Observation of magnetoelectric effect in the S 5 1/2 spin chain compound CoSe 2 O 5 single crystal
Cite as: Appl. Phys. Lett.120, 052901 (2022);doi: 10.1063/5.0077698 Submitted: 5 November 2021
.
Accepted: 18 January 2022.
Published Online: 31 January 2022
L.Lin,1,2,a) Y. S.Tang,2L.Huang,2W. J.Zhai,2G. Z.Zhou,2J. H.Zhang,2 M. F.Liu,3 G. Y.Li,2X. Y.Li,2Z. B.Yan,2 and J.-M.Liu2
AFFILIATIONS
1Department of Applied Physics, College of Science, Nanjing Forestry University, Nanjing 210037, China
2Laboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China
3Institute for Advanced Materials, Hubei Normal University, Huangshi 435002, China
a)Author to whom correspondence should be addressed:[email protected]
ABSTRACT
The antiferromagnetic structure in theS¼1/2 zigzag spin chain compound CoSe2O5was recently revealed by neutron scattering. Herein, we provide clear evidence for the linear ME coupling through systematic investigations on magnetic, dielectric, and ferroelectric properties. The simultaneous responses of theb-axis electric polarization (Pb) and dielectric anomaly (eb) against magnetic stimuli along thec-axis are revealed. In addition, both the ferroelectric transition and dielectric anomaly shift from the magnetic Neel temperatureTN8.5 K toward the low temperature under increasingHapplied along thec-axis, providing clear evidence for the magnetism-driven ferroelectricity. The observed off diagonal linear ME effect is in accordance with the prediction based on ME tensor analysis for the magnetic space groupPb0cn.
Consequently, our results may allow an interesting opportunity to further exploration of intriguing phenomena and physics of ferrotoroidic- ity in this linear-ME compound CoSe2O5due to the existence of the off diagonal term in the ME tensor, similar to the case for LiCoPO4. Published under an exclusive license by AIP Publishing.https://doi.org/10.1063/5.0077698
Multiferroic materials in which more than one ferroic order coexists have triggered a variety of emerging physics and great poten- tial applications in devices with functionalities.1–5Among them, enor- mous efforts have been devoted to predict and design magnetoelectric (ME) materials with cross coupling between the magnetic order and ferroelectric order.6–9In this regard, the electric polarization (P) can be induced by specific types of a long range magnetic order, which breaks space inversion and time-reversal symmetries simultaneously, giving rise to non-trivial ME effects. Nevertheless, such a magnetic order usually stems from the complex competition with strong mag- netic frustration. Consequently, the onset temperature (T) of the mag- netic order is extremely limited, and the ME coupling is usually weak since the mechanisms via spin–orbit or spin-lattice couplings are the second-order effects.10,11
Given the restricted nature of multiferroics, a useful way to search for ME materials would be to reexamine the magnetic order from symmetry analysis. There exists another kind of spin-driven ferroelec- trics called linear ME materials, in which their magnetic space groups could change into polar ones driven by external magnetic stimuli (H), giving rise to the linearly increasingPin response toH, while their
crystal structures could be centrosymmetric or polar, like the time- honored Cr2O312 and Fe2Mo3O8.13,14 Due to the strong symmetry restrictions, 58 out of 90 magnetic point groups permit the linear ME effect, while 66 magnetic point groups allow the quadratic ME effect.
Thus, the symmetry analysis provides a good approach to search for ME materials.15
Spin chain magnets with reduced crystallographic dimensionality have attracted great attention due to their intriguing anisotropic elec- tric and magnetic properties.16,17 So far, very limited multiferroics with spin chains have been found, as exemplified by Ca3CoMnO6,18 MnWO4,19LiCu2O2,17SrCuTe2O6,20and Fe4Nb2O9.21CoSe2O5is a less touched spin chain compound, whose structure was reported by Harrison et al.22 CoSe2O5 consists of one-dimensional ribbons of edge-sharing CoO6octahedra bound together by Se2O52-polyanionic subunits via shared oxygen atoms at the corners of octahedral, as shown inFig. 1(a). Melotet al.then reported a transition to the long- range antiferromagnetic (AFM) order below T TN¼8.5 K.23 By using powder neutron scattering, they resolved the magnetic moments that are aligned perpendicular to thec-axis but tilting alternately by þ14and –14with respect to thea-axis.23In contrast to the canted
AFM arrangement, Rodriguezet al.recently clarified that the Co2þ moments only point along thea-axis without any canting and are anti- ferromagnetically coupled along the chains parallel to thec-direction in the single crystal.24Unfortunately, since then, no reports on other properties have been made.
Motivated by aforementioned discussion, CoSe2O5draws strong attention due to its specific magnetic structure. CoSe2O5belongs to the Pb0cnmagnetic space group, which allows nonzero ME tensor compo- nentsayzandazy,25similar to the previous report on LiCoPO4,26a rare ME material that has been experimentally confirmed to exhibit ferrotor- oidicity.27In particular, only 14 out of the 58 magnetic point groups exhibit the order parameters of torodization in addition to the ME cou- pling. Therefore, CoSe2O5 may perform as a potential candidate to explore these ferroic orders simultaneously in the single phase.
In this work, we elucidate CoSe2O5as a member of the linear ME family by performing magnetic, magnetodielectric, and ME measure- ments on the single crystals. We have found theb-axis electric polariza- tion (Pb) and dielectric anomaly (eb), emerging at the magnetic Neel temperatureTN8.5 K, in response toHapplied along thec-axis, and providing clear evidence for the magnetism-driven ferroelectricity. We corroborate our experimental results based on the ME tensor analysis, thereby interpreting the ME coupling effect. Therefore, our results pro- vide a unique platform on which the ME coupling and ferrotoroidicity in the presence of low dimensional magnetic chains can be explored.
CoSe2O5single crystals were grown using the hydrothermal pro- cedure, as described previously.22The obtained dark purple crystals with a size of 1.0 mm were checked by an x-ray diffractometer (XRD) (D8 ADVANCE, Bruker) in theh-2hmode with CuKasource (k¼1.5406 A˚ ). The back-reflection Laue detector (MWL120, Multiwire Laboratrories, Ltd.) was used to check the quality of obtained single crystals, and thereby the crystallographic orientations were determined.
The well-prepared and aligned crystals were cut into thin plates for the measurements of magnetization and electric polarization. The temperature (T) dependence of magnetic susceptibility (v) was mea- sured using the quantum design superconducting quantum interfer- ence device magnetometer (SQUID) from 2 to 300 K under the zero field cooled (ZFC) and field cooling (FC) modes with cooling magnetic fieldH¼200 Oe along different crystal axes. Simultaneously, the mag- netization (M) as a function ofHwas measured at selectedTalong the different directions.
The well-aligned and polished crystal (b-plane) was submitted to electrical measurements. To probe the ME effect, the sample went through the ME annealing procedure from 30 to 2 K before the pyro- electric current measurements. The sample was electrically poled with þE¼10 kV/cm and þH¼8 T in the E//b andH//c arrangement.
Then the ME poling fields were removed, followed by sufficiently long short-circuit for 1 h to exclude other extrinsic current, e.g., trapped charges during electric field poling. When the background of an elec- trical current was less than 0.1 pA, theIb(T) curves were collected by sweepingTfrom 2 to 30 K on warming at a rate of 4 K/min under selectedHvalues. The polarization (Pb) was obtained by integration of Ibwith respect to time using the Keithley 6514 electrometer integrated with the physical property measurement system (PPMS, Quantum Design). TheHdependence of the ME current (IME) andH-induced polarization (DPb) were measured under selectedTuponHramping fromþH!–H! þHat a rate of 100 Oe/s using the same type as the ME poling procedure. The dielectric constant (e) as a function ofT was probed under selectedHusing the Agilent HP4294A impedance analyzer integrated with PPMS.
As shown in the inset ofFig. 1(b), the as-grown dark purple crys- tals are naturally quadrilateral in geometry with 1.0–1.2 mm in diame- ter and 0.5 mm in thickness. The room-temperature slow-scan XRD pattern onto the naturally developed quadrilateral plane is presented inFig. 1(b), which shows very sharp diffraction peaks well indexed by the (0k0) reflections. For the reference, another narrow face is checked by XRD, and excellent diffraction peaks from the (00l) reflections are clarified as in the inset. InFig. 1(c), we show the Rietveld refined XRD pattern of the crushed crystals. Consistent with previous reports, the lattice parametersa¼6.880 78 A˚ ,b¼10.504 15 A˚ , andc¼6.154 98 A˚ with space groupPbcnare identified.23
FIG. 1.(a) Schematics of the crystal structure of CoSe2O5. (b) The XRD patterns onto the naturally developed quadrilateral plane and the other face (inset). The pic- ture of the as-grown dark purple crystal was presented with each crystallographic axisa,b, andccarefully determined by the back-reflection Laue detector. (c) The refined XRD pattern of the crushed crystals collected at room temperature.
Figure 2(a)presents theTdependence of magnetic susceptibilities va,vb,andvcparallel to thea-,b-, andc-axes with the cooling field of 200 Oe under the FC mode, respectively. In agreement with earlier measurements, thev(T) curves exhibit significant anisotropy, which originate from the strong magnetic anisotropy of Co2þions.24With decreasingT,vashows a prominent jump belowTN, while a traditional cusp associated with the long-range AFM ordering sequences along theb-axis andc-axis is well demonstrated. We fit the inverse magnetic susceptibility 1/v(T) betweenT¼100 and 300 K to the Curie–Weiss
law, and three Weiss temperatures hcw (//a) 14.9 K, hcw (//b) 17 K, andhcw(//c)–62 K, respectively, are found, indicative of much stronger AFM interaction along thec-axis. The detailed values fitted from the Curie–Weiss law are listed in Table I. The effective moments are in close agreement with the expected value of 5.2lB/Co, typically observed for octahedral coordinated Co2þ(S¼3/2,L¼3), in which the spin and orbital contribution to the effective moment are completely decoupled. Moreover, the frustration factor f¼hcw/TN
along thec-axis is 7.6, much larger than the other two crystallographic axes, indicating a relatively high level of magnetic frustration.
Subsequently, the field dependent magnetizationMa,Mb, andMc
at selectedTare presented inFigs. 2(b)–2(d). Apparently,Mashows a sharp upturn around the critical fieldHSF6 T atT¼2 K, as shown inFig. 2(a)andHSFevidently decreases with increasingTand disap- pears nearTN. In fact, such an upturn suggests the appearance of the spin-flop transition, a common feature of complicated AFM orders performed in transition metal oxides.28Contrast to theMacase,Mb
increases monotonically withH, as shown inFig. 2(b), and no evident field-induced transition has been observed up toH¼8 T. Thec-axis magnetization belowTNshows weak deviation from a linear relation.
While the previous report shows that two steep changes occur at higher fields10 T and13.5 T along thec-axis, the weak deviation at low fields may suggest spin canting toward H. In particular, the magnetizations along the three axes are far from saturated, and an ultrahigh-field is highly desirable to unveil the possible magnetic tran- sition in CoSe2O5. Unfortunately, such a measurement is not accessi- ble to us at present but deserves further investigation using the pulse magnetic field facility.
Considering the similar ME tensor as that in LiMPO4(M¼Fe, Mn, Co, and Ni), we focus on checking the existence of the ME cou- pling. Fortunately, our desiredb-plane is perfect for subsequent elec- tric measurement, while it is difficult to cut out thea-orc-planes due to the much thinner thickness. The ME effect along the b-axis has been carefully examined under selected H in parallel to thec-axis, according to the prediction of the symmetry analysis. First, we check theT-dependence of the dielectric constantebin the absence of the magnetic field shown inFig. 3(a). No anomaly is detected within our experimental accuracy, implying no ferroelectric phase transition.
When a magnetic field is applied, a clear dielectric anomaly emerges at TN, evident in theeb(T) curve, consistent with the previous report on MnTiO3.29The dielectric peak increases in intensity with magnetic fields up to 8 T and moves toward lowTat the same time. All the results indicate strong electric response to an external magnetic field, and the concomitant AFM transition and dielectric anomaly show strong evidence of the magnetodielectric effect.
FIG. 2.TheTdependence of the magnetic susceptibility (a)va, (b)vb,and (c)vc
with the cooling field of 200 Oe under the FC mode. The magnetization as a func- tion ofHapplied along (b)H//a, (c)H//b, and (d)H//cat selected temperatures. The black solid arrows denote the critical field (HSF) for spin flop transition withH//a.
TABLE I.The Curie–Weiss temperature (hcw), frustration factor (f), Curie–Weiss constant (C), and effective moment (leff) for CoSe2O5in the geometry aligned paral- lel toa-axis,b-axis, andc-axis, respectively.Peff(S) andPeff(SþL) are theoretical effective magnetic moment and spin-only effective moment, respectively.
CoSe2O5 TN(K) hcw(K) f C leff(lB) Peff(S) Peff(SþL) a-axis 8.5 14.9 1.7 3.1 4.92 3.873 5.2 b-axis 8.5 17 1.8 2.9 4.84 3.873 5.2 c-axis 8.5 62 7.6 3.4 5.33 3.873 5.2
To further explore the ME coupling effect, we measured theIband Pbas a function ofTin the selectedHparallel to thec-axis, as shown in Figs. 3(b) and 3(c). No spontaneous polarization was observed for H¼0, consistent with the fact that the centrosymmetric space group Pbcnforbids spontaneous polarization. It is common knowledge that the ME effect can be generated in nonpolar or polar magnets, in which the magnetic space group meets the requirement of breaking the space inversion and time-reversal symmetries, as exemplified in Ni3TeO6,30 Co4Nb2O9,31 and PbCu3TeO7.32 Here, once the magnetic field is applied, sharp peaks coinciding with the dielectric anomalies one by one are well demonstrated, indicative of an intrinsic coupling between mag- netism and ferroelectricity. In addition, the gradually enhanced intensity of the pyroelectric peaks is in good accordance with the change in dielectric constant, giving rise to growing polarization upon magnetic fields, e.g., up to134lC/m2under field 8 T.
To further examine the ME coupling, we carried out more detailed measurements in terms of the ME current (IME) and H-induced polarization (DPb) againstHatT¼4 K uponHramping
fromþH! H! þHat a rate of 100 Oe/s, as shown inFigs. 4(a) and4(b), respectively, where the arrows indicate the direction of mag- netic field sweeping. A clear change in the ME current sign is shown once the field flips from positive to negative values. The most striking feature is the discrepancy of induced polarization for forward and reverse magnetic fields nearH¼60.3 T, giving rise to the two evident peaks ofDPb, as shown inFig. 4(b). In fact, this effect is very common in recently studied ME materials.26,33,34In addition, theDPb(H) data can be well fitted by a linear dependence, as denoted by the dashed red curve inFig. 4(b). The linear ME coefficientacan reach21.3 ps/m, larger than most typical linear ME materials. In addition, it is worth noting that the existence of off diagonal components may imply the occurrence of the ferrotoroidal order in CoSe2O5considering its simi- larity with LiCoPO4, calling for future experimental investigations using second harmonic generation spectroscopy or polarized neutron analysis.35–37
Finally, let us discuss the origin of ME effects observed in CoSe2O5. Combined with the data of magnetization and electric polar- ization, it is of great worth to analyze the magnetic structure to deter- mine the ME tensor, which is considered as a useful approach to explore the mechanism of ME effects. The single crystal neutron dif- fraction suggests that the magnetic ground state of CoSe2O5belongs to thePb0cnmagnetic space group.24Here, we employ the IEEE standard setting, where the physical property axis (x,y,z) corresponds to the crystallographic axis (a,b,c), respectively. As the mirror operations of thePbcnspace group are glide mirrors, the location of three mirrors is plotted inFig. 4(c), whereasmx,my, andmzmirrors lie in thex¼1/4 plane, y¼1/2 plane, and z¼1/4 plane, respectively. According to Neumann’s principle, the matrix form of the ME tensor for Pb0cn magnetic space group should be15,25
aij¼
0 0 0
0 0 ayz 0 azy 0 0
B@
1
CA; (1)
whereayzandazyrepresent linear ME coefficients. Therefore, the exis- tence of off diagonal terms should allow emergence ofPbwhenHis applied along thec-axis, or vice versa, while in fact, our experimental results exactly support the prediction.
Subsequently, let us discuss the sign reversal ofanearHc¼0.
There are two possible AFM states, AFM-a (a< 0 orL//aþ) and AFM-b(a>0 orL//a-) states, as seen inFig. 4(d). The red and blue arrows are local magnetic moments and electric dipoles, respectively.
The staggered momentLas an order parameter in an antiferromanget is defined as L¼maþ ma-, in whichmaþandma-are magnetic moments of the two sublattices. In principle, the magnetic field induced linear ME effect can be expressed asP/gLH¼aH, where the parametergdescribes the ME coupling.33Under the ME poling procedure, the single-domain AFM-a state is selected as shown in Fig. 4(e). In this case, there is no energy barrier, and the ME state with a<0 (þL) is stable. The linear ME coefficientaorLusually does not change sign when H is reversed. However, some compounds, e.g., LiCoPO4and our case here are some exceptions.26,33,34The stability of two ME states is highly dependent of not only the ME poling proce- dure (HandE) but also temperature (T). When a mall magnetic field is reversed, the free energy of the AFM-bstate is decreased, and the energy barrier arising between AFM-aand AFM-bstates is small, as FIG. 3.TheTdependence of theb-axis (a) dielectric constant (eb), (b) pyroelectric
current (Ib), and (c) electric polarization (Pb) with respect to the magnetic field for fieldsH//cup to 8 T.
seen inFig. 4(f), but the AFM-astate is still stable. However, when large enoughHis applied, the energy barrier between two states van- ishes, and the ME state witha>0 (L) is stabilized, as presented in Fig. 4(g), thereby interpreting the sign reversal of the ME coefficient near60.3 T.
In conclusion, we have presented our systematic investigation on the magnetic, magnetodielectric, and magnetoelectric properties of CoSe2O5 single crystals with S¼1/2 antiferromagnetic chain. Our results suggest that CoSe2O5is a member of the linear ME material in the spin chain family as demonstrated by the simultaneous responses of polarization and dielectric anomaly against magnetic stimuli. The observed off diagonal linear ME effect is in accordance with the pre- diction of the ME tensor analysis for the ground magnetic space group Pb0cn. Consequently, our results indicate that the ferrotoroidic order may form in CoSe2O5due to the existence of the off diagonal term in the ME tensor as that in LiCoPO4. This might propose an interesting chance to further exploration of the intriguing phenomena and phys- ics of ferrotoroidicity in the potential member CoSe2O5.
This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 92163210, 11874031, 11834002, 11774106, 12074111, 51721001, and 11974167).
AUTHOR DECLARATIONS Conflict of Interest
The authors have no conflicts to disclose.
Author Contributions
L.L. and Y.S.T. contributed equally to this work.
DATA AVAILABILITY
The data that support the findings of this study are available within the article.
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FIG. 4.(a) The ME current (IME) and (b)H-induced polarization (DPb) probing againstHatT¼4 K uponHramping fromþH! H! þHat a rate of 100 Oe/s. The arrows indicate the direction of the magnetic field scan. The red dashed line is the linear fit byDPbaH. (c) Schematic Co2þmagnetic structure ordering at the ground AFM state.
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