http://sces.phys.utk.edu/mostcited/mc1_1114.pdf

Colossal Magnetoresistant Materials: The Key Role of Phase Separation

Colossal magnetoresistance1—an unusually large change of resistivity observed in certain materials following application of magnetic field—has been extensively researched in ferromagnetic perovskite manganites. But it remains unclear why the magnetoresistive response increases dramatically when the Curie temperature (T C) is reduced. In these materials, T C varies sensitively with changing chemical pressure; this can be achieved by introducing trivalent rare-earth ions of differing size into the perovskite structure2,3,4, without affecting the valency of the Mn ions. The chemical pressure modifies local structural parameters such as the Mn–O bond distance and Mn–O–Mn bond angle, which directly influence the case of electron hopping between Mn ions (that is, the electronic bandwidth). But these effects cannot satisfactorily explain the dependence of magnetoresistance on T C. Here we demonstrate, using electron microscopy data, that the prototypical (La,Pr,Ca)MnO3 system is electronically phase-separated into a sub-micrometre-scale mixture of insulating regions (with a particular type of charge-ordering) and metallic, ferromagnetic domains. We find that the colossal magnetoresistive effect in low-T C systems can be explained by percolative transport through the ferromagnetic domains; this depends sensitively on the relative spin orientation of adjacent ferromagnetic domains which can be controlled by applied magnetic fields.

Percolative phase separation underlies colossal magnetoresistance in mixed-valent manganites

We review recent experimental work falling under the broad classification of colossal magnetoresistance (CMR), which is magnetoresistance associated with a ferromagnetic-toparamagnetic phase transition. The prototypical CMR compound is derived from the parent compound, perovskite LaMnO 3. When hole doped at a concentration of 20–40% holes/Mn ion, for instance by Ca or Sr substitution for La, the material displays a transition from a high-temperature paramagnetic insulator to a low-temperature ferromagnetic metal. Near the phase transition temperature, which can exceed room temperature in some compositions, large magnetoresistance is observed and its possible application in magnetic recording has revived interest in these materials. In addition, unusual magneto-elastic effects and charge ordering have focused attention on strong electron–phonon coupling. This coupling, which is a type of dynamic extended-system version of the Jahn–Teller effect, in conjunction with the double-exchange interaction, is also viewed as essential for a microscopic description of CMR in the manganite perovskites. Large magnetoresistance is also seen in other systems, namely Tl 2Mn2O7 and some Cr chalcogenide spinels, compounds which differ greatly from the manganite perovskites. We describe the relevant points of contrast between the various CMR materials.

REVIEW ARTICLE: Colossal magnetoresistance

This article discusses fluctuating order in a quantum disordered phase proximate to a quantum critical point, with particular emphasis on fluctuating stripe order. Optimal strategies are derived for extracting information concerning such local order from experiments, with emphasis on neutron scattering and scanning tunneling microscopy. These ideas are tested by application to two model systems---an exactly solvable one-dimensional (1D) electron gas with an impurity, and a weakly interacting 2D electron gas. Experiments on the cuprate high-temperature superconductors which can be analyzed using these strategies are extensively reviewed. The authors adduce evidence that stripe correlations are widespread in the cuprates. They compare and contrast the advantages of two limiting perspectives on the high-temperature superconductor: weak coupling, in which correlation effects are treated as a perturbation on an underlying metallic (although renormalized) Fermi-liquid state, and strong coupling, in which the magnetism is associated with well-defined localized spins, and stripes are viewed as a form of micro phase separation. The authors present quantitative indicators that the latter view better accounts for the observed stripe phenomena in the cuprates.

How to detect fluctuating stripes in the high-temperature superconductors

Recent computational studies of models for manganese oxides have revealed a rich phase diagram, which was not anticipated in early calculations in this context performed in the 1950s and 1960s. In particular, the transition between the antiferromagnetic insulator state of the hole-undoped limit and the ferromagnetic metal at finite hole density was found to occur through a mixed-phase process. When extended Coulomb interactions are included, a microscopically charged inhomogeneous state should be stabilized. These phase separation tendencies, also present at low electronic densities, influence the properties of the ferromagnetic region by increasing charge fluctuations. Experimental data reviewed here by applying several techniques for manganites and other materials are consistent with this scenario. Similarities with results previously discussed in the context of cuprates are clear from this analysis, although the phase segregation tendencies in manganites appear stronger.

http://science.sciencemag.org/content/sci/283/5410/2034.full.pdf

Phase Separation Scenario for Manganese Oxides and Related Materials

FERROMAGNETIC perovskites of the form La1–XMexMnO3–Y (where Me is Ca or Sr) have been known1 since 1950, but there has been a recent resurgence of interest following the discovery of giant magnetoresistance in this class of compounds2,3. The compounds contain both Mn3+ and Mn4+ ions; as the electronic ground state of the Mn3+ ions is degenerate, their energy is lowered by a spontaneous distortion of the surrounding lattice—the Jahn–Teller effect4. The charge carriers in these materials are strongly coupled to (and mediate the ferromagnetic interaction between) the manganese ions5, suggesting that localized lattice distortions could also play an important role in determining the electronic and magnetic properties of these compounds. Here we investigate this possibility by examining the effect on the ferromagnetic transition temperature of varying the oxygen isotope mass (replacing 16O with 18O). For La0.8Ca0.2MnO3+y, we measure an isotope shift of >20 K, significantly larger than that found for any magnetic or electronic phase transition in other oxides. In contrast, we observe no significant isotope shift for the structurally related ferromagnet SrRuO3, in which the Jahn–Teller effect is negligible. These results imply that the large isotope shift arises from coupling of the charge carriers to Jahn–Teller lattice distortions, and we suggest that such Jahn–Teller 'polarons' may also be responsible for the magnetoresistive properties of these materials.

Giant oxygen isotope shift in the magnetoresistive perovskite La1-xCaxMnO3+y

High-temperature superconductivity is known to involve the pairing of charge carriers, but the precise nature of these carriers and the mechanism of their pairing remain unclear. The copper oxides are known to exhibit a strong Jahn–Teller effect — in which spontaneous lattice distortions remove the degeneracy of the electronic ground state — and it has been suggested that the charge carriers are Jahn–Teller polarons (bare charge carriers accompanied by local lattice distortions). In fact, the demonstration1 that a strong Jahn–Teller effect can lead to the formation of such polarons led to the original discovery of high-temperature superconductivity2. Still, direct evidence that Jahn–Teller polarons exist in the superconducting state of the copper oxides has been lacking, although some indirect evidence comes from their recent discovery3 in the structurally similar but non-superconducting manganite La1–xCaxMnO3. Here we report the results of magnetization and thermal expansion measurements on samples of the copper oxide superconductor La2–xSrxCuO4 which characterize the oxygen-isotope effects on the carrier density and on the in-plane penetration depth. We find a negligible isotope effect on the former, but a large effect on the latter. Specific quantitative features of the results show that polaronic charge carriers exist and condense into Cooper pairs in the copper oxide superconductors.

Evidence for polaronic supercarriers in the copper oxide superconductors La 2–x Sr x CuO 4

The electron paramagnetic resonance (EPR) signal in the mixed valence perovskite ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3+y}$ was investigated in the paramagnetic regime for ${}^{16}$O and ${}^{18}$O isotope substituted compounds. The characteristic differences observed in EPR intensity and linewidth for the two isotope samples can be explained by a model in which a bottlenecked spin relaxation takes place from the exchange-coupled constituent ${\mathrm{Mn}}^{4+}$ ions via the ${\mathrm{Mn}}^{3+}$ Jahn-Teller ions to the lattice. For $x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.2$ the ferromagnetic exchange energy $J$ exhibits a ${}^{16}\mathrm{O}{/}^{18}\mathrm{O}$ oxygen isotope effect of $\ensuremath{\sim}\ensuremath{-}10%$. The observed isotope effects suggest the presence of Jahn-Teller polarons in these materials.

EPR Evidence of Jahn-Teller Polaron Formation in La 1-x Ca x MnO 3+y

EPR in La 1 − x Ca x MnO 3 : Relaxation and bottleneck

A very large oxygen-isotope shift of the charge-stripe ordering temperature in the high-temperature superconductors La2-xSrxCuO4 has been observed using x-ray absorption spectroscopy, a fast (~10-15 seconds) and local probe (~5 A). Upon replacing 16O with 18O, the charge-stripe ordering temperature in La1.94Sr0.06CuO4 increases from about 110 K to 180 K. The present results provide compelling evidence for a strong electron-phonon interaction in this compound, and thus place important constraints on the microscopic origin of the stripe formation.

https://iopscience.iop.org/article/10.1088/0953-8984/11/48/103/pdf

Guo-meng Zhao

Papers

Giant oxygen isotope shift in the magnetoresistive perovskite La1-xCaxMnO3+y

Evidence for polaronic supercarriers in the copper oxide superconductors La 2–x Sr x CuO 4

EPR Evidence of Jahn-Teller Polaron Formation in La 1-x Ca x MnO 3+y

EPR in La 1 − x Ca x MnO 3 : Relaxation and bottleneck

Oxygen-isotope shift of the charge-stripe ordering temperature in La2-xSrxCuO4 from x-ray absorption spectroscopy