Large magnetoelectric effect in the polar magnet Sm 2 BaCuO 5
Cite as: Appl. Phys. Lett.115, 252902 (2019);doi: 10.1063/1.5127893 Submitted: 13 September 2019
.
Accepted: 7 December 2019.
Published Online: 17 December 2019
GuanzhongZhou,1,a)JingwenGong,2,a)XiangLi,1,2,a) MeifengLiu,1,2,b) LeiyuLi,3YuWang,2JiahuaMin,2JuLiu,2 DiCai,2FeiLiu,2ShuhanZheng,1 YongsenTang,1ZaichunXu,4YunlongXie,1,2LunYang,2MinZeng,3
ZhiboYan,1BiwenLi,2XiuzhangWang,2and Jun-MingLiu1 AFFILIATIONS
1Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China
2Institute for Advanced Materials, Hubei Normal University, Huangshi 435002, China
3Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China
4Department of Physics, Jiangsu Second Normal University, Nanjing 210013, China
a)Contributions:G. Zhou, J. Gong, and X. Li contributed equally to this work.
b)Electronic mail:[email protected]
ABSTRACT
Polar magnets are a family of promising candidates for future magnetoelectric (ME) applications. In particular, the ME effect is recognized for its potential use in low-power electronic devices. However, searching for materials with a sufficiently strong ME effect remains the core issue. In this work, we present a systematic investigation on oxide compound Sm2BaCuO5, including the characterization on structural, mag- netic, specific heat, dielectric, and pyroelectric behaviors. The intrinsic linear ME effect is revealed, and two successive antiferromagnetic phase transitions occurring at temperatures ofTN15 K andTN223 K are found, respectively. While a magnetic field induced electric polarization emerges belowTN2and this polarization increases with the magnetic field in an almost linear manner, another weak pyroelectric current peak is detected aroundTN1. Furthermore, remarkable responses of both electric polarization to the magnetic field and magnetization to the electric field are demonstrated. It is suggested that the intriguing magnetic interactions play the core roles in generating the ferroelec- tricity and ME effect in this Sm2BaCuO5compound.
Published under license by AIP Publishing.https://doi.org/10.1063/1.5127893
The magnetoelectric (ME) effect is one of the highly concerned topics in materials science due to its technological applications in the future. Multiferroics represent an important class of ME materials, and they allow the coexistence of multiple ferroic orders, which have attracted growing attention for not only fundamental interest but also potential applications.1–6However, the low transition temperatures for the ferroic orders and weak ME coupling are the major issues for most multiferroics.7,8 Generally, multiferroics can be classified into two types.2,9,10The type-I multiferroics refer to those materials where mag- netic and ferroelectric orders have different origins, and thus, the ME effect is usually weak. The type-II multiferroics include those systems where polarization generation is related to specific spin orders, and these systems usually have small polarization. It is known that sponta- neous electric polarization can occur in some magnetic structures such as noncollinear spin orders [e.g., cycloidal order of Fe3þ spins in LiFe(WO4)2],11 collinear spin orders (e.g., the up-up-down-down
arrangement of Co2þand Mn4þspins in Ca3CoMnO6),12,13and com- plex configurations in other manganites (e.g.,RMnO3andRMn2O5 withRthe rare earths, CaMn7O12, etc.).14–21These magnetic structures can break the spatial inversion symmetry, while magnetic orders natu- rally break the time reversal symmetry. Consequently, these materials exhibit strong ME coupling.
By considering the two aspects highlighted above, one under- stands that an urgent issue is to search for new materials with intrinsi- cally strong ME coupling, high ferroic-ordering temperatures, and large ferroelectric polarization. Recently, it has been reported that some polar oxides with crystallographically noncentrosymmetric structures, e.g., Co4Nb2O9,22 GaFeO3,23 MnSb2O6,24,25 Ni3TeO6,26 Mn2MnWO6,27and Fe2Mo3O8,28,29show strong ME coupling below the magnetic transition temperatures. Given the broken lattice symme- try, these compounds can be classified into the catalog of the so-called polar magnets, where the long-range magnetic order with broken time
reversal symmetry is incorporated into the polar structure with broken spatial inversion symmetry. The nontrivial ME effects are expected to appear in polar magnets.29,30One typical example is the green-colored oxide familyR2BaCuO5(R¼Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, and Y). This family of oxides was originally found as impurities in the synthe- sis of high-temperature superconductingRBa2Cu3O7xoxides.31–34They crystallize in the polarPnmaspace group, as systematically discussed by Michel and Raveau in 1982, based on the powder X-ray diffraction (XRD) studies.34–36 Indraet al.investigated the physical properties of R2BaCuO5(R¼Sm, Dy, and Er)37and observed one or two magnetic phase transitions ascribed to the antiferromagnetic (AFM) ordering of Cu2þ andR3þ spins. The lattice structure in the paramagnetic phase belongs to the polarPna21space group. As a consequence, various mag- netic orders with the polar lattice structure provide the opportunity to explore a possibly strong ME effect in theR2BaCuO5family. Motived by the above discussions, we choose compound Sm2BaCuO5as the object system of the present study, noting that no report on the ME effect of this compound has been available, to the authors’ best knowledge.
Herein, we investigate Sm2BaCuO5as a promising polar magnet, and detailed characterization on its structure, magnetization (M), fer- roelectric polarization (P), and ME effect will be presented. We observe two successive AFM transitions occurring at temperatures of TN15 K andTN223 K, respectively. A weak electric polarization driven by Sm3þspin order, emerging belowTN1, is identified and can be suppressed by the external magnetic field. Moreover, another mag- netic field induced large electric polarization appears belowTN2, and the linear ME effect is shown. In addition, the temporal evolution ofP andMin response to the applied magnetic field and electric field is probed, respectively, further confirming the remarkable ME effect.
In our experiments, polycrystalline Sm2BaCuO5 samples were synthesized by the conventional solid-state reaction method in air, using the highly purified powder of oxides Sm2O3and CuO as well as carbonates BaCO3as raw materials. The mixed powder in stoichio- metric ratio was ground and fired at 850C for 24 h in air. The resul- tant powder was reground and then pelletized under a hydrostatic pressure of 1000 psi into disks of 2.0 cm in diameter. The as-prepared pellets were sintered at 950C for 24 h in air again. Finally, these pel- lets were cut into thin disks for subsequent structural, magnetic, and electrical measurements.
It is believed that for orthorhombic Sm2BaCuO5, the Cu ions occupy the center positions of distorted square pyramids CuO5. These pyramids are connected to the monocapped trigonal prism SmO7
polyhedra by sharing triangular faces, as shown inFigs. 1(a)and1(b).
We first look at the crystallinity of the as-prepared samples.Figure 1(c)shows theh-2hX-ray diffraction (XRD) spectrum obtained by using a Rigaku Smartlab SE (Cu-Karadiation) system with a scanning step of 0.02at room temperature. The structure refinement was per- formed using the standard Rietveld method,38 based on the lattice structure shown inFigs. 1(a)and1(b). All the peaks can be properly indexed by the standard Bragg reflections, and no impurity phase has been identified. The high quality Rietveld fitting indicates an ortho- rhombic structure with thePnmaspace group, and the eventual reli- ability parameters for the fitting areRWP¼8.35%,RP¼5.89%, andv2
¼ 2.22. The corresponding lattice parameters are a¼12.404 A˚ , b
¼5.760 A˚ , and c ¼7.274 A˚ , in good agreement with the earlier results.32,35The Ba2þions are eleven-coordinated with oxygen, giving rise to highly irregular polyhedra.
To understand the magnetic behaviors of the as-prepared sam- ples, we investigate the dc magnetic susceptibility (v) as a function of temperature (T) under different measuring magnetic fields (H) over theT-range from 2 K to 300 K in the zero-field cooled (ZFC) and field cooling (FC) modes, using a Quantum Design Superconducting Quantum Interference Device (SQUID). In fact, the obtained v(T) FIG. 1.(a) and (b) Schematic illustration of the lattice structure of Sm2BaCuO5with space groupPnma. (c) Theh-2hXRD pattern measured at room temperature and the data from the corresponding Rietveld fitting. The black dots and red curve represent the experimental and calculated (fitted) data, respectively. The blue curve indicates the differences between them, and the olive bars denote the Bragg positions.
curves in the ZFC and FC modes overlap in the whole covered T-range, and thusFig. 2(a)only presents the ZFCv(T) curves under different H. First, each curve shows two sharp peaks around TN1
5 K andTN2 23 K, revealing the long-range AFM ordering of Sm3þand Cu2þmoments, respectively. In order to further investigate the effect of the magnetic field on the magnetic transitions, we plot the measured specific heat curvesCP(T) under differentH, as plotted in Fig. 2(b), noting that the CP(T) curves were measured using a Quantum Design Physical Property Measurement System (PPMS) with the heat relaxation method. TheCP(T) curves also exhibit two sharp peaks atTN1andTN2and coincide well with the features ofv(T) curves.TN1andTN2show no remarkable shifting with the increasing magnetic field.
Here, it should be mentioned that the applied magnetic field has only an insignificant effect on the AFM transition temperatures (TN1
andTN2). This effect has been often observed in those highly frustrated AFM oxides due to the strong AFM interactions. We believe that Sm2BaCuO5 also belongs to this class. Moreover, the measured
H-dependent magnetizationM(H) curves atT¼5 K, 10 K, and 23 K in the cycle of first increasingHand then decreasingHare presented inFig. 2(c). No isothermal field hysteresis can be observed in each H-cycle and a nearly linear M(H) relationship without saturating tendency is identified, also suggesting the strong AFM interaction of Sm2BaCuO5.
For addressing the ME effect of Sm2BaCuO5, it is useful to inves- tigate the dielectric and ferroelectric properties. For the electrical mea- surements, Au electrodes were deposited on each side of the disklike sample of 3.0 mm in diameter and 0.3 mm in thickness, constituting a parallel-plate capacitor. The dielectric constant er(T) under different signal frequencies and magnetic fields is shown in Fig. S1 of thesup- plementary material. No anomaly was observed under zero magnetic field, but a clear dielectric anomaly aroundTN223 K was induced by increasing the magnetic field. In the low-Trange, a magnetic field as low as 1.0 T seems to be sufficient to enhance the dielectric constant remarkably, suggesting the strong ME coupling. In addition, the dielectric loss is quite small, implying the negligible leakage.
In order to check possible ferroelectric polarization in Sm2BaCuO5, we present in Fig. 3(a)the measured pyroelectric currentI(T) curves under different magnetic fieldsH. Before probingI(T), the sample was prepoled from 50 K to 2 K under a poling electric field ofE¼7.3 kV/cm offered by a source-meter (Keithley 2400) and simultaneously under an
FIG. 2.(a) The magnetic susceptibilityvmeasured under different magnetic fields.
(b) The specific heatCP(T) data measured under different magnetic fields. (c) The magnetization as a function of magnetic fieldHmeasured atT¼5 K, 10 K, and 23 K, respectively.
FIG. 3.(a) The pyroelectric currentI(T) data measured under different magnetic fields, and the inset shows the data aroundTN1, suggesting that the peak disap- pears aroundH¼Hc8.0 T. (b) The evaluated polarizationP(T) data under differ- ent applied magnetic fields. The inset shows theP(T¼10 K) data at different magnetic fields.
assignedHthat can be fixed between 0 and 9 T. After the sample was cooled down to 2 K, the poling electric field was removed but the mag- netic field remained unchanged. The sample was then electrically short- circuited for several hours at 2 K in order to exclude possible trapped charge during the poling process. Subsequently, the electric current released from the capacitor under the magnetic field was determined using a Keithley 6514A electrometer during the sample warming at a rate of 2 K/min. Before proceeding with the discussion, the fact that the measured currentI(T) does come from the pyroelectric effect is con- firmed in thesupplementary material, as shown in Figs. S2 and S3.
Herein, two major features deserve highlighting inFig. 3. First, a weak pyroelectric current peak appears atTN1 5 K if the applied magnetic field is set to zero, implying an extremely weak spontaneous polarization ensuing at this temperature. This current peak can be gradually suppressed by the external magnetic field, as shown in the inset ofFig. 3(a). A critical magnetic fieldHc8 T was found to be suf- ficient to fully suppress the pyroelectric peak aroundTN1. This peak can be ascribed to the ordering of Sm3þspins, and its dependence on the magnetic field implies the magnetic field control of the Sm3þspin order induced polarization. Second, a remarkable pyroelectric peak gradually emerges aroundTN2upon application of a magnetic field, as shown inFig. 3(a), suggesting clearly the magnetic field induced electric polarization. The peak height increases with the magnetic field. As a consequence, we evaluate the electric polarizationPas a function ofT from the pyroelectric currentI(T) data, and the obtainedP(T) curves under differentHare plotted inFig. 3(b). Clearly, the emerging temper- atureTN2of the polarization remains roughly unchanged against differ- entH, although the polarization magnitude increases with increasing H. Taking the data of polarizationPatT¼10 K as a function ofH, one obtains theP(H) dependence as shown in the inset ofFig. 3(b), sugges- ting a roughly linear ME behavior. It is noted that theP(H) value can reach up to35lC/m2forH¼9 T, implying the strong ME coupling in this system, although this polarization is yet much smaller than that of those conventional ferroelectrics (104lC/m2).
In order to further investigate the linear ME effect, we measured the currentI induced by a modulated magnetic field H. Similar to the pyroelectric measurement, the sample was first poled under E¼7.3 kV/cm andH¼5 T and then cooled down toT¼10 K fol- lowed by the short-circuit process. Subsequently, the sample tempera- ture was fixed and the current data were recorded in response to the varying magnetic field between 0.5 T and0.5 T, i.e.,DH¼60.5 T. It is noteworthy that the variationDH¼60.5 T was chosen to minimize the magnetic hysteresis. As seen fromFig. 4(a), the measured current changes periodically betweenI1 0.03 pA andI2 0.32 pA, which was induced by varying the magnetic field between 4.5 T and 5.5 T. The integratedP(H) plotted inFig. 4(b)oscillates linearly withH, and the maximum variation is0.3lC/m2, indicating a stable ME coupling in Sm2BaCuO5with a ME coefficient defined asDP/DH3.5 ps/m.
In addition, we also performed the electrocontrol of magnetiza- tion in thedcmode with the SQUID. By applying a magnetic field of 5 T and an electric field of 7.3 kV/cm, the sample was cooled from 50 K to 10 K. After the short-circuit process, bothE and H were removed. Subsequently, a periodically varying electric field between E¼0 and 7.3 kV/cm was applied to the sample, during which the magnetizationMwas measured with a measuring field of 2000 Oe. It is seen that the measuredMchanges rapidly to3.770103lB/f.u.
asEis set to zero and restores to3.815103lB/f.u. asEis set to
7.3 kV/cm [Fig. 4(c)]. The magnitude ofDMis 4.5105lB/f.u., and the coefficient of the so-called inverse linear ME effect defined as DM/DEis up to8.1 ps/m.
Finally, we give a brief discussion on the possible magnetic struc- ture of Sm2BaCuO5, which is helpful for one to understand the linear magnetic field dependence of the electric polarization. However, it is difficult to perform the neutron diffraction measurement because Sm (samarium) is a strongly neutron-absorbent element. It has been reported that the magnetic structures of differentR2BaCuO5(R¼Dy, Ho, Er, Tm, Yb, Y,…) members are not the same.39–41Specifically, for Gd2BaCuO5, an incommensurate-commensurate transition occurs at about5 K. Owing to the variety of magnetic structures ofR2BaCuO5, it is hard to assume a reasonable magnetic structure for Sm2BaCuO5
and its variation with the magnetic field, although the AFM nature would be hardly changed unless the applied magnetic field is extremely large. Nevertheless, it is reasonable to assume that the magnetic struc- ture of Sm2BaCuO5is similar to one of the reported structures, while other possibility cannot be ruled out in the absence of the neutron dif- fraction measurement.
In addition, the mutual ME control in Sm2BaCuO5can be under- stood as follows. Although the measured macroscopic magnetization remains less sensitive to the magnetic field [as shown inFig. 4(c)], owing to its AFM nature, the spin configuration can be changed from one to another in response to applied HandE since the magnetic structure is highly frustrated. This property is responsible for the FIG. 4.The measured temporal evolution of (a) pyroelectric currentIand (b) elec- tric polarization variationDPin response to the periodically varied magnetic fieldH atT¼10 K. (c) The temporal evolution of magnetizationMin response to periodi- cally varied electric fieldEbetweenE¼0 and 7.3 kV/cm atT¼10 K. For probing M, the measuring magnetic field is 2.0 kOe.
mutual ME effect, and thus the magnetocontrol of electric polarization can be observed on one hand. On the other hand, the applied electric field forces the electric polarization originating from the magnetic order to arrange in its direction, which will indirectly result in reorder- ing of the magnetic structure.
In conclusion, the physical properties of Sm2BaCuO5, in particu- lar, the ME effect, have been systematically investigated. Two succes- sive AFM phase transitions, one atTN15 K and the other atTN2
23 K, have been observed. BelowTN1, a weak electric polarization arising from the Sm3þorder emerges, which is suppressed by the mag- netic field. With the increasing magnetic field, another electric polari- zation is observed atTN2, which varies in an almost linear manner.
The ME and inverse ME effects have been observed separately, and the two ME coefficients reach 3.5 ps/m and 8.1 ps/m, respectively. Our experimental finding of the linear ME effect with compound Sm2BaCuO5should shed light on the understanding of the ME mutual control in polar magnets.
See thesupplementary materialfor more details of the dielectric and pyroelectric measurements on Sm2BaCuO5.
The authors acknowledge financial support from the National Key Research Projects of China (Grant No. 2016YFA0300101), the National Natural Science Foundation of China (Grant Nos. 11704109, 11804088, 11704139, 11574091, 11974167, and 51801059), and the Research Project of Hubei Provincial Department of Education (Grant No. B2018146).
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