Ferromagnetic shape memory Ni-Mn-Ga alloys: A new synergy between structure and properties

The family of Ni-Mn based Heusler alloys provides an extended playground of physical properties. The interplay between a reversible martensitic transformation (MT) and magnetically ordered states gives rise to a series of functional properties that can be exploited for developing innovative devices [1] which originate from the possibility to dramatically change the materials properties by an applied external stimulus, such as magnetic field, stress of pressure. One of the most interesting property rely on the occurrence of large strains activated by external magnetic field. This phenomenon which takes the name of Magnetic Shape Memory (MSM) is mainly related to the particular crystalline structure assumed by the martesntic phase characterizing such family of this alloy. The martensitic transformation induced by the temperature change (corresponding to TM) consists of a deformation of the lattice of the austenitic phase having a structure type L21 with cubic symmetry. In Ni-Mn-Ga alloys the austenite is associated to a ferromagnetic state which is retained during the martensitic transformation. Upon the application of a magnetic field the martensite is magnetized by sliding the twinning boundaries typically featuring the product phase. The macroscopic effect of this process is represented by giant strains and this special synergy between the crystal structure of martensite and magnetic properties is at the basis of several international studies focused on the possible technological applications especially in the field of micromechanics. The cooperative aspect between crystal structure and magnetism is also manifested in the giant magnetocaloric effect. This property has triggered attention to the possible applications in the refrigeration industry (green technology). Basically, the observed magnetocaloric effect is closely related to the large difference in entropy between the martensitic and austenitic phase. In this paper we present the main physical properties of the Ni-Mn-Ga Heusler alloys potentially suitable for the design of a new generation of multifunctional smart devices.

Il composto intermetallico Ni 2 MnGa appartiene alla famiglia delle leghe di Heusler, con formula generale X 2 YZ. In generale le composizioni ternarie di tipo Ni-Mn-Z con Z appartenente agli elementi dei gruppi IIIA-VA, hanno recentemente catalizzato un crescente interesse nel campo scientifico internazionale. Questi materiali multifunzionali hanno infatti dimostrato di possedere un'ampia varietà di proprietà intrinseche che li rende attrattivi per svariati campi di applicazioni [1]. Si annoverano infatti diversi effetti che possono essere controllati con l'applicazione di un campo magnetico; deformazioni giganti (MFIS - Magnetic Field Induced Strain o MSM - Magnetic Shape Memory) [2], proprietà magnetocaloriche (MCE) [3, 4], comportamenti magnetoelastici e magnetoresistivi [5,6]. L'interazione tra struttura e magnetismo è all'origine di questa straordinaria fenomenologia. In questo contributo vengono presentati i principali fenomeni alla base dei comportamenti polifunzionali e una panoramica dei più recenti avanzamenti in campo scientifico con l'obiettivo di portare le leghe Ni-Mn-Z ad una fase matura per uno sviluppo pre-industriale. In questa memoria verrà dato risalto a due fondamentali proprietà : l'effetto a memoria di forma magnetico e l'effetto magnetocalorico