Krešo Zadro

Synthesis and magnetic properties of NiFe2−xAlxO4 nanoparticles

Nanocrystalline Al-doped nickel ferrite powders have been synthesized by sol-gel auto-ignition method and the effect of non-magnetic aluminum content on the structural and magnetic properties has been studied. The X-ray diffraction (XRD) revealed that the powders obtained are single phase with inverse spinel structure. The calculated grain sizes from XRD data have been verified using transmission electron microscopy (TEM). TEM photographs show that the powders consist of nanometer-sized grains. It was observed that the characteristic grain size decreases from 29 to 6 nm as the non-magnetic Al content increases, which was attributed to the influence of non-magnetic Al concentration on the grain size. Magnetic hysteresis loops were measured at room temperature with a maximum applied magnetic field of 1T. As aluminum content increases, the measured magnetic hysteresis curves become more and more narrow and the saturation magnetization and remanent magnetization both decreased. The reduction of agnetization compared to bulk is a consequence of spin non-collinearity. Further reduction of magnetization with increase of aluminum content is caused by non-magnetic Al^{3+} ions and weakened interaction between sublattices. This, as well as the decrease in hysteresis was understood in terms of the decrease in particle size.

Temperature induced magnetic bistability in a crystal of tetrachlorosemiquinone anion radical

The anion radicals with nonaromatic π–systems, such as semiquinones, are interesting in the design of functional materials. Magnetic properties of solvates of alkali salts of tetrachlorosemiquinone (chloranil) radical anion are tuned by crystal engineering using different solvents (2-butanone and acetonitrile) and/or cations (potassium and ammonium) in crystals. The structural and magnetic characteristics of two salts were studied by variable-temperature single crystal X-ray diffraction and magnetic susceptibility measurements. DFT and CAS-MP2 calculations were used to correlate magnetic and structural properties of radical anions. The solid-state structure of potassium tetrachlorosemiquinone 2-butanone solvate, influenced by temperature, switches between a paramagnetic-like monomeric radical form at high temperature (200 K) and diamagnetic dimeric one at low temperature (100 K). The reversible transition is controlled by solvent and cation types which affect intrastack distance and magnetic properties.

Single crystals of DPPH grown from diethyl ether and carbon disulfide solutions - crystal structures, IR, EPR and magnetization studies.

Single crystals of the free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) obtained from diethyl ether (ether) and carbon disulfide (CS₂) were characterized by the X-ray diffraction, IR, EPR and SQUID magnetization techniques. The X-ray structural analysis and IR spectra showed that the DPPH form crystallized from ether (DPPH1) is solvent free, whereas that one obtained from CS₂ (DPPH2) is a solvate of the composition 4DPPH·CS₂. Principal values of the g-tensor were estimated by the X-band EPR spectroscopy at room and low (10 K) temperatures. Magnetization studies revealed the presence of antiferromagnetically coupled dimers in both types of crystals. However, the way of dimerization as well as the strength of exchange couplings are different in the two DPPH samples, which is in accord with their crystal structures. The obtained results improved parameters accuracy and enabled better understanding of properties of DPPH as a standard sample in the EPR spectrometry.

Reduction of loss in composite magnetic material

Abstract Composite material consisting of amorphous Co 70.3 Fe 4.4 Si 15 B 10 ribbon with both surfaces covered with thin layers containing SmCo 5 powder has been prepared. For composites with layers magnetically oriented in a such way that their magnetizations are in the plane of the layer but perpendicular to the length of the ribbon and antiparallel at opposite surfaces of the ribbon, large reductions in coercive field ( H c ) and loss ( E ), in comparison to those of clean ribbon, has been obtained. The experiments with direct current flowing along the ribbon show that the decrease of H c and E is associated with the perpendicular field ( H ′ p ) which is formed at the ribbon surfaces due to SmCo 5 deposits, and can therefore be explained in the same way as the decrease of H c and E due to current flow along the clean ribbon. The results prove that new magnetic materials with improved soft magnetic properties can be made.

Temperature induced reversible structural and magnetic changes in a crystal of tetrachlorosemiquinone anion radical

The anion radicals with nonaromatic π-systems, such as semiquinones, are interesting in the design of functional materials. Magnetic properties of solvates of alkali salts of tetrachlorosemiquinone (chloranil) radical anion are tuned by crystal engineering using different solvents (2-butanone and acetonitrile) and/or cations (potassium and ammonium) in crystals. The structural and magnetic characteristics of two salts were studied by variable-temperature single crystal X-ray diffraction and magnetic susceptibility measurements. DFT and CAS-MP2 calculations were used to correlate magnetic and structural properties of radical anions. The solid-state structure of potassium tetrachlorosemiquinone 2-butanone solvate, influenced by temperature, switches between a paramagnetic-like monomeric radical form at high temperature (200 K) and diamagnetic dimeric one at low temperature (100 K). The reversible transition is controlled by solvent and cation types which affect intrastack distance and magnetic properties.

Correlation between structural, physical and chemical properties of three new tetranuclear NiII clusters

Based on a new family of NiII cubane-like clusters, this work addresses current challenges in synthesis, analysis and dynamics of single-molecule-magnet (SMM) systems. Investigation of various synthetic routes and desorption-sorption processes yielded series of isomorphous compounds: [Ni4L4(CH3OH)4]·xsolv, [Ni4L4(CH3OH)4] and [Ni4L4]. In order to analyze these deceivingly simple materials, several analytical and quantum mechanical methods had to be used. This revealed materials with mixed lattice solvents, statistical disorder of the solvent, and disordered [Ni4L4] cores, what offered an insight into risks of the self-assembly process and interconversion dynamics of the investigated NiII family. These findings also allowed structural-magnetic relationships to be established, and the outcomes were exploited in two turns: in first, effect of the coordinated and lattice solvent on the magnetic properties was examined, and in second, magnetic properties were used to facilitate crystal structure determination.

Effect of magnetic NiCoB nanoparticles on superconductivity in MgB2 wires

A systematic study of the influence of doping MgB2 with single domain magnetic nanoparticles of NiCoB alloy, uncoated and coated with SiO2, has been performed. Electrical resistivity, transport critical current density, Jc(B,T), and magnetization of well characterized undoped and doped with 1.38 and 2.67 wt% of NiCoB particles (both uncoated and coated) MgB2 wires have been investigated in the temperature interval 2–300 K and in magnetic field B ≤ 16T. The superconducting transition temperature, Tc, decreases approximately linearly with the amount of dopand and the intergranular connectivity (the active cross-sectional area fraction, AF) is also reduced upon doping. Reduction of critical fields (irreversibility field, Birr, and upper critical field, Bc2) of doped wires was observed in the whole temperature interval, but an enhancement of Jc of doped wires with respect to the undoped one was observed at low temperature (5 K). Common scaling of Jc(B,T) curves, Birr(T) and volume pinning force, Fp, for doped and undoped wires indicates that the main mechanism of flux pinning is the same in both types of samples.

Critical behavior of Fe15Ni65B18Si2 ferromagnetic glass

AC susceptibility, magnetization and electrical resistivity around the Curie temperature (Tc) were measured for Fe15Ni65B18Si2 glass. The results yield Tc = (307.6±0.1) K and the following critical exponents γ = 1.50±0.03, β = 0.375±0.01, δ = 5.1±0.1 andα = -0.29±0.05. These values were obtained in the reduced temperature interval 1×10-3 ⩽|T−Tc|Tc⪕5 ×10-2. In spite of the fact that these values for the critical exponents were obtained from different measurements they obey the equality relations γ = β(δ−1) and γ+2β+α = 2. Reduced magnetisation and field follow a magnetic equation of state derived for a second-order phase transition over a wide temperature range. This set of critical exponents is compared with those derived from the Heisenberg model as well as with the usual ones for a pure crystalline ferromagnets. The comparison shows that the values of |α| and γ, for our alloy, are considerably larger than those from the model and the usual crystalline ones. A similar difference is also observed in some other amorphous and dilute crystalline ferromagnets and is probably due to magnetic inhomogeneities.

Investigation of the spin glass transition in a low U doped YRu2Si2 sample

We have made extensive studies of a dilute U doped YRu2Si2 alloy using several experimental techniques such as ac and dc magnetic susceptibility, dc resistivity and microstructural investigations. The real part of the ac susceptibility shows a maximum at around 6.75 K for f = 7 Hz. With increasing frequency, the temperature of this maximum increases while, at the same time, the magnitude of the maximum decreases, which is a typical spin glass behaviour. The magnitude of the imaginary part shows frequency-dependent behaviour too. We show that most of the imaginary part of the susceptibility, at least over the range of the frequencies we used, comes from the conduction electrons. Zero-field cooling and field cooling dc magnetization measurements also indicate the presence of a spin glass transition at about the same temperature. We also found that relaxation processes below the freezing temperature show ln(t + tO) behaviour. We discuss our findings in light of the origin of the spin glass behaviour and compare them with other similar studies.

Critical behaviour of an amorphous ferromagnet close to the percolation threshold

Abstract A detailed study of the magnetisation, electrical resistivity and ac magnetic susceptibility of the amorphous Fe 3 Ni 77 B 18 Si 2 alloy has been performed. This alloy is ferromagnetic with the Curie temperature T c = 19,5 K . Critical exponents δ = 4,9 and α = −0,51 were obtained from the experiment while γ = 1,66 and β = 0,43 were calculated from the scaling relations. However the effective critical exponent γ ∗ deduced from the initial magnetic susceptibility exhibits a non-monotonic variation with temperature reaching a maximum of about 1.8 around 43 K. The results are discussed in terms of recent theoretical predictions.