Linear magnetoelectric effect as a signature of long-range collinear antiferromagnetic ordering in the frustrated spinel CoAl2O4

Abstract

The ground state of the frustrated A-site magnetic spinel CoAl 2 O 4 has been a controversial issue whether it is a collinear antiferromagnetic ordering or a spiral spin -liquid state, as the ratio of the two competing interactions, J 2 /J 1 lies close to the boundary between these two ground states.Here, we address the magnetic ground state in CoAl 2 O 4 with different amount of Co 2+ /Al 3+ site disorder from the study of magnetoelectric effect and Monte Carlo simulations. CoAl 2 O 4 with low site disorder exhibits linear magnetoelectric effect below the magnetic ordering temperature.With increasing disorder, the magnetoelectric effect is suppressed and the sample with 14% disorder exhibits a spin glass behavior without the magnetoelectric effect. Monte Carlo simulations support the experimental findings and suggest that the site disorder suppresses longrange antiferromagnetic order and induces a spin glass state. Since the linear magnetoelectric effect requires a long -range magnetic ordering, we suggest that the ground state of CoAl 2 O 4 with low site disorder is a collinear antiferromagnet. PACS number(s): 75.47.Lx, 75.50.Ee, 75.85.+t *E-mail: sundaresan@jncasr.ac.in spin -liquid states [1,2]. Many contradicting experimental results have been reported regarding the true nature of the magnetic ground state of CoAl 2 O 4 . In an earlier report by W. L. Roth, using neutron diffraction experiments on a polycrystalline sample of Co 1-x Al x [Al 2-x Co x ]O 4 with an antisite disorder of x = 0.05, which hereafter will be represented as CAO:0.05, a collinear antiferromagnetic ordering of Co 2+ spins at 4.2 K was suggested from the observation of a weak magnetic Bragg peak at (200) position in neutron diffraction pattern, similar to the arrangement of Co 2+ spins in Co 3 O 4 [5]. Later, a spin glass state was proposed using magnetic, electron spin resonance and heat capacity measurements in CAO:0.08 [8]. Recently, Kenataro Hanashima et al. investigated a series of compounds with different degree of disorder (x) and have shown that the spin glass state is present with higher degree of disorder (x  0.10), while the magnetic state changes to spiral spin -liquid state as it becomes more ordered (x  0.06) [9]. Based on inelastic neutron diffraction experiments on CAO:0.07-0.08, Krimmel et al. have shown that J 2 /J 1 ~ 0.17 > 1/8 and concluded the presence of spiral spin -liquid state with a very strong but short -range spin correlations [3]. The presence of spin -liquid like state is also supported by the results of the neutron diffraction experiments on single crystal of CAO:0.08 by Zaharko et al.[2]. Later, they reported that the magnetic ordering is unconventional and its bulk magnetic properties are isotropic [10]. On the other hand, it has been shown that these properties are anisotropic microscopically as inferred from neutron diffraction experiments under magnetic fields [10]. On the same sample, using local spin probe techniques such as electron spin resonance, nuclear magnetic resonance and muon spin relaxat...

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