Electronic structure and magnetic properties of Gd3Co11B4 compound

The electronic structure of hexagonal Gd₃Co₁₁B₄ compound has been studied by X-ray photoemission spectroscopy (XPS) and ab initio self-consistent tight binding linear muffin tin orbital (TB LMTO) method. We have found a good agreement between the experimental XPS valence band spectra and theoretical LMTO calculations. Results showed that the Gd₃Co₁₁B₄ compound is ferrimagnetic with the calculated total magnetic moment M=14.29 μ_B/f.u. The values of the magnetic moments on Co atoms strongly depend on the local environment. We have also compared the electronic structure and magnetic properties of Gd₃Co₁₁B₄ compound with those of Nd₃Co₁₁B₄ compound.     

Characterization of porous silicon prepared by powder technology

In this study, we have proposed the powder technology as new method for preparation of bulk porous silicon. Formation of porous silicon by high-energy ball milling followed by pressing and sintering was studied by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy (XPS). A crystalline wafer with (1 1 1) orientation was extensively ball milled up to 72 h leading to a decrease in average crystallite size up to 15 nm. The most significant reduction of crystallite size was observed after milling process for about 24 h. The nanopowders were then pressed into pellets at a pressure up to 400 MPa and sintered at 1173 K for 60 min in a high purity argon atmosphere. Results showed that after sintering the material became porous with uniform porosity in whole volume, independently of the sinter size. It is not possible to prepare such porous materials using the conventional electrochemical etching, where the porous structure depth usually does not exceed tens of micrometers. Core-level XPS studies showed very good agreement between peak positions of the sintered porous silicon and in-situ prepared polycrystalline 20 nm-Si thin film or single-crystalline Si (1 1 1) wafer. Furthermore, the valence band spectra measured for sintered samples are broader compared to those measured for the Si (1 1 1) wafer or polycrystalline Si thin film. On the other hand, the shape and broadening of the valence bands measured for the sintered samples are in very good agreement with those reported for electrochemically prepared porous silicon.     

Biquadratic exchange interactions in the Europium monochalcogenides

We have examined the biquadratic exchange interaction strengths in the Europium monochalcogenides EuO, EuS, EuSe and EuTe using magnetization data of the paramagnetic phase and elaborate the consequences this additional interaction mechanism has on the magnetic phase diagrams of EuSe and EuTe. It is shown that the cubic susceptibility χ₃ obeys a Curie-Weiss law at suffciently high temperatures and that the associated Curie-Weiss temperature θ₃ is a measure for the biquadratic interaction strength. For all these materials the biquadratic interaction is ferromagnetic (θ₃ > 0). This leads to a conflicting situation in the case of EuTe for which θ₁ < 0. We attribute the peculiar observation, that the MnO superstructure reflection intensities as observed with neutron scattering correspond only to 0.6 of that moment expected for perfect magnetic order, to the presence of biquadratic interactions. The critical field B_c follows a T² law in the spin-wave regime (T < 0.8 K) for EuTe and EuSe but for these two materials with an antiferromagnetic ground state the cubic susceptibility χ₃ diverges at a temperature T* which is 2.5 K and 1.2 K above the ordering temperature, respectively. In the temperature range T_c < T < T* the magnetization curves exhibit some weak but definite anomaly which might be interpreted as a field-induced transition into the ferromagnetic state. A new multicritical point has been identified along the critical field curve B_c of EuSe.     

Interface magnetic anisotropy in metallic superlattices

We have prepared Pd/M and Cu/M (M = Fe, Co, Ni) magnetic superlattices using a dc sputtering and standard UHV e-beam evaporation. Sample quality was confirmed by the intense satellite peaks in the θ−2θ X-ray diffraction patterns. For all the systems studied, torque measurements revealed positive intrinsic uniaxial anisotropy but less than 2πM²_s. Results showed that the main source of the interface anisotropy is Néel surface anisotropy.     

Exchange coupling effects in NiO/Co and NiO/permalloy bilayers

NiO/Co and NiO/Ni₈₀Fe₂₀ bilayers were prepared at 293 onto SiO₂(1 0 1)/Si(1 1 1) and glass substrates using UHV (5×10⁻¹⁰ mbar) RF/DC magnetron sputtering. Results on magnetic measurements showed that the exchange biasing and coercive fields are inversely proportional to the Co and Ni₈₀Fe₂₀ (Py) layer thickness down to 2 nm. A maximal RT coupling energy for the NiO–Co and NiO–Py interface was estimated as 0.04 and 0.03 mJ/m² for the samples prepared onto SiO₂(1 0 1)/Si(1 1 1) substrates.     

Structure and Electronic Properties of La(Ni,Al)5 Alloys

Nanocrystalline and polycrystalline La(Ni,Al)₅ alloys were prepared by mechanical alloying (MA) followed by annealing and arc melting method, respectively. The amorphous phase of MA samples forms directly from the starting mixture of the elements, without other phase formation. Heating the MA powders at 800 °C for 1 h resulted in the creation of hexagonal CaCu₅‐type nanocrystalline compound with mean crystallite size less than 80 nm. XPS studies showed that the shape of the valence band measured for the arc melted (polycrystalline) LaNi₅ is practically the same compared to that reported earlier for the single crystalline sample. The substitution of Ni in LaNi₅ by Al leads to significant modifications of the electronic structure of the polycrystalline sample. On the other hand, the XPS valence band of the MA nanocrystalline LaNi_4.2Al_0.8 alloy is considerably broader compared to that measured for the polycrystalline sample. The strong modifications of the electronic structure of the nanocrystalline LaNi_4.2Al_0.8 alloy could significantly influence on its hydrogenation properties.     

Magnetic properties of electrolytic Co-Pd alloy films

A method has been developed to produce Co_100−xPd_x (40 < x < 90) alloy films by electrodeposition from a single bath. In Pd-rich composition (70 < x < 85 at %) we have observed strong perpendicular magnetic anisotropy, attributed to the stress-induced anisotropy due to tensile stresses introduced into the magnetostrictive films during electrodeposition.     

Structure and magnetic properties of wedged Co/Ti multilayers

Co/Ti multilayers with wedge-shaped Co or Ti sublayers were prepared using UHV (5×10⁻¹⁰ mbar) DC/RF magnetron sputtering. The planar growth of the Co and Ti layers was confirmed in situ by X-ray photoelectron spectroscopy. Cobalt sublayers grow on sufficiently thick titanium sublayers in the soft magnetic nanocrystalline phase up to a critical thickness d_crit3.0 nm. For a thickness greater than d_crit, the Co sublayers undergo a structural transition to the polycrystalline phase with much higher coercivity. Furthermore, for the Co/Ti multilayers with nanocrystalline Co sublayers with d_Co=2.7 nm we have observed a significant drop of the coercivity — typically from H_c3.5 kA/m to H_c0.2 kA/m — for Ti thickness d_Ti0.35 nm. The above effect could be explained by the existence of a minimum Ti sublayer thickness (d_min0.35 nm), which is required for the nanocrystalline growth of Co, and/or the formation of quasi-continuous non-magnetic layers for d_Tid_min giving rise to a decrease of the exchange energy between Co sublayers. Magnetic domains and walls studies revealed the structural transitions of the Co sublayers.     

Interlayer Exchange Coupling Across Quasi-Amorphous Ti-Co and Zr-Co Spacers

Co/Ti and Co/Zr multilayers with wedge-shaped and constant-thickness Ti and Zr sublayers were prepared using UHV DC/RF magnetron sputtering. Results showed that the Co sublayers are ferromagnetically coupled up to Ti and Zr spacer thickness of about 1.9 and 2.4 nm, respectively. Furthermore, a weak antiferromagnetic coupling of the Co sublayers was observed for a Ti (Zr) thickness range between 1.9 and 2.7 nm (2.4 and 3.2 nm). The Co sublayers are very weakly exchange coupled or decoupled for d _Ti 2.7 nm and d _Zr 3.2 nm. The rapid decrease of the interlayer exchange coupling could be explained by its strong damping due to formation of a non-magnetic quasi-amorphous Ti-Co and Zr-Co alloy layer at the interfaces.     

Influence of the Regression Technique on the Extrapolated Retention Values of Single-Point Adsorption Compounds on Silica Gel

JPC – Journal of Planar Chromatography – Modern TLC - The retention (RM) values of nine one-point adsorption model compounds: diphenylamine, indol, 2-naphtol, 1-naphtol, 1-naphtylamine,...