New class of magnetic refrigerants based on the Cr(V) tetraperoxides

Abstract

We have recently investigated a series of compounds based on the Cr(V) tetraperoxides (general formulas M3CrO8 where M=Li, Na, K, Rb, Cs) that exhibit novel magnetic and heat capacity properties. Chief among them is the promise of superior performance for magnetic refrigeration. Currently, adiabatic demagnetization refrigeration (ADR) compounds are generally limited in their effective cooling temperature range. These limitations are a function of the magnetic spin ordering temperature, which limits the lowest attainable temperature, and the spin concentration, which limits the refrigeration power. Our investigations indicate that the peroxychromates can be tuned to possess maximal refrigeration power at a desired operating temperature. Evidence for this claim is suggested by the heat capacity coefficient (b). The lower the magnitude of b, the greater the ADR potential. Experimental evidence shows introducing larger cations significantly lowers b, while basically maintaining the spin concentration. This paper focuses on explaining the trend in terms of contributions to the internal fields, which are tentatively assigned to the exchange and dipolar interactions. It is predicted that the controlled modification of these fields by molecular structural alterations tunes these compounds to have maximum refrigeration power over a broad range of temperatures, i.e., 0.010 to 20 K.

Thermomagnetic properties of the Finland trityl radical

“…We attribute this to g-anisotropy and inhomogeneous broadening due to the buildup of internal fields at low temperatures. This is consistent with the magnetization data that showed the Curie-Weiss temperature of Ϫ0.7 K. In terms of magnetic cooling, this magnitude of internal field theoretically indicates that the concentrated trityl system would be of use in the temperature range of ϳ1-10 K. 7 Diluted samples would be expected to provide convenient systems that balance cooling loads with desired temperature below 1 K. 7 T 1 is much shorter for this concentrated system than for dilute systems, which is attributed to spectral diffusion and/or spin-spin interaction. For quantum computing purposes, these results would be termed spin entanglement.…”

“…An earlier report proposed a new class of tunable refrigerants based on the variable spin concentrations of a series of Cr͑V͒ tetraperoxides. 7 The ease with which the trityl spin concentration is controlled via glassy mixtures may render them as viable, tunable, adiabatic demagnetization materials. However, an understanding of the magnetization and internal fields is necessary for such conclusions.…”

“…13 A trade-off exists between the lowest attainable temperature and the refrigeration power. 7 A high spin concentration will have a high refrigeration power, but the presence of spin-spin exchange and dipolar interactions will limit the lowest temperature. An earlier report proposed a new class of tunable refrigerants based on the variable spin concentrations of a series of Cr͑V͒ tetraperoxides.…”

“…6,7 Magnetic cooling is important due to limited helium supplies as well as in space applications with low gravitational fields. 13 A trade-off exists between the lowest attainable temperature and the refrigeration power.…”

“…These applications include dynamic nuclear polarization ͑DNP͒ for enhancement of nuclear magnetic resonance and magnetic resonance imaging ͑MRI͒, 1-5 magnetic cooling, 6,7 electron paramagnetic resonance imaging ͑EPRI͒, 8,9 and quantum computing. 10,11 Paramagnetic systems include transition metal ions and organic radicals.…”