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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.

Electron spin resonance was applied on samples of Gd 5.09 Ge 2.03 Si 1.88 . The results are discussed under the scope of magnetization measurements, optical metallography, and wavelength dispersive spectroscopy. Polycrystalline arc-melted samples submitted to different heat treatments were investigated. The correlation of the electron spin resonance and magnetization results permitted a characterization of the present phases and their transitions. Two coexisting phases in the temperature range between two phase transitions have been identified and associated to distinct crystallographic phases. Additionally, the magnetic moment at high temperatures has been estimated from the measured effective g factor. A peak value of 21.5 J / kg K for the magnetocaloric effect was obtained for a sample heat treated at 1500°C for 16 h.

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Fe x Cu 100 − x metastable alloys were prepared by pulsed electrodeposition for 5<x<85. The Fe-rich alloys crystallize in the bcc structure of α-Fe and the Fe-poor ones in the fcc structure of Cu. The magnetic properties of the ferromagnetic Fe-rich alloys (x>50) are reminiscent of those observed in Fe–Cu alloys prepared by other methods. The Curie temperature decreases regularly with decreasing x. In the Fe-poor alloys (x≤30), the observed properties indicate that Fe-rich clusters form within a Cu-rich matrix. In the x=10 alloy, the Fe clusters are found to be superparamagnetic at room temperature, but a superferromagnetic order develops below a critical temperature of about 120 K. It is suggested that the intercluster coupling is mediated by Ruderman–Kittel–Kasuya–Yosida interactions which are predominantly positive due to the very short intercluster distances.

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a b s t r a c tThe development of devices based on magnetic tunnel junctions has raised new interests on the structural and magnetic properties of the interface Co/MgO. In this context, we have grown ultrathin Co films ( r 30Å) by molecular-beam epitaxy on MgO(0 0 1) substrates kept at different temperatures (T S ). Their structural and magnetic properties were correlated and discussed in the context of distinct magnetic anisotropies for Co phases reported in the literature. The sample characterization has been done by reflection high energy electron diffraction, magneto-optical Kerr effect and ferromagnetic resonance. The main focus of the work is on a sample deposited at T S ¼ 25 1C, as its particular way of growth has enabled a bct Co structure to settle on the substrate, where it is not normally obtained without specific seed layers. This sample presented the best crystallinity, softer magnetic properties and a four-fold in-plane magnetic anisotropy with Co/1 1 0S easy directions. Concerning the samples prepared at T S ¼ 200 and 5001 C, they show fcc and polycrystalline structures, respectively and more intricate magnetic anisotropy patterns.