  Disorder can have a dominating influence on correlated and quantum materials
leading to novel behaviors which have no clean limit counterparts. In magnetic
systems, spin and exchange disorder can provide access to quantum criticality,
frustration, and spin dynamics, but broad tunability of these responses and a
deeper understanding of strong limit disorder is lacking. In this work, we
demonstrate that high entropy oxides present an unexplored route to designing
quantum materials in which the presence of strong local compositional disorder
hosted on a positionally ordered lattice can be used to generate highly tunable
emergent magnetic behavior--from macroscopically ordered states to
frustration-driven dynamic spin interactions. Single crystal
La(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3 films are used as a structurally uniform model
system hosting a magnetic sublattice with massive microstate disorder in the
form of site-to-site spin and exchange type inhomogeneity. A classical
Heisenberg model is found to be sufficient to describe how compositionally
disordered systems can paradoxically host long-range magnetic uniformity and
demonstrates that balancing the populating elements based on their discrete
quantum parameters can be used to give continuous control over ordering types
and critical temperatures. Theory-guided experiments show that composite
exchange values derived from the complex mix of microstate interactions can be
used to design the required compositional parameters for a desired response.
These predicted materials are synthesized and found to possess an incipient
quantum critical point when magnetic ordering types are designed to be in
direct competition; this leads to highly controllable exchange bias sensitivity
in the monolithic single crystal films previously accessible only in
intentionally designed bilayer heterojunctions.
