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

Abstract

Understanding the underlying mechanism and phenomenology of colossal magnetoresistance in manganites has largely focused on atomic and nanoscale physics such as double exchange, phase separation, and charge order. Here we consider a more macroscopic view of manganite materials physics, reporting on the ferromagnetic domain behavior in a bilayer manganite sample with a nominal composition of La 2−2x Sr 1+2x Mn 2 O 7 with x = 0.38, studied using in-situ Lorentz transmission electron microscopy. The role of magnetocrystalline anisotropy on the structure of domain walls was elucidated. On cooling, magnetic domain contrast was seen to appear first at the Curie temperature within the a − b plane. With further reduction in temperature, the change in area fraction of magnetic domains was used to estimate the critical exponent describing the ferromagntic phase transition. The ferromagnetic phase transition was accompanied by a distinctive nanoscale granular contrast close to the Curie temperature, which we infer to be related to the presence of ferromagnetic nanoclusters in a paramagnetic matrix, which has not yet been reported in bilayer manganites.

Coexistence of Merons with Skyrmions in the Centrosymmetric Van Der Waals Ferromagnet Fe5–xGeTe2

“…Our results suggest that unconventional topological spin textures, [ 10b ] that is, those distinct from merons or skyrmions, might exist in atomically thin vdW layers and their properties have yet to be unveiled and explored for spintronic real applications. To support this assertion, we estimated through Lorentz microscopy [ 35 ] the domain wall width among planar domains obtaining a remarkably wide average width df = (25 ± 5) nm (Figure S19, Supporting Information). As such a wide domain wall meanders between planar domains, it is likely to locally acquire either Néel or Bloch character likely explaining the apparent discrepancies among the different reports on Fe 5– x GeTe 2 .…”

Challenges and Applications to Operando and In Situ TEM Imaging and Spectroscopic Capabilities in a Cryogenic Temperature Range

“…To study low-temperature transition phenomena on the local scale in a TEM, accurate and tunable control of the temperature is needed, typically to follow in situ via TEM the microstructural reorganization happening across phase transitions. − In addition, thermally activated processes and their interactions with device performance need to be studied operando , i.e., with the application of external stimuli. Moreover, the in situ and operando TEM experiments require the observation of devices in their native state and under operating conditions.…”

“…There are an increasing number of reports on atomic-resolution imaging at cryogenic temperatures in transmission electron microscopes (TEMs) to delay the structural changes induced by electron beam irradiation − or to study low-temperature-induced reorganization. − For the former, the main challenge in studying electron-beam-sensitive emerging materials is to maintain TEM resolution while cooling the specimen to cryogenic temperature to delay electron-beam-induced damage (or radiation damage). For the latter, the imaging of low-temperature phases and charge transitions on the local scale is related to ongoing efforts to further understand low-temperature phenomena such as quantum materials’ phase diagrams ,− ,,, and the local pinning of domain walls in ferroelectric and ferromagnetic , materials. The imaging of temperature-dependent phenomena requires not only accurate continuous and variable temperature control but also the application of external stimuli (electric, magnetic, light, etc.)…”

“…9−18 For the former, the main challenge in studying electron-beam-sensitive emerging materials is to maintain TEM resolution while cooling the specimen to cryogenic temperature to delay electron-beam-induced damage (or radiation damage). For the latter, the imaging of lowtemperature phases and charge transitions on the local scale is related to ongoing efforts to further understand low-temperature phenomena such as quantum materials' phase diagrams 9,[11][12][13]15,17,18 and the local pinning of domain walls in ferroelectric 10 and ferromagnetic 14,16 materials. The imaging of temperature-dependent phenomena requires not only accurate continuous and variable temperature control but also the application of external stimuli (electric, magnetic, light, etc.)…”

Domain behavior in functional materials studied using Lorentz microscopy

“…One of the proposed mechanisms for the CMR effect is that small ferromagnetic regions are formed which are connected in a percolative manner as the material undergoes a phase transition [1]. We have explored the in-situ phase transition from room temperature to below the Curie temperature of T c = 125 K for bilayer manganite with a nominal composition of La 1.24 Sr 1.76 Mn 2 O 7 [2]. We will show that from quantitative analysis of domain images parallel and perpendicular to the 001 anisotropy axis (Figure 1(a) and (b)) enabled us to estimate the exchange stiffness, A = 15.2 pJ/m from the domain wall width and to show that domain walls are tilted with respect to 001 .…”