Hints on the origin of the thermal hysteresis suppression in giant magnetocaloric thin films irradiatied with highly charged ions

Abstract

In a recent experiment we demonstrated the possibility to suppress the thermal hysteresis of the phase transition in giant magnetocaloric MnAs thin film by interaction with slow highly charged ions (Ne 9+ at 90 keV) [1]. This phenomenon has a major impact for possible applications in magnetic refrigeration and thus its reproducibility and robustness are of prime importance. Here we present some new investigations about the origin and the nature of the irradiation-induced defects responsible for the thermal hysteresis suppression. Considering in particular two samples that receive different ion fluences (two order of magnitude of difference), we investigate the reliability of this process. The stability of the irradiation-induced defects with respect to a soft annealing is studied by X-ray diffraction and magnetometry measurements, which provide some new insights on the mechanisms involved.

Low energy Ne ion beam induced-modifications of magnetic properties in MnAs thin films

“…As previously reported [31,32], the ion irradiation does not affect the coercivity field values. Changes of the saturation magnetization are however visible.…”

“…2, the domain direction flip takes place at H ∼ 50 Oe for low temperatures, but increases up to H ∼ 200 Oe during the phase coexistence. This singular behavior is characteristic of MnAs thin films and it has already observed previously [32,42,43] and successfully modeled [44] in the past.…”

“…Irradiation of the thin film with slow ions produce new defects in the MnAs crystal. As reported in [31,32], these new defects act as nucleation seeds of one phase with respect to the other during the magneto-structural transition leading to the elimination of the thermal hysteresis.…”

“…Investigations with x-ray diffraction for possible structural changes show that the α and β phases structure of irradiated samples, up to a fluence of 10 15 ions cm −2 , are unchanged as presented in [31,32]. The film remains monocrystalline and only a small increase of the average lattice spacing is noticed (less than 0.3% for a fluence of × 1.6 10 15 ions cm −2 ).…”

“…Recently it has been demonstrated that slow ions bombardment can interestingly change the thin film properties [31,32]. New irradiation-induced defects facilitate the nucleation of one phase with respect to the other one in the first-order magneto-structural MnAs transition, with a consequent stable suppression of the thermal hysteresis without any significant perturbation of the other properties.…”