Spin order and lattice frustration in optimally doped manganites: A high-temperature NMR study

Abstract

The physics underlying the complex glassy phenomena, which accompany the formation of polarons in optimally doped manganites (ODM) is a cumbersome issue with many unexplained aspects. In this article we present 139 La and 55 Mn NMR in the temperature range 80K -900K of ODM La0.67Ca0.33MnO3. We show that local lattice distortions, established in the Paramagnetic (PM) phase for T < 700K, induce a genuine spin-glass state, which for T < Tc consolidates with the Ferromagnetic (FM) state into a single thermodynamic phase. Comparative NMR experiments on La0.77Ca0.23MnO3, La0.59Ca0.41MnO3, and La0.70Sr0.30MnO3 demonstrate the dominant role of lattice distortions, which appear to control (i) the stability of the spin glass phase component and (ii) the kind (1st or 2nd order) of the PM-FM phase transition. The experimental results are in agreement with the predictions of the compressible random bond -random field Ising model, where consideration of a strain field induced by lattice distortions, is shown to invoke at Tc a discontinuous (1st order like) change of both the FM and the "glassy" Edwards-Anderson (EA) order parameters.

Disorder-driven phase transition in La0.37D0.30Ca0.33MnO3 (D = Bi, Sm) manganites

“…1-3 The significant changes in structure, magnetic and magnetoresistive (MR) properties of manganites can be achieved by varying particle size, 4,5 A/B-site doping [6][7][8][9] and oxygen stoichiometry. 10 The doping of trivalent rare-earth ions of different sizes into the perovskite structure can cause disorder in the sample because of the change in the chemical pressure.…”

“…[6][7][8] In the hole-doped La 1−x Ca x MnO 3 (x = 0.33) manganite, the nature of FM-PM transition at critical point changes from first-order to second-order when the grain size is reduced. 9 The disorder induced second-order FM-PM phase transition is reported in the literature.…”

“…49 Whereas, La 1−x Ca x MnO 3 manganite undergoes first-order FM-PM transition in the composition range 0.3 ≤ x ≤ 0.4. [6][7][8] This difference between the nature of FM-PM transition of the two systems is presumably due to the increased average A-site ionic radius of LSMO [Sr 2+ (1.32A o )]system compared to LCMO [Ca 2+ (1.12 A o )] system. However, the present doped samples show second order transition at critical point, although the average A-site ionic radius is reduced when Sm/Bi is substituted at La site.…”

Polaron freezing and the quantum liquid-crystal phase in the ferromagnetic metallic La0.67Ca0.33MnO3

“…By decreasing temperature, at T ≈ 710 K, an abrupt increase of the SFD width is observed, which marks the transition from the R3c to the Pnma crystal structure and the onset of strong incoherent JT displacements. 39,40 Neutron scattering experiments indicate that the appearance of the JT distortions is related with the formation of uncorrelated dynamic JT polarons right below T RO , which by further decreasing temperature freeze into a CE-type polaron glassy state with correlation length of ≈1 nm. 9 Most notable, for T < 100 K spectra start to broaden and shift to higher frequency, which might indicate the onset of an until now unnoticed novel FM phase.…”

“…It is furthermore noticed that for T < T c the magnetic field B at the site of the La nuclei is equal to the sum of the applied external field B and the transferred hyperfine field B hf ¼ ð1=γ hÞA S h i, where A is the hyperfine coupling constant and S h i the average electronic spin of the eight nearest Mn neighbors. 38,39 Figure 1a demonstrates 139 La NMR spectra of the LCMO(0.33) system in the temperature range 3.2-900 K. LCMO(0.33) undergoes two major phase transitions: a structural one from the high temperature R3c rhombohedral phase to the low temperature orthorombic Pnma phase at T RO ≈ 710 K, 39,40 and a paramagnetic to FM (PM-FM) one at T c = 267 K, as shown in Figures S3.1, S3.2 and S5.1 of the Supplementary Information. The critical temperature of the PM-FM phase transition T c , defined as the inflection point of the frequency vs. T plot, in the magnetic field of 9.4 T is found to be T c ≈ 300 K (right bottom inset), sufficiently higher than in zero external magnetic field.…”