Magnetic nanoparticles and superparamagnetic resonance (2) "Structure and electronic states of magnetic nanoparticles"

When we assume the particle as a sphere, a parameter S_s, which is the ratio of the surface area (S=4𝜋𝑟²) to its volume , is expressed as

Surface area is in inverse proportion to the particle radius. S_s is the specific surface area, and 𝑟 is the radius of the particle. Figure 1 shows the relation between sphere's diameter and calculated specific surface area according to eq. (1). This plot lets us know that the surface area of the mesoscopic matter is to be very large.
This unique largeness of the specific surface area significantly affects the physical properties of nano-ordered particle materials. This is because the surface poses discontinuous interface, and can generate different electron states from uniform and periodic crystal in the inner region of the particle.

Fig. 3 SPR spectra of Fe₃O₄ magnetic nanoparticles dispersion in toluene (0.625 mg / mL), images obtained using TEM (JEM-2100Plus, The accelerating voltage is 200 kV) and size distribution of the particles, (a) with diameter of 5 nm, (b) 10 nm, and (c) 20 nm.

Electron spins located in the different environments have different energy level spacing and intrinsic interaction. Therefore, each relaxation path or velocity by microwave excitation is not the same, the respective spin has a unique relaxation time (spin-lattice relaxation : T₁, spin-spin relaxation : T₂). Since the spin relaxation time of the usual ferromagnets is very short, direct observation of relaxation time is very difficult. However, measurements of microwave power dependence (saturation property) make it possible to distinguish different spins.
Spectra shown in Fig. 4 are irradiated power variations of superparamagnetic resonance by MNPs (Fe₃O₄). It can be seen that every spectrum consists of two components, which are broad one and narrow (g ~ 2) one. As the diameters of the particles are getting smaller, the narrow components ratio is increasing. The narrow components are easy to saturate, but its g-value and line width are constant. On the contrary, the broad components do not saturate even over 160 mW, and its line widths increase with power levels. These spectral properties imply two distinct different spins in a particle, and it leads to support the core-shell structure model of the MNPs^[2].

Fig. 4 Microwave power dependence of SPR spectra of Fe₃O₄ magnetic nanoparticles dispersion in toluene (0.625 mg / mL) and electron diffraction, with a diameter of (a) 5 nm, (b) 10 nm, and (c) 20 nm.

* Electron diffraction patterns were obtained using JEM-2100Plus (the accelerating voltage is 200 kV). Three diffraction patterns of the magnetic nanoparticles as shown in (a) – (c) were coincident with the known diffraction pattern of Fe₃O₄.

Characterization magnetic nanoparticles by multi-frequency FMR/SPR spectra

Fig. 5 ESR spectra of magnetite (Fe₃O₄) particles with different diameters using multi-frequency.

(a) X-band FMR spectrum of Fe₃O₄ powder with diameter of 50 – 100 nm. (b) X-band SPR spectra of Fe₃O₄ magnetic nanoparticles dispersion in toluene (0.625 mg / mL). (c) Q-band spectrum of (a). (d) Q-band spectra of (b). Q-band spectra were obtained using ES-SQ5 (as shown above picture).

We show multi-frequency ESR spectra of MNPs (Fe₃O₄) in Fig.5. These were obtained with X-band (9.4 GHz) and Q-band (35 GHz) spectrometers. Microwaves can not penetrate into the conductive materials over the skin depth, which is dependent on the frequency. The shorter the skin depth is, the more distorted the ESR spectral pattern in Dysonian. As shown in Figs.5 (a) and (c), a Q-band spectrum is more distorted Dysonian pattern than an X-band one. That is, magnetite with this diameter’s range has the electrical conductivity like a bulk crystal as well as the ferrimagnetism. In the case of MNPs, as shown in Figs.5 (b) and (d), the characteristic narrow component ratio of MNPs decreases drastically in Q-band spectra. Similar spectral characters of maghemite　(γ-Fe₂O₃) MNPs, which is iron oxide consisted of different composition, are reported in the previous literature^[4], too. The exciting energy (hv) of Q-band microwave is 4 times larger than that of X-band. Therefore, it is interpreted that a unique superparamagnetism might weaken due to a decrease of thermal fluctuation^[4].