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Segregation and speromagnetic order in amorphous Tb0.18Si0.82 detected by neutron scattering at small and large angles
Journal of Non-Crystalline Solids 163 (1993) 43-48 North-Holland
jo~RNa t or l ~ 0 N ~ l ~ ~0III~
Segregation and speromagnetic order in amorphous Yb0.1sSi0.s2 detected by neutron scattering at small and large angles B. B o u c h e r a n d R. T o u r b o t Seruice de Physique de l'Etat Condensd, CEA-Saclay, F-91191 Gif-sur-Yt,ette cddex, France
G. A n d r 6 Laboratoire Ldon Brillouin (Laboratoire mixte CEA-CNRS), Centre d'Etude de Saclay, F-91191 Gif-sur-Yt,ette, France Received 30 March 1993 Revised manuscript received 19 May 1993
A m o r p h o u s Tbo.lsSi0.82 has been investigated at low temperatures by neutron diffraction (ND) and small angle neutron scattering (SANS). Below the magnetic ordering temperature, magnetic ND rings are observable between the nuclear rings, but no magnetic SANS is detectable at low q (q = 4w sin 0 / A ) values where nuclear SANS is intense. From these observations and the comparison with the antiferromagnetic order in crystalline TbSik67 alloy, a phase segregation and a speromagnetic order in a m o r p h o u s Tbo.lsSi0.s2 with magnetic m o m e n t values of about 4k~B/Tb and compensation of the magnetic m o m e n t s over very small distances are proposed.
I. Introduction
It has been shown that the amorphous alloys RExM1_x (RE = Gd, Tb, Er, Ho . . . . M --Ag, Cu, Zr, S i . . . ) have a well defined nuclear medium range order (N-MRO) [1]. This order is described either by the juxtaposition of domains of different composition with sharp boundaries, or by the existence of fluctuations of composition. In both cases, the size of domains or the correlation length is of the order of few tens or few hundreds of nanometers. At low temperature, the alloys containing at least 20 at.% R E show an asperomagnetic order * [2-4] and a magnetic medium range order (M-MRO) with either magnetization fluctuations of 0.1-2p, B / R E , or domains with magnetizations varying in magnitudes and orientations. Whatever the model, numerous examples
Correspondence to: Dr B. Boucher, Service de Physique de l'Etat Condense, CEA-Saclay, F-91191 Gif-sur-Yvette c6dex, France. Tel.: +33-1 69 08 73 85. Telefax: +33-1 69 08 81 20.
prove that the asperomagnetic order is strongly influenced by the nuclear order which determines the magnetic domain limits or the characteristic distances of magnetization fluctuations. On a smaller scale, small composition inhomogeneities of ~ 2 nm are distributed either uniformly in the sample with exclusion distances of 3-5 nm or at random. At low temperature, these inhomogeneities create local peaks or depressions in the mean magnetization and give rise to a very characteristic magnetism called 'seedy magnetic order' [5]. This nuclear and magnetic M R O has been detected by small angle neutron scattering (SANS). The inhomogeneities give rise to intensities of the order of 50 barns per (RExM ~ ,)
* T h e asperomagnetic order is defined as a magnetic ordering in which the m o m e n t s tend to be parallel without reaching a good alignment. The speromagnetic order corresponds to a m o m e n t configuration in which the m o m e n t s are distributed in all directions [ref. [4], p.33].
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
D. Boucher et al. / Segregation and speromagnetic order in a-Tbo.lsSio.82
44
formula unit, while the small angle scattering, due to domains or fluctuations, is intense and can reach 1 0 4 - 1 0 6 barns per formula unit. For large angle scattering, neutron diffraction (ND) patterns show rings and the Fourier transformation (FT) of the spectra gives the interatomic distances and coordination numbers. At low temperatures, the asperomagnetic order [2,3] leads to magnetic diffraction rings which are superimposed on the nuclear rings. This model of nuclear and magnetic order is very general and has been found in several R E x M I _ x systems with x > 0.2 [1]. The a-Tb0.18Si0.82 ( a - amorphous), which has less R E than in previous cases, has distinctive properties. In his thesis work, Simonnin [5,6] detected a negative asymptotic Curie temperature ( - 1 0 K), transport properties of semi-conductor type, nuclear order governed by Si-Si tetrabounding and the existence of Si-Si chains. We recall that the Tb-rich SiTb alloy has a nuclear order controlled by T b - T b coordination and a metallic type conductivity [6-8]. The atomic structure parameters, given in table 1 and table 2, are interatomic distances, coordination numbers, and nuclear ring location. At low temperature, preliminary ND measurements showed magnetic rings appearing between the nuclear rings [9], while in the case of Tb-rich alloys the magnetic rings are superimposed on the nuclear rings. Figure 1 shows the nuclear SANS pattern [6,10]. We observe a q - 4 (q = 4at sin 0/A) scattering dependence at low q-values and a ring at q = 1.3 nm-1. The high intensities at low q-values are due to the sharp variation of the scattering contrast ( b l / U 1 - b z / u 2) (b i being the average scattering length in volume, ui) which is related to
Table 2 C o m p a r i s o n b e t w e e n locations of n u c l e a r and m a g n e t i c rings of a m o r p h o u s Tbo.lsSi0.s2 a n d of n u c l e a r and m a g n e t i c p e a k s of crystalline TbSil.67 ( o r t h o r h o m b i c p h a s e ) qN ( n m - 1 )
-
-
-
23.8 30.5 35.8 40.0 -
Si-Si Si-Tb Tb-Si Tb-Tb
Coordination number, N
0.238 0.305 0.305 0.355
3.83 1.90 10.8 ~ 1
1)
amorphous
24.1 28.0 31.0 36.4 42.0
crystall. ortho.
7.6
7.5
19.5 29.5 39.5 -
19.5 29.5 -
changes in composition. Simonnin [6,8] has presented evidence for the coexistence of two phases, one of a-Si containing several impurities (H, O, Ar) and a-TbSi z. Each phase occupies about the same volume fraction of a sample and is divided in domains ~ 105 nm 3 or spheres of 30 nm radius. This segregation is in agreement with the results of differential thermal analysis (DTA) measurements [6,7]. In addition, Simonnin [5,6] shows that the ring observed at q ~ 1.3 n m is due to inhomogeneities. From the position, shape and intensity of this ring, he estimates the volume, dispersion in the sample and composition of the inhomogeneities following the method used in ref. [5]. He concludes that the inhomo-
125 oo
0o
~"
Distance, r (nm)
crystall, ortho,
-
0
Table 1 E x p e r i m e n t a l v a l u e s e x t r a c t e d from the n e u t r o n diffraction m e a s u r e m e n t s [6].
qM (rim
amorphous
E c~ × k~
T=5OOK 100
75
5O
25
b 0
_ _ _
I
I
I
I
I
1
2
,3
4
5
q ( n m -1 )
Fig. 1. S A N S of Tb0.18Si0.82 at 300 K (after ref. [6]). W e observe at low q-values a q - 4 b e h a v i o u r a n d at q ~ 1.3 n m - I a n i n t e r f e r e n c e ring extensively s t u d i e d by S i m o n n i n [6].
D. Boucher et al. / Segregation and speromagnetic order in a-Tbo.isSio.s2
geneities, assumed to be spherical, have 0.7 nm radius, are separated by 3.7 nm and occupy 5.6% of the total volume of sample. The impurities can be either terbium hydrides (TbHx, 2 < x < 3), or Silly (2 < y < 4), or a mixture of the two. From the existence of an a-TbSi 2 phase and negative asymptotic Curie temperature, we expect a speromagnetic order. We want to observe this type of magnetic M R O using SANS measurements and N D data.
2. Experimental details and data analysis The samples of amorphous Tb0.18Si0.82 were obtained by sputtering as sheets which were crushed into powder [5,6]. From chemical analysis and incoherent neutron scattering, the formula Tb0.1sSi0.szH0.oTO0.o4Ar0.03 was deduced [6]. The various magnetic, neutron diffraction and small angle neutron scattering m e a s u r e m e n t s were performed on the same samples. For small angle neutron scattering, the powder was contained in a cylindrical cell with its axis parallel to the incident beam, while for neutron diffraction patterns the axis was parallel to the section of the incident beam. All measurements were performed in a cryostat in the t e m p e r a t u r e range 4.2-300 K for SANS and 1.6 to 300 K for ND. We used three different spectrometers at the Laboratoire L6on Brillouin (LLB, C E A Saclay, France): (i) P A X Y for SANS with a planar 64 × 64 cells detector, allowing us to test the isotropy of scattering in a q-range (q = 4 ~ sin 0/A) 3.5 × 10 2 < q < 2 n m - 1 ; (ii) 7C2 in the q-range 8 < q < 10 n m - l ; (iii) G4.1 used at a large wavelength (A = 0.245 nm) in 1 < q < 26 nm -], i.e., presenting a good resolution for amorphous materials enabling us to measure the ring location and the ring widths directly. On each spectrometer the counting time was chosen to have a statistical accuracy of about 3 per mille. All these measurements were normalized either by comparison with a vanadium plate scattering or by m e a s u r e m e n t s of the incident b e a m intensity [11] allowing us to merge them in an unique pattern for 3.5 × 10 -2 < q < 100 n m - l . The patterns taken at room t e m p e r a t u r e were corrected for the cryostat, paramagnetism, hydro-
45
0 0
x?02 co
-5
. ".... • ..'
o ,~
.
.
-
.
--5
:&
. )..-;.i.:...:...:.:.....::.:,v::......-....v.. ....."
-10
• .
E
~.'- .
-lo
-15
I
o b'~
-2o o
I
1
I
0.5
1.0
1.5
I
-15
2.0
q(nm 1) Fig. 2. SANS difference pattern (4.2 300 K). The cross-section difference is constant, negative and equal to - 9 . 2 7 barns per Yb0.18Si0.~;2, e.g., ~ 710/zi~/Tb.
gen and background scattering following standard methods [5,12] so as to obtain the nuclear crosssection. Below the magnetic ordering temperature, the magnetic scattering was directly obtained in a differential way after subtraction of the high temperature (300 or 176 K) raw data corrected for paramagnetic scattering.
3. Results 3.1. Small angle neutron scattering At 300 K, the sample is magnetically disordered; the SANS (fig. 1) is large, exceeding at low q several thousand barns per chemical unit formula. When the temperature decreases to 4.2 K, the diffraction pattern shows magnetic ordering but at very low q ( q < 0 . 4 nm -~) the SANS diminishes only slightly and at higher q its decrease does not exceed 35%. Figure 2 shows the difference between low (4.2 K) and high temperature (300 K) patterns. This difference is invariant with q, negative and ~ 7 1 0 / ~ / T b . At low q, the dispersion of the points results from the difference between two large quantities but the experimental values remain very small by comparison with magnetic intensities usually observed and are distributed around the mean value deduced from high q measurements. Therefore, the magnetic ordering does not introduce any contrast variation; the reduction of the intensity corresponds roughly to that of the paramagnetic scat-
D. Boucher et al. / Segregation and speromagnetic order in
46
200
2.6 eq O0
O -100
1.3ix:)
:i/(!
g
)o
m
t! 0~
e~ I ~ I
-1.3
-2.6
111
• LT= 1.5K LT=15K I
16
21
26
q(nrn -1)
Fig. 3. Difference patterns: 1.5-176 K, datapoints; 15-176 K, dotted line for the smoothed datapoints. We clearly see the two first magnetic rings at q = 7.6 nm I and q = 19.5 nm-1 as indicated by the arrows. The patterns taken at 176 and 300 K are similar.
tering (788/x~/Tb) which has been lost in favour of the magnetic m o m e n t ordering. We note that no magnetic scattering related to inhomogeneities is detectable. Therefore no magnetic contrast between a-TbSi z and a-Si phases or between inhomogeneities and amorphous material exists, although a magnetic order is present.
3.2. Neutron diffraction The location of nuclear and magnetic rings at 1.5 K has been given [9]. Figure 3 shows a part of the difference patterns taken on G4.1 spectrometer. It confirms the location of the two first magnetic rings and allows us to obtain the cross-section of the first one (or = l l 5 # ~ / T b ) . Unfortunately, the second one (or= 18/z~/Tb) is measured with a 30% error. The two intensities have been corrected for the magnetic form factor. It is interesting to compare these results with the patterns described in ref. [13] for crystalline TbSil.67 especially with that corresponding to the orthorhombic structure which was not known at the time of our first studies• In table 2, we compare the position of the rings and the Bragg
a-Tb0.18Si0.az
peaks of orthorhombic compound patterns. At 300 K the three first rings of the amorphous material correspond to three big peaks of TbSil.67 obtained at 60 K. In the same way, the positions of the three magnetic rings detectable at 1.6 K correspond to those of the strongest magnetic peaks, showing imperfect magnetic order, observed in ref. [13] at 1.2 K. So the phase separation detected by Simonnin as a-Si,a-TbSi 2 may be a-Si,a-TbSi].67 (or Tb3Sis). The width at half height of the first magnetic ring observed at 1.6 K is 4.5 nm-1, a value which is a good order of magnitude for an amorphous state [14]. If we were considering the sample as microcrystalline, the Scherrer formula ( L = 0.9A//3 cos 0 with standard notations [15])would lead to L = 1.4 nm. This distance can be assumed to be the correlation length between magnetic moments. L is ~ four times the unit cell parameters of TbSil.67 and thus the correlation would affect ~ 60 magnetic atoms.
3.3. Temperature dependence The SANS measurements show a variation of the paramagnetic cross-section from 710~z2/Tb at 4.2 K, to 680/x2/Tb at 15 K and zero at 300 K. N D measurements show magnetic rings at 1.5 and 15 K and they are not detected at 176 K. The magnetic ordering t e m p e r a t u r e is between these two last temperatures. These results are to be compared with the transition temperatures given in ref. [13] for a crystallized sample (T N = 16 or 39 K depending on chemical composition). The variations of intensities between 1.5 and 15 K are weak (about 15%) (fig. 3) and we suggest a magnetic ordering t e m p e r a t u r e of order of 50 K.
4. Discussion The location of rings in amorphous Tb0.185i0.82 with the experimental patterns given in ref. [13] for crystallized TbSi1.67 are in agreement, particularly, with the orthorhombic phase• Such a comparison between amorphous and Bragg peak pattern is only possible at a qualitative level; as a matter of fact, we are comparing the ring and
47
D. Boucher et al. / Segregation and speromagnetic order in a-Tbo.lsSio.82
peak location. However we obtain some indication of the composition in the segregated phase of the amorphous sample. This phase separation (a-Si,a-TbSil.67) would be a little different from the one (a-Si,a-TbSi 2) [6,7] proposed by Simonnin from D T A and SANS measurements. We checked that the previous SANS data are compatible with the new proposed composition; the volume fraction of phases changes only slightly. Moreover at low temperature the appearance of magnetic diffraction rings between the nuclear rings and the disappearance of paramagnetic scattering is similar to the antiferromagnetic behaviour of crystallized YbSil.67 [13] and we suggest a speromagnetic ordering. The small variation of the SANS over the entire small q range observed between 300 and 4.2 K is due to disappearance of the paramagnetic scattering. The agreement in sign and quantity is correct. The absence of another scattering shows that the magnetization of a-YbSil.67, which is zero at 300 K, remains zero at 4.2 K below the magnetic ordering temperature. If this zero magnetization had not been the case, a magnetic contrast between the magnetization of YbSil.67 and the zero magnetization of a-Si would have been observed. Thus the magnetic order of aYbSil.67 is speromagnetic and over small distances the resulting magnetization is approximately zero; that is confirmed by the absence of scattering for the highest small angle values corresponding to correlations over small distances (1 nm). The absence of SANS for high small angles, in addition, provides information on the magnetic nature of the inhomogeneities. Embedded in non-magnetized matrices (a-Si or a-TbSi1.67), these inhomogeneities have to be either intrinsically non-magnetic or with no significant net magnetization. The magnetic order of these inhomogeneities, if it exists, is speromagnetic. These results are in agreement with the negative value of the asymptotic Curie temperature. It is possible to measure with a reasonable accuracy the two first magnetic ring intensities, the incoherent scattering being small compared to these intensities. But the statistical error on difference patterns taken on G4.1 and 7C2 spectrometers and the inaccuracy of merging the
spectra performed with two different spectrometers do not allow us to satisfactorily Fourier transform and we are obliged to discuss the data in reciprocal space, using a procedure established for microcrystalline structures. With the standard notations, for an amorphous alloy, the measured intensity, l(q), of a magnetic ring is written [5]:
I( q) = 2 ( e2y/2mc2)2( 2/sin 0
sin
20) f 2( q )
× ( Y'~MiMj expi( qrij) ), ij where M i is the magnetic moment of atom i and riy is the distance vector of atoms i and j. From the two measured ring intensities, we find at 1.6 K the value (EijMiM j expi(qrij)) reported per atom as equal to 16.3/~2/Tb and 15.4/z2/Tb, e.g., a moment of at least 4/xB/Tb. Such a value is relatively important for an amorphous alloy, but is still smaller than the value observed for crystallized compound (6.5/.tB/Tb) [13].
5. Conclusion
This study identifies the phase separation, detected by Simonnin in amorphous Tbo.lsSio.sz, as a-Si-TbSil.67. For the first time in an amorphous RExM~_ x system, we observe a magnetic order of speromagnetic type. The magnetic SANS which usually superimposed on nuclear SANS does not exist here because of the absence of magnetization due to a compensation of moments over small distances (1 nm). The phase a-YbSil.67 is magnetically well ordered. Magnetic diffraction rings appear between nuclear rings with relatively high intensities in spite of nuclear disorder which decreases the interference effects, and tends to disperse the moments. The authors gratefully acknowledge A. Brulet and E. Le Coz (LLB, CE Saclay, France) for their help during the measurements on PAXY. They are grateful to P. Chieux (Institut L a u e - L a n gevin, Grenoble, France) for very helpful discussions.
48
D. Boucher et al. / Segregation and speromagnetic order in a-Tb0.1sSi0.82
References [1] B. Boucher and P. Chieux, J. Phys.: Condens. Matter 3 (1991) 2207. [2] J.M.D. Coey and P.W. Readman, Nature 246 (1973) 476. [3] J. Chappert, in: Magnetism of Metals and Alloys, ed. M. Cyrot (North-Holland, Amsterdam, 1982) p. 487. [4] K. Moorjani and J.M.D Coey, eds., Magnetic Glasses (Elsevier, Amsterdam, 1984). [5] B. Boucher, P. Chieux, P. Convert, R. Tourbot and M. Tournarie, J. Phys. F16 (1986) 1821. [6] P. Simonnin, th6se, Universit6 P. et M. Curie, Paris VI (1984). [7] P. Simonnin, R. Tourbot, B. Boucher, M. Perrin and J. Vanhaute, Phys. Status Solidi (a)98 (1986) 551.
[8] P. Simonnin, R. Tourbot, B. Boucher and R. Bellissent, J. Phys. F17 (1987) 559. [9] B. Boucher, R. Tourbot and R. Bellissent, J. Phys.: Condens. Matter 3 (1991) 2843. [10] P. Simonnin, R. Tourbot, M. Tournarie and B. Boucher, J. Phys. F15 (1985) L189. [11] J.P. Cotton and A. Brulet, private communication (LLB, Saclay). [12] B. Boucher and R. Tourbot, J. Phys.: Condens. Matter 4 (1992) 9697. [13] P. Schobinger-Papamantellos, D.B. Mooij and K.H.J. Buschow, J. Magn. Magn. Mater. 79 (1989) 231. [14] B. Boucher, Q. Chen, R. Tourbot and R. Bellissent, J. Non. Cryst. Solids 159 (1993) 101. [15] A. Guinier, Th6orie et Technique de la Radiocristallographie (Dunod, Paris, 1956) p. 462.
jo~RNa t or l ~ 0 N ~ l ~ ~0III~
Segregation and speromagnetic order in amorphous Yb0.1sSi0.s2 detected by neutron scattering at small and large angles B. B o u c h e r a n d R. T o u r b o t Seruice de Physique de l'Etat Condensd, CEA-Saclay, F-91191 Gif-sur-Yt,ette cddex, France
G. A n d r 6 Laboratoire Ldon Brillouin (Laboratoire mixte CEA-CNRS), Centre d'Etude de Saclay, F-91191 Gif-sur-Yt,ette, France Received 30 March 1993 Revised manuscript received 19 May 1993
A m o r p h o u s Tbo.lsSi0.82 has been investigated at low temperatures by neutron diffraction (ND) and small angle neutron scattering (SANS). Below the magnetic ordering temperature, magnetic ND rings are observable between the nuclear rings, but no magnetic SANS is detectable at low q (q = 4w sin 0 / A ) values where nuclear SANS is intense. From these observations and the comparison with the antiferromagnetic order in crystalline TbSik67 alloy, a phase segregation and a speromagnetic order in a m o r p h o u s Tbo.lsSi0.s2 with magnetic m o m e n t values of about 4k~B/Tb and compensation of the magnetic m o m e n t s over very small distances are proposed.
I. Introduction
It has been shown that the amorphous alloys RExM1_x (RE = Gd, Tb, Er, Ho . . . . M --Ag, Cu, Zr, S i . . . ) have a well defined nuclear medium range order (N-MRO) [1]. This order is described either by the juxtaposition of domains of different composition with sharp boundaries, or by the existence of fluctuations of composition. In both cases, the size of domains or the correlation length is of the order of few tens or few hundreds of nanometers. At low temperature, the alloys containing at least 20 at.% R E show an asperomagnetic order * [2-4] and a magnetic medium range order (M-MRO) with either magnetization fluctuations of 0.1-2p, B / R E , or domains with magnetizations varying in magnitudes and orientations. Whatever the model, numerous examples
Correspondence to: Dr B. Boucher, Service de Physique de l'Etat Condense, CEA-Saclay, F-91191 Gif-sur-Yvette c6dex, France. Tel.: +33-1 69 08 73 85. Telefax: +33-1 69 08 81 20.
prove that the asperomagnetic order is strongly influenced by the nuclear order which determines the magnetic domain limits or the characteristic distances of magnetization fluctuations. On a smaller scale, small composition inhomogeneities of ~ 2 nm are distributed either uniformly in the sample with exclusion distances of 3-5 nm or at random. At low temperature, these inhomogeneities create local peaks or depressions in the mean magnetization and give rise to a very characteristic magnetism called 'seedy magnetic order' [5]. This nuclear and magnetic M R O has been detected by small angle neutron scattering (SANS). The inhomogeneities give rise to intensities of the order of 50 barns per (RExM ~ ,)
* T h e asperomagnetic order is defined as a magnetic ordering in which the m o m e n t s tend to be parallel without reaching a good alignment. The speromagnetic order corresponds to a m o m e n t configuration in which the m o m e n t s are distributed in all directions [ref. [4], p.33].
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
D. Boucher et al. / Segregation and speromagnetic order in a-Tbo.lsSio.82
44
formula unit, while the small angle scattering, due to domains or fluctuations, is intense and can reach 1 0 4 - 1 0 6 barns per formula unit. For large angle scattering, neutron diffraction (ND) patterns show rings and the Fourier transformation (FT) of the spectra gives the interatomic distances and coordination numbers. At low temperatures, the asperomagnetic order [2,3] leads to magnetic diffraction rings which are superimposed on the nuclear rings. This model of nuclear and magnetic order is very general and has been found in several R E x M I _ x systems with x > 0.2 [1]. The a-Tb0.18Si0.82 ( a - amorphous), which has less R E than in previous cases, has distinctive properties. In his thesis work, Simonnin [5,6] detected a negative asymptotic Curie temperature ( - 1 0 K), transport properties of semi-conductor type, nuclear order governed by Si-Si tetrabounding and the existence of Si-Si chains. We recall that the Tb-rich SiTb alloy has a nuclear order controlled by T b - T b coordination and a metallic type conductivity [6-8]. The atomic structure parameters, given in table 1 and table 2, are interatomic distances, coordination numbers, and nuclear ring location. At low temperature, preliminary ND measurements showed magnetic rings appearing between the nuclear rings [9], while in the case of Tb-rich alloys the magnetic rings are superimposed on the nuclear rings. Figure 1 shows the nuclear SANS pattern [6,10]. We observe a q - 4 (q = 4at sin 0/A) scattering dependence at low q-values and a ring at q = 1.3 nm-1. The high intensities at low q-values are due to the sharp variation of the scattering contrast ( b l / U 1 - b z / u 2) (b i being the average scattering length in volume, ui) which is related to
Table 2 C o m p a r i s o n b e t w e e n locations of n u c l e a r and m a g n e t i c rings of a m o r p h o u s Tbo.lsSi0.s2 a n d of n u c l e a r and m a g n e t i c p e a k s of crystalline TbSil.67 ( o r t h o r h o m b i c p h a s e ) qN ( n m - 1 )
-
-
-
23.8 30.5 35.8 40.0 -
Si-Si Si-Tb Tb-Si Tb-Tb
Coordination number, N
0.238 0.305 0.305 0.355
3.83 1.90 10.8 ~ 1
1)
amorphous
24.1 28.0 31.0 36.4 42.0
crystall. ortho.
7.6
7.5
19.5 29.5 39.5 -
19.5 29.5 -
changes in composition. Simonnin [6,8] has presented evidence for the coexistence of two phases, one of a-Si containing several impurities (H, O, Ar) and a-TbSi z. Each phase occupies about the same volume fraction of a sample and is divided in domains ~ 105 nm 3 or spheres of 30 nm radius. This segregation is in agreement with the results of differential thermal analysis (DTA) measurements [6,7]. In addition, Simonnin [5,6] shows that the ring observed at q ~ 1.3 n m is due to inhomogeneities. From the position, shape and intensity of this ring, he estimates the volume, dispersion in the sample and composition of the inhomogeneities following the method used in ref. [5]. He concludes that the inhomo-
125 oo
0o
~"
Distance, r (nm)
crystall, ortho,
-
0
Table 1 E x p e r i m e n t a l v a l u e s e x t r a c t e d from the n e u t r o n diffraction m e a s u r e m e n t s [6].
qM (rim
amorphous
E c~ × k~
T=5OOK 100
75
5O
25
b 0
_ _ _
I
I
I
I
I
1
2
,3
4
5
q ( n m -1 )
Fig. 1. S A N S of Tb0.18Si0.82 at 300 K (after ref. [6]). W e observe at low q-values a q - 4 b e h a v i o u r a n d at q ~ 1.3 n m - I a n i n t e r f e r e n c e ring extensively s t u d i e d by S i m o n n i n [6].
D. Boucher et al. / Segregation and speromagnetic order in a-Tbo.isSio.s2
geneities, assumed to be spherical, have 0.7 nm radius, are separated by 3.7 nm and occupy 5.6% of the total volume of sample. The impurities can be either terbium hydrides (TbHx, 2 < x < 3), or Silly (2 < y < 4), or a mixture of the two. From the existence of an a-TbSi 2 phase and negative asymptotic Curie temperature, we expect a speromagnetic order. We want to observe this type of magnetic M R O using SANS measurements and N D data.
2. Experimental details and data analysis The samples of amorphous Tb0.18Si0.82 were obtained by sputtering as sheets which were crushed into powder [5,6]. From chemical analysis and incoherent neutron scattering, the formula Tb0.1sSi0.szH0.oTO0.o4Ar0.03 was deduced [6]. The various magnetic, neutron diffraction and small angle neutron scattering m e a s u r e m e n t s were performed on the same samples. For small angle neutron scattering, the powder was contained in a cylindrical cell with its axis parallel to the incident beam, while for neutron diffraction patterns the axis was parallel to the section of the incident beam. All measurements were performed in a cryostat in the t e m p e r a t u r e range 4.2-300 K for SANS and 1.6 to 300 K for ND. We used three different spectrometers at the Laboratoire L6on Brillouin (LLB, C E A Saclay, France): (i) P A X Y for SANS with a planar 64 × 64 cells detector, allowing us to test the isotropy of scattering in a q-range (q = 4 ~ sin 0/A) 3.5 × 10 2 < q < 2 n m - 1 ; (ii) 7C2 in the q-range 8 < q < 10 n m - l ; (iii) G4.1 used at a large wavelength (A = 0.245 nm) in 1 < q < 26 nm -], i.e., presenting a good resolution for amorphous materials enabling us to measure the ring location and the ring widths directly. On each spectrometer the counting time was chosen to have a statistical accuracy of about 3 per mille. All these measurements were normalized either by comparison with a vanadium plate scattering or by m e a s u r e m e n t s of the incident b e a m intensity [11] allowing us to merge them in an unique pattern for 3.5 × 10 -2 < q < 100 n m - l . The patterns taken at room t e m p e r a t u r e were corrected for the cryostat, paramagnetism, hydro-
45
0 0
x?02 co
-5
. ".... • ..'
o ,~
.
.
-
.
--5
:&
. )..-;.i.:...:...:.:.....::.:,v::......-....v.. ....."
-10
• .
E
~.'- .
-lo
-15
I
o b'~
-2o o
I
1
I
0.5
1.0
1.5
I
-15
2.0
q(nm 1) Fig. 2. SANS difference pattern (4.2 300 K). The cross-section difference is constant, negative and equal to - 9 . 2 7 barns per Yb0.18Si0.~;2, e.g., ~ 710/zi~/Tb.
gen and background scattering following standard methods [5,12] so as to obtain the nuclear crosssection. Below the magnetic ordering temperature, the magnetic scattering was directly obtained in a differential way after subtraction of the high temperature (300 or 176 K) raw data corrected for paramagnetic scattering.
3. Results 3.1. Small angle neutron scattering At 300 K, the sample is magnetically disordered; the SANS (fig. 1) is large, exceeding at low q several thousand barns per chemical unit formula. When the temperature decreases to 4.2 K, the diffraction pattern shows magnetic ordering but at very low q ( q < 0 . 4 nm -~) the SANS diminishes only slightly and at higher q its decrease does not exceed 35%. Figure 2 shows the difference between low (4.2 K) and high temperature (300 K) patterns. This difference is invariant with q, negative and ~ 7 1 0 / ~ / T b . At low q, the dispersion of the points results from the difference between two large quantities but the experimental values remain very small by comparison with magnetic intensities usually observed and are distributed around the mean value deduced from high q measurements. Therefore, the magnetic ordering does not introduce any contrast variation; the reduction of the intensity corresponds roughly to that of the paramagnetic scat-
D. Boucher et al. / Segregation and speromagnetic order in
46
200
2.6 eq O0
O -100
1.3ix:)
:i/(!
g
)o
m
t! 0~
e~ I ~ I
-1.3
-2.6
111
• LT= 1.5K LT=15K I
16
21
26
q(nrn -1)
Fig. 3. Difference patterns: 1.5-176 K, datapoints; 15-176 K, dotted line for the smoothed datapoints. We clearly see the two first magnetic rings at q = 7.6 nm I and q = 19.5 nm-1 as indicated by the arrows. The patterns taken at 176 and 300 K are similar.
tering (788/x~/Tb) which has been lost in favour of the magnetic m o m e n t ordering. We note that no magnetic scattering related to inhomogeneities is detectable. Therefore no magnetic contrast between a-TbSi z and a-Si phases or between inhomogeneities and amorphous material exists, although a magnetic order is present.
3.2. Neutron diffraction The location of nuclear and magnetic rings at 1.5 K has been given [9]. Figure 3 shows a part of the difference patterns taken on G4.1 spectrometer. It confirms the location of the two first magnetic rings and allows us to obtain the cross-section of the first one (or = l l 5 # ~ / T b ) . Unfortunately, the second one (or= 18/z~/Tb) is measured with a 30% error. The two intensities have been corrected for the magnetic form factor. It is interesting to compare these results with the patterns described in ref. [13] for crystalline TbSil.67 especially with that corresponding to the orthorhombic structure which was not known at the time of our first studies• In table 2, we compare the position of the rings and the Bragg
a-Tb0.18Si0.az
peaks of orthorhombic compound patterns. At 300 K the three first rings of the amorphous material correspond to three big peaks of TbSil.67 obtained at 60 K. In the same way, the positions of the three magnetic rings detectable at 1.6 K correspond to those of the strongest magnetic peaks, showing imperfect magnetic order, observed in ref. [13] at 1.2 K. So the phase separation detected by Simonnin as a-Si,a-TbSi 2 may be a-Si,a-TbSi].67 (or Tb3Sis). The width at half height of the first magnetic ring observed at 1.6 K is 4.5 nm-1, a value which is a good order of magnitude for an amorphous state [14]. If we were considering the sample as microcrystalline, the Scherrer formula ( L = 0.9A//3 cos 0 with standard notations [15])would lead to L = 1.4 nm. This distance can be assumed to be the correlation length between magnetic moments. L is ~ four times the unit cell parameters of TbSil.67 and thus the correlation would affect ~ 60 magnetic atoms.
3.3. Temperature dependence The SANS measurements show a variation of the paramagnetic cross-section from 710~z2/Tb at 4.2 K, to 680/x2/Tb at 15 K and zero at 300 K. N D measurements show magnetic rings at 1.5 and 15 K and they are not detected at 176 K. The magnetic ordering t e m p e r a t u r e is between these two last temperatures. These results are to be compared with the transition temperatures given in ref. [13] for a crystallized sample (T N = 16 or 39 K depending on chemical composition). The variations of intensities between 1.5 and 15 K are weak (about 15%) (fig. 3) and we suggest a magnetic ordering t e m p e r a t u r e of order of 50 K.
4. Discussion The location of rings in amorphous Tb0.185i0.82 with the experimental patterns given in ref. [13] for crystallized TbSi1.67 are in agreement, particularly, with the orthorhombic phase• Such a comparison between amorphous and Bragg peak pattern is only possible at a qualitative level; as a matter of fact, we are comparing the ring and
47
D. Boucher et al. / Segregation and speromagnetic order in a-Tbo.lsSio.82
peak location. However we obtain some indication of the composition in the segregated phase of the amorphous sample. This phase separation (a-Si,a-TbSil.67) would be a little different from the one (a-Si,a-TbSi 2) [6,7] proposed by Simonnin from D T A and SANS measurements. We checked that the previous SANS data are compatible with the new proposed composition; the volume fraction of phases changes only slightly. Moreover at low temperature the appearance of magnetic diffraction rings between the nuclear rings and the disappearance of paramagnetic scattering is similar to the antiferromagnetic behaviour of crystallized YbSil.67 [13] and we suggest a speromagnetic ordering. The small variation of the SANS over the entire small q range observed between 300 and 4.2 K is due to disappearance of the paramagnetic scattering. The agreement in sign and quantity is correct. The absence of another scattering shows that the magnetization of a-YbSil.67, which is zero at 300 K, remains zero at 4.2 K below the magnetic ordering temperature. If this zero magnetization had not been the case, a magnetic contrast between the magnetization of YbSil.67 and the zero magnetization of a-Si would have been observed. Thus the magnetic order of aYbSil.67 is speromagnetic and over small distances the resulting magnetization is approximately zero; that is confirmed by the absence of scattering for the highest small angle values corresponding to correlations over small distances (1 nm). The absence of SANS for high small angles, in addition, provides information on the magnetic nature of the inhomogeneities. Embedded in non-magnetized matrices (a-Si or a-TbSi1.67), these inhomogeneities have to be either intrinsically non-magnetic or with no significant net magnetization. The magnetic order of these inhomogeneities, if it exists, is speromagnetic. These results are in agreement with the negative value of the asymptotic Curie temperature. It is possible to measure with a reasonable accuracy the two first magnetic ring intensities, the incoherent scattering being small compared to these intensities. But the statistical error on difference patterns taken on G4.1 and 7C2 spectrometers and the inaccuracy of merging the
spectra performed with two different spectrometers do not allow us to satisfactorily Fourier transform and we are obliged to discuss the data in reciprocal space, using a procedure established for microcrystalline structures. With the standard notations, for an amorphous alloy, the measured intensity, l(q), of a magnetic ring is written [5]:
I( q) = 2 ( e2y/2mc2)2( 2/sin 0
sin
20) f 2( q )
× ( Y'~MiMj expi( qrij) ), ij where M i is the magnetic moment of atom i and riy is the distance vector of atoms i and j. From the two measured ring intensities, we find at 1.6 K the value (EijMiM j expi(qrij)) reported per atom as equal to 16.3/~2/Tb and 15.4/z2/Tb, e.g., a moment of at least 4/xB/Tb. Such a value is relatively important for an amorphous alloy, but is still smaller than the value observed for crystallized compound (6.5/.tB/Tb) [13].
5. Conclusion
This study identifies the phase separation, detected by Simonnin in amorphous Tbo.lsSio.sz, as a-Si-TbSil.67. For the first time in an amorphous RExM~_ x system, we observe a magnetic order of speromagnetic type. The magnetic SANS which usually superimposed on nuclear SANS does not exist here because of the absence of magnetization due to a compensation of moments over small distances (1 nm). The phase a-YbSil.67 is magnetically well ordered. Magnetic diffraction rings appear between nuclear rings with relatively high intensities in spite of nuclear disorder which decreases the interference effects, and tends to disperse the moments. The authors gratefully acknowledge A. Brulet and E. Le Coz (LLB, CE Saclay, France) for their help during the measurements on PAXY. They are grateful to P. Chieux (Institut L a u e - L a n gevin, Grenoble, France) for very helpful discussions.
48
D. Boucher et al. / Segregation and speromagnetic order in a-Tb0.1sSi0.82
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[8] P. Simonnin, R. Tourbot, B. Boucher and R. Bellissent, J. Phys. F17 (1987) 559. [9] B. Boucher, R. Tourbot and R. Bellissent, J. Phys.: Condens. Matter 3 (1991) 2843. [10] P. Simonnin, R. Tourbot, M. Tournarie and B. Boucher, J. Phys. F15 (1985) L189. [11] J.P. Cotton and A. Brulet, private communication (LLB, Saclay). [12] B. Boucher and R. Tourbot, J. Phys.: Condens. Matter 4 (1992) 9697. [13] P. Schobinger-Papamantellos, D.B. Mooij and K.H.J. Buschow, J. Magn. Magn. Mater. 79 (1989) 231. [14] B. Boucher, Q. Chen, R. Tourbot and R. Bellissent, J. Non. Cryst. Solids 159 (1993) 101. [15] A. Guinier, Th6orie et Technique de la Radiocristallographie (Dunod, Paris, 1956) p. 462.