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

Exchange bias properties of MnFe[Formula: see text]O[Formula: see text]@[Formula: see text]-Fe[Formula: see text]O[Formula: see text] 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.

Sporadic increases of radioactive aerosols as a possible reason for heavy nuclides enhancements recorded with the en-detectors.

Some unexpected sporadic increases of an environmental radioactive background have been recorded at mountain level at Baksan Neutrino Observatory (BNO, 1700 m above sea level) using electron-neutron detectors (en-detectors), which could be explained by radioactive aerosol enhancements. The large area inorganic scintillator en-detectors developed for cosmic ray study are continuously monitoring environmental thermal neutron fluxes at various geophysical conditions. Application of the pulse shape discrimination method allows us to select and separately measure both thermal neutrons and radioactive beta-decay nuclides being products of radon decays in air (mostly Rn-222 and Rn-220). There are two en-detector setups running now at BNO, one deep underground while another one at surface. Both installations had recorded some strange sporadic increases of radioactive nuclides in air. In this paper, we present results and the most probable explanation of the significant increases joint by radioactive aerosols production but caused by different reasons: Baksan river floods or nearby underground experiment with powerful Cr-51 radioactive source.

Power losses in a suspension of magnetic dipoles under a rotating field.

Energy absorption due to viscous friction in a dilute suspension of single-domain ferromagnetic particles subjected to a rotating field is considered. The problem is treated in the framework of kinetic approach. The behavior of specific loss power (SLP) as a function of the field amplitude and frequency is studied. It is shown that for either of these parameters (while the other is kept constant) SLP first grows quadratically and then saturates. The cases of a rotating field and oscillating fields are compared, and the essential differences are revealed. The results obtained enable one to assess the allowable or optimal field parameters for a given magnetic suspension intended for rotational magneto-inductive heating.

Magneto-orientational behavior of a suspension of antiferromagnetic particles.

A theory describing magneto-orientational properties of suspensions containing antiferromagnetic nanoparticles is developed. Due to their small size, these particles possess, apart from an anisotropic magnetic susceptibility pertinent to antiferromagnets, a spontaneous magnetic moment caused by sublattice decompensation. In a colloid subjected to a DC field of increasing strength an orientational crossover takes place: the particle magnetic moments, initially aligned along the field, turn to the transverse orientation. This behavior considerably changes the observable characteristics of the system: the spectrum of linear dynamic susceptibility and the integral time of magnetic relaxation under a pulse field.

Orientational dynamics of ferrofluids with finite magnetic anisotropy of the particles: relaxation of magneto-birefringence in crossed fields.

Dynamic birefringence in a ferrofluid subjected to crossed bias (constant) and probing (pulse or ac) fields is considered, assuming that the nanoparticles have finite magnetic anisotropy. This is done on the basis of the general Fokker-Planck equation that takes into account both internal magnetic and external mechanical degrees of freedom of the particle. We describe the orientation dynamics in terms of the integral relaxation time of the macroscopic orientation order parameter. To account for an arbitrary relation between the bias (external) and anisotropy (internal) fields, an interpolation expression for the integral relaxation time is proposed and justified. A developed description is used to interpret the measurements of birefringence relaxation in magnetic fluids with nanoparticles of high (cobalt ferrite) and low (maghemite) anisotropy. The proposed theory appears to be in full qualitative agreement with all the experimental data available.

Noise- and force-induced resonances in noisy rotary oscillations of classical spins.

As an important example of noisy rotary oscillations, the dynamic magnetization of an assembly of superparamagnetic particles is considered. In the presence of a bias field, there exists a mechanism that causes selective suppression of higher harmonics in the response spectrum of the system. Manifestation of this effect at temperature variation is known as the noise-induced resonance. Its manifestation at the change of excitation intensity, formerly unknown, we term the force-induced resonance.