Solid State Communications, Vol.49,No.3, pp.24]-244, |984. 0038-|098/84 $3.00 + .00 Printed in Great Britain. Pergamon Press Ltd.

TRANSPORT PROPERTIES OF YTTRIUM ALLOYS WITH DILUTE RARE EARTH SOLUTES

Naushad All and S.B. Woods

Department of Physics, University of Alberta, Edmonton, Alberta, Canada T6G 2Jl

(Received 12 September 1983 by M. F. Collins)

The temperature dependence of electrical resistivity and absolute thermoelectric power (TEP) of dilute YI RE (RE=Ce, Sm, Dy and Tb)

i-x alloys has been investigated in the temperature range of 2 to 40 K. YCe (Ce = 3%) shows only a typical Kondo effect whereas YCe (Ce = 15%) shows spln-glass behaviour at lower temperatures with a Kondo resistance minimum at ~ 7 K. For YDy alloys, 2% Dy is sufficient consentratlon to produce spln-glass behavlour with resistance varying as T 3"2 at lower temperatures and a broad peak in the TEP with a sign change from positive to negative at a higher temperature. We observe an antiferro- magnetic to paramagnetlc phase transition in YSm (Sm = 3%) and YTb (Tb = 3%) with a well defined N~el temperature (T N) in the resistance versus temperature curve. No such sharp phase transition is evident in the TEP data. YDy (Dy = 10%) does not show a well defined TN; rather it behaves more like a spln-glass of lower temperatures.

Dilute alloys containing transition ele- ments in non-magnetic host metals exhibit Kondo effect and spin-glass behavlour at low temperatures. These phenomena are believed to be associated with the magnetic moment of the transition metal ions. An s-d exchange inter- action between the conduction electrons and the localized magnetic moments is supposed to be the origin of this behaviour. The localization of the magnetic moments of the transition metals has been found to be quite complicated whereas the magnetic moments of rare-earth ions dis- solved in non-magnetlc metals are close to that of the free ions and a clearly localized state is produced. For this reason there has been growing interest in dilute magnetic alloys containing rare-earth ions.

Earlier work I on dilute yttrium-rare-earth alloys shows that YCe (2%,% represent atomic percentage in this paper) exhibits the Kondo effect, while the other dilute yttrium-rare- earth alloys order antlferromagnetlcally at low temperatures. More recent work of Sarklsslan and Coles 2 has shown that alloys llke YI_xRE x (RE= Dy, T5 and Gd) exhibit spln-glass behavlour for lower RE concentrations, while for higher concentrations of the magnetic moments there is a helical ordering at low temperatures.

Kondo 3, who set out to explain the decreas- ing resistance with increasing temperature seen in dilute alloys with magnetic impurities at low temperatures, did so on the basis of second order as well as first order scattering pro- cesses between conduction electrons and isolated ions with magnetic moments. Combined with phonon scattering, which becomes important with increasing temperature a resistance mini- mum is produced at an intermediate temperature, T m. Kondo alloys frequently exhibit a 'giant' thermoelectric power (TEP), of the order of i0 -s VK -I at low temperatures; such TEP

requires not only a strong energy dependence of the scattering but a large asymmetry in the energy dependence about the Fermi energy. The complex nature of the higher order processes involved in the Kondo alloys appears to be able to explain the giant TEP found in dilute alloys that show the resistance minimum. Certainly the occurrence of these two properties together defines a Kondo alloy.

Spin-glass behaviour, which is usually associated with higher concentrations of the magnetic ions, is characterized by locking of the magnetic moments in random orientations below a well-defined freezing temperature, Tf. There have been numerous theoretical ~-s as well as experimental 9-13 studies of transport pro- perties of spin-glass systems. A.C. suscepti- bilities of these systems show a cusp at T=. The electrical resistivity varies as ~ T 3/~ and sometimes as T 2 at the lowest temperatures. The TEP seems to be more complicated. It has a broad peak as in the Kondo effect but it is much reduced in absolute height and there is a change in sign at higher temperatures. Theoretical investigations by Matho and B~al- Monod e, Cooper et al. 12 and Fischer ? suggest that there are at least two magnetic mechanisms and possibly one spln-independent interaction between the magnetic impurities and the conduction electrons responsible for the TEP behaviour in spin-glass systems. Of the two magnetic mechanisms, one is the "Kondo" term

S~I)(T)- (which reduces for vanishing spin interaction to the TEP of a Kondo system) and

the other is the "resonant" term S(2)(T), which vanishes in the Kondo limit. It turns out that for spin-glass systems the TEP changes sign at some characteristic temperature T O if the interaction is more strongly antiferromagnetic than ferromagnetic.

In this communication we present an

241

242 TRANSPORT PROPERTIES OF YTTRIUM ALLOYS WITH DILUTE RARE EARTH SOLUTES Vol. 49, No. 3

experimental study of resistivity and absolute thermoelectric power in the temperature range from 2 to 40 K on dilute YI_xRE x alloys (RE= Ca, Sm, Tb and Dy). Yttrium was chosen because it forms solid solutions wlth all the rare-earth metals and has the same number of valence electrons as most of the rare-earth metals. The alloys were prepared by melting the constituent elements in an arc furnace and annealing the ingots at ~ 800°C in an argon atmosphere. Specimens of 15mm×2mmx2mm slze we cut from the ingots using a diamond saw. The resistance was measured using a conventional four-terminal method wlth the help of a direct current comparator. A specimen current of 20 mA was used in all the resistance measure- ments without any significant joule heating. A galvanometer amplifier with a sensitivity of ~ 10 -9 volts was used as a null detector. A standard potentlometer and galvanometer amplifier were used for the TEP measurements enabling us to measure voltage with an accuracy of 2 × i0 -9 volts. The temperature gradient was always kept below 2% of the specimen tempera- ture and was measured with a AuFe (0.07 at.%)- Cu thermocouple, To obtain absolute values of the TEP, the specimen was replaced by lead (Pb) and a superconductor (V3Ga) wlth T c = 17 K enabling us to find the TEP of the leads. The specimen temperature was measured with a calibrated Ge resistance thermometer.

The residual resistance ratio (RRR), that is the ratio of the resistance at 300 K to that at 4.2 K of dilute YI_xREx alloys is given in Table i. The variation of resistance (r =R~/R s,

TABLE I

Sample RRR

Y0.97Ce0.03 6

Y0.85Ce0.15 4.2

Y0.97Smo.03 7

Y0.97Tb0.03 7.7

Y0.98DY0.02 13.8

Y0.9oDY0.10 6.2

where R x is the specimen resistance and R s= 0.i ~ standard resistance) with tempera- ture (T) for YCe (Ce = 3% and 15%) is shown in Fig. i. For 3% Ce alloys we observe a typical Kondo behavlour with a resistance minimum at

22.2 K. But as we increase the concentration of Ce the resistivity behavlour changes. For 15% Ce we observe (Fig. i) that at the lowest termperatures the resistance first increases with T, goes through a small peak at ~ 4.5 K then to a minimum at ~ 7 K beyond which it monotonically rises with temperature. Below 4.5 K the alloy is showing a spin-glass behaviour and above that, a Kondo effect. A similar effect has been observed in concentra- ted Lal_xCe x alloys 14 and dilute AuFe alloys 13 Figure 2 shows the TEP (S) as a function of temperature (T). There is a broad peak at about ~ 19 K for 3% Ce with Sma x = 13 ~V/Deg. A similar broad peak is found for the 15% Ce alloy at ~ 17 K with Sma x = 2.1 ~V/Deg. There is a change of sign in the TEP at ~ 41 K for

629

O ',- 628 X

X (/) r~ Ii ~ 627

II 626

630 , , , , , , ,

/ Y o . 8 5 G e o . 1 5

624 II ' l i ~ ' ~ i - ~ I i 2 5 10 15 20 25 30

T(K)

127 ×

125 ×l " 123 ~ r r

II 121 ~-

119

117 35 40

Fig. I Normalized resistance (r =Rx/Rs), where Rx is specimen resistance and Rs = 0.i~, as a function of temperature (T) of YCe (Ce = 3% and 15%) alloys.

2.0

O~ (D 1.0

~ 0

-I.0

0

Fig. 2

-- T f ~ I l i

-- " . . ,

s

/

I I i i [ 0 5 I0 15 20 25 30

T(K)

Absolute thermoelectric power (S) of YCe (Ce= 3% and 15%) and YDy (Dy=2% and 10%) alloys as a function of temperature (T).

15

10

v

5 O9

15% Ce but no such sign change was found for 3% Ce alloys. In Fig. 3 r vs T and in Flg. 2 S vs T curve is shown for Yl_xDYx (x = 0.02 and 0.I0). The resistance increases with T but no sharp phase transition Is evident. At the

lowest temperatures the resistance behaviour is like a spin-glass, which is also evident in Fig. 2 where a broad peak in TEP and a change in sign from positive to negative is seen. For 10% Dy the resistance increase with T is faster below 6 K but no explicit helical ordering is evident as suggested by Sarkissian and Coles 2.

Results for YSm (3% Sm and YTb (3% Tb) alloys are shown in Fig. 4 and Fig. 5. The resistance versus temperature curves for these alloys show clearly a sharp transition from an antiferromagnetic to paramagnetic phase (TN=5.0 K and 5.2 K for YSm (3%) and for YTb (3%) respectively). Below T N the resistance increases very rapidly with temperature whereas the slope of the curve decreases above T N and becomes almost constant at higher temperatures. This rapid increase in resistance below T N is due to spin-wave excitations in an antiferro- magnetic metal. The effect of dilution down to

Vol. 49,

84 ! i

83

82 !

Y 811

,o

II 79 L , .

78

77

76 2

Fig. 3.

No. 3 TRANSPORT PROPERTIES OF YTTRIUM ALLOYS WITH DILUTE RARE EARTH SOLUTES

O 38 T-

X

36 H 35

34

33

7 J Yo.9o Oyo.~o

5 lO 15

T(K)

51

50

"L 2

O

x 49

H

48

Fig. 4

Normalized resistance (r =Rx/Rs) as a function of temperature (T) of YDy (Dy = 2% and 10%) alloys.

TN I I

7 Smo.03

I I T

43

42

o

41 X

II 40 ~-

I I i 5 10 15

r(N

Normalized resistance (r = Rx/Rs) as a function of temperature (T) of YSm (3% Sm) and YTb(3% Th) alloys.

3% Sm and Th in Y is a reduction of T N as the strength of the interaction of the magnetic moments (RKKY type) is reduced. Sugarawa ~ has observed a similar behaviour from resistivity measurements on dilute YTb alloys and obtained

T N= 5K for an alloy with 2% Tb. Nagasawa and Sugarawa 15 performed susceptibility measure- ments on a series of dilute YTb alloys and obtained the temperature T o of the susceptibi- lity maximum. For a 2.716 atom%Tb alloy they obtained T o = 11.5 K and noted that in dilute alloys T o does not always correspond to T N. The sensitivity of their resistivity data was not sufficient for them to locate T~ from the slope change. Sarkissian and Coles ~ saw a susceptibility maximum at T O = 12 K for a YTb (3% Tb) alloy but, because the resistance showed no increase with decreasing temperature below T o , they identified this as a spin-glass material. Their specimens were chill cast after arc melting, which may have had an effect on the low temperature resistivity and they may have missed a slope change over a narrow temperature range with normal potentiometric measurements. It should also be noted that the TEP shown in Fig. 5 has a broad peak for the YSm (3% Sm) and YTb (3% Tb) alloys and does not show any evidence of the phase transition.

243

3.0

20 O~

"0 1.0

oo o

-I.0

0

l I I I I

Yo.97 Sm 0.03

I L L I I

5 I0 15 20 25 30

T(K)

Fig. 5 Absolute thermoelectric power (S) of YSm (3% Sm) and YTb (3% Tb) alloys as a function of temperature (T).

In conclusion, we have observed Kondo effect in a dilute YCe alloy and spin-glass behaviour in more concentrated YCe alloys. Spin-glass behaviour is also found in dilute YDy alloys, but we clearly observe an antifer- romagnetic phase at low temperatures in the resistivity of YSm (3% Sm) and YTb (3% Tb). The thermoelectric power on the other hand shows no sharp change associated with the phase changes.

Acknowledgements: We are thankful to Mr. T. Valian for his technical assistance. This work was supported in part by the Natural Sciences and Engineering Research Council of Canada.

244 TRANSPORT PROPERTIES OF YTTRIUM ALLOYS WITH DILUTE RARE EARTH SOLUTES

REFERENCES

Vol. 49, No. 3

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