Stripe antiferromagnetism in van der Waals metal HoTe3 decoupled from charge density wave order

Abstract

The $R\mathrm{Te}_3$ ($R = \text{rare earth}$) family of layered van der Waals (vdW) compounds hosts coexisting magnetic and charge density wave (CDW) orders, yet the interplay between these degrees of freedom remains little explored. Combining polarized and unpolarized neutron diffraction on single-crystal $\mathrm{HoTe}_3$, we identify two distinct antiferromagnetic (AFM) phases, both exhibiting a collinear $\uparrow\uparrow\downarrow\downarrow$ motif within individual vdW layers. The two phases are distinguished by the vdW stacking of magnetic layers: ferromagnetic (FM) stacking in the higher-temperature AFM-II phase, here termed ``vertical-stripe'', and AFM stacking in the AFM-I ground state, here termed ``tilted-stripe''; the two phases have propagation vectors $\boldsymbol{q}_{\mathrm{m2}} = (0.48, 0, 0)$ and $\boldsymbol{q}_{\mathrm{m1}} = (0.5, 0.5, 0)$, respectively. In contrast to the CDW-driven exotic magnetism in $\mathrm{DyTe}_3$, $\mathrm{TbTe}_3$, and $\mathrm{GdTe}_3$, we find no evidence for coupling between magnetism and CDW in $\mathrm{HoTe}_3$. The relative alignment between AFM and CDW propagation vectors, as well as single-ion anisotropy, are likely essential for generating coupled spin/charge orders in layered vdW systems.

Figures (4)

• Figure 1: (color online). Magnetism in $R$Te$_3$ ($R=$ Ho, Dy) with charge density wave (CDW) order. (a) Orthorhombic crystal structure with space group Cmcm (No. 63) in layered HoTe$_3$, showing covalently bonded Ho--Te$_\mathrm{A}$ bilayers and conducting Te$_\mathrm{B}$ square nets footnote1. Note that the $b$-axis is perpendicular to the vdW layers. (b) Charge order in DyTe$_3$ is predominantly unidirectional along $c$. Its conical magnetic texture is modulated along the $c$-axis through coupling to the CDW-1. (c) HoTe$_3$ has a more strongly double-$\bm q$ character of the CDW, with two propagation vectors. Its AFM-I ground state has moments parallel to the $c$-axis and is independent of the double $\boldsymbol{q}$-CDW. Te$_\mathrm{A}$ atoms and CDW-induced lattice distortions are not depicted. (d) $B$--$T$ magnetic phase diagram of HoTe$_3$ with field applied along the $b$-axis. Phase boundaries are determined from magnetization measurements. The light gray region between the PM and AFM-II phases hosts short-range magnetic order (SRO). (e) Temperature dependence of magnetic reflection intensities in HoTe$_3$ corresponding to the AFM-I and AFM-II propagation vectors, measured using a triple-axis diffractometer at JRR-3 in zero magnetic field.
• Figure 2: (color online). Polarized neutron scattering in phase AFM-I of HoTe$_3$. (a) The incoming and outgoing wavevectors of the neutron beam, $\bm{k}_\mathrm{i}$ and $\bm{k}_\mathrm{f}$, span the $hk0$ scattering plane (gray). The total momentum transfer is $\boldsymbol{Q}$. In the Ho sublattice, moments are collinear along the $c$-axis (perpendicular to the scattering plane), yielding finite non-spin flip (NSF) and zero spin-flip (SF) scattering intensity. Spin directions of polarized neutrons are shown as blue balls with arrows. (b,c) $h$-scans at $\boldsymbol{Q}=(0.5,8.5,0)$ and $(1.5,3.5,0)$. Only NSF scattering is observed. Insets: schematic of $\bm Q$ in the scattering plane. (d) Flipping ratio $P = (I_\mathrm{NSF}-I_\mathrm{SF})/(I_\mathrm{NSF}+I_\mathrm{SF})$ at $T = 2.2$ K for magnetic reflections lying in the scattering plane $hk0$, measured as a function of $\omega$, the angle between each reflection's momentum transfer $\boldsymbol{Q}$ and the $a$-axis ($h00$). $P$ is normalized to $P_0$ to account for imperfect beam polarization (see text). When all points are on the red (blue) line, the moments are fully parallel (fully perpendicular) to the scattering plane.
• Figure 3: (color online). Symmetry analysis for phase AFM-I. We show the highest-symmetry magnetic subgroups (mSGs) of the paramagnetic space group $Cmcm1'$ and the corresponding candidate spin textures. (a,b) Magnetic structures in the $P_a2_1/m$ mSG exhibit Ising-like collinear AFM order along the $c$-axis, consistent with polarized neutron scattering in Fig. \ref{['Fig2']}. (c,d) Magnetic structures in the $P_c2_1/c$ mSG have their moments perpendicular to $c$, inconsistent with polarized neutron scattering. In each mSG, the Ho atoms at the Wyckoff $4c$ positions are divided into two sets, termed inner and outer set, which are separated by interlayer spacings of approximately $b/3$ and $2b/3$, respectively. In Model 1 (Model 2), the outer (magenta) and inner (gray) Ho sets are AFM (FM) and FM (AFM) coupled, respectively. The structures (c) and (d) can be discussed on the same grounds. Only Model 1 [blue box in (a)] is consistent with the analysis in Fig. \ref{['Fig4']}.
• Figure 4: (color online) Refinement of the magnetic stricture in phase AFM-I of HoTe$_3$. (a) Geometry of SENJU, a time-of-flight (TOF) Laue diffractometer at J-PARC. Red and blue arrows indicate the ingoing and outgoing neutron wavevectors, $\bm k_\mathrm{i}$ and $\bm k_\mathrm{f}$. (b) Reconstructed RSM in the $hk0$ plane at $T = 1.5$ K. In AFM-I, there is a pronounced suppression of intensity at $k = -3/2, -9/2, -15/2$ when $l=0$, consistent with Eq. \ref{['extinction']}. (c) Two domains of Model 1 for AFM-I, showing alternating tilted-stripe spin textures. Only Ho sites are shown. The gray box indicates the conventional magnetic unit cell, four times larger than the conventional crystallographic unit cell (blue box). Moments align along the $c$-axis with propagation vector $\boldsymbol{q}_{\mathrm{m1}}$ (purple arrow). The tilted grey and violet boxes with round edges are guides to the eye, indicating planes of parallel spins -- i.e., spin stripes. (d) Magnetic refinement of unpolarized neutron intensity using Model 1. $F_\mathrm{obs}$, $F_\mathrm{calc}$ and $R$ denote observed and calculated magnetic structure factors, as well as the agreement factor.