Magnetic interactions in Pr:Eu alloys

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Magnetic interactions in Pr:Eu alloys

F. Götz, G. Czjzek, J. Fink, H. Schmidt, P. Fulde

To cite this version:

F. Götz, G. Czjzek, J. Fink, H. Schmidt, P. Fulde. Magnetic interactions in Pr:Eu alloys. Journal de

Physique Colloques, 1979, 40 (C5), pp.C5-19-C5-21. �10.1051/jphyscol:1979506�. �jpa-00218878�

JOURNAL DE PHYSIQUE Colloque C5, supplkment au no 5, Tome 40, Mai 1979, page C5-19

Magnetic interactions in B : E u alloys

F. Gotz, G. Czjzek, J. Fink, H. Schmidt

Kernforschungszentrum Karlsruhe GmbH, Institut fiir Angewandte Kernphysik I D-7500 Karlsruhe, Postfach 3640, F.R.G.

and P. Fulde

Max-Planck-Institut fiir Festkorperforschung, D-7000 Stuttgart, F.R.G.

R6sumB. - Nous avons CtudiC l'effet Mossbauer de 151Eu et l'aimantation dans des alliages diluCs Pr : Eu (0,s at. % $ c,, 6 6 at. %). Ces mesures ont montrk que le moment magnetique de l'europium a uneyrienta- tion antiferromagnktique par rapport au moment des atomes de Pr voisin. La dkcomposition du spectre que I'on observe pour de faibles concentrations d'europium 9 basse tempkrature semble &tre due 9 des phbnom8nes de lente relaxation du spin Clectronique de I'europium. Nous proposons un mod8le qualitatif dans lequel autour de chaque atome d'europium une zone de polarisation est couplke antiferromagnktiquement au spin de I'euro- pium.

~bstract. - We have studied dilute & : Eu alloys (0.5 at. % ,< c,, ,< 6 at. %) by measurements of bulk magne- tization and by 1 5 ' ~ u Miissbauer spectroscopy. From both bulk magnetization and Mossbauer measurements we found that the Eu moment orients itself antiferromagnetically to neighbouring Pr moments. For low Eu concentrations magnetically split Mossbauer spectra are observed, which seem to be caused by slow electronic relaxation. A qualitative model is proposed featuring a polarization cloud around each Eu ion, coupled anti- ferromagnetically to the generating spin.

1 . Introduction. - In the singlet ground state system Pr, the ratio of exchange interactibns to crystal field splitting has been determined to be close to critical (0.92, [I]). Small changes of the exchange interactions are expected to push the Pr ion over the polarization instability. he exchange interactions can be locally changed by alloying magnetic ions into praseodymi~m. Such magnetic impurities are expected to produce a long range polarization on neighbouring Pr sites.

Eu and G d are well suited as magnetic impurities, both having a big 4f moment without orbital contri- bution. Besides, the 21.5 keV transition of the 15'Eu nucleus allows an investigation of this system with Mossbauer spectroscopy. In this paper we will present first results of bulk magnetization and l5'Eu MOSS- bauer spectroscopy measurements on the alloy system Pr : Eu.

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2. Experimental results. - Magnetization measure- ments in fields u p to 60 kOe were done on three alloys Pr : Eu (1.2,4.1 and 6 at. % Eu) and Pr metal (99.99 %

purity). The measurements were performed with a Faraday balance of conventional design in the tempe- rature range between 1.5 and 250 K. Above 50 K, the susceptibilities of all alloys follow a Curie-Weiss law.

At low temperatures, the results of bulk magnetization measurements were similar to those found for Pr - : Nd,

where Lebech et al. [I] observed an amplitude modu- lated antiferromagnetic structure with neutron scat- tering. As in the case of Pr : Nd, no indication of magnetic order is found F o m bulk magnetization measurements. The difference between the magne-

I I

10 20 30 LO 50

APPLIED FIELD (kOe)

Fig. 1. -Difference of the magnetization of the alloys Pr,,,,,Eu,,,,,

and pure Pr - i.e. the part of the magnetization caused by the

presence of Eu-moments - scaled to the Pr concentration, M = M,,,, - c,,M,,. The right-hand scale gives the resulting moment per Eu atom. The solid lines are guides for the eye.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979506

tization of the alloy and that of pure Pr is shown in figure 1. This difference which is the additional magnetization caused by the presence of Eu moments, is positive at low temperatures in fields up to 10 kOe.

The negative difference at higher fields indicates that the Eu-moments are then aligned antiparallel to the applied field. At higher temperatures (> 10 K) the positive difference at low fields disappears and the magnetization difference is negative for all fields.

'51Eu-Mossbauer transmission spectra of the alloys with 0.5, 1.2, 4.1 and 6 at. % Eu were taken in the temperature range 1.5 to 290 K and in fields up to 50 kOe using a spectrometer of conventional design.

In the most dilute samples (0.5 at. % l5'Eu resp.

1.2 at. % Eu) a magnetically split spectrum corres- ponding to a unique hyperfine field of 168 kOe was observed at 1.5 K in zero external field. In the range 3 to 10 K the spectra can be described by a superposition of a broad single line and a magnetically split compo- nent. The relative intensities of both components vary with temperature. The spectra are very similar to those observed in the case of slow electronic relaxation.

Above 10 K, only a single line with natural line- width is found. Spectra taken at various temperatures are shown in figure 2.

For the sample with 1.2 % Eu, we also took Moss-

bauer spectra in external fields of 20 and 50 kOe applied in the direction of observation. As the effective field increases with external field, the hyperfine- field is parallel to the applied field. This confirms our conclusion from bulk magnetization measurements that the Eu-moment is oriented antiparallel to the applied field because the hyperfine field of Eu is known to be negative (i.e. antiparallel to its moment) in almost all its alloys. We conclude that the exchange interaction of Eu with Pr is negative.

At Europium concentrations of 4.1 and 6.0 at. %

all spectra were composed of two subspectra with different isomer shifts, but almost equal relative intensities. The first subspectrum was similar to the spectra obtained at low Eu concentrations and was centered at the same isomer shift of - 6.1 mm!s.

The second subspectrum is centered at - 8 mm/s and is magnetically split up to very high temperatures corresponding to a unique hyperfine field of

- 250 kOe. The splitting temperature Tsp depends strongly on the Eu concentration (T,, - ^{90 K at}

4.1 %, Tsp 2 290 K at 6 %).

3. Discussion. ^- The observation of a magneti- cally split Mossbauer spectrum does not imply magne- tic order in all cases. If the relaxation of the local

I

⁰

0 -3(10 -225 -150 -75 0.0 75 150 225 -30.0 -225 -15.0 -75 0.0 75 15.0 225

VELOCmY (rnm/sec) VELOCITY Irnm/sec)

Fig. 2. - Mossbauer spectra of the alloy Pro,,,,Euo~o,, a t a) 1.5 K, b) 4.2 K, c ) 5.6 K, d) 30 K, in zero external field ; above the spectra

the relative intensities and positions of the lines are indicated.

MAGNETIC INTERACTIONS I N Pr - : Eu ALLOYS

surroundings ^- in our case the 4-f moment - is slower than the measuring time, a magnetically split spectrum is observed. In the case of Pr : Eu, no indi- cation of magnetic order was detezed from bulk magnetization measurements. The similarity of the low concentration Mossbauer spectra to the slow electronic relaxation case as well as the decrease of the splitting temperature with increasing Eu-concentration support the explanation of the magnetic splitting by slow electronic relaxation.

Our results indicate the same behaviour of all Eu ions. Thus, either Eu impurities occupy only sites of hexagonal symmetry or the difference between the two sites is not reflected in the magnetic properties of the Eu ions.

Mais [2] has calculated the magnetization at the lattice sites around an impurity in a selfconsistent manner, simulating Pr by a simple two-singlet system and the impurity by a Kramers ion with twofold degenerate ground state. He found a large increase in the polarization as the system approaches the critical value for the exchange interaction. Adding the experimental information that the exchange integral is negative, a polarization cloud coupled antiferromagnetically to the central moment is expected around each Eu atom. The Eu moment and the surrounding polarization cloud combine to a small net moment in the direction of the Eu spin.

These dwarf moments might qualitatively explain the observed magnetization and spectroscopic results.

Small external fields tend to align the dwarf moments, leading to a magnetization higher than that of the Pr matrix. At higher fields, a ferromagnetic moment is induced on the Pr sites, the dwarf moments are broken up and the Eu moment is aligned antiferromagnetically to the Pr matrix, leading to anet magnetization smaller than that of the matrix. The fact that the Eu ions are coupled to a polarization cloud might be responsible for the slow relaxation, which is much slower than expected for a bare Eu moment.

The source for the occurence of a second Mossbauer subspectrum at higher Eu concentration remains obscure. A phase admixture cannot be excluded, but the strong concentration dependence of the splitting temperatures makes this explanation doubtful.

4. Conclusions. - In dilute Pr : Eu alloys, the Eu moment orients itself antiferro~agnetically to neigh- bouring Pr moments. Based on this fact, a qualitative model featuring a polarization cloud around each Eu ion coupled antiferromagnetically to the generating spin is able to explain almost all experimental results obtained from bulk magnetization measurements and Mossbauer spectroscopy. The observed magne- tically split Mossbauer spectra seem to be caused by slow electronic relaxation of these dwarf moments.

References

[ I ] LEBECH, B., MCEWEN, K. A., LINDGARD, P. A., J. Phys. C 8 (1975) 1684.

[2] MAIS, H., Verunreinigungen in Van Vleck Paramagneten,

Dissertation, Universitat des Saarlandes, Saarbriicken

1975.