Highly Reliable Magnetic Memory-Based Physical Unclonable Functions.

Magnetic random-access memory (MRAM), which stores information through control of the magnetization direction, offers promising features as a viable nonvolatile memory alternative, including high endurance and successful large-scale commercialization. Recently, MRAM applications have extended beyond traditional memories, finding utility in emerging computing architectures such as in-memory computing and probabilistic bits. In this work, we report highly reliable MRAM-based security devices, known as physical unclonable functions (PUFs), achieved by exploiting nanoscale perpendicular magnetic tunnel junctions (MTJs). By intentionally randomizing the magnetization direction of the antiferromagnetically coupled reference layer of the MTJs, we successfully create an MRAM-PUF. The proposed PUF shows ideal uniformity and uniqueness and, in particular, maintains performance over a wide temperature range from -40 to +150 °C. Moreover, rigorous testing with more than 1584 challenge-response pairs of 64 bits each confirms resilience against machine learning attacks. These results, combined with the merits of commercialized MRAM technology, would facilitate the implementation of MRAM-PUFs.

Field-free spin-orbit torque switching in ferromagnetic trilayers at sub-ns timescales.

Current-induced spin torques enable the electrical control of the magnetization with low energy consumption. Conventional magnetic random access memory (MRAM) devices rely on spin-transfer torque (STT), this however limits MRAM applications because of the nanoseconds incubation delay and associated endurance issues. A potential alternative to STT is spin-orbit torque (SOT). However, for practical, high-speed SOT devices, it must satisfy three conditions simultaneously, i.e., field-free switching at short current pulses, short incubation delay, and low switching current. Here, we demonstrate field-free SOT switching at sub-ns timescales in a CoFeB/Ti/CoFeB ferromagnetic trilayer, which satisfies all three conditions. In this trilayer, the bottom magnetic layer or its interface generates spin currents with polarizations in both in-plane and out-of-plane components. The in-plane component reduces the incubation time, while the out-of-plane component realizes field-free switching at a low current. Our results offer a field-free SOT solution for energy-efficient scalable MRAM applications.

Spintronic Physical Unclonable Functions Based on Field-Free Spin-Orbit-Torque Switching.

Physical unclonable function (PUFs) utilize inherent random physical variations of solid-state devices and are a core ingredient of hardware security primitives. PUFs promise more robust information security than that provided by the conventional software-based approaches. While silicon- and memristor-based PUFs are advancing, their reliability and scalability require further improvements. These are currently limited by output fluctuations and associated additional peripherals. Here, highly reliable spintronic PUFs that exploit field-free spin-orbit-torque switching in IrMn/CoFeB/Ta/CoFeB structures are demonstrated. It is shown that the stochastic switching polarity of the perpendicular magnetization of the top CoFeB can be achieved by manipulating the exchange bias directions of the bottom IrMn/CoFeB. This serves as an entropy source for the spintronic PUF, which is characterized by high entropy, uniqueness, reconfigurability, and digital output. Furthermore, the device ensures a zero bit-error-rate under repetitive operations and robustness against external magnetic fields, and offers scalable and energy-efficient device implementations.

Current-induced manipulation of exchange bias in IrMn/NiFe bilayer structures.

The electrical control of antiferromagnetic moments is a key technological goal of antiferromagnet-based spintronics, which promises favourable device characteristics such as ultrafast operation and high-density integration as compared to conventional ferromagnet-based devices. To date, the manipulation of antiferromagnetic moments by electric current has been demonstrated in epitaxial antiferromagnets with broken inversion symmetry or antiferromagnets interfaced with a heavy metal, in which spin-orbit torque (SOT) drives the antiferromagnetic domain wall. Here, we report current-induced manipulation of the exchange bias in IrMn/NiFe bilayers without a heavy metal. We show that the direction of the exchange bias is gradually modulated up to ±22 degrees by an in-plane current, which is independent of the NiFe thickness. This suggests that spin currents arising in the IrMn layer exert SOTs on uncompensated antiferromagnetic moments at the interface which then rotate the antiferromagnetic moments. Furthermore, the memristive features are preserved in sub-micron devices, facilitating nanoscale multi-level antiferromagnetic spintronic devices.