We have solved a long-standing problem in the statistical physics of disordered magnetic systems, which often defy conventional critical behavior and challenge our understanding of phase transitions. An experimental challenge has arisen for the random-field Heisenberg model (RFHM), as the Harris criterion suggests that weak quenched disorder should be irrelevant for Heisenberg magnetic systems. Here, we report that this criterion can be violated by correlated disorder, thereby enabling the experimental realization of the RFHM. Our findings establish ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{MnO}}_3$ $(0.175\le x\le 0.300)$ as a prototypical experimental system for the RFHM. ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{MnO}}_3$ exhibits correlated disorder with a bimodal distribution of random fields, which fundamentally alters the critical phenomena by inducing a crossover from characteristic Heisenberg behavior to tricritical mean-field behavior and challenging the order of the phase transition.

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• Magnetic interactions
• Magnetic order
• Magnetic phase transitions
• Spin dynamics