Showing papers by "Stéphane Fusil published in 2006"

Investigation on the origin of the magnetic moment of BiFeO 3 thin films by advanced x-ray characterizations

Abstract: Single phase (001)-oriented ${\mathrm{BiFeO}}_{3}$ (BFO) thin films grown by pulsed laser deposition can only be obtained in a narrow window of deposition pressure and temperature and have a low magnetic moment. Out of the stability window Fe- or Bi-rich impurity phases form, which has a strong impact on the physical and structural properties of the films, even for impurity concentrations hardly detectable by standard x-ray diffraction measurements. By using more sensitive tools such as x-ray absorption spectroscopy and x-ray magnetic circular dichroism and performing advanced x-ray diffraction characterization, we show that in nonoptimal conditions Fe forms ferrimagnetic $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ precipitates that are responsible for virtually all the ferromagnetic signal measured on such BFO films by standard magnetometry. This confirms that the BFO phase has a very low intrinsic moment that does not depend on strain. We also study the influence of film thickness on the nucleation of parasitic phases and find that epitaxial strain can stabilize the pure BFO phase in slightly overoxidizing growth conditions.

Tunnel magnetoresistance and robust room temperature exchange bias with multiferroic BiFeO3 epitaxial thin films

Abstract: The authors report on the functionalization of multiferroic BiFeO3 epitaxial films for spintronics. A first example is provided by the use of ultrathin layers of BiFeO3 as tunnel barriers in magnetic tunnel junctions with La2∕3Sr1∕3MnO3 and Co electrodes. In such structures, a positive tunnel magnetoresistance up to 30% is obtained at low temperature. A second example is the exploitation of the antiferromagnetic spin structure of a BiFeO3 film to induce a sizable (∼60Oe) exchange bias on a ferromagnetic film of CoFeB at room temperature. Remarkably, the exchange bias effect is robust upon magnetic field cycling, with no indications of training.

Combining half-metals and multiferroics into epitaxial heterostructures for spintronics

Abstract: We report on the growth of epitaxial bilayers of the La2∕3Sr1∕3MnO3 (LSMO) half-metallic ferromagnet and the BiFeO3 (BFO) multiferroic, on SrTiO3(001) by pulsed laser deposition. The growth mode of both layers is two dimensional, which results in unit-cell smooth surfaces. We show that both materials keep their properties inside the heterostructures, i.e., the LSMO layer (11 nm thick) is ferromagnetic with a Curie temperature of ∼330K, while the BFO films shows ferroelectricity down to very low thicknesses (5 nm). Conductive-tip atomic force microscope mappings of BFO/LSMO bilayers for different BFO thicknesses reveal a high and homogeneous resistive state for the BFO film that can thus be used as a ferroelectric tunnel barrier in tunnel junctions based on a half-metal.

Spin injection in a single metallic nanoparticle: a step towards nanospintronics

Abstract: We have fabricated nanometer sized magnetic tunnel junctions using a new nanoindentation technique in order to study the transport properties of a single metallic nanoparticle. Coulomb blockade effects show clear evidence for single electron tunneling through a single 2.5 nm Au cluster. The observed magnetoresistance is the signature of spin conservation during the transport process through a non magnetic cluster.

Spintronics with Multiferroics

Abstract: Multiferroics belong to a class of compounds that show simultaneous electric and magnetic orders. Multiferroics could be useful for a broad range of applications and have a strong potential for spintronics. Magnetoelectric effect can be used to change the resistance state of a spin-valve or magnetic tunnel junction (MTJ) without applying an external magnetic field. In this regard, both ferroelectric ferromagnets and ferroelectric antiferromagnets can be useful. The oxide heterostructures for spintronics combining multiferroic materials like BFO and LBMO, and ferromagnetic metals like SRO and the LSMO half-metal experimental results are presented. The influence of the ferroelectric nature of these barriers on the conductance and tunnel magnetoresistance is illustrated.

Probing enhanced spin lifetime through magneto-Coulomb effects in isolated nanometer size metallic clusters

Abstract: We experimentally study spin injection and spin dependent transport in a single non magnetic nanoparticle contacted by ferromagnetic leads through alumina barriers. Theoretical studies on the interplay of discrete charging and spin polarised current in such a structure predict spin accumulation effects in the nanoparticle, leading to novel effects such as tunnel magnetoresistance oscillations versus bias voltage. We report the first experimental observation of spin accumulation in Au and Cu single nanoparticles of few nanometers in diameter. Fabrication is carried out using a conductive tip AFM with real time monitoring of the conductance during the nanoindentation process. With this technique, we can contact a single isolated particle of a few nanometer in size. Because the capacitance of the nanoparticle is very small, the energy needed to add an electron on it becomes larger than the thermal energy leading to discrete voltage jumps of e/C in the I-V characteristics called Coulomb blockade "staircases". The 4.2K I-V curves, showing typical Coulomb blockade transport through a unique nanoparticle, are in excellent agreement with simulations.