Critical Self Organization of Antiferromagnetic Domains in Perovskite Nickelates

Rare earth nickelates are strongly correlated oxides who offer rich phenomenology due to interplay of structural, electronic and spin degrees of freedom. Here we study a thin film of PrNiO3 grown on a (LaAlO3)0.3(Sr2TaAlO6)0.7 (LSAT) substrate (tensile strain). This material exhibits concurrent structural (orthorhombic to monoclinic) electronic (metal to insulator) and magnetic (paramagnetic to antiferromagnetic) transitions. The phase transition is first order in nature and happens at ~115K with a hysteresis of 30K. Here we performed coherent resonant soft x-ray scattering tuned to the Ni L3 edge to study the emergence of the magnetic order parameter as the transition happens. Coherent magnetic x-ray scattering produces a speckle pattern that serves as a ‘fingerprint’ of a particular magnetic domain configuration. We observed a novel small-q periodic modulation of the magnetic Bragg peak intensity which only occurs at temperatures near the onset of transition. We argue coherent magnetic domain ordering as the origin of the small-q modulation when the transition is critically fluctuating. Furthermore we provide experimental evidence that this novel magnetic texture is decoupled from any structural origin.