HOLEBURNING AND ODNMR STUDY OF A Eu3+ SITE IN CaF2

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HOLEBURNING AND ODNMR STUDY OF A Eu3+

SITE IN CaF2

A. Silversmith, A. Radlinski, N. Manson

To cite this version:

A. Silversmith, A. Radlinski, N. Manson.

HOLEBURNING AND ODNMR STUDY OF A

HOLEBURNING AND ODNMR STUDY OF

A

EU~'

SITE IN

C a F 2

A . J . Silversmith, A.P. Radliiiski and N.B. Manson

Department of Sotid State Physics, Research School of Physical Sciences, Australian National University, Canberra ACT 2601, Australia

.4bstract

-

Holeburning and optically detected nuclear magnetic resonance techniques were used to characterize the hyperfine coupling in a Eu3+ center of C?, symmetry.The hyperfine splittinge and effective magnetic moments in

the F0 and 5D0 states, a s well a s the quadratic Zeeman shift of the 7 ~ 0 ~ 5 ~ 0 transitian have been satisfactorily analy~ed.

I. INTRODUCTION

We have used holeburning and optically detected nuclear magnetic resonance (ODNMR)

to

study hyperfine structure in the 7 ~ 0 and 5 ~ 0 states of trivalent europium. Theoretical calculations about the hyperfine structure in E U ~ + were published by Elliott /l/ in 1957 and he made two major predictions: 1) Electric q u a d r u p l e coupling should dominate the hyperfine structure. Pseudo-quadrupole (second order magnetic) effects are about two orders of magnitude smaller. 2) In the

7 ~ 0 ground state, the nuclear magnetic moment is largely quenched by fields set up by the 4f electrons. Little quenching is expected in the 5D0 optically excited state. Recently, optical holeburning and ODNMR experiments on Eu3+ systems have provided good agreement with Elliott's original calculations /2-5/.

The present work deals with a trigonal E U ~ + center in CaF2 called G1 /6/. The charge compensation is achieved by an 02' replacing a nearest neighbor

F-

in the < I l l > direction. The tightly bound nature of this center causes larger crystal field splittings than a r e usually observed for ieolated E U ~ + ions and anomalous transition intensities /7/.

The ODNMR experiments provided data which superficially seemed to disagree with both of Elliott's predictions, showing both large pseudo-quadruple effects in the ground state, and little apparent quenching of the magnetic moment. Our question was: Could the hyperfine coupling in the G1 center be described using Elliott's approach, or a r e other effects responsible for the anomalous behavior?

C7-532 JOURNAL DE PHYSIQUE

11. THEORETICAL BACKGROUND

A brief summary of Elliott's calculations /l/ concerning the hyperfine coupling in E U ~ + is presented here. All hyperfine coupling in the 7 ~ 0 and 5 ~ 0 (both J=0) states is second order in nature. Electric quadrupole coupling dominates, and the appropriate spin Hamiltonian is:

(All expressions in this paper are suitable for axial systems only.) I is the nuclear

spin angular momentum, and P is proportional to the electric q u a d r u p l e moment of the nucleus Q and to the electric field gradient (EFG) a t the nucleus.

In addition, there is a pseudo-quadruple term which comes from the square of the magnetic hyperfine interaction:

B,

h,

and

4

a r e the electronic and nuclear Bohr magnetons and the nuclear magnetic moment. is an operator which gives the magnetic field a t the origin (nucleus) set up by the electrons. The pseudo-quadrupolar interaction (E& in second order) can be added to the quadrupole Iiamiltonian to give a total effective zero-field Hamiltonian of the form:

88NLk<r -3 2 I < O I N ~ I ~ > I ~

where

P S =

2 1 3I

>]

[h,

g12-

kzl2]

lad

[nil2=

Ai ; i=x,y,z (3) The major contribution to P ' arises from the interaction with the nearest J=1 level. For negligible pseudo-quadruple contribution (P'=O), the hyperfine splittings would be directly proportional to Q. Then, the magnitude of the splittings in 1 5 3 ~ u and 1513u would form a ratio precisely equal to 1 5 3 ~ / 1 5 1 ~ . The deviation of the observed ratio from this value determines the relative size of the pseudo-quadrupolar coupling.

When an external magnetic field is present, two terms are added which describe the electronic and nuclear Zeeman interactions:

A cross term between the electronic Zeeman and

HM

adds to the the nuclear Zeeman term, but has opposite sign, and the effective Zeeman Hamiltonian is:

(ii mc+2S)

q

is c d e d the quenching parameter and was calculated t o be -0.9 in europium ethyl sulphate /l/.

Experimentally the 7 ~ O w 5 ~ 0 transition energy is measured, some of the shift is due to the 5 ~some to the ~ , 7 ~ 0 . The three experiments discussed in the next section were designed to measure the parametere in the expressions 3, 5, and 6 and to see if a consistent fit of all three were possible. In Elliott'a original paper the matrix elements were assigned their free-ion values. This assumption was not made here a s it yielded inwnsietencies.

111. EXPERIMENTAL RESULTS AND DISCUSSION

The hyperfine structure in zero-field (see eq. 3) was obtained from the ODNMR spectrum shown in the bottom trace in Fig. 1. Positive signals correspond to ground state hyperfine eplittings, and negative signals to excited state structure. Information about both of the naturally abundant isotopes 1 5 1 ~ ~ and 1 5 3 ~ ~ is present; the 1533, hyperfine splittings are larger because of its larger quadrupole moment. The energy level diagram (expressed in MHz) shown in Fig. 2

1 5 l ~ u l53811 I,

Fig. 1) (c) ODNMR spectra in zero f i e l d (bottom trace) and for H (1.07kG) along the major crystal axes. Fi

.

2) ( +

_k

Energy level diagram sharing the qFo anand DO hyperfine structure. Because the center is axial, the energy levels are I, eigenstates. summarizes the results. (The excited state transitions in 1 5 3 ~ u are outside the range shown.) The degree of accuracy quoted in Fig. 2 i s obtainable from slow scans over a lMHz region about the resonance.

C7-534 JOURNAL DE PHYSIQUE

7 ~ 0 : 1531 P+P '

1

/ l5l1 P+P'J

=

2.41 4.01 ( 151

1

PtP '

l

=8.26~Hz; 153

1

P+P'J =19,93MHz) 5D0: 153

1

P+P

I

/

1

P+P '

l

=

2.54 4.02 (

1

P+P

'l

Z 2 2 . 2 3 ~ ~ ~ ;

1

P+P

'I

=56.5MHz) The 5~~ i s well isolated from its nearest J=1 state, and the size of the pseudo-quadruple coupling can be calculated with the result that P' is negligible compared to P. Therefore the 5D0 ratio must reflect the true quadrupole moment ratio, and it is indeed in agreement with other measurements of 1 5 3 ~ / 1 5 1 ~ /2/. However, the measured ratio in the 7 ~ 0 state implies significant pseudo-quadrupole coupling.

Fig. 1 also shows the behavior of the hyperfine levels under an external magnetic field

.

We concentrated on the 1 5 1 3 ~ ion because of i t s larger magnetic moment (151~/153~2.26) /3/. The spectra verify that the <Ill> axes a r e q u a d r u p l e e-axes, and accurate values for the quenching parameters (eq. 5) of 1513u are obtained by analyzing these splitting patterns versus field magnitude and direction. The results (for 1 5 1 3 ~ ) are:

These values of (l-acil are the second apparent 5 ~ 0 'F0 contradiction t o E l l i o t t ' s calculation. The l 0.95 0.92 quenching appears t o be not only small, but

I l % , y l

0.90 0.85 isotropic; t h i s is not expected in the 7 ~ 0 as

the A2 and E s t a t e s of 7 ~ 1 are f a r apart in energy /7/. The apparent contradiction is immediately explained when one realizes that (l*) can be either positive or negative, with no observed change in the experimental results. Thus, it is possible that %,y and

oc,

are in fact quite different, and it is accidental that the quenching appears to be isotropic. Indeed, this turns out to be the case, a s seen by completing the analysis with the quadratic Zeeman data.

Soleburning was used to measure the nonlinear shift (eq. 6) of the 7 ~ O i + 5 ~ 0

transition. The optical hole is burned in a field and scanned in zero field, and the shift of the hole gives the shift of the optical transition. Fig. 3 shows sample data taken along one crystal axis. The experimental shifts must be assigned to the correct centers.

e (angle between Observed Quadratic 7 ~ 0 s h i f t (=observed

z and 8 ) s h i f t (MHz/k~2) + 5 ~ o >

(w/w2)

O0 not seen

-

c 0 s - l ( 2 / 3 ) ~ ~ 3 5 ~ 0.72 0.79 cos-I ( 1/3)% r55O 1.36 1.43 cos-l(1/3) a710 1.85 1.93 90° 2.00 2.08

The only consistent fit to the quadratic Zeeman data is shown above. Since the 5D0 is isolated in energy, it's interactions are assumed to be well described by free-ion matrix elements, and its quadratic shift is calculated theoretically. Then, i t is added a s a small correction to the observed shift to produce the 7F0 shift. The data in the third column overdetermine the values of [Y,y]2 and [&l2 which are (+'S in cmq1) [px,y]2

=

3 . 2 ~ 1 0 - ~ and rk12

=

2.2~10-~*

because there are two inequivalent centers.

8.48

7.34

are used to determine the values of parameters in (5)

C and (6) which then determine the pseudo-quadrupole

0

-

6.84

E parameter P', and finally the quadrupole moment ratios.

.D

4 6.34 _{Different ratios result depending on which of the two} ff

values is chosen for both and

o;.

The physically reasonable answer is chosen, thus determining which of the two a's is correct. Of eight possible combinations (2 choices of sign for three parameters: (l%,,), (l%), and (PtP'), only one gives agreement with the observed results. I t is:

(l%,y)<O

=>

%,y=1.85, ( l a z ) > O

=>

%=0.08, (PtP')>O

4.19

Using these values, one obtains 153~/151~=2.54 for the -200 0 +zoo b

laser frequency (MHZ)- H(kG) 7~~ state. All other combinations give values of

153p/151~ outside the accepted range. IV. CONCLUDING REMARKS

By measuring the ODNMR spectra under magnetic field and the nonlinear Zeeman shift of the 7 ~ O w 5 ~ 0 transition, we have been able to explain the observed results using Elliott's approach, but allowing for more deviation from free-ion wavefunctions. The axial symmetry of this center allows more definitive analysis than for lower symmetry centers, and this is the first time that the theory has been tested so completely in a E U ~ + center.

REFERENCES

/l/ R.J. Elliott, Proc. Phys. Soc. London, Sect B 70, 119 (1957).