Abstract
Quantum fluctuations are ubiquitous in physics. Ranging from conventional examples like the harmonic oscillator to intricate theories on the origin of the universe, they alter virtually all aspects of matter -- including superconductivity, phase transitions and nanoscale processes. As a rule of thumb, the smaller the object, the larger their impact. This poses a serious challenge to modern nanotechnology, which aims total control via atom-by-atom engineered devices. In magnetic nanostructures, high stability of the magnetic signal is crucial when targeting realistic applications in information technology, e.g. miniaturized bits. Here, we demonstrate that zero-point spin-fluctuations are paramount in determining the fundamental magnetic exchange interactions that dictate the nature and stability of the magnetic state. Hinging on the fluctuation-dissipation theorem, we establish that quantum fluctuations correctly account for the large overestimation of the interactions as obtained from conventional static first-principles frameworks, filling in a crucial gap between theory and experiment [1,2]. Our analysis further reveals that zero-point spin-fluctuations tend to promote the non-collinearity and stability of chiral magnetic textures such as skyrmions -- a counter-intuitive quantum effect that inspires practical guidelines for designing disruptive nanodevices.
Highlights
Matter is constituted by a collection of ions and a surrounding cloud of electrons
By adapting the coupling constant integral formalism [1,7,34] to the modern framework of time-dependent density functional theory (TDDFT) [35,36,37], we show that the reduction of the magnetic exchange interaction (MEI) magnitude induced by local and nonlocal zero-point spin fluctuations (ZPSFs) results in very good agreement with previous experimental results [27,28]
An optimal shield against quantum spin fluctuations can be achieved by a combination of large ferromagnetic Heisenberg exchange interaction (HEI) coupling, relatively low Gilbert damping, and a strong out-of-plane magnetic anisotropy energy (MAE)
Summary
Matter is constituted by a collection of ions and a surrounding cloud of electrons. These microscopic entities obey quantum mechanical laws that, in addition to thermal fluctuations, involve intrinsic quantum fluctuations, a direct consequence of Heisenberg’s uncertainty principle. Interactions, local probing techniques generally find no stable magnetic signal in the very same systems To resolve this apparent contradiction, first-principles theory has successfully invoked ZPSF as a mechanism that destroys the magnetic bistability by locally reducing the MAE barrier; the larger the fluctuations, the larger the local reduction, clarifying many of the observed trends [25,26]. Our analysis reveals that antisymmetric spin interactions of Dzyaloshinskii-Moriya type are robust against ZPSFs, implying that quantum fluctuations favor the emergence of chiral magnetic textures. These findings highlight the importance of quantum effects in the study of nanoscale magnets and their future applications
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