Magnetic domain characterization

1
Microstructure Design of Advanced Multi-Domain
Magnetic Materials under Applied
FieldsKhachaturyan (Rutgers), Laughlin(CMU) and
Wang (OSU), DMR (FRG) Award 9905725

• Magnetic domain characterization
• Lorentz transmission electron microscopy (LTEM)
  imaging techniques are used to image the magnetic
  domain structure of Fe-55atPd alloy. Three
  different types of microstructure are identified.
  
• In situ Lorentz TEM imaging techniques are used
  to image the magnetization process in polytwinned
  microstructure.
• Magnetic properties design
• The effect of atomic ordering on Curie
  temperature (Tc) of FePd and FePt bulk alloy is
  investigated.
• A molecular field model is being developed to
  understand the different responses of Curie
  temperature to atomic ordering for FePt and FePd
  alloys.
• ????? u u u
• Lisha Wang, David. E. Laughlin, Y. Wang, A. G.
  Khachauryan, Magnetic domain structure of
  Fe-55atPd alloys at different stages of
  ordering, J. Appl. Phys. 93, 7984 (2003)

Tc changes with composition for fcc and L10 Fe-Pd
alloys.
2
Microstructure Design of Advanced Multi-Domain
Magnetic Materials under Applied
FieldsKhachaturyan (Rutgers), Laughlin(CMU) and
Wang (OSU), DMR (FRG) Award 9905725

• Microstructure Engineering
• We are investigating novel processing routes for
  microstructure engineering of advanced magnetic
  materials.
• Effects of dislocation patterning on spinodal
  decomposition are studied. Novel two-phase
  microstructrural patterns (figures on the right)
  are predicted, which may have unique magnetic
  properties.
• Solute segregations at both dislocations and
  grain boundaries are studied. A segregation
  transition is predicted in both cases, where
  solute concentration, cd, at a dislocation or a
  grain boundary experiences a sharp transition as
  the temperature changes. The transition
  temperatures during cooling and heating are
  different, leading to a hysteresis (right bottom
  figure).
• ? ??u u u
• Ning Ma, S. A. Dregia and Y. Wang, Segregation
  Transition and Drag Force at Grain Boundaries,
  Acta mater. 51, 3687-3700 (2003).

Microstructural evolution during spinodal
decomposition with the presence of an array of
edge dislocations.
c0 0.02
cd
T(K)
Solute segregation transition at a dislocation.
3
Microstructure Design of Advanced Multi-Domain
Magnetic Materials under Applied
FieldsKhachaturyan (Rutgers), Laughlin(CMU) and
Wang (OSU), DMR (FRG) Award 9905725

• Microstructures in Ferromagnetic Films
• The phase field microelasticity (PFM) theory is
  developed for phase transformations near free
  surfaces and in thin filmsa. It enables us to
  simulate the formation of underlying
  microstructures of ferromagnetic film in magnetic
  storage device. The tetragonal lattice
  ferromagnetics are studied (L1o FePt is one
  example).
• Free standing films made of ferromagnetic
  shape-memory alloys exhibiting giant
  magnetostriction have important application as
  actuators and sensors. Being such new smart
  materials, FePd and Fe3Pt undergo cubic to
  tetragonal martensitic transformations. The
  proposed PFM is capable of taking into account
  magnetic and elastic coupling and simulating the
  complex microstructure evolution to provide
  critical information for these novel materials.
• The multi-variant and multi-phase domains
  together with polycrystal textures provide plenty
  of structural flexibilities for tailoring
  microstructures in thin films, which allows us to
  design magnetic properties.
• ?? ?u u u
• aY.U. Wang, Y.M. Jin and A.G. Khachaturyan, The
  Effects of Free Surfaces on Martensite
  Microstructures 3D Phase Field Microelasticity
  Simulation Study, Acta Mater. (submitted, 2003).

(a)
(b)
The microstructures in y-z cross-section (film
plane) for different tetragonalities.
4
Microstructure Design of Advanced Multi-Domain
Magnetic Materials under Applied
FieldsKhachaturyan (Rutgers), Laughlin(CMU) and
Wang (OSU), DMR (FRG) Award 9905725

• Microstructure in Thin Film
• The microelasticity theory is developed for
  dislocation dynamics in epitaxial thin film.
• Magnetic recording devise is used in the form of
  epitaxial films. The epitaxial stress introduces
  misfit dislocations and changes the underlying
  microstructure.
• The introduction of misfit dislocations could be
  used to design special texture of the underlying
  microstructure to obtain desired magnetic
  property. But the improper operation of the
  misfit dislocations may degrade its magnetic
  property.
• ?? ?u u u
• Y.U. Wang, Y.M. Jin and A.G. Khachaturyan,
  Phase Field Microelasticity Modeling of
  Dislocation Dynamics near Free Surface and in
  Heteroepitaxial Thin Films, Acta Mater. 51,
  4209, 2003. ).

Education Three graduate students (two female)
and a postdoctoral fellow are trained extensively
in both theory, computation and experiment. In
addition, the project has enabled us to have
three high school students work in our
laboratories.
The dislocation evolution in epitaxial thin film.
Movement of the threading dislocation deposits
the misfit dislocations at the interface of the
film and substrate. These dislocations change the
magnetic property of the ferromagnetic film.