2D crystal structure and anisotropic magnetism of GdAu6.75-xAl0.5+x (x ≈ 0.54)

Exploration of the gold-rich part of the ternary Gd–Au–Al system afforded the intermetallic compound GdAu_6.75−xAl_0.5+x (x ≈ 0.54) which was structurally characterized by single crystal X-ray diffraction (Pnma, a = 18.7847(4) Å, b = 23.8208(5) Å, c = 5.3010(1) Å). GdAu_6.75−xAl_0.5+x crystallizes in a previously unknown structure type featuring layers of Gd₂(Au, Al)₂₉ and Gd₂(Au, Al)₂₈ clusters which are arranged as in a close-packing parallel to the ac plane. The Gd substructure corresponds to slightly corrugated 3⁶ nets (d_Gd–Gd = 5.30–5.41 Å) which are stacked on top of each other along the b direction with alternating short (5.4, 5.6 Å, within layers) and long distances (6.4 Å, between layers). The title compound has been discussed with respect to a quasicrystal approximant (1/1 AC) GdAu_5.3Al in the same system. The magnetic properties of GdAu_6.75−xAl_0.5+x were found to be reminiscent to those of some ternary ACs, with sharp peaks in the temperature dependent magnetization, and metamagnetic-like transitions. The material becomes antiferromagnetic below 25 K; magnetometry results suggest that the antiferromagnetic state is composed of ferromagnetic ac planes, coupled antiferromagnetically along the b direction.

Memory and superposition in a superspin glass

The non-equilibrium dynamics of the superspin glass state of a dense assembly of ~ 2 nm MnFe₂O₄ nanoparticles was investigated by means of magnetization, ac susceptibility and Mössbauer spectroscopy measurements and compared to the results of Monte Carlo simulations for a mesoscopic model that includes particles morphology and interparticle interactions. The zero-field cooled (ZFC), thermoremanent (TRM), and isothermal remanent magnetization (IRM) were recorded after specific cooling protocols and compared to those of archetypal spin glasses and their dimensionality. The system is found to display glassy magnetic features. We illustrate in detail, by a number of experiments, the dynamical properties of the low-temperature superspin glass phase. We observe that these glassy features are quite similar to those of atomic spin glasses. Some differences are observed, and interestingly, the non-atomic nature of the superspin glass is also reflected by an observed superspin dimensionality crossover. Monte Carlo simulations—that explicitly take into account core and surface contributions to the magnetic properties of these ultrasmall nanoparticles in direct contact, as well as interparticle interactions—evidence effects of the interplay between (intraparticle) core/surface exchange coupling and (interparticle) dipolar and exchange interactions.

Random fields and apparent exchange bias in the dilute Ising antiferromagnet Fe0.6Zn0.4F2

Random field induced spontaneous excess moments appear in field cooled single crystals of diluted Ising antiferromagnets. Here we report results from low temperature measurements of field cooled (including zero field) magnetic hysteresis loops parallel and perpendicular to the c-axis of a single crystal of composition Fe0.6Zn0.4F2. We find that weak static ferromagnetic excess moments attained on field cooling give rise to an apparent exchange bias of the magnetic hysteresis loops, whose magnitude is controlled by temperature and the strength and direction of the cooling field. Random field induced temporal excess moments only become observable in cooling fields larger than 1 T applied along the c-axis direction of the Fe0.6Zn0.4F2 single crystal.

Ferromagnetic excess moments and apparent exchange bias in FeF2 single crystals

The anisotropic antiferromagnet FeF2 has been extensively used as an antiferromagnetic layer to induce exchange bias effects in ferromagnetic/antiferromagnetic bilayers and heterostructures. In this work, an apparent exchange bias occurring in the low temperature hysteresis loops of FeF2 single crystals is investigated. A detailed investigation of the hysteresis and remnant magnetization indicates that the observation of an apparent exchange bias in FeF2 stems from an intrinsic excess moment associated with a distortion of the antiferromagnetic structure of piezomagnetic origin.

Near-room-temperature colossal magnetodielectricity and multiglass properties in partially disordered La2NiMnO6

We report magnetic, dielectric, and magnetodielectric responses of the pure monoclinic bulk phase of partially disordered La2NiMnO6, exhibiting a spectrum of unusual properties and establish that this compound is an intrinsically multiglass system with a large magnetodielectric coupling (8%-20%) over a wide range of temperatures (150-300 K). Specifically, our results establish a unique way to obtain colossal magnetodielectricity, independent of any striction effects, by engineering the asymmetric hopping contribution to the dielectric constant via the tuning of the relative-spin orientations between neighboring magnetic ions in a transition-metal oxide system. We discuss the role of antisite (Ni-Mn) disorder in emergence of these unusual properties.

Competing exchange interactions in magnetic multilayers

We have studied alloying of the nonmagnetic spacer layer with a magnetic material as a method of tuning the interlayer coupling in magnetic multilayers. We have specifically studied the Fe/V(100) system by alloying the spacer V with various amounts of Fe. For some Fe concentrations in the spacer, it is possible to create a competition between antiferromagnetic Ruderman-Kittel-Kasuya-Yoshida exchange and direct ferromagnetic exchange coupling. The exchange coupling and transport properties for a large span of systems with different spacer concentrations and thicknesses were calculated and measured experimentally and good agreement between observations and theory was observed. A reduction in magnetoresistance of about 50% was observed close to the switchover from antiferromagnetic to ferromagnetic coupling.

Symmetrical temperature-chaos effect with positive and negative temperature shifts in a spin glass

Aging in a Heisenberg-like spin glass Ag(11 at. % Mn) is investigated by measurements of the zero-field-cooled magnetic relaxation at a constant temperature after small temperature shifts absolute value of Delta T/T(g) < 0.012. A crossover from fully accumulative to nonaccumulative aging is observed, and by converting time scales to length scales using the logarithmic growth law of the droplet model, we find quantitative evidence that positive and negative temperature shifts cause an equivalent restart of aging (rejuvenation) in terms of dynamical length scales. This result supports the existence of a unique overlap length between a pair of equilibrium states in the spin-glass system.

Domain growth by isothermal aging in 3D Ising and Heisenberg spin glasses

Nonequilibrium dynamics of three-dimensional model spin glasses, the Ising system Fe0.50Mn0.50TiO3 and the Heisenberg-like system Ag(11 at % Mn), has been investigated by measurements of the isothermal time decay of the low frequency ac susceptibility after a quench from the paramagnetic to the spin-glass phase. It is found that the relaxation data measured at different temperatures can be scaled according to predictions from the droplet scaling model, provided that critical fluctuations are accounted for in the analyses.

Aging effect in ceramic superconductors

A three-dimensional lattice of the Josephson junctions with a finite self-conductance is employed to model ceramic superconductors. By using Monte Carlo simulations it is shown that the aging disappears in the strong screening limit. In the weak screening regime, aging is present even at low temperatures. For intermediate values of the self-inductance, aging occurs in an intermediate temperature interval but is suppressed entirely at high and low temperatures. Our results are in good agreement with experiments.

Complementary Mössbauer and EPR studies of iron(III) in diferric human serum transferrin with oxalate or bicarbonate as synergistic anions

The spectral and magnetic properties of iron(III) bound in the metal binding sites of human serum transferrin with oxalate or bicarbonate as synergistic anions has been studied with Mössbauer spectroscopy and electron paramagnetic resonance (EPR). The Mössbauer spectra of the iron(III) in diferric transferrin with oxalate have been described using a spin Hamiltonian with the values of the zero field splitting parameter, D = 0.55 +/- 0.05 cm-1, and the rhombicity of the crystal field, E/D = 0.045 +/- 0.005. The EPR spectrum can be described with D = 0.58 cm-1 and E/D = 0.057, using a g-strain model for the lineshape that is based on a Gaussian distribution of D and E/D with Gaussian widths sigma(D) = 0.35 cm-1 and sigma(E/D) = 0.013, respectively. The rhombicity of the iron surroundings for the transferrin-oxalate complex is almost one order of magnitude smaller than for the bicarbonate complex and the zero field splitting parameter is twice as large in the oxalate as in the bicarbonate complex. We conclude that the crystal field symmetry of the iron site is almost tetragonal in the oxalate complex but rhombohedral in the bicarbonate complex, reflecting the different geometries of the oxalate and bicarbonate coordination. The isomer shift delta = 0.56 +/- 0.01 mm s-1 and the quadrupole splitting delta EQ = 0.2 +/- 0.1 mm s-1, on the other hand, are very close to the values found for the bicarbonate complex. No differences between the Mössbauer spectra of the two iron(III) ions in diferric transferrin with oxalate were found. The homogeneity of the diferric transferrin samples was controlled by capillary zone electrophoresis in the presence of urea.