresistance that is characteristic of ε-FeCoSi below their magnetic ordering temperatures $T_\mathrm{ord}$, as well as the huge anomalous Hall effect of order several μΩcm. The ordering temperatures are higher than those observed in bulk, up to 77 K for x = 0.4. The saturation magnetic moment of the films varies as a function of Co doping, with a contribution of ~1 μ_{B}/ Co atom for x \lesssim 0.25. When taken in combination with the carrier density derived from the ordinary Hall effect, this signifies a highly spin-polarised electron gas in the low x, semiconducting regime.

cond-mat.mtrl-sci↗

arXiv1997

Search for TeV Counterparts in Gamma-Ray Bursts

Based on BACODINE network notification the Whipple Observatory gamma-ray telescope has been used to search for the delayed TeV counterpart to BATSE-detected gamma-ray bursts. In the fast slew mode, any point in the sky can be reached within two minutes of the burst notification. The search strategy, necessary because of the uncertainty in burst position and limited FOV of the camera, is described.

astro-ph↗

arXiv1997

A search for correlations of TeV gamma-rays with ultra-high energy cosmic rays

A search was conducted for TeV gamma-rays emitted from the direction of the ultra-high energy cosmic ray detected by the Fly's Eye Experiment with E ~ 3 x 10**20 eV. No enhancement was found at a level of 10**-10 gamma/cm**2-sec for E>350 GeV. This upper limit is consistent with theoretical estimates based on topological defects as sources of UHE cosmic rays. An upper limit was also set for the flux of TeV gamma rays from 3C147, the most prominent AGN in the error box.

astro-ph↗