Purely Electrical Controllable Spin–Orbit Torque‐Based Reconfigurable Physically Unclonable Functions

In the field of information hardware security, random variations obtained during device manufacturing process play a key role in generating unique and unclonable security keys. Existing physical unclonable functions (PUFs) utilizing these static randomnesses usually exhibit static challenge‐response pairs, which cannot be refreshed and limit their application prospects. Here, it is demonstrated that purely electrical controllable spin–orbit torque (SOT)‐based reconfigurable PUFs (rPUFs) can be realized in Pt/IrMn/Co/Ru/CoPt heterojunctions. By applying current pulses along two orthogonal directions, both the exchange bias between IrMn and Co and the SOT switching polarity (clockwise or counterclockwise) of perpendicularly magnetized CoPt can be reversibly controlled, which enables the realization of four rPUF states within a single device. By taking advantages of the sample growth and micro‐nanofabrication induced variations in the critical switching current density of SOT‐driven magnetization switching, the rPUF device with relatively good uniqueness and low bit error rate is achieved. Our work provides an approach to information hardware security and broadens the application of spintronics.

DOI: 10.1002/aelm.202201268 physical entity; and therefore, is vulnerable to being copied. [1] Another approach is to use physical unclonable functions (PUFs), where inherent random variations introduced during sample preparation and device manufacturing process are used to generate secret keys. These uncontrollable variations in turn randomly change the characteristics of fabricated PUF device, making it impossible to create an identical physical "copy." However, most PUFs exhibit static challenge-response pairs (CRPs) behavior once prepared, [2][3][4] which cannot be refreshed and limit the application of PUF in reusable scenarios. Hence, reconfigurable PUFs (rPUFs) are proposed to introduce an update mechanism to refresh the CRPs, [5] which can be used to protect nonvolatile storage from intrusive attacks and malicious manufacturers. Accordingly, many rPUFs have been explored through various approaches, [6,7] including optical rPUFs, [8] phase change memory rPUFs, [9] resistive random access memory rPUFs, [10,11] spin-transfer torque magnetic random access memory rPUFs, [12] and spinorbit torque (SOT) based rPUFs. [13,14] Especially, SOT-based rPUFs have attracted increasing attention because SOT-induced magnetization switching has the advantages of low power consumption, fast switching speed, and high endurance. [15,16] For example, Zhang et al. [13] demonstrated two types of rPUFs in Ta/CoFeB/MgO heterojunction based on process-induced SOT switching current variations and SOT-induced nonlinear domain wall dynamics, where an in-plane assistant magnetic field was needed to achieve deterministic SOT switching. Lee et al. [14] further demonstrated highly reliable spintronic PUFs based on field-free SOT switching in IrMn/CoFeB/Ta/ CoFeB heterojunction, where randomizing the magnetization direction of the exchange-biased bottom CoFeB layer through field annealing was crucial. The presence of an external magnetic field could make the device design more complex. Hence, from the application point of view, a fully electrical controllable SOT switching based rPUF is more suitable for potential realistic applications because it has better compatibility with today's complementary metal oxide semiconductor technology. In order to achieve magnetic field-free deterministic perpendicular magnetization switching, researchers have explored various methods, including the utilization of inplane exchange bias field, [17] interlayer exchange coupling, [18][19] In the field of information hardware security, random variations obtained during device manufacturing process play a key role in generating unique and unclonable security keys. Existing physical unclonable functions (PUFs) utilizing these static randomnesses usually exhibit static challenge-response pairs, which cannot be refreshed and limit their application prospects. Here, it is demonstrated that purely electrical controllable spin-orbit torque (SOT)based reconfigurable PUFs (rPUFs) can be realized in Pt/IrMn/Co/Ru/CoPt heterojunctions. By applying current pulses along two orthogonal directions, both the exchange bias between IrMn and Co and the SOT switching polarity (clockwise or counterclockwise) of perpendicularly magnetized CoPt can be reversibly controlled, which enables the realization of four rPUF states within a single device. By taking advantages of the sample growth and micronanofabrication induced variations in the critical switching current density of SOT-driven magnetization switching, the rPUF device with relatively good uniqueness and low bit error rate is achieved. Our work provides an approach to information hardware security and broadens the application of spintronics.

Introduction
With the rapid development of modern electronic devices, security has become one of the most pressing needs for the emerging information technology. In order to ensure safety, modern electronic systems usually use digital keys that are stored in a nonvolatile memory. However, the digital key is assigned to a www.advelectronicmat.de wedged structure, [20] titled anisotropy, [21] spin current manipulation, [22,23] multiple symmetry broken, [24] and Néel chiral symmetry breaking. [25] Despite all these significant progresses, the application prospect or suitability of a purely electrical controllable SOT switching in the field of rPUFs has been seldomly explored.
In this work, we demonstrate that fully electrical controllable rPUFs can be realized in the Pt/IrMn/Co/Ru/CoPt heterojunctions by using the changes in the critical SOT switching current density. Moreover, each heterojunction can be manipulated into four different magnetic configurations by applying orthogonal current pulses along x-and y-axes, which largely expand the design flexibility of the prepared rPUFs. Our work provides a fresh approach to the information hardware security and expands the potential applications of spintronics.

Sample Property Characterization
The multilayered heterojunction with a core structure of Pt(5)/ IrMn(8)/Co(2)/Ru(0.8)/CoPt(3.3) (numbers are thickness in nm) is designed to achieve easily integrated spin-based rPUFs. In this structure, the magnetization easy axis of the Co layer is along in-plane direction (x-axis) and that of the CoPt layer is along out-of-plane direction (z-axis) ( Figure S1, Supporting Information). Figure 1a shows the schematic of the studied 2D ferromagnetic array rPUF device, where 60 Hall bars (X n ) are divided into ten units (X = A-J) and each unit has six heterojunctions (n = 1-6). The anomalous Hall resistances of 60 Pt/IrMn/Co/Ru/CoPt heterojunctions are shown in Figure 1b, where good perpendicular magnetic anisotropy is observed for all the studied heterojunctions. Moreover, due to uncontrollable variations in the sample preparation and Hall bar fabrication process, the coercivity of 60 Hall bars varies in the range of 57 to 139 Oe. The coercive field distribution is extracted as shown in Figure S2, Supporting Information. Micromagnetic simulation suggests that the anisotropy constant of CoPt is ≈70-80 kJ m −3 , and defects can cause anomalous Hall loops to deviate from the ideal square shape as given in Figure S3, Supporting Information. According to our previous work, [26] the exchange bias between IrMn and Co can be manipulated by applying current pulses along y-axis, where the current-induced Oersted field and Joule heating play a dominant role while the spin current generated by IrMn might also contribute. [27] Measurements shown in Figure S4, Supporting Information confirm that the field-free SOT switching of CoPt is counterclockwise after the application of I y = +35 mA while it is clockwise after the application of I y = −35 mA. This is because the opposite current pulse applied along y-axis would set the magnetization of Co layer as well as the exchange bias at IrMn and Co interface into opposite direction along the x-axis, leading to the reversal of effective field felt by the CoPt layer. This fully electrically controllable SOT switching provides a fresh platform for the investigation of reconfigurable physically unclonable functions.

Electrically Controllable Random Distribution of Magnetic Moments
In the following, we show that random distribution of CoPt magnetic moments can be realized in a fully electrical way. Figure 2a shows that after the application of I y = +35 mA, counterclockwise field-free SOT switching is observed for all the studied heterojunctions. The critical switching current varies in the range from 12 to 20 mA, which is directly related to the coercivity variations observed in Figure 1b as a result of the uncontrollable variations in both sample preparation and manufacturing process. In this case, deterministic magnetization switching occurs when a large enough current pulse along x-axis is applied (e.g., I x = ±24 mA) while partial magnetization switching occurs when the applied current pulse along xaxis is not large enough (e.g., I x = ±15 mA) depending on the specific coercivity of particular Hall bar. Taking unit J 1-6 as an example, the extracted negative and positive critical switching currents after the application of I y = ±35 mA are shown in Figure S5, Supporting Information, from which the magnitude of the setting current pulse is chosen to be 15 mA. Accordingly, we choose to use the current pulse combination of I x = +24 mA/−15 mA (Figure 2b) or I x = −24 mA/+15 mA (Figure 2c) to RESET/SET these devices. In detail, after the application of I x = +24 mA (I x = −24 mA), the magnetization of CoPt layer  Figure 2b (Figure 2c). In such a way, the random distribution of CoPt magnetization can be realized. Similarly, after the application of I y = −35 mA, a clockwise SOT switching is observed as shown in Figure 2d. In that case, the application of I x = +24 mA/−15 mA (Figure 2e) and I x = −24 mA/+15 mA (Figure 2f), respectively leads to the random partial magnetization switching toward  z + and  z − direction. Hence, there are four types of combinations of the applied current pulses along both y-axis and x-axis to realize random distribution of CoPt magnetization, which enables the flexible design of rPUFs.

Reconfigurable Physically Unclonable Functions
As summarized in Figure 3, every heterojunction can be set into four different nonvolatile magnetic configurations by controlling the applied I y = ± 35 mA and I x = ± 24 mA. Moreover, these four magnetic configurations can be transferred into each other in a fully electrical way as schematically illustrated by the arrows in Figure 3. In addition, a proper "SET" current pulse can be used to achieve random distribution of CoPt magnetizations as indicated in Figure 2b. For instance, a "SET" current pulse of I x = −15 mA can be used for magnetic states shown in Figure 3a,d, while I x = +15 mA can be used for magnetic states shown in Figure 3b,c. This great tunability makes Pt/IrMn/Co/ Ru/CoPt heterojunction a possible building block for the rPUF applications, where the PUFs need to be regularly updated to resist the exhaustive CRP access attacks, or when all CRPs are exhausted and the PUFs need to be refreshed.
For practical PUFs applications, there are two critical parameters, namely inter Hamming Distance (HD inter ) and intra Hamming Distance (HD intra ), which respectively correspond to the safety and stability of the PUFs. On the one hand, the uniqueness and unclonable nature of PUFs lead to completely different responses to the same challenge from different PUF devices. In addition, HD inter is used to present the difference between the test results of different devices. The ideal HD inter value is 0.5, and the closer to this value, the stronger the randomness of the PUF device. On the other hand, the HD intra represents the difference between the results obtained from different tests of the same device due to unavoidable external factors such as environmental noise and measurement uncertainty. The ideal value of HD intra is 0, which means the high stability of the PUF device. In order to obtain the HD inter and the HD intra of our designed rPUF device, the distribution of normalized Hamming Distance is studied. By comparing the anomalous Hall resistance of each unit one by one, [28] the bitmap of a rPUF device with ten units (units A-J) is obtained in  bar (X n ) is smaller than the resistance of latter Hall bar (from X n+1 to X 6 ), the output is "0," otherwise the output is "1". Hence, a PUF unit with six Hall bars can produce 15 C 6 2 ( ) binary bits.
By analyzing the bitmap shown in Figure 4a, we obtain the distribution of the normalized Hamming Distance as revealed in Figure 4b. The HD inter shows a Gaussian distribution that is centered at 0.51402 with a standard deviation of 0.01844. In the meanwhile, we perform a second measurement on all the 60 Hall bars using the same measurement condition as the first one, where quite similar random distribution can be achieved as proven in Figures S6 and S7, Supporting Information. In addition, the deduced HD intra is 0.03662 as given in Figure 4b. These results suggest that our designed rPUF has a relatively good performance regarding both uniqueness and reliability, which should have great anti-cloning and antireplication abilities. Moreover, the same rPUF device can be easily manipulated into another PUF with distinct responses to the same challenge. For instance, Figure 4c,d shows the bitmap and normalized Hamming Distance when the device is initialized by I y = −35 mA and RESET/SET current is fixed to I x = +24 mA/−15 mA. The obtained HD inter is centered at 0.52237 ± 0.01448, while the HD intra is centered at 0.03873, indicating relatively good PUF properties after reconfiguration. Similarly, when the device is initialized by I y = ± 35 mA and the RESET/SET current combinations of I x = −24 mA/+15 mA, which are used to randomize the magnetization distribution of CoPt layers, another two types of rPUFs are obtained with HD inter = 0.47141 ± 0.02402, HD intra = 0.03873, HD inter = 0.48516 ± 0.01081, and HD intra = 0.04196 (More details can be found in Figure S8, Supporting Information). In such a way, four different types of rPUFs with relatively good uniqueness, reliability, and reconfigurability are unambiguously demonstrated based on the fully electrical controllable field-free SOT switching.

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It is worth mentioning that the studied rPUF device has several advantages. First, more rPUFs could be realized in the studied device by applying different initialization I y to selected Hall bars. In addition, this great tunability was beneficial for the flexible design of PUF devices. Second, our previous study indicates that all 16 Boolean logic functions could be realized in a single Pt/IrMn/Co/Ru/CoPt heterojunction; [26] hence, part of the Pt/IrMn/Co/Ru/CoPt heterojunctions could be used as rPUF device while others could be used to perform logic-in-memory operations. In this sense, Pt/IrMn/Co/Ru/ CoPt heterojunction was a potential building block for future integrated circuits. Third, due to the good repeatability of the field-free SOT switching, the security key could be refreshed by applying RESET current pulse after each operation to avoid invasive physical attacks. Fourth, field-free SOT-based PUFs could have the common advantages from SOT-induced magnetization switchings, such as nonvolatile, low power consumption, and fast magnetization switching. Last, the performance of the studied rPUFs could be further optimized by choosing proper SET current pulse to realize more randomization of the CoPt magnetization or by integrating nano-scale magnetic tunnel junctions through the growth of another perpendicular magnetic layer on top of MgO as a pinned layer [29] to increase the resistance difference of the output signal toward better intra Hamming Distance. In practice, when rPUFs are used as cryptographic key generators, the combination with extraction techniques is needed to obtain reliable cryptographic keys. Methods such as fuzzy extractors [30][31][32][33] have been proposed to increase the reliability. Considering the fast development of information security area, the realization of fully electrical controllable rPUF with good stability and reconfigurability provides a promising approach to hardware information security and broadens the potential applications aspects of spintronics.

Conclusion
In summary, based on purely electrical controllable exchange bias and field-free SOT switching, the Pt/IrMn/Co/Ru/CoPt heterojunction can be manipulated into four different magnetic configurations, which can be used as a building block for programmable spintronic devices. In addition, using The corresponding normalized inter-hamming distance HD inter (orange) and intra-hamming distance HD intra (pale blue). c) The corresponding results for the same applied RESET/SET current combination of I x = +24 mA/−15 mA after the pre-application of I y = −35 mA. d) The distribution of HD inter (orange) and HD intra (pale blue) extracted from (c).

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60 Pt/IrMn/Co/Ru/CoPt heterojunctions, a highly reliable and reconfigurable PUF device has been fabricated, which has been engineered into four typical rPUFs. Moreover, the realization of SOT-based fully electrical controllable rPUFs in Pt/IrMn/Co/ Ru/CoPt heterojunctions, which are also capable for nonvolatile memory and spin logic applications, provides a fresh platform to develop highly secure multifunctional integrated circuits.

Experimental Section
The multilayered structure of Ru(2)/Pt(5)/IrMn(8)/Co(2)/Ru(0.8)/ CoPt(3.3)/MgO(2) (thickness in nanometers) was deposited on thermally oxidized Si(001)/SiO 2 substrates by using magnetron sputtering. The CoPt sublayer has a nominal structure of Pt(0.7)/Co(0.3)/ Pt(0.5)/Co(0.5)/Pt(0.3)/Co(1), as previously reported. [19] The bottom Ru layer was the buffer layer and the top MgO layer was the capping layer. The magnetic properties had been characterized by using the Superconducting Quantum Interference Device (MPMS-XL 7). For the electrical transport measurements, the sample was patterned into 6 × 10 conventional Hall bar structures with a channel width of 5 µm by using optical lithography and Ar-ion beam etching. In addition, the electrical transport measurements were conducted by using Oxford physical property measurement system. For SOT switching measurement, a current pulse of 50 ms width was provided by Keithley 6221, and after 3 s, the anomalous Hall resistance was detected by Keithley 2182A with a small reading current of 0.1 mA. All the measurements were conducted at room temperature and current-induced SOT switchings were measured without magnetic field.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.