Indirect Exchange Interaction Leads to Large Lattice Contribution to Magnetocaloric Entropy Change.

Materials with a large magnetocaloric response are highly desirable for magnetic cooling applications. It is suggested that a strong spin-lattice coupling tends to generate a large magnetocaloric effect, but no microscopic mechanism has been proposed. In this work, we use spin lattice dynamics simulation to examine the lattice contribution to the magnetocaloric entropy change in bcc iron (Fe) and hcp gadolinium (Gd) with exchange interaction parameters determined from ab initio calculations. We find that indirect Ruderman Kittel Kasuya Yosida (RKKY) exchange interaction in hcp Gd leads to longer range spin lattice coupling and more strongly influences the low frequency long wavelength phonons. This results in a higher lattice contribution towards the total magnetocaloric entropy change as compared to bcc Fe with short range direct exchange interactions. Our analysis provides a framework for understanding the magnetocaloric effect in magnetic materials with strong spin lattice couplings. Our finding suggests that long range indirect RKKY type exchange gives rise to a larger lattice contribution to the magnetocaloric entropy change and is, thus, beneficial for magnetocaloric materials.

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