⁵⁷Fe Mössbauer spectroscopy measurements are presented in the underdoped Ba(Fe_{1- x}Co_x)₂As₂ series for x=0.014 (T_c<1.4 K) and x=0.03 and 0.045 (T_c 2 and 12 K, respectively). The spectral shapes in the so-called spin density wave (SDW) phase are interpreted in terms of incommensurate modulation of the magnetic structure, and allow the shape of the modulation to be determined. In undoped BaFe₂As₂, the magnetic structure is commensurate, and we find that incommensurability is present at the lowest doping level (x=0.014). As Co doping increases, the low-temperature modulation progressively loses its "squaredness" and tends to a sine-wave. The same trend occurs for a given doping level, as temperature increases. We find that a magnetic hyperfine component persists far above the SDW transition, its intensity being progressively tranferred to a paramagnetic component on heating.

We present an interpretation of zero-field energy integrated (``diffuse'') neutron scattering and of high-field magnetization data at very low temperature in the frustrated pyrochlore system ${\text{Tb}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$. This material has antiferromagnetic exchange interactions, and it is expected to have an Ising character at low temperature. Contrary to expectations, it shows no magnetic ordering down to 0.05 K, being thus labeled a ``spin liquid.'' However, the ground state in ${\text{Tb}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ is not a mere fluctuating moment paramagnet but, as demonstrated by very recent experiments, a state where the electronic degrees of freedom are hybridized with the phononic variables in an unconventional way. We show here that, by approximating this complex and still unraveled electron-phonon interaction by a dynamic Jahn-Teller coupling, one can account rather well for the diffuse neutron scattering and the low-temperature isothermal magnetization.

We present a model which accounts for the high-field magnetization at very low temperature in the frustrated pyrochlore compound Er2Ti2O7. In Er2Ti2O7, the Er(3+) ion has a planar crystal field anisotropy and the material undergoes a transition to antiferromagnetism at TN = 1.2 K. Our model is a mean-field self-consistent calculation involving the four rare earth sites of a tetrahedron, the building unit of the pyrochlore lattice. It includes the full crystal field Hamiltonian, the infinite range dipolar interaction and anisotropic nearest neighbour exchange described by a 4-component tensor. We discuss the equivalence of our treatment of the exchange tensor, taken to be diagonal in a frame linked to a rare earth-rare earth bond, with the pseudo-spin Hamiltonian recently developed for Kramers doublets in a pyrochlore lattice.

Inexpensive simple medical devices allowing fast and reliable counting of whole cells are of interest for diagnosis and treatment monitoring. Magnetic-based labs on a chip are one of the possibilities currently studied to address this issue. Giant magnetoresistance (GMR) sensors offer both great sensitivity and device integrability with microfluidics and electronics. When used on a dynamic system, GMR-based biochips are able to detect magnetically labeled individual cells. In this article, a rigorous evaluation of the main characteristics of this magnetic medical device (specificity, sensitivity, time of use and variability) are presented and compared to those of both an ELISA test and a conventional flow cytometer, using an eukaryotic malignant cell line model in physiological conditions (NS1 murine cells in phosphate buffer saline). We describe a proof of specificity of a GMR sensor detection of magnetically labeled cells. The limit of detection of the actual system was shown to be similar to the ELISA one and 10 times higher than the cytometer one.

We report ⁵⁷Fe Mössbauer spectral results in pure and doped Ba(Fe_1−xNi_x)₂As₂ with x = 0.01 and 0.03. We show that all these materials present a first-order magnetic transition towards a magnetically ordered state. In the doped compounds, a broad distribution of Fe hyperfine fields is present in the magnetic phase. We successfully fit the Mössbauer data in Ba(Fe_1−xNi_x)₂As₂ in the framework of two different models: (i) an incommensurate spin density wave (IC-SDW); (ii) a dopant-induced perturbation of the Fe polarization, recently proposed to interpret ⁷⁵As NMR data in Ba(Fe_1−xNi_x)₂As₂, which is valid only in the very dilute limit x = 0.01. Moreover, we show here that these NMR data can also be successfully analysed in terms of the 'incommensurate model' for all doping contents by using the parameters obtained from the Mössbauer spectral analysis. Therefore it is not possible to rule out the presence of an IC-SDW on the basis of the ⁷⁵As NMR data.