Frustrated antiferromagnetic interactions in the new, b-Mn related, compounds Mn₃IrGe and Mn₃CoSi

T. Eriksson¹, L. Bergqvist², P. Nordblad³, O. Eriksson² and Y. Andersson¹

¹ Dept. Materials Chemistry, Uppsala Univ., Box 538, SE-751 21 Uppsala, Sweden

² Dept. Physics, Uppsala Univ., Box 530, SE-751 21 Uppsala, Sweden

³ Dept. Materials Science, Uppsala Univ., Box 534, SE-751 21 Uppsala, Sweden

Two new compounds, Mn₃IrGe and Mn₃CoSi, have been synthesised. Both order magnetically at low temperatures, but although they are isostructural, the magnetic structures are completely different. The observation of magnetic order is very interesting, since the compounds have a crystal structure similar to that of b‑manganese [1]. The b-Mn phase has been the focus of much attention, since it does not order magnetically. This is suggested to be an effect of geometric frustration of antiferromagnetic interactions on a triangular network of Mn atoms (a 3D analogue of the 2D kagomé net) [2]. However, the same triangular Mn network exists in Mn₃IrGe and Mn₃CoSi, and our results therefore shed new light on the understanding of b-Mn.

The crystal structure of Mn₃IrGe and Mn₃CoSi is of the AlAu₄-type [3], an ordered form of the b-manganese structure. The compounds are thus isostructural with the recently reported phase Mn₃IrSi [4]. Structure refinements by the Rietveld method, using the program FULLPROF [5], for neutron powder diffraction data collected at room temperature, gave the final agreement factors R_profile=4.23%, R_Bragg=4.04% for Mn₃IrGe and R_profile=3.56%, R_Bragg=5.84% for Mn₃CoSi.

Powder neutron diffraction results show noncollinear antiferromagnetic order for Mn₃IrGe at temperatures below 225 K, with a magnetic unit cell of the same size as the crystallographic cell. Structure refinements by the Rietveld method gave the final agreement factors R_profile=4.74%, R_Bragg=5.54%, R_magnetic=5.55%, for data collected at 10 K, with the magnetic moment 3.39(4) µ_B on Mn. The magnetic structure is very similar to what was previously reported for Mn₃IrSi [4].

(a)	(b)
Fig. 1. (a) The crystal structure of Mn3IrGe, with the network of corner linked triangles (shaded) of near neighbour Mn atoms. One Mn atom outside the unit cell (smaller) is added for clarity. (b) The orientations of the magnetic moments on a unit of three corner linked Mn triangles. Interatomic distances at 10 K are indicated.

In the crystal structure, the near neighbour Mn atoms are found on a network of corner linked triangles, where each corner (=Mn atom) is shared between three triangles, see Fig. 1a. The magnetic moments on Mn are oriented so that their projections onto the triangle planes have 120-degree angles between them, see Fig. 1b. In accord with the argument in Ref. 4, we suggest this to be the result of geometrically frustrated antiferromagnetic interactions.

A complete range of solid solubility exists for Mn₃IrSi_1‑xGe_x, as demonstrated by a linear variation of the unit cell parameter (obtained from powder x-ray diffraction films, recorded by a Guinier-Hägg camera). The magnetic susceptibility, measured by SQUID-magnetometry, is weakly temperature dependent, and the antiferromagnetic transition temperature, 225±10 K, does not show any large variation with the silicon content for the solid solution Mn₃IrSi_1‑xGe_x, see Fig. 2. This could be explained by suppression of the transition temperature, caused by geometric frustration of strong antiferromagnetic interactions on the triangular network of Mn atoms.

Fig. 2. Magnetic susceptibility (c) vs. temperature for the substitution series Mn₃IrSi_1‑xGe_x (x=0.00; 0.10; 0.30; 0.50; 0.75; 1.0), measured in an applied field of 250 Oe.

Measurements of the magnetic susceptibility vs. temperature for Mn₃CoSi show a maximum at 110 K, indicating an antiferromagnetic transition. In contrast to the magnetic structures of Mn₃IrGe and Mn₃IrSi, low temperature neutron powder diffraction patterns for the isostructural compound Mn₃CoSi show satellite peaks characteristic of an incommensurate magnetic structure. The transition from commensurate magnetic order in Mn₃IrSi to incommensurate order in Mn₃CoSi is currently under study for the solid solution Mn₃Ir_1‑xCo_xSi. Interestingly, by a suitable choice of x, a compound with the same cell parameter as b-Mn can be produced, which may provide some insight into the governing parameters behind the observed lack of magnetic order in b-Mn.

1. G. D. Preston, Phil. Mag. 5 (1928) 1207; C. B. Shoemaker et al., Acta Crystallogr. B34 (1978) 3573.

2. H. Nakamura, et al., J. Phys.: Condens. Matter 9 (1997) 4701.; J. R. Stewart, et al. Phys. Rev. B 59 (1999) 4305; J. R. Stewart, et al., J. Magn. Magn. Mater., in press.

3. O. E. Ullner, Arkiv Kemi, Mineral., Geol. 14A (1940) 1.

4. T. Eriksson et al., Phys. Rev. B 69 (2004) 054422.

5. J. Rodríguez-Carvajal, FULLPROF computer program, version 2.45, LLB, Saclay (2003).