Available on-demand - *S.NM07.04.02
Magnetic Field Induced Berezinskii-Kosterlitz-Thouless Correlations in Three-Dimensional Manganites
Subray Bhat1,Bhagyashree KS1,Arjun Ashoka2
Indian Institute of Science1,University of Cambridge2
Show Abstract
Ideal two-dimensional (2D) Heisenberg magnets lack long range order [1]. 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 [2,3]. 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 spin resonance (ESR) linewidth ΔH(T) of Cr3+ doped bismuth strontium manganite Bi0.5Sr0.5Mn1-xCrxO3 (x= 0.04, 0.1) (BSMCO).
BSMCO [4] belongs to the family of mixed valent manganites of the type ReAMnO3 where Re is a trivalent rare earth ion or Bi3+ and A is a divalent alkaline earth ion, which are intensely studied in the last few years [5]. ΔH(T) provides the most important probe to study the spin interactions in these strongly correlated spin systems. We find that ΔH(T) observed in BSMCO (current work), as well as in La.05Ca.95MnO3 and CaMnO3 [6] is better (than the usually adopted critical state model) described by the BKT scenario which predicts [7]
ΔHBKT(T) = ΔH∞ exp [ 3b/(T/TBKT — 1)0.5 ] + mT + ΔHind , T > TBKT
where TBKT is the BKT transition temperature and b = π/2 for a square lattice and the last two terms are included to account for the high temperature and temperature independent behaviour. This is unexpected as these manganites have a 3D structure and the BKT model addresses systems with spin and spatial dimensions of two. We understand this result in terms of an effective symmetry reduction induced by i) the magnetic field applied in the ESR experiment and ii) the intrinsic anisotropy arising from microscopic details such as doping and phase separation - contributing to an effectively 2-dimensional XY easy plane anisotropy. Nanometric scale spin clusters similar to the ones observed in La doped CaMnO3 [8] could conceivably play the role of vortices in these 3D materials. The sensitivity of the BKT behaviour to applied field is also supported by a re-analysis of the field dependence of the ΔH(T) in the quasi-2D antiferromagnetic compound BaNi2V2O8 reported by Heinrich at al., [9]. For undoped manganites we find TBKT is of the order of the magnetic interaction energy, suggesting that the applied field could be the sole origin of the BKT behaviour [10]. We shall also address the interesting observation that in BSMCO (x=0.04, 0.1), TBKT is composition-independent while the magnetic properties are quite sensitive to composition. <span style="font-size:10.8333px">.</span>
SVB and AA thank the Indian National Science Academy, the National Academy of Sciences, India and the Indian Academy of Sciences for support.
References:
[1] N. D. Mermin and H. Wagner, Phys. Rev. Lett. 17, 1133 (1966)
[2] V. L. Berezinskii, Sov. Phys. JETP 32, 493 (1971)
[3] J. M. Kosterlitz and D. J. Thouless, Journal of Physics C: Solid State Physics 6, 1181 (1973)
[4] K. S. Bhagyashree, L. R. Goveas, and S. V. Bhat, Applied Magnetic Resonance 50,1049 (2019)
[5] Y. Tokura, Rep. Progr. Phys., 69, 797 (2006)
[6] E. Granado et al., Phys. Rev. Lett. 86, 5385 (2001)
[7] M. Hemmida et al., Phys. Rev. B95, 224101 (2017)
[8] E. Granado et al., Phys. Rev. B68, 134440 (2003)
[9] M. Heinrich et al., Phys. Rev. Lett. 91, 137601 (2003)
[10] A. Ashoka, K. S. Bhagyashree and S. V. Bhat, arXiv:1809.07635v3