Magnetic Field Induced Berezinskii-Kosterlitz-Thouless Correlations in 3-Dimensional Manganites

Ideal two-dimensional (2D) Heisenberg magnets lack long range magnetic order. However, the XY model with spins confined to a plane shows a topological phase transition at a finite temperature corresponding to binding and unbinding of vortices. Experimental evidence for such Berezinskii-Kosterlitz-Thouless (BKT) transitions has been difficult to obtain in condensed matter systems, where, even a weak interlayer coupling that is invariably present leads to long-range order, pre-empting the BKT transition. The BKT signatures are still discernible above the long-range ordering temperature, however, in the characteristic exponential temperature dependence of the coherence length of the fluctuations. In this work we report that an applied magnetic field can induce such BKT correlations not only in quasi 2-dimensional systems but also in nominally 3-dimensional manganites undergoing antiferromagnetic transitions. We arrive at this unexpected conclusion based on our studies of temperature dependence of electron paramagnetic resonance (EPR) linewidth ΔH(T) of Cr3+ doped bismuth strontium manganite Bi0.5Sr0.5Mn1-xCrxO3 (x= 0.04, 0.1) (BSMCO04 and BSMCO10).