Effect of sample preparation on the magnetic and magnetocaloric properties of amorphous Gd70Ni30

Abstract

Certain applications of magnetic materials require the knowledge of the magnetocaloric effects, i.e., the adiabatic change of temperature or the isothermal change of the entropy upon application or removal of magnetic field. While the isothermal magnetic entropy change can be calculated from magnetic data only, the calculation of the adiabatic temperature change requires the knowledge of the field-dependent specific heat as well. To compare magnetocaloric effect results obtained from magnetic-only, thermal-only, or combined methods requires the evaluation of the mutual reliability of all methods. In crystalline materials it usually does not present a problem to perform thermal and magnetic measurements on the same sample or at least on samples of identical state. For amorphous materials, unfortunately, thermal measurements are extremely difficult to perform on the ribbons themselves so usually pellets pressed from powdered ribbons, with silver as a binder, are applied. After the magnetic entropy change calculated from thermal measurements performed on such a composite did not agree with the data obtained from magnetic measurements performed on the original ribbon, magnetic measurements were carried out on the pellet and its predecessor powders. Comparison of the results revealed that the pellet’s properties do not reflect the properties of the original ribbon. However, comparison of magnetic and thermal measurements carried out on the pellet itself show good agreement if proper numerical methods are used for evaluation. The study concludes that amorphous ribbons are far more competitive for certain applications than the thermal measurements, performed on the pellet only, would indicate.

Magnetocaloric Materials

“…The major effort in studying the MCE of amorphous materials focused on various alloys containing lanthanide and transition metals, which were prepared by rapid solidification (melt spinning) of arc-melted alloy buttons (76,(123)(124)(125)(126)(127). This class of materials generally shows quite broad magnetic ordering transitions, which result in much lower |∂M/∂T| at constant magnetic field, and therefore, the observed peak S M (T ) H and T ad (T ) H are considerably lower (Equations 4 and 9) than those of their crystalline alloy counterparts.…”

Achieving tailorable magneto-caloric effect in the Gd-Co binary amorphous alloys

“…[4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] Metallic glasses undergo a second order magnetic transition and exhibit a broadened magnetic entropy change peak. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] Therefore, although the peak values of magnetic entropy change (-∆S m peak ) for the metallic glasses are not as high as some of the crystalline alloys, the amorphous alloys still have evoked intensive interests because their refrigeration capacity (RC) are usually several times higher than that of the crystalline alloys. Our recent results have shown that the RC can reach an ultra-high value of above 800 J·kg −1 under 5 T, accompanied with a relatively high -∆S m peak value of nearly 10 J·kg −1 ·K −1 under 5 T in some of the Gd-based metallic glasses.…”

The influence of disorder on magnetocaloric effect and the order of transition in ferromagnetic manganite (La1−x Nd x )2/3(Ca1−y Sr y )1/3MnO3

“…The deviation observed in the n values, for (J3), (J4,) and (J5) samples, from that predicted by the mean field model (n = 2/3) [42], confirms the critical behavior studied previously for our samples and the invalidity of the mean field model in the description of our materials near the transition temperature for all samples. In the case of (J3), (J4), and (J5) samples, n increases to reach a values between 1 and 2, suggesting the magnetization depends on the temperature but weaker than a linear temperature dependence case [43]. This behavior can well be understood, because in this high-temperature limit range, the magnetization has a linear field dependence.…”

“…The increase in disorder with increasing Sr and Nd doping content enhances the inhomogeneity of the samples. In this case, Shen et al [41,43] show that in single-phase materials, n is field-independent especially at the peak temperature, and when the system is multiphase, n is field-dependent at any temperature. Thus, the variation of n (T, H) indicates the inhomogeneous character for all specimens, among the various forms of the inhomogeneous phase is the existence of some alterations of shortrange magnetic order of FM clusters [44,45].…”