Damir Pajić

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.

Superparamagnetic relaxation in CuxFe3-xO4 (x = 0.5 and x = 1) nanoparticles

Abstract The scope of this article is to report very detailed results of the measurements of magnetic relaxation phenomena in the new Cu 0.5 Fe 2.5 O 4 nanoparticles and known CuFe 2 O 4 nanoparticles. The size of synthesized particles is (6.5±1.5) nm. Both samples show the superparamagnetic behaviour, with the well-defined phenomena of the blocking of magnetic moment. This includes the splitting of zero-field-cooled and field-cooled magnetic moment curves, dynamical hysteresis, slow quasi-logarithmic relaxation of magnetic moment below blocking temperature. The scaling of the magnetic moment relaxation data at different temperatures confirms the applicability of the simple thermal relaxation model. The two copper-ferrites with similar structures show significantly different magnetic anisotropy density and other magnetic properties. Investigated systems exhibit the consistency of all obtained results.

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.

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.

Competing antiferromagnetism and local magnetic order in the bulk ceramic PZT-PFW multiferroic system: searching for the most promising ratio between PZT and PFW

Bulk ceramic compounds of formula xPZT+(1 − x)PFW, where PZT = Pb(Zr0.575Ti0.425)O3 and PFW = Pb(Fe2/3W1/3)O3, for x = 0.15; 0.20; 0.25; 0.37; 0.50; 0.60; 0.65; 0.70 and 0.80, were synthesized. In these single phase perovskites a change from rhombohedral to cubic structure is observed as the proportion of PFW increases. Magnetic investigations including zero-field cooled and field-cooled magnetization versus temperature measurements, magnetic hysteresis loops measurement and ac susceptibility measurements were performed. At high temperatures a clear antiferromagnetic (AFM)-like transition was observed for high PFW concentrations. The low temperature behaviour indicated spin freezing and thermal activation of magnetic moments and measurements confirmed the existence of a barrier maximum for x = 0.37. Competition between the long-range AFM order and short-range magnetic ordering of small nano-sized regions with finite magnetic moments was observed in the studied system. (Some figures may appear in colour only in the online journal)

Slow magnetic dynamics and hysteresis loops of the bulk ferromagnet Co7(TeO3)4Br6

Magnetic dynamics of a bulk ferromagnet, a new single crystalline compound Co-7(TeO3)(4)Br-6, was studied by ac susceptibility and related techniques. Very large Arrhenius activation energy of 17.2 meV (201 K) and long attempt time (2 x 10(-4) s) span the full spectrum of magnetic dynamics inside a convenient frequency window, offering a rare opportunity for general studies of magnetic dynamics. Within the experimental window, the ac susceptibility data build almost ideally semicircular Cole-Cole plots. A comprehensive study of experimental dynamic hysteresis loops of the compound is presented and interpreted within a simple thermal-activation-assisted spin-lattice relaxation model for spin reversal. Quantitative agreement between the experimental results and the models prediction for dynamic coercive field is achieved by assuming the central physical quantity, the Debye relaxation rate, to depend on frequency, as well as on the applied field strength and sample temperature. Crossover between minor to major hysteresis loops is carefully analyzed. Low-frequency limitations of the model, relying on domain-wall pinning effects, are experimentally detected and appropriately discussed.

Nanocrystalline hybrid inorganic–organic one-dimensional chain systems tailored with 2- and 3-phenyl ring monocarboxylic acids

Two novel nanosized hybrid inorganic–organic frameworks, VO(C14H9COO)2, and VO(C10H7COO)2 have been solvothermally synthesized and their structures elucidated using a combination of powder XRD and DFT geometry optimization. They contain one-dimensional chains of corner-sharing tetrahedra in the case of VO(C10H7COO)2, and corner-sharing octahedra for VO(C14H9COO)2 oriented along orthorhombic/monoclinic c-axis, respectively. While VO(C14H9COO)2 exhibits bidentate bridging binding of organic moiety to the metal center, VO(C10H7COO)2 shows a monodentate mode as evidenced from DFT and infrared spectroscopy. Both hybrids exhibit fiber-like morphology, consisting of smaller individual single crystals aligned in parallel to the growth direction along the c-axis. They are thermally stable up to 350 °C having even more stable impurities containing vanadium in its highest oxidation state. The magnetic properties have also been investigated and indicate antiferromagnetic ordering along the chains characterized by rather low spin exchange parameters.