Giant and isotropic magnetoresistance as huge as −53% was observed in magnetic manganese oxide La0.72Ca0.25MnOz films with an intrinsic antiferromagnetic spin structure. We ascribe this magnetoresistance to spin‐dependent electron scattering due to spin canting of the manganese oxide.

31p-S-4 Giant Magnetoresistance in Magnetic Manganese Oxide with Intrinsic Antiferromagnetic Spin Structure

The manganese oxides of general formula RE1−xMxMnO3 (RE = rare earth, M = Ca, Sr, Ba, Pb) have remarkable interrelated structural, magnetic and transport properties induced by the mixed valence (3+–4+) of the Mn ions. In particular, they exhibit very large negative magnetoresistance, called colossal magnetoresistance (CMR), in the vicinity of metal–insulator transition for certain compositions. In this review paper, we summarize the most important features of the physics of the CMR manganites. The growth techniques for manganese oxide thin films, which are the basic material for potential applications, are reviewed and their structure and morphology examined in relation to growth parameters. The effects of epitaxial strains on the physical properties are discussed. Early works on superlattices and devices are presented.

http://iopscience.iop.org/article/10.1088/0022-3727/36/8/201/pdf

CMR manganites: physics, thin films and devices

We present a neutron scattering study of stripe correlations measured on a single crystal of La$_{1.875}$Ba$_{0.125}$CuO$_{4}$. Within the low-temperature-tetragonal (LTT) phase, superlattice peaks indicative of spin and charge stripe order are observed below 50 K. For excitation energies $\hbar\omega\le12$ meV, we have characterized the magnetic excitations that emerge from the incommensurate magnetic superlattice peaks. In the ordered state, these excitations are similar to spin waves. Following these excitations as a function of temperature, we find that there is relatively little change in the {\bf Q}-integrated dynamical spin susceptibility for $\hbar\omega\sim10$ meV as stripe order disappears and then as the structure transforms from LTT to the low-temperature-orthorhombic (LTO) phase. The {\bf Q}-integrated signal at lower energies changes more dramatically through these transitions, as it must in a transformation from an ordered to a disordered state. We argue that the continuous evolution through the transitions provides direct evidence that the incommensurate spin excitations in the disordered state are an indicator of dynamical charge stripes. An interesting feature of the thermal evolution is a variation in the incommensurability of the magnetic scattering. Similar behavior is observed in measurements on a single crystal of La$_{1.875}$Ba$_{0.075}$Sr$_{0.050}$CuO$_{4}$; maps of the scattered intensity in a region centered on the antiferromagnetic wave vector and measured at $\hbar\omega=4$ meV are well reproduced by a model of disordered stripes with a temperature-dependent mixture of stripe spacings. We discuss the relevance of our results to understanding the magnetic excitations in cuprate superconductors.

Stripe order, depinning, and fluctuations in La 1.875 Ba 0.125 CuO 4 and La 1.875 Ba 0.075 Sr 0.050 CuO 4

The magnetic properties and the low-temperature magnetic structures of the orthorhombic perovskite HoMnO3 (space group Pnma) have been studied on polycrystalline samples by magnetization, specific ...

Complex Magnetism and Magnetic Structures of the Metastable HoMnO3 Perovskite

A basic concept in solid-state physics is that when some kind of symmetry in a solid is spontaneously broken, collective excitations will arise1. For example, phonons are the collective excitations corresponding to lattice vibrations in a crystal, and magnons correspond to spin waves in a magnetically ordered compound. Modulations in the relative shape of the electronic clouds in an orbitally ordered state2,3,4,5,6,7,8,9 could in principle give rise to orbital waves, or ‘orbitons’, but this type of elementary excitation has yet to be observed experimentally. Systems in which the electrons are strongly correlated—such as high-temperature superconductors and manganites exhibiting colossal magnetoresistivity—are promising candidates for supporting orbital waves, because they contain transition-metal ions in which the orbital degree of freedom is important10,11. Orbitally ordered states have been found in several transition-metal compounds12,13, and orbitons have been predicted theoretically for LaMnO3 (refs 4, 5). Here we report experimental evidence for orbitons in LaMnO3, using Raman scattering measurements. We perform a model calculation of orbiton resonances which provides a good fit to the experimental data.

Observation of orbital waves as elementary excitations in a solid

Neutron-scattering experiments have been performed on a LaMnO 3 single crystal. The spin waves exhibit two-dimensional anisotropic dispersion, that is, strong planar ferromagnetic coupling and weak antiferromagnetic interplane coupling with a finite gap. The critical exponent β of the order parameter is 0.25 ±0.01, which is much smaller than values obtained for the three-dimensional Ising (0.326) and Heisenberg (0.367) models. Ferromagnetic spin-wave-like dispersion remains for ω> 10 meV at T c ≈140 K, though the spectrum near the zone center is entirely overdamped around ω= 0.

Two-dimensional planar ferromagnetic coupling in LaMnO3

We grew large single crystals of reproducibly over a wide doping range using an improved travelling solvent floating zone (TSFZ) method. We performed a simulation on the lamp-image focusing to find an optimum condition for a steep temperature gradient along the rod. Based on the results of simulation, we adapted smaller lamps and shielded the partial area of the mirror surfaces. The improvement of the focusing had a great effect on keeping the molten zone stable for more than 100 h. Stabilization of the molten zone improves both the reproducibility of the crystal growth and the quality of the grown crystals.

https://iopscience.iop.org/article/10.1088/0953-2048/11/9/011/pdf

Growth of large single crystals using the improved lamp-image floating-zone furnace: application to

Submillimeter wave ESR measurement of LaMnO3

We have found a strong planar ferromagnetic and a weak antiferromagnetic interplane couplings in the spin waves of LaMnO3. Upon doping holes, the anisotropic character initially becomes enhanced. However, it is taken over by the isotropic ferromagnetic spin waves when the Sr concentration x reaches ∼0.1, much lower than the compositional metal-insulator transition range (x ∼ 0.17).

Neutron-scattering studies of magnetic excitations in La1−xSrxMnO3: A dimensional crossover

Quasielastic neutron scattering experiments from La 2 NiO 4+δ revealed a substantial hole doping effect on the 2 dimensional spin correlations. Thermal evolution of correlation length clearly shows the reduction of the exchange stiffness constant by hole doping, which is in marked contrast to that of La 2 CuO 4+δ . The present result is reflected by the hole character in La 2 NiO 4+δ , which is different from that in the cuprate.

Nobuhisa Kaneko

Papers

Two-dimensional planar ferromagnetic coupling in LaMnO3

Growth of large single crystals using the improved lamp-image floating-zone furnace: application to

Submillimeter wave ESR measurement of LaMnO3

Neutron-scattering studies of magnetic excitations in La1−xSrxMnO3: A dimensional crossover

Hole Doping Effect on 2D Spin Correlations in La2NiO4+δ