Magnetoelectric mutual-control in collinear antiferromagnetic NdCrTiO
5XiangLi,1,2,a)MeifengLiu,1,2,a),b)YuWang,1LimanTian,1RuiShi,1LunYang,1QiyunPan,1 JuanjuanHan,1BoXie,1NianZhao,1XiuzhangWang,1ShaozhenLi,3LinLin,2ZhiboYan,2 and Jun-MingLiu1,2
1Institute for Advanced Materials, Hubei Normal University, Huangshi 435002, China
2Laboratory of Solid State Microstructures and Innovative Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China
3School of Mathematics and Physics, Hubei Polytechnic University, Huangshi 435003, China
(Received 5 July 2018; accepted 3 September 2018; published online 19 September 2018)
Strong magnetoelectric (ME) coupling has been one of the dreaming goals in magnetoelectric and multiferroic materials. In particular, the electro-control of magnetic ordering and magnetization is of high interest. In this work, we synthesize NdCrTiO5and perform a set of characterization studies on the multiferroic properties and the linear ME effect. It is revealed that NdCrTiO5exhibits a magnetic phase transition atTN20 K, below which a remarkable ME response is observed. On one hand, it is non-ferroelectric at zero magnetic field and a magnetic field as low as 1.0 T is sufficient to induce remarkable pyroelectric current below TN, demonstrating the magnetism-induced ferroelectricity.
On the other hand, the remarkable magnetic control of electric polarization and electro-control of magnetization are recorded. At 10 K, a magnetic field of 1.0 T can lead to a change in polarization as large as 20%. Moreover, magnetization M can be significantly modulated by an electric field, with the estimated inverse ME coefficient as large as1.84 ps/m. The temporal evolution of electri- cal polarization and magnetization indicates the stable ME mutual control, suggesting potential applications of NdCrTiO5as a promising multiferroic.Published by AIP Publishing.
https://doi.org/10.1063/1.5047077
The magnetoelectric (ME) effect that denotes the controls of either magnetization by an electric field or polarization by a magnetic field in a material has attracted widespread interest owing to its potential technological applications in high-density data storage or ME switching devices.1,2Since the theoretical prediction of the ME effect in 1959 and experimental observa- tion of the linear ME effect in Cr2O3, huge effort has been paid to the design and synthesis of effective ME or multiferroic materials over the past century.3–6Subsequently, the linear ME effect has been observed in many antiferromagnetic (AFM) systems such as MnTiO3, Co4Nb2O9, and Ni0.4Mn0.6TiO3, and it is believed that this effect originates from properly broken inversion symmetry.7–9
Besides, giant ME coupling can be obtained in single- phase multiferroics in which electric polarization is generated in a properly spin-ordered phase. Along this line, type-II mul- tiferroics in which the electric polarization originates from the spatial inverse symmetry breaking induced by asymmetric and symmetric striction mechanisms could be one category of promising candidates.10,11 For the past decade, the long- sought control of electric properties by a magnetic field has been achieved in the so-called type-II multiferroics, for exam- ple, RMnO3, RMn2O5 (R¼rare earths), LiCu2O2, CuFeO2, Ba2CoGe2O7, CaMn7O12, and LiFe(WO4)2.12–16 Recent research on type-II multiferroics with strong ME coupling demonstrated that it is beneficial to discuss the linear ME effect from the viewpoint of multiferroicity.17,18 It can help promote our understanding of additional ME coupling modes and further find the application-driven ME operations such as
magneto-control of polarization or even electro-control of magnetization that has been observed in Cr2O3, noting that any electro-control of magnetization is highly concerned due to its advantages over magneto-control of electric polariza- tion.19 Moreover, the large energy barrier between different ferroic states in most multiferroics produces hysteresis and a large coercive field, which brings about deleterious effects such as low precision or asymmetrical oscillations in ME devices. For such reasons, we discuss one class of collinear antiferromagnetic oxide NdCrTiO5 that has been concerned due to the uncertain mechanism wobbling between the linear ME effect and multiferroicity. It is our motivation to realize the ME mutual control in this oxide compound and achieve the stable ME response without significant hysteresis.
As one of the first known ME materials possessing two distinct magnetic sublattices, NdCrTiO5 was preliminarily investigated to identify the lattice and magnetic structures in 1970s.20,21It is now known that NdCrTiO5crystallizes in the orthorhombic structure with thePbamspace group, as shown in Fig.1(a). There exist cross-site occupations between Cr3þ and Ti4þions. The 4hsites on the bases of oxygen square pyr- amids are occupied by Cr3þions with a probability of 0.95 and Ti4þions with a probability of 0.05. The 4f sites in the center of oxygen octahedra are filled with Cr3þand Ti4þions in a probability of 0.05 and 0.95, respectively. The 4gsites are occupied by Nd3þions alone. The crystal structure shown in Fig. 1(a), for simplicity, has ignored the 5% cross-site occupation of Cr3þand Ti4þions. The octahedra centered at 4fsites form the infinite chains with shared edges along thec- axis. In the ab-plane, the pairing square pyramids connect each other with their base sharing oxygen edges, and the octa- hedra and pyramids are thus linked with corner-sharing
a)X. Li and M. Liu contributed equally to this work.
b)Electronic address: [email protected]
0003-6951/2018/113(12)/122903/5/$30.00 113, 122903-1 Published by AIP Publishing.
APPLIED PHYSICS LETTERS113, 122903 (2018)
oxygen atoms of either apex or bases. The Nd3þions locate on the alternative layers in the octahedral and pyramidal network.
Such a specific lattice structure allows multifold exchange interactions, and thus, the magnetic structure of NdCrTiO5seems to be a bit complex. As shown in Fig.1(b), the magnetic structure consists of two sublattices. One is the Cr3þspin sublattice where the spins are collinearly aligned along thec-axis and antiferromagnetically ordered in theab plane below 13 K, forming the G-type antiferromagnetic order. The other is the Nd3þsublattice where the spins order in theab plane with the spin-rotation away from theb-axis by 12.20
Nevertheless, several major issues with this compound in terms of linear ME and multiferroic responses remain unsolved. First, it was argued that the ordering of Nd3þspins is driven by the neighboring Cr3þspins via the Cr3þ-Nd3þ exchange coupling rather than the independent Nd3þ spin exchange. This issue remains open yet. It is also uncertain whether the antiferromagnetic order is the consequence of Cr3þspin exchanges or the coupling of these two magnetic
sublattices.22Second, it was further confirmed that the emer- gence of the antiferromagnetic order and even ferroelectric polarization (driven by magnetic field) is around Neel point TN¼18–21 K rather than 13 K deduced from the neutron dif- fraction data by Buisson.20Since no further magnetic phase transition has been confirmed between these two tempera- tures, they may imply the same phase transition, i.e., the ordering of Cr3þspins. Nevertheless, debatable opinions on the origin of electric polarization were raised.23–25 Third, magnetic substitution and doping in NdCrTiO5 brought no enhanced performance in terms of electric polarization and ME effect.22,25–27 In fact, no detailed data on the ME response, especially on the electro-control of magnetism, have been available so far. These issues thus raise substantial interest in revisiting the ME and multiferroic properties in NdCrTiO5belowTN.
Herein, we experimentally demonstrate the non- hysteretic magneto-control of polarization and robust electro- control of magnetization in NdCrTiO5. Detailed investigation of the temperature dependences of magnetizationM, specific heat CP, and electric polarization P induced by a magnetic field below TN will be reported in detail. Furthermore, we probe the temporal evolution of PandM in response to the applied magnetic field and electric field. The stable response indicates a fascinating ME operation in NdCrTiO5.
The single-phase polycrystalline NdCrTiO5was prepared with a conventional solid-state reaction method. The highly purified powder of oxides Nd2O3, Cr2O3, and TiO2in stoi- chiometric ratio was mixed and ground, followed by reaction in an alumina crucible at 1200C for 24 h. The resultant pow- der was fully re-ground and pelletized under 5000 psi pressure to a disk of 20 mm in diameter and 1 mm in height. The pellet was sintered at 1350C for 24 h, followed by the natural cool- ing down to room temperature in air.
The crystal structure of NdCrTiO5was characterized by X-ray diffraction (XRD) in theh-2hmode using a Bruker D8 Advanced diffractometer (Cu-Karadiation) at room tempera- ture, as shown in Fig. 1(c). All the peaks can be properly indexed by the standard Bragg reflections without identifiable impurity phases. For a quantitative evaluation of the phase purity and lattice distortion, the Rietveld refinement was adopted to fit the measured XRD data. The high quality of Rietveld fitting is guaranteed by the obtainedRwp¼13.60%, Rp¼9.88%, and v2¼1.076. The refined lattice parameters are a¼7.332 A˚ , b¼8.508 A˚ , andc¼5.662 A˚ , in agreement with earlier results.20
Subsequently, we look at the magnetic behaviors of the as-prepared samples. Thedcmagnetic susceptibilitiesvas a function of temperatureTare depicted in Fig.2(a), under the measuring (cooling) field H¼1000 Oe in zero-field cooled (ZFC) and field cooling (FC) modes using the Quantum Design Superconducting Quantum Interference Device mag- netometer (SQUID). The two measured curves are almost overlapped, indicating strong antiferromagnetic interactions in NdCrTiO5. The evaluateddv=dTTcurve drawn in Fig.
2(b) shows a sharp peak near TN 20 K, corresponding to the tiny kink of thevT curve aroundTN, which indicates the long-range antiferromagnetic ordering of Cr3þspins. The broad peak in thevTcurve at aroundT010 K might be ascribed to the ordering of Nd3þspins.
FIG. 1. (a) A schematic drawing of the lattice structure of NdCrTiO5with 5% cross-site occupying ions ignored. (b) The orientation of Nd3þand Cr3þ spins in NdCrTiO5 denoted by green and red arrows, respectively. (c) Measuredh-2hXRD spectrum of the NdCrTiO5polycrystalline sample and the refined results using the Rietveld method.
A further study was carried out to disclose the possible phase transitions by measuring the specific heatCP(normal- ized by T) with the thermal relaxation method using the Quantum Design Physical Properties Measurement Systems (PPMS), as drawn in Fig. 2(b). A clear anomaly at TN is seen, confirming the paramagnetic to antiferromagnetic tran- sition. Also, a broad shoulder aroundT010 K is observed in accord with our assumption of the magnetic ordering of Nd3þmoments. There might be two possible origins, one for the independent Nd3þspin interactions and the other for the induction by Cr3þ-Nd3þ exchange coupling, which cannot be distinguished in this work. Moreover, the ZFC vT curves under different cooling/measuring magnetic fields are shown in Fig. S1 in thesupplementary material. No distinct change can be identified between these curves, which further confirms the strong antiferromagnetic interactions of mag- netic ions in NdCrTiO5.
Before checking possible ME coupling, we first investi- gate whether spontaneous ferroelectric polarization exists in NdCrTiO5 or not. We employ the ultra-high sensitive pyro- electric current method to detect the relatively weak ferroelec- tricity. For the electrical measurements, a disk-like sample of 3.0 mm in diameter and 0.2 mm in thickness was deposited with Au electrodes on each side, in order to form a parallel plate capacitor geometrical structure. First, the sample was poled with an electric poling field of 10 kV/cm offered by a source-meter, as well as different applied external magnetic fields H (H¼0 if no magnetic field was applied). By this scheme, the sample was cooled down from a given tempera- ture (up to room temperature) to 2 K. Then, the electric field was removed, and the sample was electrically short-circuited for sufficient time at 2 K, followed by a slow heating until the temperature became higher thanTN, during which the electric current released from the capacitor was recorded using a Keithley 6514A electrometer. The heating rate could be 2–4 K/min under different measuring cycles.
We present in Fig.3(a) the measured pyroelectric cur- rent IpyroðTÞcurves under E¼10 kV/cm and different mag- netic fields l0H¼0–9 T. It is noted that the measured IpyroðTÞdata atl0H¼0 are almost zero, which indicates no ferroelectric polarization if the magnetic field is absent. It suggests that NdCrTiO5 is not an intrinsic ferroelectric.
Upon increasing magnetic field H, the current peak takes sharp around19 K and broadens, which gradually exhibits a slight downshift of the peak temperature. Given that the IpyroðTÞ signals are purely from the pyroelectric effect (we discussed it in the supplementary material as shown in Fig.
S2), one has the polarizationP(T) data evaluated by integrat- ing the current plotted in Fig. 3(b). It is seen that the ferro- electric polarization becomes larger with increasing H and reaches13lC/m2atl0H¼9 T. It is worth noting that the onset of electric polarization is just around the temperature TN where the paramagnetic to antiferromagnetic transition occurs, indicating that the ferroelectricity in NdCrTiO5does have the magnetic origin ascribed to the induction of the magnetic field. Moreover, to check whether this magneti- cally induced ferroelectricity comes from the linear ME effect, we plotted the H-induced polarization P(T¼10 K) in the inset of Fig.3(b). As evidently seen,Pis proportional to the applied H, suggesting that the linear ME effect plays a major role in NdCrTiO5. The linear ME effect with a rough coefficient of 2.01 ps/m defined bydP/dHis on the same order of magnitude as that reported earlier.23
For the characterization of the electro-response to the magnetic field, the sample was first poled under E¼10 kV/
cm andl0H¼5 T, similar to the earlier, and cooled down to
FIG. 2. (a) Thedcmagnetic susceptibilitiesvin the ZFC and FC modes with a measuring field of 1000 Oe. (b) The measured derivativedv=dT(left) and theT-normalized specific heatCP=T(right) as the functions of temperatureT.
FIG. 3. (a) Measured pyroelectric currentIpyroðTÞcurves atE¼10 kV/cm under different applied magnetic fields with heating ratev¼4 K/min. (b) The polarizationP(T) curves evaluated from the pyroelectric current data, with the inset clarifying the polarizationPðT¼10 KÞas a function of the applied magnetic field.
10 K followed by the short-circuit process. Then, the magnetic fieldHlinearly changes betweenHminandHmax, during which the electric current was measured using the electrometer. We observed a repeatable variation ofIinduced by a modulated external magnetic field varying between l0H¼4.5 T and 5.5 T atT¼10 K, as depicted in Fig.4(a). The variation of l0DH¼1 T was chosen to minimize the magnetic hystere- sis.28 It is seen from Fig. 4(a) that the collected current changes rapidly between I1 0.33 pA and I2 0.54 pA without any delay as H changes linearly. The deduced DP
¼PðHÞ Pð5 TÞshown in Fig.4(b)exhibits that in the pres- ence of a modulated magnetic field, the electric polarizationP oscillates linearly with the variation of 1.6 lC/m2, up to 20% relative toP(5 T) at 10 K. It is noted that no phase shift occurs between the external magnetic field and the deduced electric polarization. In other words, with the increasing (decreasing) magnetic field, the electric polarization increases (decreases). It is obviously revealed thatPis almost linearly modulated by applied H without hysteresis. The temporal evolution ofPdependent onHindicates a stable ME control.
Furthermore, we investigated the inverse ME effect, that is, the magneto-response to the electric field. Prior to this characterization, the sample was poled under the electric field E¼10 kV/cm and magnetic fieldl0H¼4 T and cooled down to 10 K. After the short-circuit process, bothE and H were removed. Subsequently, E¼0 and 10 kV/cm were applied periodically, during which the magnetization was measured using the SQUID VSM (measuring field 2000 Oe). Figure 4(c)shows our experimental demonstration of theE-induced magnetization’s variety without alteringHat 10 K. The mea- sured M remains to be 1:740103 lB/f.u. while E¼0
and changes rapidly to 1:726103 lB/f.u. while E¼10 kV/cm, with the magnitude ofDMbeing1:4105 lB/f.u. The inverse line ME effect with the coefficient of 1.84 ps/m was defined as DM=DE, which has never been reported earlier. The temporal evolution of M induced byE indicates a stable magnetic response to the electric field.
As for the magnetic field control of polarization, the applied magnetic field could influence the spin orders and thus modulate the inherent coupled electric orders. To the contrary, the applied electric field could force the polariza- tion induced by ME cooling to re-arrange in its direction.
Noting that the polarization originates from the magnetic order, the variation of polarization would lead to the re- ordering of antiferromagnetic regions, which accounts for the electric-field control of magnetism.
In summary, we revisited the classical linear ME effect in NdCrTiO5 by measuring electric polarizationP induced by a magnetic field H and magnetization M induced by an electric field E belowTN. We experimentally demonstrated the electric polarization responding toHand the magnetiza- tion responding toEat 10 K. The observed ME effect shows an obvious and stable ME mutual control by magnetic and electric fields. The obtained coefficients of ME and inverse ME effects are 2.01 ps/m and1.84 ps/m, respectively. Our experimental results could provide an appropriate contribu- tion to a comprehensive understanding of the electro-control of magnetism on linear ME materials or multiferroics.
Seesupplementary materialfor more details of the mag- netization, pyroelectric measurements, and magnetic control of electric polarizations.
This work was supported by the National Key Research Projects of China (Grant No. 2016YFA0300101), the National Natural Science Foundation of China (Grant Nos.
11704109, 51431006, 51332006, and 11804088), and the Research Project of Hubei Provincial Department of Education (Grant Nos. Q20172501 and B2018146).
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