Stray‐Field‐Induced Actuation of Free‐Standing Magnetic Shape‐Memory Films

Abstract

Magnetic shape-memory (MSM) alloys, such as NiMnGa, [1] reach maximum strains of close to 10% in single crystals. [2] This exceeds the strain obtainable from piezoelectric or magnetostrictive materials, currently used in actuators, by more than one order of magnitude, and opens new opportunities for applications. Advanced materials have been developed based on new compositions [3][4][5] or on innovative routes of fabrication resulting in foams, [6] fibers, [7] textured polycrystals, [8] or composites.[9] Thin MSM films with these high strains would be of significant benefit for use in microactuators. However, preparation of films exhibiting such high strains has not been achieved so far. In the present work, we demonstrate a new and enhanced thermal actuation mode, which is achieved by the use of freestanding, epitaxially grown NiMnGa films. This new mode utilizes a variant selection by magnetic stray-field energy within the martensite state. It allows for reversible actuation similar to the conventional two-way shape-memory effect, yet neither training nor an external magnetic field is required. A novel path to the realization of sub-micrometer, miniaturized microactuators based on MSM films is established. Due to the finite size of magnetic domains, this actuation mode is a unique feature of small systems.The novel mode uses both the ferromagnetic and martensitic properties of MSM alloys. Since it involves features of known actuation modes, these are described in the following. Shapememory alloys exhibit a diffusionless transition from a hightemperature cubic austenite phase to a low-temperature martensite phase of lower symmetry.[10] The martensitic phase may, as in the present case, exhibit a tetragonal structure with two identical long crystal a axes and one short c axis. Consequently, three different orientations of the martensitic c axis with respect to the cubic austenite cell are possible. Neighboring unit cells of identical orientation form so-called martensitic variants. Adjacent variants with different orientations are connected by twin boundaries. This special microstructure allows an easy deformation by changing the fractions of martensitic variants through twin-boundary motion. Heating to the cubic austenite state restores the original shape, which is the so-called one-way shape-memory effect. To obtain the two-way shape-memory effect, several cycles of mechanical training and heating are required. This presumably leads to the formation of microstructural defects, which can act as nucleation sites during martensite formation, memorizing the uneven variant distribution and hence the macroscopic shape.[11] For this actuation mode, temperature is the control parameter.Martensitic materials, which also exhibit ferromagnetic order, allow two additional actuation modes. The first type utilizes the coupling between crystal structure and spontaneous magnetization. In a magnetic field, the phase with the higher magnetic moment is energetically favored, leading to a field-dependent shift of the martensitic p...

Magnetically induced martensite transition in freestanding epitaxial Ni–Mn–Ga films

“…A detailed analysis revealed that this is due to an alternative thermal actuation mode, only possible in thin magnetic shape memory films. 17 Since variants with a perpendicular aligned easy magnetization axis are energetically disfavored due to the high stray-field energy, this effect was called stray-field-induced microstructure ͑SFIM͒. In this letter we show that these freestanding films also exhibit a magnetically induced martensitic transition.…”

Magnetic Shape Memory Microactuators

“…Actuation mechanism Reference Magnetic field-induced reorientation (MIR) of martensite Magnetic shape memory (MSM) actuation [4] Temperature gradient-induced martensitic transformation Single phase boundary actuation [34] Thermally induced ferromagnetic transition and martensitic transformation Bidirectional magnetostatic and thermoelastic actuation [35] Thermally induced metamagnetic transition Thermomagnetic actuation [36] Magnetic stray-field-induced microstructure Magnetic stray-field induced actuation [37] …”

“…Epitaxial FSMA films show another actuation mechanism based on the magnetic stray-field-induced microstructure [37]. When transforming a ferromagnetic austenite film to the martensite phase, variants with their easy axis in the film plane are preferentially formed in order to avoid magnetostatic energy contributions.…”

Recent Progress in FSMA Microactuator Developments

“…The MIR effect enables contactless control of large deformations at relatively large bandwidths, which are important prerequisites for applications in small space [6]. Recently, also other mechanisms for FSMA microactuation have been explored such as the induction of phase transformation in Ni-Mn-Ga thin films under high magnetic fields (magnetically induced martensite, MIM) [7,8], the antagonism of thermally induced shape recovery force and ferromagnetic attraction force in a magnetic field gradient [9][10][11], or utilizing a film's magnetic stray field energy to induce a preferential variant orientation [12]. Recent efforts mostly concentrate on the development of FSMA microactuators based on the MIR effect since MIR does not require a phase transformation, which has advantages in terms of power requirements and actuation frequencies.…”