Microscopic and macroscopic effect of substitution of Dy in DyAl2 by trivalent rare-earth ions (Sc, Y, La, Lu)

HAL Id: jpa-00218957

https://hal.archives-ouvertes.fr/jpa-00218957

Submitted on 1 Jan 1979

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Microscopic and macroscopic effect of substitution of Dy in DyAl2 by trivalent rare-earth ions (Sc, Y, La, Lu)

Y. Berthier, R. Devine

To cite this version:

Y. Berthier, R. Devine. Microscopic and macroscopic effect of substitution of Dy in DyAl2 by trivalent rare-earth ions (Sc, Y, La, Lu). Journal de Physique Colloques, 1979, 40 (C5), pp.C5-116-C5-117.

�10.1051/jphyscol:1979540�. �jpa-00218957�

J O U R N A L

DE PHYSIQUE Colloque C5, supplément au n° 5, Tome 40, Mai 1979, page C5-116

Microscopic and macroscopic effect of substitution of Dy in DyAl

by trivalent rare-earth ions (Sc, Y, La, Lu)

Y. Berthier (*) and R. A. B. Devine (*) C)

(*) Laboratoire de Spectrometrie Physique, B.P. 53X, 38041 Grenoble Cedex, France (*) Laboratoire Louis-Neel, C.N.R.S., B.P. 166X, 38042 Grenoble Cedex, France

Résumé. — Le champ hyperfin sur le noyau

Dy a été étudié par résonance magnétique nucléaire dans les composés intermétalliques Dy

M

_iAl

(M = Se, Y, La, Lu) dans la phase ferromagnétique. Cette étude a pour but de déterminer la perturbation produite par la substitution, sur le champ moléculaire et le champ cristallin.

Les effets observés sont analysés en tenant compte des mesures de rayons X ainsi que des températures de Curie.

Abstract. — The hyperfine field at the

Dy nucleus in ferromagnetic Dy

M

Al

(M = Sc, Y, La, Lu) inter- metallic compounds has been studied by nuclear magnetic resonance, with the aim of determining the perturbation of the molecular and crystal fields produced by substitution. The observed perturbations are discussed taking into account X ray and Curie temperature measurements.

In the following we report the results of susceptibi- lity and nuclear magnetic resonance (NMR) measu- rements on DyAl

and Dy

M

jAl

(M = Sc, Y, La, Lu). The aim of these experiments was twofold;

firstly, since NMR spectra are sensitive to changes in local environment [1], to observe the influence of Sc and Lu substitution and secondly, to look for correla- tion between the NMR spectral linewidths and the macroscopic behaviour of the magnetisation.

1 1 -pr 1

Dy„„ M „ . AI„ / ^-A

0.9 0.1 2 / / \ \

M = La ' I \ \

MaSc —"^ / A \ \

•g KI = Lu - " ' \ \ TO / fl\

"J / " V / 1 a / ^~* 11

= / I \

•= M = Y '

s

4 [

D y A I2 /

. \ . . , , \ , . , , \ . ,

1175 1180 1185 FREQUENCY (MHZ)

Fig. 1. — Experimentally observed NMR spectra of 163Dy in ferromagnetic DyAI2 and Dy0 SM0 iAl2 (M = Sc, Y, La, Lu) alloys at 1.4 K. The transition observed is the 1/2 *-* — 1/2.

(*) Laboratoire associe au C.N.R.S.

Powdered samples were studied at 1.4 K using pulsed techniques [1]. The experimental results for the

1/2 <-* — 1/2 nuclear transition are shown in figure 1.

A satellite structure is observed for substituted alloys correlated with the modification of the molecular field due to first nearest neighbour substitution by an M ion. Statistical probability excludes observation of other satellites due to two, three or four substitu- tions [1] for a concentration of 0.1. The resonance frequencies for the central lines and satellites as well as the widths at half peak height of the central lines are given in table I.

Table I. — Resonance frequencies for central lines and satellites and linewidth at half peak height of the central line. For the La case the satellite was not resolved due to the large linewidth.

Compound Resonance Frequency (MHz) Central Central line Satellite linewidth (MHz)

DyAl2 1 182.5 ± 0.25 0.5

D y^{0 9}S c^{0 1}A l² 1181.0 + 0.5 1174.5 + 0.5 2.3 D y0 9Y0 !A12 1 182.5 + 0.5 1 177.5 + 0.5 1.2 D y0 9L a0 1A l2 1 1 8 0 . 5 + 0 . 5 Not extracted 2.5 Dyo'gLUo^Alj 1 182.5 + 0.5 1 176.0 + 0.5 1.6

— The susceptibility in low field was measured using a mutual induction method. The curves of l/x versus Tare given, in arbitrary units, for Dy

.

Y

Al

and D y

L u

A l

in figure 2. For DyAl

, Dy

.

Y

Al

and Dy

.

La

Al

the susceptibility is Curie Weiss like leading to the paramagnetic Curie temperatures given in table II. For the Lu and Sc alloys there is a change in slope of l/x at 63 K and

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979540

MICROSCOPIC AND MACROSCOPIC EFFECT OF SUBSTITUTION OF Dy IN DyA12 C5-117

TEMPERATURE ( K ) Fig. 2. - Inverse susceptibility (in arbitrary units) versus tempe- rature for Dyo~,Luo~,Al, and Dyo~9Yo~,AI,. The inset for the Y curve shows the determination of the paralferro transition width.

57 K respectively. The slopes at higher temperatures lead to the Curie temperatures given in table 11.

The width of the paramagnetic/ferromagnetic transi- tion is crudely lorentzian and can be characterised by a width at half peak height (figure 2). The fractional widths AT/Op, are given in table 11. Because of the change in slope of the Sc and Lu alloys we use the low temperature slopes to determine AT.

Table 11.

Paramagnetic Curie temperatures, frac- tional transition widths and equivalent NMR linewidths for the compounds measured.

Equivalent Compound 8 p (K) AT/&

(A)

- - - -

DyA12 62 0.64 0.40

DY o.,Sco.~Al, 50.0 2.10 1.30 Dyo.,Yo 1'412 53.4 1.85 1.22 DYO.&~O I-% 53.9 3.52 2.34 DYO.~LUO.IAI~ 53.0 1.0 0.65

Discussion. - The resonance frequencies quoted in table I indicate less than 0.17 % difference between the central lines of all the alloys, suggesting that the Dy moment (g,

J , )) varies by this same figure. The magnetic and electric internal fields are hence little

influenced by effects due to M ions at further than nearest neighbour proximity. In the case of electric fields, screening within the unit cells outside of the central ion and four nearest neighbbur Dy ions would account for this. The satellite lines reflecting first nearest neighbour substitution do not signifi- cantly differ either, and one is lead to the conclusion that although the crystal fields seen by rare-earths in ScAl, and LuAl, differ from YAl, and LaAl,, the influence of Sc or Lu substitution in the ferromagnetic phase is negligeable (Note the paramagnetic suscep- tibility results do suggest differences for Sc and Lu, figure 2). Although no ready explanation is possible, we believe exchange effects may lead to dominance of the conduction electrons in the crystal field in the ferromagnetic REAL, compounds so that substitu- tion effects are unimportant 121.

We come back to the problem of the resonance linewidths. Two mechanisms seem plausible, either the linewidths represent a distribution of magnetic field or crystalline electric field. Because of screening arguments presented previously it seems likely that the distribution is one of magnetic field. X-ray linewidth measurements on high reflection planes indicate no discernable difference in atomic distributions within the limits of resolution and hence do not help in interpretation. However, if one compares the diffe- rence in lattice parameter between MAl, and DyAl, one finds both the NMR linewidth and magnetic transition width increase as a function of misfit of M in DyAl,.

Since both NMR and transition widths correlate with lattice distortion it is interesting to look for correlation between the magnetization and NMR.

We take the fractional widths AT/@ given in table I1 and estimate the equivalent molecular field distribu- tion. The values of the equivalent NMR linewidths corresponding are given in table 11.

For other than the cases of Sc and Lu where the transition was ill defined we find good agreement indicating the origin of the magnetic transition width to be a function of molecular field distribution rather than a critical fluctuation effect.

In conclusion, Sc and Lu seem to cause no perturba- tion of the crystal field in the ferromagnetic phase although anomalies exist in the paramagnetic phase.

From lattice parameter arguments it seems that a molecular field distribution arises due to lattice misfit which both governs the paramagnetic/ferromagnetic transition width and dominates the NMR linewidth

at low temperatures.

References

[I] BERTHIER, Y., DEVINE, R. A. B. and BARBARA, B., Phys. Rev.

B 16 (1977) 1025.

[2] DEVINE, R. A. B. and

RAY,