Spintronic magnetic anisotropy

Abstract

An attractive feature of magnetic adatoms and molecules for nanoscale applications is their superparamagnetism, the preferred alignment of their spin along an easy axis preventing undesired spin reversal. The underlying magnetic anisotropy barrier -a quadrupolar energy splitting -is internally generated by spin-orbit interaction and can nowadays be probed by electronic transport. Here we predict that in a much broader class of quantum-dot systems with spin larger than one-half, superparamagnetism may arise without spin-orbit interaction: by attaching ferromagnets a spintronic exchange field of quadrupolar nature is generated locally. It can be observed in conductance measurements and surprisingly leads to enhanced spin filtering even in a state with zero average spin. Analogously to the spintronic dipolar exchange field, responsible for a local spin torque, the effect is susceptible to electric control and increases with tunnel coupling as well as with spin polarization.The growing interest in nanomagnets, e.g., magnetic adatoms 1 and single-molecule magnets 2 is fueled by prospects of their application in novel spintronic devices whose functionality derives from their unique magnetic features 3 . A key property of such systems is their strong magnetic anisotropy leading to magnetic bistability, required for building blocks for nanoscale memory cells 4,5 and non-trivial quantum dynamics, useful for quantum information processing 6,7 . In either case, operational stability of such devices hinges heavily on the height of the energy barrier opposing the spin reversal. Though recently progress in the control over the magnetic anisotropy by synthesis 8 , mechanical straining 9 , atomic manipulation 10 or electrical gating 11 has been made, achieving of a high spin-reversal barrier still remains a challenge. Incorporating a nanomagnet into an electronic circuit may significantly alter its magnetic properties [12][13][14] , but may also be advantageous. One possible, spintronic route for manipulation of nanomagnets entails ferromagnetic electrodes and uses the spin torque due to spin-polarized scattering 15 or Coulomb interaction 16 , magnetic analogs of the proximity effect in superconducting junctions. In this article, we present another route that combines spintronics with molecular magnetism: high-spin quantum dots can acquire a significant magnetic anisotropy that is purely of spintronic origin, instead of deriving from the spin-orbit interaction, as the tunneling to ferromagnets induces a local, quadrupolar exchange field. Besides providing an alternative approach to electrical manipulation and engineering of superparamagnetic nanomagnets, this new quantity is of key importance for the analysis of experiments that probe atoms or molecules using highly spin-polarized electrodes. DIPOLAR VS. QUADRUPOLAR EXCHANGE FIELDThe origin of superparamagnetism, usually dominating the magnetic behaviour of a nanomagnet, is a magnetic anisotropy energy barrier. For instance, an adatom with a spin-degenerate ground multiplet (q...

QmeQ 1.0: An open-source Python package for calculations of transport through quantum dot devices

“…(C.5)] are not important, because this approach preserves positivity. At the same time we note that the principal parts are required to catch important physics such as renormalization of energy levels in some systems [66,67,68,69]. By simulating different systems we also observed that with neglected principal parts the 1vN and the Redfield approaches give the same results.…”

Voltage-Induced Switching Dynamics of a Coupled Spin Pair in a Molecular Junction

“…28 here becomes vanishingly small. Therefore the effective electron mediated spin-spin interactions dominates the properties and control of the molecular dimer.…”

“…Arrays of magnetic molecules inserted between conducting leads, moreover, provide an important forum to investigate fundamental magnetic properties of finite one-dimensional Ising or Heisenberg chains [14][15][16] as well as potential for electrical and thermal control of the magnetic state. Certain classes of molecules, e.g., metal-phthalocyanines (MPc) and metal-porhyrins (MP) present chemical stability with specific optical and electrical properties make them highly appreciated for technological applications including organic field effect transistors [17,18], light emitting devices [19,20] and photovoltaic cells [21], and for fundamental studies [7,[22][23][24][25][26][27].While incorporation of magnetic elements in molecular compounds can have a significant effect on the overall molecular transport properties [7], the main established route to spintronics manipulations entails external magnetic fields [2] or ferromagnetic electrodes [28,29], often exploiting spin transfer torques from spin-polarized scattering [30] or Coulomb interaction [31]. Here, we propose a different route to molecular spintronics based on voltage induced control of magnetic interactions that allows for all electrical control of the transport properties.…”

Ferromagnetism and Half‐Metallicity in a High‐Band‐Gap Hexagonal Boron Nitride System

“…[1,2] The synthesis of graphene and its different analogues,t he main-groupbased planar 2D materials, has therefore attracted significant attentiono wing to their unique and tunable electronic, optoelectronic, and spin-electronic properties. [3][4][5][6][7][8][9][10][11][12][13] Furthermore, main-group-based systemss how long spin relaxationt imes owing to weak spin-orbit coupling.…”