M. A. Salgueiro da Silva

Anomalous magnetoresistance behavior of CoFe nano-oxide spin valves at low temperatures

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.

Comparative study of magnetoresistance and magnetization in nano-oxide specular and nonspecular MnIr/CoFe/Cu/CoFe spin valves from 10 to 300 K

The temperature and magnetic field dependences of the magnetoresistance (MR), superconducting quantum interference device magnetization M, its electrical resistance R and temperature derivative dR/dT (10–300 K) 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 ∼175 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 T∼150 K, which is discussed in terms of electron scattering in the nano-oxide layers. The application of a saturating magnetic field suppresses most of ...

Transport properties and magnetic phases of NdRu2Si2

The tetragonal NdRu2Si2 compound exhibits a sequence of different magnetic phases below TN = 23.5 K: (i) first, a purely sine-wave modulation with q = (2π/a)(0.13, 0.13, 0), (ii) below T∗ = 15 K this structure gradually squares-up (3 q-harmonics) and (iii) below Tc ≈ 9 K the ferro-magnetic phase sets in. Accurate measurements of the electrical resistivity (p, dp/dT), thermoelectric power (S, dS/dT) and high resolution differential thermal analysis (DTA) are reported. Sharp singularities occur in dp/dT and dS/dT at Tc and TN. The onset of the square-modulated phase (T < 15 K) produces a pronounced decrease in p(T). The DTA data show that the TN-anomaly is of 2nd order. Also, for the order-order transition at Tc (modulated-ferro) the DTA data give no evidence for a latent heat. A theoretical account is given on the dominant effects observed in each magnetic phase. The anomalous behaviour of p and S at high temperature are here associated to crystal field effects.

Specific heat of Gd4Co3

The specific heat (C(T)) of Gd₄Co₃ was measured in the temperature range 2-300 K and its magnetic contribution (C(m)(T)) was determined using a new method that fits the electronic specific heat coefficient (γ) and the Debye temperature (θ(D)) by constraining the resulting magnetic entropy (S(m)(T)) to saturate at temperatures far above the Curie temperature (T(C)). C(m)(T) exhibits a low-temperature bump originating from thermal excitation of gapped spin waves, which is responsible for pronounced peaks, at ≈35 K, in both C(m)/T and the temperature derivative of the magnetic contribution to electrical resistivity (dρ(m)/dT). Apart from in the vicinity of T(C), an excellent global correlation was found between C(m)/T and dρ(m)/dT. Our results provide strong support for the consistency of the new method proposed for the determination of C(m)(T) and rule out any major role of short-range order on Gd moments or d-electron spin fluctuation effects in the paramagnetic phase. A comparative analysis with other methods used in similar compounds points to the need for a better evaluation of C(m)(T) in such compounds, especially in the magnetically ordered phase, where a deficient evaluation of C(m)/T has a larger impact on the S(m)(T) curve.

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).

Magnetic behavior and phase diagram of Gd4(Co1−xCux)3 compounds

In order to study the effects caused by substituting Co for Cu on the magnetic properties of Gd4(Co1−xCux)3 compounds with x=0.05, 0.1, 0.2, and 0.3, superconducting quantum interference device magnetization measurements were performed on polycrystalline samples in the temperature range of 5–300 K using applied magnetic fields from 0 to 5 T. Experimental results show that the substitution of Co for Cu produces an increase in magnetic anisotropy, which is accompanied by important changes in the magnetic ordering of these ferrimagnetic compounds with respect to that of Gd4Co3. A magnetic T-x phase diagram comprised of c-axis, canted, and possibly partially disordered ferrimagnetic phases, is proposed. The calculated average saturation magnetic moment of Co atoms decreases exponentially with increasing Cu content. Such behavior of MCo(x) may originate from the combined effect of the 3d band filling by 4s electrons from Cu atoms and a strong dependence of Co moment on the number of local Co neighboring atoms.

The magnetic properties of a Gd72Sc28 single crystal

We present a discussion of the magnetic properties and magnetic phase transitions of Gd72Sc28 monocrystalline alloys based on experimental results of the magnetization (M) and transport properties ( rho , d rho /dT, Delta rho / rho ). In particular, a spin reorientation transition is observed in the canted ferromagnetic phase. Using dM/dH and d( Delta rho / rho )/dH data we have obtained the H-T magnetic phase diagram of this particular alloy composition as a function of basal plane applied magnetic field, H.

Magnetic phase diagrams of NdRu2Si2 and TbRu2Si2 compounds

Here we report a detailed study providing complete H‐T phase diagrams for both NdRu2Si2 and TbRu2Si2 intermetallic compounds, based on high resolution SQUID magnetization measurements, in the temperature range 2–300 K and in applied magnetic fields up to 5 T, using c‐axis magnetically oriented powders. For NdRu2Si2, our results are in good agreement with recent neutron diffraction work under applied magnetic fields, confirming the appearance of an intermediate (temperature and field) ferrimagnetic phase responsible for the two‐step metamagnetic process observed in the temperature range 16–22.5 K. Our isofield magnetization curves provide unambiguous evidence on the decomposition of this ferrimagnetic phase into ferromagnetic and antiferromagnetic components. Furthermore, at low temperatures and low fields, we observe, in this compound, the presence of thermal irreversibility effects. For TbRu2Si2, we present an experimental account, on its H‐T phase diagram, which shows striking differences with respect t...

Thermoelectric power of Gd4(Co1-xCux)3 compounds

We report results of thermoelectric power measurements (S(T)) in ferrimagnetic Gd4(Co1-xCux)3 compounds with x = 0.05, 0.1, 0.2, and 0.3, in the temperature range 8–300 K. Whereas in Gd4Co3, S(T) is always negative, for x > 0 the substitution of Co for Cu gives rise to the appearance of a low temperature positive maximum in S(T) at around 30 K. Based on our previous study of these compounds, we argue that this maximum in S(T) originates from electron-magnon scattering and is sensitive to electron band structure changes resulting from the substitution of Co for Cu and the accompanying reduction in the ratio between the electron-magnon and the electron-phonon scattering strengths. The decreasing role of Co 3d electrons with the progressive substitution of Co for Cu, evidenced by a strong reduction in the spin disorder resistivity and the Co magnetic moment, is seen to be crucial for the existence of such a low temperature maximum in S(T) for x > 0.

Magnetoresistance and SQUID measurements in Gd70.5Sc29.5 single crystals

Abstract Gd1−xScx random alloys exhibits a complexH-T magnetic diagram, with several multicritical points. Magnetic transitions and the corresponding critical fields are accurately indentified through the field derivatives d ( Δϱ ρ ) d H . Extensive Δϱ ϱ data in conjucntion with SQUID measurements allow a thorough analysis of the different characteristics of the field induced magnetic phases.