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ELECTRONIC, MAGNETIC AND STRUCTURAL PROPERTIES OF AMORPHOUS Eu80Au20 ALLOYS
FROM HYPERFINE INTERACTIONS AT EUROPIUM
J. Friedt, M. Maurer, J. Sanchez, A. Berrada, A. Qachaou, P. Panissod, J.
Durand
To cite this version:
J. Friedt, M. Maurer, J. Sanchez, A. Berrada, A. Qachaou, et al.. ELECTRONIC, MAGNETIC AND STRUCTURAL PROPERTIES OF AMORPHOUS Eu80Au20 ALLOYS FROM HYPERFINE INTERACTIONS AT EUROPIUM. Journal de Physique Colloques, 1980, 41 (C8), pp.C8-638-C8-641.
�10.1051/jphyscol:19808160�. �jpa-00220261�
JOURNAL DE PHYSIQUE CoZZoque C8, suppl6ment au n08, Tome 41, a o c t 1980, page C8-638
ELECTRONIC, MAGNETIC AND STRUCTURAL PROPERTIES OF AMORPHOUS
E u g 0 A u 2 ~ALLOYS FROM HYPERFINE INTERACTIONS AT EUROPIUN
J.M. Friedt, M. Maurer, J.P. Sanchez, A. Berrada+, A. ~achaou+, P. ~anissod+and~. ~urand' Centre de Recherches NucZ6aires, 67037 Strasbourg Cedex, France
+L M S E S, 4 , rue BZaise PascaZ, U L P 67000 Strasbourg, France.
Abstract.
-
Amorphous EusoAuzo has been investigated by means of MEssbauer spectroscopy and N M R at "'Eu.A large electric field gradient (E F G) is detected. The E F G and isomer shift at ions reveal unusu- ally large temperature dependences between 4.2 and 245 K. This feature is tentatively attributed to tunne- ling states, typical of amorphous materials. All the results are consistent with a strong structural short range order. The alloy orders asperomagnetically below Tc=85 K. The spontaneous magnetization, the hyper- fine field distribution and the reduced magnetization temperature dependence are consistently discussed in terms of distributions of moments directions and of exchange interactions.
INTRODUCTION
Electronic and magnetic properties of amorphous neous magnetization M(T)/M is significantly flat- alloys are closely connected with the structural tened as compared to the S = 712 Brillouin beha- and chemical short range order [I, 21. Therefore, vior (fig. 1 ) .
combined macroscopic (magnetization measurements
131) and microscopic TMZssbauer spectroscopy and
NMR) investigations are powerful tools for un- derstanding the magnetic groperties. The series of a-RE 80A~20 alloys is particularly attractive because magnetic properties and crystal field effects vary with R E element, whereas one expects
the structural short range order to be roughly
1 I
constant on the basis of the strong similarities
between R E - A u phase diagrams [41. In these
I I
a-RE80A~20 alloys, only the R E atoms carry a ma-
gnetic moment, while the Au atoms are non-magne- T/TC
tic. The present paper is devoted to the study of Fig. 1 : Temperature dependence of magnetization a-Eu80A~20 which is an exemplary system in view
of the following features : a) Eu is divalent;
thus, anisotropy and crystal field are negligible b) Eu nuclei are suitable for both MEssbauer and
of a-Eus Auzo (circles) normalized to the spontaneous saturation magnetization, compared to Brillouin function (dashed line). The solid line corresponds to a calculation with ai and J distributions.
i j
N M R spectroscopies c) this amorphous material 15'3u MEssbauer spectroscopy confirms the or- is magnetically ordered. dering temperature : indeed, the paramagnetic ab- sorption spectra broaden suddenly below 9 2 f 5 K.
EXPERIMENTAL RESULTS
Above this temperature, spectra are satisfacto;ily Bulk magnetization measurements were previous- fitted assuming single values for both isomer
ly reported F.31. A Curie temperature of T = 2
shift 61S and quadrupole interaction e q Q (Table 1,
--
8 5 k 5 K was determined, consistently from AC-~us- fig. 2). In the fits, the linewidth was constrai- ceptibility and from Arrott plots. The saturation ned to 2.7 mm/s. Fits are insensitive to an E F G moment per Eu 2+ atom (6.7 -+ 0.15 pB) is slightly asymmetry parameter and attempts of including dis- reduced with respect to the theoretical value. tributions of either 61S or (and) e qQ were unsa- Z The temperature dependence of the reduced sponta- tisfactory.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19808160
The value of e L q ~ is unusually large for the Eu 2+ configuration in an intermetallic [53 and the positive sign is unambiguous. Both and e qQ 2 display very large reversible temperature depen- dences (Table 1).
VELOCITY ( m m l s )
Fig. 2 : "?EU Mzssbauer spectra of a-EwoAuzo at 185(a) and 4.2 K (b). Fits are represented by solid lines.
T SIS mm/s e2qQ MHZ
K vs Sm203 at 300 K ( I mm/s = 17.4 MHz) 245 -10.0 (2) +315 (6)
185
-
9.6 (1) +278 (4)100
-
9.3 (1) +261 (4)4.2
-
8.3 (2) +270 (cons trained) Table 1 :'"
EU ~Essbauer hyperfine parameters.Errors on last figure in parentheses.
In the ordered state (T
<
T ) , the ~Essbauer absorption function is satisfactorily simulated assuming : a) a single 61S value, b) a value of +I6 m / s for e qQ (consistent with paramagnetic 2 blijssbauer and low temperature N M R results), c) a broad asymmetric distribution of modulus of the hyperfine field Hhf d ) an independent sphe- rical distribution of the polar angle between E F G and I$f axes. The best distribution for Hhf( P ( Y l f ) ) corresponds to a convolution of a narrow
gaussian with a truncated parabola (fig. 3). The deduced first and second moments of the distribu- tion are respectively 157 kOe and 1800 kOe 2
.
Notice that the temperature dependence of the ave- rage Hhf follows the reduced magnetization.
Fig. 3 : Hyperfine field distribution (arbitrary unit) at '"EU in a-Eu8~Auto at 4.2 K.
Spin echo N M R measurements were carried out at 1.4 K on same samples. The frequency corrected spectrum arises mainly from "'EU nuclei (fig. 4).
Signals corresponding to the lS3~u and I g 7 ~ u iso- topes are possibly detected on the low frequency side.
ylf
and e qQ values of 160f5 kOe and 2 330f70 lfHz are deduced from numerical analysis of the high frequency spectral side (Is1~u). Owing to the large quadrupole interaction, the perturbation treatment used here is of limited validity; this explains the large unaccuracy in the determination of e qQ. In any case, the agreement between the 2 N M R and kf6ssbauer hyperfine parameters is very satisfactory and gives confidence in the indepen- dently concluded results.DISCUSSION
The unique values (or narrow distributions) measured for both 8 and e2*Q indicate that
IS
fluctuations of local (chemical) environment of Eu atoms are small. Indeed, the hyperfine para- meters of Eu in crystalline compounds are known to depend significantly on the number and the na- ture of neighbors [ S - 6 1 .
The large E F G arises from charge disturbances caused by the asymmetry of the nearest neighbors atomic surrounding since E U ~ + is an orbital S state ion. Refering to the structural studies on a-La80A~20 [7], each Eu atom has on average 8Eu and 3Au atoms as nearest neighbors. The E F G axes are likely to be randomly distributed with respect to any macroscopic axis.
JOURNAL DE PHYSIQUE
Fig.
0 al
3
0
e
0w
0 I
w X
90
Frequency (MHz)
4 : Spin echo N M R spectrum of a-EusoAuzo at 1.4 K. The bar diagram represents the computed resonance lines or cut-off frequencies.
The temperature dependence of (-1.7 m / s ) between 4.2 and 245 K is unusually large (Table I ) . It exceeds by an order of magnitude the-second order Doppler shift (-6.6 x 10-~nun/s from 4.2 to 245 K). The hypothesis of an Eu fluctuating va- lence is ruled out by the sign of the temperature dependence of This indicates a decreasing electron density at increasing temperature, whe- reas thermal excitation of an E U ~ + state would induce an effect of opposite sign. Also, photoe- mission measurements [8] at 300 K confirm a pure 3u2' valence and the effective paramagnetic moment from the Curie-Weiss law is 8.220.2 y /at, which B is close to the lZu2+ ionic value (7.94 yB/at). The anomalous behaviour of fjIS cannot either be accoun- ted for by thermal expansion. Indeed, by reference to crystalline Eu systems, the volume expansion over this range of temperature should be roughly 15 % in order to account for the observed change
of
l91.
We suggest tentatively that the anomalous ther- ma1 dependences of and e qQ might be a speci- 2 fic consequence of the amorphous structure. some properties of covalent glasses and of metallic amorphous systems (e.g. sound velocity, ultrasonic
attenuation) have been explained in the frame of a two-levels-systems (TLS) model [ l o - 131. In the T L S, the elastic potential curve versus generali- zed coordinates of atoms presents several minima.
Thus, inequivalent atomic positions can be occu- pied through a dynamic thermal process. Assuming that these inequivalent positions have slightly different structural environments, and therefore different bIS and e qQ values, it is expected that 2 the time-averaged and e qQ measured by 2 MEssbauer spectroscopy may shift with changing temperature.
The saturation moment at 4.2 K , as obtained from an 1/H extrapolation, is 6.7 +0.15 y /Eu at. This
7 B
rather low valuefor a4f configuration can be in- terpreted as a result of misalignement of moments
(asperomagnetism). The computation of the cone angle requires the knowledge of the saturation mo- ment in case of perfect spin alignement. The moment of cd3' is known to be 7.55 pB/at, which is inier-
9
preted as the sum of the 4f' contribution (7 pB) and of the 5d16s2 conduction electrons contribu- tions (0.55 p ) .
A
comparison of the 4.2 K highB
field isotherm curves of a-Gd80A~20 and a-Eu Au 80 20 shows that the moment of Gd exceeds by
0.15f0.05 y B the one of Eu, under the reasonable assumption of identical magnetic structures. This difference originates from different conduction electron polarizations. Thus, Eu atoms should carry a moment of 7.3
uB.
Consequently, the lowe- ring of the macroscopic Eu moment by asperomagne- tism is 0.6-+0.1 1-1 /at.B
The whole of the present magnetization and hy- perfine field distributions can well be accounted for assuming such an asperomagnetic order [2]. At low temperature, spin directions are not collinear, owing to structural disorder which induces canting of the local easy axes of magnetization. However, this anisotropy is weak because EU" is an S
-
stateion. Let
a.
be the angle between the spin Si and+
the reference axis defined by the macroscopic ma- gnetization direction, and let J be the exchange
-+
+ ijinteraction between S and S ; the Heisenberg
i j
Hamiltonian expresses as :
The reduction of saturation moment per E U ~ + atom is understood assuming a gaussian distribution of
a.
with a F W H M equal to 60". The 6s net density being constant, it is reasonable to assume that Jij has a narrow (gaussian) distribution around the mean value.The hyperfine field at Eu nuclei arises from three main contributions :
H is the core polarization, which reflects the CP
susceptibility of the inner shells via intra-ato- mic exchange with the localized moment. H is the
OP ion's own polarization of the 6s electrons. Accor- ding to Nowik et a1 [14], the 6s polarization is proportional to the 6s net density and thus, a monotonic correlation between H and 61s is ob-
OP
served. In this picture, if the
Hhf
distribution would only arise from H,
the 61S distribution should be several m / s w ~ d e . OP This is not experi- mentally observed. Therefore, we conclude that, in~ - E U ~ ~ A U ~ ~ , both H and H are roughly constant.
OP CP
H is the contribution of the neighbors to H
n hf at
the central site. The
Hhf
distribution must arise from this term. Under the assumption that the Hhf distribution reflects the distribution of exchange field, the ~Zssbauer P(Hhf) (which is also consis- tent with N M R data) is interpreted as resulting from bothai
and J distributions. The gaussiani j
distribution of
ai
leads to an exchange field dis- tribution of truncated parabolic shape. The gaus- sian J distribution ( F W H M = 25 %) is narrow,i j
in relation with the well defined 61S. Assuming that ai and Jij are independent variables, P (Hhf) will be given by a convolution of these two dis- tributions. Since long range magnetic interactions are dominant (Tc is well defined), we have com- puted the reduced saturation magnetization versus TIT in a mean field approximation taking both
a.
and J.. distributions into account. The agreement
1 J
with experimental data is indeed satisfactory (fig. I). A ~Gssbauer investigation of the thermal variation of the width of P(Hhf) is impracticable because of fitting ambiguities above 40 K. Such data would have allowed to confirm that the main part of the magnetic coupling has long range
character [15].
of exchange interaction between the localized mo- ments, which are asperomagnetically ordered at low temperature. These magnetic properties, the unique value of the isomer shift and of the quadrupole interaction for all the Eu 2+ atoms con£ irm that there is a strong short range order. The isomer shift and the quadrupole interaction display ano- malously large reversible temperature dependences, which are tentatively described in terms of a
two- levels model, characteristic of amorphous states. Nsssbauer spectroscopy experiments at both rare earth and gold atoms in other a-RE
8oAU20 are in progress. These combined studies should provide detailed information with respect to magnetic order, short-range order and crystal field effects in such amorphous intermetallics.
References
1. R.W. COCHRANE, R. HARRIS and M.J. ZUCKERMANN, Physics Reports 48, 1 (1978)
2. J.M.D. COEY, J. Appl. Phys. 49, 1646 (1978) 3. A. BERRADA, J. DURAND, N. HASSANAIN and
B. LOEGEL, J. Appl. Phys. 50, 7621 (1979) 4. W.G. IIOFFATT, The Handbook of Binary Phase
Diagrams, General Electric Cop. Vol.1 (1978) 5. K.H.J. BUSCHOW, Rep. Prog. Phys. 42, 1373
(1979)
6. F.J. Van STEENWIJK and K.H.J. BUSCHOW, Physica 85B, 327 (1977)
7. J. LOGAN, Scripta. Met. 9, 379 (1975) 8. A. BERRADA, G. KRILL, N. HASSANAIN and J.
DURAND (to be published)
9. G.M. KALVIUS, U.F. KLEIN and G. WORTMANN, J. Physique Colloques 35, C6, 139 (1974) 10. P.W. ANDERSON, B.I. HALPERIN and C.M. VARMA,
Philos. Mag. 25, 1 (1972)
1 1 . W.A. PHILLIPS, J. Low. Temp. Phys. 7, 351 (1 972)
12. M. BANVILLE and R. HARRIS, Phys. Rev. Letters 44, 1136 (1980) and ref. therein
13. J.L. BLAKE, Bull. Am. Phys. Soc. 25, 195 (1980) E. SIEGEL, Bull. Am. Phys. Soc. 25, 243 (1980) 14. I. NOWIK, B.D. DUNLAP and J.H. WERNICK, Phys.
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15. J. BALOGH, I. VINCZE, Sol. Stat. Corn. 25, 695
( 1978)
CONCLUSION
The magnetic properties of a-Eu 80A~20 are con- sistently depicted assuming a narrow distribution
