Abstract: The magnetically weakly anisotropic cubic Ni-Mn-Ga Heusler alloys exhibit
martensitic transformation resulting in martensitic phases with elastically soft crystal lattices and
strong magnetocrystalline anisotropies. The magnetic state of these martensites is coupled with a
highly mobile twin structure through the ordinary magnetoelastic interactions giving rise to a giant
magnetic-field-induced-strain effect. This effect is the key ingredient of a new scientific field. In the
present article, the basic phenomena and concepts of this field, such as lattice instability, soft-mode
behavior, electron concentration, ferromagnetic shape memory effect, magnetic-field-induced
superelasticity, twinning strain-induced change of magnetization, and magnetoelastic mechanism of
magnetostress are briefly reviewed.
Abstract: Rapid solidification route by melt spinning has been adopted for preparation of a Ni52.5Mn24.5Ga23
(at %) ferromagnetic shape memory alloy in the form of ribbons. In the as-spun state, the ribbon
revealed a predominant austenitic L21 structure in combination with martensitic feature as observed
from x-ray diffraction studies. Transmission electron microscopic (TEM) evaluation showed these
features in the form of martensitic plates. At low temperature, martensite to austenite transformation
was exhibited by an increase in magnetization during heating cycle. The reverse effect was
observed during cooling cycle. Annealing temperature and magnetising field was also found to
effect this transformation.
Abstract: Textured polycrystalline NiMnGa alloys were prepared by directional solidification.
Alloys were chosen to have either the 7M or the 5M modulated martensitic structure after proper
heat treatment. Mechanical training allowed to reduce the twin boundary pinning stress to below the
magnetically induced stress. Thus, magnetic field induced changes in the mechanical behaviour
could be demonstrated. The conditions of preparation and mechanical training will be discussed
together with their influence on structure, microstructure, and the magneto-mechanical behaviour.
Abstract: A series of Ni51.4Mn28.3Ga20.3 films sputter-deposited on Si(100) wafer (with 500 nm
thick buffer layer of SiNx) and annealed at 800 oC for 1h. are investigated with respect to their
transformation behavior and magnetic properties. The film thickness, d, varies from 0.1 to 5.0 μm.
Resistivity measurements reveal martensitic transformation above room temperature for all the films
except for 0.1μm-thick film which is transforming at much lower temperature. The magnetic
characteristics of martensitic films such as susceptibility and anisotropy field extracted from the inplane
and out-of-plane magnetization curves show film thickness dependence likewise Curie
temperature obtained from the resistivity curves. The surface topography and micromagnetic
structure are studied by scanning probe microscopy. A stripe magnetic domain pattern featuring a
large out-of-plane magnetization component is found in the films. The domain width, δ, depends on
the film thickness, d, as δ ~ d .
Abstract: Off-stoichiometric Ni2MnGa ferromagnetic shape memory alloys undergo a martensitic
transformation (MT) from the L21 cubic phase to a martensitic crystal lattice consisting, in the
majority of cases, of a periodic stacking sequence of nearly closed-packed planes with periodicity
of 5, 7 or 2 planes, denoted as 10M (five layered tetragonal), 14M (seven layered orthorhombic) and
2M (non-modulated tetragonal).
In addition to the parent to martensite transformation, Ni-Mn-Ga alloys tend to show stress or
temperature induced intermartensitic transformations (IMTs) towards the most stable 2M phase,
through the sequences 10M→14M→2M or 14M→2M depending on the first formed martensite.
The IMTs reported in the literature show a variety of characteristics such as reversibility,
completeness, hysteresis and temperature of occurrence, but, as a general trend, the role of internal
stresses in favouring the occurrence of IMTs is recognised.
Recently it has been shown that the L21 order degree favours the occurrence of the
intermartensitic transformation from 14M to 2M martensite, stabilising the non modulated
martensite through a decrease of its free energy with respect to the layered martensite. From this
point of view, the occurrence of intermartensitic transformations in Ni-Mn-Ga alloys appears as a
“chemical“ free energy effect. Aiming to go deeply into this aspect, in this work the occurrence of
IMTs and their properties have been examined for an extensive set of off-stoichiometric Ni2MnGa.
The results show the existence of a relationship between the IMTs temperatures and the alloys
composition, as well as the dependence of all observed IMTs (i.e., 10M→14M, 14M→2M and their
corresponding reverse transformations) on the L21 order degree.
Abstract: The effect of annealing temperature on martensite transformation temperature and
magnetic properties has been investigated in a polycrystalline Ni53.69Mn26.06Ga20.25 alloy. The
significant variation of the martensite transformation temperature and the magnetic properties is
observed as a function of annealing temperature.