Collective magnetism of a single-crystalline nanocomposite FeCoCrMnAl high-entropy alloy

Abstract

We have investigated the nature of the magnetic state of a single-crystalline FeCoCrMnAl nanocomposite high-entropy alloy (HEA), composed of crystallographically oriented magnetic nanoplatelets embedded in a magnetic matrix of different magnetic order. The two-phase nanocomposite was formed by a bcc-B2 spinodal decomposition. Due to the single-crystalline nature of the material, there is no symmetry breaking of the surface atomic monolayer at the borders between the two phases and there are no interface regions between the nanoplatelets and the matrix. The material also does not exhibit grain structure, allowing for the observation of the true intrinsic magnetism of a nanocomposite HEA. Upon cooling, the predominantly Fe‒Cr‒Mn chemically disordered bcc matrix orders first at TC1≈ 425 K in an asperomagnetic-type magnetic state. Below TC2≈ 370 K, the B2 nanoplatelets that are predominantly an Al30(Co,Mn)70 pseudo-binary intermetallic compound, start to order in a ferromagnetic (FM)-type manner. We have focused to the question whether the magnetic state of the nanocomposite below TC2 is a collective state of the interacting nanoplatelets and the matrix or their coupling is weak enough that the magnetic ordering of each of them can be treated independently. Experimental results support the development of a single collective, disordered FM-type magnetic state upon cooling due to the exchange coupling between the nanoplatelets and the matrix. The nanocomposite is magnetically soft and the strong variation of the magnetization with the temperature in a large interval ΔT≈ 125 K just above room temperature due to two successive magnetic phase transitions make this material promising for the application in magnetocaloric refrigeration.