Fabrication of 109° Periodic Domain Walls with a Bottom Electrode in BiFeO3 Thin Films

Summary

109° periodic domain walls can successfully be fabricated by introducing a dielectric La-BiFeO₃ layer between a bottom electrode and a ferroelectric BiFeO₃ layer, enabling the study of the switching behavior of a 109° domain structure and the investigation of the electric field control of exchange bias in a ferromagnet/BiFeO₃ system.

Abstract

A variety of exciting phenomena have been discovered using 109° domain walls in BiFeO₃ thin films, such as domain wall conductivity, photovoltaic effects, and magnetoelectric coupling effects. The control of these physical properties with an electric field plays a key role in the development of nanoelectric devices. Therefore, it is critical to fabricate 109° periodic domain walls with a bottom electrode. However, the introduction of a bottom electrode favors the formation of a 71° domain structure due to the electrostatic boundary conditions. In this study, pulsed laser deposition (PLD) is used to produce multilayer epitaxial thin films. A 25% La doping BiFeO₃ layer is inserted as the dielectric spacer between the bottom electrode SrRuO₃ layer and the pure BiFeO₃ layer, enabling the fabrication of 109° periodic domain walls engineered by an interface effect-depolarization field. Moreover, the fabrication of the 109° domain structure with a bottom electrode enables the study of its switching behavior. This protocol provides a novel route to produce 109° periodic domain walls and opens a new pathway to explore fascinating phenomena, such as the room-temperature electric field control of exchange bias in a ferromagnet/BiFeO₃ system and room-temperature multiferroic vortices in BiFeO₃.

Introduction

Domain wall functionalities in BiFeO₃ thin films, such as domain wall conductivity¹, photovoltaic effects², magnetism³, and magnetoelectric coupling⁴, have inspired many studies on the fabrication and manipulation of the domain structures^5,6,7. Periodically ordered 71°, 109°, and 180° stripe domains have been obtained by tuning the film thickness effects, misfit strain effects, and electrostatic boundary conditions^8,9,10,11. The periodic 71° stripe domain structure can be produced on thick SrRuO₃ bottom electrodes and can be reversibly controlled under an electric field⁸, which has promoted a series of seminal works on the electric-field control of magnetism in the ferromagnet/multiferroic BiFeO₃ system^{4,12,13,14,15}. However, the pure 109° stripe domain structure can only exist without a (or with an ultra-thin) SrRuO₃ bottom electrode⁸, making it typically unstable or unresponsive under an applied electric field and inhibiting the study of the switching behavior of 109° domains. Moreover, a large exchange bias has been demonstrated in the ferromagnet/BiFeO₃ system with the 109° BiFeO₃ domain walls^16,17. Thus, the successful fabrication of a 109° domain structure with a bottom electrode is promising for controlling exchange bias using an electric field. This has great potential application to low-energy-consumption, non-volatile magnetoelectronic memory devices.

Our previous study¹⁸ revealed a novel approach to precisely control the domain structure in BiFeO₃ thin films by tuning the depolarization field with a dielectric layer. In the BiFeO₃/SrRuO₃/DyScO₃ stack without La-BiFeO₃, the screening effects at the ferroelectric (BiFeO₃)/metallic (SrRuO₃) interface enable the formation of 71° domains, while the introduction of the La-BiFeO₃ dielectric space layer enables an increase in the distance between the screening charges from the SrRuO₃ and BiFeO₃. This leads to a reduction of the screening effects and thus an increase in the depolarization field. Consequently, the 71° stripe domain is destabilized and a 109° stripe domain structure forms to decrease the energy cost due to the strong depolarization field.

In this work, we focus on the fabrication of 109° periodic domain walls with a bottom electrode in BiFeO₃ thin films by introducing a La-BiFeO₃ dielectric layer. The detailed protocol describes how to grow the multilayer thin films using PLD and how to use piezoresponse force microscopy (PFM) to measure the 109° domain structure and study its switching behavior. The typical PLD system is shown in Figure 1. This protocol can help to increase the reproducibility of the fabrication of 109° periodic domain walls and promote the study of domain wall functionality.

Protocol

1. Substrate Preparation

1. Clean a 5 mm × 5 mm × 0.5 mm single-crystal DyScO₃ (110) substrate with acetone in an ultrasonic cleaner for 5 min.
2. Rinse the substrate with acetone for 5 s and transfer it to isopropyl alcohol. Clean it for 5 min in the ultrasonic cleaner.
3. Rinse the substrate with isopropyl alcohol for 5 s and dry it using N₂ flow.
4. Mount the substrate on a heater with silver paint and then put the heater on a hot plate to dry the silver paint for 10 min at 100 °C.
5. After the heater cools down to room temperature, blow it with N₂ flow. Mount it into the PLD chamber.
   NOTE: The protocol can be paused here.

2. PLD Setup

1. Mount a SrRuO₃ target, a BiFeO₃ target, and a 25% La doping BiFeO₃ (La-BiFeO₃) target in the PLD chamber.
2. Set the target-to-substrate distance to 50 mm.
3. Align the laser light path to focus the laser spot on the target. Attach a piece of sensitive paper on the target and move the focusing lens to a proper position to obtain uniform laser spots, which will ensure the homogeneity of the laser energy.
   NOTE: The target will rotate with roto-translational motion to avoid overheating the target during film growth.
4. Measure the laser energy outside and inside the laser window using a power meter to calculate the energy loss rate.
   NOTE: The loss rate increases with increasing growth time due to the coating on the laser window during deposition. Here, the laser window is cleaned every other day.
5. Pump the chamber with a mechanical pump until the vacuum is less than 10 Pa. Use the turbo pump to obtain a high vacuum (< 5 × 10^-4_Pa).
   NOTE: The protocol can be paused here.

3. Selection of Deposition Parameters

1. Adjust the pumping speed by tuning the gate valve. Inflate oxygen into the chamber to create oxygen pressure of 13 Pa.
2. Heat up the heater to 700 °C at a rate of 20 °C/min and anneal the substrate for 10 min.
3. Start the pulsed laser and set the desired laser energy for SrRuO₃ growth by changing the voltage. Measure the laser energy using a power meter at a laser frequency of 2 Hz.

4. Growth of Multilayer Films

1. Clean the SrRuO₃ target surface at a frequency of 10 Hz for 3 min.
2. Grow SrRuO₃ epitaxial thin film on a (110)-oriented DyScO₃ substrate at 10 Hz for 6 min using PLD.
   NOTE: The growth temperature should be 700 °C and the oxygen partial pressure should be 13 Pa.
3. Tune the heater temperature to 690 °C and clean the La-BiFeO₃ target surface at a frequency of 10 Hz for 3 min.
4. Start the growth of the La-BiFeO₃ layer for 10 min at 10 Hz.
   NOTE: The growth temperature should be 690 °C and the oxygen partial pressure should be 13 Pa.
5. Clean the BiFeO₃ target surface at a frequency of 10 Hz for 3 min.
6. Adjust the frequency of the pulsed laser to 5 Hz and start to grow the BiFeO₃ layer for 40 min at 690 °C under oxygen partial pressure of 13 Pa.
7. Close the gate valve and inflate oxygen to into the chamber. Cool down the sample at 10 °C/min in a 10,000-Pa oxygen atmosphere.
8. Open the chamber after the heater temperature is below 80 °C and remove the sample from the heater.

5. PFM Measurement

1. Attach the sample to a thin metal plate and contact the bottom electrode with the plate using silver paint.
2. Take the PFM measurement of the sample with the cantilever along (110) under the PFM mode to capture 109° domain structure images.
   NOTE: The PFM scanning size should be 5 µm × 5 µm.
3. Apply -6 V in a 3 µm × 3 µm box to study the switching behavior of the 109° domain structure.
4. Capture the 5 µm × 5 µm PFM images after -6-V switching.

Results

The multilayer BiFeO₃/La-BiFeO₃/SrRuO₃films on the DyScO₃ (110) substrate are produced by PLD, and the heterostructure stack is shown in Figure 2. To obtain the 109° domain structure with a bottom electrode, a thin dielectric layer of La-BiFeO₃ is inserted between the SrRuO₃ bottom electrode and the ferroelectric BiFeO₃ layer.

Discussion

PLD is a powerful technique to fabricate complex oxide epitaxial thin films¹⁹. Using this technique, many investigations have been carried out on BiFeO₃ thin films^13,20,21,22. As one of the most striking aspects, domain walls are widely studied due to a wealth of fascinating phenomena^1,2,

Materials

Name	Company	Catalog Number	Comments
Pulsed Laser	Coherent	Complex Pro 205 F
Power Meter	Coherent	FieldMaxII
Target	Home Made	Made on request
Piezoresponse Force Microscopy	Oxford Insturments	MFP-3D Infinity Asylum Research AFM
Isopropyl Alcohol	Fisher Chemical	A415-4
Actone	Fisher Chemical	A949-4