Effect of nonmagnetic substituent Zn on the phase competition and multiferroic properties in the
polar magnet Fe 2 Mo 3 O 8
Cite as: Appl. Phys. Lett.118, 112901 (2021);doi: 10.1063/5.0044565 Submitted: 17 January 2021
.
Accepted: 1 March 2021.
Published Online: 15 March 2021
W.Wang,1,2P. Z.Li,2Y. T.Chang,1M. F.Liu,3 C. L.Lu,1,a)X. B.Lu,2 M.Zeng,2,a) and J.-M.Liu2,4 AFFILIATIONS
1School of Physics and Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan 430074, China
2Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials and Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China
3Institute for Advanced Materials, Hubei Normal University, Huangshi 435002, China
4Laboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 21009, China
a)Authors to whom correspondence should be addressed:cllu@hust.edu.cnandzengmin@scnu.edu.cn
ABSTRACT
The polar magnet, Fe2Mo3O8(FMO), with linear magnetoelectric (ME) coupling, is a promising candidate for multiferroic applications in advanced spin devices. However, a giant magnetic bias (Hb) is needed for optimizing the inverse ME effect, i.e., electric field (E) modulation of magnetization (M), which is still a core issue. Herein, we utilize the chemical doping route to enhance the sensitivity of controlling the competitive magnetic interactions and/or multiferroic phases by means of introducing nonmagnetic Zn2þions into FMO crystals. Compared with FMO, the Zn-doped composition (Fe0.95Zn0.05)2Mo3O8(FZMO) generates three metastable magnetic states in the middle of antiferro- magnetic and ferrimagnetic states, along with obvious ferroelectric polarization. The inverse ME effect of FZMO is intact with a relative change ofDM0.06lB/f.u. responding to anEvalue of620 kV/cm at 52 K. Most interestingly, the excitingHbis dramatically dropped to 0.8 T for FZMO from 5.1 T for FMO, which is in favor of the application of ME coupling. It is suggested that the perturbation of magnetic interactions via substituting specific sites by nonmagnetic ions plays a key role in decreasing the excitingHbwithout deteriorating the inverse ME coupling in this polarM2Mo3O8family.
Published under license by AIP Publishing.https://doi.org/10.1063/5.0044565
The magnetoelectric material with intrinsic cross control between electric polarizationPand magnetizationMhas triggered much atten- tion for the past few decades due to its potential application for multi- function electronic devices.1–3 Among them, the spin-driven ferroelectrics exhibiting strong ME coupling have been receiving spe- cial attention. Since the seminal discovery of TbMnO3,4a large num- ber of spin-driven ferroelectrics were found and the related ME mechanisms were revealed, such as the inverse Dzyaloshinskii-Moriya (iDM) interaction due to the antisymmetric exchange (Si Sj of neighboring spins Si,j),5–9 the Heisenberg-like exchange striction owing to the symmetric exchange (Si Sj),10–14 and the spin- dependentp-dorbital hybridization mechanism.15,16
It has been reported the iDM interaction in noncollinear magnetic order can generate a strong ME effect operating at higher
temperature (T) and a lower magnetic field (H), such as in Y-type and Z-type hexaferrites,17–20while a magnetic/electric poling procedure is necessary and the spin-driven P is relatively small. In contrast, the Heisenberg-like exchange striction in collinear spin texture usually produces a relatively large P, experimentally verified by the ortho- rhombic TbMnO3 that crosses a multiferroic phase transition from a spiral order (Ps 0.1lC/cm2) to a collinear AFM order (Ps1lC/cm2) under pressure.21,22 Recently, a considerable P (0.3lC/cm2) was discovered in the collinear polar magnet Fe2Mo3O8 (FMO), accompanied by a pronounced linear ME effect.23,24In addition, an enormous advantage in FMO is that the pol- ing process is avoidable due to its single polar domain, making it more possible for E-controllable spin devices.23 Thus, it is worthwhile to make further studies in this system.
The polar magnetM2Mo3O8(M¼Mn, Fe, Co, Ni) has a noncen- trosymmetric hexagonal structure with a space group of P63mc, allowing the crystallographic polarity along thec-axis.25–28It was sug- gested that two nonequivalentM2þions with different spin, such as Fe2þ, occupy the octahedral and tetrahedral oxygen cages, respectively [Fig. 1(a)], forming a ferrimagnetic (FIM) honeycomb structure in the ab-plane. The Mo4þ ions are combined into unique spin-singlet trimers without contribution to the magnetism.24The early neutron powder diffraction study verified the collinear antiferromagnetic (AFM) order along thec-axis [Fig. 1(b)], which is stabilized in FMO and Co2Mo3O8below 60 and 39 K, respectively,24–27while an FIM ground state [Fig. 1(c)] is demonstrated in Mn2Mo3O8.25It should be noted that the macroscopic FIM state is hidden in FMO due to the off- set stacking of neighboring honeycomb layers, which can be produced by an applied field along thec-axis.23,24As for Ni2Mo3O8, a more com- plicated AFM spin structure with a mixture of stripy and zigzag char- acter was illustrated recently.28
Furthermore, the low-T ME properties are abundant in the M2Mo3O8family. The linear ME effect of FMO is attractive with a large coupling coefficient ofa3¼ 16.2 ps/m in the FIM state,24also detected in Mn2Mo3O8,25 but it is forbidden in Co2Mo3O8.27 Specifically, the excellent inverse ME effect (EcontrollingM) was con- firmed in FMO.23Unfortunately, a giant excitingHbis necessary for achieving an evident change ofMby applyingE, unlike the noncollin- ear spin configurations where the moments are easier to be manipu- lated by electric perturbations.18Interestingly, it is reported that the Zn2þ ions are preferential to reside into the oxygen tetrahedrons [Fig. 1(a)], selectively diluting the competing magnetic interactions in FMO.24,29–31Moreover, the diluted effect leads to the linear ME coeffi- cienta1varying from142 to 107 ps/m when enlarging the Zn con- tent from 12.5% to 50%.24In other words, the ME coupling is still robust in spite of introducing nonmagnetic Zn2þions. However, it is extremely possible to reduce the biasHbfor the inverse ME effect due to the diluted magnetic interactions. Therefore, it is urgently needed to make comprehensive investigations of this issue.
In this paper, the magnetic and dielectric peculiarities of (Fe1-xZnx)2Mo3O8 (x¼0, 0.05) single crystals were systematically investigated. Upon chemical doping, it was confirmed that replacing
Fe2þ(1) by the nonmagnetic Zn2þions enables us to generate three fresh metamagnetic states in the middle of AFM and FIM states.
While the competing magnetic interactions are diluted by introducing a little of Zn2þ dopants, its ME coupling is still robust. Here, the inverse ME effect, a significant change ofDM0.06lB/f.u. produced by an externalE varying fromþ20 to20 kV/cm, was achieved in both FMO and (Fe0.95Zn0.05)2Mo3O8(FZMO) at 52 K. However, the necessary excitingHbis dramatically dropped, i.e., one order lower in FZMO (0.8 T) compared with that in FMO (5.1 T). The lower biasHb
in FZMO, accompanied by a remarkable inverse ME coefficient aE 838 ps/m, makes it possible for applying the ME coupling to the next-generation spintronic devices conceptionally.
The (Fe1-xZnx)2Mo3O8(x¼0, 0.05) single crystals were fabricated by the chemical vapor transport technique.32The high-purity raw mate- rials (Fe, Fe2O3, MoO2, and ZnO) were mixed isochemically and ground in an agate mortar. Then, the milled powders along with a small amount of transporting agents TeCl4were placed into an aluminum crucible, which was sealed in a quartz tube with high vacuum. Finally, the tubes were placed into a two-zone furnace for crystals growing over 10 days.
For structural characterization, the crystallinity of as-grown crys- tals was checked by the standard X-ray diffraction (D8 advanced, Bruker) with CuKaradiation. The magnetism including thedcmag- netic susceptibility (v) and isothermal magnetization was measured using the vibrating sample magnetometer (VSM) integrated with the physical property measurement system (PPMS). For electrical mea- surements, Au electrodes were deposited on the parallel end surfaces of the crystal with the diameter of 3 mm and the thickness of 0.15 mm. The polarized currentIwas measured with constant rates ofT-sweep (2 K/min) andH-sweep (80 Oe/sec) using an electrometer (Keithley Model 6430). The electric polarization Pwas obtained by integratingIwith respect to timet.
The XRD pattern of the FZMO crystal along thec-axis is shown inFig. 1(d). It can be found that the characteristic diffraction peaks are well overlapped with the standard patterns of FMO, revealing that the samples used in our measurements are single phase with the space groupP63mc. The inset ofFig. 1(d)exhibits the morphology of one typical as-grown crystal of FZMO with large dimensions of 3 mm 3 mm1 mm. The hexagonal shape further verifies the high qual- ity of our crystals.
Figure 2(a)shows thedcmagnetic susceptibilityvof FMO and FZMO as a function ofTmeasured with theH//c-axis under the field- cooling (FC) process. The sharp peaks inv(T) curves indicate the Neel pointTNfor the long-range AFM ordering. As for FMO, the value of TN is 59.4 K, which is in agreement with the previous work,23,24 while the slightly declinedTNof53.9 K for FZMO suggests that the Zn dopant leads to the diluted magnetic interaction. Note that an addi- tional bump peak in thev(T) curve for FZMO under the zero-field cooling (ZFC) process is emerged near 30 K, as shown in Fig. S1, supplementary material. These differences in theM(H) dependence between the two crystals are more remarkable, and the results are dis- played inFigs. 2(b)and2(c). It is found that only a sharp jump occurs inM(H) curves for FMO [Fig. 2(b)], corresponding to the AFM to FIM transition.23,24That is to say, the two magnetic phases are closely competed with each other. In contrast, this competition becomes more complex in FZMO, producing a series of intermediate metasta- ble magnetic states, as marked by the black arrows in Fig. 2(c). In FMO, the frustrated sublattice couplings, i.e., the interlayer Fe2þ(1)- FIG. 1.(a) The crystalline structure of (Fe1-xZnx)2Mo3O8(x¼0, 0.05). The sche-
matic representation of (b) AFM and (c) FIM configurations of FMO in one unit cell.
(d) Thec-axis X-ray diffraction pattern of FZMO with the inset showing its morphol- ogy. The red lines below the diffraction peaks represent the standard peak positions of FMO. The blue, purple, gray, and red spheres represent Fe2þ(1)/Zn2þ, Fe2þ(2), Mo4þ, and O2-ions. The blue and purple arrows mean the spins of Fe2þions.
Applied Physics Letters
ARTICLE scitation.org/journal/aplFe2þ(1) and Fe2þ(1)-Fe2þ(2) couplings, induce the metamagnetic transition from AFM to FIM.25It is distinct that these couplings are perturbed because a number of Fe2þ(1) ions are substituted by non- magnetic Zn2þions, therefore giving rise to the multiple metastable states. This multi-fold stepwise character inM(H) curves has also been reported in Mn2þ doped FMO compound (Mn0.25Fe0.75)2Mo3O8, which is different from FZMO due to Mn2þions occupying the two Fe2þsites.25The magnetic anisotropy in this system is recognized by the comparisons between the in-plane and out-of-planev(T) curves, as shown inFigs. 2(a)and S2.
The T dependence of dielectric constant er(T) presented in Fig. 3(a)displays a cleark-shaped peak aroundTNfor FZMO, consis- tent with the magnetic anomaly in the v(T) curve, revealing the appearance of an intrinsic FE phase.Fig. 3(b)exhibits the polarized currentIof FZMO as a function ofTunderl0H¼0.1 T. A deep valley in theI(T) curve can be found nearTN, implying the inherent mag- netic origin of the FE polarization. The corresponding polarization DP(T) curve is depicted inFig. 3(c). It can be seen thatDP(T) gradu- ally increases whenTis cooled from 100 K due to the pyroelectric effect.24Upon further cooling, a dramatic drop ofDP(T) is observed when crossingTN. This indicates that FZMO is entered into a stable FE state with a significantDP 0.1lC/cm2, which is comparable to the reported value in pristine FMO.24It is recognized that the polariza- tion is induced by the collinear AFM spin texture by virtue of the exchange striction mechanism for FMO in both theory and experi- ment.23,24 This mechanism has the major responsibility for the
ferroelectric property in FZMO, although the magnetic interaction is slightly diluted by the Zn2þdopant.
To illustrate the correlation between the metamagnetic phase and polarization behavior in FZMO, theHdependence ofIandPunder theH//c-axis was measured at different temperatures, and the results are exhibited in Figs. 4(a) and 4(b). Obviously, each I(H) curve [Fig. 4(a)] exhibits three clear current peaks, and the corresponding jumps inDP(H) curves [Fig. 4(b)] are also clearly identified regardless of T. These peaks and/or jumps are excellently consistent with the magnetic anomalies inM(H) curves, confirming the fact that the varia- tion ofPstems from the metamagnetic transitions. Note that the FIM phase presents a linear ME response under theH//c-axis. The linear ME trend appears at lowerTin spite of the occurrence of a small kink indicated by the black arrows, which is gradually broken as T approachesTNdue to the thermal fluctuation. Compared with FMO that only exhibited a sharp jump in isothermal magnetization and polarization curves, the two additional transitions ofDP(H) in FZMO, accompanied by three metastable magnetic phases, reveal that three new multiferroic states are born owing to the fixed-site doping by Zn2þions. According to the magnetic and dielectric anomalies inT andHscans ofMandPunder theH//c-axis, we summarize theH-T phase diagram of FZMO, as shown inFig. 4(c). Obviously, the phase boundaries fromM(T)/M(H) andP(T)/P(H) measurements are con- sistent. WithHparalleling thec-axis, FZMO undergoes five distinct major phases, labeled as low-HAFM and high-HFIM phases, as well as three new metastable multiferroic phases (M1, M2, and M3) between them. The research of exploring the magnetic structures for FIG. 2.The magnetic properties of (Fe1-xZnx)2Mo3O8(x¼0, 0.05) along thec-axis.
(a) The temperature dependence of magnetic susceptibility under the FC condition for FMO and FZMO. The isothermal magnetization curves for (b) FMO and (c) FZMO with maximuml0H¼9 T. Data for (a) are shifted for clarity.
FIG. 3.(a) The dielectric thermal spectra under the 0.1 T field withf¼50 kHz along thec-axis. Thev(T) curve is added here for clearly comparing the changes in magnetism and electricity nearTN. (b) The polarized current curveI(T) and (c) the integral resultDP(T) (DP(T)¼P(T) –P(120 K)) measured withl0H¼0.1 T along thec-axis.
the metamagnetic states will be down in the future. From the legible H-Tphase diagram, one can infer that the magnetic transition has a strong influence on the ME coupling in the FZMO system.
To further explore the intrinsic ME effect, the variation of M responding to the external pulsedE(620 kV/cm) was measured at 52 K under an excitingHbfor the two crystals.Hbis chosen according to the phase boundary where neighboring multiferroic phases are strongly competed. As shown inFig. 5(a)for FMO, Mis changed against pulsedEwith perfect synchronization and without any obvious decay, demonstrating remarkableE-modulatedM. The varying mag- nitude ofDMis0.06lB/f.u., but an enormous bias fieldl0H¼5.1 T is needed to activate the strong coupled AFM moments. As for FZMO [Fig. 5(b)], a similar modulation ofMdepending on the pulsed Eis confirmed, and the variationDMproduced by6Eis almost equal to that in FMO. This means that the strong ME coupling is maintained even though the competing magnetic interactions are diluted with doping Zn2þions. It is worth noting that the excitingHbin FZMO is low with a value of 0.8 T, which is almost one order smaller than that in FMO. The prominent decrease in the exciting field is essential for the application of ME coupling. Moreover, the inverse ME coefficient aE, defined asaE¼l0DM/DE,17can be estimated to be838 ps/m in
FZMO. Considering that the ME effect is greatly depended on exciting Hb, the inverse ME effect is dramatically deteriorated when Hb is declined to 0.5 T, as shown in Fig. S3, although the inverse ME effect is still held.
In conclusion, the polar magnet (Fe1-xZnx)2Mo3O8(x¼0, 0.05) single crystals were studied systematically. A detailedH-Tphase dia- gram of FZMO was summarized through the magnetic and dielectric measurements. The introduction of 5% Zn2þions to the oxygen tetra- hedra enables us to generate three fresh multiferroic states, where the strong ME coupling is ascertained from the highly consistent anoma- lies of magnetism and FE polarization. Moreover, the inverse ME effect is robust with a very small excitingHb. Therefore, the chemical doping is a valid approach to control the ME coupling in multiferroic materials.
See thesupplementary material for the details of the in-plane v(T) curves and out-of-plane ZFCv(T) curves for FMO and FZMO and the inverse ME effect of FZMO at 52 K with the biasHbdropping to 0.5 T.
This work was supported by the National Key Research Program of China (No. 2016YFA0201004), the Science and Technology Program of Guangzhou (No. 2019050001), the National Natural Science Foundation of China (Grant Nos. 11774106, 12074111, and 11574091), and the Natural Science Foundation of Guangdong Province of China (No. 2019A1515011128).
DATA AVAILABILITY
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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FIG. 4.(a) The isothermal polarized current I(H) and (b) polarization DP(H) (DP(H)¼P(H) –P(0)) measured with a maximum field ofl0H¼9 T from 10 K to 40 K along thec-axis. The black arrows indicate small peaks ofI(H) and small changes inDP(H) induced by magnetic transformation. (c) TheH-Tphase diagram of FZMO. Data for (a) are shifted for clarity.
FIG. 5.The inverse ME effect of (a) FMO and (b) FZMO measured with a bias field ofl0H¼5.1 T and 0.8 T, respectively, at 52 K.
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