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Submitted on 1 Jan 1988
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MAGNETIZATION OF CsFeCl3 UNDER MAGNETIC
FIELD UP TO 40 T
T. Tsuboi, M. Chiba, H. Hori, I. Shiozaki, M. Date
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Supplement au no 12, Tome 49, dhcembre 1988
MAGNETIZATION OF CsFeClg UNDER MAGNETIC FIELD UP TO
40T
T. Tsuboi (I), M. Chiba (2), H. Hori (3), I. Shiozaki (3) and M. Date (3)
(I) Physics Department, Kyoto Sangyo University, Kamigamo, Kyoto 603, Japan
(2) Institute of Atomic Energy, Kyoto University, Kyoto 611, Japan
(3) Department of Physics, Osaka University, Osaka 560, Japan
Abstract.
-
Magnetization process of CsFeCls is investigated using high magnetic fields B. In addition to magnetic ordering due to level-crossing within fictitious S' = 1 states, new spin ordering is observed to appear at 32 T under B//
c.No field-induced ordering is observed under B l c .
~ e ' + ion of triangular antiferromagnet CsFeCls has a singlet ground state. The spin Hamiltonian describ- ing this system is given, using fictitious spin S' = 1, by
where D is the anisotropy energy, J's the ferro- magnetic exchange energy along hexagonal oaxis [I]; D/]c~=20 K, Jl/Ica=5.3 K, Jll/]c~=3.5 K [2]. Unlike RbFeCls, CsFeCls does not exhibit a long-range spin ordering (LRO) at zero magnetic field. Because, mag- nitude of exchange interaction which leads to spin or- dering is smaller than energy of uniaxial anistropy which leads t o quench spin moment in the ground state.
When magnetic field is applied parallel t o c-axis, the ground level crosses one of the excited, doublet level. Then LRO is brought about since the exchange inter- action induces non-vanishing transverse spin compo- nents (S,) and (S,) ( z
//
c ) in the vicinity of the level crossing point [3]. Appearance of LRO was observed in 4-12 T region [4]. Recently we observed a new meta- magnetic phase transition around 33 T [5]. Present work was done t o obtain the detailed information of the phase transition and magnetization process below 40 T.1. Low field spin ordering
When a pulsed magnetic field is applied parallel t o c-axis below 4 K, a linear increase of magnetization is observed between 4 and 12 T. The linear increase reflects the appearance of 3D LRO which is caused by level crossing in the fictitious S' = 1 spin states
[5]. Magnetic susceptibility dM/dB is measured t o know the detailed magnetization process. As shown
Fig. 1. - Magnetic susceptibility (dM/dBf of CsFeC13 at
1.3 K under magnetic fields (B) applied parallel to c-axis.
Inset shows a fine structure observed in 32-33 T region.
Haseda et al. [4] found a sharp peak followed two broad humps in the temperature dependence curve of specific heat and suggested that it is caused by suc- cessive magnetic phase transitions. The observed step structure in the d M / d B curve is consistent with the magnetic phase diagram [4] although we observed a structure of not three-steps but two-steps. Thus, the step structure observed -at 3.8 and 4.6 T is concluded to be caused by successive phase transitions from non- magnetic phase into thermally frustrating, incommen- surate phase and into commensurate three-sublattice antiferromagnetic phase.
Figure 2 shows the spin structure of the field-induced
3D antiferromagnetic ordering in CsFeCls, where both the transverse and longitudinal spin components of the
by arrows in figure step is observed at 3'8 and 4'6
Fig. 2. - Triangle spin structure of CsFeC13 antiferromag- when LRO appears. The same step structure is also net appeared by 5-11 T longitudinal fields. Arrows repre- observed when the LRO disappears by increasing the sent directions of spin moments. Spins are ferroma~neti-
-
field: step appears a t 11.2 and 11.6 T. cally coupled along c-axis.C8 - 1444 JOURNAL DE PHYSIQUE
ground state, (S,)
,
(S,) and (S,),
are not zero and the components of spin moment in c-plane make an angle of 120' to each other by the spin frustration.2. High field spin ordering
When magnetic field is applied parallel to c-axis, an additional abrupt increase of magnetization is ob- served between 32 and 33 T. As shown in figure 1, a two-step magnetization process is observed. A close examination of susceptibility reveals a fine structure. This is shown in the insert of figure 1: at least six peaks are observed a t 3.206; 3.222, 3.237, 3.247, 3.268 and 3.273 T. This suggests presence of successice.phase transitions as the case of low field spin ordering.
The high field magnetization cannot be explained within a frame of the fictitious S' = 1 spin, which was used to explain the low field magnetization. Since magnitude of magnetization at 33 T is large, i.e. M =
4.1 l . l ~ / ~ e 2 + [5], it seems the anomalous increase of magnetization is caused by the upper excited S' = 2 spin state. Thus, the S' = 2 state has a level cross- ing with the ground S' = 1 state. In fact, one of the present authors explains the fine structure of fig-
ure 1 using a model of exchange interaction between the S' = 1 and S' = 2 spins [6].
3. Magnetization in transverse fields
Unlike the case of the longitudinal field (B
//
c),
no anomalous increase of magnetization is observed. This suggests non-appearance ofLRO
under the transverse field. As shown in figure 3, the magnetization increases with increasing field below 3 T, while above 10 T it increases quite slowly reflecting the Van Vleck suscep- tiblity. A saturation of magnetization is obtained in the high field region after subtracting the Van Vleck paramagnetism. From the saturation, the transverse gvalue is estimated as g ~ . = 3.6.
Non-appearance of
LRO
is understood from energy level diagram of Fe2+ in CsFeCls. The energy dia- gram is obtained usingt
= 0.9, ] X I = 103 cm-l and0
0
-*
MAGNET l C Fl ELD (
T
) Fig. 3. - Magnetization measured at 1.3 K under the trans- verse fields. Inset shows a calculated electron spin energy diagram of ~ e in CsFeCls. Lowest three levels belong to ~ + the fictitious S' = 1 spin, whereas upper five levels theS' = 2 spin.
I S I k X I = 2 for orbital reduction factor, spin orbit cou- pling constant and magnitude of trigonal distortion, respectively [5], which is shown in figure 3. As seen in the diagram, the singlet ground level is away from all the excited levels with increasing field and never crossed by any level. Therefore no
LRO
is expected under the transverse field. The observed field depen- dence of transverse magnetization can be explained us- ing a molecular Geld approximation. The details will be published elsewhere.[I] Yoshizawa, H., Kozukue, W. and Hirakawa, K., J. Phys. Soc. Jpn 49 (1980) 1.44.
[2] Chiba, M., Ajiro, Y., Adachi, K. and Morimoto, T., J. Phys. Soc. Jpn 57 (1988) 3178.
[3] Tsuneto, T. and Murao, T., Physica 51 (1971) 186.
[4] Haseda, T., Wada, N., Hata, :M. and Amaya, K., Physica 108B (1981) 841.
[5] Chiba, M., Tsuboi, T., Hori, 11., Shiozaki, I. and Date, M., Solid State Commwn. 63 (1987) 427.
