Study on the mechanism of tunable ferromagnetic composites with different rare earth ions

Size-controlled Fe3O4 nanoparticles doped with rare earth (RE) ions (La3+, Ce3+, and Dy3+) varying from 15 nm to 30 nm were successful synthesized by a hydrothermal method for potential applications in the fields of biomedicine, environmental protection and magnetic memory devices. They possessed good dispersibility, adjustable particle size and nearly spherical shape. The particle grain size was uniformly distributed and showed a low degree of agglomeration in comparison with undoped Fe3O4 nanoparticles. The FTIR results showed that the RE elements partially replaced Fe2+, occupied the octahedral position, and enhanced the vibration of the Fe–O bond. The XPS study further revealed that the valence states of La, Ce, and Dy are both positive trivalent. The XPS Fe 2p valence band spectra observed a shift in the peak position toward higher binding energy after RE doping, confirming the existence of RE ions in the octahedral position. This paper explains the mechanism of rare earth doping with Fe3O4, and clarifies the influence of the doping of different RE ions on its magnetic properties. The detailed analysis of RE-doped ferrite materials can open a new perspective in designing biomedical and spintronics materials with tailored properties by choosing suitable cation substitution.


Introduction
Superparamagnetic Fe 3 O 4 nanoparticles have attracted much attention for their potentials application in biomedicine, nanomedicine, environmental science and chemical production because of their chemical stability, innocuousness, high saturation magnetization (M s ), and inexpensiveness. 1-4 The magnetic properties of nanoparticles are strongly inuenced by their size distribution, chemical composition and magnetic interaction. It is worth noting that on the nanometer scale, due to the surface spin effect, the decrease of particle size usually means the reduction of saturation magnetization. In this case, Fe 3 O 4 nanoparticle clusters represent a promising system with high M s , size control and magnetism control by rare earth (RE) doping. [5][6][7] RE elements have a special electronic structure with 4f orbitals buried inside the atom and shielded by 5s and 5p electrons, making the 4f electrons in the inner layer unaffected by the surrounding environment. The orbital contribution to the magnetic moment is not suppressed by the electrostatic eld of other surrounding ligand atoms. Therefore, when calculating magnetic moment, we should consider the contribution of both spin and orbit. RE magnetism depends on the number of unpaired 4f electrons, and hence, their optical and electronic phenomena have been applied to introduce magnetic eld effects. 8,9 The 4f orbital of RE ions are in an unlled state, which can be lled with electrons, offering broad research in magnetic and optical elds. 10,11 By reducing the size of materials to the nanoscale, it can help promote many special properties of the materials, due to the small size effect, quantum size effect, surface effect and macroscopic quantum tunnel effect. When the size of the magnetic material reaches about 100 nm, the magnetic material will become a single domain particle, i.e., a particle that contains only one single magnetic domain. 12,13 Relevant research shows that the physical and chemical properties of Fe 3 O 4 can be affected by doping modication. Lastovina et al. prepared Sm-doped magnetic Fe 3 O 4 nanoparticles by a solvothermal polyol method. By introducing 2,2 0bipyridine in the synthesis process, the average particle size was reduced to about 9 nm. The M s of Sm-doped MNPs and MNPs-Bpy samples were 71.6 and 68.8 emu g À1 , respectively. 14 Paransa prepared Nd-Ce doped Fe 3 O 4 -chitosan nanocomposites with a core-shell structure by coprecipitation and cross-linking method. The results showed that the magnetic response and catalytic activity were improved aer doping Nd 3+ and Ce 3+ . 15 16 Many studies have reported that RE elements replace Fe 3+ or Fe 2+ in Fe 3 O 4 crystals to inuence their physical and magnetic properties. [17][18][19][20][21][22] However, relatively little attention has been paid to the effects of different RE elements doping and the doping amount on the structure of magnetite Fe 3 O 4 nanoparticles. In this paper, Fe 3 O 4 nanoparticles doped with various RE elements, i.e., La 3+ , Ce 3+ and Dy 3+ , were synthesized via hydrothermal technique. The morphologies and structure of each doping sample will be investigated. The size control of magnetite Fe 3 O 4 nanoparticles by RE doping and their magnetic properties will be reported, and promoted a new direction for new medical materials and magnetic memory devices. Not only that, but this research will also be benecial to environmental protection (such as adsorption of industrial and urban wastewater, and degradation of organic polluted water). 23 O) were dissolved and mixed in 80 mL of ethylene glycol according to the ratios of 10 : 1 and 15 : 1, respectively, and then 7.2 g of sodium acetate and 2.0 g of polyethylene glycol were added. The mixture was mechanical stirred in a water bath at 70 C for 1.0 h, and further sealed in a 100 mL stainless steel autoclave lined with Teon. Aer that, the sealed autoclave was heated and maintained at 200 C for 12.0 h, then naturally cooled down to room temperature to form a black precipitate. The obtained black precipitate was washed several times with ethanol and deionized water. The black particles were collected by centrifugation and dried overnight at 60 C in an oven to obtain RE (La 3+ , Ce 3+ , Dy 3+ ) doped Fe 3 O 4 particles. The ratios of Fe to RE elements of 10 : 1 and 15 : 1 are respectively marked as 10Fe 3

Characterization
An X-ray diffractometer (XRD, Rigaku D/MAX 2500 V) was used to analyze the crystal phase of the as-prepared products. The wavelength of the copper target was l ¼ 1.5406, the working current and accelerating voltage were 100 mA and 40 kV, respectively. The average crystallite size of all samples was estimated by the Scherrer equation. 25 The morphology and nanostructure of the samples were analyzed by transmission electron microscope (TEM, Tecnai F20, operated at 200 kV). In the range of 400-4000 cm À1 , the chemical bonds in the samples were studied by Fourier transform infrared spectroscopy (FTIR, Spectrum Two, PerkinElmer). The material was characterized by vibrating sample magnetometer (VSM, PARC 155) at room temperature with a maximum applied magnetic eld of 2 T. Xray photoelectron spectroscopy (XPS, ESCALAB 250Xi) was used to analyze the chemical properties of the material surface, the surface element composition, and valence state of the material. Binding energies of all elements were corrected in reference to the C 1s peak at 284.6 eV.

Results and discussion
3.1. Structural and morphology analysis 3.1.1. XRD analysis. Fig. 1 is the XRD pattern of the all asprepared samples. It can be seen from the pattern that the position of the diffraction peak appearing in the Fe 3 O 4 test curve corresponds to the face-centered cubic magnetite structure (JCPDS no. 75-1372), indicating that the Fe 3 O 4 crystal has a good inverse spinel structure and the diffraction peak shape is sharp. The particle size distribution is narrow, which proves the successful preparation of the  Fig. 1(a), we also found that the XRD pattern of the RE-doped Fe 3 O 4 sample is wider than that of pristine Fe 3 O 4 , indicating that doping RE elements does not change the original inverse spinel structure of Fe 3 O 4 , while it does affect the particle size. 26 Moreover, the XRD patterns of the La 3+ -and Ce 3+ -doped nanoparticles shied to a higher angle compared with pristine Fe 3 O 4 ( Fig. 1(b)), which is probably due to La 3+ and Ce 3+ (ionic radius of 106.1 pm and 103.4 pm, respectively) replacing either Fe 2+ (61.78 pm) or O atoms (140 pm), 19 resulting in smaller lattice parameters and smaller interplanar spacing. No obvious shiing of peaks can be detected in the Dy 3+ -doped sample, which is probably because the ionic radius of Dy 3+ (91.2 pm) is closer to the size of Fe 2+ (61.78 pm). 27 Hence, there are more substituted Fe atoms and fewer substituted O atoms. The peak position presents no obvious changes, while the diffraction peaks become broader as the amount of doping increases. An observed broad XRD peak can lead to the decrease of grain size and the increase of the lattice stress caused by the existence of RE atoms in the Fe 3 O 4 lattice. 7 The average crystallite diameter of the samples was calculated from the strongest peaks based on the Debye-Scherrer formula: where D is the crystallite size inÅ, l is the wavelength inÅ, b is the FWHM in radian and q is the scattering angle in degree, k ¼ 0.89.
As can be seen from Table 1, the average particle size of the doped nanoparticles is about 27 nm, which is half smaller than that of pristine Fe 3 O 4 (56 nm). The size increases with the increase of doping concentration, which may be that RE ion (La 3+ , Ce 3+ , Dy 3+ ) doping inhibits the growth of Fe 3 O 4 grains. The concentration of oxygen vacancies (V O ) in the material is caused by the continuous decrease of RE doping. Grain growth is affected by grain boundary movement, which is mainly affected by the diffusion of V O . 28 Therefore, the existence of V O was conducive to ion diffusion, promotes grain growth, reduced the concentration of V O , reduced the rate of grain boundary movement, slowed down grain growth and inhibited grain growth. From defect chemistry, the concentration of iron vacancies (V Fe ) increased when RE ions (La 3+ , Ce 3+ , Dy 3+ ) are doped into ferrite. Thus, the concentration of V O decreased, slowing down the movement rate of grain boundaries, and thus inhibiting grain growth. 29 In addition, aer RE doping, some of the RE ions (La 3+ , Ce 3+ , Dy 3+ ) could not enter the crystal lattice, although they were distributed at or near the grain boundary, which increased the stress of the grain boundary, hindered the movement of the grain boundary and inhibited the growth of the grain.
3.1.2. SEM analysis. The morphology analysis of the REdoped Fe 3 O 4 system was conrmed by TEM images (Fig. 2(af)). As shown in the inset of Fig. 2(a), pristine Fe 3 O 4 is uniformly dispersed and spherical, uniform in morphology, large in size, about 200 nm in size, and serious in particle agglomeration.
The TEM images show that RE doping greatly reduced the particle size to about 20 nm, which is in good agreement with the XRD results, and no round clusters are formed among the particles ( Fig. 2(a-f)). It also shows that with the increase of the doping amount, the degree of sample agglomeration decreases.
3.1.3. FTIR analysis. Fig. 3(a) shows the infrared spectrum of all samples. The characteristic peak of Fe 3 O 4 appears at 534 cm À1 , which is attributed to the tensile vibration of the Fe-O bond. A weak peak at 434 cm À1 is probably due to the existence of the Fe 3+ -O 2À bond at the octahedral position, which conrmed the formation of spinel Fe 3 O 4 . The broad, weak band found at about 3446 cm À1 was attributed to the stretching and bending of the hydroxyl vibration of absorbed water molecules on the surface of Fe 3 O 4 . The 3446 cm À1 peak gets narrower and sharper aer doping, but the peak becomes narrower and the peak intensity becomes larger aer doping, which indicates that there is superposition of the OH stretching band in ethylene glycol and the degree of water absorption increases. 30 The stretching vibration of the carbonyl group has a strong infrared absorption peak at 1564, 1410 and 1076 cm À1 , which can also be related to CH 3 COONa. 31 Compared with pristine Fe 3 O 4 , there is no signicant shi in peak position of the infrared spectrum in the RE-doped Fe 3 O 4 . However, the transmittance of the characteristic peak is obviously enhanced, which proves that the RE elements were successfully doped into the Fe 3 O 4 lattice, partially replaced Fe 2+ , occupied the octahedral position, and enhanced the vibration of the Fe-O bond. 19 Fig. 3(b) is a ball stick model diagram of Fe 3 O 4 , in which red represents the O atom and blue represents the Fe atom, in which Fe 2+ occupies half of the octahedral voids, and Fe 3+ occupies the remaining half of the octahedral voids and all tetrahedral voids.

XPS analysis.
In order to study the surface chemical properties and valence states of the RE-doped nanoparticles, XPS analysis was carried out on the samples. Fig. 4(a) shows the XPS spectrum of Fe 2p. The peaks at 724.23 eV and 710.63 eV correspond to Fe 2p 1/2 and Fe 2p 3/2 , respectively, which are mainly attributed to the Fe-O bond and are very close to the reported data for Fe 3 O 4 in the literature (724.63 eV and 710.97 eV, respectively). 20 Compared with the literature, with the increase of the doping amount, the Fe 2p 1/2 and Fe 2p 3/2 of the doped samples shi towards low binding energy, which may be due to the RE ions replacing Fe 2+ and changing the ratio of Fe 3+ :Fe 2+ . 22 The observation of the peak shiing in Fe 2p 3/2 of the La 3+ and Ce 3+ -doped samples being larger than that of the Dy 3+ -doped samples could be due to the fact that the ionic radius of Dy 3+ is closer to Fe 2+ , resulting in a small change of the Fe 2p 3/2 peak position. Satellite peak appears near 718 eV, which proves the successful doping of RE elements in Fe 3 O 4 . 32,33 In Fig. 4(b), the characteristic peak at 529.94 eV corresponding to the O 1s orbital in Fe 3 O 4 was observed, which is in agreement with the previous reported results in the literature (530.0 eV). 34 Among them, O 1s of the Dy 3+ -doped sample shis toward a lower binding energy, while the La 3+ -and Ce 3+ -doped samples shi toward higher binding energy. We considered that the doping elements substitution of V O by doping elements, which is consistent with XRD pattern analysis. The XPS spectrum of La 3d, the peaks are observed at 851.79 eV and 834.95 eV correspond to La 3d 3/2 and La 3d 5/2 , respectively (Fig. 4(c)), indicating that La 3+ is doped and La exists in the La 3+ valence state. 35 XPS spectrum of Ce 3d in Fig. 4(d) conrms the successful incorporation of Ce ions in the Fe 3 O 4 lattice. The two main peaks at 904.05 eV and 886.59 eV were assigned to Ce 3d 1/2 and Ce 3d 3/ 2 . 20 Fig. 4(e) shows the XPS spectrum of Dy 4d. The peaks at 156.86 eV and 153.66 eV correspond to Dy 4d 3/2 and Dy 4d 5/2 , respectively, indicating the successful incorporation of Dy inside the Fe 3 O 4 structure framework with the main chemical state of Dy 3+ . 36,37 It is clear from Fig. 4(a and b) that RE doping would affect the surface properties of the original Fe 3 O 4 . However, the increase of the doping amount does not change its  surface chemical state (Fig. 4(c-e)), indicating that the doping amounts are within a reasonable range and had little effect on its surface crystal properties.
The atomic concentration values of each element from each sample are summarized in Table 2

Magnetic performance analysis
In order to study the magnetic properties of the samples, VSM analysis was carried out. Fig. 5(a)  14 and 59.79 emu g À1 , respectively (as shown in Fig. 5(b)). The results indicate that the M s of pristine Fe 3 O 4 is lower than that of standard block Fe 3 O 4 (96.43 emu g À1 ). 19 Aer doping RE elements, the corresponding M s decreases signicantly, and the magnetization decreases with the increase of doping concentration. Compared with the pristine Fe 3 O 4 nanoparticles (M s value of 79.59 emu g À1 ), the La 3+ -doped sample shows the lowest decrease (11.1%) in the M s , whereas the highest decrease (24.9%) is found for the Dy 3+ -doped sample. As for nanoparticles, their ultrane particle properties, surface disorder and cation distribution are the main reasons for the decrease of M s . The decrease of M s is due to the lack of oxygen-mediated super exchange mechanism between the surface iron ions, which leads to the decrease of the exchange coupling and the inclined spin of the surface layer, so there will be a magnetic dead surface layer. 38,39 Another reason may be that Fe 3 O 4 can easily absorb water aer doping (as shown by infrared spectrum), thus reducing the volume fraction of the magnetic materials. The magnetic moment on the octahedral position of Fe 3 O 4 is antiferromagnetic. Meanwhile, the magnetic moment on the tetrahedral position is ferromagnetic. Moreover, the magnetic moment of the unit cell only comes from the Fe 2+ ions. XRD and FTIR data suggested that RE ions were more inclined to the octahedral position and replaced some Fe 2+ ions. These RE ions are nonmagnetic (without unpaired d electrons),  :Dy nanomaterials with superparamagnetic properties were synthesized by hydrothermal method with uniform particle size distribution and dispersibility. XRD data conrmed that the sizes of Fe 3 O 4 were adjusted by doping. RE elements doping can inhibit crystal growth and reduce the particle size of Fe 3 O 4 . With the increase of the doping amount, the particle sizes decreased accordingly. The XRD pattern of the doped nanoparticles shied to a high angle, indicating that part of the doped atoms replaced O atoms, resulting in a smaller lattice parameter and a smaller interplanar spacing. XRD and XPS analyses conrmed the successful doping of RE elements into the framework of

Conflicts of interest
There are no conicts to declare.