Current oscillation and low-field colossal magnetoresistance effect in phase-separated manganites

Current-induced switching from a metallic to an insulating state is observed in phase-separated states of (La(1-y)Pr(y))0.7Ca0.3MnO3 (y=0.7) and Nd(0.5)Ca(0.5)Mn(1-z)Cr(z)O3 (z=0.03) crystals. The application of magnetic fields to this current-induced insulating state causes a pronounced low-field negative magnetoresistance effect [rho(H)/rho(0)=10(-3) at H=1 kOe]. The application of a constant voltage also causes the breakdown of the Ohmic relation above a threshold voltage. At voltages higher than this threshold value, oscillations in currents are observed. This oscillation is well reproduced by a simple model of local switching of a percolative conduction path.     

Exchange field induced magnetoresistance in colossal magnetoresistance manganites.

The effect of an exchange field on the electrical transport in thin films of metallic ferromagnetic manganites has been investigated. The exchange field was induced both by direct exchange coupling in a ferromagnet/antiferromagnet multilayer and by indirect exchange interaction in a ferromagnet/paramagnet metallic superlattice. The electrical resistance of the metallic manganite layers was found to be determined by the magnitude of the vector sum of the effective exchange field and the external magnetic field.     

Magnetorefractive effect in manganites with a colossal magnetoresistance in the visible spectral region

The magnetotransmission, magnetoreflection, and magnetoresistance of the La_0.7Ca_0.3MnO₃ and La_0.9Ag_0.1MnO₃ epitaxial films have been investigated. It has been found that the films exhibit a significant magnetorefractive effect in the case of reflection and transmission of light in the fundamental absorption region both in the vicinity of the Curie temperature and at low temperatures. It has been shown that the magnetorefractive effect in the infrared spectral region of the manganites is determined by a high-frequency response to magnetoresistance, whereas the magnetorefractive effect in the visible spectral region of these materials is associated with a change in the electronic structure in response to a magnetic field, which, in turn, leads to a change in the electron density of states, the probability of interband optical transitions, and the shift of light absorption bands. The obtained values of the magnetotransmittance and magnetoreflectance in the visible spectral region are less than those observed in the infrared region of the spectrum, but they are several times greater than the linear magneto-optical effects. As a result, the magnetorefractive effect, which is a nongyrotropic phenomenon, makes it possible to avoid the use of light analyzers and polarizers in optical circuits.     

A new theory of doped manganites exhibiting colossal magnetoresistance

Rare earth manganites doped with alkaline earths, namely Re_1-xA_xMnO₃, exhibit colossal magnetoresistance, metal insulator transitions, competing magnetic, orbital and charge ordering, and many other interesting but poorly understood phenomena. In this article I outline our recent theory based on the idea that in the presence of strong Jahn-Teller, Coulomb and Hund’s couplings present in these materials, the low-energy electronic states dynamically reorganize themselves into two sets: one set (ℓ) which are polaronic, i.e., localized and accompanied by large local lattice distortion, and another (b) which are non-polaronic and band-like. The coexistence of the radically different ℓ andb states, and the sensitive dependence of their relative energies and occupation upon dopingx, temperatureT, magnetic fieldH, etc., underlies the unique effects seen in manganites. I present results from strong correlation calculations using dynamical mean-field theory and simulations on a new 2-fluid model which accord with a variety of observations.     

Theory of insulator metal transition and colossal magnetoresistance in doped manganites

The persistent proximity of insulating and metallic phases, a puzzling characteristic of manganites, is argued to arise from the self-organization of the twofold degenerate e(g) orbitals of Mn into localized Jahn-Teller (JT) polaronic levels and broad band states due to the large electron-JT phonon coupling present in them. We describe a new two band model with strong correlations and a dynamical mean-field theory calculation of equilibrium and transport properties. These explain the insulator metal transition and colossal magnetoresistance quantitatively, as well as other consequences of two state coexistence.     

Multicritical end point of the first-order ferromagnetic transition in colossal magnetoresistive manganites

We have studied the bandwidth-temperature-magnetic-field phase diagram of RE0.55Sr0.45MnO3 colossal magnetoresistance manganites with ferromagnetic metal (FM) ground state. The bandwidth was controlled both via chemical substitution and hydrostatic pressure with a focus on the vicinity of the critical pressure p;{*} where the character of the zero-field FM transition changes from first to second order. Below p;{*} the first-order FM transition extends up to a critical magnetic field. It approaches zero on the larger bandwidth side where the surface of the first-order FM phase boundary is terminated by a multicritical end point. The change in the character of the transition and the decrease of the colossal magnetoresistance effect is attributed to the reduced charge-order and orbital-order fluctuations.     

Effects of correlated disorder on the magneto‐transport in colossal magnetoresistance manganites

Monte‐Carlo simulations predict that a local correlated disorder is responsible for many of the novel transport and magnetic properties of colossal magnetoresistance (CMR) materials such as manganites. One important prediction of these models is that the resistivity at the metal–insulator transition (MIT) in manganites depends strongly on the correlated quenched disorder. However, experimental confirmation has been challenging since it is difficult to control the amount of disorder in these compounds. We carried out experiments on Sm_0.55Sr_0.45MnO₃, a prototypical CMR manganite with a sharp MIT, whereby the oxygen‐related disorder is systematically enhanced by low temperature thermal activation. We observe dramatic changes in the temperature dependence of resistivity at the MIT as the amount of quenched disorder is increased, occurring in a manner that is in agreement with theoretical predictions.

     

Interplay of charge, spin, orbital and lattice correlations in colossal magnetoresistance manganites

We derive a realistic microscopic model for doped colossal magnetoresistance manganites, which includes the dynamics of charge, spin, orbital and lattice degrees of freedom on a quantum mechanical level. The model respects the SU(2) spin symmetry and the full multiplet structure of the manganese ions within the cubic lattice. Concentrating on the hole doped domain ( 0≤x≤0.5) we study the influence of the electron-lattice interaction on spin and orbital correlations by means of exact diagonalisation techniques. We find that the lattice can cause a considerable suppression of the coupling between spin and orbital degrees of freedom and show how changes in the magnetic correlations are reflected in dynamic phonon correlations. In addition, our calculation gives detailed insights into orbital correlations and demonstrates the possibility of complex orbital states. Received 4 September 2002 / Received in final form 8 November 2002 Published online 31 December 2002     

Ferromagnetic, A-type, and charge-ordered CE-type states in doped manganites using jahn-teller phonons

The two-orbital model for manganites with both noncooperative and cooperative Jahn-Teller phonons is studied at hole density x = 0.5 using Monte Carlo techniques. The phase diagram is obtained by varying the electron-phonon coupling and the t(2g)-spins exchange. The insulating CE-type charge- and orbital-ordered state with the z-axis charge stacking observed in narrow-bandwidth manganites is stabilized in the simulations. Its charge gap Delta(CO) is much larger than the critical temperature k(B)T(CO). Metalliclike A-type and ferromagnetic states are also obtained in the same framework, and the phase boundaries among them have first-order characteristics.     

Occurrence of re-entrant ferromagnetic transitions in rare-earth manganates on cooling the charge-ordered states

Some of the compositions of the half-doped rare-earth manganates, La_0.5−xLn_xCa_0.5MnO₃ (Ln=Nd, Pr) and Nd_0.5Ca_0.5−xSr_xMnO₃ with relatively small A-cation radii, 〈r_A〉, show an unusual behavior wherein they become ferromagnetic (FM) on cooling the charge ordered (CO) state (T_CO>T_C). With increase in 〈r_A〉, however, the T_C becomes greater than T_CO. Thus, plots of T_C and T_CO against 〈r_A〉 for La_0.5−xLn_xCa_0.5MnO₃ (Ln=Nd, Pr) and Nd_0.5Ca_0.5−xSr_xMnO₃ show cross-over from the T_CO>T_C regime to the T_C>T_CO regime around 〈r_A〉 values of 1.195±0.003 and 1.200±0.005 Å, respectively. Between T_C and T_CO, the CO and FM phases are likely to coexist. In Nd_0.5Ca_0.5Mn_1−xM_xO₃ (M=Cr, Ru), T_CO>T_C when x≤0.10, suggesting the re-entrant nature of the FM transition.     

Analysis of colossal magnetoresistance effect in perovskite oxide heterostructures

A systemic study of magnetoresistance (MR) in manganite perovskite oxide p-n junction is performed with experiment and theoretical calculation. The spin-dependent tunneling current is calculated with a model of double-band barrier and MR with reverse bias is explained as a result of competition between tunneling currents with different spins. The reduction of recombination rate at the interface of heterojunction with magnetic field is proposed to explain positive MR at forward bias. Furthermore, negative MR is predicted to be observed in oxide heterostructure without electron filling in t2g↓ band of manganite at the interface region with both forward and reverse bias.     

Bound ferromagnetic and paramagnetic polarons as an origin of the resistivity peak in ferromagnetic semiconductors and manganites

A theory of resistivity is developed for ferromagnetic semiconductors, possibly, including manganites. The theory is based on analysis of the interaction of the free and bound charge carriers with the magnetization of the crystal. The temperature dependence of free energy for nonionized donors and free electrons is calculated for the spin-wave and paramagnetic regions. In addition to the trapping by the ferromagnetic fluctuations (the ferromagnetic polarons), the electron trapping by the random magnetization fluctuations as T → is taken into account (the paramagnetic polarons). For the nondegenerate semiconductors, the theory makes it possible to explain a nonmonotonic temperature dependence of the activation energy, with the value for T = 0 being lower than that for T → ∞. For degenerate semiconductors, the theory explains a metal-insulator transition that occurs with increasing temperature in samples with relatively low charge carrier density. If the density is larger, a reentrant metal-insulator transition should take place, so that the crystal is highly conductive as T → ∞.     

Understanding the insulating phase in colossal magnetoresistance manganites: shortening of the Jahn-Teller long-bond across the phase diagram of La1-xCaxMnO3

The detailed evolution of the magnitude of the local Jahn-Teller (JT) distortion in La(1-x)Ca(x)MnO3 is obtained across the phase diagram for 0< or =x< or =0.5 from high-quality neutron diffraction data using the atomic pair distribution function method. A local JT distortion is observed in the insulating phase for all Ca concentrations studied. However, in contrast with earlier local structure studies, its magnitude is not constant, but decreases continuously with increasing Ca content. This observation is at odds with a simple small-polaron picture for the insulating state.     

Percolation and the colossal magnetoresistance of Eu-based hexaboride

Upon substituting Ca for Eu in the local-moment ferromagnet EuB6, the Curie temperature T(C) decreases substantially with increasing dilution of the magnetic sublattice and is completely suppressed for x相似文献   

Non-Markovian theory of electron paramagnetic resonance in localized and quasi-localized electron spins via an example of manganites with colossal magnetoresistance

An approach based on the memory functions formalism is applied to derive non-Markovian equations of motion for the magnetization components of localized and quasi-localized electron spins under electron paramagnetic resonance (EPR) conditions using the example of manganites with colossal magnetoresistance. General Hasegawa-Bloch-type equations are applied to describe certain experimental data concerning the shape and the width of EPR lines and the longitudinal and transverse relaxation rates. Particular cases of these equations reproduce well-known theoretical results concerning EPR in manganites with colossal magnetoresistance. The results obtained explain certain well-known experimental phenomena and may stimulate further research.     

Glass transition in the polaron dynamics of colossal magnetoresistive manganites

Neutron scattering measurements on a bilayer manganite near optimal doping show that the short-range polaron correlations are completely dynamic at high T, but then freeze upon cooling to a temperature T(*) approximately equal 310 K. This glass transition suggests that the paramagnetic/insulating state arises from an inherent orbital frustration that inhibits the formation of a long-range orbital- and charge-ordered state. Upon further cooling into the ferromagnetic-metallic state (T(C) = 114 K), where the polarons melt, the diffuse scattering quickly develops into a propagating, transverse optic phonon.     

Size distribution effect on the magnetization and magneto-resistance in ferromagnetic nanostructured manganites

We present experimental data of magnetization and magneto-resistance of nanostructured La_2/3B_1/3MnO₃ with B=Ca, Sr, which present difference between the coercive field in the magnetization loop with their corresponding maximum value in the magneto-resistance. This difference is described by a model that include, size distribution of magnetic particles, randomly oriented anisotropy axis and electronic transfer between the particles, which is mediated by spin-polarized tunneling process. Also, the model predicts that the maximum magneto-resistance can be, in the magnetic disorder state, two times larger than the experimental value. The model results can be used to estimate the size dispersion of nanoparticles in similar systems.     

Tunneling magnetoresistance and Hall effect of granular ferromagnetic metals

It is shown that two circumstances must be taken into account in order to describe the tunneling magnetoresistance and Hall effect in granular ferromagnetic metals: 1) the size variance of the metallic granules and 2) the percolation character of the tunneling conductivity of the system, determining the optimal (temperature-dependent) size of the granules through which current transport occurs. This complicates the dependences of the magnetoresistance and Hall resistance of the system on its magnetization and temperature. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 8, 579–584 (25 April 1999)