Whether a molecular transport junction can exhibit useful spintronic function when every spin-dependent element is supplied by the molecule itself — the magnetic transition-metal centers of paramagnetic molecules such as binuclear ferric phthalocyanine, or stable organic radical spins — while all electrodes are nonmagnetic and no external magnetic field or ferromagnetic component is used. First-principles calculations predict perfect spin filtering and large magnetoresistance in a binuclear Fe phthalocyanine junction between gold electrodes, nonequilibrium theory predicts voltage-controlled switching of an exchange-coupled pair of molecular spins, and related work predicts optically driven pure spin currents; experiments have demonstrated zero-field Kondo and spin-flip readout of radical spins with nonmagnetic gold contacts, magnetoresistance from purely organic radical molecules (but only under fields of several tesla), and bias-voltage-controlled spin-state switching in scanning-probe settings. What remains undemonstrated is field-free, ferromagnet-free device-level function — writable magnetic configurations, reversible all-electrical switching, or write-plus-read of a molecular spin — in a transport junction, together with the exchange-control and decoherence physics that would make such function useful.

Background

A spin valve is a device whose conductance depends on the relative orientation of two magnetic elements; in a magnetic tunnel junction two ferromagnetic electrodes separated by a tunnel barrier give magnetoresistance (a conductance difference between parallel and antiparallel magnetizations). Molecular spintronics seeks to build such function from single molecules wired between electrodes, using mechanically controlled break junctions (a bendable substrate that opens and closes a nanometer-scale gap bridged by one molecule), scanning-tunneling-microscope break junctions, electromigrated nanogaps, or planar device structures such as silicon metal-oxide-semiconductor double-tunnel junctions. Ferromagnetic electrodes and externally applied magnetic fields, the workhorses of conventional spin valves, are a serious constraint at the molecular scale: they add poorly controlled molecule-electrode spinterfaces, stray fields, and incompatibility with standard silicon integration, and they are awkward for downsizing. This motivates the question of whether the spin physics can instead be carried entirely by the molecule, contacted only by nonmagnetic leads.

Three molecular ingredients are central. First, paramagnetic transition-metal complexes: in a phthalocyanine (a planar macrocyclic ligand surrounding a metal ion), the metal center carries a local magnetic moment, and in a binuclear ferric phthalocyanine (Fe2Pc2) two Fe(III) centers sit in one molecule. Density-functional-theory transport calculations (DFT combined with nonequilibrium Green's functions) by Liu et al. showed that magnetic proximity — the induction of spin polarization in the molecular framework by the Fe moments — yields perfect spin filtering and large magnetoresistance in an Au/Fe2Pc2/Au junction even though the electrodes are nonmagnetic, with the response switchable by reorienting the two Fe moments. Second, exchange-coupled molecular spin pairs: two molecular spins coupled by an exchange interaction JJ can in principle be written and read electrically; Saygun et al. predicted that tuning a junction between the ferromagnetic and antiferromagnetic exchange regimes with transport bias modulates the current by at least an order of magnitude, motivating all-electrical control that avoids both magnetic fields and ferromagnetic contacts. Third, stable organic radicals — open-shell molecules such as the perchlorotriphenylmethyl (PTM) radical, TEMPO, nitronyl nitroxides, and bianthracene-based diradicals — whose unpaired electron spins combine chemical stability with weak spin-orbit and hyperfine coupling, promising long spin coherence. Frisenda et al. demonstrated the Kondo effect (the many-body screening of a local spin by conduction electrons, visible as a zero-bias conductance resonance) in a break junction of a neutral, stable all-organic radical between nonmagnetic gold electrodes, establishing spectroscopic spin readout with nonmagnetic contacts and posing open questions about spin coherence, spin-transport efficiency, and gate control in such junctions. Related theory predicts pure spin currents (opposing equal spin-up and spin-down currents carrying angular momentum but no net charge) in binuclear phthalocyanine junctions driven by the photogalvanic effect, in which noncentrosymmetric junctions rectify optical excitation.

Adjacent results delimit the field: chiral-induced spin selectivity (CISS), where helical molecules filter spins in contact with nonmagnetic electrodes, achieves field- and ferromagnet-free spin polarization but through molecular chirality rather than molecular magnetic moments; and single-molecule electron spin resonance implemented with scanning tunneling or atomic force microscopy now allows coherent control of individual molecular spins on surfaces, but typically with applied static fields and, for spin-polarized readout, magnetic probe tips. No reported experiment demonstrates spintronic device function from molecular magnetic centers or radical spins in a transport geometry that simultaneously satisfies: nonmagnetic contacts only, zero applied field, and controllable (writable or switchable) spin-dependent response.

Problem Statement

Can useful spintronic function be experimentally demonstrated in a molecular transport junction — in any platform: single-molecule break junction, electromigrated junction, scanning-probe junction, or planar molecular device such as a semiconductor double-tunnel structure — satisfying all three constraints: (i) every electrode and contact is nonmagnetic, with no ferromagnetic component anywhere in the device; (ii) no external magnetic field is applied during operation; and (iii) the spin-dependent function derives entirely from the molecule's own magnetic moments, either the magnetic transition-metal centers of paramagnetic molecules (for example the two Fe(III) centers of binuclear ferric phthalocyanine) or stable organic radical spins? Useful function means at least one of the following, each with a quantified figure of merit and stated operating conditions (temperature, bias, gate): a spin-valve-type response, a conductance or magnetoresistance contrast between two magnetically distinct configurations of the molecular spins that are written electrically or optically within the experiment; reversible all-electrical switching of a molecular spin state or of the exchange interaction JJ between coupled molecular spins, with a quantified on/off ratio or switching fidelity; or write-plus-read of a molecular spin configuration through the junction, with readout contrast for a spin state the experiment has set or gated — passive zero-field equilibrium spectroscopy of an unperturbed spin alone does not close the question. A complete answer comprises the experimental demonstration together with mechanistic evidence attributing the function to the molecular spins (configuration or molecular-structure dependence, Kondo or spin-flip spectroscopy, comparison against diamagnetic closed-shell control molecules, isotope substitution, or quantitative theory), rather than to spurious interface magnetism or purely charge-based conductance effects. The question is open across molecular platforms and control mechanisms (bias, gate, charging, optical excitation), and it is deliberately one-sided: it asks for a demonstrated capability, so any single demonstration that satisfies the three constraints above, with a quantified figure of merit, stated operating conditions and mechanistic evidence, counts as a complete answer whatever its temperature range or which molecule it uses, and only such a demonstration resolves the question — a result that relaxes one of the three constraints or lacks the mechanistic evidence, and likewise a no-go argument or a design or feasibility analysis, is partial progress that narrows the problem without resolving it.

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