Ferromagnetic Metallic State Research Articles

Current-induced persistent magnetization in a relaxorlike manganite

A single crystal of 7% Fe-doped (La0.7Pr0.3)0.65Ca0.35MnO3 shows up as a typical relaxor ferromagnet, where ferromagnetic metallic (FM) and charge-orbital-ordered insulating clusters coexist with controllable volume fraction by external stimuli. There, the persistent FM state can be produced by an electric-current excitation as the filamentary region, the magnetization in which is increased by ∼0.4μB per Mn. A clear distinction from the current heating effect in a magnetic field, which conversely leads to a decrease in FM fraction, enables us to bidirectionally switch both the magnetization and resistance by applying the voltages with different magnitudes.

Effect of Ti4+ substitution on structural, transport and magnetic properties ofLa0.67Sr0.33Mn1−xTixO3

La0.67Sr0.33Mn1−xTixO3 (0≤x≤0.20) polycrystalline materials are prepared by employing lower annealing temperaturecompared to the temperatures reported for the materials of the same composition. Thetransport and magnetic properties of these materials are significantly differentfrom those compounds prepared at higher annealing temperature. Samples withx<0.10 show metal–insulator transition and those withx≥0.10 exhibit insulating behavior over the entire temperature range investigated. A gradualtransition occurs from the ferromagnetic-metallic state to the ferromagnetic-insulator statewith increasing Ti substitution. Lattice parameters and bond lengths of Mn andits near neighbors however do not change appreciably with the dopant contentx in these materials.It is shown that Ti4+ doping in the low-temperature annealed samples is inhomogeneous, resulting inisolated Mn rich regions that are connected by a variable range hopping polaron.

Phase separation in Eu0.58Sr0.42MnO3 under pressure

Resistivity measurements of Eu0.58Sr0.42MnO3 were carried out at different magnetic fields under ambient pressure and 1 GPa, respectively. The critical temperature TC of the ferromagnetic metallic state is 128.5 K under 1 GPa and 7 T. The antiferromagnetic insulating phase can be transformed into the ferromagnetic metallic phase by either magnetic field or pressure. Different from the effect of the magnetic field, a plateau is observed in the temperature dependence of the resistivity curve in the lower temperature under pressure, which indicates a phase separation. Magnetic phase diagrams at ambient pressure and 1 GPa are established.

Fe-doping-induced evolution of charge-orbital ordering in a bicritical-state manganite

Impurity effects on the stability of a ferromagnetic-metallic state in a bicritical-state manganite, ${({\mathrm{La}}_{0.7}{\mathrm{Pr}}_{0.3})}_{0.65}{\mathrm{Ca}}_{0.35}\mathrm{Mn}{\mathrm{O}}_{3}$, on the verge of metal-insulator transition have been investigated by substituting a variety of transition-metal atoms for Mn ones. Among them, Fe doping exhibits the exceptional ability to dramatically decrease the ferromagnetic transition temperature. Systematic studies on the magnetotransport properties and x-ray diffraction for the Fe-doped crystals have revealed that charge-orbital ordering evolves down to low temperatures, which strongly suppresses the ferromagnetic-metallic state. The observed glassy magnetic and transport properties as well as diffuse phase transition can be attributed to the phase-separated state where short-range charge-orbital-ordered clusters are embedded in the ferromagnetic-metallic matrix. Such a behavior in the Fe-doped manganites form a marked contrast to the Cr-doping effects on charge-orbital-ordered manganites known as the impurity-induced collapse of charge-orbital ordering.

Elastic and magnetic properties of the bilayer manganese oxide(Pr0.6La0.4)1.2Sr1.8Mn2O7

The elastic and magnetic properties of a single crystal of the bilayer manganese oxide ${({\mathrm{Pr}}_{0.6}{\mathrm{La}}_{0.4})}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ have been investigated by means of ultrasonic and high-field magnetization measurements. Remarkable changes in the elastic constants ${C}_{11}$, ${C}_{33}$, ${C}_{44}$, and ${C}_{66}$ as a function of temperature and magnetic field have been observed. In particular, a distinct elastic anomaly was observed at low temperatures and in magnetic fields when crossing the phase boundary between the paramagnetic insulating and the field-induced ferromagnetic metallic state. A pronounced elastic softening as a function of magnetic field $(H)$ appears across the boundary of the low-temperature magnetic phase below around $40\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, accompanied by a distinct hysteresis. In the high-field region, however, these elastic constants exhibit a monotonic increase upon increasing the magnetic field. The high-field magnetization measurements characterizing the magnetic state point out a strong coupling between the elastic strain and the magnetic moment. The data can be described reasonably well considering a strong coupling between elastic strain and magnetic susceptibility ${\ensuremath{\chi}}_{m}=\ensuremath{\partial}M∕\ensuremath{\partial}H$.

Vacancy-induced enhancement of magnetic interactions in (Ca, Na)-doped lanthanum manganites

Structural, electric, and magnetic properties of the bulk polycrystalline samples with nominal composition La0.7Ca0.3−xNaxMnO3 (x=0−0.10) have been investigated in the work. As follows from the results of chemical and x-ray structural analyses, the increase in x brings about the rise in the number of structural vacancies in both lanthanum and oxygen sublattices. The influence of the variation of the vacancy number on peculiar features of the transition from ferromagnetic metallic state to paramagnetic state with activated conductivity has been studied in detail. The conclusion is made that the rise in the number of structural vacancies gives rise not only to the enhancement of magnetic inhomogeneity and broadening of the magnetic transition, but also to the increase in the temperature of magnetic transition. It is demonstrated that the changes in magnetic parameters also result in the increase of the temperatures of resistivity and magnetoresistance peaks as well as in the change of the character of the temperature dependence of electric resistance.

Epitaxial growth and surface properties of half-metal NiMnSb films

We present, herein, an extended study of the half-Heusler alloy NiMnSb, starting with thedeposition technique, continuing with the basic structural and magnetic properties of thethin films, and finishing with the electronic and compositional properties of their surfaces.The experimental methods we apply combine magnetization and magnetoresistivitymeasurements, atomic force microscopy, ferromagnetic resonance, x-ray and neutrondiffraction, low energy electron diffraction, angle resolved x-ray photoemission, extendedx-ray absorption fine structure spectroscopy, soft x-ray magnetic circular dichroism andspin polarized inverse photoemission spectroscopy. We find that stoichiometric surfacesexhibit close to 100% spin polarization at the centre of the surface Brillouin zone at theFermi edge at ambient temperatures. There is strong evidence for a moment reorderingtransition at around 80 K which marks the crossover from a high polarization state(T<80 K) to a more representative metallic ferromagnetic state(T>80 K). The results from the different experimental techniques are successively reviewed, withspecial emphasis on the interplay between composition and electronic structure of theNiMnSb film surfaces. Surface segregation, consistent with a difference in free enthalpybetween the surface and the bulk, is induced by annealing treatments. This surfacesegregation greatly reduces the surface polarization.

Correlation between structural, transport, and magnetic properties in Sm1−xAxMnO3 (A=Sr,Ca)

Transport, magnetic, and structural properties of Sm1−xAxMnO3, where A is Ca and Sr, have been investigated systematically over the range of doping 0⩽x⩽0.52. For x&amp;lt;0.30, both systems are ferromagnetic (FM) insulator, and the resistivity (ρ) can be described well by polaron hopping model. Above x=0.30, Sr-doped compounds exhibit large negative magnetioresistance (MR) in the FM metallic state and charge∕orbital ordering over a narrow doping range around x=0.50. On the other hand, Ca-doped system does not show any insulator to metal transition and MR, possibly due to the smaller average A-site cationic radius ⟨r⟩ than that for Sr-doped one. The magnetic ground state of Ca-doped system changes from FM to canted antiferromagnetic above x=0.30. A strong coupling between transport, magnetic, and structural properties has been established from this study. The results are summarized in (x−T) phase diagrams.

Charge state modification in Mn site substituted CMR manganites: strong deleteriousinfluence on the ferromagnetic–metallic state

The effect of charge state modification at the Mn site on the physicalproperties of CMR manganites is reported. With a view to avoidingadditional complexity of local spin coupling effects, Mn site substitution ofLa0.67Ca0.33MnO3 is carried out with appropriate diamagneticions—Zn2+, Zr4+, Ta5+ and W6+—of different valence states. The substitution results in size changes of the unit cell andenhanced local structural distortions, which increase in the order Zn, Zr, Ta and W.The ground state is ferromagnetic–metallic below a certain critical concentrationxc of the substituents, beyond which the magnetic ground stateshows a glassy behaviour. The phase transition temperatures(TMI andTc) decreasewith substitution, but to different extents. The observed suppression rates of the Curie temperature,Tc, of∼39 K/at.% and∼45 K/at.% respectivelyfor Ta5+ and W6+ substituted compounds are the highest reported in the Mn site substituted CMR manganites.Besides the modification of majority carrier concentration due to the increased (decreased)Mn3+ concentration and enhanced local structural effects, the local electrostatic potential of thesubstituents seems to contribute to the unusually strong reduction in the itinerantferromagnetism and the observed glassy states.

Electron spin resonance study of bulk and nanosized La 0.875Sr 0.125MnO 3

The structural, transport and electron spin resonance properties of bulk and nanosized La 0.875Sr 0.125MnO 3 prepared by a sol–gel method have been investigated. The bulk sample has an orthorhombic structure and a ferromagnetic insulating ground state. The ESR spectra indicate the coexistence of the ferromagnetic insulating and ferromagnetic metallic phases below T C . In addition to a sharp peak in the vicinity of T C , another sharp peak close to T OO is clearly observed in the intensity of the spectra, which may be correlated with the structural transition and orbital ordering at this temperature. For the nanosized sample, a drastically different behavior is found. With a rhombohedral structure down to 70 K, the nanosized sample shows a ferromagnetic metallic ground state. The ESR studies reveal the coexistence of the paramagnetic and ferromagnetic resonance signals. The resonance intensity shows a broad peak around 200 K, which may be due to the wide ferromagnetic transition in the nanoparticle.

Effect of strain and electric field on the electronic soft matter in manganite thin films

We have studied the effect of substrate-induced strain on the properties of the hole-doped manganite (La$_{1-y}$Pr$_{y}$)$_{0.67}$Ca$_{0.33}$MnO$_{3}$ ($y$ = 0.4, 0.5 and 0.6) in order to distinguish between the roles played by long-range strain interactions and quenched atomic disorder in forming the micrometer-scale phase separated state. We show that a fluid phase separated (FPS) state is formed at intermediate temperatures similar to the strain-liquid state in bulk compounds, which can be converted to a metallic state by applying an external electric field. In contrast to bulk compounds, at low temperatures a strain stabilized ferromagnetic metallic (FMM) state is formed in the $y$ = 0.4 and 0.5 samples. However, in the $y$ = 0.6 sample a static phase separated (SPS) state is formed similar to the strain-glass phase in bulk compounds. Hence, we show that long-range strain interaction plays a dominant role in forming the micrometer-scale phase separated state in manganite thin films.

The antisymmetric double exchange spin state

A reconsideration of the lowest antisymmetric spin state in the Anderson–Hasegawa double exchange (DE) model removes several complications which are manifest in the understanding of DE manganite systems. The new approach (1) describes the antiferromagnetic insulating charge ordered state as an excited state of the symmetric DE metallic ferromagnetic state. (2) Mends the discrepancies in the deGennes and Anderson–Hasegawa DE descriptions and can be readily modified to incorporate polaron and Jahn–Teller effects. (3) Reduces the DE binding energy, so that level crossings between symmetric and antisymmetric states become more likely. (4) Asks for a second (orientational) quantization in addition to the standard k-quantization in the case of DE magnetic lattices. (5) Opens a new way of calculating spin-dependent bandstructures simultaneously with the local spin structure. (6) Allows a straightforward interpretation of many so-called peculiar and puzzling experimental results such as sequences of giant positive/negative magnetoresistances, electroresistive effects, giant magnetoabsorption etc.

Transport properties of Cs doped Pr 2/3(Ba 1− xCs x) 1/3MnO 3 manganites

Magneto-transport behaviour of Pr 2/3Ba 1/3MnO 3 manganite (PBMO) doped with Cs at Ba-site is reported here. The observed decrease in the Curie temperature T C and the two insulator–metal like transitions (at T P1 and T P2) in PBMO with Cs doping have been discussed in the light of the decrease in the carrier concentration emanating due to the change in the Mn 3+/Mn 4+ ratio. While the peak magnetoresistance (MR) value at T P1 (reminiscent of the intra-granular behaviour) remains nearly invariant, the inter-granular (grain boundary) behaviour is clearly reflected through a substantial higher value of MR below T P2. The insulating behaviour above T P1 has been co-related to the decrease in the density of states at the Fermi level and is substantiated by the observed decrease in the resistivity with magnetic field. Electron–magnon scattering process has been invoked to consider the ferromagnetic metallic state below T P2.

Numerical Analysis of Heat Transport Behavior in the Ferromagnetic Metallic State of La0.80Ca0.20MnO3 Manganites

The lattice contribution to the thermal conductivity (κph) in La0.80Ca0.20 MnO3 manganites is discussed within the Debye-type relaxation rate approximation in terms of the acoustic phonon frequency and relaxation time. The theory is formulated when heat transfer is limited by the scattering of phonons from defects, grain boundaries, charge carriers, and phonons. The lattice thermal conductivity dominates in La–Ca–MnO manganites and is an artifact of strong phonon-impurity and -phonon scattering mechanisms in the ferromagnetic metallic state. The electronic contribution to the thermal conductivity (κe) is estimated following the Wiedemann–Franz law. This estimate sets an upper bound on κe, and in the vicinity of the Curie temperature (240 K) κe is about 1% of total heat transfer of manganites. Another important contribution in the metallic phase should come from spin waves (κm). It is noticed that κm increases with a T2 dependence on the temperature. These channels for heat transfer are algebraically added and κtot develops a broad peak at about 55 K, before falling off at lower temperatures. The behavior of the thermal conductivity in manganites is determined by competition among the several operating scattering mechanisms for the heat carriers and a balance between electron, magnon, and phonon contributions. The numerical analysis of heat transfer in the ferromagnetic metallic phase of manganites shows similar results as those revealed from experiments.

Magnetic structure and transport properties of atomically disordered crystals of two-dimensional manganites La2−2x Sr1+2x Mn2O7

The magnetic structure and transport properties of partially disordered crystals of two-dimensional manganites La2−2x Sr1+2x Mn2O7 (x = 0.3, 0.4) are studied over a wide range of temperatures. The crystals are transformed into an atomically disordered state under irradiation with fast neutrons at a dose of 2 × 1019 cm−2. The average concentration of substitutional defects in the crystal is ≈4%. It is found that substitutional defects are responsible for the transition of these manganites from the ferromagnetic metal state to the insulator state with a spin glass structure. The results obtained are discussed in terms of the ratio between the kinetic energy of charge carriers and the exchange energy of localized spins.

Structure and magnetic transport properties of Ge doped La(Co,Si)13 intermetallics

The crystal structure and the magnetic and electronic transport properties of Ge doped LaCo10.5Si2.5−xGex (x=0–0.3) intermetallics have been investigated using powder x-ray diffraction and magnetic and electrical measurements. According to the x-ray diffraction pattern analysis, Ge as a dopant decreases the stability of the cubic LaCo 1:13 phase and stabilizes the Si based La(Co,Si) 1:13 tetragonal phase. From the authors’ structural refinement, the authors found that Ge prefers to occupy the 16l(2) position in the tetragonal phase. LaCo10.5Si2.2Ge0.3 shows ferromagnetic order with a metamagnetic state around 350K. An anomaly in resistivity is observed at around 367K, which is associated with the metamagnetic transition. The resistivity follows ρ(T)=ρo+aTn rule in the temperature range of 4–400K with n=1.17(3) for the half metallic state and n=2 for the normal metallic state, respectively. This type of discontinuity can be interpreted as a crossover from a low temperature metamagnetic half metallic state to the normal ferromagnetic-metallic state.

Nanoscale ferromagnetism in phase-separated manganites

Magnetic domain structures in phase-separated manganites were investigated by low-temperature Lorentz electron microscopy, in order to understand some unusual physical properties such as a colossal magnetoresistance (CMR) effect and a metal-to-insulator transition. In particular, we examined a spatial distribution of the charge/orbital-ordered (CO/OO) insulator state and the ferromagnetic (FM) metallic one in phase-separated manganites; Cr-doped Nd 0.5 Ca 0.5 MnO 3 and ( La 1 - x Pr x ) 5 / 8 Ca 3 / 8 MnO 3 with x = 0.375 , by obtaining both the dark-field images and Lorentz electron microscopic ones. It is found that an unusual coexistence of the CO/OO and FM metallic states below a FM transition temperature in the two compounds. The present experimental results clearly demonstrated the coexisting state of the two distinct ground states in manganites.

A -site disorder induced collapse of charge-ordered state and phase separated phase in manganites

The effects of A-site cational size mismatch (A-site disorder) on the stability of charge-ordered states and phase separated phase in a series of manganites with constant A-site ionic average radii ⟨rA⟩=1.18Å but different A-site ionic size mismatches σ2 are experimentally investigated. It is revealed that the charge/orbital ordered antiferromagnetic ground state becomes destabilized and eventually collapses into coexisting of the predominant ferromagnetic metal (FMM) state and short-range charge/orbital ordered state with increasing σ2, resulting in enhanced colossal magnetoresistance. However, further increasing A-site disorder will suppress the FMM state and seem to favor a cluster-glass insulating state due to the severe electronic localization.

Magnetic and electronic transitions in charge-ordered Nd 0.50Ca 0.47Ba 0.03MnO 3 manganite

The ABO 3 type charge-ordered antiferromagnetic Nd 0.50Ca 0.50MnO 3 (NCMO) manganite is doped at A-site by 3%of Ba 2+ for Ca 2+. The resulting system, Nd 0.50Ca 0.47Ba 0.03MnO 3 (NCBMO), is studied for the effects of Ba doping on the magnetic and electronic properties. On application of magnetic field to NCBMO, strongly correlated successive sharp metamagnetic and electronic transitions are observed from antiferromagnetic-insulating to ferromagnetic-metallic state at 2.5 K. The critical magnetic field ( H c) required for metamagnetism is found to reduce drastically from 15 T for undoped NCMO to 3 T for NCBMO. On increasing the temperature, the H c of NCBMO passes through a minimum. This behavior of H c of NCBMO contrasts to that of NCMO. The results are discussed in context of A-site cation disorder and size.

First-principles study on the metallic antiferromagnet Co[PhPO 3]·H 2O

The electronic structure and the magnetic properties of transition metal phosphonate Co(PhPO 3)·H 2O have been studied by first-principles within the density-functional theory (DFT) and the full potential linearized augmented plane wave (FP-LAPW) method. The total energy, the total magnetic moment, the atomic spin magnetic moments and the density of states(DOS) of Co(PhPO 3)·H 2O were all calculated. The calculations reveal that the title compound is a metallic antiferromagnet and has a metallic ferromagnetic metastable state, which are in good agreement with the experiment. The spin magnetic moment of Co(PhPO 3)·H 2O is about 4.93 μ B per molecule, and it is mainly assembled at the cobalt atom, at the same time, with a little contribution from the P, O1, O2, O3.