On Ferro- and Antiferro-Spin-Density Waves Describing the Incommensurate Magnetic Structure of NaYNiWO₆

Abstract

The incommensurate magnetic structure (0.47, 0, 0.49) of NaYNiWO6 exhibits unconventional spin-density waves (SDWs) along the [100] direction, in which up and down spins alternate in each half-wave. This is in contrast to conventional SDWs, in which only one type of spin is present in each half-wave. We probed the formation of these unconventional SDWs by evaluating the spin exchanges of NaYNiWO6 based on density functional theory calculations and analyzing the nature of the spin frustration in NaYNiWO6 and by noting that a SDW is a superposition of two cycloids of opposite chirality. The unconventional SDWs along the [100] direction originate from the spin-frustrated antiferromagnetic chains of Ni2+ ions along that direction, leading to conventional SDWs along the [101] direction and unconventional SDWs along the [001] direction.

Spin Hamiltonians in Magnets: Theories and Computations

“…For example, the FM effects of double exchange resulting from mobile electrons in some antiferromagnetic lattices give rise to a distortion of the ground-state spin arrangement and lead to a canted spin configuration [92]. A magnetic solid with moderate spin frustration lowers its energy by adopting a noncollinear superstructure (e.g., a cycloid or a helix) in which the moments of the ions are identical in magnitude but differ in orientation or a collinear magnetic superstructure (e.g., a spin density wave, SDW) in which the moments of the ions differ in magnitude but identical in orientation [93,94]. For a cycloid formed in a chain of magnetic ions, each successive spin rotates in one direction by a certain angle, so there are two opposite ways of rotating the successive spins hence producing two cycloids opposite in chirality but identical in energy.…”

“…For a cycloid formed in a chain of magnetic ions, each successive spin rotates in one direction by a certain angle, so there are two opposite ways of rotating the successive spins hence producing two cycloids opposite in chirality but identical in energy. When these two cycloids occur with equal probability below a certain temperature, their superposition leads to a SDW [93,94]. On lowering the temperature further, the electronic structure of the spin-lattice relaxes to energetically favor one of the two chiral cycloids so that one can observe a cycloid state.…”

Spin Exchanges between Transition Metal Ions Governed by the Ligand p-Orbitals in Their Magnetic Orbitals

“…In general, when the temperature is lowered below a certain temperature, T SDW , a moderately spin-frustrated magnetic system gives rise to two cycloids of opposite chirality with equal probability. The resulting superposition of the two (Figure 15a-c) leads to a state known as a spin density wave (SDW) [27,28]. The latter becomes transverse if the preferred spin orientation at each magnetic ion is perpendicular to the SDW propagation direction, but becomes longitudinal if the spin orientation prefers the SDW propagation direction (Figure 15d-f).…”

Factors Governing the Propagation Direction and Spin-Rotation Plane of Noncollinear Magnetic Structures: A Helix vs Cycloid in Doubly Ordered Perovskites NaYMnWO6 and NaYNiWO6

“…One obtains a SDW that has up-and down-spins alternating in each half-wave (i.e., an antiferro-SDW) from two cycloids in which the spin rotation starts from a chain of antiferromagnetically coupled spins (Figure 1c). 7 For a magnetic system described by a spin Hamiltonian of isotropic spin exchanges, 8,9 the reduction in spin frustration by spin rotation is not dependent on whether the rotation occurs in a plane parallel or perpendicular to the propagation direction, namely, whether the noncollinear structure is a cycloid or a helix (see Section S2 in the Supporting Information). In contrast to a cycloid, a helix arises from a spin rotation in the plane perpendicular to the propagation vector direction (Figure 1d).…”

“…Recent neutron powder diffraction (NPD) studies 19 of NaYNiWO 6 showed that, upon reducing the temperature, it undergoes a phase transition at 20 K to a transverse antiferro-SDW with wave vector k = (0.47, 0, 0.49). 7 At 18 K, this SDW structure changes to a collinear, commensurate structure with k = ( 1 / 2 , 0, 1 / 2 ) in which the spins have an antiferromagnetic (AFM) coupling along the a-and c-directions but a ferromagnetic (FM) coupling along the b-direction. The latter observation shows that the spin frustration in NaYNiWO 6 is not strong, so it can be readily converted to a collinear phase by a slight relaxation in the electronic structure.…”