No abstract available

[en] We report very detailed measurements of the magnetoresistance (MR) in the vicinity (±5 K) of the spin reorientation temperature (T_SR) for pure Gd and Gd_1-xY_x single crystals with x=0.025 and 0.075. The temperature dependence of the a-axis magnetoresistance in pure Gd is always negative in the temperature range studied. In the dilute Gd-Y samples the low-field MR changes from negative above T_SR to positive for T< T_SR, becoming again negative below some lower temperature T^*, where another phase transition takes place

[en] We study the magnetic effects of the nano-oxide layers of specular spin valves, based on zero-field-cooling and field-cooling (under H=H₀) experiments in the 320-15 K temperature range. One finds a small increase (decrease) of ΔR/R at 15 K after H₀>0 (<0) cooling, but a sharp ΔR/R minimum occurs when H₀=-500 Oe. Also ΔR/R(T,H₀) departs from the ZFC curve at T∼175 K. These features are related to magnetic ordering in the oxide layers

[en] The temperature- and field-induced spin reorientation transitions in uniaxial ferromagnetic Pr₂(Cosub(1-x)Fesub(x))₁₇ pseudo-compounds are studied using high-accuracy measurements of the electrical resistivity derivatives drho/dT and drho/dH. This technique avoids the need for single crystal samples. For x = 0.2, a first-order spin reorientation transition is precisely located at Tsub(sr) = 190 K, where a sharp peak occurs in drho/dT. The spontaneous magnetisation Msub(s) then rotates abruptly from the basal plane to the c axis. A minute discontinuity was found in the resistivity at Tsub(sr), Δrho/rho approximately equal to 0.003. For x = O, no spin reorientation transition is observed, confirming previous (single crystal) information that Msub(s) always sits in the basal plane. For x = 0.2 and T = 77 K, a field-induced spin reorientation transition was observed for a critical value Hsub(c) = 5.0 +- 0.2 kG, with a pronounced anomaly in drho/dH. This just corresponds to the first-order Msub(s) rotations inside those particular crystallites with c parallel to H (hard direction). Theoretical considerations are developed to characterise the anomalies associated with the spin reorientation transitions in polycrystalline samples. (author)

[en] A detailed study of the combined effect of divalent ion and A-site vacancy self-doping in ferromagnetic La_1-x-yCa_xOe_yMnO₃ manganite system is presented. It is found that the system can accommodate high vacancy content, with a strong lattice distortion for y>0.12. Near the stoichiometric maximum T_c (x=0.33, y=0), vacancies do not degrade T_c, and the magnetoresistance can even be enhanced

[en] Fairly detailed and accurate data is presented for the Hall resistivity (rhosub(H)), temperature derivative (drhosub(H)/dT), electrical resistivity (rho), and derivative drho/dT in a ferromagnetic Tb₇₅-Gd₂₅ polycrystalline alloy, over a wide range of temperatures (77-360 K). Magnetic susceptibility data(chi) near and above the Curie point is also included. The different contributions to the electrical resistivity are separated out, including the spin-disorder, residual and phonon terms, the results being compared with theory. For the Hall resistivity, the normal (rhosub(H)sup(0)) and the extraordinary (rhosub(H)sup(s)) terms are obtained, with the corresponding Hall constants (R₀, Rsub(s)). Very sharp anomalies are observed both in drho/dT and drhosub(H)/dT in the transition region, indicative of a well defined critical point in our concentrated random magnetic alloy. We Obtain Tsub(c)=244,79 K. For drhosub(H)/dT the situation is complex, due to the effect of the applied magnetic field. Our analysis shows the existence of important fluctuation effects near Tsub(c), as given by the derivative dRsub(s)/dT. The extraordinary Hall resistivity in Tb₇₅-Gd₂₅ is extensively analysed here, in order to separate out the different contributions, i.e. the skew and side-jump terms, including the effects of several possible scattering Hamiltonians (Kondo, Smit). We show that rhosub(H)sup(s) in Tb₇₅-Gd₂₅ can be accounted for in terms of a Kondo (skew and side-jump) and a side-jump Smit contribution. Finally, the Curie temperature of Tb₇₅-Gd₂₅ is related to the transition points of Tb and Gd, using the theory of binary rare earth random magnetic alloys

[en] The electrical resistivity rho and its temperature derivative dsub(rho)/dT of two TbZn monocrystals have been studied over a large temperature range encompassing the Curie temperature (Tsub(c) approximately 199.56 K) and the spin reorientation temperature (Tsub(r) approximately 61.9 K). Different experimental conditions have been used to sort our the interesting features. Particular emphasis is put on the critical behaviour analysis. A detailed study shows that a scaling law behaviour characterised by a finite but fairly small critical exponent cannot be experimentally distinguished from a logarithmic dependence above Tsub(c). The cross-over temperature to the mean field behaviour is also pointed out. Outside the critical region, the anomalously large dsub(rho)/dT values observed over most of the ferromagnetic range are explained using a mean field model. Finally, the critical behaviour of dsub(rho)/dT near Tsub(r) is analysed, and the resemblance to the anomaly observed in the specific heat is pointed out. (author)

No abstract available

[en] Experimental observation of the first-order nature of the spin reorientation transition in polycrystalline Er₂Fe₁₄B through high resolution resistivity, thermopower and ⁵⁷Fe Moessbauer data is reported. Both a steplike decrease in resistivity and an increase in thermopower over a very narrow temperature interval around 323 K indicate a discontinuous spin reorientation transition. This is consistent with the change in sign of the quadrupolar shifts at all Fe sites at this temperature, derived from the Moessbauer spectra, which are indicative for the transition from planar to uniaxial anisotropy with increasing temperature. The symmetrical lineshape of the sharp singularity recorded in the temperature derivative of resistivity provides unambiguous evidence for the first-order nature of the spin reorientation. copyright 1997 American Institute of Physics

[en] A detailed investigation and characterization is made of the magnetic phase transitions that occur in the cubic CeFe₂ Laves phase compound using high resolution direct measurements of the temperature derivative of the electrical resistivity (drho/dT). The results confirm the existence of a spin reorientation transition (SRT) in the ferromagnetic phase of CeFe₂ at T/sub sr/ approximately (132 +- 1) K, of second order character. This transition can be understood in terms of a [100] → [uuw] spin reorientation transition due to a non-negligible K₃ term in the expression of the magnetic anisotropy energy. Near the Curie temperature, critical analysis of drho/dT data shows that CeFe₂ behaves as an isotropic Heisenberg ferromagnet with a critical temperature T/sub c/ approximately (227.5 +- 0.1) K and a critical exponent α = 0.10 +- 0.02. This value is consistent with the theoretical expectancy based on the magnetic specific heat. (author)