1. IMEM-CNR, Parco Area Delle Scienz 37/A, Parma, Italy

1. IMEM-CNR, Parco Area Delle Scienz 37/A, Parma, Italy

Topic Code:

Ferromagnetic shape memory materials turn to nano:microstructure engineering of thin films and nano-disks

F. Albertini1, S. Fabbrici1,2, F. Casoli 1, L. Nasi 1, P. Ranzieri1, R. Cabassi1, M. Campanini3,

C. Magén4, F. Celegato5, G. Barrera5, P. Tiberto 5,1

1. IMEM-CNR, Parco Area delle Scienz 37/a, Parma, Italy

2. Mist ER laboratory, Via Gobetti 101, Bolgna, Italy

3. EMPA, Uberland- str. 129, Dübendorf, Switzerland

4. Instituto de Nanociencia de Aragón, calle M. Esquillor, Zaragoza , Spain

5. INRIM, Strada delle Cacce 91, Torino, Italy

Email:

Abstract:We present a multiscale study of ferromagnetic shape memory thin films and nano-disks and report some noticeable examples of microstructure engineering in view of their possible exploitation as micro/nano actuators.

1. INTRODUCTION

Magnetic shape memory materials show remarkable multifunctional properties (e.g. magnetomechanical, magnetocaloric) arising from the presence of a martensitic transformation and magnetic states [1].

Low-dimensional materials, mainly thin films, have recently attracted much interest for their great potential in applications (e.g. microactuators, valves, solid-state microrefrigerators) [2]. With respect to the bulk, they offer the further possibility of tuning properties by exploiting the epitaxial growth on suitable substrates and underlayers [3].

Patterned films and 2D nanostructures are nowadays a vast and unexplored field. The present talk will be focused on microstructure engineering of NiMnGa thin films and nano-disks.

2. Experimental

Epitaxial thin films of NiMnGaof thickness from 10 to 400 nmwere grown by sputtering r.f. on MgO substrates with and without a Cr underlayer. Patterned thin films were obtained by polystyrene-nanosphere lithography. Freestanding nano-disks (d=160, 650nm) were subsequently obtained by removing the Cr underlayer by a selectivechemical etching.

A multiscale structural and magnetic study was performed by means of electron microscopy (HREM, STEM-HAADF, electron diffraction, Lorentz microscopy), X-ray diffraction, AFM/MFM, and AGFM and SQUID magnetometry and susceptometry.

3. Results and discussion

We have found that the microstructure of thin films and nanodisks can be suitably tuned by thickness, lateral confinement, and substrate/underlayer.

As an example, films of 200 nm grown on Cr presenttwo different families of martensitic variants (X and Y). X-type regions twins inclined at 45° with respect to the film plane and easy magnetization direction (EMD) alternatively in-plane and out-of plane; Y-type regions show twins perpendicular to the film plane and EMDalong two different directions of the film planeAdditionally, by stressing the substrate during the martensitic transformation it is possible to alter the distribution of X and Y regions.

Fig.1. Cross sectional (a) and in plane (b) HRTEM image showing X- and Y-regions of martensitic twin variants. (c), (d) Lorentz microscopy maps.White arrows indicate the easy magnetization directions.

These films show huge jumps in the magnetization curve (up to 55%)that can be related tomagnetic field induced reorientation of twin variants. Remarkably, the intensity and anisotropic character of the effect can be tuned by changing the microstructural patterns under the application of external stress.

It was also found that free-standing nano-disks preserve the salient martensitic and magnetic properties of continuous films and are thus exploitable in multifunctional applications. In addition, twin microstructure of nano-disks, can also be modified by lateral confinement.

These features, mainly exploitable in shape memory and magnetic shape memory applications, pave the way to the realization of shape-controlled micro/nanoactuators.

REFERENCES

[1].M. Acet, et al., Handbook of Magnetic Materials vol. 19, Elsevier, Amsterdam, 2001.

[2] A. Backen et al., Adv. Eng. Mat. 14,(2012) 696.

[3] P. Ranzieri et al., Acta Mater. 61(2013)263.

[4] P. Ranzieri et al., Adv. Mater. 32,(2015) 4760.