Spin correlations in the frustrated ferro-antiferromagnet $\mathrm{SrZnVO}({\mathrm{PO}}_4{)}_2$ near saturation

Single-crystal elastic and inelastic neutron-scattering experiments are performed on the frustrated ferro-antiferromagnet $\mathrm{SrZnVO}({\mathrm{PO}}_4{)}_2$ in high magnetic fields. The fully polarized state, the presaturation phase and the columnar-antiferromagnetic phase just below the presaturation phase were investigated. The observed renormalization of spin-wave bandwidths, re-distribution of intensities between different branches and nonlinearities in the magnetization curve are all indicative of strong deviations from classical spin-wave theory. The previously observed presaturation transition is attributed to a staggered pattern of Dzyaloshinskii-Moriya interactions.

Figure 1

(a) Heisenberg exchange constants in the ${\mathrm{V}}^{4+}$ layers of SrZnVO(${\mathrm{PO}}_4{)}_2$. Yellow circles are ${\mathrm{V}}^{4+}$ ions. The gray rectangle represents the unit cell. (b) A partially magnetized ${\mathrm{CAF}}_{\text{b}}$-type spin structure with canting angle $\theta$ and the staggered magnetization pointing along $\mathbf{a}$. The two antiferromagnetic sublattices are labeled as “A” and “B,” respectively. The blue arrows indicate crystal symmetry-compatible Dzyaloshinskii-Moriya vectors $\mathbf{D}$ on the respective directional $J_{2,1}$ bonds (dark yellow). The cross products of spins interacting via the corresponding $\mathbf{D}({\mathbf{S}}_A\times {\mathbf{S}}_B)$ Dzyaloshinskii terms are indicated by orange arrows. (c) The same as in (b) but for the ${\mathrm{CAF}}_{\text{a}}$ spin structure.

Figure 2

Measured magnetization in a SrZnVO(${\mathrm{PO}}_4{)}_2$ single crystal in magnetic fields applied along $\mathbf{c}$ (symbols). The orange dashed line is the linear-SWT result. The red solid line includes first-order $1/S$ corrections for the $J_1\text{−}J2$ model following Ref. [19].

Figure 3

Peak intensities of several potentially magnetic neutron Bragg peaks measured as a function of magnetic field applied along the $\mathbf{c}$ axis. The background due to multiple scattering and higher-order beam contamination has been subtracted as described in the text.

Figure 4

False color plot of neutron-scattering intensities measured in SrZnVO(${\mathrm{PO}}_4{)}_2$ just above saturation for a field of 14.9 T applied along the $\mathbf{c}$ axis. The horizontal axes show wave-vector transfer along high-symmetry crystallographic directions. The vertical axis shows the energy transfer. Solid lines represent the spin-wave theory- (SWT-) calculated dispersion of magnons, computed using the exchange constants tabulated in the first column of Table 1. Line thickness corresponds to the SWT-calculated scattering intensity.

Figure 5

Typical constant-$Q$ scans measured in SrZnVO(${\mathrm{PO}}_4{)}_2$ at ${\mu }_{0}H=14.8$ T and $T=100$ mK, just above the saturation transition (symbols). The solid lines are results of a global fit to the data as described in the text. The gray area is the modeled background contribution.

Figure 6

Neutron-scattering intensities measured in the TOF experiment. The false color plots are energy-momentum slices along (a) and (b) $(0,k,0)$ and (c) and (d) $(1,k,0)$. The data are fully integrated along $l$ and $\pm 0.2$ relative lattice unit (r.l.u.) along the $h$ direction. The data are taken (a) and (c) just below and (b) and (d) just above the presaturation transition at $H_c$ at $T=250$ mK. The red rectangles indicate the data used for the constant-$Q$ cuts shown in Fig. 7.

Figure 7

Symbols: energy scans cut from the TOF data obtained by momentum integration of intensities in areas bordered in red in Fig. 6. The dashed line is a SWT simulation based on exchange constants measured above saturation. The solid colored line is the same simulation performed at a fictitious value of the magnetic field in order to obtain the correct magnetization value in SWT. The solid straight line is a linear background estimate.