Physica B 186-188 (1993) 538-540 North-Holland PHYSICA

Magnetic-field-induced phase transitions in the cerium monochalcogenides CeS and CeSe

A. D6nni a, A. Oyamada a, T. Suzuki a, K. Sugiyama b, M. Date b, G. Kido c, P. Fischer d, A. F u r r e r d and F. Hu l l i ge r e

aDepartment of Physics, Faculty of Science, Tohoku University, Sendai, Japan bDepartment of Physics, Faculty of Science, Osaka University, Japan Clnstitute for Materials Research, Tohoku University, Katahira, Sendai, Japan aLaboratory for Neutron Scattering, ETH Ziirich, Villigen PSI, Switzerland ~Laboratory for Solid State Physics, ETH Ziirich, Switzerland

Magnetic phase diagrams of CeS and CeSe have been studied by neutron scattering, magnetoresistance and magnetiza- tion experiments. Along the easy axis [110] the magnetization curve of CeS at 4.2 K shows two steps at 5.6 and 9 T. In CeSe at 1.7 K a first-order phase transition occurs at 5.7 T in fields along [110] and [111].

1. Introduction

The rocksalt-type cerium monochalcogenides CeSe and CeS undergo a second-order magnetic phase tran- sition to an antiferromagnetic type-II structure [1,2] with the ordered moments p oriented parallel to the propagation vector k = [1/2, 1/2, 1/2]. Presumably due to Kondo hybridization the magnetic saturation moments of 0.57(5)/z B (CeSe) and 0.57(3)/~ (CeS) are considerably reduced below the value of 0.71/x B expected from the CEF ground state doublet F 7.

We have investigated magnetic phase diagrams of CeSe and CeS and present results of neutron scatter- ing, magnetoresistance and magnetization experi- ments.

2. Experiments

On a CeSe single crystals with T N =5.12(2) K, neutron scattering experiments were performed up to 4.2 T at ETH Ziirich and up to 6 T at ILL Grenoble. Magnetoresistance and magnetization of CeSe were measured up to 10 T at Tohoku University. Magneti- zation measurements on a CeS single crystal with T N = 8.48(2) K were done in permanent fields up to 23T using the hybrid magnet at the Institute for Materials Research, Tohoku University. Free powder

Correspondence to: A. D6nni, Department of Physics, Facul- ty of Science, Tohuku University, Sendai 980, Japan.

magnetization experiments in pulsed fields up to 60 T (CeSe) and 40T (CeS) were performed at Osaka University.

3. Results and discussion

3.1. Neutron scattering

The antiferromagnetic structure of CeSe gives rise to four magnetic domains with different spin direc- tions. In an external magnetic field along the [110] direction, a magnetic phase transition occurs in CeSe at H = Hcrit(T) ~< 5.7(1) T. For H < Hcrit the magnetic peaks correspond to an antiferromagnetic type-II structure with t, llk and intensity is observed for all four magnetic domains. For H > ncrit the magnetic peaks correspond to the same antiferromagnetic type- II structure with p II k but intensity is observed only for two of the four magnetic domains. The domains [111] and [111] with the magnetic moments perpendicular to the direction of the external field are favoured at the cost of the domains [111] and [111] where the mag- netic moments form an angle of 35.26 ° with the direc- tion of the magnetic field. Figure l(c) shows the field-dependent intensities of magnetic peaks corre- sponding to the two field favoured domains at T = 1.6 K at saturation.

The canting of the magnetic moments due to the ferromagnetic alignment along the direction of the external magnetic field leads to a reduction of the

0921-4526/93/$06.00 (~) 1993 - Elsevier Science Publishers B.V. All rights reserved

A. D6nni et al. I Magnetic-field-induced phase transitions 539

0.05

0.00, ¢?.

-0.05

-0.10

-0,15

0.4

o 0.3

-~- 0.2

0.1

0.0 3

._= 2 u

1=

0

CeSe, 1.7 K

I I l i l i i l I i I | i i ~ A & ~ & & & , ,

a

I s a[100] I • H[110] inc I m H[110]dec I • ~ • • •

| = = =

. b

i u i n

H[1,-1,01, 1.6 K o o ( - l f 2 , - 1 / 2 , 3/2)1 o Ora, v2, 3r2) i

C 0 0 0 Q ° Q ~ I

[ ]

I I I 2 4 6 8 10

H [Tesla]

Fig. 1. Magnetoresistance (a), magnetization (b) and field- dependent magnetic neutron intensities (c) of CeSe at satura- tion. #: data from ref. [3].

zero-field antiferromagnetic ordered moment. Increas- ing the field H[110] from 0 to 6 T, we observe at 1.6 K a decrease of 10% in the sum of the magnetic elastic peak intensities (fig. lc) which gives rise to a reduction of the ordered antiferromagnetic moment from 0.56/z a at zero field to 0.53/x B at 6 T. The magnetization in fig. l(b) gives rise to a field-induced ordered ferromag- netic moment along [170] of 0.26/z a at 6T. With increasing external magnetic field the increasing cant- ing of the magnetic moments leads to an instability of the two magnetic domains, which disappear in a fully reversible first-order phase transition at Hcdt = 5.7(1) T.

3.2. Magnetoresistance

Figure l(a) shows the transverse magnetoresistance Ap/p o of CeSe measured at 1.7K. Neglecting the phonon part at this low temperature the electric resis- tivity of Po = 23.15 i~I~ cm basically contains contribu- tions from conduction electrons scattered at im- purities, magnetic domain walls and spin fluctuations. The impurity scattering independent of magnetic field seems to be rather high (between 10 and 20 i ~ cm). In magnetic fields along [110] the magnetoresistance is

almost constant for H < H , , and exhibits a big step- like decrease of 12% at He,i,, which we interpret as a decrease in domain wall scattering. The magnetic phase transition is absent in magnetic fields along [100], because the [100] direction is symmetrical to the directions of the ordered moments of all four anti- ferromagnetic domains The observed additional small negative bending of the magnetoresistance towards higher magnetic fields corresponds to the decrease in the spin fluctuations, which are depressed in high magnetic fields.

3.3. Magnetization

At 1.7 K the magnetization of CeSe shown in fig. l(b) behaves isotropically for H < He,it. A step in the magnetization at H , , indicates the first-order phase transition which occurs in magnetic fields along [110] and [111] and is absent along the [100] direction.

Figure 2 shows the magnetization of CeS measured at 4.2 K at saturation. Along the easy axis of magneti- zation [110] two steps are observed in CeS at 5.6 and at 9 T. But as are more clearly shown in fig. 3, the steps in CeS are much less pronounced than the step observed in CeSe. From the present experiments it is not clear whether the different behaviors of the mag- netization of CeSe (fig. lb) and CeS (fig. 2) are intrinsic or due to inferior sample quality in the case of CeS.

Figure 3 shows the free powder magnetization M(H) and derivative d M / d H up to 60 T for CeSe at 1.3 K and up to 40 T for CeS at 4.2 K. The kink in the

0.7

0.6 CeS, 4.2 K + ~ *

0.5

0.4 m

• 0.3

0.2

0.1

0.0 0 5 1'0 1; ~ 25

H [Tesla] Fig. 2. Magnetization of CeS at 4.2 K up to 23 T along the cubic main symmetry directions [100], [110] and [111].

540 A. D6nni et al. / Magnetic-field-induced phase transitions

1.5

1.2

o 0.9

0.6

0.3

0,0

~" 0.06

0.03

CeSe, 1.3 K

i i i i i

0.00 0 I; 210 310 4; 510 60

H [Tesla]

Fig. 3. High-field free powder magnetization M(H) and derivative d M / d H of CeSe up to 60 T and of CeS up to 40 T.

magnetization curve of CeSe around 25 T indicates the saturation of the CEF ground state doublet F 7. The high-field behavior of the magnetization of CeSe is discussed in ref. [4].

Financial support by the Japan Society for the Pro- motion of Science is gratefully acknowledged.

References

[1] H.R. Ott, J.K. Kjems and F. Hulliger, Phys. Rev. Lett. 42 (1979) 1378.

[2] P. Schobinger-Papamantellos, P. Fischer, A. Niggli, E. Kaldis and V. Hildebrandt, J. Phys. C 7 (1974) 2023.

[3] F. Hulliger, B. Natterer and H.R. Ott, J. Magn. Magn. Mater. 8 (1978) 87.

[4] A. D6nni, A. Oyamada, Y. Ohe, T. Suzuki, K. Sugiyama, M. Date, P. Fischer, A. Furrer and F. Hul- liger, J. Alloys Comp., to be published.