Phase Diagram of Multi-ferroic MnWO4 determined by Neutron
Diffraction in 40 T Pulsed Magnetic Fields
T. Kihara1, Y. Narumi1, K. Ohoyama1, K. Kuwahara2, T. Kimura3, K. Oikawa4,M. Harada4, F. Maekawa4, and H. Nojiri1
1

Institute for Materials Research, Tohoku University, Sendai-shi, Miyagi 980-8577, Japan
2
Ibaraki University, Mito-shi, Ibaraki 320-8512, Japan
3
Osaka University, Toyonaka-shi, Osaka 560-8531, Japan
4
Japan Proton Accelerator Research Complex, Naka-gun 319-1195, Japan

Multi-ferroic phenomena found in frustrated antiferromagnets have been investigatedintensively for their cross-correlation between magnetism and dielectric properties. Various
novel phase transitions arise in high magnetic fields and the magnetic structures are highly
non-trivial. Thus, it is essential to observe the magnetic structure directly by using neutrondiffraction. MnWO4 is one of the most well-known multi-ferroic compounds, which shows
the memory effect in high magnetic fields. For the strong spin frustration, three different
magnetic phases;AF1(uudd), AF2 (non-collinear, incommensurate cycloidal), AF3(incommensurate sinusoidal) appear at zero field [1] (Fig. 1). There are at least three phases inhigh magnetic fields: HF, IV and V. When a magnetic field is applied along the easy-axis, adistinct memory effect is found. Namely, the polarization between the AF2 and IV are always
opposite irrespective of the initial polarization. Interestingly, the polarization of the phase IV
reverses when it enters into the phase V. These facts show that the memory is kept while thesystem is passing through the HF and V phases, where no polarization exists. The key
experiment to understand the mechanism of the multi-ferroic behavior and the distinct
memory effect in MnWO4 is the determination of magnetic structures in the high magnetic
field phases HF, IV and V. We have investigated this compound at SNS in Oakridge and at
J-PARC in last three years and found following results [2]. (1) At 4.2 K, the reentrant
transitions occur in the sequence of AF1-AF2-HF-PhaseIV. (2) HF is close to commensurate,
but there is a small splitting in the magnetic Bragg peak showing possible two phases mixture
or long-pitch incommensurate magnetic structure. The intensity of two components varies
with the magnetic field intensity and with the hysteresis (arrow A in Fig. 1). (3) Phase IV isincommensurate and there is about 1 % change of incommensurate wave vector between AF2(10 K at zero field) and Phase IV (4.2 K at 40 T) (arrow B in Fig. 1). It is not clear if theJ. Phys. Soc. Jpn. 81(2012) 054705
change is continuous in temperature or there is a new phase boundary
withinthe phase IV. (4)At the phase boundaries of the reentrant transitions, twosuccessive phases coexist showing that the transitions are 1st(a)order. (5) The magnetic structure of Phase V at 10 K and at 35
T (blue circle in Fig. 1) is incommensurate and the wave
(10K, 35T)
vector is nearly identical with that of Phase IV. If one assumes
that Phase V persists between 4 and 12 K, the phase V isAB
incommensurate phase without polarization. In another word,
the origins of the absence of the polarization are different
between phase V and HF phase. Magnetic structure models
will be discussed considering the results of neutron
diffractions.
[1] H. Mitamura et al., J. Phys. Soc. Jpn. 81 054705 (2012)[2] H. Nojiri et al., Phys. Rev. Lett. 106 237202 (2011)

Fig. 1 Phase diagram of MnWO4 [1,2].
(b)

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