A. Mougin

High resolution polar Kerr magnetometer for nanomagnetism and nanospintronics

A new high resolution polar magneto-optical (MO) Kerr magnetometer, devoted to the study of nanometer sized elements with perpendicular magnetic anisotropy, is described. The unique performances of this setup in terms of sensitivity (1.2x10(-15) emu), stability (lateral drift +/-35 nm over 3 h), and resolution (laser spot full width at half maximum down to 470 nm) are demonstrated, and illustrated by Kerr hysteresis loop measurements on a unique ultrathin magnetic nanodot, and over small segments of ultranarrow magnetic tracks. Large scanning MO Kerr microscopy images were also obtained with the same performances.

Spatially periodic domain wall pinning potentials: Asymmetric pinning and dipolar biasing

Domain wall propagation has been measured in continuous, weakly disordered, quasi-two-dimensional, Ising-like magnetic layers that are subject to spatially periodic domain wall pinning potentials. The potentials are generated non-destructively using the stray magnetic field of ordered arrays of magnetically hard [Co/Pt]m nanoplatelets, which are patterned above and are physically separated from the continuous magnetic layer. The effect of the periodic pinning potentials on thermally activated domain wall creep dynamics is shown to be equivalent, at first approximation, to that of a uniform, effective retardation field, Hret, which acts against the applied field, H. We show that Hret depends not only on the array geometry but also on the relative orientation of H and the magnetization of the nanoplatelets. A result of the latter dependence is that wall-mediated hysteresis loops obtained for a set nanoplatelet magnetization exhibit many properties that are normally associated with ferromagnet/antiferromagne...

Hexagonal surface structure during the first stages of niobium growth on sapphire (1120)

A detailed reflection high-energy electron diffraction study of the first stages of the niobium growth on (1120)s sapphire is presented for several substrate temperatures. It is shown that the niobium film exhibits an hexagonal surface structure when the deposited thickness is smaller than a critical value, which, depending on the substrate temperature, varies between 5 and 15 A. For thicknesses larger than this critical thickness, the surface hexagonal structure relaxes to the (110) bcc niobium structure. The hexagonal surface structure is observed for high substrate temperatures (820-410oC) but does not appear when the substrate temperature is 270oC. The epitaxial relationships between the substrate, the surface hexagonal structure of niobium and the cubic niobium phase are presented.

Exploration of the ultimate patterning potential of focused ion beams

We aim to explore the nanostructuring potential of a highly focused pencil of ions. We show that focused ion beam technology (FIB) is capable of overcoming some basic limitations of current nanofabrication techniques and allowing innovative patterning schemes for nanoscience. In this work, we first detail the very high resolution FIB instrument developed specifically to meet nanofabrication requirements. Then we introduce and illustrate some new patterning schemes for next-generation FIB processing. These patterning schemes are: 1. nanoengraving of membranes as a template for nanopores and nanomask fabrication; 2. local defect injection for magnetic thin film direct patterning; 3. function of graphite substrates to prepare 2-D organized arrays of clusters; and 5. selective epitaxy of III-V semiconductors on FIB patterned surfaces. Finally, we show that FIB patterning allows bottom-up or organization processes.

Epitaxial growth of Fe on Nb(1 1 0)

Abstract The epitaxial growth of Fe on a bcc Nb(1xa01xa00) buffer has been studied at room temperature and at 550 °C, using reflection high energy electron diffraction and Auger electron spectroscopy. At room temperature, the growth occurs in several steps that have been clearly identified in increasing the Fe deposited thickness: (i) two-dimensional growth of a strained Fe layer (ii) intermixing between Fe and Nb with the formation of an alloy (iii) stabilization of a pure Fe layer. After an annealing process up to 550 °C, another alloy is formed, that exhibits a rectangular surface mesh with twins in the (1xa01xa00) plane. A single domain alloy, with a similar surface structure, can be also obtained by directly growing Fe on the Nb(1xa01xa00) buffer maintained at 550 °C.

Stabilization of the helical phase in Tb and in alloys films grown epitaxially on Y

Neutron diffraction experiments and macroscopic magnetization measurements show evidence of the stabilization of the helical magnetic phase over large temperature ranges for Tb and alloy films grown epitaxially on yttrium. In particular, the temperature of the transition between the helical and the ferromagnetic states is shifted from 220 K for bulk terbium to 160 K for a pure terbium film grown epitaxially on yttrium, despite the low stability of the helical phase in the bulk element. The decrease of the Curie temperature is due to the negative c-axis strain induced by the epitaxial growth on yttrium. The epitaxial strains also induce modifications of the Fermi surface, which leads to an increase of the turn angle even at . At low temperature, a long-wavelength-modulated phase, whose origin still remains to be explained, has been observed.

DyFe2(110) nanostructures: Morphology and magnetic anisotropy

Single-crystalline DyFe2(110) nanosystems have been obtained by molecular-beam epitaxy. From reflection high-energy electron diffraction observations, the systems have been shown to grow in a Stranski–Krastanov mode. Depending on elaboration conditions (substrate temperature and nominal thickness), dots with anisotropic shape or continuous films with low surface roughness are obtained. Compared to the bulk compounds, the epitaxial systems are strained because of thermal differential contraction and exhibit modifications of easy-magnetization direction compared to bulk. The magnetization reversal process is correlated to the morphology of the layers.

Large wavelength magnetic modulation in (0001)Tb films

A large wavelength magnetic modulation, which has never been observed up to now, has been evidenced by neutron scattering experiments in (0001) terbium single crystal films. With decreasing temperature, this magnetic arrangement appears when the helical one vanishes and before the long range ferromagnetic order sets in. The large wavelength magnetic modulation is squared and it varies between 180 and 700 A depending on the temperature and the film thickness. We assume that it is due to the stacking of ferromagnetic blocks whose net magnetization points along a basal plane direction and rotates by a given angle between two consecutive blocks. The occurrence of this phase seems to be independent of the strains induced by epitaxy but it would be related to the lattice clamping between epitaxied layers. A temperature-magnetic field phase diagram of a terbium film is presented.

Magnetic anisotropy in (1 1 0) DyFe2 films

Abstract The magnetic properties of (1xa01xa00) epitaxial DyFe2 films have been studied by SQUID magnetometry at various temperatures, for thicknesses between 50 and 1000xa0A. At low temperature, the easy magnetization direction is the same as in the bulk compound, whereas it rotates towards [ 1 1 0] at room temperature, because of the occurrence of epitaxial strains. The influence of the film thickness on the magnetic anisotropy and on the coercive filed is also presented. A large increase of the coercive field is observed when the thickness decreases.

Exploration of the Ultimate Patterning Potential Achievable with Focused Ion Beams

Decisive advances in the fields of nanosciences and nanotechnologies are intimately related to the development of new instruments and of related writing schemes and methodologies. Therefore we have recently proposed exploitation of the nano-structuring potential of a highly Focused Ion Beam as a tool, to overcome intrinsic limitations of current nano-fabrication techniques and to allow innovative patterning schemes urgently needed in many nanoscience challenges. In this work, we will first detail a very high resolution FIB instrument we have developed specifically to meet these nano-fabrication requirements. Then we will introduce and illustrate some advanced FIB processing schemes. These patterning schemes are (i) Ultra thin membranes as an ideal template for FIB nanoprocessing. (ii) Local defect injection for magnetic thin film direct patterning. (iii) Functionalization of graphite substrates to prepare 2D-organized arrays of clusters. (iv) FIB engineering of the optical properties of microcavities.