NMR investigation of spin spiral $ZnCr_2Se_4$

Abstract

Incommensurate spin spiral of the bond frustrated $ZnCr_2Se_4$ has attracted interest for its strong spin-lattice coupling evidenced by magnetostriction, negative thermal expansion, and simultaneous magnetic and structural phase transitions occurring at 21 K. However, from the pure ionic point of view, half-filled $t_{2g}$ orbital of $Cr^{3+}$ ions at octahedral centers is Jahn-Teller inactive and carries no orbital angular momentum implying zero spin-orbit coupling. Instead of the conventional origins of the spin-lattice coupling, the spin driven Jahn-Teller effect during which magnetic frustration is released through distortion has been suggested. For better understanding of the spin-lattice coupled behavior of this material, information on exact spin state is essential. In this thesis, we report estimation of the coupling constants between Cr ion spins and the spin spreading from a Cr ion to neighboring Se ions observed by NMR. We estimated the coupling constants by comparing temperature dependence of the resonance frequency with theoretical prediction of sublattice magnetization, M(T)~ $T^2$, calculated by the linear spin wave theory for incommensurate spiral spin order. The comparison shows that ferromagnetic $J_1$ coupling competes with antiferromagnetic $J_3$ coupling resulting in the bond frustration. The isotropic and anisotropic hyperfine fields of a Cr ion provide the evidences that the electronic configuration is different from $d^3$ and the magnetic moment is reduced below 3$\mu_B$. The hyperfine field of a Se ion strongly implies that the spin polarization of a Cr ion is significantly transferred to a Se ion due to the strong covalent bond between them. These observations indicate that the Jahn-Teller effect is not completely missing in $ZnCr_2Se_4$. The NMR spectra and $T_2$ relaxation time of both ions at the temperatures lower than 6 K shows smaller signal intensities and faster relaxation rates at low frequency halves of spectra, due to the gradual spin state change from the isotropic incommensurate to the anisotropic spirals. This schange is because of the magnetic anisotropy change due to the temperature dependent dipole-dipole interaction.