Approaching the true ground state of frustratedA-site spinels: A combined magnetization and polarized neutron scattering study

Abstract

We re-investigate the magnetically frustrated, diamond-lattice-antiferromagnet spinels FeAl${}_{2}$O${}_{4}$ and MnAl${}_{2}$O${}_{4}$ using magnetization measurements and diffuse scattering of polarized neutrons. In FeAl${}_{2}$O${}_{4}$, macroscopic measurements evidence a ``cusp'' in zero field-cooled susceptibility around 13 K. Dynamic magnetic susceptibility and 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 FeAl${}_{2}$O${}_{4}$. 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-neighbor shell. The estimated value of the Land\'e $g$ factor points towards orbital contributions from Fe${}^{2+}$. This is also supported by a Curie-Weiss analysis of the magnetic susceptibility. MnAl${}_{2}$O${}_{4}$, on the contrary, undergoes a magnetic phase transition into a long-range ordered state below $\ensuremath{\approx}$40 K, which is confirmed by macroscopic measurements and polarized neutron diffraction. However, the polarized neutron studies reveal the existence of prominent spin fluctuations co-existing with long-range antiferromagnetic order. The magnetic diffuse intensity suggests a similar short-range order as in FeAl${}_{2}$O${}_{4}$. Results of the present work support 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.