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Pronounced noise signatures enabling one to discriminate metastable resistivity states in La0.86Ca0.14MnO3 single crystals have been observed. The normalized noise spectra for metastable resisitivity differ both in shape and magnitude, indicating that the metastable state is associated with transition of the electronic system into another local minimum of the potential landscape. Such scenario is consistent with freezing of the electronic system into a Coulomb glass state. [ABSTRACT FROM AUTHOR]

Magnetic properties of CaMn1-xWxO3 (0 ≤ x ≤ 0.1) have been investigated, and the research was focused on the exchange bias (EB) phenomenon in CaMn0.93W0.07O3. Magnetic ground state was found to be dependent on tungsten doping level and the following states were distinguished: (i) G-type antiferromagnetic (AFM) state with a weak ferromagnetic (FM) component at x=0 and 0.04; (ii) mostly orbitally ordered C-type AFM at x=0.07 and 0.1. For the studied manganites, spontaneous magnetization increases sharply with increasing doping level reaching M0 ≈ 9.5 emu/g at T=10K for x=0.04, and then decreases rapidly reaching zero for x=0.1. Exchange bias effect, manifested by vertical and horizontal shifts in the hysteresis loop for field cooled sample, has been observed in CaMn0.93W0.07O3. Exchange bias field, coercivity, remanence asymmetry, and magnetic coercivity depend strongly on temperature, cooling field, and maximal measuring field. Horizontal and vertical shifts of magnetization loop sharply decrease with increasing temperature and vanish above 70K, whereas coercivity and magnetic coercivity disappear only above 100K (temperature of transition to the G-type AFM state). The exchange bias field HEB increases with increasing cooling field Hcool and goes through a broad maximum at 40 kOe, while the remanence asymmetry increases monotonously in the entire investigated cooling field range, up to 50 kOe. For the compound with x=0.07, the size of the FM regions 2 nm was estimated from the dependence of exchange bias field HEB upon Hcool. It is suggested that the exchange bias originates from interface exchange coupling between small FM clusters and the G-type AFM phase inside the primary C-type orbitally ordered AFM phase. [ABSTRACT FROM AUTHOR]

High temperature X-ray powder diffraction technique was employed to study the orthorhombic to rhombohedral (O-R) structural transition (ST) in single crystal and ceramics of La0.7Ca0.3MnO3. The data obtained evidence that such ST in both samples occurs in the vicinity of temperature T ∼ 700 K and demonstrates O and R phases coexistence within the T-intervals of about 20 K in crystal and ∼60 K in ceramics. This first order transition is hysteretic on heating and cooling procedures and it is shifted on ΔT ∼ 10 K to higher temperatures in crystal as compared to the ceramics of the same composition. We have described these features in the terms of the difference between 'technologically driven' chemical disorder characteristic for considered samples. Thus, it seems that spatial (up to the micron size scale) fluctuations of the Ca-dopant content in crystal and 'mesoscopic scale disorder' within grain boundary regions in ceramics are responsible for both: above noted difference of ST parameters and reported earlier difference in magnetic ordering of these samples. [ABSTRACT FROM AUTHOR]

X-band electron magnetic resonance and dc/ac magnetic measurements have been employed to study the effects of chemical disorder on magnetic ordering in bulk and nanometer-sized single crystals and bulk ceramics of optimally-doped La0.7Ca0.3MnO3 manganite. The magnetic ground state of bulk samples appeared to be ferromagnetic with the lower Curie temperature and higher magnetic homogeneity in the vicinity of the ferromagnetic-paramagnetic phase transition in the crystal, as compared with those characteristics in the ceramics. The influence of technological driven 'macroscopic' fluctuations of Ca-dopant level in crystal and 'mesoscopic' disorder within grain boundary regions in ceramics was proposed to be responsible for these effects. Surface spin disorder together with pronounced inter-particle interactions within agglomerated nano-sample results in well defined core/shell spin configuration in La0.7Ca0.3MnO3 nano-crystals. The analysis of the electron paramagnetic resonance data enlightened the reasons for the observed difference in the magnetic order. Lattice effects dominate the first-order nature of magnetic phase transition in bulk samples. However, mesoscale chemical disorder seems to be responsible for the appearance of small ferromagnetic polarons in the paramagnetic state of bulk ceramics. The experimental results and their analysis indicate that a chemical/magnetic disorder has a strong impact on the magnetic state even in the case of mostly stable optimally hole-doped manganites. [ABSTRACT FROM AUTHOR]

Magnetic and transport properties of 100 nm thick La0.9MnO3-δ self-doped manganite films have been investigated in the temperature range 5–300 K. The films exhibit a paramagnetic to ferromagnetic transition at TC=194 K. The temperature dependence of the resistivity shows a metal-insulator transition at 204 K and a strong resistivity increase below 160 K. The magnetoresistance was always negative and slightly bias dependent. Variations in resistivity with magnetic field and current are nonhysteretic, while the temperature dependence of the resistivity exhibits unusual inverse thermal hysteresis. The magnetic field independent inverse thermal hysteresis is strongly influenced by a thermal history of the sample. The data suggest that nonlinear low temperature transport is dominated by inelastic tunneling through intrinsic tunnel junctions formed by phase-separated ferromagnetic metallic domains and insulating antiferromagnetic matrix. [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]

Experimental manifestations of current enforced metastable conductivity in La[sub 0.82]Ca[sub 0.18]MnO[sub 3] single crystals are discussed. Metastable conductivity manifests itself by complex current dependence, a pronounced bias dependent structure, resistance hysteresis, enhanced noise with non-Gaussian components, and long relaxation times. Spin polarized tunneling through intrinsic tunnel junctions associated with a network of twin boundaries has been tentatively associated with the phenomena observed. [ABSTRACT FROM AUTHOR]

Magnetic properties of electron-doped LaCaMnO manganite nanoparticles, with average size of 12 and 60 nm, prepared by the glycine-nitrate method, have been investigated in the temperature range 5-300 K and magnetic fields up to 90 kOe. It is suggested that weak ferromagnetic moment results from ferromagnetic shells of the basically antiferromagnetic nanoparticles and from domains of frustrated disordered phase in the core. Assumption of two distinct sources of ferromagnetism is supported by the appearance of two independent ferromagnetic contributions in the fit of the T Bloch law to spontaneous magnetization. The ferromagnetic components, which are more pronounced in smaller particles, occupy only a small fraction of the nanoparticle volume and the antiferromagnetic ground state remains stable. It is found that the magnetic hysteresis loops following field cooled processes, display size-dependent horizontal and vertical shifts, namely, exhibiting exchange bias effect. Time-dependent magnetization dynamics demonstrating two relaxation rates were observed at constant magnetic fields upon cooling to T < 100 K. [ABSTRACT FROM AUTHOR]

Magnetic and transport properties of LaCaMnO nanoparticles with average size of 18 nm and Curie temperature T≈231 K have been studied. Pronounced irreversibility of magnetization below T≈208 K has been observed. Studied particles have shown memory effects in zero-field-cooled and field-cooled magnetization. The resistivity has a semiconducting character at 150< T<300 K and exhibits relaxation and memory effects below the Curie temperature. The results suggest that superspin-glass features in ensembles of interacting 18 nm LaCaMnO particles appear along with superferromagnetic-like features. [ABSTRACT FROM AUTHOR]

Abstract: Transport and magnetic properties of LaMnO3+ δ nanoparticles with average size of 18nm have been investigated. The ensemble of nanoparticles exhibits a paramagnetic to ferromagnetic (FM) transition at T C∼246K, while the spontaneous magnetization disappears at T≈270K. It was found that the blocking temperature lies slightly below T C. The temperature dependence of the resistivity shows a metal–insulator transition at T≈192K and low-temperature upturn at T<50K. The transport at low temperatures is controlled by the charging energy and spin-dependent tunnelling through grain boundaries. The low temperature I–V characteristics are well described by indirect tunnelling model while at higher temperatures both direct and resonant tunnelling dominates. [Copyright &y& Elsevier]

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]

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]

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]

Presents a study that examined the current induced telegraph noise in colossal magnetoresistive (CMR) manganites. Background on CMR manganites; Methodology; Determination of the amplitude and the lifetime of the two-level random telegraphic noise.

Vortex motion in ferromagnetic-superconducting bilayers constituted by thin manganite La 0.67 Sr 0.33 MnO 3 film deposited on the top of thin YBa 2 Cu 3 O 7 − δ film has been investigated. Both films were deposited on uniformly twinned single-crystalline substrates and patterned into strips directed at different angles to the twin lines pattern. These conditions lead to the guided vortex motion at different angles to the periodic pinning potential. It has been established that critical current and flux flow resistance in such an anisotropic pinning landscape-scale follow the elliptic dependence on the angle of the direction of the strip. • Anisotropic properties of YBaCuO/LaSrMnO3 bilayers. • Vortex motion properties of YBaCuO/LaSrMnO3 bilayers. • YBaCuO/LaSrMnO3 bilayers on twinned substrates. [ABSTRACT FROM AUTHOR]

Magnetic investigations, focused on the uncommon behavior of the exchange bias (EB) effect, have been performed for basically antiferromagnetic (AFM) electron doped manganite Sm 0.1 Ca 0.6 Sr 0.3 MnO 3 . The studied system in the ground state exhibits a heterogeneous spin configuration consisting of the C -type antiferromagnetic phase with the Néel temperature T N−C ≈ 150 K, the G -type AFM phase with the Néel temperature T N−G ≈ 70 K, and a FM-like phase with very weak spontaneous magnetic moment. The phase separation, into two different AFM phases and a FM-like phase at the temperatures below T N–G , leads to unusual magnetic properties, such as: narrowing of magnetic hysteresis loops in field cooling process, unconventional EB effect associated with spontaneous magnetization at temperatures below T N–C , strong magnetic field dependence of the negative exchange bias at fields below 100 Oe turning into practically field independent one for fields above 0.5 kOe, significant shift of EB with temperature with a change of the sign from negative at 10 K to positive above 40 K. The atypical magnetic properties are discussed and related to two different interfaces appearing between coexisting magnetic phases. [ABSTRACT FROM AUTHOR]