Ferromagnetic Metallic State Research Articles

Orbital ordering induced by high impurity levels: Does long-range orbital ordering exist in manganites?

Substitutions at the Mn site by cations M with oxidation states higher than 4+, such as Mo6+ and Ru5+, are shown to stabilize SrMn1−xMxO3 cubic perovskite manganites. This effect is explained by the induced Mn valency shift, which corresponds to electron doping by Mn3+ in the Mn4+ matrix. Interestingly, these electron-doped manganites, characterized by large A-site cationic size, exhibit coupled structural, magnetic, and transport transitions close to or beyond 300 K for M=Ru5+ and M=Mo6+, respectively. These transitions are characteristic of the dz2 orbital ordering generally associated with C-type antiferromagnetism. This result contrasts with the inhomogeneous ferromagnetic metallic state induced by Ru in CaMnO3. The possibility to induce orbital/charge ordering in CaMn1−xMoxO3 or orbital ordering in SrMn1−xMxO3 (M=Ru, Mo) with M contents superior to 10% makes it more difficult to explain based on long-range cooperative Jahn–Teller phenomena in mixed-valency manganites.

Impurity effect on charge-ordered and ferromagnetic–metallic state in manganese perovskites: Comparison between Cr and Al doping

The role of impurities in melting of the charge-ordered (CO) state in half-doped manganese perovskites is discussed based on the systematic study of Cr and Al doping on the CO state as well as on the ferromagnetic–metallic (FM) state. The CO state in Nd0.5Sr0.5MnO3 is destroyed and the FM state is restored by 2% high-spin (S=3/2) Cr3+, whereas the CO state survives doping of spinless Al3+. On the other hand, the doping effects on the FM state in La1−xSrxMnO3 are almost identical between Cr and Al, except that Cr degrades Curie temperature less. The slight resistance of the CO state against Cr doping can be explained by the magnetic interaction between the Cr and the Mn t2g spins.

Magnetic contributions to the low-temperature specific heat of the ferromagnetic insulator Pr 0.8 Ca 0.2 MnO 3

The Pr 1-x CaxMnO3 system exhibits a ferromagnetic insulating state for the composition range x ? 0.25. A metallic ferromagnetic state is never realized because of the low hole concentration and the very small averaged A-site cation radius. In the present study, the nature of the magnetic excitations at low temperature has been investigated by specific heat measurements on a Pr 0.8 Ca0.2MnO3 single crystal. The decrease of the specific heat under magnetic field is qualitatively consistent with a suppression of ferromagnetic spin waves in a magnetic field. However, at low temperature, the qualitative agreement with the ferromagnetic spin waves picture is poor. It appears that the large reduction of the specific heat due to the spin waves is compensated by a Schottky-like contribution possibly arising from a Zeeman splitting of the ground state multiplet of the Pr3+ ions.

Some open issues in correlated electron systems

Numerical computations and simulations are important tools in the investigation of correlated electron systems. Two examples are discussed. First, monovalent (Li +) and divalent (Zn 2+) dopants in a spin liquid are treated within a t– J model. Experiments that show a similar local moment for both dopants in some underdoped cuprates find a simple explanation only if Li + does not bind a hole in these hosts. The second example illustrates the success of the generalizations of density functional method to include onsite correlation (the so-called LDA+U method) in explaining the transition between singlet semiconducting and ferromagnetic metallic states in the isostructural transition metal compounds, FeSi and FeGe.

Stabilization of Ferromagnetic Metallic Phase and Anomalous Magnetotransport in Pr1-XCaX(Mn1-ZCoZ)O3

The magnetization M ( T ) and the electrical resistivity ρ( T ) of Pr 1- X Ca X (Mn 1- Z Co Z )O 3 were measured and the magnetic and the electronic phase diagram of this system was determined as a function of Ca and Co concentrations and the magnetic fields. For 0.30 ≤ X ≤0.55, where the charge and orbital ordered phase is the ground state in the Z =0 system, the ferromagnetic metallic (FM-M) state is materialized by the Co substitution and the T c - X curves take a maximum around X =0.4-0.45. The results were compared with those of the Cr- and Ni-doped systems.

Metallic ferromagnetism in a generalized Hubbard model

The possibility of realization of a metallic ferromagnetic state in a generalized Hubbard model with correlated hopping and exchange interaction integrals is investigated. The single-electron energy spectrum recently obtained by means of the mean-field approximation is applied for the description of ground-state and finite-temperature properties of the system. An expression for the Curie temperature is found, and the behavior of the temperature dependences of the magnetization and paramagnetic susceptibility is analyzed. Taking the correlated hopping into account allows one to explain certain peculiarities of the ferromagnetic behavior of transition metals and their compounds.

Phase separation region in the phase diagram of polycrystalline Pr 0.65Ca 0.35− xSr xMnO 3

We have performed measurements of thermoelectric power, electrical resistivity and DC magnetization in polycrystalline samples of Pr 0.65Ca 0.35− x Sr x MnO 3 with x=0, 0.02, 0.05, 0.10 and 0.20. The system presents an evolution from a charge-ordered manganite at x=0, with an insulating-like resistivity and low magnetization values, towards a ferromagnetic (FM) metallic state for x=0.20. We show that in the intermediate region a strong competition between the FM double-exchange and the localized charge-ordering occurs, inducing the phase segregation of the two competing states in a wide temperature range. We estimate the temperature ( T) evolution of the FM phase fraction of the x=0.10 sample and construct a complete T− x phase diagram.

Microwave properties of Nd 0.5Sr 0.5MnO 3: a key role of the ( x2− y2)-orbital effects

Transmittance of the colossal magnetoresistive compound Nd 0.5Sr 0.5MnO 3 showing metal–insulator phase transition has been studied by means of the submm- and mm-wavelength band spectroscopy. An unusually high transparency of the material provided direct evidence for the significant suppression of the coherent Drude-weight in the ferromagnetic metallic state. Melting of the A-type antiferromagnetic states has been found to be responsible for a considerable increase in the microwave transmission, which was observed at the transition from the insulating to the metallic phase induced by magnetic field or temperature. This investigation confirmed a dominant role of the ( x 2− y 2)-orbital degree of freedom in the low-energy optical properties of Nd 0.5Sr 0.5MnO 3 and other doped manganites with planar ( x 2− y 2)-orbital order, as predicted theoretically. The results are discussed in terms of the orbital-liquid concept.

X-ray study of Nd0.5Sr0.5MnO3 manganite structure above and below the ferromagnetic metal–antiferromagnetic insulator spontaneous phase transition

The crystal structure of the manganite Nd0.5Sr0.5MnO3 is studied at temperature T=300 and 77.3 K by means of an x-ray diffractometer. It is shown that the transition from the ferromagnetic metallic state to the antiferromagnetic insulating charge-ordered state is accompanied by a lowering of the symmetry of the structure from orthorhombic to monoclinic. The space-group symmetry of the orthorhombic and monoclinic phases is identified as Imma and P21/m, respectively. Twinning of the crystal and the formation of a twin domain structure with coherent boundaries in the (00l) crystallographic planes are found.

Magnetic-field-dependent optical studies of Pr 0.69Ca 0.31MnO 3

We investigated magnetic-field-dependent optical conductivity spectra σ( ω) of Pr 0.69Ca 0.31MnO 3 at 4.2 K, which was in a charge-ordered insulating state at 0 T. With increasing the magnetic field up to 12 T, we observed an insulator–metal transition around 4 T and large spectral weight changes up to 4 eV. Especially, we found that the spectral weight above 0.5 eV moved to a lower energy region and that a Drude-like peak appeared clearly at 12 T. By analyzing dielectric response changes, we suggested that the field-dependent insulator–metal transition might occur through a percolation process of ferromagnetic metal domains. On the other hand, with decreasing the magnetic field from 12 to 0 T, σ( ω) showed little changes, which showed that the ferromagnetic metallic state did not return to the insulating state.

Effects of Cr doping on the structure, charge ordering, transport and spin ordering state in Nd 0.5Sr 0.5Mn 1− xCr xO 3

The effects of cation substitution in the manganese site of charge-ordered Nd 0.5Sr 0.5MnO 3 have been investigated to elucidate how the properties of this material change with chromium doping. Nd 0.5Sr 0.5Mn 1− x Cr x O 3 ( x=0, 0.05, 0.1, 0.15, 0.2, 0.25) compounds have been studied by X-ray diffraction and transport and magnetic measurements. All samples were determined to be single phase and crystallized in an orthorhombic structure. Magnetic measurements showed that chromium doping decreases the charge-ordering temperature T co as x values are increased from 0 to 0.05. The charge ordering is completely destroyed for all the higher Cr doping levels ( x>0.05), and the material changes from an antiferromagnetic insulator to the ferromagnetic metallic (FMM) state for 0.05< x<0.2. The resistivity decreases as the result of a metal–insulator transition for x<0.2, but the resistivity increases, and the material becomes an insulator if x>0.2. At the same time, the magnetization increases with increasing Cr content, reaches a maximum for x=0.15 and then decreases for x>0.15. It is proposed that Cr destroys the charge ordering and facilitates double exchange between Mn 3+–O–Mn 4+. It is thus responsible for the FMM state seen at low Cr doping levels, while an antiferromagnetic interaction between Mn 4+–O–Cr 3+ or Cr 3+–O–Cr 3+ is responsible for the insulator state at high Cr doping levels.

Strain effect and the phase diagram ofLa1−xBaxMnO3thin films

We report magnetic and transport properties of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ba}}_{x}{\mathrm{MnO}}_{3}$ $(x=0.05--0.33)$ epitaxial thin films. Compared with the corresponding bulk materials, the ferromagnetic transition temperature is reduced in the compressive strained ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ba}}_{x}{\mathrm{MnO}}_{3}$ thin films with $x=0.3$ and 0.33, but enhanced significantly in the tensile strained thin films with $x&lt;~0.2.$ Especially, ferromagnetism and low field colossal magnetoresistance effect were observed around room temperature in $x=0.1$ thin film, and as $x=0.05,$ a spin-canting insulating state in bulk shifts to ferromagnetic metallic state in thin film. The phase diagram of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ba}}_{x}{\mathrm{MnO}}_{3}$ thin films was obtained, and strain effect on these novel properties was discussed.

Specific heat of single-crystallinePr0.63Ca0.37MnO3in the presence of a magnetic field

We present the results of a study of specific heat on a single crystal of Pr$_{0.63}$Ca$_{0.37}$MnO$_3$ performed over a temperature range 3K-300K in presence of 0 and 8T magnetic fields. An estimate of the entropy and latent heat in a magnetic field at the first order charge ordering (CO) transition is presented. The total entropy change at the CO transition which is $\approx$ 1.8 J/mol K at 0T, decreases to $\sim$ 1.5 J/mol K in presence of 8T magnetic field. Our measurements enable us to estimate the latent heat $L_{CO}$ $\approx$ 235 J/mol involved in the CO transition. Since the entropy of the ferromagnetic metallic (FMM) state is comparable to that of the charge-ordered insulating (COI) state, a subtle change in entropy stabilises either of these two states. Our low temperature specific heat measurements reveal that the linear term is absent in 0T and surprisingly not seen even in the metallic FMM state.

Enhanced Phonon Heat Conduction Correlated with Induced Ferromagnetic Metallic Phase in Pr0.65Ca0.35(Mn1-ZCoZ)O3

The thermal conductivity κ( T ), the diffusivity α( T ) and the thermal dilatation d L ( T )/ L have been measured for Pr 0.65 Ca 0.35 MnO 3 and Pr 0.65 Ca 0.35 (Mn 1- Z Co Z )O 3 ( Z =0.02–0.10) crystals under the magnetic field of up to 5 T. By application of high magnetic fields or Co substitution for the Mn site, the ferromagnetic metallic (FM–M) state are induced and κ( T ) and α( T ) are drastically enhanced below the ferromagnetic transition temperature T F . The enhancement is closely related to the relaxation of the local Jahn–Teller lattice distortion due to the increased hole mobility in the FM–M phase.

Temperature dependent memory effects in the bilayer manganite(La0.4Pr0.6)1.2Sr1.8Mn2O7

We report on the low-temperature resistance and magnetization of a single crystal of the bilayer manganite $({\mathrm{La}}_{0.4}{\mathrm{Pr}}_{0.6}{)}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}.$ In zero field the sample is an insulator at all temperatures while at fields larger than 3 T a transition to a ferromagnetic metallic state is observed. Below 50 K the sample shows a pronounced memory effect, in which both resistivity and magnetization become dependent on the magnetic history and clearly demonstrate hysteretic behavior. At these low temperatures a difference in zero-field resistivity before and after magnetization of six orders of magnitude has been measured. The relaxation of the memory effect follows a stretched exponential function with a characteristic decay time of 5 hours at 10 K, which decreases to less than 1 ms above 40 K. These low-temperature properties can be related to magnetostriction data, indicating the presence of a field-induced change in the orbital occupancy of the ${e}_{g}$ electrons of ${\mathrm{Mn}}^{3+}.$

Anomalies of magnetoelastic properties and insulator-metal phase transitions induced by a magnetic field in substituted praseodymium manganites

The magnetostriction of Pr0.6Ca0.4MnO3 and Pr0.65Ca0.28Sr0.07MnO3 manganite crystals is investigated in pulsed magnetic fields up to 250 kOe. The field-induced phase transitions from the charge-ordered antiferromagnetic (paramagnetic) state to the metallic ferromagnetic state are studied. It is found that these transitions are accompanied by abrupt negative jumps in the longitudinal and transverse magnetostrictions and a strong hysteresis at low temperatures. The threshold fields of transitions are determined, and the H-T phase diagrams are constructed.

Competing orders and multicritical phenomena in strongly correlated electronic systems

Phase diagrams and the critical phenomena in strongly correlated electronic systems are often understood in terms of competing orders. Here we present the theoretical survey of the multicritical phenomena in the following examples. (i) The high- T c cuprates with hidden order in the underdoped region. (ii) The bicritical phenomenon of superconductivity (SC) and antiferromagnetism (AF) in organic conductors κ-BEDT salts. (iii) The coexistence the SC and AF in heavy fermion system CeCu 2(Si 1− x Ge x ) 2. (iv) The competing two phases of AF/orbital order/charge order and metallic ferromagnetic state in manganites showing the colossal magnetoresistance. It is stressed that the structure of the phase diagram itself gives an important clue to the underlying physical model for each system.

Magnetic phase diagram of the charge ordered manganite Pr0.8Na0.2MnO3

Pr 1−x Na x MnO 3 solid solutions up to x=0.25 were prepared. Irrespective of magnetic order all samples are electrically insulating. The Curie temperature is maximum at TC=125 K for x=0.15; the samples with x⩾0.2 are charge-ordered antiferromagnets. The x=0.2 antiferromagnetic charge-ordered insulator (AFCO) can be switched to the ferromagnetic metallic state (FMM) by application of a relatively weak external magnetic field. This is accompanied by a huge CMR effect, irreversible at low temperatures. The neutron diffraction data, which show a decoupling of charge order (TCO=215 K) and long range AF order of the “pseudo” CE (TN=175 K), are supported by specific heat measurement. The specific heat at 14 T confirmed the transformation of the x=0.2 sample to FMM state and the nature of the AFCO–FMM transition was probed by magnetic moment and electrical resistivity relaxation measurements. This comprehensive characterization of the sample led to a complete determination of the magnetic phase diagram which demonstrated the near degeneracy of the ferro- and antiferromagnetic ground states in Pr0.8Na0.2MnO3.

Jahn-Teller phonon anomaly and dynamic phase fluctuations in La0.7Ca0.3MnO3.

Inelastic neutron scattering was used to study the temperature (T) dependence of the lattice excitations in La 0.7Ca 0.3MnO (3). An optical Jahn-Teller phonon exhibits continuous but anomalous damping with increasing temperature in the ferromagnetic-metallic phase and collapses above the Curie temperature T(C) (240 K). We attribute this anomaly to the growing dynamic phase segregation as T-->T(C), thus providing evidence of local fluctuations associated with the short-range polaron or charge/orbital ordering in the ferromagnetic-metallic state.

Strain-driven charge-ordered state inLa0.67Ca0.33MnO3

We present evidence for the coexistence of ferromagnetic metallic and charge-ordered insulating phases in strained thin films of ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}$ at low temperatures. Such a phase-separated state is confirmed using low-temperature magnetic force microscopy and magnetotransport measurements. This phase separated state is not observed in the bulk form of this compound and is caused by the structural inhomogeneities due to the nonuniform distribution of strain in the film. The strain weakens the low-temperature ferromagnetic metallic state and a charge-ordered insulator is formed at the high strain regions. The slow dynamics of the transport properties of the mixed phase is illustrated by measurements of the long-time scale relaxation of the electrical resistance.