Mesoscopic thermodynamics and kinetics of an inhomogeneous colossal magnetoresistive phase in a bulk crystal

Role of disorder in phase coexistence in manganites: Noise in layered films

“…7,15,16 Metastable magnetic domain orientation noise 17 is suppressed by ͉H͉ Ͼ H C , the coercive field at which the magnetization collapses to a history-independent value. Hysteresis in R͑H͒ curves showed H C Ͻ 100 Oe ͑in plane͒ in both ALL films throughout the relevant range of T. Although much of the excess noise near T C is suppressed at H = 1 kOe ͑in plane͒, a clear peak remained as seen in Fig.…”

“…The temperature range of the noise regime in these low-strain films is similar to that of a bulk single crystal. 16 Replacing the standard random alloy with ordered superlattice layers has little effect on this noise, other than to broaden the noisy transition region a bit. We conclude that strain interactions are more important than intrinsic chemical disorder in stabilizing the mixed phase regime near T C , at least in films on substrates.…”

“…We have no precise interpretation of that fact, but the prevalence of a noise peak near T C in materials in which the disorder does not determine a discrete set of fluctuators generally also supports the idea that thermodynamic constraints rather than quenched disorder are the key to understanding the mixed regime in these films. In the bulk, some residual disorder may be required ͑and persistent fluctuators are found even in a good crystal 16 ͒, in order to nucleate regions with different preferred axes, within which the strain constraints from neighbors 9 might play a similar role to the substrate constraint for films.…”

“…16 ͓Here v N is defined as v S fS R ͑f͒ / R 2 where v S is the sample volume and S R ͑f͒ is the spectral density of the fluctuations in R at frequency f. Since the spectrum is close to a 1 / f form and the fluctuations lack long-range correlations, v N is nearly independent of f and not dependent on the area of the sample.͔ Films on NGO are thus well suited for testing whether the intrinsic alloy disorder plays a major role in stabilizing the coexistence regime.…”

“…In some films 7,15 and bulk crystals 16 the noise can directly be seen to comprise fluctuations of domains between FM and some weakly magnetic, poorly conducting states. PI is the most obvious but not the only candidate for those states.…”