Magnon Spin Hall Magnetoresistance of a Gapped Quantum Paramagnet

Abstract

Motivated by recent experimental work, we consider spin transport between a normal metal and a gapped quantum paramagnet. We model the latter as the magnonic Mott-insulating phase of an easy-plane ferromagnetic insulator. We evaluate the spin current mediated by the interface exchange coupling between the ferromagnet and the adjacent normal metal. For the strongly interacting magnons that we consider, this spin current gives rise to a spin Hall magnetoresistance that strongly depends on the magnitude of the magnetic field, rather than its direction. This Letter may motivate electrical detection of the phases of quantum magnets and the incorporation of such materials into spintronic devices. DOI: 10.1103/PhysRevLett.120.177202 Introduction.-Spin transport through magnetic insulators and its actuation and detection via adjacent normal metals have been attracting a great deal of attention from the spintronics community. These developments yield the possibility to transport spin angular momentum without an accompanying charge current and thus without Joule heating. In addition to raising scientific interest, this opens the possibility of using magnetic insulators to transport information, with the long-term goal of replacing electronics with a more energy-efficient solution.There are several experimental manifestations of the coupling, across an interface, between the magnetic order in the insulating ferromagnet (FM) with the electron spins in the normal metal (NM). The first class of experiments involves static magnetic order. Here, a prime example is spin Hall magnetoresistance. This is the observation that the resistance of a heavy normal metal, typically Pt, depends on the relative orientation of the current and the magnetization direction of an adjacent magnetic insulator-typically, Yttrium-Iron-Garnet (YIG) [1]. The second class of experiments involves coherent dynamics of the magnetic order, e.g., in response to a microwave field. This leads, e.g., to pumping of spin current from the ferromagnet into the normal metal [2,3] or vice versa [4]. The final class of experiments involves incoherent dynamics of the magnetic insulator [5]. In most experiments with magnetic insulators this means that the magnetic dynamics is described in terms of magnonsquantized spin waves of the magnetic order parameter. The spin Seebeck effect, where a magnon spin current is induced by a thermal gradient and detected via the inverse spin Hall effect in an adjacent normal metal [6][7][8], belongs to this final class of experiments and recently has been used to probe short-ranged order in classical spin liquids [9]. Another typical experiment involving incoherent magnons is a nonlocal transport measurement that involves two

Quantum magnonics: when magnon spintronics meets quantum information science

“…It is then sufficient to diagonalize the quadratic component  (2) to derive the eigenspectrum. Note that this approach works for both collinear and noncollinear magnetic structures [81][82][83][84][85].…”

“…In addition to the states relevant for BEC, magnons may enter more exotic quantum-many body states. The Hamiltonian for the easy-plane magnetic insulator, for example, features a zero-temperature Mott-insulator-to-superfluid transition [290] for large enough | | [85]. That is, for | | ≫ , the magnons are localized on each lattice site because of the interactions and the system has a gap ∼ | |.…”

Green's function formalism for nonlocal elliptical magnon transport

“…Such phenomena are expected to arise whenever the ferromagnet under consideration has sufficiently strong anisotropy, e.g. in iron thin films [20] or exotic quantum magnets [22]. We develop a non-equilibrium Green's function (NEGF) formalism, also known as Keldysh formalism [23,24], to study the anomalous or off-diagonal correlations that are generated by the anisotropy terms, and as a proof of concept, apply it to determine whether magnon ellipticity gives rise to observable effects in local-and nonlocal transport experiments.…”

Dynamic spin-charge coupling: ac spin Hall magnetoresistance in nonmagnetic conductors

“…This effect was experimentally demonstrated in platinum by Vélez et al [10].Earlier, a similar effect was discovered and studied by Nakayama et al [11] in layered structures ferromagnetnormal metal. The magnetization in the ferromagnet can be rotated by an applied magnetic field which results in a change in the normal metal resistivity.In recent years, the ac spin Hall effect in ferromagnetnormal metal structures has also been studied both experimentally [12][13][14][15] and theoretically [16][17][18]. The precession of the magnetization in a ferromagnet leads to a time-varying injection of spin into the normal metal.…”

“…In recent years, the ac spin Hall effect in ferromagnetnormal metal structures has also been studied both experimentally [12][13][14][15] and theoretically [16][17][18]. The precession of the magnetization in a ferromagnet leads to a time-varying injection of spin into the normal metal.…”