Magnetic Shape Memory films
In some ferromagnetic and martensitic materials high reversible strains can be induced by magnetic fields. In particular Magnetic Shape Memory (MSM) alloys like Ni-Mn-Ga or Fe-Pd can reach strains of up to 10%.
Since no additional levers are required the high strain of these materials will allow miniaturization of integrated positioning systems (actuators) as well as a significant increase of the dynamic range of strain sensors. For both MSM application routes – actuators and sensors – different effects are utilized. In actuators, magnetic field induced reorientation (MIR) is used. Driven by the reduction of total magnetic anisotropy energy, the reorientation of martensitic variants causes straining of the materials already in moderate magnetic fields. In contrast, for sensor applications the inverse MSM effect is utilized. By mechanically straining a film the reorientation is forced and the change of magnetic state is measured. This will allow measuring strains up to 10%.
Due to the reduced size of microactuators and sensors the growth of films exhibiting the MSM effect is essential. Since in bulk material the highest strain is obtained in single crystals, we focus on the epitaxial (single-crystal-like) film growth. Freestanding films are required in order to allow a movement of the MSM film since a rigid substrate blocks the MSM effect. Thus the substrate has to be removed by chemical or physical methods.
We examine the growth of MSM films and adequate methods to release films from the substrate. Using pulsed laser deposition as well as sputter deposition we succeeded in growing epitaxial films of both key materials, Ni-Mn-Ga and Fe-Pd, which are ferromagnetic and martensitic at room temperature. Structure, (martensitic) microstructure and texture were investigated using X-ray diffraction, electron diffraction (RHEED, EBSD) and microscopy methods. The martensite-austenite transformation orientation relationship of the Ni-Mn-Ga 7M martensite was derived from theory and verified by thin film experiments. In both, Ni-Mn-Ga and Fe-Pd films stress-induced martensite formation stabilizing the martensitic phases at room temperature was verified by in-situ and ex-situ stress measurements. In Fe-Pd films an enhanced stability of the fct martensite was achieved, which only exists in a very narrow composition range in bulk. In addition, due to the reduced volume constraint of a thin film, we observed reversibility of the fct-bct transformation. In Ni-Mn-Ga films near the stoichiometric composition Ni2MnGa, the 7M phase could be stabilized above room temperature (315 K). Preparation and physical properties of freestanding films for actuator applications have been investigated using films peeled of from MgO or NaCl substrates. We could show that twin boundary motion as well as the classical shape memory effect occurs in Ni-Mn-Ga films. For disordered Fe-Pd films DFT calculations and magnetization measurements promise an increase in anisotropy when increasing the tetragonal distortion and Pd content of the “fct”- phase. A significant tetragonal distortion could be achieved in epitaxial fct Fe-Pd films. Due the nanoscale twinned microstructure the anisotropy however is reduced, an effect which could be explained by the random anisotropy model.
1)
Nanometer scale martensitic microstructure observed by SEM on a 7M Ni-Mn-Ga
film (a). Due to film stress, selection of martensitic variants occurs in the
films. All observed variants have there twinning plane lying along the [101]
direction of the film (b), while the other variants with the twinning plane
along [110] are not observed (c).
2) Magnetic evidence for magnetic field induced reorientation at room temperature in Ni-Mn-Ga martensite films on MgO(100) (for two different substrate orientation). The jumps in the magnetization curves are a typical feature of MIR.
3) Tetragonal
distortion (c/a ratio) of as-deposited Fe-Pd films measured by XRD at room
temperature. In comparison to bulk, expanded composition range of the fct
martensite (c/a ratio <1) is observed in as-deposited Fe-Pd films.
[J. Buschbeck et al., PRB 2007]
4) 1.5
µm thin bridge actuator made of a Ni-Mn-Ga film (left) and the temperature
dependent deflection caused by the martensitic transformation of the material
(right).
[F. Khelfaoui et al., EPJ 2008]
5) SEM picture of a freestanding, thin, polycrystalline Fe-Pd film with film thickness of 300 nm layered on a Si-substrate
Key publications
J. Buschbeck, I. Opahle, M. Richter, U. K. Rößler, P. Klaer, M. Kallmayer, H. J. Elmers, G. Jakob, L. Schultz, S. Fähler
Full Tunability of Strain along the fcc-bcc Bain Path in Epitaxial Films and Consequences for Magnetic Properties
Phys. Rev. Lett. 103 (2009) 216101
Stray-Field-Induced Actuation of Free-Standing Magnetic Shape-Memory Films
Adv. Mat. 21 (2009) 3708
Magnetically induced reorientation of martensite variants in constrained epitaxial Ni-Mn-Ga films grown on MgO (001)
New J. of Phys. 10 (2008) 023040
2010
M. Kohl, Srinivasa Reddy Y., F. Khelfaoui, B. Krevet, A. Backen, S. Fähler, T. Eichhorn, G. Jakob, A. Mecklenburg
Recent Progess in FSMA Microactuator Develepments
Mat. Sci. Forum 635 (2010) 145
2009
O. Heczko, L. Straka, V. Novak, S. Fähler
Magnetic anisotropy of non-modulated Ni-Mn-Ga martensite revisited
J. Appl. Phys. (2009) submitted
Stray-Field-Induced Actuation of Free-Standing Magnetic Shape-Memory Films
Adv. Mat. 21 (2009) 3708
S. Fähler
Über die Faszination von Magnetischen Formgedächtnislegierungen
Werkstofftechnologien Ingenieurwissenschaften (2009) in print
Full tunability of strain along the fcc-bcc Bain path in epitaxial films and consequences on magnetic properties
Phys. Rev. Lett. 103 (2009) 216101
The effect of post annealing on structure, microstructure and magnetic properties of thin Ni-Mn-Ga films
ESOMAT (2009) 04002
S. Kaufmann, U. K. Rößler, O. Heczko, M. Wuttig, J. Buschbeck, L. Schultz, S. FählerAdaptive martensite in Ni-Mn-Ga
Phys. Rev. Lett. (2009) submitted
Magnetically induced martensite transition in freestanding epitaxial Ni-Mn-Ga films
Appl. Phys. Lett. 94 (2009) 152513
In-situ studies of the martensitic transformation in epitaxial Ni-Mn-Ga films
Acta Mat. 57 (2009) 2516
2008
S.
Kaufmann, J. Buschbeck, O. Heczko, L. Schultz, S. Fähler
Epitaxial growth of Ni-Mn-Ga: Consequences of
Magnetron configuration on martensitic behavior
Proc.
ICOMAT 2008, Santa Fe (2008), submitted
S. Fähler
Magnetic Shape Memory Alloys: Achievements of the
German SPP 1239
Proc. Actuator 2008,
11th Int. Conf. on New Actuators, Bremen, Germany (2008) 267
J. Buschbeck, L.
Schultz, S. Fähler
Epitaxial Fe-Pd Magnetic Shape Memory Films - Issues
for Preparation and Applications
Proc. Actuator 2008,
11th Int. Conf. on New Actuators, Bremen, Germany (2008) 750
S. Fähler, O. Heczko, M.
Thomas, R. Niemann, J. Buschbeck, L. Schultz
Recent Progress towards active epitaxial Ni-Mn-Ga Magnetic
Shape Memory Films
Proc. Actuator 2008,
11th Int. Conf. on New Actuators, Bremen, Germany (2008), 754
M.
Thomas, O. Heczko, J. Buschbeck, L. Schultz, S. Fähler
Stress induced martensite in epitaxial Ni-Mn-Ga
films deposited on MgO (001)
Appl. Phys. Lett. 92 (2008) 192515
J. Buschbeck, I. Opahle,
S. Fähler, L. Schultz, M. Richter
Magnetic Properties of Fe-Pd Magnetic Shape Memory Alloys
- Density Functional Calculations and Epitaxial Films
Phys. Rev. B. 77 (2008) 174421
M. Thomas, O. Heczko, J. Buschbeck, U. K. Rößler, J. McCord, N. Scheerbaum,
L. Schultz, S. Fähler
Magnetically induced reorientation of martensite variants in
constrained epitaxial Ni-Mn-Ga films grown on MgO (001)
New J. of Phys. 10 (2008) 023040
O. Heczko, S. Fähler,
T.M. Vasilchikova, T.N. Voloshok, K.V. Klimov, Yu.I. Chumlyakov, A.N. Vasiliev
Thermodynamic, kinetic and magnetic properties of a
Ni54Fe19Ga27 magnetic shape memory single
crystal
Phys. Rev. B. 77 (2008) 174402
J. Buschbeck, O. Heczko, A.
Ludwig, S. Fähler, L. Schultz
Magnetic Properties of epitaxial Fe-Pd
films measured at elevated temperatures
J. Appl. Phys. 103
(2008) 07B334
T. Edler, J. Buschbeck,
C. Mickel, S. Fähler, S. G. Mayr
Mechanisms of stress generation and relaxation
during pulsed laser deposition of epitaxial Fe-Pd magnetic shape memory alloy
New J. of Phys. 10 (2008) 063007
O. Heczko, M. Thomas, J. Buschbeck, L. Schultz, S. Fähler
Epitaxial Ni-Mn-Ga films deposited on SrTiO3 and evidence of
magnetically induced reorientation of martensite variants at room
temperature
Appl. Phys. Lett. 92 (2008) 072502
F. Khelfaoui, M. Kohl, J.
Buschbeck, O. Heczko, S. Fähler, L.
Schultz
A Fabrication Technology for Epitaxial
Ni-Mn-Ga Microactuators
Eur. Phys. J. Special Topics 158 (2008) 167-172
2007
S. Fähler
An Introduction to Actuation Mechanisms of Magnetic Shape Memory Alloys
ECS Trans. 3 (25) (2007) 155
J. Buschbeck, I. Lindemann, L. Schultz, S. Fähler
Growth, structure, and texture of epitaxial Fe100-xPdx films deposited on MgO(100)
at room temperature: An x-ray diffraction study
Phys. Rev. B 76 (2007) 205421