First Order Phase Transitions of Magnetic Materials

Praveen Chaddah joined BARC through their Training School, in 1973. His Ph D work involved setting up a Compton Profile Spectrometer with a γ-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 glasses to form. Praveen Chaddah received the INSA Young Scientist medal in 1978, and the MRSI -ICSC Prize for Superconductivity in 1993. He is a Fellow of the Indian National Science Academy, a Fellow of the Indian Academy of Sciences, and also a Fellow of the National Academy of Sciences of India. He is also an elected Member of the Asia Pacific Academy of Materials. He has authored over 200 research papers. He was the Director of the UGC-DAE Consortium for Scientific Research for over eight years, where he established internationally competitive facilities for experimental research that were open to researchers from universities across India. He retired from the Department of Atomic Energy in December 2013.

Phase transitions and rigorous definitions





Phase transitions in Nature, and a meandering introduction



Introduction: back to basics



The Ehrenfest classification
1.3.1 First order transitions




Studying phase transitions with two control variables



Van der Waals gas as an exactly solvable model: isotherms and isobars



Metastable states across a phase transition: limitations of the Ehrenfest classification.

References




Modern classification of phase transitions





First order transitions, and the rest



Metastable states are specific to first order transitions



Limits of metastability



Hysteresis across first order phase transitions



Metastable to stable transformations



First order phase transitions with two control variables

References




Defining characteristics of first order transitions.





Necessary and sufficient characteristics following the Ehrenfest classification
3.1.1 Melting of the vortex lattice




Need for other characteristic tests beyond the Clausius-Clapeyron relation



Necessary and sufficient characteristics following the modern classification





Hysteresis as an indicator



Hysteresis together with metastable to stable transformation



References




Metastable and ‘arrested unstable’ states across first order transitions





Conceptual difference between metastable and unstable states



Relaxation rates for metastable, and for arrested unstable, states



Manifestations of kinetic arrest in studies using two control variables





Temperature variations in various fields



The CHUF protocol



CHUF for supercooled states



Isothermal variations of field



References




Disorder-broadened transitions





Broadened supercooling and superheating bands



Kinetic arrest and interrupted transitions: tuning by cooling field



Kinetic arrest and interrupted transitions: isothermal variation of field



The CHUF protocol



Measuring Tk(H) using CHUF



What causes kinetic arrest?



References

Subject Index