Revisiting the ground state of CoAl2O4 : Comparison to the conventional antiferromagnet MnAl2O

Abstract

The A-site spinel material, CoAl2O4, is a physical realization of the frustrated diamond-lattice antiferromagnet, a model in which unique incommensurate or 'spin-spiral liquid' ground states are predicted. Our previous single-crystal neutron scattering study instead classified it as a 'kineticallyinhibited' antiferromagnet, where the long ranged correlations of a collinear Néel ground state are blocked by the freezing of domain wall motion below a first-order phase transition at T* = 6.5 K. The current paper provides new data sets from a number of experiments, which support and expand this work in several important ways. We show that the phenomenology leading to the kinetically-inhibited order is unaffected by sample measured and instrument resolution, while new low temperature measurements reveal spin correlations are unchanging between T = 2 K and 250 mK, consistent with a frozen state. Polarized diffuse neutron measurements show several interesting magnetic features, which can be entirely explained by the existence of short-ranged Néel order. Finally, and crucially, this paper presents some of the first neutron scattering studies of single crystalline MnAl2O4, which acts as an unfrustrated analogue to CoAl2O4 and shows all the hallmarks of a classical antiferromagnet with a continuous phase transition to Néel order at TN = 39 K. Direct comparison between the two compounds indicates that CoAl2O4 is unique, not in the nature of high-temperature diffuse correlations, but rather in the nature of the frozen state below T * . The higher level of cation inversion in the MnAl2O4 sample indicates that this novel behavior is primarily an effect of greater next-nearest-neighbor exchange.PACS numbers: 75.30.Ds, 75.50.Lk, 78.70.Nx The A-site spinels, AB 2 X 4 with A magnetic, have seen a surge of interest in the past decade, due to a series of interesting experimental observations 1-15 and theoretical predictions of novel spin-liquid ground states [16][17][18][19][20] . Magnetic cations in these materials comprise a bi-partite diamond lattice, and novel behavior is argued to be the result of a competition between nearest (J 1 ) and nextnearest (J 2 ) neighbor superexchange interactions 2,21 . This is demonstrated explicitly by the calculations of Bergman et al. 16 , who have shown for spin-only materials that the collinear Néel structure favored by J 1 is progressively destabilized with increasing J 2 , until a Lifshitz point is encountered at J2 J1 = 1 8 . For greater J 2 , the ground state is predicted to be a novel 'spiral spin liquid' (SSL) state characterized by fluctuations between an infinitely degenerate set of incommensurate spin spirals, whose propagation wavevectors form a series of calculable manifolds in reciprocal space ('spiral surfaces'). Further calculations [16][17][18] predict that these mass degeneracies are lifted by low-lying thermal or quantum fluctuations, driving first-order phase transitions via the order-by-disorder mechanism 22 to either unique spin spiral or Néel ordered states, depending o...

Fractional antiferromagnetic skyrmion lattice induced by anisotropic couplings

“…Strong INS intensities are observed, emanating from the magnetic Bragg reflections that correspond to the propagation vector q = (0.75, 0.75, 0) and reaching a maximal energy of E ≈ 0.9 meV at wavevector Q ≈ 0.9 Å −1 . Compared to other similar spinel compounds [33][34][35] , the magnon dispersion bandwidth in MnSc 2 S 4 is narrower, consistent with its relatively low ordering temperature of T N = 2.3 K (refs. 28,29 ).…”

Chemical and physical pressure effects in the A-site spinel antiferromagnets CoM2O4 (M = Al, Co, and Rh)

“…Remarkably, the magnetic transition temperature of the cobalt aluminate CoAl 2 O 4 , which is a partially inverted A-site spinel oxide, is strongly suppressed to T m =8.5 K compared with the Weiss temperature θ=−95 K [4][5][6][7]. It is revealed by neutron diffraction measurements [8,9] that the magnetic state of CoAl 2 O 4 is characterized to be in the Néel state but in the vicinity of the phase boundary at J 2 /J 1 =1/8. The recent discovery that the T m for CoAl 2 O 4 increases with increasing pressure with a pressure coefficient (1/T m )(dT m /dp) equal to 0.044 GPa −1 [7] suggests that T m (i.e., the degree of frustration for the A-site cobalt spinel system) can be controlled by application of pressure.…”

Evolution of magnetic and orbital properties in the magnetically diluted A -site spinel Cu1−xZnxRh2Zakrzewski1, Gangopadhyay2, MacDougall3 et al. 2018Phys. Rev. BSelf Cite11160View full textAdd to dashboardCite

“…Although the preferred method for exploring fluctuations and exotic spin physics in A-site spinels has been detailed measurements of neutron diffuse correlations [13,14,22,23], the lack of large single crystals of CuRh 2 O 4 has eliminated that route of exploration as a viable option. In the current Article, we instead study the effect of systematic site dilution on spin, orbital and thermodynamic properties of this compound.…”