Growth and magnetic characterization of Co nanoparticles obtained by femtosecond pulsed laser deposition.

We present a detailed study on the morphology and magnetic properties of Co nanostructures deposited onto oxidized Si substrates by femtosecond pulsed laser deposition. Generally, Co disks of nanometric dimensions are obtained just above the ablation threshold, with a size distribution characterized by an increasingly larger number of disks as their size diminishes, and with a maximum disk size that depends on the laser power density. In Au/Co/Au structures, in-plane magnetic anisotropy is observed in all cases, with no indication of superparamagnetism regardless of the amount of material or the laser power density. Magnetic force microscopy observations show coexistence of single-domain and vortex states for the magnetic domain structure of the disks. Superconducting quantum interference device magnetometry and x-ray magnetic circular dichroism measurements point to saturation magnetization values lower than the bulk, probably due to partial oxidation of the Co resulting from incomplete coverage by the Au capping layer.

Observation of phonon-induced magnetic deflagration in manganites

Very fast magnetic avalanches in (La, Pr)-based manganites are the signature of a phase transition from an insulating blocked charge-ordered antiferromagnetic state to a charge-delocalized ferromagnetic (CD-FM) state. We report here the experimental observation that this transition does not occur either simultaneously or randomly in the whole sample but there is instead a spatial propagation with a velocity of the order of tens of m/s. Our results show that avalanches originate from the inside of the sample, move to the outside, and occur at values of the applied magnetic field that depend on the CD-FM fraction in the sample. Moreover, upon application of surface acoustic waves at constant magnetic fields, we are able to trigger avalanches at very well-determined values of the temperature and magnetic field. Due to the interaction with the acoustic waves, the number of isolated ferromagnetic clusters in La0.225Pr0.40Ca0.375MnO3 starts to grow across the entire sample in the same way as if it were a magnetic deflagration.

Quantum magnetic deflagration in Mn12 acetate

We report controlled ignition of magnetization reversal avalanches by surface acoustic waves in a single crystal of Mn12 acetate. Our data show that the speed of the avalanche exhibits maxima on the magnetic field at the tunneling resonances of Mn12. Combined with the evidence of magnetic deflagration in Mn12 acetate [Y. Suzuki et al., Phys. Rev. Lett. 95, 147201 (2005)], this suggests a novel physical phenomenon: deflagration assisted by quantum tunneling.

Excitation modes of vortices in submicron magnetic disks

Classical and quantum theory of spin waves in the vortex state of a mesoscopic submicron magnetic disk have been developed with account of the finite mass density of the vortex. Oscillations of the vortex core resemble oscillations of a charged string in a potential well in the presence of the magnetic field. A conventional gyrotropic frequency appears as a gap in the spectrum of spin waves of the vortex. The mass of the vortex has been computed, and the result agrees with experimental findings. The finite vortex mass generates a high-frequency branch of spin waves. The effects of an external magnetic field and dissipation have been addressed.

Magnetic, thermal and spectral characterization of 2,6-dimethoxybenzoates of Co(II), Ni(II) and Cu(II)

The complexes of 2,6-dimethoxybenzoic acid anion with ions of Co(II), Ni(II), and Cu(II) have been synthesized as polycrystalline solids, and characterized by elemental analysis, spectroscopy, magnetic studies, and also by X-ray diffraction and thermogravimetric measurements. The analysed complexes have following colours: pink for Co(II), green for Ni(II), and blue for Cu(II) compounds. The carboxylate group binds as monodentate, and bidentate bridging and chelating ligands. On heating in air to 1173 K the complexes decompose in four, three or two steps. At first, they dehydrate in one or two steps to anhydrous salts, that next decompose to oxides of the respective metals. The solubility of the investigated dimethoxybenzoates in water at 293 K is of the order of 10-2 mol/dm3. Their magnetic moments were determined in the temperature range of 76-303 K. The results reveal the compounds of Co(II) and Ni(II) to be high-spin complexes and that of Cu(II) to form dimer.

Magnetic, thermal and spectral characterization of 2,4-dimethoxybenzoates of Mn(II), Co(II) and Cu(II)

2,4 - Dimethoxybenzoates of Mn(II), Co(II) and Cu(II) have been synthesized as hydrated or anyhydrous polycrystalline solids and characterized by elemental analysis, IR spectroscopy, magnetic studies and X-ray diffraction measurements. They possess the following colours: Mn(II) - white, Co(II) - pink and Cu(II) - blue. The carboxylate groups bind as monodentate, or a symmetrical bidentate bridging ligands and tridentate. The thermal stabilities were determined in air at 293-1173K. When heated the hydrated complexes dehydrate to from anhydous salts which are decomposed to the oxides of respective metals. The magnetic susceptibilites of the 2,4-dimethoxybenzoates were measured over the range 76-303 K and their magnetic moments were calculated. The results reveal the complexes of Mn(II), Co(II) to be high-spin complexes and that of Cu(II) to form dimer.

The physico-chemical properties of 4-chlorophenoxyacetates of Mn(II), Co(II), Ni(II) and Cu(II)

The complexes of 4-chlorophenoxyacetates of Mn(II), Co(II), Ni(II) and Cu(II) have been synthesized as polycrystalline solids, and characterized by elemental analysis, spectroscopy, magnetic studies and also by X-ray diffraction and thermogravimetric measurements. The analysed complexes have the following colours: pink for Co(II), green for Ni(II), blue for Cu(II) and a pale pink for Mn(II) compounds. The carboxylate group binds as monodentate and bidentate ligands. On heating to 1173K in air the complexes decompose in several steps. At first, they dehydrate in one step to anhydrous salts, that next decompose to the oxides of respective metals. Their magnetic moments were determined in the range of 76-303K. The results reveal them to be high-spin complexes of weak ligand fields.

Comparison of some properties of 2,3 - and 3,4 - dimethoxybenzoates of Cu(II), Co(II) and Nd(III)

The physicochemical properties of 2,4-, and 3,4- dimethoxybenzoates of Cu(II), Co(II) and Nd(III) were studied and compared to observe the -OCH3 substituent positions in benzene ring on the character of complexes. The analysed compounds are crystalline hydrated or anhydrous salts with colours depending on the kind of central ions: blue for Cu(II), pink for Co(II) and violet for Nd(III) complexes. The carboxylate groups bind as monodentate, bidentate bridging or chelating and even tridentate ligands. Their thermal stabilities were studied in air at 293-1173K. When heated the hydrated complexes release the water molecules and form anhydrous compounds which are then decomposed to the oxides of respective metals. Their magnetic moment values were determined in the range of 76-303K. The results reveal the compounds of Nd(III) and Co(II) to be the high-spin and that of Cu(II) forms dimer. The various positions of -OCH3 groups in benzene ring influence some of physicochemical properties of analysed compounds.

The physico-chemical properties of 2-methoxyphenoxyacetates of Mn(II), Co(II), Ni(II) and Cu(II)

The complexes of 2-methoxyhenoxyacetates of Mn(II), Co(II), Ni(II) and Cu(II)with the general formula: M(C9H9O4)3·4H2O, where M(II) = Mn, Co, Ni and Cu have been synthesized and characterized by elemental analysis, IR spectroscopy, magnetic and thermogravimetric studies and also X-ray diffraction measurements. The complexes have colours typical for M(II) ions (Mn(II) - a pale pink, Co(II) - pink, Ni(II) - green, and Cu(II) - blue). The carboxylate group binds as monodentate and bidentate ligands. On heating to 1273K in air the complexes decompose in the same way. At first, they dehydrate in one step to anhydrous salts, that next decompose to the oxides of respective metals with the intermediate formation of the oxycarbonates. Their solubility in water at 293K is of the order of 10-5 mol·dm-3. The magnetic moments of analysed complexes were determined in the range of 76-303K. The results reveal them to be high-spin complexes of weak ligand fields.

Ferromagnetic properties of pyrolytic nanocarbon

The work reported in this thesis is dedicated to irreversible magnetic properties in pyrolytic nanocarbon samples. Based on atomic force microscope images, the samples consist of carbon clusters with radius 30..120 nm. These are treated as single-domain nanoparticles. Magnetic hysteresis, field cooled, zero field cooled and thermoremanent magnetization measurements were performed using an RF SQUID magnetometer and ferromagnetic behaviour was observed. Analysis suggests that the ferromagnetic ordering is associated with defects in a thin surface layer, whose thickness is independent of particle size. Critical radius for single-domain particles, critical radius for coherent rotation, magnetic layer thickness, distance between elementary magnetic moments, saturation magnetization, exchange stiffness constant and anisotropy energy density are also presented.

Structure-related magnetic fabric studies : implications for deformed and undeformed Precambrian rocks

The aim of this thesis research was to gain a better understanding of the emplacement of rapakivi granite intrusions, as well as the emplacement of gold-bearing hydrothermal fluids in structurally controlled mineralizations. Based on investigations of the magnetic fabric, the internal structures could be analysed and the intrusion mechanisms for rapakivi granite intrusions and respectively different deformation stages within gold-bearing shear and fault zones identified. Aeromagnetic images revealed circular structures within the rapakivi granite batholiths of Wiborg, Vehmaa and Åland. These circular structures represent intrusions that eventually build up these large batholiths. The rapakivi granite intrusions of Vehmaa, Ruotsinpyhtää within the Wiborg batholith and Saltvik intrusions within the Åland batholith all show bimodal magnetic susceptibilities with paramagnetic and ferromagnetic components. The distribution of the bimodality is related to different magma batches of the studied intrusions. The anisotropy of magnetic susceptibility (AMS) reveals internal structures that cannot be studied macroscopically or by microscope. The Ruotsinpyhtää and Vehmaa intrusions represent similar intrusion geometries, with gently to moderately outward dipping magnetic foliations. In the case of Vehmaa, the magnetic lineations are gently plunging and trend in the directions of the slightly elongated intrusion. The magnetic lineations represent magma flow. The shapes of the AMS ellipsoids are also more planar (oblate) in the central part of the intrusion, whereas they become more linear (prolate) near the margin. These AMS results, together with field observations, indicate that the main intrusion mechanism has involved the subsidence of older blocks with successive intrusion of fractionated magma during repeated cauldron subsidence. The Saltvik area within the Åland batholith consists of a number of smaller elliptical intrusions of different rapakivi types forming a multiple intrusive complex. The magnetic fabric shows a general westward dipping of the pyterlite and eastward dipping of the contiguous even-grained rapakivi granite, which indicates a central inflow of magma batches towards the east and west resulting from a laccolitic emplacement of magma batches, while the main mechanism for space creation was derived from subsidence. The magnetic fabric of structurally controlled gold potential shear and fault zones in Jokisivu, Satulinmäki and Koijärvi was investigated in order to describe the internal structures and define the deformation history and emplacement of hydrothermal fluids. A further aim of the research was to combine AMS studies with palaeomagnetic methods to constrain the timing for the shearing event relative to the precipitation of ferromagnetic minerals and gold. All of the studied formations are dominated by monoclinic pyrrhotite. The AMS directions generally follow the tectonic structures within the formations. However, internal variations in the AMS direction as well as the shapes of the AMS ellipsoids are observed within the shear zones. In Jokisivu and Satulinmäki in particular, the magnetic signatures of the shear zone core differ from the margins. Furthermore, the shape of the magnetic fabric in the shear zone core of Jokisivu is dominated by oblate shapes, whereas the margins exhibit prolate shapes. These variations indicate a later effect of the hydrothermal fluids on the general shear event. The palaeo-magnetic results reveal a deflection from the original Svecofennian age geomagnetic direction. These results, coupled with correlations between the orientation of the NRM vectors and the magnetic and rock fabrics, imply that the gold-rich hydrothermal fluids were emplaced pre/syntectonically during the late stages of the Svecofennian orogeny.

Hyperfine interactions in the new Fe-based superconducting structures and related magnetic phases

This thesis is devoted to the study of the hyperfine properties in iron-based superconductors and the synthesis of these compounds and related phases. During this work polycrystalline chalcogenide samples with stoichiometry 1:1 (FeTe1-χSχ, FeSe1-x) and pnictide samples with stoichiometry 1:2:2 (BaFe2(As1-χPχ)2, EuFe2(As1-x Px)2) were synthesized by solid-state reaction methods in vacuum and in a protecting Ar atmosphere. In several cases post-annealing in oxygen atmosphere was employed. The purity and superconducting properties of the obtained samples were checked with X-ray diffraction, SQUID and resistivity measurements. For studies of the magnetic properties of the investigated samples Mössbauer spectroscopy was used. Using low-temperature measurements around Tc and various values of the source velocity the hyperfine interactions were obtained and the magnetic and structural properties in the normal and superconducting states could be studied. Mössbauer measurements together with XRD characterization were also used for the detection of impurity phases. DFT calculations were used for the theoretical study of Mössbauer parameters for pnictide-based ᴻsamples BaFe2(As1-xPx)2 and EuFe2(As1-xPx)2.

Emission, kinetic and magnetic phenomena in rare-earth and transition metal doped ZnSe single crystals

In this work emission, optical, electrical and magnetic properties of the d- and f- elements doped zinc selenide crystals were investigated within a wide temperature range. Doping was performed in various technological processes: during the growth by chemical vapor transport method; by thermal diffusion from the Bi or Zn melt. Concentration of the doping impurity in the crystals was controlled by amount of the dopant in the source material or by its concentration in the doping media. Special interest in the work was paid to the influence of the different concentrations of Cr and Yb impurities on ZnSe crystals’ properties, correlations between observed effects and similarities with the Ni, Mn and Gd dopants are analysed. Possibility of formation of the excitons bound to the doping d-ions was shown. In contrast to this, it was observed that f-elements do not bound excitons, but prevent formation of excitons bound to some uncontrolled impurities. A mechanism of Cr doping impurity interaction with background impurities and zinc selenide structural defects was proposed based on experimental data. An assumption about resonant energy transfer between double charged chromium ions and complexes based on crystals’ vacancy defects was made. A correlation between emission and magnetic properties of the d- ions doped samples was established. Based on this correlation a mechanism explaining the concentration quench of the emission was proposed. It was found that f-ions bind electrically active shallow and deep donor and acceptor states of background impurity to electrically neutral complexes. This may be observed as “purification” of ZnSe crystals by doping with the rare-earth elements, resulting i tendency of the properties of f-ion doped crystals to the properties of intrinsic crystals, but with smaller concentration of uncontrolled native and impurity defects. A possible interpretation of this effect was proposed. It was shown that selenium substituting impurities decrease efficiency of the Yb doping. Based on this experimental results an attempt to determine ytterbium ion surroundings in the crystal lattice was made. It was shown that co-doping of zinc selenide crystals with the d- and f- ions leads to the combination of the impurities influence on the material’s properties. On the basis of obtained data an interaction mechanism of the d- and f-elements co-dopants was proposed. Guided by the model of the ytterbium ion incorporation in the selenide sublattice of the ZnSe crystals, an assumption about stabilization of single charged chromium ions in the zinc sublattice crystal nodes, by means of formation of the local charge compensating clusters, was made.