J. H. Belo

On the Curie temperature dependency of the magnetocaloric effect

We investigate the magnetocaloric effect dependency on the most important microscopic parameters of ferromagnetic materials, such as the Curie temperature (TC), the spin value (J), and the magnetic field change (ΔH). Second- and first-order phase transition systems are considered, using the Bean-Rodbell model [C. P. Bean and D. S. Rodbell, Phys. Rev. 126, 104 (1962)] of magnetovolume interactions on the Weiss mean-field model [P. Weiss, J. Phys. Theory Appl. 6, 661 (1907)]. The magnetocaloric effect simulations show a surprising TC−2/3 linear dependence of the maximum entropy change (ΔSmmax), which is observed for all simulated systems. An approximate state equation establishing the dependence of ΔSmmax on TC, ΔH, J, and the magnetic atoms density (N) is presented. The dependence of maximum magnetic entropy change on TC−2/3 is validated by a wide set of experimental results of second- and first-order phase transition materials that are promising for magnetic refrigeration applications at room temperature.

Unveiling the (De)coupling of magnetostructural transition nature in magnetocaloric R5Si2Ge2 (R 5Tb, Gd) materials

We present a detailed study on the magnetization under high magnetic fields of Tb5Si2Ge2 and Gd5Si2Ge2 compounds. From the Arrott plot construction (A. Arrott, Phys. Rev. 108, 1394 (1957)), we were able to estimate the TC of each structure (M and O(I)) experimentally and found that the TC of the O(I) phase can be directly obtained by extrapolating the TC curve of this phase in the respective phase diagram. Using a physical model based on free energy considerations, one explains the (de)coupling of the magnetic and structural transitions in R5(SixGe1−x)4 (R = Tb, Gd) compounds.

Cryogenic Temperature Response of Reflection-Based Phase-Shifted Long-Period Fiber Gratings

In this study, we investigated the temperature behavior of phase-shifted long-period fiber gratings (PS-LPFGs) inscribed in two types of optical fiber: B/Ge and SMF28. The experiments were carried out from 5 to 305 K using a superconducting quantum interference device magnetometer. The average temperature sensitivity obtained of -0.43 nm/K for PS-LPFGs inscribed in the B/Ge fiber is one order of magnitude larger than for PS-LPFGs inscribed in the SMF28 fiber, in the 60-240 K range. Values ranging from -0.08 nm/K up to 0.2 nm/K were obtained in the 5-35 K temperature range, which are considerably better than previous results achieved for metal-coated FBGs and also for LPFGs inscribed in a similar B/Ge codoped fiber. Nevertheless, further work is required in order to correctly address sensor reliability.

Critical magnetic behavior of magnetocaloric materials with the Gd5Si4-type structure

An extensive investigation on the magnetism of Gd5(SixGe1−x)4 compounds which stabilize in the Gd5Si4-type structure is here presented, focusing on the Gd5Si4 and Gd5Si2Ge2 compositions. Through Arrott plot analysis, the temperature dependencies of the spontaneous magnetizations are retrieved and the magnetic interaction coefficients (Jexch) are estimated. The obtained Jexch value for Gd5Si4 is 15% higher than that of Gd5Si2Ge2. Such enhancement cannot be attributed to a magneto-volume coupling only, being mainly associated with the magnetic polarization of the (Si,Ge)-(Si,Ge) interslab connections. The critical exponents were estimated (δ=3.1 and 2.9, γ=1.01 and 1.06, β=0.48 and 0.56, and n=0.67 and 0.63 for Gd5Si4 and Gd5Si2Ge2, respectively) demonstrating that both compositions belong to the same universal class. This conclusion is reinforced by the construction of a universal logarithmic scaling plot of the M(H) isothermal curves. Hence, we conclude that the magnetism of the Gd5Si4-type structure comp...