L. Morellon

Structural, magnetic and transport properties of Sr2Fe1−xCrxMoO6−y

Abstract The series of Sr 2 Fe 1− x Cr x MoO 6− y has been synthesized and studied by means of a wide set of experimental techniques. These samples belong to the family of A 2 BB′O 6 double perovskites with a ferrimagnetic ground state. The replacement of Fe by Cr leads to samples with a high oxygen deficiency and to increasing cation disorder between B and B′ sites. The magnetic moments of these samples decrease as the iron content does. Electrical properties change as well strongly along the series: while Sr 2 FeMoO 6 is metallic, the rest of the samples show a semiconducting behavior with an activation energy that increases as the content of Cr does. Only Fe-rich samples exhibit large negative magnetoresistance with the low-field response characteristic of a half-metallic system. The Cr-rich samples display instead a small linear negative magnetoresistance.

Magnetoelastic behaviour of Gd5Ge4

A complete investigation of the complex magnetic behaviour of Gd5Ge4 by means of linear thermal expansion and magnetostriction measurements (5–300 K, 0–120 kOe) has been carried out. Our results support the suggested existence in this system of a coupled crystallographic–magnetic transition from a Gd5Ge4-type Pnma (antiferromagnetic) to a Gd5Si4-type Pnma (ferromagnetic) state. Strong magnetoelastic effects are observed at the field-induced first-order magnetic–martensitic transformation. A revised magnetic and crystallographic H– T phase diagram is proposed.

Structural and magnetic characterization of the new ternary phase Tb3(Fe1-xTix)29

We report the existence and characterization of the stable ternary phase Tb3(Fe1-xTix)29. The structural characterization by X-ray powder diffraction is evidence for a monoclinic structure (P21/c space group) with refined lattice parameters a=10.583(1) AA, b=8.5116(7) AA, c=9.6736(9) AA and beta =97.018(5) degrees . A large magnetovolume effect has been observed in the volume thermal expansion at the order temperature Tc=455 K. The anisotropy has been measured by using the singular point detection technique. Two distinct anisotropy fields of relatively high intensity have been detected. The HA values measured at 6.4 T and 1.9 T at 293 K and both increase markedly with decreasing temperature. Below 200 K, the approach to saturation corresponding to the highest HA develops into a first-order magnetization process.

Pressure effects in the giant magnetocaloric compounds Gd5(SixGe1?x)4

We report a study of the effect of hydrostatic pressure up to 9 kbar on selected compounds of the Gd5(SixGe1−x)4 series (x = 0.8, 0.45, 0.1) by means of ac magnetic susceptibility, compressibility, and linear thermal expansion measurements. The pressure-induced increase of the transition temperatures at the second-order boundaries of the phase diagram is rather moderate: (x = 0.8) and (x = 0.1). This effect is stronger in the range, where (x = 0.45,0.1), indicating that the ferromagnetic ordering can be simultaneously driven through a pressure-induced structural transformation. The values of d lnTC/d lnV calculated with the use of the measured value of compressibility () are significantly lower than those estimated from the concentration dependence of the lattice cell volume, thus demonstrating that the dependence of the transition temperatures upon changing the Si/Ge ratio across the series cannot be explained by a pure volume effect.

Anisotropy and magnetic ordering in the new phase Nd3(FeTi)29

Recently, a new rare-earth iron-rich phase 3:29 has been found to be stabilized using Ti. The Curie temperature TC=413 K is considerably higher than in the related Nd2Fe17 compound. The easy magnetization direction lies in the basal plane of the monoclinic structure at room temperature. A magnetic anomaly has been detected at TSR=233 R in the thermal dependence of the AC initial susceptibility, which has been related to a spin reorientation transition from an easy-plane phase towards an easy cone at low temperatures. Thermal expansion measurements clearly show an extra magnetic contribution at TC, a weak anomaly being also perceptible at TSR. The anisotropy field measurements using the SPD technique have confirmed the existence of these two magnetic phases.

Effect of pressure on the magnetocrystalline anisotropy of (ErxR1-x)2Fe14B intermetallics

A systematic study of the effect of pressure up to 1 GPa on the spin reorientation transition (SRT) phenomena has been performed on several series of pseudoternary compounds (ErxR1-x)2Fe14B for R identical to Nd, Gd, Dy and Y using a low-field AC susceptibility technique. The variation in the SRT temperature TSR under pressure has been used as an indication of the effect of the pressure on the crystal electric field (CEF) at the rare-earth ion site. The authors have observed that the pressure effect on the Er2Fe14B compound ( Delta TSR/ Delta p=-20 K GPa-1) was not affected by the substitution of Er by Gd and Y ions. A completely different high-pressure behaviour has been observed for the (ErxDy1-x)2Fe14B and (ErxNd1-x)2Fe14B compounds, in which the variation in TSR under pressure decreases for increasing content of Dy and Nd. This different behaviour has been attributed to the strong influence of pressure on the CEF interactions. Using these results the authors have determined the variation in the CEF parameters under applied pressure.

Transport and magnetic study of the spin reorientation transition in the Tb5(Si0.5Ge0.5)4 magnetocaloric compound

Detailed measurements of the electrical resistivity ?(T), thermopower S(T) and magnetization of Tb5(Si0.5Ge0.5)4 in the vicinity of the spin reorientation transitions observed in this compound are reported. Our results indicate a complex spin reorientation process associated with three different lattice sites occupied by the Tb ions. We identify two critical transition temperatures: one at TSR1 = 57?K, as previously reported, and a new one at TSR2 = 40?K. A simple model based on an approximate magnetic anisotropy energy is presented; it gives a satisfactory qualitative description of the main features of the reorientation processes.

Epitaxial stabilization of the perovskite phase in (Sr1-xBax)MnO3 thin films

A novel mechanism of ferroelectricity driven by off-centering magnetic Mn(4+) ions was proposed in (Sr1-xBax)MnO3, in its ideal perovskite phase, which yields enormous expectations in the search for strong magnetoelectric materials. Still, the desired perovskite phase has never been stabilized in thin films due to its extremely metastable character. Here, we report on a thorough study of the perovskite phase stabilization of (Sr1-xBax)MnO3 thin films, 0.2 ≤ x ≤ 0.5, grown by pulsed laser deposition onto (001)-oriented perovskite substrates. X-ray diffraction measurements and scanning transmission electron microscopy reveal that, under appropriate deposition conditions, the perovskite phase is fully stabilized over the nonferroelectric hexagonal phase, despite the latter being increasingly favored on increasing Ba-content. Moreover, we have managed to grow epitaxial coherent cube-on-cube (Sr1-xBax)MnO3 films upon strains ranging from 0% to 4%. Our results become a milestone in further studying perovskite (Sr1-xBax)MnO3 thin films and pave the way for tailoring ferroic and magnetoelectric properties either by strain engineering or Ba-doping.

1s2p resonant inelastic x-ray scattering-magnetic circular dichroism: A sensitive probe of 3d magnetic moments using hard x-ray photons

We investigated the magnetic properties of thin magnetite and iron films grown on MgO(001) by means of hard x-ray photon-in photon-out probe, namely, 1s2p resonant inelastic x-ray scattering-magnetic circular dichroism (RIXS-MCD). A comparison of the spectra acquired from bulk and thin layer magnetite samples reveals their nearly identical shape. Hysteresis loops measured with RIXS-MCD also show a close similarity to the vibrating sample magnetometer profiles supporting the conclusion that the technique can be applied for the quantitative analysis of element and site specific magnetization in buried films containing transition metal elements. We show that Fe 1s2p RIXS-MCD is insensitive to the magnetic signal of iron impurities naturally dispersed in monocrystalline MgO substrates. The latter, combined with a unique feature of resonant inelastic x-ray scattering, namely the ability to tune incident and emitted photon (transfer) energy, allows us to separate the dichroic signal of metal layers from that of...

MAGNETOSTRICTION IN MIXED VALENT MAGNETIC OXIDES

Since the early discovery of the magnetostriction effects on iron by Joule [1] and the Invar alloys by Ch. E. Guillaume [2], a vast discipline emerged in solid state physics. From the basic point of view based on thermodynamic and symmetry considerations, the first phenomenological explanation of the magnetostriction was established by Becker R., Doring W. (1939) [3], Lee E. W. (1955) [4]. An important contribution to the explanation of the magnetostriction incorporating a Quantum-Mechanics formalism is due to Callen E.R.&Callen H.B. (1963) [5, 6] who really established the grounds of the modern magnetostriction theory for localised magnetic moment systems. In a broad sense, it is considered that magnetostriction has as origin the spin-orbit coupling, which is manifested in two different kinds of magnetostriction behaviours. One is isotropic, giving rise to volume effects, and the other of anisotropic nature, having its origin in a local distortion of the lattice, due to a preferential orientation of the angular moment of the magnetic electronic charge cloud. Those mechanisms are well explained in systems with localised magnetic moments. Good examples of this effect are observed in rare earth metals [7, 8], and their alloys, mainly with 3d metals (Fe, Co, Ni), in the Laves phase structure (RM2) [9, 10, 11] and also in cubic RZn [12, 13]. The largest room temperature effect was found in Terfenol (Tb-Dy)Fe2 alloy [9], which is the base of the commercial materials for magnetostrictive applications. In these systems, the isotropic effect, i.e. volume magnetostriction, is small and originated either by field induced change of the intrinsic magnetisation (paraprocess), giving rise to the forced magnetostriction, or by the change with the distance of the exchange interaction between localised magnetic moments.