Discrete evolution of the crystal structure during the growth of Ba-hexaferrite nanoplatelets.

An understanding of the adaptation of the crystal structure of materials confined at the nanoscale, the influences of their specific structures on the evolution of their morphologies and, finally, their functional properties is essential not only for expanding fundamental knowledge, but also for facilitating the designs of novel nanostructures for diverse technological and medical applications. Here we describe how the distinct structure of barium-hexaferrite nanoplatelets evolves in a stepwise manner in parallel with the development of their size and morphology during hydrothermal synthesis. The nanoplatelets are formed by reactions between Ba- and Fe-hydroxides in an aqueous suspension at temperatures below 80 °C. Scanning-transmission electron microscopy showed that the structure of the as-synthesized, discoid nanoplatelets (∼2.3 nm thick, ∼10 nm wide) terminates at the basal surfaces with Ba-containing planes. However, after subsequent washing of the nanoplatelets with water the top two atomic layers dissolve from the surfaces. The final structure can be represented by a SRS* sequence of the barium-hexaferrite SRS*R* unit cell, where S and R represent a hexagonal (BaFe6O11)2- and a cubic (Fe6O8)2+ structural block, respectively. Due to the stable SRS* structure, the thickness of the primary nanoplatelets remains unchanged up to approximately 150 °C, when some of the primary nanoplatelets start to grow exaggeratedly and their thicknesses increase discretely with the addition of the RS segments to their structure. The SRS* structure of the primary nanoplatelets is too thin for the complete development of magnetic ordering. However, the addition of just one RS segment (SRS*R*S structure) gives the nanoplatelets hard magnetic properties.

Novel Ba-hexaferrite structural variations stabilized on the nanoscale as building blocks for epitaxial bi-magnetic hard/soft sandwiched maghemite/hexaferrite/maghemite nanoplatelets with out-of-plane easy axis and enhanced magnetization.

Atomic-resolution scanning-transmission electron microscopy showed that barium hexaferrite (BHF) nanoplatelets display a distinct structure, which represents a novel structural variation of hexaferrites stabilized on the nanoscale. The structure can be presented in terms of two alternating structural blocks stacked across the nanoplatelet: a hexagonal (BaFe6O11)2- R block and a cubic (Fe6O8)2+ spinel S block. The structure of the BHF nanoplatelets comprises only two, or rarely three, R blocks and always terminates at the basal surfaces with the full S blocks. The structure of a vast majority of the nanoplatelets can be described with a SR*S*RS stacking order, corresponding to a BaFe15O23 composition. The nanoplatelets display a large, uniaxial magnetic anisotropy with the easy axis perpendicular to the platelet, which is a crucial property enabling different novel applications based on aligning the nanoplatelets with applied magnetic fields. However, the BHF nanoplatelets exhibit a modest saturation magnetization, MS, of just over 30 emu g-1. Given the cubic S block termination of the platelets, layers of maghemite, γ-Fe2O3, (M), with a cubic spinel structure, can be easily grown epitaxially on the surfaces of the platelets, forming a sandwiched M/BHF/M platelet structure. The exchange-coupled composite nanoplatelets exhibit a remarkably uniform structure, with an enhanced MS of more than 50 emu g-1 while essentially maintaining the out-of-plane easy axis. The enhanced MS could pave the way for their use in diverse platelet-based magnetic applications.

Magnetic inhomogeneity on a triangular lattice: the magnetic-exchange versus the elastic energy and the role of disorder.

Inhomogeneity in the ground state is an intriguing, emergent phenomenon in magnetism. Recently, it has been observed in the magnetostructural channel of the geometrically frustrated α-NaMnO2, for the first time in the absence of active charge degrees of freedom. Here we report an in-depth numerical and local-probe experimental study of the isostructural sister compound CuMnO2 that emphasizes and provides an explanation for the crucial differences between the two systems. The experimentally verified, much more homogeneous, ground state of the stoichiometric CuMnO2 is attributed to the reduced magnetoelastic competition between the counteracting magnetic-exchange and elastic-energy contributions. The comparison of the two systems additionally highlights the role of disorder and allows the understanding of the puzzling phenomenon of phase separation in uniform antiferromagnets.

Frustration-induced nanometre-scale inhomogeneity in a triangular antiferromagnet.

Phase inhomogeneity of otherwise chemically homogenous electronic systems is an essential ingredient leading to fascinating functional properties, such as high-Tc superconductivity in cuprates, colossal magnetoresistance in manganites and giant electrostriction in relaxors. In these materials distinct phases compete and can coexist owing to intertwined ordered parameters. Charge degrees of freedom play a fundamental role, although phase-separated ground states have been envisioned theoretically also for pure spin systems with geometrical frustration that serves as a source of phase competition. Here we report a paradigmatic magnetostructurally inhomogenous ground state of the geometrically frustrated α-NaMnO2 that stems from the system's aspiration to remove magnetic degeneracy and is possible only due to the existence of near-degenerate crystal structures. Synchrotron X-ray diffraction, nuclear magnetic resonance and muon spin relaxation show that the spin configuration of a monoclinic phase is disrupted by magnetically short-range-ordered nanoscale triclinic regions, thus revealing a novel complex state of matter.

Persistent spin dynamics intrinsic to amplitude-modulated long-range magnetic order.

An incommensurate elliptical helical magnetic structure in the frustrated coupled-spin-chain system FeTe(2)O(5)Br is surprisingly found to persist down to 53(3) mK (T/T(N)~1/200), according to neutron scattering and muon spin relaxation. In this state, finite spin fluctuations at T→0 are evidenced by muon depolarization, which is in agreement with specific-heat data indicating the presence of both gapless and gapped excitations. We thus show that the amplitude-modulated magnetic order intrinsically accommodates contradictory persistent spin dynamics and long-range order and can serve as a model structure to investigate their coexistence.