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TEM, XRD and DSC analysis of thin films and foils of FeSiBNb alloys doped with Mn
Vacuum 83 (2009) S182–S185
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
TEM, XRD and DSC analysis of thin films and foils of FeSiBNb alloys doped with Mn J. Balcerski a, *, R. Brzozowski a, M. Wasiak b, K. Polan´ski a, M. Moneta a a b
´dz´, Department of Solid State Physics, Pomorska 149, PL 90-236 Ło ´dz´, Poland University of Ło ´dz´, Department of Physical Chemistry, Pomorska 163, PL 90-236 Ło ´dz´, Poland University of Ło
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 June 2008 Received in revised form 28 January 2009 Accepted 30 January 2009
The FINEMET based amorphous alloys in forms of thin films of 20 O 40 nm thickness and 20 mm thick foils of Fe73.5xSi13.5B9Cu1Nb3Mnx with Mn doping (x ¼ 11 O 15 at.%), as-quenched (a-q) and after annealing (a) were analyzed with transmission electron microscopy (TEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The structure and composition of the films differ from those of the foils, first of all in size of crystallites and the lattice constants. Ó 2009 Published by Elsevier Ltd.
Keywords: FINEMET Mn doping TEM SEM XRD DSC
1. Introduction
2. Experiment
Magnetic properties of nanocrystalline FeSiB alloys are due to the fact that size of crystallites, w15 nm, may be shorter than the exchange correlation length [1,2]. The as-quenched metal alloys, like Fe73.5xSi13.5B9Cu1Nb3, consist primarily of amorphousmagnetic phase which, after appropriate thermal or mechanical treatment, are transformed first into amorphous-paramagnetic phase and next into the phase where iron silicide and boride magnetic nanocrystals are embedded in a ferro–paramagnetic amorphous residual matrix [3,4]. The temperature dependence of structure and magnetic properties of the basic FINEMET, x ¼ 0, were analyzed in many previous papers [5–7]. TEM and EDX pictures, XRD patterns and differential scanning calorimetry (DSC) scans allow us to determine the compositions, structure and phase transitions. It is known that Cu atoms segregate and act as nucleation centers of nanocrystals during annealing. Nb reduces atomic mobility, retards grain growth and decreases the Curie temperature. The results of experiments related to low Mn doped FINEMET published recently [8–10] show the decrease of the Curie temperature and activation energy and suggest magnetic hardening of the nanocrystalline phase as a function of Mn concentration x.
The films of Fe73.5xSi13.5B9Cu1Nb3Mnx alloys of a quartz crystalcontrolled 40 nm thickness were prepared by means of physical vapour deposition (PVD). The poly-crystalline powder composite was thermally evaporated in HV (108 Tr) and the metal vapour was deposited on the NaCl surface. The salt was removed by dissolution in H2O. Next the films were heated at the rate w20 C/min to the annealing temperature Ta and then annealed for 1 h. The annealing temperatures for subsequent x were determined from DSC scan of the foils (understood as massive materials) as related to crystallization peak temperature ti, shown in Fig. 2. The transmission electron microscopy (TEM) diffraction patterns from thin films were measured at the room temperature T0 with a TESLA BS-540 microscope operating at 80 kV with l ¼ 0.00418 nm. The baseline was calibrated with Au (a0 ¼ 0.407 nm). The pictures were taken in the diffraction mode and in the bright field mode. The Fe73.5xSi13.5B9Cu1Nb3Mnx¼11O15 foils of approximately 2 mm 20 mm were prepared from basic composite alloys by a melt-spinning technique, commonly used for producing such amorphous metals. The Cu-wheel surface speed over 25 m/s resulting in the cooling rate of 106 K/s assures amorphous alloy. For lower speeds we get even micro-crystalline structure, determined by cooling conditions. The X-ray diffraction (XRD) patterns were measured with a Philips PW 1830 instrument with a CuKa generator (Eg ¼ 8.05 keV,
* Corresponding author. E-mail address: [email protected] (J. Balcerski). 0042-207X/$ – see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.vacuum.2009.01.058
J. Balcerski et al. / Vacuum 83 (2009) S182–S185
S183
Fig. 1. OM picture of outer surface of the foil with the Mn content x ¼ 15 annealed at 700 C for 1 h. Size z55 95 mm2.
l ¼ 0.15405 nm). The differential scanning calorimetry (DSC) measurements were carried out with a Setaram TG DSC-111 instrument. The heating rates 1 O 10 K/min up to the maximum temperature of 900 C in N2 atmosphere were used. The scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) with Link300 and a Zeiss optical microscope (OM) with up to 1000 times magnification were used.
3. Results and discussion OM pictures of both the as-quenched and the annealed samples show irregular interface suitable rather for bulk analysis of the sample than for surface analysis. Objects of the size smaller than 1 mm can be easily resolved on the surface, Fig. 1. The average thin film composition against foil composition was checked at the room temperature T0 with EDX, since differences caused by the evaporation and deposition processes were predicted. The averaged compositions of the alloys scaled to the Fe nominal atomic content
Fig. 2. DSC scans for 20 mm thick FINEMET foils as functions of Mn doping x. Peak positions, enthalpy and masses of the samples are indicated.
are reasonably well correlated with the nominal composition of the glasses. DSC temperature scans of w1 mg foils were performed at the heating rate ranging from 1 up to 10 K/min. They allow us to determine temperatures Ti for exothermic phase transformation and calculate the corresponding enthalpy change DHi. The scans are shown in Fig. 2. Only one crystallization temperature ti w600 C can be resolved and this value is rather independent of mass of the sample and of Mn content. However, as shown in Fig. 2, the crystallization enthalpy DHi decreases linearly with Mn doping. Since decreasing of temperature and subsequent DSC scanning did not give crystallization peaks, the measured enthalpy turned out to be sufficient for final transformation of the sample into the crystalline phase. It is known that Ti can be associated with the final crystallization temperature for iron silicides and also for borides. Decrease of DHi with Mn doping x, even against constant Ti, can be interpreted in terms of easier penetration of Mn into the grains, which should reduce their magnetization.
Fig. 3. XRD patterns from 20 mm thick Fe73.5xSi13.5B9Cu1Nb3Mnx alloys annealed at Ta ¼ 700 C (grey), Ta ¼ 580 C (dark gey) and Ta ¼ 505 C (black) for 1 h. Reflections from bcc-a-Fe(Si), t-Fe3B and fcc-Fe23B6 are indicated.
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J. Balcerski et al. / Vacuum 83 (2009) S182–S185
Table 1 Fe73.5xSi13.5B9Cu1Nb3Mnx. The lattice constant a0 [nm] (c0 [nm]) and average crystal diameter D [nm] of thin film (TEM) annealed at Ta ¼ 540 C and foil (XRD Eq. (1)) annealed at Ta ¼ 580 C for 1 h. Ref. [3]
x
11-film
11-foil
13-film
13-foil
15-film
15-foil
Fe3Si (0.2851) Fe23B6 (1.059) Fe3B (0.8844) (0.4397)
D a0 D a0 D a0 c0
30 0.2852
31 0.2846 18 1.0655 22 0.880 0.442
29 0.2855
30 0.2839 17 1.0635 21 0.879 0.440
29 0.2858
26 0.2835 17 1.0613 20 0.880 0.437
XRD angular scans measured at T0 for the foils annealed at Ta ¼ 700 C, 580 C and 505 C for 1 h are shown in Fig. 3. The XRD structure is well shaped even at 580 C. Basically no more than three crystal structures can be identified in the scans: bcc-aFe(Si), t-Fe3B and fcc-Fe23B6, as in the case of FINEMET. The average lattice parameters were determined with a PowderX code and the average size of crystallites D can be determined with the Scherrer relation:
Fig. 5. TEM diffraction pattern (dots) from a few crystallites embedded in the 40 nm thin film made from Fe73.5xSi13.5B9Cu1Nb3Mnx¼11 annealed at Ta ¼ 540 C for 1 h. The upper-left picture corresponds to a large number of crystallites and the upper-right picture refers to the film before annealing.
D ¼
0:9l ; s cos q
(1)
where l is the X-ray wavelength, 2q is the angle of the dominant Bragg maximum and the s[rad] is the FWHM of the Lorenzian distribution fitted to the peak. The results are shown in Table 1. Shifts of lattice constants in reference to pure FINEMET were interpreted as due to the influence of Mn. It should be noted that the crystallites grain size decreases with Mn doping, and the lattice constants for Fe3Si, Fe23B6 and Fe3B crystallites are systematically smaller than those determined for pure FINEMET [3]. After annealing at Ta z ti z 600 C for 1 h it can be seen in Table 1 that the grain size D of nanocrystals calculated with Eq. (1) decreases with the increase of Mn content x. The lattice constants are smaller than those for pure FINEMET. For Ta ¼ 700 C the average D for Fe(Si) is D z 30 nm, whereas for Fe3B is D z 22 nm and for Fe23B6 is D z 18 nm in diameter. The results of TEM analysis of thin film structure are shown in Figs. 4 and 6 for alloys with x ¼ 11 and x ¼ 15 Mn content, annealed
Fig. 4. TEM diffraction pattern (SAD) (upper) and analysis of the structure (lower) obtained from 40 nm self-supporting thin film evaporated from Fe73.5xSi13.5B9Cu1Nb3Mnx¼15 alloy, subsequently annealed at Ta ¼ 540 C for 1 h.
Fig. 6. TEM picture of crystallites grown in the 40 nm film made from Fe73.5xSi13.5B9Cu1Nb3Mnx¼13 after annealing at Ta ¼ 540 C for 1 h. The average crystal size D z 30 nm.
J. Balcerski et al. / Vacuum 83 (2009) S182–S185
at Ta ¼ 540 C for 1 h. The films before annealing yield the standard diffraction shape, shown in Fig. 5 (upper right), with broad maximums like in the XRD case. After annealing a sharp diffraction pattern appears, as can be seen in the upper-left part of Fig. 5. The selected area diffraction (SAD) pattern in Fig. 4 allows us to determine dominating structure (FeMn)3Si, as shown in Table 1. The lattice parameter a0 ¼ 0.2852 O 0.2858 nm is calculated to be shorter than for a-Fe (0.2866 nm) and it is rather independent of Mn doping x. The absence of borides (Fe3B and Fe23B6) in the TEM diffraction pattern can only be attributed to the absence of B in the vapour due to low temperature of the melt. Although the annealing temperature Ta z 540 C is much lower than the DSC crystallization temperature ti z 600 C, the diffraction pattern is very well shaped. The same takes place in the case of XRD diffraction, Fig. 3, where sharp lines appear after annealing even at Ta ¼ 505 C. It means that crystallization can start locally in the material at temperatures lower than that determined by DSC, which accounts for the global phase transformation. The distribution of grain size was determined by scanning of the bright field pictures, like in Fig. 6. The determined TEM results for the films, presented in Table 1, are consistent (within the statistical error) with those for the foils determined by means of XRD. 4. Conclusions FINEMET doped with Mn at w15%(at.) was analyzed by means of different methods before and after annealing. The
S185
lattice constants a and c were shown to decrease with x in the case of all identified crystalline structures. Only one crystallization temperature, ti z 600 C for both iron silicides and iron borides was found. The grain size was determined to be smaller than that for pure FINEMET and it was shown to decrease with Mn doping. Acknowledgements Financial support from Uq505/881 project has been acknowledged. Many thanks to Mrs. Zosia Fijarczyk for help in preparing the manuscript. References [1] [2] [3] [4] [5] [6] [7]
Herzer G. IEEE Trans Magn 1990;26:1397. Shahri F, Beitollahi A, Shabestari SG, Kamali S. Phys Rev B 2007;76:024434. Chen WZ, Ryder PL. Mater Sci Eng B1995;34:204. Lefebrve W, Morin-Grognet S, Danoix F. J Magn Magn Mater 2006;301:343. Okumura H, Laughlin DE, McHenry ME. J Magn Magn Mater 2003;267:347. Jjang JZ. J Magn Magn Mater 1996;154:375. Hujsheng Y, Guochao T, Xiotao X, Zuxiong X, Ruzhang M. J Magn Magn Mater 1994;138:94. [8] Gomez-Polo C, Perez-Landazabal JI, Recarte V, Mendoza-Zelis P, Li YF, Vazquez M. J Magn Magn Mater 2005;290–291:1517. [9] Tho ND, Chau N, Yu SC, Lee HB, Tuan LA, Hoa NQ. J Magn Magn Mat 2006; 157:868. [10] Chau N, Thanh PQ , Hoa NQ , The ND. J Magn Magn Mat 2006;304:36.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
TEM, XRD and DSC analysis of thin films and foils of FeSiBNb alloys doped with Mn J. Balcerski a, *, R. Brzozowski a, M. Wasiak b, K. Polan´ski a, M. Moneta a a b
´dz´, Department of Solid State Physics, Pomorska 149, PL 90-236 Ło ´dz´, Poland University of Ło ´dz´, Department of Physical Chemistry, Pomorska 163, PL 90-236 Ło ´dz´, Poland University of Ło
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 June 2008 Received in revised form 28 January 2009 Accepted 30 January 2009
The FINEMET based amorphous alloys in forms of thin films of 20 O 40 nm thickness and 20 mm thick foils of Fe73.5xSi13.5B9Cu1Nb3Mnx with Mn doping (x ¼ 11 O 15 at.%), as-quenched (a-q) and after annealing (a) were analyzed with transmission electron microscopy (TEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The structure and composition of the films differ from those of the foils, first of all in size of crystallites and the lattice constants. Ó 2009 Published by Elsevier Ltd.
Keywords: FINEMET Mn doping TEM SEM XRD DSC
1. Introduction
2. Experiment
Magnetic properties of nanocrystalline FeSiB alloys are due to the fact that size of crystallites, w15 nm, may be shorter than the exchange correlation length [1,2]. The as-quenched metal alloys, like Fe73.5xSi13.5B9Cu1Nb3, consist primarily of amorphousmagnetic phase which, after appropriate thermal or mechanical treatment, are transformed first into amorphous-paramagnetic phase and next into the phase where iron silicide and boride magnetic nanocrystals are embedded in a ferro–paramagnetic amorphous residual matrix [3,4]. The temperature dependence of structure and magnetic properties of the basic FINEMET, x ¼ 0, were analyzed in many previous papers [5–7]. TEM and EDX pictures, XRD patterns and differential scanning calorimetry (DSC) scans allow us to determine the compositions, structure and phase transitions. It is known that Cu atoms segregate and act as nucleation centers of nanocrystals during annealing. Nb reduces atomic mobility, retards grain growth and decreases the Curie temperature. The results of experiments related to low Mn doped FINEMET published recently [8–10] show the decrease of the Curie temperature and activation energy and suggest magnetic hardening of the nanocrystalline phase as a function of Mn concentration x.
The films of Fe73.5xSi13.5B9Cu1Nb3Mnx alloys of a quartz crystalcontrolled 40 nm thickness were prepared by means of physical vapour deposition (PVD). The poly-crystalline powder composite was thermally evaporated in HV (108 Tr) and the metal vapour was deposited on the NaCl surface. The salt was removed by dissolution in H2O. Next the films were heated at the rate w20 C/min to the annealing temperature Ta and then annealed for 1 h. The annealing temperatures for subsequent x were determined from DSC scan of the foils (understood as massive materials) as related to crystallization peak temperature ti, shown in Fig. 2. The transmission electron microscopy (TEM) diffraction patterns from thin films were measured at the room temperature T0 with a TESLA BS-540 microscope operating at 80 kV with l ¼ 0.00418 nm. The baseline was calibrated with Au (a0 ¼ 0.407 nm). The pictures were taken in the diffraction mode and in the bright field mode. The Fe73.5xSi13.5B9Cu1Nb3Mnx¼11O15 foils of approximately 2 mm 20 mm were prepared from basic composite alloys by a melt-spinning technique, commonly used for producing such amorphous metals. The Cu-wheel surface speed over 25 m/s resulting in the cooling rate of 106 K/s assures amorphous alloy. For lower speeds we get even micro-crystalline structure, determined by cooling conditions. The X-ray diffraction (XRD) patterns were measured with a Philips PW 1830 instrument with a CuKa generator (Eg ¼ 8.05 keV,
* Corresponding author. E-mail address: [email protected] (J. Balcerski). 0042-207X/$ – see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.vacuum.2009.01.058
J. Balcerski et al. / Vacuum 83 (2009) S182–S185
S183
Fig. 1. OM picture of outer surface of the foil with the Mn content x ¼ 15 annealed at 700 C for 1 h. Size z55 95 mm2.
l ¼ 0.15405 nm). The differential scanning calorimetry (DSC) measurements were carried out with a Setaram TG DSC-111 instrument. The heating rates 1 O 10 K/min up to the maximum temperature of 900 C in N2 atmosphere were used. The scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) with Link300 and a Zeiss optical microscope (OM) with up to 1000 times magnification were used.
3. Results and discussion OM pictures of both the as-quenched and the annealed samples show irregular interface suitable rather for bulk analysis of the sample than for surface analysis. Objects of the size smaller than 1 mm can be easily resolved on the surface, Fig. 1. The average thin film composition against foil composition was checked at the room temperature T0 with EDX, since differences caused by the evaporation and deposition processes were predicted. The averaged compositions of the alloys scaled to the Fe nominal atomic content
Fig. 2. DSC scans for 20 mm thick FINEMET foils as functions of Mn doping x. Peak positions, enthalpy and masses of the samples are indicated.
are reasonably well correlated with the nominal composition of the glasses. DSC temperature scans of w1 mg foils were performed at the heating rate ranging from 1 up to 10 K/min. They allow us to determine temperatures Ti for exothermic phase transformation and calculate the corresponding enthalpy change DHi. The scans are shown in Fig. 2. Only one crystallization temperature ti w600 C can be resolved and this value is rather independent of mass of the sample and of Mn content. However, as shown in Fig. 2, the crystallization enthalpy DHi decreases linearly with Mn doping. Since decreasing of temperature and subsequent DSC scanning did not give crystallization peaks, the measured enthalpy turned out to be sufficient for final transformation of the sample into the crystalline phase. It is known that Ti can be associated with the final crystallization temperature for iron silicides and also for borides. Decrease of DHi with Mn doping x, even against constant Ti, can be interpreted in terms of easier penetration of Mn into the grains, which should reduce their magnetization.
Fig. 3. XRD patterns from 20 mm thick Fe73.5xSi13.5B9Cu1Nb3Mnx alloys annealed at Ta ¼ 700 C (grey), Ta ¼ 580 C (dark gey) and Ta ¼ 505 C (black) for 1 h. Reflections from bcc-a-Fe(Si), t-Fe3B and fcc-Fe23B6 are indicated.
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J. Balcerski et al. / Vacuum 83 (2009) S182–S185
Table 1 Fe73.5xSi13.5B9Cu1Nb3Mnx. The lattice constant a0 [nm] (c0 [nm]) and average crystal diameter D [nm] of thin film (TEM) annealed at Ta ¼ 540 C and foil (XRD Eq. (1)) annealed at Ta ¼ 580 C for 1 h. Ref. [3]
x
11-film
11-foil
13-film
13-foil
15-film
15-foil
Fe3Si (0.2851) Fe23B6 (1.059) Fe3B (0.8844) (0.4397)
D a0 D a0 D a0 c0
30 0.2852
31 0.2846 18 1.0655 22 0.880 0.442
29 0.2855
30 0.2839 17 1.0635 21 0.879 0.440
29 0.2858
26 0.2835 17 1.0613 20 0.880 0.437
XRD angular scans measured at T0 for the foils annealed at Ta ¼ 700 C, 580 C and 505 C for 1 h are shown in Fig. 3. The XRD structure is well shaped even at 580 C. Basically no more than three crystal structures can be identified in the scans: bcc-aFe(Si), t-Fe3B and fcc-Fe23B6, as in the case of FINEMET. The average lattice parameters were determined with a PowderX code and the average size of crystallites D can be determined with the Scherrer relation:
Fig. 5. TEM diffraction pattern (dots) from a few crystallites embedded in the 40 nm thin film made from Fe73.5xSi13.5B9Cu1Nb3Mnx¼11 annealed at Ta ¼ 540 C for 1 h. The upper-left picture corresponds to a large number of crystallites and the upper-right picture refers to the film before annealing.
D ¼
0:9l ; s cos q
(1)
where l is the X-ray wavelength, 2q is the angle of the dominant Bragg maximum and the s[rad] is the FWHM of the Lorenzian distribution fitted to the peak. The results are shown in Table 1. Shifts of lattice constants in reference to pure FINEMET were interpreted as due to the influence of Mn. It should be noted that the crystallites grain size decreases with Mn doping, and the lattice constants for Fe3Si, Fe23B6 and Fe3B crystallites are systematically smaller than those determined for pure FINEMET [3]. After annealing at Ta z ti z 600 C for 1 h it can be seen in Table 1 that the grain size D of nanocrystals calculated with Eq. (1) decreases with the increase of Mn content x. The lattice constants are smaller than those for pure FINEMET. For Ta ¼ 700 C the average D for Fe(Si) is D z 30 nm, whereas for Fe3B is D z 22 nm and for Fe23B6 is D z 18 nm in diameter. The results of TEM analysis of thin film structure are shown in Figs. 4 and 6 for alloys with x ¼ 11 and x ¼ 15 Mn content, annealed
Fig. 4. TEM diffraction pattern (SAD) (upper) and analysis of the structure (lower) obtained from 40 nm self-supporting thin film evaporated from Fe73.5xSi13.5B9Cu1Nb3Mnx¼15 alloy, subsequently annealed at Ta ¼ 540 C for 1 h.
Fig. 6. TEM picture of crystallites grown in the 40 nm film made from Fe73.5xSi13.5B9Cu1Nb3Mnx¼13 after annealing at Ta ¼ 540 C for 1 h. The average crystal size D z 30 nm.
J. Balcerski et al. / Vacuum 83 (2009) S182–S185
at Ta ¼ 540 C for 1 h. The films before annealing yield the standard diffraction shape, shown in Fig. 5 (upper right), with broad maximums like in the XRD case. After annealing a sharp diffraction pattern appears, as can be seen in the upper-left part of Fig. 5. The selected area diffraction (SAD) pattern in Fig. 4 allows us to determine dominating structure (FeMn)3Si, as shown in Table 1. The lattice parameter a0 ¼ 0.2852 O 0.2858 nm is calculated to be shorter than for a-Fe (0.2866 nm) and it is rather independent of Mn doping x. The absence of borides (Fe3B and Fe23B6) in the TEM diffraction pattern can only be attributed to the absence of B in the vapour due to low temperature of the melt. Although the annealing temperature Ta z 540 C is much lower than the DSC crystallization temperature ti z 600 C, the diffraction pattern is very well shaped. The same takes place in the case of XRD diffraction, Fig. 3, where sharp lines appear after annealing even at Ta ¼ 505 C. It means that crystallization can start locally in the material at temperatures lower than that determined by DSC, which accounts for the global phase transformation. The distribution of grain size was determined by scanning of the bright field pictures, like in Fig. 6. The determined TEM results for the films, presented in Table 1, are consistent (within the statistical error) with those for the foils determined by means of XRD. 4. Conclusions FINEMET doped with Mn at w15%(at.) was analyzed by means of different methods before and after annealing. The
S185
lattice constants a and c were shown to decrease with x in the case of all identified crystalline structures. Only one crystallization temperature, ti z 600 C for both iron silicides and iron borides was found. The grain size was determined to be smaller than that for pure FINEMET and it was shown to decrease with Mn doping. Acknowledgements Financial support from Uq505/881 project has been acknowledged. Many thanks to Mrs. Zosia Fijarczyk for help in preparing the manuscript. References [1] [2] [3] [4] [5] [6] [7]
Herzer G. IEEE Trans Magn 1990;26:1397. Shahri F, Beitollahi A, Shabestari SG, Kamali S. Phys Rev B 2007;76:024434. Chen WZ, Ryder PL. Mater Sci Eng B1995;34:204. Lefebrve W, Morin-Grognet S, Danoix F. J Magn Magn Mater 2006;301:343. Okumura H, Laughlin DE, McHenry ME. J Magn Magn Mater 2003;267:347. Jjang JZ. J Magn Magn Mater 1996;154:375. Hujsheng Y, Guochao T, Xiotao X, Zuxiong X, Ruzhang M. J Magn Magn Mater 1994;138:94. [8] Gomez-Polo C, Perez-Landazabal JI, Recarte V, Mendoza-Zelis P, Li YF, Vazquez M. J Magn Magn Mater 2005;290–291:1517. [9] Tho ND, Chau N, Yu SC, Lee HB, Tuan LA, Hoa NQ. J Magn Magn Mat 2006; 157:868. [10] Chau N, Thanh PQ , Hoa NQ , The ND. J Magn Magn Mat 2006;304:36.