Zvonko Jagličić

Diluted magnetic semiconductors: Mn/Co-doped ZnO nanorods as case study

The structure and the magnetic properties of 3 and 5 mol% (based on the starting concentrations) Co- and Mn-doped ZnO nanorods, synthesized by a straightforward and experimentally simple nonaqueous sol–gel route based on benzyl alcohol as solvent, have been investigated by various characterization techniques, including X-ray diffraction with Rietveld refinement, high-resolution transmission electron microscopy, selected area electron diffraction, energy dispersive X-ray spectroscopy, magnetization measurements and electron paramagnetic resonance. The doped as-synthesized ZnO nanocrystals retain the wurtzite structure with a morphology in the form of nanorods grown along the [001] direction, whose dimensional parameters as well as degree of agglomeration depend on the type and level of doping. The Co-doped ZnO powders are ferromagnetic with a Curie temperature exceeding room temperature. Conversely, the Mn-doped samples show antiferromagnetic correlations with a possible transition to an antiferromagnetic ground state below TN = 10 K. The results suggest that the magnetic ground state is extremely sensitive to the type of dopant, which is in agreement with previous studies.

Soft-templating synthesis of mesoporous magnetic CuFe2O4 thin films with ordered 3D honeycomb structure and partially inverted nanocrystalline spinel domains

Combining sol-gel chemistry with polymer templating strategies enables production of CuFe(2)O(4) thin films with both an ordered cubic network of 17 nm diameter pores and tunable spinel domain sizes. These nanocrystalline materials contain only minor structural defects with λ = 0.85 ± 0.02 and exhibit multiple functionalities, including superparamagnetic behavior (T(B)≈ 310 K) and redox- and photoactivity.

Ferromagnetism in a cobaltocene-doped fullerene derivative below 19 K due to unpaired spins only on fullerene molecules

Abstract We report the discovery and physical properties of a new organic ferromagnet, 3-aminophenyl-methano-fullerene[60]-cobaltocene, with a Curie temperature T C of 19 K. Near- and mid-infrared spectroscopy together with electron spin resonance experiments show that spins reside entirely on the fullerene units, from which we deduce that the ferromagnetism is a consequence solely of exchange interactions between distributed p-electrons on the 3-aminophenyl-methano-fullerene[60] molecules. The ferromagnetic state of the compound is characterized by a very rapid transition to ferromagnetic order below T C , a low saturation field of H s =40 Oe and no measurable hysteresis.

A comparative study of magnetic properties of LiFePO4 and LiMnPO4

A detailed comparative study of the magnetic properties of LiFePO4 and LiMnPO4 samples is presented. Magnetic susceptibility, electron paramagnetic resonance and 7Li NMR experiments were performed on samples as prepared for electrochemical studies. The ground state of LiFePO4 seems to be that of a collinear antiferromagnet and very robust against crystal imperfections. On the other hand, our LiMnPO4 samples possess a weak ferromagnetic ground state with a transition temperature TN = 42 K. We suggest that solitons may be very important magnetic excitations in these systems and that pinning of solitons below TN together with frustration plays a decisive role in the formation of the weak ferromagnetic state in LiMnPO4. The differences between the magnetic properties of these two samples reflect also the differences between their electronic structures and may thus be important for the electrochemistry of LiFePO4 and LiMnPO4.

Surface-spin magnetism of antiferromagnetic NiO in nanoparticle and bulk morphology

The surface-spin magnetism of the antiferromagnetic (AFM) material NiO in nanoparticle and bulk morphology was investigated by magnetic measurements (temperature-dependent zero-field-cooled (zfc) and field-cooled (fc) dc susceptibility, ac susceptibility and zfc and fc hysteresis loops). We addressed the question of whether the multisublattice ordering of the uncompensated surface spins and the exchange bias (EB) effect are only present in the nanoparticles, originating from their high surface-to-volume ratio or if these surface phenomena are generally present in the AFM materials regardless of their bulky or nanoparticle morphology, but the effect is just too small to be detected experimentally in the bulk due to a very small surface magnetization. Performing experiments on the NiO nanoparticles of different sizes and bulk NiO grains, we show that coercivity enhancement and hysteresis loop shift in the fc experiments, considered to be the key experimental manifestations of multisublattice ordering and the EB effect, are true nanoscale phenomena only present in the nanoparticles and absent in the bulk.

Weak ferromagnetism and ferroelectricity in K3Fe5F15

Here, we report on the observation of a weak ferromagnetic transition at TN=122K in a K3Fe5F15 system which is ferroelectric and ferroelastic below Tc=490K. The magnetization and the susceptibility continuously increase with decreasing T down to 2K. The Mossbauer spectra show a spontaneous magnetic ordering and at least three sites corresponding to Fe2+ and Fe3+. The ratio between Fe2+ and Fe3+ is 60:40. At 6K, there are two magnetically ordered sextets with internal fields of 585 and 263kOe. The “slim” hysteresis loops observed are as well characteristic of weak ferromagnetism. At 122K and at low frequencies, the system shows dielectric anomaly characteristic of magnetoelectric behavior.

Synthesis, structure and magnetic properties ofβ-MnO2nanorods

We present synthesis, structure and magnetic properties of structurally well-ordered single-crystalline β-MnO2nanorods of 50–100 nm diameter and several µm length. Thorough structural characterization shows that the basic β-MnO2material is covered by a thin surface layer (∼2.5 nm) of α-Mn2O3phase with a reduced Mn valence that adds its own magnetic signal to the total magnetization of the β-MnO2nanorods. The relatively complicated temperature-dependent magnetism of the nanorods can be explained in terms of a superposition of bulk magnetic properties of spatially segregated β-MnO2and α-Mn2O3constituent phases and the soft ferromagnetism of the thin interface layer between these two phases.

A Pyridazine-Bridged Sandwiched Cluster Incorporating Planar Hexanuclear Cobalt Ring and Bivacant Phosphotungstate

A planar hexanuclear cobalt ring was clamped by two bivacant α1-[PW10O37](9-) with the assistance of the pyridazine bridges to form a novel sandwiched Co(II)-polyoxometalate cluster compound, [Na(H2O)6][Co3(OH) (pydz)4(H2O)7][Co6(PW10O37)2(pydz)4(H2O)6]·43H2O (1; pydz = pyridazine).This cluster was identified by X-ray single-crystal diffraction, elemental analysis, Fourier transform IR and UV-visible spectroscopies, and cyclic voltammetry (CV). Structural analysis reveals that 1 comprises a hexahydrated sodium, a trinuclear [Co3(OH) (pydz)4(H2O)7](5+) cationic cluster, and an anionic [Co6(PW10O37)2(pydz)4(H2O)6](6-) sandwiched cluster, thus giving an intrinsical intercluster compound. The isolation of such cluster was dependent on the in situ transformation of trivacant [α-P2W15O56](12-) to α1-[PW10O37](9-) under the hydrothermal condition. The CV shows reversible multielectron waves from the redox of W(VI) in 1. Cluster 1 exhibits remarkable electrocatalytic activity toward the reduction of nitrite. Magnetism studies indicated a weak anti-ferromagnetic exchange interaction between Co(II) ions within 1.

KAgF3, K2AgF4 and K3Ag2F7: important steps towards a layered antiferromagnetic fluoroargentate(II),

Crystal structure and magnetic properties of K2AgF4, related to recently studied Cs2AgF4, have been scrutinized. It crystallizes orthorhombic (Cmca No.64) with a = 6.182(3) A, b = 12.632(5) A, c = 6.436(3) A (Z = 4, V = 502.6(7) A3). K2AgF4 exhibits slightly puckered [AgF2] sheets and a compressed octahedral coordination of Ag(II) and it is not isostructural to related Cs2AgF4. Violet–coloured K2AgF4 orders ferromagnetically below 26 K. The DFT calculations reproduce semiconducting properties and ferromagnetism of K2AgF4 at the LSDA + U level but only if substantial values of Mott–Hubbard on-site electron–electron repulsion energies for Ag and F are used in calculations. We have also succeeded to solve the crystal structure of a brown KAgF3 (1D antiferromagnet below 64 K; GdFeO3–type, PnmaNo. 62, a = 6.2689(2) A, b = 8.3015(2) A, c = 6.1844(2) A, Z = 4, V = 321.84(2) A3) and to prepare K3Ag2F7, a novel KAgF3/K2AgF4 intergrowth phase and a member of the Ruddlsden–Popper KnAgFn+2 series (n = 1.5). Dark brown K3Ag2F7 crystallizes orthorhombic (K3Cu2Cl7-type, CccaNo. 68, setting 2) with a = 20.8119(14) A, b = 6.3402(4) A, c = 6.2134(4) A (Z = 4, V = 819.87(9) A3).

nuclear magnetic resonance and electron spin resonance of amorphous hydrogenated carbon

The temperature dependences of the and magnetization recovery and spin-lattice relaxation times as well as the NMR spectra have been studied between 300 K and 4 K. The observed short and nearly temperature independent proton and spin-lattice relaxation times demonstrate that the dominant spin-lattice relaxation mechanism is spin diffusion to paramagnetic impurities. The fact that the magnetization recovery curves clearly deviate from the single-exponential form expected for the case of spin diffusion and randomly distributed paramagnetic centres demonstrates that the paramagnetic centres aggregate in clusters. Superparamagnetic freezing effects expected for such an inhomogeneous distribution are indeed seen below 50 K in the temperature dependence of the electron spin-resonance (ESR) intensity, the NMR spectra and the SQUID magnetization measurements which show a distinct magnetic hysteresis loop.