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

Colossal Magnetoresistant Materials: The Key Role of Phase Separation

Correlated electron materials can undergo a variety of phase transitions, including superconductivity, the metal-insulator transition and colossal magnetoresistance. Moreover, multiple physical phases or domains with dimensions of nanometres to micrometres can coexist in these materials at temperatures where a pure phase is expected. Making use of the properties of correlated electron materials in device applications will require the ability to control domain structures and phase transitions in these materials. Lattice strain has been shown to cause the coexistence of metallic and insulating phases in the Mott insulator VO(2). Here, we show that we can nucleate and manipulate ordered arrays of metallic and insulating domains along single-crystal beams of VO(2) by continuously tuning the strain over a wide range of values. The Mott transition between a low-temperature insulating phase and a high-temperature metallic phase usually occurs at 341 K in VO(2), but the active control of strain allows us to reduce this transition temperature to room temperature. In addition to device applications, the ability to control the phase structure of VO(2) with strain could lead to a deeper understanding of the correlated electron materials in general.

/pdf/strain-engineering-and-one-dimensional-organization-of-metal-4v4091wn8w.pdf

Strain engineering and one-dimensional organization of metal–insulator domains in single-crystal vanadium dioxide beams

The coexistence of distinct metallic and insulating electronic phases within the same sample of a perovskite manganite1,2,3,4,5,6, such as La1-x-yPryCaxMnO3, presents researchers with a tool for tuning the electronic properties in materials. In particular, colossal magnetoresistance7 in these materials—the dramatic reduction of resistivity in a magnetic field—is closely related to the observed texture owing to nanometre- and micrometre-scale inhomogeneities1,2,3,4,5,6,8. Despite accumulated data from various high-resolution probes, a theoretical understanding for the existence of such inhomogeneities has been lacking. Mechanisms invoked so far, usually based on electronic mechanisms and chemical disorder9,10,11, have been inadequate to describe the multiscale, multiphase coexistence within a unified picture. Moreover, lattice distortions and long-range strains12,13 are known to be important in the manganites14. Here we show that the texturing can be due to the intrinsic complexity of a system with strong coupling between the electronic and elastic degrees of freedom. This leads to local energetically favourable configurations and provides a natural mechanism for the self-organized inhomogeneities over both nanometre and micrometre scales. The framework provides a physical understanding of various experimental results and a basis for engineering nanoscale patterns of metallic and insulating phases.

https://web.njit.edu/~kenahn/AhnNature04.pdf

Strain-induced metal–insulator phase coexistence in perovskite manganites

We present the results of a comprehensive investigation of the dc magnetization, ac susceptibility, and magnetotransport properties of the glassy ferromagnet ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}.$ The compositions studied span the range from the end-member ${\mathrm{LaCoO}}_{3}$ $(x=0.0)$ through to $x=0.7.$ These materials have attracted attention recently, primarily due to the spin-state transition phenomena in ${\mathrm{LaCoO}}_{3}$ and the unusual nature of the magnetic ground state for finite x. In this paper we present a consistent picture of the magnetic behavior of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}$ in terms of short-range ferromagnetic ordering and intrinsic phase separation. At high Sr doping $(xg0.2)$ the system exhibits unconventional ferromagnetism (with a Curie temperature up to 250 K), which is interpreted in terms of the coalescence of short-range-ordered ferromagnetic clusters. Brillouin function fits to the temperature dependence of the magnetization as well as high-temperature Curie-Weiss behavior suggest that the ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}$ ions are both in the intermediate spin state. At lower Sr doping $(xl0.18)$ the system enters a mixed phase that displays the characteristics of both a spin glass and a ferromagnet. A cusp in the zero-field-cooled dc magnetization, a frequency-dependent peak in the ac susceptibility and time-dependent effects in both dc and ac magnetic properties all point towards glassy behavior. On the other hand, field cooling results in a relatively large ferromagneticlike moment, with zero-field-cooled and field-cooled magnetizations bifurcating at an irreversibility point. Even in the region above $x=0.2$ the out-of-phase component of the ac susceptibility shows frequency-dependent peaks below the Curie temperature (indicative of glassy behavior) which have previously been interpreted in terms of the freezing of clusters. All of the results are consistent with the existence of a strong tendency towards magnetic phase separation in this material, a conclusion which is further reinforced by consideration of the electronic properties. The metal-insulator transition is observed to be coincident with the onset of ferromagnetic ordering $(x=0.18)$ and has a behavior in the doping dependence of the low-temperature conductivity which is strongly suggestive of percolation. This can be interpreted as a percolation transition within the simple ferromagnetic cluster model. On the metallic side of the transition the system exhibits colossal magnetoresistance-type behavior with a peak in the negative magnetoresistance (\ensuremath{\sim}10% in 90 kOe) in the vicinity of the Curie temperature. As the transition is approached from the metallic side we observe the onset of a negative magnetoresistance that increases in magnitude with decreasing temperature, reaching values as large as 90% in a 90-kOe field. This magnetoresistance is enhanced at the metal-insulator transition, where it persists even to room temperature.

Glassy ferromagnetism and magnetic phase separation in La 1 − x Sr x CoO 3

Colossal-magnetoresistance materials: manganites and conventional ferromagnetic semiconductors

Scanning tunneling spectroscopy was used to investigate single crystals and thin films of La1– x Ca x MnO3(with x of about 0.3), which exhibit colossal magnetoresistance. The different spectroscopic signatures of the insulating (paramagnetic) and metallic (ferromagnetic) phases enable their spatial extent to be imaged down to a lateral scale of the order of 10 nanometers. Above the bulk transition temperature T c, the images show mostly insulating behavior. Below T c, a phase separation is observed where inhomogeneous structures of metallic and more insulating areas coexist and are strongly field dependent in their size and structure. Insulating areas are found to persist far below T c. These results suggest that the transition and the associated magnetoresistance behavior should be viewed as a percolation of metallic ferromagnetic domains.

https://science.sciencemag.org/content/sci/285/5433/1540.full.pdf

M. Fäth

Papers

Spatially Inhomogeneous Metal-Insulator Transition in Doped Manganites