Papers by Keyword: Spin Glass

Abstract: Disordered and frustrated magnetization of different surface coated (Cr₂O₃, Co₃O₄, ZrO₂, and SiO₂) MnFe₂O₄ nanoparticles have been studied using SQUID-magnetometry. Magnetic measurements, such as ZFC/FC and ac-susceptibility evidence surface spin-glass behavior. ZFC/FC curves were also compared with numerical simulation to get information about effective anisotropy constants. Frequency dependent ac susceptibility results were analyzed by using Arrhenius, Vogel Fulcher and dynamic scaling laws to further confirm the spin-glass behavior. It is observed that the strength of surface spins disorder and frustration strongly depends upon the type of the coating material. All these analyses signify that disordered and frustrated surface magnetization in MnFe₂O₄ nanoparticles greatly depend on the type of the surface coating materials and are useful for controlling the nanoparticle’s magnetism for different practical applications.

Abstract: There has been carried out comparative analysis of energy minimizing methods in systems of magnetic Ising dipoles, randomly distributed on the nonmagnetic plane. The considered methods of ground state search are based on alignment of a magnetic moment of dipole in direction of an internal magnetic field. It is found out that the rate of convergence is low in the used methods, based on collective flipping of group of particles

Abstract: β-Cu₃Fe₄(VO₄)₆ single crystals were synthesized. The X-ray, magnetic, Mössbauer, and EPR investigations of the crystals were fulfilled. It is established that single-crystal β-Cu₃Fe₄(VO₄)₆ is a spin glass at low temperatures. Freezing temperature varies considerably depending on the orientation of the magnetic field relative to the crystal axes. Spin-glass state is confirmed by the relaxation of the magnetization.

Abstract: Conditions of phase transitions in systems of identical ferromagnetic spherical nanoparticles randomly distributed in metal nonmagnetic matrix and superlattices of small number of nanoparticles with the Ruderman-Kittel-Kasuya-Yosida interaction are researched. In the framework of the Ising model the behavior of superspins is well described with the random interaction field method. The alteration of the effective magnetic moment due to the change in volume affects the choice of the magnetic state of the system: ferromagnetic spin glass or antiferromagnetic spin glass. The ground state of superlattice depends on the distance between particles.

Abstract: The magnetization, ac susceptibility and magnetic relaxation of Nd_0.90Sr_0.10CoO₃ polycrystalline sample were systematically investigated in this paper. The experimental studies of susceptibility and magnetic relaxation evidence the existence of a low-temperature spin-glass. A dynamic analysis of ac susceptibility implies a spin-glass transition temperature TSG =12.17 K and the dynamical exponent zv=8. Moreover, low-temperature zero-field cooling and field cooling magnetic relaxation show perfectly mirror symmetry, and field cooling processes relaxation obeys a stretched exponential form. Therefore, our study confirms that the phase separation in Nd_0.90Sr_0.10CoO₃ originates from both the ferromagnetic clusters interaction and the spin glasslike phase at low temperature.

Abstract: Disordered multiferroic materials (type-III multiferroics) escape the conventional schematics of type-I and type-II multiferroics, where two types of ferroic long-range order are expected to coexist under different interdependences and promise to attain a maximized bilinear (α or EH) magnetoelectric effect under special symmetry conditions. Nevertheless sizable higher order ME response occurs also in disordered systems such as in the simultaneous dipolar and spin glasses (multiglass) Sr_0.98Mn_0.02TiO₃ and K_0.94Mn_0.03TaO₃, the quantum paraelectric antiferromagnet EuTiO₃, the spin glass and relaxor ferroelectric PbFe_0.5Nb_0.5O₃, and the antiferroelectric antiferromagnetic dipole glass CuCr_1-xIn_xP₂S₆. They have in common to show large quadratic magneto-capacitance effects, Δε H², which are related to dominating third-order E²H² terms in their free energies and do not require special symmetry conditions. The polarization controlled exchange coupling can achieve giant fluctuation-enhanced values in the vicinity of critical magnetic fields as observed, e.g., in EuTiO₃. Exceptionally, even the first-order EH-type magnetoelectric effect is observed whenever metastable homogeneous order parameters are induced by field cooling as in EuTiO₃, or in the spin glass phase of the relaxor multiferroic Pb (Fe_0.5Nb_0.5)O₃ at T < T_g = 10.6 K.

Abstract: Magnetic and 57Fe Mössbauer measurements are reported on Taylor-phase compounds T-Al3Mn(Pd,Fe), which are complex metallic alloys containing 156 atoms in the unit cell. These investigations are extended to Al71Mn19Fe10 which crystallize as decagonal quasicrystal. Common for all samples spin glass behavior is present at low temperatures with freezing temperatures Tf increasing (decreasing) with Fe (Pd) content. Below Tf rejuvenation and memory effects are observed in aging investigations. Above Tf the 57Fe Mössbauer spectra were analysed by a superposition of two subspectra with intensity ratio around 75:25, which might be allocated to Fe substituted on Mn sites surrounded either by Al and Mn or completely by Al in the nearest neighborhood. Below Tf broad hyperfine field distributions are present.

Abstract: We have discovered a novel compound Na0.12CrO2Ge0.18Ox•yH2O using a hydrothermal method. Its powder X-ray diffraction reveals a monoclinic unit cell with lattice parameters: a = 7.260 Å, b = 2.950 Å, c = 5.117 Å and β = 103.7°. A Rietveld analysis shows that this compound has a layered structure based on CrO2 sheets consisting of edge-shared CrO6 octahedra. Such a layered structure is commonly seen in many hexagonal Cr oxides such as NaCrO2. A heat treatment at 100 °C modifies the crystal structure without destruction of a layered structure based on Cr triangular-lattice, because H2O molecules among the CrO2 sheets are removed at high temperature. Magnetization measurements reveal a spin-glass behavior below Tg = 3 K.

Abstract: Amorphous oxide thin films of Fe2O3-R2O3 (R = La, Gd and Tb) systems have been deposited on silica glass substrates by using a radio frequency sputtering method, and magnetic properties of the thin films have been examined. The Fe2O3-La2O3 thin films exhibit cusp-like maxima of dc magnetic susceptibility in their temperature dependences as well as magnetic aging and memory effects characteristic of typical spin glasses. For Fe2O3-Gd2O3 and Fe2O3-Tb2O3 systems, magnetic moments of iron ions take part in formation of a spin glass state, as indicated by the magnetic aging effects, while those of rare-earth ions remain to be in a paramagnetic state even at very low temperatures.

Abstract: MZFC, MFC magnetizations, χac susceptibility and resistance measurements have been carried out for various polycrystalline compounds of (AA’)(BB’)O3 perovskites with A = La, (Nd,Y); B = Mn,Co; A’ = Sr,Ca and B’= Fe, Ti. Analyses have been done to elucidate the metallic ferromagnetism (FM) established by Double Exchange interaction between B+3 ion and the B+4 ion that is induced by the substitution of A’ for A. It is shown that there is no ‘pure’ FM ordering existing in the compounds, in other words the materials consist of a FM and a non-FM phase. In case when the latter is an AFM phase, the coexistence and competition of the two phases result in spin-glass like appearances. Such phenomena have been observed in several compounds of both single- and/or mixed-element at B site. The studies on substitution of Mn for Co in La0.7Sr0.3Co1- yMnyO3 show no DE interaction between the two magnetic ions, which results in not only SG behavior at y = 0.1 concentration but a clear evidence on a phase separation in the high y region. An analysis of M(H,T) curves for this region suggests a structural scenario that the FM phase forms as islets (of volume fraction as small as 2.7 – 8.6%) embedded in isolating AFM matrix. For the case while B’= Ti the inhomogeneity evidence appears as an enhancement of the Low Field MagnetoResistance, which is commonly understood as an intergranular effect.