In this paper we present a simple method for the determination of the total magnetocaloric effect based on the acoustic detection of the adiabatic temperature rise caused by the application of an ac magnetic field of small amplitude. The continuous scanning of a superimposed dc magnetic field allows, by numerical integration, the determination of large temperature variations caused by magnetic field steps from zero to tens of kOe. Absolute values of temperature rise are easily acquired after the calibration of the microphone signal using an appropriate reference sample. Once the calibration is done, no further information about the sample's thermal properties is necessary since the measured signal is directly proportional to the temperature variation. Measurements were made in Gd and Gd 5.09 Ge 2.03 Si 1.88 samples in the temperature range from 240 to 320 K. The technique shows to be suitable for the investigation of materials undergoing both purely magnetic phase transitions, as in the case of Gd, and magnetic-crystallographic first-order ones, as observed for Gd 5.09 Ge 2.03 Si 1.88 . Besides the ability to determine the temperature variation due to a large magnetic field step through the continuous scanning of the magnetic field, the technique is also very suitable for measuring the magnetocaloric effect under very small magnetic field steps since it has sensitivity below millikelvin. Moreover, it is able to detect temperature variations in very small amount of sample, leading to its potential application in magnetocaloric thin films.

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.

In this paper we explore the acoustic detection method applied to the investigation of the magnetocaloric effect in Gd and Gd 5 ͑Ge 1−x Si x ͒ 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 Gd 5 Ge 2 Si 2 that underwent different thermal treatments. Small differences were observed when comparing powder and pellet samples of Gd and Gd 5 ͑Ge 1−x Si x ͒ 4 compounds with 0.500Ͻ x Յ 1.00. For the alloys with composition around Gd 5 Ge 2 Si 2 , 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.

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.

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In this paper we demonstrate the use of the acoustic detection as an alternative way to determine the entropy variation, ΔS T , 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, ΔS T , was found as a function of the temperature for several magnetic field steps.

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