Physica B 163 (1990) 201-204

North-Holland

LOW TEMPERATURE MAGNETIC BEHAVIOR IN CePtGa,

J. TANG”, K.A. GSCHNEIDNER, Jr.“, R. CASPARYb and F. STEGLICHb “Ames Laboratory* and Departments of Materials Science and Engineering, and Physics, Iowa State Universiry, Ames, IA 50011, USA ‘lnstitut fiir Festktirperphysik, Technische Hochschule, D-6100, Darmstadt, FRG

A low temperature study of CePtGa, shows a peak in the heat capacity of -2.8 J/mol Ce K at 1.7 K, which is shifted to

higher temperatures (-6 K at 9.8T) in magnetic fields. Furthermore, the heat capacity data are consistent with

spin-glass-like behavior. The 8 T field data below 0.2 K suggests a y of -120 mJ/mol Ce K’. AC magnetic susceptibility

data as a function of the magnetic field (up to 2T) suggests some small fraction of the Ce spins are involved with

percolating antiferromagnetic clusters. Unlike the ordinary spin glasses, the It Ce. ions in CePtGa, occupy periodic lattice

sites. The random RKKY interactions between spins which provides the basis for spin-glass-like behavior is induced by the

random distribution of Pt and Ga atoms around the Ce’+ ions.

1. Introduction

“Classical” spin glass behavior is found in dilute metallic alloys &Fe or QMn containing -1 at% of magnetic impurities Fe or Mn [l, 21. Unlike the fer- romagnetic or antiferromagnetic system, in which the RKKY interaction between the nearest neighboring spins are either positive or negative, the RKKY inter- action between the impurity spins in a spin glass system is random due to the random distribution of magnetic impurities in the lattice. It is because of this random interaction that the spins can be frustrated in trying to align parallel or antiparallel to each other, and upon cooling below a critical temperature spins are locked together (frozen) in a configuration where they are oriented in random directions.

In the case mentioned above, the random distribu- tion of magnetic impurities in the lattice is responsible for the spin glass behavior. Recently Rauchschwalbe et al. [3] found a different kind of spin glass behavior in CeCu,,Al,,. In this system the Ce3+ moments form a periodic array, and the randomness in the system is introduced by the statistical occupation of non-magnetic sites in the cage surrounding each Ce7+ ion. A similar situation was also found in CePd,B,, [4] where Ce 3+ ions and Pd atoms occupy regular

periodic sites in the lattice, the corners and faces, respectively, but the B atom only partially (-l/3) occupies the body-centered site. In this compound the random distribution of boron atoms and vacancies,

* Operated for the US Department of Energy by Iowa State University under contract no. 4-7405ENG-82. This work

was supported by the Office of Basic Energy Sciences.

0921-4526/90/$03.50 0 Elsevier Science Publishers B.V. (North-Holland)

which changes the electronic environment around Ce3+ ions, is responsible for the spin glass behavior in

CePd,B,,. In this paper we report on the low temperature

magnetic behavior in CePtGa,, and because of the random distribution of Pt and Ga atoms around the Ce3+ ions we expect a similar type behavior as noted above for CeCu,,Al,, and CePd,B,, 3.

2. Sample preparation and characterization

CePtGa, was prepared by arc-melting, and then heat treated at 600°C for about 2 weeks. Metallo- graphic examination of CePtGa, showed the presence of an impurity phase, in the form of parallel black strips.

The crystal structure of CePtGa, was first reported by Grin et al. [S]. They found that CePtxGa4_x (0.2 < x < 1.15) crystallizes in the body-centered tetragonal BaAI,-type structure. In this structure the Ce atoms occupy 2(a) (O,O, 0) positions; and Pt and Ga atoms randomly occupy the 4(e) ?(O, 0, z) and the 4(d) t(O, l/2, l/4) positions. X-ray diffraction showed that CePtGa, crystallized in BaAI,-type structure.

3. Heat capacity

The heat capacity measurements were carried out using an adiabatic heat pulse type calorimeter [6]. A magnetic field (0~ H< 1OT) was applied during the measurements. The heat capacity of CePtGa, as a

.I. Tang et al. I Low temperature magnetic behavior in CePtGa, 202

3

G 2

z > 0

I

0

CePtGo,

0.5 I Temperature (Kl

5

20 I 1 I I I

15

Y

z E 10 \

-3

t;

5

0 0 5 10 15 20

Temperature(K)

Fig. 1. (a) Heat capacity of CePtGa, from 0.06 to 5 K in fields of H = 0, 2 and 8 T. (b) The heat capacity from 1.5 to 20 K in fields of H = 0, 5.3 and 9.8 T.

function of temperature is shown in fig. 1. The heat capacity at zero field starts to increase with decreasing temperature when temperature passes through 7 K

(see fig. I(b)), and then it reaches a maximum at -1.7 K. The maximum shifts to a higher temperature when a magnetic field is applied. This suggests that the maximum is due to a magnetic phase change and not due to heavy fermion behavior. The significant feature for this compound is that this maximum in zero field heat capacity is not a sharp but a broad one, which suggests that the system undergoes a spin glass type transition since a rounded peak is a characteristic feature of a spin glass system [7]. For a ferromagnetic or antiferromagnetic phase transition, a sharp A-type

O-

i xx CePtGa,

O8T

0 0.5

Temperature (K)

Fig. 2. (C ~ C “U/I )/ T versus T in fields of H = 0, 2 and 8 T.

peak is normally found. The shift of the peak with increasing applied field is also typical of spin glass behavior [7], although a shift to higher temperature also occurs in ferromagnetic materials. As will be seen in the magnetic susceptibility, the peak is not due to ferromagnetic ordering.

Below 4 K, the Ga nuclear hyperfine contribution (a T-* term) to the heat capacity was subtracted out. The resultant AC/T = (C - Cnuc,)/ T versus T plot is shown in fig. 2. An estimate of the electronic specific heat coefficient can be obtained from the limiting AC/T value at H = 8 T below 0.2 K. Unfortunately, y cannot be extracted for the low-field (H = 2 T) and zero field cases, since the magnetic contribution is still dominant at the lowest temperature. Generally the spin glass derived linear contribution to the low tem- perature heat capacity is strongly depressed by a magnetic field [8]. Therefore, y taken at H = 8 T yields an upper bound of the electronic specific heat constant of 120 mJ/mol K’. This large value suggests the presence of electronic quasiparticles with strongly renormalized mass caused by a Kondo type mechan- ism. As was found previously, the latter can be opera- tive in the presence of both antiferromagnetic [9] and spin glass [4] order.

The entropy associated with the peak in C for zero field data seems to saturate at 90% of the expected value, Rln2, for the doublet ground state of Ce’+. This 10% entropy loss is typically found in other spin glass systems [lo, 111. It is believed that at temperatures far above the freezing temperature, T,, the spins in the system are already somewhat correlated with each other, and that accounts for the loss of the entropy.

J. Tang et al. I Low temperature magnetic behavior in CePtGa, 203

4. Magnetic susceptibility

The ac magnetic susceptibility x,, of CePtGa, from -0.4 K to 3 K is shown in fig. 3. The absence of a peak around 1.7 K in x,, suggests that the peak in heat capacity at the same temperature is not due to an ordinary magnetic phase transition, e.g. ferromagnetic phase transition, since the peaks in x,, and heat capacity would occur at about the same temperature for an ordinary magnetic phase transition, but is not inconsistent with the spin glass-like behavior. Unfortu- nately, no cusp is observed in the x,, versus T plot at -1 T, (i.e. -0.8 K). Instead, an apparent plateau in x,, is found at -0.5 K. In a field of 2 T a maximum is observed at a temperature, which is slightly lower than the point at which the plateau develops. This suggests the existence of some antiferromagnetic ordering near

-10 ,- 0 2 3

Temperature(K)

IO

5

,- : Ii G

0

x x

_E

-IC

Fig. 3. (a) The ac magnetic susceptibility x,, of CePtGa,. (b) ac susceptibility at 0, 1 and 2 T.

0.5 K. This contrasts our heat capacity data which does not show evidence for any long range (i.e. antiferromagnetic) ordering.

5. Conclusion and summary

Combining the data on ac susceptibility and heat capacity, the following conclusion can be made. Whereas the heat capacity data suggest a spin-glass- like ordering, the susceptibility results indicates the occurrence of some antiferromagnetic ordering. From the entropy the majority of the Ce moments must participate in the spin-glass-type ordering. The minor portion of Ce ions is involved in the antiferromagnetic ordering. One possibility is that the long range mag- netic ordering is due to the second phase noted previ- ously in section 2. A second possibility is that the long range ordering is due to a few percolating antifer- romagnetic clusters in the matrix phase itself. New samples, free of second phase, have been prepared and are under further studies to distinguish between these two explanations.

Unlike the ordinary spin glasses, the spin-carrying Ce3+ ions in CePtGa, occupy the periodic lattice sites. The random RKKY interaction between the spins which provides the basis for spin glass-like behavior, is induced by the random distribution of Pt and Ga atoms around Ce” ions. The large y value (-120 mJ/ mol K’) suggests that the Kondo effect can coexist with a spin glass-like state.

Acknowledgment

The authors wish to thank B.J. Beaudry and N.M. Beymer for preparing the samples.

References

[ll

PI

[31

[41

PI

V. Cannella, J.A. Mydosh and J.I. Budnick, J. Appl.

Phys. 42 (1971) 1689.

C.A.M. Mulder, A.J. van Duyneveldt and J.A. Myd-

osh, Phys. Rev. B 23 (1981) 1384.

U. Rauchschwalbe, U. Gottwick, U. Ahlheim, H.M.

Mayer and F. Steglich, J. Less-Common Metals 111

(1985) 265. S.K. Dhar, K.A. Gschneidner, Jr., CD. Bred1 and F.

Steglich, Phys. Rev. B 39 (1989) 2439.

Yu.N. Grin, P. Rogl and K. Hiebl, J. Less-Common

Metals 136 (1988) 329.

204 J. Tang et al. I Low temperature magnetic behavior in CePtGa,

[6] K. Ikeda, K.A. Gschneidner, Jr.. B.J. Beaudry and U.

Atzmony, Phys. Rev. B 25 (1982) 4604.

171 K. Binder and A.P. Young, Rev. Mod. Phys. 58 (1986)

801.

[8] C.D. Bredl, F. Steglich, H.V. Ldheysen and K. Matho.

J. de Phys. 39 (1978) C6-925.

(91 C.D. Bredl, F. Steglich and K.D. Schotte, Z. Phys. B 29

(1978) 327.

[lo] L.E. Wenger and P.H. Keesom, Phys. Rev. B 13 (1976) 4053.

1111 J.G. Sereni, T.E. Huber and C.A. Luengo, Solid State

Commun. 29 (1979) 671.