EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

The CoCrFeMnNi high-entropy alloy (HEA) is a magnetically concentrated crystalline system with all lattice sites magnetic, containing randomness (five different types of spins are randomly positioned on the lattice) and frustration (a consequence of mixed ferromagnetic and antiferromagnetic interactions). We have studied experimentally the nature of the magnetic ground state and found out that upon cooling, no long-range magnetic ordering takes place, but the spin system undergoes a kinetic freezing transition to a spin glass phase, where below the spin freezing temperature T_f ≈ 20 K, ergodicity of the system is broken [1]. The observed broken-ergodicity phenomena include zero field cooled – field cooled magnetization splitting in low magnetic fields, frequency-dependent cusp in the ac susceptibility, ultraslow time-decay of the thermoremanent magnetization and the memory effect, where a state of the spin system reached upon isothermal aging at a certain temperature can be retrieved after a negative temperature cycle. All these phenomena are associated with the out-of-equilibrium dynamics of a nonergodic, frustrated system of coupled spins that approach thermal equilibrium, but can never reach it on a finite experimental time scale, so that we are observing only transient effects of partial equilibration within localized spin domains. The observation of the memory effect prompts for the application of the CoCrFeMnNi HEA as a thermal memory cell, where a byte of digital information can be stored into the material by pure thermal manipulation, in the absence of any external magnetic or electric field.

[1] P. Koželj, S. Vrtnik, et al., J. Magn. Magn. Mater. 523, 167579 (2021).