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We have designed and constructed a torsional oscillator magnetometer for use in magnetic fields up to 25 T. The anisotropic component of the magnetization is detected by measuring the shift in the resonant frequency of the oscillator, which is fabricated from a single‐crystal silicon wafer using micromachining techniques. The frequencies for the oscillators described here are between 100 Hz and 1 kHz and can be measured with a resolution of order 1 part in 108 in a 10 s averaging time, allowing for the detection of magnetic moments of 2×10−11 J/T at 1 T. Our oscillators are optimized for experiments on GaAs‐AlGaAs heterostructures, but the method is suitable for any sample with an anisotropic susceptibility. We have applied the technique to two systems, a quasi‐one‐dimensional spin chain and a two‐dimensional electron gas.

International audience; Prussian blue analogues magnetic nanoparticles (of radius R0 = 2.4–8.6 nm) embedded in PVP (polyvinylpyrrolidone) or CTA+ (cetyltrimethylammonium) matrices have been studied using neutron diffraction and small angle neutron scattering (SANS) at several concentrations. For the most diluted particles in neutral PVP, the SANS signal is fully accounted for by a “single-particle” spherical form factor with no structural correlations between the nanoparticles and with radii comparable to those inferred from neutron diffraction. For higher concentration in PVP, structural correlations modify the SANS signal with the appearance of a structure factor peak, which is described using an effective “mean-field” model. A new length scale R* ≈ 3R0, corresponding to an effective repulsive interaction radius, is evidenced in PVP samples. In CTA+, electrostatic interactions play a crucial role and lead to a dense layer of CTA+ around the nanoparticles, which considerably alter the SANS patterns as compared to PVP. The SANS data of nanoparticles in CTA+ are best described by a core-shell model without visible inter-particle structure factor.

Magnetic nanowires could be the building bricks in the fabrication of composite magnetic materials because of their large intrinsic shape anisotropy. We investigate the relation between the detailed shape of a magnetic nanowire and its magnetic coercivity. We have performed three-dimensional micromagnetic simulations on various types of nanowires synthesized during the past few years such as cylinders, dumbbells, or diabolos. The calculations, performed on individual model objects, show that the wire tip plays a key role in the reversal mechanism and on the magnitude of the coercive field and that the aspect ratio plays a much lesser role. Ellipsoidal or cylindrical shapes favor a coherent rotation of the magnetization and thus large coercive fields. Complex tip shapes act as nucleation points and significantly reduce the coercive field. Thus, in order to optimize the shape of magnetic nanowires for permanent magnet applications, the focus should be put on the detailed shape of the wire tips and thus on the growth mechanism rather than on the aspect ratio. The numerical modeling results on individual wires are compared with the experimental data obtained on various types of wires synthesized by soft chemistry methods.