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为了明确La0.9Pr0.1Fe12B6合金的变磁相变属性和对应的晶体结构特征，以及伴随的磁热效应，本文研究了该合金在磁场诱导和温度诱导下的变磁相变过程及其对应的X射线衍射图谱（X-ray diffraction spectrum，XRD）变化，并对不同测量模式下磁热性能进行了深入对比。结果表明，La0.9Pr0.1Fe12B6合金主相在低场升温过程中，温度诱导的磁相变顺序为反铁磁态→铁磁态→顺磁态；在等温磁化过程中，在不同温度区间呈现出了三种磁场诱导的变磁相变，即在低温时的两种反铁磁态（Antiferromagnetic，AFM）与铁磁态（ferromagnetic，FM）之间的相变，以及高温的顺磁态（paramagnetic state，PM）与FM态之间的相变，且对应的临界磁场（critical magnetic field，HC）要比LaFe12B6母合金的低得多。零场和加场变温XRD图谱显示La0.9Pr0.1Fe12B6合金的主相在磁无序和有序态间的转变过程中，会伴随磁晶耦合现象，其结果是XRD图谱中除原有主相的衍射峰外，还会出现一些PM态下无法观察到的新衍射峰，并且其强度随着温度的降低或磁场的增加而增强。另外，在基于连续测量模式下的等温磁化数据所计算的磁熵变随温度变化曲线中，可在居里温度附近观察到因磁场诱导PM-FM一级变磁相变而导致的大磁熵变（ΔSM），如在70kOe的磁场下，在50K附近的最大磁熵变可达19J/kg·K，相对制冷量约为589.1J/kg。然而在同样的测量模式下，却没有观察到因AFM-FM变磁相变所期望的大磁熵变。但采用非连续测量模式，则同样观察到AFM-FM变磁相变过程伴随的大磁熵变，如在70kOe的磁场下，8K附近的最大磁熵变可达-12J/kg·K。
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关键词:
- La0.9Pr0.1Fe12B6合金 /
- 磁相变 /
- 磁热效应
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.-
Keywords:
- La0.9Pr0.1Fe12B6alloy /
- magnetic transition /
- magnetocaloric effect
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