Inorganic Functional Materials

Magnetic Shape Memory Thin Films and Sensors

Magnetic shape memory (MSM) materials have received a lot of attention in recent years due to their interesting properties for actuator and sensor applications. Strains of up to 10% can be obtained in magnetic fields by the redistribution of differently oriented martensite variants. The orientation of these variants or twins is determined by the orientation of the short c-axis of the tetragonal unit cell. Two adjacent variants are separated by a twin boundary which in case of MSM materials is highly mobile. The maximal strain that these materials can undergo is therefore determined by the c/a ratio of the tetragonal unit cell which in case of the alloys NiMnGa and FePd is 0.94 (hence 6% strain can be achieved in these materials).

In comparison to other magnetostrictive materials such as Terfenol-D (0.24%) or Galfenol (0.03%), these strains are unmatched, though fields of up to 1 Tesla are necessary to attain them.


Fig. 1: Schematic of conventional magnetostriction (left) and magnetic shape memory (right) (M. Wuttig, E. Quandt: Comparison of joule and twin induced magnetostriction, Actuator 2004, Bremen, S. 359-362) 

Prerequisites for the MSM effect are ferromagnetic martensites that have high magnetocrystalline anisotropy and low twinning stress. Since the reorientation of the martensite variants takes place in external magnetic fields and is reversible in case of an applied compressive load, cycle times in the kHz range can be obtained which is advantageous compared to thermally activated shape memory materials.

The DFG priority programme SPP1239 "Modification of microstructure and shape of solid materials by external magnetic fields" contains a subproject that is focussed on the development of composites consisting of ferromagnetic shape memory thin films and polymers for the use in sensor applications. The envisaged sensor is intended to measure large strains based on the inverse MSM effect. The change in magnetic properties (permeability) with strain stems from the magnetic anisotropy along the crystallographic axis of the MSM material. When martenstic variants redistribute by twin boundary motion due to external stress, those variants having the long axis directing in the strain direction are preferred over the others. Thus the shape of the magnetization curve along the strain direction or perpendicular to it changes with strain.

In case of polycrystalline materials, grain boundaries hinder the twin boundary movement and limit the maximal achievable strain to less than 1%. The most pronounced MSM effect is therefore achieved in single crystalline materials which are up to today only available as bulk. For the integration into microsystem technology thin films with a single crystalline microstructure are needed. One possibility to obtain such films is sputtering on single crystalline substrates so that the FePd thin film grows epitaxially with a defined orientation towards the substrate. In his project these thin films are fabricated by rf magnetron sputtering on MgO (001) oriented substrates that are coated with a metallic buffer layer. The buffer layers foster epitaxial growth and act at the same time as a sacrificial layer, so that freestanding thin films can be obtained and used for sensor applications.

The combination with soft substrates, e.g. polymers, allows for strain sensors with a range of several % strain. In addition, this sensor can be read out in a non contact mode since strain dependent magnetic properties are detected. The realization of such a MSM thin film sensor is the aim of this project.