EFFECTIVE MOLECULAR FIELD AT S O M E RARE EARTH IONS IN ORDERED INTERMETALLIC C O M P O U N D S RE I _xGdxA12

Y. B E R T H I E R Laboratoire de Spectrometric Physique BP 53 X, 38041 Grenoble-c~dex, France

and R. A. B. DEVINE Physics Department, University of Miami PO Box 248046, Coral Gables, FL 33124, USA

Hyperfine fields at rare earth nuclei 167Er and t59Tb have been measured by N M R in ordered REI_xGdxAI2 compounds as a function of x (0 < x < 1). The results suggest a lower value of the effective molecular field than expected and a decrease of the conduct ion band polarization in Er Alloys.

1. I n t r o d u c t i o n

It has been demonstrated recently that nuclear magnetic resonance (NMR) measurements of rare earth nuclei in ordered intermetallics compounds such as RE A12 can yield microscopic information on various aspects of the magnetism in these alloys [1, 2]. Previous measurements on the 163Dy nucleus in ordered compounds of Dy1_xM~A12 (M = Y, Gd) demonstrated that it is possible to observe resonances from Dy nuclei surrounded by 0, 1, 2, 3 or 4 nearest neighbour M (M = Gd, Y) ions. In a recent study of Dy in Dyl_xGdxAl 2 across the whole range of Gd concentration, it was found that the resonance frequencies did not increase as ex- pected with increasing molecular field, and it was assumed that in part the anomalous behaviour originated from a rotation of the easy axis of magnetization from (100) in OyAl 2 to (l 1 l) in GdA12. In the following we report the results of measurements of 167Er in Ert_~GdxA12 and 1 5 9 T b

in Tb,_xGdxA12, two series of compounds where the easy axis of magnetization remains oriented along (111) throughout.

2. E x p e r i m e n t a l a n d r e s u l t s

Samples were prepared by levitation melting of stochiometric amounts of RE and A1, crushing and sieving to a powder of < 80/~ diameter and then annealing in a vacuum better than 10-5 Torr for 60 h at 850°C. Measurements were made at 1.4 K using a pulsed spin echo spectrometer continuously tuneable in the range 300-4000 MHz. In Erl_,,GdxA12, the observed spectra, for 0 < x <

1 1 0.95, both for the 3,--> - ~ and other quadrupolar

transitions were composed of a multiple resonance pattern consistent with resonances from Er nuclei surrounded by 0, 1, 2, 3 or 4 first nearest neighbours Gd ions [4]. The resonance frequencies

I 1 for the 3*-->- ~ transitions and the quadrupolar splitting (__+1~.> ___ ~3 transition) are plotted as a function of Gd concentration in fig. 1. In Tbt_xGd,,A12 our investigations have been limited to the Tb rich alloys (x = 0) and Gd rich alloys (x = 0.9). For x --- 0 the hyperfine parameters cor- responding to the main line are ~,0(½~-.- ½)= 3248 MHz; i 3 ~,q(i~-~i) -- 710 MHz, for x -- 0.9 we obtained t, 0 = 3264 MHz and Pq -- 728 MHz. Pre- cision of all measurements is _+ 1 MHz.

. r

940

9 2 0

O uJ

~w 9001 U Z

Z

8 8 0

I I

o -@

• •

l, ErAI 2

1 , 1 , 1 , 1 1

0 0 0 0 0 0 e e o e e e e e

A ~ : A A A •

~t Z o II g o o e "

A N Z

o Z

1 4 o ~_. l.- ..J a.

1 3 0

-

120 ~ 0

I t I I I 1 I I I 1 1 0 0

0 . 2 0 , 4 0,6 0.8 GdAI 2

x

- ~ transition) and Fig. I. N M R frequencies in MHz (½<--~ quadrupolar splitting in MHz of ~67Er, corresponding to dif- ferent environment around the Er ion, in Er I _xGdxA12 as a function of x, at 1.4 K. ( , , A, A, • and O refer respectively to

0, 1, 2, 3 and 4 nm Gd).

Journal of Magnetism and Magnetic Materials 15-18 (1980) 703-705 ©North Holland 703

704 Y. Berthier, R. A. B. Devine/Effective molecular f ield in ordered alloys RE I _,, Gd,,AI 2

TABLE 1

Estimated molecular field, H m (in kOe), theoretical resonance frequency (in MHz) and measured resonance frequency for the various configuration of Er and [Tb] surrounded by 0, 1, 2, 3 or 4 Gd nearest neighbors in Er [Tb] rich or Gd rich alloys

Configuration 0 1 2 3 4

HmEr[Tb I 50 [436] 71 92 113 134 Er[Tb] rich v Theor. 890 [3248] 933 964 988 1007

Exp. 890 [3248] 918 927 934 939

HmEr[Tb ] 194 215 236 257 278 [556] Gd rich ~ Theor. 1042 1049 1054 1058 1060 [3288]

Exp. - - 923 928 935 938 [3264]

3. Discussion

The hyperfine field H T at 167Er o r 159Tb is a sum of the 4f, self polarization and transferred hyper- fine field, and may be written in the form H T = ,4(J~ > + A nn(S'), where .,~ is an effective hyper- fine field per unit of total angular momentum of the 4f electrons, which is different from the free ion A4f of Bleaney [3] because we have incorporated the self polarization term Asp which might be im- portant in RE Al 2 due to the orbital polarization of the 5d electrons. A n ( S ; ) is the transferred hyper- fine field from the neighbouring ions. This last term is generally small, and it has been shown previously that in these compounds, where the 4f moment is reduced due to crystal field effect, the major modification to H T arises from variation in (J~) due to variations in the molecular field. Noting that the molecular field seen by an ion of type fl in an alloy of ions of type a is

- - ] < s , > o ,

and assuming H ~ b (in GdAI2) = 834 kOe we ob- tain: H~ r (in GdA12) --- 278 kOe and Hm Tb (in GdA12) = 556 kOe. In the case of Dyl_xGdxA12 it was found that a first nearest neighbour ion pro- duced 8.7% of Hm Dy in GdAI 2 and DyAI 2. Assuming the same situation applies for the case of Er in ErA12 or in GdA12, we have calculated the effective molecular field seen by an Er ion surrounded by 0, 1, 2, or 4 Gd first nn in either Er or Gd rich alloys of Erl_xGdxAl 2. Taking these fields and assuming the crystal field parameters given in ref. [5] for ErA12 we have calculated the corresponding ( J , ) values. Then using the known values of A and a

"bare" ( J z ) in pure ErA12 of 6.08 we have deduced the resonance frequencies for the various config- urations. A comparison with the experimental re- sults in table 1 shows that the effective molecular field variations must be lower than those deduced from our simple model. From the maximum (J~) value of 7.26 calculated for Er in rich Gd alloys we deduced: A -- 938/7.26 = (129.5)(J~) MHz. The corresponding Asp value is (4.5) (J~) MHz [taking A4t(Er ) = (125)(J~) MHz]. A comparison with the value of this parameter in pure ERA12:Asp(ErA12) = (21.4)(J~) MHz, would indicate that the orbital contribution to Hsp which was important in ErA12 decreases strongly for Er in the GdAI 2 host.

The results concerning Tbt_xGdxA12 alloys can be analysed following the same model, but we must note that in these alloys the molecular field varia- tion is lower than for Erl_xGdxA1: when x in- creases. Assuming the crystal field parameters given in ref. [6] and O c = 108 K, we get a "bare" ( Jz ) of 5.69 corresponding to the initial value of of (571) ( Jz ) MHz in pure TbA12. For H m = 556 kOe in Gd rich alloys the computed value of (J~) is 5.76. This gives a theoretical frequency for a Tb ion in a sea of GdA12 which is again higher than the experimental frequency, but the value of (566) ( Jz ) MHz for A in this limiting case stays in good agreement with an important orbital contribution in Asp (a pure spin model would give ,4 --- (28) (J~) MHz.

In conclusion the present results demonstrate that the effective molecular field seen by RE ions in these alloys increases less than predicted by a simple scaling deduced from the Curie tempera- ture, and shows that the orbital polarization of the conduction band decreases when the molecular

Y. Berthier, R. A. B. Devine~Effective molecular field in ordered alloys REt_xGdxAl 2 705

field increases substantially (Er alloys). This can be correlated with a modification of the band struc- ture.

References

[1] Y. Berthier, R. A. B. Devine and B. Barbara, Phys. Rev. BI6 (1977) 1025.

[2] Y. Berthier, R. A. B. Devine and E. Belorizky, Phys. Rev. BI7 (1978) 4137.

[3] B. Bleaney, in: Magnetic Properties of Rare-Earth Metals, ed. R. J. Elliot (Plenum, New York, 1972) Ch. 8.

[4] Y. Berthier and R. A. B. Devine, J. Appl. Phys. 50 (1979) 2321.

[5] H. G. Purwins, E. Walker, B. Barbara, M. F. Rossignol and A. Furrer, J. Phys. C9 (1976) 1025.

[6] H. G. Purwins, E. Walker, B. Barbara and M. F. Rossignol, Phys. Lett. 45A (1973) 427.