Giant Anisotropic Magneto-Resistance in ferromagnetic atomic contacts

Magneto-resistance is a physical effect of great fundamental and industrial interest since it is the basis for the magnetic field sensors used in computer read-heads and Magnetic Random Access Memories. As device dimensions are reduced, some important physical length scales for magnetism and electrical transport will soon be attained. Ultimately, there is a strong need to know if the physical phenomena responsible for magneto-resistance still hold at the atomic scale. Here, we show that the anisotropy of magneto-resistance is greatly enhanced in atomic size constrictions. We explain this physical effect by a change in the electronic density of states in the junction when the magnetization is rotated, as supported by our ab-initio calculations. This stems from the "spin-orbit coupling" mechanism linking the shape of the orbitals with the spin direction. This sensitively affects the conductance of atomic contacts which is determined by the overlap of the valence orbitals.Comment: latex AAMR.tex, 6 files, 5 figures, 4 pages (http://www-drecam.cea.fr/spec/articles/S06/011

Nano-Hall sensors with granular Co-C

We analyzed the performance of Hall sensors with different Co-C ratios, deposited directly in nano-structured form, using $Co_2(CO)_8$ gas molecules, by focused electron or ion beam induced deposition. Due to the enhanced inter-grain scattering in these granular wires, the Extraordinary Hall Effect can be increased by two orders of magnitude with respect to pure Co, up to a current sensitivity of $1 \Omega/T$. We show that the best magnetic field resolution at room temperature is obtained for Co ratios between 60% and 70% and is better than $1 \mu T/Hz^{1/2}$. For an active area of the sensor of $200 \times 200 nm^2$, the room temperature magnetic flux resolution is $\phi_{min} = 2\times10^{-5}\phi_0$, in the thermal noise frequency range, i.e. above 100 kHz.Comment: 5 pages, 4 figure

Phase Diagram of Half Doped Manganites

An analysis of the properties of half-doped manganites is presented. We build up the phase diagram of the system combining a realistic calculation of the electronic properties and a mean field treatment of the temperature effects. The electronic structure of the manganites are described with a double exchange model with cooperative Jahn-Teller phonons and antiferromagnetic coupling between the $Mn$ core spins. At zero temperature a variety of electronic phases as ferromagnetic (FM) charge ordered (CO) orbital ordered (OO), CE-CO-OO and FM metallic, are obtained. By raising the temperature the CE-CO-OO phase becomes paramagnetic (PM), but depending on the electron-phonon coupling and the exchange coupling the transition can be direct or trough intermediate states: a FM disorder metallic, a PM-CO-OO or a FM-CO-OO. We also discus the nature of the high temperature PM phase in the regime of finite electron phonon coupling. In this regime half of the oxygen octahedra surrounding the $Mn$ ions are distorted. In the weak coupling regime the octahedra are slightly deformed and only trap a small amount of electronic charge, rendering the system metallic consequentially. However in the strong coupling regime the octahedra are strongly distorted, the charge is fully localized in polarons and the system is insulator.Comment: 10 pagses, 9 figures include

Magnetotransport properties of iron microwires fabricated by focused electron beam induced autocatalytic growth

We have prepared iron microwires in a combination of focused electron beam induced deposition (FEBID) and autocatalytic growth from the iron pentacarbonyl, Fe(CO)5, precursor gas under UHV conditions. The electrical transport properties of the microwires were investigated and it was found that the temperature dependence of the longitudinal resistivity (rhoxx) shows a typical metallic behaviour with a room temperature value of about 88 micro{\Omega} cm. In order to investigate the magnetotransport properties we have measured the isothermal Hall-resistivities in the range between 4.2 K and 260 K. From these measurements positive values for the ordinary and the anomalous Hall coefficients were derived. The relation between anomalous Hall resistivity (rhoAN) and longitudinal resistivity is quadratic, rhoAN rho^2 xx, revealing an intrinsic origin of the anomalous Hall effect. Finally, at low temperature in the transversal geometry a negative magnetoresistance of about 0.2 % was measured

Giant orbital moments are responsible for the anisotropic magnetoresistance of atomic contacts

We study here, both experimentally and theoretically, the anisotropy of magnetoresistance in atomic contacts. Our measurements on iron break junctions reveal an abrupt and hysteretic switch between two conductance levels when a large applied field is continuously rotated. We show that this behaviour stems from the coexistence of two metastable electronic states which result from the anisotropy of electronic interactions responsible for the enhancement of orbital magnetization. In both states giant orbital moments appear on the low coordinated central atom in a realistic contact geometry. However they differ by their orientation, parallel or perpendicular, with respect to the axis of the contact. Our explanation is totally at variance with the usual model based on the band structure of a monatomic linear chain, which we argue cannot be applied to 3d ferromagnetic metals