Magnetic domain characterization

1. (b) (a) Magnetic domain wall image of (a) tweed and (b) polytwinned microstructure. 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-55at%Pd 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-55at%Pd 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. variant 3 variant 2 variant 1 cubic 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.