Foundering of mafic lower continental crust into underlying convecting mantle has been proposed as one means to explain the unusually evolved chemical composition of Earth's continental crust, yet direct evidence of this process has been scarce. Here we report that Late Jurassic high-magnesium andesites, dacites and adakites (siliceous lavas with high strontium and low heavy-rare-earth element and yttrium contents) from the North China craton have chemical and petrographic features consistent with their origin as partial melts of eclogite that subsequently interacted with mantle peridotite. Similar features observed in adakites and some Archaean sodium-rich granitoids of the tonalite-trondhjemite-granodiorite series have been interpreted to result from interaction of slab melts with the mantle wedge. Unlike their arc-related counterparts, however, the Chinese magmas carry inherited Archaean zircons and have neodymium and strontium isotopic compositions overlapping those of eclogite xenoliths derived from the lower crust of the North China craton. Such features cannot be produced by crustal assimilation of slab melts, given the high Mg#, nickel and chromium contents of the lavas. We infer that the Chinese lavas derive from ancient mafic lower crust that foundered into the convecting mantle and subsequently melted and interacted with peridotite. We suggest that lower crustal foundering occurred within the North China craton during the Late Jurassic, and thus provides constraints on the timing of lithosphere removal beneath the North China craton.

Recycling lower continental crust in the North China craton

The interaction of subpicosecond laser pulses with magnetically ordered materials has developed into a fascinating research topic in modern magnetism. From the discovery of subpicosecond demagnetization over a decade ago to the recent demonstration of magnetization reversal by a single 40 fs laser pulse, the manipulation of magnetic order by ultrashort laser pulses has become a fundamentally challenging topic with a potentially high impact for future spintronics, data storage and manipulation, and quantum computation. Understanding the underlying mechanisms implies understanding the interaction of photons with charges, spins, and lattice, and the angular momentum transfer between them. This paper will review the progress in this field of laser manipulation of magnetic order in a systematic way. Starting with a historical introduction, the interaction of light with magnetically ordered matter is discussed. By investigating metals, semiconductors, and dielectrics, the roles of nearly free electrons, charge redistributions, and spin-orbit and spin-lattice interactions can partly be separated, and effects due to heating can be distinguished from those that are not. It will be shown that there is a fundamental distinction between processes that involve the actual absorption of photons and those that do not. It turns out that for the latter, the polarization of light plays an essential role in the manipulation of the magnetic moments at the femtosecond time scale. Thus, circularly and linearly polarized pulses are shown to act as strong transient magnetic field pulses originating from the nonabsorptive inverse Faraday and inverse Cotton-Mouton effects, respectively. The recent progress in the understanding of magneto-optical effects on the femtosecond time scale together with the mentioned inverse, optomagnetic effects promises a bright future for this field of ultrafast optical manipulation of magnetic order or femtomagnetism.

/pdf/ultrafast-optical-manipulation-of-magnetic-order-2ms2t5k1zo.pdf

Ultrafast optical manipulation of magnetic order

This article reviews static and dynamic interfacial effects in magnetism, focusing on interfacially-driven magnetic effects and phenomena associated with spin-orbit coupling and intrinsic symmetry breaking at interfaces. It provides a historical background and literature survey, but focuses on recent progress, identifying the most exciting new scientific results and pointing to promising future research directions. It starts with an introduction and overview of how basic magnetic properties are affected by interfaces, then turns to a discussion of charge and spin transport through and near interfaces and how these can be used to control the properties of the magnetic layer. Important concepts include spin accumulation, spin currents, spin transfer torque, and spin pumping. An overview is provided to the current state of knowledge and existing review literature on interfacial effects such as exchange bias, exchange spring magnets, spin Hall effect, oxide heterostructures, and topological insulators. The article highlights recent discoveries of interface-induced magnetism and non-collinear spin textures, non-linear dynamics including spin torque transfer and magnetization reversal induced by interfaces, and interfacial effects in ultrafast magnetization processes.

https://link.aps.org/accepted/10.1103/RevModPhys.89.025006

Interface-induced phenomena in magnetism

Our knowledge of the structure of matter is largely based on X-ray diffraction studies of periodic structures and the successful transformation (inversion) of the diffraction patterns into real-space atomic maps. But the determination of non-periodic nanoscale structures by X-rays is much more difficult. Inversion of the measured diffuse X-ray intensity patterns suffers from the intrinsic loss of phase information, and direct imaging methods are limited in resolution by the available X-ray optics. Here we demonstrate a versatile technique for imaging nanostructures, based on the use of resonantly tuned soft X-rays for scattering contrast and the direct Fourier inversion of a holographically formed interference pattern. Our implementation places the sample behind a lithographically manufactured mask with a micrometre-sized sample aperture and a nanometre-sized hole that defines a reference beam. As an example, we have used the resonant X-ray magnetic circular dichroism effect to image the random magnetic domain structure in a Co/Pt multilayer film with a spatial resolution of 50 nm. Our technique, which is a form of Fourier transform holography, is transferable to a wide variety of specimens, appears scalable to diffraction-limited resolution, and is well suited for ultrafast single-shot imaging with coherent X-ray free-electron laser sources.

/pdf/lensless-imaging-of-magnetic-nanostructures-by-x-ray-spectro-1onlexme9b.pdf

Lensless imaging of magnetic nanostructures by X-ray spectro-holography

In this review we propose to address the question: for the life-science researcher, what does X-ray microscopy have to offer that is not otherwise easily available?We will see that the answer depends on a combination of resolution, penetrating power, analytical sensitivity, compatibility with wet specimens, and the ease of image interpretation.

Soft X-ray microscopes and their biological applications

Circularly polarized soft x-rays have been used with an imaging photoelectron microscope to record images of magnetic domains at a spatial resolution of 1 micrometer. The magnetic contrast, which can be remarkably large, arises from the fact that the x-ray absorption cross section at inner-shell absorption edges of aligned magnetic atoms depends on the relative orientation of the photon spin and the local magnetization direction. The technique is element-specific, and, because of the long mean free paths of the x-rays and secondary electrons, it can record images of buried magnetic layers.

Element-Specific Magnetic Microscopy with Circularly Polarized X-rays

The development of electron spectromicroscopy

A photoemission microscope with a hemispherical capacitor energy filter

We investigate the β‐SiC(100)3×2 surface oxidation by core level and valence band photoemission spectroscopies using synchrotron radiation. Low molecular O2 exposures on the (3×2) surface reconstruction leads to direct SiO2/β‐SiC(100)3×2 interface formation already at room temperature (RT). To our best knowledge, this is the first example of RT oxidation leading directly to dominant silicon dioxide growth by O2 chemisorption only. The amount of SiO2 is enhanced when the surface temperature is raised by few hundred degrees only (<500 °C) during O2 exposures leading to ‘‘bulk oxide’’ formation already at small thicknesses. These findings are also relevant in low‐temperature semiconductor oxide processing.

Direct SiO2/β‐SiC(100)3×2 interface formation from 25 °C to 500 °C

The structure of the $\mathrm{Si}$-rich $3C\text{\ensuremath{-}}\mathrm{SiC}(001)\char21{}(3\ifmmode\times\else\texttimes\fi{}2)$ surface reconstruction is determined using soft x-ray photoelectron diffraction. Photoelectrons are detected along a full hemispherical sector for different photon energies. A comparison between the experimental data and multiple scattering calculations of the competing models favors a modified version of the two-adlayer asymmetric dimer model. An $R$-factor analysis has been employed to refine this model. We determine the interlayer spacings of the last six atomic layers and find a corrugation of $(0.25\ifmmode\pm\else\textpm\fi{}0.10)$ $\mathrm{\AA{}}$ for the atoms in the outermost dimer. Atoms in the second layer dimerize as well, forming rows of long and short dimers.

D. Dunham

Papers

Element-Specific Magnetic Microscopy with Circularly Polarized X-rays

The development of electron spectromicroscopy

A photoemission microscope with a hemispherical capacitor energy filter

Direct SiO2/β‐SiC(100)3×2 interface formation from 25 °C to 500 °C

Photoelectron diffraction study of the Si-rich 3C-SiC(001)-(3×2) structure