Observation of the Er3+ Ɖ8 groundstate ESR-resonance in YAl2 single crystals

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Observation of the Er3+ D8 groundstate ESR-resonance in YAl2 single crystals

K. Baberschke, B. Bachor, H. Luft, J. Pellisson

To cite this version:

K. Baberschke, B. Bachor, H. Luft, J. Pellisson. Observation of the Er3+ D8 groundstate ESR- resonance in YAl2 single crystals. Journal de Physique Colloques, 1979, 40 (C5), pp.C5-51-C5-53.

�10.1051/jphyscol:1979520�. �jpa-00218937�

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JOURNAL DE PHYSIQUE Colloque C5, supplkment au no 5 , Tome 40, Mai 1979, page C5-51

Observation of the Er3+ T, groundstate ESR-resonance in YAl, single crystals

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K. Baberschke, B. Bachor, H . Luft

Institut fiir Atom- und Festkorperphysik, Freie Universitst Berlin, Boltmannstr. 20, D-1000 Berlin, 33, F.R.G.

and J. Pellisson

DPMC, Ecole de Physique, 32, bd d'Yvoy, CH-1211 Geneva, Switzerland

R6sumB. - On montre que le niveau fondamental de Er3 ⁺dans YAI, est un niveau T,. Les exptriences ont t t t mentes pour une concentration d'environ 1 000 ppm. O n obtient le param6tre de champ cristallin, x, Cgal a +0,23.

Abstract. - It is shown that the groundstate of Er3+ in YAI, is a T i 3 ) . The experiments were performed for a concentration of approx. 1 000 ppm. The crystal field parameter yields x =

+

0.23.

The cubic Laves phases X, ^-,Er,Al, (X = Y, La) have been the subject of many investigations for the determination of the crystal field (CF) splitting of Er3+. Samples with different concentrations, x, have been investigated by susceptibility and specific heat measurements as well as inelastic neutron scattering [I]

and electron spin resonance (ESR) [2]. The experimental results published up to now do not show a consistent levelscheme - or in the notation of LLW [3], the x and U.' values are completely inconsis- tent. The ESR measures magnetic dipole transitions within the groundstate manifold. This determines the groundstate properties very precisely. T,, T, or the anisotropic T , resonance can be distinguished easily.

For the latter, single crystals are needed, because of the anisotropy of the resonance spectrum [4,5]. Most of the experimental technique mentioned above are limited to a concentration of magnetic impurities down to 1 O/, to 10 O/,. At these concentrations interaction effects are still present. and may mask the pure CF.

Here we report ESR results on samples doped with x = 1.000 and 1.500 ppm. This low concentration justifies the assumption of non interacting ions.

.The single crystals were grown from the melt. The experiments were performed at 10 and 35 GHz.

Figure 1 shows two resonance transitions : the field for resonance, linewidth and intensity depending on the orientation. At g ⁼2 a background signal is produced by the empty cavity. The dashed lines are single line fits using a Dysonian lineshape. At 10 GHz similar signals were recorded, but because of some crystallographic imperfections for larger size crystals, the resonance signal became broader and the signal to noise ratio was worse.

(*) Work is part of the research program of the Sonder for- schungsbereich 161 der DFG.

I

^{Er, Y,.,} Alz s~ngle crystal x = 1500 ppm

--- flt

0 = oO ^S[I001

! !

Fig. 1. - ESR-Signal for Er concentration at 1 500 ppm. 8 being the angle between the applied field and the cubic axis.

Figure 2 shows the possible level scheme for J = 15/2 and cubic symmetry. From our results in figure 1 there are only two solutions : T i 1 ) W > 0 and 0.6 5 x

<

1 or T i 3 ) W < 0 and - 1

<

^x5 0.8.

Furthermore we can exclude low lying (few deg. K) excited levels. In such a case the Zeeman interaction would not be linear in the frequency because of an

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979520

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C5-52 K. BABERSCHKE, B. BACHOR, H. LUFT AND J. PELLISSON

Taking the conduction electrons into consideration the complete Hamiltonian is given by [6] :

Here we assume an isotropic exchange interaction J e f f . It acts on the magnetic impurities as an effective field and can be described within the groundstate manifold as an efSecti1.e change of the g-value. The fit

0 .23 .36

1 in figure 3 yields g,,p/g,,eor = 1.086 8

Fig. 2. - Energy scheme of J = 1512 in a cubic sylrihnetry [lo].

This work : x = 0.23, Ref. [lo] s =

+

0.36, W < 0. ^{g J}-

g e x p l g t h e o r = 1 + - ^'

SJ N(Ei=) J e f f .

admixture into the ground state. In figure 3 the full angular dependence of the field for resonance is shown. The full and dashed lines are theoretical fits of a full diagonalisation of the T 8 crystal field Hamil- tonian and the Zeeman interaction. The fit yields a

rr)

^{and x}⁼

⁺

0.23. The x-value also determines the eigenstates of the

r8

quadruplet and therefore the relative intensities of the different transitions.

These are in reasonable agreement with the experi- ment, however the experimentally determined intensities show large error bars because of inhomogeneous broadening of the individual transitions.

Fig. 3. - Field for resonance as a function of orientation. The dashed line was fitted for-an 8 degree mi orientation between the axis of rotation and 1 101 1.

Assuming a

f i t )

groundstate no consistent fit for figure 3 was possible. The only other possibility was

rA3)

^{and x}^z^-0.4. But this x-value is unlikely because of the nearby lying

rg)

level for a reasonable value of W.

This means a positive g-shift of N(E,) Jeff ⁼0.5.

The value is quite large compared to Gd : YAI, N(E,) J e f f = 0.07 [7]. In our analysis we have assumed an isotropic exchange. The difference at a factor 7 for Er and Gd may indicate that an anisotropic exchange analysis is needed. Pellisson [8] has shown that this is needed for Er doped into Pt.

Our results are in rough agreement with recent inelastic neutron scattering data on an 8 "/, Er sample [lo], the experiments yield .u =

+

0.36 and W = - 0.046 meV. The difference in the two x-values is out of the experimental error bars and can have several reasons : 1) there exists a real concentration dependence of the CF parameters, given by the change of the lattice constant and/or the change of the effective screening charge by substituting a RE ion on an Y-site. 2) For concentration of several % RE ions an internal field distribution surely is produced as well as an RKKY coupling of the RE ions will be present. (In earlier neutron scattering work [9] a quasielastic peak was detected.) This will affect the

r ,

ground state and the analysis.

We have shown that ESR is a convenient technique to determine C F ground states at very low impurity concentrations. This in accord with neutron scattering for moderate RE concentrations yields a unique set of CF parameters. Susceptibility and other bulk measurements are at least in this system very insen- sitive (see figure 5 in Ref. [9]) to different levels schemes.

Further experiments to determine the thermal broadening, a possible concentration dependent x-value and the system Er : LaAl, are under current investigation.

We thank Dr. H. Happel for sending us the neutron scattering data prior publication.

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OBSERVATION OF THE Er3+ r8 GROUNDSTATE ESR-RESONANCE I N YA12 SINGLE CRYSTALS C5-53

References [I] See the different contributions in : Crystal Field Effects m

Metals and Alloys, Furrer, A., ed. (Plenum Press, New York, London) 1977.

[2] DEVINE, R. A. B., ZINGG, W., MORET, J. H., SHALTIEL, D., Solid State Commun. 12 (1973) 515 and

DEVINE, R. A. B., POIRIER, M., CYR, T., J. Phys. F 5 (1975) 1407.

[3] LEA, K. R., LEASK, M. J. M. and WOLF, W. P., J. Phys. Chem.

Solids 23 (1962) 1381.

[4] ABRAGAM, A., BLEANEY, B., EPR of Transition Ions (Clarendon, Oxford) 1970.

[5] DEVINE, R. A. B., ZINGG, W., MORET, J. M., Solid Stare Commun. 11 (1972) 233.

[6] For notation see ref. [4] page 721 ff and ref. [3].

[7] SCH~~FER, W., SCHMIDT, H. K., ELSCHNER, B., BUSCHOW, K. H., Z . Phys. 254 (1972) 1 .

[8] PELLISSON, J., Thesis, Geneve 1977, unpublished.

[9] HEER, H., FURRER, A,, WALKER, E., TREYVAUD, A., PUR- WINS, H. G., KJEMS, J., J. Phys. C 7 (1974) 1207.

[lo] BLANKENHAGEN, P., HAPPEL, H., KNORR, K., J. Magnetism Mag. Materials, in press.