Frustrated Magnetism of Pharmacosiderite Comprising Tetrahedral Clusters Arranged in the Primitive Cubic Lattice

Abstract

We show that pharmacosiderite is a novel cluster antiferromagnet comprising frustrated regular tetrahedra made of spin-5/2 Fe 3+ ions that are arranged in the primitive cubic lattice. The connectivity of the tetrahedra and the inter-cluster interaction of 2.9 K, which is significantly large compared with the intra-cluster interaction of 10.6 K, gives a unique playground for frustration physics. An unconventional antiferromagnetic order is observed below TN ~ 6 K, which is accompanied by a weak ferromagnetic moment and a large fluctuation as evidenced by Mössbauer spectroscopy. A q = 0 magnetic order with the total S = 0 for the tetrahedral cluster is proposed based on the irreducible representation analysis, which may explain the origin of the weak ferromagnetism and fluctuation.

Dimensional reduction by geometrical frustration in a cubic antiferromagnet composed of tetrahedral clusters

“…Figure 3d shows the magnetic contribution at 1.6 K; a nuclear contribution has been subtracted using the 10 K data as a reference. No structural transition was observed down to 1.6 K (Supplementary Note 1); hence, the cubic symmetry was preserved, which is in agreement with previous X-ray diffraction experiments 16 . The magnetic peaks have nearly the same widths as the nuclear peaks (Supplementary Note 4), indicative of a long-range magnetic order; the magnetic correlation length estimated is 360 Å, which is 45 times larger than the a-axis length.…”

“…Interestingly, however, previous Mössbauer spectroscopy measurements depicted an broad spectrum even in the LRO state (Fig. 3b) 16 . Such broadening could arise from structural inhomogeneity (Supplementary Note 1).…”

“…The intra-and intercluster magnetic interactions, J and J′, are estimated to be AF with magnitudes of 10.6 and 2.9 K, respectively, from the fitting of the magnetic susceptibility (Fig. 3a) 16 . Notably, there is a strong frustration in both the tetrahedral cluster and the intercluster couplings because the four J′ paths form an elongated tetrahedron along the cube edges.…”

“…However, we should note that the large nuclear R wp factor of~27%, which mostly comes from partial deuteration, suggests that the standard deviation of the ordered moment value derived from the Rietveld analysis is somewhat underestimated (Supplementary Note 1). 16 . The black line is a fit to a Blume-Tjon model 35 taking into account fluctuating local magnetic fields (Supplementary Note 2).…”

“…In this paper, we report a unique 3D tetrahedral-cluster antiferromagnet, pharmacosiderite ((H 3 O)Fe 4 (AsO 4 ) 3 (OH) 4 •5. 5H 2 O) 15,16 , in which low dimensionality is not present in the crystal structure but emerges purely by geometrical frustration without lattice distortions.…”