Effect of NiZn Ferrite Nanoparticles upon the ... - ACS Publications

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Effect of NiZn Ferrite Nanoparticles upon the Structure and Magnetic and Gyromagnetic Properties of Low-Temperature Processed LiZnTi Ferrites Tingchuan Zhou,*,† Dainan Zhang,‡ Lijun Jia,† Feiming Bai,† Lichuan Jin,† Yulong Liao,† Tianlong Wen,† Cheng Liu,† Hua Su,† Ning Jia,† Zongliang Zheng,† Vincent G. Harris,§ Huaiwu Zhang,*,† and Zhiyong Zhong† †

State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, People’s Republic of China ‡ Department of Electrical and Computer Engineering, University of Delaware, Newark, Delaware 19716, United States § Center for Microwave Magnetic Materials and Integrated Circuits and Department of Electrical and Computer Engineering, Northeastern University, Boston, Massachusetts 02115-5000, United States S Supporting Information *

ABSTRACT: NiZn ferrite nanoparticles (2−20 wt %) of composition Ni0.4Zn0.6Fe2O4 were introduced into LiZnTi ferrite of composition Li0.42Zn0.27Ti0.11Fe2.2O4 and sintered at a temperature of 920 °C for 2 h, well below that of the Ag melting point. Here, LiZnTi ferrites were prepared by a solidstate reaction method, and NiZn ferrite nanoparticles were fabricated by a hydrothermal chemical technique at 180 °C. A low ferromagnetic resonance (FMR) line width, low coercivity, and high magnetic moment were achieved after refinement of the heat treatment conditions of the mixture. Riveted full profile refinement of the X-ray powder diffraction patterns and analysis of Mössbauer spectra were employed to study the structure and caption distribution. The results confirm a pure spinel phase after processing. A narrow FMR line width of 152.5 Oe, a reduced coercivity of 132.9 A/m, and an improved saturation magnetization of 74.23 emu/g were obtained by way of the addition of 8 wt % NiZn ferrite nanoparticles.

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INTRODUCTION

size of latching microwave devices requires miniaturization and their operational frequencies continue to increase, the magnetic materials applied to these microwave devices employing LTCC technology should have a sintering temperature lower than the melting point of silver (i.e., 961 °C) and good gyromagnetic properties, that is, a narrow ferromagnetic resonance (FMR) line width (the narrower the better), high saturation magnetization (4πMs > 3000 G), and low coercivity (Hc < 160 A/m). NiZn ferrite is one of the best magnetic materials for highfrequency microwave devices due to its low coercivity, low magnetic and dielectric loss, and low production cost.15,16 Meanwhile, NiZn ferrite nanoparticles have been the subject of current interest in scientific research from both basic science and application viewpoints.17−30 The properties of nanoparticles are very sensitive to the preparation methods and sintering conditions. Therefore, it is very important to choose a suitable method to obtain ferrite nanoparticles. Recently,

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Ferrites, having been studied for several decades, are attractive materials for rf applications in which the sending, receiving, and manipulation of electromagnetic signals is paramount. Their relatively high permeability and permittivity, low electrical conductivity, moderate magnetization, narrow ferromagnetic resonance line width, and low magnetic and dielectric losses provide them with unparalleled application potential.2 Zinc- and titanium-substituted lithium ferrites are low-cost materials that have been investigated and found application in latching microwave devices such as phase shifters and switches.3−8 Zinc substitution is effective in lowering anisotropy fields and decreasing the ferromagnetic resonance line width, but also in promoting densification and grain growth.3,6 Titanium of a high doping level (about 0.7 ion/formula unit) is mainly chosen to lower the magnetization toward applications at various frequencies, especially below the X band;3,6,9 however, it concomitantly decreases the coercivity for small quantities of substitution.10,11 Meanwhile, miniaturization of these microwave devices benefits from the use of lowtemperature cofired ceramic (LTCC) processes.12−14 Since the © 2015 American Chemical Society

Received: December 18, 2014 Revised: May 12, 2015 Published: May 18, 2015 13207

DOI: 10.1021/jp512608z J. Phys. Chem. C 2015, 119, 13207−13214

Article

The Journal of Physical Chemistry C

dodecylbenzenesulfonate (SDBS) was added as a surfactant to the mixed solution of metal salts. NaOH aqueous solution was slowly added until the hydrothermal solution pH value reached 10. The suspensions were transferred to a Teflon-lined autoclave filled with 1 MPa of nitrogen gas and heated to 180 °C for 6 h. After processing, the particles were washed using deionized water and ethanol until the pH value of the cleaning solution reached 7 and then dried at 80 °C for 12 h. Thereafter, various ratios of the NiZn ferrite particles were introduced into the precalcined LiZnTi ferrite powders and the BBSZ glass powders such that (Li0.42Zn0.27Ti0.11Fe2.2O4 + x wt % Ni0.4Zn0.6Fe2O4) + 1 wt % BBSZ, where x = 0, 2, 4, 8, 12, and 20 wt %, were wet-milled for 6 h. The dried mixtures were then mixed with 10 wt % polyvinyl binder, sieved through a mesh of 100 μm, and pressed at 9 MPa to form toroidal samples (18 mm ⌀ × 8 mm). Finally, the samples were sintered in air at 920 °C for 2 h. Materials Characterization. The ferrite phase was investigated at room temperature using a Philip X’Pert Pro X-ray powder diffractometer with Cu Kα radiation (λ = 1.5406 Å). The X-ray powder diffraction (XRPD) data for Rietveld refinement were collected in the range of 2θ = 16−120° at a scan step of 0.02°. The Rietveld refinement was performed using the Rietica program.37,38 Transmission electron microscopy (TEM) was carried out on a Tecnai G2 F20 S-TWIN at 200 kV. The NiZn ferrite particles were dispersed in ethanol by ultrasonication for 5 min. Transmission Mössbauer spectroscopy employing a 57Co radiation source in a Rh matrix was used to study the local magnetic properties of ferrous and ferric ions on the γ-ray absorption spectra. The velocity of the radiation source was calibrated with a standard α-Fe foil. The Mössbauer spectra were least-squares fitted using the software WinNormos-For-Igor. The microstructures of the sintered samples were observed by scanning electron microscopy (SEM; JEOL JSM-6490). M−H loops were measured using a BHV525 vibrating sample magnetometer (VSM) at an external applied field ranging from +2500 to −2500 Oe at room temperature for bulk samples with a mass of about 0.5 g. Coercivity and saturation induction were measured using an Iwatsu BH analyzer (SY8232) in an alternating magnetic field of 1800 A/m at 1 kHz. Part of the sintered toroids were crushed and shaped to small spheres having a diameter of ∼1 mm for the measurement of the ferromagnetic resonance line width (ΔH) in a TE106 cavity at 9.5 GHz by applying the perturbation method.

several methods have been successfully used to synthesize NiZn ferrite nanoparticles, such as sol−gel,17,18 autocombustion,19 coprecipitation,20 hydrothermal,21,22 ball milling,23 reverse micelle synthesis,24 and the polyol process.25 Among these methods, hydrothermal synthesis has attracted great interest due to its potential to produce highly crystallized, high-purity, chemically homogeneous, lightly agglomerated powders with narrow size distribution and controlled morphologies. It is wellknown that abnormal grain growth occurs in low-temperature liquid-phase sintered ceramics. Thus, this kind of nanoparticle could make contributions to suppressing abnormal grain growth of the microsize grains and obtaining a more uniform grain size distribution especially for those ferrites sintered at low temperature in the liquid phase because uniformity is important to obtain a low FMR line width.31 Here, we introduce optimal amounts of Ni0.4Zn0.6Fe2O4 ferrite nanoparticles to a Li0.42Zn0.27Ti0.11Fe2.2O4 ferrite with the expectation to obtain a high saturation magnetization, a low coercivity, and a narrow FMR line width, whereas a certain amount of H3BO3−Bi2O3−SiO2−ZnO (BBSZ) glass was chosen as the fluxing agent to reduce the sintering temperature.32 Regarding the occupancy of cations in LiZnTi ferrites, Li+ ions would be expected to remain in the octahedral [B] sites up to 0.5 ion/formula unit, whereas Ti4+ ions occupy the [B] sites and nonmagnetic Zn2+ ions substitute for Fe3+ ions on the tetrahedral (A) sites.4,8,33 For NiZn ferrites, Zn2+ ions prefer the (A) sites and Ni2+ ions occupy the [B] sites,24,27,34,35 but it is also possible for Zn2+ and Ni2+ ions to partially mix their occupancy in NiZn ferrite nanoparticles.26,34,35 Additionally, when small amounts of Li+ (less than 0.5 ion/formula unit) and Ti4+ ions are codoped into NiZn ferrites, it was reported that Li+ ions occupy the (A) sites while Ni2+ ions occupy both the (A) sites and [B] sites.36 In the present work, we have carried out careful characterization of the samples by Rietveld refinement of the X-ray powder diffraction patterns and Mössbauer spectroscopy to determine with a high degree of certainty the cation distributions. We also studied the effect of the NiZn ferrite nanoparticle addition and the cation site occupancy on the gyromagnetic properties of the ferrite materials that are key to realizing practical rf magnetic devices such as phase shifters, filters, and antenna substrates to name a few.

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EXPERIMENTAL SECTION Materials Synthesis. LiZnTi ferrites of chemical composition Li0.42Zn0.27Ti0.11Fe2.2O4 were synthesized by a solid-state reaction method. High-purity Li2CO3 (99%), ZnO (99%), TiO2 (98%), and Fe2O3 (99.5%) were used as precursors. According t o the stoichio metric formulation of Li0.42Zn0.27Ti0.11Fe2.2O4, the mixture of precursors was ballmilled (the ball to powder ratio was 2:1) in deionized water for 4 h using a planetary mill with steel balls (the balls were 10 mm in diameter and 6 mm in diameter) at a speed of 220 rpm. The powder mixture was dried and calcined under an oxygen gas atmosphere at 800 °C for 2 h. BBSZ glass was prepared by a conventional glass fabrication process. Powders of H3BO3, Bi2O3, SiO2, and ZnO (molar ratio of 6:5:2:7) were mixed, calcined at 1000 °C for 1 h, and then quenched in deionized water and crushed to a fine powder. The Ni0.4Zn0.6Fe2O4 ferrite nanoparticles were fabricated by a hydrothermal method. Nickel nitrate [Ni(NO3)2·6H2O], zinc nitrate [Zn(NO3)2· 6H2O], and iron nitrate [Fe(NO3)3·9H2O] (molar ratio of 0.2:0.3:1) were dissolved in deionized water (300 mL). Sodium

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RESULTS AND DISCUSSION The hydrothermal method is very reproducible. The lattice parameter of synthesized Ni0.4Zn0.6Fe2O4 ferrite nanoparticles (8.404 ± 0.001 Å) is close to published values in the JCPDS database (i.e., JCPDS-47-0023). The NiZn ferrite nanoparticles have an average crystallite size of 11.3 ± 0.3 nm, which was calculated by the Debye−Scherrer formula for the main peaks of the X-ray powder diffraction (XRPD) pattern (see Figure S1, Supporting Information). Figure 1 shows a TEM image of Ni0.4Zn0.6Fe2O4 ferrite nanoparticles. It shows a largely monodispersed collection of nanoparticles having an average size of 12.8 ± 0.1 nm, which is in close agreement (∼11.7% deviation) with the size determined by the Debye−Scherrer analysis. An optimal amount of NiZn ferrite nanopartilces and glass powder, when introduced to LiZnTi ferrite, will enhance the quality of the crystal structure as described in detail below. 13208


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seen that all the fitted curves match well with the experimental data and the positions of the Bragg reflections are almost consistent with the indexed peaks of the spinel structure. LiZnTi ferrites and NiZn ferrites belong to the same space group, Fd3m ̅ . However, LiZnTi, LiZnTiNi, and NiZn ferrite phases coexist after the addition of the NiZn ferrite nanoparticles to the LiZnTi ferrite. Figure 4 shows the Rietveld refinement of the crystal structure of the LiZnTi ferrite samples sintered after introduction of various amounts of NiZn ferrite nanoparticles. The calculated atomic coordinates, reliability factors, and lattice parameters are listed in Table S2 (see the Supporting Information). For LiZnTi ferrite, both Li+ and Ti4+ ions occupy the octahedral [B] sites, while Zn2+ ions occupy the tetrahedral (A) sites. The Fe3+ ions occupy both the (A) and [B] sites. For different amounts of NiZn ferrite nanoparticles (2−20 wt %) added to the LiZnTi ferrites, Ni2+ and Fe3+ ions partly substitute for Li+ and Ti4+ ions on the [B] sites of LiZnTi ferrites. Additionally, Zn2+ and Fe3+ ions from the (A) sites of NiZn ferrite nanoparticles enter the (A) sites of the LiZnTi ferrites, whereupon Fe3+ ions of LiZnTi ferrites partially substitute for Zn2+ ions. The LiZnTi ferrite sintered with 8 wt % addition of NiZn ferrite nanoparticles may have the greatest amount of ion substitution, as can be seen from the occupancy information provided in Table S2. Mössbauer spectra were also collected to further study variations of the crystal structure, chemical compositions, and coordination of Fe cations in the spinel ferrites. Figure 5 shows the room temperature Mössbauer spectra of the LiZnTi ferrite cosintered with various amounts of NiZn ferrite nanoparticles. The Mössbauer spectra reveal a well-defined Zeeman pattern consisting of two separate hyperfine magnetic sextets corresponding to Fe3+ ions at the (A) sites and the [B] sites of a typical spinel crystal structure. The corresponding hyperfine interaction parameters, such as the isomer shift (IS), line width (WID), quadrupole splitting (QS), and average magnetic hyperfine field (B), obtained by fitting the spectra of all samples, are listed in Table 1. The QS values for both the (A) and [B] sites of the Zeeman sextets were nearly zero within the experimental error, which is consistent with the spinel crystal structure as confirmed by XRD. The WID values corresponding to the (A) and [B] sites vary with the content of NiZn ferrite nanoparticles, which was attributed to the change in the environment of Fe3+ ions in the same sublattice. Meanwhile, the line width of the outermost [B] sites was smaller than that of the (A) sites, which indicates that the field gradients at the [B] site ions are more uniform when the (A) sites are occupied to a greater extent by Fe ions. The room temperature IS values were 0.16−0.25 mm/s relative to the Fe metal, which is consistent with the high-spin Fe3+ charge state.24,39 The average magnetic hyperfine field B at the (A) sites decreases first and then increases with increasing content of NiZn ferrite nanoparticles, where a minimum is achieved at x = 8. Meanwhile, the average magnetic hyperfine field B at the [B] sites increases first upon 2 wt % addition of NiZn nanoparticles, decreases thereafter, and then increases again as a function of NiZn ferrite nanoparticle addition with a minimum at x = 8. The magnetic hyperfine fields at the 57Fe nucleus in magnetically ordered nonmetals are primarily due to the Fermi contact interaction between the nuclei and the spin-polarized selectron shells.40 The average magnetic nucleus fields of Fe3+ ions in the (A) and [B] sites are proportional to their average magnetization. After a small amount of NiZn ferrite nanoparticles (x = 2) were introduced, the A−A sublattice

Figure 1. TEM image of Ni0.4Zn0.6Fe2O4 ferrite nanoparticles produced at 180 °C using a hydrothermal method.

XRD patterns of the LiZnTi ferrite samples sintered with various amounts of NiZn ferrite nanoparticles are shown in Figure 2. The X-ray diffraction peaks correspond to the (111),

Figure 2. X-ray powder diffraction patterns of LiZnTi ferrite samples sintered with various amounts of NiZn ferrite nanoparticles. The insets show the main peaks for the (220), (311), and (511) reflections.

(220), (311), (222), (400), (422), (511), and (440) reflections, indicating that the powders were well crystallized and only the spinel structure was detected for all samples. The XRD peaks have the same shape for addition of various amounts of NiZn ferrite nanoparticles, as shown in the insets to Figure 2, indicating the spinel structure of Li0.42Zn0.27Ti0.11Fe2.2O4 is preserved in the ferrite materials. Furthermore, a gradual shift of the XRD peaks toward lower angles is observed upon increasing the amount of NiZn ferrite nanoparticles. The lattice parameters calculated from the XRD data (see Table S1, Supporting Information) exhibit a slight increase as the content of NiZn ferrite nanoparticles increases, which slowly approaches that of the Ni0.4Zn0.6Fe2O4 ferrite. Here Rietveld refinement of the XRD patterns was performed to further investigate the crystal structure and atomic site distribution of the cations. Figure 3 shows the Rietveld refinement of the LiZnTi ferrite samples sintered with (a) 0 wt %, (b) 4 wt %, (c) 8 wt %, and (d) 20 wt % NiZn ferrite nanoparticles. It can be 13209


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Figure 3. Rietveld refinement of the LiZnTi ferrite samples sintered with (a) 0 wt %, (b) 4 wt %, (c) 8 wt %, and (d) 20 wt % NiZn ferrite nanoparticles. The difference (Yobsd − Ycalcd) pattern is displayed in green. The positions of the Bragg reflections are represented by blue vertical bars (the first two lines are for LiZnTi ferrite, the second two lines are for LiZnTiNi ferrite, and the third two lines are for NiZn ferrite).

interaction of LiZnTi ferrites became weaker due to the decreased concentration of magnetic Fe3+ ions in the (A) sites by partial substitution of Fe3+ ions with Zn2+ ions from NiZn ferrites (confirmed by Rietveld refinement), which caused a decrease of the magnetic hyperfine field B at the (A) sites. Meanwhile, the B−B sublattice interaction of LiZnTi ferrites became stronger due to the partial substitution of Li+ and Ti4+ ions with Ni2+ and Fe3+ ions from the NiZn ferrite (confirmed by Rietveld refinement), which caused an increase of the magnetic hyperfine field B at the [B] sites. As the amount of NiZn ferrite nanoparticles increases, Zn2+ ions continue to occupy the (A) sites of LiZnTi ferrites, with the maximum being achieved at x = 8 (see Table S2, Supporting Information). As a result, the magnetic species of the A−A sublattice become so diluted that the A−B superexchange interaction is reduced at x = 8 to the point where the B spins no longer remain parallel but slightly cant due to the antiferromagnetic B−B exchange.41,42 Furthermore, since the 2− 2− 2+ 3+ 3+ Fe3+ A −O −NiB interaction is weaker than the FeA −O −FeB interaction, the magnetic hyperfine fields derive largely from 2− 3+ the Fe3+ A −O −FeB interaction. In this case, the average magnetic hyperfine fields B at the (A) and [B] sites both decrease. Here, the variation of magnetic hyperfine fields due to NiZn ferrite nanoparticle addition is not negligible at x > 12 wt % since LiZnTi, LiZnTiNi, and NiZn ferrite phases coexist as confirmed by Rietveld refinement. Saturation magnetization (Ms) values of the LiZnTi ferrite samples sintered with various amounts of NiZn ferrite nanoparticle additives, calculated from the room temperature magnetic hysteresis loops (see Figure S3, Supporting Information), are shown in Figure 6. As can be seen from Figure 6, saturation magnetization increases from 69.67 to 77.56 emu/g after small amounts (x = 2) of NiZn ferrite nanoparticles are added and then decreases as the nanoparticle content increases. The variation of saturation magnetization as a function of the content of NiZn ferrite nanoparticles is consistent with that of the average grain size (see Figure S2 and Table S1, Supporting Information). The initial increase of

Figure 4. Crystal structure of the LiZnTi ferrite samples sintered with various amounts of NiZn ferrite nanoparticles based on the Rietveld refinements.

Figure 5. Room temperature Mössbauer spectra of the LiZnTi ferrite samples sintered with (a) 0 wt %, (b) 2 wt %, (c) 4 wt %, (d) 8 wt %, and (e) 12 wt % NiZn ferrite nanoparticles. 13210


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Table 1. Mössbauer Parameters of the Sintered Samples, Measured at Room Temperature: Isomer Shift (IS), Average Magnetic Hyperfine Field (B), Line Width (WID), Quadrupole Splitting (QS), and Relative Area (%), Obtained by Fitting the Zero-Field Spectra sample

subspectrum

WID (mm/s)

IS (mm/s)

QS (mm/s)

B (T)

area (%)

0.0 wt %

(A) [B] (A) [B] (A) [B] (A) [B] (A) [B] (A) (B)

0.62(2) 0.41(1) 0.74(9) 0.41(2) 0.49(2) 0.37(1) 0.51(3) 0.40(2) 0.59(4) 0.40(1) 0.61(3) 0.41(1)

0.232(7) 0.184(2) 0.251(32) 0.197(6) 0.196(5) 0.165(2) 0.201(12) 0.167(4) 0.233(12) 0.188(4) 0.231(11) 0.192(2)

−0.045(15) −0.016(4) −0.058(32) −0.005(12) −0.018(11) −0.001(4) −0.036(23) −0.019(8) −0.040(21) −0.013(7) −0.037(22) −0.016(8)

43.46(17) 45.97(2) 42.69(39) 46.45(5) 36.15(14) 38.17(2) 36.06(26) 38.14(3) 43.21(29) 45.72(3) 43.29(24) 45.81(3)

30.9 69.1 31.1 68.9 31.5 68.5 35.0 65.0 34.3 65.7 33.2 66.8

2.0 wt % 4.0 wt % 8.0 wt % 12.0 wt % 20.0 wt %

Figure 6. Saturation magnetization (Ms) of the LiZnTi ferrite samples sintered with various amounts of NiZn ferrite nanoparticles calculated from the magnetic hysteresis loops.

Figure 7. Coercivity (Hc) and saturation induction (Bs) of the LiZnTi ferrite samples sintered with various amounts of NiZn ferrite nanoparticles.

saturation magnetization is caused by the increase of Fe3+ ions (magnetic moment 5 μB) and Ni2+ ions (magnetic moment 2 μB) on the [B] sites and the decrease of Fe3+ ions on the (A) sites. The subsequent decrease is mainly caused by the decrease of the A−B superexchange interaction and saturation polarization due to the gradual increase of Zn2+ ions on the (A) sites of the LiZnTi ferrite phase.10,43 The further decrease of saturation magnetization when at x > 8 is mainly caused by the larger amount of randomly dispersed NiZn ferrite phase. The saturation magnetization of this magnetic second phase (see Figure S4d, Supporting Information) is much lower than that of the LiZnTi ferrite (see Figure S4b). The resulting mixture would in practice fit the conditions described in the magneticinhomogeneity model.44 B−H measurements were also conducted to better understand the coercivity properties. Figure 7 shows the coercivity (Hc) and saturation induction (Bs) of the LiZnTi ferrite samples sintered with various amounts of NiZn ferrite nanoparticles. The coercivity decreases rapidly after small amounts (x = 2) of NiZn ferrite nanoparticles are added, which is due to the increase of the average grain size, the more uniform grain size distribution (see Figure S2 and Table S1), and partial substitution of Fe3+ ions with Zn2+ ions from the NiZn nanoparticle ferrites.3,45 Then the coercivity increases slightly with increasing content of NiZn ferrite nanoparticle addition, which could be attributed to the fact that the decreasing average grain size has more influence on the coercivity compared to the Zn2+ ions that continue to

occupy the (A) sites of the LiZnTi ferrites (see Figure S2 and Table S1). Thereafter, the coercivity rapidly decreases again with increasing content of NiZn ferrite nanoparticle addition, mainly due to the substitution of Fe3+ ions with Zn2+ ions from the NiZn ferrite additive. As the content of NiZn ferrite nanoparticles increases to x > 12 wt %, the NiZn ferrite phase with very low coercivity makes a contribution to the further decrease of the coercivity.46−48 Additionally, it is observed that the variation of saturation induction with the content of NiZn ferrite nanoparticles shows a tendency similar to that of saturation magnetization. Saturation induction increases rapidly after small amounts of NiZn ferrite nanoparticles are added and achieves its maximum at x = 4. The FMR spectra of the LiZnTi ferrite samples, sintered with various amounts of NiZn ferrite nanoparticles, are shown in Figure 8. The experimental data were fitted better with a Lorentzian than Gaussian distribution at high frequencies (5.25−12 GHz).49 However, the line shape of the samples with a low line width is more Lorentzian-like, and high line width samples are more Gaussian-like.50,51 In addition, the spectra for x = 4−20 wt % (Figure 8c−f) were fitted better to Lorentzian curves than the spectra for x = 0−2 wt % (Figure 8a,b), which indicated that a lower line width would be obtained for LiZnTi ferrite samples sintered with 4−20 wt % NiZn ferrite nanoparticles. The FMR line width (ΔH) calculated from the experimental data is shown in Figure 9a. It can be seen that ΔH first decreases linearly from 194.1 to 152.5 Oe as the NiZn 13211


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Figure 8. X-band FMR spectra with fitted Lorentzian and Gaussian curves for LiZnTi ferrite samples sintered with (a) 0 wt %, (b) 2 wt %, (c) 4 wt %, (d) 8 wt %, (e) 12 wt %, and (f) 20 wt % NiZn ferrite nanoparticles.

Figure 9. (a) FMR line width (ΔH) calculated from the experimental data of FMR spectra. (b) Normalized anisotropy-broadened line width ΔHa and porosity-broadened line width ΔHp of the LiZnTi ferrite samples sintered with various amounts of NiZn ferrite nanoparticle additives.

ΔHi + ΔHa + ΔHp = ΔHi + 2.07(Ha2/4πMs) + 1.5(4πMs)P, where ΔHi, ΔHa, and ΔHp are the intrinsic, random anisotropy field, and porosity contributions, respectively. Figure 9b shows the normalized anisotropy-broadened line width ΔHa and porosity-broadened line width ΔHp of the LiZnTi ferrite samples sintered with various amounts of NiZn ferrite nanoparticles. ΔHa decreases first as a function of NiZn ferrite nanoparticle addition (x), attaining a minimum at x = 8, and

ferrite nanoparticle content increases, reaching a minimum at x = 8, and then increases again thereafter for x > 8. ΔH for x = 4−20 wt % is smaller than that for x = 0−2 wt %, which is almost consistent with the experimental data. The variation of ΔH with the content of NiZn ferrite nanoparticles is proportional to the porosity (see Table S1, Supporting Information). As discussed in our prior work,5 ΔH values of polycrystalline ferrites have three contributions, namely, ΔH = 13212


The Journal of Physical Chemistry C then increases thereafter for x > 8. The decrease of ΔHa was caused by partial substitution of Fe3+ ions with Zn2+ ions from NiZn ferrites (confirmed by Rietveld refinement), which lowers the anisotropy fields.3,6 The thereafter increase of ΔHa may be caused by magnetic inhomogeneities due to the larger amount of randomly dispersed NiZn ferrite phase, which enhances the anisotropy fields. ΔHp is mainly affected by saturation magnetization and porosity. ΔHp is 4−9 times larger than ΔHa as a function of the content of NiZn ferrite nanoparticles. Moreover, the minimum ΔH achieved at x = 8 is attributed to the relatively large cation redistribution (see Table S2, Supporting Information) and low porosity (see Table S1).

ACKNOWLEDGMENTS

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CONCLUSIONS The roles of NiZn ferrite nanoparticle additives in the structure and magnetic and gyromagnetic properties have been systematically investigated. Pure spinel phase was detected after the LiZnTi ferrite and NiZn ferrite nanoparticle mixture was sintered at a temperature of 920 °C for 2 h. Meanwhile, Zn2+ ions from the (A) sites of NiZn ferrite nanoparticles substituted for Fe3+ ions at the (A) sites of LiZnTi ferrites, and Ni2+ and Fe3+ ions from the [B] sites of NiZn ferrite nanoparticles substituted for Li+ and Ti4+ ions at the [B] sites of the LiZnTi ferrite. Optimized additions of NiZn ferrite nanoparticles not only improved the saturation magnetization but also reduced the FMR line width (ΔH) and coercivity. The LiZnTi ferrite sample sintered with 8 wt % NiZn ferrite nanoparticles exhibited a saturation magnetization (Ms) of 74.23 emu/g, a coercivity (Hc) of 132.9 A/m, and an X-band FMR line width (ΔH) of 152.5 Oe, suggesting that it is promising for LTCC applications including those for phase shifters, filters, and miniaturized antenna substrates. ASSOCIATED CONTENT

* Supporting Information S

X-ray powder diffraction pattern of Ni0.4Zn0.6Fe2O4 ferrite nanoparticles (Figure S1), SEM micrographs of the LiZnTi ferrite samples sintered with (a) 0 wt %, (b) 2 wt %, (c) 8 wt %, and (d) 12 wt % NiZn ferrite nanoparticles (Figure S2), room temperature magnetic hysteresis loops of the LiZnTi ferrite samples sintered with various amounts of NiZn ferrite nanoparticles (Figure S3), room temperature magnetic hysteresis loops of (a) Li0.42Zn0.27Ti0.11Fe2.2O4 ferrite prepared by the conventional method sintered at 1050 °C and (b) Ni0.4Zn0.6Fe2O4 ferrite prepared by the conventional method sintered at 1150 °C (Figure S4), effects of the NiZn ferrite content on the lattice parameter, average grain size, bulk density, and porosity of the sintered samples (Table S1), and Rietveld refinement results of the LiZnTi ferrite samples sintered with (a) 0 wt %, (b) 4 wt %, (c) 8 wt %, and (d) 20 wt % NiZn ferrite nanoparticles (Table S2). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jp512608z.

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We are indebted to Prof. Hao Fu (School of Physical Electronics, University of Electronic Science and Technology of China (UESTC)) for access to the Rietica program. This work is partly supported by the National Natural Science Foundation of China (Grants 51132003, 51402041, and 61171047), the National Basic Research Program of China (Grant 2012CB933104), and the Outstanding Doctoral Fund of UESTC (Grant A1098524023901001014).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86-158-82257068. Fax: +86-28-83202556. *E-mail: [email protected]. Phone: +86-028-8320-7656. Fax: +86-28-83202556. Notes

The authors declare no competing financial interest. 13213


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