Numerical simulation of magnetic interactions in polycrystalline YFeO3

The magnetic behavior of polycrystalline yttrium orthoferrite was studied from the experimental and theoretical points of view. Magnetization measurements up to 170 kOe were carried out on a single-phase YFeO3 sample synthesized from heterobimetallic alkoxides. The complex interplay between weak-ferromagnetic and antiferromagnetic interactions, observed in the experimental M(H) curves, was successfully simulated by locally minimizing the magnetic energy of two interacting Fe sublattices. The resulting values of exchange field (H-E = 5590 kOe), anisotropy field (H-A = 0.5 kOe) and Dzyaloshinsky-Moriya antisymmetric field (H-D = 149 kOe) are in good agreement with previous reports on this system. (C) 2007 Elsevier B.V. All rights reserved.

Magnetic interaction in exchange-biased bilayers: a first-order reversal curve analysis

The distributions of coercivities and magnetic interactions in a set of polycrystalline Ni(0.8)Fe(0.2)/FeMn bilayers have been determined using the first-order reversal curve (FORC) formalism. The thickness of the permalloy (Py) film was fixed at 10 nm (nominal), while that of the FeMn film varied within the range 0-20 nm. The FORC diagrams of each bilayer displayed two clearly distinguishable regions. The main region was generated by Py particles whose coercivities were enhanced in comparison with those in which the FeMn film was absent (sample O). The minor region was produced by Py particles with coercivities similar to or slightly higher than those of particles in the Py film of sample O. Each sample presented two distributions of interaction fields, one for each region, and both were centred slightly below the exchange-bias field, thus indicating a prevalence of magnetizing interactions. These results are consistent with a grain size distribution in the Py layer and the presence of uncompensated antiferromagnetic moments.

Magnetic resonance study of a vanadium pentoxide gel

In this work we report results from continuous-wave (CW) and pulsed electron paramagnetic resonance (EPR) and proton nuclear magnetic resonance (NMR) studies of the vanadium pentoxide xerogel V2O5:nH(2)O (n approximate to 1.6). The low temperature CW-EPR spectrum shows hyperfine structure due to coupling of unpaired V4+ electron with the vanadium nucleus. The analysis of the spin Hamiltonian parameters suggests that the V4+ ions are located in tetragonally distorted octahedral sites. The transition temperature from the rigid-lattice low-temperature regime to the high temperature liquid-like regime was determined from the analysis of the temperature dependence of the hyperfine splitting and the V4+ motional correlation time. The Electron Spin Echo Envelope Modulation (ESEEM) data shows the signals resulting from the interaction of H-1 nuclei with V4+ ions. The modulation effect was observed only for field values in the center of the EPR absorption spectrum corresponding to the single crystals orientated perpendicular to the magnetic field direction. At least three protons are identified in the xerogel by our magnetic resonance experiments: (I) the OH groups in the equatorial plane, (ii) the bound water molecules in the axial V=O bond and (iii) the free mobile water molecules between the oxide layers. Proton NMR lineshapes and spin-lattice relaxation times were measured in the temperature range between 150 K and 323 K. Our analysis indicates that only a fraction of the xerogel protons contribute to the measured conductivity.

Emplacement mechanism of the main granite pluton of the Lavras do Sul intrusive complex, South Brazil, determined by magnetic anisotropies

Magnetic fabric and rock magnetism studies were performed on apparently isotropic granite facies from the main intrusion of the Lavras do Sul Intrusive Complex pluton (LSIC, Rio Grande do Sul, South Brazil). This intrusion is roughly circular (similar to 12 x 13.5 km), composed of alkali-calcic and alkaline granitoids, with the latter occupying the margin of the pluton. Magnetic fabrics were determined by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). The two fabrics are coaxial. The parallelism between AMS and AARM tensors excludes the presence of a single domain (SD) effect on the AMS fabric of the granites. Several rock-magnetism experiments performed in one specimen from each sampled site show that for all sites the magnetic susceptibility is dominantly carried by ferromagnetic minerals, while mainly magnetite carries the magnetic fabrics. Lineations and foliations in the granite facies were successful determined by applying magnetic methods. Magnetic lineations are gently plunging and roughly parallel to the boundaries of the pluton facies, except at the few sites in the central facies which have a radial orientation pattern. In contrast, the magnetic foliations tend to follow the contacts between the different granite facies. They are gently outerward-dipping inside the pluton, and become either steeply southwesterly dipping or vertical towards its margin. The lack of solid-state and subsolidus deformations at outcrop scale and in thin sections precludes deformation after full crystallization of the pluton. This evidence allows us to interpret the observed magnetic fabrics as primary in origin (magmatic) acquired when the rocks were solidified as a result of processes reflecting magma flow. The foliation pattern displays a dome-shaped form for the main LSIC-pluton. However, the alkaline granites which outcrop in the southern part of the studied area have an inward-dipping foliation, and the steeply plunging magnetic lineation suggests that this area could be part of a feeder zone. The magma ascent probably occurred due to ring-diking. (C) 2008 Elsevier B.V. All rights reserved.

Tectonic fabric revealed by AARM of the proterozoic mafic dike swarm in the Salvador city (Bahia state): Sao Francisco Craton, NE Brazil

Magnetic fabric and rock magnetism studies were performed on 25 unmetamorphosed mafic dikes of the Meso-Late Proterozoic (similar to 1.02 Ga) dike swarm from Salvador (Bahia State, NE Brazil). This area lies in the north-eastern part of the Sao Francisco Craton, which was dominantly formed/reworked during the Transamazonian orogeny (2.14-1.94 Ga). The dikes crop out along the beaches and in quarries around Salvador city, and cut across both amphibolite dikes and granulites. Their widths range from a few centimeters up to 30 m with an average of similar to 4 m, and show two main trends N 140-190 and N 100-120 with vertical dips. Magnetic fabrics were determined using both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). The magnetic mineralogy was investigated by many experiments including remanent magnetization measurements at variable low temperatures (10-300 K), Mossbauer spectroscopy, high temperature magnetization curves (25-700 degrees C) and scanning electron microscopy (SEM). The rock magnetism study suggests pseudo-single-domain magnetite grains carrying the bulk magnetic susceptibility and AARM fabrics. The magnetite grains found in these dikes are large and we discard the presence of single-domain grains. Its composition is close to stoichiometric with low Ti substitution, and its Verwey transition occurs around 120 K. The main AMS fabric recognized in the swarm is so-called normal, in which the K(max)-K(int) plane is parallel to the dike plane and the magnetic foliation pole K(min)) is perpendicular to it. This fabric is interpreted as due to magma flow, and analysis of the K m inclination permitted to infer that approximately 80% of the dikes were fed by horizontal or sub-horizontal flows (K(max) < 30 degrees). This interpretation is supported by structural field evidence found in five dikes. In addition, based on the plunge of K(max), two mantle sources could be inferred; one of them which fed about 80% of the swarm would be located in the southern part of the region, and the other underlied the Valeria quarry. However, for all dikes the AARM tensors are not coaxial with AMS fabrics and show a magnetic lineation (AARM(max)) oriented to N30-60E, suggesting that magnetite grains were rotated clockwise from dike plane. The orientation of AARM lineation is similar to the orientation of a system of faults in which the Salvador normal fault is the most important. These faults were formed during Cretaceous rifting in the Reconcavo-Tucano-jatoba assemblage that corresponds to an aborted intra-continental rift formed during the opening of the South Atlantic. Therefore, the AARM fabric found for the Salvador dikes is probably tectonic in origin and suggests that the dike swarm was affected by the important tectonic event responsible for the break-up of the Gondwanaland. (C) 2008 Elsevier B.V. All rights reserved.

Magnetic fabric and zircon U-Pb geochronology of the Itaoca pluton: Implications for the Brasiliano deformation of the southern Ribeira Belt (SE Brazil)

The Itaoca pluton consists of porphyritic monzogranite that intruded the upper crust into low-grade metasedimentary rocks of the Apiai Dornain (Ribeira Belt). Anisotropy of magnetic susceptibility and zircon U-Pb (Shrimp) geochronology were combined to determine pluton emplacement mechanisms and its chronology relative to the collision structures of the Paranapiacaba (Brasiliano II) orogenic system. Magnetic susceptibility ranges between 4 and 38 x 10(-3) SI, and thermomagnetic measurements indicate multidomain magnetite is the main carrier of anisotropy. The pluton shows an ""onion-skin"" structure roughly elongated to the northeast with its hinge zone including kilometer-wide roof-pendants. Magnetic lineations are variable in orientation in consistency with the dominant oblate symmetry of the magnetic fabric. A distinct NE-trending point-maxima, however, indicates the mean lineation is parallel to the stretching direction of the transpressive deformation that affected the regional host rocks. Prismatic zircon from the monzogranite, both in the core and in the finely-zoned margins, yielded an age of 623 +/- 10 Ma. These results suggest the magmatic fabric recorded the earlier strain increments of the regional shear deformation. It may correspond to the transition from continental arc to collision tectonics of the southern Ribeira Belt. (C) 2008 Elsevier Ltd. All rights reserved.

Fabrics of pre- and syntectonic granite plutons and chronology of shear zones in the Eastern Borborema Province, NE Brazil

We used the fabrics of two granite plutons and U/Pb (SHRIMP) zircon ages to constrain the tectonic evolution of the E-trending Patos shear zone (Borborema Province, NE Brazil). The pre-tectonic Teixeira batholith consists of an amphibole leucogranite locally with aegirine-augite. Zircons from a syenogranite yielded crystallization ages of 591 +/- 5 Ma. The batholith fabrics were determined by anisotropy of magnetic susceptibility (AMS) and mineral shape preferred orientation. The fabrics support pre-transcurrent batholith emplacement, as evidenced by: (i) magmatic/magnetic fabrics in low susceptibility (<0.35 mSI) leucogranites highly discordant to the regional host rock structure, and (ii) concordant magnetic fabrics restricted to high susceptibility (>1 mSI) corridors connected to shear zones branching off from Patos. One of these satellite shear zones controlled the syntectonic emplacement of the Serra Redonda pluton, which yields a crystallization age of 576 +/- 3 Ma. This late shearing event marks the peak regional deformation that, south of Patos, was coupled to crustal shortening nearly perpendicular to the shear belt. The chronology of the deformational events indicates that the major shear zones of the eastern Borborema are late structures active after the crustal blocks amalgamated. (C) 2007 Elsevier Ltd. All rights reserved.

Efeitos da interação dipolar na nucleação de vórtices em nano-cilindros magnéticos

The effect of confinement on the magnetic structure of vortices of dipolar coupled ferromagnetic nanoelements is an issue of current interest, not only for academic reasons, but also for the potential impact in a number of promising applications. Most applications, such as nano-oscillators for wireless data transmission, benefit from the possibility of tailoring the vortex core magnetic pattern. We report a theoretical study of vortex nucleation in pairs of coaxial iron and Permalloy cylinders, with diameters ranging from 21nm to 150nm, and 12nm and 21nm thicknesses, separated by a non-magnetic layer. 12nm thick iron and Permalloy isolated (single) cylinders do not hold a vortex, and 21nm isolated cylinders hold a vortex. Our results indicate that one may tailor the magnetic structure of the vortices, and the relative chirality, by selecting the thickness of the non-magnetic spacer and the values of the cylinders diameters and thicknesses. Also, the dipolar interaction may induce vortex formation in pairs of 12nm thick nanocylinders and inhibit the formation of vortices in pairs of 21nm thick nanocylinders. These new phases are formed according to the value of the distance between the cylinderes. Furthermore, we show that the preparation route may control relative chirality and polarity of the vortex pair. For instance: by saturating a pair of Fe 81nm diameter, 21nm thickness cylinders, along the crystalline anisotropy direction, a pair of 36nm core diameter vortices, with same chirality and polarity is prepared. By saturating along the perpendicular direction, one prepares a 30nm diameter core vortex pair, with opposite chirality and opposite polarity. We also present a theoretical discussion of the impact of vortices on the thermal hysteresis of a pair of interface biased elliptical iron nanoelements, separated by an ultrathin nonmagnetic insulating layer. We have found that iron nanoelements exchange coupled to a noncompensated NiO substrate, display thermal hysteresis at room temperature, well below the iron Curie temperature. The thermal hysteresis consists in different sequences of magnetic states in the heating and cooling branches of a thermal loop, and originates in the thermal reduction of the interface field, and on the rearrangements of the magnetic structure at high temperatures, 5 produce by the strong dipolar coupling. The width of the thermal hysteresis varies from 500 K to 100 K for lateral dimensions of 125 nm x 65 nm and 145 nm x 65 nm. We focus on the thermal effects on two particular states: the antiparallel state, which has, at low temperatures, the interface biased nanoelement with the magnetization aligned with the interface field and the second nanoelement aligned opposite to the interface field; and in the parallel state, which has both nanoelements with the magnetization aligned with the interface field at low temperatures. We show that the dipolar interaction leads to enhanced thermal stability of the antiparallel state, and reduces the thermal stability of the parallel state. These states are the key phases in the application of pairs of ferromagnetic nanoelements, separated by a thin insulating layer, for tunneling magnetic memory cells. We have found that for a pair of 125nm x 65nm nanoelements, separated by 1.1nm, and low temperature interface field strength of 5.88kOe, the low temperature state (T = 100K) consists of a pair of nearly parallel buckle-states. This low temperature phase is kept with minor changes up to T= 249 K when the magnetization is reduced to 50% of the low temperature value due to nucleation of a vortex centered around the middle of the free surface nanoelement. By further increasing the temperature, there is another small change in the magnetization due to vortex motion. Apart from minor changes in the vortex position, the high temperature vortex state remains stable, in the cooling branch, down to low temperatures. We note that wide loop thermal hysteresis may pose limits on the design of tunneling magnetic memory cells

SPH simulations of clumps formation by dissipative collisions of molecular clouds - II. Magnetic case

We performed computer simulations of interstellar cloud-cloud collisions using the three-dimensional smoothed particle magnetohydrodynamics method. In order to study the role of the magnetic field on the process of collision-triggered fragmentation, we focused our attention on head-on supersonic collisions between two identical spherical molecular-clouds. Two extreme configurations of the magnetic field were adopted: parallel and perpendicular to the initial clouds motion. The initial magnetic field strength was approximately 12.0 muG. In the parallel case, much more of the collision debris were retained in the shocking region than in the non-magnetic case where gas escaped freely throughout the symmetry plane. Differently from the non-magnetic case, eddy-like vortices were formed. The regions of highest vorticity and the the regions of highest density are offset. We found clumps formation only in the parallel case, however, they were larger, hotter and less dense than in the analogous non-magnetic case. In the perpendicular case, the compressed field works as a magnetic wall, preventing a stronger compression of the colliding clouds. This last effect inhibits direct contact of the two clouds. In both cases, we found that the field lines show a chaotic aspect in large scales. Also, the field magnitude is considerably amplified in the shock layer. However, the field distribution is almost coherent in the higher density regions.

Temperature dependence and magnetocrystalline anisotropy studies of self-assembled L1(0)-Fe55Pt45 ferromagnetic nanocrystals

Temperature dependence and uniaxial magnetocrystalline anisotropy properties of the chemically synthesized 4 nm L1(0)-Fe55Pt45 nanoparticle assembly by a modified polyol route are reported. As-prepared nanoparticles are superparamagnetic presenting fcc structure, and annealing at 550 degrees C converts the assembly into ferromagnetic nanocrystals with large coercivity (H-C>1 T) in an L1(0) phase. Magnetic measurements showed an increasing in the ferromagnetically ordered fraction of the nanoparticles with the annealing temperature increases, and the remanence ratio, S=M-R/M-S congruent to 0.76, suggests an (111) textured film. A monotonic increase of the blocking temperature T-B, the uniaxial magnetocrystalline anisotropy constant K-U, and the coercivity H-C with increasing annealing temperature was observed. Magnetic parameters indicate an enhancement in the magnetic properties due to the improved Fe55Pt45 phase stabilizing, and the room-temperature stability parameter of 67, which indicates that the magnetization should be stable for more than ten years, makes this material suitable for ultrahigh-density magnetic recording application.(c) 2007 American Institute of Physics.

ANISOTROPY OF ELECTRICAL-PROPERTIES IN ALPHA-FE2O3 CERAMICS

Electrical conductivity and thermoelectric power measurements carried out in a heamatite ceramic showed a strong anisotropy in directions normal and parallel to the uniaxial pressing direction. This behaviour is similar to that verified in alpha-Fe2O3 single crystal. The results suggest that the extended structural defects, generated during sintering, disturb the magnetic order on the (001) planes of alpha-Fe2O3 and limit the mobility of n type carriers.

Magnetic susceptibility of an exactly solvable anisotropic spin ladder system

We investigate the thermodynamics of an integrable spin ladder model which possesses a free parameter besides rung and leg couplings. The model is exactly solvable by means of the Bethe ansatz and exhibits a phase transition between a gapped and a gapless spin excitation spectrum. The magnetic susceptibility is obtained numerically and its dependence on the anisotropy parameter is determined. The spin gap obtained from the susceptibility curve and the one obtained from the Bethe ansatz equations are in very good agreement. Our results for the magnetic susceptibility fit well the experimental data for the organometallic compounds (5IAP)(2)CuBr4 . 2H(2)O (Landee C. P. et al., Phys. Rev. B, 63 (2001) 100402(R)) Cu-2(C5H12N2)(2)Cl-4 (Hayward C. A., Poilblanc D. and Levy L. P., Phys. Rev. B, 54 (1996) R12649, Chaboussant G. et al., Phys. Rev. Lett., 19 ( 1997) 925; Phys. Rev. B, 55 ( 1997) 3046.) and (C5H12N)(2)CuBr4 (Watson B. C. et al., Phys. Rev. Lett., 86 ( 2001) 5168) in the strong-coupling regime.

Magnetic field of an in-plane vortex inside and outside a layered superconducting film

In the present work we study an anisotropic layered superconducting film of finite thickness. The film surfaces are considered parallel to the be face of the crystal. The vortex lines are oriented perpendicular to the film surfaces and parallel to the superconducting planes. We calculate the local field and the London free energy for this geometry. Our calculation is a generalization of previous works where the sample is taken as a semi-infinite superconductor. As an application of this theory we investigate the flux spreading at the super conducting surface.

Fractal dimension and anisotropy of soil CO2 emission in an agricultural field during fallow

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Magnetic field dependence of the magnetic susceptibility and the specific heat of the doped plasticized polyaniline (PANI-DB3EPSA)0.5

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)