MnO3 Single Crystal Research Articles

Elastic and magnetic properties of the La0.6Pr0.1Ca0.3MnO3 single crystal

The temperature dependences of the velocity of longitudinal sound, internal friction, and magnetization of the single crystal with the nominal composition La0.6Pr0.1Ca0.3MnO3 have been measured. It has been found that the substitution of praseodymium for lanthanum in La0.7Ca0.3MnO3 leads to a decrease in the velocity of sound and to an increase in the spontaneous magnetization. The method of determining the Curie temperature distribution function during a first-order transition has been proposed. It has been shown that, in the crystal under study, this function is asymmetric.

Giant anisotropic magnetoresistance in bilayered La1.2Sr1.8Mn2O7 single crystals

We report an observation of anomalous anisotropic magnetoresistance (AMR) in bilayered La1.2Sr1.8Mn2O7 single crystals. A giant AMR is found to be 80% under a magnetic field of 1 T near the metal-insulator transition temperature, where AMR is defined as AMR=[ρ(H⊥c)−ρ(H∥c)]/ρ(H⊥c)×100%, and ρ(H⊥c) and ρ(H∥c) are the resistivity with the magnetic field perpendicular and parallel to c-axis, respectively. The AMR effect shows strong temperature and magnetic field dependences, and indicates a close interrelation with the anisotropic field-tuned metal-insulator transition.

The zero-field glassy ground state and field-induced ferromagnetic transition in(La0.4Pr0.6)1.2Sr1.8Mn2O7

We have investigated glassy magnetic freezing in(La0.4Pr0.6)1.2Sr1.8Mn2O7 single crystals together with the field-induced transition to a metastable ferromagneticphase using ac magnetic susceptibility and heat capacity measurements. The magnetizationmeasurements show evidence for the development of a zero-field glassy groundstate below 45 K along with a hysteretic, field-induced change in susceptibilityassociated with the transition to the ferromagnetic phase above 5 T. The heatcapacity develops a clear peak at higher temperatures with the application oflarge magnetic fields, consistent with the development of a ferromagnetic order,while at low temperatures the Sommerfeld coefficient is monotonically reduced byan applied field, suggesting suppression of spin fluctuations. The heat capacityshows hysteretic behaviour, accompanied by a sharp decrease at a critical field,when held at fixed temperature, which does not recover on reducing the fieldback to zero. These measurements suggest that the zero-field ground state for(La0.4Pr0.6)1.2Sr1.8Mn2O7 consists of frozen disordered spin clusters, which develop into a metastable ferromagneticstate in modest magnetic fields.

Bias dependent 1/f conductivity fluctuations in low-doped La1−xCaxMnO3 manganite single crystals

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.

Anisotropic magnetoresistance in low-doped La0.78Ca0.22MnO3 crystals

Unusual behavior of anisotropic magnetoresistance (AMR) has been encountered in low-doped La0.78Ca0.22MnO3 single crystals. In contrast with previous studies of AMR in manganites, as the maximal effect was observed around TC, the AMR of La0.78Ca0.22MnO3 single crystals (TC ≈ 189 K) increases monotonously with decreasing temperature, reaches a maximum around 140 K, and then decreases with further temperature decrease. Moreover, around the maximum, AMR increases almost linearly with magnetic fields and only at fields exceeding H ∼ 7 kOe does it start to saturate. The observed unusual enhancement of AMR may be ascribed to the changes in magnetic domain structure at temperatures below 150 K as observed previously by us using a magneto-optical imaging technique.

Anisotropic magnetoresistance in low-doped La0.79Ca0.21MnO3 crystals

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.

Behavior of Clustered State above the Curie Temperature in La<sub>0.78</sub>Ca<sub>0.22</sub>MnO<sub>3</sub> Single Crystal

The results of structural neutron diffraction study and the data on the transport and magnetic properties (the linear and nonlinear (second and third order) susceptibilities) are presented for the La0.88Ca0.22MnO3 single crystal. This compound has the orthorhombic Pbnm space group, showing no structural transformations between 90 K and 300 K. It exhibits the paramagnet-ferromagnet (P-F) (TC  186 K) and insulator – metal (I-M) (ТIM  201 K > TC) phase transitions. The measurements of the second harmonic of the magnetization in the parallel ac and dc H magnetic fields, M2(H,T), showed that below T* 252 K, the FM clusters appear in the PI matrix undergoing the second-order transition. The concentration of the FM clusters increases with cooling so that it is above the percolative threshold value below TIM, and the sample exhibits metallic behavior. The M2(H) hysteresis loops possess a high sensitivity to the I-M transition. Their usual central symmetry is broken in the vicinity of TIM and the loops become asymmetric. This effect is attributed to cluster link nucleation-and-growth process.

Nonlinear properties and paramagnet-to-ferromagnet transition in Nd0.7Ba0.3MnO3 single crystal with metallic ground state

We present the results from studying the magnetic properties (linear and nonlinear susceptibilities and the depolarization of polarized neutrons) of Nd1 − xBaxMnO3 manganite, x = 0.3, with Curie temperature TC ≈ 140 K and dielectric-to-metal transition temperature TDM ≈ 129 K. Its critical behavior corresponds to that of an isotropic 3-D ferromagnet at temperatures above T*≈ 144 K, but a strong nonlinear response in weak magnetic fields and depolarization are observed at temperatures below T*. It is shown that this nontraditional behavior is related to the generation of ferromagnetic clusters in the paramagnetic matrix that form a conducting percolative network at temperatures near TDM.

Negative differential resistance in Gd0.5Sr0.5MnO3: A consequence of Joule heating

Negative differential resistance (NDR) in current-voltage (I-V) characteristics and apparent colossal electroresistance were observed in Gd0.5Sr0.5MnO3 single crystals at low temperatures. The continuous dc I-V measurements showed a marked thermal drift. In addition, temperature of the sample surface was found to be significantly higher than that of the base at high applied currents. Two different strategies namely estimation and diminution of the Joule heating (pulsed I-V measurements) were employed to investigate its role in the electric transport properties. Our experiments reveal that the NDR in Gd0.5Sr0.5MnO3 is a consequence of Joule heating rather than the melting of charge order.

Raman study of the antiferromagnetic phase transitions in hexagonalY MnO3 andLuMnO3

The A1,E1 andE2 Raman activemodes in hexagonal Y MnO3 and LuMnO3 single crystals are studied as a function of temperature and compared with previousmeasurements. In addition to anharmonicity, some phonon frequencies show belowTN anomalous temperature dependences that reflect the atomic displacements whilesome other phonon frequencies are more sensitive to the spin–phonon coupling.

Memory effect in Dy0.5Sr0.5MnO3 single crystals

We have performed a series of magnetic aging experiments on single crystals ofDy0.5Sr0.5MnO3. The results demonstrate striking memory and chaos-like effects in this insulating half-dopedperovskite manganite and suggest the existence of strong magnetic relaxation mechanismsof a clustered magnetic state. The spin-glass-like state established below a temperatureTsg≈ 34 K originates from quenched disorder arising due to the ionic-radii mismatch at the rareearth site. However, deviations from the typical behavior seen in canonical spin glassmaterials are observed which indicate that the glassy magnetic properties are due tocooperative and frustrated dynamics in a heterogeneous or clustered magnetic state. Inparticular, the microscopic spin flip time obtained from dynamical scaling near the spinglass freezing temperature is four orders of magnitude larger than microscopictimes found in atomic spin glasses. The magnetic viscosity deduced from the timedependence of the zero-field-cooled magnetization exhibits a peak at a temperatureT < Tsg and displays a marked dependence on waiting time in zero field.

Effect of Electrical Current on Magnetic and Transport Properties of Single-Crystalline La$_{0.82}$Ca$_{0.18}$MnO$_{3}$

AC susceptibility and voltage relaxation behaviors after current pulses were studied at various passing through currents for La0.82Ca0.18MnO3 single crystals. It was found that a current of 10 mA caused a significant temperature shift towards lower temperatures on the ac-susceptibility versus temperature curve when measured in a cryostat, while the current induced effect was insignificant when the sample is immersed in liquid N2. Exponential decay of voltage was observed after current pulses when measured in the cryostat at 77 K, but the effect was greatly suppressed when measured in liquid N2. The observed results suggest that Joule heating is associated with electrical currents passing through the sample and is the main contribution to the current induced effects.

Effect of hydrostatic pressure on magnetic phase transition and magnetocaloric properties of (Sm0.8Nd0.2)0.52Sr0.48MnO3

We have investigated the effect of hydrostatic pressure (P) on ferromagnetic (FM) phase transition and magnetocaloric properties of (Sm0.8Nd0.2)0.52Sr0.48MnO3 single crystal. At ambient pressure, the system undergoes a first order FM transition associated with large magnetic entropy change (ΔSM). The temperature distribution of ΔSM exhibits an asymmetric behavior with respect to TC. The application of pressure increases magnetization, shifts the FM transition to higher temperature, and weakens the metamagnetism. As a result, |ΔSM| decreases and its thermal distribution becomes more symmetric as compared to P=0.

Coexistence of colossal magnetoresistance, a Griffiths-like phase, and a ferromagnetic insulating ground state in single crystal La0.73Ba0.27MnO3

Detailed temperature/field-dependent magnetic and transport behaviors are reported for a nominally doped La0.73Ba0.27MnO3 single crystal. The magnetic analysis demonstrates an intriguing coincidence of critical exponents consistent with Heisenberg model values (γ=1.39, β=0.38, δ=4.83) and the presence of a Griffiths-like phase. The transport measurements yield a ferromagnetic insulating ground phase, but one complemented by the appearance of colossal magnetoresistance near the ferromagnetic to paramagnetic (FM-PM) phase transition temperature TC=245 K.

Anomalous magnetic field dependence of magnetocaloric effect at low temperature in Pr0.52Sr0.48MnO3 single crystal

We report the magnetocaloric effect (MCE) in a Pr0.52Sr0.48MnO3 single crystal. A peak in the temperature dependence of magnetic entropy change (ΔSM) with a fairly large negative value (≈3.8 J/kg K) is observed at 275 K close to Curie temperature. MCE is spread over a wide temperature range resulting in a considerable refrigerant capacity (≈293 J/kg). At low temperature the sign of ΔSM changes, below which anomalous field dependence of ΔSM is observed indicating the coexistence of ferromagnetic and antiferromagnetic interactions. Interplay between the interactions is strongly influenced by direction and magnitude of applied magnetic field in the ordered state.

Effect of hydrostatic pressure on ferromagnetic phase transition in (Sm0.7Nd0.3)0.52Sr0.48MnO3 single crystal

We report on the pressure dependence of the order of ferromagnetic (FM) to paramagnetic (PM) phase transition in (Sm0.7Nd0.3)0.52Sr0.48MnO3 single crystal. At ambient condition, the system exhibits clear evidences of first-order FM-PM phase transition at (Tc ≈ 146 K). The application of pressure (P = 12.1 kbar) makes the FM transition second-order in nature with Tc≈ 176K.

An investigation of first-order transition across charge ordered and ferromagnetic phases inGd0.5Sr0.5MnO3 single crystals by magnetic and magnetotransport studies

Gadolinium strontium manganite single crystals of the compositionGd0.5Sr0.5MnO3 were grown using the optical float zone method. We report here the magneticand magnetotransport properties of these crystals. A large magnetoresistance∼109% was observed at 45 K under the application of a 110 kOe field. We have observednotable thermomagnetic anomalies such as open hysteresis loops across thebroadened first-order transition between the charge order insulator and theferromagnetic metallic phase while traversing the magnetic field–temperature (H–T)plane isothermally or isomagnetically. In order to discern the cause of these observed anomalies, theH–T phasediagram for Gd0.5Sr0.5MnO3 is formulated using the magnetization–field (M–H), magnetization–temperature (M–T) andresistance–temperature (R–T) measurements. The temperature dependence of the critical field (i.e. Hup, the field required for transformation to the ferromagnetic metallic phase) is non-monotonic.We note that the non-monotonic variation of the supercooling limit is anomalousaccording to the classical concepts of the first-order phase transition. Accordingly,Hup valuesbelow ∼20 K are unsuitable to represent the supercooling limit. It is possible that the nature of themetastable states responsible for the observed open hysteresis loops is different from that ofthe supercooled ones.

Orientation-dependent x-ray Raman scattering from cubic crystals: Natural linear dichroism in MnO and CeO2

Information on valence orbitals and electronic interactions in single crystal systems can be obtained through orientation-dependent x-ray measurements, but this can be problematic for a cubic system. Polarisation-dependent x-ray absorption measurements are common, but are dominated by dipole transitions which, for a cubic system, are isotropic even though a cubic system is not. Many edges, particularly for transition metals, do have electric quadrupole features that could lead to dichroism but proximity to the dipole transition can make interpretation challenging. X-ray Raman Spectroscopy (XRS) can also be used to perform orientation-dependent near-edge measurements – not only dependent on the direction of the momentum transfer but also its magnitude, q. Previous XRS measurements on polycrystalline materials revealed that multipole (higher order than dipole) transitions are readily observable in the pre-threshold region of rare earth N4,5 edges, actually replacing the dipole at high-q. We have extended these studies to examine orientation-dependent XRS for CeO2 and MnO single crystals, as prototype systems for theoretical treatment. Dichroism is observed at both the Ce N4,5 and Mn M2,3 edges in these cubic materials.

Ultrafast quasi-particle dynamics of charge/orbital ordered and ferromagnetic clusters in La0.7Ca0.3MnO3

We report on the quasi-particle dynamics of the charge/orbital ordered (COO) and ferromagnetic clusters in the optimally doped manganite, La0.7Ca0.3MnO3 (LCMO) single crystal by time-resolved two-color pump–probe spectroscopy. Pump photons with energies of ∼1.55 and ∼0.21 eV were employed in our transient optical spectroscopy to investigate the percolative phase separation including the COO and ferromagnetic clusters from 4 to 480 K. At 1.55 eV, the transient reflectivity change, ΔR/R, at Δt=0 shows a similar temperature dependence as that in resistivity and in neutron scattering intensity. We attribute the reflectivity signal to the characteristic optical response of the COO domains. We identify a new temperature scale T*∼400 K for the clean limit to the formation of COO clusters in LCMO. In contrast, the temperature-dependent amplitude of the transient reflectivity change in the mid-infrared (IR) absorption band (∼5 μm) scales with the volume fraction of the ferromagnetic metallic (FM) phases. Our results suggest ultrafast optical spectroscopy to be a powerful probe to reveal the correlated polarons and charge disorders in phase-separated manganites.

Effect of pressure on lattice distortion, transport and magnetic properties of Pr-substitutedLa1.2Sr1.8Mn2O7 bilayered manganite

We have studied the effect of pressure on the anomalous lattice striction, both in theab-plane andalong the c-axis, of (La,Pr)1.2Sr1.8Mn2O7 single crystals over the temperature region where the paramagnetic insulator toferromagnetic metal transition takes place. We have examined the temperature dependenceof the resistivity and the magnetization under applied pressure. The chemical pressureeffect due to Pr-substitution at the La site suppresses the transition temperature of theparent crystal, while the application of external pressure on Pr-substituted crystalsenhances the double exchange driven metallic state, resulting in a stable rise ofTc.