Competing magnetic interactions in the extended Kagomé systemYBaCo4O7

Abstract

YBaCo4O7 belongs to a new class of geometrically frustrated magnets like the pyrochlores, in which Co-spins occupy corners of tetrahedra. The structure can be viewed as an alternating stacking of Kagomé and triangular layers. Exactly half of the triangular units of the Kagomé plane are capped by Co ions to form columns running perpendicular to the Kagomé sheets. Neutron powder diffraction reveals a broad temperature range of diffuse magnetic scattering, followed by long range magnetic ordering below 110K. A unique low-temperature magnetic structure simultaneously satisfies an S=0 arrangement in the uncapped triangular units and antiferromagnetic coupling along the columns. A spin reorientation above 30K tracks the relative strengths of the in-plane and out-of-plane interactions.PACS numbers: 25.40. Dn, 75.25.+z, Magnetic frustration has attracted considerable interest over the last 50 years [1,2,3]. Whether the frustration arises from competing exchange interactions or the peculiar geometry of the spin lattice, it invariably leads to unconventional magnetic states at low temperatures. Frustrated systems commonly exhibit the persistence of strong spin fluctuations at low temperatures and, at least, partial suppression of the magnetic order. Various spin states can be stabilized below the cooperative paramagnetic regime, such as spin liquid, spin glass or spin ice. In some cases, long range magnetic order is established, either by structural distortions that lift the ground state degeneracy, or by the "order-bydisorder" mechanism [4]. Kagomé antiferromagnets have been widely studied, in particular because a spin-liquid state is predicted at low temperatures for the S=1/2 Heisenberg system. However, real Kagomé lattices display either spin glass behavior or long range order with the well known propagation vectors k=0 and k=( 1 3 , 1 3 ) (often referred to as k=( √ 3, √ 3)) structures [5,6]. More exotic ground states, including incommensurate spin density waves and cycloidal structures, have been found in related lattices such as the so-called Kagome-staircase [7].Recently, a new class of geometrically frustrated magnets, with formula RBaCo 4 O 7 (R=Y,rare-earth ion) has been reported [8,9,10]. The crystal structure is built up of alternating Kagomé and triangular cobalt lattices, in a similar way as SrCr 9x Ga 12−9x O 19 (SCGO) [11], but with magnetic Co 2+ /Co 3+ ions in a 3:1 ratio on both crystallographic sites (S=1.625 on average). The magnetic network in RBaCo 4 O 7 is reminiscent of that of hexagonal ice 1h [12], but with one important difference: here, half of the triangles in the Kagomé sheets are bi-capped by Co atoms of the adjacent triangular layers, providing magnetic super-exchange interactions along the third direction. An added element of interest is that this network is built of corner sharing CoO 4 tetrahedra, as opposed to the more common octahedral or pyramidal arrangement of other frustrated magnetic oxides. The ground state of this new topology is yet unknown, although one would suspect that th...

spinvert: a program for refinement of paramagnetic diffuse scattering data

“…Perhaps most significant is a study reporting single-crystal neutron scattering [43]. Just above T N , at T = 130 K, broad diffuse features were observed in the (hk0) reciprocal-space plane, indicating short-range magnetic order within the kagome planes.…”

Spin-liquid phase and order by disorder of classical Heisenberg spins on the swedenborgite lattice

“…The magnetic Co atoms in this structure reside on the so-called swedenborgite lattice and at present there exist examples of systems that order magnetically and some that do not [15][16][17][18][19][20][21][22]. In this Rapid Communication, we undertake an in-depth study of a minimal model for these compounds and show evidence that both classical spin-liquid phases as well as entropic order-by-disorder are potentially existent in these systems.…”

Controlling structural distortion in the geometrically frustrated layered cobaltate YBaCo4O7+δby Fe substitution and its role on magnetic correlations

“…The compound YBaCo 4 O 7+δ crystallizes in the trigonal structure (space group P31c) [13]. With decreasing temperature, a crystal structural transition occurs from the trigonal phase (P31c) to orthorhombic phase (Pbn2 1 ) at ~ 310 K and gives rise to a regime of short-range magnetic order [5,18]. With further cooling, a magnetic transition to long-range antiferromagnetic (AFM) order state occurs at ~ 110 K [5].…”

“…The geometry of the lattice structures causes spin frustrations allowing to tune magnetic properties by controlling the lattice geometry. In this regards, the new class of geometrically frustrated layered mixed-valence cobaltate YBaCo 4 O 7+δ and its derivatives with an extended Kagomé structure, belonging to the swedenborgite compound family [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16], are of interest to us.…”