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Giant enhancement and anomalous thermal hysteresis of saturation moment in magnetic nanoparticles embedded in multiwalled carbon nanotubes Guo-meng Zhao, Jun Wang, and Yang Ren Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl401685h • Publication Date (Web): 23 May 2013 Downloaded from http://pubs.acs.org on May 24, 2013
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Giant enhancement and anomalous thermal hysteresis of saturation moment in magnetic nanoparticles embedded in multiwalled carbon nanotubes Guo-meng Zhao1,2,∗ , Jun Wang2 , Yang Ren3 , and Pieder Beeli1 1
Department of Physics and Astronomy,
California State University at Los Angeles, Los Angeles, CA 90032, USA
2
Department of Physics,
Faculty of Science, Ningbo University, Ningbo, P. R. China
3
X-Ray Science Division,
Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA
Abstract we report high-energy synchrotron x-ray diffraction spectrum and high-temperature magnetic data for multiwalled carbon nanotubes embedded with Fe and Fe3 O4 nanoparticles. We unambiguously show that the saturation moments of the embedded Fe and Fe3 O4 nanoparticles are enhanced by a factor of about 3.0 compared with what would be expected if they would be unembedded. More intriguingly the enhanced moments were completely lost when the sample was heated up to 1120 K and the lost moments were completely recovered through two more thermal cycles below 1020 K. These novel results cannot be explained by magnetism of the Fe and Fe3 O4 impurity phases, magnetic proximity effect between magnetic nanoparticles and carbon, and ballistic transport of multiwalled carbon nanotubes.
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Graphene is a sheet of carbon atoms distributed in a honeycomb lattice and is the building block for graphite and carbon nanotubes. Originating from conical valence and conduction bands that meet at a single point in momentum space, the massless charge carriers of graphene, known as Dirac fermions, exhibit relativistic behavior. Strong electron-electron correlation of the Dirac fermions in graphene can lead to the formation of a short-range resonating-valence-bond (RVB) liquid [1] and/or to a ferromagnetic instability [2]. On the basis of the RVB theory of superconductivity originally proposed by Anderson [3], heavily doped graphene can exhibit ultrahigh temperature d-wave superconductivity [4]. These theoretical works appear to agree with experimental observations of the intrinsic hightemperature ferromagnetism in graphite and carbon-based materials [5–9] and possible hightemperature superconductivity in carbon films [10, 11], highly oriented pyrolithic graphite [5], carbon nanotubes [12–14], and amorphous carbon [15]. Here we report high-energy synchrotron x-ray diffraction spectrum and high-temperature magnetic data for MWCNTs embedded with Fe and Fe3 O4 nanoparticles. We unambiguously show that the saturation moments of the embedded Fe and Fe3 O4 nanoparticles are enhanced by a factor of about 3.0 compared with what would be expected if they would be unembedded. More intriguingly, the thermal hysteresis of the saturation moments is anomalous and cannot be explained by any magnetism, magnetic proximity effect, and ballistic transport. MWCNT powder sample embedded with Fe and Fe3 O4 nanoparticles was obtained from SES Research of Houston (Catalog No. RS0657). The sample was synthesized by chemical vapor deposition under catalyzation of Fe nanoparticles. During the purification process, some Fe nanoparticles were oxidized into the Fe3 O4 and α-Fe2 O3 phases and were removed partially by HCl. The metal-based impurity concentrations of the sample were determined by a Perkin-Elmer Elan-DRCe inductively coupled plasma mass spectrometer (ICP-MS), which yielded the metal-based magnetic impurity concentrations (by weight): Fe = 0.69±0.02%, Co = 0.0036±0.0002%, Ni = 0.0021±0.0001%. The morphology of the sample was checked by scanning electron microscopy, which shows that the tubes are quite uniform and their mean outer diameter is about 70 nm (see Supporting Information, Fig. 1a). Transmission electron microscopic image indicates that the mean wall thickness of the MWCNTs is about 10 nm (see Supporting Information, Fig. 1b). The impurity concentrations can also be determined precisely from high-energy synchrotron x-ray diffraction (XRD) data [17]. Fig. 1 shows a synchrotron XRD spectrum for the MWCNT sample along with the standard spectra of α-Fe, Fe3 O4 , and α-Fe2 O3 phases.
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300
Intensity (arb. units)
250
MWCNTs α-Fe2O3 F e3O4 α-Fe
200 150 100 50 0 -50 1
2
3
4
5
6
7
2θ (degree)
FIG. 1: High-energy synchrotron x-ray diffraction spectrum of a MWCNT sample along with the standard spectra of α-Fe, Fe3 O4 , and α-Fe2 O3 phases. The wavelength of the high-energy x-ray is 0.1078 ˚ A. In addition to the major peaks corresponding to the diffraction peaks of MWCNTs [16], there are many minor peaks, which match well with all the peaks of the α-Fe, Fe3 O4 , and α-Fe2 O3 phases. This indicates that the visible impurity phases are α-Fe, Fe3 O4 , and α-Fe2 O3 , in agreement with the impurity analysis using the ICP-MS.
Using monochromated radiation with a wavelength of λ = 0.1078 ˚ A, the XRD spectrum was taken on a high-energy synchrotron x-ray beam-line 11-ID-C at the Advanced Photon Source, Argonne National Laboratory. In addition to the major peaks corresponding to the diffraction peaks of MWCNTs [16], there are many minor peaks, which match all the peaks of the α-Fe, Fe3 O4 , and α-Fe2 O3 phases. This indicates that the visible impurity phases are α-Fe, Fe3 O4 , and α-Fe2 O3 , in agreement with the impurity analysis using the ICP-MS. Detailed quantitative analyses (see Supporting Information, section 2) of the XRD spectrum with the same method presented in Ref. [17] show that the sample contains (by weight) 0.241±0.004% of Fe, 0.216±0.015% of α-Fe2 O3 , and 0.250±0.010% of Fe3 O4 . These impurity phases contribute to a metal-based Fe concentration of 0.58±0.02%, which is about 0.11±0.04% lower than the total Fe concentration (0.69±0.02%) determined from the ICPMS. This 0.11±0.04% discrepancy is due to the presence of a minor phase of Fe3 C, which is too little to be unresolved in the XRD spectrum but can be clearly seen from the magnetization data below (also see Supporting Information, Figure 5b). From the widths of the XRD peaks and using Scherrer’s formula, we can determine the mean diameters of the nanoparticles and the mean wall thickness of the MWCNTs (see Supporting Information, section 2). The mean diameters for Fe, α-Fe2 O3 , and Fe3 O4 3
nanoparticles are found to be 46, 23, and 18 nm, respectively. Figure 2 shows low-field (50 Oe) magnetic thermal hysteresis for a MWCNT sample of RS0657. Magnetic moment was measured using a Quantum Design vibrating sample magnetometer (VSM). The low-field magnetic data clearly show two magnetic transitions at 850 K and 1047 K, which are associated with the ferrimagnetic transition of the Fe3 O4 impurity phase and the ferromagnetic transition of the α-Fe phase, respectively. What is unusual is that the field-cooled-cooling (FCC) susceptibility is always lower than the fieldcooled-warming (FCW) susceptibility (see curve 2). At 1000 K the FCW susceptibility is larger than the FCC susceptibility by a factor of about 1.8, and at 600 K the FCW susceptibility is larger than the FCC susceptibility by a factor of about 1.7. 1.5 FCW
χ (10-3emu/g)
2 1.0 1
0.5 FCC 2 1
H = 50 Oe
0.0 300 400 500 600 700 800 900 1000 1100
T (K)
FIG. 2: Low-field (50 Oe) magnetic thermal hysteresis for a MWCNT sample of RS0657. At 1000 K the FCW susceptibility is larger than the FCC susceptibility by a factor of about 1.8, and at 600 K the FCW susceptibility is larger than the FCC susceptibility by a factor of about 1.7.
Figure 3 shows temperature dependence of high-field (10 kOe) magnetization for another virgin (unmagnetized) MWCNT sample of RS0657. Since the magnetization in 10 kOe is close to the saturation magnetization Ms (see Supporting Information, Fig. 3), the temperature dependence of the saturation magnetization is approximated by the temperature dependence of the magnetization in 10 kOe. The initial warm-up curve in Fig. 3 clearly demonstrates three magnetic transitions at about 500 K, 860 K, and 1060 K, which should be associated with the magnetic transitions of the Fe3 C, Fe3 O4 , and α-Fe impurity phases, respectively. What is remarkable is that when the sample was cooled from 1120 K, the cool-down magnetization was reduced dramatically compared with the initial warm-up magnetization. 4
1.6 1.4
H = 10 kOe
M (emu/g)
1.2 1.0 2.9
0.8 0.6
Fe+Fe3O 4
0.4 0.2
3.0 Fe
0.0 300 400 500 600 700 800 900 10001100
T (K)
FIG. 3: Temperature dependence of high-field (10 kOe) magnetization for another virgin (unmagnetized) MWCNT sample of RS0657. The solid blue line represents the expected contribution from the Fe impurity phase if it would be isolated from MWCNTs. The solid green line is the expected contributions of both Fe and Fe3 O4 impurity phases below the Curie temperature of the Fe3 O4 phase if they would be isolated from MWCNTs. The initial warm-up magnetization is larger than the expected magnetization by a factor of about 3 at both 600 K and 1000 K, as indicated by the numbers next to the vertical arrows.
The solid blue line represents the expected contribution from the Fe impurity phase if it would be isolated from MWCNTs. The solid green line is the expected contributions of both Fe and Fe3 O4 impurity phases below the Curie temperature of the Fe3 O4 phase if they would be isolated from MWCNTs. These expected curves are calculated according to the measured concentrations of the Fe and Fe3 O4 impurity phases and their expected saturation magnetizations (see Supplemental Information, section 3). Comparing the initial warm-up curve with the expected solid curves, one can clearly see that the initial warm-up magnetization is larger than the expected magnetization by a factor of 2.9 at 600 K and 3.0 at 1000 K (marked by the numbers next to the black vertical arrows). This implies that the moment enhancement factors for both Fe and Fe3 O4 impurity phases are similar (about 3). It is interesting that the enhancement factors are also very close to that (about 3.4) for 11-nm nickel nanoparticles embedded in MWCNTs [17], suggesting a common origin. Another striking feature is that the solid blue line in Fig. 3 almost overlaps with the cool-down data points between 960 and 1060 K. This implies that there is no moment enhancement in this temperature range upon cooling from 1120 K. This in turn implies that the initally enhanced moment at 300 K (via applying a magnetic field of 10 kOe) was 5
completely lost by heating the sample up to 1120 K and not recovered until 960 K upon cooling from 1120 K. There is a sharp increase in the magnetic moment at about 960 K, indicating an onset of the moment enhancement.
M (emu/g)
1.6 1.4
3
1.2
2
1
1 2 3
1.0
3
0.8 2
0.6 0.4
1
H = 10 kOe
0.2 0.0 300 400 500 600 700 800 900 10001100
T (K)
FIG. 4: Thermal hysteresis of saturation magnetization for the MWCNT sample. There are three thermal cycles, labeled by 1, 2 and 3. It is striking that through two more thermal cycles (curves 2 and 3) below 1020 K, the lost moment during the first heating measurement (up to 1120 K) was recovered almost completely.
Figure 4 show three thermal cycles of the saturation magnetization including the first cycle already presented in Fig. 3. It is striking that through two more thermal cycles (curves 2 and 3) below 1020 K, the lost moment during the first heating measurement (up to 1120 K) was recovered almost completely. Curve 3 also implies that the enhanced moment of the embedded Fe3 O4 nanoparticles is lost completely at 920 K and starts to gain back upon cooling down to 700 K. In contrast, the enhanced moment of the embedded Fe nanoparticles starts to get lost above 1000 K and starts to gain back upon cooling down to 980 K (see curve 2). This indicates that the onset temperature of the moment enhancement strongly depends on the Curie temperatures of magnetic impurity phases. Moreover, the significant moment enhancement always occurs in the temperature range between 600 and 700 K, independent of whether the sample is heated or cooled. In Figure 5, we show the temperature dependence of the field-cooled magnetization, which was measured in a magnetic field of 20 kOe and with different cooling rates. The high-field (saturation) magnetization is independent of the cooling rate in the temperature range above 700 K while the magnetization at lower temperatures decreases significantly with increasing cooling rate. Since the moment of the Fe3 O4 phase between 700 K and 850 K is close to the 6
1.6 10 K/min 20 K/min 30 K/min
1.4
M (emu/g)
1.2 1.0 H = 20 kOe
0.8 0.6 0.4 300
400
500
600
700
800
900 1000
T (K)
FIG. 5: Temperature dependence of the field-cooled magnetization measured with different cooling rates and in a field of 20 kOe. The vertical arrow shows the increasing direction of the cooling rate. It is apparent that the moment enhancement increases significantly with decreasing cooling rate.
value expected for unembedded Fe3 O4 nanoparticles (compare the solid green line in Fig. 3), the sudden moment increase below 700 K is caused by moment enhancement. Therefore, the result clearly demonstrates that the moment enhancement increases significantly with decreasing cooling rate. In section 5 of Supporting Information, we have provided 6 possible explanations to our intriguing experimental results in terms of ferrimagnetism/ferromagnetism of embedded nanoparticles, interactions between two nanoparticle components, superparamagnetism, strong diamagnetism of MWCNTs due to ballistic transport, magnetic proximity effect between spins of magnetic nanoparticles and conducting electrons of doped MWCNTs, and high-field paramagnetic Meissner effect (HFPME). Since the high-field paramagnetic Meissner effect requires our doped MWCNTs to exhibit ultrahigh temperature superconductivity (UHTSC) and there is no direct experimental evidence for UHTSC in MWCNTs, the HFPME interpretation still shows a lack of foundation. Regardless of interpretations, our intriguing experimental results will stimulate researchers to develop new theoretical models to understand the strong interplay between magnetic nanoparticles and carbon-based materials. Moreover, the strong interplay between spin degree of freedom in magnetic nanoparticles and electronic degree of freedom in MWCNTs will also open new perspectives for spin-based high-speed and high-sensitivity devices, where logic and memory elements are integrated at the molecular level. Acknowledgment: We thank M. Du and F. M. Zhou for the elemental analyses using
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ICP-MS. We also thank the state of California for providing a fund for a Quantum Design Physical Property Measurement System (PPMS). Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. This work was partly supported by the National Natural Science Foundation of China (Grants No. 11174165). We would also like to thank the MBRS-RISE program and the NIH for their support under grant No. NIH RISE GM61331. ∗
[email protected] [1] Meng, Z. Y.; Lang, T. C.; Wessel, S.; Assaad, F. F.; Muramatsu, A. Nature (London) 2010, 464, 847. [2] Baskaran, G.; Jafari, S. A. Phys. Rev. Lett. 2002, 89, 016402. [3] Anderson, P. W. Science 1987, 235, 1196. [4] Black-Schaffer, A. M.; Doniach, S. Phys. Rev. B 2007, 75, 134512. [5] Kopelevich, Y.; Esquinazi, P.; Torres, J. H. S.; Moehlecke, S. J. Low Temp. Phys. 2000, 119, 691. [6] Moehlecke, S.; Ho, C.; Maple, M. B. Phil. Mag. B 2002, 82, 1335. [7] Mombru, A. W.; Pardo, H.; Faccio, R.; de Lima, O. F.; Leite, E. R.; Zanelatto, G.; Lanfredi, A. J. C.; Cardoso, C. A.; Araujo-Moreira, F. M. Phys. Rev. B 2005, 71, 100404(R). [8] Esquinazi, P.; Spemann, D.; Hohne, R.; Setzer, A.; Han, K. H.; Butz, T. Phys. Rev. Lett. 2003, 91, 227201. [9] Cervenka, J.; Katsnelson, M. I.; Flipse, C. F. J. Nature Physics 2009, 5, 840. [10] Antonowicz, K. Nature (London), 1974, 247, 358. [11] Lebedev, S. G. Nucl. Instr. Meth. 2004, A521, 22. [12] Zhao, G. M. Molecular Nanowires and Other Quantum Objects, Alexandrov, A. S., Demsar, J. & Yanson, I. K. (Ed.), (Nato Science Series, Kluwer Academic Publishers, Netherlands, 2004) 95-106. [13] Zhao, G. M. Trends in Nanotubes Research, Delores A. Martin, D. A., (Ed.), (Nova Science Publishers, New York, bf 2006) 39-75. [14] Zhao, G. M. Carbon nanotubes and their applications, edited by Zhang, Qing (Pan Stanford
8
Publishing, Singapore, 2011) 319-354. [15] Felner, I.; Kopelevich, Y. Phys. Rev. B 2009, 79, 233409. [16] Reznik, D.; Olk, C. H.; Neumann, D. A.; Copley, J. R. D. Phys. Rev. B 1995, 52, 116. [17] Wang, J.; Beeli, P.; Ren, Y.; Zhao, G. M. Phys. Rev. B 2010, 82, 193410.
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