The magnetic and electrical properties of high quality single crystals of A-site disordered (solid solution) Ln0.5Ba0.5MnO3 are investigated near the phase boundary between the spin glass insulator and colossal-magnetoresistive ferromagnetic metal, locating near Ln = Sm. The temperature dependence of the ac-susceptibility and the x-ray diffuse scattering of Eu0.5Ba0.5MnO3 are analyzed in detail. The uniformity of the random potential perturbation in Ln0.5Ba0.5MnO3 crystals with small bandwidth yields, rather than the phase separation, an homogeneous short ranged charge/orbital order which gives rise to a nearly-atomic spin glass state. Remarkably, this microscopically disordered "CE-glass" state alone is able to bring forth the colossal magnetoresistance.PACS numbers: 75.47. Gk, 75.50.Lk, 75.40.Gb The phase diagram of colossal magnetoresistive (CMR) perovskite manganites, as well as many strongly correlated electron systems, is multicritical, involving competing spin, charge/orbital, and lattice orders [1,2]. For example in the half-doped Ln 0.5 Ba 0.5 MnO 3 (Ln being a rare earth cation), the charge/orbital ordered (CO/OO) insulating (favored by small Ln cations) and ferromagnetic (FM) metallic (larger Ln cations) states compete with each other, and bicritically meet near Ln = Nd [3]. The meeting point is actually multicritical, as the Atype antiferromagnetic (AFM) instability also exists [3]. In the presence of quenched disorder, namely when the perovskite A-sites are solid solution of Ln and Ba (termed "A-site disorder"), the phase diagram becomes asymmetric. The FM phase transition is still observed near the critical point, even though the Curie temperature T c is steeply diminished. The long-range CO/OO state is, on the other hand, completely suppressed and only shortrange CO/OO correlation is observed. This phase corresponds in the spin sector to a spin glass (SG) state, which, as we will show in this article, is not related to some macroscopic phase separation [4,5], but results from the frustration and magnetic disorder microscopically introduced within this "CE-glass"[6] state. The degree of quenched disorder can be controlled by modifying the mismatch of the constituent A-site cations [7]. Recent pressure experiments show that the asymmetric phase diagram of disordered Ln 0.5 Ba 0.5 MnO 3 (LnBMO) is solely determined by the bandwidth W variation, for a fixed degree of disorder [8]. It thus implies that quenched disorder affects the CO/OO phase more aggressively than the FM state. These experimental findings could be accounted for by the theoretical model considering multicritical fluctuations between the FM and CO/OO phases [9], in presence of disorder. The calculations also predict an enhanced electronic localization above T c near the critical region, suggesting that large CMR effects may emerge [9].We thus investigate the magnetic and electrical properties of high-quality single crystals of A-site disordered Ln 0.5 Ba 0.5 MnO 3 , around the SG insulator/FM metal phase boundary (near Ln = Sm). The...
