Exotic Magnetic Phases Studied Using In-Field Neutron Diffraction

Exotic magnetic phases studied using in-field neutron diffraction

Vasudeva Siruguri

UGC-DAE Consortium for Scientific Research Mumbai Centre,

R-5 Shed, BARC Campus, Mumbai – 400 085, INDIA

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First order phase transitions (FOPT) in solids are usually broadened due to presence of quenched disorder. One can supercool or superheat across the FOPT, resulting in metastable states and hysteresis. There exists a spatial distribution of transition temperatures within the correlation length. A supercooled metastable would transform into an equilibrium state on lowering of temperature below the supercooling limit. It is also possible to interrupt the kinetics of the transition to form a glass, resulting in a phase coexistence of metastable and equilibrium phases, a phenomenon termed as kinetic arrest. These concepts, typical of structural glasses, can be extended to magnetic systems that exhibit first order magnetic phase transitions. Magnetic materials like manganites, magnetic shape memory alloys, multiferroics and magnetocalorics undergo FOPT whose kinetics can be strongly influenced by an external magnetic field, resulting in a phase separated coexistence of metastable and equilibrium phases. Magnetic field is used as an effective second thermodynamic variable to interrupt the magnetic FOPT and form a magnetic glass. The phase coexistence can be quantified by measurement of physical properties like magnetization and microscopic magnetic structure under novel thermo-magnetic histories in the (H,T) space termed as CHUF (Cooling and Heating in Unequal Fields) protocol. Since neutron diffraction is an ideal technique to probe magnetic and chemical structures of materials, it can be used to track changes in magnetic and chemical structures across phase transitions. In-field neutron diffraction is used in conjunction with the CHUF protocol to establish the kinetic arrest of FOPT in the above magnetic materials. The devitrification of the arrested phases into their low temperature equilibrium phases at the characteristic temperature TK(H) and their subsequent reentrant transition to the high temperature equilibrium phases, unambiguously demonstrates the glassy nature of the arrested states. In this talk, I discuss the existence of some exotic magnetic phases in some selected intermetallic alloys which exhibit kinetic arrest [1-5]. It is seen that analogies can be drawn between the dynamics involved in such kinetically arrested systems and those observed for structural glasses.

References

[1] R. Rawat, P. Chaddah, P. Bag, P. D. Babu, and V. Siruguri, J. Phys.: Condens Mater 25, 066011 (2013)

[2] V Raghavendra Reddy, R Rawat, Ajay Gupta, Pallab Bag, V Siruguri and P Chaddah J. Phys.: Condens Mater. 25, 316005 (2013)

[3] Pallab Bag, R. Rawat, P. Chaddah, P.D. Babu and V. Siruguri, Phys. Rev. B 93, 014416 (2016)

[4] V. Siruguri, P.D. Babu, S.D. Kaushik, A. Biswas, S.K. Sarkar, K. Madangopal and P. Chaddah, J. Phys.: Condens. Matter 25, 496011 (2013) and references therein.

[5] S. Rayaprol, V. Siruguri, A. Hoser, C. Ritter and E.V. Sampathkumaran, Phys. Rev. B90, 134417 (2014)