Abstract Exchange bias properties of MnFe $$_2$$ 2 O $$_4$$ 4 @ $$\gamma$$ γ –Fe $$_2$$ 2 O $$_3$$ 3 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.

The gamma-ray background problem is known to be acute in any low-background underground experiment. The variations of this background depend on many parameters and should be taken into account when interpreting the results of experiments combined under the term "underground physics". This paper is devoted to studying the long-term variations of the gamma-ray background in an underground laboratory with a scintillation detector based on a CsI crystal. Our studies have revealed a new effect in underground physics-a delayed nonlinear pumping effect for the gamma-ray background in an underground room that can lead to a significant rise of this background at an anomalously low atmospheric pressure.

The paper is devoted to the emergence of digital diplomacy in the informational space and its public side. Talking into account the development of information technology and the accelerated growth of social networks, a new reality is being considered in the development of the informational space not only for organizations and citizens, but also for states. The idea is substantiated that along with changes in the sphere of public life, the transformation of diplomacy can also take place in the digital age. It is shown how foreign affairs agencies use social networks and other tools to communicate with the public. The author comes to the conclusion that with the objective development of means of communication in the informational space, the evolution of diplomacy takes place, while maintaining its significance in interstate interaction.Problem statement in general and its relation to important scientific and practical tasks. With the development of new technologies in diplomatic practice and with the advent of the concept of “digital diplomacy”, works began to appear in many information sources that raise questions of the broad understanding of the potential of the digital network for the needs of foreign policy. The diplomats had the opportunity to directly, live, monitor various international events, in contrast to the traditional models of diplomatic communication between countries and heads of state. There is an opportunity to analyze foreign audiences, handle and exchange data, develop new formats for multilateral cooperation to provide international assistance to developing countries. One of the main tasks for digital diplomacy is the task of adapting the ministries of foreign affairs to new achievements in the informational and technological field. This problem has been little studied and requires modern research.Formation of research objectives (goals). To trace how the development of digital communication in the information space affects the evolution of diplomacy in the interstate interaction.Statement of the main results and their justification. To indicate an innovative way of influencing foreign society using the Internet, the following is used: Internet diplomacy, social networks diplomacy (Twitter diplomacy), public diplomacy Web 2.0 Public diplomacy, using new technologies, is constantly improving foreign policy mechanisms both to meet the country's foreign affairs needs and to strengthen its position in the information space. It is about improving the interaction of diplomats with foreign Internet users using digital diplomacy, the main tool of which are social networks.Initially, the term “digital diplomacy” was used in relation to US public policy, relying on digital technologies to promote foreign policy interests, the impact of information propaganda via the Internet, social networks and mobile phones on the mass consciousness and the political elite . At the same time, social media make it possible to carry out this task in real time and cover a wide radius of action.For the future of digital diplomacy, the issue of adapting the state information policy to the new tasks and challenges of the information technology sphere is important. It is important to understand that modern diplomacy has evolved and will change along with the objective development of means of communication in the informational space, while retaining its significance in the interstate interaction.Conclusions and prospects for further research. Information technologies can transform and change, but at the same time they represent one of the key technological products of current society, introducing into the global world order the increasingly pronounced element of the concentration of the Network. For effective and innovative development of Ukraine, new foreign policy projects in the digital sphere of the informational space are needed. When implementing modern projects, the state should take into account the opportunities offered by the informational environment. These opportunities are provided by the promising toolkit of digital diplomacy.It is necessary to consider the potential of digital diplomacy for Ukraine taking into account the potential for further development of the informational sector for the domestic economy, national security, and public administration. It is important to try to find a balance between informational security and active foreign policy in the informational space aimed at increasing the attractiveness of Ukraine in the world. It is necessary to involve civil society representatives in the implementation of foreign policy goals with the coordinating role of the state, which is a prospect for further research.

The mass composition of primary cosmic rays remains an unresolved problem of physics in the region of energies above the knee. Results from different experiments are contradictory in assessments of the mean mass number and its variation as the primary energy grows. The PRISMA project is designed to study the energy spectrum and mass composition of cosmic rays at energies of 1015–1017 eV. The project is based on a detector capable of simultaneously detecting the electromagnetic and hadron components of a shower. The results are presented from measurements with the PRISMA-YBJ prototype located at 4300 m a.s.l., made over 3.5 years of operation. A new way of estimating the average mass composition from the electron/neutron ratio is tested.

The dynamic susceptibility of concentrated ferrofluids of magnetite-kerosene type is studied experimentally to clarify the effect of interparticle interactions on the magnetization reversal dynamics and the ferrofluid relaxation time spectrum. We synthesize six ferrofluid samples, four of which have the same wide particle size distribution with a high (more than 2kT) average energy of magnetic dipole interactions. These samples differ in particle concentration and dynamic viscosity. The two remaining samples have a lower content of large particles and a moderate energy of magnetic dipole interactions. For all samples, we measure the dynamic susceptibility in the weak probing field at frequencies up to 160 kHz and the field amplitude dependence of the susceptibility at a frequency of 27 kHz. The results show that the susceptibility dispersion at frequencies up to 10 kHz is due to the rotational diffusion of colloidal particles and aggregates. Steric and hydrodynamic interparticle interactions are the main reason for the strong concentration dependence of the viscosity and so they also strongly influence the frequency dependence of the susceptibility. The influence of van der Waals and magnetic dipole interactions on the susceptibility is manifested indirectly, through the formation of multiparticle clusters. The contribution of clusters to the low-frequency susceptibility reaches 80%. Their large sizes (about 100 nm) shift the dispersion region to frequencies of 1-100 Hz, depending on the temperature and particle concentration. Experiments at 27 kHz demonstrate the increase in the dynamic susceptibility with increasing field amplitude. This growth is unexpected since all spectral amplitudes in the Debye function expansion of the dynamic susceptibility decrease monotonically with increasing field. To clarify the situation, the auxiliary problem of the magnetodynamics of a uniaxial particle in the alternating field is solved numerically. The Fokker-Planck-Brown rotational diffusion equation is used. It is shown that an increase in the field amplitude reduces the anisotropy barrier and the Néel relaxation time of particles and increases the dynamic susceptibility by one to two orders of magnitude compared to the weak-field limit. The calculation results are in qualitative agreement with the experimental data and allow us to propose a consistent interpretation of these data. We find that the increase in dynamic susceptibility with increasing amplitude is observed when two necessary conditions are met: (i) The suspension viscosity and the field frequency are high enough to cause the blocking of the rotational degrees of freedom of particles and aggregates and (ii) particles with a large magnetic anisotropy are present in the ferrofluid.

Interesting results are obtained using a setup with electron-neutron detectors (EN-detectors) developed for the PRISMA (PRImary Spectrum Measuring Array) project to study extensive atmospheric showers. A small installation of four EN-detectors (PRISMA-YBJ) has been in operation for three and a half years on the Earth’s surface in Tibet, 4300 m above sea level, constantly measuring the natural flux of thermal neutrons. Neutrons are produced in soil during (α, n )-reactions with naturally radioactive α-particles, mainly from the decay of radon and daughter heavy nuclides. The neutrons are thermalized in the medium and, once in equilibrium with it, become sensitive to its state. They then can indicate many geophysical processes in the crust, including earthquakes. Results illustrating the sensitivity of EN-detectors to earthquakes are considered.

The mass composition of primary cosmic rays remains an unresolved problem of physics in the region of energies above the knee. Results from different experiments are contradictory in assessments of the mean mass number and its variation as the primary energy grows. The PRISMA project is designed to study the energy spectrum and mass composition of cosmic rays at energies of 10 15 –10 17 eV. The project is based on a detector capable of simultaneously detecting the electromagnetic and hadron components of a shower. The results are presented from measurements with the PRISMA-YBJ prototype located at 4300 m a.s.l., made over 3.5 years of operation. A new way of estimating the average mass composition from the electron/neutron ratio is tested.

We demonstrate that a ferronematic (a dilute suspension of ferromagnet nanoparticles in a nematogenic matrix) may display a direct analog of Néel superparamagnetism entailed by the orienting effect of the matrix on the embedded particles. The latter are assumed to be the objects with pronounced anisometricity; in magnetic aspect, they are single-domains with strong magnetic hardness (e.g. imposed by their rod-like shape) so that the magnetic moment is fixed inside the particle body. Above the isotropic–nematic transition point, the considered system is just a ferrofluid with random distribution of the particle axes. Below the transition, the particle–matrix coupling emerges that sets the axis of each particle (and, hence, its magnetic moment) under the same angle to the director. If this alignment is along the director, then each particle falls under the action of orientational potential with two equal wells (0° and 180°) separated by the energy barrier whose height is defined by the intensity of the surface particle–matrix interaction. Provided the thermal energy is of the order of the barrier height (the material estimates readily admit that), the Brownian motion makes the particle to randomly rotate between the orientational minima. This mechanism entails spontaneous inversions of the magnetic moment as well, thus ensuring relaxation of any initially established magnetization of the ferronematic; the reference time of this process depends exponentially on the hight of the energy barrier scaled with thermal energy. Treating the nematogenic matrix with the aid of mean field model and using the linear response theory to describe the magnetodynamics of the particles, we show that the “liquid-crystalline” superparamagnetism produces an easily identifiable signature in the dynamic magnetic susceptibility spectrum of a ferronematic.

A consistent theory of ferromagnetic resonance in a dilute suspension of superparamagnetic particles with uniaxial anisotropy of arbitrary strength is presented. The developed approach is used for studying the high-frequency response of a magnetic fluid at different temperatures. It is shown that in a certain temperature interval the absorption line splits into two components. The width of this interval is essentially dependent on the magnitude of the particle anisotropy.

Some results on the EAS neutron component measurements by means of the PRISMA-32 array are presented. The array consists of 32 electron-neutron detectors (en detectors) capable to detect two main EAS components: electromagnetic one consisting of charged particles, and hadronic one by measuring delayed thermal neutrons accompanying the showers. For thermal neutrons detection, a compound of a well-known inorganic scintillator ZnS(Ag) and LiF, enriched to 90 % with Li-6 isotope is used. The setup allows us to record neutron component over the whole array area.

EAS (extensive air shower) thermal neutron measurement gives advantages to study energy and mass composition of primary cosmic rays especially in the knee region. After the success of the PRISMA-YBJ experiment, we build a new EAS thermal neutron detection array at Tibet University, Lhasa, China (3700m a.s.l.) in March, 2017. This prototype array so called "PRISMA-LHAASO-16" consists of 16 EAS EN-detectors ("EN" is abbreviation for electron and neutron) measuring two main EAS components: hadronic and electromagnetic ones. Different from PRISMA-YBJ, these detectors use a thin layer of a novel type of ZnS(Ag) scintillator alloyed with natural boron compound for thermal neutron capture. PRISMA-LHAASO-16 will be moved to the LHAASO site in the near future. In this paper, we introduce principle of the detection technique, deployment of the array, and the test results of the array.

Primary cosmic ray mass composition above 1 PeV has been measured with PRISMA-YBJ being a prototype of PRISMA array at altitude of 4300 m a.s.l. It realized a novel type of EAS recording method measuring hadronic EAS component over the total array area through thermal neutron detection with a specially developed so-called en-detectors sensitive to electron and thermal neutron EAS components. Primary c. r. mass composition was recovered through measurements of two EAS components with the en-detectors: electromagnetic (energy deposit) and the number of secondary thermal neutrons produced locally by high energy hadrons. Monte-Carlo simulations of the experiments allowed us to find a parameter highly sensitive to the primary particle mass (atomic number) A above 1 PeV. The preliminary obtained mass composition is consistent with light composition and does not show any significant change at higher energies.

Long-term variations in the natural thermal neutron flux in Tibet at an altitude of 4300 m above sea level are studied using scintillation en-detectors developed at the Institute for Nuclear Research, Russian Academy of Sciences. Substantial growth (on the level of several per cent each year over the last three years) in both the thermal neutron flux and the background recorded by the detectors is observed. This growth is associated with the intensity of cosmic rays. The effect is explained by an increase in the low-energy cosmic ray flux, due to reduced solar activity in the current solar cycle.

The exchange bias effect in nano-objects is modelled by adding to the magnetic energy two terms allowing for two kinds of anisotropy, viz. a fixed-axis and a rotatable one. The signature of the second contribution is an isotropic shift H-RA of the ferromagnetic resonance field. The temperature dependence of H-RA that is due to the superparamagnetic theory is obtained in the framework of kinetic theory. Our analysis predicts that the temperature behaviors of H-RA are qualitatively different depending on the possible symmetry type of the rotatable anisotropy energy term: unidirectional or uniaxial.

The paper is devoted to the results of the EAS neutron component investigations by means of the PRISMA-32 array. The array consists of 32 en-detectors and enables to record delayed thermal neutrons accompanying showers. For registration of thermal neutrons, the scintillator based on $^{6}Li$ isotope as a target is used in the detectors. Some results of the processing of data accumulated over a long period of time are presented: the lateral distribution function of neutrons in EAS and preliminary results on EAS neutron multiplicity spectrum and distribution of showers in e/n ratio.

EAS array of novel type have been constructed on the base of ARGO-YBJ experiment (Tibet, China). It consists of the four specially designed scintillator en-detectors capable to measure two main EAS components: hadrons through thermal neutrons (n) and electrons (e). The results of simulation for these arrays using CORSIKA and GEANT4 codes are presented. Simulated thermal neutron and electron lateral distributions are compared with experimental data. Obtained distributions are compared with those obtained by other arrays.