Neutron Laue Diffraction Study on the Magnetic Phase Diagram of Multiferroic ${\mathrm{MnWO}}_4$ under Pulsed High Magnetic Fields

We have combined time-of-flight neutron Laue diffraction and pulsed high magnetic fields at the Spallation Neutron Source to study the phase diagram of the multiferroic material ${\mathrm{MnWO}}_4$. The control of the field-pulse timing enabled an exploration of magnetic Bragg scattering through the time dependence of both the neutron wavelength and the pulsed magnetic field. This allowed us to observe several magnetic Bragg peaks in different field-induced phases of ${\mathrm{MnWO}}_4$ with a single instrument configuration. These phases were not previously amenable to neutron diffraction studies due to the large fields involved.

Figure 1

(a) Reciprocal space map showing Bragg diffraction in the Laue method. The shaded area can be simultaneously captured, and circular arcs correspond to constant field line in a pulsed field. (b) An example of the delay time control for half-sinusoidal field pulses. The start time of field-pulse I (solid line) is set so that the maximum field coincides with the TOF (i) in (a). In the case II (dashed line), two reflections at (ii) and (iii) can be observed in a single field-pulse time-frame. (c) Schematic cut-away view of the magnet coil and the experimental arrangement. (d) The configuration of magnetic field $B$, and scattering vectors ${\mathit{Q}}_C$ and ${\mathit{Q}}_{\mathrm{IC}}$.

Figure 2

(a) The magnetic phase diagram of ${\mathrm{MnWO}}_4$ for $B$ parallel to magnetic easy axis [13]. Ferroelectric phases are shaded in green. The present experiment has been performed along the arrows $A$ and $B$. (b) A schematic diagram of the magnetic structures in the AF1 and AF2 phases. In AF1, the magnetic easy axis lies in the $ac$ plane and is tilted by 35° from the $a$ axis.

Figure 3

(a) Color contour map of Laue diffraction spanned by [$-1$,2,2] and [2,0,1] axes in reciprocal space and taken at $B=0\mathrm{T}$. The $C$ and IC peaks are observed at 2 K and 9 K, respectively. (b) The pixel-integrated TOF spectrum at 2 K with $B_{\max }=25\mathrm{T}$. Scattering from the $C$ and IC positions are in red and blue, respectively. Inset: time integrated Laue diffraction shows the simultaneous measurement of the $C$ and IC reflections. (c) The magnetic field dependence of integrated $C$ and IC Bragg intensities at 2 K.

Figure 4

(a) The right panels show magnified Laue images around the ${\mathit{Q}}_C$ position within the AF1 and HF phases. The left panels show the TOF spectrum with the $x$ axis converted to $d$ spacing. The red and blue lines depict intensity as a function of $d$ at the $A$ and $B$ areas, respectively, of the Laue image in the bottom right panel. (b) The TOF spectrum for IC reflections in AF2 (red line) and IV (blue line) phases. The $x$ axis has been converted to $d$ spacing.