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Anisotropic magnetoresistance (AMR) in La0.79Ca0.21MnO3 single crystals has been investigated. In contrast with previous studies, in our experiments AMR increases with decreasing temperature, reaches the maximum below TC and decreases with further temperature decrease. Variations of MR while rotating the sample around the [110] crystalline direction may be associated with uniaxial magnetocrystalline anisotropy. The unusual enhancement of AMR below TC is ascribed to changes in the magnetic domain structure revealed by magneto-optical techniques. [ABSTRACT FROM AUTHOR]

ac and dc magnetic measurements have been employed to probe and compare magnetic ordering in bulk and nanometer-sized La0.9Ca0.1MnO3 single crystals. About the same cationic content was determined in both types of the samples. However, the magnetic ground state of the bulk crystal is characterized by a complex mixture of antiferromagnetically and ferromagnetically ordered phases, while the ground state of the nanosample seems to be ferromagnetic and coexisting with the superparamagnetic state at T<240 K. The results are discussed in terms of the different randomness of indirect ferromagnetic exchange interactions mediated by the bound holes. [ABSTRACT FROM AUTHOR]

Transport properties of self-doped La0.89MnO3 single crystals with Néel temperature of TN ≈139 K have been investigated in wide temperature range 10–300 K. Data suggests that current at low temperature is conducted through a strongly temperature-dependent, but almost bias independent channel operating in parallel with a bias controlled but temperature independent channel. The first channel is associated with transport across an insulating antiferromagnetic matrix while the latter one represents tunnel conductivity through intrinsic tunnel junctions appearing due to interruption of conducting percolating paths by phase separated insulating inclusions. Tunnel character of the conductivity manifests itself in nonlinear current-voltage characteristics and appearance of a zero-bias anomaly in the form of a prominent conductance peak in the vicinity of zero bias. Zero bias anomaly and V-shaped characteristics of the differential conductance at high voltages are ascribed to the formation of local magnetic states in the insulating region of the tunneling junction. [ABSTRACT FROM AUTHOR]

Transport properties of phase separated La0.8Ca0.2MnO3 crystals in the aged highly resistive metastable state were studied. It was found that the coexistence of different ferromagnetic phases at low temperatures is sensitive to electric current/field. In a contrast with the previously studied low resistivity metastable states the high resistivity state exhibits positive magnetoresistance and significant current dependence of the resistivity even at temperatures much higher than the Curie temperature. Application of current pulses results in appearance of zero bias anomaly in the current dependent conductivity. Similarly to the low resistivity metastable states the memory of the resistivity can be erased only after heating of the sample to Te ≈360 K. After one year storage at room temperature the La0.8Ca0.2MnO3 samples show clear signatures of aging. The aged samples spontaneously evolute towards high resistivity states. The results are discussed in the context of a coexistence of two ferromagnetic phases with different orbital order and different conductivity. The metallic ferromagnetic phase seems to be less stable giving rise to the experimentally observed electric field effects and aging. [ABSTRACT FROM AUTHOR]

Abstract: Ferromagnetic and twin domains in lightly Ca-doped LaCaMnO single crystals have been visualized and investigated by means of the magneto-optical technique. Both types of domains became visible below the Curie temperature. The dominant structures seen in applied magnetic field are associated with magneto-crystalline anisotropy and twin domains. In a marked difference to the twin domains which appear only in applied magnetic field, ferromagnetic domains show up in zero applied field and are characterized by oppositely oriented spontaneous magnetization in adjacent domains. Ferromagnetic domains take form of almost periodic, corrugated strip-like structures. The corrugation of the ferromagnetic domain pattern is enforced by the underlying twin domains. [Copyright &y& Elsevier]

In this study, Zr-Ti-Cu-Ni-Be bulk metallic glass (BMG) alloys containing a small amount of crystalline phase particles were fabricated by strip casting, and their improvement of mechanical properties and fracture toughness was explained by direct observation of the microfracture process. The compressive and fracture toughness test results indicated that strength, strain to failure, and fracture toughness of the strip-cast BMG alloy containing coarse crystalline particles were higher than those of the as-cast monolithic BMG alloy or the strip-cast BMG alloy containing fine crystalline particles. From in-situ microfracture observations, the improvement of overall mechanical properties of the strip-cast BMG alloy containing coarse crystalline particles could be interpreted by taking consideration of both the existence of coarse crystalline particles and the role of the particles to block crack propagation and to form multiple shear bands. Such property improvement suggests new applicability of the strip-cast BMG alloys containing coarse crystalline particles, which can work as toughening and strengthening reinforcements, to structures and components requiring excellent mechanical properties. [ABSTRACT FROM AUTHOR]

Effects of crystalline particles formed in the strip cast Zr-base bulk metallic glass (BMG) alloy on ductility and strength were investigated in this study by directly observing microfracture processes. The compressive and fracture toughness test results indicated that ductility, strength, and fracture toughness of the strip cast BMG alloy were higher than those of the die cast monolithic BMG alloy, although the strip cast alloy contained a considerable amount (4.5 vol.%) of hard, brittle crystalline particles. The improvement of mechanical properties in the strip cast BMG alloy could be plausibly explained by mechanisms of (1) blocking of shear bands or crack propagation, (2) formation of multiple shear bands, and (3) crack deflection by crystalline particles. [Copyright &y& Elsevier]

The resistive and magnetic properties of La1-xCaxMnO3 single crystals are investigated. In particular, properties of close-to-critically doped crystals with x = 0.22 are confronted with those of underdoped crystals with x = 0.18 and x = 0.2 and optimally doped crystals with x = 0.3. A systematic investigation of nonlinear transport shows that the critically doped crystal does not simply constitute an intermediate case between strongly nonlinear underdoped and linear, optimally doped one. The observed low-temperature resistivity increase can be interpreted in terms of orbital ordering or as a manifestation of an intrinsic tunnelling mechanism. The transport measurements have been completed by magneto-optic investigations which suggest that the low temperature resistivity of the underdoped x = 0.18 compound is dominated by tunnelling through intrinsic barriers associated with twin domains, while phase separation dominates in the critically doped x = 0.22 compound. [ABSTRACT FROM AUTHOR]

1/f noise in current biased La0.82Ca0.18MnO3 crystals has been investigated. The temperature dependence of the noise follows the resistivity changes with temperature, suggesting that resistivity fluctuations constitute a fixed fraction of the total resistivity, independently of the dissipation mechanism and magnetic state of the system. The noise scales as a square of the current as expected for equilibrium resistivity fluctuations. However, at 77 K at bias exceeding some threshold, the noise intensity starts to decrease with increasing bias. The appearance of nonequilibrium noise is interpreted in terms of bias dependent multistep indirect tunneling. [ABSTRACT FROM AUTHOR]

Conductivity noise in dc current biased La0.82Ca0.18MnO3 single crystals has been investigated in different metastable resistivity states enforced by applying voltage pulses to the sample at low temperatures. Noise measured in all investigated resistivity states is of 1/f-type and its intensity at high temperatures and low dc bias scales as a square of the bias. At liquid nitrogen temperatures for under bias exceeding a threshold value, the behavior of the noise deviates from above quasi-equilibrium modulation noise and depends in a non monotonic way on applied bias. The bias range of nonequilibrium 1/f noise coincides with the range at which the conductance increases linearly with bias voltage. This feature is attributed to a broad continuity of states enabling indirect inelastic tunneling across intrinsic tunnel junctions. The nonequilibrium noise has been ascribed to indirect intrinsic tunneling mechanism while resistivity changes in metastable states to variations in the energy landscape for charge carriers introduced by microcracks created by the pulse procedures employed. [ABSTRACT FROM AUTHOR]

Low frequency noise in current biased La0.82Ca0.18MnO3 single crystals has been investigated in a wide temperature range from 79 to 290 K. Despite pronounced changes in the magnetic properties and dissipation mechanisms of the sample with changing temperature, the noise spectra were found to be always of the 1/f type, and their intensity (except for the lowest temperature studied) scaled as the square of the bias. At liquid nitrogen temperatures and under a bias exceeding some threshold value, the behavior of the noise deviates from the quasiequilibrium modulation noise and starts to depend in a nonmonotonic way on the bias. It has been verified that the observed noise obeys the Dutta and Horn model of 1/f noise in solids. The appearance of nonequilibrium 1/f noise and its dependence on bias have been associated with changes in the distribution of activation energies in the underlying energy landscape. These changes have been correlated with bias induced changes in the intrinsic tunneling mechanism dominating dissipation in La0.82Ca0.18MnO3 at low temperatures. [ABSTRACT FROM AUTHOR]

Abstract: Spontaneous ferromagnetic domain structure in La0.78Ca0.22MnO3 single crystals has been investigated by means of magneto-optical technique. The magnetic domain topology was found to be correlated with the intrinsic twin structure of the investigated crystal. With decreasing temperature the regular network of ferromagnetic domains undergoes significant changes resulting in apparent rotation of the domain walls in the temperature range of 70–150K. The observed modifications of the domain structure are reversible and the original domain structure can be restored upon heating. The rotation of the domain walls is interpreted in terms of additional twinning of the crystals resulting from the possible low-temperature Jahn–Teller (JT) distortion of the orthorhombic cell. [Copyright &y& Elsevier]