55Mn NMR investigation of electronic phase separation in La1-xCaxMnO3 for 0.2 < or = x < or = 0.5

Magnetic phase separation in double layer ruthenates Ca3(Ru(1-x)Ti(x))2O7

A phase transition from metallic AFM-b antiferromagnetic state to Mott insulating G-type antiferromagnetic (G-AFM) state was found in Ca₃(Ru_1−xTi_x)₂O₇ at about x = 0.03 in our previous work. In the present, we focused on the study of the magnetic transition near the critical composition through detailed magnetization measurements. There is no intermediate magnetic phases between the AFM-b and G-AFM states, which is in contrasted to manganites where a similar magnetic phase transition takes place through the presence of several intermediate magnetic phases. The AFM-b-to-G-AFM transition in Ca₃(Ru_1−xTi_x)₂O₇ happens through a phase separation process in the 2–5% Ti range, whereas similar magnetic transitions in manganites are tuned by 50–70% chemical substitutions. We discussed the possible origin of such an unusual magnetic transition and compared with that in manganites.

Nanoscale phase boundaries: a new twist to novel functionalities

In functional materials, nanoscale phase boundaries exhibit exotic phenomena that are notably absent in their parent phases. Over the past two decades, much of the research into complex oxides (such as cuprate superconductors, CMR manganites and relaxor ferroelectrics) has demonstrated the key role that nanoscale inhomogeneities play in controlling the electronic and/or ionic structure of these materials. One of the key characteristics in such systems is the strong susceptibility to external perturbations, such as magnetic, electric and mechanical fields. A direct consequence of the accommodation of a large number of cationic substitutions in complex oxides is the emergence of a number of physical phenomena from essentially the same crystal framework. Recently, multiferroic behavior, which is characterized by the co-existence and potential coupling of multiple ferroic order parameters, has captured considerable worldwide research interest. The perovskite, BiFeO(3), exhibits robust ferroelectricity coupled with antiferromagnetism at room temperature. A rather unique feature of this material system is its ability to "morph" its ground state when an external mechanical constraint is imposed on it. A particularly striking example is observed when a large (~4 to 5%) compressive strain is imposed on a thin film through the epitaxial constraint from the underlying substrate. Under these conditions, the ground state rhombohedral phase transforms into a tetragonal-like (or a derivative thereof) phase with a rather large unit cell (c/a ratio of ~1.26). When the epitaxial constraint is partially relaxed by increasing the film thickness, this tetragonal-like phase evolves into a "mixed-phase" state, consisting of a nanoscale admixture of the rhombohedral-like phase embedded in the tetragonal-like phase. Such a system gives us a new pathway to explore a variety of mechanical, magnetic and transport phenomena in constrained dimensions. This article reviews our progress to date in this direction and also captures some possible areas of future research. We use the electromechanical response and the magnetic properties as examples to illustrate that its novel functionalities are intrinsically due to the phase boundaries and not the constituent phases. The possible origin of the giant piezoelectric response and enhanced magnetic moment across the boundaries is proposed based on the flexoelectric and flexomagnetic effects.

Persistent photoinduced magnetization and oxygen non-stoichiometry in La(0.9)Ca(0.1)MnO(3) films

The influence of thermal annealings on La(0.9)Ca(0.1)MnO(3) (LCMO) films in oxygen and in vacuum with low hole doping is investigated in the phase separation region where competition between AFM and FM phases is high. Measurements by x-ray diffractometry, atomic force microscopy and magnetometry reveal changes in the lattice parameters and magnetic properties of the films, depending on the oxygen content. All films show magnetic cluster glass properties with similar freezing temperatures of around 45 K. Clearly the highest increase of the magnetization is observed in the films annealed in vacuum. We attribute this effect to trapping of unpaired electrons at oxygen vacancies where they can form rigid self-trapped magnetic polarons in potential wells of local moments. As a result long-range spin distortions with local ferromagnetic order may be realized. In conformity with these results, photoinduced persistent magnetization showing different mechanisms of generation, depending on the method of thermal annealing, is observed.

Magnetism, transport, and specific heat of electronically phase-separated Pr(0.7)Pb(0.3)MnO(3) single crystals

Magnetization, resistivity, and specific heat were studied systematically in the absence and presence of magnetic field in Pr(0.7)Pb(0.3)MnO(3) single crystals, in which electronic phase separation occurs near the ferromagnetic/metallic-paramagnetic/insulating phase transition and the metal-insulator transition temperature is much higher than the Curie temperature. These measurements allow us to extract some fundamental physical parameters such as Fermi energy, density of states at the Fermi energy, Debye temperature, and interaction among electrons, phonons, and magnons. Furthermore, the magnetic entropy was studied around the phase transition temperature regime. It was found that a magnetic entropy change associated with the transition from the connected ferromagnetic phase to isolated superparamagnetic clusters appeared near the metal-insulator transition temperature following a large magnetic entropy change near the ferromagnetic-paramagnetic phase transition.

Phase separation in A-site-ordered perovskite manganite LaBaMn2O6 probed by 139La and 55Mn NMR

139La- and 55Mn-NMR spectra demonstrate that the ground state of the A-site-ordered perovskite manganite LaBaMn2O6 is a spatial mixture of the ferromagnetic and antiferromagnetic regions, which are assigned to the metallic and the insulating charge ordered state, respectively. This exotic coexisting state appears below 200 K via a first-order-like formation of the antiferromagnetic charge ordered state inside the ferromagnetic metal one. The Mn spin-spin relaxation rate indicates that the ferromagnetic region coexisting with the antiferromagnetic one in LaBaMn2O6 is identical to the bulk ferromagnetic metal phase of the disordered form La0.5Ba0.5MnO3 in spite of the absence of A-site disorder. This suggests a mesoscopic rather than nanoscopic nature of the ferromagnetic region in LaBaMn2O6.

Orbital domain state and finite size scaling in ferromagnetic insulating manganites

55Mn and 139La NMR measurements on a high quality single crystal of ferromagnetic (FM) La0.80Ca0.20MnO3 demonstrate the formation of localized Mn(3+,4+) states below 70 K, accompanied by a strong cooling-rate dependent increase of certain FM neutron Bragg peaks. (55,139)(1/T(1)) spin-lattice and (139)(1/T(2)) spin-spin relaxation rates are strongly enhanced on approaching this temperature from below, signaling a genuine phase transition at T(tr) approximately 70 K. The disappearance of the FM metallic signal by applying a weak external magnetic field, the different NMR radio-frequency enhancement of the FM metallic and insulating states, and the observed finite size scaling of T(tr) with Ca (hole) doping, as observed in powder La(1-x)CaxMnO3 samples, are suggestive of freezing into an inhomogeneous FM insulating and orbitally ordered state embodying "metallic" hole-rich walls.

Magnetic phase separation in La1-xSrxCoO3 by 59Co nuclear magnetic resonance

59Co NMR measurements on La1-xSrxCoO3 reported here establish unequivocally, for the first time, the coexistence of ferromagnetic regions, spin-glass regions, and hole-poor low spin regions at all x values from 0.1 to 0.5. A zero external field NMR spectrum, which is assigned to the ferromagnetic regions, has a spectral shape that is nearly x independent at 1.9 K, as are the relaxation times, T1 and T2. The integrated spectral area increases rapidly with x up to x = 0.2 and then decreases slightly for larger x. In a field of 9.97 T, a narrow NMR line is observed at 102 MHz, identical to that found in x = 0 samples in previous work. The integrated intensity of this spectrum decreases rapidly with increasing x, and is ascribed to hole-poor low spin regions. Beneath this spectrum, a third broad line, with a peak at 100 MHz, is assigned to a spin- or cluster-glass-like phase.

(139)La NMR investigation of quasistatic orbital ordering in La(1-x)Ca(x)MnO(3)

We report (139)La nuclear magnetic resonance in ferromagnetic and insulating (FMI) La(1-x)Ca(x)MnO(3), 0.10< or =x< or =0.20, which at low temperatures shows the formation of Mn octants with enhanced Mn-O wave function overlapping and electron-spin alignment. The rapid increase of the relaxation rates and the "wipeout" of the (139)La NMR signal intensity on heating, imply a quasistatic character for the Mn octant cells in the FMI phase, which freeze below a transition temperature T(f).

Two-phase character of metallic ferromagnetism in manganites

(55)Mn NMR spectra of several ferromagnetic manganites, including La(0.7)Ca(0.3)MnO(3) and La(0.7)Sr(0.3) MnO(3), were measured as a function of temperature. A thorough analysis of the spin-spin relaxation and the NMR line shape revealed that in these compounds two different ferromagnetic phases coexist below T(C) in a certain temperature interval which depends on the composition. The phase with lower hyperfine field has faster nuclear relaxation, its characteristic dimension is a few nm, and its volume decreases with decreasing temperature. The NMR lines of both phases are motionally narrowed, i.e., the electron holes are mobile.

Nanoscale multiphase separation at La(2/3)Ca(1/3)MnO3/SrTiO3 interfaces

55Mn nuclear magnetic resonance experiments are reported on a series of fully strained epitaxial La(2/3)Ca(1/3)MnO3 thin films on SrTiO3. We have found evidence of multiple phase segregation into ferromagnetic metallic and nonmetallic regions as well as regions that are nonferromagnetic and insulating. These insulating regions are mainly located close to interfaces and may have a significant impact on the performance of spin-tunnel devices. As a result of phase segregation, the ferromagnetic coupling within the metallic regions is depressed. This accounts for the reduction of the Curie temperature and conductivity in nanometric thin films.