Mn3TeO6 - A New Multiferroic Material with Two Magnetic Substructures
L.Zhao, Z. Hu, C.Y. Kuo, T.-W. Pi, M.-K. Wu, L.H. Tjeng, A.C. Komarek
MMn TeO – a new multiferroic material with two magnetic sub-structures Li Zhao , Zhiwei Hu , Chang-Yang Kuo , Tun-Wen Pi , Maw-Kuen Wu , Liu Hao Tjeng , and Alexander C. Komarek* ,1 Max-Planck-Institute for Chemical Physics of Solids, Nöthnitzer Strasse 40, 01187 Dresden, Germany National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu 30077, Taiwan Institute of Physics, Academia Sinica, 128 Sec. 2, Academia Road, Nankang, Taipei 11529, Taiwan
Keywords multiferroics, ferroelectricity, magnetoelectric effects, Mn TeO Abstract From magnetic susceptibility, dielectric permittivity, electric polarization and specific heat measurements, we discover spin-induced ferroelectricity and magnetoelectric coupling in Mn TeO and observe two successive magnetic transitions at low temperatures. A non-ferroelectric intermediate magnetic state occurs below 23 K and a multiferroic ground state emerges below 21 K. Moreover, Mn TeO is a candidate for a multiferroic material where two types of incommensurate spin structures, cycloidal and helical, coexist. Theoretically, both spin substructures may contribute to the macro electric polarization via different mechanisms. This could open new ways of manipulating the ferroelectric polarization in a multiferroic material. Multiferroic materials have attracted enormous attention in the last decade because of their potential application in future devices [1–6]. The intrinsic cross-coupling between magnetic and electric properties opens up the possibility to manipulate the electric polarization by an external magnetic field and the magnetic structure by an applied electric field. The ferroelectric polarization in these materials can often be explained by the inverse Dzyaloshinskii–Moriya (DM) interaction P ij = A eˆ ij ×( S i × S j ) [7–9] or by the P ij ∂ ( S i · eˆ ij ) S i –( S j · eˆ ij ) S j term in the Arima model [10, 11]. Most multiferroic materials are based on oxides. Several halide and chalcogenide compounds [12–14] are also known to exhibit multiferroicity. Very recently, the transition metal orthotellurates M TeO (M = Mn , Co , and Ni ) with a corundum-related structure have attracted considerable attention [15–21]. A magnetic field induced spontaneous electric polarization has been discovered in Co TeO [15] and a colossal magnetoelectric (CME) effect below its AFM ordering temperature of about 52 K was reported in Ni TeO [19]. Here we focus on Mn TeO which was found to exhibit a complex spin structure with both helical and cycloidal components [20, 21]. We have carried out a comprehensive study of the magnetic usceptibility, specific heat, local electronic structure, dielectric permittivity, and electric polarization, and discovered that Mn TeO is multiferroic. The coexistence of a helical and of a neighboring cycloidal spin structure in the ferroelectric phase makes this multiferroic material unique among other multiferroic materials. Polycrystalline Mn TeO samples were prepared by solid-state reaction. The mixture of high-purity MnO and TeO in the stoichiometric ratio of 3:1 was thoroughly ground and pressed into pellets. Subsequently it was sintered in a tubular furnace with an Ar flow at 750–800 ° for 72 hours with several intermediate grindings. The light brown bulk samples have been characterized by X-ray powder diffraction, confirming that our samples are single phase. Magnetic properties were measured in a SQUID magnetometer (MPMS-5XL, Quantum Design). To measure the dielectric properties of Mn TeO , the polycrystalline samples were polished to thin plates with thickness of 0.2–0.5 mm. Silver paint was applied to both sides as electrodes to form parallel plate capacitors whose capacitance are proportional to the dielectric constant ( ε ). The samples are glued on the cryogenic stage of a homemade probe inserted into a Quantum Design 9T PPMS. A high-precision capacitance bridge (AH 2700A, Andeen-Hagerling, Inc.) was used for dielectric measurement. We tried various excitation levels (from 1 V to 15 V) and sweeping rates, and no apparent difference was found in the different measuring conditions. The electric polarization has been obtained by measurements of the pyroelectric current. Firstly we polarized the specimens with a static electric field of 300–800 kV/m during the cooling process, then removed the electric field and short-circuited both sides of the sample at low temperature for about one hour to remove the possible trapped interfacial charge carriers. The pyroelectric current was measured during the warming process at a heating rate of about 3 K/min. The electric polarization ( P ) has been obtained from the integration of the measured pyroelectric current. Furthermore, soft X-ray absorption spectroscopy (XAS) at the Mn-L edge has been performed in the total electron yield mode with a photon energy resolution of 0.2 eV at the BL08B beamline of National Synchrotron Radiation Research Centre in Taiwan. Clean sample surfaces were obtained by fracturing the samples in situ just before collecting the data in an ultrahigh vacuum chamber with the pressure below 10 –9 mbar. The Mn-L XAS spectrum of a single crystal of MnO was measured simultaneously as a standard reference.
We have synthesized single phase polycrystalline Mn TeO samples by solid-state reaction. Details can be found in Section 2. The temperature dependence of the magnetic susceptibility ( c ) of Mn TeO measured in the field of H = 1000 Oe is shown in Fig. 1a. The zero-field-cooling (ZFC) and field-cooling (FC) curves exhibit the typical behavior for an antiferromagnet. The Curie–Weiss emperature q CW from a fit to c ( T ) = C /( T – q CW ) amounts to –112 K, indicating that the interactions in Mn TeO are antiferromagnetic (AFM). Furthermore, the effective magnetic moment m eff amounts to 5.8 m B, indicating that the Mn ion is divalent with the S = 5/2 spin state. For temperatures below about 130 K, c ( T ) deviates clearly from the Curie–Weiss behavior, suggesting that short-range correlations start to develop. At ~
23 K we observe a sharp kink in the susceptibility (see also the sharp peak in d c /d T ( T ) as shown in the inset of Fig. 1a. We can identify this temperature as the Néel temperature of Mn TeO , since the temperature dependent specific heat as displayed in Fig. 1b shows a sharp λ -shaped peak indicating the release of substantial amount of entropy. Here we will use the notation T N1 for this 23 K Néel temperature to differentiate it from a second spin transition at a lower temperature T N2 described later. We note that T N1 is much lower than q CW . The q CW / T N1 ratio is about ~ 4.9, indicating considerable magnetic frustration in Mn TeO [22]. To understand better the origin of the 5.8 m B effective magnetic moment, we have carried soft X-ray absorption experiments at the Mn L B edge. Figure 1c shows the spectrum of Mn TeO together with those of MnO and LaMnO as reference for Mn and Mn ions, respectively. The features and their energy positions of the Mn
2+ 3+3
TeO spectrum resemble very much those of the MnO but are very different from the ones of LaMnO . We can therefore safely conclude that the Mn ion in Mn TeO is divalent and in the high spin ( S = 5/2) state [23]. Figures 2a,b show the temperature-dependence of the dielectric constant ( ε ) and the dielectric loss (tan d ) measured in zero field using different measurement frequencies (ranging from 1 kHz to 20 kHz). The most apparent anomaly in ε and tan d is a sharp l -like peak at T N2 ~
21 K, which is indicative for the emergence of a ferroelectric transition in Mn TeO at T N2 . The frequency independence of this sharp peak excludes possible spurious artefacts arising from grain boundaries, contact etc. We have performed complementary pyroelectric measurements. As shown in Fig. 2c a non-zero electric polarization ( P ) develops below T N2 and saturates at a value of about 2.7 μ C/m . The observed polarization can be inverted with opposite poling of the applied electric field unambiguously demonstrating the ferroelectric nature of the transition at T N2 . Hence, Mn TeO is a multiferroic material. The size of P is comparable to that of many other known multiferroics in which the electric polarization is induced by spin structures at low temperatures [5, 6]. In addition to the sharp peak, a kink-like feature also appears at T N1 in ε ( T ), whereas no corresponding anomalies can be observed in tan d ( T ). This demonstrates that Mn TeO is not ferroelectric between T N1 and T N2 . An anomalous drop in ε ( T ) at the Néel temperature (denoted as T N1 here) has been observed also in other non-polar AFM systems [24, 25]. The occurrence of an electric polarization at a temperature lower than the Néel temperature is often observed in multiferroic materials with an intrinsic coupling between dielectric and magnetic properties [26–28]. Hence, Mn TeO is indeed a new magnetically driven multiferroic material in which a sizeable magnetoelectric coupling can be expected due to the close coupling between magnetism and ferroelectric properties. The magnetoelectric coupling effects in Mn TeO were measured as a function of external magnetic field ( H = ε ( H ) – ε ( H= ε ( H = T = 25 K above T N1 , there is almost no field-induced change in ε . Below T N1 the magnetodielectric coefficient increases quadratically with H within the low field region – which is consistent with the empirical analysis based on Landau theory [24] – and tends to saturate at higher fields. In order to investigate the magnetic field effects in Mn TeO systematically, we plot ε as function of T under different external fields ( H = 0–9 T). Figure 2e shows that between T N2 and T N1 , the ε ( T ) curves are almost unaffected by H . Around T N1 there are not any discernable anomalies in dielectric loss up to the highest fields H = 9 T indicating no field induced FE above T N2 . But around T N2 , the dielectric peak is suppressed by H and is shifted to lower temperatures with increasing field. The field suppression of ferroelectricity in Mn TeO can be also observed within electric polarization measurements, see Fig. 2f. At 5 K, the saturated P decreases gradually from 2.8 μ C/m at zero field to 1.9 μ C/m at 9 T. The ferroelectric transition temperature T N2 deduced from the peak position in ε ( T ) decreases continuously from ~
21 K for H = 0 T to ~ H = 9 T. The strong field-dependent behavior of ε ( T ) and P indicates the magnetic origin of ferroelectricity in Mn TeO . Our results reveal two successive phase transitions in Mn TeO . This can also be seen in the specific heat measurements. Close to the large lambda-peak at T N1 we can detect a weak hump-like anomaly of C p / T around the ferroelectric transition T N2 (marked with the vertical dashed line in Fig. 3a. We now can draw a magnetic phase diagram of Mn TeO as shown in Fig. 3. The two successive magnetic phase transitions found in Mn TeO resemble on the observations in other magnetically driven multiferroic materials with spiral spin structures, e.g. in LiCu O (~ 24 K and ~22 K) [26], TbMnO (~41 K and ~28 K) [1] and NaFeSi O (~ 9 K and ~ 6 K) [28]. These materials first undergo a transition from a paramagnetic state to a sinusoidal SDW with collinear spin structure, which does not give rise to any ferroelectricity. As temperature decreases further, the order parameter grows and the systems undergo a second transition from SDW to the spiral state [27]. Similar to these known multiferroic materials, the FE polarization in Mn TeO occurs only below a second transition temperature T N2 , which is slightly lower than the first AFM transition temperature T N1 . Remarkably, the spin structure in the ground state of Mn TeO , reported by Ivanov et al. [20] is much more complex than in other known multiferroic materials. In Mn TeO the AFM state hosts two sets of incommensurate non-collinear spin configurations of fundamentally different types – one is a helical spin structure and the other one is basically a cycloidal one. Below T N2 , both helical and cycloidal spin sub-structures in Mn TeO are theoretically able to induce macroscopic electric polarization, but via different mechanisms. The cycloidal-like spin structure hosted in the Mn(2) chains can produce an electric polarization P perpendicular to c -axis via the inverse DM interaction, while the ‘screw-type’ helical spin sub-structure hosted within the Mn(1) chains is expected to induce a polarization along the c -axis according to the Arima model. Studies of the interplay between both spin substructures in single crystal Mn TeO would be highly interesting since the presence of two different kind of coexisting spiral spin configurations – helical and cycloidal – is an unprecedented situation in a multiferroic material. To summarize, in this work, we presented dielectric and pyroelectric measurements with high resolution on our bulk polycrystalline Mn TeO samples. For the first time, we observe strong magnetoelectric coupling and spin-induced ferroelectricity in Mn TeO . Hence, Mn TeO is a new multiferroic material with complex magnetic structure and with promising magnetoelectric properties. We observed successive two-step magnetic transitions: a non-FE intermediate magnetic state occurs below 23 K and a multiferroic ground state emerges below 21 K. In Mn TeO . Each of the two spin sub-structures might be able to produce macro electric polarization - either via inverse DM interaction for the cycloidal spin sub-structure or via the Arima mechanism for the helical spin sub-structure. The presence of two spin sub-structures might open new ways of manipulating the ferroelectric polarization in a multiferroic material. Acknowledgements
We would like to thank H. Borrmann and his team for powder X-ray diffraction measurements and Ch. Becker,T. Mende and S. Wirth for their support on the construction of the dielectric measurement devices at MPI CPfS. We also acknowledge Z.W. Li, A. Keil and H.J. Guo for their help.
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