J.M. Moreira

The role of the Ti surface roughness in the self-ordering of TiO2 nanotubes: a detailed study of the growth mechanism

Highly ordered TiO2 nanotubes (NTs) were synthesized by electrochemical anodization of Ti foils. We investigated the effect of the Ti surface roughness (applying different pre-treatments prior to the anodization) on the length, growth rate and degree of self-organization of the obtained NT arrays. The mechanisms related to the TiO2 NT formation and growth were correlated not only with the corresponding anodization curves but also with their appropriate derivatives (1st order) and suitable integrated and/or obtained parameters, to reveal the onset and end of different electrochemical regimes. This enables an in-depth interpretation (and physical–chemical insight), for different levels of surface roughness and topographic features. We found that pre-treatments lead to an extremely small Ti surface roughness, offer an enhanced NT length and also provide a significant improvement in the template organization quality (highly ordered hexagonal NT arrays over larger areas), due to the optimized surface topography. We present a new statistical approach for evaluating highly ordered hexagonal NT array areas. Large domains with ideally arranged nanotube structures represented by a hexagonal closely packed array were obtained (6.61 μm2), close to the smallest grain diameter of the Ti foil and three times larger than those so far reported in the literature. The use of optimized pre-treatments then allowed avoiding a second anodization step, ultimately leading to highly hexagonal self-ordered samples with large organized domains at reduced time and cost.

Temperature dependence and critical behaviour of the thermoelectric power in ferromagnetic TbZn compound

High-accuracy data on the thermoelectric power (S) are obtained for ferromagnetic single crystals of TbZn (CsCl structure), over a wide range of temperatures (80-300K). The general temperature dependence S(T) is analysed in the context of s-f scattering models. Information is obtained on such parameters as the s-f exchange constant G, the Fermi level EF, and the effect of simultaneous spin disorder and phonon scattering on the thermopower. Experiment suggests the possible existence of extra S contributions in TbZn, besides that arising from s-f scattering. Finally, the critical behaviour of dS/dT near the Curie point (Tc=199.6K) is investigated in detail, and related to that previously observed in the temperature derivative of the electrical resistivity (d rho /dT) in TbZn.

Magnetostriction of the NdCo5 uniaxial permanent magnet

Abstract Magnetostriction measurements, between ≈5 and 300 K, of powder magnetically aligned samples of the hexagonal compound NdCo 5 , in strong pulsed magnetic fields up to 15 T are reported. The measurement have been performed parallel, perpendicular and at 45° to the alignment c -axis, for the field also applied in those directions. This set of measurements allows us to determine the six irreducible magnetoelastic modes: λ α 11 , λ α 21 , related to the strain dependence of the isotropic exchange, and λ α 12 , λ α 22 , λ γ , λ ϵ , related to the strain dependence of the crystalline field anisotropy energy. Anomalies on the strains, associated with the spin reorientation (SR) regime are observed, in particular at the temperatures of beginning, T SR1 , and end, T SR2 , of the SR. The thermal variation of the strains is explained by the standard magnetostriction model, including an ingredients: exchange striction (varying as m 2 Nd ), single-ion anisotropy (varying as m 3 Nd ), (where m Nd is the reduc ed Nd sublattice magnetization), as well as an angular dependence of the form {sin 2 θ( H, T )−sin 2 θ(0, T )}, where θ( H, T ) and θ(0, T ), respectively, are the SR angles under field and spontaneous. The irreducible magnetoelastic coupling constants at 0 K have been estimated, although the point charge model is far from agreement with those results.

Spin-reorientation transitions in NdCo5 and critical effects on the electrical resistivity temperature derivative

High resolution electrical resistivity measurements ( rho d rho /dT) are used to investigate in detail the two spin-reorientation transitions of the hexagonal ferromagnetic NdCo5 compound which occur at TSR1=242 K and TSR2=283 K (T>TSR2 gives spontaneous magnetisation, Ms, parallel to the c axis; TSR1<T<TSR2 gives a conical structure; T<TSR1 produces Ms in the basal plane). The authors have obtained the temperature dependence of the angle theta (TSR1<T<TSR2) using rho (T) data. The results being in fair agreement with theta (T) derived from the anisotropy constants K1 and K2. However, the temperature derivative d rho /dT exhibits new (and very localised) delta -type singularities near TSR1 and TSR2. These new anomalies in d rho /dT are interpreted in terms of lattice softening effects at TSR1 and TSR2 and associated anomalies in the C33 elastic constant.

Electrical resistivity singularities and spin reorientation transitions in Pr2(Co1-xFex)17

The temperature- and field-induced spin reorientation transitions in uniaxial ferromagnetic Pr2(Co1-xFex)17 pseudo-compounds are studied using high-accuracy measurements of the electrical resistivity derivatives d rho /dT and d rho /dH. This technique avoids the need for single crystal samples. For x=0.2, a first-order spin reorientation transition is precisely located at Tsr=190K, where a sharp peak occurs in d rho /dT. The spontaneous magnetisation Ms then rotates abruptly from the basal plane to the c axis. A minute discontinuity was found in the resistivity at Tsr, Delta rho / rho approximately=0.003. For x=0, no spin reorientation transition is observed, confirming previous (single crystal) information that Ms always sits in the basal plane. For x=0.2 and T=77K, a field-induced spin reorientation transition was observed for a critical value H.c=5.0+or-0.2 kG, with a pronounced anomaly in d rho /dH. This just corresponds to the first-order Ms rotations inside those particular crystallites with c parallel to H (hard direction). Theoretical considerations are developed to characterise the anomalies associated with the spin reorientations in polycrystalline samples.

Aspects on the critical behaviour of transport properties in ferromagnetic metals

Abstract New experimental results are presented to extend information into less well known aspects concerning the behaviour of transport properties in ferromagnetic systems. We study in particular the critical behaviour of the temperature derivative of the electrical resistivity (dϱ/d T ) in the vicinity of a spin-reorientation transition ( T s ). In addition, the extreme sensitivity of dϱ/d T is used to investigate the role of magnetic domains in the vicinity of a ferro-paramagnetic phase transition ( T c ).

Universal behaviour of thermal and electrical transport coefficients near the néel temperature of Cr99.94Al0.06

Abstract Measurements of electrical and thermal transport coefficients in the antiferromagnetic alloy Cr 99.94 Al 0.06 reveal an anomaly near the Neel temperature T N . Temperature derivatives of the electrical resistivity and thermo-power are analysed and are shown to be proportional to each other above and below T N in the reduced temperature range 6 × 10 −2 ⩾| t |⩾ 4 × 10 −4 . This is consistent with the theoretical prediction of a universal critical behaviour of all transport coefficients.

Helimagnetism and field-induced phases in random Gd64Sc36 single crystals

We report magnetization, neutron diffraction and elastic constant measurements of the single-crystal rare earth alloy Gd64Sc36 as a function of magnetic field. For an a-axis field the material exhibits transitions from helix to fan to basal plane ferromagnet. At low temperatures the looked-in magnetic structure is very sensitive to the application of small fields (~5 mT).

Critical behaviour of the electrical resistivity at the spin reorientation transition of pure gadolinium

The behaviour of the temperature derivative of the electrical resistivity rho near a spin reorientation transition is discussed along a Landau-like theory. The strong diminution of the role of critical fluctuations leads to a sharp break in d rho /dT. Evidence is shown by new results on d rho /dT in polycrystalline gadolinium with very high purity ( Gamma approximately=85).

Complex magnetic phase behavior in Gd-rich Gd-Y single crystal alloys

The magnetic properties of single crystal gadolinium-rich Gd/sub 100-x/Y/sub x/ alloys are investigated using ac susceptibility, magnetization, and ultrasonic techniques, to determine the magnetic phase behavior in the region x<10. For Gd/sub 92.5/Y/sub 7.5/, observations are consistent with a canting of the magnetic moment away from the c-axis toward the basal plane at 243 K, and a return toward the c-axis at lower temperatures. The precedence of a phase change at 116 K is confirmed.