Spin Transfer Torques Achieve Low-Current Stabilisation of Magnetisation in Nearly Isotropic Magnets

Spin transfer torques, which manipulate magnetisation using electric currents, underpin a growing number of nanoscale spintronic devices. Hidekazu Kurebayashi, Joseph Barker, Takumi Yamazaki, et al. from the London Centre for Nanotechnology, University College London, the University of Leeds, and the Institute for Materials Research, Tohoku University, have demonstrated a method to enhance this effect in nearly isotropic magnetic thin films. By carefully controlling the growth and annealing of cobalt-iron-boron films, the researchers have minimised competing magnetic forces, achieving a state where magnetisation can be dynamically stabilised against an applied field. This breakthrough realises a spintronic equivalent of the Kapitza pendulum and opens avenues for exploring previously inaccessible spin dynamics, including anti-magnonics, with potential applications in probabilistic computing and novel hardware paradigms. The ability to drive large, continuous fluctuations in magnetisation direction represents a significant step towards harnessing far-from-equilibrium spin states.

The research team maximised the STT effect through a carefully designed growth-annealing protocol applied to CoFeB thin films, effectively cancelling out magnetic anisotropies from both the interface and the shape of the material. This precise control resulted in magnets exhibiting near-isotropic properties, enabling the low-current stabilisation of magnetisation opposing an applied magnetic field, a phenomenon analogous to the behaviour of a Kapitza pendulum. Experiments show that in an intermediate current range, the STT induces substantial fluctuations in the magnetisation vector, spanning the entire Bloch sphere, and establishing a continuous variable linked to the stochastic direction of magnetisation.

The study unveils a novel platform for exploring far-from-equilibrium spin dynamics, including the emerging field of anti-magnonics, with potential implications for unconventional computing paradigms. Researchers achieved this by establishing isotropic magnets as ideal systems for investigating complex, non-linear dynamics, mirroring the behaviour of a mechanically vibrating pendulum that maintains an upright position through continuous actuation. This work establishes a direct parallel between the mechanical Kapitza pendulum and the electrical control of magnetisation, where the STT acts as the driving force and the magnetic field defines the potential energy landscape. The continuous variable derived from the stochastic magnetisation direction offers a potential resource for probabilistic computing and the development of neuromorphic hardware, opening avenues for energy-efficient information processing.

This breakthrough reveals that the team’s approach allows for the observation of dynamical stability at significantly lower current densities than previously possible, overcoming limitations encountered in hard magnetic layers. By meticulously controlling the magnetic anisotropy of the CoFeB films, the researchers created a system where the equilibrium magnetisation state can be actively manipulated and sustained against magnetic relaxation. The resulting large magnetisation vector fluctuations, covering the entire Bloch sphere, represent a unique opportunity to study fundamental spin dynamics and explore new functionalities beyond traditional magnetic memory applications. This innovative approach establishes a foundation for investigating uncharted territories in spin physics and developing advanced spintronic devices.

The research establishes a clear analogy between the behaviour of a rigid pendulum with a vibrating pivot and the electrical control of magnetisation in these specially engineered materials. The team’s findings demonstrate that the STT can effectively counteract magnetic dissipation, enabling the magnetisation to be stabilised at a potential maximum, much like the bob of a Kapitza pendulum defying gravity. This dynamical stability, achieved through precise material engineering and current control, represents a significant advancement in the field of spintronics and paves the way for exploring novel concepts such as anti-magnonics and unconventional computing architectures. The ability to drive and control these complex spin dynamics holds immense promise for future technological innovations.

Isotropic CoFeB Films for Enhanced Spin Dynamics

Researchers engineered a novel approach to maximise spin transfer torque (STT) effects in CoFeB thin films, crucial for advancing nano-scale spintronic devices. The study pioneered a dedicated growth-annealing protocol to nearly eliminate magnetic anisotropies stemming from both the interface and the film’s shape, creating nearly isotropic magnets. This precise control enables low-current dynamical stabilisation of magnetisation opposing an applied magnetic field, effectively realising a spintronic analogue of the Kapitza pendulum, a system exhibiting counterintuitive energy transfer. The team’s work establishes these isotropic magnets as an ideal platform for exploring previously uncharted spin dynamics, including anti-magnonics, with potential for unconventional computing paradigms.

The fabrication process began with growing multilayer stacks of MgO(3nm)|CoFeB(2nm)|W(3nm) using ultrahigh vacuum sputtering onto thermally oxidised silicon substrates at room temperature. As-grown films initially exhibit easy-plane magnetism due to demagnetising fields, with perpendicular anisotropy arising from the CoFeB|MgO interface. Scientists carefully tuned the system via growth and post-annealing to achieve an effective magnetisation (Meff) close to zero, minimising the influence of these anisotropies. This meticulous control is essential, as the STT-induced critical current is inversely proportional to Meff, allowing for significantly reduced energy consumption.

Experiments employed the spin Hall effect to initiate spin transfer, converting charge current in the x-direction into a spin current polarised along the y-direction. The resulting spin-orbit torque (SOT) destabilises the equilibrium magnetisation at a critical current (Ic) determined by a complex equation incorporating the Gilbert damping constant, external field, and a phenomenological parameter (β) proportional to the spin Hall angle. Researchers demonstrated that current-induced self-torque within the CoFeB layer is negligible, as electrons predominantly flow through the tungsten layer, simplifying the analysis. Spatial variations in material parameters were accounted for by introducing a constant offset.

Dynamical stability of the anti-parallel magnetisation state was detected by measuring the spin-Hall magnetoresistance (SMR) in both static and dynamic regimes. The angular dependence of electrical resistance was modelled using spin-diffusion theory, allowing for accurate calculation of the SMR change. By modulating the direct current with a microwave signal and monitoring the resulting rectified voltage via spin-torque ferromagnetic resonance (ST-FMR), the team mapped out resonance fields and linewidths. A double Lorentzian fit to the ST-FMR data, detailed in the methods, confirmed a linear relationship between linewidth and current up to Ic, with a measured current density of 2x 10 11 Am -2, approximately one order of magnitude lower than observed in anisotropic magnets.

Kapitza-Like Stabilisation in CoFeB Thin Films

Scientists achieved dynamical stabilisation of magnetisation, mirroring the behaviour of a Kapitza pendulum, through meticulous control of CoFeB thin film growth and annealing. The research demonstrates that nearly isotropic magnets enable low-current stabilisation of magnetisation direction opposite to an applied magnetic field. Experiments revealed that in an intermediate current regime, spin transfer torques drive substantial magnetisation vector fluctuations, effectively mapping the entire Bloch sphere. Measurements confirm that the continuous variable describing the stochastic magnetisation direction holds potential as a resource for probabilistic computing and neuromorphic hardware development.

The team measured magnetic anisotropies in the CoFeB films, successfully minimising both interface and shape contributions to achieve near-isotropy. This precise control allows for the realisation of a spintronic analogue of the Kapitza pendulum, a system where a driven pendulum appears to defy gravity through dynamic stability. Results demonstrate that the applied current induces a steady state at the free energy maximum, a departure from typical auto-oscillatory behaviour observed under high spin transfer torque. Data shows that the system’s response is not simply switching between energy minima, but rather a sustained, dynamically stable state at a point of potential instability.

Further experiments involved solving the stochastic Landau-Lifshitz-Gilbert equation for a macrospin within the isotropic magnet. Scientists recorded that, starting from the south pole, the magnetisation precesses away from the field direction due to the anti-damping spin transfer torque exceeding the Gilbert damping torque. This precession ultimately settles around the inverted state, indicated by a change in potential energy values on the Bloch sphere. The breakthrough delivers a platform for studying far-from-equilibrium spin dynamics, including anti-magnonics, and opens avenues for unconventional computing paradigms.

Measurements establish that the system’s behaviour is fundamentally different from traditional magnetic switching used in memory applications, where bistability and energy barriers are crucial for data retention. Instead, the research focuses on exploiting the dynamic stability at a potential maximum, driven by the spin transfer torque. This work establishes isotropic magnets as a key material platform for exploring uncharted territories in spin dynamics and potentially revolutionising future computational technologies.

Isotropic Magnets Enable Novel Spin Dynamics

Researchers have demonstrated a method for enhancing spin transfer torque (STT) effects in cobalt-iron-boron (CoFeB) thin films through a specific growth and annealing process. This protocol yields nearly isotropic magnets, where internal magnetic forces are balanced, allowing for low-current stabilisation of magnetisation against an applied field, a phenomenon analogous to the Kapitza pendulum. Investigations reveal that these magnets facilitate substantial fluctuations in magnetisation direction, spanning the entire Bloch sphere, and establish a continuous variable linked to this stochastic direction. This work establishes isotropic magnets as a viable platform for exploring previously unstudied spin dynamics, including anti-magnonics, with potential for novel computational paradigms.

The authors acknowledge limitations related to the specific materials and fabrication techniques employed, and note that further research is needed to fully characterise the behaviour of these systems under varying conditions. Future work could focus on exploring the potential of these materials in probabilistic computing and unconventional magnetic devices, building on the demonstrated control of magnetisation dynamics. The team also developed a continuous restricted Boltzmann machine (cRBM) architecture utilising these isotropic magnets, which consistently outperformed a conventional restricted Boltzmann machine (RBMrate) in generating realistic and diverse image data. This improvement is attributed to the cRBM’s use of continuous visible units, enabling higher fidelity data processing compared to the binary units of the RBMrate. These findings suggest a pathway towards physically realising a cRBM using a magnetic system, a capability beyond the reach of current binary spintronic technology.