K. Dumesnil

Epitaxial growth of (110) DyFe2, TbFe2 and Dy0.7Tb0.3Fe2 thin films by molecular beam epitaxy

We present the first epitaxial growth of some (110) rare earth-Fe2 (DyFe2, TbFe2 and Dy0.7Tb0.3Fe2 known as terfenol-D) thin films on (110) Nb(11−20) sappire by molecular beam epitaxy. The epitaxy is initiated by the deposition of a thin layer of iron on niobium. The structures are investigated by RHEED and X-ray scattering. Depending on the thermal treatment of the iron thin layer, the films of RE-Fe2, epitaxially grown on it, are either single crystals or present twins related by a 110° rotation about the surface normal. The growth of epitaxial thin films of these compounds is of interest because of the magnetic and magnetostrictive properties these materials may exhibit.

Room temperature ferrimagnetic thin films of the magnetoelectric Ga2−xFexO3

(0k0) oriented films of the magnetoelectric material Ga2−xFexO3 (0.8 ≤ x ≤ 1.4) have been grown by pulsed laser deposition on various substrates: the non conducting yttrium stabilized zirconia (YSZ) (001) and the conducting indium tin oxide (ITO) buffered YSZ(001) and single crystalline Pt(111) buffered YSZ(111). The films are ferrimagnetic for all compositions and their Curie temperature increases with x. For x = 1.4, their Curie temperature is above room temperature (370 K) and their room temperature saturation magnetization is 90 emu/cm3. The effect of the conducting substrates on both the crystalline and electrical properties of the films has been studied. The single crystalline Pt(111) buffered YSZ(111) substrates allow substantial improvements both on the crystallographic and electrical points of view with a reduction of the number of in-plane variants down to 3 and a decrease of the leakage current down to 10−5 A at 10 V. This work opens new perspectives for the integration of a room temperature ferrimagnetic magnetoelectric material in spintronic devices.

Magnetoelastic and Exchange Contributions to the Helical-Ferromagnetic Transition in Dysprosium Epitaxial Films

The Curie temperature of dysprosium films epitaxially grown on yttrium and on erbium is shown to increase continuously with the epitaxial strain induced along the c-axis. This variation is correlated to modifications of the α strains, of the γ distortion and of the exchange energy barrier. The evolution of these different terms is discussed from the experimental thermal variations of the lattice parameters and turn angles determined by neutron diffraction.

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.

Magnetocrystalline anisotropy in a (110)?(Tb0.27Dy0.73)Fe2 thin-film

Magnetic anisotropy measurements performed in a (110) (Tb0.27Dy0.73)Fe2 (Terfenol-D) film epitaxially grown on a sapphire substrate are presented. The magnetic torque curves have been determined by using a vectorial vibrating sample magnetometer, which allows us to measure the angular dependence of magnetization components parallel, , and perpendicular, , to the applied field up to 2 T. The fourfold symmetry associated with the cubic structure within the (110) plane is clearly observed. The analysis of the experimental torque has been carried out considering magnetocrystalline anisotropy up to sixth order and magnetoelastic energy up to second order; so, the magnetocrystalline anisotropy constants in the (110) plane of the film, K1 and K2, have been obtained. This allows us to determine the direction of the magnetization easy axis for (110) Terfenol-D thin-film: it is at RT, passes through at 140 K and then changes to at 40 K. It was completely impossible to explain the angular dependence of the experimental magnetic torque without including shear and tetragonal magnetoelastic stress parameters, b2 and b1, respectively. This confirms the paramount role of the strain in the determination of the magnetic properties in this kind of Terfenol-D thin film.

Clamping effects in the Al2O3(112¯0)∕Nb(110)∕Eu(110) epitaxial system

In-plane and out-of-plane lattice parameters of Nb and Eu have been measured as a function of temperature between 10 and 300 K in the Al2O3(112¯0)∕Nb(110)∕Eu(110) epitaxial system. It is shown that the Nb lattice is clamped in the growth plane to the sapphire substrate, in the whole temperature range. On its own, the Eu lattice is totally free to expand isotropically above a clamping temperature Tcl, below which in-plane parameters also freeze to an almost constant value. This well-defined clamping temperature strongly depends on the Eu thickness and likely is related to the thermal mobility of interface defects.

Thermal dependence of magnetic springs location in a DyFe2/YFe2 superlattice

Element selective x-ray magnetic circular dichroism measurements have been performed to unravel the complex magnetization reversal process occurring in antiferromagnetically exchange-coupled Laves phase superlattices. The separate investigation of each magnetic compound in a [DyFe2 (50 A)/YFe2 (200 A)] superlattice has enlightened a drastic thermal variation towards an unexpected high-temperature regime, where the magnetization in the hard layers first reverses in positive field, whereas the magnetization in the soft ones remains stuck along the field direction. This transfer of the interface magnetic twists towards the hard layers permits us to explain the thermal evolution of the coercive field.

Magnetoelastic coupling in TbFe2 (110) thin films

We have determined the rhombohedral magnetoelastic stress of a Laves phase TbFe2 (110) single-crystal film, grown by molecular-beam epitaxy. The film thickness was 1300 A. The magnetoelastic stress was directly measured by using a low-temperature cantilever capacitive method, between 300 and 10 K. The isotherms clearly display the coercive field but, unlike bulk alloy behavior, do not saturate even at the maximum field of 12 T. The determined rhombohedral magnetoelastic parameter of the film is Be,2=−0.43 GPa, at 0 K and 12 T, which is 0.67 times the value for bulk TbFe2. Be,2 follows a power m3 of the reduced magnetization m, indicating a single-ion volume origin for the rhombohedral magnetoelastic stress of this film. Measurements performed in a 300 A TbFe2 (110) film deposited onto a YFe2 buffer show that the coercive field is drastically lowered and that the magnetoelastic distortion is negligible.

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.

Magnetoelastic stresses in rare-earth thin films and superlattices

A study of the magnetoelastic behavior of some rare-earth based thin films and superlattices (SLs) is reported. Magnetoelastic stress (MS) measurements are made by a cantilever capacitive technique over a wide range of temperatures (10–30 K) and magnetic fields (up to 12 T). Expressions are derived which relate the cantilever curvatures and the magnetoelastic stresses in anisotropic thin films and SLs (of cubic symmetry) deposited on crystalline substrates. The magnetoelastic energy associated with the interfaces and the epitaxial stress dependence of the volume MS are investigated by studying the basal plane MS in Hon/Lu15 and Ho10/YmSLs: interface MSs even higher than the volume ones are obtained, and the effect of the epitaxial strain on the bulk MSs is large. The MS contributions are also deduced for Dy/Y and Er/Lu SLs, but for ER/Lu incomplete saturation leads to inconclusive results. Although the latter also happens in Ho/Tm SLs, the effect of the epitaxial strain on the bulk MSs is large. The MS cl...