## 2 Experimental Section  
Polymeric gels filled with ferromagnetic nanoparticles were obtained by radical copolymerization of acrylamide (main monomer) and N,N'-methylenebisacrylamide (cross linking agent) in an aqueous dispersion of magnetite (Fe\({}_{3}\)O\({}_{4}\)) nanoparticles. The reaction medium contained 10 wt.% of acrylamide. The weight ratio of cross linking agent to main monomer was 0.02. To initiate copolymerization, hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) in combination with Fe\({}^{2+}\) ions was used. The surface of the magnetite particles was the source of Fe\({}^{2+}\) ions. The concentration of hydrogen peroxide was equal to 0.001 M. The copolymerization was performed at room temperature in air. Magnetite dispersion was obtained by coprecipitation of Fe\({}^{2+}\) and Fe\({}^{3+}\) salts in an alkaline medium (NH\({}_{4}\)OH, 10 wt.%). The process was carried out at reduced temperatures (\(\sim\)2 \({}^{\circ}\)C) in order to increase the nanoparticles size. The average diameter of nanoparticles in the magnetic dispersion was 30(1) nm as measured by quasielastic light scattering (Figure 1). The concentration of nanoparticles in the dispersion was 0.630(25) vol.%. The dispersion was stabilized by an electrical double layer in a phosphate-citrate buffer with pH 4.0. The magnetization of the magnetic liquid was 2.05(10) G.  
It decreased to 1.49(10) G after hydrogen peroxide addition. The reduction probably resulted from oxidation of the magnetite particles by hydrogen peroxide accompanied by the formation of non-ferromagnetic Fe\({}^{3+}\) oxide (\(\alpha\)-Fe\({}_{2}\)O\({}_{3}\)) on the particle surface. The solution of acrylamide and N,N'-methylenebisacrylamide was mixed with the magnetic liquid in the volume ratio of 1:1. Then hydrogen peroxide was added to the mixture. Hydrogel was formed within several minutes. Gel formation was carried out in a static magnetic field of 1.5 kG as well as without a magnetic field. Gels obtained in the presence of the field are referred to as \"_magnetoanisotropic gels_\" and those obtained without field as \"_magnetoisotropic ones_\". After preparation, the magnetic gels were placed in an aqueous solution of paramagnetic sensor. The water soluble stable nitroxide radical TEMPOL (2,2,6,6-tetramethyl-4-hydroxypiperidine-1-oxyl) was used as paramagnetic sensor. The radical concentration was 8.5 mM. The gels swelled in the radical solution, and radical molecules penetrated into them. In this process, the weight of the gels increased by a factor of 1.2 to 1.7. The concentration of nanoparticles in the gels after swelling was 0.19(2) and 0.27(3) vol.% for magnetoisotropic and magnetoanisotropic gels respectively. The EPR spectrum of the dilute solution of the nitroxide radical showed three lines of hyperfine structure (HFS) of equal intensity [11] that resulted from the interaction of unpaired electrons with the nuclear spin of nitrogen (\(I_{\mathrm{N}}=1\)). The experimental values of the hyperfine interactions and isotropic \(g\)-factor of the TEMPOL radical were \(A_{\mathrm{iso}}=16.9(1)\) G and \(g_{\mathrm{iso}}=2.00532(5)\), respectively. The intrinsic line shape of the radical (instrumental line shape) was presented by a convolution \(g_{\mathrm{o}}\big{(}\Delta\big{)}=\int dhg_{L}\big{(}\Delta-h\big{)}g_{G} \big{(}h\big{)}\), where \(g_{L}\big{(}\Delta\big{)}=\frac{1}{\pi}\frac{\Gamma_{L}}{\Delta^{2}+\Gamma_{ L}^{2}}\) and \(g_{G}\big{(}\Delta\big{)}=\big{(}2\pi\Gamma_{G}^{2}\big{)}^{1/2}\exp\!\left(- \frac{\Delta^{2}}{2\Gamma_{G}^{2}}\right)\) with \(\Gamma_{L}=1.021(45)\) G and \(\Gamma_{G}=0.714(28)\) G.  
Electron magnetic resonance (EPR and FMR) spectra of the magnetic gels were recorded at room temperature using the X-band spectrometer Bruker EMX-8/2.7 with microwave radiation power of 1 mW. The parameters of the magnetic field modulation applied in the experiments were 1 G in amplitude and 100 kHz in frequency. The first derivative of the adsorption signal was recorded.