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Microstructural and magnetic properties of Ni–Ce nanocomposite particles
Materials Science and Engineering C 16 Ž2001. 95–98 www.elsevier.comrlocatermsec
Microstructural and magnetic properties of Ni–Ce nanocomposite particles Xiangcheng Sun ) , M.J. Yacaman ININ, Amsterdam 46-202, Col. Hipodromo Condesa, 06100 Mexico D.F., Mexico ´
Abstract A new type of magnetic core–shell Ni–Ce nanocomposite particles Ž15–50 nm. are presented. Scanning electron microscope ŽSEM. images and X-ray energy-dispersive analysis ŽEDX. spectra indicate that these nanoparticles are strongly magnetic interacting order with chain-like features. Typical HREM images show that many planar defects Ži.e., nanotwins and stacking faults. exist in large Ni core zone Ž10–45 nm.; the shell layers Ž3–5 nm. are consisted of innermost NiCe alloy and outermost NiO oxide. Selected area electron diffraction ŽSAED. patterns show an indication of well-defined spots characteristic of core–shell nanocomposite materials. Magnetization measurements as a function of magnetic fields and temperatures were performed in a SQUID magnetometer. Superparamagnetic behavior above average blocking temperature ŽTB . 170 K was exhibited, this superparamagnetic relaxation behavior was found to be modified by interparticle interactions, which depend on the applied field and size distribution. In addition, antiferromagnetic order occurred with a Neel ´ temperature TN of about 11 K. A spin-flop transition was also observed below TN at a certain applied filed. In particular, electron paramagnetic resonance ŽEPR. spectra at low and room temperature reflected this magnetic order nature associated with this type of core–shell microstructure, coupling with the strong interparticle interaction. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Ni–Ce nanocomposite particles; Core–shell; Superparamagnetic behavior
1. Introduction Magnetic nanoparticles are an active subject of intense research due to their unique magnetic properties which made them appealing from both scientific and technological points of view w1–3x. Specifically, metallic ŽFe, Ni, etc.. w3x and oxide Ž g-Fe 2 O 3 , etc.. w4x magnetic nanoparticles have attracted considerable attention as high-density magnetic storage media because the high coercive force and the high saturation magnetization can be achieved. Recently, spontaneous magnetization reversal in some magnetic ferritin ŽNiFe 2 O4 , etc.. w5x nanoparticles due to surface disorder is also of significant interest, since this effects will determine the stability of stored information and limit the ultimate storage density. Moreover, experimental evidences for low temperature superparamagnetism relaxation w6x and the ordering of spin-glass like behavior w4x in some single domain magnetic particle Ži.e., g-Fe 2 O 3 , etc.. have been reported. In this study, the Ni–Ce nanocomposite particles with NiCe alloy and NiO oxide
)
Corresponding author. Tel.: q52-5-53297296; fax: q52-5-53297296. E-mail address: [email protected] ŽX. Sun..
shell layers have successfully been prepared. Our study emphasis will be involved in the microstructure features and their magnetic order properties as well as their correlation.
2. Experimental These types of Ni–Ce nanocomposite particles were prepared by the hydrogen plasma-metal reaction techniques w7x. A JEOL-4000EX high-resolution transmission electron microscope ŽHRTEM., operated at 400 KeV, was used to determine the detailed core–shell phase and average grain size of the particles. It also allowed us to record selected area electron nanodiffraction ŽSAED.. A Phillips XL30 scanning electron microscope ŽSEM. equipped with an X-ray energy-dispersive analysis ŽEDX. system was employed to provide the morphology of the samples and composition analysis. The magnetization measurements were performed by the following useful methods: zerofield-cooling ŽZFC. and field-cooling ŽFC. using a SQUID magnetometer. From the curves of M vs. T ŽZFC and FC cases., the average blocking temperature ŽT B . was defined as the maximum of ZFC curve. The electron paramagnetic
0928-4931r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 8 - 4 9 3 1 Ž 0 1 . 0 0 2 8 1 - 8
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X. Sun, M.J. Yacamanr Materials Science and Engineering C 16 (2001) 95–98
resonance ŽEPR. measurements were conducted using a JEOL JES-RE3X spectrometer.
3. Results and discussion SEM image and EDX analysis ŽFig. 1a,b. clearly indicate that these particles are nearly spherical with negligible shape anisotropy, and with strong tendency of forming chain-like features. Such chain-like behavior can be attributed to the tendency of reducing the specific surface energy of system. It is very interesting to note from the typical HREM image ŽFig. 2. of this Ni–Ce nanocomposite particles that some foot-like feature and clear lattice images of core–shell type structure are observed. Each particle consists of a large core and a thin outer shell; there exits a very clear boundary between the core and the outer shell. In addition, many structural defects are found, such as nanotwins and
stacking faults, which form the kinks and steps on the core zones of the particles. Most particles are covered with ca. 3–5-nm thick crystalline phase, which has some discontinuities Žas shown in arrow of Fig. 2.. This indicates that the shell phases are polycrystalline. The outermost layer spacing is 0.24 nm, this value corresponds to NiO Ž111. plane, and the innermost layers spacing of 0.278 and 0.219 nm have been indicated, which correspond to NiCe Ž111. and Ni 2 Ce Ž311. planes, respectively. Selected area electron diffraction ŽSAED. patterns ŽFig. 3. show well-defined spots characteristic of nanocomposite grained materials, where some crystal relationships among orthorhombic w 110 x of NiCe and cubic w 311x of Ni 2 Ce, face-center crystal w22 2 x of NiO, cubic w 111x of nickel can be identified owing to the core–shell nature of particles. On the other hand, stacking faults that existed in nickel core region will greatly give rise to distortions for crystal surface, and further affect the crystal parameters, for instance, Ni 2 Ce and NiO lattice spacing, which lead to the diffuse scattering in the SAED patterns.
Fig. 1. EDX spectra Ža. and SEM morphology Žb. of Ni–Ce nanocomposite particles.
X. Sun, M.J. Yacamanr Materials Science and Engineering C 16 (2001) 95–98
Fig. 2. HREM image of Ni–Ce nanocomposite particle with innermost NiCe compounds Žlattice spacing, 0.278 and 0.219 nm, for NiCe and Ni 2 Ce, respectively. and outermost NiO layer Žlattice spacing, 0.24 nm. shell structure. Also, planar defects, i.e. stacking faults, are shown by arrows.
The variations of zero-field-cooled ŽZFC. and fieldcooled ŽFC. magnetization with temperature at different applied magnetic field Žfrom 100 G to 1 T. are shown in Fig. 4a–d. It is evident as a typical blocking behavior of superparamganetic nanoparticles that the Ni–Ce nanocomposite particles show a different magnetizatization process when the sample is cooled below the blocking temperature with an applied magnetic field w8x. The ZFC magnetization
97
Fig. 3. SAED patterns of Ni–Ce nanocomposite particles, indicating the crystal relationship among orthorhombic w110x of NiCe and cubic w311x of Ni 2 Ce, face-center crystal w222x of NiO, cubic w111x of nickel.
curves have a broad maximum at about 170 K, such a maximum is also a characteristic feature of superparamganetic relaxation and blocking behavior. Actually, the broad maximum of MZFC ŽT . suggest a distribution of the grain sizes of the particle assemblies. Here, the maximum of ZFC curve is defined as the average blocking temperature ŽT B s 170 K.. The most prominent feature is the blocking temperature varies as the strength of applied magnetic filed. This indicates that such a superparamagnetic behavior can be modified by the applied field and size distribu-
Fig. 4. Ža, b, c, d. Magnetization as function of temperatures for Ni–Ce nanocomposite particles.
98
X. Sun, M.J. Yacamanr Materials Science and Engineering C 16 (2001) 95–98
pled with the strong spin–spin interaction, is the possible reason for the increased line width for EPR spectra. We can suppose that it is an experimental facts that the spin associated with the 4f electron of cerium ŽCe., which is in an intermediate valence state of the orthorhombic NiCe compound ŽCrB-type structure., will give rise to a large spin–spin interaction, which lead to the broadening EPR lines. Consequently, this assumption makes magnetic magnetization and EPR measurements consistent and also further reconfirms the HREM microstructure analysis.
4. Conclusion
Fig. 5. EPR spectra at different temperatures for Ni–Ce nanocomposite particles.
tion, and coupling with the strong interparticle interaction w9x among Ni–Ce nanocomposite particles assemblies. On the other hand, the magnetization for Ni–Ce nanocomposite particles, which is shown in Fig. 4, exhibit a sharp maximum around Neel ´ temperature TN s 5–11 K suggesting an antiferromagnetic transition. This can be attributed to the appearance of magnetic ordering of Ce ions w10x in the shell layer of Ni–Ce nanocomposite particles. At low temperature, the magnetization of Ni–Ce nanocomposite particles exhibit complicated behavior Žsee Fig. 4a–d., which suggests a spin-flop occurring below TN in the antiferromagnetic state of this Ni–Ce nanocomposite particle assemblies. Fig. 5a,b shows the EPR spectra at 100 K and room temperature ŽRT.. We can see, at low temperature, that the spectral line with Gaussian shape is centered at g s 1.4338 " 5 and has a linewidth D H ( 160 mT; at RT, the EPR spectrum has a tetragonal symmetry with g H s 2.0609 " 5 and g ´ s 1.4355 " 5 as well as a linewidth D H ( 168 mT. The spectra resemble the EPR line usually observed for an S s 1r2 spin system. It can readily be expected that the S s 1r2 spin state might arise in our case from the antiferromagnetic coupling of one high-spin Ce 3q Ž S s 1r2. ion with one Ni 2q Ž S s 1. ion, then the state with S s 1r2 becomes the ground state of the particle. In the case of low temperature, the EPR spectrum also shows a very broad resonance line. The increased magnetic field produced by the large magnetocrystalline anisotropy, cou-
Novel magnetic core–shell nanocomposite Ni–Ce particles, with an average grain size of 15–50 nm, have been extensively studied in this study. Microstructural analysis ŽSEM, EDX, HREM, SAED. showed that microstructural defects Žnanotwins and stacking faults. existed in large Ni core zone; the shell layers consisted of innermost NiCe alloy as well as outermost NiO oxide. Modified superparamagnetic behavior above average blocking temperature ŽTB s 170 K. have been exhibited for Ni–Ce nanocomposite particles assemblies. In addition, a spin-flop transition and the antiferromagnetically order with a Neel ´ temperature TN of about 5–11 K have been observed at low temperature. EPR spectra measurements reconfirmed the magnetization results and HREM microstructure analysis.
Acknowledgements The authors gratefully acknowledge the support by CONACyT projects of ANanoparticles, Quantum ColloidsB in Mexico, and thank Dr. R. Escudero and Dr. F. Morales for SQUID measurements in IIM of UNAM.
References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x
C.P. Gibson, K.J. Putzer, Science 267 Ž1995. 1338. C.L. Chien, J. Appl. Phys. 69 Ž1991. 5267. R.D. Shull, L.H. Bennett, Nanostruct. Mater. 1 Ž1992. 83. B. Martinez, X. Obradors, Li. Balcells, A. Rouanot, C. Monty, Phys. Rev. Lett. 80 Ž1998. 181. M.E. McHerry, S.A. Majetich, E.M. Kirkpatrick, Mater. Sci. Eng., A 204 Ž1995. 19. X.X. Zhang, J. Tejeade, J.M. Hernandez, R.F. Ziolo, Nanostruct. Mater. 9 Ž1997. 301. Z.L. Cui, L.F. Dong, Z.K. Zhang, Nanostruct. Mater. 5 Ž7r8. Ž1995. 829. Q. Chen, Z.J. Zhang, Appl. Phys. Lett. 73 Ž1998. 3165. M. Hanson, C. Johansson, M.S. Pederson, S. Morup, J. Phys.: Condens. Matter 7 Ž1995. 9269. F. Fourgerot, B. Chevalier, J. Etourneau, Physica B 230r232 Ž1997. 256.
Microstructural and magnetic properties of Ni–Ce nanocomposite particles Xiangcheng Sun ) , M.J. Yacaman ININ, Amsterdam 46-202, Col. Hipodromo Condesa, 06100 Mexico D.F., Mexico ´
Abstract A new type of magnetic core–shell Ni–Ce nanocomposite particles Ž15–50 nm. are presented. Scanning electron microscope ŽSEM. images and X-ray energy-dispersive analysis ŽEDX. spectra indicate that these nanoparticles are strongly magnetic interacting order with chain-like features. Typical HREM images show that many planar defects Ži.e., nanotwins and stacking faults. exist in large Ni core zone Ž10–45 nm.; the shell layers Ž3–5 nm. are consisted of innermost NiCe alloy and outermost NiO oxide. Selected area electron diffraction ŽSAED. patterns show an indication of well-defined spots characteristic of core–shell nanocomposite materials. Magnetization measurements as a function of magnetic fields and temperatures were performed in a SQUID magnetometer. Superparamagnetic behavior above average blocking temperature ŽTB . 170 K was exhibited, this superparamagnetic relaxation behavior was found to be modified by interparticle interactions, which depend on the applied field and size distribution. In addition, antiferromagnetic order occurred with a Neel ´ temperature TN of about 11 K. A spin-flop transition was also observed below TN at a certain applied filed. In particular, electron paramagnetic resonance ŽEPR. spectra at low and room temperature reflected this magnetic order nature associated with this type of core–shell microstructure, coupling with the strong interparticle interaction. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Ni–Ce nanocomposite particles; Core–shell; Superparamagnetic behavior
1. Introduction Magnetic nanoparticles are an active subject of intense research due to their unique magnetic properties which made them appealing from both scientific and technological points of view w1–3x. Specifically, metallic ŽFe, Ni, etc.. w3x and oxide Ž g-Fe 2 O 3 , etc.. w4x magnetic nanoparticles have attracted considerable attention as high-density magnetic storage media because the high coercive force and the high saturation magnetization can be achieved. Recently, spontaneous magnetization reversal in some magnetic ferritin ŽNiFe 2 O4 , etc.. w5x nanoparticles due to surface disorder is also of significant interest, since this effects will determine the stability of stored information and limit the ultimate storage density. Moreover, experimental evidences for low temperature superparamagnetism relaxation w6x and the ordering of spin-glass like behavior w4x in some single domain magnetic particle Ži.e., g-Fe 2 O 3 , etc.. have been reported. In this study, the Ni–Ce nanocomposite particles with NiCe alloy and NiO oxide
)
Corresponding author. Tel.: q52-5-53297296; fax: q52-5-53297296. E-mail address: [email protected] ŽX. Sun..
shell layers have successfully been prepared. Our study emphasis will be involved in the microstructure features and their magnetic order properties as well as their correlation.
2. Experimental These types of Ni–Ce nanocomposite particles were prepared by the hydrogen plasma-metal reaction techniques w7x. A JEOL-4000EX high-resolution transmission electron microscope ŽHRTEM., operated at 400 KeV, was used to determine the detailed core–shell phase and average grain size of the particles. It also allowed us to record selected area electron nanodiffraction ŽSAED.. A Phillips XL30 scanning electron microscope ŽSEM. equipped with an X-ray energy-dispersive analysis ŽEDX. system was employed to provide the morphology of the samples and composition analysis. The magnetization measurements were performed by the following useful methods: zerofield-cooling ŽZFC. and field-cooling ŽFC. using a SQUID magnetometer. From the curves of M vs. T ŽZFC and FC cases., the average blocking temperature ŽT B . was defined as the maximum of ZFC curve. The electron paramagnetic
0928-4931r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 8 - 4 9 3 1 Ž 0 1 . 0 0 2 8 1 - 8
96
X. Sun, M.J. Yacamanr Materials Science and Engineering C 16 (2001) 95–98
resonance ŽEPR. measurements were conducted using a JEOL JES-RE3X spectrometer.
3. Results and discussion SEM image and EDX analysis ŽFig. 1a,b. clearly indicate that these particles are nearly spherical with negligible shape anisotropy, and with strong tendency of forming chain-like features. Such chain-like behavior can be attributed to the tendency of reducing the specific surface energy of system. It is very interesting to note from the typical HREM image ŽFig. 2. of this Ni–Ce nanocomposite particles that some foot-like feature and clear lattice images of core–shell type structure are observed. Each particle consists of a large core and a thin outer shell; there exits a very clear boundary between the core and the outer shell. In addition, many structural defects are found, such as nanotwins and
stacking faults, which form the kinks and steps on the core zones of the particles. Most particles are covered with ca. 3–5-nm thick crystalline phase, which has some discontinuities Žas shown in arrow of Fig. 2.. This indicates that the shell phases are polycrystalline. The outermost layer spacing is 0.24 nm, this value corresponds to NiO Ž111. plane, and the innermost layers spacing of 0.278 and 0.219 nm have been indicated, which correspond to NiCe Ž111. and Ni 2 Ce Ž311. planes, respectively. Selected area electron diffraction ŽSAED. patterns ŽFig. 3. show well-defined spots characteristic of nanocomposite grained materials, where some crystal relationships among orthorhombic w 110 x of NiCe and cubic w 311x of Ni 2 Ce, face-center crystal w22 2 x of NiO, cubic w 111x of nickel can be identified owing to the core–shell nature of particles. On the other hand, stacking faults that existed in nickel core region will greatly give rise to distortions for crystal surface, and further affect the crystal parameters, for instance, Ni 2 Ce and NiO lattice spacing, which lead to the diffuse scattering in the SAED patterns.
Fig. 1. EDX spectra Ža. and SEM morphology Žb. of Ni–Ce nanocomposite particles.
X. Sun, M.J. Yacamanr Materials Science and Engineering C 16 (2001) 95–98
Fig. 2. HREM image of Ni–Ce nanocomposite particle with innermost NiCe compounds Žlattice spacing, 0.278 and 0.219 nm, for NiCe and Ni 2 Ce, respectively. and outermost NiO layer Žlattice spacing, 0.24 nm. shell structure. Also, planar defects, i.e. stacking faults, are shown by arrows.
The variations of zero-field-cooled ŽZFC. and fieldcooled ŽFC. magnetization with temperature at different applied magnetic field Žfrom 100 G to 1 T. are shown in Fig. 4a–d. It is evident as a typical blocking behavior of superparamganetic nanoparticles that the Ni–Ce nanocomposite particles show a different magnetizatization process when the sample is cooled below the blocking temperature with an applied magnetic field w8x. The ZFC magnetization
97
Fig. 3. SAED patterns of Ni–Ce nanocomposite particles, indicating the crystal relationship among orthorhombic w110x of NiCe and cubic w311x of Ni 2 Ce, face-center crystal w222x of NiO, cubic w111x of nickel.
curves have a broad maximum at about 170 K, such a maximum is also a characteristic feature of superparamganetic relaxation and blocking behavior. Actually, the broad maximum of MZFC ŽT . suggest a distribution of the grain sizes of the particle assemblies. Here, the maximum of ZFC curve is defined as the average blocking temperature ŽT B s 170 K.. The most prominent feature is the blocking temperature varies as the strength of applied magnetic filed. This indicates that such a superparamagnetic behavior can be modified by the applied field and size distribu-
Fig. 4. Ža, b, c, d. Magnetization as function of temperatures for Ni–Ce nanocomposite particles.
98
X. Sun, M.J. Yacamanr Materials Science and Engineering C 16 (2001) 95–98
pled with the strong spin–spin interaction, is the possible reason for the increased line width for EPR spectra. We can suppose that it is an experimental facts that the spin associated with the 4f electron of cerium ŽCe., which is in an intermediate valence state of the orthorhombic NiCe compound ŽCrB-type structure., will give rise to a large spin–spin interaction, which lead to the broadening EPR lines. Consequently, this assumption makes magnetic magnetization and EPR measurements consistent and also further reconfirms the HREM microstructure analysis.
4. Conclusion
Fig. 5. EPR spectra at different temperatures for Ni–Ce nanocomposite particles.
tion, and coupling with the strong interparticle interaction w9x among Ni–Ce nanocomposite particles assemblies. On the other hand, the magnetization for Ni–Ce nanocomposite particles, which is shown in Fig. 4, exhibit a sharp maximum around Neel ´ temperature TN s 5–11 K suggesting an antiferromagnetic transition. This can be attributed to the appearance of magnetic ordering of Ce ions w10x in the shell layer of Ni–Ce nanocomposite particles. At low temperature, the magnetization of Ni–Ce nanocomposite particles exhibit complicated behavior Žsee Fig. 4a–d., which suggests a spin-flop occurring below TN in the antiferromagnetic state of this Ni–Ce nanocomposite particle assemblies. Fig. 5a,b shows the EPR spectra at 100 K and room temperature ŽRT.. We can see, at low temperature, that the spectral line with Gaussian shape is centered at g s 1.4338 " 5 and has a linewidth D H ( 160 mT; at RT, the EPR spectrum has a tetragonal symmetry with g H s 2.0609 " 5 and g ´ s 1.4355 " 5 as well as a linewidth D H ( 168 mT. The spectra resemble the EPR line usually observed for an S s 1r2 spin system. It can readily be expected that the S s 1r2 spin state might arise in our case from the antiferromagnetic coupling of one high-spin Ce 3q Ž S s 1r2. ion with one Ni 2q Ž S s 1. ion, then the state with S s 1r2 becomes the ground state of the particle. In the case of low temperature, the EPR spectrum also shows a very broad resonance line. The increased magnetic field produced by the large magnetocrystalline anisotropy, cou-
Novel magnetic core–shell nanocomposite Ni–Ce particles, with an average grain size of 15–50 nm, have been extensively studied in this study. Microstructural analysis ŽSEM, EDX, HREM, SAED. showed that microstructural defects Žnanotwins and stacking faults. existed in large Ni core zone; the shell layers consisted of innermost NiCe alloy as well as outermost NiO oxide. Modified superparamagnetic behavior above average blocking temperature ŽTB s 170 K. have been exhibited for Ni–Ce nanocomposite particles assemblies. In addition, a spin-flop transition and the antiferromagnetically order with a Neel ´ temperature TN of about 5–11 K have been observed at low temperature. EPR spectra measurements reconfirmed the magnetization results and HREM microstructure analysis.
Acknowledgements The authors gratefully acknowledge the support by CONACyT projects of ANanoparticles, Quantum ColloidsB in Mexico, and thank Dr. R. Escudero and Dr. F. Morales for SQUID measurements in IIM of UNAM.
References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x
C.P. Gibson, K.J. Putzer, Science 267 Ž1995. 1338. C.L. Chien, J. Appl. Phys. 69 Ž1991. 5267. R.D. Shull, L.H. Bennett, Nanostruct. Mater. 1 Ž1992. 83. B. Martinez, X. Obradors, Li. Balcells, A. Rouanot, C. Monty, Phys. Rev. Lett. 80 Ž1998. 181. M.E. McHerry, S.A. Majetich, E.M. Kirkpatrick, Mater. Sci. Eng., A 204 Ž1995. 19. X.X. Zhang, J. Tejeade, J.M. Hernandez, R.F. Ziolo, Nanostruct. Mater. 9 Ž1997. 301. Z.L. Cui, L.F. Dong, Z.K. Zhang, Nanostruct. Mater. 5 Ž7r8. Ž1995. 829. Q. Chen, Z.J. Zhang, Appl. Phys. Lett. 73 Ž1998. 3165. M. Hanson, C. Johansson, M.S. Pederson, S. Morup, J. Phys.: Condens. Matter 7 Ž1995. 9269. F. Fourgerot, B. Chevalier, J. Etourneau, Physica B 230r232 Ž1997. 256.