PRL 113, 137602 (2014)
PHYSICAL REVIEW LETTERS
week ending 26 SEPTEMBER 2014
Orbital-Ordering-Driven Multiferroicity and Magnetoelectric Coupling in GeV 4 S8
1
Kiran Singh,1,‡ Charles Simon,1,2,† Elena Cannuccia,2 Marie-Bernadette Lepetit,2,3 Benoit Corraze,4 Etienne Janod,4 and Laurent Cario4,*
Laboratoire CRISMAT, CNRS UMR 6508, ENSICAEN, 6 Bd. du Maréchal Juin, 14050 Caen Cedex 4, France 2 Institut Laue Langevin, 71 avenue des Martyrs, 38000 Grenoble, France 3 Institut Néel, CNRS UPR 2940 Département MCBT, 25 avenue des Martyrs, BP 166, 38042 Grenoble Cedex 9, France 4 Institut des Matériaux Jean Rouxel, Université de Nantes, CNRS, 2 rue de la houssinière, BP32229, 44322 Nantes Cedex 3, France (Received 23 January 2014; revised manuscript received 14 July 2014; published 23 September 2014) We report here the discovery of multiferroicity and large magnetoelectric coupling in the type I orbital order system GeV4 S8 . Our study demonstrates that this clustered compound displays a para-ferroelectric transition at 32 K. This transition originates from an orbital ordering which reorganizes the charge within the transition metal clusters. Below the antiferromagnetic transition at 17 K, the application of a magnetic field significantly affects the ferroelectric polarization, revealing thus a large magnetoelectric coupling. Our study suggests that the application of a magnetic field induces a metamagnetic transition which significantly affects the ferroelectric polarization thanks to an exchange striction phenomenon. DOI: 10.1103/PhysRevLett.113.137602
PACS numbers: 77.55.Nv, 75.25.Dk, 77.84.-s
Magnetism and ferroelectricity coexist in a restricted class of compounds called multiferroics (MF). In MF materials with strong magnetoelectric coupling one can tune the electric order parameter by magnetic field and vice versa. In the last few years, MF compounds have experienced a resurgence of interest as controlling the electric polarization by an external magnetic field might give the opportunity to build up multifunctional devices with interesting applications [1–4]. MF compounds are usually classified into two classes. In type I MF compounds the microscopic mechanisms responsible for ferroelectricity (often a second-order Jahn-Teller distortion, or the presence of lone pair ions) and magnetism are physically very different and basically originate from different ions or subsystems [2,3]. Therefore, the ferroelectric and (anti-) ferromagnetic ordering occur at different temperatures, and generally these systems do not exhibit strong magnetoelectric coupling. In contrast, a large magnetoelectric coupling is achieved in type II MF compounds for which the ferroelectric and magnetic ordering occur at the same temperature. In these systems the ferroelectricity is directly induced by the spin ordering (very often a spiral magnetic ordering resulting from magnetic frustration) signifying that an intrinsic magnetoelectric coupling occurs between the ferroelectric and magnetic order parameters [1]. However, the magnitude of the ferroelectricity in type II MF compounds is rather weak (10−2 μC=cm2 ) compared to type I MF compounds (1–100 μC=cm2 ) [2,4]. In this context, a new kind of type I MF compounds, i.e., charge order MF compounds, has recently received much attention, as it might get together a high magnitude of ferroelectricity and a large magnetoelectric coupling [5]. Examples of materials where this mechanism is believed to occur are scarce and include Pr1−x Cax MnO3 [6], Fe3 O4 0031-9007=14=113(13)=137602(5)
[5], LuFe2 O4 [7], and ðTMTTFÞ2 X [8]. But none of these systems are fully understood. For example, recent studies affirm that LuFe2 O4 does not show a polar charge ordering [9]. Moreover, in many of these systems, as, for example, in Pr1−x Cax MnO3 , the buildup of a macroscopic polarization is hindered by a too high conductivity, [5] which explains why direct experimental observations of ferroelectric polarization or of magnetoelectric coupling are often missing. In the present work we report that the orbital ordering is an alternative way to the charge ordering to reach ferroelectricity and large magnetoelectric coupling in a type I MF system. Indeed, the clustered compound GeV4 S8 exhibits an orbital ordering which reorganizes both the charge and the spin within the magnetic tetrahedral transition metal clusters. This leads to the direct observations of ferroelectricity and of large magnetoelectric coupling in this type I orbital order MF system. The AM4 Q8 ðA ¼ Ga;Ge;M ¼ V;Nb;Ta;Q ¼ S;SeÞ compounds exhibit a lacunar spinel structure with tetrahedral transition metal clusters M4 containing one or two unpaired electrons [10]. The large separation of these M 4 clusters explains the Mott insulating state of these compounds and their narrow Mott-Hubbard gap (a few tenths of eV) [11]. The AM4 Q8 compounds display a variety of unusual transport and magnetic properties, such as, for example, Mott transition [12] and superconductivity under pressure [11,13], ferromagnetic half-metal behavior [14], colossal magnetoresistance [15], or resistive switching [16]. These systems also exhibit intrinsic lattice degrees of freedom leading to lattice instabilities and/or structural phase transitions. For example, GeV4 S8 with two unpaired localized electrons per cluster presents two successive structural phase transitions [17,18]. The first one, from cubic F − 43m to orthorhombic Imm2 (space group
137602-1
© 2014 American Physical Society
PRL 113, 137602 (2014)
PHYSICAL REVIEW LETTERS
compatible with ferroelectricity), occurs at T S ¼ 32 K, while the second transition is associated to a commensurate antiferromagnetic ordering (space group Pmn21 compatible with ferroelectricity) at T N ¼ 17 K [18,19]. This compound is highly insulating at low temperature and hence appears as a very interesting candidate for multiferroicity and magnetoelectric coupling. The synthesis of GeV4 S8 was first reported by D. Johrendt in 1998 [20]. Single crystals were prepared by a gas transport technique that proved to be efficient to grow lacunar spinel compounds (see Supplemental Material [21]) [14,16]. This synthesis yielded mostly 50 to 300 μm large and well-shaped tetrahedral crystals. However, a few tetrahedral crystals were truncated perpendicularly to the [111] direction and appeared as platelet shaped. One of these platelet shaped crystals of about 17 μm thickness was then selected for dielectric measurements. 100 μm diameter electrodes were painted with carbon paste on each parallel (111) faces of the crystal [see Fig. 1(b)]. These crystals are highly insulating at low temperature. Dielectric measurements under zero and different magnetic fields were measured using Agilent 4284A LCR meter (see Supplemental Material [21]). Figure 1(a) displays the temperature dependence of the dielectric permittivity between 10 to 60 K during warming and cooling (1 K= min) at four different frequencies (5–100 kHz). Two strong frequency independent anomalies are observed at 32 and at 17 K, both in the real and imaginary part. The tan δ value is small (
PHYSICAL REVIEW LETTERS
week ending 26 SEPTEMBER 2014
Orbital-Ordering-Driven Multiferroicity and Magnetoelectric Coupling in GeV 4 S8
1
Kiran Singh,1,‡ Charles Simon,1,2,† Elena Cannuccia,2 Marie-Bernadette Lepetit,2,3 Benoit Corraze,4 Etienne Janod,4 and Laurent Cario4,*
Laboratoire CRISMAT, CNRS UMR 6508, ENSICAEN, 6 Bd. du Maréchal Juin, 14050 Caen Cedex 4, France 2 Institut Laue Langevin, 71 avenue des Martyrs, 38000 Grenoble, France 3 Institut Néel, CNRS UPR 2940 Département MCBT, 25 avenue des Martyrs, BP 166, 38042 Grenoble Cedex 9, France 4 Institut des Matériaux Jean Rouxel, Université de Nantes, CNRS, 2 rue de la houssinière, BP32229, 44322 Nantes Cedex 3, France (Received 23 January 2014; revised manuscript received 14 July 2014; published 23 September 2014) We report here the discovery of multiferroicity and large magnetoelectric coupling in the type I orbital order system GeV4 S8 . Our study demonstrates that this clustered compound displays a para-ferroelectric transition at 32 K. This transition originates from an orbital ordering which reorganizes the charge within the transition metal clusters. Below the antiferromagnetic transition at 17 K, the application of a magnetic field significantly affects the ferroelectric polarization, revealing thus a large magnetoelectric coupling. Our study suggests that the application of a magnetic field induces a metamagnetic transition which significantly affects the ferroelectric polarization thanks to an exchange striction phenomenon. DOI: 10.1103/PhysRevLett.113.137602
PACS numbers: 77.55.Nv, 75.25.Dk, 77.84.-s
Magnetism and ferroelectricity coexist in a restricted class of compounds called multiferroics (MF). In MF materials with strong magnetoelectric coupling one can tune the electric order parameter by magnetic field and vice versa. In the last few years, MF compounds have experienced a resurgence of interest as controlling the electric polarization by an external magnetic field might give the opportunity to build up multifunctional devices with interesting applications [1–4]. MF compounds are usually classified into two classes. In type I MF compounds the microscopic mechanisms responsible for ferroelectricity (often a second-order Jahn-Teller distortion, or the presence of lone pair ions) and magnetism are physically very different and basically originate from different ions or subsystems [2,3]. Therefore, the ferroelectric and (anti-) ferromagnetic ordering occur at different temperatures, and generally these systems do not exhibit strong magnetoelectric coupling. In contrast, a large magnetoelectric coupling is achieved in type II MF compounds for which the ferroelectric and magnetic ordering occur at the same temperature. In these systems the ferroelectricity is directly induced by the spin ordering (very often a spiral magnetic ordering resulting from magnetic frustration) signifying that an intrinsic magnetoelectric coupling occurs between the ferroelectric and magnetic order parameters [1]. However, the magnitude of the ferroelectricity in type II MF compounds is rather weak (10−2 μC=cm2 ) compared to type I MF compounds (1–100 μC=cm2 ) [2,4]. In this context, a new kind of type I MF compounds, i.e., charge order MF compounds, has recently received much attention, as it might get together a high magnitude of ferroelectricity and a large magnetoelectric coupling [5]. Examples of materials where this mechanism is believed to occur are scarce and include Pr1−x Cax MnO3 [6], Fe3 O4 0031-9007=14=113(13)=137602(5)
[5], LuFe2 O4 [7], and ðTMTTFÞ2 X [8]. But none of these systems are fully understood. For example, recent studies affirm that LuFe2 O4 does not show a polar charge ordering [9]. Moreover, in many of these systems, as, for example, in Pr1−x Cax MnO3 , the buildup of a macroscopic polarization is hindered by a too high conductivity, [5] which explains why direct experimental observations of ferroelectric polarization or of magnetoelectric coupling are often missing. In the present work we report that the orbital ordering is an alternative way to the charge ordering to reach ferroelectricity and large magnetoelectric coupling in a type I MF system. Indeed, the clustered compound GeV4 S8 exhibits an orbital ordering which reorganizes both the charge and the spin within the magnetic tetrahedral transition metal clusters. This leads to the direct observations of ferroelectricity and of large magnetoelectric coupling in this type I orbital order MF system. The AM4 Q8 ðA ¼ Ga;Ge;M ¼ V;Nb;Ta;Q ¼ S;SeÞ compounds exhibit a lacunar spinel structure with tetrahedral transition metal clusters M4 containing one or two unpaired electrons [10]. The large separation of these M 4 clusters explains the Mott insulating state of these compounds and their narrow Mott-Hubbard gap (a few tenths of eV) [11]. The AM4 Q8 compounds display a variety of unusual transport and magnetic properties, such as, for example, Mott transition [12] and superconductivity under pressure [11,13], ferromagnetic half-metal behavior [14], colossal magnetoresistance [15], or resistive switching [16]. These systems also exhibit intrinsic lattice degrees of freedom leading to lattice instabilities and/or structural phase transitions. For example, GeV4 S8 with two unpaired localized electrons per cluster presents two successive structural phase transitions [17,18]. The first one, from cubic F − 43m to orthorhombic Imm2 (space group
137602-1
© 2014 American Physical Society
PRL 113, 137602 (2014)
PHYSICAL REVIEW LETTERS
compatible with ferroelectricity), occurs at T S ¼ 32 K, while the second transition is associated to a commensurate antiferromagnetic ordering (space group Pmn21 compatible with ferroelectricity) at T N ¼ 17 K [18,19]. This compound is highly insulating at low temperature and hence appears as a very interesting candidate for multiferroicity and magnetoelectric coupling. The synthesis of GeV4 S8 was first reported by D. Johrendt in 1998 [20]. Single crystals were prepared by a gas transport technique that proved to be efficient to grow lacunar spinel compounds (see Supplemental Material [21]) [14,16]. This synthesis yielded mostly 50 to 300 μm large and well-shaped tetrahedral crystals. However, a few tetrahedral crystals were truncated perpendicularly to the [111] direction and appeared as platelet shaped. One of these platelet shaped crystals of about 17 μm thickness was then selected for dielectric measurements. 100 μm diameter electrodes were painted with carbon paste on each parallel (111) faces of the crystal [see Fig. 1(b)]. These crystals are highly insulating at low temperature. Dielectric measurements under zero and different magnetic fields were measured using Agilent 4284A LCR meter (see Supplemental Material [21]). Figure 1(a) displays the temperature dependence of the dielectric permittivity between 10 to 60 K during warming and cooling (1 K= min) at four different frequencies (5–100 kHz). Two strong frequency independent anomalies are observed at 32 and at 17 K, both in the real and imaginary part. The tan δ value is small (
