## 3 Results and Discussion  
As noted above, the paramagnetic sensor technique was previously tested on magnetic hydrosols (inviscid dispersions of magnetic particles). Aggregates of magnetic particles, formed in the liquids in a magnetic field, fell apart immediately after the field was switched off. In the magnetoanisotropic gel, the aggregates formed in course of the gel synthesis (when the particles' mobility was rather high) should be fixed by a polymer network. They should not disintegrate even after removing the gel out of the magnetic field. In the magnetoisotropic gel, individual magnetic particles immobilized by polymer network seem to be unable to form long aggregates under magnetic field influence. These assumptions are qualitatively supported by FMR data (Figure 2). It is known that aggregation of the particles leads to FMR spectra transformation [12]. When linear aggregates are formed and aligned along magnetic fields of the spectrometer their FMR spectrum is shifted significantly to low fields due to dipole-dipole interaction between the aggregated particles [12]. In our experiment, the main axes of the aggregates in the magnetoanisotropic gel were orientated along and across the spectrometer field (due to corresponding orientation of the gel sample). In case of parallel orientation of the aggregates, the FMR spectrum of the magnetoanisotropic gel (as well as of the magnetic liquids) should be shifted to lower fields relative to the spectrum of the magnetoisotropic gel. While in case of transversal orientation of the aggregates, the FMR spectrum should shift to higher magnetic fields. It can be seen from Figure 2 that the expectation is justified. The FMR spectrum of the magnetoisotropic gel (curve 1) is observed in stronger fields than the spectra of magnetoanisotropic gel (curve 2) and magnetic liquid (curve 4). The rotation of the magnetoanisotropic gel sample by 90\({}^{\circ}\) in the spectrometer field, results in a FMR spectrum shift to higher fields (curve 3). Thus the aggregates formed in magnetoanisotropic gel rotate with the sample and are not influenced by the magnetic field of spectrometer. This confirms the strong aggregate fixation in the gel matrix. The FMR spectrum of  
Figure 2: Ferromagnetic resonance (FMR) spectra of magnetite nanoparticles in polyacrylamide gel (1, 2, 3) and in water (4). 1—gel synthesized without magnetic field; 2—gel synthesized in a magnetic field of \(\sim\)1.5 kG and orientated by its magnetic axis along the spectrometer field; 3—gel synthesized in a magnetic field of \(\sim\)1.5 kG and orientated by its magnetic axis across the spectrometer field. the magnetic liquid has a maximal shift to low fields mainly because of average magnetization of the nanoparticles in the magnetic liquid. It is higher than the particles magnetization in the magnetic gels (the magnetization decreases during polymerization, see Section 2). To get quantitative information on the fraction of aggregated particles and aggregate's aspect ratio, the paramagnetic sensor technique is used. As it was shown earlier [9], the EPR line of the sensor in magnetically diluted medium is inhomogeneously broadened by the local magnetic fields of the nanoparticles and their aggregates, and the line shape is defined by the local field configuration. If point-like or spherical dipoles act as the sources of the local fields, the paramagnetic sensor line should be described by [PERSON] with the line width  
\[D_{\mathrm{A}}=\frac{8\pi^{2}}{9\sqrt{3}}M_{\mathrm{G}} \tag{1}\]  
where \(M_{\mathrm{G}}\) is the gel magnetization. Such a line is called \"Anderson line\" [13, 14, 15]. Here and below full polarization of the nanoparticles is assumed. The above description is adequate when the gyromagnetic ratio of a sensor and of the absolute majority of particles is sufficiently different to omit flip-flop terms of their interaction.