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PARAMAGNETIC HYPERFINE STRUCTURE OF
DILUTE Dy-IMPURITIES IN THORIUM
Winfried Wagner
To cite this version:
Winfried Wagner. PARAMAGNETIC HYPERFINE STRUCTURE OF DILUTE
JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 12, Tome 37, Décembre 1976, page C6-133
PARAMAGNETIC HYPERFINE STRUCTURE
OF DILUTE Dy-IMPURITIES IN THORIUM (*)
Winfried WAGNER
Physik Department, Technische Universitat Miinchen, D-8046 Garching, FRG
Résumé. — On étudie par effet Môssbauer des propriétés dynamiques de l'interaction hyperfine du Dy dilué en impuretés paramagnétiques dans du thorium métallique cubique. L'échange des électrons de conduction avec le moment localisé est étudié par sa variation en fonction de la tempé-rature. On trouve que la polarisation locale des électrons de conduction, responsable d'une augmen-tation du champ hyperfin au site du noyau de Dy, est proportionnelle à la densité d'états au niveau de Fermi.
Abstract. — The study of the dynamical hyperfine interaction of paramagnetic Dy impurities in
the cubic actinide metal Thorium by means of the Mossbauer effect is reported. The exchange of the conduction electrons with the local moment is analysed through the temperature dependence of the electronic relaxation time. The local polarisation of the conduction electrons which is respon-sible for the enhancement of the hyperfine field at the Dy nucleus is found to be proportional to the density of states at the Fermi level.
1. Introduction.—The Mossbauer resonance of
Dy1 6 0 with its (2+-0+) E 2 nuclear transition is highly
favourable for the study of dynamical hyperfine (hf) interactions in the Dy3 +(6H1 5 / 2) ion in highly dilute
alloys, because of its simple hyperfine pattern. In cubic host metals like the actinide Thorium the spin orbit multiplet (2S+1LJ) of the rare earth (RE) ion
splits into magnetically isotropic Kramers doublets ( r6, r7) and into magnetically anisotropic quartets
(( i )r8) due to the interaction with the crystalline electric
field (CEF). In the heavy RE ions (Tb ->• Yb) the CEF splitting is not able to mix the different / multiplets. The alloys do not order magnetically in the tempera-ture range where this investigation was carried out (*S 4.2 K).
The coupling of the localized magnetic moment to the conduction electron bath produces spin flips whose rate is comparable to both the hf interaction frequency and the nuclear life time. This leads to a so-called relaxation line broadening of the hf spectrum. Its temperature dependence yields information on the strength of the exchange coupling Jsf between the
magnetic moment and the conduction electrons [1]. In addition, the presence of conduction electrons enhances the hf coupling constant A of the Dy ion by about 5 percent when compared to the value in a non-conducting, magnetically dilute compound.
2. Sample preparation. — A Th : Tb1 6 0 (500 ppm)
alloy was prepared by argon arc melting of a small piece of neutron activated Tb160(jT1/2 = 72 d) metal
(*) Work supported in part by the Gesellschaft fur Kern-forschung mbH, Karlsruhe.
with an ingot of Thorium metal. This sample was then used as the source without any further treatment. The hyperfine structure of the 86.79 keV gamma transi-tion in Dy1 6 0 was analysed with an absorber of
Dy0 4Sc0.6H2 which possesses a single absorption
line of FWHM = 5.2 mm/s at 4.2 K [2].
Mossbauer effect measurements in dilute alloys are extremely sensitive to clustering of the magnetic impurity in the bulk of the sample [3], Since the heavy RE metals have good solubility in Thorium, this problem does not arise in the present case.
3. Magnetic Hf coupling. — In figure 1 the ME spectra at 1.4 and 4.2 K show a twoline pattern reflecting the hyperfine splitting in the source. From the line positions at — 3.Ag/2g> and Ag/gj [1] it can be concluded that the electronic ground state of the CEF multiplet is a pure T7 Kramers doublet. The
solid line through the data points is a least square fit using a relaxation theory for hyperfine interaction in cubic symmetry as described earlier [4, 5]. In our previous investigation of Th : Er1 6 6 (550 ppm) [6] we
observed a very small intensity at zero velocity, caused by some magnetic contamination of other ele-ments. In the Th : Dy alloy no noticeable additional intensity besides the T7 pattern is observed in the
spectra down to 1.4 K. From this fact one may conclude that the CEF multiplet of Tb3 + (7F6) has a
non-magnetic groundstate and excited non-magnetic CEF states are not populated in the temperature region under consideration. Assuming a well isolated f7 for
Dy3 +, the groundstate for Tb3 + must be a F2 singlet [7].
EPR studies [8] of Dy in Th gave a value of g = 7.61 for the isotropic g-factor, but because of excessive
C6-134 W. WAGNER
.60 - 4 0 -20 0 20 40 60 VELOCITY ( m m l s )
FIG. 1. - Mossbauer resonance spectra of Dy160 in Th metal
(500 ppm) at 1.4 and 4.2 K. The solid curves through the data points are the least-square fits using the relaxation theory
discussed in the text.
linewidth the hyperfine coupling between the
T,
electronic state and the nuclear groundstates of the two isotopes Dyl6l(I = 512) and D y 1 6 3 ( ~ = 512) could not be resolved.
Analyzing the present data within the frame work of the relaxation theory already mentioned, gives a hyperfine coupling constant of
A(Dy160) = 223
+
3 MHz (3.2 mm/s).
Using the eigenfunctions
I
Kri
>
of the CEF ground- state [7] the magnetic hf-splitting of a RE ion (Hund's rule groundstate J and nuclear spin I ) can be described by the eigenvalues of the spin Hamiltonian [9] :The hf-tensor A reduces for a magnetical isotropic electronic state
I
KTi>
in cubic symmetry to the scalar coupling constant A which can be separated into two terms :A = A.
+
ACE.Provided that J is a good quantum number A, repre- sents the hf coupling constant [9] in insulator host matrices. The extra contribution ACE arises from the polarization of conduction electrons in the neigh- bourhood of the impurity spin.
Following the arguments of Hirst and Tao et al. 110-121 we expect ACE to be roughly proportional to the total density of states near the Fermi level N(EF). Using the values of the hf-coupling constant A, listed by Bleaney [13] one obtains A,(D~'~') = 209 MHz [14]. This value should be correct within 2
%
for various insulator hosts. In figure 2 A(Dy160) is plotted1 2 3 4
TEMPERATURE [K]
FIG. 2. - Relaxation rate W from hyperfine spectra of the r7
groundstate as a function of temperature.
against N(E,). One sees that within the present accu- racy ~ ( D y l ~ ' ) increases linearily with N(EF). Such a behaviour has been shown previously [15] to exist also for Er166.
4. Exchange coupling.
-
The relaxation process [16] arises from the interaction of the conduction electron spin of Thorium with the local magnetic moment J at the Dy3 + ion. For a pure exchange type interactionThe spectral density J(T) of the conduction electron bath is given by the Korringa law [5]
From figure 3 it is seen that in the temperature range covered by this experiment the relaxation rate W(T) = $ g 2 J(T) follows the Korringa law. From the observed ratio A WIAT = 122
+
10 MHz/K the value J,, = 0.022+
0.002 eV for the exchange coupling constant was derived. It is in good agreementN(EF) ( e V atom spin)-'
FIG. 3. - Hyperfine coupling constants A for Dy160 in various metal hosts as determined by Mossbauer experiments against the conduction electron density of states at the Fermi level per spin direction N(Ew). The values for A are taken from Stohr and Cashion [2] (YH2 : Dy), Stohr et al. [17] (Zr : Dy) and Bleaney 1131 ( ~ n s u l a m . The values for N(EF) are from Stohr and Cashion [2] (YHz), Loucks [18] (Zr) and Gupta and
PARAMAGNETIC HYPERFINE STRUCTURE OF DILUTE Dy-IMPURITIES IN THORIUM C6-135
with EPR data in ref. [8] where the same value is obtained when J,, is related to XeX,, as given above. In the analysis of the spectra an experimental uncer- tainty arises as to the absolute value of W(T). Com- pared to a static line pattern (He,,, = 0) the resonance lines are somewhat broadened even in the slow relaxa- tion limit (Hex,,
<
H,,). Unfortunately the linewidth of the source in the static limit is not well known. This additional linebroadening effects of unknown origin put an uncertainty on the evaluation of the relaxation parameters in the slow regime. This experi-mental difficulty, however, does not affect the size of the coupling constant
Jsf
as long as the Korringa relaxation is strictly valid.The calculations of W(T) shown in the figure were performed using the experimental linewidth
Acknowledgement.
-
I would like to thankProf. G. M. Kalvius and Dr. L. Asch for valuable discussions.
References
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(1974) 583.
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[12] TAO, L. J., DAVIDOV, D., ORBACH, R. and CHOCK, E. P.,
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