Spin-Lattice Order in FrustratedZnCr2O4

Abstract

Using synchrotron X-rays and neutron diffraction we disentangle spin-lattice order in highly frustrated ZnCr2O4 where magnetic chromium ions occupy the vertices of regular tetrahedra. Upon cooling below 12.5 K the quandary of anti-aligning spins surrounding the triangular faces of tetrahedra is resolved by establishing weak interactions on each triangle through an intricate lattice distortion. The resulting spin order is however, not simply a Néel state on strong bonds. A complex co-planar spin structure indicates that antisymmetric and/or further neighbor exchange interactions also play a role as ZnCr2O4 resolves conflicting magnetic interactions. PACS numbers:While tetrahedral atomic clusters are a natural consequence of close packing, they are particularly inconvenient for antiferromagnetically interacting spins. This is because no spin configuration can simultaneously satisfy all six antiferromagnetic interactions amongst spins on the vertices of a tetrahedron [1,2,3,4,5]. The consequence of such "geometrical frustration" is deep suppression of magnetic order and a range of temperatures where spins remain fluctuating despite interactions that far exceed thermal energies [6,7]. Indeed for spins on a lattice of corner-sharing tetrahedra, it appears there is no conventional order in the quantum limit (S = 1/2, T = 0) [5]. Because they entail higher energy spin configurations, geometrically frustrating lattices however typically do not survive in the low temperature limit. Instead a compromise between spin and lattice energy is reached through a first order phase transition that freezes the spin liquid and distorts the lattice [8,9,10,11,12,13]. Such phase transitions challenge conventional theories of magnetism because they involve strongly correlated spins and the collapse of the rigid lattice approximation [14,15,16].A case in point is ZnCr 2 O 4 . At room temperature, it has a cubic F d3m crystal structure where Cr 3+ (S = 3/2) ions form a network of corner-sharing tetrahedra [9]. The Curie-Weiss temperature is -390 K indicating strong antiferromagnetic frustration, yet chromium spins remain in a cooperative paramagnetic phase down to T C = 12.5 K [6,9]. There, a first order phase transition from a cubic paramagnet to a tetragonal antiferromagnet signals the end of distinct spin and lattice degrees of freedom. Tetragonal strain energy alone does not account for the difference between magnetic energy gain and overall latent heat and this was a first indication of a more comprehensive rearrangement of the lattice [9]. Subsequently X-ray superlattice peaks were detected at ( 2 ) c type reflections (see Fig. 1 (a)) [17]. This indicates that below T N the tetragonal lattice has I4m2 symmetry and a √ 2 × √ 2 × 2 chemical unit cell [18]. Theoretical efforts to understand the nature of the phase transition have focused on magneto-elastic couplings that involve symmetric isotropic nearest neighbor (NN) exchange interactions [14,15,16].Here we report a combined synchrotron X-ray and magnetic neutron diffraction study ...

Structure and magnetism in the bond-frustrated spinelZnCr2Se4

“…Further studies conducted between the Curie-Weiss and Néel temperatures revealed the presence of negative thermal expansion below 100 K [16,23], shifting of the magnetic resonance line [20], anomalous behavior of eigenfrequencies of phonon modes [16], and deviation from Curie-Weiss behavior [1], making further investigation with muon spin-relaxation measurements attractive. The presence of local magnetic correlations is well established in the analogous oxide, ZnCr 2 O 4 [24][25][26], but it possesses, in contrast to selenide, a large negative Curie-Weiss constant. Therefore, the suppression of long-range magnetic order in ZnCr 2 O 4 (θ CW /T N = 32) is the result of inherent geometric frustration and not only from bond frustration.…”

Ground State Selection and Spin-Liquid Behaviour in the Classical Heisenberg Model on the Breathing Pyrochlore Lattice

“…As a consequence these materials have emerged as canonical examples of the phenomenon of "order-by-distortion" in which magnetic frustration is relieved by a structural transition. [16][17][18][19] Calculations based on a classical effective model for the spins with coupling to the lattice predict a rich phase diagram in an applied magnetic field [20][21][22][23][24] which was observed in a series of remarkable experiments at fields up to 600T. [25][26][27][28][29] owen.benton@oist.jp Recently, a new class of magnetic materials has been synthesised which promises an interesting variation on this picture.…”

Competing Jahn-Teller distortions and ferrimagnetic ordering in the geometrically frustrated systemNi1−xCuxCReehuis1, Tovar2, Többens3 et al. 2015Phys. Rev. B3710450View full textAdd to dashboardCite

“…However, in the highly frustrated chromites MgCr2O4, ZnCr2O4, CdCr2O4, and HgCr2O4 antiferromagnetic ordering sets in at the Néel temperatures TN = 12.9, 12.5, 7.8, and 5.8 K, respectively. [6][7][8][9] Due to the presence of magnetoelastic effects a change to a lower crystal structure symmetry was found at these temperatures which relieve the frustration and enabling the magnetic order. 7,8 by high-resolution x-ray synchrotron powder diffraction that the change from the tetragonal to an orthorhombic structure (space group Fddd) occurs concurrently with the onset of ferrimagnetic order.…”