Mesoscopic Percolating Resistance Network in a Strained Manganite Thin Film

Abstract

Many unusual behaviors in complex oxides are deeply associated with the spontaneous emergence of microscopic phase separation. Depending on the underlying mechanism, the competing phases can form ordered or random patterns at vastly different length scales. Using a microwave impedance microscope, we observed an orientation-ordered percolating network in strained Nd0.5Sr0.5MnO3 thin films with a large period of 100 nm. The filamentary metallic domains align preferentially along certain crystal axes of the substrate, suggesting the anisotropic elastic strain as the key interaction in this system. The local impedance maps provide microscopic electrical information of the hysteretic behavior in strained thin film manganites, suggesting close connection between the glassy order and the colossal magnetoresistance effects at low temperatures. PACS numbers:Doped cuprate superconductors and colossal magnetoresistive (CMR) manganites, the two most studied complex oxides, exhibit rich phase diagrams as a result of the simultaneously active charge, spin, orbital, and lattice degrees of freedom [1,2]. Recent work on these strongly correlated materials has shown that multiple states can coexist near certain phase boundaries, a scenario known as microscopic phase separation [1]. The configurations of these spatially inhomogeneous phases reflect the underlying interactions. When the long-range Coulomb interaction prevails, the competing phases usually form nanometer-scale orders, because of the electrostatic energy penalty for macroscopic phase separation [3][4][5][6]. For self-organized patterns at larger length scales, weaker long-range interactions, such as the elastic strain arising from either the cooperative lattice distortions or lattice mismatch between substrates and epitaxial thin films, become the dominant factors [7][8][9]. Finally, the unavoidable quenched disorders in real materials always introduce short-range potential fluctuations, which usually smear out the orders or even result in micrometersized clusters with random shapes [10].Many physical properties affected by the phase separation, such as the local density of states [4-6, 11, 12], the local magnetization [13][14][15], and the atomic displacement [16], can be spatially mapped out by established microscopy tools. For CMR manganites with drastic resistance changes at different temperatures (T) and magnetic fields (H), the local resistivity (ρ) has a large span that makes spatially resolved DC measurements challenging. Imaging with high-frequency AC-coupled local probes is thus desirable. We carried out a microwave impedance microscopy (MIM) study [17,18] on manganite thin films. Unlike other GHz microscopes [19], the cantilever probe is well shielded to reduce the stray fields [18]. In the microwave electronics, the high-Q resonator [20] susceptible to environmental conditions is eliminated so that the system can be implemented under variable temperatures (2-300K) and high magnetic fields (9T). Our cryogenic MIM is therefore a versatile tool to investigat...

Phase transition in bulk single crystals and thin films ofVO2by nanoscale infrared spectroscopy and imaging

“…In addition, the nontrivial charge and orbital orderings in TMOs are highly susceptible to local and external stimuli such as thermal excitation [4], strain [5][6][7] or light illumination [8][9][10]. The resultant phase transitions are often accompanied by inevitable mesoscopic electronic and/or magnetic inhomogeneities [11][12][13][14], making it extremely difficult to distinguish emergent single phase properties from area-averaging phenomena originating from a mixed state [13,[15][16][17]. As a result, experimental observations strongly depend on the particularities of the samples and measurement techniques, posing difficulties in interpreting the results obtained with complementary experimental approaches.…”

Speed limit of the insulator–metal transition in magnetite

“…Fluctuations to this trimeron order have been observed up to 80 K above T V in neutron scattering studies [11,12]. 3 In many oxide materials [13,14] the insulator-metal transition has been discussed in terms of freezing in fluctuating lattice and electronic (charge, spin and orbital) order, sometimes in combination with the occurrence of phase separation [15,16]. However, due to the many competing degrees of freedom, the close energetic proximity of the different phases often obscures the exact nature of these phase transitions as they are probed in thermal equilibrium [17].…”

Dynamics of multiple phases in a colossal-magnetoresistive manganite as revealed by dielectric spectroscopy

“…The electronic phase separation has been revealed at first by magnetic and electrictransport characterization and verified consequently by a variety of structural probes, such as X-ray and neutron diffraction measurements [3][4][5] . Recently, studies using microscopies have provided us with the real space image of phase separation in correlated electron oxides 3,6 . Nevertheless, knowledge of dynamic transport in multiple phases is lacking up to now, although it is quite important both for understanding of underlying physics as well as promising applications 1 .…”