Abstract
Aiming to predict new materials for magnetic refrigeration from high-throughput calculations asks for an accurate, transferable, and resource-wise balanced approach. Here, we analyze the influence of various approximations on the calculation of key properties of magnetocaloric materials, while revisiting the well-known FeRh system for benchmarking our approach. We focus on the entropy change and its contributions from the electronic, lattice, and magnetic degrees of freedom. All approximations considered are based on first-principles methods and have been tested, and compared for FeRh. In particular, we find that in this context, the Debye approximation for the lattice entropy fails, due to the presence of soft phonon modes in the AFM phase. This approximation is frequently used in the literature as a simple alternative to full phonon calculations. Since soft modes are likely to occur also among promising magnetocaloric materials where structural transformations are common, the use of the Debye approximation should be discarded for these systems treatment. This leaves the calculations of the lattice contribution the most demanding task from the computational point of view, while the remaining contributions can be approximated using more efficient approaches. The entropy change ΔS shows a peak around 370 K, for which the total entropy change is given by 24.8 JK−1kg−1 (ΔSele = 7.38, ΔSlat = 7.05, ΔSmag = 10.36 JK−1kg−1) in good agreement with previous theoretical and experimental findings.
Highlights
The idea of replacing convectional room temperature cooling devices by solid-state magnetic devices, which have the potential for better energy efficiency without producing harmful greenhouse gases, has promoted the interest in magnetocaloric materials
Analogous to Ref. [8], we propose the use of the entropy variation between the involved magnetic phases (DS) as an approximation of DSiso
The magnetocaloric effect is in general small unless it is operated at temperatures in the vicinity of a phase transition, whereas it is strongly enhanced by the entropy variation of a transition, which justifies our approach [2,9]
Summary
The idea of replacing convectional room temperature cooling devices by solid-state magnetic devices, which have the potential for better energy efficiency without producing harmful greenhouse gases, has promoted the interest in magnetocaloric materials. The existence of an orthorhombic low-temperature phase of FeRh has been predicted from first-principles calculations [12,14,36] as well as a martensitic transformation under strain [14,15,37] The existence of such broad knowledge and detailed information in the literature together with the complex metamagnetic behaviour that demands a careful treatment makes FeRh an ideal test system for our purpose to identify a method that can be applied in a high-throughput study for finding new magnetocaloric materials. We discuss thoroughly the single entropy contribution in terms of electronic, lattice and magnetic components This is done for FeRh using different approximations, albeit without considering thermal effects on the structure. From this we are able to conclude which is the most viable approach to be applied in high-throughput calculations
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