T. M. Seixas

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.

Electrical resistivity of Gd4(Co1−xCux)3 compounds

In order to study the effects caused by substituting Co for Cu on the electronic and transport properties of Gd4(Co1−xCux)3 compounds with x=0.05, 0.1, 0.2, and 0.3, electrical resistivity [ρ(T)] measurements were performed in the temperature range 12–300K. The paramagnetic-ferrimagnetic and spin-reorientation transitions are marked by clear anomalies in dρ∕dT. For x=0.2, however, the onset of ferrimagnetic order at TC produces just a comparatively small peak in dρ∕dT. This fact is compatible with a hypothetical mixed ferrimagnetic phase exhibiting partial disorder of the magnetic moments, as suggested by magnetic measurements [T. M. Seixas, J. M. Machado da Silva, H. F. Braun, and G. Eska (unpublished)]. The spin disorder resistivity shows a pronounced decrease with increasing x that can be attributed to the observed reduction of the magnetic moment of Co atoms. This stresses the important role played by 3d-band electrons in both magnetic state and in the strong s‐d electron scattering. Magnon-induced el...

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 Importance of Measurement Data Spacing

When conducting experiments involving the measurement of physically related quantities, choosing an appropriate spacing for the experimental independent variable is a crucial procedure whose consequences may go beyond data graphical visualization. This is particularly true if the measured quantities are nonlinearly related and experimental errors are nonuniform. In this work, we show that measurement procedures should provide a higher density of data points in the range where experimental errors are larger, to more effectively average out random errors and thus mitigate accuracy errors in parameter determination. Knowing the effective experimental independent variable helps in defining an adequate data spacing.

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.

Paramagnetic behaviour of Ce1−xGdxCo2 compounds

Abstract Magnetic susceptibility studies have been made on the cubic C15 (MgCu2-type) Laves phases Ce1−xGdxCo2 in the temperature range 300–900 K. Well above Tc, the inverse susceptibility follows a linear temperature dependence, as predicted by a phenomenological mean-field model describing a three-component ferrimagnetic alloy with localized magnetic moments. The values of the Curie constant determined in this way decrease regularly with x from x = 1 down to x = 0.4. Around x = 0.3, the Curie constant exhibits a small local maximum. This suggests the onset of a spin-fluctuation regime at low x, possibly due to the reduction of the internal field with the increase of cerium content.

The Role of Data Range in Linear Regression

Measuring one physical quantity as a function of another often requires making some choices prior to the measurement process. Two of these choices are: the data range where measurements should focus and the number (n) of data points to acquire in the chosen data range. Here, we consider data range as the interval of variation of the independent variable (x) that is associated with a given interval of variation of the dependent variable (y). We analyzed the role of the width and lower endpoint of measurement data range on parameter estimation by linear regression. We show that, when feasible, increasing data range width is more effective than increasing the number of data points on the same data range in reducing the uncertainty in the slope of a regression line. Moreover, the uncertainty in the intercept of a regression line depends not only on the number of data points but also on the ratio between the lower endpoint and the width of the measurement data range, reaching its minimum when the dataset is ce...

Formative assessment of laboratory work

We propose a method for formative assessment of laboratory work which provides a running updated diagnostic of the student performance and skills in all predefined formative areas and sub-areas. The method is easily adaptable to any type of report or laboratory performance assessment, provided that a graded assessment profile can be defined.

Spin Disorder Resistivity and Electronic Specific Heat of Gd4(Co1-xCux)3 Compounds

In this work, we present a study of the spin disorder resistivity ( ρm∞) and the electronic specific heat coefficient ( γ) in Gd4(Co1-xCux)3 compounds, with x = 0, 0.05, 0.10, 0.20, 0.30. The experimental results show a strongly non-linear dependence of ρm∞ on the de Gennes factor which, in similar intermetallic compounds, is usually attributed to the existence of spin fluctuations on the Co 3d bands and its amplification by the thermal disorder of the Gd magnetic moments through the Gd-Co exchange coupling. Using a novel combined analysis of ρm∞ and γ, we show, however, that only electron band structure changes are involved in the anomalous behaviour of ρm∞ and that a linear dependence of ρm∞ on the de Gennes factor is obtained when the variation of the effective mass is properly taken into account.