M. E. Soffner

Magnetocaloric effect in GdGeSi compounds measured by the acoustic detection technique: Influence of composition and sample treatment

In this paper we explore the acoustic detection method applied to the investigation of the magnetocaloric effect in Gd and Gd5(Ge1−xSix)4 compounds, in the temperature range from 230 to 360 K and for magnetic fields up to 20 kOe. Measurements were performed in as-cast materials, both for powder and pellet samples, and in tree samples with compositions around Gd5Ge2Si2 that underwent different thermal treatments. Small differences were observed when comparing powder and pellet samples of Gd and Gd5(Ge1−xSix)4 compounds with 0.500<x≤1.00. For the alloys with composition around Gd5Ge2Si2, which exhibit giant magnetostriction and coexistence of distinct phases, expressive changes were observed when comparing powder and pellet samples. Based on these cases, it is easy to see that the acoustic method can distinguish a second-order phase transition from a first-order magnetic-crystallographic one, and that it presents good sensitivity to detect spurious material phase in small quantities.

Anisotropic magnetocaloric effect in gadolinium thin films: Magnetization measurements and acoustic detection

In this letter, it is demonstrated the ability of the magnetoacoustic technique in detecting the magnetocaloric effect in gadolinium thin films (1.0 μm and 3.0 μm thick), which is not accessible through conventional temperature sensors because of the reduced mass of the samples. The method, which detects the direct effect of the sample temperature variation, proved to be sensitive to the anisotropy of the films, making possible for the investigation of the anisotropic magnetocaloric effect. Magnetization measurements were also carried out, and from these measurements both the adiabatic temperature and the isothermal entropy variations were calculated. The acoustically detected magnetocaloric effect shows very good agreement with these calculations.

Determination of the entropy change using the acoustic detection technique in the investigation of the magnetocaloric effect

In this paper we demonstrate the use of the acoustic detection as an alternative way to determine the entropy variation, ΔST, a parameter normally used to characterize the magnetocaloric effect. The measurements were performed for a Gd sample in the 252–316 K temperature range for magnetic fields from zero up to 50 kOe. The reversible adiabatic curves were built in a T versus H diagram, and specific heat data obtained at zero-magnetic field were employed to assign the entropy values of each curve. Subsequently, the entropy was plotted as a function of temperature for fixed magnetic fields, and therefore the isothermal entropy variation, ΔST, was found as a function of the temperature for several magnetic field steps.

Large magnetocaloric effect and refrigerant capacity near room temperature in as-cast Gd5Ge2Si2−xSnx compounds

Large values of isothermal entropy change (ΔST) and refrigerant capacity have been found in Gd5Ge2Si2−xSnx compounds. Values of the order of 20 J kg−1 K−1 for −ΔST were obtained in as-cast samples when submitted to a magnetic field variation of 2 T. First-order-magneto-structural transition is induced by the substitution of silicon by tin and it is shifted to lower temperatures with the tin content. It means that the magnetocaloric effect on this series can be properly tuned to a specific practical thermodynamic cycle, including near room temperature range.

Thin films of gadolinium investigated by photothermally modulated magnetic resonance

The photothermally modulated magnetic resonance (PM-MR) technique was used to study thin films of Gd grown on fused quartz substrates. With this technique it was possible to observe the magnetic phase transitions for samples with different thickness and thermal treatments. Results were correlated with both magnetization and ESR measurements. The effect of the stress induced by the substrate was clearly observed through peak of the PM-MR signal.

Photothermally modulated magnetic resonance using a light access microwaves cavity: Influence of skin depth and of photo-injected carriers

This study reports on the modulation frequency dependence of the photothermally modulated magnetic resonance signal of a set of magnetic samples in the form of foils, layers, and thin films: 50 μ m Fe and Ni foils; 5 μ m magnetic layer of γ - F e 2 O 3 in a cassette tape; and 150 nm Co and Permalloy films deposited on glass and Si (111) substrates, besides the naked Si substrate. It is shown, both by analytical calculation and by measurements, that the skin depth of the microwaves deeply influences the signal behavior by selecting the portion of the sample that is probed. Clear differences in the frequency dependence are observed between the metallic Ni and Fe foils and the dielectric γ - F e 2 O 3 cassette tape. Furthermore, the thermal mismatch between the magnetic films (Co and Permalloy) and substrates (glass and Si) also plays a crucial role, once the modulation of the temperature is strongly dependent on the substrate thermal parameters at low modulation frequencies. The non-resonant signal from the diamagnetic Si is also analyzed. It is produced by the absorption of microwaves by the photo-injected free carriers and presents characteristic behavior in the investigated frequency range.This study reports on the modulation frequency dependence of the photothermally modulated magnetic resonance signal of a set of magnetic samples in the form of foils, layers, and thin films: 50 μ m Fe and Ni foils; 5 μ m magnetic layer of γ - F e 2 O 3 in a cassette tape; and 150 nm Co and Permalloy films deposited on glass and Si (111) substrates, besides the naked Si substrate. It is shown, both by analytical calculation and by measurements, that the skin depth of the microwaves deeply influences the signal behavior by selecting the portion of the sample that is probed. Clear differences in the frequency dependence are observed between the metallic Ni and Fe foils and the dielectric γ - F e 2 O 3 cassette tape. Furthermore, the thermal mismatch between the magnetic films (Co and Permalloy) and substrates (glass and Si) also plays a crucial role, once the modulation of the temperature is strongly dependent on the substrate thermal parameters at low modulation frequencies. The ...