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We present ab initio calculations of the evolution of anisotropic magnetoresistance (AMR) in Ni nanocontacts from the ballistic to the tunnel regime. We find an extraordinary enhancement of AMR, compared to bulk, in two scenarios. In systems without localized states, like chemically pure break junctions, large AMR only occurs if the orbital polarization of the current is large, regardless of the anisotropy of the density of states. In systems that display localized states close to the Fermi energy, like a single electron transistor with ferromagnetic electrodes, large AMR is related to the variation of the Fermi energy as a function of the magnetization direction.
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We report point-contact measurements of anisotropic magnetoresistance (AMR) in a single crystal of antiferromagnetic (AFM) Mott insulator Sr2IrO4. The point-contact technique is used here as a local probe of magnetotransport properties on the nanoscale. The measurements at liquid nitrogen temperature revealed negative magnetoresistances (MRs) (up to 28%) for modest magnetic fields (250 mT) applied within the IrO2 a-b plane and electric currents flowing perpendicular to the plane. The angular dependence of MR shows a crossover from four-fold to two-fold symmetry in response to an increasing magnetic field with angular variations in resistance from 1-14%. We tentatively attribute the four-fold symmetry to the crystalline component of AMR and the field-induced transition to the effects of applied field on the canting of AFM-coupled moments in Sr2IrO4. The observed AMR is very large compared to the crystalline AMRs in 3d transition metal alloys/oxides (0.1-0.5%) and can be associated with the large spin-orbit interactions in this 5d oxide while the transition provides evidence of correlations between electronic transport, magnetic order and orbital states. The finding of this work opens an entirely new avenue to not only gain a new insight into physics associated with spin-orbit coupling but also better harness the power of spintronics in a more technically favorable fashion.
The effects of the spin-orbit interaction on the tunneling magnetoresistance of ferromagnet/semiconductor/normal metal tunnel junctions are investigated. Analytical expressions for the tunneling anisotropic magnetoresistance (TAMR) are derived within an approximation in which the dependence of the magnetoresistance on the magnetization orientation in the ferromagnet originates from the interference between Bychkov-Rashba and Dresselhaus spin-orbit couplings that appear at junction interfaces and in the tunneling region. We also investigate the transport properties of ferromagnet/semiconductor/ferromagnet tunnel junctions and show that in such structures the spin-orbit interaction leads not only to the TAMR effect but also to the anisotropy of the conventional tunneling magnetoresistance (TMR). The resulting anisotropic tunneling magnetoresistance (ATMR) depends on the absolute magnetization directions in the ferromagnets. Within the proposed model, depending on the magnetization directions in the ferromagnets, the interplay of Bychkov-Rashba and Dresselhaus spin-orbit couplings produces differences between the rates of transmitted and reflected spins at the ferromagnet/seminconductor interfaces, which results in an anisotropic local density of states at the Fermi surface and in the TAMR and ATMR effects. Model calculations for Fe/GaAs/Fe tunnel junctions are presented. Furthermore, based on rather general symmetry considerations, we deduce the form of the magnetoresistance dependence on the absolute orientations of the magnetizations in the ferromagnets.
In this paper we report an exhaustive experimental work on magnetoresistance effects found in a system in which a large number of nanocontacts are produced between oxidized Fe fine particles. We have obtained the following performances: i) Huge low field room temperature magnetoresistance (over 1000%). ii) Non-linear I-V at different applied fields and temperatures. iii) Large thermal stability and reproducible resistance value under thermal cycles from room temperature down to 5 K. iv) Easy to fabricate with an almost 100% success. v) Heavy duty and transportable samples with reproducibility tested in several laboratories. We realized that the extraordinary effect found is related to the oxygen content at the particles surface
Colloidal probe Atomic Force Microscopy (AFM) allows to explore sliding states of vanishing friction, i.e. superlubricity, in mesoscopic graphite contacts. In this respect, superlubricity is known to appear upon formation of a triboinduced transfer layer, originated by material transfer of graphene flakes from the graphitic substrate to the colloidal probe. It was suggested that friction vanishes due to crystalline incommensurability at the sliding interface thus formed. However several details are missing, including the roles of tribolayer roughness and of loading conditions. Hereafter we gain deeper insight into the tribological response of micrometric silica beads sliding on graphite under ambient conditions. We show that the tribotransferred flakes increase interfacial roughness from tenths to several nanometers, in fact causing a breakdown of adhesion and friction by one order of magnitude. Furthermore, they behave as protruding asperities dissipating mechanical energy via atomic-scale stick-slip instabilities. Remarkably, such contact junctions can undergo a load-driven transition from continuous superlubric sliding to dissipative stick-slip, that agrees with the single-asperity Prandtl-Tomlinson model. Our results indicate that friction at mesoscopic silica-graphite junctions depends on the specific energy landscape experienced by the topographically-highest triboinduced nanoasperity. Superlubricity may arise from the load-controlled competition between interfacial crystalline incommensurability and contact pinning effects.
We report the effects of variation in length on the electronic structure of CdSe nanorods derived from atomic clusters and passivated by fictitious hydrogen atoms. These nanorods are augmented by attaching gold clusters at both the ends to form a nanodumbbell. The goal is to assess the changes at nanolevel after formation of contacts with gold clusters serving as electrodes and compare the results with experimental observations [PRL, 95, 056805 (2005)]. Calculations involving nanorods of length 4.6 Angstrom to 116.6 Angstrom are performed using density functional theory implemented within plane-wave basis set. The binding energy per atom saturates for nanorod of length 116.6 Angstrom. It is interesting to note that upon attaching gold clusters, the nanorods shorter than 27 Angstrom develop metallicity by means of metal induced gap states (MIGS). Longer nanorods exhibit a nanoscale Schottky barrier emerging at the center. For these nanorods, interfacial region closest to the gold electrodes shows a finite density of states in the gap due to MIGS, which gradually decreases towards the center of the nanorod opening up a finite gap. Bader charge analysis indicates localized charge transfer from metal to semiconductor.
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