Study on magnetic transition, X-ray diffraction spectrum changes, and magnetocaloric effect in La0.9Pr0.1Fe₁₂B₆ Alloy

In order to clarify the metamagnetic transition properties and corresponding crystal parameter characteristics of La0.9Pr0.1Fe₁₂B₆ alloy, as well as the accompanying magnetocaloric effects, we studied the magnetic phase transition process of the alloy induced by magnetic field and temperature, and the corresponding X-ray diffraction patterns changes, and conducted in-depth comparisons of the magnetocaloric properties under different measurement modes. The results indicate that La0.9Pr0.1Fe₁₂B₆ sample mainly consists of about 90 wt.% SrNi₁₂B₆ type structural main phase and about 10 wt.% Fe2B and α-Fe,which is consistent with references. During the zero-field increasing temperature process, the magnetic state sequence of the main phase of La0.9Pr0.1Fe₁₂B₆ alloy is antiferromagnetic (AFM)→ferromagnetic (FM)→paramagnetic (PM); during the isothermal magnetization process, three types of magnetic field-induced metamagnetic transitions occur in different temperature ranges, namely, two different transitions between AFM and FM states at low temperatures, and a transition between PM and FM states above the Curie temperature(TC). The corresponding critical magnetic field (HC) is much lower than that of the LaFe₁₂B₆ parent alloy. On the contrary, the main phase of La0.9Pr0.1Fe₁₂B₆ alloy exhibits only PM-FM transition present. This indicates that after the alloy transitions from PM state to FM state during the cooling process, even after the temperature drops to a certain value, it will not transition to AFM state. Similar phenomena also exist in other alloys of LaFe₁₂B₆ system. Based on the Néel temperature(TN) and TC obtained the ZFCW modeM-T curves, the magnetic state phase diagram of La0.9Pr0.1Fe₁₂B₆ alloy was plotted. The results indicate that as the external magnetic field increases, TC moves linearly towards higher temperatures at a rate of almost 0.48 K/kOe. Conversely, TN1 and TN2 gradually move towards lower temperatures at rates of 0.48 K/kOe and 0.26 K/kOe, respectively. The zero-field and field-variable temperature XRD patterns show that during the magnetic transition between disorder and order states of the main phase in La0.9Pr0.1Fe₁₂B₆ alloy, there is a phenomenon of magnetocrystalline coupling. As a result, in addition to the original diffraction peaks of the main phase, some new diffraction peaks that are not observable in the PM state also appear, and their intensity increases with decreasing temperature or increasing magnetic field. Through Retveld refinement XRD patterns under different conditions, it was found that the atomic occupancy of La/Pr and Fe is very stable under different environments, but the atomic occupancy of B varies greatly, which may be the main factor leading to the appearance of new diffraction peaks. In addition, in the temperature dependent magnetic entropy change curve calculated based on isothermal magnetization data in continuous measurement mode, a large magnetic entropy change can be observed near TC due to the magnetic field induced first-order metamagnetic transition of PM-FM. For example, under a magnetic field of 70kOe, the maximum magnetic entropy change near 50K can reach 19J/kg·K, and the relative cooling capacity is about 589.1J/kg. However, under the same measurement mode, the expected large magnetic entropy change due to the AFM-FM metamagnetic transition was not observed. But, when using a discontinuous measurement mode, the large magnetic entropy change accompanying the AFM-FM transition process is also observed. For example, under a magnetic field of 70kOe, the maximum magnetic entropy change near 8K can reach -12J/kg·K.