Absence of ferroelectricity in double-perovskite Y 2 CoMnO 6 single crystals

Absence of ferroelectricity in double-perovskite Y ₂ CoMnO ₆ single crystals

Cite as: J. Appl. Phys.126, 084102 (2019);doi: 10.1063/1.5111790

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Submitted: 30 May 2019 · Accepted: 6 August 2019 · Published Online: 26 August 2019

S. M. Wang,¹S. H. Zheng,¹ L. Lin,^1,a)Y. S. Tang,¹J. H. Zhang,¹ R. Chen,²J. F. Wang,²C. L. Lu,²Z. B. Yan,¹ X. P. Jiang,³and J.-M. Liu^1,3

AFFILIATIONS

1Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China

2Wuhan National High Magnetic Field Center and School of Physics, Huazhong University of Science and Technology, Wuhan 430074, China

3School of Materials Sciences, Jingdezhen Ceramic Institute, Jingdezhen 333403, China

a)Author to whom correspondence should be addressed:llin@nju.edu.cn

ABSTRACT

The rare-earth double-perovskite R₂CoMnO₆family has been reported to be promising multiferroics where the predicted ferroelectric polar- ization along theb-axis is induced by the Co²⁺/Mn⁴⁺collinear magnetic order. In this work, we report our characterizations on the struc- tural, magnetic, dielectric, and ferroelectric behaviors offlux-grown Y2CoMnO6single crystals. A weak ferromagnetic transition with the Curie pointTC∼70 K is evident in combination with a remarkable magnetic hysteresis loop along thec-axis. The earlier reported dielectric anomaly at the magnetic transition is not detected in our single crystals. In addition, the peak of each electric current curve as a function of temperature, probed along different crystallographic directions, shows serious shifting with the temperature ramping rate, excluding the intrinsic ferroelectric contribution. The measured data are not in conformity with theoretical predictions and earlier experiments on poly- crystalline samples. In addition, no reliable sign for magnetoelectric coupling can be detected. The present work seems to exclude the exis- tence of magnetism-driven ferroelectricity in Y2CoMnO6.

Published under license by AIP Publishing.https://doi.org/10.1063/1.5111790

I. INTRODUCTION

In the past ten or more years, type II multiferroics, in which magnetism and ferroelectricity coexist and couple in a single phase, have experienced a resurgence of interest for promising potential applications in new magnetoelectric devices.^1–4In these systems, fer- roelectric polarization (P) appears intrinsically in phases of specific spin orders, e.g., spiral order or collinear order.^5–8Many prototypes of multiferroics belong to the former case^9–13where spiral spin order drives long-range ferroelectric ordering through competition between the nearest-neighbor and further-neighbor magnetic exchanges of comparable strength.⁵ Hence, the spin-orbit coupling or in other words the Dzyaloshinskii-Moriya (DM) interaction governs the mech- anism of the magnetoelectric effect.^13,14However, it is rare that these interactions [usually antiferromagnetic (AFM)] are comparable in a magnitude of room temperature. In fact, small but similar nearest- neighbor and further-neighbor exchanges are more frequently found, and thus the multiferroic temperature is considerably low.

In parallel to the magnetoelectric coupling based on spin-orbit coupling, the symmetric exchange striction, which essentially stems from the spin-lattice coupling,⁵is also concerned. Prototypes of this class of materials include the Ising chain magnet Ca3CoMnO6with a collinear up-up-down-down (↑↑↓↓) magnetic order¹⁵ and ortho- rhombic HoMnO3 with an E-type spin order.¹⁶ Recently, double- perovskite R2CoMnO6compounds, where R is rare-earth ions, such as Lu₂CoMnO₆and Yb₂CoMnO₆offering the↑↑↓↓spin order, were reported to exhibit such type II multiferroicity too.^17–20Nevertheless, the existing contradictory results on ferroelectricity in this com- pound family become more and more questionable. For example, remarkable magnetoelectric response in such double-perovskites was first reported in polycrystalline Lu₂CoMnO₆, with a simultaneous coexistence of the E-type magnetic order and dielectric anomaly at the Néel point TN∼50 K.¹⁷Subsequently, a study on single crystals revealed the electric polarization aligned along theb-axis in spite of sharing the same magnetic order and Co²⁺/Mn⁴⁺charge order along thec-axis, as understood in Ca3CoMnO6.^18,19

Nonetheless, another work on several R2CoMnO6compounds in the polycrystalline form indicated no clear ferroelectric transition occurring atT_N, discarding the existence of intrinsic magnetoelec- tric effect and ferroelectricity.²¹From this point of view, the results are somehow sample-dependent or author-dependent, thus appeal- ing for additional checking of this claim by careful investigation.

Here, several issues along with the reliability of measured data using different techniques should be highlighted. First, the predicted elec- tric polarization in these compounds is usually small, making a reli- able probing of the polarization hard unless the high-resolution pyroelectric current method is utilized. Second, these compounds have a relatively narrow bandgap, thus allowing easy trapping/detrap- ping of charges during the electric poling process necessary for pyro- electric measurements. Thus, thermally activated charge detrapping produces a current release that is somehow much larger than the pyroelectric current. Third, the existence of 4f-R ions in these R2CoMnO6systems may also bring complexity into the ferroelectric- ity generation, due to the 4fmagnetism of these R ions.

Y₂CoMnO₆ is an unusual member in this double-perovskite family, noting that Y³⁺is nonmagnetic and does not add more com- plexity to the magnetism. Therefore, this system has been well inves- tigated earlier due to this advantage. Y2CoMnO6 crystallizes in a monoclinic symmetry with space group P2₁/n.²² The Co²⁺ and Mn⁴⁺ ions stack along the c-axis in distorted corner-sharing oxygen octahedral, as shown inFig. 1(a). Thefirst evidence for fer- roelectric transition atTFE∼80 K was reported by Sharmaet al. on polycrystalline samples.²³ A partial suppression (∼8%) of electric polarization by applying a magnetic field of ∼5 T was observed, indicating a moderate magnetoelectric effect. They claimed that the ferroelectricity is associated with magnetic ordering of Co²⁺ and Mn⁴⁺moments in the↑↑↓↓arrangement, which could be explained by the exchange striction mechanism.

Subsequently, these measured results were explained by the first-principles calculations that the magnetic ground state is the E-type order in competition with the ferromagnetic (FM) and A-type AFM orders.^24,25The recent calculations predicted that the polariza- tion is aligned along theb-axis rather than thec-axis, as shown in Fig. 1(b), different from aforementioned results.²⁴Therefore, we have two possibilities of predicted polarization alignment: thec-axis and

b-axis. However, later on, neutron scattering revealed that Y2CoMnO6

undergoes ferromagnetic transition, and it was claimed that the pyroelectric current should arise from thermally stimulated depo- larization current (TSDC) instead of an intrinsic effect.²⁶Therefore, based on the above controversial results, it is highly recommended to recheck this issue and answer the question whether Y2CoMnO6is ferroelectric or not.

Here, it should be mentioned that data available so far on fer- roelectricity have been obtained from polycrystalline samples instead of single crystals. This shortcoming leaves uncertainty for a conclusion. In addition, it should be highly preferred to check every aspect of multiferroicity of Y₂CoMnO₆, as a representative example of such double-perovskite oxides. In this work, we report our characterizations on Y2CoMnO6single crystals we grew fortu- nately and check several aspects of the multiferroicity by revisiting simultaneously the structural, magnetic, dielectric, and ferroelectric properties. Our results allow us to conclude that the Y2CoMnO6

single crystals we grew do not have intrinsic ferroelectricity and magnetoelectric coupling, while the magnetic and thermodynamic behaviors remain similar to earlier reports.

II. EXPERIMENTAL DETAILS

After substantial efforts, in particular, the Y₂CoMnO₆ single crystals were successfully grown utilizing the flux method with Bi2O3flux in air under well-controlled conditions, learning much from an earlier report.¹⁹The high purity powder of Y₂O₃, Mn₂O₃, and Co₃O₄were ground and homogeneously mixed in stoichiomet- ric ratios in a mortar. The obtained mixture was calcined at 1000 °C for 12 h followed by regrinding and sintering at 1200 °C for 24 h. The synthesized polycrystalline sample was mixed with Bi₂O₃flux with a ratio of 1:12. The mixture was heated to 1300 °C in a platinum crucible, cooled slowly down to 950 °C at a rate of 2 °C per hour, and then cooled down to room temperature after the furnace was turned off. This is an optimized procedure of sample preparation. Diluted hydrochloric acid was added into the platinum crucible to dissolve part of Bi2O3 to separate the flux from the crucible. Then, the as-grown crystals were peeled offfrom theflux by mechanical means (gentle knocking).

For the as-prepared samples, the crystallographic structure in the single crystal form and reground powder form was checked by a powder X-ray diffractometer (XRD, D8 Advance, Bruker, Inc.) using Cu-K_αradiation at room temperature. The detailed character- ization for the structure was performed by the Rietveld refinement by applying FullProf software to the measured data. Chemical homogeneity was checked using the scanning electron microscopy (SEM, Quanta 200, FEI, Inc.) in the EDX mode, and the evaluated data show a roughly good stoichiometry and the valent states of Co and Mn were determined using the X-ray photoelectron spectro- scopy (XPS, PHI 5000 Versaprobe, ULVCA, Inc.) with no remark- able trace of states other than Co²⁺and Mn⁴⁺states.

The magnetic susceptibility (χ) as a function of temperature (T) under the zero-field cooling (ZFC) and field cooling (FC) modes were measured by the superconducting quantum interfer- ence device magnetometer (SQUID, Quantum Design, Inc.), with field cooling and measuring fields of 1000 Oe. The specific heat (CP) was also measured using the Quantum Design physical FIG. 1. (a) The theoretical monoclinic crystal structure of Y2CoMnO6. (b) Two

neighboring Mn-Co spin chains of the theoretical E-type order and the symmet- ric exchange striction induced O ionic displacements projected on thebc-plane.

property measurement system (PPMS) in the standard procedure.

High-field magnetization was measured in pulsed fields using a coaxial pickup coil in the Wuhan National High Magnetic Field Center (WHMFC) facility. To ensure the accuracy of data, the baseline was measured under the same discharge voltage and the signals from the sample were calibrated by a comparison with the low-field data measured by SQUID.

For the electrical measurement, the crystals were polished into slices of 2 mm in the in-plane dimension and 0.3 mm in thickness and deposited with Au electrodes on the top/bottom surfaces. The dielectric constant as a function of T was measured using the HP4294A impedance analyzer integrated with PPMS. The spontane- ous polarization P was checked by probing the electric current released in the sample warming sequence via the Keithley 6517 elec- trometer connected with PPMS. In detail, the sample was electrically poled under an electricfieldE_p= 6.67 kV/cm and cooled down from 100 K to 2 K. Then, the sample at T= 2 K was electrically short- circuited isothermally for 0.5–2.0 h after removing the polingfield.

Typically, the discharging process continues until the background is less than 0.1 pA to exclude other extrinsic contributions, such as the detrapped charges.²⁷Finally, theTdependence of the current released from the sample was collected at a warming rate from 2 K/min to 6 K/min during the whole process.

For the isothermal magnetoelectric measurement, the current released from the sample upon the cycling of magneticfieldHwas collected. The crystal wasfirst electrically poled and cooled down to 2 K without a magnetic field, and then the electric field was removed, followed by sufficient time for electric short-circuiting.

The magnetoelectric current IH upon the magnetic field cycling betweenH= ±3.0 T at a rate of 100 Oe/s was collected. TheP(T) andP(H) data were obtained by integrating the probed current as a function ofTand/orH.

III. RESULTS AND DISCUSSION A. Structural characterizations

Wefirst present the structural characterizations on the as-grown single crystals. The crystal size is about 2 × 2 × 2 mm³, and most of them have severalflat faces. The slow-scan XRD patterns focusing on the four naturally developed planes are plotted inFig. 2, noting that the baselines come from the clay used to mount the samples. It can be seen that strong diffraction signals are detected from the selected faces, as indicated by the four color arrows pointing to the crystal imaged in the inset. Compared with the standard diffraction spec- trum, the four planes are well indexed by (00l), (hh0), and (h0h).

There is no (h00) or (0k0) plane detected probably because the areal atomic density of the (110) plane is higher than that of the (100) or (010) plane. Usually, the most prominent faces of a crystal are those with the greatest density of lattice nodes according to the Bravais law, and thus the (100) and (010) planes disappear gradually during the growth. In the following step, to verify the ferroelectricity, we chose the (hh0) and (00l) planes as the in-plane and out-of-plane orienta- tions for ferroelectric polarization measurement.

It is well known that the antisite disorder (ASD), namely the degree of Co/Mn atomic mixing commonly exists in each R₂CoMnO₆ member. It was argued that the ASD intensity is highly dependent on the cooling rate during the crystal growth,^26,28 but we believe that

more complicated factors determine the ASD degree. Previous reports showed that the ASD has a strong impact on magnetic prop- erties, whereas the magnetization at highfield decreases with increas- ing ASD degree.²⁸ In the perfectly ordered double-perovskite state, Co and Mn ions stack alternately along thec-axis, forming the mono- clinic phase with the P21/n space group, while the orthorhombic Pbnmspace group will be adopted if Co and Mn ions are distributed randomly at the 6c site. Therefore, the ASD degree can be qualita- tively reflected by the content of thePbnmphase in the sample.²⁹

To further check the contents of theP21/nphase and Pbnm phase in the crystals, we performed the data refinement using the P2₁/nandPbnmspace groups respectively, as shown inFig. 3. At the same time, the refined results are listed inTable I. It is seen that the refinement reliability parameters are almost the same for the two models. Neither of the two space groups can reproduce the experimental data perfectly, indicating that the ASD in our crystals cannot be neglected. In addition, it should be noted that the X-ray diffraction is difficult to determine the exact Co/Mn distribution on the crystallographic sites since both cations have similar scattering factors. Therefore, neutron scattering is more appropriate to detect the ASD degree if the magnetic structure and moments can be rea- sonably extracted, which is a challenging issue.

B. Magnetic properties

Subsequently, we concentrate on the magnetic property both in the dc mode for low magnetic field and pulse mode for high field. The measured magnetizationM(T) curves in the geometry of dcfieldHparallel and perpendicular to thec-axis were measured and presented in Figs. 4(a)and4(b). In agreement with previous results, a magnetic phase transition is evident at the reported Curie temperature T_C∼70 K, coinciding with the anomaly of specific heatC_P(T), which is plotted inFig. 4(c). We also observe a kink at Tfin the ZFCM(T) curve, which might be related to spin freezing, FIG. 2. Room temperature XRD patterns of different planes of as-grown Y2CoMnO6single crystal, and the peaks marked with♦ come from the clay used to mount the samples. Inset: A photo of an as-grown crystal, where the arrows correspond to the four planes.

similar to the case observed in Lu₂CoMnO₆.³⁰ In addition, the obvious difference in the M(T) curve between the H//c and H⊥c modes reveals the existence of magnetic anisotropy with strong magnetic easy axis along the [001] direction, in agreement with the previous neutron diffraction results.²⁶

The magnetic susceptibility allows us to check the dominant exchanges andFig. 5(a)presents the inverse magnetic susceptibility χ⁻¹ as a function of T. The Curie-Weiss law fitting of the data above 100 K yields the Curie-Weiss temperature T_CW∼60.96 K, and 29.73 K for theH//candH⊥cmodes, respectively. The effective momentsμeff= 6.57μB(H//c) and 6.415μB(H⊥c) are derived from the Curie constant, consistent with the theoretically predicted spin

only moment of Co²⁺(S= 3/2) and Mn⁴⁺(S= 3/2). These data do suggest that the dominant magnetic exchanges in our crystals are ferromagnetic, although theoretically proposed ground state should be antiferromagnetic. Earlier data from other groups show similar behaviors, and the first-principles calculations suggest a small energy difference between the antiferromagnetic state and ferro- magnetic state, although the former is the ground state.

It is noted that theχ⁻¹(T) curves exhibit a downturn between TCWandTC, afingerprint of a Griffiths phase, although we do not have sufficient evidence for such a claim. It is understandable that the long-range ferromagnetic order would be partially suppressed by the existence of nonnegligible ASD to some extent. Physically, the Griffiths singularity is described by equationH/M= (T−T0)^1−λ, where 0≤λ< 1,³¹ and here T0 can be set as TCW. Figure 5(b) shows the logarithm (H/M) curves along with corresponding λ values:λabandλcforH//ab-plane andH//c-axis, respectively. It is seen that λab is slightly bigger than λc, indicating the stronger structural disordering in theab-plane. In fact, the Griffiths phase- like behavior was not recognized in polycrystalline Y₂CoMnO₆ samples but well identified in other double-perovskite oxides^32–34 and Ca3CoMnO6.³⁵It is also argued that this behavior in those Ising spin-chain compounds may originate from the competition between Mn-Co ferromagnetic exchange and Mn-Mn or Co-Co antiferromagnetic exchange.³⁵

We may also check the isothermal magnetization of the as-grown single crystal in theH//candH⊥ccases at various tem- peratures, as shown inFigs. 6(a)and6(b). For theH//ccase at 5 K, the initial magnetization curve is not sharply different from an earlier report²⁴ifH< 50 kOe. Besides, the magnetic hysteresis loop presents a coercive fieldHC=±12.7 kOe, and the maximum mag- netic moment was found to be 3.1μB/f.u. at 50 kOe, which is signifi- cantly smaller than the expected value of 6.0μB/f.u. for a perfectly ordered sample, indicating the presence of ASD.³⁶For theH⊥ccase at 5 K, magnetization reveals a weaker hysteresis loop with remnant moment M_r= ±1.4μB/f.u. and coercivefieldH_C= ±1.25 kOe, which

TABLE I.Unit cell parameters, positional parameters, and reliability factors for structuralfitting using the groupsP21/nandPbnm,respectively.

Structure Monoclinic Orthorhombic

Space group P2₁/n Pbnm

a (Å) 5.2444(2) 5.2436(3)

b (Å) 5.6432(8) 5.6434(4)

c (Å) 7.4633(4) 7.4623(3)

β(deg.) 89.9671(1) 90

V (ų) 220.8833(9) 220.8260(8)

Y (x, y, z) [0.9836(6), 0.0734(2), 0.2505(3)] [0.9825(4), 0.0741(7), 0.25]

Co (x, y, z) (0.5, 0, 0) (0.5, 0, 0)

Mn (x, y, z) (0, 0.5, 0) (0.5, 0, 0)

O1(x, y, z) [0.1060(7), 0.4636(3), 0.2510(7)] [0.1008(4), 0.4742(7), 0.25]

O2(x, y, z) [0.6691(9), 0.2918(4) 0.0587(1)] [0.6868(6), 0.3085(0), 0.0524(4)]

O₃(x, y, z) [0.7065(0), 0.3421(6), 0.4472(1)] …

R_p(%) 6.55 6.53

Rwp(%) 8.94 8.84

Rexp(%) 6.04 6.04

χ² 2.19 2.14

FIG. 3.Room temperature XRD of ground crystal powder and corresponding Rietveld refinement as (a)P21/nand (b)Pbnmspace groups.

may originate from the canted magnetic moment revealed by neutron scattering.

The magnetic hysteresis curves at 5 K to higher magnetic field were measured in pulsed magneticfields (PMF) up to 55 T, as shown inFig. 6(c). Here, the whole PMF data are normalized by the low-field SQUID data. According to the PMF data, the saturation magnetic moment of our single crystal is found to be about 4.0μB/f.u. at H= 12 T, which is significantly lower than the expected 6μB/f.u., indicating the presence of about (6−4)/6/2 = 16.67% antisite disor- der. The maximum moment in theH⊥c mode is smaller than the expected value. No more peak or hump is observed at a highH range, indicating no more magnetic plateau.

To sum up the above magnetic results, it seems that the ground state of partially disordered Y₂CoMnO₆crystal is ferromagnetic. As proposed in previous works, some theoretical studies predict an E-type magnetic structure for these compounds, but the experimental evidence shows that this ordering is only produced for the heavier rare-earth atoms (Lu and Yb) while the ground state is ferromagnetic for the rest rare-earth atoms (from Tm to La, and also for Y).^21,26

C. Dielectric behaviors and absence of ferroelectricity Finally, as an important issue, we pay attention to the dielec- tric and ferroelectric properties of Y2CoMnO6 crystals. We first

look at the dielectric constantεand dielectric loss tanδas a func- tion ofT, measured in two different modes: (1)E//candH//cmode and (2)E⊥candH//cmode. The data at signal frequencyf= 10⁵Hz are plotted in Figs. 7(a)and 7(b)forH= 0 andH= 3.0 T, respec- tively. Several features deserve for highlighting. First, there is an absence of any sharp anomaly in dielectric constant across the magnetic phase transitionTC∼70 K, revealing the absence of ferro- electric transition in our crystals. It should be noticed that the dielectric loss tanδ is as small as 10⁻³on the order of magnitude, indicating that the sample is highly insulated. Second, a weak hump in theε∼Tand tanδ∼Tcurves aroundT∼20 K–30 K can be evidenced, and this hump position is close to the kink tempera- ture Tf inFig. 4(c) but is highly frequency-dependent, suggesting its relaxation nature. The measured loss data at different frequen- cies (f) for the E⊥c andH//cmode as an example are plotted in Fig. 7(c). We use the thermally activated relaxation model to describe the hump dispersions,²⁶

τ¼τ0exp Ea

kBT

, (1)

whereτis the inverse frequency (1/f) andτ0is the characteristic time factor,E_ais the activation energy, andk_Bis the Boltzmann constant. Here, the temperature corresponding to the hump is defined by the valley point of function d²(tanδ)/dT². The FIG. 4.Magnetization vs temperatureTmeasured on warming in a 1000 Oefield

after zero-field cooling (ZFC) orfield cooling (FC) from room temperature for (a)H//c and (b)H⊥c. (c)T-dependent specific heat divided byT(CP/T) of Y2CoMnO6.

FIG. 5. (a)T-dependent inverse magnetic susceptibility 1/χand Curie-Weiss fits from 100 K. (b) The log(H/M) of FC vs log(T−TCW) curves. Solid lines are linearfits to establishλ.

evaluatedτandT data sets are plotted inFig. 7(c)for theE⊥c andH//c mode. It can be clearly seen that the data are in the linear relation in the low-Trange and we apply the linearfitting to the data. The estimated activation energy is Ea∼11.3 meV.

This activation energy is quite small, corresponding to the local dielectric response, while charge trapping and detrapping, if any, should need much larger activation energy. Second, this energy is smaller than that evaluated from polycrystalline samples (E_a> 50 meV) due to the substantial exclusion of capacitance effect from grain boundaries.²⁶Third, the activation energy seems to be slightly enhanced by the magnetic field, probably due to the

magnetic field enhanced stability of ferromagnetic state, allowing the dielectric relaxation to be weaker.

Even though the dielectric data nearly exclude the existence of ferroelectricity in our single crystals, it is still deserved to present our data on the ferroelectric testing. The collected currents released from the sample during the warming processes of different sweep- ing rates are plotted in Figs. 8(a) and 8(b) for the two modes (E//c and E⊥c), where the warming rates are labeled. It is noted that similar measurements on polycrystalline samples were per- formed.²⁶For the two modes, clearly current peaks appear around T∼60 K–65 K, similar to earlier work on polycrystalline samples.²³ It is seen that the peak profile shifts remarkably toward the high-T FIG. 7. Temperature dependence of the εr and tanδ atH= 0 and 3 T for (a) E//c, H//c and (b)E⊥c,H//c, respectively. (c) Tdependence of tanδfor different frequencyE//c. (d) Lnτvs 1000/Tplot and the linearfit.

FIG. 6. (a) and (b) Magnetic hysteresis loops measured under different temper- ature forH//candH⊥c, respectively. (c) Hysteresis loops measured in pulsed magneticfields (PMF). (d) ThedM/dHof PMF hysteresis loops vsHcurves.

side with the increasing warming rate from 2 K/min to 6 K/min, with a shift as large as 6 K. This large shift raises concern whether the current is from the pyroelectric effect or not. Before checking this issue, we assume that the current would be from the pyroelectric effect and thus the evaluated electric polarizations as a function of TunderH= 0 andH= 3.0 T in theE//candH//cmode is plotted in Fig. 8(c). The obtained value is∼0.1μC/cm², reasonably larger than the values reported in polycrystalline samples.²³ Also, there is a polarization suppression of ∼4% upon a magnetic field of 3.0 T, similar to the results on polycrystalline samples.²³

Now we come to discuss the nature of the measured current.

Most likely, it comes from the thermally stimulated depolarization current (TSDC) that is ascribed to the reorientation of charged defect dipoles. These charges are basically injected into the sample during the electric poling sequence during the sample cooling, noting that such a current peak did not appear when the sample was cooled down without electric poling. This process can be described by the well-defined thermal activation mode,^26,37

Ea

kB¼ 1 βτ0

e^(Ea=kBTm)T_m², (2)

whereTmis the temperature of current peak,τ0is the relaxation time constant, andEais the activation energy. TheT⁻¹-dependences of ln (T²/β) in theE//candE⊥cmodes are plotted inFig. 8(d). The evalu- ated values forτ0andE_aare 47μs and 53.5 meV for theE//cmode and 242μs and 24.2 meV for theE⊥cmode. These values are consis- tent with the data from the polycrystalline samples. This seems to hint that the current presented in Figs. 8(a)and8(b)should be the TSDC rather than the pyroelectric effect induced current.^21,26

In addition, we performed measurement on the isothermal magnetoelectric response in the two modes. It was identified that the collected current upon the magnetic field cycling between

±3.0 T is on the level of 0.1 pA, the noise background. Clearly, no sign from the magnetically stimulated magnetoelectric current can be confirmed, providing further evidence that magnetoelectric cou- pling in Y₂CoMnO₆is absent. Surely, this conclusion is based on the fact that our single crystals contain nonnegligible Co/Mn occu- pation disorder and no possibility for absolutely excluding the ferroelectricity in Y2CoMnO6.

IV. CONCLUSIONS

In conclusion, we have touched a contradictory issue that the R₂CoMnO₆compound family is ferroelectric by performing exten- sive investigation on Y2CoMnO6single crystals grown using theflux method, noting that this system has no 4fmagnetism. It is revealed that the structural and magnetic behaviors including the Griffiths phaselike behaviors are qualitatively similar to earlier works on this family of compounds. It seems that long-range ferromagnetic order is partially suppressed by structurally disordered occupation of Co and Mn ions in our single crystals, as confirmed by structural refine- ment results. However, our data on the dielectric and pyroelectric behaviors allow us to conclude that no ferroelectric polarization and magnetoelectric coupling are available within the measuring uncertainties. The absence of ferroelectricity may be ascribed to the Co/Mn occupation disorder inevitable in such a compound family, and the absence of E-type magnetic order, but the high-resolution pyroelectric current and magnetoelectric current measurements allow us to raise questions on the symmetric exchange striction origi- nating from the collinear spin order, which seems to be inapplicable to the double-perovskite Y2CoMnO6.

ACKNOWLEDGMENTS

This work was financially supported by the National Key Research Program of China (Grant No. 2016YFA0300101) and the National Science Foundation of China (NNSFC) (Grants Nos.

11874031, 11834002, 11774106, 51721001, and 11974167).

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