Role of structurally and magnetically modified nanoclusters in colossal magnetoresistance

Abstract

It is generally accepted that electronic and magnetic phase separation is the origin of many of exotic properties of strongly correlated electron materials, such as colossal magnetoresistance (CMR), an unusually large variation in the electrical resistivity under applied magnetic field. In the simplest picture, the two competing phases are those associated with the material state on either side of the phase transition. Those phases would be paramagnetic insulator and ferromagnetic metal for the CMR effect in doped manganites. It has been speculated that a critical component of the CMR phenomenon is nanoclusters with quite different properties than either of the terminal phases during the transition. However, the role of these nanoclusters in the CMR effect remains elusive because the physical properties of the nanoclusters are hard to measure when embedded in bulk materials. Here we show the unexpected behavior of the nanoclusters in the CMR compound La 1−x Ca x MnO 3 (0.4 ≤ x < 0.5) by directly correlating transmission electron microscopy observations with bulk measurements. The structurally modified nanoclusters at the CMR temperature were found to be ferromagnetic and exhibit much higher electrical conductivity than previously proposed. Only at temperatures much below the CMR transition, the nanoclusters are antiferromagnetic and insulating. These findings substantially alter the current understanding of these nanoclusters on the material's functionality and would shed light on the microscopic study on the competing spin-lattice-charge orders in strongly correlated systems.nanoscale inhomogeneities | superlattice I t is widely believed that electronic inhomogeneities due to the complex spin-lattice-charge interplay (1-9) and the percolation of the conducting phase in manganites (10-18) are essential to the understanding of the colossal magnetoresistance (CMR) mechanism (19-21). However, the competing magnetic, lattice, and charge orders result in very intricate phase-separation scenarios in the materials and, therefore, it is difficult to unravel the role of individual phases in the CMR effect. Firstly, the dimension of the percolative phases that yields the CMR effect is under intensive debate. Submicron percolating network was observed using several imaging techniques (10-12), although evidence by various diffraction techniques and scanning tunneling microscopy (STM) supports nanoscale phase separation to be the key to study the CMR (3-9, 15-18). Secondly, among the nanoscale phases, a phase in form of nanoclusters with unique structural characteristic has attracted great attention because its appearance coincides with the CMR effect in a wide range of doped manganites without exception (3-9). This nanoscale phase was found to have satellite-superlattice reflections detected by neutron scattering, synchrotron X-ray, and electron diffractions during a paramagnetic (PM) to ferromagnetic (FM) phase transition upon cooling (3-9). The superstructure modulation exhibits in the nanoclusters with maximum density in...

Ferromagnetic domain behavior and phase transition in bilayer manganites investigated at the nanoscale

“…The first appearance of magnetic domain wall contrast was observed at T = 118 K. This temperature is about 7 K lower than the Curie temperature of the same TEM sample as measured from magnetometry to be T c = 125 K. This difference can attributed to the fact that at temperatures very close to T c , the ferromagnetic domain signal is too weak to be detected using Lorentz TEM. Similar differences between temperature at which magnetic contrast is observed and the Curie temperature have previously been reported 13,20 . The area fraction of the sample that was ferromagnetic was calculated as a function of temperature from this series of images.…”

“…Only recently there have been some efforts towards direct real space visualization of the way in which the small FM regions form and become connected, leading to the formation of FM domains within the material below T c . Lorentz transmission electron microscopy (LTEM) has also been used to study cubic manganites since it offers high spatial resolution and a direct visualization of the magnetic domains [11][12][13] . Furthermore with current advanced in-situ capabilities, LTEM offers unique possibilities to study the magnetic phase transitions as a function of temperature, while simultaneously obtaining information about structural and charge ordering using electron diffraction.…”

Origin of colossal magnetoresistance in LaMnO 3 manganite

“…The presence of Mn 3+ and Mn 4+ ions together with the site-site double-exchange (DE) mechanism (1) appear to capture the essence of this phenomenon. A plethora of experimental and theoretical investigations have recently suggested that the ground states of manganites are intrinsically inhomogeneous and characterized by the presence of competing phases (2)(3)(4)(5)(6)(7)(8) extending over domains at nanoscale/mesoscale. High pressure (P) has a triggering effect for phase separation because either magnetic or structural domains have been observed in compressed manganites (9)(10)(11)(12).…”

Nanoclustering phase competition induces the resistivity hump in colossal magnetoresistive manganites

“…The strong coupling between charge, spin, orbital, and lattice degrees of freedom, supplemented by weak disorder, leads to unusual colossal response phenomena in manganites [1][2][3][4] . There has been intense focus over last two decades to validate the phase coexistence/competition scenario [5][6][7][8][9][10][11][12][13][14][15][16][17][18] , which is believed to be a necessary ingredient to explain the resistivity hump in colossal magnetoresistive (CMR) manganites. Furthermore, understanding the phase competition in bulk manganites helps in designing low-dimensional manganite nanostructures with emergent physical phenomena [19][20][21] .…”