Propriedades estruturais, magnéticas e magnetocalóricas do composto DyMn2Si2 na forma nanoestruturada

Abstract

In this study, we propose the high-energy ball milling technique to synthesize the DyMn2Si2 intermetallic compound on a microscopic scale. This approach rejects the conventional idea that atomic disorder should be minimized to maximize magnetic and magnetocaloric properties. A controlled atomic disorder is induced in this material by reducing its physical dimensions through the mechanical milling process. As a result, it is expected to promote small changes in the system microscopic interactions to change its magnetic properties and increase the magnetocaloric properties. DyMn2Si2 polycrystalline sample was mechanical milled (between 2 and 32 hours) to obtain small particles. Subsequently, the obtained particles are thermally annealed to relieve residual strain. X-ray diffraction technique was used to characterize the particle's structural properties, the measurements were performed before and after the particles thermal annealing. The results show an increase in the particle micro-deformation as the milling time increases, indicating atomic disorder. However, the micro-deformation level is reduced for annealed particles. A reduction of 83% in the average crystallite size was observed for 32h samples compared to the bulk sample; however, this reduction decrease to 67% for 32h sample submitted to thermal annealing. The magnetic properties investigation indicates that the milling process induces changes in the DyMn2Si2 magnetic interactions. The isothermal magnetic entropy change (-ΔSM) responds to the crystallites reduction, i.e. a gradual widening of the -ΔSM peaks is observed as the average crystallite size decrease, promoting a table-like magnetocaloric effect. Particularly, the samples milled for 2 and 4 hours showed an appreciable widening of the -ΔSM peaks without a significant reduction of the maximum values of -ΔSM peaks. It promotes an increase of 63% and 72% respectively, in the relative cooling power of these samples, compared with bulk DyMn2Si2. This result can be related to the strengthening of the ferrimagnetic transition induced by the moderated level of atomic disorder in these samples. On the other hand, the possible existence of a superparamagnetic state exhibited by the 32 hours sample can be attributed to a fraction of the crystallites that reached a critical size to enter the blocked state. In this study, we propose the high-energy ball milling technique to synthesize the DyMn2Si2 intermetallic compound on a microscopic scale. This approach rejects the conventional idea that atomic disorder should be minimized to maximize magnetic and magnetocaloric properties. A controlled atomic disorder is induced in this material by reducing its physical dimensions through the mechanical milling process. As a result, it is expected to promote small changes in the system microscopic interactions to change its magnetic properties and increase the magnetocaloric properties. DyMn2Si2 polycrystalline sample was mechanical milled (between 2 and 32 hours) to obtain small particles. Subsequently, the obtained particles are thermally annealed to relieve residual strain. X-ray diffraction technique was used to characterize the particle's structural properties, the measurements were performed before and after the particles thermal annealing. The results show an increase in the particle micro-deformation as the milling time increases, indicating atomic disorder. However, the micro-deformation level is reduced for annealed particles. A reduction of 83% in the average crystallite size was observed for 32h samples compared to the bulk sample; however, this reduction decrease to 67% for 32h sample submitted to thermal annealing. The magnetic properties investigation indicates that the milling process induces changes in the DyMn2Si2 magnetic interactions. The isothermal magnetic entropy change (-ΔSM) responds to the crystallites reduction, i.e. a gradual widening of the -ΔSM peaks is observed as the average crystallite size decrease, promoting a table-like magnetocaloric effect. Particularly, the samples milled for 2 and 4 hours showed an appreciable widening of the -ΔSM peaks without a significant reduction of the maximum values of -ΔSM peaks. It promotes an increase of 63% and 72% respectively, in the relative cooling power of these samples, compared with bulk DyMn2Si2. This result can be related to the strengthening of the ferrimagnetic transition induced by the moderated level of atomic disorder in these samples. On the other hand, the possible existence of a superparamagnetic state exhibited by the 32 hours sample can be attributed to a fraction of the crystallites that reached a critical size to enter the blocked state.