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CRYSTAL FIELD OF Dy IMPLANTED IN A1 OBTAINED BY MÖSSBAUER SPECTROSCOPY
P. Kikkert, L. Niesen
To cite this version:
P. Kikkert, L. Niesen. CRYSTAL FIELD OF Dy IMPLANTED IN A1 OBTAINED BY MÖSS- BAUER SPECTROSCOPY. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-203-C1-206.
�10.1051/jphyscol:1980163�. �jpa-00219732�
JOURNAL DE PHYSIQUE Colloque C1, suppl&ment au n o 1 , Tome 41, janvier 1980, page C1-203
CRYSTAL FIEUI OF Dy IMPLANTED I N Al OBTAINED BY M~SSBAUER SPECTROSCOPY
P . J . Kikkert and L. Niesen
Luboratorium voor AZgemene Natuurkunde, University of Groningen, The Netherlands.
The l m tapraturemagnetic properties of rare earth ions in cubic metallic environments are de- tennined to a great extent by the crystalline electric field (CEF) acting on the rare earth impurities. Various methods have been used to determine the parameters of the cubic CEF in these systems, e.g. ESR, magnetic susceptibility, speci- fic heat loeasu~enents /1,2,3/. In most of these cases single crystals of dilute alloys have to be prepared but often low solubility and clus- tering of the rare earth impurities prevent the fo~mation of well defined dilute allays.
In this paper we shm that l m temperature ion implantation is a very useful alternative production method. We have investigated the hyper- fine interaction of 161Dy SUbstit;Utionally h- planted inAl by means of ~ssbauer spectro- scopy in an external field. A preliminary analysis of the measurements yields already unique infor- mation about the CEF acting on the rare earth electron spin.
In the presence of a magnetic field the in- teraction between the D y 4f electron cloud and its cubic neighbowhood can be described by the HBniltonian /4/:
%?= &,-=r4raj
(08 +
50t)+
%<r6ryj (0g-
2102)+ gjVB
3.3
(1)where the quantization axis is chosen a l q the
<loo> direction of the crystal. <r4> and <r6>
are radial integrals, the 0; are the so called -@tor equivalents and 6 . and y
.
are mlti-I I
p%iq+ive factors. The coefficients iI4<r4> and
%a6> can be calculated if the lattice is sup- psed to consist of point charges but since there are other (more important) contributions that can- not be calculated, it is customary to regard
&<r4> and &<r6> as parmneters to be detemhed by expriment. In the case of Dy ( 6 ~ 1 5/2) the qualitative effect of the cubic field Hamiltonian is a splitting of the lcrwest J multiplet into ttm doublets (r6 and
r,)
three quartets (re(=), r8(3) and In the presence of an external,
magnetic field these levels are mixed by the Zeanan interaction and any degeneracy is remwed.
Using first order perturbation theory the magnetic and electric part of the 4f contribution to the hyperfine interaction in a certain electronic state can be written as /5/:
eJZ>
-
with
Bhf
= Bhf(max) =%
Bhf(max)In these expressions
Bhf
(max) and q4£ (max) are the hyperfine field and eledric field gradient, res- pectively, w h m the 4f contribution is m x i m l(<JZ> = J)
.
For the case of Dy their values are well k n m /5,6/. Fran our low tgnperature%ssbauer experiments we can cbtain the reduction factors
%
andRp
in the electronic ground state as a function of the e x t d magnetic field. By diagonalizing eg. (1) we can find values for <J,>and 4~>(or, u s h eq. (2b) and (3b) for
flM
ani!15
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980163
JOURNAL DE PHYSIQUE
161T!b (TI = 7d) is the decay prcduct of 161~d (T = 4 m i n )
.
l ~ b was separated £ran the irra-h
diated oxide with the Groningen Isotope Separator and inplanted with an energy of 116 keV i n an A l single crystal of 5N purity, cut perprdicular to the
<loo>
axis. During the implantation the A l crystal was kept a t liquid helium -awe. The implanted dose was between 1 and 4 x 1 0 1 4 a t an -2.
After transprtation of this source a t liquid nitrogen t a p r a t u r e
,
-%ssbauer effect msasurements were performed a t 4.2 K i n another ccyostat. Mag- netic fields up t o 3.2 T were produced by a super--20 -10 0 1 0 2 0 VELOCITY ( C M I S )
Fig. 1: IGssbauer spectra £ran DyRJ in different ly'
magnetic fields
.
r6W
5 0
G'
'-t
R ) in terms of the parameters &+<r4> and %<r6>. r7
Q
These parameters can now be adjusted so as to ob- tain a least squares f i t t o the measured
%
and%-
Experimental.- The source for the -iment was obtained by irradiating 12 mg of Gd203 enriched to 96% in 60Gd, for 18 days in a t h d neutron flux of 2 x 10' ncm-2s-1. The desired activity
L I I I
1.0 2.0 3.0
MAGNETIC FIELD I T )
Fig. 2: Measured reduction factors
%
and calm-lated curve, using & + a 4 > = -29 K and %<r6> = 14.0 K, without relaxation effects (a) and includ- inrj relaxation effects (b)
.
-60 -LO -20 0 2 0
IKI
Fig. 3: Allmed values for the crystal field para- m t e r s
&'X+<X~>
and ~ ~ < r ~ > for (one standard deviation criterion). The insert shows the derived energy level sch- using the best CEF parameters(dot)
.
conducting s p l i t coil. The magnetic.field was oriented along the
<loo>
direction of the crystal.The a b s o r b (11.2 mg
an2
'61Dy~3) was mwedsinusoidally a t roan t a p r a t m e with a frequen- cy of 37 Hz. Velocity calibration was performed by the
mire
technigue /7/. G a m a rays m e de- tected parallel t o the magnetic field using a Kr-Co2 f i l l e d proprtional counter. Neaaranents were performed i n magnetic fields of 0.69, 1.29,1.76, 2.25, 2.73 and 3.21 T.
Results arid discussion.- The Plijssbauer spectra, two of which are given in fig. 1, &OW the pre- sence of t m ccqmnents, one of which (70%) is associated with Dy ions in a substitutional site
(cubic synmetry), while the other one (30%) is associated with Dy ions in a strong a x i d CEF
(<JZ> = J)
.
This was also verified at Bext = 0 where the substitutional ccmpnent shows up as a relaxation broadened single line. The spectrum of the first ccqonent can be fitted by a sum of 10 brentzians (no nIZ = 0 transitions) while the other one can be fitted by a surn of 16 Lorentzians, the positions of which can be found £ran q. (2) and (3) by taking JZ =T .
15 A good fit for the firstcapnent can only be obtained by introducing a large Lorentzian spread in the interaction (this spread is larger when the magnetic field is lower).
As will be pointed out, this broadening is ar- tificially caused by fitting the spectra using eq. (2) and (3). F r m these fits the reduction factorspiven by q. (2b) and (3b), can be calcu- lated. These reduction factors and the reduction factors calculated f r m the best CEF parmters, lA4<r4> = -29 K and %<r6> = 14.0 K, are sham
infig. 2. As can be seen f r m this figure the high field values coincide best. Fig. 3 s h m the
and %<r6>
region of values of A4<r > that give acceptable fits to the data (one standard devia- tion criterion). The energy level scheme derived is also &am in fig. 3. A lattice sum point charge calculation would predict 50 K arid 6 K respectively (Al 3 )
.
About the discrepancy has been written by Williams and Hirst /3/ and D i x o n /9/. It follm that at 4.2 K the first excited level is popilated to sane extent. Consequently relaxation phencanena will be present in the spec- tra,especially in lay fields. These can be treated in a two level apprcximation using the rate equa-tion methcd / 6 / . Using a relaxation rate obtained f r m /1/ and the values of <Jz> and
<J?
in thelmest two levels we calculated the relaxation spectrum of the substitutional canponent. One of the results is sham in fig. 1. Parameters in the fit of the spctmm are only the intensities of the substitutional canponent and the intensities arid line positions of the non-substitutional canpnent.
The fairly good agreaent, even at low fields, suggests that the 'broadening' effects are indeed due to relaxation. The parameters needed to cal- culate the relaxation spectrum had to be found frcm a preliminary fit of the m m e d spectrum.
This fit was obtained by introducing a spread in the magnetic (or quadruple) interaction but in this way a systematic deviation occurs in the value of obtained. This deviation is indicated in fig.
2. It twns out that the agreement between the ' ~ i m e n t a l '
%
values and the calculated curve at the laxer fields is inproved considerably by the relaxation calculation. An analysis by fitting the spctra directly to the relaxation &el is in progress. Also additional experiments at l m taperatures are being performed. The relaxation effects do not affect the best values of the CEE' parmters significantly.
The values of &+<r% and%<r6> obtained give rise to a
r7
ground state as observed by ESR /1,9/. Frcm the enerqy level dia- gram we calculate an effective g value,g = AE/u B = 7.555 (5) where AE is the distance B
between the lcwest two electron states, in the field B used in the ESR maswments. This is equal to the vdue for a pure r7 doublet. The difference between this value and the measured values, g = 7.58(5) /1/ and g = 7.59 (3) /8/
indicates a probable ferramagnetic coupling between the 4f electron spin and the conduction electrons with a couplincj amstant Jsf = +0.06 (6) eV.
References
JOURNAL DE PHYSIQUE
/1/ Rettori, C., Davidav, D., Orbach, R.
,
Ricks, B., Chock, E.P., Phys. Rev.87
(1973) 1./2/ oseroff, S., Passeggi, M., Wohlleben, D-, Schultz, S., Phys. Rar.
B15
(1977) 1283./3/ Williams, G., H i r s t , L.&.
,
Phys. Rev.185
(1969) 407.
/4/ Baker, J.M., Bleaney, B., Hayes, W., Proc.
Roy. Sm.
A247
(1958) 141./5/ Abragam, A., Bleaney, B., Electron Paramagnetic Resonance of Transition Ions
(C~arerdon Press, Oxford, 1970).
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(1978) 233./7/ De Waard, H., Rev. Sci. I n s t r .
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