First Order Phase Transitions of Magnetic Materials (ePub)

Praveen Chaddah joined BARC through their Training School, in 1973. His Ph D work involved setting up a Compton Profile Spectrometer with a ¿-ray source (the 1st in India) for electron momentum density measurements. His work focused on electron states in structurally disordered materials, and on electron correlation effects. As a post-doc at University of Illinois at Urbana- Champaign, he initiated measurements of nuclear momentum densities in the quantum solid 4He, using the spallation neutron source IPNS at Argonne. Dr Chaddah also worked on correlating the superconducting and martensitic transitions in A-15 superconductors. He worked on the development of superconducting magnets and of multifilamentary NbTi wires at BARC during 1982-87, and later made important contributions to the development and extension of Bean's Critical State model for the high TC superconductors. He reformulated this as a 'minimum flux-change' hypothesis, and contributed to its application to sample-shapes having finite demagnetization factor. He then worked on 1st order phase transitions in vortex-matter in superconductors, as also in magnetic materials. His recent emphasis has been on understanding metastabilities associated with supercooling and superheating, as also those associated with glass-like arrest of kinetics. His work showing tunability of coexisting phases in halfdoped manganites by varying the cooling field, was followed up on many materials exhibiting magnetic field induced first order transitions. This introduced the idea of "kinetic arrest" as a broad first order transition that is interrupted before completion. He developed the protocol CHUF (cooling and heating in unequal fields) that has provided rather visual evidence of kinetic arrest resulting in a glass-like arrested state. The observation of such behavior across magnetic 1st order transitions in various magnetic materials, where diffusive motions are not apparent, may lead to newer understanding of what causes