Magnetism in high-entropy alloys is a non-trivial problem even in HEAs which are a single solid solution due to the several types of atoms and interactions present. In reality, most HEAs that are being actively researched today are multiphase systems – sometimes as a side product of the synthesis procedure but more often intentionally as a means to optimize their properties, e.g. to combine the high hardness of one phase and the ductility of the other. A recent publication has outlined the connection between hierarchical phase separation and the complex magnetism in the $\mathrm{AlCo_{0.5}Cr_{0.5}FeNi}$ HEA as analysed via electronic microscopy and APT in combination with magnetic imaging. This contribution will present our attempt at complementing the available information about magnetism in Al-Co-Cr-Fe-Ni HEAs through a systematic study of the bulk magnetism on a large temperature range of 2 K to 400 K.

Our sample with composition $\mathrm{Al_{28}Co_{20}Cr_{11}Fe_{15}Ni_{26}}$ was synthesized with the Czochralski method and has grains of cca $1\,\mathrm{mm^3}$ volume. Due to the slightly different composition and different synthesis method (compared to the recent publication), we obtained a nanocomposite consisting of a B2 matrix with the A2 phase taking the form of spherical nanoparticles of dimension 64 ± 10 nm. The matrix of the nanocomposite sample is without the antiferromagnetic Cr and thus expected to be ferromagnetic. The spherical nanoparticles are Co-Cr-Fe, likely with the outer shell enriched in Co and Fe. Measurements of the temperature dependence of the magnetization indicate a ferromagnetic transition at cca 390 K, which is due to the disordered ferromagnetism of the B2 matrix. A non-ergodic regime occurs below 15 K as can be seen from the zfc-fc splitting of the temperature-dependent magnetization and also through the frequency dependence of the AC susceptibility. It can be inferred that this non-ergodic regime is due to the mixed ferro- and antiferromagnetic interactions in the A2 nanoparticles, which through the inherent disorder and frustration give rise to an asperomagnetic magnetic state. We have also carried out a thermoremanent magnetization experiment, which directly demonstrates the broad distribution of fluctuation times below 15 K – as expected for the asperomagnetic state. Results up to now indicate that the magnetism of $\mathrm{Al_{28}Co_{20}Cr_{11}Fe_{15}Ni_{26}}$ is not a collective state of the B2 matrix and A2 nanoparticles with significant interactions between them, but a simple sum of their independent magnetisms.