Yu. G. Chukalkin

Magnetic state of a Zn1 −xCrxSe bulk crystal

The spin system of a Zn1 −xCrxSe bulk crystal (x = 0.045) was studied using thermal-neutron diffraction and magnetic measurements. Previously, it was reported in the literature that thin films (∼200 nm thick) of this type of semiconductors exhibit a ferromagnetic order. In this study, the ferromagnetic order is found to be absent in the bulk crystal.

Peculiairities of magnetic properties of the garnet Y2.25Gd0.75Fe5O12 irradiated by fast neutrons

Polycrystalline yttrium-gadolinium iron garnet samples irradiated by different fluences of fast neutrons ((0 to 2.0)×10 24 m -2 ) were investigated by magnetic and X-ray methods.. Irradiation was found to increase the temperature of sublattice magnetization compensation from 5 to 47 K. The increase was attributed to the different behaviour of gadolinium and iron sublattices under irradiation. In the irradiated samples negative magnetization states were found to arise in the vicinity of the compensation point. The phase H-T diagram of such states was plotted based on the measurements performed. The cause of a negative magnetization occurrence was shown to be the intersection of a spin reorientation with respect to temperature in the compensation point and a freezing of magnetic moments of ferrimagnetic clusters in a spin-glass matrix.

Structure and Magnetic Properties of ZnxNi1—xFe2O4 Ferrites in Structurally Ordered and Disordered States

Polycrystal nickel-zinc ferrites in the structurally ordered and disordered states were investigated by neutron diffraction, X-ray and magnetic methods. In the ordered ferrites at zinc concentrations above 0.45 substantial high-field magnetic susceptibility, abnormal temperature dependencies of the dynamic susceptibility and intensive small-angle neutron scattering were observed. The set of experimental data available indicates the occurrence of local canted spin structures at high zinc concentration. The disordered state was attained after fast neutron irradiation of ferrites. The disordered samples of all compositions were found to be ferrimagnetic. Sublattice magnetic moments, the magnetic moment of the tetrahedrally coordinated Ni 2+ ion and its contribution to the magnetic anisotropy were determined.

Structure and magnetism of the manganite LaMnO3 with defects

The structural and magnetic states of LaMnO3 samples containing various concentrations of antisite defects created by fast-neutron irradiation were studied by neutron diffraction, x-ray diffraction, and magnetic measurements. It is found that the static uncorrelated displacements of oxygen ions induced by the formation of antisite defects break the Jahn-Teller Q2 mode and the initial orthorhombic O′-type structure transforms into the pseudocubic O* modification. As a result, the indirect ferromagnetic exchange caused by the overlap of the eg orbitals of manganese ions and the pσ orbitals of oxygen ions becomes three-dimensional and the initial A-type antiferromagnetic phase transforms into a canted ferromagnetic phase. The magnetic phase diagram of LaMnO3 is plotted for various defect concentrations.

Magnetic state of nickel-zinc ferrites at high zinc concentrations

Neutron diffraction and magnetic methods are used to investigate ferrites from the system ZnxFe1−x [Ni1−xFe1+x]O4. In these investigations, no diffraction effects were observed that would indicate ordered positions for the perpendicular projections of spins at 4.2 K over the entire ferrimagnetic range of concentrations x. However, the high-field magnetic susceptibility and intense small-angle scattering of neutrons observed at helium temperatures in samples with x>0.45 are evidence of local angular structures with effective sizes of 1–10 nm. The temperatures at which these local angular structures are disrupted are determined.

Magnetism of LiMn2O4 manganite in structurally ordered and disordered states

The specific features of the crystal structure and the magnetic state of stoichiometric lithium manganite in the structurally ordered Li[Mn2]O4 and disordered Li1 − δMnδ[Mn2 − δLiδ]O4 (δ = 1/6) states have been investigated using neutron diffraction, X-ray diffraction, and magnetic methods. The structurally disordered state of the manganite was achieved under irradiation by fast neutrons (Eeff ≥ 1 MeV) with a fluence of 2 × 1020 cm−2 at a temperature of 340 K. It has been demonstrated that, in the initial sample, the charge ordering of manganese ions of different valences arises at room temperature, which is accompanied by orthorhombic distortions of the cubic spinel structure, and the long-range antiferromagnetic order with the wave vector k = 2π/c(0, 0, 0.44) is observed at low temperatures. It has been established that the structural disordering leads to radical changes in the structural and magnetic states of the LiMn2O4 manganite. The charge ordering is destroyed, and the structure retains the cubic symmetry even at a temperature of 5 K. The antiferromagnetic type of ordering transforms into ferrimagnetic ordering with local spin deviations in the octahedral sublattice due to the appearance of intersublattice exchange interactions.

Effects of structural disordering in the manganite La0.825Ba0.175MnO3

Neutron and X-ray diffraction have been used to study the structural state of polycrystalline samples of the perovskite-like manganite La0.825Ba0.175MnO3 with different concentrations of antisite defects produced by fast-neutron irradiation. It has been established that the concentration of antisite defects depends linearly on the fraction of the sample volume affected by cascades of atom-atom displacements, reaching at high fluences a value equal to 0.0875, which is “limiting” for a given composition. The formation of antisite defects is accompanied by the appearance of significant static uncorrelated ion displacements, which for oxygen ions become greater than their tilting displacements. It has been shown that in this case, the tilting mode ϕ, which forms the rhombohedral structure of the manganite, is not destructed completely, although the magnitude of correlated tilting displacements decreases noticeably.

Amorphization of oxides under irradiation with fast neutrons

A variant of the solid-state radiation amorphization as a result of accumulation of the critical concentration of defects in the crystal has been considered using the example of oxides with the garnet and perovskite structures irradiated by fast neutrons. It has been shown that such defects can be antisite defects, the formation of which leads to considerable static displacements from the equilibrium sites of nearest ions and, consequently, to the loss of stability of the crystalline structure. The dependences of the root-mean-square displacements of oxygen ions on the concentration of the antisite defects are constructed based on the analysis of the experimental data. It has been established that the so-called critical concentrations of antisite defects, at which the spontaneous amorphization occurs, differ for oxides with the garnet and perovskite structures. As the criterion of the spontaneous radiation amorphization, it is proposed to consider the critical static displacement of the ions, which is identical for studied oxides and equal to ∼0.28 Å, or ∼0.14 in fractions of interatomic distances, which is close to the well-known Lindemann melting criterion.

Defects and the structure of a manganite La0.85Sr0.15MnO3 crystal

Defects in a ferromagnetic crystal of manganite La0.85Sr0.15MnO3 were created by irradiation with fast neutrons (E > 0.1 MeV). Fast neutrons produce defect clusters in a crystal lattice. The volume fraction of the clusters in the crystal after irradiation to a dose F = 2 × 1019 cm−2 (Tirr = 340 K) was ≅40%. The structural and magnetic states of the modified manganite were studied using thermal-neutron diffraction and magnetic measurements. It was revealed that neutron irradiation of a crystal suppresses the cooperative Jahn-Teller effect and the initial charge modes and decreases the temperature of ferromagnetic ordering. Under irradiation with fast neutrons, the crystalline structure of the manganite changes from the orthorhombic O′ to the pseudocubic O* phase. Arguments are advanced in favor of the specific features of the irradiated-manganite structural state being determined by long-wavelength strains induced in the crystal by antisite defects.

Evolution of Magnetic Properties of Cladding Austenitic Steel under Irradiation in a Reactor

Magnetic properties of samples of austenitic steel ChS-68 cut from the cladding of a fuel element, which was irradiated in a BN-600 fast-neutron reactor to a maximal damage dose of ∼80 displacements per atom (dpa) at temperatures of 370–587°C, have been investigated. It has been established that irradiation with fast neutrons leads to the formation of ferromagnetic microregions, the effective sizes and concentration of which depend on the damage dose. It has been shown that, at damage doses higher than ∼55 dpa, small spontaneous magnetization and magnetization hysteresis, which are characteristic of the ferromagnetic state, appear in the samples. It is assumed that the ferromagnetic microregions are the nuclei of the α′ phase and the radiation-induced segregation microregions, in which the spacing between the nearest iron atoms exceeds the critical distance that determines the change in the sign of exchange interaction. Arguments in favor of this assumption are presented.