Spin-glass to Magnetic-glass via Bragg-glass and Vortex-glass

The concept of spin-glass, which came into the forefront in mid 1970s, initially dealt with the competition between antiferromagnetic (AFM) and ferromagnetic (FM) interactions in the metallic alloys with relatively dilute magnetic impurities. This competition is so strong that none of the long-range magnetic orders is established. Instead, the competition gives rise to a random spin configuration frozen in time, with the typical experimental signature of thermomagnetic irreversibility and metastability. In analogy with the frozen in liquid-like structure in structural glasses and the associated metastability, the name spin-glass was attached with the observed phenomena. However, unlike in structural glasses, in early 1980s a theoretical model of second order thermodynamic phase transition with definite spin-glass order parameter was introduced, which eventually took the concept of spin-glass well beyond the realm of dilute magnetic alloys to divergent areas including neural networks.

With the advent of high T_C superconductors, the flux-line lattice of type-II superconductors came under scrutiny, so much so that a new term ‘vortex matter’ was introduced to represent the flux-line lattice. The thermomagnetic irreversibility and metastability are typical characteristics of the vortex matter, and in the initial days the idea of spin-glass was imported to explain the experimental results. Soon it was established that there existed two distinctly resolved solid phases of vortex matter, which are distinguished from the ‘high-temperature high magnetic field’ vortex liquid. These two vortex solid phases are referred to as low-field quasi-ordered solid or Bragg-glass and high-field disordered solid or vortex-glass, and the transition between various vortex matter phases were found to be a first order thermodynamic phase transition.

Exporting back the various ideas developed in the study of vortex matter to magnetic systems, it was realized in many magnetic systems a kinetic arrest of the first-order FM to AFM phase transition leads to a nonequilibrium magnetic state with a configuration of FM and AFM clusters frozen randomly in experimental time scale. The dynamics of this nonequilibrium magnetic state is very similar to that of a structural glass, and analogically, this new magnetic state is named magnetic-glass. The results emerging from disparate classes of magnetic systems starting from alloys and intermetallic compounds to various other classes of magnetic systems including manganite systems showing colossal magnetoresistance, suggest that this magnetic-glass phenomenon is distinctly different from spin-glass, and it is also independent of the underlying microscopic nature of magnetic interactions.

This seminar will give a glimpse of the author’s journey form spin-glass to magnetic-glass through vortex-matter, over the span of last four decades.