Ferromagnetic Double Exchange Research Articles

Structural study on the phase separation in Sm1−xCaxMnO3 (0.8≤x≤0.92)

We have investigated the details of phase separation in the electron-doped manganites Sm1−xCaxMnO3 (0.8≤x≤0.92) by using temperature dependent x-ray diffraction. The lattice of the low-temperature orthorhombic phase is distorted to various extents according to the strength of the coexisting monoclinic phase. Meanwhile, the room-temperature lattice becomes more distorted due to the increasing Jahn–Teller active Mn3+ ions by Sm3+ doping. On the other hand, the substitution of Ca2+ with Sm3+ introduces the Mn3+–Mn4+ pairs, which favor the double exchange ferromagnetism. As a result of the competition between the lattice distortion and electron doping level, the modified phase diagram have been presented, including the high-temperature Griffiths phase and the ground state with competing multiphases.

Electron-doped Sm 1−xSr xMnO 3 perovskite manganites: Crystal and magnetic structures and physical properties

We present the results of a study of electron-doped Sm 1− x Sr x MnO 3 ( x>0.5) perovskite manganites by combining high-resolution neutron powder diffraction with measurements of resistivity, magnetization and magnetic susceptibility. Although investigated Sm 0.45Sr 0.55MnO 3 and Sm 0.37Sr 0.63MnO 3 compounds belonging to the same phase diagram area differ significantly in the strontium content, they are homogeneous antiferromagnetic (AF) insulators and do not exhibit CMR. They have different crystallographic symmetries (orthorhombic Pbnm and tetragonal I4 /mcm, respectively) in the entire temperature range under study (1.5–288 K), differ in the type of spin ordering at low temperatures (AF -A and AF -C), are characterized by different orbital polarizations ( d x 2 − y 2 and d 3 z 2 − r 2), and possess two- and one-dimensional magnetic properties, respectively. The lack of magnetoresistance for these compositions is explained by the lack of coexisting magnetic phases involving double exchange ferromagnetism, in contrast to what is observed for the magnetoresistive Sm 1− x Sr x MnO 3 compounds, that is with x⩽0.52.

Effect of Ga doping on charge transport mechanism of La0.85Zr0.15Mn1−xGaxO3

Structural properties, temperature-dependent resistivity ρ(T) and thermoelectric power S(T) of La0.85Zr0.15Mn1−xGaxO3 (LZMGO) manganites with 0.0≤x≤0.06 have been extensively investigated. It is found that the Mn–O–Mn bond angle decreases and the Mn–O bond length increases with increasing Ga content, indicative of a significant distortion of MnO6 octahedra in LZMGO. The local lattice distortion and suppression of double exchange ferromagnetism induced by the Ga doping shift both Curie temperature (TC) and metal-insulator transition temperature (TMI) toward lower temperatures. Followed by the metal-insulator transition, ρ(T) exhibits an insulatinglike behavior accompanied by a relatively small value of S(T) below T∗ at low temperatures. An anomalous peak observed in S(T) is attributed to an enhancement of electron-magnon interaction caused by the Ga doping and a decrease in magnetic entropy near TMI. Three different charge transport mechanisms were identified in three temperature regions based on detailed analyses of ρ(T) and S(T) data. The small-polaron hopping mechanism governs the charge transport in the high-temperature paramagnetic region (T>TMI). The polaron binding energy determined from the ρ(T) and S(T) data increases with increasing Ga content, suggesting that polaron in La0.85Zr0.15MnO3 has a magnetic nature. In addition, the electron-magnon scattering dominates the charge transport in the intermediate-temperature metallic region (T∗<T<TMI), whereas the transport behavior in the low-temperature insulating region (T<T∗) can be described by the three-dimensional variable-range-hopping model.

Mean-field model for the CE-phase of manganites

The CE phase of La0.5Ca0.5MnO3 displays long-range magnetic, charge and orbital order of the eg states of the Mn ions. The magnetic order of the t2g spins of Mn is determined by the antiferromagnetic superexchange and the ferromagnetic double-exchange. The checkerboard charge and the orbital order is the consequence of the Jahn-Teller coupling of the eg orbitals to the lattice. Using a mean-field slave-boson approach for the eg electrons in two orbitals per site with excluded multiple occupancy and Hund's rule coupling between the eg and t2g states, we obtain the tight-binding band structure of the CE phase. The unit cell of the CE phase consists of 16 sites. The 32 eg bands in the Brillouin zone are grouped into two sets of 16 bands separated by a charge order gap. The charge gap does not directly affect the properties of the compound. If long-range orbital order is included an additional gap opens at the Fermi level.

Magnetic and electric transport properties of Nd0.75Sr1.25Co1−xMnxO4

We present the results of a systematic investigation of crystal structure, dc magnetization, ac susceptibility, resistivity, and magnetoresistance (MR) of Nd0.75Sr1.25Co1−xMnxO4 (0≤x≤0.3) polycrystals. All synthesized specimens are indexed in the same tetragonal space group I4/mmm with random occupation of Co and Mn ions at the identical site. The refinement result confirms the tetragonal distortion of the CoO6 octahedron with elongation along the c axis. The substitution of the Mn ions at Co site brings about the suppression of ferromagnetism as well as the enhancement of antiferromagnetism. The coexistence of ferromagnetic double exchange interactions and antiferromagnetic superexchange interactions and the suppression of ferromagnetism with increasing Mn doping are substantiated by the isothermal magnetization hysteresis loops. The result suggests that the substitution creates more antiferromagnetic bonds with superexchange interactions at the expense of the existing Co–O–Co bonds with ferromagnetic double exchange interactions. At low temperature, a crossover from ferromagnetic cluster-glass phase to spin-glass phase is shown in the dc and ac magnetic measurements. For all the specimens, the resistivity ρ(T) follows semiconducting behavior (dρ/dT<0) in the whole measured temperature region. The substitution induces an obvious increase in resistivity, which originates from the diminishing of ferromagnetic double exchange interactions and the localization of charge carriers caused by the disorder for the substitution. The system presents negative MR due to tunneling effect at low temperatures and positive MR at high-temperature range.

Anomalous ground states at the interface between two transition-metal compounds

The effects of strong covalency across a strongly correlated interface between two transition-metal compounds are studied. Since the charge transfer is directional, the lowest electron-removal and -addition states are often not involved in the formation of covalent bonds across the interface. This paper shows that this can lead to the formation of unusual ground states not found in the bulk. For cuprates, the formation of ``Zhang-Rice triplets'' is observed. For nickelates, we demonstrate the possibility of in-plane or out-of-plane orbital switching, whereas cobaltates are prone to spin switching. For Co and Fe compounds, a change between antiferromagnetic superexchange and ferromagnetic double exchange is found. Calculations of x-ray magnetic dichroism are presented, which could provide insight into the presence of these unusual ground states.

Magnetization relaxation and collective spin excitations in correlated double-exchange ferromagnets

We study spin relaxation and dynamics of collective spin excitations in correlated double-exchange ferromagnets. For this, we introduce an expansion of the Green's functions equations of motion that treats nonperturbatively all correlations between a given number of spin and charge excitations and becomes exact within a subspace of states. Our method treats relaxation beyond Fermi's golden rule while recovering previous variational results for the spin-wave dispersion. We find that the momentum dependence of the spin-wave dephasing rate changes qualitatively due to the on-site Coulomb interaction, in a way that resembles experiment, and depends on its interplay with the magnetic exchange interaction and itinerant spin lifetime. We show that the collective spin relaxation and its dependence on the carrier concentration depend sensitively on three-body correlations between a spin excitation and a Fermi sea electron and hole. The above spin dynamics can be controlled via the itinerant carrier population.

Temperature evolution of structure and magnetic properties in the perovskite Sr 2MnSbO 6

The structural and magnetic properties of the perovskite Sr 2MnSbO 6 have been studied. Combining neutron and X-ray powder diffraction data, the temperature evolution of the structural parameters was investigated with the Rietveld method between 2 and 1000 K. The crystal structure is tetragonal (space group I4/ m) within the temperature interval of 2–750 K and cubic (space group Fm-3 m) above 750 K. Both octahedral B-site positions were found to be partially occupied by Mn and Sb, but with different Mn/Sb ratios. The magnetic susceptibilities showed irreversibility between field cooled and zero-field cooled (ZFC) conditions and spin glass like magnetic dynamics including aging and memory phenomena at temperatures below 30 K; all appearing well above a broad maximum at 13 K in the ZFC susceptibility curves. This suggests that the material reaches an unconventional spin-glass state at low temperatures, possibly arising from a competitive situation between the double exchange (ferromagnetism) and the super-exchange (antiferromagnetism). Neutron diffraction patterns showed no evidence of a long-range magnetic ordering at 2 K which is consistent with spin glass behavior. The factors governing the observed structural and magnetic properties of Sr 2MnSbO 6 are discussed and compared with those of other quaternary Mn- and Sb-containing perovskites. Graphs of the temperature of magnetic phase transitions as functions of the cation size were constructed and are discussed for the AB 3+ 1/2B 5+ 1/2O 3 series with isomorphous substitution of B 3+ and B 5+ cations. Possible influence of the A-cation sublattice on magnetic properties is also shortly considered.

Effects of strain on orbital ordering and magnetism at perovskite oxide interfaces:LaMnO3/SrMnO3

We study how strain affects orbital ordering and magnetism at the interface between SrMnO$_3$ and LaMnO$_3$ from density-functional calculations and interpret the basic results in terms of a three-site Mn-O-Mn model. Magnetic interaction between the Mn atoms is governed by a competition between the antiferromagnetic superexchange of the Mn t$_{2g}$ core spins and the ferromagnetic double exchange of the itinerant e$_g$ electrons. While the core electrons are relatively unaffected by the strain, the orbital character of the itinerant electron is strongly affected, which in turn causes a large change in the strength of the ferromagnetic double exchange. The epitaxial strain produces the tetragonal distortion of the MnO$_6$ octahedron, splitting the Mn-e$_g$ states into x$^2$-y$^2$ and 3z$^2$-1 states, with the former being lower in energy, if the strain is tensile in the plane, and opposite if the strain is compressive. For the case of the tensile strain, the resulting higher occupancy of the x$^2$-y$^2$ orbital enhances the in-plane ferromagnetic double exchange owing to the larger electron hopping in the plane, causing at the same time a reduction of the out-of-plane double exchange. This reduction is large enough to be overcome by antiferromagnetic superexchange, which wins to produce a net antiferromagnetic interaction between the out-of-plane Mn atoms. For the case of the in-plane compressive strain, the reverse happens, viz., that the higher occupancy of the 3z$^2$-1 orbital results in the out-of-plane ferromagnetic interaction, while the in-plane magnetic interaction remains antiferromagnetic. Concrete density-functional results are presented for the (LaMnO$_3$)$_1$/(SrMnO$_3$)$_1$ and (LaMnO$_3$)$_1$/(SrMnO$_3$)$_3$ superlattices for various strain conditions.

Effects of Selective Dilution on the MagneticProperties of La0.7Sr0.3Mn1-xM'xO3 (M' = Al, Ti)

The magnetic lattice of mixed-valence Mn ions in La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ is selectively diluted by partial substitution of Al or Ti for Mn. The ferromagnetic transition temperature $T_\mathrm{c}$ and the saturation magnetization $M_\mathrm{s}$ both decrease with substitution. By presenting the data in terms of selective dilution, $T_\mathrm{c}$ in the low-doping region is found to follow the relation $T_\mathrm{c}=T_\mathrm{c0}(1-n_\mathrm{p})$, where $T_\mathrm{c0}$ refers to the undiluted system and $n_\mathrm{p}$ is the dilution concentration defined as $n_\mathrm{p}=x/0.7$ or $n_\mathrm{p}=x/0.3$ for $M^\prime=$ Al or Ti, respectively. The scaling behavior of $T_\mathrm{c}(n_\mathrm{p})$ can be analyzed in the framework of the molecular-field theory and still valid when Mn is substituted by both Al and Ti. The results are discussed with respect to the contributions from ferromagnetic double exchange and other possible antiferromagnetic superexchange interactions coexisting in the material.

Half-metallic monolayer superlattices with no net magnetization

We propose a viable approach to artificial half-metallic antiferromagnets (HM-AFMs) with no spontaneous magnetization (or fully compensated ferrimagnets) through introduction of compositional periodicity into a tetrahedrally coordinated magnetic compound, which consists of alternating monolayers of transition-metal chalcogenides: $TX/MX$ ($X=\text{S}$, Se, and Te). By using a full-potential augmented plane-wave plus local-orbitals method within density-functional theory, we find two promising series of monolayer superlattices (MSLs), $\text{Cr}X/\text{Fe}X$ and $\text{V}X/\text{Co}X$, where two constituent magnetic ions in a supercell have antialigned local moments that exactly cancel by virtue of the integer filling of one spin channel. All of these MSLs with [001] orientation are predicted to be bulk HM-AFMs, whereas CrS/FeS and CrSe/FeSe are the only systems that show half-metallicity among MSLs with [111] orientation. Spin-orbit coupling of spin-polarized conduction electrons can be neglected and never destroy the half-metallicity. The relation between bonding nature and magnetic interaction is discussed in terms of the ``ghost-bond-orbital model''; the coexisting covalency in $T\text{\ensuremath{-}}X$ bonds and ionicity in $M\text{\ensuremath{-}}X$ bonds would stabilize and enhance the ferromagnetic double exchange and antiferromagnetic superexchange interactions within each local bond, respectively, realizing the half-metallic antiferromagnetism of the $TX/MX$ systems.

Structural and magnetic properties of polycrystalline La0.77Sr0.23Mn1−xCuxO3 (0≤x≤0.5) manganites

The effect of the substitution for Mn with Cu in the polycrystalline La0.77Sr0.23Mn1−xCuxO3 (0 ≤ x ≤ 0.5) manganites upon the structural and magnetic properties has been investigated by means of X-ray diffraction (XRD) and magnetization measurements. The results indicate that replacing Mn by Cu in La0.77Sr0.23Mn1−xCuxO3 system led to a change in crystal structure at room temperature from rhombohedral phase (R-3c space group) to orthorhombic phase (Pmna space group) at x ≥ 0.3. Paramagnetic to ferromagnetic phase transition at Curie temperature, TC, is observed. It was found that substitution of Cu on the Mn site causes a reduction in TC. The analysis of the crystallographic data suggested a strong correlation between structural and magnetism, for instance a relationship between a distortion of the MnO6 octahedron and the reduction in the Curie temperature. At x ≥ 0.3 the spin glass-like behaviour is evidence by a cup in the temperature-dependent magnetization, M(T), curves due to the appearance of antiferromagnetic (AFM) superexchange interaction imposed with ferromagnetic (FM) double exchange (DE) interaction. The variation of the effective magnetic moment, μeff, upon Cu-doping level at B-site is analyzed in terms of structure phase transition. Magnetization at lower temperatures is less in the doped samples and decreases with increasing the Cu contents. A reduction in the saturated magnetic moments with increasing of the Cu substitution is observed in the concentration range x = 0–0.2.

Effect of Co doping on the magnetic properties of La 0.85Ag 0.15(Mn 1−yCo y)O 3

The mixed valent manganites (La 0.85Ag 0.15)MnO 3 with perovskite structure has been prepared by doping up to 50% of Co at the Mn site. Paramagnetic (PM) to ferromagnetic (FM) transitions have been observed in all the prepared materials. However, the long-range magnetic ordering observed in (La 0.85Ag 0.15)MnO 3 is systemically reduced to cluster glass-type (short-range) of FM ordering due to the introduction of Co. The FM transition temperature was found to decrease with increase in Co doping up to 20% and for further increase in Co doping, the T c was found to increase. They are explained on the basis of competition between FM double exchange interactions in Mn–O–Mn and Co–O–Co networks. In addition to PM–FM transition, evidences of FM to antiferromagnetic (AFM), and AFM to reentrant spin-glass transitions have been observed. The shift in spin-glass freezing temperature, T f has been observed from the frequency variation of ac susceptibility measurements. The observed magnetic transitions are explained on the basis of magnetic interactions in different Mn–O–Mn, Mn–O–Co and Co–O–Co networks and such transitions are also observed from the measurement of third harmonic susceptibility. Metal–insulator transition and colossal magneto-resistivity have been observed up to 10% of Co doping.

Zener Double Exchange from Local Valence Fluctuations in Magnetite

Magnetite (Fe3O4) is a mixed valent system where electronic conductivity occurs on the B site (octahedral) iron sublattice of the spinel structure. Below T(V)=123 K, a metal-insulator transition occurs which is argued to arise from the charge ordering of 2+ and 3+ iron valences on the B sites (Verwey transition). Inelastic neutron scattering measurements show that optical spin waves propagating on the B site sublattice (approximately 80 meV) are shifted upwards in energy above T_{V} due to the occurrence of B-B ferromagnetic double exchange in the mixed valent phase. The double exchange interaction affects only spin waves of Delta(5) symmetry, not all modes, indicating that valence fluctuations are slow and the double exchange is constrained by short-range electron correlations above T(V).

Analysis of the electronic configuration of the pulsed laser deposited La 0.7Ca 0.3MnO 3 thin films

The electronic properties of La 0.7Ca 0.3MnO 3− δ thin films grown by the pulsed reactive crossed beam laser ablation method are investigated. The effects of post-deposition annealing of epitaxial La 0.7Ca 0.3MnO 3− δ thin films have been investigated using X-ray photoelectron spectroscopy (surface sensitive) and hard X-ray absorption spectroscopy (bulk sensitive). The films deposited in the high vacuum are oxygen deficient and contain mostly Mn 3+. High temperature annealing in a flowing oxygen atmosphere partially changes the Mn oxidation state from +3 towards +3.4. These changes should favor a metal-like conduction and a ferromagnetic double exchange transport mechanism in the annealed thin films.

Electron Leakage and Double-Exchange Ferromagnetism at the Interface between a Metal and an Antiferromagnetic Insulator:CaRuO3/CaMnO3

Density-functional electronic structure studies of a prototype interface between a paramagnetic metal and an antiferromagnetic (AFM) insulator (CaRuO(3)/CaMnO(3)) reveal the exponential leakage of the metallic electrons into the insulator side. The leaked electrons in turn control the magnetism at the interface via the ferromagnetic (FM) Anderson-Hasegawa double exchange, which competes with the AFM superexchange of the bulk CaMnO3. The competition produces a FM interfacial CaMnO3 layer (possibly canted); but beyond this layer, the electron penetration is insufficient to alter the bulk magnetism.

Non-Heisenberg spin dynamics of double-exchange ferromagnets with Coulomb repulsion

With a variational three--body calculation we study the role of the interplay between the onsite Coulomb, Hund's rule, and superexchange interactions on the spinwave excitation spectrum of itinerant ferromagnets. We show that correlations between a Fermi sea electron--hole pair and a magnon result in a very pronounced zone boundary softening and strong deviations from the Heisenberg spinwave dispersion. We show that this spin dynamics depends sensitively on the Coulomb and exchange interactions and discuss its possible relevance to experiments in the manganites.

Magnetic and superconducting properties of RuSr2GdCu2O8

Magnetization measurements of the magnetic (TM≈138K) superconductor (TS≈42K) RuSr2GdCu2O8 in pulsed magnetic fields up to 47T reveal at 48K and the highest available magnetic field a Ru contribution to the measured magnetic moment of about 2.2μB/formula unit. This value is indicative of a mixed-valence state of the Ru ions involving Ru5+(S=3/2) and Ru4+(S=1) ions. We propose that the mixed-valence state combined with charge transport in the RuO2 planes results in a competition between ferromagnetic double exchange involving Ru5+ and Ru4+ ions and antiferromagnetic superexchange involving Ru5+ ions. This causes magnetic-phase separation in ferromagnetic (non-superconducting) and antiferromagnetic (superconducting) domains. We conclude that the magnetic and superconducting properties of the ruthenocuprates critically depend on the Ru5+/Ru4+ ratio which can be affected by the preparation conditions.

Spin disorder in Fe-doped manganites

Ln0.7M0.3MnO3 compounds are well-known ferromagnets mediated by a double exchange mechanism. As Mn atoms are substituted by Fe in the ratio Mn1−xFex the magnetic structure dramatically changes, because the ferromagnetic double exchange chain is broken. At low Fe concentrations all compounds are magnetically ordered. For intermediate values ferro (FM), antiferro (AF) and paramagnetic (PM) phases co-exist in a large temperature range, and at x≈0.2 spin or cluster-glass behavior is found. Magnetization, Mössbauer, polarized and low angle neutron scattering as well as muon spin relaxation experiments have been performed on 0⩽x⩽0.30 compounds showing the transit from long range ferromagnetism to spin glass. Co-existence of FM and AF clusters of different size has been found for all doped compounds.

Vacancy-enhanced ferromagnetism in Fe-doped rutileTiO2

Based on a series of supercell density functional calculations of Fe-doped $\mathrm{Ti}{\mathrm{O}}_{2}$ both with and without O vacancy $({V}_{\mathrm{O}})$, we show that ${V}_{\mathrm{O}}$ plays an important role in determining the magnetic properties of the dilute magnetic semiconductors (DMS). Without ${V}_{\mathrm{O}}$, two Fe atoms in rutile lattice are ferromagnetically coupled except at a separation distance of $3.57\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, where they are antiferromagnetically coupled. The ${V}_{\mathrm{O}}$ introduces two electrons into the conduction bands of rutile, which are either captured by the Fe dopants or form a shallow impurity state. The ferromagnetic (FM) coupling $J$ between two Fe atoms is enhanced, through the enhancement of the FM double exchange if ${V}_{\mathrm{O}}$ is sufficiently close.