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Magnetic study of fine and ultrafine particles of amorphous Nd-Fe-B oxides
/intO'
Journal of Magnetism and Magnetic Materials 101 (1991) 403-404 North-Holland
Magnetic study of fine and ultrafine particles of amorphous N d - F e - B oxides J. Rivas
a, M.A. Ldpez-Quintela a R.J. D u r o a, G. Barault b and J.M. G r e n e c h e b
,1 Dpto de Fisica Aplicada and Quimica F(sica, Univer~idad de Santiago de Compostela, 15706 Santiago de Compostela, Spain b Laboratoire de Physique des Matgriaux, URA-CNRS 807, Universiti du Maine, 72017 Le Mans Cedex, France
Nd-Fe-B oxides have been obtained by chemical reaction in solution. Using M6ssbauer spectroscopy and magnetic measurements we have observed the existence of several iron based phases and an order-disorder magnetic transition in the temperature range 25 K < T < 30 K. The magnetic data were fitted assuming a superposition of superparamagnetic and ferromagnetic phases. 1. Introduction The study of N d - F e - B compounds has been the object of many investigations [1]. Among them, the study of fine and ultrafine amorphous particles is important, firstly because of their peculiar properties [2], and secondly, because of their role in the new chemical synthesis processes for the production of hard magnets based on N d - F e - B [3]. Although N d - F e - B compounds have been extensively studied, far less attention has been paid to their oxides [4,5] whose magnetic properties are also very interesting [2]. In this work we discuss the experimental results of 57Fe M6ssbauer spectroscopy and magnetization measurements, in the temperature range 2-300 K, of amorphous N d - F e - B oxides in the form of fine and ultrafine particles obtained by chemical synthesis [3]. From M6ssbauer measurements we infer the existence of an order-disorder magnetic transition at approximately 25 K < T < 30 K, depending on the composition of the samples. We have also observed and confirmed by means of magnetic measurements the existence of superimposed superparamagnetic and ferromagnetic behaviours of several iron based amorphous phases. 2. Experimental The magnetic powders were prepared by chemical reduction/coprecipitation of Fe(II) and Nd(III) salts using NaBH 4 either in aqueous solution (fine particles) or in microemulsion (ultrafine particles: size < 10 nm) following a procedure similar to the one described in the literature [3,6]. As the main objective of this work was the production of N d - F e - B oxides, the reaction was carried out at room temperature in an air atmosphere. The 57Fe M6ssbauer absorption spectra were recorded in a transmission geometry using a conventional constant acceleration spectrometer and a 57Co source diffused in a rhodium matrix. The samples made of powdered N d - F e - B oxides contained 5 mg
F e / c m 3. The present experiments were performed over the temperature range 4.2-300 K using a bath cryostat. Magnetic measurements were carried out in the temperature interval 2 K < T < 300 K with a Quantum Design SQUID magnetometer using pressed powder samples of cylindric shapes. The magnetic field applied was H < 50 kOe. 3. Results and discussion With respect to the Mfssbauer measurements, the most significant spectra are reported in fig. 1. They were recorded on samples 1 and 2 at 77 and 4.2 K. The 77 K spectra consist of two parts: (i) a quadrupolar component at the center, attributed to an amorphous ferric phase because of the presence of broadened lines and of the isomer shift mean value, and (ii) a magnetic contribution with very slightly resolved lines superposed to a sextet characteristic of t~-metallic iron phase. At liquid helium temperature, three main magnetic contributions have been considered in order to take into account the rather complex hyperfine structure observed on both spectra. The ~-metallic iron contribution was clearly confirmed. The outer sextet with broadened lines is due to the amorphous ferric iron phase which is magnetically ordered; furthermore, -12
o
mrn.~-~ 12
-8
0
b
rnm.s~ ,8
.rv
b" /?a fIl o.o,,., A ;v ,-,,., &.2K
.77K
Fig. 1. M6ssbauer spectra recorded on samples 1 and 2 at 4.2 and 77 K.
0312-8853/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved
J. Rivas et al. / Fine particles o f amorphous N d - F e - B oxides
404 30
Table 1 Coefficients for the fits to the data in figs. 3a and 3b
25
Sample 1 Sample 2
]
C2
C3
C4
C5
10.27 36.21
13.12 8.06
0.41 1.92
-0.28 -0.07
-22.19×10 5 - 5 . 5 1 x 10 5
°~°°°°°OOooooooooo
03 20
° ° ° ° ° ° ° ° ° ° ° ° o o o
v
C1
s a t u r a t i o n m a g n e t i z a t i o n vs. t e m p e r a t u r e data of two r e p r e s e n t a t i v e samples. T h e data fit a global superp a r a m a g n e t i c b e h a v i o u r with a s u p e r i m p o s e d ferrom a g n e t i c c o m p o n e n t quite well. In table 1 we show the coefficients for t h e fitting function. All of this coincides with the results o b t a i n e d above using M 6 s s b a u e r spectroscopy. Summarizing, in this work we have seen t h a t the N d - F e - B oxides o b t a i n e d by chemical synthesis in air are not magnetically h a r d c o m p o u n d s a n d they p r e s e n t a superposition of s u p e r p a r a m a g n e t i c a n d ferromagnetic phases.
D,.
15
°:'Z:"'2 i .............. °
I0
0
o I o /
I
o o i
i
i
I
o /
0
I
1
I
I
t
50
100
150
200
250
T(K) Fig. 2. Specific saturation magnetization ( H = 20 kOe) for different samples of amorphous N d - F e - B oxides.
W e would like to acknowledge the financial s u p p o r t for this work by the CICYT, Spain, in project M A T 8 9 0425-C03.
the spin-freezing t e m p e r a t u r e was o b s e r v e d over the r a n g e 2 5 - 3 0 K. Finally, a metallic N d - F e - B amorp h o u s p h a s e was evidenced t h r o u g h the t h i r d m a g n e t i c c o n t r i b u t i o n which is located in the central part of b o t h 4.2 K spectra: such a p h a s e p r o b a b l y originates from s u p e r p a r a m a g n e t i c clusters b e c a u s e of the line s h a p e of t h e m a g n e t i c c o n t r i b u t i o n o b s e r v e d at 77 K. W e p o i n t out t h a t the r o o m t e m p e r a t u r e spectra would look like the 77 K spectra. As to the m a g n e t i c c h a r a c t e r i z a t i o n , t h e s e materials t u r n e d out not to b e hard, with coercive field values of H c < 150 O e a n d s a t u r a t i o n m a g n e t i z a t i o n s of M S < 30 e m u / g which were achieved at fields of 1 5 - 2 0 kOe. In fig. 2 we show the s a t u r a t i o n m a g n e t i z a t i o n vs. t e m p e r a t u r e curves for several samples. W e observe t h a t the s a t u r a t i o n m a g n e t i z a t i o n does not follow a simple T 3/2 law for any of t h e m . As an example of the m a g n e t i c behaviour, in figs. 3a a n d b we p r e s e n t some fits for the
13.00
oExp©rim©ntalData --
'"
References
[1] Concerted European Action on Magnets (CEAM), eds. I.V. Mitchell, J.M.D. Coey, D. Givord, I.R. Harris and R. Hanitsh (Elsevier Applied Science, Barking, 1989). [2] T. Miyahara and K. Kawakami, IEEE Trans. Magn. MAG-23 (1987) 2877. [3] M.A. Ldpez-Quintela, J. Rivas and J. Quiben, Spanish Patent No. 2009404 (1989); US Patent No. 4983217 (1991); European Patent Application No. 89500115.4 (1990). [4] A.S. Kim, J. Appl. Phys. 64 (1988) 5571. [5] C.N. Cbristodoulou, J. Schulp and G.C. Hadjipanayis, J. Appl. Phys. 61 (1987) 3760. [6] M.A. L6pez Quintela and J. Rivas, in: Structure, Dynamics and Equilibrium Properties of Colloidal Systems, eds. D.M. Bloor and E. Wyn-Jones (Kluwer, Dordrecht, 1990) p. 773.
i
12.00
~
~
o Experimental _
F~t
Data "
~ 12.oo
I0,00
-
, .oo
10.00
9.00
6.00 0
50
100
T
150 200 250 0 50 100 150 200 250 T(K) T(K) Fig. 3. Two representative examples ofsaturationmagnetization vs. temperature fitted t o : M s = C 2 + C l / ( T + C3)+ C4 T l / 2 + C5 T3/2.
Journal of Magnetism and Magnetic Materials 101 (1991) 403-404 North-Holland
Magnetic study of fine and ultrafine particles of amorphous N d - F e - B oxides J. Rivas
a, M.A. Ldpez-Quintela a R.J. D u r o a, G. Barault b and J.M. G r e n e c h e b
,1 Dpto de Fisica Aplicada and Quimica F(sica, Univer~idad de Santiago de Compostela, 15706 Santiago de Compostela, Spain b Laboratoire de Physique des Matgriaux, URA-CNRS 807, Universiti du Maine, 72017 Le Mans Cedex, France
Nd-Fe-B oxides have been obtained by chemical reaction in solution. Using M6ssbauer spectroscopy and magnetic measurements we have observed the existence of several iron based phases and an order-disorder magnetic transition in the temperature range 25 K < T < 30 K. The magnetic data were fitted assuming a superposition of superparamagnetic and ferromagnetic phases. 1. Introduction The study of N d - F e - B compounds has been the object of many investigations [1]. Among them, the study of fine and ultrafine amorphous particles is important, firstly because of their peculiar properties [2], and secondly, because of their role in the new chemical synthesis processes for the production of hard magnets based on N d - F e - B [3]. Although N d - F e - B compounds have been extensively studied, far less attention has been paid to their oxides [4,5] whose magnetic properties are also very interesting [2]. In this work we discuss the experimental results of 57Fe M6ssbauer spectroscopy and magnetization measurements, in the temperature range 2-300 K, of amorphous N d - F e - B oxides in the form of fine and ultrafine particles obtained by chemical synthesis [3]. From M6ssbauer measurements we infer the existence of an order-disorder magnetic transition at approximately 25 K < T < 30 K, depending on the composition of the samples. We have also observed and confirmed by means of magnetic measurements the existence of superimposed superparamagnetic and ferromagnetic behaviours of several iron based amorphous phases. 2. Experimental The magnetic powders were prepared by chemical reduction/coprecipitation of Fe(II) and Nd(III) salts using NaBH 4 either in aqueous solution (fine particles) or in microemulsion (ultrafine particles: size < 10 nm) following a procedure similar to the one described in the literature [3,6]. As the main objective of this work was the production of N d - F e - B oxides, the reaction was carried out at room temperature in an air atmosphere. The 57Fe M6ssbauer absorption spectra were recorded in a transmission geometry using a conventional constant acceleration spectrometer and a 57Co source diffused in a rhodium matrix. The samples made of powdered N d - F e - B oxides contained 5 mg
F e / c m 3. The present experiments were performed over the temperature range 4.2-300 K using a bath cryostat. Magnetic measurements were carried out in the temperature interval 2 K < T < 300 K with a Quantum Design SQUID magnetometer using pressed powder samples of cylindric shapes. The magnetic field applied was H < 50 kOe. 3. Results and discussion With respect to the Mfssbauer measurements, the most significant spectra are reported in fig. 1. They were recorded on samples 1 and 2 at 77 and 4.2 K. The 77 K spectra consist of two parts: (i) a quadrupolar component at the center, attributed to an amorphous ferric phase because of the presence of broadened lines and of the isomer shift mean value, and (ii) a magnetic contribution with very slightly resolved lines superposed to a sextet characteristic of t~-metallic iron phase. At liquid helium temperature, three main magnetic contributions have been considered in order to take into account the rather complex hyperfine structure observed on both spectra. The ~-metallic iron contribution was clearly confirmed. The outer sextet with broadened lines is due to the amorphous ferric iron phase which is magnetically ordered; furthermore, -12
o
mrn.~-~ 12
-8
0
b
rnm.s~ ,8
.rv
b" /?a fIl o.o,,., A ;v ,-,,., &.2K
.77K
Fig. 1. M6ssbauer spectra recorded on samples 1 and 2 at 4.2 and 77 K.
0312-8853/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved
J. Rivas et al. / Fine particles o f amorphous N d - F e - B oxides
404 30
Table 1 Coefficients for the fits to the data in figs. 3a and 3b
25
Sample 1 Sample 2
]
C2
C3
C4
C5
10.27 36.21
13.12 8.06
0.41 1.92
-0.28 -0.07
-22.19×10 5 - 5 . 5 1 x 10 5
°~°°°°°OOooooooooo
03 20
° ° ° ° ° ° ° ° ° ° ° ° o o o
v
C1
s a t u r a t i o n m a g n e t i z a t i o n vs. t e m p e r a t u r e data of two r e p r e s e n t a t i v e samples. T h e data fit a global superp a r a m a g n e t i c b e h a v i o u r with a s u p e r i m p o s e d ferrom a g n e t i c c o m p o n e n t quite well. In table 1 we show the coefficients for t h e fitting function. All of this coincides with the results o b t a i n e d above using M 6 s s b a u e r spectroscopy. Summarizing, in this work we have seen t h a t the N d - F e - B oxides o b t a i n e d by chemical synthesis in air are not magnetically h a r d c o m p o u n d s a n d they p r e s e n t a superposition of s u p e r p a r a m a g n e t i c a n d ferromagnetic phases.
D,.
15
°:'Z:"'2 i .............. °
I0
0
o I o /
I
o o i
i
i
I
o /
0
I
1
I
I
t
50
100
150
200
250
T(K) Fig. 2. Specific saturation magnetization ( H = 20 kOe) for different samples of amorphous N d - F e - B oxides.
W e would like to acknowledge the financial s u p p o r t for this work by the CICYT, Spain, in project M A T 8 9 0425-C03.
the spin-freezing t e m p e r a t u r e was o b s e r v e d over the r a n g e 2 5 - 3 0 K. Finally, a metallic N d - F e - B amorp h o u s p h a s e was evidenced t h r o u g h the t h i r d m a g n e t i c c o n t r i b u t i o n which is located in the central part of b o t h 4.2 K spectra: such a p h a s e p r o b a b l y originates from s u p e r p a r a m a g n e t i c clusters b e c a u s e of the line s h a p e of t h e m a g n e t i c c o n t r i b u t i o n o b s e r v e d at 77 K. W e p o i n t out t h a t the r o o m t e m p e r a t u r e spectra would look like the 77 K spectra. As to the m a g n e t i c c h a r a c t e r i z a t i o n , t h e s e materials t u r n e d out not to b e hard, with coercive field values of H c < 150 O e a n d s a t u r a t i o n m a g n e t i z a t i o n s of M S < 30 e m u / g which were achieved at fields of 1 5 - 2 0 kOe. In fig. 2 we show the s a t u r a t i o n m a g n e t i z a t i o n vs. t e m p e r a t u r e curves for several samples. W e observe t h a t the s a t u r a t i o n m a g n e t i z a t i o n does not follow a simple T 3/2 law for any of t h e m . As an example of the m a g n e t i c behaviour, in figs. 3a a n d b we p r e s e n t some fits for the
13.00
oExp©rim©ntalData --
'"
References
[1] Concerted European Action on Magnets (CEAM), eds. I.V. Mitchell, J.M.D. Coey, D. Givord, I.R. Harris and R. Hanitsh (Elsevier Applied Science, Barking, 1989). [2] T. Miyahara and K. Kawakami, IEEE Trans. Magn. MAG-23 (1987) 2877. [3] M.A. Ldpez-Quintela, J. Rivas and J. Quiben, Spanish Patent No. 2009404 (1989); US Patent No. 4983217 (1991); European Patent Application No. 89500115.4 (1990). [4] A.S. Kim, J. Appl. Phys. 64 (1988) 5571. [5] C.N. Cbristodoulou, J. Schulp and G.C. Hadjipanayis, J. Appl. Phys. 61 (1987) 3760. [6] M.A. L6pez Quintela and J. Rivas, in: Structure, Dynamics and Equilibrium Properties of Colloidal Systems, eds. D.M. Bloor and E. Wyn-Jones (Kluwer, Dordrecht, 1990) p. 773.
i
12.00
~
~
o Experimental _
F~t
Data "
~ 12.oo
I0,00
-
, .oo
10.00
9.00
6.00 0
50
100
T
150 200 250 0 50 100 150 200 250 T(K) T(K) Fig. 3. Two representative examples ofsaturationmagnetization vs. temperature fitted t o : M s = C 2 + C l / ( T + C3)+ C4 T l / 2 + C5 T3/2.