Cells of magnetotactic bacteria are used as model systems for studying the magnetic properties of ferrimagnetic nanocrystals. Each individual bacterial strain produces magnetosomes (membrane-bounded magnetic crystals) that have distinct sizes, shapes, crystallographic orientations and spatial arrangements, thereby providing nanoparticle systems whose unique magnetic properties are unmatched by synthetic chemically-produced crystals. Here, we use off-axis electron holography in the transmission electron microscope to study the magnetic properties of isolated and closely-spaced bullet-shaped magnetite (Fe3O4) magnetosomes biomineralized by the following magnetotactic bacterial strains: the cultured Desulfovibrio magneticus RS-1 and the uncultured strains LO-1 and HSMV-1. These bacteria biomineralize magnetite crystals whose crystallographic axes of elongation are parallel to <100> (RS-1 and LO-1) or <110> (HSMV-1). We show that the individual magnetosome crystals are single magnetic domains and measure their projected in-plane magnetization distributions and magnetic dipole moments. We use analytical modelling to assess the interplay between shape anisotropy and the magnetically preferred <111> magneto-crystalline easy axis of magnetite.

The lattice parameters and the crystal and magnetic structures of Fe2SiO4 have been determined from 10 K to 1453 K by high-resolution time-of-flight neutron powder diffraction. Fe2SiO4 undergoes two antiferromagnetic phase transformations on cooling from room temperature: the first, at 65.4 K, is to a collinear antiferromagnet with moments on two symmetry-independent Fe ions; the second transition, at ~23 K, is to a structure in which the moments on one of the sets of Fe ions (those on the ‘M1 site’) become canted. The magnetic unit cell is identical to the crystallographic (chemical) unit cell and the space group remains Pbnm throughout. The magnetic structures have been refined and the results found to be in good agreement with previous studies; however, we have determined the spontaneous magnetostrictive strains, which have not been reported previously. In the paramagnetic phase of Fe2SiO4, at temperatures of 70 K and above, we find that the temperature dependence of the linear thermal expansion coefficient of the b axis takes an unusual form. In contrast to the behaviour of the expansion coefficients of the unit-cell volume and of the a and c axes, which show the expected reduction in magnitude below ~300 K, that of the b axis remains almost constant between ~70 K and 1000 K.

The distribution of hot Jupiters, for which star-planet interactions can be significant, questions the evolution of exosystems. We aim to follow the orbital evolution of a planet along the rotational and structural evolution of the host star by taking into account the coupled effects of tidal and magnetic torques from ab initio prescriptions. It allows us to better understand the evolution of star-planet systems and to explain some properties of the distribution of observed close-in planets. To this end we use a numerical model of a coplanar circular star-planet system taking into account stellar structural changes, wind braking and star-planet interactions, called ESPEM (Benbakoura et al. (2019)). We find that depending on the initial configuration of the system, magnetic effects can dominate tidal effects during the various phases of the evolution, leading to an important migration of the planet and to significant changes on the rotational evolution of the star. Both kinds of interactions thus have to be taken into account to predict the evolution of compact star-planet systems.

The Sun is the archetype of magnetic star and its proximity coupled with very high accuracy observations has helped us understanding how solar-like stars (e.g with a convective envelope) redistribute angular momentum and generate a cyclic magnetic field. However most solar models have been so fine tuned that when they are applied to other solar-like stars the agreement with observations is not good enough. I will thus discuss, based on theoretical considerations and multi-D MHD stellar models, what can be considered as robust properties of solar-like star dynamics and magnetism and what is still speculative. I will derive scaling laws for differential rotation and magnetic energy as a function of stellar parameters, discuss recent results of stellar dynamo models and define the new concept of spot-dynamo, e.g. global dynamo that develops self-consistent magnetic buoyant structures that emerge at the surface.

A new ternary phase, Er2Cu10.9Ga6.1, has been found in the Er-Cu-Ga phase diagram. X-ray powder diffraction data were successfully refined based on a rhombohedral Th2Zn17 structure type (space group R-3m, No. 166) with cell parameters a=8.6627(1) Å, c=12.6824(2) Å, and Z=3. Er2Cu10.9Ga6.1 exhibits a paramagnetic Curie-Weiss behaviour down to 2 K with a trivalent character of Er atoms.

Le Fe et le Cr ont un comportement magnétique complexe. Cette complexité se retrouve déjà dans certaines phases de volume telles que les spirales de spin du Fe cubique faces centrées ou les ondes de densité de spin du Cr. Ces dernières peuvent être décrites qualitativement par des calculs de théorie de la fonctionnelle de la densité (DFT) ou de liaisons fortes (TB) qui prédisent une forme et une polarisation de l’onde de spin en accord avec les mesures expérimentales. L’énergie des défauts tels que les surfaces ou les lacunes ou encore la stabilité de l’alliage FeCr sont influencées également par le magnétisme. Et réciproquement, les défauts peuvent modifier la structure magnétique locale et créer des motifs magnétiques non colinéaires complexes.

The solar prolateness (also known as Ovalisation, a french origin name) of the extended dynamical chromosphere is established from measurements performed above 2 Mm heights during the years of solar minimum, using the H$\alpha$, Ca II K and HeII 304 line emissions from both ground-based and space-based observations. Coronal X-EUV emissions usually penetrate deep enough into the chromosphere to completely mask this effect on transition region lines and produce the so-called coronal hole effect. However, cool lines like H$\alpha$ and Ca II lines, do NOT show this Coronal Hole (CH) effect. Coronal lines and HeI (D3; 1083 nm) do show CHs but do not show the prolateness effect. We first briefly review different methods which can potentially be used to measure the prolateness. Further we note the similarity of the geometric behaviour of the prolateness and its variation along the solar cycle compared to the behaviour of the fast solar wind. It suggests the same origin possibly related to the emergence of the small scale network and internetwork magnetic field towards the corona and small scale magnetic reconnections. A simple geometric model was proposed to explain the effect of the prolateness of the solar chromosphere by considering that the specific dynamical part of the solar atmosphere above the 2 Mm level, being a mixture of up and down moving jets of chromospheric matter with the coronal plasma between them, is responsible for the solar prolateness (Filippov and Koutchmy, 2000). We however note that polar regions are also showing different types of activity in the low corona, including small prominence eruptions seen e.g. in H$\alpha$ and linear jets seen in SXR and EUV as well as in W-L (eclipses). Some kind of dynamical dissipation of the newly emerged magnetic field is needed. More systematic measurements should be done to build a more complete, possibly 3D, picture to explain the extended in the horizontal direction lifting effect of a large part of the polar chromosphere.

This article examines the role that Arthur von Hippel played in magnetism work in the 1950s.Von Hippel understood that the ferrimagnetic insulators represented by the ferrospinels, magnetoplumbites, and ferrogarnets were critical for the high-frequency technology that was being developed after World War II. At the Laboratory for Insulation Research at MIT, he and his students concentrated on the response of these materials to electric and magnetic excitations over a wide frequency range that extended, with gaps, from dc to the ultraviolet. For magnetic studies, he used microwave frequencies to obtain resonance and relaxation data that could be interpreted because the magnetic spins are relatively loosely coupled to their surroundings. He supplemented these resonance studies with classical magnetometer, transport, and x-ray diffraction measurements on single-crystal samples in order to obtain fundamental information that would aid in the design of materials for technical applications.

This article introduces the November 2005 issue of MRS Bulletin on the life and works of Arthur Robert von Hippel, who pioneered the interdisciplinary approach to materials research. This issue of MRS Bulletin celebrates his long life, his large volume of work, and the overall impact he had on materials research as practiced today. This introductory article summarizes the start and progression of the various fields presented in this issue, and how many were inspired directly by von Hippel's work and ways, and how new fields continue to emerge based on the same foundations of interdisciplinarity. The articles in this issue cover research areas in which von Hippel was involved, namely, ferroelectrics and magnetism; fields that thrived on an interdisciplinary approach that von Hippel represented, such as semiconductors; and areas that reflect his own vision about materials research and interests later in life, including molecular design and biomaterials. Before the scientific work is presented, the issue begins with a personal sketch of von Hippel, contributed by his son Frank N. von Hippel.

This brief article introduces the July 2004 issue of MRS Bulletin, focusing on Scanning Probe Microscopy in Materials Science.Those application areas of scanning probe microscopy (SPM) in which the most impact has been made in recent years are covered in the articles in this theme.They include polymers and semiconductors, where scanning force microscopy is now virtually a standard characterization method; magnetism, where magnetic force microscopy has served both as a routine analytical approach and a method for fundamental studies;tribology, where friction force microscopy has opened entirely new vistas of investigation;biological materials, where atomic force microscopy in an aqueous environment allows biosystems to be imaged and measured in a native (or near-native) state;and nanostructured materials, where SPM has often been the only approach capable of elucidating nanostructures.