Strong limit spin and exchange disorder effects on magnetism

Exchange and spin disorder provide access to quantum criticality, frustration, and spin dynamics, but broad tunability and a deeper understanding of strong limit disorder is lacking, as traditional enthalpy-driven synthesis leads to unintended secondary crystal phases or defects when more than one or two elements are combined on a sublattice. To overcome this, a range of single crystal high entropy oxide La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ films are synthesized to probe the role of site-to-site spin and exchange interaction variances in stabilizing magnetic responses. The complexity of the system provides tunability and functionality not present in any of the ternary or half-doped quaternary parents or as a sum of parent properties. Neutron diffraction and magnetometry show that the compositionally disordered systems can paradoxically host long-range magnetic order, while manipulation of the S and J parameters through cation ratio permits continuous control of magnetic phase from antiferromagnetism (AFM), to degenerate, to ferromagnetism (FM). Tuning of the coexisting magnetic phase composition allows design of exchange bias behaviors in monolithic single crystal films, which have, until now, only been observable in AFM-FM bilayer heterojunctions or 2D layered bulk systems.  Despite the extraordinary levels of microstate complexity, a classical Heisenberg model can predict astonishingly accurate magnetic phase diagrams when highest probability local microstate populations are considered.