Study of negative thermal expansion in the frustrated spinel ZnCr2Se4

Abstract

The origin of negative thermal expansion (NTE) in the bond frustrated ZnCr 2 Se 4 has been explored. ESR and FTIR document an ideal paramagnetic state above 100 K, below which ferromagnetic clusters coexist with the paramagnetic state down to T N . By fitting the inverse susceptibility above 100 K using a modified paramagnetic Curie-Weiss law, an exponentially changeable exchange integral J is deduced. In the case of the variable J, magnetic exchange and lattice elastic energy couple with each other effectively via magnetoelastic interaction in the ferromagnetic clusters, where NTE occurs at a loss of exchange energy while a gain of lattice elastic one.

Interactions in the bond-frustrated helimagnet ZnCr2Se4 investigated by NMR

“…This means that either there should be an unquenched L or that the orbital is not exactly spherical. The g-factor measured along the [111] direction of a single crystal 8 is identical to that obtained from poly-crystals 12 , 1.996. Therefore, the unquenched orbital angular momentum is negligible and the most plausible cause of the anisotropic hyperfine field is the spin in the non-spherical d orbital.…”

“…Therefore, the simultaneous transition of the structure and magnetic orders does not originate from the conventional Jahn-Teller (JT) distortion and magnetostriction effect is not due to spin-orbit coupling. The observed g-factor is in fact quite close to the spin-only value 8,12 . On the other hand, spin-lattice coupling in frustrated magnets can be generated by the spin driven JT effect 13 that the frustration is released by structural distortion.…”

“…Supporting evidences for the reduced magnetic moment of Cr 3+ ions are also provided by the Se NMR and the anisotropic hyperfine field at a Cr ion. When macroscopic measurements of the magnetic moment gives 3 μ B per Cr ion 12,16 , and one Cr ion has less than 3 μ B , the remaining moment should be found in the other ions of ZnCr 2 Se 4 . In general, the covalence increases in metal-ligand bonding as the oxygen ligand is replaced by sulfur, and sulfur by selenium.…”

“…The magnetic moment of Cr 3+ ions has also been the subject of some debate. The fit of susceptibility data to the Curie-Weiss law produced 3 μ B per Cr ion 12,16 . Contrary to macroscopic measurements, neutron diffraction studies have reported that the magnetic moment of a Cr 3+ ion is not greater than 2 μ B 17,18 and have suggested that spin fluctuation reduces the average magnetic moment.…”

Structural transition in the magnetoelectricZnCr2Se4spinel under pressure

“…Previous studies at ambient pressure show that ZnCr 2 Se 4 undergoes a coupled magneto-structural transition below ~20 K [17][18][19][20][21][22] , whereas another structural transformation of the cubic structure into an unknown phase occurs at high temperatures 23 . Given this structural instability with varying temperature along with the strong spin-lattice coupling active in this system, the application of external pressure presents an appealing variable for further exploring and tuning the physical properties of this compound.…”