Phase-separated Manganites Research Articles

Coexistence of volatile and nonvolatile memristive effects in phase-separated La0.5Ca0.5MnO3-based devices

In this work, we have investigated the coexistence of volatile and nonvolatile memristive effects in epitaxial phase-separated La0.5Ca0.5MnO3 thin films. At low temperatures (50 K), we observed volatile resistive changes arising from self-heating effects in the vicinity of a metal-to-insulator transition. At higher temperatures (140 and 200 K), we measured a combination of volatile and nonvolatile effects arising from the synergy between self-heating effects and ferromagnetic-metallic phase growth induced by an external electrical field. The results reported here add phase separated manganites to the list of materials that can electrically mimic, on the same device, the behavior of both neurons and synapses, a feature that might be useful for the development of neuromorphic computing hardware.

Multifunctionality of Phase-separated Manganites

Multifunctionality of Phase-separated Manganites

Controlling thermal cycling effect in phase separated manganites with high temperature thermal treatments

Several phase separated manganites present a peculiar effect each time they go through a phase transition within the range characterized by phase separation. The effect is known as the thermal cycling effect (TCE) and is characterized by monotonous changes in the relative content of the coexisting phases.In this work, we analyze a way to control the effects induced by TCE, performing thermal treatments at high temperature. Our results revealed a complex interplay between the dynamic and static characteristics of the phase separated state, which can be analyzed in terms of three simple parameters. One related to the static properties, another to the dynamic properties and a last one that acts as a link between both features.

Evolution of the magnetic and charge orders in La0.15Pr0.45Ca0.40MnO3: Assessing the role of particle size and magnetic field

The evolution of the ferromagnetic (FM)/antiferromagnetic (AFM) orders and charge order (CO) as a function of the particle size and applied magnetic field (H) has been studied in detail in the canonical phase-separated system La1−x−yPryCaxMnO3 with larger Pr content (La0.15Pr0.45Ca0.40MnO3). At lower particle size (~ 45 nm) a low TC FM phase dominates with no direct evidence of the AFM and CO phases in the magnetization (M) data. As the particle size increases, the AFM and CO grow with a concomitant decrease in the FM fraction, as shown by a decrease in the magnetization and explicit transitions. All three electronic orders become distinctly visible at particle size ~ 139 nm, with TCO ~ 213 K, TN ~ 173 K, and TC ~ 58 K. At larger sizes, e.g., ~ 370 nm, FM appears as the robust phase while the AFM and CO are quenched. However, the presence of the AFM/CO clusters in this sample is signed by the hysteresis in the FCC and FCW magnetization in the FM regime. The paramagnetic (PM) region susceptibility as a function of temperature exhibits scaling of the type χ=C(T-TCON)γ, where TCON is the temperature at which the FM response becomes visible. The exponent remains γ ≈ 1.17 for samples with less dominant CO. At the ~ 139 nm, the CO is the most robust, and the exponent takes a value γ ≈ 1.31. The effect of the magnetic field in the case of the ~ 139 nm samples shows that the AFM melts at HAFM ~ 40 kOe, while the CO melts at HCO ~ 50 kOe. The scaling of the paramagnetic regime susceptibility (H/M) at different H shows that γ remains confined to the range 1.30–1.34 up to 20 kOe, and then decreases slightly to ≈1.24 at 30 kOe. At the AFM melting magnetic field ~ 40 kOe, it drops to ≈1.13, and at further higher fields, the above scaling breaks down. Thus TCON remains a good substitute for the canonical TC in the scaling law χ=C(T-TCON)γ in strongly phase-separated manganite. Our results show that a phase-separated system with FM dominance deviates from the Curie-Weiss law valid for canonical ferromagnets. However, the presence of explicit CO makes the H/M – T scaling akin to a 3D Heisenberg ferromagnet.

Nonmonotonic crossover in electronic phase separated manganite superlattices driven by the superlattice period

Studying manganite superlattices ${[{(\mathrm{LCMO})}_{2n}/{(\mathrm{PCMO})}_{n}]}_{t}$ made of ${\mathrm{La}}_{0.625}{\mathrm{Ca}}_{0.375}\mathrm{Mn}{\mathrm{O}}_{3}$ (LCMO) and ${\mathrm{Pr}}_{0.625}{\mathrm{Ca}}_{0.375}\mathrm{Mn}{\mathrm{O}}_{3}$ (PCMO), we found an unexpected behavior varying the period $n$. At small $n$, the ensemble is a three-dimensional ferromagnetic metal due to interfacial charge transfer. At large $n$, the LCMO layers dominate transport. However, rather than a smooth interpolation between these limits a sharp transport and magnetic anomaly is found at an intermediate critical PCMO thickness ${n}^{*}$. Magnetic force microscopy reveals that the phase-separation length scale also maximizes at ${n}^{*}$ where, unexpectedly, it becomes comparable to that of the ${({\mathrm{La}}_{1--y}{\mathrm{Pr}}_{y})}_{0.625}{\mathrm{Ca}}_{0.375}\mathrm{Mn}{\mathrm{O}}_{3}$ (LPCMO) alloy. We conjecture the phenomenon originates in a disorder-related length scale: Large charge-ordered clusters as in LPCMO can only nucleate when Pr-rich regions reach a critical size related to ${n}^{*}$.

Dynamic percolation of ferromagnetic regions in phase separated manganites using non-uniform electric fields

Thin films of the manganite (La1−yPry)1−xCaxMnO3 exhibit dynamic phase coexistence with micrometer scale, fluid-like ferromagnetic metallic (FMM) regions interspersed in a charge-order insulating background. It has been previously reported that a uniform electric field realigns the fluid-like FMM regions due to a phenomenon similar to dielectrophoresis. Here, we report that non-uniform electric fields have a stronger effect on the FMM regions as expected from the dielectrophoresis model. The dynamic percolation of the FMM regions is observed over a wider range of temperatures compared to the results in a uniform field. Additionally, in a non-uniform electric field, the time required for dynamic percolation along the magnetic hard axis (tB) decreased with increasing applied voltage (VA) as a power law, VA−δ, with δ≈5 while δ<2 for a uniform electric field. Our results in a non-uniform electric field provide strong evidence in favor of the dielectrophoresis model and a unique method for manipulating micrometer-sized ferromagnetic regions using electric fields.

Irreversible heat flow across phase boundaries in phase-separated manganites

We have investigated the change in entropy with direct measurements of heat flow as a function of magnetic field at fixed temperatures across the entire phase diagram of the phase-separated (PS) compound La 0.25 Pr 0.375 Ca 0.375 MnO 3 (LPCMO). At this composition, the compound shows competing charge-ordered/antiferromagnetic (CO/AF) ground states. Under a fixed temperature, we observed an increase in hysteresis in the entropy as a function of the applied field. The heat flux shows progressively irreversible hysteresis, which characterizes the energy barriers between the two competing ground states, as the temperature is lowered. The increase in the heat loss correlates with the increase in magnetic viscosity in the phase-separated state. • Differential scanning calorimetry was used to study the magnetic phase diagram of La 0.25 Pr 0.375 Ca 0.375 MnO 3 . • The irreversible heat flow of the phase transitions in a manganite was investigated and compared to the magnetic dynamics. • The heat loss at the strain glass transition closely followed an increase in magnetic viscosity.

Suppression of Spin Glass Behavior in Phase Separated La0.22Pr0.40Ca0.38MnO3 by Nanostructuring

Spin glass behavior of phase-separated (PS) manganite La0.22Pr0.40Ca0.38MnO3 is investigated as a function of crystallite size. Various sizes are obtained by heating the chemically synthesized compound at 700 °C (S7) and 1100 °C (S11) for 12 h. Rietveld refinement confirms the single-phase orthorhombic structure. The HRTEM analysis yield typical crystallite size in the range of ~10–20 nm for S7 and ~300–500 nm for S11, respectively. At larger crystallite size (S11) charge ordering (CO), antiferromagnetic (AFM), and ferromagnetic (FM) transitions are observed sequentially at $T_{\mathrm {CO}} \sim 230$ K, $T_{N} \sim ~175$ K, and $T_{C} \sim 130$ K, respectively. Spin freezing appears at $T K while a weak reentrant magnetic order is seen at further lower temperatures. At nanoscale (S7) CO and AFM transitions do not remain explicit and $T_{C}$ and $T_{P}$ are lowered to ~107 and ~96 K, respectively. In S11, the frequency dispersion of imaginary part spin freezing temperature ( $T_{f} \approx 104$ K) in the Vogel–Fulcher framework yields a relaxation time $\tau \approx 2.5 \times 10 ^{-9}$ s, activation energy $E_{a} \approx 25$ meV and VF parameter $T_{0} \sim 87$ K. These results point to the canonical spin-glass behavior. It is interesting to note that the nanostructured sample S7 did not show any frequency dispersion. This confirms the suppression of the spin-glass behavior due to the nanostructuring, which is attributed to the surface/interface enhanced spin disorder.

Meyer-Neldel rule in the conductivity of phase separated manganites

Abstract Meyer-Neldel behaviour of the conductivity of phase separated La1−xCaxMnO3 manganite system in the low Ca-doping range has been investigated. Evolution of the isokinetic temperature of the conductivity, modified by Ca-doping, hydrostatic pressure and current bias has been determined. In addition, the evolution of the isokinetic temperature with ageing has also been studied. It is found that the Meyer-Neldel behaviour of the manganite system stems from multi-excitation entropy mechanism. The isokinetic temperatures estimated from pressure and doping effects coincide but differ from those determined using current and ageing controlled conductivity changes. It is concluded that in the presence of a detailed theoretical model of the excitations coupling in manganites, the investigations of the Meyer-Neldel effect may became a powerful tool for characterization and investigation of transport mechanisms in phase separated manganites.

Tuning of Al doping on martensitic-like transition in phase separated manganites La0.6Ca0.4Mn1-xAlxO3-δ

The tuning effects of Al3+ doping on the magnetic and magnetotransport properties of La0·6Ca0·4MnO3-δ have been studied by preparing series La0·6Ca0·4Mn1-xAlxO3-δ (x = 0, 0.1, 0.125 and 0.15). Al3+ doping suppresses the ferromagnetic metal phase of La0·6Ca0·4MnO3-δ by weakening the double-exchange interaction of Mn3+-O2--Mn4+, leading to a phase separation state. Al3+ doping dependent irreversible metamagnetic transitions are observed at lower temperatures, especially a martensitic-like transition appears in x = 0.125 and 0.15 system below 4.2 K. Phase diagrams of La0·6Ca0·4Mn1-xAlxO3-δ (0.1 ≤ x ≤ 0.15) in the B-T plane are presented, in which the antiferromagnetic insulator and ferromagnetic metal phases boundary is figured out. The antiferromagnetic matrix and magnetic disorders caused by the random distribution of Al3+ ions and the spontaneous oxygen vacancies grow into a pinning center to pin the Mn moments. As magnetic field is increased to the critical field, the localized magnetic moments rotate to parallel to the magnetic field direction, leading to a martensitic-like transition.

Thermal cycling memory in phase separated manganites

Fil: Sievers, Bernardo Patricio. Comision Nacional de Energia Atomica. Centro Atomico Constituyentes; Argentina. Comision Nacional de Energia Atomica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. - Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia; Argentina

Unexpected Intermediate State Photoinduced in the Metal-Insulator Transition of Submicrometer Phase-Separated Manganites.

At ultrafast timescales, the initial and final states of a first-order metal-insulator transition often coexist forming clusters of the two phases. Here, we report an unexpected third long-lived intermediate state emerging at the photoinduced first-order metal-insulator transition of La_{0.325}Pr_{0.3}Ca_{0.375}MnO_{3}, known to display submicrometer length-scale phase separation. Using magnetic force microscopy and time-dependent magneto-optical Kerr effect, we determined that the third state is a nanoscale mixture of the competing ferromagnetic metallic and charge-ordered insulating phases, with its own physical properties. This discovery bridges the two different families of colossal magnetoresistant manganites known experimentally and shows for the first time that the associated states predicted by theory can coexist in a single sample.

Thermal cycling effects on static and dynamic properties of a phase separated manganite

In this work we address the interplay between two phenomena which are signatures of the out-of-equilibrium state in phase separated manganites: irreversibility against thermal cycling and aging/rejuvenation process. The sample investigated is La0.5Ca0.5MnO3, a prototypical manganite exhibiting phase separation. Two regimes for isothermal relaxation were observed according to the temperature range: for T > 100 K, aging/rejuvenation effects are observed, while for T < 100 K an irreversible aging was found. Our results show that thermal cycles act as a tool to unveil the dynamical behavior of the phase separated state in manganites, revealing the close interplay between static and dynamic properties of phase separated manganites.

Evolution of Intrinsic and Magnetic Field-Induced Magnetic Anisotropies in Strongly Phase-Separated Manganite Thin Films

Single crystalline thin film (∼ 100 nm) of La0.18Pr0.40Ca0.42MnO3 is grown on (001) oriented LaAlO3 substrates and the evolution of anisotropy associated with the supercooling behaviour of the magnetic liquid is studied as a function of temperature and magnetic field. The angle-dependent magnetization measurements ascertain that the easy magnetic axis lies in the plane of the film while the hard axis is along the plane normal. The ratio of the easy and hard axes magnetizations (M|| and M⊥) measured at 10 K, viz., M||/M⊥ (10 K) = 2.6, confirms the strongly anisotropic nature of the film. The easy axis ferromagnetic (FM) transition temperature (TC) is smaller than that along the hard axis. The giant hysteresis in FCC–FCW M–T, which manifests the magnetic liquid behaviour of the strongly phase-separated manganites is appreciably narrowed along the hard axis. The strain glass state is also less dominant along the easy axis. The analysis of the M–H data brings out that the easy axis remanence (Mr||) shows a nonlinear temperature dependence, while the one corresponding to the hard axis (Mr⊥) shows a nearly linear behaviour. The coercivity (HC) follows a law of the type \(H_{\mathrm {C}}\left (T \right )\mathrm {=}H_{\mathrm {C}}\mathrm {(0)(1-}{\text {AT}}^{b}\mathrm {)}\). The value of exponent b ∼ 0.5 for single-domain particles, but in the present case, the best fit to the experimental data yields the exponent b ≈ 0.25 for both HC|| and HC⊥. The smaller value of the exponent is attributed to the non-canonical nature of the ferromagnetic state and the associated strongly phase-separated nature of the LPCMO thin film.

Quantum Percolation and Magnetic Nanodroplet States in Electronically Phase-Separated Manganite Nanowires.

One-dimensional (1D) confinement has been revealed to effectively tune the properties of materials in homogeneous states. The 1D physics can be further enriched by electronic inhomogeneity, which unfortunately remains largely unknown. Here we demonstrate the ultrahigh sensitivity to magnetic fluctuations and the tunability of phase stability in the electronic transport properties of self-assembled electronically phase-separated manganite nanowires with extreme aspect ratio. The onset of magnetic nanodroplet state, a precursor to the ferromagnetic metallic state, is unambiguously revealed, which is attributed to the small lateral size of the nanowires that is comparable to the droplet size. Moreover, the quasi-1D anisotropy stabilizes thin insulating domains to form intrinsic tunneling junctions in the low temperature range, which is robust even under magnetic field up to 14 T and thus essentially modifies the classic 1D percolation picture to stabilize a novel quantum percolation state. A new phase diagram is therefore established for the manganite system under quasi-1D confinement for the first time. Our findings offer new insight into understanding and manipulating the colorful properties of the electronically phase-separated systems via dimensionality engineering.

Dynamics of the lattice and spins in the phase-separated manganite (Eu1−xGdx)0.6Sr0.4MnO3

We investigated slow relaxations of the magnetostriction and residual magnetostriction of the phase-separated system (Eu1−xGdx)0.6Sr0.4MnO3, in which the metamagnetic transition from a paramagnetic insulating state to a ferromagnetic metallic state is accompanied by a lattice shrinkage. The relaxations are well fitted by a stretched exponential function, suggesting the strong frustration between the double exchange interaction and Jahn-Teller effect. We have revealed that the Gd substitution suppresses the frozen phase-separated phase at low temperatures and stabilizes the paramagnetic insulating state in the dynamic phase-separated phase at intermediate temperatures. The former origin would be the randomness effect and the latter would be the suppression of the double exchange interaction.

Magnetically induced nonvolatile magnetoresistance and resistance memory effect in phase-separated manganite thin films

We report the observation of magnetically induced resistance memory effect in a typical electronic phase-separated manganite La5/8−xPrxCa3/8MnO3 (x = 0.3) thin film. In the hysteresis region of metal-to-insulator transition, the resistance exhibits a sharp drop with the application of magnetic field and maintains the low resistance state after the removal of field, showing a nonvolatile magnetoresistance effect. The high resistance state can be recovered until the temperature is warmed. More explicit measurements at the hysteresis region exhibit the non-volatility and irreversibility of magnetoresistance, which can be ascribed to the percolative feature in the electronic phase-separated manganite. The origin and potential applications of these interesting effects are discussed.

Tailoring transport properties of phase-separated manganite films with ordered magnetic nanostructures

The magnetotransport properties of thin manganite films (${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$) coupled with arrays of permalloy (Py) nanodots deposited on the surface of the film are studied as a function of temperature, magnetic field, and the size of the dots. In the presence of the magnetic dots, a reduction of the electrical resistivity is observed, especially at the insulator-to-metal transition, as well as a shift of the transition peak towards higher temperatures. This indicates that, due to local interface exchange coupling, highly conductive ferromagnetic domains are nucleated in the manganite film underneath the Py nanodots. The use of a simplified resistor network model allows us to estimate the size of the metallic regions induced by exchange coupling. At low temperatures, these regions extend $\ensuremath{\sim}70\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$ beyond the edge of the nanodots, a length scale comparable to the correlation length of the ferromagnetic clusters in the phase-separated state of ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$.

Normal and inverse magnetocaloric effect and short-range ferromagnetic interaction in (Pr,Sm)0.5Sr0.5MnO3 phase separated manganite

The dependence of critical phenomenon on disorder induced by R/A-site cational size mismatch in polycrystalline manganite Pr0.5-xSmxSr0.5MnO3 (x = 0.25) has been investigated by means of bulk magnetization M (H, T) in the vicinity of the ferromagnetic-paramagnetic phase-transition. The critical exponents characterizing this second order phase transition, have been obtained using different methods such as modified Arrott plot, Kouvel-Fisher plot, critical isotherm analysis and field dependence of magnetocaloric effect. Moreover, the self-consistency and reliability of obtained critical exponents are verified by the Widom scaling law and scaling equations. We demonstrate that at x = 0.25 doping level, the β value match well with those theoretically predicted for the 3D-Heisenberg model, while the γ value is close to that expected for the 3D-Ising model. This suggests that the presence of R/A-site disorder in the system must be taken into account to fully describe the magnetism of phase separated manganites. Hence, the short-range ferromagnetic originates from phase inhomogeneity and evolves with addition of disorder depending on Sm content. Moreover, this sample also shows a coexistence of normal and inverse magnetocaloric effect.

Multimodal Responses of Self‐Organized Circuitry in Electronically Phase Separated Materials

Confining an electronically phase separated manganite film to the scale of its coexisting self-organized metallic and insulating domains allows resistor-capacitor circuit-like responses while providing both electroresistive and magnetoresistive switching functionality.