Effect of magnetic field and temperature on the ferroelectric loop in MnWO4
EEffect of magnetic field and temperature on the ferroelectric loop in MnWO Bohdan Kundys, * Charles Simon, and Christine Martin
Laboratoire CRISMAT, CNRS UMR 6508, ENSICAEN, 6 Boulevard du Maréchal Juin, 14050 Caen Cedex, France
The ferroelectric properties of MnWO single crystal have been investigated. Despite a relatively lowremanent polarization, we show that the sample is ferroelectric. The shape of the ferroelectric loop of MnWO strongly depends on magnetic field and temperature. While its dependence does not directly correlate with themagnetocapacitance effect before the paraelectric transition, the effect of magnetic field on the ferroelectricpolarization loop supports magnetoelectric coupling. PACS number (cid:1) s (cid:2) : 72.55. (cid:1) s, 75.80. (cid:1) q, 75.30.Kz The mutually exclusive nature of magnetism and electricpolarization phenomena in most solids has recently at-tracted attention in the scientific community. This is essen-tially due to both the basic physics challenges posed and thepossible magnetoelectric (cid:1) ME (cid:2) applications for memory stor-age and electric field-controlled magnetic sensors. The ideaof having the two order parameters (cid:1) magnetic and electric (cid:2) atthe same temperature and magnetically controlled electricalpolarization (cid:1) or vice versa (cid:2) has stimulated a vast research ofnew materials, as well as reinvestigation of previouslyknown compounds. It becomes evident that many cantedantiferromagnets may develop electric polarization as a re-sult of the overlap of the electronic wave functions and as aresult of the spin orbit interaction. Among materials inwhich magnetoelectric effects have been recently reported,MnWO is a particularly interesting material as the electricpolarization in a single crystal may be switched from the b to a direction when a strong magnetic field is applied. Asimilar phenomenon has been observed in rare-earthmanganites.
The antiferromagnetic (cid:1) AF (cid:2) phase transi-tions of MnWO were already studied a long time ago. With decreasing temperature, MnWO undergoes a collinearantiferromagnetic state (cid:1) AF1 phase (cid:2) at T N (cid:3) (cid:1) AF2 phase (cid:2) at 12.7 K, and finally tocollinear incommensurate antiferromagnetic phase (cid:1)
AF3phase (cid:2) at 7.6 K. Among the three antiferromagnetic states,only the noncollinear one (cid:1)
AF2 phase (cid:2) appears to develop anelectric polarization that can be explained within the frame-work of the phenomenological and microscopicmodels. Polarity alone, however, does not guarantee ferro-electricity that is sometimes difficult to experimentallydemonstrate.
For example, the reversibility of the electricdipoles could require electric fields larger than the break-down field, or it might be due to asymmetric irreversiblearrangements of the atoms. In this Brief Report, the electricfield-induced dipole reversibility (cid:1) ferroelectricity (cid:2) ofMnWO is shown, along with the measures of its depen-dence on magnetic field and temperature. dc ferroelectricmeasurements were performed in a Physical Property Mea-surement System (cid:1) PPMS (cid:2)
Quantum Design cryostat by usinga Keithley 6517A electrometer. The used technique is ouradaptation of the already known method, wherein program-ming technology has been applied to the Keithley 6517Aelectrometer and PPMS to provide the possibility oftemperature- and magnetic field-dependent studies of the ferroelectric polarization loops. Its quasistatic (cid:1) dc (cid:2) opera-tional nature allows ferroelectric loops to be observed with asmall electric fatigue risk at ultralow frequency measuringsignals. A 0.37 mm sized along the b axis single crystal ofMnWO , which is grown by the floating zone method, hasbeen cut for ferroelectric loop measurements. The magneticand electric fields were applied parallel to the direction of thecrystalline b axis. The electrical contacts with the samplewere made by using a conductive silver paint. Figure 1 pre-sents the ferroelectric hysteresis loops (cid:4) P (cid:1) E (cid:2)(cid:5) as a result ofcurrent-voltage (cid:4) I (cid:1) E (cid:2)(cid:5) integration with respect to time at dif- FIG. 1. (cid:1)
Color online (cid:2) (cid:1) a (cid:2) Ferroelectric loops obtained as a resultof current integration at different temperatures. (cid:1) b (cid:2) Correspondingvoltage-current characteristics taken at different temperatures.1 erent temperatures recorded after a zero electric and mag-netic field cooling procedure. The remanent polarization ofabout 39 (cid:2) C / m at 10 K agrees well with the reportedforced polarization in this material. Remanent polarization (cid:1)
Fig. 2 (cid:2) and ferroelectric coerciveforce (cid:1) inset of Fig. 2 (cid:2) extracted from ferroelectric loops gothrough a maximum and decrease to zero for temperaturesclose to the magnetic transitions (cid:1) i.e., 7.6 and 12.7 K (cid:2) . Wehave also measured the forced polarization upon heating (cid:4) electric field (cid:1)
520 kV/m (cid:2) cooling procedure (cid:5) (cid:1)
Fig. 2 (cid:2) . Near7.6 K, the forced polarization more rapidly increases than theremanent one (cid:1)
Fig. 2 (cid:2) and the maximum is reached at 8 and10 K for the forced and remanent polarizations, respectively.This experimental result indicates that the ability to switchthe ferroelectric polarization with electric field more quicklyvanishes than the forced electric polarization in the sample inthis temperature region.The effect of the external magnetic field applied along the b axis on ferroelectric switching processes (cid:4) I (cid:1) E (cid:2) and P (cid:1) E (cid:2) loops (cid:5) at 10 K is shown in Fig. 3. The ferroelectric coerciveforce and the remanent polarization decreased upon externalmagnetic field application, and the ferroelectric loop is nomore observed at 12 T. The magnetic field dependence of thedielectric permittivity at 10 K and the magnetic field depen-dence of the ferroelectric coercive force (cid:1) along the b axis (cid:2) are shown in Fig. 4. The position of the peak in the dielectricpermittivity shows no hysteresis with respect to the magneticfield and agrees well with the magnetic field dependence ofthe both the ferroelectric coercive force and the remanentpolarization (cid:1) not shown (cid:2) . While the peak in the magneticfield dependence of dielectric permittivity is very narrowalong the a axis, a rather broad anomaly in the dielectricpermittivity is observed along the b crystallographic direc-tion (cid:1) Fig. 4 (cid:2) . It is also worth noting that practically no mag-netodielectric effect is seen in magnetic fields up to 9 T (cid:1)
Fig.4 (cid:2) , while the magnetic field-induced change in the shape of aferroelectric loop (cid:1) values of the remanent polarization and ofthe ferroelectric coercive force (cid:2) indicates a magnetoelectriccoupling in this magnetic field range (cid:4) see Fig. 3 (cid:1) a (cid:2)(cid:5) . This behavior is in agreement with the identical slope of ferro-electric loops near zero electric field for magnetic fields lessthan 9 T (cid:4) Fig. 3 (cid:1) a (cid:2)(cid:5) . These results, therefore, imply that mag-netoelectric interactions are present without noticeable mag-netodielectric effects in magnetic field region of 0–9 T. Mag- FIG. 2. Remanent polarization extracted from ferroelectric P (cid:1) E (cid:2) loops and the forced polarization recorded at heating with the pre-vious electric (cid:1)
520 kV/m (cid:2) cooling procedure. The inset shows theferroelectric coercive force as a function of temperature. FIG. 3. (cid:1)
Color online (cid:2) (cid:1) a (cid:2) Ferroelectric loops obtained as a resultof the ferroelectric current integration at different magnetic fields at10 K. (cid:1) b (cid:2) Voltage-current characteristics taken at 10 K at differentmagnetic fields.FIG. 4. (cid:1)
Color online (cid:2)
The magnetic field dependence of thedielectric permittivity at 500 kHz (cid:1) left scale (cid:2) and ferroelectric coer-cive force (cid:1) right scale (cid:2) at 10 K. The electric and magnetic fieldsapplied parallel to the crystallographic b axis.2 etocapacitance effects may also be accompanied with straycontributions that do not necessarily reflect intrinsic magne-toelectric interactions. Therefore, observing a magneticfield effect on the ferroelectric polarization loop may be aneffective alternative method for studying in depth magneto-electric coupling. In support of this, the ferroelectric loop ata magnetic field of 11 T (cid:4)
Fig. 3 (cid:1) a (cid:2)(cid:5) (cid:4) region where magneto-dielectric effect is big (cid:1) see Fig. 4 (cid:5) has a different slope nearzero electric field compared to the other loops for magneticfields less than 9 T, where the magnetodielectric effect issmall (cid:1) Fig. 4 (cid:2) . It has to be noted that, similarly, magneticfield induced ferroelectric loop has recently been found in Srsubstituted BiFeO accompanied with no magnetocapaci-tance effect in this compound. In conclusion, a quasistatic technique has been used toinvestigate ferroelectric properties of a single crystal of MnWO . It was shown that the sample is indeed ferroelectricand that the shape of its ferroelectric loop strongly dependson both temperature and magnetic field. Increasing the exter-nal magnetic field along the b axis decreased both the rem-anent polarization and ferroelectric coercive force. These ef-fects are not accompanied by any noticeable changes in themagnetic field dependence of dielectric permittivity beforethe transition to the paraelectric state at about 10.5 T. There-fore, our results also suggest that magnetoelectric couplingmay be present without obvious magnetodielectric effects inmagnetic and ferroelectric solids.We thank M. L. Hervé for crystal growth and samplepreparation. We also acknowledge the French ANR SEMONE research program. * [email protected] N. A. Hill, J. Phys. Chem. B , 6694 (cid:1) (cid:2) . D. I. Khomskii, J. Magn. Magn. Mater. , 1 (cid:1) (cid:2) . N. A. Hill, Annu. Rev. Mater. Sci. , 1 (cid:1) (cid:2) . N. A. Spaldin, Phys. World , 20 (cid:1) (cid:2) . N. Hur, S. Park, P. A. Sharma, J. S. Ahn, S. Guha, and S.-W.Cheong, Nature (cid:1)
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