In the NVFFs, data are first stored in the NVMs when the system is in the standby state, and they are then restored back into the regular FFs when the system is powered on [8; 9; 10; 11; 12; 13; 14; 15; 16]. In this case, run-time data can be maintained even if a power interruption happens. Nevertheless, most of the current nonvolatile FFs (NVFFs) suffer from long delays and high energy in the data backup and restore operations. Many efforts have been made and many NVFF designs have been proposed to minimize the latency needed for data transportation. [PERSON] et al. presented two bipolar ReRAM-based NVFFs [17]. In this design, both the endurance and the energy efficiency are improved. In addition, an area-efficient NVFF based on a spin-torque transfer magnetic tunnel junction (STT-MTJ) was proposed [18]. It has an improvement of nearly a factor of 4-23 in terms of area overhead compared to previous NVFF architectures. To reduce the energy, [PERSON] et al. proposed four NVFF designs based on either DFF or SRFF architecture and perpendicular MTJs (pMTJs) [19]. These designs can achieve a fast reading speed (<200 ps), low read power (<10 f), and high area efficiency. However, when the power supply is unstable or the system is frequently powered off, the above designs cannot work normally. Some NVFFs [10; 11; 15; 20] involve a power supply state detection circuit, so that data can be backed up into NVMs in time when the system is powered off. Nevertheless, these additional circuits not only increase the complexity but also degrade the performance. To achieve shorter delay and better area efficiency, [PERSON] et al. proposed a memristor-based NV-DFF [21]. In ref. [21], the functions are accomplished by reading and writing RRAM devices. However, the repeated reading and writing operations of RRAM devices bring high delay and high power consumption. On the other hand, NVFFs using magnetic tunnel junctions (MTJs) are the most popular candidates. MTJ has the advantages of nonvolatility, low power consumption and high speed. It can be programmed using different methods, such as field-induced magnetization (FIM) reversal, spin transfer torque (STT), and spin-orbit torque (SOT). FIM requires an external magnetic field to program the MTJ, which increases energy consumption and hinders its scalability. STT needs a large drive current, which brings high write power consumption, and the same read-write operation path will also bring read-write crosstalk problems. Recently, a new MTJ device with an in-plane exchange bias (EB) called voltage-controlled spin-orbit torque (VC-SOT) has received exten  
Figure 1: (**a**) Circuit structure of the conventional CMOS-based SR-FF; (**b**) state diagram of SR-FF (CP = “1”); (**c**) circuit structure of the conventional CMOS-based D-FF; (**d**) state diagram of D-FF (CP = “1”). sive attention from academia and industry [22]. With the aid of an in-plane exchange bias, the VC-SOT MTJ can be switched without an additional field. By controlling the bias voltage applied to the MTJ and the bidirectional SOT current through the anti-ferromagnetic (AFM) layer, the MTJ can be switched at a fast speed. In addition, the read-write current of the VC-SOT MTJ device is lower than that of STT-MTJ and SOT-MTJ devices. The lower current makes the VC-SOT MTJ device consume less energy during the read and write operations. In this paper, based on the VC-SOT-MTJ device, we propose two novel NVFFs, making the following main contributions:  
1. We proposed a symmetric complementary scheme, employing two back-to-back connected VC-SOT-MTJ devices to enable fast data configuration and a high reading threshold;
2. We proposed a simple working method for the NVFF to enable traditional FF functions and nonvolatility;
3. We proposed two high-speed, energy-efficient VC-FF designs with simpler circuit structures and lower area overhead.