We re-investigate the magnetically frustrated, {\it diamond-lattice-antiferromagnet} spinels FeAl2O4 and MnAl2O4 using magnetization measurements and diffuse scattering of polarized neutrons. In FeAl2O4, macroscopic measurements evidence a "cusp" in zero field-cooled susceptibility around 13~K. Dynamic magnetic susceptibility and {\it memory effect} experiments provide results that do not conform with a canonical spin-glass scenario in this material. Through polarized neutron scattering studies, absence of long-range magnetic order down to 4~K is confirmed in FeAl2O4. By modeling the powder averaged differential magnetic neutron scattering cross-section, we estimate that the spin-spin correlations in this compound extend up to the third nearest-neighbour shell. The estimated value of the Land\'{e} g factor points towards orbital contributions from Fe2+. This is also supported by a Curie-Weiss analysis of the magnetic susceptibility. MnAl2O4, on the contrary, undergoes a magnetic phase transition into a long-range ordered state below ≈ 40~K, which is confirmed by macroscopic measurements and polarized neutron diffraction. However, the polarized neutron studies reveal the existence of prominent spin-fluctuations co-existing with long-range antiferromagnetic order. The magnetic diffuse intensity suggests a similar short range order as in FeAl2O4. Results of the present work supports the importance of spin-spin correlations in understanding magnetic response of frustrated magnets like A-site spinels which have predominant short-range spin correlations reminiscent of the "spin liquid" state.
@article{arxiv.1405.4694,
title = {Approaching the Ground State of Frustrated A-site Spinels: A Combined Magnetization and Polarized Neutron Scattering Study},
author = {Harikrishnan S. Nair and Zhendong Fu and Jörg Voigt and Yixi Su and Th. Brückel},
journal= {arXiv preprint arXiv:1405.4694},
year = {2015}
}