Bulk Manganite Research Articles

Epitaxial growth mechanism of heterogeneous catalytic oxidation of Mn(II) on manganite under oxic conditions

Manganese oxides are ubiquitous in soils and sediments, and their formation and transformation processes affect the migration and transformation of heavy metals and organic pollutants. Most previous work has been focused the oxidation of Mn(II) on Mn(IV) oxide surface, but it remains poorly understood how Mn(II) interacts with manganite under oxic conditions. Here, the interactions between manganite and dissolved Mn(II) within pH 5.0–9.0 under oxic conditions were systematically studied. The results indicated that adsorbed Mn(II) was directly oxidized to manganite on the surface, and the oxidation rate increased with increasing pH and oxygen partial pressure. Electrons are likely transferred in bulk manganite during the oxidation process of Mn(II). Although manganite induced Mn(II) oxidation and the epitaxial growth of bulk particles, there was no obvious change in mineral phase. Dissolved oxygen accelerated the growth of manganite in Mn(II) solution. Compared with the absence of manganite, the presence of manganite inhibited the formation of hausmannite from Mn(II) oxidation by oxygen in air. This study reveals the catalytic oxidation process and mechanism of Mn(II) on manganite surface, and improves the understanding of manganite crystal growth in supergene environments.

Synthesis and Magnetic properties of Bulk and NanoPr0.57Ca0.43Mn0.98Ga0.02O3 Manganites

In this paper we report the synthesis and magnetic properties of bulk and nanoPr0.57Ca0.43Mn0.98Ga0.02O3 (PCMGO) manganites. Bulk and nano samples of PCMGO were prepared by sol gel method. All the samples were characterized by X-ray diffraction (XRD) technique, scanning electron microscopy (SEM) and energy dispersive X-ray diffraction analysis (EDAX). The temperature dependent magnetic properties of the samples were studied using Vibrating Sample Magnetometer (VSM). We have observed an interesting feature in the magnetic properties of both bulk and nano PCMGM manganite. Bulk PCMGO was charge ordering around 245 K whereas charge ordering has been disappeared in PCMGO nano samples. Both bulk and nano PCMGO show ferromagnetic transition at low temperature. Hysteresis measurements confirm the ferromagnetic transition of the nanosamples. The effect of size reduction and the substitution of Gallium in Mn site, upon the magnetic properties of PCMGO will be discussed.

Cation Substitution Effect on a Molecular Analogue of Perovskite Manganites

A recently synthesized Mn-containing molecule Ce2IIICeIVMn8IIIO8(O2CPh)18(HO2CPh) (Ce3Mn8III) structurally resembles the repeating unit of a bulk perovskite manganite. This resemblance brings forth the intriguing possibility of studying physical properties and effects that appear in both the molecule and the far more complex bulk perovskites, for example, effects of cation substitution, which in bulk manganites may lead to phase transition from metallic-ferromagnetic to insulating-paramagnetic as well as the colossal magnetoresistance effect. We investigate divalent and trivalent cation substitution in Ce3Mn8III molecules and its effects on ground-state magnetic configuration using first-principles-based approaches. One MnIII ion changes its valence to MnIV after trivalent (including La, Gd) cation substitution, while four MnIII ions become MnIV upon divalent (including Ca, Sr, Ba, and Pb) cation substitutions, all accompanied by vanishing local Jahn–Teller distortion around MnIV. The valence state of MnIV induced by cation substitutions can hop among Mn sites in molecule, and the calculated energy barrier for such state hopping is 0.25 to 0.6 eV/molecule. In addition, the charging energies of the Ce3Mn8III molecule and its derivatives are found to be strongly dependent on the spin direction of added electron.

Structural and phase transition of Mg-doped on Mn-site in La0.7Sr0.3MnO3 bulk/nanostructured perovskite characterised through online ultrasonic technique

Bulk and nanoshaped perovskite manganite materials were blended to form samples of La0.7Sr0.3Mg1−xMnxO3 (LSMMO) with x = 0.05, 0.075, and 0.100 employing solid state and sonochemical methods, respectively. The well-formed LSMMO component was characterized by comprehensive characterization techniques and rhombohedral structure was observed. The mean size of the bulk and nano LSMMO manganite perovskites ranges between 260–850 nm and 23–86 nm, respectively. The structural and phase transition of the manganite perovskites are explored by on-line ultrasonic velocity and attenuation measurements between 300 and 400 K. A deliberate change in the ultrasonic velocity, attenuation and elastic moduli shows an abnormal behavior with temperature in perovskites and was associated with the occurrence of ferromagnetic–paramagnetic (FM–PM) transition temperature (TC). In addition, the shifts in TC magnitude and width by increase in x are used to study the etiquette of TC and the change in the nanostructure of the bulk perovskites.

Oxygen Isotope Evidence for Mn(II)-Catalyzed Recrystallization of Manganite (γ-MnOOH)

Manganese is biogeochemically cycled between aqueous Mn(II) and Mn(IV) oxides. Aqueous Mn(II) often coexists with Mn(IV) oxides, and redox reactions between the two (e.g., comproportionation) are well known to result in the formation of Mn(III) minerals. It is unknown, however, whether aqueous Mn(II) exchanges with structural Mn(III) in manganese oxides in the absence of any mineral transformation (similar to what has been reported for aqueous Fe(II) and some Fe(III) minerals). To probe whether atoms exchange between a Mn(III) oxide and water, we use a (17)O tracer to measure oxygen isotope exchange between structural oxygen in manganite (γ-MnOOH) and water. In the absence of aqueous Mn(II), about 18% of the oxygen atoms in manganite exchange with the aqueous phase, which is close to the estimated surface oxygen atoms (∼11%). In the presence of aqueous Mn(II), an additional 10% (for a total of 28%) of the oxygen atoms exchange with water, suggesting that some of the bulk manganite mineral (i.e., beyond surface) is exchanging with the fluid. Exchange of manganite oxygen with water occurs without any observable change in mineral phase and appears to be independent of the rapid Mn(II) sorption kinetics. These experiments suggest that Mn(II) catalyzes manganese oxide recrystallization and illustrate a new pathway by which these ubiquitous minerals interact with their surrounding fluid.

Giant enhancement of magnetocaloric effect at room temperature by the formation of nanoparticle of La0.48Ca0.52MnO3 compound

Magnetocaloric effect (MCE) is the change in temperature or entropy of a material due to the application of external magnetic field. The temperature dependence of MCE shows maximum value near the ferromagnetic transition of the material. Material with large MCE near room temperature will make revolutionary change in cooling technology and will have large impact in human society. We have transformed antiferromagnetic bulk manganite material La0.48Ca0.52MnO3 to ferromagnetic material by the reduction of the particle size in nanometer scale. MCE, as well as the Relative cooling power (RCP) of the material at room temperature (RT) systematically increase with the reduction of the particle sizes. The RCP value for the 45 nm particle size sample at RT is about 400% larger compared with the bulk counterpart. Our detail study on the nano form of the compound highlights an effective way to enhance the MCE. It opens up a vast possibility of achieving giant enhancement of MCE of several already known materials just by the reduction of the particle size in nanometer scale.

Reversible modulation of orbital occupations via an interface-induced polar state in metallic manganites.

The breaking of orbital degeneracy on a transition metal cation and the resulting unequal electronic occupations of these orbitals provide a powerful lever over electron density and spin ordering in metal oxides. Here, we use ab initio calculations to show that reversibly modulating the orbital populations on Mn atoms can be achieved at ferroelectric/manganite interfaces by the presence of ferroelectric polarization on the nanoscale. The change in orbital occupation can be as large as 10%, greatly exceeding that of bulk manganites. This reversible orbital splitting is in large part controlled by the propagation of ferroelectric polar displacements into the interfacial region, a structural motif absent in the bulk and unique to the interface. We use epitaxial thin film growth and scanning transmission electron microscopy to verify this key interfacial polar distortion and discuss the potential of reversible control of orbital polarization via nanoscale ferroelectrics.

Field induced ferromagnetic phase transition and large magnetocaloric effect in Sm0.55Sr0.45MnO3 phase separated manganites

(a) Magnetic field and particle size dependent T C of nanometric (black and red dot) and bulk (blue and wine dot) measured upon cooling (full dots) and warming (open dots). (b) Temperature dependence magnetic entropy change Δ S at different magnetic field calculated from the magnetization data of S1250 bulk sample. • Large magnetocaloric effect is reported based on the first order phase transition. • The value of |Δ S M | reaches a maximum value of 8.48 J/kg K at 60 kOe field. • A large relative cooling power (∼574 J/kg) around T C –172 K is reported. • The adiabatic temperature change is found to be 2.38 K for magnetic field of 10 kOe. • Noticeable |Δ S M | at low field makes the system useful for magnetic refrigeration. This paper reports about the magnetocaloric effect and relative cooling power based on the first order magnetic phase transition in Sm 0.55 Sr 0.45 MnO 3 polycrystalline phase separated manganites. Upon 60 kOe applied magnetic field, the magnetic entropy change (|Δ S M |) of bulk sample reaches a maximum value of 8.48 J/kg-K with a large relative cooling power (RCP) value of 574 J/kg around its Curie temperature ( T C )–172 K after the correction for hysteretic losses caused by the first order magnetic phase transition. The corresponding adiabatic temperature change is 2.38 K for magnetic field of 10 kOe. The magnetic field induced change of entropy and specific heat vary with temperature and have their maximum around the first order magnetic phase transition. We also report the magnetic field dependence of the order of the ferromagnetic (FM) to paramagnetic (PM) phase transition in bulk and nanometric manganites. It has been shown that bulk to nanometric samples exhibit first order FM → PM phase transition under low magnetic field accompanied by magnetization with thermal hysteresis in the field cooled cooling and warming cycle. However, the samples exhibit a second order magnetic phase transition above a critical field H CR . All the signatures of the first-order magnetic phase transition in bulk and nanometric sample disappear above the critical field H CR . The magnetocaloric effect is thus modified by the field induced order of magnetic phase transition. The field induced paramagnetic to ferromagnetic transition is confirmed to be first order in nature from dc magnetization measurements and Arrott plots using a criteria given by Banerjee. The magnetic phase transition is also accompanied by a large change in resistivity with thermal hysteresis. The observed value of magnetic entropy change in bulk sample is much higher than the value generally observed in other perovskite manganites of comparable T C . This large change mainly originates from a sharp magnetization jump, associated with a first-order metamagnetic transition and coalescence of ferromagnetic clusters in the paramagnetic state. Such noticeable magnetic entropy change at low magnetic field makes this material useful for the application of active magnetic refrigerant (AMR) materials.

Phase transitions of bulk and nanocrystalline La1−xSrxMnO3 (x = 0.35 and 0.37) perovskite manganite materials using in situ ultrasonic studies

La1−xSrxMnO3 perovskite manganite materials with two compositions, namely x=0.35 and 0.37, both at bulk and nanoscale, were prepared by solid-state reaction and sonochemical reactor methods, respectively. The magnetic phase transition temperature of the prepared bulk and nanosamples was evaluated by in situ ultrasonic velocity and attenuation measurements. A home-made experimental setup was used for in situ measurement of ultrasonic velocities and attenuation over a wide range of temperatures (from 300 to 400K). The observed anomalous lattice-softening behavior in the ultrasonic parameters was used to study the phase transition temperature (Curie temperature, TC), i.e., from paramagnetic to ferromagnetic phase, both in bulk and nanostructured perovskite samples. Further, the ultrasonic measurements confirmed that sharp and broad transitions occur in bulk and nanostructured perovskite manganite materials, respectively. The Curie temperature for nanostructured perovskite samples was lower than that for the corresponding bulk perovskite sample, which was clearly identified by ultrasonic measurements.

On-line phase transitions of bulk and nanocrystalline La1−xPbxMnO3 (x = 0.3, 0.4, and 0.5) perovskite manganite materials using ultrasonic measurements

Bulk and nanostructured La1−xPbxMnO3 perovskite manganite samples for different Pb contents (x=0.30, 0.40 and 0.50) were synthesised using solid-state reaction and sonochemical methods, respectively. XRD, FTIR, SEM, EDAX, BET and TEM studies were carried out to characterise the prepared bulk and nanosamples. The XRD characterization studies confirmed that the prepared bulk and nano perovskite manganite samples were crystalline in nature. The SEM and TEM studies revealed that the particle sizes of bulk perovskite samples is in the range of 0.75–85μm, whereas it ranges from 50 to 90nm in the case of nano perovskite samples. Further, it has been confirmed that there is an increase in particle size with an increase in Pb content both in bulk and nano perovskite samples. In addition, ultrasonic velocities and attenuations of bulk and nanosamples were measured using the on-line ultrasonic through transmission method. The temperature-dependent ultrasonic parameters showed an interesting anomaly in both bulk and nano perovskite samples. The observed marked softening and hardening in sound velocities and attenuation were related to ferromagnetic to paramagnetic phase transition temperature (TC). In nano perovskites samples, the existence of diffused phase transition took place at a lower temperature compared with the corresponding bulk samples. The observed broad ferromagnetic (FM)–paramagnetic (PM) transition in nano perovskites confirms the absence of sharp transition, which is attributed to the coexistence of orbitally, ordered and disordered phases. The spin–phonon and electron–phonon couplings are known to exist because of single ion magnetostriction and dynamic Jahn–Teller effect, respectively, are evidenced by the observed anomalies in both velocities and attenuations.

Metal insulator transition of bulk and nanocrystalline La1−xCaxMnO3 perovskite manganite materials through in-situ ultrasonic measurements

Abstract Bulk and nanocrystalline La 1 − x Ca x MnO 3 ( x = 0.75, 0.80 and 0.85) perovskite manganite samples were prepared by using the solid-state reaction and sonochemical reactor methods, respectively. The X-ray diffraction studies confirmed that the prepared bulk and nanoperovskite manganite samples were crystalline in nature with an orthorhombic crystal structure. The microscopic studies revealed that the particle sizes of the bulk and nanoperovskite samples were 0.75–0.90 μm and 120–160 nm, respectively. In addition, ultrasonic velocities and attenuations of the bulk and nanosamples were measured by using the in-situ ultrasonic through transmission method. The temperature-dependent ultrasonic parameters showed an interesting anomaly in both bulk and nanoperovskite samples. The observed marked softening and hardening in sound velocities or attenuation were related to metal–insulator phase transition temperature. In nanosamples, the metal insulator transition took place at a lower temperature compared with the corresponding bulk samples. The observed results confirmed that nanoperovskite samples have a broad transition compared with the corresponding bulk samples. The observed broad peak at transition temperature in nanoperovskite samples was used to confirm the nanocrystalline nature of perovskites.

Impact of nanostructuring on the magnetic and magnetocaloric properties of microscale phase-separated La5/8−yPryCa3/8MnO3manganites

Bulk manganites of the form La${}_{5/8\ensuremath{-}}$${}_{y}$Pr${}_{y}$Ca${}_{3/8}$MnO${}_{3}$ (LPCMO) exhibit a complex phase diagram due to coexisting charge-ordered antiferromagnetic (CO/AFM), charge-disordered paramagnetic (PM), and ferromagnetic (FM) phases. Because phase separation in LPCMO occurs on the microscale, reducing particle size to below this characteristic length is expected to have a strong impact on the magnetic properties of the system. Through a comparative study of the magnetic and magnetocaloric properties of single-crystalline (bulk) and nanocrystalline LPCMO ($y$$\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}$3/8) we show that the AFM, CO, and FM transitions seen in the single crystal can also be observed in the large particle sizes (400 and 150 nm), while only a single PM to FM transition is found for the small particles (55 nm). Magnetic and magnetocaloric measurements reveal that decreasing particle size affects the balance of competing phases in LPCMO and narrows the range of fields over which PM, FM, and CO phases coexist. The FM volume fraction increases with size reduction, until CO is suppressed below some critical size, \ensuremath{\sim}100 nm. With size reduction, the saturation magnetization and field sensitivity first increase as long-range CO is inhibited, then decrease as surface effects become increasingly important. The trend that the FM phase is stabilized on the nanoscale is contrasted with the stabilization of the charge-disordered PM phase occurring on the microscale, demonstrating that in terms of the characteristic phase separation length, a few microns and several hundred nanometers represent very different regimes in LPCMO.

Ferroelectric control of magnetization in La1−xSrxMnO3manganites: A first-principles study

We present a first-principles study on the interface between perovskite ferroelectrics (PbTiO${}_{3}$) and conducting magnetic manganites (La${}_{1\ensuremath{-}x}$Sr${}_{x}$MnO${}_{3}$). We show that by switching the ferroelectric polarization, additional carriers are accumulated or depleted at the interfacial region of the manganite and that this change in carrier density can modify the magnetic spin configuration of the interfacial Mn, which is consistent with the experimentally observed anomalously large change in the magnetization. We also describe an unexpected purely interfacial phenomenon whereby the ferroelectric polarization of the interfacial region changes the magnetic energetics---a degree of freedom not present in bulk manganites. Theoretically, we show the the ground-state magnetic structure depends sensitively on the precise choice of Hubbard $U$ parameter within the widely used DFT+$U$ class of exchange correlation functionals. We provide a simple Ising-like model that explains the evolution of the magnetic structure with $U$ in tandem with a discussion of various different ways in which one might try to choose an appropriate $U$ parameter.

Extrinsic electronic transport in La0.7(Sr,Ca)0.3MnO3 films deposited on step edges and bicrystal substrates

In this paper, the electronic transport of La0.7(Sr,Ca)0.3MnO3 films grown by pulsed laser deposition on a LaAlO3(001) substrate with deep parallel structured steps and a 30° symmetric bicrystal SrTiO3(001) substrate have been discussed. The electronic transport properties have been related to the well-known extrinsic transport of bulk manganite compounds. The spin-glass-like behavior with a characteristic peak at 20 K and domain formation at the grain boundary is observed. Further, it has been quantified from the resonant tunneling model that mixed magnetic interactions play a significant role in the manganite films deposited on step edges.

Electronic tuning of La2/3Sr1/3MnO3 thin films via heteroepitaxy

Epitaxial La2/3Sr1/3MnO3 films grown on LaAlO3 substrates of various orientations exhibit a range of magnetoresistive properties, demonstrating the utility of strain as an electronic tuning parameter for manganites. Large magnetoresistance over a broad range of temperatures—highest (−64% at 50 kOe) at the lowest temperatures measured—is observed in a coherently strained La2/3Sr1/3MnO3 film on a (001) LaAlO3 substrate. In addition to higher magnetoresistance, its reduced magnetization and conductance suggest the stabilization of a more insulating ground state and the possibility of strain-induced phase coexistence. Similar field-dependent magnetotransport features at low temperatures, distinct from those exhibited by bulk manganites, are also seen in a partially strained film on a (110) LaAlO3 substrate, but bulk-like magnetoresistive behavior is observed in a relaxed La2/3Sr1/3MnO3 film on a (111) LaAlO3 substrate.

Metallicity and Ferromagnetism in Nanosystem of Charge Ordered Nd0.5Sr0.5MnO3

The fascinating phenomenon of destabilization of charge/orbital order in Nd0.5Sr0.5MnO3 with the reduction of grain size is critically investigated. Based on our magnetic and transport experiments followed by a theoretical analysis, we analyze various possible mechanisms and try to delineate a universal scenario behind this phenomenon. We revisit this issue carefully and discuss various evidences from experiments in nano and bulk manganites on the absence of correlation between size reduction and pressure effects on manganites. We propose a phenomenological model based on enhanced surface disorder to explain the appearance of weak ferromagnetism and metallicity in nanosize Nd0.5Sr0.5MnO3 system. All evidence seems to suggest that the transport is mediated through the surface via enhanced density of states in the nanometric grains. We provide theoretical support for this by performing an ab-initio electronic structure calculation as well as from a recent numerical simulation and argue that the mechanism is likely to be general in all nanosize charge ordered manganites.

Magnetic and electrical properties of La1−cSrcMn1−xGaxO3 manganites

The bulk manganites La1−cSrcMn1−xGaxO3 (c = 0.15; 0.17; 0.19; 0.025≤ x ≤0.125) were prepared by solid state reactions. The regularities of the influence of Ga3+ and Sr2+ concentrations on saturation magnetization at different temperatures, Curie point (Tc), semiconducting-metallic transition and magnetoresistance have been established. It was found that magnetization at 80 K and Tc rise with c increasing at each fixed value of x, and decrease as a functions of x at fixed c. Absolute value of negative magnetoresistance for some chemical compositions exceeds 80% at low temperatures for magnetic field strength ∼9 kOe. Ga substitution for Mn shifts phase boundary "rhombohedral-orthorhombic structure" to lower value of c, moreover, excess of oxygen content over stoichiometric one promotes the existence of rhombohedral phase at c = 0.15. Diamagnetic dilution of octahedral sublattice by Ga3+ ions and cation vacancies induce semiconducting behaviour in rhombohedral phase, even at c = 0.19. Metallic type of temperature dependence of resistivity observes in appropriate low temperature range in sintered manganites with c = 0.17; 0.19 only at small x.

Suppression of Charge and Antiferromagnetic Ordering in La0.5Ca0.5MnO3 Nanoparticles

The intermediate band-width manganite La0.5Ca0.5MnO3 has a CE type charge, orbital and antiferromagnetic order below 195 K owing to strong electronic correlation and commensurability effect. We have critically investigated in details all the possible mechanisms behind the destabilization of charge/orbital ordering in La0.5Ca0.5MnO3 with the reduction of grain size in the nanometric regime as evidenced from our various experimental results. We have tried to found out a universal scenario to explain such behavior in nanometric manganites in general. This study shows that the room temperature Pnma perovskite structure, correlated with the lattice distortion, Mn-O distances and octahedral tilts, is practically unaffected with the reduction of particle size. However, in these nanocrystalline La0.5Ca0.5MnO3, the stable charge ordered state with antiferromagnetic ordering becomes significantly suppressed and FM ordering spontaneously appears below T(c) = 260 K as compared to its bulk counterpart. From strong experimental evidences on nano and bulk manganites, we revisit this issue and also discuss the absence of correlation between size reduction and pressure effects on manganites. We argue from a phenomenological analysis that the above observations could be best attributed to enhanced surface disorder in the nanosized half-doped manganite grains which is likely to be general in all nano size charge ordered manganites.

Little volume change in orbital ordering transition in manganite thin films

We have investigated the structural feature of orbital-ordering manganite thin films fabricated on the perovskite (011) substrates by means of synchrotron x-ray diffraction technique. Like bulk manganites, these films show large jumps in lattice parameters at the orbital ordering transition temperature, which clearly show the transitions are first order. However, the magnitude of the volume change at the transition temperature for the films is much smaller than that for the orbital ordering transition in bulk manganite. This difference is understood by taking into account the stress from the substrate.

Finite size effect and phase diagram of ultra-thin La0.7Sr0.3MnO3

La0.7Sr0.3MnO3 (LSMO) ultra-thin films have been grown by pulsed laser deposition in conditions optimized for cation stoichiometry and bulk-like physical properties in the thick limit. Through electrical transport and magnetic measurements, a phase diagram is constructed as a function of film thickness. With decreasing thickness, the LSMO films cross over at high temperatures from a paramagnetic metal to a paramagnetic insulator, and at low temperatures from a ferromagnetic metal to a ferromagnetic insulator, in close similarity to that observed for varying the electronic bandwidth in bulk manganites.