Low-doped Manganites Research Articles

Insight into phase transition temperatures enhancement induced by oxygen post-annealing in low doped manganite films

Oxygen post-annealed manganite films with low doped concentration generally exhibit drastically enhanced phase transition temperatures compared with the bulk, but the specific mechanism has not been fully understood. In this study, low-doped La0.9Sr0.1MnO3 (LSMO) films were post-annealed at a wide temperature range (Tpa) of 600–1000 °C, and oxygen post-annealing effect on the structure, magnetic and transport properties were investigated. The metal-insulator transition (MIT) and a significant enhancement of Curie temperature (TC) are observed in LSMO films (Tpa = 700–1000 °C). The analysis of Mn3+/Mn4+ ratio indicates that post-annealing induces oxygen excess state in these films, and Mn3+-Mn4+ double exchange interaction is enhanced and Jahn-Teller distortion is reduced due to the decrease of Mn3+/Mn4+ ratio. In addition, the excess oxygen increases with increasing post-annealing temperature, while a drop in phase transition temperatures TMI and TC is observed after they reach the maximum at Tpa = 800 °C, probably due to the weakened double exchange interaction induced by the dropping of Mn3+/Mn4+ ratio from the optimum value. Our study has revealed that double exchange interaction and Jahn-Teller distortion decided by Mn3+/Mn4+ ratio play crucial roles in regulating phase transition temperatures in low doped manganite films.

Signature of Ferromagnetic Phase at Low Temperature in Low-Doped La0.88Ca0.12MnO3 Single Crystal

The confirmation of ferromagnetism at low temperature in a low-doped manganites system is always debatable. It is for the first time, electrical, magnetic, and thermal properties have been measured for manganites single crystal of La0.88Ca0.12MnO3 by using a floating-zone method. The electrical resistivity shows a perceptible change of resistance at a low temperature which can be viewed as a signature of the ferromagnetic phase. The ferromagnetic phase of the investigated La0.88Ca0.12MnO3 single crystal with Curie temperature of TC = 120 K which is in closed agreement with ac-susceptibility measurement. An analysis of the specific heat suggests the transition peak (TP) coincides with the TC and attains 1.2 J mol−1 K at 116 K. The magnetoresistance (MR) and temperature coefficient of resistance (TCR) were found to be 69% and 10% respectively, which expands its subtle potential for applications in spintronics and thermistor.

Tuning the interfacial charge, orbital, and spin polarization properties in La0.67Sr0.33MnO3/La1−xSrxMnO3 bilayers

The possibility of controlling the interfacial properties of artificial oxide heterostructures is still attracting researchers in the field of materials engineering. Here, we used surface sensitive techniques and high-resolution transmission electron microscopy to investigate the evolution of the surface spin-polarization and lattice strains across the interfaces between La0.66Sr0.33MnO3 thin films and low-doped manganites as capping layers. We have been able to fine tune the interfacial spin-polarization by changing the capping layer thickness and composition. The spin-polarization was found to be the highest at a critical capping thickness that depends on the Sr doping. We explain the non-trivial magnetic profile by the combined effect of two mechanisms: On the one hand, the extra carriers supplied by the low-doped manganites that tend to compensate the overdoped interface, favouring locally a ferromagnetic double-exchange coupling. On the other hand, the evolution from a tensile-strained structure of the inner layers to a compressed structure at the surface that changes gradually the orbital occupation and hybridization of the 3d-Mn orbitals, being detrimental for the spin polarization. The finding of an intrinsic spin-polarization at the A-site cation observed in x-ray magnetic circular dichroism (XMCD) measurements also reveals the existence of a complex magnetic configuration at the interface, different from the magnetic phases observed at the inner layers.

Phase separation and locally induced states in manganites

ABSTRACTLocal charged states have been induced at the surface of lanthanum strontium manganite single crystals as a result of the local bias application by a conducting tip of atomic force microscope. These results show the dependence of the surface potential of induced area on the writing time similar to that for ferroelectrics. Phenomenon of phase separation with charge segregation is considered for low-doped manganites. It indicates the tendency of manganites toward charge segregation stimulated by the magnetic ordering.

Phase Separation and Magnetoelectric Properties of Locally Induced States in Manganites

Phenomenon of phase separation with charge segregation is considered for low-doped manganites. The effects of giant dielectric permittivity and colossal magnetocapacitance due to phase separation accompanied by charged inhomogeneities are discussed. Long-range Coulomb interaction is responsible for the formation of inhomogeneous charged states and determines their characteristic length scales. Polar nanoscale areas are also formed at room temperature by applying electric field via the tip of scanning probe microscope to the surface of La0.9Sr0.1MnO3 and La0.89Sr0.11MnO3 single crystals. These results show the dependence of surface potential of induced area on the writing time, which is similar to that for ferroelectrics.

Giant dielectric permittivity and magneto-capacitance effects in low doped manganites

The effect of giant dielectric permittivity due to phase separation accompanied by charged inhomogeneities in low-doped manganites is discussed. The effect appears in the vicinity of the second-order magnetic phase transition and is caused by long-range Coulomb forces. The long-range Coulomb interaction is responsible for the formation of inhomogeneous charged states and determines their characteristic length scales. We derive the phase diagram of the inhomogeneous charged states in the framework of the phenomenological theory of phase transitions. The large value of the static dielectric function reduces the characteristic value of the Coulomb energy of the inhomogeneous state and makes the appearance of the magnetoelectric effect possible. We discuss the formation of a state with giant dielectric permittivity and magneto-capacitance effects in that case.

Zero-bias anomalies in low-doped La1−xCaxMnO3 manganite single crystals

Conductivity of current-biased La0.82 Ca0.18 MnO3 low-doped manganite single crystals has been investigated in wide temperature and magnetic field ranges. Strong zero-bias anomalies in the form of conductance minima and maxima have been observed below the Curie temperature TC. Applied magnetic field and temperature strongly affect the anomaly and cause its transition from the conductance minimum to the conductance maximum. The observed anomalies are ascribed to Anderson localisation and combined Kondo and Fano effects.

Electron resonance and magnetic response of low-doped La0.88Ca0.12MnO3 and La0.9Sr0.1MnO3 manganite single crystals

Ferromagnetic resonance and ac/dc magnetic properties of the low-doped La0.88Ca0.12MnO3 and La0.9Sr0.1MnO3 manganite single crystals were studied below (down to 5 K) their paramagnetic-ferromagnetic phase transition. It appears that technologically driven chemical/structural disorder inherent to low-doped manganites influences strongly on both resonance and magnetic properties of these crystals. It is suggested that a special complexity of such disorder, which manifests itself in Sr-doped crystal on the “meso-” and “macroscopic” scales, may cause additional (as compared to the more structurally homogeneous Ca-doped sample) orbital ordering at lowest temperatures.

Intrinsic inhomogeneities of low-doped lanthanum manganites in the paramagnetic temperature range

The nature of the electrical resistivity for low-doped lanthanum manganites is elucidated. The electrical resistivity is described by the Efros-Shklovskii law (lnρ √ (T0/T)−1/2, where T0 √ 1/Rls) in the temperature range from T* ≈ 300 K ≈ TC (TC is the Curie temperature for conducting manganites) to their TC and is explained by the tunneling of carriers between localized states. The magnetoresistance is explained by a change in the size of localized states Rls in a magnetic field. The patterns of change in Rls with temperature and magnetic field strength determined from magnetotransport properties are satisfactorily described in the model of phase separation into small-radius metallic droplets in a paramagnetic matrix. The sizes Rls and their temperature dependence have been estimated through magnetic measurements. The results confirm the existence of a Griffith phase. The intrinsic inhomogeneities produced by thermodynamic phase separation determine the electrical resistivity and magnetoresistance of lanthanum manganites.

‘Griffiths phase’ versus chemical disorder in low-doped manganites: La0.9Sr0.1MnO3 crystal revisited

Magnetic properties, electron paramagnetic and ferromagnetic resonance were studied in the vicinity and above ferromagnetic-paramagnetic (FM-PM) phase transition of the La0.9Sr0.1MnO3 crystal. It appears that complex chemical/structural disorder inherent to manganites influences strongly on both magnetic and resonance properties of this crystal. In particular, it changes the nature of a FM–PM transition to the first- or mixed-order one and induces FM clustering in their PM state. The latter effect results in coexistence of resonance signals, and non-linearity of the PM inverse magnetic susceptibility versus temperature. This model seems to be much more realistic for description of the PM state in La0.9Sr0.1MnO3 than an idealized Griffiths phase approach.

Size-induced metallic state in nanoparticles of ferromagnetic insulatingNd0.8Sr0.2MnO3

A detailed investigation of the electronic- and magneto-transport properties ofNd0.8Sr0.2MnO3 with the variation of grain size (down to 42 nm) is presented here. Interestingly, we observethat the ferromagnetic insulating state is suppressed and a metallic state is stabilized asthe grain size of the sample is reduced. As a result, a metal–insulator transition is observedin this low doped manganite which is insulating in nature in its bulk form. Destabilizationof polaronic order in the ferromagnetic insulating state due to enhanced surface disorderon grain size reduction is attributed to this effect. A phenomenological modelis proposed to represent the concept of destabilization of polaron formation inthe surface region of the nanograins. Resistivity and magnetoresistance data arecarefully analysed employing different suitable models. Electrical third harmonicresistance is measured to directly probe the electrical nonlinearity in the samples.

Selective substitution in orbital domains of a low doped manganite: an investigation fromGriffiths phenomenon and modification of glassy features

An effort is made to study the contrast in magnetic behavior resulting fromminimal disorder introduced by substitution of 2.5% Ga or Al in Mn site ofLa0.9Sr0.1MnO3. It is considered that Ga or Al selectively create disorder within the orbital domains oron its walls, causing enhancement of Griffiths phase (GP) singularity for theformer and disappearance of it in the latter case. It is shown that Ga replacesMn3+, which is considered to be concentrated within the domains, whereas Al replacesMn4+, which is segregated on the hole-rich walls, without causing any significant effect onstructure or ferromagnetic transition temperatures. Thus, it is presumed that the effect ofdisorder created by Ga extends across the bulk of the domain having correlation over asimilar length scale, resulting in enhancement of the GP phenomenon. In contrast, theeffect of disorder created by Al remains restricted to the walls, resulting in themodification of the dynamics arising from the domain walls and suppresses the GP.Moreover, contrasting features are observed in the low temperature region of thecompounds; a re-entrant spin-glass-like behavior is observed in the Ga-dopedsample, while the observed characteristics for the Al-doped sample are ascribedonly to modified domain wall dynamics with the absence of any glassy phase.Distinctive features in third-order susceptibility measurements reveal that themagnetic ground state of the entire series comprises of orbital domain states. Theseobservations bring out the role of the nature of disorder on the GP phenomenonand also reconfirms the character of self-organization in low doped manganites.

Paramagnetic spin dynamics in the nonhomogeneous crystals of low-doped lanthanum manganites

The X-band electron paramagnetic resonance (EPR) technique and the model analysis of the EPR linewidth versus temperature were employed to characterize the dopant distribution in a number of La-manganite crystals, low doped with Ca, Sr, and Ba. Such distribution appears to be inhomogeneous in accordance with the results of previous study of EPR susceptibility. The technological originated competition between the quenching and annealing effects during the cooling of the crystals from the high temperatures seems to be the reason of their inherent inhomogeneity. It depends strongly on the dopant nature and, in the crystal doped with 15% of Ba, the inhomogeneity is small as compared to those in the Ca- and Sr-doped samples. It is emphasized that such chemical disorder influences strongly the crystal structure, magnetic state, and transport of these objects.

Metal-insulator transition induced by postdeposition annealing in low doped manganite films

We studied the transport and magnetic properties of low-doped manganite films after different oxygenation processes. The oxygen content was adjusted by postdeposition annealing at different oxygen pressures and annealing times. For all the samples we observed an increase in the Curie temperature and the remnant magnetization with the oxygen content. In general, for decreasing number of oxygen vacancies, samples under expansive strain become more homogeneous and their electrical resistivity decreases. A metal-insulator transition is induced in highly oxygenated films grown on SrTiO3, probably related to a shift of the mobility edge crossing below the Fermi energy. We found that the oxygenation dynamics depend critically on the strain field induced by the substrates and also on the Sr doping concentration.

Inherent inhomogeneity in the crystals of low-doped lanthanum manganites

The X-band electron paramagnetic resonance (EPR) technique and the model analysis of the EPR susceptibility versus temperature were employed to characterize the dopant distribution in a number of La-manganite crystals, low doped with Ca, Sr, and Ba in a La site. Such distribution appears to be inhomogeneous as a result of technological-driven effect. It is emphasized that the above chemical disorder influences strongly both magnetic state and transport of the low-doped manganite crystals.

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 77K 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.

Anomalous thermal properties and local lattice effects in low doped manganites La0.875Ca0.125MnO3 and La0.875 Ca0.0625 Sr0.0625MnO3

The results of a comparative study of the magnetic, transport and thermal properties of two ferromagnetic La0.875Ca0.125MnO3(LCMO) and La0.875Ca0.0625Sr0.0625MnO3 (LCSMO) samples with the same Mn3+/Mn4+ ratio and identical crystallographic structure have been presented. While LCSMO exhibits magnetic transition accompanied by an insulator to metal transition, no such transition is observed in LCMO. Moreover, unlike LCMO, only in the case of LCSMO, is a sudden drop in the thermoelectric power S(T) along with an enhancement in the value of thermal conductivity λ(T) observed below the Curie temperature (TC). This confirms that not only the Mn3+/Mn4+ ratio but also local lattice effects and carrier transport play a vital role in determining in the physical properties of these low doped manganite systems.

Magnonic crystal theory of the spin-wave frequency gap in low-doped manganites

A theory of three-dimensional (3D) magnonic crystal (conceived as the magnetic counterpart of the photonic crystal) is developed and applied to explain the existence of the spin-wave frequency gap, recently revealed by neutron scattering experiments, in low-doped La1−xCaxMnO3 manganites. A confrontation of the theory with experimental results proves that certain manganites can be regarded as 3D magnonic crystals existing in the nanoscale in nature.

Intrinsic metastability of low doped manganites: La0.8Ca0.2MnO3 case

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.

Correlated magnetism in low-doped manganites

In this article, resonant and high-energy X-ray diffraction studies together with magnetization and transport measurements on lightly doped manganites will be presented. The main focus is put on the metal–insulator transition in La 1 - x Sr x MnO 3 with 0.10 < x < 0.15 and, in particular, on the stabilization of the ferromagnetic insulating (FMI) phase of these compounds. The experimental results reveal an orbital rearrangement at the metal–insulator transition which is tightly connected to the magnetic degrees of freedom. More precisely, the double exchange mechanism is found to be essential for the stabilization of the FMI phase. This conclusion is further supported by the studies on ( La 1 - y Pr y ) 7 / 8 Sr 1 / 8 MnO 3 and the layered compound La 7 / 8 Sr 9 / 8 MnO 4 .