| 1. |
- Pogorily, A. N., et al.
(författare)
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Magnetic anisotropy of epitaxial Co2Fe-Ge Heusler alloy films on MgO (100) substrates
- 2017
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Ingår i: AIP Advances. - : American Institute of Physics Inc.. - 2158-3226. ; 7:5
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Tidskriftsartikel (refereegranskat)abstract
- Films of Co2Fe-Ge Heusler alloy with variable Ge concentration deposited on monocrystalline MgO (100) substrates by magnetron co-sputtering are investigated using microstructural, morphological, magnetometric, and magnetic resonance methods. The films were found to grow epitaxially, with island-like or continuous-layer morphology depending the Ge-content. The ferromagnetic resonance data versus out-of-plane and in-plane angle indicate the presence of easy plane and 4-fold in-plane anisotropy. The magnetometry data indicate additional weak 2-fold in-plane anisotropy and pronounced at low fields rotatable anisotropy. The observed magnetic anisotropy properties discussed in correlation with the microstructure and morphology of the films.
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| 2. |
- Timopheev, A. A., et al.
(författare)
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The influence of intergranular interaction on the magnetization of the ensemble of oriented Stoner-Wohlfarth nanoparticles
- 2009
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 105:8
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Tidskriftsartikel (refereegranskat)abstract
- The influence of interparticle interaction on the processes of magnetization reversal is considered for an ensemble of oriented Stoner-Wohlfarth nanoparticles. This is done through a solution of a kinetic equation describing the relaxation of the total magnetization to its equilibrium value in an effective mean field which includes a term proportional to the instantaneous value of the magnetization. It is shown that the interparticle interaction influences the temperature dependence of a coercive field. Under certain conditions, the presence of the interparticle interaction can lead to the formation of the so-called superferromagnetic state with the correlated directions of the magnetic moments of the particles. If the system is unable to come to the equilibrium during the time interval necessary to perform measurements, some measured quantities become dependent on the measurement time. It is shown that the blocking temperature T-b and the temperature dependence of coercive field at T < T-b are strongly dependent on a measurement time. At T>T-b, however, the coercivity, if exists, does not depend on the measurement time. The data of magnetostatic measurements, carried out on the (CoFeB)(x)-(SiO2)(1-x) nanogranular films with the concentration of ferromagnetic particles slightly lower than a percolation threshold, are in compliance with the results of the calculations.
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| 3. |
- Ryabchenko, S. M., et al.
(författare)
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Rotatable magnetic anisotropy in Si/SiO2/(Co2Fe)(x)Ge1-x Heusler alloy films
- 2013
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 25:41, s. 416003-
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Tidskriftsartikel (refereegranskat)abstract
- Polycrystalline (Co2Fe)(x)Ge1-x Heusler alloy films are fabricated by sputtering on amorphous substrates and shown to possess three types of magnetic anisotropy. The nearly stoichiometric composition of x = 50 m.f.% shows a rectangular hysteresis loop and isotropic coercive and ferromagnetic resonance fields when the film is field-magnetized along any in-plane direction, thus predominantly possessing rotatable in-plane magnetic anisotropy. Higher-x compositions show evidence of two-and fourfold in-plane anisotropy superposed on the rotatable one. A qualitative model of the observed anisotropic magnetic properties is proposed. The model explains the rotatable anisotropy by taking into account dry friction for the in-plane rotation of the magnetization direction in a fine-grained polycrystalline film with the magnetic grain size smaller than the correlation length of the inter-grain exchange interaction. The observed two-and fourfold magnetic anisotropy contributions are attributed to partial texturing of the fine-grained films, even though the films are grown on amorphous SiO2 substrates. These results should be valuable for understanding and controlling the magnetic behaviour of highly spin-polarized Heusler alloy films used in various magnetic nanodevices.
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| 4. |
- Kulyk, M. M., et al.
(författare)
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Magnetic properties and anisotropic coercivity in nanogranular films of Co/Al2O3 above the percolation limit
- 2014
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Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 47:34, s. 345002-
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic properties of nanogranular ferromagnetic Co/Al2O3 films with 74.5 at% Co, which is above the percolation limit, are investigated. It is established that the films have perpendicular magnetic anisotropy and a weaker in-plane anisotropy. The magnetization curves show that the film consists of two magnetic components: a dominating contribution from magneto-anisotropic isolated grains with the anisotropy axis perpendicular to the film plane and a weaker contribution from the percolated part of the film. This two-component magnetic composition of the films, with the dominating contribution from the nanograins, is confirmed by transmission electron microscopy as well as by ferromagnetic resonance spectroscopy. It is further established that the coercive field of the film is almost entirely determined by the percolated part of the film. In this, the angular dependence of the coercive force, H-c(theta(H)), is essentially proportional to sin(-1)theta(H), where theta(H) is the angle between the applied field and the film's normal. However, for theta(H) -> 0, H-c(theta(H)) there is a narrow minimum with H-c approaching zero. Such non-linear dependence agrees well with our modelling results for a two-component magnetic system of the film, where the non-percolated nanograins have a distinct perpendicular anisotropy. The reported results should be important for in-depth characterization and understanding the magnetism and anisotropy in inhomogeneous systems as well as for applications, specifically in perpendicular magnetic recording.
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| 5. |
- Timopheev, A. A., et al.
(författare)
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Effect of interaction in the magnetization reversal relaxation of superparamagnetic granulaR CoFeB -SiO2 FILMS
- 2009
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Ingår i: Magnetism and Magnetic Materials. - : Scientific.Net. ; , s. 213-216
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Konferensbidrag (refereegranskat)abstract
- In order to study the magnetic interactions influence on the relaxation processes in superparamagnetic media, the magnetostatic measurements were carried out for the granular (CoFeB)x-(SiO2)1-x films with x values near percolation threshold. The observed temperature dependences of the coercivity for the samples magnetized along in-plane easy axis have shown two parts linear on √T with different inclination angles, which cross each other at the blocking temperature (Tb). The first part of the curve, which corresponds to the temperature range below T b, depends on the measuring time and is related to the thermal activation nature of magnetization reversal process in the blocked superparamagnetic state. Second part of the curve, which corresponds to the temperature range above Tb, is practically independent on the measuring time and is related to existence of correlated state of superparamagnetic grains moments, i.e. to superferromagnetic state. The method of the magnetization reversal curves simulation based on the numerical solving of kinetic equation has been applied to explain the experimental results.
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| 6. |
- Isaev, A. V., et al.
(författare)
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Model estimates of the impact of bioirrigation activity of Marenzelleria spp. on the Gulf of Finland ecosystem in a changing climate
- 2017
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Ingår i: Journal of Marine Systems. - : Elsevier BV. - 0924-7963 .- 1879-1573. ; 171, s. 81-88
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Tidskriftsartikel (refereegranskat)abstract
- Drastic changes have occurred in the Eastern Gulf of Finland ecosystem after recent invasion and establishment of polychaete Marenzelleria spp. Possible mechanisms of these changes are explored with the help of three-dimensional ecosystem model SPBEM. Relative significance of bioirrigation activity is studied by comparison of two climate change scenario simulations, which include or disregard Marenzelleria effects. The novel results obtained with this approach demonstrate that on a system level biogeochemical consequences of both implemented climate changes scenario and polychaete activity are equivalent to a weakening of vicious circle of the Baltic Sea eutrophication. The eutrophication-mitigating effects of the Marenzelleria invasion into the Eastern Gulf of Finland, revealed by the long-term field measurements, are explained by simulation-based considerations.
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| 7. |
- Ryabchenko, V. A., et al.
(författare)
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Model estimates of the eutrophication of the Baltic Sea in the contemporary and future climate
- 2016
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Ingår i: Oceanology. - 0001-4370 .- 1531-8508. ; 56:1, s. 36-45
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Tidskriftsartikel (refereegranskat)abstract
- The St. Petersburg Baltic eutrophication model (SPBEM) is used to assess the ecological condition of the sea under possible changes in climate and nutrient loads in the 21st century. According to model estimates, in the future climate water quality will worsen, compared to modern conditions. This deterioration is stronger in the climate warming scenario with a stronger change in future near-surface air temperature. In the considered scenarios of climate change, climate warming will lead to an increase in the area of anoxic and hypoxic zones. Reduction of nutrient loading, estimated in accordance with the Baltic Sea Action Plan, will only be able to partially compensate for the negative effects of global warming.
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| 8. |
- Kulyk, Mykola, et al.
(författare)
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Magnetotransport properties of nanogranular composites with low-field positive magnetoresistance
- 2020
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Ingår i: Low temperature physics (Woodbury, N.Y., Print). - : American Institute of Physics (AIP). - 1063-777X .- 1090-6517. ; 46:8, s. 792-797
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Tidskriftsartikel (refereegranskat)abstract
- A low-temperature feature is detected in nanogranular magnetic films with perpendicular anisotropy that have a low-field positive magnetoresistance: positive magnetoresistance is observed during the initial magnetization of the demagnetized sample, but is absent when the magnetic field is removed and/or during subsequent magnetizations. This effect is studied using a Co-x(Al2On)(1-x)film withx = 0.60, consisting of Co metal nanogranules in an Al(2)O(n)insulating matrix withnclose to 3. It is found that together with perpendicular anisotropy, the film has local in-plane anisotropy with randomly oriented easy axes, which prevents the reorientation of the granules' magnetic moment projections in the film plane below a certain blocking temperature. The restoration of the local short-range order in the orientation of the magnetic moments of the demagnetized film's neighboring granules, which is optimal for conductivity and is destroyed by the introduction of a magnetic field, is prevented at temperatures below the blocking temperature and is not restored during subsequent cycles of introducing/removing the magnetic field without heating to temperatures above the blocking temperature.
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| 9. |
- Vladimirova, Oksana M., et al.
(författare)
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Modelling dissolved organic nutrients in the Gulf of Finland
- 2018
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Ingår i: Fundamental and Applied Hydrophysics. - 2073-6673. ; 11:4, s. 90-101
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Tidskriftsartikel (refereegranskat)abstract
- St.-Petersburg model of eutrophication (SPBEM) has been modified for improving description of organic matter as apart of the nutrient biogeochemical cycles. The dynamics of labile and refractory fractions of dissolved organic nitrogenand phosphorus are now described with four additional equations. The modification was tested at the Gulf of Finland in anumerical experiment made with plausible initial and actual boundary conditions for the years 2009—2014. Comparisonof simulation with the available field observations indicates quite reasonable reproducibility of seasonal and inter-annualvariations of spatial distribution of hydrophysical and biogeochemical characteristics, including almost a perfect matchbetween simulated and observed dynamic of organic nutrients. The most important distinction from natural prototypesis the overestimated total amounts of inorganic nitrogen and phosphorus, which can be caused by the deficiencies inthe prescription of initial and boundary conditions as well as in the current parameterizations of pathways and rates ofmineralization fluxes. The finer tuning of SPBEM-2 requires more extensive sensitivity analysis.
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