Electric field driven destabilization of the insulating state in nominally pure LaMnO3

We report an electric field driven destabilization of the insulating state in nominally pure LaMnO3 single crystal with a moderate field which leads to a resistive state transition below 300 K. The transition is between the insulating state in LaMnO3 and a high resistance bad metallic state that has a temperature independent resistivity. The transition occurs at a threshold field (Eth) which shows a steep enhancement on cooling. While at lower temperatures the transition is sharp and involves large change in resistance but it softens on heating and eventually absent above 280K. When the Mn4+ content is increased by Sr substitution up to x=0.1, the observed transition though observable in certain temperature range, softens considerably. The observation has been explained as bias driven percolation type transition between two coexisting phases, where the majority phase is a charge and orbitally ordered polaronic insulating phase and the minority phase is a bad metallic phase. The mobile fraction f of the bad metallic phase deduced from the experimental data follows an activated kinetics with the activation energy nearly equal to 200 meV and the prefactor fo is a strong function of the field that leads to a rapid enhancement of f on application of field leading to the resistive state transition. We suggest likely scenarios for such co-existing phases in nominally pure LaMnO3 that can lead to the bias driven percolation type transition.Comment: Accepted in JPC

Nonequilibrium 1/f Noise in Low-doped Manganite Single Crystals

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 multi-step indirect tunneling.Comment: 4pages, 3figures,APL accepte

Magnetic and Optical properties of strained films of multiferroic GdMnO3

The effects of strain on a film of mulitferroic GdMnO3 are investigated using both magnetometry and magneto-optic spectroscopy. Optical spectra, in the energy range 1.5eV - 3.5eV, were taken in Faraday geometry in an applied magnetic field and also at remanence. This yielded rich information on the effects of strain on the spin ordering in these films. Epitaxial films of GdMnO3 were grown on SrTiO3 and LaAlO3 substrates. The LaAlO3 was twinned and so produced a highly strained film whereas the strain was less for the film grown on SrTiO3. The Ne\'el temperatures and coercive fields were measured using zero field data and hysteresis loops obtained using a SQUID magnetometer. Optical absorption data agreed with earlier work on bulk materials. The two well known features in the optical spectrum, the charge transfer transition between Mn d states at ~2eV and the band edge transition from the oxygen p band to the d states at ~3eV are observed in the magnetic circular dichroism; however they behaved very differently both as a function of magnetic field and temperature. This is interpreted in terms of the magnetic ordering of the Mn spins.Comment: 9 pages of text including figure

Transport, Magnetic, and Thermal Properties of La 0.7

We report the transport, magnetic, and thermal properties of La0.7Ca0.24Sr0.06MnO3 single crystal. It was prepared using floating zone technique used under oxygen pressure of 1 bar with a typical growth rate of 1 mm/hr. The resistivity data shows the metal to insulator transition (M-I) occuring at TMI=290 K along the c-axis and at TMI=280 K along the ab-plane. It is observed that the TMI is higher along the c-axis as compared to that in the ab-plane, thus indicating that more favorable hoping of electrons is along the c-axis. From bolometric application point of view, the temperature coefficient of resistance (TCR) is found to be ~28% K−1. The ac-susceptibility measurement shows that this material exhibits ferromagnetic to paramagnetic transition temperature (TC) 277 K. Sharp peak around this temperature in heat capacity data indicates the onset of long-range ordering. The entropy change associated with this transition is found to be 2.3 J/mol K

Ferromagnetic Domain Structure of La0.78Ca0.22MnO3 Single Crystals

The magneto-optical technique has been employed to observe spontaneous ferromagnetic domain structures in La0.78Ca0.22MnO3 single crystals. The magnetic domain topology was found to be correlated with the intrinsic twin structure of the investigated crystals. 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 70-150 K. The apparent rotation of the domain walls can be understood in terms of the Jahn-Teller deformation of the orthorhombic unit cell, accompanied by additional twinning.Comment: 7 pages, 5 figures, to be published in PR

Three terminal capacitance technique for magnetostriction and thermal expansion measurements

An instrument has been constructed to measure a large range of magnetostriction and thermal expansion between room temperature and 4 K in a superconductive split-coil magnet, that allows investigation in magnetic fields up to 12 T. The very small bulk samples (up to 1 mm in size) as well as big ones (up to 13 mm) of the irregular form can be measured. The possibility of magnetostriction investigation in thin films is shown. A general account is given of both electrical and the mechanical aspects of the design of capacitance cell and their associated electronic circuitry. A simple lever device is proposed to increase the sensitivity twice. The resulting obtained sensitivity can be 0.5 Angstrom. The performance of the technique is illustrated by some preliminary measurements of the magnetostriction of superconducting MgB2, thermal expansion of (La0.8Ba0.2)0.93MnO3 single crystal and magnetoelastic behavior of the Ni/Si(111) and La0.7Sr0.3CoO3/SAT0.7CAT0.1LA0.2(001) cantilevers.Comment: 6 pages, 6 figures, journal pape

Structural, magnetic and magneto-resistance properties of La₀․₈₈Ca₀․₁₂MnO₃ single crystals

484-487La₀․₈₈Ca₀․₁₂MnO₃ single crystals of high quality have been prepared by using floating zone technique. Their composition and structure are verified using X-ray diffraction and EDAX. The temperature dependence of the resistance along ab-plane and c-axis has been measured in the temperature range 77 K to room temperature in the magnetic fields up to 8 T. The R-T data along ab-plane and c-axis shows a similar behaviour and it is the characteristics of insulating behaviour having the resistance 0.7 M Ω at 90 K and 16.1 Ω at 300 K. The insulating behaviour persists under the magnetic field up to H = 8 T. There is no resistance peak or electronic transition over the whole temperature range studied. However, there is a notable change of resistance at about 120 K, seemingly related to paramagnetic insulator to ferromagnetic insulator transition and is confirmed by magnetization measurement transition at a temperature Tc = 122 K. The sharp peak around the same temperature in the heat capacity measurement indicates the onset of long range ordering. The magnetoresistance measured at different magnetic fields shows similar kind of behaviour with some marginal change in the magnitude along ab-plane and c-axis

Colossal electroresistance and current induced switching in ferromagnetic insulating state of La0.82Ca0.18MnO3

Colossal electroresistance and current induced resistivity switching have been measured in the ferromagnetic insulating (FMI) state of single crystal manganite La0.82Ca0.18MnO3. The sample has a Curie transition temperature TC = 165 K and the FMI state is realized for temperatures T<100 K. The electroresistance (ER), arising from a strong nonlinear resistivity, attains a large value ( ≈ 100%) in the FMI state. However, this is accompanied by a collapse of the magnetoresistance (MR) to a small value even in magnetic field (H) of 10 T. This demonstrates that the mechanisms that give rise to ER and MR are effectively decoupled

Magnetoelastic Properties of La0.744\text{}_{0.744}Ba0.186\text{}_{0.186}MnO3\text{}_{3} Single Crystals

Magnetoelastic properties of La$\text{}_{0.744}$Ba$\text{}_{0.186}$MnO$\text{}_{3}$ single crystals have been studied. The basic magnetoelastic constants of the crystal have been estimated using the data of magnetostriction and magnetization measurements, which have been carried out in [010] and [110] crystallographic directions in a wide temperature range from 10 K to 150 K. The anomaly of the thermal expansion has been observed at about 185 K. Near this anomaly two almost degenerated phase transitions have been observed. One of these transitions may be related to the structural phase transition, while the second one to the occurrence of spontaneous strains. The magnetostriction of La$\text{}_{0.744}$ Ba$\text{}_{0.186}$MnO$\text{}_{3}$ single crystal seems to arise due to the effect of applied field on the spontaneous strains