Manipulating the magnetism and resistance state of Mn:ZnO/
Pb(Zr
0.52Ti
0.48)O
3heterostructured films through electric fields
Yong-ChaoLi,1JunWu,1Hai-YangPan,1JueWang,1Guang-HouWang,1,2Jun-MingLiu,1,2 and Jian-GuoWan1,2,a)
1National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China
2Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China
(Received 7 February 2018; accepted 14 May 2018; published online 23 May 2018)
Mn:ZnO/Pb(Zr0.52Ti0.48)O3 (PZT) heterostructured films have been prepared on Pt/Ti/SiO2/Si wafers by a sol-gel process. Nonvolatile and reversible manipulation of the magnetism and resis- tance by electric fields has been realized. Compared with the saturation magnetic moment (Ms) in theþ3.0 V case, the modulation gain of Mscan reach 270% in the3.0 V case at room tempera- ture. The resistance change is attributed to the interfacial potential barrier height variation and the formation of an accumulation (or depletion) layer at the Mn:ZnO/PZT interface, which can be regu- lated by the ferroelectric polarization direction. The magnetism of Mn:ZnO originates from bound magnetic polarons. The mobile carrier variation in Mn:ZnO, owing to interfacial polarization cou- pling and the ferroelectric field effect, enables the electric manipulation of the magnetism in the Mn:ZnO/PZT heterostructured films. This work presents an effective method for modulating the magnetism of magnetic semiconductors and provides a promising avenue for multifunctional devices with both electric and magnetic functionalities.Published by AIP Publishing.
https://doi.org/10.1063/1.5025198
Nonvolatile and reversible manipulation of magnetiza- tion with electric fields, rather than electric currents or mag- netic fields, allows faster and lower power operations and has become one of the central issues in recent years due to its fun- damental importance and potential applications. The converse magnetoelectric (ME) effect makes it possible to control magnetism through electric fields.1–5 Owing to weak ME coupling and low operation temperatures in single phase mul- tiferroics,6 most studies on electrical control of magnetism have focused on composite multiferroics materials, and some remarkable progress has been made to date.7–13 Strain- mediated, exchange-mediated, and charge-mediated converse ME effects are the major mechanisms for controlling these manipulations.4However, the strain-mediated converse ME effect usually changes the extrinsic magnetism, such as mag- netic anisotropies.11,14 The strain in the ferroelectric (FE) layer is transferred to the ferromagnetic (FM) layer, which then tunes the lattice strain, the coercive field, magnetization, and the resistance of the films upon ferroelectric poling or polarization switching. However, maintaining the strain after removing the electric field remains a great challenge.
The ferroelectric field effect offers another available method for reversible and nonvolatile switching of magneti- zation, even in the absence of an external electric field.4The remanent polarization in the ferroelectric layer can attract (or repel) charge carriers, creating a thin charge accumulation (or depletion) layer at the interface such that the charge- related electronic transport and magnetic properties of the films can be regulated.15 For instance, in La1xSrxMnO3 (LSMO)/FE and La1xCaxMnO3 (LCMO)/FE systems, the ferroelectric field effect can induce a change in the valence
state of Mn ions and interfacial spin reconfiguration, such that electric-field modulation of the magnetism is achieved.10,16–19 However, this type of regulation usually requires low temper- ature or other severe conditions,15which restrict their practi- cal applications. It is well known that some transition metal (TM) doped ZnO materials are diluted magnetic semiconduc- tors with a high ferromagnetic Curie temperature.20,21In pre- vious experimental and theoretical research, it has been found that the magnetism of TM-doped ZnO originates from carrier- mediated exchange or bound magnetic polarons (BMPs), which are closely related to the carrier density.21–23If the car- rier density in TM-doped ZnO can be controlled by the elec- tric fields, electric manipulation of the magnetism in that system can be achieved.
In this letter, we propose a feasible approach to realizing a significant electric manipulation of magnetism at room temper- ature, i.e., constructing a heterostructure by combining a FE Pb(Zr0.52Ti0.48)O3 (PZT) layer with a FM semiconductor Mn:ZnO layer. Using the interface polarization coupling and ferroelectric field effect, we have realized electric manipulation of both the magnetism and resistance. The modulation gain of saturated magnetization (Ms) can reach 270%. Moreover, this manipulation is repeatable and shows good endurance and retention. We also reveal that the resistive switching (RS) effect is related to the variation of the interfacial potential bar- rier height and the accumulation (or depletion) formation at the Mn:ZnO/PZT interface. The magnetism variation origi- nates from the change of carrier density in the Mn:ZnO layer.
The Mn:ZnO (5 at. % Mn doped)/Pb(Zr0.52Ti0.48)O3 (PZT) bilayer films were prepared by a sol-gel process and spin coating technique. The PZT and Mn:ZnO layers in turn were grown on the Pt/Ti/SiO2/Si(100) wafer. Before deposit- ing the next layer, each layer was dried at 280C for 10 min
a)Author to whom correspondence should be addressed: [email protected]
0003-6951/2018/112(21)/212902/5/$30.00 112, 212902-1 Published by AIP Publishing.
and then annealed at 650C for 5 min by a rapid annealing process in an oxygen atmosphere. The details of the prepara- tion can be found elsewhere.24 For electric measurements, 100-nm-thick Au top electrodes with 200lm in diameter were deposited onto the film surface using the ion sputtering technique.
The phase structures of the film were characterized by X-ray diffraction (XRD) on a D/MAXRD diffractometer using Cu Karadiation, as shown in Fig.1(a). The film con- sists of perovskite PZT and wurtzite ZnO phases. No diffrac- tion peaks indicative of impure phases are observed. The cross-sectional scanning electron microscopy (SEM) image, shown in the inset of Fig. 1(a), indicates that the upper Mn:ZnO layer and bottom PZT layer are approximately 75 nm and 125 nm in thickness, respectively. A clear inter- face between the Mn:ZnO layer and the PZT layer is defined.
The average Mn content determined by energy dispersive X- ray spectrometry (EDS) is about 6.6 at. % and close to 5 at.
% in Mn:ZnO solution, as shown in Fig. S1 in thesupple- mentary material. We further measured the elemental distri- bution in the film, as shown in Fig. S2 in thesupplementary material. These results demonstrate that Mn is uniformly dis- tributed in the ZnO matrix.
Figure1(b)presents the ferroelectric hysteresis loop mea- sured by a standard ferroelectric test system (Precision Multiferroic, Radiant, Inc.). The film exhibits an asymmetrical ferroelectric hysteresis with the remanent polarization 2Pr 33.1lC/cm2and ferroelectric coercive voltageþVc2.7 V and Vc 2.0 V. Such asymmetry is mainly caused by interface polarization coupling25,26 (the polarization of the PZT layer couples with the polarization of the ZnO layer) and
asymmetry remanent polarization (more details can be found in part II of thesupplementary material).27–29A spontaneous polarization of7.0 4.1lC/cm2exists along the (0001)h direction in wurtzite ZnO, and the remanent polarization of ZnO is in practice electrically irreversible.20,25 The in-plane magnetization vs. magnetic field (M-H) curve of the as- grown film was measured at room temperature (300 K) on a superconducting quantum interference device magnetometer (VSM-SQUID, Quantum Design), as shown in Fig.2(a). The film exhibits evident ferromagnetic characteristics. The Ms and coercive field (Hc) values are 3.90 emu/cm3 and 75 Oe, respectively.
The interaction between the PZT layer and the Mn:ZnO layer can produce a converse ME coupling,26which provides us with the possibility of manipulating the magnetism of the Mn:ZnO layer by electric fields.23,30–32 To demonstrate this, we treated the samples with different electric fields (more details can be found in part III of thesupplementary material) and then measured the M-H loops at room temperature (300 K), zero-field cooling (ZFC), and field cooling (FC) curves to observe the change in magnetic features. During the electrical treatments,þ3.0 V or3.0 V electric voltages with a 0.1 ms retention were applied to the Mn:ZnO/PZT film (the Au top electrode was grounded). Since 3.0 V exceeds the fer- roelectric coercive voltage of the film, the ferroelectric domains in the PZT layer can be adequately reversed. Figure 2(a) presents two M-H loops after the film was treated by þ3.0 V and 3.0 V, respectively. One observes that the film has an evident change in the Msvalue after being subject to electric treatments. The film has a much larger Ms value (6.54 emu/cm3) after being treated by 3.0 V but only 1.75 emu/cm3in the case of theþ3.0 V treatment. The ratio of the change in Msreaches 270%. We repeatedly treated the
FIG. 1. (a) XRD patterns of the Mn:ZnO/PZT film. The inset shows the cor- responding cross-sectional SEM image. (b) Ferroelectric polarization vs.
voltage (P-U) loop. For measurements of the P-U loop, a standard bipolar triangular waveform with a measuring frequency of 10 kHz was used.
FIG. 2. (a) In-plane magnetization vs. magnetic field (M-H) loops of the Mn:ZnO/PZT film at the as-grown state andþ3.0 V and3.0 V treat- ment states. The inset shows the expansion of the curves in (a). (b) The dependence of Mson the cycle number of alternately applyingþ3.0 V and 3.0 V.
films by alternately applyingþ3.0 and 3.0 V electric vol- tages and measured the dependence of Ms on the cycling number, as shown in Fig.2(b). One can see that the Msvalue almost has no change after the film is subject to several cycles of the electric treatments. These results indicate that the mag- netism in the present films can be well manipulated by the electric fields, and the manipulation is nonvolatile and stable.
Figure S3 in the supplementary material gives the ZFC and FC curves after the film was treated by different electric voltages. The measurements were performed over the range of 10–300 K. During the measurements, a magnetic field of 500 Oe was applied. As shown in Fig. S3, the magnetic moment (M) gradually decreases as the temperature is increased. The M values obtained from the ZFC measure- ments are consistent with those from the FC measurements for both cases of þ3.0 and 3.0 V. The M value after the 3.0 V treatment is much larger than that of the þ3.0 V treatment. These results are consistent with the results of the M-H measurements, which further suggest that the magne- tism of the Mn:ZnO/PZT bilayer can be well manipulated by the electric fields, even after removal of the electric fields.
In the present films, the electric fields can not only manipulate the magnetism but also switch the resistance state. Figure3(a)shows the current vs. voltage (I-V) curves with consecutive 200 cycles measured using a Keithley 2400 source meter. The voltages were applied to the Pt electrode, and the Au electrode was grounded. The sweeping direction of the voltage was 0.0 V! þ3.0 V ! 3.0 V!0.0 V, the sweeping step was 0.1 V with a 0.1 ms duration, and the cur- rent compliance was set to be 1.0lA. The I-V curves exhibit typical bipolar RS features, and an electroforming process is not necessary. When sweeping the electric voltages from 0.0 to þ3.0 V, the current increases and rapidly reaches the
current compliance at 2.2 V. After finishing this “set” pro- cess, the resistance of the film is switched to a low resistance state (LRS) from a high resistance state (HRS). The film can maintain the LRS in the sweeping procedure from þ3.0 to 0.0 V. In the reverse sweeping procedure, the resistance of the film is switched from the LRS to the HRS at2.6 V.
Besides, after 200 consecutive cycles, the I-V curve is almost as the same as that of the first measurement. We also exam- ined the repeatability and stability of such a RS effect (a more detailed content can be found in part V of thesupple- mentary material), as shown in Figs.3(b)and3(c). It is clear that the resistances of HRS and LRS almost do not change even after being subject to 500 consecutive switching cycles.
The ratio of R(HRS) to R(LRS) is about 150. The resistance values in both the HRS and LRS are quite stable, and almost no degradation appears even after 104s continuous reading.
Thus, it is confirmed that the resistance of the films is non- volatile and allows for non-destructive reading.
To understand the mechanism of the electrical manipu- lation of both the resistance state and magnetism for the pre- sent Mn:ZnO/PZT films, we drew the schematic diagrams of Mn:ZnO/PZT at different states, as shown in Fig.4. We first consider the origin of the RS effect. Two factors, i.e., the change in the interface potential barrier height at the Mn:ZnO/PZT interface and the formation of the charge accu- mulation (or a depletion) layer at the interface between the PZT and Mn:ZnO layers, may contribute to the RS effect.
The ferroelectric polarization direction in the PZT layer has a considerable influence on both factors.32–34 We first ana- lyze the situation forþ3.0 V. After the bilayer is treated byþ3.0 V, the ferroelectric polarization in the PZT layer points to the Mn:ZnO layer; hence, positive polarization charges gather at the Mn:ZnO/PZT interface, as shown in Fig. 4(a). Owing to the interface polarization coupling and ferroelectric field effect, the carrier density increases in the Mn:ZnO layer,35,36 and an accumulation layer is formed at the Mn:ZnO/PZT interface. Thus, the resistance of the total film is mainly produced by the PZT layer, and the film is in LRS. The situation becomes different after the film is treated by3.0 V, as shown in Fig. 4(c). The ferroelectric polariza- tion direction in the PZT layer is reversed completely, and consequently, the negative polarization charges assemble at the Mn:ZnO/PZT interface. The carrier density decreases in the Mn:ZnO layer, and a depletion layer is formed at the inter- face. At the same time, the potential barrier height /bat the Mn:ZnO/PZT interface increases.32,37 In this situation, the voltage drop is mainly applied across the PZT layer and the depletion layer. Accordingly, the film is switched from the LRS to the HRS. Therefore, the current value afterþ3.0 V is larger than that after3.0 V, as shown in Fig.3(a).
Further capacitance vs. frequency measurement results also support the above analysis. From Fig. S5 in thesupple- mentary material, one observes that the capacitance value of the film treated byþ3.0 V is larger than that of the film treated by3.0 V, which should be caused by the formation of an accumulation layer.32,34 More comprehensive results and analysis can be found in part VI of the supplementary material.
Electrically manipulating the magnetism in the present Mn:ZnO/PZT films originates from the carrier density
FIG. 3. (a) The current vs. voltage (I-V) loops on a logarithmic scale. (b) The dependence of current and ratio of R(HRS) to R(LRS) on the cycling number of switching between HRS and LRS. (c) Retention capability of HRS and LRS.
variation in the Mn:ZnO layer, which can be controlled by the PZT polarization direction. The magnetism in the Mn:ZnO layer can be understood based on the bound mag- netic polaron (BMP) model.21,23,35,38 In the BMP model, each local magnetic moment (local moment bound carriers) forms a magnetic polaron.38,39 The electrons trapped by defects (e.g., oxygen vacancies) are confined in a hydrogenic orbital. The overlap between the hydrogenic electron and d- shell orbitals of magnetic ions leads to a cooperative percola- tion ordering of such magnetic ions. The magnetic exchange interaction is mediated by the occupied polaron orbits. When the polaron density reaches the percolation limit, ferromag- netism occurs. Hence, if the polaron density changes, the exchange field flipping the spin of magnetic ions and the magnetism will be changed to some degree.36 In this sense, localized carriers are beneficial for ferromagnetism, while mobile carriers are harmful to ferromagnetism. It has been observed that magnetism increases as the carrier density decreases in Co-doped ZnO films.40–42 Hence, if the carrier density in Mn:ZnO could be regulated by electric fields, the magnetism of Mn:ZnO will be changed correspondingly.
Accordingly, we explain the electrical manipulation of the magnetism in the present Mn:ZnO/PZT system as fol- lows: After being subject to theþ3.0 V treatment, the positive polarization charges assemble at the Mn:ZnO/PZT interface, as shown in Fig. 4(a). So, the mobile electron density increases in the Mn:ZnO layer due to the interface polariza- tion coupling and the ferroelectric field effect. In this situa- tion, oxygen vacancies can trap more electrons and form V••O (oxygen vacancy trapping two electrons). However, only VO• (one electron trapped) can activate bound magnetic polarons to induce the formation of ferromagnetic domains.43 Thus, the probability of overlapping Mn ions reduces, as shown in Fig. 4(b). Differently, after the film being subject to3.0 V treatment, the negative polarization charges assemble at the Mn:ZnO/PZT interface, as shown in Fig. 4(c). The mobile electron density decreases in the Mn:ZnO layer, while the
number of VO• increases. Thus, the probability of overlapping Mn ions increases compared with the case of þ3.0 V, as shown in Fig. 4(d). Therefore, the Ms value after3.0 V treatment is larger than that of þ3.0 V. The electric field is mainly applied on the PZT layer in the present system, and the possibility of the magnetism variation23,30–32 (due to the valence state change of Mn ions or the migration of oxygen vacancy in the Mn:ZnO layer caused by electric fields) can be ruled out in principle. The combination of the magnetism modulation and RS effect gives rise to integration of the elec- trical and magnetic properties into a simple bilayer film struc- ture, which has great potential applications in multifunctional devices such as multi-channel logic devices and multilevel memory and storage.
In conclusion, the Mn:ZnO/Pb(Zr0.52Ti0.48)O3 hetero- structured films have been prepared on Pt/Ti/SiO2/Si wafers using a sol-gel process, and reversible manipulation of both the magnetism and resistance states by electric fields has been realized. The modulation gain of the Ms value can reach 270% at room temperature. The resistance change can be attributed to the variation of the interface potential bar- rier height and the formation of an accumulation (or deple- tion) layer at the Mn:ZnO/PZT interface, which can be regulated by the ferroelectric polarization direction. The electrical manipulation of magnetism is attributed to the variation of the carriers in the Mn:ZnO layer, which causes the changes in the density of bound magnetic polarons. This work presents an effective method to modulating the mag- netism of magnetic semiconductors and provides us with a promising avenue for developing multifunctional electric devices.
Seesupplementary material for the distribution of ele- ments for the Mn:ZnO/PZT films, the explanation about the P-E loop, the details of the measurements, the ZFC-FC curves, the measurement sequence, and the capacitance data.
FIG. 4. Sketches of the Mn:ZnO/PZT heterostructures after different electric treatments. (a) and (b) after þ3.0 V treatment. (c) and (d) after 3.0 V treatment. Plus and minus symbols represent the positive and negative polarization charges, respectively. Pale green arrows express the polarization direction in the PZT layer. Black solid spheres, red arrows, yellow solid spheres, and white solid spheres repre- sent the Mn ions, the spins of Mn ions, the trapped electrons, and the mobile electrons, respectively.
This work was supported by the National Key Projects for Basic Research of China (Grant No. 2015CB921203), the National Key Research Programme of China (Grant No. 2016YFA0201004), the National Natural Science Foundation of China (Grant Nos. 51472113 and 11134005), and Jiangsu Planned Projects for Postdoctoral Research Funds (Grant Nos. 1701093C).
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