Observation of dipole-dipole interaction in a degenerate quantum gas

We have investigated the expansion of a Bose-Einstein condensate (BEC) of strongly magnetic chromium atoms. The long-range and anisotropic magnetic dipole-dipole interaction leads to an anisotropic deformation of the expanding Cr-BEC which depends on the orientation of the atomic dipole moments. Our measurements are consistent with the theory of dipolar quantum gases and show that a Cr-BEC is an excellent model system to study dipolar interactions in such gases.Comment: 4 pages, 2 figure

Exchange bias properties and surface spin freezing in magnetic nanoparticles

AbstractZFC and FC magnetization measurements versus field are carried out on manganese ferrite based nanoparticles with a mean diameter of 3.3 nm. The exchange bias field determined from the field shift of hysteresis loops, decreases as the cooling field increases. Magnetization measurements performed at constant applied field H as a function of temperature allows us to separate two H-dependent contributions. One is associated to the well ordered core which inflates as the field increases and the other is related to surface spins frozen in a disordered structure. The thermal dependence of this disordered surface contribution decreases exponentially with a freezing temperature Tf, which decreases as the applied field increases

Rare earth doped maghemite EDL-MF: a perspective for nanoradiotherapy?

Abstract We report on electric double layered magnetic fluids based on samarium-doped maghemite nanoparticles. The nanostructures chemical composition is carefully checked and X-ray diffraction patterns provide both their mean size and a structural characterization. Magnetization results are presented. Since these particles can become radioactive after neutron activation, they could therefore represent a new perspective for biomedical applications.

Exchange-bias and magnetic anisotropy fields in core–shell ferrite nanoparticles

Exchange bias properties of MnFe2O4@γ–Fe2O3 core–shell nanoparticles are investigated. The measured field and temperature dependencies of the magnetization point out a well-ordered ferrimagnetic core surrounded by a layer with spin glass-like arrangement. Quasi-static SQUID magnetization measurements are presented along with high-amplitude pulse ones and are cross-analyzed by comparison against ferromagnetic resonance experiments at 9 GHz. These measurements allow one to discern three types of magnetic anisotropies affecting the dynamics of the magnetic moment of the well-ordered ferrimagnetic NP’s core viz. the easy-axis (uniaxial) anisotropy, the unidirectional exchange-bias anisotropy and the rotatable anisotropy. The uniaxial anisotropy originates from the structural core–shell interface. The unidirectional exchange-bias anisotropy is associated with the spin-coupling at the ferrimagnetic/spin glass-like interface; it is observable only at low temperatures after a field-cooling process. The rotatable anisotropy is caused by partially-pinned spins at the core/shell interface; it manifests itself as an intrinsic field always parallel to the external applied magnetic field. The whole set of experimental results is interpreted in the framework of superparamagnetic theory, i.e., essentially taking into account the effect of thermal fluctuations on the magnetic moment of the particle core. In particular, it is found that the rotatable anisotropy of our system is of a uniaxial type. © 2021, The Author(s)

Nanoparticle chain-like formation in electrical double-layered magnetic fluids evidenced by small-angle X-ray scattering

Small-angle X-ray scattering (SAXS) was performed on a series of Electric Double-Layered Magnetic Fluids (EDL-MF) composed of ferrite type-\ensuremath${CoFe_2O_4}$, \ensuremath${MnFe_2O_4}$, \ensuremath${ZnFe_2O_4}$, \ensuremath${NiFe_2O_4}$ and \ensuremath${CuFe_2O_4}$-nanoparticles of different crystalline sizes (D_\ab{XR} ranging from 40 to 139Å, as determined by X-ray diffraction). The information concerning the scattering objects was obtained through the analysis of the distance distribution function $p(r)$ and of the size distribution function $D(R)$, both retrieved from SAXS data. The results show that EDL-MF, in the absence of an applied magnetic field, are composed of small magnetic particle aggregates in solution. These agglomerates are elongated in one direction (chain-like) with the longest dimension varying from 240 to 330Å. The cross-section size is of the order of D_\ab{XR}. The data also demonstrate that the maximum dimension of these aggregates is independent of the ferrite type. On the other hand, the number of aggregated magnetic particles is nanoparticle-size-dependent. Accordingly, larger ferrite-type nanoparticles as those with D_\ab{XR}=139Å form aggregates composed of 2-3 magnetic particles, whereas smaller ones with D_\ab{XR} \cong40Å form agglomerates of about 6 magnetic particles in solution. As the nanoparticle size is reduced, it might increase the particle surface defects. Such occurrence would affect the particle surface charge density, which could reduce the electrostatic screening, favoring the agglomeration phenomenon