- Email: [email protected]
Magnetic properties of nanocrystalline Ni0.7Mn0.3Gd0.1Fe1.9O4 ferrite at low temperatures
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 309 (2007) 11–14 www.elsevier.com/locate/jmmm
Magnetic properties of nanocrystalline Ni0.7Mn0.3Gd0.1Fe1.9O4 ferrite at low temperatures Lijun Zhaoa, Zhaoyang Hana, Hua Yanga,, Lianxiang Yua, Yuming Cuia, Weiqun Jina, Shouhua Fengb a College of Chemistry, Jilin University, Changchun 130023, PR China State Key Laboratory of Inorganic Synthesis and Prepartive Chemistry, Jilin University, Changchun 130023, PR China
b
Received 12 November 2005; received in revised form 29 December 2005 Available online 19 April 2006
Abstract Mo¨ssbauer spectra and magnetic measurement of Ni0.7Mn0.3Gd0.1Fe1.9O4 ferrite were investigated by Oxford MS-500 Mo¨ssbauer spectrometer and superconducting quantum interference device (SQUID) magnetometer with a field 5 T. Ni0.7Mn0.3Gd0.1Fe1.9O4 nanoparticles have a considerable coercivity of 1040 Oe when the test temperature is reduced to 2 K. Mo¨ssbauer spectra show that Ni0.7Mn0.3Gd0.1Fe1.9O4 nanoparticles exhibit superparamagnetism at room temperature and ferrimagnetism at 77 K. r 2006 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Superparamagnetism; Ferrimagnetism
1. Introduction In ferrite MFe2O4, the choice of rare-earth ions allows a relative tenability of the magnetic properties such as magnetization or anisotropy. The octahedral and tetrahedral sublattice magnetizations are antiparallel and therefore a noncompensated magnetic moment occurs. This structure is called ferrimagnetic. Due to their small size, nanoparticles exhibit novel materials properties, which largely differ from the bulk solid state [1]. It is known that rare-earth ions play an important role in determining the magnetocrystalline anisotropy in 4f–3d intermetallic compounds [2–4]. The presence of Gd3+ ions influences mainly the magnetic anisotropy of the system. The room-temperature (RT) Mo¨ssbauer spectrum of the small nanoparticles changes from a superparamagnetic doublet to a well-resolved sextet with a weak doublet. The magnetic properties of ferrites can be changed by the substitution of various kinds of M2+ among divalent cations (Zn2+, Mg2+, Cu2+, Mn2+, Ni2+, Co2+, Fe2+y), or by introducing a relatively small amount of rare-earth ions. In our experiment, the structural and Corresponding author. Tel./fax: +86 431 5167712.
E-mail address: [email protected] (H. Yang). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.03.054
magnetic properties of Ni–Mn ferrite doped with Gd3+ ions were investigated. 2. Experimental Nanocrystalline samples of the nominal formula Ni0.7Mn0.3GdxFe2xO4 where x ¼ 0 and 0.1 were prepared by emulsion method, using analytically pure-grade Ni (NO3)2 6H2O, Fe(NO3)3 9H2O, Mn(NO3)2 and Gd2O3 as starting materials. PEG (molecular weight 20,000) was used as the surfactant. The nitrates were mixed with PEG to form the solution, then NH4OH with the concentration of 2 M was dropped into the solution until pH ¼ 9.0 to form the precipitate. The precipitate was washed with distilled water for three times and dried at 90 1C for 7 h to prepare the precursor. The precursor was calcined at 873 K for 2 h, respectively. The structure and crystallite sizes are tested by X-ray diffractometer (XRD) in the 2y range 25–651 using Cu-Ka radiation (l ¼ 0:15405 nm). The type of XRD is SHIMADZU Co, Tokyo Japan. The database of the Joint Committee on Powder Diffraction Data was used for the interpretation of XRD spectra. The crystalline sizes are calculated using Scherrer’s relationship D ¼ kl=B cos y,
ARTICLE IN PRESS 12
L. Zhao et al. / Journal of Magnetism and Magnetic Materials 309 (2007) 11–14
where ‘D’ is the average diameter in nm, ‘k’ is the shape factor, and is B the half intensity width of the relevant diffraction peak and instrumental broadening, respectively. ‘l’ is the X-ray wavelength and y is the Bragg’s diffraction angle. The broadening of the (3 1 1) diffraction line of the ferrite materials was considered after computer fit of the X-ray data using the Gaussian line shape. The broadening of the diffraction line due to reduction of crystallite dimensions, i.e. B, was estimated by the relation, B2 ¼ B2m B2s , where Bm is the measured width of the diffraction line at its half maximum and Bs is the measured breadth of the line for the standard at its maximum. Mo¨ssbauer spectrum was recorded at 295 and 77 K by using a computerized Oxford MS-500 Mo¨ssbauer spectrometer of the electromechanical type in constant acceleration mode. A 57Co source in a palladium matrix was used in a continuously distributed hyperfine magnetic field. A 25 mm-thick high-purity alpha iron foil was used for calibration. The experimental data were analyzed with a standard least-square fitting program assuming Lorentzian line shapes. Magnetic measurements were carried out in a superconducting quantum interference device (SQUID) magnetometer with a field 5 T. 3. Results and discussion The diffraction peaks shown in Fig. 1 indicate that nanocrystalline Ni0.7Mn0.3GdxFe2xO4 (x ¼ 0, 0.1) ferrites with no extra reflections are FCC structure of spinel ferrite. However, the diffraction peaks in Ni0.7Mn0.3Gd0.1Fe1.9O4 ferrite appears to be broadened as a result of incorporation of the Gd3+ ions. Hence the crystallite size of Ni–Mn ferrite is larger than that of Ni0.7Mn0.3Gd0.1Fe1.9O4 ferrite. The values of Ms and Hc of samples at different temperatures are listed in Table 1. To understand the
Fig. 1. XRD patterns of nanocrystalline Ni0.7Mn0.3GdxFe2xO4 (x ¼ 0, 0.1) ferrite calcined at 873 K.
Table 1 Effects of Gd3+ ions on the magnetic properties of Ni–Mn ferrite at different temperatures Re3+
D (nm)
Temperature (K)
Ms (emu/g)
Hc (Oe)
None
13.8
300 150 2
41.4 41.5 42.4
0 0 330
Gd
7.8
300 150 2
17.8 18.4 26.9
0 0 1040
low-temperature magnetic properties, the theory of lowtemperature spin-wave is introduced [5,6]. The spin of atom system is completely parallel at absolute zero. Due to the increase of temperature, the inverse numbers of spin will be increased (Fig. 2). The thermal decrease of the magnetization between 2 and 300 K was fitted to a spinwave-type dependence Ms (T) ¼ Ms (0)(1bTb), where Ms (0), b and b are the saturation magnetization, Bloch constant and the Bloch exponent, respectively. So the value of Ms increases with the decreasing temperatures. The coercivity of nanoparticles at low temperature decided by irreversible domain rotation can be defined by the equation H c ¼ 2K 1 =m0 M s , where K1 is the magnetocrystalline anisotropy constant and m0 is vacuum susceptibility. In general, the magnetocrystalline anisotropy constant increases with the decreasing temperature, so the coercivity increase rapidly at 2 K. Furthermore, rare-earth ions have stronger s-l coupling and weaker crystal field, so they have stronger magnetocrystalline anisotropy. Table 1 demonstrated that the Ms value of Ni–Mn ferrite is larger than that of Ni0.7Mn0.3Gd0.1Fe1.9O4 ferrite. According to the above relationship, we can explain why Ni0.7Mn0.3Gd0.1Fe1.9O4 nanoparticles have a considerable coercivity at 2 K. Moreover, the radii of Gd3+ ions are larger than that of Fe3+ ions. Hence, the symmetry of crystal will be decreased after the sample was substituted by Gd3+ ions. The low symmetry of crystal will lead to strong magnetocrystalline anisotropy, which is one reason for high coercivity of Ni0.7Mn0.3Gd0.1Fe1.9O4 ferrite relative to the Ni–Mn ferrite. Fig. 3 shows the Mo¨ssbauer spectra of Ni0.7Mn0.3Gd0.1 Fe1.9O4 ferrite nanocrystal measured at 295 K (a) and 77 K (b). As evident from Fig. 3(a), the spectrum consists of a weak sextet pattern superposed on a distinct central doublet, which indicates that the sample has a ferrimagnetic and superparamagnetic nature, simultaneously. The appearance of the quadruple doublet is due to the fact that a fraction of Fe3+ ions have few nearest neighbors at the A-sites, which are magnetic having ordered spins and giving rise to the quadruple doublet C [7]. Fig. 3(b) shows the magnetic state of the powders at RT change from a superparamagnetic to a ferritmagnetic one with decrease of the testing temperature; however, the superparamagnetism do not disappear completely even if the testing temperature decreases to 77 K. The spectrum b is composed of a
ARTICLE IN PRESS
Magnetization (emu/g)
L. Zhao et al. / Journal of Magnetism and Magnetic Materials 309 (2007) 11–14
30
2K
20
150K
10
293K
Table 2 The Mo¨ssbauer parameters for Ni0.7Mn0.3Gd0.1Fe1.9O4 ferrite nanocrystal with the crystallite sizes of 7.8 nm
20000
40000
60000
Fig. 2. The hysteretic curves of nanocrystalline Ni0.7Mn0.3Gd0.1Fe1.9O4 ferrite tested at different temperatures.
101 100 99 98
Relative Transmission %
97 96 95 94
(a)
100.5 100.0 99.5 99.0 98.5 98.0 97.5 97.0 -15
(b) -10
-5
0 5 Velocity (mm/s)
10
15
Fig. 3. Mo¨ssbauer spectra of Ni0.7Mn0.3Gd0.1Fe1.9O4 nanoparticles tested at 295 K (a) and 77 K (b).
magnetic phase of six hyperfine lines and a weak central superparamagnetic phase of a quadruple doublet. But the appearance of the magnetic pattern is due to the exchange interaction between the magnetic ions at A- and B-sites [8–10]. This shows that the exchange interaction becomes stronger between the magnetic ions at A- and B-sites with decreasing temperature. d, DE, B and A0 represent the isomer shift, quadrupole split, hyperfine magnetic fields and fractional area of the
A0
A1 B1 B2 CA CB
0.32 0.38 0.37 0.37 0.39
0.04 0.04 0.06 0.59 1.12
343.9 441.3 157.2 — —
0.15 0.17 0.22 0.26 0.20
LT
A1 B1 B2 CA CB
0.40 0.64 0.51 0.49 0.50
0.02 0.06 0.05 1.24 0.58
476.3 369.5 514.4 — —
0.436 0.301 0.206 0.013 0.044
70.02
70.02
71.0
-30 0 H (Oe)
B (kOe)
RT
-20
-20000
DE (mm s1)
Sublattice
-10
-40000
d (mm s1)
Sample
0
-60000
13
Error
70.02
pattern, respectively. CA and CB represent the superparamagnetic phase. Table 2 illustrates that the isomer shift of A-sites is less than that of B-sites. Due to the larger bond separations and smaller overlapping of the orbits of Fe3+ ions and oxygen ions for octahedral sites as compared to that for tetrahedral sites, it is obvious that there is smaller covalency, and hence larger isomer shift should be expected at the octahedral sites. The isomer shift (d) of A- and B-sites show some increase at 77 K. This indicates that the s electron density of Fe3+ ions on A-sites decreases because of the increment of shielding effect and/or the increment of covalency. The quadruple splitting (DE) of A- and B-sites appears to show no significant changes with temperatures, which indicates the symmetry of the iron sites is not disturbed although the temperature is changed. The value of B at 77 K is larger than that of B at 295 K. This shows that ferrimagnetism increases with decreasing temperature, i.e. superparamagnetism gradually disappears with decreasing temperature. At RT, the hyperfine magnetic splitting has not fully collapsed. The onset of superparamagnetic effect will act to collapse the sextet structure since fixed Mo¨ssbauer interaction time becomes greater than the relaxation times of local magnetization. Meanwhile, collective magnetic excitations arising from moment fluctuations about the local easy axis will not collapse the sextet, but will reduce the measured hyperfine magnetic field (B). The obvious difference in hyperfine field for sample tested at 295 and 77 K could be due to strong spin fluctuations, which vanish at very low temperature. Especially, the absorption area of A-sites at 77 K is larger than that of A-sites at 295 K. This indicates that Fe3+ ions immigrate from B- to A-sites with decreasing temperature. 4. Conclusions Ni0.7Mn0.3Gd0.1Fe1.9O4 nanoparticles have a considerable coercivity of 1040 Oe when the tested temperature is
ARTICLE IN PRESS 14
L. Zhao et al. / Journal of Magnetism and Magnetic Materials 309 (2007) 11–14
reduced to 2 K. Mo¨ssbauer spectra show that the Ni0.7Mn0.3Gd0.1Fe1.9O4 nanoparticles exhibit superparamagnetism with a small quantity of ferrimagnetism at room temperature and absolute ferrimagnetism at 77 K. VSM and Mo¨ssbauer spectra got the same conclusion that the ferrimagnetic phase increases with decreasing temperature.
Acknowledgment This work is supported by the National Natural Science Foundation of China (NSFC).
References [1] C. Petit, M.P. Pileni, J. Phys. Chem. 92 (1988) 3505. [2] Y. Ying-chang, K. Lin-shu, Z. Yuan-bo, S. Hong, P. Xie-di, J. Phys. Coll. 49 (1988) C8543. [3] L. Hong-shuo, H. Bo-ping, J.M.D. Coey, Solid-State Commun. 66 (1988) 133. [4] Y. Ying-chang, K. Lin-shu, S. Shu-he, G. Dong-mei, J. Appl. Phys. 63 (1988) 3702. [5] J.F. Hochepied, M.P. Pileni, J. Appl. Phys. 87 (5) (2000). [6] A.T. Ngo, P. Bonville, M.P. Pileni, J. Appl. Phys. 89 (6) (2001). [7] D.C. Dobson, J.W. Linnet, M.M. Rahman, J. Phys. Chem. Solids 31 (1970) 727. [8] H.N. Ok, Y.K. Kim, Phys. Rev. B 36 (1987) 5120. [9] H.N. Ok, K.S. Baek, E.J. Choi, Phys. Rev. B 40 (1989) 84. [10] H.N. Ok, K.S. Baek, H.S. Lee, C.S. Kim, Phys. Rev. B 41 (1990) 62.
Journal of Magnetism and Magnetic Materials 309 (2007) 11–14 www.elsevier.com/locate/jmmm
Magnetic properties of nanocrystalline Ni0.7Mn0.3Gd0.1Fe1.9O4 ferrite at low temperatures Lijun Zhaoa, Zhaoyang Hana, Hua Yanga,, Lianxiang Yua, Yuming Cuia, Weiqun Jina, Shouhua Fengb a College of Chemistry, Jilin University, Changchun 130023, PR China State Key Laboratory of Inorganic Synthesis and Prepartive Chemistry, Jilin University, Changchun 130023, PR China
b
Received 12 November 2005; received in revised form 29 December 2005 Available online 19 April 2006
Abstract Mo¨ssbauer spectra and magnetic measurement of Ni0.7Mn0.3Gd0.1Fe1.9O4 ferrite were investigated by Oxford MS-500 Mo¨ssbauer spectrometer and superconducting quantum interference device (SQUID) magnetometer with a field 5 T. Ni0.7Mn0.3Gd0.1Fe1.9O4 nanoparticles have a considerable coercivity of 1040 Oe when the test temperature is reduced to 2 K. Mo¨ssbauer spectra show that Ni0.7Mn0.3Gd0.1Fe1.9O4 nanoparticles exhibit superparamagnetism at room temperature and ferrimagnetism at 77 K. r 2006 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Superparamagnetism; Ferrimagnetism
1. Introduction In ferrite MFe2O4, the choice of rare-earth ions allows a relative tenability of the magnetic properties such as magnetization or anisotropy. The octahedral and tetrahedral sublattice magnetizations are antiparallel and therefore a noncompensated magnetic moment occurs. This structure is called ferrimagnetic. Due to their small size, nanoparticles exhibit novel materials properties, which largely differ from the bulk solid state [1]. It is known that rare-earth ions play an important role in determining the magnetocrystalline anisotropy in 4f–3d intermetallic compounds [2–4]. The presence of Gd3+ ions influences mainly the magnetic anisotropy of the system. The room-temperature (RT) Mo¨ssbauer spectrum of the small nanoparticles changes from a superparamagnetic doublet to a well-resolved sextet with a weak doublet. The magnetic properties of ferrites can be changed by the substitution of various kinds of M2+ among divalent cations (Zn2+, Mg2+, Cu2+, Mn2+, Ni2+, Co2+, Fe2+y), or by introducing a relatively small amount of rare-earth ions. In our experiment, the structural and Corresponding author. Tel./fax: +86 431 5167712.
E-mail address: [email protected] (H. Yang). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.03.054
magnetic properties of Ni–Mn ferrite doped with Gd3+ ions were investigated. 2. Experimental Nanocrystalline samples of the nominal formula Ni0.7Mn0.3GdxFe2xO4 where x ¼ 0 and 0.1 were prepared by emulsion method, using analytically pure-grade Ni (NO3)2 6H2O, Fe(NO3)3 9H2O, Mn(NO3)2 and Gd2O3 as starting materials. PEG (molecular weight 20,000) was used as the surfactant. The nitrates were mixed with PEG to form the solution, then NH4OH with the concentration of 2 M was dropped into the solution until pH ¼ 9.0 to form the precipitate. The precipitate was washed with distilled water for three times and dried at 90 1C for 7 h to prepare the precursor. The precursor was calcined at 873 K for 2 h, respectively. The structure and crystallite sizes are tested by X-ray diffractometer (XRD) in the 2y range 25–651 using Cu-Ka radiation (l ¼ 0:15405 nm). The type of XRD is SHIMADZU Co, Tokyo Japan. The database of the Joint Committee on Powder Diffraction Data was used for the interpretation of XRD spectra. The crystalline sizes are calculated using Scherrer’s relationship D ¼ kl=B cos y,
ARTICLE IN PRESS 12
L. Zhao et al. / Journal of Magnetism and Magnetic Materials 309 (2007) 11–14
where ‘D’ is the average diameter in nm, ‘k’ is the shape factor, and is B the half intensity width of the relevant diffraction peak and instrumental broadening, respectively. ‘l’ is the X-ray wavelength and y is the Bragg’s diffraction angle. The broadening of the (3 1 1) diffraction line of the ferrite materials was considered after computer fit of the X-ray data using the Gaussian line shape. The broadening of the diffraction line due to reduction of crystallite dimensions, i.e. B, was estimated by the relation, B2 ¼ B2m B2s , where Bm is the measured width of the diffraction line at its half maximum and Bs is the measured breadth of the line for the standard at its maximum. Mo¨ssbauer spectrum was recorded at 295 and 77 K by using a computerized Oxford MS-500 Mo¨ssbauer spectrometer of the electromechanical type in constant acceleration mode. A 57Co source in a palladium matrix was used in a continuously distributed hyperfine magnetic field. A 25 mm-thick high-purity alpha iron foil was used for calibration. The experimental data were analyzed with a standard least-square fitting program assuming Lorentzian line shapes. Magnetic measurements were carried out in a superconducting quantum interference device (SQUID) magnetometer with a field 5 T. 3. Results and discussion The diffraction peaks shown in Fig. 1 indicate that nanocrystalline Ni0.7Mn0.3GdxFe2xO4 (x ¼ 0, 0.1) ferrites with no extra reflections are FCC structure of spinel ferrite. However, the diffraction peaks in Ni0.7Mn0.3Gd0.1Fe1.9O4 ferrite appears to be broadened as a result of incorporation of the Gd3+ ions. Hence the crystallite size of Ni–Mn ferrite is larger than that of Ni0.7Mn0.3Gd0.1Fe1.9O4 ferrite. The values of Ms and Hc of samples at different temperatures are listed in Table 1. To understand the
Fig. 1. XRD patterns of nanocrystalline Ni0.7Mn0.3GdxFe2xO4 (x ¼ 0, 0.1) ferrite calcined at 873 K.
Table 1 Effects of Gd3+ ions on the magnetic properties of Ni–Mn ferrite at different temperatures Re3+
D (nm)
Temperature (K)
Ms (emu/g)
Hc (Oe)
None
13.8
300 150 2
41.4 41.5 42.4
0 0 330
Gd
7.8
300 150 2
17.8 18.4 26.9
0 0 1040
low-temperature magnetic properties, the theory of lowtemperature spin-wave is introduced [5,6]. The spin of atom system is completely parallel at absolute zero. Due to the increase of temperature, the inverse numbers of spin will be increased (Fig. 2). The thermal decrease of the magnetization between 2 and 300 K was fitted to a spinwave-type dependence Ms (T) ¼ Ms (0)(1bTb), where Ms (0), b and b are the saturation magnetization, Bloch constant and the Bloch exponent, respectively. So the value of Ms increases with the decreasing temperatures. The coercivity of nanoparticles at low temperature decided by irreversible domain rotation can be defined by the equation H c ¼ 2K 1 =m0 M s , where K1 is the magnetocrystalline anisotropy constant and m0 is vacuum susceptibility. In general, the magnetocrystalline anisotropy constant increases with the decreasing temperature, so the coercivity increase rapidly at 2 K. Furthermore, rare-earth ions have stronger s-l coupling and weaker crystal field, so they have stronger magnetocrystalline anisotropy. Table 1 demonstrated that the Ms value of Ni–Mn ferrite is larger than that of Ni0.7Mn0.3Gd0.1Fe1.9O4 ferrite. According to the above relationship, we can explain why Ni0.7Mn0.3Gd0.1Fe1.9O4 nanoparticles have a considerable coercivity at 2 K. Moreover, the radii of Gd3+ ions are larger than that of Fe3+ ions. Hence, the symmetry of crystal will be decreased after the sample was substituted by Gd3+ ions. The low symmetry of crystal will lead to strong magnetocrystalline anisotropy, which is one reason for high coercivity of Ni0.7Mn0.3Gd0.1Fe1.9O4 ferrite relative to the Ni–Mn ferrite. Fig. 3 shows the Mo¨ssbauer spectra of Ni0.7Mn0.3Gd0.1 Fe1.9O4 ferrite nanocrystal measured at 295 K (a) and 77 K (b). As evident from Fig. 3(a), the spectrum consists of a weak sextet pattern superposed on a distinct central doublet, which indicates that the sample has a ferrimagnetic and superparamagnetic nature, simultaneously. The appearance of the quadruple doublet is due to the fact that a fraction of Fe3+ ions have few nearest neighbors at the A-sites, which are magnetic having ordered spins and giving rise to the quadruple doublet C [7]. Fig. 3(b) shows the magnetic state of the powders at RT change from a superparamagnetic to a ferritmagnetic one with decrease of the testing temperature; however, the superparamagnetism do not disappear completely even if the testing temperature decreases to 77 K. The spectrum b is composed of a
ARTICLE IN PRESS
Magnetization (emu/g)
L. Zhao et al. / Journal of Magnetism and Magnetic Materials 309 (2007) 11–14
30
2K
20
150K
10
293K
Table 2 The Mo¨ssbauer parameters for Ni0.7Mn0.3Gd0.1Fe1.9O4 ferrite nanocrystal with the crystallite sizes of 7.8 nm
20000
40000
60000
Fig. 2. The hysteretic curves of nanocrystalline Ni0.7Mn0.3Gd0.1Fe1.9O4 ferrite tested at different temperatures.
101 100 99 98
Relative Transmission %
97 96 95 94
(a)
100.5 100.0 99.5 99.0 98.5 98.0 97.5 97.0 -15
(b) -10
-5
0 5 Velocity (mm/s)
10
15
Fig. 3. Mo¨ssbauer spectra of Ni0.7Mn0.3Gd0.1Fe1.9O4 nanoparticles tested at 295 K (a) and 77 K (b).
magnetic phase of six hyperfine lines and a weak central superparamagnetic phase of a quadruple doublet. But the appearance of the magnetic pattern is due to the exchange interaction between the magnetic ions at A- and B-sites [8–10]. This shows that the exchange interaction becomes stronger between the magnetic ions at A- and B-sites with decreasing temperature. d, DE, B and A0 represent the isomer shift, quadrupole split, hyperfine magnetic fields and fractional area of the
A0
A1 B1 B2 CA CB
0.32 0.38 0.37 0.37 0.39
0.04 0.04 0.06 0.59 1.12
343.9 441.3 157.2 — —
0.15 0.17 0.22 0.26 0.20
LT
A1 B1 B2 CA CB
0.40 0.64 0.51 0.49 0.50
0.02 0.06 0.05 1.24 0.58
476.3 369.5 514.4 — —
0.436 0.301 0.206 0.013 0.044
70.02
70.02
71.0
-30 0 H (Oe)
B (kOe)
RT
-20
-20000
DE (mm s1)
Sublattice
-10
-40000
d (mm s1)
Sample
0
-60000
13
Error
70.02
pattern, respectively. CA and CB represent the superparamagnetic phase. Table 2 illustrates that the isomer shift of A-sites is less than that of B-sites. Due to the larger bond separations and smaller overlapping of the orbits of Fe3+ ions and oxygen ions for octahedral sites as compared to that for tetrahedral sites, it is obvious that there is smaller covalency, and hence larger isomer shift should be expected at the octahedral sites. The isomer shift (d) of A- and B-sites show some increase at 77 K. This indicates that the s electron density of Fe3+ ions on A-sites decreases because of the increment of shielding effect and/or the increment of covalency. The quadruple splitting (DE) of A- and B-sites appears to show no significant changes with temperatures, which indicates the symmetry of the iron sites is not disturbed although the temperature is changed. The value of B at 77 K is larger than that of B at 295 K. This shows that ferrimagnetism increases with decreasing temperature, i.e. superparamagnetism gradually disappears with decreasing temperature. At RT, the hyperfine magnetic splitting has not fully collapsed. The onset of superparamagnetic effect will act to collapse the sextet structure since fixed Mo¨ssbauer interaction time becomes greater than the relaxation times of local magnetization. Meanwhile, collective magnetic excitations arising from moment fluctuations about the local easy axis will not collapse the sextet, but will reduce the measured hyperfine magnetic field (B). The obvious difference in hyperfine field for sample tested at 295 and 77 K could be due to strong spin fluctuations, which vanish at very low temperature. Especially, the absorption area of A-sites at 77 K is larger than that of A-sites at 295 K. This indicates that Fe3+ ions immigrate from B- to A-sites with decreasing temperature. 4. Conclusions Ni0.7Mn0.3Gd0.1Fe1.9O4 nanoparticles have a considerable coercivity of 1040 Oe when the tested temperature is
ARTICLE IN PRESS 14
L. Zhao et al. / Journal of Magnetism and Magnetic Materials 309 (2007) 11–14
reduced to 2 K. Mo¨ssbauer spectra show that the Ni0.7Mn0.3Gd0.1Fe1.9O4 nanoparticles exhibit superparamagnetism with a small quantity of ferrimagnetism at room temperature and absolute ferrimagnetism at 77 K. VSM and Mo¨ssbauer spectra got the same conclusion that the ferrimagnetic phase increases with decreasing temperature.
Acknowledgment This work is supported by the National Natural Science Foundation of China (NSFC).
References [1] C. Petit, M.P. Pileni, J. Phys. Chem. 92 (1988) 3505. [2] Y. Ying-chang, K. Lin-shu, Z. Yuan-bo, S. Hong, P. Xie-di, J. Phys. Coll. 49 (1988) C8543. [3] L. Hong-shuo, H. Bo-ping, J.M.D. Coey, Solid-State Commun. 66 (1988) 133. [4] Y. Ying-chang, K. Lin-shu, S. Shu-he, G. Dong-mei, J. Appl. Phys. 63 (1988) 3702. [5] J.F. Hochepied, M.P. Pileni, J. Appl. Phys. 87 (5) (2000). [6] A.T. Ngo, P. Bonville, M.P. Pileni, J. Appl. Phys. 89 (6) (2001). [7] D.C. Dobson, J.W. Linnet, M.M. Rahman, J. Phys. Chem. Solids 31 (1970) 727. [8] H.N. Ok, Y.K. Kim, Phys. Rev. B 36 (1987) 5120. [9] H.N. Ok, K.S. Baek, E.J. Choi, Phys. Rev. B 40 (1989) 84. [10] H.N. Ok, K.S. Baek, H.S. Lee, C.S. Kim, Phys. Rev. B 41 (1990) 62.