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1.
J Phys Condens Matter ; 31(37): 375801, 2019 Sep 18.
Artículo en Inglés | MEDLINE | ID: mdl-31163402

RESUMEN

Different processes governing magnetic properties of an ensemble of magnetic nanoparticles in the temperature region close to a transition from superparamagnetic to paramagnetic state are analyzed and the ways to separate them are suggested. Enhanced role of paraprocess in magnetization behavior near Curie temperature is stressed. A procedure to isolate paraprocess contribution and adequately determine spontaneous magnetization of the ensemble of superparamagnetic nanoparticles is proposed. Critical behavior of the spontaneous magnetization is experimentally determined for the ensemble of nanoparticles of lanthanum-strontium manganites, which are considered as promising materials for self-controlled magnetic nanohyperthermia. Effect of dispersion of magnetic parameters on effective magnetic characteristics of nanoparticles and their critical behavior is discussed. Theoretical background for the use of the 'effective Curie temperature for the ensemble of nanoparticles' concept is proposed for ensembles of particles with dispersion of their Curie temperature. Based on the results obtained, various strategies to develop novel biomedical applications, in particular those suitable for noninvasive temperature monitoring, are discussed.

2.
Phys Chem Chem Phys ; 19(39): 27015-27024, 2017 Oct 11.
Artículo en Inglés | MEDLINE | ID: mdl-28956568

RESUMEN

Magnetic nanoparticles constitute promising tools for addressing medical and health-related issues based on the possibility to obtain various kinds of responses triggered by safe remote stimuli. However, such richness can be detrimental if different performances are not adequately differentiated and controlled. The aim of this work is to understand and systemize different kinds of magnetic-field-induced response for an ensemble of lanthanum-strontium manganite nanoparticles, which are considered as promising materials for self-controlled magnetic hyperthermia. A complex set of static and dynamic magnetic measurements accompanied by a numerical simulation of DC and AC magnetic behavior has been carried out. It is shown that to achieve adequate results, the dispersion of particle sizes and/or magnetic parameters should necessarily be taken into account. A quantitative description of the magnetic behavior of the ensemble should comprise two groups of nanoparticles differentiated according to the regime of their magnetization reversal: one group, which demonstrates non-hysteretic behavior similar to a superparamagnet and another one, which shows magnetic hysteresis characteristic of blocked particles. The fraction of nanoparticles in each group depends not only on the nanoparticles' parameters (in particular, their size), but also on the parameters of the external AC magnetic field (amplitude and frequency) used for remagnetization. The main outcome of this work is the development of a procedure which allows one to separately analyze contributions from different groups of nanoparticles and find the regularities of the redistribution of nanoparticles between these groups on changing the parameters of the external AC magnetic field. The results show the directions to enhance the heating efficiency of ensembles of magnetic nanoparticles and pave the way for further optimization of their characteristics and the parameters of the external field.

3.
J Phys Condens Matter ; 27(44): 446003, 2015 Nov 11.
Artículo en Inglés | MEDLINE | ID: mdl-26471166

RESUMEN

Ferromagnetic resonance properties of F1/f/F2/AF multilayers, where weakly ferromagnetic spacer f is sandwiched between strongly ferromagnetic layers F1 and F2, with F1 being magnetically soft and F2-magnetically hard due to exchange pinning to antiferromagnetic layer AF, are investigated. Spacer-mediated exchange coupling is shown to strongly affect the resonance fields of both F1 and F2 layers. Our theoretical calculations as well as measurements show that the key magnetic parameters of the spacer, which govern the ferromagnetic resonance in F1/f/F2/AF, are the magnetic exchange length (Λ), effective saturation magnetization at T = 0 (m0) and effective Curie temperature (T(C)(eff)). The values of these key parameters are deduced from the experimental data for multilayers with f = Ni(x)Cu(100-x), for the key ranges in the Ni-concentration (x = 54 ÷ 70 at. %) and spacer thickness (d = 3 ÷ 6 nm). The results obtained provide a deeper insight into thermally-controlled spin precession and switching in magnetic nanostructures, with potential applications in spin-based oscillators and memory devices.

4.
Phys Chem Chem Phys ; 17(27): 18087-97, 2015 Jul 21.
Artículo en Inglés | MEDLINE | ID: mdl-26100102

RESUMEN

The ability to controllably tune the heating efficiency of magnetic nanoparticles in an AC magnetic field is highly desirable for their application as mediators of magnetic hyperthermia. Traditional approaches to understand and govern the behavior of hyperthermia mediators include a combination of quasistatic and high-frequency (∼100 kHz) magnetic measurements with subsequent simulation of underlying processes. In this paper, we draw attention to the frequently overlooked fact that for an ensemble of magnetic nanoparticles, there is no straightforward complementarity between the dynamic characteristics obtained under different experimental conditions, as well as between corresponding underlying processes. This paper analyzes mechanisms of AC losses in a fluid based on magnetic nanoparticles, with special emphasis on the domains of their validity, and shows that the mechanisms may become qualitatively different as experimental conditions change from magnetostatic to high-frequency ones. Further, the work highlights new important features which can result from the employment of the refined approaches to interpret experimental results obtained on magnetic fluids based on La1-xSrxMnO3 (x = 0.22) nanoparticles. The gained knowledge provides necessary guidelines for tailoring the properties of magnetic nanoparticles to the needs of self-controlled magnetic hyperthermia.


Asunto(s)
Compuestos de Manganeso/química , Campos Magnéticos , Modelos Teóricos , Nanopartículas/química , Nanopartículas/ultraestructura , Difracción de Rayos X
5.
Nat Mater ; 12(1): 52-8, 2013 Jan.
Artículo en Inglés | MEDLINE | ID: mdl-23104152

RESUMEN

Large thermal changes driven by a magnetic field have been proposed for environmentally friendly energy-efficient refrigeration, but only a few materials that suffer hysteresis show these giant magnetocaloric effects. Here we create giant and reversible extrinsic magnetocaloric effects in epitaxial films of the ferromagnetic manganite La(0.7)Ca(0.3)MnO(3) using strain-mediated feedback from BaTiO(3) substrates near a first-order structural phase transition. Our findings should inspire the discovery of giant magnetocaloric effects in a wide range of magnetic materials, and the parallel development of nanostructured bulk samples for practical applications.

6.
J Phys Condens Matter ; 21(38): 386003, 2009 Sep 23.
Artículo en Inglés | MEDLINE | ID: mdl-21832381

RESUMEN

Electron spin resonance spectra have been studied in partially crystallized films of La(1-x)Na(x)MnO(3) (x = 0.16) in the vicinity of the para- to ferromagnetic transition. The objects of investigation were obtained by magnetron sputtering on polycrystalline Al(2)O(3) substrates held at different temperatures. It is shown that, in a regime where the paramagnetic and ferromagnetic phases coexist, the resonance conditions for one phase strongly depend on the parameters of the other phase. As a result, the resonance field of the paramagnetic phase becomes dependent on the shape of the sample, the saturation magnetization and the fraction of the ferromagnetic phase. A simple model is developed to predict the character of the changes in the resonance field of the paramagnetic phase upon the nucleation of the ferromagnetic phase.

7.
J Phys Condens Matter ; 19(24): 246212, 2007 Jun 20.
Artículo en Inglés | MEDLINE | ID: mdl-21694055

RESUMEN

In this work, we carry out the analysis of the resonance absorption of electromagnetic radiation for the system in which para- and ferromagnetic phases coexist over a wide temperature region. It is found that taking account of the mutual influence of coexisting phases gives rise to the appearance of substantial changes in the curves of resonance absorption and values of resonance fields, as well as to making the geometry of a phase distribution dependent on an external magnetic field. Near the temperature boundaries of the phase coexistence region, the expressions for description of the curves of the dispersive absorption of electromagnetic radiation are obtained and the rules of the behaviour of the resonance fields for each of the phases are specified. As follows from the calculations, the resonance field for the paramagnetic phase becomes dependent on the shape of the sample, the saturation magnetization and the fraction of ferromagnetic phase. It is shown that the character of magnetic resonance spectra and the features of their temperature change agree well with the experimental data, obtained by various groups of researchers on the single crystalline and polycrystalline samples of doped perovskite manganites.

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