Magnetic proximity effect and shell-ferromagnetism in metastable Ni50Mn45Ga5

Abstract

The present study on magnetic and structural properties of Ni50Mn45Ga5 confirms that structural metastability is an inherent property of Ni50Mn50–xXx Heusler alloys with X as In, Ga, and Sn. The ternary alloy transforms during temper-annealing into a dual-phase composite alloy. The two phases are identified to be cubic L21, Ni50Mn25Ga25, and tetragonal L10 Ni50Mn50. Depending on the annealing temperature, the magnetic-proximity effect giving rise to shell-ferromagnetism has been observed when annealing is carried out under an external magnetic field. The upper and lower remanence values MR+ and MR− have the same sign even at high temperatures. Such alloys can be promising candidates for heat- and magnetic-field-resistant magnetic recording media.

Properties and Decomposition of Heusler Alloys

“…This brief discussion illustrates that time‐dependent effects in combination with the martensitic transformation and effects of annealing on Ni–Co–Mn–In and other ferrous shape‐memory alloys are important and need understanding. In the following we will briefly discuss the effects of slow and fast annealing of Heusler alloys, which finally leads to decomposition (segregation) of the Heusler alloys as observed in the experiments of Acet . The experiments support the findings of decomposition of the alloys into a dual phase type of composite alloy, which consists of stoichiometric Ni 2 MnIn precipitates embedded in a tetragonally distorted NiMn‐phase.…”

“…The structural metastability of Ni 50 Mn 45 Ga 5 alloys is an inherent property of the Ni 50 Mn 50− x X x alloys with as Ga, In, and Sn . The alloys transform during temper‐annealing to a dual‐phase composite alloy, cubic L2 1 Heusler and L1 0 Ni 50 Mn 50 .…”

“…In this article, we review mostly Ni–Mn–In and Ni–Co–Mn–Sn and the corresponding metamagnetic effects following their large magnetocaloric effect properties and high level of sensitivity of transformation characteristics (e.g., B2–L2 1 and martensitic details in dependence of the microstructural changes, anti‐site defects, and potential role of vacancies), which have been discussed recently . This article is based on reviewing, in particular, details of decomposition, segregation, and time‐dependent effects reported previously …”

“… B2/L2 disorder–order transformation of Ni 50 Mn 50− x Z x as a function of x (upper axis) and e / a (lower axis) for Z=Al, Ga, In . The asterisks mark the alloy systems for which segregation has been investigated . They are all very close to the martensitic transformation.…”

Shell-ferromagnetism in Ni-Mn-based Heuslers in view of ductile Ni-Mn-Al

“…Increasing M with progressing time demonstrates that Ni 50 Mn 45 Al 5 is magnetically not stable at the temper-annealing temperature 650 K. Analogous to the arguments in our previous studies on Ni-Mn-X (X: Ga, In, and Sn), Ni 50 Mn 45 Al 5 tends to decompose into Ni 2 MnAl and and NiMn. 4,13,16 NiMn is AF, and consequently, it remains for Ni 2 MnAl to become FM. Therefore, we find that ferromagnetism can indeed be stabilized in Ni 2 MnAl precipitates, contrary to the fact that it is difficult to introduce FM interactions in this material when prepared in bulk form.…”