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Room-temperature multiferroic properties in NiBi2O4 Kai Chen, Feng Gao, Weiwei Lin, Hongling Cai, Guolin Li, Xingwei Dong, Song Peng, Xiaoshan Wu, Mao Yang, Jun Du, Xiaomei Lu, Junming Liumybreak mybreak and Jinsong Zhu EPL, 89 (2010) 27004
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January 2010 EPL, 89 (2010) 27004 doi: 10.1209/0295-5075/89/27004
www.epljournal.org
Room-temperature multiferroic properties in NiBi2O4 Kai Chen1,2,3(a) , Feng Gao1,2 , Weiwei Lin1,2 , Hongling Cai1,2 , Guolin Li1,2 , Xingwei Dong1,2 , Song Peng1,2 , Xiaoshan Wu1,2 , Mao Yang1,2 , Jun Du1,2 , Xiaomei Lu1,2 , Junming Liu1,2 and Jinsong Zhu1,2(b) 1
National Laboratory of Solid State Microstructures, Nanjing University - Nanjing 210093, PRC Department of Physics, Nanjing University - Nanjing 210093, PRC 3 Department of Physics, Nanjing University of Science and Technology - Nanjing 210094, PRC 2
received 6 August 2009; accepted in final form 21 December 2009 published online 1 February 2010 PACS PACS
75.30.-m – Intrinsic properties of magnetically ordered materials 77.84.-s – Dielectric, piezoelectric, ferroelectric, and antiferroelectric materials
Abstract – Magnetism and ferroelectricity at room temperature are observed in the NiBi2 O4 ceramics. Both the time reversal and the inversion symmetry of the structure (space group F-43m) are broken. The saturation magnetization is 0.028 emu/g and the saturation polarization 2Ps ∼ 4.0 µ C/cm2 . NiBi2 O4 also shows other room-temperature multiferroic properties, e.g. the piezoelectric coefficient (d33 ), the polarized dielectric character, the magneto-dielectric response and the magnetoelectric effect. c EPLA, 2010 Copyright
Multiferroic single-phase compounds (MSPC), which simultaneously show a spontaneous magnetization and polarization, are rare [1]. These materials, for instance the perovskite-BiFeO3 and the manganite-HoMn2 O5 , are currently of great interest, due to the potential applications in some of the most important technology advances [2]. Whereas most multiferroic materials show the coexistence of multiple order parameters at low temperature, few are the room temperature (RT) MSPC [3]. Although the RT-MSPC remain elusive, some fundamental physics behind the multiferroic scarcity have found to date that, in general, the transition metal 3d-electrons, which are essential for the magnetism, reduce the tendency for ionic off-center displacements, which are traditionally regarded as the primary contribution to the electrical polarization [4,5]. Thus the coexistence of the magnetic order and the ferroelectric polarization depends on the delicate balance of two competing factors, e.g. the 3d-electrons and the off-center structural distortion. In the perovskite-type BiFeO3 or BiMnO3 multiferroics, these two orderings are favored by the existence in the A sublattice of a Bi3+ ion having a sterochemically active unshared pair of 6s-electrons, with the transition metal ions Fe3+ or Mn4+ owning 3d -electrons in the B sublattice [6]. This structure-chemical criterion indicates that for the RT-MSPC oxides, the transition metal (a) E-mail: (b) E-mail:
[email protected] [email protected]
cations with 3d-electrons create the net spin while the lattice distortion is induced by such ions, e.g., Bi3+ [6]. Although much effort has been devoted to design new multiferroics, such as NiTiO3 [7], how and under what circumstances a new MSPC could be discovered is still a challenge that provides concrete examples to structure a unified physics picture. Our strategy is to transform this problem into pursuing the spinel-related AB2 O4 with these characteristic ions mentioned above and to expect that the strong spin-lattice coupling weakens the thermal disruption on the magnetic order and the ferroelectric one. The previous work indicates that both the time reversal and the inversion symmetry of the spinel structure might be broken, and the multiferroic could exist [9–12]. In this letter, we report that the NiBi2 O4 (NBO) bulk exhibits the magnetism, the ferroelectricity and the piezoelectric property at RT. This material also has a high dielectric constant but a low dissipation factor while showing both the obvious magneto-dielectric and magnetoelectric effect. High-purity nickel nitrate ( 99%) and bismuth nitrate ( 99%) in stoichiometric proportions (1 : 2 molar ratios) were dissolved in an appropriate amount of analytically pure nitric acid. The solution was stirred for 72 hours at 30 ◦ C to get a clear solution and then heated to form the gel. The dried gel was mixed for three hours. The final powder was calcined at 800 ◦ C for 2 hours in air. The powder was ground once again for 1 h and pressed
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Kai Chen et al. Table 1: The refined crystal structure parameters of NB2 O4 bulk.
Spinel F-43m Atomic position Ni(1) Bi(1) Ni(2) Bi(2) Ni(3) Bi(3) O(1) O(2) Lattice parameter a = 8.04617(6) ˚ A
Site 4a 4a 4c 4c 16e 16e 16e 16e
x 0 0 0.25 0.25 0.6205(2) 0.6205(2) 0.8499(3) 0.3850(5)
Fig. 1: X-ray diffraction patterns of the NiBi2 O4 sample.
into pellets prior to the final sintering in air at 1280 ◦ C for 3 h, and then subsequently quenched to RT. The crystalline phase was characterized by a Rigaku X-ray Diffractometer (D/MAX-RB) with Cu-Kα radiation. The morphology was determined using scanning electron microscopy (SEM 1530YP, Leo Co., Germany) with energy dispersive spectrometer (EDS). Magnetic and ferroelectric measurements were carried out using a vibrating-sample magnetometer (VSM) (EV7, ADE Co., USA), and a RT6000 standard ferroelectric test system (Radiant Technologies, USA), respectively. Dielectric spectroscopy and the magnetic-dielectric response were performed using a Hewlett-Packard Impedance Analyzer, model 4294 (Agilent Co., USA). The RT-magnetoelectric equipment was used in ref. [8]. X-ray diffraction data is plotted in fig. 1. The X-ray analysis reveals that the samples are single phase and no impurity phases are observed. The structure at RT is cubic spinel (space group F-43m), so the time reversal and the inversion symmetry are broken [13]. The structure parameters are summarized in table 1. The typical scanning electron microscopy (SEM) image of our
y 0 0 0.25 0.25 0.6205(2) 0.6205(2) 0.8499(3) 0.3850(5)
z 0 0 0.25 0.25 0.6205(2) 0.6205(2) 0.8499(3) 0.3850(5)
Occupancy 0.762(2) 0.236(2) 0.764(2) 0.236(2) 0.118(1) 0.882(1) 1 1
Fig. 2: The magnetic-field dependence of magnetization (M -H ) of the NBO samples at 286 K and 77 K. The left inset shows the coercive field at room temperature, and the right inset exhibits the temperature dependence of the magnetization with the 15 kOe magnetic field.
NBO ceramics as shown in the inset, reveals that the grain shape is almost triangular prism and the grain is about 1 µm. No impurity elements are found. Room-temperature magnetization-magnetic field (M -H ) curves of the NBO ceramic (fig. 2) were measured with an applied magnetic field of 15 kOe and the sample shows strong hysteresis behavior. The shape of the magnetic hysteresis loops indicates that there are two-dimensional spin planes and the sublattices of the Ni2+ ions with the 3d-electrons are in accordance with the XRD conclusion. Although the M -H loop has a typical S shape, it does not show the characteristic ferromagnetic (FM) behavior until the slim hysteresis and the low magnetic coercivity are distinguished from a signature of G-type antiferromagnetism with weak FM due to the spin canting [12]. The saturation magnetization is ∼0.028 emu/g at RT and the coercive field ∼48 Oe (the left inset). With decreasing temperature the sample exhibits an enhancement in magnetization and it is the typical result of the weak thermal disturbance on the spin order (the right inset). At T = 77 K, the magnetization increases to
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Room-temperature multiferroic properties in NiBi2 O4
Fig. 3: The P -E hysteresis loops of the samples at room temperature.
∼ 0.04 emu/g. The magnetization at higher temperatures shows that the Curie temperature of the samples is higher than the one of Ni (630 K), and thus rules out that the magnetism is due to a Ni impurity phase. As for the multiferroic bulk ceramics, the low resistivity of the samples is always an obstacle to observe the well-saturated polarization vs. the electric field (P -E ) hysteresis loop; therefore, our sample was prepared by a conventional solid-state reaction followed immediately by a quenching process to decrease the leakage current [14]. Figure 3 shows the typical RT P -E hysteresis loop of the NBO ceramics. With the electric field ∼200 kV/cm, the NBO shows a well-saturated loop with saturation polarization 2Ps ∼ 4.0 µC/cm2 . The sample will break down with further increasing electric field and the resisitivty of the samples bears strong correlation with processing conditions. The piezoelectric coefficient, d33 , of the polarized samples is 2.8 × 10−12 C/N. These results indicated that the NBO is an improper ferroelectric [12] in which the magnetic order may create the weak ferroelectricity [4,12,15,16]. It is noticed that the dielectric spectroscopy of polarized samples at RT (fig. 4) evidences the insulating character of the bulk, which is also indicated by the P -E hysteresis loop. At RT, there is a resonant dielectric peak ∼569 at the frequency of 204 Hz. The real part of complex dielectric value, ε′r , decreases rapidly with increasing frequencies and finally ends with 468 at f = 1 MHz. In the tested frequencies, the dissipation factor is less than 0.08 and there is a loss peak at the frequency of ∼ 977 Hz while it reaches the minimum value at f ∼ 1.86 MHz. The nonlinear dielectric response excludes the effect of the leakage current [17]. The response does not change significantly on the polishing faces. As shown in the inset, there are a small section of larger arc at the high frequencies and nearly half of semicircular arc (fitted with the solid line in the inset) in the complex plane. This phenomenon can often be illustrated by the Maxwell-Wagner relaxation [18,19]. The dielectric response suggests that the NBO ceramics are electrically inhomogeneous. Recent experiments reported
Fig. 4: The frequency dependence of ε′r and the dissipation factor of the polarized samples at room temperature. The inset shows the Maxwell-Wagner relaxation and the fitted curve (red line).
Fig. 5: The temperature dependence of the ε′r change as the magnetic field is 7 T. The right inset shows the temperaturedependent dielectric response at 1 MHz when the applied magnetic field is 0 T or 7 T. The left inset shows that the Hbias dependence of the induced magnetoelectric voltage increment |ΔVME | at a given d.c. magnetic frequency f = 1.0 kHz for the NiBi2 O4 ceramic.
the observation on the room-temperature conducting features of ferroelectric domain walls in the insulating MSP CBiFeO3 [20]. This suggests that the electrical inhomogeneity is due to the electrical difference between ferroelectric domains and domain boundaries. We also measured the magneto-dielectric behavior at different temperatures and the ME effect of the NBO ceramics at room temperature, as shown in fig. 5. Without the magnetic field, the dielectric response of NBO is similar to that of the MSPC TbMnO3, as shown in the right inset [21]. After a magnetic field ∼ 7 T is applied the dielectric response at 1 MHz is changed as shown in the right inset, which excludes the extrinsic contribution from the sample-electrode interfaces. The
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Kai Chen et al. most dielectric change ∼ 57% at 108 K is observed and the non-linear dielectric changes in the whole temperature range indicate that the spin-lattice coupling may be related to the high-order term [15]. The magneto-dielectric response indicates that the electro-active magnons (or electromagnons) and spin waves can be excited by an a.c. electric field [22]. On the other hand, the magnetic field slows down the “freezing” process of the electrical dipoles. The left inset plots the Hbias -dependence of the induced voltage increment |ΔVME | at a given a.c. magnetic-field frequency f = 1.0 kHz. The NBO exhibits a clear signature of magnetoelectric effects. With increasing Hbias , the |ΔVME | value increases, reaching the maximum value of 1.9 mV at Hbias = 1.9 kOe. It is noted that |ΔVME | drops to the zero when the applied Hbias is 3.6 kOe, which indicates the net polarization is zero even though the ferroelectric domains are orderly distributed by the spin-lattice coupling, in other words, the electronic polarization from the valence electrons is oppositely counteracting the ionic polarization from the lattice displacements [5]. After the Hbias larger than 3.6 kOe is applied, the value of the induced voltage increment rapidly increases to 11.2 mV when Hbias is 6.34 kOe. This trend could be attributed to the more ordered ferroelectric domains which provide an ionic polarization larger than the electronic polarization. The calculated maximum increment of the magnetoelectric voltage coefficient is as high as 10 mV cm−1 Oe, which is larger than that of the reported MSPC ceramics. The Hbias -dependence of the ME effect indicates that the spinlattice coupling is not linear but strong. In summary, a new multiferroic NiBi2 O4 is discovered. Notably, the sample obviously shows the magnetization and the ferroelectricity at RT, simultaneously exhibiting the magneto-dielectric behavior and the ME effect. This could give rise to a new generation of devices based on the multiferroic behavior. ∗∗∗ The authors are grateful for the financial support of the National Natural Science Foundation of China (Grant Nos. 50672034, 50832002), the State Key Program for Basic Research of China (Grant Nos. 2009CB623303 and 2009CB929501), the Open Project of National Laboratory of Solid State Microstructures, and the Outstanding Foundation of Nanjing University of Science and Technology. REFERENCES [1] Wang J., Neaton J. B., Zheng H., Nagarajan V., Ogale S. B., Liu B., Viehland D., Vaithyanathan V., Schlom D. G., Waghmare U. V., Spaldin N. A., Rabe K. M., Wuttig M. and Ramesh R., Science, 299 (2003) 1719.
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