We report magnetoresistance curves of CoFe nano-oxide specular spin valves of MnIr/CoFe/nano-oxidized CoFe/CoFe/Cu/CoFe/nano-oxidized CoFe/Ta at different temperatures from 300 to 20 K. We extend the Stoner–Wolfarth model of a common spin valve to a specular spin valve, introducing the separation of the pinned layer into two sublayers and their magnetic coupling across the nano-oxide. We study the effect of different coupling/exchange (between the antiferromagnetic layer and the bottom sublayer) field ratios on the magnetization and magnetoresistance, corresponding with the experimentally observed anomalous bumps in low temperature magnetoresistance curves.

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The temperature and magnetic field dependences of the magnetoresistance ͑MR͒, superconducting quantum interference device magnetization M, its electrical resistance R and temperature derivative dR/dT ͑10-300 K͒ are reported for nonspecular CoFe/Cu/CoFe spin valves and specular spin valves formed by nano-oxidation of the pinned and free CoFe layers. The MR(T) increases linearly with decreasing temperature on both spin valves, and data extrapolation converges to zero MR practically at the same Curie temperature. At temperatures below ϳ175 K the specular spin valve MR(H) curves present two anomalous bumps not seen in the nonspecular curves. Also, for the nonspecular spin valve a clear relation is visible between M and MR curves, which is not the case for the specular spin-valve curves. In the specular spin valve, dR/dT presents an anomalous enhancement after Tϳ150 K, which is discussed in terms of electron scattering in the nano-oxide layers. The application of a saturating magnetic field suppresses most of the anomaly, indicating its magnetic origin.

Low-temperature specific-heat measurements on the tetragonal TbRu2Si2 intermetallic compound show the existence of a magnetic transition at T, =4.7 K, where a [ -", 0,0] square modulated phase with the magnetic moments directed along the c axis was generally accepted to exist. Such a transition, which exhibits all the characteristic features of a ) -type order-disorder transition with extended fluctuation effects on both sides of T, can be explained through a. phase shift creating (100) nodal magnetic planes separated by 13 atomic spacings. The observed entropy change AS=0.4 J mol ' K ' is very close to the value R/13 ln2=0. 44 predicted for a -,~molar fraction of Ising 2 spins. A two-dimensional isotropic Ising model is shown to reproduce well the main aspects of the reported transition.

We present a study of the spin disorder resistivity ([Formula: see text]) and the electronic specific heat coefficient (γ) in Gd(4)(Co(1-x)Cu(x))(3) compounds, with x = 0.00, 0.05, 0.10, 0.20 and 0.30. The experimental results show a strongly nonlinear dependence of [Formula: see text] on the average de Gennes factor (G(av)) which, in similar intermetallic compounds, is usually attributed to the existence of spin fluctuations on the Co 3d bands. Values of γ were found around 110 mJ mol(-1) K(-2) for the Gd(4)(Co(1-x)Cu(x))(3) compounds, much larger than 38.4 mJ mol(-1) K(-2) found for the isostructural nonmagnetic Y(4)Co(3) compound. Using a novel type of analysis we show that the ratio [Formula: see text] follows a well-defined linear dependence on G(av), which is expected when appropriate dependencies with the effective electron mass are taken into account. This indicates that band structure effects, rather than spin fluctuations, could be the main cause for the strong electron scattering and γ enhancement observed in the Gd(4)(Co(1-x)Cu(x))(3) compounds. A discussion on relevant features of magnetization and electrical resistivity data, for the same series of compounds, is also presented.