Magnetic transition in the CePd3Bx system

Journal of Magnetism and Magnetic North-Holland, Amsterdam

MAGNETIC

Materials

TRANSITION

IN THE CePd,B,

J.P. KAPPLER,

M.J. BESNUS,

UniversitP Strasbourg

I, L.M.S.E.S.,

J. SERENI Centro Atomico,

111

478~48 (1985) 111-114

SYSTEM

E. BEAUREPAIRE,

A. MEYER

Institut de Physique, 67084 Stmsbourg,

France

and G. NIEVA Bnriloche, Argentina

Susceptibility, high-field magnetisation, resistivity and heat capacity data for the CePd,B, system are reported. The observed valence transition of Ce is the first example of a transition from an IV state system to a stable magnetic 4f’ state system, whose properties are tentatively ascribed to an aleatory cooperative state, due to the inhomogeneous character of the valence transition, rather than to a Kondo effect.

1. Introduction

CePd, is known to be an archetypical intermediate valent material for which alloying effects are widely studied. For instance, the dilution of Ce by Y [l] or the substitution of Pd by Rh [2] leads to a non-magnetic state for cerium while in the pseudo binaries Ce(Pd, Ag), [3] a Kondo-like behaviour is reported. Boron-induced valence change of Ce in CePd, towards a trivalent state was first proposed by Dhar et al. [4]. Recently spectroscopy measurements (XPS, BIS) due to Wuilloud et al. [6] lead to a 4f occupation increasing from n r = 0.9 for CePd, to n, = 1 for CePd,B,,,. We present here the results of a thorough study of the low temperature properties (susceptibility, high-field magnetisation, resistivity, specific heat) of CePd,B, alloys (0 c: x I 1) which show that the addition of boron in the octahedral sites of CePd, induces a transition of the electronic state of the neighbouring Ce atoms from the IV state with a M = 2 J + 1 = 6 degeneracy to a 4f’ state with M = 2.

2. Results In the whole concentration range, high-temperature susceptibility data [5] show C-W behaviour with a Curie constant close to the free tripositive ion value. The first indication, in addition to the lattice parameter increase, of a drastic change in the valence of Ce when going from CePd, to CePd,B is given by a linear 0304~8853/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

decrease of the extrapolated C-W temperature ( 19~ 1with a limit value of about 10 K for x 2 0.3. The second indication is given by the change in the thermal variation of the low temperature susceptibility (T i 20 K). In the low concentration range the measured susceptibility can be described by x(T) = x(O) + C/T, where x(O) is the T independent susceptibility characteristic of IV compounds and C the Curie constant of the paramagnetic contribution. In the high concentration range the temperature independent term is the Van Vleck contribution, due to the crystal field splitting A( r, - r,). In fig. 1 we represent both the Curie constant and the temperature independent term as a function of boron concentration. As shown on the figure there is a strong increase in both C and x, which for x > 0.4 reach the respective values of = 0.2 and = 2.6 X lo-* cm3/Ce-at, evidencing the F, doublet as the ground state with an overall splitting A of about 60 K. Correlatively, when measurable, the temperature of the IV susceptibility maximum decreases rapidly with increasing boron content (insert to fig. 1). Magnetisation curves calculated within this crystal field scheme are in good agreement with our experimental data for H 6 150 kOe and T > 1 K. Susceptibility data for T < 1 K (fig. 2) show however that a simple spin paramagnetism does not work in this temperature range characterised by the appearance of irreversible phenomena and thermal history effects. These irreversible effects are also observed in high field magnetisation measurements (up to 50 kOe) and thus exclude a possible impurity contribution. In addition, specific heat measurements

J.P. Kappler et al. / Magnetic

3-

transition in CePd, B,

CePd,Bx -_+----+/+-/ ,p’--T--

I

I

I

I

CePd,

I

I

0.5

Fig. 1. Low temperature vs. x.

(temperature

independent)

I

I

cd

X susceptibility,

x, and Curie constant,

C, vs boron concentration.

I i

+++ +++++ + +

x = 0.5 /**

. l

l

-•..

+

l

CePd3Bx

+ :

.* :

+++

1 Tt.

1 Fig. 2. Susceptibility

vs. T for CePd,B,

2 (0,

0) zero field cooled:

3

T[K]

( + ) field cooled.

4

H = 500 Oe.

Insert:

T( x,,,)

J. P. Kappler et al. / Magnetic transition in CePd, B,

I

8T[Kl

Fig. 3. Specific heat vs. T for CePd,B,.

(0.4 K I T 5 30 K, H s 30 kOe) show broad

concentration dependent anomalies in the OS-2 K range. The observed C,(T) variations with maxima reaching 2.5 J/KCe-at are given in fig. 3. For CePd,B the entropy associated with this anomaly is 0.9R In 2 in agreement with a I, ground state. With applied magnetic field, these maxima appear also in a C,/T representation; further the C,(T) anomalies are shifted towards higher temperatures (e.g. for CePd,B T,,, = 2 K, for H = 30 kOe) with a correlative increase in the values of C,,,,, (= 30% at H = 30 kOe).

(),~-~---o C&d,

.25

SO

Fig. 4. Characteristic temperatures T(x_,) and T(x~,,,) (0) in the CePd,B, system.

1

.75

Y (0).

CeW,B T(C,,,,,)

(0)

I

113

1 ’

8

3. Discussion

The analysis of these results leads to describe this transition as inhomogeneous, the Ce atoms not involved in it retaining the typical properties of the IV state with however a decrease of their characteristic temperature T(x,._) (insert fig. 1). This transition, almost achieved for x = 0.35 (i.e. for = 0.3 added boron atom per cerium atom) results in a strong modification of the electronic structure in this system. This interpretation is in agreement with the NMR results [5]. The partially filled 4f level located in close proximity to E, in CePd, moves below with increasing B content with a correlative increased 4f’ localisation. As shown in fig. 4, the low temperature anomalies in C,(T) and x(T) coexist for small B concentrations with the typical IV susceptibility maxima. This leads to ascribe these anomalies to the part of Ce atoms already in their stable 4f’ state; as a matter of fact, the associated entropies scale with the concentration of magnetic Ce atoms as derived from the values of the Curie constant C. However, the low temperature behaviour of both resistivity and specific heat may reflect a Kondo regime. Effectively, the thermal variation of the resistivity presents (for x > 0.3) a large saturated value of about 250 l&cm at low temperature (down to 0.1 K) and the symmetric shape on a logarithmic T scale of C,(T) could suggest such an interpretation. But the experimen-

114

J.P. Kappier t-1 al. / Mqnerrc

tal values of the specific heat maxima are much larger than the theoretical value of 1.5 J/KCe-at for a r, (S = l/2) state [7], and the susceptibility is not decreased in the way predicted by theories which however concern the impurity problem, while we deal here with a dense system. Alternatively, one may ascribe the low temperature properties to some kind of cluster glass behaviour which results in Schottky-like anomalies for the specific heat and in irreversible phenomena for the low temperature susceptibility. Further experimental data may help to clarify the problem.

transimn

rn CePd., B,

References

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