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Article Dans Une Revue Physical Review Letters Année : 2016

Quantum Stoner-Wohlfarth Model

Takuya Hatomura
  • Fonction : Auteur
Bernard Barbara
  • Fonction : Auteur
  • PersonId : 954215
Seiji Miyashita

Résumé

The quantum mechanical counterpart of the famous Stoner-Wohlfarth model—an easy-axis magnet in a tilted magnetic field—is studied theoretically and through simulations as a function of the spin size S in a sweeping longitudinal field. Beyond the classical Stoner-Wohlfarth transition, the sweeping field-induced adiabatic change of states slows down as S increases, leading to a dynamical quantum phase transition. This result gives us new insights to describe the collapse of the metastability from the viewpoint of a critical phenomenon associated with the Landau-Zener tunneling gaps. Furthermore, a beating of the amplitude of the magnetization (the spin-length fidelity) is discovered after the Stoner-Wohlfarth transition. The period of the beating, confirmed analytically, arises from a new type of quantum phase factor. The Stoner-Wohlfarth (SW) model [1] is a classical model giving the field values at which a spin with uniaxial anisotropy switches under a tilted magnetic field applied in the opposite hemisphere. This is a discontinuous metastable to stable transition, in the limit of infinite spin. The quantum counterpart of this model has been studied from the viewpoint of quantum tunneling by making use of the WKB approach in zero longitudinal field [2,3]. The well-known phenomena of the stepwise magnetization curves observed in the single molecule magnet Mn 12-ac demonstrated quantum tunneling at avoided crossings of opposite magnetization levels [4,5]. Observed discreteness was a result of the finite value of the molecule spin (S ¼ 10). The phenomena have been studied from the viewpoint of quantum tunneling, including theoretical approaches based on the Landau-Zener (LZ) transition [6,7], which was also observed in experiments [8]. Some quantum aspects of the spinodal transition have been studied in the transverse Ising chain with a sweeping longitudinal field [9] in which a relatively fast sweeping case was studied. The present study gives a more direct quantum effect on the collapse of the metastable state. We study the quantum SW model numerically in the presence of a fixed transverse field and a sweeping longitudinal field in the limit of large spins (going from S ¼ 20 to 320). When S increases, we find, from a microscopic viewpoint, how the densification of the energy level structure, leading to a suppression of discreteness, affects the collapse of metastability and modifies the spin dynamics. We study more particularly the quantum critical properties appearing near the end of metastability of the classical model. Furthermore, a pre-cession beating is discovered which is confirmed analytically and interpreted in terms of a new type of quantum phase factor. In order to catch the properties of our model in the S → ∞ limit properly, we introduce the normalized quantum spin operators with a modified commutation relation: s α ¼ S α S ; ðα ¼ x; y; zÞ; ½s α ; s ⠊ ¼ i S ϵ αβγ s γ : ð1Þ The corresponding SW Hamiltonian, with uniaxial anisotropy, transverse field (fixed), and longitudinal field (sweeping at the time rate c), is written as [10]
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Dates et versions

hal-01656835 , version 1 (06-12-2017)

Identifiants

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Takuya Hatomura, Bernard Barbara, Seiji Miyashita. Quantum Stoner-Wohlfarth Model. Physical Review Letters, 2016, 116 (3), pp.037203. ⟨10.1103/PhysRevLett.116.037203⟩. ⟨hal-01656835⟩

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