Synthesis and Size-Selective Precipitation of Monodisperse

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Synthesis and Size Selective Precipitation of Monodisperse Nonstoichiometric MFe O (M=Mn, Co) Nanocrystals and Their DC and AC Magnetic Properties. x

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Dichen Li, Hongseok Yun, Benjamin T. Diroll, Vicky V. T. DoanNguyen, James M. Kikkawa, and Christopher B. Murray Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03280 • Publication Date (Web): 28 Dec 2015 Downloaded from http://pubs.acs.org on January 3, 2016

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Chemistry of Materials

Synthesis and Size Selective Precipitation of Monodisperse Nonstoichiometric MxFe3-xO4 (M=Mn, Co) Nanocrystals and Their DC and AC Magnetic Properties. Dichen Li,† Hongseok Yun,† Benjamin T. Diroll,† Vicky V. T. Doan-Nguyen,‡ James M. Kikkawa,§ Christopher B. Murray*,†,‡ †

‡

§

Department of Chemistry, Department of Materials Science and Engineering, Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States. ABSTRACT: Spinel ferrite nanocrystals (NCs) have shown great promise for a wide variety of electromagnetic and medical applications. In this work, the AC magnetic properties of nonstoichiometric manganese and cobalt ferrites (MxFe3-xO4, M = Mn, Co) NCs are systematically studied as a function of composition. Samples of very similar average size and shape, but different Mn to Fe and Co to Fe ratios are prepared to study the effect of composition. Conventional syntheses are combined with a size selective precipitation method using oleic acid as an anti-solvent yielding nearly monodisperse samples. DC and AC magnetic measurements shows that introducing Co to the ferrite crystal increases the blocking temperatures and magnetic anisotropies of the nanocrystals with corresponding shifts in AC magnetic susceptibilities, while manganese ferrites are relatively insensitive to the variation in compositions as size and shape dominate over crystal anisotropy. The systematic AC-magnetic characterizations of superparamagnetic MnxFe3-xO4 and CoxFe3-xO4 NCs raise the importance of controlling chemical composition of ferrite NCs for AC magnetic applications.

study the radio frequency AC magnetic properties of NCs as it reveals information about the materials, especially the magnetic moment relaxation and ferromagnetic resonance effect, 11, 31-33 which are closely related to the AC magnetic susceptibility and thermal energy dissipation efficiencies critical to the applications mentioned above.8,

INTRODUCTION Monodisperse magnetic nanocrystals (NCs) with controlled size and shape have been the focus of intense study for more than a decade. 1-7 Among them, spinel ferrites with chemical formula MxFe3-xO4 (M=Mn, Fe, Co, Ni, Zn, x=0-1) have shown great potential for various high frequency applications, such as on-chip electromagnetic devices, 8-11 hyperthermia treatment, 12-15 magnetic resonance imaging (MRI), 6, 7, 13, 16-18 and biosensors. 19, 20 Various synthesis methods have been developed for the production of monodisperse ferrite NCs. 5, 21, 22 In some cases, size selective precipitation has been employed to further narrow the size distribution of the particles. 2 These developments have enabled extensive study of the DC23 and AC24, 25 magnetic properties of ferrite NCs with fine control over their sizes and shapes.

34-41

The concentration of substitutional cations within the spinel ferrite NCs also has a significant effect on their magnetic properties. 23, 42-44 For cobalt ferrite, the magnetocrystalline anisotropy is very sensitive to the doping level of Co2+ with larger amounts of Co2+ resulting in higher magnetic anisotropy and coercivity, until a maximum at approximately x = 0.6 for CoxFe3-xO4. Then it decreases with more Co added.23, 42 This is due to the spinorbit coupling effect on the electrons of Co2+, originating from their strong orbital angular momentum. 23, 42, 45 In contrast, Mn2+ has no net orbital angular momentum, and the spin-orbital coupling effect is absent, resulting in very low magnetocrystalline anisotropy. 46 As a result, the magnetocrystalline anisotropy of bulk manganese ferrite crystals decreases as the doping level of Mn2+ increases. 46

The magnetic properties of ferrite NCs are very sensitive to the metal elements added, varying from soft magnets (such as manganese and zinc ferrite) with low magnetocrystalline anisotropy, to hard magnets (such as cobalt ferrite) with high magnetocrystalline anisotropy and coercivity. 26, 27 The distinct responses of soft and hard spinel ferrite NCs to AC magnetic fields are the bases for two different application areas: electromagnetic devices and thermoablation. For the application of magnetic core materials in electronic devices, soft magnetic materials with high magnetic susceptibility and low power loss at high operating frequencies are needed. 9-11 Conversely, for radio frequency thermoablation, such as cancer treatment and drug delivery, hard magnetic materials with high energy dissipation efficiency under AC magnetic field are desired. 28-30 In both circumstances, it is necessary to

It has been established that the magnetic properties of ferrite NCs are strongly affected by their shapes39, 47, 48 and sizes. 39, 48-51 To study the effect of M:Fe ratio (M=Mn, Co) on the AC magnetic properties of spinel ferrite, it is important to keep similar average size, shape, and monodispersity for all samples of different compositions. This brings a significant challenge to the synthesis of ferrite NCs with precise control over size, shape while varying the chemical compositions. However, such fine control has been absent in most studies on the

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under 8000 RPM for 10 minutes to remove large particles in the distribution. Next, 5 mL oleic acid was added to the supernatant and centrifuged to precipitate all remaining particles. The precipitate was dispersed in hexane, and washed one more time with ethanol to remove extra oleic acid.

composition dependence of DC magnetic properties of ferrite NCs. 43, 44, 52, 53 Also, similar studies on the frequency dependent AC magnetic properties of ferrite NCs at radio frequencies is scarce as well. We present the synthesis, size selective precipitation, and systematic DC and AC magnetic properties characterizations of MnxFe3-xO4 (x=0.11-0.49) and CoxFe3xO4 (x=0.06-0.55) NCs, as model systems for hard and soft magnetic ferrites. Manganese and cobalt ferrite samples of similar average size (~ 6 nm in diameter) and shape were synthesized. In addition, size selective precipitation is used to further narrow the size distributions.

By replacing cobalt (II) acetylacetonate with manganese (II) acetylacetonate while keeping all the other experimental conditions the same, nonstoichiometric manganese ferrite NCs were prepared. The precursor ratios and the corresponding products metal ratios are listed in Table 1. Characterizations. Transmission electron microscopy images were taken using JEM 2100 microscope with a Gatan Orius CCD camera. Wide Angle X-ray Scattering (WAXS) data were collected with a Rigaku Smartlab highresolution diffractometer using Cu Kα radiation (λ=1.5416 Å). Small-angle X-ray scattering (SAXS) spectra were acquired at the Multi-Angle X-ray Scattering Facility, also using Cu Kα radiation. The data were fitted using Datasqueeze software. 54

EXPERIMENTAL METHODS Chemicals. Iron (III) acetylacetonate (99%) was purchased from Strem Chemicals. Manganese (II) acetylacetonate (99%) was purchased from Acros Organics. Cobalt (II) acetylacetonate (97%), 1, 2tetradecanediol (90%), oleic acid (90%), oleylamine (70%), benzyl ether (98%) were purchased from Sigma Aldrich. All chemicals were used without further purification.

Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). For ICP-AES analysis the NC samples were digested in concentrated aqua regia (3 to 1 ratio of hydrochloric acid to nitric acid) overnight. The resulting solutions were diluted to 0.5-10 ppm. The metal calibration standards (0.1-50 ppm) were prepared by diluting aliquots from Inorganic Ventures stock solutions of 1000 ppm metal content (iron, cobalt). The measurements were done using a Spectro Genesis spectrometer with a concentric nebulizer. The operating conditions are described in the Supporting Information.

Synthesis of manganese and cobalt ferrites NCs. The nonstoichiometric cobalt ferrite NCs were synthesized by methods reported in the literature25 with minor modifications.5 First, iron (III) acetylacetonate and cobalt (II) acetylacetonate with molar ratios varying from 23:1 to 1.6:1 and a total of 12 mmol were added to a three neck flask, together with 8 mmol (2.6 mL) oleic acid, 40 mmol (12.8 mL) oleylamine, 24 mmol (5.53 g) 1,2tetradecanediol, and 12 mL of benzyl ether. The mixture was heated to 110 °C under fast N2 flow and kept for 60 minutes and then heated at a rate of 8 °C/minute to 205 °C and kept for 90 minutes. Next, the reaction was heated to 295 °C with 8 °C/minutes ramp rate and kept at this temperature for 1 h to yield narrow size distribution, 5 then cooled to room temperature. To purify samples, oleic acid was added to the reaction mixture with 1:1 volume ratio to precipitate NCs. The particles were then re-dispersed in hexane solution, and washed with ethanol one time.

DC magnetic characterizations. Zero-field cooling, field cooling and hysteresis curves were conducted on a Quantum Design MPMS 7 Tesla superconducting quantum interference device (SQUID). Zero-Field Cooling (ZFC) curves were collected by first cooling down samples from 300K to 15K under zero magnetic field, then the magnetic moment was measured as the temperature was increased from 15K to 300K under an applied magnetic field of 0.03T. Field Cooling (FC) curves were collected by first cooling down samples from 300K to 15K under a 0.03T field, and then the magnetic moment was measured as the temperature was increased from 15K to 300K under the same field. Hysteresis curves were collected from -7.00 to 7.00 T magnetic field at 15 K and 300 K.

Size selective precipitation of NCs. To further reduce the size distribution of particles (from 12 % to 6-8 % in the case of Mn0.44Fe2.56O4 NCs), size selective precipitation was conducted using oleic acid as the precipitant. To 20 mL of hexane dispersion of NCs was added 10 mL of oleic acid dropwise and mixed by shaking the solution after each drop. The precipitate was observed in the solution during the process. Then the solution was centrifuged under 8000 RPM for 10 minutes, the liquid phase is discarded removing the smaller particles in the dispersion. The precipitate was collected, re-dispersed in 20 mL of hexane, and 7.5 mL of oleic acid was added dropwise, shaking after each after each drop to partially precipitate the NCs. The solution was centrifuged again

AC magnetic characterizations. AC magnetic susceptibility measurement was conducted using a 4395A Agilent Network Analyzer with a 16454A Agilent Magnetic Material Test Fixture. A toroidal shaped sample holder (8 mm of outer diameter, 3.2 mm of inner diameter, 3 mm of height, and 2.5 mm of depth) was filled with NCs by slowly depositing a hexane dispersion of NCs. Then the holder was placed in the test fixture. The reactance and resistance data were collected under sweeping frequency

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from 10MHz to 500MHz in log frequency. The real and imaginary parts of susceptibility were calculated from reactance and resistance data by Equation (3).

adjusting the amount of oleic acid, also produces controllable average sizes within a small range. Using the method described above, the average sizes of different samples were narrowed down and brought closer.

RESULTS AND DISCUSSIONS Size selective precipitation using oleic acid as a precipitant Cobalt ferrites (CoxFe3-xO4) with x values from 0.06 to 0.55 and manganese ferrites (MnxFe3-xO4) with x values from 0.11 to 0.49 are synthesized by the literature method with minor modifications25. The as-synthesized products have relatively wide size distributions. Therefore size selective precipitation was conducted by adding oleic acid as a precipitant. In conventional methods, 2, 55 polar antisolvents such as ethanol and methanol have been used for size selective precipitation. However, it has been reported that excessive use of short chain protic solvents can strip off the protecting oleate capping ligand on NC surfaces, 56 thus destabilizing the particles. Therefore, oleic acid was employed in our study to avoid this problem. Although oleic acid is a common capping ligand for NCs, it was reported that excessive amount of oleic acid functioned as an anti-solvent to precipitate and purify PbS NCs. 57, 58 This can be explained by the increased solvent polarity of the hexane – oleic acid mixture. In general, it is observed that as the amount of oleic acid increases, larger particles precipitate first. Thus, oleic acid can serve as an alternative choice for size selective precipitation. For example, the size selection of the Mn0.44Fe2.56O4 NCs was conducted as described in the experimental section. First, 10 ml oleic acid was added to 20 ml of manganese ferrite hexane dispersion and centrifuged to collect particles in the precipitate and to remove smaller particles remaining in solution. Next, the solid particles were re-dispersed to 20 ml hexane. Then 7.5 ml oleic acid was added to remove the larger particles in precipitates, and the particles remaining in solution had similar sizes after the selection. On the other hand, when 5 ml & 7.5 ml of oleic acid were used for size selection, larger average sized particles were obtained. The samples before size selection (A), after size selection by 5 ml - 7.5 ml oleic acid (to remove largest and smallest particles, respectively) (B), and by 7.5 ml - 10 ml oleic acid (C) were collected, and characterized by Small Angle X-ray Scattering (SAXS) to measure their average sizes and size distributions. Figure 1 shows the TEM images and SAXS results of samples fitted using spherical form factors59. It is shown that the size distribution of both Samples B and C are significantly narrowed after size selection. Moreover, the average size of Sample C (6.3 nm) is obviously smaller than those of Samples A & B (both 7 nm). The size distribution histogram collected from TEM images (Figure 1.e) shows a similar trend, except the average sizes are larger than the values determined by SAXS fitting simulation. These data prove that the method significantly narrows size distributions and, by

Figure 1. (a), (b), (c) are TEM images of Mn0.44Fe2.56O4 NCs before size selection(a), after size selection by 5 mL - 7.5 mL oleic acid (b), and by 7.5 mL - 10 mL oleic acid (c). Figure (d) shows the Small Angle X-ray Scattering (SAXS) results of the corresponding samples. The average sizes are A: 7.0 nm, B: 7.0 nm C: 6.3 nm. The standard deviations of sizes are A: 12%, B: 6% and C: 8%. Figure (e) shows the size distribution histograms of the three samples with their average diameter d and standard deviation σ.

TEM, ICP, SAXS and WAXS results Figure 2 shows the TEM and corresponding HRTEM images of representative cobalt ferrite and manganese ferrite samples after size selective precipitations. The TEM images in Figure S1 show that the samples show similar sizes and spherical shapes. Good crystallinity is observed in the HRTEM (Figure 2b and 2d) of the NCs. The lattice distances shown in the figures are about 0.257 nm and 0.253 nm for cobalt ferrite NCs and manganese ferrite NCs, respectively, which are close to the reported values60 of (311) plane spacing in inverse spinel structure.

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Figure 2. TEM and HRTEM images of representative ferrite samples. (a) TEM and (b) HRTEM images of Co0.42Fe2.58O4. (c) TEM and (d) HRTEM images of Mn0.44Fe2.56O4.

Figure 3. Small Angle X-ray Scattering (SAXS) (left) and Wide Angle X-ray Scattering (WAXS) (right) plots of (a) cobalt ferrites and (b) manganese ferrites samples. For SAXS plots, the lines show fitting results by Rayleigh function. For WAXS plots, all the peaks correspond to the diffractions from the faces of spinel ferrite type crystals. (220): 30.1°, (311): 35.5°, (400): 43.0°, (422): 53.3°, (511) and (333): 56.9°, (440): 62.5°.

Table 1. Average and standard deviation of sample diameters, determined by SAXS fitting results. Composition

Average size (nm)

Standard deviation size (σ)

Mn0.11Fe2.89O4

6.4

11%

Mn0.18Fe2.82O4

6.3

8%

Mn0.29Fe2.71O4

6.3

7%

Mn0.44Fe2.56O4

6.5

7%

Mn0.49Fe2.51O4

6.4

9%

Co0.06Fe2.94O4

6.3

18%

Co0.12Fe2.88O4

5.9

13%

Co0.19Fe2.81O4

6.0

15%

Co0.37Fe2.63O4

6.0

12%

Co0.42Fe2.58O4

6.4

10%

Co0.55Fe2.45O4

6.4

10%

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The samples were further characterized by x-ray scattering. Figure 3 shows the SAXS (left) and WAXS (right) patterns of (a) cobalt ferrites and (b) manganese ferrites samples. The WAXS patterns indicate typical spinel crystal structures in good agreement with reported values6, 23. The SAXS plots are fitted using Rayleigh function. The average particle sizes and standard size deviations are derived from the fitting results, and summarized in Table 1. It can be seen that the NCs with different x values have similar average sizes (5.9nm to 6.5nm). The signals from 0.5° to 1.0° were depressed comparing to the fitted curves, this is mainly caused by strong interparticle interactions or even aggregation. The size distributions of samples vary with different x values. In synthesis, higher concentration of Fe(acac)3 precursor tends to cause wider size distribution in products. For cobalt ferrite, the samples of x = 0.55, 0.42, 0.37 have relatively narrow size distributions (standard deviation σ ≈ 10%), but as the ratio of Fe continues to increase, the x = 0.19, 0.12, 0.06 samples have larger size distributions (σ = 13% to 18%). In contrast, all the manganese ferrite samples show good monodispersity (σ ≤ 9 %) except the sample of x = 0.11 (σ = 11%). In general, because of the similar average size, shape and surface coating (oleic acid), the composition dependence of magnetic properties of cobalt and manganese ferrites can be independently studied.

The compositions of the particles are controlled by adjusting the precursor ratios (M(acac)2 : Fe(acac)3, M=Co or Mn) in synthesis. The M:Fe ratios in the NCs were characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES). In general, higher M:Fe ratios in precursor give rise to higher ratios in products, but the M2+ concentrations in products are lower than those in precursors. The Co(acac)2:Fe(acac)3 and Mn(acac)2:Fe(acac)3 molar ratios from synthesis, and the corresponding ratios in NCs are summarized in Table S1.

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DC magnetic characterizations

The TB values of cobalt ferrites are summarized in Figure 4 (h). It can be seen that TB increases from 60K to 260K as x increases from 0.06 to 0.55, revealing the same trend for anisotropy values. Figure 4 (c) shows the 7 T hysteresis curves of cobalt ferrite samples at 15K. Figure 4 (h) summarizes the coercivity at 15K for all samples. The coercivity of samples increases as the ratio of Co increases in composition. This is in good agreement with the literature results23. At 300K, all samples show superparamagnetic behavior, with zero coercivity (Figure 4 (d)). In general, the increasing coercivity and blocking temperature is due to the increasing magnetocrystalline anisotropy associated with the spin-orbital coupling effect of Co2+.

Figure 4 shows the DC magnetic properties (ZFC, FC and hysteresis) on manganese and cobalt ferrites NCs. Figure 4 (a) and (b) show the ZFC-FC curves of samples measuring from 15K to 300K. The temperature at which the ZFC and FC curves converge is the blocking temperature, TB, above which the hysteresis of NCs reduces to zero and the NCs become superparamagnetic. 2 At TB, the anisotropic energy KV is proportional to the thermal energy of particles, kBTB. 26, 39 Considering that all samples have similar volume, a higher TB indicates higher magnetocrystalline anisotropy (K) of the NCs.

Figure 4 (a) also shows that the FC curves of all samples are flat at low temperature. This is typically considered an indication that strong dipolar interactions exist among particles. 8, 61-64 Such interactions will cause the ferrite NCs to have higher anisotropy values,65 and may cause an increase in blocking temperatures.26 Table 2. Magnetic moment of ferrite samples at 300K and 15K. Sample Moment at 300K Moment at 15K (emu/g) (emu/g) Mn0.11Fe2.89O4

68.7

78.2

Mn0.18Fe2.82O4

61.1

70.6

Mn0.29Fe2.71O4

59.3

69.3

Mn0.44Fe2.56O4

63.3

75.6

Mn0.49Fe2.51O4

66.3

78.3

Co0.06Fe2.94O4

74.7

81.3

Co0.12Fe2.88O4

79.8

93.7

Co0.19Fe2.81O4

65.6

71.6

Co0.37Fe2.63O4

69.6

83.4

Co0.42Fe2.58O4

75.2

80.9

Co0.55Fe2.45O4

80.5

85.4

The magnetic properties of manganese ferrites show sharp contrast with those of cobalt ferrites. Figure 4(b) shows that all manganese ferrite samples have almost the same blocking temperature (TB ~ 30K), which is much lower than that of cobalt ferrites. The blocking temperature is so low that at the minimum measurement temperature (15K), the ZFC magnetic moment is still close to the maximum. It is expected to drop if the starting temperature is lower. The spin-orbital coupling effect of Co2+ is absent in Mn2+ because the orbital momentum is quenched in Mn2+,46, 66 which explains why the blocking temperature is not affected by the amount of manganese as much as that of cobalt. Considering that all samples have similar average size, the same TB indicates that all samples have similar anisotropy coefficient. Besides, all the samples show very small coercivity values (about 6

Figure 4. DC magnetic characterizations of cobalt ferrite NCs and manganese ferrite NCs. (a) zero-field cooling and field cooling curves of cobalt ferrites. (b) zero-field cooling and field cooling curves of manganese ferrites. (c) Hysteresis curves of cobalt ferrites at 15K. (d) Hysteresis curves of cobalt ferrites at 300K. (e) Hysteresis curves of manganese ferrites at 15K. (f) Hysteresis curves of manganese ferrites at 15K, zoomed in at around zero field to show coercivity of samples. (g) Hysteresis curves of manganese ferrites taken at 300K. (h) Blocking temperature (black), and coercivity at 15K (red) plotted against x of CoxFe3-xO4, the curves are polynomial fitting results to show the trends.

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mT) even at 15 K (Figures 4(e) and 4(f)). At 300 K, all samples show almost identical hysteresis with zero coercivity (Figure 4(g)). In general, the manganese ferrite samples share similar DC magnetic properties despite differences in their compositions.

= − " , = +

AC susceptibility characterization The AC susceptibility characterization was conducted using the same method as published previously. 67, 68 In this method, a one-turn inductor type circuit is placed in the instrument with a standard size, toroidal shape sample holder filled with magnetic NC solid deposited as the inductor core. An alternating current of frequencies scanning from 10MHz to 500MHz is then applied to the inductor. The frequency-dependent reactance and resistance of the circuit are recorded. The susceptibility can be calculated using:

(1)

Here Zm is the impedance measured with sample mounted, Zsm is the blank impedance (measured with empty sample holder), f is frequency, h is the height of the toroidal sample, c is the outer diameter of sample, and b is the inner diameter of sample. The impedance Zm is equal to Rm + iXm where R is resistance and X is reactance. Zsm is calibrated before measurement. Therefore: =

−

And the frequency dependent real and imaginary parts of magnetic susceptibility are: =

, "

=

( )

(4)

Figure 5(b), (d) shows the frequency dependent real and imaginary parts of susceptibility (χ’ and χ”) for manganese ferrite samples. Again, the trend is very different from that of cobalt ferrite. The χ’ and χ” values for manganese ferrites are higher than those of cobalt ferrite samples, this is because manganese ferrite has higher saturation magnetization associated with a larger moment for Mn2+ versus Co2+.24, 46 Manganese ferrite is inverse spinel ferrite and Mn2+ replaces Fe2+ ions in the octahedral sites73, 74. Because Fe2+ has only 4 unpaired electrons, compared to 5 for Mn2+, as the ratio of Mn:Fe increases, higher saturation magnetization results,43 which explains why the χ’ and χ” values increases with increasing Mn:Fe ratios (Figure 5(b), (d)).

(2)

, "

where χ0 is zero frequency (DC) susceptibility, χ∞ is infinitely high frequency susceptibility. ω=2πf is the angular frequency of the field. χ” value reaches the maximum point when τ = 1/ωmax, where τ is the magnetic spin relaxation time of NCs, ωmax is the angular frequency of magnetic field, ωmax=2πfmax. When placed under AC magnetic field, the magnetic spin rotates through the Brownian and Néel relaxation mechanisms32, 70. In Brownian relaxation, the NCs physically rotate to change their magnetic moment directions70. In Néel relaxation, the magnetic moment rotates within the magnetic core, while the particle itself stays stationary71. Here the magnetic spin relaxation is dominated by Néel mechanism, the Brownian relaxation is hindered because the samples are solid deposits. Néel relaxation time is mainly affected by KV/kBT, 72 so a higher τ indicates higher anisotropic energy KV at a given temperature. Figure 5(c) shows that as x increases from 0.06 to 0.55 for CoxFe3-xO4 samples, fmax decreases from about 500MHz to about 10MHz, corresponding to lower KV. Also, all samples have similar average size and shape, the volumes are similar. So the decreasing fmax value shown in the experiment is mainly caused by higher K. Therefore, the experimental evidence shows that the crystal anisotropy of cobalt ferrites is highly sensitive to the doping level of Co2+ due to the strong spin-orbital coupling effect of cobalt23, 42.

The table 2 shows the magnetic moments of all samples at 300K and 15K. For manganese ferrites, it shows interesting trend that the magnetic moments first decrease then increase. It reaches the minimum for sample Mn0.29Fe2.71O4. For cobalt ferrites, the trend is similar except for Co0.12Fe2.88O4 which has high magnetic moment.

=

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(3)

Figure 5(a) shows the frequency dependent real part of susceptibility (χ’) curves for cobalt ferrite samples. The values are between 0.4-0.7 at 10 MHz and decrease as frequencies become higher. Figure 5(c) shows the imaginary part of susceptibility (χ”). Here the frequency ωmax (where the χ” values reach maximum) decreases as the concentration of cobalt increases in the samples. For example, the Co0.12Fe2.88O4 sample (red) reaches maximum at about 200 MHz. Also, a maximum is expected on frequency slightly below 10 MHz for samples Co0.55Fe2.45O4 and Co0.42Fe2.58O4, and around 500 MHz for sample x=0.06 by the trend.

Unlike the trend of cobalt ferrites, the χ” values of manganese ferrites increase monotonically with frequency (Figure 5(d)). This implies that the superparamagnetic-ferromagnetic relaxation frequencies of samples are higher than 500MHz, so the Néel relaxation time of manganese ferrites is much shorter than that of cobalt ferrite, which indicates very small anisotropic energy (KV) of particles. Also, this is originated from the 3d orbital configurations. The 5 unpaired electrons of Mn2+ occupy all 3d octahedral orbitals symmetrically, leading to zero net orbital angular momentum. As a result, the spin-orbital coupling is mostly quenched, giving rise to a much lower crystal anisotropy46.

Such results can be explained by the differences in anisotropic constant of samples. According to the Debye model31, 32, 69, the complex susceptibility of monodisperse magnetic NCs with a single relaxation time τ is as follows:

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Chemistry of Materials lower tan δ value in the frequency range of measurement. This can be attributed to its wider size distribution69. For cobalt ferrites, the loss tangent curves are very similar to the χ” curves. By changing the amount of cobalt, the maximum value of tangent loss shifts from about 10 MHz (x=0.55) to about 500MHz (x=0.06).

Figure 5. Frequency dependent real and imaginary parts of susceptibility plots of cobalt and manganese ferrites. (a) χ’ plots of cobalt ferrites. (b) χ’ plots of manganese ferrites. (c) χ” plots of cobalt ferrites. (d) χ” plots of manganese ferrites. (e) Loss tangent plots of cobalt ferrites. (f) Loss tangent plots of manganese ferrites.

χ” is closely related to the energy dissipation efficiency. According to Rosensweig’s model72, 75, the specific loss of power (SLP) is proportional to the product of AC field frequency and χ”. When other conditions are identical, it is expected that magnetic NCs with higher χ” value will have higher energy dissipation efficiency. Therefore, from the χ” plots in Figure 5 (c), (d) we can predict that the SLP of the cobalt ferrite NCs varies with different Co:Fe ratios at the frequency of measurement. On the other hand, for manganese ferrites, SLP is relatively insensitive to the Mn doping level. A very low SLP value is expected below 100MHz, but increases rapidly as frequency continue to increase, as can be seen from the trend of χ” plots.

Figure 6. Comparison of DC hysteresis curve slope at low field (-1 to 3 mT) and AC real part of susceptibility χ’ for samples (a) manganese ferrites, (b) cobalt ferrite.

Figure 6 shows a comparison of the DC and AC magnetic properties of the ferrite samples. Here the black lines represent the slopes of the hysteresis curves at 300K and -1 to 3 mT magnetic field. The slopes are directly derived from the data shown in Figure 4 (e), (g). The values of slope are the DC magnetic susceptibility of samples at low magnetic field. Therefore, the values should have some relevance to the AC susceptibility values (in red lines) which are also measured under a low magnetic field (0.3 Oe). For manganese ferrites (Figure 6 (a)), the DC susceptibility shows very similar trend to the AC χ’ at 10 MHz. In general, both curves show the increase of the values as the Mn concentration increases. On the other hand, the trend of cobalt ferrite (Figure 6 (b)) is different. Again, compared to Figure 5 (a), for samples x = 0.06, 0.12, which have the ferromagnetic resonance frequency larger than 10 MHz, their susceptibility values show a similar trend to the DC susceptibility. However, as Co2+ concentration increases and the maximum value of χ” is getting closer to 10 MHz, the DC and AC plots diverge. The AC susceptibility values of the samples with high Co2+ concentration are lowered because the measurement frequency (10 MHz) is close to the ferromagnetic resonance frequencies of the samples.

Figure 5 (e), (f) shows the loss tangent curves of cobalt and manganese ferrite samples. The loss tangent is defined as tan δ =χ”/(χ’+1). For low power loss device applications, high χ’ and low χ” are desired to enhance magnetic field strength and suppress energy loss at the same time. Therefore tan δ is a comprehensive indication of the energy efficiency of NCs as magnetic core material for electromagnetic device applications. From Figure 5 (f) we can see that the values of tan δ are very close for all samples except Mn0.11Fe2.89O4. Although the magnetic moment is slightly enhanced by introducing higher amount of manganese, the χ” value increases by the same ratio, giving rise to higher energy loss at the same time. On the other hand, the sample Mn0.11Fe2.89O4 has slightly

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In result, the magnetic moments of the NCs become discordant to the external magnetic field. This clearly shows the correlation between DC and AC magnetic properties, as well as the limitation of DC magnetic measurements for studying the AC properties and application performances of the samples.

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DE-AR0000123 for the development of the synthesis and the AC testing and the NSF Nano-Bio Interface Center DMR-0832802 for structural analysis. DC magnetic characterization was supported by NSF MRSEC DMR1120901. Notes The authors declare no competing financial interest.

CONCLUSION

ACKNOWLEDGMENT

We have introduced the systematic study on the synthesis, size selective precipitation and composition dependence of DC- and AC-magnetic properties of MnxFe3-xO4 and CoxFe3-xO4 NCs with similar average size and shape. The size selective precipitation method using oleic acid as precipitant led to the highly effective size selection, enabling us to derive two samples of different average sizes from only one batch of synthesis product. The size distribution of the samples was significantly narrowed as well.

The authors thank Dr. Douglas Yates and Dr. Jamie Ford at the Nanoscale Characterization Facility (NCF) at the University of Pennsylvania for the support on the transmission electron microscopy characterization.

REFERENCES 1. Desvaux, C.; Dumestre, F.; Amiens, C.; Respaud, M.; Lecante, P.; Snoeck, E.; Fejes, P.; Renaud, P.; Chaudret, B. FeCo Nanoparticles from an Organometallic Approach: Synthesis, Organisation and Physical Properties. J. Mater. Chem. 2009, 19, 3268-3275. 2. Murray, C. B.; Sun, S. H.; Doyle, H.; Betley, T. Monodisperse 3d Transition-Metal (Co, Ni, Fe) Nanoparticles and Their Assembly into Nanoparticle Superlattices. MRS Bull. 2001, 26, 985-991. 3. Paik, T.; Gordon, T. R.; Prantner, A. M.; Yun, H.; Murray, C. B. Designing Tripodal and Triangular Gadolinium Oxide Nanoplates and Self-Assembled Nanofibrils as Potential Multimodal Bioimaging Probes. ACS Nano 2013, 7, 2850-2859. 4. Sun, S. H.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Monodisperse FePt Nanoparticles and Ferromagnetic FePt Nanocrystal Superlattices. Science 2000, 287, 1989-1992. 5. Sun, S. H.; Zeng, H.; Robinson, D. B.; Raoux, S.; Rice, P. M.; Wang, S. X.; Li, G. X. Monodisperse MFe2O4 (M = Fe, Co, Mn) Nanoparticles. J. Am. Chem. Soc. 2004, 126, 273-279. 6. Tromsdorf, U. I.; Bigall, N. C.; Kaul, M. G.; Bruns, O. T.; Nikolic, M. S.; Mollwitz, B.; Sperling, R. A.; Reimer, R.; Hohenberg, H.; Parak, W. J.; Forster, S.; Beisiegel, U.; Adam, G.; Weller, H. Size and Surface Effects on the MRI Relaxivity of Manganese Ferrite Nanoparticle Contrast Agents. Nano Lett. 2007, 7, 2422-2427. 7. Kim, B. H.; Lee, N.; Kim, H.; An, K.; Park, Y. I.; Choi, Y.; Shin, K.; Lee, Y.; Kwon, S. G.; Na, H. B.; Park, J. G.; Ahn, T. Y.; Kim, Y. W.; Moon, W. K.; Choi, S. H.; Hyeon, T. Large-Scale Synthesis of Uniform and Extremely Small-Sized Iron Oxide Nanoparticles for High-Resolution T1 Magnetic Resonance Imaging Contrast Agents. J. Am. Chem. Soc. 2011, 133, 12624-12631. 8. Yun, H.; Liu, X.; Paik, T.; Palanisamy, D.; Kim, J.; Vogel, W. D.; Viescas, A. J.; Chen, J.; Papaefthymiou, G. C.; Kikkawa, J. M.; Allen, M. G.; Murray, C. B. Size- and CompositionDependent Radio Frequency Magnetic Permeability of Iron Oxide Nanocrystals. ACS Nano 2014, 8, 12323-12337. 9. Adam, J. D.; Davis, L. E.; Dionne, G. F.; Schloemann, E. F.; Stitzer, S. N. Ferrite Devices and Materials. IEEE Trans. Microw. Theory Tech. 2002, 50, 721-737. 10. Pardavi-Horvath, M. Microwave Applications of Soft Ferrites. J. Magn. Magn. Mater. 2000, 215, 171-183. 11. Song, N. N.; Yang, H. T.; Liu, H. L.; Ren, X.; Ding, H. F.; Zhang, X. Q.; Cheng, Z. H. Exceeding Natural Resonance Frequency Limit of Monodisperse Fe3O4 Nanoparticles Via Superparamagnetic Relaxation. Sci. Rep. 2013, 3, 5.

The composition dependent magnetic characterizations show that by introducing Co2+ to ferrite NCs, the magnetocrystalline anisotropy energy was greatly enhanced, causing shifts in superparamagneticferromagnetic relaxation frequency. On the other hand, for manganese ferrite, different amounts of Mn2+ in ferrite nanocrystals result in similar DC & AC magnetic properties, the superparamagnetic-ferromagnetic relaxation frequencies, and loss tangent curves all have identical trend. These composition dependent behaviors provide another important dimension of tuning the DC and AC magnetic properties of spinel ferrite NCs, other than changing the size, shape and various surface modifications, to meet the needs for different applications.

ASSOCIATED CONTENT Supporting Information. Details about Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) experimental conditions, composition data, TEM images and DC hysteresis characterizations of all sample. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

Author Contributions All authors have given approval to the final version of the manuscript.

Funding Sources This work was supported by the Department of Energy's Advanced Research Projects Agency-Energy (ARPA-E)

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Synthesis and Size-Selective Precipitation of Monodisperse

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