Ferromagnetic Metallic State Research Articles

Structure and electrical properties in La 2/3(Ca 1− xSr x) 1/3MnO 3 films

The thin films of La 2/3(Ca 1− x Sr x ) 1/3MnO 3 (LCSMO) with x = 0, 1/3, and 2/3 through the Ca, Sr double-doping at A site of the La 2/3A 1/3MnO 3 were prepared using the RF magnetron sputtering method. The X-ray diffraction patterns of the bulk samples show that with the increase of x, the structures exhibit the transition from the rhombohedral phase to the pseudo-cubic phase. The electrical properties show that these three thin films are the ferromagnetic and metallic state which can be fitted into the single magneton scattering formula: ρ( T) = ρ 0 + aT 2 at low temperature. The temperature range of the ferromagnetic metallic state becomes larger, and the peak resistivity temperature ( T p) increases with x. These can be qualitatively explained by the double exchange model.

Fabrication and characterization of La2/3Ca1/3MnO3/Eu2CuO4/ La2/3Ca1/3MnO3 magnetic tunneling junctions

Magnetic tunneling junctions of La2/3Ca1/3MnO3/Eu2CuO4/ La2/3Ca1/3MnO3 have been fabricated by RF-magnetron sputtering, photolithography and ion beam etching. Tunneling effect was observed in these junctions by measuring their I-V curves. Interestingly, a discontinuity was observed in the I-V curves below the metal-insulator transition temperature (Tp) of La2/3Ca1/3MnO3 By changing temperatures, the discontinuity in I-V curves happens at the same voltage of 209mV, but starts at different currents. The discontinuity starting current (Ic) decreases with increasing temperature. There is a hysteresis near the discontinuity when the current increases and decreases. The discontinuity in I-V curves could indicate a novel switching process. This switching phenomenon occurs only in the ferromagnetic metal state and strongly suggesting a magnetic related effect. The physics of the switching process has not been understood yet, but it could be of interest for potential applications in spintronic devices.

The Effect of Laser Acting On Double-doped La2/3(Ca3/4Sr1/4)1/3MnO3 Thin Films

Photo-responsive properties studied in perovskite manganese La2/3(Ca3/4Sr1/4)1/3MnO3 (LEMO) thin film with continued laser and modulated laser pulses are studied in this paper. The two valence alkaline earth elements make the Mn have two states: Mn3+(t32geg) and Mn4+(t32g). The Sr, Ca double-doped holes concentration is from 0.2 to 0.4 with maintained electric charge balance. The resistivity and temperature principle indicates that the material has CMR effect and transforms characteristics from metal to semiconductor and insulator. The results show that the phase transition from ferromagnetic metallic state to paramagnetic insulating state occurs at 341 K while the highest peak temperature is near to Tp. The maximizing photo-responsive resistivity change (ΔR/R0)max can reach 43.7%. Responsive signal intensity to the laser pulse has nonlinear relationships with the temperature and bias current. The photo-response resistivity changes with excited down-spin electrons and the exciting field. The optimum photo-responsive condition is T = 229 K, I = 24.7 mA. (ΔR/R0)max increases with temperature following exponential characteristic and there is a limited temperature for photo-response. The carrier electrons and small polarons have effects on thephoto-responsive characters.

Band Structure and the Magnetic and Elastic Properties of SrFeO[sub 3] and LaFeO[sub 3] Perovskites

The band structure and the magnetic and elastic characteristics of SrFeO3 and LaFeO3 perovskites with ferromagnetic and antiferromagnetic collinear spin configurations (of the A, C, and G types) are investigated using the ab initio pseudopotential method (the VASP program package) with the inclusion of the single-site Coulomb correlations (the LSDA + U formalism). It is shown that, in the pressure range 0–50 GPa, the most stable states are the ferromagnetic metal state for the SrFeO3 compound and the antiferromagnetic insulator state of the G type for the LaFeO3 compound.

Transient differential reflectivity of ferromagnetic and paramagnetic phases in the bilayered manganite La 1.24Sr 1.76Mn 2O 7

Photoinduced effects in a single crystal of bilayered manganites, La 2−2 x Sr 1+2 x Mn 2O 7 ( x=0.38), were investigated in a wide range of temperatures by pump-probe measurement at a photon energy of 1.6 eV. In a ferromagnetic metallic state, significant enhancement of positive rise in differential reflectivity with a slow relaxing time of 100 ps was observed just below T C=127 K, indicating that the reflectivity change with the slow relaxation time constant is induced by laser heating. We have also observed an unconventional fast relaxing component that has a time constant of the order of 10 ps. This fast relaxing component, whose absolute value has an asymmetric peak at T C, is presumably due to short-range correlation of Jahn–Teller distortion.

Magnetoelastic investigations at Pr [formula omitted]Ca [formula omitted]MnO [formula omitted] manganites

The magnetoelastic properties of the Pr 1 - x Ca x MnO 3 manganites were investigated for samples with x = 0.35 and 0.5 by magnetostriction experiments using a capacitive miniature dilatometer at high magnetic fields. The forced magnetostriction, measured at several temperatures between 2 and 240 K and at fields up to 23 T at the Grenoble high-field laboratory, is characterized by a lattice contraction of about 10 - 3 at the transition from the charged ordered insulating into the ferromagnetic metallic state. The effect is highly hysteretic. In contrast, the antiferromagnetic ordering process does not change the lattice remarkably. Our findings agree well with the results of sound velocity experiments and confirm the ( H , T ) phase diagram. The strong correlation effects between the lattice and the electronic system indicate the importance of the magnetoelastic coupling in these colossal magnetoresistance (CMR) compounds.

Ab initio investigation of the magnetic states of Ca2MnO4 and Ca2MnO3.5

Abstract An analysis of the electronic and magnetic properties of Ca 2 MnO 4 and Ca 2 MnO 3.5 is carried out within local spin density functional theory using the augmented spherical wave method. From energy differences between the hypothetic magnetic configurations both systems are found to be insulating antiferromagnets in the ground state with a ∼1 eV gap. However we identify an intermediate half metallic ferromagnetic state with the Hund’s rule expected moments for Mn IV (3 μ B ) and Mn III (4 μ B , high spin HS configuration), respectively. The latter result of moment magnitude finds support in recent experimental evidence of Mn III bismuth oxide as a ferromagnet in its ground state. This is characterized by a small density of states (DOS) magnitude of itinerant states in spin (↑) channel pointing to a metallic-like behavior as it is experimentally evidenced. For both Ca 2 MnO 4 and Ca 2 MnO 3.5 the chemical bonding characteristics are resolved for the two spin channels. Relationship to colossal magnetoresistive compounds is proposed.

Nanoscale structural correlations in magnetoresistive manganites

The colossal magnetoresistance (CMR) effect in perovskite manganites is a magnetic-field-induced transition from a paramagnetic insulating (PI) to a ferromagnetic metallic state. The large electrical resistivity of the PI state lies at the heart of the CMR effect. This enhanced resistivity stems, in part, from strong electron–lattice coupling and the associated local lattice distortions. Both uncorrelated local distortions (Jahn–Teller polarons) and correlated distortions are present in the PI state. The latter are believed to signal the presence of nanoscale orbital correlations. In this paper, we describe recent x-ray and neutron-scattering studies of the orbital correlations in pseudocubic manganites Ln1−xBxMnO3. Possible microscopic structures giving rise to these correlations are discussed. It is found that the correlations are ubiquitous in the orthorhombic PI phase of hole-doped manganites, and that their properties are defined by a single parameter—the doping level x. The correlations, however, are absent in the other paramagnetic phase exhibited by the manganites—the rhombohedral phase. The latter phase is metallic in the doping range in which the CMR effect is observed. Since the uncorrelated lattice distortions are present in both these phases, the insulating character of the PI state in CMR manganites results from the presence of the correlated lattice distortions. The orbital correlations, therefore, play the key role in the CMR effect.

Metallic Ferromagnet with Square-LatticeCoO2Sheets

A single-crystalline film of Sr2CoO4 with square-lattice CoO2 sheets (K2NiF4-type structure) was synthesized, and its electronic properties were characterized. The compound exhibited both ferromagnetic and metallic behaviors, with a fairly high Curie temperature T(C) of approximately 250 K, in contrast to the superconductivity recently found in a triangular-lattice CoO2-sheet compound. The film's large magnetoresistance with current perpendicular to the CoO2 plane showed field-hysteretic behavior analogous to tunneling magnetoresistance. The electronic structure of Sr2CoO4 was also investigated to characterize the quasi-two-dimensional ferromagnetic metal state in terms of optical spectroscopy and first-principles calculation.

Pressure effects on charge, spin, and metal-insulator transitions in the narrow bandwidth manganitePr1−xCaxMnO3

Pressure effects on the charge and spin states and the relation between the ferromagnetic and metallic states were explored on the small bandwidth manganite ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ ($x=0.25$, 0.3, 0.35). Under pressure, the charge ordering state is suppressed and a ferromagnetic metallic state is induced in all three samples. The metal-insulator transition temperature $({T}_{\mathit{MI}})$ increases with pressure below a critical point $P*$, above which $({T}_{\mathit{MI}})$ decreases and the material becomes insulating as at the ambient pressure. The ${e}_{g}$ electron bandwidth and/or band filling mediate the pressure effects on the metal-insulator transition and the magnetic transition. In the small bandwidth and low doping concentration compound $(x=0.25)$, the ${T}_{\mathit{MI}}$ and the Curie temperature $({T}_{C})$ change with pressure in a reverse way and do not couple under pressure. In the $x=0.3$ compound, the relation of ${T}_{\mathit{MI}}$ and ${T}_{C}$ shows a critical behavior: They are coupled in the range of $\ensuremath{\sim}0.8--5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and decoupledoutside of this range. In the $x=0.35$ compound, ${T}_{\mathit{MI}}$ and ${T}_{C}$ are coupled in the measured pressure range where a ferromagnetic state is present.

Field-induced ferromagnetic metallic state in the bilayer manganite (La0.4Pr0.6)1.2Sr1.8Mn2O7, probed by neutron scattering.

The bilayer manganite La1.2Sr1.8Mn2O7 exhibits a phase transition from a paramagnetic insulating (PI) to a ferromagnetic metallic (FM) state with a colossal magnetoresistance (CMR) effect. Upon 60% Pr substitution, magnetic order and PI to FM transition are suppressed. Application of a moderate magnetic field restores an FM state with a CMR effect. Neutron scattering by a single crystal of (La0.4Pr0.6)1.2Sr1.8Mn2O7, under a magnetic field of 5 T, has revealed a long-range and homogeneous ferromagnetic order. In the PI phase, under zero field, correlated lattice polarons have been detected. At 28 K, under 5 T, the spin wave dispersion curve determines an in-plane isotropic spin wave stiffness constant of 146 meV A(2). So the magnetic field not only generates a homogeneous ferromagnetic ground state, but also restores a magnetic coupling characteristic of FM CMR manganites.

Magneto-transport and specific heat behavior of Cd-doped La0.5Ca0.5MnO3

The temperature-dependent magneto-transport properties and the specific heat of La0.5(Ca0.5−xCdx)MnO3 (x=0.0–0.5) are measured for understanding the electronic phase separation behaviors in manganites. It is surprising to observe that Cd-doping enhances the ferromagnetic metallic state and suppresses the insulating state. Two insulator–metal transitions in La0.5(Ca0.3Cd0.2)MnO3 are observed, one occurs at temperature TP1 in accordance with the observed paramagnetic–ferromagnetic transition point TC and the other appears at a lower temperature TP2. The magnetoresistance of La0.5(Ca0.3Cd0.2)MnO3 as a function of temperature shows a sharp peak around TC, and remains roughly unchanged over a wide temperature range below TP1. It is argued that these fantastic properties are attributed to the coexistence of ferromagnetic metallic state and ferromagnetic insulating state under an antiferromagnetic background due to the Cd-doping effect.

Switching phenomena in La2/3Ca1/3MnO3/Eu2CuO4/La2/3Ca1/3MnO3 ramp-type junctions

Ramp-type junctions of p-typeLa2/3Ca1/3MnO3 (LCMO) electrodeswith n-type Eu2CuO4 (ECO) barriers have been fabricated by rf magnetron sputtering, photolithography andion beam etching. A tunnelling effect was observed in junctions with colossalmagnetoresistive materials. Interestingly, a discontinuity was observed in theI–V curves below the metal–insulator transition temperature(Tp) of LCMO electrodes. With increasing current, the voltage across the junction increaseslinearly at first, then deviates from linearity, then a discontinuity occurs and finally thevoltage recovers its linearity. With changing temperature, the discontinuity in theI–V curves happens at the same voltage of 209 mV, but starts at different currents. The discontinuity startingcurrent (Ic) decreases with increase of the temperature. The discontinuity disappears aboveTp.There is a hysteresis near the discontinuity when the current increases and decreases. The discontinuityin the I–V curves could indicate a novel switching process. This switching phenomenon occurs only inthe ferromagnetic metal state and strongly suggests a magnetism related effect. The physicsof the switching process has not been understood yet, but it could be of interest forpotential applications in spintronics devices.

Electron paramagnetic resonance measurements for (La,Y)2/3(Ca,Ba)1/3MnO3

We have measured the paramagnetic (PM) resonance behavior as a function of temperature for various manganese perovskites (La,Y)2/3(Ca,Ba)1/3MnO3 with an average A-site size 〈rA〉 basically covering the whole region of ferromagnetic (FM) metallic ground state. We show that at least three regions with different magnetic behaviors can be distinguished by two phase lines: Tonset−〈rA〉 and Tc−〈rA〉. For each given 〈rA〉, the complete PM and long-range FM behaviors appear above Tonset and below Tc, respectively; while some anomalous PM behaviors appear for the intermediate temperature range of Tonset<T<Tc . A possible magnetic transition process is discussed in order of decreasing temperature from high-T complete paramagnetism to low-T long-range ferromagnetism.

Comparative study of the magnetic phase diagrams of (La 1− yPr y) 0.7Ca 0.3MnO 3 with oxygen isotopes 16O and 18O

Transitions into the magnetically ordered state in function of the average A-cation radius, 〈 r A〉, have been studied by neutron powder diffraction for (La 1− y Pr y ) 0.7Ca 0.3MnO 3 perovskites (LPCM- y×100) with various oxygen isotopes [natural, p( 16O)∼99.7%, and substituted, p( 18O)∼75%]. Comparing both series of samples, qualitatively the same type of phase diagrams (PD) are found for Pr concentrations in the range 0.25⩽ y⩽1.0. This includes the homogeneous regions for small (FM metallic) and large (AFM insulating) y values and the phase-separated state between them. The most important difference is the shift in the PD of the 18O-samples towards the higher 〈 r A〉 (smaller y) border. This means that isotope substitution in LPCM causes a strong redistribution of the energy balance between the charge-ordered insulating and ferromagnetic metallic states.

Nd doping induced simultaneous magnetic and electronic transition in La 0.83− xNd xK yMnO 3

We have studied polycrystalline La 0.83− x Nd x K y MnO 3 using powder X-ray diffraction, magnetic and electrical resistance measurements. Nd doping induces a simultaneous transition on the structure, the magnetic order, and the electronic phase in La 0.83− x Nd x K y MnO 3. It exhibits a rhombohedral–orthorhombic structural transition, a ferromagnetic–paramagnetic transition associated with a metal–insulator transition, and a low temperature magnetic transition antiferromagnetic-like associated with a low-temperature insulating state in La 0.83− x Nd x K y MnO 3. Nd doping also results in the presentation of the second magnetoresistance ratio (MR) peak at low temperature. All these phenomena might be interpreted by Nd doping induced a phase separation between the ferromagnetic metallic state and the antiferromagnetic insulating state at low temperature.

Ferromagnetic insulating phase inPr1−xCaxMnO3

A ferromagnetic insulating (FM-I) state in ${\mathrm{Pr}}_{0.75}{\mathrm{Ca}}_{0.25}{\mathrm{MnO}}_{3}$ has been studied by neutron scattering experiment and theoretical calculation. The insulating behavior is robust against an external magnetic field, and is ascribed to neither the phase separation between a ferromagnetic metallic (FM-M) phase and a nonferromagnetic insulating one, nor the charge ordering. We found that the Jahn-Teller type lattice distortion is much weaker than ${\mathrm{PrMnO}}_{3}$ and the magnetic interaction is almost isotropic. These features resemble the ferromagnetic metallic state of manganites, but the spin exchange interaction J is much reduced compared to the FM-M state. The theoretical calculation based on the staggered type orbital order well reproduces several features of the spin and orbital state in the FM-I phase.

Anisotropic optical spectra of doped manganites with pseudocubic perovskite structure

To investigate the effect of charge-spin-orbital correlations on electronic structures, anisotropic optical conductivity spectra have been investigated for twin-free crystals of various doped manganites ${R}_{1\ensuremath{-}x}{A}_{x}{\mathrm{MnO}}_{3}$ $(R=\mathrm{Pr}$ and Nd, $A=\mathrm{Ca}$ and Sr; $0.4l~xl~0.7)$ with pseudo-cubic perovskite structure. In doped manganites having an antiferromagnetic ground state, a remarkable anisotropy has been observed in their optical spectra, reflecting the respective ferroic/antiferroic orders of ${e}_{g}$-orbital on Mn sites. Such orbital-order patterns give rise to characteristic optical spectra that show a strong spectral weight below $\ensuremath{\sim}2\mathrm{eV}$ with light polarization parallel to the direction of orbital lobes. Moreover, even in the ferromagnetic-metallic phases of ${\mathrm{Pr}}_{1/2}{\mathrm{Sr}}_{1/2}{\mathrm{MnO}}_{3}$ and ${\mathrm{Nd}}_{1/2}{\mathrm{Sr}}_{1/2}{\mathrm{MnO}}_{3},$ we observed substantial anisotropy in optical spectra. The results indicate the existence of orbital-polarized ferromagnetic-metallic phase in doped manganites although the ferromagnetic-metallic state attributed to the double-exchange interaction has been normally believed to accompany the orbital disordering.

Strong isotope effect in Sm 1− xSr xMnO 3 manganites near x=0.5

The effect of 16O→ 18O isotope substitution on electrical resistivity and magnetic susceptibility of Sm 1− x Sr x MnO 3 manganites is analyzed. It is shown that the oxygen isotope substitution drastically affects the phase diagram at the crossover region between the ferromagnetic metal state and that of antiferromagnetic insulator (0.4< x<0.6), and induces the metal–insulator transition at x=0.475 and 0.5. The nature of antiferromagnetic insulator phase is discussed.

Transport and structural study of pressure-induced magnetic states inNd0.55Sr0.45MnO3andNd0.5Sr0.5MnO3

Pressure effects on the electron transport and structure of ${\mathrm{Nd}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ $(x=0.45, 0.5)$ were investigated in the range from ambient to $\ensuremath{\sim}6\mathrm{GPa}.$ In ${\mathrm{Nd}}_{0.55}{\mathrm{Sr}}_{0.45}{\mathrm{MnO}}_{3},$ the low-temperature ferromagnetic metallic state is suppressed and a low-temperature insulating state is induced by pressure. In ${\mathrm{Nd}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3},$ the CE-type antiferromagnetic charge-ordering state is suppressed by pressure. Under pressure, both samples have a similar electron-transport behavior although their ambient ground states are much different. It is surmised that pressure induces an A-type antiferromagnetic state at low temperature in both compounds.