Upon cooling, the isolated ferromagnetic domains in thin films of La0.33Pr0.34Ca0.33MnO3 start to grow and merge at the metal-insulator transition temperature TP1, leading to a steep drop in resistivity, and continue to grow far below TP1. In contrast, upon warming, the ferromagnetic domain size remains unchanged until near the transition temperature. The jump in the resistivity results from the decrease in the average magnetization. The ferromagnetic domains almost disappear at a temperature TP2 higher than TP1, showing a local magnetic hysteresis in agreement with the resistivity hysteresis. Even well above TP2, some ferromagnetic domains with higher transition temperatures are observed, indicating magnetic inhomogeneity. These results may shed more light on the origin of the magnetoresistance in these materials.

Spin glasses are founded in the frustration and randomness of microscopic magnetic interactions. They are non-ergodic systems where replica symmetry is broken. Although magnetic glassy behaviour has been observed in many colossal magnetoresistive manganites, there is no consensus that they are spin glasses. Here, an intriguing glass transition in (La,Pr,Ca)MnO3 is imaged using a variable-temperature magnetic force microscope. In contrast to the speculated spin-glass picture, our results show that the observed static magnetic configuration seen below the glass-transition temperature arises from the cooperative freezing of the first-order antiferromagnetic (charge ordered) to ferromagnetic transition. Our data also suggest that accommodation strain is important in the kinetics of the phase transition. This cooperative freezing idea has been applied to structural glasses including window glasses and supercooled liquids, and may be applicable across many systems to any first-order phase transition occurring on a complex free-energy landscape.

We have studied electrostatic field-induced doping in La0.8Ca0.2MnO3 transistors using electrolyte as a gate dielectric. For positive gate bias, electron doping drives a transition from a ferromagnetic metal to an insulating ground state. The thickness of the electrostatically doped layer depends on bias voltage but can extend to 5 nm requiring a field doping of 2x10;{15} charges per cm;{2} equivalent to 2.5 electrons per unit cell area. In contrast, negative gate voltages enhance the metallic conductivity by 30%.

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STANDFIRST:Manganites are pseudo-cubic oxides of manganese that show extremes of functional behavior. Diverse magnetic and electronic phases coexist on a wide range of length scales even within single crystals. This coexistence demonstrates a complexity that inspires ever deeper study. Yet even the basic nature of the coexisting phases remains controversial. Can the ferromagnetic metallic phase provide fully spin-polarized electrons for spin electronics? Does the superlattice in the highly insulating phase represent charge order? Here we highlight recent results that demonstrate a coexistence of opinions about a field in rude health. ABSTRACT:In a material, the existence and coexistence of phases with very different magnetic and electronic properties is both unusual and surprising. Manganites in particular capture the imagination because they demonstrate a complexity that belies their chemically single-phase nature. This complexity arises because the magnetic, electronic and crystal structures interact with one another to deliver exotic magnetic and electronic phases that coexist. This coexistence is self-organized and yet readily susceptible to external perturbations, permitting subtle and imaginative experiments of the type that we describe here. Moreover, these experiments reveal that each competing phase itself remains an incompletely solved mystery.Manganites were known to show pronounced magnetoresistance 1,2 and phase separation 3 effects in the 1950s, but they really hit the heights during the 1990s for three reasons. First, magnetoresistance took prominence between the 1988 discovery 4 of giant magnetoresistance (GMR) in metallic multilayers, and the first shipment of GMR discdrive heads by IBM in 1998. Second, great advances in laboratory infrastructure permitted new approaches, e.g. the fabrication of high-quality thin films, the advent of superconducting magnets, and the imaging of magnetic and electronic texture via e.g. scanning probe techniques. Third, this infrastructure had been very productive in the study of the high-temperature cuprate superconductors, and scientists were therefore immediately able to accept the new challenges presented by the manganites.The manganites are a family of perovskite oxides in which the composition of the Asite cations may be varied using mixtures of divalent rare-earth and trivalent alkalineearth elements. The most immediate consequence of this variation is to alter the charge doping of the magnetic and electronic structures that reside in the sublattice formed by the B-site manganese and intervening oxygen atoms. Significantly charge disorder on the

We report experiments of electron spin resonance ͑ESR͒ of Cu 2+ in polycrystalline samples of CaCu 3 Ti 4 O 12 post-annealed in different atmospheres. After being synthesized by solid state reaction, pellets of CaCu 3 Ti 4 O 12 were annealed for 24 h at 1000°C under air, Ar or O 2 . Our temperature dependent ESR data revealed for all samples nearly temperature independent g value ͑2.15͑1͒͒ and linewidth for T ӷ T N Ϸ 25 K. However, the values of ESR linewidth are strongly affected by the oxygen content in the sample. For instance, argon post-annealed samples show a much larger linewidth than the O 2 or air post-annealed samples. We attribute this broadening to an increase of the dipolar homogeneous broadening of the Cu 2+ ESR lines due to the presence of oxygen vacancies which induce an S =1/2 spin inside the TiO 6 octahedra. Correlation between a systematic dependence of the ESR linewidth on the oxygen content and the high dielectric constant of these materials is addressed. Also, ESR, magnetic susceptibility, and specific heat data for a single crystal of CaCu 3 Ti 4 O 12 and for polycrystals of CdCu 3 Ti 4 O 12 are reported.

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We report electrically induced changes of 20% in both the remnant magnetization and the coercive field of a magnetoelectric heterostructure. This heterostructure is an industrially produced multilayer capacitor comprising Ni-based magnetostrictive electrodes sandwiching a BaTiO3-based dielectric that is ferroelectric and therefore piezoelectric. Both magnetization and strain are shown to be hysteretic with applied electric field. These inexpensive capacitors might find use as electrically controlled ferromagnets.