CMR Effect Research Articles

Magnetic field modulation effect of photoelectric properties in dye-sensitized solar cells with La0.67(Ca,Ba)0.33MnO3 as counter electrodes.

In recent years, many semiconductor materials with unique band structures have been used as Pt counter electrode (CE) substitutes for dye-sensitized solar cells (DSSCs), which makes the photoelectric properties of DSSCs possible to be modulated by electric field, magnetic field, and light field. In this work, La0.67(Ca Ba)0.33MnO3 (LCBMO) thin film is employed to act as CE in DSSCs. The experimental results indicate that short-circuit current density and photoelectric conversion efficiency present better stability when applying an external magnetic field to the DSSCs. Furthermore, both the exchange current density (J0) and limit diffusion current density (Jlim) are largely enhanced by an external magnetic field. J0 increases from -0.51 mA•cm-2 to -0.65 mA•cm-2, and Jlim increases from 0.2 mA•cm-2 to 0.3 mA•cm-2 when applying a magnetic field of 0.25 T. The fitting results of the impedance test verify that the magnetic field reduces the value of Rct. Both magnetic-field enhancing catalytic activity and CMR effect jointly promote the increase of photocurrent and finally improve the photovoltaic effect in DSSCs.

Electrical transport mechanism and magnetoresistive behavior of trilayer La0.7Sr0.3MnO3/γ-Fe2O3/La0.7Sr0.3MnO3 (FM/FIM/FM) manganites

The magnetization, electrical transport, and magnetoresistance (MR) behavior of Pulsed Laser Deposited polycrystalline tri-layer La0.7Sr0.3MnO3 (LSMO)/γ-Fe2O3/LSMO film and single layer LSMO film on Si (100) substrates are investigated in the temperature range of 5 K–300 K. A magnetic anomaly appears in the magnetization {M(T)} and resistivity {ρ(T)} characterizations of the trilayer film at around 100 K, which is quite robust and is present even at 7 T of the applied magnetic field. The trilayer sample exhibits metal to insulator (MIT) transition at 210 K. The transport behavior of both the films is analysed in the context of nearest-neighbour small polaron hopping model (SPH), Mott variable range hopping (VRH) mechanism as well as thermal activation (TA) model in different temperature ranges for both the films under study. The correlated polarons or less mobile bipolarons exist in the metallic regime of the trilayer sample due to the presence of magnetic inhomogeneities. The low temperature resistivity upturn is observed in LSMO film, which is attributed to the spin fluctuations (Mn3+–O–Mn4+) due to thermal energy. The trilayer sample exhibits a large negative MR of (∼45%) at low temperature (5 K). It manifests consistent negative CMR effect across the whole temperature range of 5 K–210 K (MIT). The improved MR behavior of the trilayer sample as compared to the LSMO film is due to enhanced polycrystalline character of the trilayer film owing to higher grain boundary density and spin dependent scattering at both the interfaces of LSMO/γ-Fe2O3/LSMO structure.

Structural and electrical properties of Pr1−xSrxMnO3 (x=0.25, 0.3, 0.35 and 0.4) manganites

In this work, we have studied the structural features and electrical conductivity of the polycrystalline Pr1−xSrxMnO3 (x=0.25, 0.3, 0.35, 0.4) manganites. All the compounds have orthorhombic structure with space group Pbnm. The electrical resistivity of all the samples exhibits metal–insulator transition TMI for both H=0T and H=8T. At H=8T, ρ (T) drops down considerably which indicates the existence of the CMR effect. The electron–phonon, electron–electron and electron–spin fluctuation interactions are effective to describe the resistivity behavior for temperature less than the metal–insulator transition (TMI). For temperatures, T>TMI, the insulating nature is discussed with a small polaron conduction (SPC) model. The resistivity of all the samples shows a field-dependent minimum at low temperature. The above is due to the Coulomb interaction between carriers strongly enhanced by disorder and Kondo-like spin dependent scattering.

Structural Phase Transitions in EuC2

Pure EuC(2), free of EuO impurities, was obtained by the reaction of elemental europium with graphite at 1673 K. By means of synchrotron powder diffraction experiments, the structural behavior was investigated in the temperature range from 10 to 1073 K. In contrast to former results, EuC(2) crystallizes in the ThC(2) type structure (C2/c, Z = 4) at room temperature. A tetragonal modification (I4/mmm, Z = 2) is only observed in a very small temperature range just below the transition to a cubic high-temperature modification (Fm3m, Z = 4) at 648 K. DTA/TG investigations confirm these results. According to Raman spectroscopy, EuC(2) contains C(2)(2-) ions (nu(C[triple bond]C) = 1837 cm(-1)). The divalent character of Eu is confirmed by the results of magnetic susceptibility measurements and (151)Eu Mossbauer spectroscopy. In these measurements a transition to a ferromagnetic state with T(C) = 15 K is observed, which is in reasonable agreement with literature data. Above T(C) EuC(2) is a semiconductor according to measurements of the electric resistivity vs temperature, again in contrast to former results. Around T(C) a sharp maximum of the electric resistivity vs temperature curve was observed, which collapses on applying external magnetic fields. The observed CMR effect (colossal magnetoresistance) is much stronger than that reported for other EuC(2) samples in the literature. These investigations explicitly show the influence of sample purity on the physical and even structural properties of EuC(2).

Magneto-electric Behaviors of La<SUB>0.67-x</SUB>Nd<SUB>x</SUB>Sr<SUB>0.33</SUB>MnO<SUB>3</SUB> System

The magneto-electric behaviors of La_(0.67-x)Nd_xSr_(0.33)MnO_3 system were studied by the measurement of M-T curve,ESR curve,infrared spectra,Raman spectra,p-T curve and MR-T curve of the La_(0.67-x)Nd_xSr_(0.33)MnO_3 system(x=0.00,0.10,0.20,0.30,0.40).The experiment result shows that with the Nd doping increasing,the magnetic structure of the system transforms from the long-range ferromagnetic order to spin cluster glass state;for the samples with x=0.30 and 0.40,the phase separation occurs at the temperature high above T_c;The transport property exhibits change with the Nd doping increasing.Magneto-electric behaviors of the system and CMR effect attributes to lattice effect and interface tunneling effect relating to the spin.

Electronic phase separation and the CMR effect in Pr 1− xCa xMnO 3 films on (0 0 1) vicinal SrTiO 3 substrates

Our previous work showed that Pr 1− x Ca x MnO 3 ( x = 0.2–0.5) thin films reveal an electronic phase separation, in which the low-temperature phase is orbital- and charge-ordered of the Zener-polaron type [Ch. Jooss, T. Beetz, L. Wu, M. Beleggia, R. Klie, M. Schofield, Y. Zhu, S. Schramm, J. Hoffmann, submitted for publication]. Applying a magnetic field leads to a ferromagnetic and conducting phase, which gives rise to the colossal magneto resistance effect (CMR). In this article we show that the orbital- and charge-ordered state could be significantly modified by different kind of lattice defects. Quenched disorder, present after the film deposition, can suppress long-range ordering. In addition, extended defects like twin boundaries act as a nucleation center for ordered domains, i.e. the hysteretic properties depend on the density of these defects. Well-ordered artificial anti-phase boundaries (APB) can be introduced by an epitaxial growth on vicinal substrates, leading to anisotropic transport properties with respect to the orientation of the APB.

Peculiar transport properties and CMR effect of La0.67Sr0.08Na0.25MnO3

The La0.67Sr0.08Na0.25MnO3 sample was prepared by solid-state reaction. The transport properties and CMR effect of the sample were studied through M-T curves, ρ-T curves and ρ-T fitted curves. The results show that: Both the ρ-T curves and the MR-T curve exhibit the double-peak phenomenon; the high-temperature peak corresponds to the I-M transition accompanied by the PM-FM transition, and the low-temperature peak is attributed to grain boundary effect. The two insulator phases have different transport mechanisms: in the low temperature range(248 KTρ(T) accords with variable-range-hopping model of polaron, while in the high temperature range(330 KTρ(T) accords with near- neighbor- hopping model of polaron; and the two metal-like phases also have different transport mechanisms: in the low temperature range(67 KTρ(T) accords with the ρ-T2.5 relation, and the transport mechanism is the actions of spin wave scattering and electron-magneton scattering, while in the high temperature range(292 KTρ(T) accords with the ρ-T2 relation, and the transport mechanism is single magneton scattering.

Speech recognition in coherently-amplitude modulated noise

Comodulation masking release is a phenomenon that improves the detectability of a masked pure tone or speech signal by addition of a coherently amplitude-modulated energy above and/or the signal frequency. The majority of the studies done on comodulation masking release have studied threshold detection for a pure tone, or speech identification at or near threshold levels with a favorable signal-to-noise ratio. Moreover, various studies examining comodulation masking release under conditions of reduced frequency selectivity in listeners with cochlear hearing impairment indicate absence or reduced comodulation of masking release effect (Hall et al., 1984; Hall and Grose, 1985; Moore et al., 1993). In the current study, the speech recognition task was studied for the CMR effect at supra-threshold level and unfavorable signal-to-noise ratio of −10 dB. Results indicated absent CMR effect at supra-threshold level for the word recognition task for normal-hearing individuals.

Possible quantum critical point in(La1−xDyx)0.7Ca0.3MnO3

We report a study of the possible quantum critical point (QCP) in the rare earth substituted colossal magnetoresistive manganite $({\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Dy}}_{x}{)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ with $x=0.347$. A magnetic phase diagram has been proposed in the temperature-field-plane based on the neutron diffraction, small-angle neutron scattering, and ac susceptibility results. We show the role of applied magnetic field to tune the QCP driven by quenched disorder. The implications of the observed QCP phenomenon to understand the physics of the CMR effect and Griffiths singularities in strongly correlated electron systems are discussed.

Extraordinary Transport Behaviors of La 0.67Sr 0.33Mn 1- xCr xO 3(0.00≤ x≤0.30)

M-T curves, ρ- T curves and MR-T curves of La 0.67Sr 0.33Mn 1- x Cr x O 3 (0.00≤ x≤0.30) were studied. Experimental results indicate that the substitution of Cr on Mn sites is impotent in driving T C, but with an increase of Cr content, there appear two peaks in resistivity and MR response, thus the temperature range of CMR response is significantly broadened. The extraordinary transport behavior and the abnormal CMR effect could be the results of coexistence and competition of Mn 3+ -O-Mn 4+ and Mn 3+ -O-Cr 3+ channels.

Intrinsic magnetic inhomogeneity of Eu substituted La 0.7Pb 0.3Mno 3 single crystals

The study of magnetic and magnetotransport properties of the crystals of (La 1− y Eu y ) 0.7Pb 0.3MnO 3 system has been carried out. Eu ions enter the crystals being in trivalent nonmagnetic state. Europium ions possessing of smaller ionic radius in comparison with La ions, induce local distortions of Mn–O–Mn bonds in the system that cause random distribution of magnetic exchange interactions in magnitude and, probably, in sign. The competition of magnetic interactions leads to the appearance of the inhomogeneous magnetic state in the crystals. The enhancement of concentration of Eu ions results in decrease of the Curie temperature and broadening of the inhomogeneous magnetic state area. At y=0–0.4 the coexistence of the paramagnetic phase with conductivity of the polaronic type and the ferromagnetic metallic phase is observed in a bounded temperature interval both above and below T C. Below T C the increasing of y up to 0.6 induces the magnetic state representing the coexistence of two different FM phases. These phases are spatially separated due to frustration of FM and AFM exchange interactions on phase boundaries. Above T C, up to 1.6 T C ferromagnetic clusters exist in a paramagnetic matrix similar to the case of samples with y=0–0.4. Concerning electric properties, the samples with y=0–0.4 reveal the metal–insulator transition at temperature that practically coincides with T C. The sample with y=0.6 has conductivity of insulator character up to the lowest temperatures. For all investigated compositions y=0–0.6 the CMR effect is observed in the area where the inhomogeneous magnetic state exists. The effect is determined by different conductivity of the coexisting phases, as well as by sensitivity of the inhomogeneous state to external magnetic field.

Charge order phase and reentrant spin-glass behavior in half doped Sm0.5Ca0.5MnO3 manganite

The structure, transport and magnetic properties of half doped Sm0.5Ca0.5MnO3 manganite has been systamatically studied. The results show that Sm0.5Ca0.5MnO3 compound has the O′orthorhombic structure and shows typical Jahn-Teller distortion. Throughout the temperature range used, the electric transport shows semiconducting behavior and there are no I-M transition and CMR effect. Meanwhile, it is found that the charge-order and antiferromagnetic transition temperatures of the system are around 270K and 200K, respectively, whereas at lower temperature there appears a typical spin-glass state around 41K with reentrant behavior. The negative magnetization is also observed in this compound. All these phenomena indicate that for the ground state of Sm0.5Ca0.5MnO3, there exists the competition mechanism of several complex magnetic interactions. Present study may provide abundant experimental information for understandig the mechanism of the strongly electron correlated system.

Hydrostatic pressure effect on electrical and magnetic properties of electron-doped R 0.16Ca 0.84MnO 3 (R=Pr, Gd, Eu)

Temperature dependences of resistivity and magnetization of electron-doped R 0.16Ca 0.84MnO 3 (R=Pr, Gd, Eu) orthomanganites have been studied under applied pressures up to 11 kbar. For Pr 0.16Ca 0.84MnO 3, the transition temperature between antiferromagnetic insulating and paramagnetic semi-metallic state T t decreases under pressure at the rate d T t / d P = - 0.3 K / kbar . The sign of the effect is opposite and the value is an order of magnitude lower than that for ferromagnetic metallic hole-doped R 1− x D x MnO 3 ( x C < x < 0.5 , D=Ca, Sr, x C —percolation threshold) compounds. The negative sign of the effect is in evident contradiction with a positive shift of the magnetic transition temperature of the parent compound CaMnO 3. To interpret this result, a possibility of electron delocalization and a crossover to a band antiferromagnetism by electron doping have been suggested. The compounds Eu 0.16Ca 0.84MnO 3 and Gd 0.16Ca 0.84MnO 3 below T t are in a mixed magnetic state with coexisting ferro- and antiferromagnetic exchange interactions. The weak pressure effect on T t for Eu 0.16Ca 0.84MnO 3 does not correlate with the record of the magnetoresistance effect of electron-doped compounds. Considering the obtained applied pressure data, the role of interatomic distances in the formation of CMR effect for electron-doped orthomanganites is discussed.

The Effect of Laser Acting On Double-doped La2/3(Ca3/4Sr1/4)1/3MnO3 Thin Films

Photo-responsive properties studied in perovskite manganese La2/3(Ca3/4Sr1/4)1/3MnO3 (LEMO) thin film with continued laser and modulated laser pulses are studied in this paper. The two valence alkaline earth elements make the Mn have two states: Mn3+(t32geg) and Mn4+(t32g). The Sr, Ca double-doped holes concentration is from 0.2 to 0.4 with maintained electric charge balance. The resistivity and temperature principle indicates that the material has CMR effect and transforms characteristics from metal to semiconductor and insulator. The results show that the phase transition from ferromagnetic metallic state to paramagnetic insulating state occurs at 341 K while the highest peak temperature is near to Tp. The maximizing photo-responsive resistivity change (ΔR/R0)max can reach 43.7%. Responsive signal intensity to the laser pulse has nonlinear relationships with the temperature and bias current. The photo-response resistivity changes with excited down-spin electrons and the exciting field. The optimum photo-responsive condition is T = 229 K, I = 24.7 mA. (ΔR/R0)max increases with temperature following exponential characteristic and there is a limited temperature for photo-response. The carrier electrons and small polarons have effects on thephoto-responsive characters.

CMR effect exalted by substrate-induced strains in thin films of charge ordered manganites

Strained thin films of Nd 0.5Ca 0.5MnO 3 (NCMO) have been synthesized by the pulsed-laser deposition on (1 0 0)-(SrTiO 3) (STO). These films of different thicknesses have been extensively studied with numerous techniques of investigation in order to understand the relationship between structure and transport properties in manganite thin films. This study allowed us to highlight the effects of substrate-induced strains on these materials. We observed a huge decrease of the melting magnetic field. This melting field of 5 T for thin films is much lower than the one (24 T) necessary to induce the metallic phase in the bulk sample. Films undergo two phenomena: firstly, a deformation of the cell by the substrate-induced strains and secondly a structural quenching which freezes the film's structure.

Magnetic and transport properties of (La0.7−2xEux)(Ca0.3Srx)MnO3: Effect of simultaneous size disorder and carrier density

Structural, magnetic and transport studies have been carried out on (La0.7−2xEux)(Ca0.3Srx)MnO3 (0.05⩽x⩽0.25) compounds forming in a distorted orthorhombic structure (space group Pnma, No. 62). The Eu and Sr substitutions avoid any average A-site cation size disparity throughout the series. However, increasing both the cation size mismatch at the A-site and carrier concentration induces interesting changes in structural, transport and magnetic properties. Both insulator–metal transition temperature (Tp) and Curie temperature (TC) decrease with increasing x. The resistivity of all the samples in the semiconducting regime fits to the Variable Range Hopping (VRH) of Mott type model. Carrier localization length, L, obtained from VRH plots, decreases from 4.6 Å for x=0.05 to 3.9 Å for the x=0.20 sample. In the metallic region, the n term in the resistivity fits to the Zener-Double exchange polynomial law (ρ=ρ0+ρ2T2+ρnTn) increases from n=5.5 for x=0.05 to n=7.5 for x=0.15. From magnetic susceptiblity measurements, it is observed that there is an increase in the deviation in susceptibility from Curie–Weiss behavior with increasing size disorder and Mn4+ ion concentration. Further, complex magnetic behavior appears in paramagnetic and ferromagnetic regimes for half-doped sample indicative of phase separation. A disparity between Tp and TC is also observed and is as a result of phase segregation. At low temperatures, a large CMR effect occurs with a decrease in Tp.

Transport properties of La 0.6Y 0.1Ca 0.3MnO 3 compounds with different interfaces

The electrical resistivity ρ( T), magnetoresistivity ρ( T, H), and impedance spectroscopy Z(ω, T) of polycrystalline La 0.6Y 0.1Ca 0.3MnO 3 compounds with different intergranular surfaces were investigated. The interfaces were modified through controlled addition of pores in samples prepared by the sol-gel technique. X-ray diffraction, scanning electron microscopy, and mercury porosimetry analysis showed that all specimens are single phase having similar microstructures and differing only by the volume fraction of pore. ρ( T, H) and Z( ω, T) data revealed that all samples undergo a metal-insulator transition at T∼175 K. In addition, the Z( ω, T) data indicate two well defined contributions to the electrical resistivity. The ρ( T, H) data exhibit features of colossal magnetoresistivity CMR phenomenon but were found to be independent of porosity. The combined results suggest that the CMR effect is mainly associated with percolation of metallic regions and weakly dependent on sample morphology.

Magnetization distribution in the mixed-phase state of hole-doped manganites.

The effect of 'colossal magnetoresistance' (CMR) in hole-doped manganites--an abnormal decrease of resistivity when a magnetic field is applied--has attracted significant interest from researchers in the past decade. But the underlying mechanism for the CMR phenomenon is not yet fully understood. It has become clear that a phase-separated state, where magnetic and non-magnetic phases coexist, is important, but the detailed magnetic microstructure of this mixed-phase state is so far unclear. Here we use electron microscopy to study the magnetic microstructure and development of ferromagnetic domains in the mixed-phase state of La(1-x)Sr(x)MnO3 (x = 0.54, 0.56). Our measurements show that, in the absence of a magnetic field, the magnetic flux is closed within ferromagnetic regions, indicating a negligible magnetic interaction between separated ferromagnetic domains. However, we also find that the domains start to combine with only very small changes in temperature. We propose that the delicate nature of the magnetic microstructure in the mixed-phase state of hole-doped manganites is responsible for the CMR effect, in which significant conduction paths form between the ferromagnetic domains upon application of a magnetic field.

Structural and magnetic phase diagram and room temperature CMR effect of La 1− xAg xMnO 3

The study of the structural and magnetic phase diagram of the manganites La 1− x Ag x MnO 3 shows similarity with the La 1− x′ Sr x′ MnO 3 series, involving a metallic ferromagnetic domain at relatively high temperature (≈300 K). The Ag-system differs from the Sr-one by a much smaller homogeneity range ( x≤1/6) and the absence of charge ordering. But the most important feature of the Ag-manganites deals with the exceptionally high magnetoresistance (−25%) at room temperature under 1.2 T, that appears for the composition x=1/6. The latter is interpreted as the coincidence of the optimal double exchange condition (Mn 3+:Mn 4+=2) with T max=300 K (maximum of the ρ( T) curve in zero field).

Spin-polarization dependent carrier diffusion—a study of the CMR effect in manganese perovskite

A theory based on spin-polarization dependent carrier diffusion is developed to study the transport behaviors, including colossal magnetoresistance effect, in manganese perovskites R–A–Mn–O (R: rear earth; A: metals with a valence of two or one). Starting from Hubbard–Kondo Hamiltonian and using a mean-field method, we have derived the double exchange energy between neighboring Mn ions. Then using the notion of carrier diffusion, a relation of resistivity versus temperature, concentration and normalized magnetization has been obtained for the hole-doped manganese perovskite. By the simple relation, most of the transport behaviors reported, including the magnetoresistance, in the manganese perovskite have been successfully simulated.