Plenarvorträge - DPG-Tagungen

Magnetismus Donnerstag
MA 23 Hauptvorträge - Symposium Magnetic Shape Memory Alloys (zusammen mit M)
Zeit: Donnerstag 10:15–11:45 Raum: H23
Hauptvortrag MA 23.1 Do 10:15 H23
Martensite: Introductory Remarks — •E.F. Wassermann — Experimentalphysik,Universität
Duisburg-Essen, D-47048 Duisburg,
As a general introduction to the field of magnetic shape memory alloys
(MSMA), we briefly define what is a martensitic phase transformation
(MPT) in general and what are the characteristic features. We then focus
on the metallic elements and systems, where we distinguish between
Fe-based systems, in which the austenite has fcc structure and the martensite
is bcc (or bct), and the Hume-Rothery systems, in which the
opposite structural behavior is observed. Typical for the latter systems is
the phonon softening occurring at certain q-vectors of the Brillouin Zone
at temperatures above Ms. We will briefly discuss these precursor effects
and their relevance to MPTs. We will then highlight in general the shape
memory behavior, the one way-, the two way- and the all round-effect,
and briefly touch the present microscopic knowledge of these effects. Finally,
a short overview of practical applications of SMA will be given.
Hauptvortrag MA 23.2 Do 10:45 H23
Ferromagnetische Formgedächtnis Legierungen — •Manfred
Wuttig — Stiftung Caesar, Ludwig-Erhard-Allee 2, 53175 Bonn — Department
of Materials Science, University of Maryland, MD,USA
Ferromagnetische Formengedächtnis Legierungen (FGDLn) wurden
Mitte der 80iger Jahre interessant als bekannt wurde, dass solche Materialien
eine außerordentlich große ’ Magnetostriktion‘ aufweisen. Für die
am meisten untersuchten und am besten bekannten Materialien, basierend
auf der Heusler Legierung Ni2MnGa, kann die magnetfeld-indizierte
Dehnung 10 % erreichen. Die explosionsartige Entwicklung in diesem
Feld hat dazu geführt, dass im Moment FGDLn bestehend aus fast allen
Kombinationen von (Fe,Ni,Co)2Mn(Ga,Al) bekannt und teilweise charakterisiert
sind. Andere wichtige Elemente sind Pt und In. Die Stabi-
lität der Legierungen, i.e. die martensitischen Umwandlungstemperatur,
folgt der mittleren Elektronenkonzentration und die Curie Temperatur
in etwa dem bekannten Bethe-Slater Zusammenhang. Beide Temperaturen
variieren in entgegengesetzter Richtung, wie schon seit Zener bekannt.
Die ’ Magnetostriktion‘ der FGLn beruht auf der magnetfeldinduzierten
Bewegung der Zwillingswände zwischen martensitischen Varianten.
Um diese Art der (Um)magnetisierung gegenüber Drehungen
der Sättigungsmagnetisierung zu bevorzugen ist eine große Kristallanisotropie
nötig wie auch an Ni2MnGa gemessen. Die FGDLn haben Anwendungspotential
als Sensoren und Aktoren obwohl ihre magnetische
Energiedichte besondere Vorkehrungen für die letzteren erfordert. Der
Vortrag wird diese und mehr ins Einzelne gehende Tatsachen enthalten.
Hauptvortrag MA 23.3 Do 11:15 H23
The magnetic, structural and dynamical properties of the
Heusler alloys — •Alexey Zayak and Peter Entel — Theoretische
Tieftemperaturphysik, Universität Duisburg-Essen, 47048
Duisburg
We have performed first-principles calculations of a series of Heusler
compounds. In particular, the ferromagnetic alloys Ni2MnGa and
Ni2MnAl are of interest because of their shape-memory properties. The
ab-initio methods allow an accurate description the cubic, tetragonal and
orthorhombic as well as the long-range modulated structures. The latter
were found to be critically important for the stability of the martensitic
states allowing for c/a ratios smaller then 1. In addition we have calculated
the phonon spectra. The high symmetry cubic structure L21 turned
to be unstable at zero temperature, both for Ni2MnGa and Ni2MnAl, due
to the transverse acoustic mode which softens with the wave vector close
to 1/3[110](2π/2). The obtained results are analysed in terms of the electronic
configuration and charge transfer.
MA 24 Symposium Magnetic Shape Memory Alloys (zusammen mit M)
Zeit: Donnerstag 11:45–13:15 Raum: H23
MA 24.1 Do 11:45 H23
Electronic and Magnetic Properties of Heusler-type Magnetic
Shape Memory Alloys — •Mehmet Acet, Thorsten Krenke,
and Eberhard F. Wassermann — Experimentalphysik, Universität
Duisburg-Essen, 47048 Duisburg
The presence of large magnetic field ( 1 Tesla) induced strains ( 6
percent) found in the martensitic state of the magnetic shape memory
Heusler compound Ni2MnGa is believed to be due to twin boundary
motion. The attractive technological implications of this property has
provided the motivation for intensive research in the field of magnetic
shape memory. Efforts to improve the mechanical properties of otherwise
brittle Ni2MnGa have been paralleled by theoretical and experimental
research aimed at understanding the mechanism of the magnetic shape
memory effect and predicting the existence of other Heusler based systems
that may well exhibit this effect. By examining the electron concentration
dependence of the martensitic transformation temperatures in
various Heusler systems, it is possible construct a method with which it
is possible to choose sample systems that can be potential candidates for
magnetic shape memory alloys. We show by magnetization measurements
and microscopy that a series of ferromagnetic ternary and quasiternary
Heusler based compounds such as Ni-Co-Al and Ni-Mn-In-Al can undergo
martensitic transformations and have a large magnetocrystalline
anisotropy in the martensitic state.
MA 24.2 Do 12:00 H23
Influence of composition and heat treatment on structure
and magnetic properties of NiMnGa shape memory alloys
— •Stefan Roth 1 , Norbert Mattern 1 , and Andrea Böhm 2
— 1 IFW Dresden, POB 270116, D-01171 Dresden — 2 FhG IWU,
Bamberger Strasse 7, D-01187 Dresden
Ferromagnetic Ni(100-x-y)Mn(x)Ga(y) - alloys were prepared by arc
melting pure constituents with (x,y) = (26,22) and (29 or 30, 20 or 21).
The Curie temperature and the martensitic transformation temperature
were determined by a Faraday magnetometer and differential scanning
calorimetry, respectively. Magnetisation as a function of field and temperature
was measured by a vibrating sample magnetometer and the
crystalline structure was investigated using XRD. The magnetic anisotopy
field was derived from the field dependence of the magnetisation.
Annealing above the order temperature and cooling at different rates
causes changes in the magnetic properties and the transformation behaviour
which are correlated to changes in the degree of order, changes
in the lattice parameters and in the amount of remaining austenite.
MA 24.3 Do 12:15 H23
A study of ferromagnetic NiMnGa sputtered films including
shape memory properties — •Holger Rumpf 1 , Corneliu Craciunescu
2 , Manfred Wuttig 1,3 , and Eckhard Quandt 1 — 1 Stiftung
Caesar, Ludwig-Erhard-Allee 2, 53175 Bonn, D — 2 Politehnica University
of Timisoara, Romania — 3 Department of Materials Science, University
of Maryland, College Park, MD, U.S.A.
Off-stoichiometric Ni2MnGa thin films were deposited by DC
magnetron sputtering technique. Freestanding thin films as well as
Ni50Mn30Ga20/Mo composites were fabricated. Rapid thermal annealing
at temperatures equal or higher 500C led to polycrystalline films
revealing the reversible martensitic transformation. Transformation temperatures
and work output of the transformation were strongly influenced
by the temperature of the annealing process as demonstrated by DSC results
and cantilever deflection mode measurements. Magnetization experiments
disclose a Curie temperature of 103C in accordance with results
from NiMnGa single crystals. The shape of the ferromagnetic hysteresis
is significantly effected by the annealing temperature. Magnetic field induced
strains of freestanding films biases by an external load of several
MPa revealed elongations of ca. 0.1 per cent at 200 mT.