Antiferromagnetic Charge Ordering Research Articles

Exploring complex magnetic phase transitions and magnetocaloric effect in the nanocrystalline Nd0.5Ca0.5MnO[formula omitted] compound

In this article, we have investigated the temperature and magnetic field-induced complex magnetic phases and magnetocaloric properties of the nanocrystalline Nd0.5Ca0.5MnO3 (NCMO) compound in detail. The compound exhibits charge-ordered antiferromagnetic (CO-AFM) and charge-orbital-ordered antiferromagnetic (AFM) phases at TCO≃ 225 K and TN= 160 K respectively. At the intermediate temperature range, i.e. TN≤ T ≤ TCO, an unfamiliar magnetic state is present where the orbital ordering of manganese ions sets progressively. Both the short-range and zig-zag ordered ferromagnetic (FM) chains and an AFM phase coexist within this temperature window. The effect of magnetic field and temperature drastically modifies the fragile magnetic ground state and this change is also reflected in the magnetocaloric properties of the studied compound. At the low-temperature region, an inverse magnetocaloric effect (MCE) is noticed due to the kinetically arrested magnetic moments of the AFM phase. However, a conventional MCE is noticed in the high-temperature regime. The anomalous behavior of the MCE in the nanocrystalline NCMO compound is analyzed by considering the effect of the reduction of particle size and the meta-stable nature of the magnetic ground state. In addition, we have paid attention to the hysteretic losses associated with the first-order phase transition (FOPT) in order to estimate the magnetic cooling capacity for the material. These studies carry notable importance for the usefulness of nanocrystalline half-doped manganite compounds not only for basic scientific research purpose, but also having possible practical utilization in the magnetic refrigeration industries.

Structural, magnetic and magneto-transport properties of Pr0.5Ca0.5Mn0.9V0.1O3: Indication of large field coefficient of resistance (FCR)

Structural, magnetic and magneto-transport properties of polycrystalline Pr0.5Ca0.5Mn0.9V0.1O3 have been studied. The incorporation of vanadium attributes to enhancement in the Mn3+/Mn4+ ratio as well as density of conducting eg carriers, which affects the magnetic and electrical behavior of the system. On cooling, a multi-phase separated “paramagnetic (PM) – charge-ordered antiferromagnetic (COAFM) – ferromagnetic (FM) – reentrant spin glass (RSG)” phases are observed with the characteristic charge ordering temperature TCO = 233 K and Neel temperature TN = 143 K. Field-induced first-order magnetic phase transition from a charge-ordered antiferromagnetic state to a ferromagnetic cluster glass state takes place at TC = 44 K. A 7 T magnetic field induces metal–insulator transition at TMI = 163 K. At low temperatures, strong memory-effect and colossal magnetoresistance (CMR) ~ 99.99% are observed in resistivity data. The sample shows a high field coefficient of resistance (FCR) value of about 855% in the low-temperature regime.

Magnetocaloric effect simulation by Landau theory and mean-field approximation in Pr0.5Sr0.5MnO3

The magnetocaloric effect in Pr0.5Sr0.5MnO3 (PSMO) has been successfully modeled in this work. PSMO undergoes a first-order antiferromagnetic charge ordering (AFM/CO) to a ferromagnetic (FM) transition at $ T_{CO}=T_{N}\sim 165$ K followed by a second-order ferromagnetic (FM) to paramagnetic (PM) transition at the Curie temperature, $ T_{C}\sim 255$ K. The magnetocaloric effect in PSMO has been studied by the simulation of the magnetic entropy change $ (-\Delta S_M)$ for these two transitions by developing a numerical approach based on the Landau free energy within the mean-field approximation. Simulated results agree well with the experimental ones for these two transitions.

Magnetic Properties and Charge Ordering in Polycrystalline La1-xCaxMnO3 (x = 0.2, 0.5) Manganite

In this work, we report the attractive magnetic properties of La1-xCaxMnO3 manganites with x = 0.2 (LCMO-2) and x = 0.5 (LCMO-5). X-ray diffraction (XRD) measurements show that the prepared samples are single phase and they crystallize in the orthorhombic structure with Pnma space group. Scanning electron microscope (SEM) measurements reveal that the particles of samples are homogeneous and possess the diameter of several microns. Magnetization versus temperature measurements show paramagnetic (PM) to ferromagnetic (FM) transition at TC ≈ 200 K for LCMO-2 and at TC ≈ 250 K for LCMO-5. Below TC, FM nature is observed in LCMO-2. However, LCMO-5 displays the complex magnetic phase transition. Below TC, LCMO-5 shows a FM to charge-ordered antiferromagnetic (COAFM) transition at CO temperature TCO and a novel, unexpected FM phase transition at characteristic temperature T* (near 35 K). The FM and COAFM phases coexist in LCMO-5, and the competition of FM and AFM phases lead to the unexpected FM transition at T*. Furthermore, the inverse of susceptibility versus temperature measurements confirm the similar results. Magnetic hysteresis loop measurements suggest the typical FM feature of LCMO-2. However, LCMO-5 exhibits the hysteresis loops with butterfly-type structure at 100 and 150 K, suggesting the presence of COAFM phase. These attractive magnetic properties are explained by the existence of COAFM phase and the competition between FM and COAFM phases.

Magnetocaloric effect studying by means of theoretical models in [formula omitted] manganite

In this work, we have used many theoretical models to simulate the magnetocaloric effect (MCE) in the charge ordered Pr0.5Sr0.5MnO3 (PSMO) manganite. This sample undergoes a first order antiferromagnetic charge-ordered (AFM/CO) to a ferromagnetic (FM) transition at TCO∼165 K followed by a second order FM to paramagnetic (PM) transition at Curie temperature, TC∼255 K. The Mean-field theory and the Bean-Rodbell model are called to study deeply the MCE in FM-PM transition. Based on the mean-field theory, we proposed a numerical approach to study the evolution of magnetization versus temperature for the first order AFM/CO – FM transition. By using a phenomenological model, we have compared theoretical and experimental MCE for the above two transitions.

Investigation of magnetic response of charge ordering in half-doped La0.5Ca0.5MnO3 manganite

La0.5Ca0.5MnO3 (LCMO) polycrystalline sample was synthesized by the solid-state reaction method. The result of X-ray diffraction shows that the LCMO is single phase. We have studied the magnetic response of charge-ordered (CO) state in the LCMO manganite. The magnetic hysteresis loop at 100 K displays an interesting butterfly-type structure, which indicates the existence of CO anti-ferromagnetic (AFM) structure in LCMO. The temperature-dependent magnetization measurements show a paramagnetic (PM) to ferromagnetic (FM) transition at Curie temperature (TC ≈ 130 K) and a FM to AFM transition at Neel temperature (TN ≈ 50 K), measured under different fields of 100, 500 and 5000 Oe. Moreover, there is an interesting intercross in zero-field cooling (ZFC) and filed-cooling (FC) measurements, i.e., an intercross of the ZFC/FC curves in the CO LCMO manganite. These present magnetism measurements are discussed and possible explanations are provided by the existence of CO AFM state.

Control of charge order melting through local memristive migration of oxygen vacancies

The colossal magnetoresistance (CMR) in perovskite manganites and the resistive switching (RS) effect in metal-oxide heterostructures have both attracted intensive attention in the past decades. Up to date, however, there has been surprisingly little effort to study the CMR phenomena by employing a memristive switch or by integrating the CMR and memristive properties in a single RS device. Here, we report a memristive control of the melting of the antiferromagnetic charge ordered (AFM-CO) state in ${\mathrm{La}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_{3\ensuremath{-}\ensuremath{\delta}}$ epitaxial films. We show that an in situ electrotailoring of the boundary condition, which results in layers of oxygen vacancies at the metal-oxide interface, can not only suppress the critical magnetic field for the AFM-CO state melting in the interfacial memristive domain, but also promote the one in the common pristine domain of the RS device in the high and low resistive states. Our study thereby highlights the pivotal roles of functional oxygen vacancies and their dynamics in strong correlation physics and electronics.

Doping-Dependent Magnetism and Exchange Bias in CaMn1–<italic>x</italic>Re<italic>x</italic>O3

Magnetic and structural properties of CaMn1– x Re x O3 (0.02 ≤ x ≤ 0.1) have been investigated. Substitution of Re5+ ion for the Mn4+ site of 3 generates Mn3+ ions according to the chemical formula CaMn1–2 x 4+Mn x 3+Re x 5+O3, accompanied by an increase of lattice parameters and unit-cell volume with increasing ${x}$ . With increasing doping level $x$ , the magnetic ground state evolves from an antiferromagnetic (AFM) with a weak ferromagnetic (FM) component, for $x = 0.02 - 0.06$ , to the charge ordered ${C}$ -type AFM state at $x = 0.1$ . Spontaneous magnetization at $T = 10$ K increases quickly with increasing $x$ , approaches the maximum value of 3.5 emu/g for $x = 0.04$ , and then decreases rapidly to 0.2 emu/g for $x = 0.1$ . Anomalous negative magnetization (NM) for $x = 0.02$ has been observed in the zero-field-cooled and field-cooled (FC) magnetization below the magnetic transition temperature. Exchange bias (EB) effect, manifested by horizontal shift in the hysteresis loops of FC samples, has also been observed. This effect is very small for $x = 0.02$ , almost zeroes for 0.04, and monotonously increases with increasing $x$ . The EB appears due to low-temperature phase separation into FM clusters and charge-ordered AFM phases. The effect of hydrostatic pressure for all samples revealed a significant increase of the FM phase volume under pressure, linked to both suppression of NM in $x = 0.02$ sample and reduction of the EB effect in all samples.

Effect of K-doping on the structural, magnetic and magnetocaloric properties of Pr0.8Na0.2–xKxMnO3 (0 ≤ x ≤ 0.15) manganites

In this paper, the effect of potassium “K” substitution on morphological, crystallographic and magnetic properties of the Pr0.8Na0.2MnO3 perovskite was investigated. All samples were synthesized using the Pechini sol–gel method. On the one hand, the morphological study showed that the grains are irregularly spherically-shaped and have a submicronic size. On the other hand, the chemical analysis expectedly unveiled the presence of all the predicted elements in the right composition. Through the X-ray diffraction analysis, we proved that the prepared compounds crystallize in the orthorhombic structure with the Pbnm space group. The temperature dependence of the magnetization M (T) showed a large increase of M when x increases from 0 to 0.05, as does the Curie temperature TC. The transition from ferromagnetic (FM) to charge ordered antiferromagnetic (CO-AFM) phase at high temperature is observed for x = 0, however the doped manganites exhibit only an FM metallic phase. The magnetocaloric effect (MCE) was calculated according to the Maxwell relation based on magnetic measurements. The magnetic entropy change (ΔSM) reaches a maximum value which remains constant when increasing the K content. The highest value of the relative cooling power RCP was found to be 277 J/kg for the Pr0.8Na0.05K0.15MnO3 under an applied magnetic field of 5 T.

Correlating melting and collapse of charge ordering with magnetic transitions in La0.5-xPrxCa0.5MnO3

The magnetic properties of polycrystalline La0.5-xPrxCa0.5MnO3 material are investigated at different temperatures. The existence of magnetically diverse phases associated with various relaxation modes and their modulation with temperature and doping is analyzed. La0.5Ca0.5MnO3 exhibited field induced melting and collapse of charge ordered antiferromagnetic (CO-AFM) phase into ferromagnetic (FM) state. This phenomenon results in lowering of Neel’s temperature (TN) along with changes in the slope of magnetic moment with temperature. Using normalized M(T) curves, the variation and interplay of charge ordered temperature (TCO), Curie temperature (TC) and TN is conferred. The coexistence of two magnetic modes is explained as major ingredient for the magnetic transitions as well as metal to insulator transition (MIT); where melting and collapse of charge ordering is conversed as basic feature in these Praseodymium (Pr) doped La0.5Ca0.5MnO3 materials.

Dynamical behavior of step-like transition of La0.5Sr0.5Mn1−xTixO3 in a widened field sweep rate

The magnetization behavior of Ti4+ doped perovskite manganites La0.5Sr0.5Mn1−xTixO3 (x=0.15, 0.175 and 0.2) was investigated. Experimental results show that Ti4+ dopant suppresses the antiferromagnetic charge ordering and leads to a step-like magnetization behavior below 3K. The step-like transitions and the critical magnetic fields are strongly dependent on the Ti doping level, magnetic field sweep rate and temperature. Above 3K, the step-like transitions transform to broad ones. In addition, under pulsed high magnetic fields with an ultrafast field sweep rate of ~103T/s, the sharp step-like transitions observed in the static magnetization measurements become smooth metamagnetic transition at low temperatures. This feature is correlated with the collapse of the balance between the magnetic energy and elastic energy in the phase separation system within a martensitic-like scenario.

Atomic scale studies of La/Sr ordering in colossal magnetoresistant La(2-2x)Sr(1+2x)Mn2O7 single crystals.

To date, it is unclear whether chemical order (or disorder) is in any way connected to double exchange, electronic phase separation, or charge ordering (CO) in manganites. In this work, we carry out an atomic resolution study of the colossal magnetoresistant manganite La(2-2x)Sr(1+2x)Mn2O7 (LSMO). We combine aberration-corrected electron microscopy and spectroscopy with spectroscopic image simulations, to analyze cation ordering at the atomic scale in real space in a number of LSMO single crystals. We compare three different compositions within the phase diagram: a ferromagnetic metallic material (x=0.36), an insulating, antiferromagnetic charge ordered (AF-CO) compound (x=0.5), which also exhibits orbital ordering, and an additional AF sample (x=0.56). Detailed image simulations are essential to accurately quantify the degree of chemical ordering of these samples. We find a significant degree of long-range chemical ordering in all cases, which increases in the AF-CO range. However, the degree of ordering is never complete nor can it explain the strongly correlated underlying ordering phenomena. Our results show that chemical ordering over distinct crystallographic sites is not needed for electronic ordering phenomena to appear in manganites, and cannot by itself explain the complex electronic behavior of LSMO.

Structural, magnetic and magnetocaloric properties of Ag-doped La0.5Ca0.5−Ag MnO3 compounds with 0 ⩽x⩽ 0.4

Structural, magnetic and magnetocaloric properties of manganites series with the La0.5Ca0.5−xAgxMnO3 (0.0 ⩽ x ⩽ 0.4) compounds were investigated. Samples were synthesized using the solid state reaction at high temperature and analyzed by XRD data based on the Rietveld refinement technique. Samples with x = 0.0, 0.1, 0.2, 0.3 and 0.4 crystallized in orthorhombic symmetry with Pnma space group. Also, Magnetizations measurements were performed to investigate magnetic properties. Transition from ferromagnetic (FM) to charge ordered antiferromagnetic (CO-AFM) phases at low temperature is observed for Ag-free La0.5Ca0.5MnO3. Compounds with x = 0.2, 0.3 and 0.4, do not exhibit this transition, and only FM metallic phases are observed. The Curie temperature TC increases with increasing Ag amount from 222 K (for x = 0.0) to 264 K (for x = 0.2).While x > 0.2, TC remains substantially constant. Magnetic entropy change (ΔSM) was estimated from isothermal magnetization data. Parent compound (x = 0.0) exhibits both normal (negative ΔSM) and inverse (positive ΔSM) magnetocaloric effects at TC and TCO, respectively. However, all the Ag doped samples show only normal magnetocaloric effect at TC. The Largest MCE occurs for sample with x = 0.4 (ΔSM = 3.27 J kg−1 K−1, ΔH = 5 T) for which largest relative cooling power (RCP = 229.27 J kg−1, ΔH = 5 T) is reached as well.

Comparative study of magnetic and magnetotransport properties of Sm0.55Sr0.45MnO3 thin films grown on different substrates

Highly oriented polycrystalline Sm0.55Sr0.45MnO3 thin films (thickness ∼100 nm) deposited on LaAlO3 (LAO, (001)), SrTiO3 (STO, (001)) and (La0.18Sr0.82) (Al0.59Ta0.41)O3 (LSAT, (001)) single crystal substrates by ultrasonic nebulized spray pyrolysis have been studied. The out of plane lattice parameter (OPLP) of the film on LAO is slightly larger than that of the corresponding bulk. In contrast, the OPLP of the films on STO and LSAT are slightly smaller than the corresponding bulk value. This suggests that the film on LAO is under compressive strain while LSAT and STO are under tensile strain. The films on LAO and LSAT show simultaneous paramagnetic-ferromagnetic (PM–FM) and insulator-metal transition (IMT) temperature at TC/TIM ∼ 165 K and 130 K, respectively. The PM–FM and IM transition occur at TC ∼ 120 K and TIM ∼ 105 K, respectively in the film on STO substrate. At T < TC, the zero field cooled–field cooled (ZFC–FC) magnetization of all the films shows strong bifurcation. This suggests the presence of a metamagnetic state akin to cluster glass formed due to coexisting FM and antiferromagnetic–charge order (AFM–CO) clusters. All the films show colossal magnetoresistance but its temperature and magnetic field dependence are drastically different. The films on LAO and STO show peak CMR around TC/TIM, while the film on LSAT shows MR > 99 % over a very wide temperature range of ∼40 K centred on TC/TIM. In the lower temperature region the magnetic field dependent isothermal resistivity also shows signature of metamagnetic transitions. The observed results have been explained in terms of the variation of the relative fractions of the coexisting FM and AFM–CO phases as a function of the substrate induced strain and oxygen vacancy induced quenched disorder.

Microstructural and magnetotransport properties of La1 − xCaxMnO3 (0.45 ≤ x ≤ 0.60) thin films

In the present work we focus on the coexistence of magnetic-electronic phases and hence phase separation in La1−xCaxMnO3 thin films in the composition range of 0.45≤x≤0.60. The oriented polycrystalline thin films were prepared by nebulized spray pyrolysis technique on single crystalline substrate LaAlO3 [LAO (001)]. The lattice parameters are observed to decrease with increasing in Ca content and the structural/microstructural disorder is also seen to increase concomitantly. As the Ca content is increased from x=0.45 to 0.55, the paramagnetic insulator (PMI) to ferromagnetic metal (FMM) transition temperatures (TC/TIM) are suppressed. This is accompanied by (i) appreciable suppression of the TC, (ii) sharp reduction in the magnetic moment, (iii) occurrence of a peak in the magnetization below TC, (iv) stronger bifurcation of the ZFC–FC magnetization, and (v) decline in the magnetization in the lower temperature regime. At x=0.60, the FMM disappears and a weak FM insulator like phase is observed. In the vicinity of x=0.55, the magnetic ground state resembles spin cluster glass. This is also confirmed by the occurrence of a hysteresis loop in the resistivity of the x=0.55 measured during the cooling and warming cycle. The transition broadening up to x=0.55 and the vanishing of the two transitions in x=0.60 film along with the simultaneous decrease in the magnetic moment and enhanced resistivity show that the observed variations are due to the increase in the antiferromagnetic-charge ordered (AFM-CO) phase fraction at higher Ca content. This is also confirmed by the decrease in magnitude of the magnetoresistance and its temperature dependence. Our results demonstrate that, there is strong phase separation in the overdoped region (0.50<x≤0.60) due to the presence of multiple magneto-electric phases.

Novel Excitation-Density Dependence in Strongly Spin-Charge Coupled System

Photoinduced spin-charge dynamics in strongly correlated electron systems is studied based on an extended double-exchange model. Solving a time-dependent Schrodinger equation with the Lanczos method, we trace the process of photoinduced melting of an antiferromagnetic charge order and analyze the excitation-density dependence on that. In the case of low density photoexcitation, both charge and spin orders are melted by photocarrier doping. This is interpreted with a conventional double-exchange mechanism. In the case of high density photoexcitation, however, the charge order is melted. The antiferromagnetic spin order is transiently weakened but after turnoff of the photoirradiation it recovers. This phenomenon strikingly differs from the weak photoexcitation case.

Competing two-phase coexistence in doped manganites: Direct observations byin situLorentz electron microscopy

We examined thin epitaxial films ${\text{La}}_{5/8\ensuremath{-}y}{\text{Pr}}_{y}{\text{Ca}}_{3/8}{\text{MnO}}_{3}$ $(\text{LPCMO}:y=0.275\ensuremath{-}0.3)$ in situ by Lorentz transmission electron microscopy (TEM) and other microscopy methods. Clear evidence was obtained for the competing two-phase coexistence of antiferromagnetic charge-ordered (CO) and ferromagnetic (FM) phases that exhibit mesoscale phase separation below the metal-to-insulator transition (MIT) at $\ensuremath{\sim}164\text{ }\text{K}$. In addition, we observed some regions of mixed CO- and FM-domain contrast attributed earlier to formation of the new CO-FM phase. Using in situ heating/cooling TEM experiments, we interpret this effect as the interfacial wetting phenomenon inherent to the first-order CO-FM phase transition, rather than to the formation of new CO-FM phase. It is evidenced by the partial magnetic melting of CO phase at interfaces with the FM phase, thereby creating charge-disordered spin-glass metastates. For coexisting CO- and FM-domain configurations, we directly refined the relationship between charge-orbital and spin-ordering vectors, consistent with FM moments pinned by (101)-crystal twins in LPCMO films. We also discuss the striking linear dependence observed below MIT for the log-resistance behavior and the CO fraction in LPCMO directly measured by TEM. Such linear dependence does not follow typical percolation equations, suggesting that percolation model needs further revisions for transport description of manganites.

Short-range ordering state and cluster-glass behavior in electron-doped manganite [formula omitted

In this paper, we investigated the electrical and magnetic properties of electron-doped manganite Y 0.4Ca 0.6MnO 3 with a relatively small average cationic radius 〈 r A 〉 . The small 〈 r A 〉 of the present system results in a short-range antiferromagnetic charge-ordering (AFM-CO) state in the manganite. With the decrease of temperature, a ferromagnetic super-exchange interaction begins to establish itself between the remaining disordered Mn ions after the formation of the short-range CO state. Then, a reentrant cluster-glass state originates from the frustration between the FM order in the clusters and the AFM interactions present in the background matrix.

Grain-size effects on the charge ordering and exchange bias inPr0.5Ca0.5MnO3: The role of spin configuration

The grain-size effects on the charge ordering and exchange bias in CE-type antiferromagnetic charge ordered manganite compound ${\text{Pr}}_{0.5}{\text{Ca}}_{0.5}{\text{MnO}}_{3}$ are explored by magnetometry. Reducing the size suppresses antiferromagnetism and charge ordering that does not occur any more when the size is smaller than 40 nm. At the same time ferromagnetic clusters appear and gradually become larger; their fraction increases to about 19.7% when the particle size is reduced to 20 nm. The magnetic hysteresis loops of the nanosized samples exhibit both horizontal and vertical shifts in the field-cooled process. The exchange bias field ${H}_{\text{eb}}$ and the coercivity ${H}_{\text{C}}$ do not depend monotonously on size but show a maximum at about 85 nm, which indicates that in antiferromagnetic charge-ordered manganites ${H}_{\text{eb}}$ can be effectively tuned by changing the grain size. These notable phenomena can be understood when the evolution of the spin configuration is considered that results from the surface-phase separation and surface effect.

Charge-order melting and magnetic phase separation in thin films ofPr0.7Ca0.3MnO3.

We investigate the effect of strain on the phenomenon of charge-order melting, that is the transformation of a charge-ordered insulating state to a metallic state under the influence of a magnetic field (the melting field) in thin films of ${\text{Pr}}_{0.7}{\text{Ca}}_{0.3}{\text{MnO}}_{3}$ grown on various substrates. We find that unstrained films grown on ${\text{SrLaGaO}}_{4}$ behave quite similar to bulk material, but that strained films grown on ${\text{SrTiO}}_{3}$ and ${\text{NdGaO}}_{3}$ show hugely increased melting fields. Strain relaxation by postannealing again leads to bulklike behavior. In this material the antiferromagnetic charge-order phase can coexist with a ferromagnetic insulating state. Magnetization measurements, where we demonstrate the presence of exchange bias effects, show that this is also the case in the strained films. We argue that the phase mixture in the strained films is more difficult to melt than in the unstrained case.