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Understanding the multiferroicity in TmMn2O5 by a magnetically induced ferrielectric model

  • Scientific Reports 6, Article number: 34767 (2016)
  • doi:10.1038/srep34767
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Abstract

The magnetically induced electric polarization behaviors in multiferroic TmMn2O5 in response to varying temperature and magnetic field are carefully investigated by means of a series of characterizations including the high precision pyroelectric current technique. Here polycrystalline rather than single crystal samples are used for avoiding the strong electrically self-polarized effect in single crystals, and various parallel experiments on excluding the thermally excited current contributions are performed. The temperature-dependent electric polarization flop as a major character is identified for different measuring paths. The magneto-current measurements indicate that the electric polarization in the low temperature magnetic phase region has different origin from that in the high temperature magnetic phase. It is suggested that the electric polarization does have multiple components which align along different orientations, including the Mn3+-Mn4+-Mn3+ exchange striction induced polarization PMM, the Tm3+-Mn4+-Tm3+ exchange striction induced polarization PTM, and the low temperature polarization PLT probably associated with the Tm3+ commensurate phase. The observed electric polarization flop can be reasonably explained by the ferrielectric model proposed earlier for DyMn2O5, where PMM and PTM are the two antiparallel components both along the b-axis and PLT may align along the a-axis. Finally, several issues on the unusual temperature dependence of ferroelectric polarizations are discussed.

Introduction

The discovery of magnetically induced ferroelectric (FE) polarization in a number of transition metal oxides which usually have the strong electronic correlation characters represents a milestone for multiferroic physics and materials sciences1,2,3,4,5,6. Taking perovskite ABO3 manganites (RMnO3) as the most extensively investigated multiferroic systems where R is Y or rare-earth7,8, two major microscopic mechanisms for magnetically induced ferroelectricity have been well accepted. The first mechanism is the collective ionic displacement generated by the Dzyaloshinskii-Moriya (DM) interaction (asymmetric exchange striction) associated with the spin-orbit coupling in non-collinear spin systems such as orthorhombic RMnO3 with R = Ga, Tb, Dy, and so on3,9,10,11. The second one is the collective ionic displacement generated by the spin-lattice (symmetric exchange striction) in specific collinear spin systems (e.g. ↑↑↓↓ spin order) such as orthorhombic RMnO3 with R = Ho, Y, Tm, Lu, and so on12,13,14,15. While some other mechanisms have also been proposed for individual systems16,17, the two mechanisms represent the main pillars for the physics of magnetically induced ferroelectricity in the so-called type-II multiferroics4.

Nevertheless, besides RMnO3, another class of manganites, RMn2O5, which are also orthorhombic in structure and identified as the type-II multiferroics, are believed to have different physics of multiferroicity but not yet well-understood18,19,20,21,22,23,24,25,26,27. The FE polarizations in most RMn2O5 are much larger than those in RMnO3 and the multiferroic phase transitions are more complicated23,27,28. The possible reason for such big difference is associated with the stronger spin frustration due to the coexistence of Mn3+ and Mn4+ spins and more seriously distorted lattice structure29. These characters may enable more than three, four, or even more magnetic phase transitions within a narrow range of temperature (T) below 40 K. We take DyMn2O5 as an example, which is a representative one of this RMn2O5 family of complicated magnetic structure evolution30. Upon cooling from the paramagnetic (PM) state above T = TN0 ~ 43 K, a series of incommensurate (IC) antiferromagnetic (ICM) and commensurate (C) antiferromagnetic (CM) ordering events are identified, where the moments are believed to be from the Mn spins. It is suggested that DyMn2O5 transits into a non-FE ICM phase below TN0 and then enters a FE CM phase below TN1 ~ 40 K. The magnetic phase transitions at TN2 ~ 28 K mark the gradual transition from the FE CM phase to the non-FE ICM phase until TDy ~ 9 K below the Dy3+ spins form the independent CM order.

Based on the magnetic scenario highlighted above, the FE behaviors of RMn2O5 members have been discussed in details18,19,20,29,30,31. The common characters in terms of ferroelectricity, excluding those individual features for respective members, can be summarized from several aspects11,31. First, an electric polarization P (denoted as PC) appears in the CM phase below TN1 and above TN2, which is quite large. The measured P drops rapidly down to another P (denoted as PIC) once the CM phase is gradually and partially replaced by the ICM phase below TN2. The PIC can possibly be negative in some cases and positive in others. For reference, the measured P(T) data below TN1, taken from literature for several compounds using the conventional pyroelectric current technique are schematically drawn in Fig. 1(a)31,32. Second, the two components PC and PIC can be separated by assuming various mechanisms for them. The two cases, taking R = Tm and R = Y, are plotted in Fig. 1(b,c) for a guide of eyes31,33. It has been suggested that component PC is related to the symmetric exchange striction, i.e. PC ~ (Si·Sj) with Si and Sj the spin moments at two neighboring sites i and j, while component PIC is ascribed to the asymmetric exchange striction, i.e. PIC ~ (Si×Sj). Both PC and PIC align along the b-axis. It should be mentioned here that for TmMn2O5, third electric polarization PLT along the a-axis is believed below T ~ TTm ~ 5–6 K, which is associated with a long-range commensurate (LCM) ordering of Tm3+ spins34,35. Third, it was revealed that the response of polarization below TN2, i.e. PIC, against magnetic field H, is much more remarkable than that of polarization PC between TN1 and TN2. This effect fits the scenario of PIC ~ (Si×Sj), noting that the noncollinear spin structure is usually more sensitive to varying H than the collinear structure36,38.

Figure 1: Schematic presentations of ferroelectric polarization P(T) for several members of RMn2O5 for a guide of eyes.
Figure 1

(a) The P(T) curves for R = Y, Dy, Eu, and Tm. (b) The proposed polarization components as a function of T respectively for R = Tm, where the claimed origins for PC, PIC, and PLT are described in text. (c) The proposed polarization components as a function of T respectively for R = Y. These curves are sketched from the data reported in ref. 31.

Nevertheless, one has several reasons to question the above highlighted scenario on the multiferroicity of RMn2O5. First, it is seen that the P(T) dependences of these compounds above TN2, i.e. PC(T), are roughly similar for different compounds, but the P(T) dependences below TN2, i.e. PIC(T), are very different from each other with no rational correlation between the PIC(T) behavior and the R site ionic size or magnetic moment22,24. This suggests that the measured PIC contain some unknown contributions other than PIC ~ (Si×Sj). Second, it is noted that all these data were obtained in single crystal samples grown by flux method. These single crystals are claimed to have strong self-polarized effect which would self-polarize the FE domains32,33, leading to the strong path-relevance of the measured data. This consequence was often mentioned in literature on RMn2O533,35 to account for those anomalous data on polarization below TN2. In this sense, the proposed relation PIC ~ (Si×Sj) should be questioned too. To avoid this self-polarized effect and its impact on the pyroelectric current data, polycrystalline sample may be even a better choice than single crystal since the small grain size in polycrystalline samples allows a sufficient cancellation of this self-polarized effect if any. Third, the so-called double-wave method (DWM) associated with the Sawyer-Tower circuit was used to measure the polarization-electric field hysteresis loop of these materials31. The obtained second hysteresis loops exhibit clear double-loop shape, suggesting possible ferrielectric (FIE) or antiferroelectric (AFE) characters for YMn2O5 around TN2, reviving the earlier proposed FIE model although a number of unclear issues on this model remains to be identified.

In fact, systematic investigations on the behaviors of P(T, H) in polycrystalline DyMn2O5 were performed recently using the highly sensitive pyroelectric current technique29,38,39. The main polarization can be illustrated by a FIE model, in which consists of two roughly antiparallel polarization components. This FIE model reasonably explains the main features of the measured multiferroic behaviors of DyMn2O5 below TN129,40. However, this FIE model relies on the R-Mn coupling and it becomes questionable if R is non-magnetic such as R = Y, which remains to be an issue so far. Furthermore and more importantly, additional check of this FIE model with other RMn2O5 would be necessary for a consideration of generality. In this work, we intend to investigate the multiferroicity of TmMn2O5. Earlier neutron scattering analysis of the magnetic structure and electric polarization of TmMn2O5 suggested a qualitatively similar magnetic structure and FE behaviors to those of DyMn2O521,34. The T-phase diagram in terms of dielectric permeability and IC/C-AFM phase on single crystal TmMn2O5 is indeed similar to DyMn2O5 although the values of TN0, TN1, TN2 are slightly different34. This allows us an opportunity to revisit this FIE model for understanding the complicated magnetism-induced ferroelectric behaviors of TmMn2O5.

Results

Multiferroic phase transitions

Before presenting the Multiferroic phase transitions data, we first give a set of microstructural characterizations results. The lattice structure of orthorhombic TmMn2O5 is schematically shown in Fig. 2(a), which clearly indicates the ordered Mn3+ and Mn4+ occupation in the lattice. The occupancy of Tm3+ ions can be also seen clearly. In Fig. 2(b), the measured and refinement-evaluated data X-ray diffraction (XRD) θ–2θ patterns at room temperature for a sample is presented. Pure orthorhombic structure with space group of Pbam is clearly identified, as confirmed by the high refinement quality factors shown in the inset of Fig. 2(b). The refined lattice parameters are a = 0.7216 nm, b = 0.8436 nm, and c = 0.5654 nm, well consistent with earlier reported results21. Furthermore, the scanning electron microscopy (SEM) images of the broken surface of the sample at several scales are shown in Fig. 3(a~c). It is seen that the grains are short-bar like in shape and very dense. The spatial distributions of the species Tm, Mn, and O on an area shown in Fig. 3(d) are presented in Fig. 3(e–o) respectively, revealing the high element homogeneity. The evaluated chemical composition is close to the nominal one within uncertainty of less than 5%.

Figure 2
Figure 2

(a) A schematic drawing of lattice structure of TmMn2O5. (b) Measured XRD θ-2θ spectrum of polycrystalline TmMn2O5 sample and the Rietveld method refined data for comparison. The reliability factors Rwp, Rp, and χ2 of the refinement are labeled.

Figure 3
Figure 3

SEM images of the polycrystalline sample (a–d). The planar composition distributions of Tm, Mn, and O are presented in (e–g) respectively, as obtained by the EDS imaging with the SEM.

We discuss the measured specific heat (CP), magnetization (M), and dielectric constant (ε) as a function of T respectively, and the results are summarized in Fig. 4(a~c). In order to exaggerate the features in the CP ~ T curve, the evaluated d(CP/T)/dT ~ T curve is inserted in Fig. 4(a) for a comparison. Given the measuring uncertainties from various research groups, the proposed magnetic transition points are nicely reproduced in the present work, with an error of ~ ±1 K, i.e. TN0 ~ 44 K, TN1 ~ 35 K, TN2 ~ 24 K, and TTm ~ 6 K21,34,35. As stated earlier21,35, TN0 marks the transition from the PM phase to the high-temperature ICM phase (probably mixed with small amount of CM phase), and TN1 labels the transition from the ICM phase to the CM phase, followed by the transition from the CM phase to the low temperature ICM phase at TN2. The anomaly at TTm indicates the so-called LCM phase associated with the Tm3+ spin order and the nature of this LCM ordering remains elusive.

Figure 4
Figure 4

Measured specific heat CP and its derivative d(CP/T)/dT (a) magnetization M under the FC and ZFC modes, and dielectric constant at f = 1 MHz (b) and pyroelectric current Ipyro(T) at Tend = 2 K, EP = 10 kV/cm, and a warming rate of 2 K/min (c) as a function of T respectively.

It should be mentioned that the anomaly in the d(CP/T)/dT ~ T curve around TN2 is not remarkable, while some other weak anomalies between TN1 and TTm have not yet been properly assigned. The weak feature around TN2 seems to suggest that the CM-ICM transition may not be typical and the CM and ICM coexistence in this T-range is highly possible. On the other hand, the Tm-Mn coupling should be strong although no specific indication in the d(CP/T)/dT ~ T curve can be found. The well-ordered magnetic structure including the R3+ spin ordering in the high-T range for TmMn2O5, similar to the cases of DyMn2O5 and GdMn2O5 for instance19,27, confirms the strong Tm-Mn coupling.

Nevertheless, different from the CP data, the measured M(T) curve does not show non-trivial feature but a smooth and monotonous increasing with decreasing T over the whole T-range, as shown in Fig. 4(b). No separation between the ZFC and FC curves and no remarkable anomaly at TN0, TN1, TN2, and even TTm can be identified. Obviously the magnetization signals are mainly from Tm3+ moment since it is much larger than the Mn3+/Mn4+ moment. The smooth M ~ T curve over the whole T-range also suggests that the Tm3+ spin ordering is already gradually developed far above TTm ~ 6 K, most likely induced by the strong Tm-Mn coupling. Otherwise, an anomaly around TTm should be observable if the Tm3+ spin structure is paramagnetic above TTm.

The measured ε(T) data provide additional indication of the multiferroic phase transitions and one set of data at f = 1.0 MHz are plotted in Fig. 4(b), while the ε(T) curves at different f show similar features. Besides the relatively sharp peak around TN1 and small peak right around TTm, a smeared and broad shoulder around TN2 can be found. The sharp peak at ~TN1 certainly marks the FE transition corresponding to the ICM-CM transition. The small peak at ~TTm indicates another FE transition which should be related with the LCM ordering at TTm. The intermediate shoulder (bump) seems to characterize the gradual or diffusive FE transition covering the broad T-range around TN2. It was reported that the single-crystal samples do show a weak jump around TN234,41, which becomes a shoulder here.

Before discussing the magnetically induced ferroelectricity data in details, we present one representative pyroelectric current Ipyro(T) curve as shown in Fig. 4(c) measured at Tend = 2 K with a warming rate of 2 K/min. The data reliability will be identified later. First, one sharp valley at ~TN1 and one sharp peak at ~TTm are observed, consistent with the anomalies in the ε(T) curve. The sharp valley indicates the appearance of electric polarization at TN1. The peak at ~TTm indicates another FE transition and the generated electric polarization should be assigned as PLT along the a-axis (shown in Fig. 1(b))31,34. Second, a broad peak between TN1 and TN2 is identified, suggesting the existence of third electric polarization which ensues gradually with decreasing T. Here, the most important feature is the negative current valley at ~TN1 and broad positive peak at TN2 < T < TN1, intimating the existence of two antiparallel polarizations, distinctly different from the reported data on single crystal samples32,33. This behavior is however similar to earlier data on polycrystalline DyMn2O529,39. It may come to us immediately that the two polarizations are the components of a FIE state, giving rise to the polarization flop with decreasing T at certain temperature. Below TN2, the two polarizations compete with each other, leading to the flat grade between TN2 and TTm. A detailed discussion on this FIE model will be performed later.

Pyroelectric current and electric polarization

The issue of top priority here is the reliability of measured Ipyro(T) data, and this issue is critical for a growing understanding of the underlying physics. The pyroelectric current method is sometime questioned since the measured “pyroelectric” current has been often questioned to include other current contributions such as trapped charges29,30,42. A careful clarification of these contributions if any should be made. We have performed the following measurements for this clarification.

First, we checked the Ipyro(T) data at different warming rates given the fixed electric poling field EP = 10 kV/cm and Tend = 2 K, as shown in Fig. 5(a). The three peaks/valleys against varying warming rate are non-shifted from one and another, and the under-curve area is roughly proportional to the warming rate, revealing that the measured current signals have no contribution from the de-trapped charges during the sample warming. Otherwise one will observe the shifting of these peaks/valleys towards the high-T side with increasing rate. Second, the nearly symmetric Ipyro(T) curves with respect to axis Ipyro = 0 under EP = ±10 kV/cm respectively, as plotted in Fig. 5(b), also evidence the reversible electric polarization. The as-evaluated P(T) curve is presented in Fig. 5(c), indicating clearly the appearance of a negative polarization around TN1, the polarization flop from negative value to positive one around TN2, and another FE transition around TTm, respectively. This polarization flop is the character of a typical FIE system29,38.

Figure 5
Figure 5

Measured Ipyro(T) curves at three warming rates at EP = 10 kV/cm (a) Ipyro(T) curves at EP = ±10 kV/cm and warming rate of 2 K/min (b) evaluated P(T) curve at EP = 10 kV/cm (c). Tend = 2 K for all the cases.

For further checking the pyroelectric origin for the Ipyro(T) data, we plot the Ipyro(T) curves measured at different EP in Fig. 6(a). We first look at the data at EP = 0 which are indeed very weak and can’t be identified unless the data are magnified for 30 times. The repeated measuring cycling shows the similar features: weak valley around TN1, weak bump right above TN2, and another weak valley or bump (history-dependent) around TTm, respectively. Given the fact that the sample was not pre-poled electrically, these weak features suggest the existence of FE phase transitions around TN1, TN2, and TTm respectively. The electric pre-poling treatments under increasing EP enable a set of Ipyro(T) curves with non-shifted valleys/peaks but increasing valley/peak magnitudes. This fact also suggests no remarkable contribution from the de-trapped charges which otherwise would shift the valleys/peaks with increasing EP. The corresponding P(T) curves evaluated from the Ipyro(T) data are plotted in Fig. 6(b), giving rise results consistent with the above discussion.

Figure 6
Figure 6

Measured Ipyro(T) curves at different poling fields EP as labeled (a). Evaluated P(T) curves from these Ipyro(T) curves (b). Tend = 2 K for all the cases. Measured dielectric constant ε(T) curves at several different frequencies (c).

The measured ε(T) curves under an ac-field of 0.1 V/cm and different frequencies f are also plotted in Fig. 6(c) for checking the FE phase transitions. All these curves have the anomalies at TN1 and TTm, while weak shoulders around TN2 appear only in the high-f curves. It is noted that the highest frequency used for the ε(T) measurements is 1.0 MHz which is sufficient to suppress any charge de-trapping under such a weak electric field. In the other word, these anomalies would be absent if the Ipyro(T) signals are from the non-pyroelectric contributions. Furthermore, it is found that the dielectric peak at TN1 is quite broad, suggesting that the FE phase transition is not sharp. However, this phase transition would not be diffusive since no remarkable frequency dispersion can be identified34,42. The another possible reason is that the broad peak may come from a superposition of more than one FE phase transitions such as the consecutive appearance of the two electric polarizations of the FIE system, as to be proposed below. On the other hand, it is understandable that the polarization flop as proposed at TN2 may not necessarily produce significant dielectric response. Anyhow, to this end, one may trust that the measured Ipyro(T) data are indeed contributed from the pyroelectric current and the FE phase transitions do appear at TN1 and TTm, while a polarization flop at TN2 is argued.

Path-dependent electric polarization

In order to understand the nature of the observed P(T) dependence, we have performed a set of Ipyro(T) measurements given a fixed EP = 10 kV/cm but different Tend. The measured Ipyro(T) curves and evaluated P(T) curves are plotted in Fig. 7(a,b) respectively where the curves are shifted vertically for clarification. Several critical features deserve for highlighting here. First, the major features of the Ipyro(T) curves above Tend remain essentially unchanged, showing that the data are path-independent. In other words, this set of experiments demonstrates that the pyroelectric current is only T-dependent and has nothing to do with the measurement sequences (e.g. different Tend). Second, the peaks around TTm and TN2 and the valley at TN1 are seemingly irrelevant with each other and the disappearance of one feature does not affect the others. Third, the polarization flop occurring around TN2 remains unaffected until Tend > TN2. What surprises us is that the negative polarization remains as T approaches TN1, noting that the sample is positively poled. This unusual behavior will also be discussed later.

Figure 7
Figure 7

Measured Ipyro(T) curves (a) and evaluated P(T) curves (b) for different values of Tend as marked. The warming rate is 2 K/min and EP = 10 kV/cm. For data clarification, these curves are shifted from one and another vertically.

Discussion

We would like to address that all these features support the argument that the observed P(T) dependence is the consequence of a FIE system. Before we discuss this FIE scenario, it is noted that the Ipyro(T) peak around TTm is contributed from the LCM phase generated polarization PLT aligned along the a-axis31,34,35. Due to the polycrystalline nature of the samples, its contribution can be detected here. Similar to the FIE model in DyMn2O5, see Fig. 8, Fig. 8(a) shows the ionic and spin configurations on the ab-plane of DyMn2O5, which consist of ordered Mn4+, Mn3+, and Dy3+ occupations in the lattice. Clearly, the ab-plane spin configuration can be divided into four sub-groups as shown in Fig. 8(b,c)39. If the weak noncollinear components of these spins are not considered, these sub-groups actually are the ↑↑↓ or ↓↓↑ blocks. Each of these blocks contributes one local electric dipole due to the ionic displacement, consulting to the symmetric exchange striction for ferroelectricity generation. Immediately, one recalls that the ab-plane constitutes a FIE lattice of two roughly antiparallel polarization components: One component can be generated by the symmetric exchange striction in the Mn3+-Mn4+-Mn3+ blocks, denoted as PMM, and the other can be generated by the symmetric exchange striction in the Tm3+-Mn4+-Tm3+ blocks, denoted as PTM which is antiparallel to PMM. Here, we applies it to the present case of TmMn2O5 for explaining the observed P(T) behaviors. The only revision to Fig. 8(a~c) is to replace the Dy3+ ions with the Tm3+ ions. In principle, it is believed that the strong Tm-Mn coupling enables the gradual ordering of Tm3+ spins in coherence with the Mn3+/Mn4+ spin ordering below TN1. The independent Tm3+ ordering occurs at TTm due to the competition of the Tm-Tm exchange over the Tm-Mn coupling, contributing to the LCM phase around TTm.

Figure 8
Figure 8

(a) The ab-plane projected ionic and spin configurations of DyMn2O5 and TmMn2O5 is assumed to have similar configurations, where the arrows indicate the spins, open circles mark the original ionic sites without inclusion of the magnetic interactions, and solid circles the final ionic sites with the magnetic interactions. (b,c) The four blocks each of which consists of three neighboring magnetic cation ions. The symmetric exchange striction in each block leads to shifting of the ions, generating a local electric polarization as indicated by the long arrow, and the as-generated polarizations PMM and PDM (PTM) are roughly antiparallel, forming a ferrielectric lattice.

The proposed physical sequence upon decreasing T is as the following. As T falls down to TN1, the Mn spins begin to order into the CM phase which generates polarization PMM. In spite of the claimed CM-ICM phase transition at TN2, no sufficient evidence with the absence of ferroelectricity in the CM phase is available. Therefore, it can be argued that this CM-ICM phase transition does not affect the PMM very much. Since the Mn-Mn exchange is strong, suggesting that the spin ordering at TN1 is sharp and the FE phase transition finishes in a narrow T-range. On the other hand, the Tm-Mn coupling enables the coherent Tm3+ spin ordering with the Mn spin ordering, leading to polarization PTM which is roughly antiparallel to PMM. This phase transition would be relatively diffusive since the Tm3+ spin ordering is induced by the Tm-Mn coupling as the second-order exchange. Consequently, the total polarization P = PMM + PTM as a function of T can be complex and a polarization flop event may occur.

The observed P(T) curve suggests that inequality |PTM| > |PMM| should be satisfied below TN1. The proposed two components as a function of T in a qualitative sense, are plotted in Fig. 9(a). The two FE sublattices coherently constitute the FIE lattice and the two polarization components as a function of T respectively are schematically drawn just for a guide of eyes. When the low-T polarization component PLT is summed to the total P = PMM + PTM + PLT, the observed P(T) curve (blue) can be nicely reproduced, as shown in Fig. 9(a) too. The three polarization components at different T can be schematically mapped by arrows in Fig. 9(b).

Figure 9
Figure 9

(a) Measured P(T) curve at the warming rate of 2 K/min and EP = 10 K/min, and the assumed polarization components PMM(T), PTM(T), and PLT(T), as a function of T respectively. (b) A schematic drawings of the three polarization components (arrows in colors) at different T.

It has been repeatedly confirmed that the Mn spin orders in RMn2O5 systems are highly robust against external magnetic field38,39,40. A magnetic field of several Tesla seems not to shake much the Mn spin orders. Different from this property, one is aware of the much soft Tm3+ spin orders against magnetic field, due to the fact that the 4f-4f exchange is weak with respect to the Mn-Mn exchange43,44. The Tm-Mn coupling is much stronger than the 4f-4f exchange although it is relatively weaker than the Mn-Mn exchange. Therefore, both PTM and PMM are robust against H but PMM is the highest robust. This difference allows an opportunity to check this FIE model by measuring the magnetoelectric response of TmMn2O5. Given a fixed T, applying a magnetic field certainly destabilizes the Tm3+ spin orders and thus completely suppresses polarization component PLT, while component PTM may be also partially suppressed but PMM remain less affected.

The above discussion is confirmed by the measured data, as shown in Fig. 10(a~d) respectively. On one hand, the Ipyro(T) peak and the PLT component in the P(T) curves around TTm are indeed suppressed by a field of ~5.0 T, while PMM and PTM remain less affected, as shown in Fig. 10(a,b). On the other hand, Fig. 10(c,d) show the magnetically induced current (IH) loops in response to the H-cycling at T = 2 K and 10 K. The IH ~ H loop at 2 K clearly indicates the suppression of PLT by increasing and decreasing H cycle. This feature becomes seriously weakened at T = 10 K where the weak IH response is from the partially H-suppressed PTM, consistent with the above discussed FIE scenario too.

Figure 10
Figure 10

Measured Ipyro(T) curves (a) and evaluated P(T) curves (b) under different magnetic fields H as labeled, with Tend = 2 K and EP = 10 kV/cm. The iso-thermal Ipyro(H) hysteresis loops at T = 2 K and 10 K are plotted in (c,d) respectively. The polarization components at various stages are drawn for a guide of eyes.

It should be noted that our proposed FIE model seems to reasonably explain our experimental results. However, the ceramic samples exhibit some intrinsic defects such as grain boundaries and voids, which have an influence on the polarization properties, although the use of polycrystalline may avoid the impact of self-polarized effect on polarization behaviors in our samples. Therefore, we felt that more research by testing of single crystal samples is needed to further support our claims.

Furthermore, an unsolved and puzzling issue regarding this FIE model is the appearance of negative P right below TN1 for both DyMn2O5 discussed earlier and TmMn2O5 here. In fact, the appearance of negative P right below TN1 is understandable if TendTN1, because of |PTM| > |PMM| at T TN1 and EP > 0. However, as shown in Fig. 7, for Tend ~ 32 K at which |PTM| < |PMM| and thus the positive EP would drive a positive P instead of negative P under a positive poling field. An understanding of this unusual feature is detrimental to the FIE model, which otherwise could be totally wrong.

So far, almost all of the literature discussing the multiferroicity of RMn2O5 has concentrated on the magnetically induced ferroelectricity below TN1 (TN0)18,19,20,21,22,23,24,25,26,27. This fact leaves a somehow misleading impression that RMn2O5 would be paraelectric above TN1. The only exceptional experiment was from V. Baledent et al. who demonstrated that RMn2O5 is actually a family of room ferroelectricity whose microscopic origin has nothing to do with magnetism, and it was argued that this room temperature ferroelectricity is structurally driven45. The magnetically induced FE polarization is simply an additional component to the room ferroelectricity. Unfortunately, due to the extremely large conductivity of single crystal RMn2O5 at T > 150 K, the FE polarization at high T can’t be measured. In the present work, our polycrystalline samples show better insulating property than single crystals, allowing a relatively sufficient electric poling at T up to ~300 K by a field of ~8 kV/cm, although the electrical insulativity of the samples are still far from sufficient to obtain reliable P-E hysteresis. The high-T FE phase transition temperature has not yet been determined.

Nevertheless, the claimed room ferroelectricity for TmMn2O5 allows us an opportunity to explore the origin for the negative P right below TN1. We perform the following experiments: an electric field EP(T), which is gradually increased with decreasing T, is imposed to the sample during the sample cooling. The value of EP is increased to 10 kV/cm (or −10 kV/cm) at 260 K down to T ~ 45 K which is slightly higher than TN0 = 44 K. Then the poling field is removed and the sample is further cooled down to 32 K which is below TN1. Subsequently, the sample is sufficiently short-circuited electrically and then the sample is warmed at a rate of 2 K/min up to 50 K, during which the Ipyro(T) is measured. The measured data are presented in Fig. 11(a,b) respectively, where the EP(T) data are also inserted for reference.

Figure 11: Measured Ipyro(T) curve and evaluated P(T) curve for two specific cases.
Figure 11

The sample was cooled down from room temperature with an electric poling down to 45 K, and then further down to 32 K with zero electric bias, followed by warming at a rate of 2 K/min during Ipyro(T) data are is probed: (a) EP < 0 and (b) EP > 0. The poling sequences for the two cases are marked.

Surprisingly, one observes the positive Ipyro peak (positive P) around TN1 when EP < 0 above TN0 (Fig. 11(a)), and the negative Ipyro peak (negative P) around TN1 when EP > 0 above TN0 (Fig. 11(b)), noting that no electrical bias is imposed on the sample between TN0 and T = 32 K. This implies that the negative P valley just below TN1 in our whole package of measurements is induced by the electric poling process far above TN0. This is the reason for the observed negative P valley in Fig. 7 at Tend = 32 K and below.

Assuming that TmMn2O5 is already a ferroelectric at room temperature and its polarization is P0, which is positive if EP > 0 and negative if EP < 0. This structurally induced P0 may align the magnetically induced polarization PMM in opposite direction via e.g. the ferroelastic effect or so far unknown magneto-lattice coupling mechanism46,47. In the other words, PMM is always antiparallel to P0 which is however aligned by the electric poling. Although such a coupling or ferroelastic effect has not yet been understood and will be further investigated, all of the observed phenomena in our experiments can be well explained, given this PMM↓↑P0 assumption.

Conclusion

In conclusion, we have performed extensive measurements of the magnetically induced ferroelectricity of TmMn2O5 in polycrystalline form, focusing on the ferrielectric nature and its magnetic origins. It is revealed that the ferroelectric polarization contains several components which appear respectively in various temperature ranges and is suggested to be generated by different microscopic mechanisms. These components include the polarization PMM generated by the Mn3+-Mn4+-Mn3+ collinear spin block via the symmetric exchange striction, the polarization PTM generated by the Tm3+-Mn4+-Tm3+ collinear spin block via the symmetric exchange striction, and the polarization PLT generated by long-range noncollinear Tm3+ spin ordering via the asymmetric exchange striction. The Tm3+-Mn4+-Tm3+ collinear spin ordering is driven by the strong Mn-Tm coupling. The two components PMM and PTM are antiparallel, constituting two ferroelectric sublattices of a ferrielectric system. The polarization flop with decreasing temperature is observed as a representative character of the ferrielectricity. It is suggested that this magnetically induced ferrielectricity of TmMn2O5 is likely an additional ingredient to the high-temperature ferroelectricity which is structurally driven. The present work represents a growing understanding of the complicated multiferroic behaviors in RMn2O5 compounds.

Methods

In our experiments, polycrystalline TmMn2O5 samples were prepared by the standard solid-state reaction method. Stoichiometric amounts of Tm2O3 (99.99%) and Mn2O3 (99%) powder were thoroughly ground and then fired at 980 °C for 24 h in a flowing oxygen atmosphere. The resultant powder was re-ground and granulated using 4 wt% poly vinyl alcohol (PVA) solution and then pelletized under a pressure of 9 MPa into disks of 11.5 mm in diameter. The disk samples were sintered at 1080 °C for 48 h in a flowing oxygen atmosphere in prior to natural cooling down to room temperature. The as-prepared samples were submitted to a set of microstructural characterizations. The crystallinity and lattice structure were checked using the X-ray diffraction (XRD) (PANalytical X’Pert PRO diffractometer) with the Cu-Kα radiation at room temperature. The data were refined using the Rietveld method. The scanning electron microscopy (SEM, Ultra 5, Zeiss) and the associated EDS mapping were used to check the grain morphology and chemical distribution.

The isometric specific heat (CP) of the sample as a function of T was measured in the standard procedure using a physical properties measurement system (PPMS, Quantum Design Inc.) installed inside a well-shielded space to insure extremely low electrical and thermal noise (background). The electrical noise can be as low as 0.02 pA as probed by the Keithley 6430 electrometer. The evaluated d(CP/T)/dT ~ T data are used to mark these phase transition points. The dc magnetization M(T) data at the field-cooling (FC) and zero-field-cooled (ZFC) modes were obtained using the vibrating sample magnetometer (VSM) integrated with the PPMS system. The cooling and measuring fields were both 100 Oe, sufficiently low so that the magnetic field driven side-effects if any are as weak as possible.

For the electrical measurements, disk-like samples of 11.5 mm in diameter and 0.17 mm in thickness were deposited with Au electrodes on each side. The dielectric constant ε as a function of T at various frequencies (f) covering six orders of magnitude was measured using the E4980A precision LCR meter. The electric polarization P as a function of T was evaluated from the pyroelectric current Ipyro-T data. The measurement was carried out following the procedure below. First, the sample in the plate capacitor geometry was cooled down to 150 K without electrical bias and then an electric poling field EP = ± 10 kV/cm was applied to the sample during further cooling at a rate of 2 K/min down to a given end temperature Tend. Then the sample capacitor was electrically short-circuited for sufficient time at Tend, followed by a slow sample heating until a temperature higher than TN0, during which the electric current (Itot) released from the capacitor was recorded using the Keithley 6430 electrometer.

A set of additional experiments were performed to check whether the probed current Itot from the capacitor is solely from the pyroelectric current Ipyro or not. These experiments include the measurement of dependence ε(f, T), released current curves Itot(T) at several heating rates, and isothermal magneto-current IH in the H-cycling at a rate of 150 Oe/s. The Ipyro(T) data under various magnetic fields H were also collected. The Tend was varied from 2 K to T < TN1. Furthermore, the polarization current method was also employed to qualitatively the sample’s ferroelectricity. The current passing through the sample under an electric field of 10 kV/cm, which should contain both the leaky current (IE) and polarization current (IP), was measured too. Assuming that IE(T) decreases monotonously with decreasing T, the IP can be extracted although it may not be quantitatively accurate. Unfortunately, for TmMn2O5, the current passing across the samples was too big to be possible for extracting the polarization current of tens of pA, suggesting the incapability of this method in the present case.

Additional Information

How to cite this article: Yang, L. et al. Understanding the multiferroicity in TmMn2O5 by a magnetically induced ferrielectric model. Sci. Rep. 6, 34767; doi: 10.1038/srep34767 (2016).

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Acknowledgements

This work was financially supported by the National Key Research Program of China (Grant No. 2016YFA0300101), the National Natural Science Foundation of China (Grant Nos 11234005, 51431006, 51332007 and 11574091), and the Science and Technology Project of Guangdong Province (Grant No. 2015B090927006).

Author information

Affiliations

  1. Institute for Advanced Materials and Laboratory of Quantum Engineering and Quantum Materials, South China Normal University, Guangzhou 510006, China

    • L. Yang
    • , P. L. Li
    • , M. Zeng
    • , M. H. Qin
    • , X. S. Gao
    •  & J.-M. Liu
  2. Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China

    • X. Li
    • , M. F. Liu
    • , Z. B. Yan
    •  & J.-M. Liu
  3. Institute for Advanced Materials, Hubei Normal University, Huangshi 435003, China

    • J.-M. Liu

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Contributions

J.-M.L. and M.Z. conceived the idea. J.-M.L., L.Y. and P.L.L. designed and performed the experiments. J.-M.L., M.Z., L.Y., X.L., M.F.L., Z.B.Y., M.H.Q., X.S.G. and M.F.L. analyzed the data. J.-M.L., M.Z. and L.Y. wrote the manuscript. All the authors commented on the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to M. Zeng or J.-M. Liu.

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