Examination of the magnetic hyperthermia and other magnetic properties of CoFe2O4@MgFe2O4 nanoparticles using external field Mössbauer spectroscopy

CoFe2O4@MgFe2O4 core/shell nanoparticles were synthesized by high temperature thermal decomposition with seed-mediated growth. The crystal structure and magnetic properties of the nanoparticles were investigated using X-ray diffractometry (XRD), vibrating sample magnetometry (VSM), and Mossbauer spectrometry. The magnetic hyperthermia properties were investigated using a MagneTherm device. Analysis of the XRD patterns showed that CoFe2O4@MgFe2O4 had a cubic spinel crystal structure with space group Fd-3m and a lattice constant (a0) of 8.3686 A. The size and morphology of the CoFe2O4@MgFe2O4 nanoparticles were confirmed by HR-TEM. The VSM measurements showed that the saturation magnetization (MS) of CoFe2O4@MgFe2O4 was 77.9 emu/g. The self-heating temperature of CoFe2O4@MgFe2O4 was 37.8 °C at 112 kHz and 250 Oe. The CoFe2O4@MgFe2O4 core/shell nanoparticles showed the largest saturation magnetization value, while their magnetic hyperthermia properties were between those of the CoFe2O4 and MgFe2O4 nanopartic...


I. INTRODUCTION
Magnetic nanoparticles have been intensively investigated for use in bio-applications because of their unique properties, including their synthesis methods, structural characteristics, and remarkable magnetic properties. [1][2][3][4] For example, CoFe 2 O 4 nanoparticles have high saturation magnetization and are known to be hard ferrites. [5][6][7][8] On the other hand, MgFe 2 O 4 nanoparticles are close to being soft ferrites, [9][10][11] although there have been few reports on MgFe 2 O 4 nanoparticles to date. In order to reduce the large magnetic anisotropy of CoFe 2 O 4 nanoparticles, we coated them with a shell of MgFe 2 O 4 nanoparticles to form a core/shell structure. In this study, core/shell CoFe 2 O 4 @MgFe 2 O 4 nanoparticles were prepared for use in magnetic hyperthermia, an experimental cancer treatment. The bio-applicability of nanoparticles to magnetic hyperthermia treatment was tested by examining the heating effects of the nanoparticles in an AC magnetic field. 12 In addition, we show the relationship between the spin canting angle and the hyperthermia effect of CoFe 2 O 4 @MgFe 2 O 4 nanoparticles by measuring the external magnetic field using Mössbauer spectroscopy. core/shell nanoparticles were prepared by seed-mediated growth. Co(acac) 2 (1 mmol), Fe(acac) 3 (2 mmol), oleylamine (6 mmol), and oleic acid (6 mmol) were dissolved in 30 ml of benzyl ether. The solution was first heated at 200 • C for 30 min and heated up to 298 • C for 45 min. After the solution had cooled to room temperature, 40 ml of ethanol was added. The black precipitates were collected by centrifugation, washed several times with hexane and ethanol, and dried at 80 • C. A similar synthesis procedure was used to prepare the MgFe 2 O 4 nanoparticles.
The CoFe 2 O 4 core/MgFe 2 O 4 shell nanoparticles were made using a solution of as-prepared nanoparticles in 4 ml of hexane, which was mixed with of Mg(acac) 2 (1 mmol), Fe(acac) 3 (2 mmol), oleylamine (6 mmol), and oleic acid (6 mmol) in 30 ml of benzyl ether. The solution was heated first at 200 • C for 30 min and heated up to 298 • C for a further 45 min. The collection and cleaning of the CoFe 2 O 4 @MgFe 2 O 4 nanoparticles were performed as before.

B. Measurements
The crystal structure of the CoFe 2 O 4 @MgFe 2 O 4 nanoparticles was investigated using X-ray diffraction (XRD) with a monochromatic Cu-Kα radiation source (λ = 1.5406 Å). The XRD patterns of the fabricated nanoparticles were analyzed with Fullprof software. High-resolution transmission electron microscopy (HR-TEM) was used to confirm the size and structure of the CoFe 2 O 4 @MgFe 2 O 4 particles. The magnetic measurements were carried out using vibrating sample magnetometry (VSM). Hysteresis curves were recorded at 295 K under magnetic fields of up to ±1.5 T. Zero-field-cooled (ZFC) and field-cooled (FC) curves were performed at applied magnetic fields of 100 and 1000 Oe and at temperatures of 4.2-300 K. The magnetic hyperthermia properties of the nanoparticles were confirmed using a MagneTherm system at a frequency of 112 kHz and an alternating magnetic field strength of 250 Oe. Mössbauer spectra were obtained using the constant acceleration mode with a 57 Co γ-ray source, 13 at a temperature of 4.2 K and applied field strengths ranging from 0-4.5 T.

III. RESULTS AND DISCUSSION
The XRD patterns of CoFe 2 O 4 , MgFe 2 O 4 , and CoFe 2 O 4 @MgFe 2 O 4 nanoparticles were analyzed using the Rietveld refinement method with Fullprof software, as shown in Fig. 1. It can be seen from the patterns that spinel phases were formed in all samples, with the single phases of all three crystal structures being determined to be cubic spinel with the space group Fd-3m. The lattice constants (a 0 ) of the samples were calculated to be 8 was approximately 17 nm (±3 nm) in size, and the core/shell structure of this material was also confirmed by HR-TEM.
The temperature-dependent magnetization of the core/shell ferrite was measured at temperatures ranging from 4.2-300 K under applied fields of 100 and 1000 Oe, as shown in Fig. 2. The zero-field-cooled CoFe 2 O 4 @MgFe 2 O 4 showed a sharp decrease when a field of 1000 Oe was applied, whereas there was no significant change in the slope of the field-cooled nanoparticles, so this can be considered quite independent of the temperature. The magnetization of the CoFe 2 O 4 @MgFe 2 O 4 increased steadily as the temperature decreased during zero-field-cooling, reaching a maximum at a critical temperature defined as the blocking temperature, T B . 14    at room temperature in order to obtain insights into the chemical compositions and structures of the nanoparticles, and the results are shown in Fig. 5(a). The Mössbauer spectra were analyzed two sextets for tetrahedral and octahedral. The Mössbauer spectra of CoFe 2 O 4 and CoFe 2 O 4 @MgFe 2 O 4 were analyzed with a tetrahedral A-site sextet (blue line) and an octahedral B-site sextet (red line), as two hyperfine sextet patterns were obtained in these spectra. The MgFe 2 O 4 spectrum was analyzed with a tetrahedral A-site sextet (red line), an octahedral B-site sextet (blue line), one quadruplet (green line), and one amorphous (cyan line). CoFe 2 O 4 @MgFe 2 O 4 had the largest value of H hf , with 445 kOe being obtained for the A-site and 477 kOe for the B-site. Mössbauer spectra were also measured at 4.2 K with an external field of 0-4.5T, as shown in Fig. 5(b). When the external field was 0 T, two overlapping sextets were observed. As the external field was increased, the absorption lines of the two sextets could be distinguished. Because the magnetic spin direction is parallel to the gamma rays due to the external magnetic field, the average canting angle 15,16 can be calculated using equation (1): where A 2,5 represents the area ratio of the 2 nd or 5 th absorption line and A 1,6 represents the area ratio of the 1 st or 6 th absorption lines. Thus, a decrease of the second/fifth absorption line indicates a decrease in the canting angle. The canting angle gradually decreased from 56.96 • to 17.04 • as the external magnetic field was increased from 0 to 4.5 T. The dependence of the canting angle on the magnetic field indicates that the sample had soft-magnetic properties. Furthermore, these characteristics can be confirmed through the hyperthermia behavior of the material.

IV. CONCLUSION
We have investigated CoFe 2 O 4 @MgFe 2 O 4 nanoparticles using XRD, TEM, VSM, and Mössbauer spectrometry, while its applicability to magnetic hyperthermia was also examined. According to XRD analysis with Rietveld refinement, the nanoparticles had a single phase and a cubic spinel structure with the space group Fd-3m. In the temperature-dependent magnetization curves, blocking temperature transitions were observed at 300 and 275 K at magnetic fields of 100 and 1000 Oe, respectively. The self-heating magnetic hyperthermia properties of CoFe 2 O 4 @MgFe 2 O 4 were determined to be intermediate in value, with those of MgFe 2 O 4 and CoFe 2 O 4 being lower and higher, respectively. The Mössbauer spectra at were observed room temperature, and it was found that CoFe 2 O 4 @MgFe 2 O 4 had the highest H hf . Mössbauer spectra were also measured at applied magnetic fields of 0-4.5 T at 4.2 K to allow the canting angle to be observed. It was confirmed that the canting angle decreased as the magnetic field increased. The decrease in canting angle indicates that the magnetic spin maintained weak directionality, which can be considered to show soft-magnetic properties, as confirmed by the magnetic hyperthermia experiments. The core/shell nanoparticle structure investigated in this study has high saturation magnetization and moderate thermal properties. Therefore, these nanoparticles are of sufficient value for studying magnetic hyperthermia.