Polarized SANS Study of The magnetic iron nanoparticles in mesoporous silica matrix

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The ordered magnetic iron-containing nanowires were sythesized in the mesoporous silica matrix SiO2. The method is based on the introduction of a hydrophobic metal compound, into the hydrophobic part of the as-prepared mesoporous silica-surfactant composite. In order to perform inroduction of iron into the silica matrix we have chosen iron pentacarbonyl because this non-polar molecule is expected to dissolve very well in the hydrophobic part of the SiO2/surfactant micelles and it is easily decomposed to elemental iron by UV-irradiation in vacuum. To provide better crystallinity of iron nanowires we performed additional annealing in a hydrogen flow. The samples were denoted as FeSiO2-T with the annealing temperature T = 260, 300, 350, 375, 400 °C .

Scheme of the neutron scattering experiment. The ring of neutron intensity corresponds to the scattering on the regular two-dimensional structure of nanopores. The contribution to the scattering depends strongly on nanoparticle orientation.

Obviously the nanoparticles oriented parallel to the neutron beam, make the major contribution to the diffraction ring with six reflexes, the particles oriented perpendicularly to the beam give two low-intensity reflexes, while nearly no contribution should be observed for other particles due to the absence of well-defined periodicity for this orientations.  For SAPNS, in general, the cross-section of polarized neutrons consists of nuclear (N) and magnetic (M) contributions, as well as a nuclear-magnetic (NM) interference scattering

where FN and FM are the nuclear and magnetic form factors describing diffuse small angle scattering, and ФN d(q-qB) and ФM d(q-qB) are those describing Bragg reflection.

We determine:

Polarized SANS resultsv

Q-dependence of SANS intensity for Fe-SiO2-375 sample demonstrates a presence of the diffraction peak at qB » 1.65 ± 0.002 nm-1 (Fig.3) . This corresponds to a regular structure with periodicity a0 » 4.6 ± 0.1 nm. The field induced scattering IH (q) = I(q,H) - I(q,0) is shown in Fig. 4 at T = 300K and 10K. This scattering is attributed to a formation of the magnetic structure composed of the iron magnetic nanowires. The scattering intensity is summarized over the Bragg peak and plotted as a function of the field. The measurements reveal a large hysteresis in field dependent part of the cross-section IH(q).

As the magnetic field (perpendicular to the neutron beam and to the nanowires) increases from 0 to 300 mT, the intensity of the Bragg reflection increases slightly. This is attributed to the rotation of the magnetization inside nanowires from the direction along the wire to the direction perpendicular to it. As a result, at the magnetic field 292 mT we get the ensemble of nanowires with the magnetic moments oriented perpendicular to the anisotropy axis. A decrease of the magnetic field leads to the coherent rotation of the magnetic moments of the whole ensemble toward the anisotropy axis (parallel to the beam), and therefore, to the further increase of the Bragg reflection. This coherency is decreased at the magnetic field smaller than 150¸100 mT by thermal fluctuations and dipole interactions. The magnetization of nanowires becomes randomly oriented again and the intensity of the Bragg reflection decreases.

The temperature dependences of the scattering intensity DIint integrated over the range 0.2nm-1 < q < 1nm-1 was measured for Field Heating after Zero Field Cooling (FH after ZFC) and for Field Cooling (FC) regimes at H = 300 Oe. The temperature hysteresis is observed in the range from 8 to 100 K. The observed hysteresis at low temperatures is interpreted as a formation of the complicated magnetic structure composed of the magnetic nanowires. The temperature dependence of the interference scattering demonstrate the transition from the superparamagnetic to the dipole glass states at T ~ 100 K caused by an interplay/competition of the magnetic field, dipole-dipole and thermal interactions.  SANS data are in a good agreement with the SQUID measurements.

The temperature dependent part of the integral cross section over the Bragg peak position is shown in Fig.6 for Fe-SiO2-375 as an example. The sample was cooled down in the field of H=292mT. The figure demonstrates an increase of the cross-section with saturation at lowering temperature.

The polarized SANS technique in external magnetic field was applied for characterization of structure and magnetic properties of ordered arrays of magnetic iron-containing  nanowires in SiO2 matrix. The study has shown presence of the regular ferromagnetic structure with the period, which coincides with that of the matrix. The magnetic field applied perpendicular to the long axis of the wires leads to the coherent rotation of the magnetization in the whole ensemble of the nanowires. The transition to the dipole glass state is observed upon cooling at T = 100 K caused by an interplay/competition of the magnetic field, dipole-dipole and thermal interactions.