Magnetic properties of NiMn, PdMn, and Ni–Mn–Sn after annealing in a magnetic field

In this work, the decomposition of Ni50Mn45Sn5 into full-Heusler Ni2MnSn and tetragonal NiMn is investigated with a focus on composition, microstructure, and magnetic properties. The interest for this decomposition lies in the intriguing magnetic properties of the resulting nanocomposite. These properties are a shift of M(B) along the magnetization axis, ferromagnetism with ultra-high coercive field (∼ 5 T), and ferromagnetism with low coercive field (∼ 10 mT). The first two are only present if a magnetic field was applied during decomposition. This work takes two different approaches to understand this behavior. The first approach is an extensive structural and chemical characterization of the decomposition process down to the nanoscale. This identifies the decomposition mechanism as cellular precipitation, which results in a lamellar structure with alternating nm-thick layers of ferromagnetic Ni2MnSn and antiferromagnetic NiMn. The second approach is a detailed study of the decomposition product NiMn and its close relative PdMn. Here, an excess of Ni for NiMn and Pd for PdMn also leads, if annealed in an applied magnetic field, to a vertical shift of M(B). This vertical shift is explained with the magnetic field biased diffusion model. In this model, an applied magnetic field leads to a change of the diffusive jump rate if the diffusive jump changes the net magnetic moment along the field direction. In NiMn and PdMn, this change of the diffusive jump rate leads to the accumulation of excess atoms in one of the two antiferromagnetic sublattices and results in uncompensated magnetic moments. These moments are responsible for the vertical shift of M(B). Magnetic measurements on Ni–Mn–Sn reveal many similarities to PdMn, especially in the dependencies of the vertical shift on the annealing time, temperature, and field. These results make a similar origin for the vertical shift in Ni–Mn–Sn likely. A possible explanation for the ferromagnetism with ultra-high coercivity in Ni–Mn–Sn would then be the coupling of soft ferromagnetic Ni2MnSn to the uncompensated magnetic moments in NiMn. The ultimate goal is the utilization of this effect for permanent magnets. Another discovery of this work concerns the ferromagnetism with low coercivity, which emerges after annealing in both Ni–Mn–Sn and NiMn. It is shown that for Ni–Mn–Sn it partially and for NiMn as a whole stems from surface oxidation and subsequent enrichment of Ni in the subsurface region.


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