Phase-separated Manganites Research Articles

Influence of pressure on magnetization and magnetostriction jumps in the manganite (Eu1−xGdx)0.6Sr0.4MnO3(x=0,0.1)

We have demonstrated the influence of external pressure on magnetization jump in Gd substituted Eu0.58Sr0.42MnO3. The collapse of magnetization jump of the Eu based manganite caused by the applied pressure up to 1.2 GPa is explained by a suppression of the metastable blocked state, which is inherent to the phase separated manganite system exhibiting the magnetic avalanche. On the other hand, the magnetization jump of the Gd substituted sample is enhanced under the same pressure. To further examine the nature of the magnetization jump, the influence of pressure on the isothermal magnetostriction of the parent manganite is carried out. These findings indicate the crucial role on the abrupt transition played by the frozen phase separated phase.

Evidence of glassy ferromagnetic phase and kinetic arrest of electronic phase in Sm0.35Pr0.15Sr0.5MnO3 manganites

The effect of doping of rare earth Pr3+ ion as a replacement of Sm3+ in Sm0.5Sr0.5MnO3 is investigated. Temperature dependent dc and ac magnetic susceptibility, resistivity, magnetoresistance measurements on chemically synthesized (Sm0.5−xPrx)Sr0.5MnO3 show various unusual features with doping level x=0.15. The frequency independent ferromagnetic to paramagnetic transition at higher temperature (∼191K) followed by a frequency dependent reentrant magnetic transition at lower temperature (∼31K) has been observed. The nature of this frequency dependent reentrant magnetic transition is described by a critical slowing down model of spin glasses. From non-linear ac susceptibility measurements it has been confirmed that the finite size ferromagnetic clusters are formed as a consequence of intrinsic phase separation, and undergo spin glass-like freezing below a certain temperature. There is an unusual observation of a 2nd harmonic peak in the non-linear ac susceptibility around this reentrant magnetic transition at low temperature (∼31K). Arrott plots at 10 and 30K confirm the existence of glassy ferromagnetism below this low temperature reentrant transition. Electronic- and magneto-transport measurements show a strong magnetic field—temperature history dependence and strong irreversibility with respect to the sweeping of magnetic field. These results are attributed to the effect of phase separation and kinetic arrest of the electronic phase in this phase separated manganite at low temperatures.

The dynamics of magnetization in phase separated manganite around half doping: A case study for Pr0.5Sr0.5Mn0.925Ga0.075O3

We investigate the magnetization dynamics in a phase separated half-doped manganite after introducing nonmagnetic quenched disorder Ga at Mn-site. The compound, Pr0.5Sr0.5Mn0.925Ga0.075O3, exhibits a new magnetic anomaly at low temperature. Our detailed study shows this phase arises due to thermal blocking of embedded nanometer size ferromagnetic clusters, demonstrating superparamagnetic behavior. The system, however, exhibits slow magnetic relaxation, aging and memory effect which are believed due to collective behavior induced by the intercluster interactions. Our results imply that the dynamics can be explained by the hierarchical model rather than droplet model which are commonly used for spin glass dynamics.

Magnetocapacitance of La1−xCaxMnO3/SrTiO3: Nb junctions with different Ca contents

Manganite-based heterojunctions La1−xCaxMnO3/SrTiO3: Nb (0.5 wt. %) with x = 0.2, 0.3, 0.375, 0.4, 0.5, 0.6, and 0.7 have been fabricated, and the lattice constants of La1−xCaxMnO3 films are experimentally studied. The capacitance characteristics of those junctions under a magnetic field have been investigated. Obvious magnetocapacitance was observed in La0.7Ca0.3MnO3/Nb-SrTiO3 p-n junction near the Curie temperature (TC) of La0.7Ca0.3MnO3 at 0.75 T. The result can be explained by phase separation. This study demonstrates the remarkable tunability of the capacitance for phase separated manganite heterojunctions, which may lead to some potential applications.

Magnetoelastic response of La0.7Sr0.3MnO3/SrTiO3superlattices to reversible strain

The influence of an electrically controlled biaxial in-plane strain on the magnetization of superlattices of ferromagnetic La${}_{0.7}$Sr${}_{0.3}$MnO${}_{3}$ and SrTiO${}_{3}$ was studied for single-layer thicknesses of $d$ = 1.5--13 nm. Superlattices were grown by pulsed laser deposition on both SrTiO${}_{3}$(001) and piezoelectric 0.72Pb(Mg${}_{1/3}$Nb${}_{2/3}$)O${}_{3}$--0.28PbTiO${}_{3}$(001), or PMN-PT(001), substrates and have been structurally characterized by x-ray diffraction (XRD) and transmission electron microscopy. Grazing-incidence XRD reveals the vertical homogeneity of the piezoelectrically controlled reversible in-plane strain, even in a 600-nm-thick superlattice containing 100 oxide interfaces. The as-grown strain is almost identical in all superlattices that are coherently grown, with small variations resulting from the partially relaxed growth of the first La${}_{0.7}$Sr${}_{0.3}$MnO${}_{3}$ layer on PMN-PT(001). The magnetic transition temperature decreases with the layer thickness $d$ as a consequence of the finite layer thickness, and the strain-induced response of the magnetization changes its character from that of a long-range-ordered ferromagnet to that of a magnetically disordered (possibly electronically phase-separated) manganite. The strain response of a modified interface layer (``dead layer'') of the thickness ${d}_{I}$ is distinguished from that of the layer's interior by its different temperature dependence, allowing an estimation of 10 \AA{} ${d}_{I}$ 16 \AA{} for the superlattices on PMN-PT.

Percolation transport and magnetic transitions in a phase-separated manganite La0.55Ca0.45MnO3

The structural, electronic and magnetic properties of La0.55Ca0.45MnO3 were measured. A rapid change of lattice parameters appeared around 190 K associated with the d z 2 orbital ordering and charge ordering (CO) states that were reflected by both magnetization and resistivity. Great difference of magnetizations between the field-cooling (FC) and zero-field-cooling (ZFC) modes below the charge ordering temperature T CO in high magnetic field (H > 4 T) was clearly seen. A field of 5 T (threshold field) is sufficient to completely destroy the antiferromagnetic (AFM) CO state for FC mode in magnetization while it is not the case for ZFC mode. A much larger field (larger than 10 T from ZFC resistivity data) is needed to destroy the CO state for ZFC mode. This could be explained by the coexistence and transformation of two phases reported by Huang et al. For ZFC mode, with increasing H, T CO gradually moves to low temperature regime and the relationship between the critical field H C (0) to destroy CO state and T CO conforms to a power law H C=H C(0)(1−T/T CO(0)) γ , where H C (0) is the critical field to destroy the CO state at 0 K, and T CO (0) is the CO temperature in zero field.

Out-of-plane low field anisotropic magnetoresistance in Nd 0.51Sr 0.49MnO 3 thin films

A comparative study of the out-of-plane anisotropic magnetoresistance (AMR) in single crystalline and polycrystalline thin films of phase separated manganite Nd 0.51Sr 0.49MnO 3 has been carried out. On-axis DC magnetron sputtering was used to deposit the single crystalline films (30 and 100 nm in thickness) on single crystal (0 0 1) LaAlO 3 (LAO) and polycrystalline films (100 nm) on (1 0 0) Yttrium-stabilized ZrO 2 (YSZ) substrates. The in-plane and out-of-plane magnetotransport properties of these films differ significantly. A large low field AMR is observed in all the films. AMR shows a peak below the insulator–metal transition temperature in the single crystalline films, while the same increases monotonically in the polycrystalline film. Relatively larger low field AMR (∼20% at T=78 K and H=1.7 kOe) in the polycrystalline films suggests the dominance of the shape anisotropy.

Dynamics of a first-order electronic phase transition in manganites

By reducing an electronically phase-separated manganite (${\mathrm{La}}_{1\ensuremath{-}y}{\mathrm{Pr}}_{y}$)${}_{x}{\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{MnO}}_{3}$ single-crystal thin film to dimensions on the order of the inherent phase domains, it is possible to isolate and monitor the behavior of single domains at a first-order transition. At this critical point, it is possible to study the coexistence, formation, and annihilation processes of discrete electronic phase domains. With this technique, we make several observations on the mechanisms leading to the metal-insulator transition in manganites. We observe that domain formation is emergent and random, the transition process from the metallic phase to the insulating phase takes longer than the reverse process, electric field effects are more influential in driving a phase transition than current-induced electron heating, and single domain transition dynamics can be tuned through careful application of temperature and electric field.

Single domain to multidomain transition due to in-plane magnetic anisotropy in phase-separated (La0.4Pr0.6)0.67Ca0.33MnO3thin films

Phase separated perovskite manganites have competing phases with different crystal structures, magnetic and electronic properties. Hence, strain effects play a critical role in determining the magnetic properties of manganite thin films. Here we report the effect of anisotropic stress on the magnetic properties of the phase separated manganite (La$_{0.4}$Pr$_{0.6}$)$_{0.67}$Ca$_{0.33}$MnO$_{3}$. Thin films of (La$_{0.4}$Pr$_{0.6}$)$_{0.67}$Ca$_{0.33}$MnO$_{3}$ grown under anisotropic in-plane stress on (110) NdGaO$_{3}$ substrates display in-plane mangetic anisotropy and single domain to multidomain transition as a function of temperature. Angle dependent magnetization measurements also show that the magnetization reversal occurs mainly through the nucleation $&$ propagation mechanism. By comparing the results with (La$_{0.4}$Pr$_{0.6}$)$_{0.67}$Ca$_{0.33}$MnO$_{3}$ thin films grown on (001) SrLaGaO$_{4}$ substrates, we have confirmed that the magnetic anisotropy is mainly due to substrate induced anisotropic stress. Our results suggest novel avenues for storing magnetic information in nanoscale magnetic media.

Correlation of electrical, magnetic, and thermal properties of the La0.85Ag0.15MnO3 manganite near the phase transition temperature

The correlation between electrical, magnetic, and lattice properties of manganites has been studied by analyzing the temperature dependence of the electrical resistivity of the La0.85Ag0.15MnO3 manganite. The results have been discussed within the model of a phase-separated manganite in terms of the percolation theory. It has been shown that, from analyzing the temperature and magnetic field dependences of the electrical resistivity, it is possible to predict the behavior of the magnetization and specific heat, as well as the change of magnetic entropy near the phase transition temperature, and that the obtained agreement with experimental data for this analysis is quite reasonable.

Giant magnetic domain-wall resistance in phase-separated manganite films

The top and bottom electrodes of the ferromagnetic manganite La0.7Ca0.3MnO3 may be magnetically decoupled by a La0.59Ca0.41MnO3 interlayer in which ferromagnetic and paramagnetic phases coexist. In mesas fabricated from these trilayer films, in-plane magnetic domain walls associated with the antiparallel magnetic electrode configuration show a giant resistance-area product >10−10 Ω m2. This is almost two orders of magnitude larger than our devices with interlayer composition La0.6Ca0.4MnO3 [C. Israel et al., Phys. Rev. B 78, 054409 (2008)] where phase separation is less extreme. High-field treatments modify the ferromagnetic phase fraction of the interlayer.

Highly anisotropic resistivities in the double-exchange model for strained manganites

The highly anisotropic resistivities in strained manganites are theoretically studied using the two-orbital double-exchange model. At the nanoscale, the anisotropic double-exchange and Jahn-Teller distortions are found to be responsible for the robust anisotropic resistivities observed here via Monte Carlo simulations. An unbalanced in the population of orbitals caused by strain is responsible for these effects. In contrast, the anisotropic superexchange is found to be irrelevant to explain our results. Our model study suggests that highly anisotropic resistivities could be present in a wide range of strained manganites, even without (sub)micrometer-scale phase separation. In addition, our calculations also confirm the formation of anisotropic clusters in phase-separated manganites, which magnifies the anisotropic resistivities.

Magnetothermal cooling with a phase separated manganite

We show that temperature of a current (I=20 mA) carrying manganite (Nd0.5Ca0.5Mn0.93Ni0.07O3) in presence of a magnetic field (H) decreases abruptly as much as ΔT=45 K (7 K) accompanied by a steplike decrease in magnetoresistance at a critical value of H when the base temperature is 40 K (100 K). The magnitude of ΔT and the position of magnetoresistance step decrease toward lower H with decreasing amplitude of the current. We discuss possible origins of the current and magnetic-field driven temperature change which may find applications in magnetothermal refrigeration besides magnetocaloric effect.

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.

Magnetic and transport properties of phase-separated manganite [formula omitted

We have investigated the magnetic and transport properties of Bi 0.1La 0.5Ca 0.4MnO 3 manganite, systematically. Four distinct feature temperature points in the temperature dependence of the magnetization curve M ( T ) occurring at ∼ 218 K , ∼ 181 K , ∼ 112 K and ∼ 38 K are observed; these are suggested to be related to charge ordered (CO), weak ferromagnetic (FM), strong FM, and blocked metastable state. These temperature points can be changed by applying magnetic fields. The field-cooled (FC) M ( T ) curves show an obvious thermal hysteresis between the FC cooling (FCC) and FC warming (FCW) measuring process. The large variation between the FC and zero-field-cooled (ZFC) magnetization curves at low temperatures reflects the existence of blocked metastable states separated by high energy barriers. The blocked state can be weakened or eventually destroyed by applying magnetic fields. The field dependence of the magnetization and resistivity at both 5 K and 130 K shows that the magnetic and electrical transport properties are tightly correlated with the same critical field. The results indicate that CO and FM phases coexist in the sample at low temperatures. The local lattice distortion induced by the Bi 3+ doping may play an important role in the complex magnetic and transport properties of the sample.

Elastically driven anisotropic percolation in electronic phase-separated manganites

Complex oxide films are highly anisotropic in the way they conduct electricity, which is due to phase separation. However, the origin of this metal–insulator phase coexistence has been unclear. Transport measurements now show that strain, rather than chemical inhomogeneity, is mainly responsible. The presence of electronic phase separation in complex materials has been linked to many types of exotic behaviour, such as colossal magnetoresistance, the metal–insulator transition and high-temperature superconductivity1,2,3,4; however, the mechanisms that drive the formation of coexisting electronic phases are still debated5,6,7,8. Here we report transport measurements that show a preferential orientation of electronic phase domains driven by anisotropic long-range elastic coupling between a complex oxide film and substrate. We induce anisotropic electronic-domain formation along one axis of a pseudocubic perovskite single-crystal thin-film manganite by epitaxially locking it to an orthorhombic substrate. Simultaneous temperature-dependent resistivity measurements along the two perpendicular in-plane axes show substantial differences in the metal–insulator transition temperature and extraordinarily high anisotropic resistivities, which indicate that percolative conduction channels open more readily along one axis. These findings suggest that the origin of phase coexistence is much more strongly influenced by strain than by local chemical inhomogeneity.

Enhanced magnetic refrigeration capacity in phase separated manganites

Multiple phase transitions and magnetic phase coexistence lead to a negative magnetocaloric effect in a wide temperature range of ∼10–250 K in La0.215Pr0.41Ca0.375MnO3. A large fraction of the magnetocaloric effect originates from the low temperature phase separated state, which is composed of coexisting, magnetic field dependent charge ordered, and ferromagnetic regions. While the maximum isothermal entropy change is modest, the persistence of the field-dependent phase separated state over a ∼240 K temperature span yields a refrigeration capacity of ∼3.2 J/cm3. Materials with magnetic field dependent phase separation can therefore be used to improve regenerative magnetic refrigerators.

HIGH-FREQUENCY RESPONSE AND VOLTAGE NOISE IN MAGNETIC NANOCOMPOSITES

We study the noise spectra and high-frequency permeability of inhomogeneous magnetic materials consisting of single-domain magnetic nanoparticles embedded into an insulating matrix. Possible mechanisms of 1/f voltage noise in phase-separated manganites is analyzed. The material is modelled by a system of small ferromagnetic metallic droplets (magnetic polarons or ferrons) in insulating antiferromagnetic or paramagnetic matrix. The electron transport is related to tunnelling of charge carriers between droplets. One of the sources of the 1/f noise in such a system stems from fluctuations of the number of droplets with extra electron. In the case of strong magnetic anisotropy, the 1/f noise can arise also due to the fluctuations of the magnetic moments of ferrons. The high frequency magnetic permeability of nanocomposite film with magnetic particles in insulating non-magnetic matrix is studied in detail. The case of strong magnetic dipole interaction and strong magnetic anisotropy of ferromagnetic granules is considered. The composite is modelled by a cubic regular array of ferromagnetic particles. The high-frequency permeability tensor components are found as a functions of frequency, temperature, ferromagnetic phase content, and magnetic anisotropy. The results demonstrate that magnetic dipole interaction leads to a shift of the resonance frequencies towards higher values, and nanocomposite film could have rather high value of magnetic permeability in the microwave range.

Magnetocaloric effect in phase separated manganite probed by differential thermal analysis

Differential thermal analysis is used to measure the temperature change (ΔT) due to the application of an external magnetic field in a manganite with phase separation. The temperature dependence of ΔT is compared with the expected from magnetization measurements. A phenomenological model for the enthalpy change is used to obtain ΔT during the metamagnetic transition.

Current induced electroresistance in Nd 0.5Ca 0.5Mn 0.95Ni 0.05O 3

We have investigated the dc and pulsed current induced electroresistance in phase separated manganite Nd 0.5Ca 0.5Mn 0.95Ni 0.05O 3 (NCMONi05) as a function of temperature and magnetic field. It is shown that the negative differential resistance which appears above a threshold current ( I c ) and hysteresis in the V – I progressively vanish with increasing period of the current pulses. However a strong nonlinearity in V – I exists even for a pulse period of 6 s. The peak voltage at I c decreases in magnitude and shifts towards higher current values with increasing strength of the magnetic field. The strong nonlinear behavior and the negative differential resistance in the dc current sweep are accompanied by a rapid increase of the sample surface temperature and therefore primarily arise from the Joule heating in the sample. While the Joule heating assists electroresistance in the high dc current regime, the nonlinearity in the pulsed current sweep and the resistivity switching between a high and low value induced by controlling the width and period of pulses cannot be explained solely on the basis of Joule heating.