Magnetic ordering of the frustrated 1dim-chain system Tb₂Cl₃

Low dimensional magnetism and magnetic frustration have attracted particular attention because they brought about a number of unusual and unexpected phenomena in the field of magnetism. Metal-rich rare earth compounds which exhibit highly anisotropic crystal structures with low dimensional building blocks are prime candidates to look for such phenomena.

Tb$$₂Cl$$₃ belongs to the metal-rich halides of the rare earth metals and crystallizes in a highly anisotropic structure which containing parallel chains of trans-edge sharing Tb atom octahedra with a high degree of topological frustration for the apical Tb2 atoms (Fig.1).

Figure 1: Chain of trans-edge sharing metal octahedra cut out of the structure of Tb$$₂Cl$$₃. 1 and 1' label the metal atoms in the shared edge; 2 and 2' label the apical metal atoms. The chain axis is along [010].

The bulk magnetic properties indicated long-range afm ordering below about 47K and typical features for low-dimensional magnetic behavior above the transition with a broad maximum in the magnetic susceptibility (Fig. 2). Single crystal susceptibility point to a second magnetic transition at 4K.

Figure 2: Magnetic susceptibility of a powder sample of Tb$$₂Cl$$₃. The broad shoulder indicates magnetic short range ordering.

The magnetic ordering behavior of Tb$$₂Cl$$₃ was investigated by a detailed powder neutron diffraction experiment between 1.5K and 250K in collaboration with the LNS at PSI, Villigen (Switzerland).

Figure 3: Observed (solid line) and calculated (dashed line) neutron diffraction   pattern   of Tb$$₂Cl$$₃ at 1.5K; neutron wavelength $\lambda$=1.7031$\AA$. The reflections used in the calculation are marked by the vertical bars; upper: nuclear; lower: magnetic reflections.

Below 47K the neutron powder diffraction patterns exhibit magnetic Bragg reflections with a complex temperature dependence of the intensities at lower temperature. In addition, a broad diffuse shoulder is observed between d=9.0$\AA$ and 3.9$\AA$ (Fig. 3).

Figure 4: Refined magnetic moments and angle Theta ($o$: Tb1; Tb2) of the two inequivalent Tb atoms in Tb$$₂Cl$$₃. $\Theta$: angle enclosed by the moments and the c-axis.

A satisfying Rietveld refinement of the diffraction patterns was achieved in the magnetic space group Pc2/m' by confining the moments into the $a$-$c$ plane, perpendicular to the chains. The diffuse shoulder was treated as background and subtracted.

The refined magnetic moments of the Tb1 atoms saturate towards low temperatures at 9.3(2)mu_B whereas the moments at Tb2 below 10K undergo a decrease and reorientation which is ascribed to an increasing magnetic frustration of the apical positions (Fig. 4). The origin of the diffuse shoulder is unclear at present but too strong to be attributed to magnetic defects only. There is a clear correlation between the temperature dependence of the apical moments and the intensity of the diffuse shoulder. We therefore rather ascribe the diffuse shoulder to the intrinsic magnetic behavior of Tb$$₂Cl$$₃.

( R.K. Kremer, U. Beck, A. Simon and P. Fischer)

From the yearbook of the institute ("Wissenschaftlicher Tätigkeitsbericht") 1994

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