Unconventional magnetic phase separation in γ-CoV2O6

Abstract

We have explored the magnetism in the non-geometrically frustrated spin-chain system γ-CoV2O6 which possesses a complex magnetic exchange network. Our neutron diffraction patterns at low temperatures (T T N = 6.6 K) are best described by a model in which two magnetic phases coexist in a volume ratio 65(1) : 35(1), with each phase consisting of a single spin modulation. This model fits previous studies and our observations better than the model proposed by Lenertz et al in J. Phys. Chem. C 118, 13981 (2014), which consisted of one phase with two spin modulations. By decreasing the temperature from T N , the minority phase of our model undergoes an incommensuratecommensurate lock-in transition at T * = 5.6 K. Based on these results, we propose that phase separation is an alternative approach for degeneracy-lifting in frustrated magnets.

Magnetic microphase inhomogeneity as a thermodynamic precursor of ground-state phase separation in weakly coupled spin- 32 chains

“…The k 1 phase disappears at T N1 = 6.6 K, while the Néel temperature of the k 2 phase (T N2 ) could not be unambiguously determined by NPD [13]. This is due to the prevailing magnetic diffuse scattering that emerges around the k 2 modulated Bragg positions above T and remains observable up to at least 25 K [13]. In the same region, another investigation, using magnetometry and inelastic neutron scattering (INS), has revealed strong ferromagnetic (FM) fluctuations in this compound [16].…”

“…In the ground state, our neutron powder diffraction (NPD) study showed that γCVO possesses two spatially separated magnetic order modulated by k 1 = ( 1 2 , 0, 0) and k 2 = ( 1 4 , 0, -1 4 ), respectively [13,15]. As the temperature increases, the k 2 phase undergoes a commensurate-incommensurate transition at T = 5.6 K, accompanied by the loss of the long-range spin correlations [13]. The k 1 phase disappears at T N1 = 6.6 K, while the Néel temperature of the k 2 phase (T N2 ) could not be unambiguously determined by NPD [13].…”

“…In quasi 1D magnets, magnetic phase separation is often dynamic [9,10] or appears in the critical region of a first-order transition [11,12] and therefore regarded as metastable. Static phase separation in the long-range ordered magnetic ground state, on the other hand, is much rarer [13]. Moreover, thermodynamic information about the phase separation, which is essential to extracting the fundamental physics in relevant systems [7,8], has not been understood in any quasi 1D case so far.…”

“…The triclinic cobaltate compound γ-CoV 2 O 6 (γCVO) has weakly coupled zigzag chains of Co running along the b axis; each chain is composed of two crystallographically inequivalent Co 2+ (S = 3 2 ) cations, Co (1) and Co (2), in a ratio of 1 : 2 [13,14]. In the ground state, our neutron powder diffraction (NPD) study showed that γCVO possesses two spatially separated magnetic order modulated by k 1 = ( 1 2 , 0, 0) and k 2 = ( 1 4 , 0, -1 4 ), respectively [13,15]. As the temperature increases, the k 2 phase undergoes a commensurate-incommensurate transition at T = 5.6 K, accompanied by the loss of the long-range spin correlations [13].…”

“…As the temperature increases, the k 2 phase undergoes a commensurate-incommensurate transition at T = 5.6 K, accompanied by the loss of the long-range spin correlations [13]. The k 1 phase disappears at T N1 = 6.6 K, while the Néel temperature of the k 2 phase (T N2 ) could not be unambiguously determined by NPD [13]. This is due to the prevailing magnetic diffuse scattering that emerges around the k 2 modulated Bragg positions above T and remains observable up to at least 25 K [13].…”