Electric Field Manipulated Multilevel Magnetic States Storage in FePt

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Electric Field Manipulated Multilevel Magnetic States Storage in FePt/(011) PMN-PT Heterostructure Xiaoyu Zhao, Jiahong Wen, Bo Yang, Huachen Zhu, Qingqi Cao, Dunhui Wang, Zhenghong Qian, and Youwei Du ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11015 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 29, 2017

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ACS Applied Materials & Interfaces

Electric Field Manipulated Multilevel Magnetic States Storage in FePt/(011) PMN-PT Heterostructure

Xiaoyu Zhaoa ,b, Jiahong Wena ,b, Bo Yangc, Huachen Zhud, Qingqi Caoa ,b, Dunhui Wanga,b*, Zhenghong Qiand, Youwei Dua ,b

a. National Laboratory of Solid State Microstructures and Jiangsu Key Laboratory for Nanotechnology, Nanjing University, Nanjing 210093, P.R. China b. Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, P.R. China c. Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang, Liaoning 110819, China d. Center for Integrated Spintronic Devices, Hangzhou Dianzi University, Hangzhou 310018, China

Keywords: multiferroic heterostructure, nonvolatile manipulation, magnetoelectric effect, multistate storage, electric-writing magnetic-reading.

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Abstract In the current information society, the realization of magnetic storage technique with an energy efficient design and high storage density is greatly desirable. Here, we demonstrate that, without bias magnetic field, different values of remanent magnetization (Mr) can be obtained in a FePt/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) heterostructure by applying unipolar electric field across the substrate. These multilevel magnetic signals can serve as writing data bits in a storage device, which remarkably increase the storage density. As for the data reading, these multilevel Mr values can be read nondestructively and distinguishably using commercial giant magnetoresistance magnetic sensor by converting magnetic signal to voltage signal. Furthermore, these multilevel voltage signals show good retention and switching property, which enables the promising applications in electric-writing magnetic-reading memory devices with low-power consumption and high storage density.


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1. Introduction Magnetic random access memory (MRAM) has been regarded as a dominant type of memory due to its nonvolatility, infinite endurance and fast random access.1 However, for meeting the increasing demand of consumer electronics and multimedia storages, the realization of magnetic storage technique with an energy efficient design and high storage density is still a great challenge.1-4 In magnetic data storage, magnetization is manipulated with a currentgenerated magnetic field, which leads to high energy consumption.5,6 The discovery and development of multiferroic materials which simultaneously have ferroelectric (FE) and ferromagnetic (FM) orders bring an opportunity to solve this problem, especially in the heterostructure with a magnetic thin film growing on a FE substrate.7-10 In this magnetoelectric (ME) heterostructure, the electric field control of magnetization is usually realized with strainmediated mechanism,3,7,11-13 i.e. through the electric-field-induced strain transferred from FE layer, the magnetism can be manipulated by means of altering magnetic anisotropy,14,15 modifying the exchange coupling16 or driving magnetic phase transition.17,18 Therefore, the electric field manipulation of magnetization provides a promising potential for the design of lowpower consumption memory devices. Improving storage density is a permanent valuable issue for all kinds of storage technologies.19-21 For the current MRAM devices, the binary memory cells show two different magnetic states of the recording medium which correspond to “0” and “1”, respectively.5,6 As we know, magnetization is a vector quantity and has not only magnitude but also direction.22 So in this sense, magnetization has more than two states and the binary memory may not fully utilize the advantage of magnetization.23 Anisotropy is an important feature of magnetization and by altering it, the magnetization direction can be changed correspondingly, resulting in different


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values of remanent magnetization (Mr) at zero magnetic field.15 Therefore, multiple magnetic states can be achieved in a memory cell by adjusting the magnetic anisotropy, which is expected to improve the storage density. As mentioned above, the magnetic anisotropy can be manipulated through strain-mediated ME effect.14,15 So the combination of multistate memory and electricfield-controlled magnetization in a ME heterostructure is a promising way to develop storage technique with low power consumption and high storage density. In this paper, a (011)-oriented 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN−PT) single crystal is selected as a FE substrate for its electric field controlling reversible and nonvolatile strain with the application of electric field.24-26 As for the FM component, we choose the binary alloy, FePt, due to its large magnetic anisotropy and magnetostriction constant.27,28 We demonstrate that multiple Mr states can be reversibly written as storage data bits in a FePt/PMN-PT heterostructure by electric field. These magnetic signals are nonvolatile and steady enough to retain written information. Furthermore, the multilevel magnetic signals can be read nondestructively and distinguishably using a commercial giant magnetoresistance (GMR) magnetic sensor, which enables a high-density and low-power consumption memory device. 2. Experimental Section The FePt film with the thickness of 20 nm was deposited on a 10 mm × 5 mm × 0.5 mm commercial (011)-cut PMN-PT substrate by co-sputtering Fe (99.99%) and Pt (99.99%) targets using magnetron sputtering (JZCK-400DJ). The distance between the target and substrate was 10 cm. The base vacuum of the chamber was below 2.0×10-4 Pa. During film deposition, the substrate temperature was kept at 450 °C while the argon pressure was 0.6 Pa. The sputtering power of Fe and Pt target was 20 and 30 W, respectively. The composition of the FePt film was Fe50Pt50, which was determined by SEM (JEOL 6500) equipped with EDS. A film of Au (150


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nm) was grown onto the backside of the PMN-PT substrate as an electrode. The crystal structure of the FePt film was characterized by X-ray diffraction (XRD, using Cobalt radiation) at room temperature. The surface morphology of FePt film was recorded with scanning probe microscopy (SPM, Veeco, Dimension V). A variable-voltage power supply was used to provide the electric field applying between the FePt layer and Au electrode. The strain properties of the heterostructure were measured by the resistance strain gauge technique. The magnetic properties of FePt/PMN-PT were measured using a vibrating sample magnetometer (VSM, Microsense EV7) with the resolution of 10-5 emu. During the ME coupling measurement, the electric field was vertically applied along the thickness (011) direction of the PMN-PT by a source meter (Keithley, model 2410). A commercial GMR magnetic sensor (SpinIC, China) with high sensitivity was used to read magnetic signals and output voltage signals. The GMR sensor with dimensions of 5 mm (Length) × 3 mm (Width) × 1 mm (Height) is placed on the surface of FePt film without any binder. A 5 V regulated voltage was supplied to the magnetic sensor by a Keithley 6517B source meter. The multilevel magnetic signals converted to multistate voltage signals were output by a Keithley 2182A nanovoltmeter. 3. Results and discussion

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Figure 1. (a) XRD pattern of the FePt/PMN-PT (011) heterostructure. (b) AFM image of the


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FePt film with an area of 2 × 2 um2. Figure 1a shows the XRD pattern of the FePt/PMN-PT (011) heterostructure, where the substrate stays in an unpoled strain state. In addition to the (011) and (022) peaks of PMN-PT substrate, the (022) diffraction peak of the FePt film is also observed, indicating that the film is highly oriented along the substrate direction. The AFM image demonstrates that the FePt film has a smooth surface with the root-mean-square roughness of 0.4 nm, which is shown in Figure 1b. The epitaxial growth and smooth surface morphology of FePt film provide a favorable condition for the following ME coupling measurements.

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Figure 2. (a) Schematic diagram of strain measurement for the heterostructure under applied electric field. (b) In-plane strain curves along the [01-1] direction. (c) The strain curves under the


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unipolar electric field with different positive amplitudes. (d) The retention characteristic of the remanent strain states as a function of time. The (011)-oriented PMN-PT FE single crystal is used as the FE substrate to provide the electrically-controlled strain.29-33 Figure 2a presents the schematic diagram of strain measurement for the substrate in which the electric field is applied along the thickness direction and the strain gauge is adhered to the surface of FePt film. The electric field dependence of strain (S-E) curves with different switching pathways of electric field (bipolar or unipolar) are shown in Figure 2b. By applying a symmetric bipolar electric field sweeping from −5 to 5 kV/cm, an SE curve with two large nonlinear strain jumps up to 1300 ppm near the coercive field Ec = ± 2 kV/cm is observed. After the remove of electric field, the strain state returns to zero, showing a volatile behavior.32 When a unipolar sweeping electric field is applied (the negative electric field is larger than the Ec and the positive electric field is smaller than the Ec),12 an S-E curve which has a hysteretic response with respect to sweeping electric field is exhibited. Unlike the case of symmetric bipolar electric field, the strain under the application of unipolar electric field does not switch back to its initial state when the electric field returns to zero, indicating a nonvolatile remanent strain state. Therefore, by selecting proper unipolar electric fields in the vicinity of Ec, it is possible to achieve multilevel and permanent remanent strain states in PMN-PT substrate. As shown in Figure 2c, three different remanent strain states of B, C, and D are observed in the S-E curves, corresponding to the unipolar electric fields of 1.2 kV/cm, 1.4 kV/cm and 1.6 kV/cm, respectively. From the practical application point of view, the stability of remanent strain states is very important for a device to maintain its functionality. Figure 2d displays the retention property of the remanent strain states with different values as a function of time, in which no obvious decay of strain is observed. On the contrary, the remanent strain in the [100] direction is


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near zero with the same operation.32 Thus, by precisely controlling the amplitude of unipolar electric fields, multilevel and stable remanent strains along the [01-1] direction can be achieved in the (011) PMN-PT substrate.

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Figure 3. Schematics of domain structures for the PMN-PT under various applied electric fields. (a) The unpoled state. (b) The 71˚, 109˚, and 180˚ polarization switching. (c-d) Two different remanent strain states A (E = 0 kV/cm) and D (E = 1.6 kV/cm) can be realized, respectively. The aforementioned nonvolatile strain response is mainly due to the non-180˚ FE polarization reorientation in the rhombohedral crystal structure of PMN-PT.12,29-30,32 It is reported that the (011)-oriented PMN-PT substrate has eight possible spontaneous polarization directions, which are along the body diagonals of the pseudocubic unit cell. As shown in Figure 3a, four spontaneous polarization directions point in the out-of-plane direction and the others are inplane. Under a vertically-applied electric field, there are three possible switching pathways of the polarization between the out-of-plane and the in-plane directions including 71˚, 109˚, and 180˚ domain switching, which are shown in Figure 3b. When a negative electric field of -4 kV/cm (>Ec) is applied and removed, the polarization would switch from the in-plane direction to the out-of-plane direction and a state of almost zero remanent strain (A) is obtained,12 which is shown in Figure 3c. After a positive electric field of 1.6 kV/cm (

Electric Field Manipulated Multilevel Magnetic States Storage in FePt

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