Double Exchange Process Research Articles

Colossal magnetoresistance of perovskite Nd 0.67Sr 0.33Mn 1− xFe xO 3 single crystals

The effect of Fe doping on the Mn site in the ferromagnetic phases of the perovskite Nd 0.67Sr 0.33Mn 1− x Fe x O 3 single crystals grown by the floating zone method has been studied. Conduction and ferromagnetism are suppressed by Fe doping. The material with x=0.1 shows a cluster glass-like behavior with spin freezing temperature T g =70 K instead of a long-range ferromagnetic order for mother material ( x=0) with T c =240 K . The resistivity for x=0.1 has a very large peak near T g, where a metal–insulator transition takes place. Besides, the magnetic field causes the colossal magnetoresistance near T g. The field of 7 T suppresses the resistivity by an order of magnitude 3 for x=0.1 single crystal, which is to be compared with reduction of an order of magnitude 1.5 for the sister material, and much larger than the suppression of resistivity near T c for x=0 single crystal. Further doped material with x=0.2 shows the spin-glass-like and insulating behaviors with rather small magnetoresistance. These results are ascribed to the frustration of random competing ferromagnetic and antiferromagnetic interactions with the introduction of Fe 3+ ions, which do not participate in the double-exchange process.

Magnetic and transport properties of the perovskite Nd 0.67Sr 0.33M 1− xFe xO 3 (M=Mn,Co)

The effect of Fe doping on the M site in the perovskite Nd 0.67 Sr 0.33 M 1−x Fe x O 3 (x⩽0.3) has been studied. Both mother materials are ferromagnetic metals with T c =260 K for M=Mn and 170 K for M=Co. Nd 0.67Sr 0.33Mn 0.9Fe 0.1O 3 shows a spin-glass-like behavior with T g =80 K and also a very large resistivity peak near T g with colossal magnetoresistance, which is much larger than that of mother material. Nd 0.67Sr 0.33Co 0.9Fe 0.1O 3 keeps a long-range ferromagnetic order with T c =140 K . Further doped materials with x=0.3 for both systems show the typical spin glass and insulating behaviors. These results are ascribed to the frustration of random competing ferromagnetic and antiferromagnetic interactions with the introduction of Fe 3+ ions, which do not participate in the double-exchange process.

Colossal magnetoresistance and spin-glass behavior of the perovskite Nd 0.67Sr 0.33Mn 1− xFe xO 3

The effect of Fe doping on the Mn site in the ferromagnetic phases of the perovskite Nd 0.67Sr 0.33Mn 1− x Fe x O 3 ( x⩽0.3) has been studied. Conduction and ferromagnetism are suppressed by Fe doping. The material with x=0.1 shows a spin-glass-like behavior with spin freezing temperature T g=80 K instead of a long-range ferromagnetic order for mother material ( x=0) with T c=260 K, which has been well confirmed by the DC magnetization and nonlinear AC susceptibility measurements. The resistivity for x=0.1 has a very large peak near T g, where a metal–insulator transition seems to take place. Besides, the magnetic field of 7 T causes the colossal magnetoresistance of 97%, which is much larger than 57% for x=0 observed near T c. Further doped materials with x=0.2 and 0.3 show the typical spin glass and insulating behaviors with very small magnetoresistance. These results are ascribed to the frustration of random competing ferromagnetic and antiferromagnetic interactions with the introduction of Fe 3+ ions, which do not participate in the double-exchange process.

Electrical conductivity and magnetic behavior of La 0.67Ca 0.33MnO 3 as influenced by substitution of Co

La 0.67Ca 0.33Mn 1− x Co x O 3 ( x=0.00, 0.05, 0.075, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40) have been investigated by AC susceptibility, DC magnetization and DC electrical resistivity measurements. Substitution of Co for Mn did not cause any change in crystal symmetry, yet electrical conductivity as well as ferromagnetism have been consistently suppressed by the dopant. The results can be explained on the basis of weakening of the double exchange process and an increase in anisotropy of the exchange interaction consequent upon Co doping.

Transport properties, magnetic ordering, and hyperfine interactions in Fe-dopedLa0.75Ca0.25MnO3: Localization-delocalization transition

Transport properties of ${\mathrm{La}}_{0.75}{\mathrm{Ca}}_{0.25}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{O}}_{3+y}$ ceramics are examined for ``$x$'' values from 0 to 0.05. Dramatic changes are observed in the resistivity and its temperature dependence as ``$x$'' increases above about 0.04. Temperature-dependent M\ossbauer spectroscopy and magnetization studies are also performed. These data suggest that transport in this system necessarily involves quasiparticle excitation in association with the double exchange process and it undergoes localization-delocalization-type transition at a critical dopant concentration.

Colossal magnetoresistance of spin-glass perovskite La0.67Ca0.33Mn0.9Fe0.1O3

The magnetic and magnetotransport properties of the perovskite La0.67Ca0.33Mn0.9Fe0.1O3 have been investigated, and the spin-glass behavior with a spin freezing temperature of 42 K has been well confirmed for this compound. A metal-to-insulator transition and colossal magnetoresistance have been observed near its spin freezing temperature; besides, the insulator behavior has been found to reappear at lower temperature. The formation of ferromagnetic and antiferromagnetic clusters and the competition between them with the introduction of Fe3+ ions, which do not participate in the double-exchange process, have been suggested to explain the experimental results.

Transport and magnetism in La2/3Ca1/3(Mn1−xCox)O3 and La2/3Ca1/3(Mn1−xNix)O3 (abstract)

La 2/3 Ca 1/3 MnO 3 is a strong double-exchange ferromagnet where the ratio of La3+–Ca2+ ions has been chosen to maximize its favorable magnetic and transport properties. The Mn ion is unique as an effective participant in the double-exchange process. It is of interest to add Ni or Co atoms substitutionally for Mn in the form of La2/3Ca1/3(Mn1−xCox)O3 and La2/3Ca1/3(Mn1−xNix)O3 in order to determine the relative importance of the double-exchange interaction as the Mn concentration is reduced. Powder x-ray diffraction reveals we can retain the original orthorhombic structure of La2/3Ca1/3MnO3 (with up to 50% Co or Ni substitution) by a series of anneals at 1300 °C followed by repeated grindings. We have investigated the magnetization, electrical conductivity, and thermopower of these mixed transition-metal perovskites. For La2/3Ca1/3(Mn1−xCox)O3, the magnetization decreases with added Co, but ferromagnetism is retained (at least up to 50% Co). Although metallic conductivity is still present at 10% Co concentration, these Co–Mn perovskites become insulating for x⩾0.2. For La2/3Ca1/3(Mn1−xNix)O3, the metallic state is retained at least until x=0.5. The magnetic properties exhibit spin-glasslike behavior for x⩾0.25.

Modified Two-Pion-Exchange Model for Double-Charge-Exchange Processes

The eikonal method, applied to the problem of coupled channels of different mass in potential theory, is used to derive an expansion for the scattering amplitudes in powers of the potentials between different channels. Just as single-exchange processes are given approximately by the first-order term in the Born series with absorptive modifications, double-exchange processes are given approximately by the modified imaginary part of the second-order term. This result is taken over to a relativistic theory, with one-pion-exchange "potentials" connecting different channels, and used to estimate the differential cross section for the double-charge-exchange process ${\ensuremath{\pi}}^{\ensuremath{-}}+p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}+{\ensuremath{\Delta}}^{\ensuremath{-}}$.

Interaction between thed-Shells in the Transition Metals. II. Ferromagnetic Compounds of Manganese with Perovskite Structure

Recently, Jonker and Van Santen have found an empirical correlation between electrical conduction and ferromagnetism in certain compounds of manganese with perovskite structure. This observed correlation is herein interpreted in terms of those principles governing the interaction of the $d$-shells of the transition metals which were enunciated in the first paper of this series. Both electrical conduction and ferromagnetic coupling in these compounds are found to arise from a double exchange process, and a quantitative relation is developed between electrical conductivity and the ferromagnetic Curie temperature.