Two-band Double Exchange Model Research Articles

Designing reversible multi-level resistance states in a half-doped manganite

We design reversible multi-level resistance states in a half-doped charge-ordered manganite, (SCSMO). By exploiting the electronic phase separation in SCSMO at 10 K, we show that the system can be stabilized into several metastable states, against thermal cycling, up to 62 K. The magnetization and the resistivity remain unaltered in each metastable states during the thermal cycling. Monte Carlo calculations using a two-band double-exchange model, including super-exchange, electron-phonon coupling, and quenched disorder, show that the system freezes into a phase-separated metastable state due to the disorder during the thermal cycling. We outline pathway to control the multi-level switching between four reversible metastable states. Our results may open the door for future investigations to engineer memory devices based on multi-level resistance switching using phase-separated transition metal oxides.

Systematic analysis of metamagnetic transitions in A-site disordered Sm0.5(Ca0.5−ySry)MnO3 compounds: A combined experimental and model Hamiltonian study

We investigate the ultra-sharp jump in the isothermal magnetization and the resistivity in the polycrystalline Sm0.5(Ca0.5−ySry)MnO3(y = 0, 0.1, 0.2, 0.25, 0.3, 0.5) compounds. The critical field (Hcr), required for the ultra-sharp jump, decreases with the increase of ‘Sr’ concentration, i.e. with an increase of average A-site ionic radius 〈rA〉. In addition, The magneto-transport data indicate that the phase separation increases with the increase of 〈rA〉, i.e. with ’y’. Moreover, the dependency of Hcr with magnetic field sweep rate reveals that the ultra-sharp jump from antiferromagnetic (AFM) state to the ferromagnetic (FM) state is of martensitic in nature. In parallel with experiments, we have carried out numerical calculations to understand the role of A-site disorder in the metamagnetic transition. Our two-band double exchange model Hamiltonian calculations show that the ‘Sr’ doping induces the ferromagnetic clusters in the antiferromagnetic insulating phase and in turn reduces the critical field. Overall, our combined study comprising of several materials with different composition of A-site reveal the fact that how and why certain A-type disorder produces prominent metamagnetic transition.

Collapse of ferromagnetism with Ti doping in Sm0.55Sr0.45MnO3

We have investigated the effect of Ti doping on the transport properties coupled with the magnetic ones in Sm$_{0.55}$Sr$_{0.45}$Mn$_{1-\eta}$Ti$_{\eta}$O$_3$ ($0 \leq \eta \leq 0.04$). The parent compound, Sm$_{0.55}$Sr$_{0.45}$MnO$_3$, exhibits a first-order paramagnetic-insulator to ferromagnetic-metal transition just below $T_{\rm c}$ = 128 K. With substitution of Ti at Mn sites ($B$-site), $T_{\rm c}$ decreases approximately linearly at the rate of 22 K$\%^{-1}$ while the width of thermal hysteresis in magnetization and resistivity increases almost in an exponential fashion. The most spectacular effect has been observed for the composition $\eta$=0.03, where a magnetic field of only 1 T yields a huge magnetoresistance, $1.2 \times 10^7$ $\%$ at $T_c\approx$ 63 K. With increasing magnetic field, the transition shifts towards higher temperature, and the first-order nature of the transition gets weakened and eventually becomes crossover above a critical field ($H_{cr}$) which increases with Ti doping. For Ti doping above 0.03, the system remains insulting without any ferromagnetic ordering down to 2 K. The Monte-Carlo calculations based on a two-band double exchange model show that the decrease of $T_{\rm c}$ with Ti doping is associated with the increase of the lattice distortions around the doped Ti ions.

Nanoclustering phase competition induces the resistivity hump in colossal magnetoresistive manganites

Using a two-band double-exchange model with Jahn-Teller lattice distortions and super-exchange interactions, supplemented by quenched disorder, at electron density $n=0.65$, we explicitly demonstrate the coexistence of the $n$ = 1/2-type ($\pi, \pi$) charge-ordered and the ferromagnetic nanoclusters above the ferromagnetic transition temperature $T_{\rm c}$ in colossal magnetoresistive (CMR) manganites. The resistivity increases due to the enhancement of the volume fraction of the charge-ordered and the ferromagnetic nanoclusters with decreasing the temperature down to $T_{\rm c}$. The ferromagnetic nanoclusters start to grow and merge, and the volume fraction of the charge-ordered nanoclusters decreases below $T_{\rm c}$, leading to the sharp drop in the resistivity. By applying a small external magnetic field $h$, we show that the resistivity above $T_{\rm c}$ increases, as compared with the case when $h=0$, a fact which further confirms the coexistence of the charge-ordered and the ferromagnetic nanoclusters. In addition, we show that the volume fraction of the charge-ordered nanoclusters decreases with increasing the bandwidth and consequently the resistivity hump diminishes for large bandwidth manganites, in good qualitative agreement with experiments. The obtained insights from our calculations provide a complete pathway to understand the phase competition in CMR manganites.

Phase diagrams of La1−xCaxMnO3 in double exchange model with added antiferromagnetic and Jahn-Teller interaction

The phase diagram of the multivalent manganites La1−x Ca x MnO3, in space of temperature and doping x, is a challenge for the theoretical physics. It is an important test for the model used to study these compounds and the method of calculation. To obtain theoretically this diagram for x < 0.5, we consider the two-band Double Exchange Model for manganites with added Jahn-Teller coupling and antiferromagnetic Heisenberg term. In order to calculate Curie and Néel temperatures we derive an effective Heisenberg model for a vector which describes the local orientation of the total magnetization of the system. The exchange constants of this model are different for different space directions and depend on the density of e g electrons, antiferromagnetic constants and the Jahn-Teller energy. To reproduce the well known phase transitions from A-type antiferromagnetism to ferromagnetism at low x and C-type antiferromagnetism to G-type antiferromagnetism at large x, we argue that the antiferromagnetic exchange constants should depend on the lattice direction. We show that ferromagnetic to A-type antiferromagnetic transition results from the Jahn-Teller distortion. Accounting adequately for the spin fluctuations, Curie and Néel temperatures are calculated. The results are in very good agreement with the experiment and provide values for the model parameters, which best describe the behavior of the critical temperature for x < 0.5.

Orbital degeneracy and Mott transition in Mo pyrochlore oxides

We present our theoretical results on an effective two-band double-exchange model on a pyrochlore lattice for understanding intricate phase competition in Mo pyrochlore oxides. The model includes the twofold degeneracy of e′g orbitals under trigonal field splitting, the interorbital Coulomb repulsion, the Hund's-rule coupling between itinerant e′g electrons and localized a1g spins, and the superexchange antiferromagnetic interaction between the a1g spins. By Monte Carlo simulation with treating the Coulomb repulsion at an unrestricted-type mean-field level, we obtain the low-temperature phase diagram as functions of the Coulomb repulsion and the superexchange interaction. The results include four dominant phases with characteristic spin and orbital orders and the metal-insulator transitions among them. The insulating region is characterized by a 'ferro'-type orbital ordering of the e′g orbitals along the local ⟨111⟩ axis, irrespective of the spin ordering.

Effect of Jahn-Teller coupling on Curie temperature in the double-exchange model

We consider the two-band double exchange model for manganites with Jahn-Teller (JT) coupling and explore the suppression of the ferromagnetism because of the JT distortion. The localized spins of the $\emph{t}_{2g}$ electrons are represented in terms of the Schwinger bosons, and two spin-singlet Fermion operators are introduced instead of the $e_{g}$ electrons' operators. In terms of the new Fermi fields the on-site Hund's interaction is in a diagonal form and one accounts for it exactly. Integrating out the spin-singlet fermions, we derive an effective Heisenberg model for a vector which describes the local orientations of the total magnetization. The exchange constants are different for different space directions and depend on the density $n$ of $\emph{e}_{g}$ electrons and JT energy. At zero temperature, with increasing the density of the $\emph{e}_{g}$ electrons the system undergoes phase transition from ferromagnetic phase $(0<n<n_c)$ to A-type antiferromagnetic phase $(n_c<n)$. The critical value $n_c$ decreases as JT energy is increased. At finite temperature we calculate the Curie temperature as a function of electron density for different JT energy. The results show that JT coupling strongly suppresses the spin fluctuations and decreases the Curie temperature.

Disorder-induced orbital ordering in doped manganites

We study the effect of quenched disorder on the ordering of orbital and magnetic degrees of freedom in a two-dimensional, two-band double-exchange model for ${e}_{g}$ electrons coupled to Jahn--Teller distortions. By using a real-space Monte Carlo method, we find that disorder can induce a short-range ordering of the orbital degrees of freedom near 30% hole doping. The most striking consequence of this short-range ordering is a strong increase in the low-temperature resistivity. The real-space approach allows us to analyze the spatial patterns of the charge, orbital, and magnetic degrees of freedom and the correlations among them. The magnetism is inhomogeneous on the nanoscale in the short-range orbitally ordered state.

Curie temperature of the two-band double-exchange model for manganites

We consider the two-band double-exchange model and calculate the critical temperature in the ferromagnetic regime (Curie temperature). The localized spins are represented in terms of the Schwinger bosons, and two spin-singlet Fermion operators are introduced. In terms of the new Fermi fields the on-site Hund's interactions are in a diagonal form and one accounts for them exactly. Integrating out the spin-singlet fermions we derive an effective Heisenberg-type model for a vector which describes the local orientations of the total magnetization. The transversal fluctuations of the vector are the true magnons in the theory, which is the base for Curie temperature calculation. The critical temperature is calculated employing the Schwinger-bosons mean-field theory. While approximate, this technique of calculation captures the essentials of the magnon fluctuations in the theory, and for two-dimensional systems one obtains zero Curie temperature, in accordance with the Mermin-Wagner theorem.

Effect of disorder in an orbitally ordered Jahn-Teller insulator

We study a two-dimensional, two-band double-exchange model for ${e}_{g}$ electrons coupled to Jahn-Teller distortions in the presence of quenched disorder using a recently developed Monte Carlo technique. In the absence of disorder the half-filled system at low temperatures is an orbitally ordered ferromagnetic insulator with a staggered pattern of Jahn-Teller distortions. We examine the finite-temperature transition to the orbitally disordered phase and uncover a qualitative difference between the intermediate and strongly coupled systems, including a thermally driven insulator to metal crossover in the former case. Long-range orbital order is suppressed in the presence of disorder and the system displays a tendency towards metastable states consisting of orbitally disordered stripelike structures enclosing orbitally ordered domains.

Spin wave excitations in ferromagnetic manganites with orbital order

We use a two-band double exchange model extended by Jahn–Teller terms to describe the ferromagnetic phase of cubic manganites. When the Jahn–Teller interactions stabilize orbital order, spin excitations are anisotropic and depend on the type of occupied orbitals. They exhibit a pronounced asymmetry between low- and high-hole doping.

Spin excitations in ferromagnetic manganites with orbital order

The ferromagnetic metallic phase of doped cubic manganites with orbital order is investigated in a two-band double exchange model extended by Jahn–Teller (JT) terms. Evaluating the spin wave self-energy in the lowest order 1/S expansion, one finds that spin excitations are anisotropic and depend on the type of occupied eg orbitals. In the case of a small splitting between eg orbital energies, magnon dispersion exhibits a pronounced asymmetry between low and high hole doping, while a strong JT effect leads to the alternating orbital order which can reduce the spin stiffness by one order of magnitude. For a bilayer system, the kinetic energy induces an x2 − y2 orbital character and is responsible for a drastic change in the interplane exchange coupling in the doping range 0.3 < x < 0.5.

EFFECTIVE ORBITAL INTERACTION: SPIN AND ORBITAL ORDERING IN UNDOPED MANGANITES

Including strong on-site interorbital Coulomb interaction and Jahn–Teller (J-T) coupling, we derive an effective spin and orbital (isospin) interaction from the two-band double-exchange model by using projection operator method. For the resulting Hamiltonian, anisotropic A-type antiferromagnetic (A-AF) state emerges naturally from a simple mean-field approximation in certain parameter region. We predict that the alternatingly occupied eg orbitals in the A-AF state should be (d3z2-r2-dx2-y2)/(d3z2-r2+dx2-y2) not d3x2-r2/d3y2-r2 as suggested in some other works. These results are in good agreement with experimental observations in undoped manganites. More accurate Monte Carlo simulations are performed to verify the mean-field ground-state phase diagram and study the finite-temperature spin and isospin disordering phase transitions. The main effect of the J-T coupling is to alter the effective coupling strength, quantitatively.