Spin Transfer due to Quantum Magnetization Fluctuations

Abstract

We utilize a nanoscale magnetic spin-valve structure to demonstrate that current-induced magnetization fluctuations at cryogenic temperatures result predominantly from the quantum fluctuations enhanced by the spin transfer effect. The demonstrated spin transfer due to quantum magnetization fluctuations is distinguished from the previously established current-induced effects by a non-smooth piecewise-linear dependence of the fluctuation intensity on current. It can be driven not only by the directional flows of spin-polarized electrons, but also by their thermal motion and by scattering of unpolarized electrons. This effect is expected to remain non-negligible even at room temperature, and entails a ubiquitous inelastic contribution to spin-polarizing properties of magnetic interfaces.Spin transfer [1][2][3] -the transfer of angular momentum from spin-polarized electrical current to magnetic materials -has been extensively researched as an efficient mechanism for the electronic manipulation of the static and dynamic states in nanomagnetic systems, advancing our understanding of nanomagnetism and electronic transport, and enabling the development of energy-efficient magnetic nanodevices [3][4][5][6][7][8][9][10][11][12][13][14][15]. This effect can be understood based on the argument of spin angular momentum conservation for spin-polarized electrons, scattered by a ferromagnet whose magnetization M is not aligned with the direction of polarization. The component of the electron spin transverse to M becomes absorbed, exerting a torque on the magnetization termed the spin transfer torque (STT). In nanomagnetic devices such as spin valve nanopillars [ Fig. 1(a)], STT can enhance thermal fluctuations of magnetization [ Fig. 1(b)], resulting in its reversal [5,16] or auto-oscillation [6], which can be utilized in memory, microwave generation, and spin-wave logic [17,18]. The approximation for the magnetization as a thermally fluctuating classical vector M provides an excellent description for the quasi-uniform magnetization dynamics [19]. However, the short-wavelength dynamical modes of the magnetization whose frequency extends into the THz range [20] become frozen out at low temperatures, and the effects of spin transfer on them cannot be described in terms of the enhancement or suppression of thermal fluctuations. Short-wavelength modes are not readily accessible to the common electronic spectroscopy and magneto-optical techniques, and their role in spin transfer remains largely unexplored.Here, we introduce a frequency non-selective, magnetoelectronic measurement approach allowing us to demonstrate that at low temperatures the current-dependent magnetization fluctuations arise predominantly from the enhancement of quantum fluctuations by spin transfer. The observed effect is analogous to the well-studied spontaneous emission of a photon by a two-level system, also caused by quantum fluctuations, which occurs even when there are no photons to stimulate the emission. In the studied magnetic system, the role of photons...

Temperature dependence of the effective spin-mixing conductance probed with lateral non-local spin valves

“…2(c) cannot account for the differences in λ N . On the other hand, we note that quantum magnetization fluctuations 39,40 in YIG can also play a role at low T , leading to an enhanced G s .…”

Quantum spin transfer torque induced nonclassical magnetization dynamics and electron-magnetization entanglement

“…However, as discussed above, such fluctuations are forbidden in the ground state of ferromagnets [20], and if generated as random magnetic field [38] by the nonequilibrium spin shot noise [36], they would lead to rotation of magnetization away from the initial collinear direction that was excluded from the experiments of Ref. [16]. We propose to clarify the role of spin shot noise by using a sequence of Lorentzian voltage pulses to inject a train of levitons into a collinear spin valve, where leviton as a minimal collective many-body excitation above the Fermi sea carrying a single electron charge, is free of particle-hole pairs and has vanishing current noise [39,40].…”

“…However, this well-established picture cannot explain very recent experiments [16] on collinear spin valves at cryogenic temperatures 3 K, where resistance measurements have revealed magnetization dynamics even though thermal fluctuations that could introduce noncollinearity between the free and fixed magnetizations are suppressed. This implies a mechanism where standard STT is zero, T ≡ 0 in Eq.…”

“…Nevertheless, it changes its length, thereby signaling generation of highly nonclassical magnetization states [16]. However, the proposed intuitive picture [16] where such mechanism would amplify quantum spin fluctuations, for both fixed-to-free and free-to-fixed spin current directions, cannot be rigorously justified. That is, although quantum fluctuations of the local spin operators [17] (or, equivalently, zero-point energy of magnons as bosonic particles to which spin operators can be mapped) play an important role in lowering the energy of classical ground states of antiferromagnets [18] or noncollinear spin textures [19], they vanish in a FM with uniaxial anisotropy because the collinear state of local magnetic moments is also a ground eigenstate of the exact Hamiltonian [20].Aside from few disparate attempts [21-23], a general framework for describing current-driven quantum dynamics of magnetization is lacking.…”

Quantum magnetization fluctuations via spin shot noise

“…Therefore, measuring the magnetoresistance reflects the total magnetization fluctuations. Reference [11] finds an asymmetric enhancement of the magnetic fluctuations as a function of current, which is in contradiction to semiclassical STT theory.…”