A. Wolska

Homogeneous and heterogeneous magnetism in (Zn,Co)O : From a random antiferromagnet to a dipolar superferromagnet by changing the growth temperature

For more than a decade ZnO doped with Mn and Co has remained as one of the most prospected diluted magnetic semiconductor for spintronic applications with conflicting outcome concerning the genuineness of its room temperature ferromagnetism. In order to clarify this issue we investigate (Zn,Co)O layers grown by atomic layer deposition at low temperatures. We employ and relay on wide range of extensive material characterization, which in combination with superconducting quantum interference device magnetometry allow us decisively exemplify the growth temperature as the key factor discriminating between paramagnetic (obtained at 160 °C) and various forms of ferromagnetic responses, seen when the grows is carried out at 200 °C and above.

Paramagnetism of cobalt-doped ZnO nanoparticles obtained by microwave solvothermal synthesis.

Summary Zinc oxide nanopowders doped with 1–15 mol % cobalt were produced by the microwave solvothermal synthesis (MSS) technique. The obtained nanoparticles were annealed at 800 °C in nitrogen (99.999%) and in synthetic air. The material nanostructure was investigated by means of the following techniques: X-ray diffraction (XRD), helium pycnometry density, specific surface area (SSA), inductively coupled plasma optical emission spectrometry (ICP-OES), extended X-ray absorption fine structure (EXAFS) spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and with magnetometry using superconducting quantum interference device (SQUID). Irrespective of the Co content, nanoparticles in their initial state present a similar morphology. They are composed of loosely agglomerated spherical particles with wurtzite-type crystal structure with crystallites of a mean size of 30 nm. Annealing to temperatures of up to 800 °C induced the growth of crystallites up to a maximum of 2 μm in diameter. For samples annealed in high purity nitrogen, the precipitation of metallic α-Co was detected for a Co content of 5 mol % or more. For samples annealed in synthetic air, no change of phase structure was detected, except for precipitation of Co3O4 for a Co content of 15 mol %. The results of the magentometry investigation indicated that all as-synthesized samples displayed paramagnetic properties with a contribution of anti-ferromagnetic coupling of Co–Co pairs. After annealing in synthetic air, the samples remained paramagnetic and samples annealed under nitrogen flow showed a magnetic response under the influences of a magnetic field, likely related to the precipitation of metallic Co in nanoparticles.

Synthesis, structural studies and biological activity of new Cu(II) complexes with acetyl derivatives of 7-hydroxy-4-methylcoumarin.

The new Cu(II) complexes with 6-acetyl-7-hydroxy-4-methylcoumarin (HL1) and 8-acetyl-7-hydroxy-4-methylcoumarin (HL2) have been obtained by the electrochemical method. The density functional theory calculations and X-ray absorption spectroscopy techniques have been used to geometrically describe a series of new compounds. The studies have been focused on the coordination mode of the hydroxy ligands to the metallic centre. The complexes, Cu(HL1)2 and Cu(HL2)2⋅0.5H2O, have flat square geometry with oxygen atoms in the first coordination sphere. Two bidentate anionic coumarins are bonded to the metal cation via the acetyl and deprotonated hydroxyl O atoms. Biological activity, including microbiological and cytotoxic, has been evaluated and found to be enhanced in comparison with the parent ligands. Moreover, the Cu(II) complex with 8-acetyl-7-hydroxy-4-methylcoumarin shows similar antifungal activity as commercially used fluconazole.

Understanding Chloroquine Action at the Molecular Level in Antimalarial Therapy: X-ray Absorption Studies in Dimethyl Sulfoxide Solution

X-ray absorption spectroscopy is used to determine the local atomic structure around the iron atom from a soluble synthetic analogue of malaria pigment (hemozoin), cf. ferrimesoporphyrin IX of mesohematin anhydride, in the absence or presence of chloroquine (CQ) in dimethyl sulfoxide (DMSO). Of particular note are the CQ-induced changes in the structure of mesohematin anhydride, which might confirm the formation of CQ-ferrimesoporphyrin IX complex. Examination of solutions of mesohematin anhydride dissolved in DMSO reveals preservation of the dimerlike structure with the central iron atoms of the ferric porphyrin IX reciprocally linked by propionate side chains, which is typical for hematin anhydride (β-hematin). In the presence of CQ, additional light atoms, such as nitrogen, carbon, and oxygen, were detected surrounding the iron in a distance ranging from 2.48 to 3.77 Å. The changes introduced by CQ in DMSO are different from that observed in the acetic acid solution.

Toward understanding the chloroquine action at the molecular level in antimalarial therapy--X-ray absorption studies in acetic acid solution.

The local atomic structure around the central iron of the synthetic soluble analog of malarial pigment in acetic acid solution and with addition of chloroquine as found by X-ray absorption spectroscopy is reported. The special interest was drawn to the axial linkage between the central iron atom of the ferriprotoporphyrin IX (FePPIX) coordinated axially to the propionate group of the adjacent FePPIX. This kind of bonding is typical for hematin anhydride. Detailed analysis revealed differences in oxygen coordination sphere (part of dimer linkage bond) between synthetic equivalent of hemozoin in the powder state and dissolved in acetic acid and water at different concentrations mimicking the physiological condition of the parasites food vacuole. The results of performed studies suggest that the molecular structure of synthetic analogue of hemozoin is no longer dimer-like in acidic solution. Further changes in atomic order around Fe are seen after addition of the antimalarial drug chloroquine.

ZnO, ZnMnO and ZnCoO films grown by atomic layer deposition

Despite many efforts, the origin of a ferromagnetic (FM) response in ZnMnO and ZnCoO is still not clear. Magnetic investigations of our samples, not discussed here, show that the room temperature FM response is observed only in alloys with a non-uniform Mn or Co distribution. Thus, the control of their distribution is crucial for the explanation of contradicted magnetic properties of ZnCoO and ZnMnO reported till now. In this paper, we discuss advantages of the atomic layer deposition (ALD) growth method, which enables us to control the uniformity of ZnMnO and ZnCoO alloys. Properties of ZnO, ZnMnO and ZnCoO films grown by the ALD are discussed.

Magnetic properties of MnSb inclusions formed in GaSb matrix directly during molecular beam epitaxial growth

Despite of intensive search for the proper semiconductor base materials for spintronic devices working at room temperature no appropriate material based on ferromagnetic semiconductors has been found so far. We demonstrate that the phase segregated system with MnSb hexagonal inclusions inside the GaSb matrix, formed directly during the molecular beam epitaxial growth reveals the ferromagnetic properties at room temperature and is a good candidate for exploitation in spintronics. Furthermore, the MnSb inclusions with only one crystalline structure were identified in this GaMn:MnSb granular material. The SQUID magnetometry confirmed that this material exhibits ferromagnetic like behavior starting from helium up to room temperature. Moreover, the magnetic anisotropy was found which was present also at room temperature, and it was proved that by choosing a proper substrate it is possible to control the direction of easy axis of inclusions’ magnetization moment between in-plane and out-of-plane; the latter is ...

Structural and magnetic properties of the molecular beam epitaxy grown MnSb layers on GaAs substrates

The structural and magnetic properties of MnSb layers grown on two differently oriented GaAs substrates are reported. The MnSb compounds grow nonhomogenously both on GaAs (111) B and on GaAs (100) substrates. In x-ray diffraction studies the formation of two epitaxial domains is observed depending on the crystallographic orientation of the substrate. The observed diffusion of Ga atoms from the substrate to the layers results in the formation of an additional Mn-rich cubic phase of GaMnSb. In the case of the (100) oriented substrate, the diffusion of Mn into the substrate was additionally found. Traces of other phases were also noticed. The complex morphology of the layers is found to influence their magnetic properties. Magnetic force microscopy images revealed an inhomogenous distribution of the magnetic force gradient on the surface and the formation of magnetic domains in the samples. X-ray absorption studies of the chemical bonding and local atomic structure around Mn atoms confirmed high structural and chemical disorder in the samples. The chemical bonding of the dominating fraction of Mn atoms is found, however, similar to that in the reference MnSb powder. The x-ray magnetic circular dichroism measurements reveal an enhanced orbital moment and a reduced spin moment, which is most likely caused by the presence of different phases and a Mn-rich surface in the investigated samples

Structural and magnetic properties of nanoclusters formed in III-V semiconductors

Studies of X-ray magnetic circular dichroism (XMCD) were performed for a set of GaMnAs films with different Mn concentrations priory and after high temperature annealing (500 and 600 oC). After thermal treatment, GaMnAs samples with zinc blende structure and MnAs hexagonal nano-clusters were formed. In most of the samples, both types of clusters were detected by EXAFS studies. Dependence of the orbital and the spin moments on magnetic field were calculated from XMCD data by applying the sum rule. It was shown that both moments were much larger for MnAs nano-clusters. When these inclusions are formed even in a small amount, they dominate the XMCD signal. Interestingly, in some of samples the zinc blende GaMnAs nano-clusters were observed at a surface while in the bulk of hexagonal MnAs. Therefore, the location of magnetic ions in the host matrix is crucial for their magnetic properties. This unique information can be provided by XAS and XMCD.

Synthesis, structural studies and biological activity of novel Cu(II) complexes with thiourea derivatives of 4-azatricyclo[5.2.1.02,6]dec-8-ene-3,5-dione

The new Cu(II) complexes of 1/2/3-(bromophenyl)-3-(1,7,8,9-tetramethyl-3,5-dioxo-4-azatricyclo[5.2.1.02,6]dec-8-en-4-yl)thiourea derivatives have been synthesized. The spectroscopic studies together with density functional theory calculations of Cu(II) complexes revealed that two parent ligands coordinate to the copper cation in bidentate fashion via thiocarbonyl S and deprotonated N atoms forming rarely observed four-membered chelate ring, with nearly planar [CuN2S2] moiety. In solid state, the mononuclear complex is formed for thiourea derivative with 3-bromophenyl, whereas for Cu(II) connection with 2- and 4-bromophenyl-thioureas the formation of dinuclear complexes is observed, the latter formed by the stacking of mononuclear complexes. The microbiological activity of novel compounds has been evaluated. The Cu(II) complex with 4-bromophenyl ring connected to the thiourea moiety showed significant inhibition against standard strains of S. aureus and S. epidermidis. The range of minimal inhibitory concentration values is 2-4μg/mL. That compound exhibited antibiofilm potency and effectively inhibited the formation of biofilm of methicillin-susceptive strain of S. epidermidis ATCC 12228. Moreover, the cytotoxicity against the MT-4 cells of all obtained complexes has been evaluated. The complexes turned out to be non-cytotoxic for exponentially growing MT-4.