ELSEVIER Journal of Magnetism and Magnetic Materials 131 (1994) 224-228

Metamagnetism and quenching of spin fluctuation scattering in the Laves phase intermetallic compounds

N g u y e n H u u D u c

Cryogenic Laboratory, Department of Physics, University of Hanoi, Dongda, Hanoi, I/iet Nam

(Received 9 November 1992; in revised form 13 July 1993)

Abstract

For a number of Y(Co, A1) 2 and (R, Y)Co 2 compounds, the effects of external and internal (molecular) fields on the resistivity are accentuated. The results show that the resistivity is initially enhanced by applied field. A sharp decrease in resistivity, which is possibly due to the quenching of the spin fluctuations, can only be observed just at the metamagnetic transition. The different roles of the external and molecular fields on this phenomenon are also discussed.

1. Introduction

Quenching of spin fluctuations has been found by specific heat measured in a magnetic field of 10 T at low temperature for the strongly Pauli paramagnetic RCo 2 ( R = Sc, Lu and Y) com- pounds [1]. Upon the application of the molecu- lar field (by substitution of the magnetic rare earth atoms on Y(Lu) sites), the initial increase in the values of the electronic specific heat coeffi- cients (y) with increasing x in RxYI_xCo 2 com- pounds was observed [2,3]. This phenomenon is usually related to the enhancement of the spin fluctuations [3]. However, it can also be thought as a clear indication for the rising induced Co moment which finally reaches 1 ~B in RxY 1_xCO2 compounds [2,4]. In some studies of the influence of the spin fluctuations on the transport proper- ties of (R, Y)C02 and R(Co, A1) 2 compounds it was claimed that the spin fluctuations which are present in all investigated compounds above the ordering temperature (T¢), are enhanced, particu-

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larly around T c by the presence of 4f-moments [4] or by volume expansion [5,6]. This is the case of the resistivity minima around Tc in (R, Y)Co 2 compounds [5] and the upturn in the resistivity below T~ for a number of R(Co, AI) 2 compounds [6,7]. Recently, this enhancement of resistivity has also been connected to the scattering on the induced 3d-magnetic moment [7]. For itinerant electron magnets, spin fluctuations are thought to be present; they contribute a negative magnetore- sistance for all cases [8]. However, the effects of magnetic fields on the spin fluctuation scattering is usually compensated by the magnetic scatter- ing. The spin fluctuation scattering may be ob- served at low temperature or, in particular, at the metamagnetic transitions (MT), where the spin fluctuations are completely quenched.

An MT was found directly in the magnetiza- tion measurements at the critical magnetic field of 70 T for YCo 2 and LuCo 2 [9]. By substituting A1 for Co, the transition occurs below 40 T [10] for the pseudo-binary Y(Lu)(Co, A1) 2 com-

reserved

N.H. Duc /Journal of Magnetism and Magnetic Materials 131 (1994) 224-228 225

pounds. With these compounds, it is thus possible to study experimentally the quenching of the spin fluctuations under high fields. However, up to the present, no direct calorimetric measurements have been made in such high fields where the MT takes place. For this series of compounds, evi- dence of the field effect on spin fluctuation was indirectly obtained from a thermodynamic analy- sis of the high-field magnetization data [11]. This result clearly shows that a substantial reduction of 3, occurs at the MT, which is possibly due to a quenching of the spin fluctuations.

The aim of the present paper is to investigate the metamagnetism and quenching of the spin fluctuations by means of the resistance measure- ments for a number of Y(Co, A1) 2 and (R, Y)Co 2 compounds.

2. Experimental results and analysis

Z 1. External magnetic field effect

The magnetization and magnetoresistance measurements in fields up to 38 T have been performed in the Amsterdam high-field installa- tion. Data are presented in Fig. l(a, b) for the Y(Co0.925AI0.075) 2 compound. A dearer compari- son of the transition in these two measurements is illustrated in Fig. l(c) in the plots of dM/dH versus H and d[AR/R(O)]/dH versus H. A sharp drop in the resistance at the magnetic field (of 30.3 T) corresponding to the metamagnetic tran- sition is observed. The reduction of resistance started at the field of about 20 T. Below this field, the resistance increases with increasing ap- plied magnetic field. We obtained the value of AR/R(O) of + 1.25% at 10 T, whereas the de- pression of the electronic specific heat constant by applied magnetic field showed a value of A y / y of -10.8% for LuCo 2 [1]. In a thermodynamic analysis of the magnetization data, the same re- duction of y was also observed for Y(Co0.s9 A10.11) 2 [11]. For itinerant electron magnets, the resistivity can be given as [1]:

AR(H, T) = ARc(H, T) + ARsF(H, r),

where ARc(H, T) is the positive contribution due

I"5 i

= 1"0 I

0.5

I I I

0 10 20 H(T)30

0.15

0.10 0.O5

r Y

"-- 0.0 re"

-0.05

- 0.10

I+0

- 0.15

I I I

- b

, iL-7~ . . . . .

I I I

10 20 30 /,0 H(T)

-1-

0

60 H(T)

i , i , i ' ' ' ' I i l i i I , , i I

- - - . . . . . . . . . . . . ~ - - - ' ~ z , - - 7 - = - -

l l i I i I I i I I l t ' l I ' ' I

10 20 30

" 0

O

Fig. 1. (a) High-field magnetization, (b) magnetoresistance data, and (c) plots of d M / d H versus H and d [AR/R(O)] /dH versus H at 4.2 K for Y(Coo.925Alo.075) 2.

226 N.H. Duc /Journal of Magnetism and Magnetic Materials 131 (1994) 224-228

to cyclotron motion of conduction electrons and ARsF(H, T) is the negative one due to spin fluc- tuations. Following our measurement, a positive contribution such as AR c seems to be important at low applied fields. However, it was indicated recently that the field effect is to induce a mag- netic moment of the fluctuating electrons [1,3,7]. A linear increase in AR/R with increasing mag- netization (AM/M) was found for ferromagnetic Y(Co, AI) 2 compounds in an applied magnetic field up to 12 T [7]. At present, this magnetic scattering may also be the reason for the initial increase in magnetoresistance. In this context, the magnetic scattering is thought to be larger at MT where a jump of magnetization occurs. A

sharp decrease in resistance, therefore cannot be related to the stability of Co magnetic moment, but can be considered as an indication of the quenching of the spin fluctuation at MT.

2.2. Internal (molecular) field effect

With regard to the molecular field, the investi- gation has been focused on (R, Y)Co 2 com- pounds where the variation of internal field can be considered as a function of rare earth concen- tration and/or as a function of temperature [3]. For the first case, the residual resistivity (P0) collected for RxY 1_xC02 compounds with R = Er, Ho and Dy is presented in Fig. 2(a). By relating

100~a, , , , i , , , ,

~ D ~ I oRxYl-xC°2 E

{ I I , l i i I , , , ,

"0 0.5 x 1.0

1.0

0 .8

100t b, t ~0.6 R xYI_x C°2 =

0 ,

501- ~ --Ho

I 0 .0

0 100 H(T} 200

r o

I I I

100 200 300

H(TI

Fig. 2. Variation of the residual resistivity as a function of (a) R concentration and (b) molecular field; (c) variation of Co moments as a function of the molecular field (after Refs. [9,12]) for a number of RxYl_xCo2 compounds (R = Gd (®), Tb (D), Dy ( I ) , Ho ( o ) and Er (o)).

N.H. Duc ~Journal of Magnetism and Magnetic Materials 131 (1994) 224-228 227

the x-value to the molecular field H R (",x(gj- 1)Jt~) arising from rare earth moments [12], we can present these different variations of P0 in an identical plot, as shown in Fig. 2(b). It nicely reproduces the above observed picture for AR(H)/R(O) in an external field. The enhance- ment effects of 90 at low molecular field strengthen, although a complete quenching still occurs at the field (of about 70 T), corresponding to the appearance of a CO magnetic moment in these compounds (see Fig. 2c).

Regarding the thermal effect, the temperature dependence of the magnetic scattering in RCo 2

30

A 20 E L..;

c 10 .-..l

at 0 <:3

-10 !

m t I

' ~ . , , I

.oco2

I I

too T(K) 200

a

300

30

A E 20 t . J

-10 b

I I I

0 50 H(T)100 15(

Fig. 3. Excess resistivity Ap as a function of (a) temperature, and (b) molecular field for HoCo 2.

compounds is also affected by the thermal varia- tion of the rare earth spins. In these compounds, the resistivity originating from the influence of the rare earth atoms, interacting with the itiner- ant Co matrix has been estimated by subtracting the resistivity of YCo2 [5]. The resulting curve is shown in Fig. 3(a) for HoCo 2. It is dearly seen that the curve is characterized by a sharp change at the magnetic ordering temperature T e. Above T c the excess resistivity Ap decreases with in- creasing temperature up to 250 K. In attempt to focus attention on the influence of molecular field, we identified the temperature variation with the variation of rare earth magnetic moments reported in Re£ [13]. Finally, an analogous be- haviour is presented in Fig. 3(b) for Ap(H).

3. Discussion

Ikeda et al. [1] derived from their experiments evidence for the existence of the spin fluctuations and their quenching in applied magnetic fields of 10 T for YCo 2 and LuCo 2. Sakakibara et al. [11] also pointed out that this quenching of spin fluc- tuations is continued with increasing fields and becomes strongest at the MT. However, in our analysis a clear indication of the quenching of spin fluctuations can only be observed at the MT. At low applied magnetic fields, the observed magnetoresistance is positive. It is due to the scattering on the induced moments, which is strongly field dependent in the nearly ferromag- netic compounds. Nevertheless, here the question is why the effect is strongly enhanced by the magnetic rare earth substitution.

In some previous studies of the anomalies in transport phenomena and magnetic properties in RxY l_xCo 2 compounds, various effects have been considered [3,4]: (i) the random freezing of the diluted rare earth moments; (ii) the presence of spin fluctuations; and (iii) the onset of Co mag- netism. The random freezing of localized R mo- ments is usually thought to be dominant for small x-values, it must be independent of the rare earth elements. However, as mentioned above, the ob- served effects of the critical concentration are

228 N.H. Duc /Journal of Magnetism and Magnetic Materials 131 (1994) 224-228

strongly dependent on the kind of the rare earth. Moreover, a similar behaviour of resistivity also occurs in the non-substituted HoCo 2 compound in the paramagnetic state. Thus, the present phe- nomena can be related mainly to the influence of the molecular field on the onset of Co mag- netism. We have shown in Fig. 2(c) that the enhanced Co susceptibilities in the magnetic rare earth-Co 2 compounds are higher than the corre- sponding YCo 2 susceptibility. This has also been confirmed by means of polarized neutron diffrac- tion on TmCo 2, HoCo 2 and T h C o 2 [13], which suggests a slight variation in the d-band structure in these compounds.

4. Acknowledgement

It is a pleasure to thank Dr. I.A. Campbell for his discussions and encouragement. This work was partly supported by the National Basic Re- search Program in Natural Sciences.

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