Papers by Author: Lluís Mañosa

Abstract: Magnetic shape-memory properties refer to the ability of certain materials to show strong response in strain to an applied magnetic field. This strain is caused by either inducing the martensitic transition or rearranging martensitic variants. In the first, case a superelastic effect is possible, while in the second, the system is able to show the shape-memory effect. The complex behaviour displayed by these materials is mainly a consequence of a strong interplay between magnetism and structure which is driven by a martensitic transition. This interplay is the source of many other observed effects such as giant magneto-resistance, exchange bias and magnetocaloric effects. In this paper, we will overview the present state of the art, discuss present challenges and outline some future perspectives in the field.

Abstract: Different experimental procedures for the location of sources of Acoustic Emission (AE) avalanches during Martensitic Transformations are discussed. A first example corresponds to the 1D location of AE events during stress-induced martensitic transitions in a Cu-Zn-Al shape memory alloy (3.5 cm length). The obtained data allows monitoring of the interface advancement with a spatial resolution of less than 1 mm. Secondly, we discuss two different ideas that have significant potential for improving this resolution in the case of thermally induced transitions in small single crystalline samples (~1 cm): the use of elastodynamic simulations based on finite element methods and the simultaneous detection of AE and Barkhausen noise in ferromagnetic samples.

Abstract: In this paper, we discuss the magnetocaloric behavior of Ni-Mn-based Heusler alloys in rela- tion to their shape-memory and superelastic properties. We show that the magnetocaloric effect in these materials originates from two different contributions: (i) the coupling that is related to a strong uniaxial magnetic anisotropy and takes place at the length scale of martensite variants and magnetic domains (extrinsic effect), and (ii) the intrinsic microscopic magnetostructural coupling. The first contribution is intimately related to the magnetically induced rearrange- ment of martensite variants (magnetic shape-memory) and controls the magnetocaloric effect at small applied fields, while the latter is dominant at higher fields and is essentially related to the possibility of magnetically inducing the martensitic transition (magnetic superelasticity). The possibility of inverse magnetocaloric effect associated with these two contributions is also considered.

Abstract: In this paper, we discuss the possibility of inducing a martensitic transition by means of an applied magnetic field or hydrostatic pressure in Ni-Mn based Heusler shape memory alloys. We report on the shift of the martensitic transition temperatures with applied magnetic field and applied pressure and we show that it is possible to induce the structural transformation in a Ni50Mn34In16 alloy by means of both external fields due to: (i) the low value of the entropy change and (ii) the large change of magnetization and volume, which occur at the martensitic transition.

Abstract: We report on strain measurements in Ni-Mn-Ga, Ni-Mn-In, Ni-Mn-Sn and Ni-Mn-Sb polycrystalline alloys as a function of temperature and magnetic field. Experiments are carried out in the austenitic and martensitic phases of the alloys. It is shown that in the cubic phase the magnetostrain is similar for all systems but by contrast very different behaviour is found when a field is applied in the martensitic phase. In the latter case, magnetic shape memory and magnetic superelasticity is obtained for Ni-Mn-Ga and Ni-Mn-In, respectively. The physical reasons for the different behaviour are discussed.

Abstract: The magnetic shape-memory effect is a consequence of the coupling between magnetism and structure in ferromagnetic alloys undergoing a martensitic transformation. In these materials large reversible strains can be magnetically induced by the rearrangement of the martensitic twin-variant structure. Several Heusler and intermetallic alloys have been studied in connec- tion with this property. In this paper we will focus on the Ni-Mn-Ga Heusler alloy which is considered to be the prototypical magnetic shape-memory alloy. After a brief summary of the general properties of this class of materials, we will present recent results of relevance for the understanding of the effect of magnetism on the martensitic transformation. Finally, we will discuss the requirements for the occurrence of the magnetic shape-memory effect.

Abstract: Elastic constants of solids were, until recently, evaluated predominantly by pulse-echo ultrasonic methods which are based on measuring the speed of ultrasonic waves propagation in solids. Resonant ultrasound spectroscopy (RUS) is a relatively novel method in which all components of elastic tensor are determined from measured resonance frequencies of a freely vibrating specimen. The RUS technique has been employed in this work to investigate temperature dependence of the elastic properties of the parent austenite phase in CuAlNi shape memory alloy single crystals. This phase exhibits very high elastic anisotropy (anisotropy factor A  12) and softening the shear coefficient C0 upon cooling when approaching the Ms transformation temperature. The complications (need for large number of resonant frequencies) emerging when one tries to determine all elastic constants of highly elastically anisotropic materials by the RUS technique are discussed. It is concluded that the shear coefficients C0 and C44, which are the most important for shape memory alloys, are, nevertheless, determined reliably.