Direct measurements of the adiabatic temperature change of rapidly quenched ribbon samples Ni50-xCoxMn50-yAly (x = 5, 6; y = 18, 19) in magnetic fields up to 8 T

Abstract

In this paper, we present the results of direct measurements of the adiabatic temperature change (ΔT_ad) in Ni₄₄Co₆Mn₃₂Al₁₈ and Ni₄₅Co₅Mn₃₁Al₁₉ ribbon samples in the temperature range of 100–350 K in a cyclic magnetic field of 1.8 T using the magnetic field modulation method, as well as in a magnetic field of 8 T using the classical extraction method. It is shown that near the magnetostructural phase transition (MSPT), the value of the inverse effect in a cyclic magnetic field of 1.8 T with a frequency of 0.2 Hz decreases significantly for both samples compared to the value of the effect with a single turn on of the magnetic field. For both samples, the behavior of ΔT_ad(H) near the MSPT in magnetic fields up to 8 T is irreversible. When the magnetic field is turned on, an inverse magnetocaloric effect (MCE) is observed with values of −1.8 K and −2.0 K at T = 250 and 264 K, respectively, for the Ni₄₄Co₆Mn₃₂Al₁₈ and Ni₄₅Co₅Mn₃₁Al₁₉ samples. Turning off the magnetic field leads to the irreversibility of the MCE for both samples, and the final temperature of the samples remains below the initial level. This is due to the fact that a magnetic field of 8 T is insufficient to induce MSPT in the entire volume of the sample, only part of the sample transitions from the martensite phase to the austenite phase. It is shown that by regulating the concentration of Co in Ni-Mn-Al alloys we can control the kinetics of the magnetostructural phase transition, which in the future will allow us to obtain alloys with specified characteristics for magnetic cooling technology.