Showing papers by "Albert Fert published in 2002"

Mechanisms of Spin-Polarized Current-Driven Magnetization Switching

Abstract: The mechanisms of the magnetization switching of magnetic multilayers driven by a current are studied by including exchange interaction between local moments and spin accumulation of conduction electrons. It is found that this exchange interaction leads to two additional terms in the Landau-Lifshitz-Gilbert equation: an effective field and a spin torque. Both terms are proportional to the transverse spin accumulation and have comparable magnitudes.

Switching the magnetic configuration of a spin valve by current-induced domain wall motion

Abstract: We present experimental results on the displacement of a domain wall by injection of a dc current through the wall. The samples are 1-μm-wide long stripes of a CoO/Co/Cu/NiFe classical spin-valve structure. The stripes have been patterned by electron-beam lithography. A neck has been defined at 1/3 of the total length of the stripe and is pinning center for the domain walls, as shown by the steps of the giant magnetoresistance curves at intermediate levels (1/3 or 2/3) between the resistances corresponding to the parallel and antiparallel configurations. We show by electric transport measurements that, once a wall is trapped, it can be moved by injecting a dc current higher than a threshold current of the order of magnitude of 107 A/cm2. We discuss the different possible origins of this effect, i.e., local magnetic field created by the current and/or spin transfer from spin-polarized current.

Experimental evidence of the ferrimagnetic ground state of Sr2FeMoO6 probed by X-ray magnetic circular dichroism

Abstract: We have investigated the magnetic structure of a Sr2FeMoO6 single crystal by X-ray magnetic circular dichroism at the L2,3 edges of Fe and Mo sites. The spin magnetic moments we find on Fe (3.05μB) and Mo ( − 0.32μB) give, for the first time, a direct confirmation of their ferrimagnetic ordering. The presence of a finite spin moment on Mo together with only very small orbital moments on both Fe and Mo confirms that the predicted half-metallicity of the Sr2FeMoO6 compound is due to a configuration with five localized d electrons forming a high-spin moment on Fe and one s antiparallel delocalized electron shared between the Mo and the other sites.

Nanoscale analysis of a SrTiO 3 / La 2 / 3 Sr 1 / 3 MnO 3 interface

Abstract: The aim of the present work is an atomic scale characterization of the ${\mathrm{SrTiO}}_{3}/{\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_{3}$ interface involved in ${\mathrm{C}\mathrm{o}/\mathrm{S}\mathrm{r}\mathrm{T}\mathrm{i}\mathrm{O}}_{3}/{\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_{3}$ tunnel junctions. Our high-resolution transmission electron microscopy experiments show the quality of the pseudo-morphic growth of ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_{3}$ (LSMO) and ${\mathrm{SrTiO}}_{3}$ (STO), but do not allow us to determine the termination of LSMO. On the other hand, we have carried out an energy-loss near-edge structure study of the ${\mathrm{Mn}}_{2p}$ and ${\mathrm{O}}_{1s}$ edges. The main result is that the valence of Mn keeps its intrinsic value of the bulk material till the last ${\mathrm{MnO}}_{2}$ layers at the interface, which is consistent with the observation of magnetoresistance up to practically the Curie temperature of bulk LSMO in our junctions. These results suggest a ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}\mathrm{O}$ termination of LSMO.

Magnetization reversal in GaMnAs layers studied by Kerr effect

Abstract: In this letter, magnetization reversal is investigated in (Ga,Mn)As thin films using the magneto-optical Kerr effect. A Stoner–Wohlfart model, modified to take into account the existence of magnetic reversal discontinuities associated with nucleation and propagation phenomena allows us to estimate most of the characteristic constants. These results demonstrate a reversal behavior analogous to that observed in metallic magnetic layers (coherent rotation followed by a nucleation propagation process). The dynamic study at T=20 K shows a strong increase of the coercivity with the increase of dH/dt. This effect is related to the random distribution of Mn magnetic ions in the lattice.

Spin injection from a ferromagnetic metal into a semiconductor

Abstract: We calculate the spin polarization of electrons injected from a ferromagnetic metal (F) into a semiconductor (SC) or a two-dimensional semiconductor electron gas (2-DEG) and the resulting magnetoresistance (MR) of a F/SC/F structure. Due to the conductivity mismatch between F and SC, efficient spin-injection can be obtained only by inserting a tunnel junction at the F/N interfaces. We find that a F1/SF/F2 structure can present a nonzero MR if the junction resistance is chosen in a relatively narrow range determined by the spin-diffusion length and resistivity of F and SC, the distance between the ferromagnets and the width of the conduction channels. The case of a 2-DEG is particularly favorable.

Temperature and voltage dependence of the resistance and magnetoresistance in discontinuous double tunnel junctions

Abstract: We have studied the electric and magnetotransport properties of $\mathrm{Co}//{\mathrm{Al}}_{2}{\mathrm{O}}_{3}/{\mathrm{Co}}_{\mathrm{I}}/{\mathrm{Al}}_{2}{\mathrm{O}}_{3}//(\mathrm{Co}or\mathrm{Cu})$ tunnel double junctions, in which the intermediate ${\mathrm{Co}}_{I}$ layer is either granular or percolated clusters and the top electrode is either Co or Cu. In granular junctions, spin-dependent tunneling with Coulomb-blockade effect was observed. The temperature dependence of the resistance can be attributed to two conduction channels in parallel between electrodes via granules and via other channels such as metallic impurities in the insulating layer. Voltage and temperature dependences of the tunnel magnetoresistance are discussed. The Coulomb blockade and the improvement of the antiparallel alignment at low temperature both enhanced the TMR effects.

Spin injection and experimental detection of spin accumulation

Abstract: Spin injection is involved in many experiments in the field of spin electronics. Examples of spin injection effects can be found in several types of transport experiments in metallic structures, in the problem of spin injection from a ferromagnetic metal into a semiconductor or also in the reversal of magnetization by transverse spin injection, as reviewed in the oral presentation by one of us (AF) at ICMFS. Here we will limit our presentation to a general description of the physics of spin injection and to an illustration of this physics by experiments in which the spin accumulation generated by spin injection in metallic structures is directly detected by voltage measurements.

Switching the magnetic configuration of a spin valve by current induced domain wall motion

Abstract: We present experimental results on the displacement of a domain wall by injection of a dc current through the wall. The samples are 1 micron wide long stripes of a CoO/Co/Cu/NiFe classical spin valve structure. The stripes have been patterned by electron beam lithography. A neck has been defined at 1/3 of the total length of the stripe and is a pinning center for the domain walls, as shown by the steps of the giant magnetoresistance curves at intermediate levels (1/3 or 2/3) between the resistances corresponding to the parallel and antiparallel configurations. We show by electric transport measurements that, once a wall is trapped, it can be moved by injecting a dc current higher than a threshold current of the order of magnitude of 10^7 A/cm^2. We discuss the different possible origins of this effect, i.e. local magnetic field created by the current and/or spin transfer from spin polarized current.

Sputter growth and magnetic properties of exchange-biased La1/4Ca3/4MnO3–La2/3Sr1/3MnO3 epitaxial bilayers

Abstract: We have grown La1/4Ca3/4MnO3–La2/3Sr1/3MnO3 (LCMO–LSMO) thin films by reactive facing target radio frequency sputtering on SrTiO3(001) substrates. After 1 bar oxygen annealing at 800 °C, the films have the desired oxygen stoichiometry and perovskite structure. Transport properties of single LCMO films reveal the charge order (CO) transition between 100 and 180 K depending on the LCMO layer thickness while LSMO films have a Curie temperature greater than 330 K. Kerr effect measurements show evidence of an increase of the LSMO coercive field HC from ∼50 Oe at room temperature up to 110 Oe at 40 K and a significant decrease in the squareness of the hysteresis loop. This effect is accompanied by a slight shift of the hysteresis loop HE due to exchange bias between the antiferromagnetic charge ordered LCMO layer and the LSMO ferromagnet which reaches 20 Oe at 40 K. Both the onset of HE and the drop in squareness are observed to be close to the charge order transition of a similar LCMO single layer and are assu...

Nanoscale Analysis of a Co-SrTiO3 Interface in a Magnetic Tunnel Junction

Abstract: We use High Resolution Electron Microscopy together with Electron Energy Loss Spectroscopy to analyze the crystallography and the chemical configuration of a Co/SrTiO 3 interface in a Co/SrTiO 3 /La 2/3 Sr 1/3 MnO 3 magnetic tunnel junction. PACS: 75.47.-m, 75.70.Cn, 68.37.Lp, 79.20.Uv

Coulomb blocking variable capacitance having first/second armature insulating layer separated and having conductor aggregate one/next separated.

Abstract: The voltage variable capacitance has a first (L) and second (R) armature separated by an insulating layer (D). The insulating layer has a number of conductor aggregates separated one from the other.