706 NATURE NANOTECHNOLOGY | VOL 8 | OCTOBER 2013 | www.nature.com/naturenanotechnology

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Since the discovery of the giant magnetoresistance effect in the late 1980s1,2, spintronics, or spin-

based electronics, has led to the dramatic improvement in the hard-disk storage capacity and, more recently, is contributing to the development of next-generation computer memories. As the performance of the traditional complementary metal–oxide–semiconductor (CMOS)-based architectures is starting to stagnate, spintronics has emerged as an alternative approach for designing versatile integrated circuits, where the charge degrees of freedom are replaced with spin — a quantum-mechanical angular momentum associated with the intrinsic spinning motion of elementary particles

or collective excitations. Now, writing in Physical Review B, van Hoogdalem and Loss from the University of Basel, Switzerland propose to control the flow of spins in a transistor-like device, which is based on molecular magnets contacted by spin chains (that is, strings of localized spins)3. This work hints at a possible new direction for the development of integrated circuits in the future.

At present, spin-based technologies for memories and logic are based on ferromagnetic metals, such as iron, nickel, cobalt, or their alloys, which are made up of electrons that can conduct electricity nearly as well as copper and whose spins become ordered due to their strong quantum-mechanical exchange correlations.

Ferromagnetic spin ordering results in the existence of two distinct species of electrons: majority electrons, which are aligned with the ordering direction, and minority electrons, which are aligned opposite to the ordering direction. This motivates a loose analogy with particles and holes in semiconductors, two species of electronic excitations that underlie CMOS technology. Indeed, two neighbouring ferromagnets with opposite ordering orientations are similar to a p–n junction, as one ferromagnet’s majority electrons are the minority electrons of the other ferromagnet. Switching the relative magnetic orientation of the ferromagnets by an external magnetic field can thus change conductance of the entire structure, termed a spin valve.

SPINTRONICS

An insulator-based transistorA theoretical study proposes the use of molecular magnets in a type of transistor in which the flow of collective spin excitations transports and processes information.

Yaroslav Tserkovnyak

in an as-grown sample3, in much thinner samples4 or in a sample manufactured from an FeNi alloy with a lower Curie temperature than permalloy5.

The artificial kagome spin ice displays a much more complex behaviour than artificial square ice, which can be understood by considering not only the magnetic moments but also the charge order. The charge order can be visualized by replacing each magnetic dipole with a charge dumbbell that has a positive and negative charge at its ends (inset in Fig. 1b); the net charge at a lattice vertex should then be considered6,7. It has been theoretically predicted that several phases occur when the artificial kagome spin ice is cooled down8,9, starting from a paramagnetic phase, then passing through a phase with disordered magnetic charges (kagome ice I), followed by a phase with ordered charges (kagome ice II, Fig. 1b) and finally, at low temperature, a phase with both spin and charge order. Whereas approaches that use an alternating magnetic field have led to the formation of the kagome ice I phase10, Schiffer and colleagues have now been able to observe the charge ordering associated with the kagome ice II phase. As the charge order is two-fold degenerate, they observed crystallites of the ordered magnetic charges, whose size can be controlled by adjusting

the separation of the nanomagnets. They also compared the experimental results to Monte Carlo simulations that incorporate vertex charge interactions and found good agreement.

In the future, the method of Schiffer and colleagues could reveal details of relaxation processes and the phase transitions in thermally active frustrated arrangements11. Furthermore, the relative availability of the experimental techniques needed for these experiments (nanofabrication facilities and magnetic force microscopy) suggests that the approach could be easily followed by other research groups. The study of such artificial spin-ice systems is also promising for applications. One possibility is to use the transport of magnetic charges, referred to as emergent magnetic monopoles, to create low-power logic circuits and construct novel reconfigurable magnonic crystals12. Replacing the arrays of nanomagnets with connected wires may provide opportunities for devices based on the motion of magnetic domain walls in complex networks. It should also be feasible to fabricate artificial spin-ice systems in three dimensions, which would better mimic their crystal counterparts. Moreover, the planar configurations are not restricted to kagome and square lattices, and there are analogous systems that can be manufactured from

mesoscopic materials such as colloids or patterned superconducting thin films. Ultimately, arrays of nanomagnets in any geometry could be designed and the thermodynamics of the systems studied with methods that allow the precise magnetic configurations to be determined in both space and time. ❐

Laura J. Heyderman is at the Laboratory for Mesoscopic Systems, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland, and Laboratory for Micro- and Nanotechnology, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland. e-mail: Laura.heyderman@psi.ch

References1. Wang, R. F. et al. Nature 439, 303–306 (2006).2. Zhang, S. et al. Nature 500, 553–557 (2013).3. Morgan, J. P., Stein, A., Langridge, S. & Marrows, C. H.

Nature Phys. 7, 75–79 (2011).4. Farhan, A. et al. Phys. Rev. Lett. 111, 057205 (2013).5. Porro, J. M., Bedoya-Pinto, A., Berger, A. & Vavassori, P.

New J. Phys. 15, 055012 (2013).6. Castelnovo, C., Moessner, R. & Sondhi, S. L. Nature

451, 42–45 (2008).7. Mengotti, E. et al. Nature Phys. 7, 68–74 (2011).8. Möller, G. & Moessner, R. Phys. Rev. B 80, 140409(R) (2009).9. Chern, G. W., Mellado, P. & Tchernyshyov, O. Phys. Rev. Lett.

106, 207202 (2011).10. Qi, Y., Brintlinger, T. & Cumings, J. Phys. Rev. B

77, 094418 (2008).11. Farhan, A. et al. Nature Phys. 9, 375–382 (2013).12. Heyderman, L. J. & Stamps, R. L. J. Phys. Condens. Matter

25, 363201 (2013).

Published online: 15 September 2013

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NATURE NANOTECHNOLOGY | VOL 8 | OCTOBER 2013 | www.nature.com/naturenanotechnology 707

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Such spintronic technologies have certain advantages over their electronic counterparts, most notably non-volatility of their elementary operations: a ferromagnet that is switched remains so even in the absence of a power supply. However, even though an electron’s spin plays an important facilitating role, the core functionality still relies on the transfer of charge through the electronic circuit. Alternative ideas also exist for designing spintronic circuits based on non-magnetic semiconductors, where the spin-dependent charge transport is controlled by electric rather than magnetic fields via the relativistic spin–orbit interaction. However, the most radical departure from the traditional paradigms of semiconductor-based electronics is offered by harnessing spin transport in magnetic insulators, as pursued by van Hoogdalem and Loss.

Magnetic insulators are inert from the point of view of charge transfer. They do, however, possess low-energy excitations, which can readily transport spin through interconnects and across active elements. The authors propose to use ferromagnetic or antiferromagnetic non-conducting spin chains as the interconnects for spin transfer and triangular molecular magnets as the active elements, resulting in transistors that can switch and amplify spin signals. Elementary spin excitations in antiferromagnetic chains are known as spinons, which turn out to have a fermionic nature and are very similar to free electrons. Spin transport in these chains is thus similar to electron transport through high-quality wires. The ferromagnetic chains possess bosonic excitations called magnons, which could at low temperatures conduct spin signals essentially without any losses4. It is this potential of spintronic circuits to mitigate heating in operating circuits that fundamentally sets them apart from their electronic counterparts. Furthermore, the versatility afforded by an ability to interface diverse insulating materials, which possess collective spin excitations, in the same circuit, should bolster much enthusiasm for exploring other ideas for spintronic interconnects and transistor-like elements.

It may be challenging to test the authors’ proposal experimentally, let alone produce a commercially viable device. However, it does illustrate a proof of principle for full-Boolean-logic based on spin chains and molecular magnets derived from existing materials. For fully magnetic operation of their magnon transistor, one ferromagnetic spin chain (source) provides a flux of out-of-equilibrium magnons, once biased by a magnetic field pulse4. This flux then impinges on a molecular magnet (gate),

whose energy levels are controlled by a locally applied magnetic field5, which can in turn block or transmit spin current to the second ferromagnetic chain (drain). Because both the energy gap of the spin excitations in the spin chains and the energetics of the molecular magnets can be reliably controlled by magnetic fields, this approach may allow the efficient switching of spin transfer. Furthermore, the transmitted signal can be used to control the state of the gate in another switch, as the associated accumulation of magnons results in its own dipolar magnetic field. The authors show that three such source–gate–drain switches can be combined to form a NAND gate, a universal gate for classical computation.

The dipolar and other relativistic interactions discussed by the authors as means to control and measure magnonic spin currents in their molecular magnet/spin chain circuits seem innately weak and ultimately limit practical functionality. At the same time, the existence of the magnons, as well as the associated spin fluxes, rely only on the exchange interactions, which are rooted in quantum mechanics rather than relativity. A natural question that then arises is whether it is possible to design spintronic circuits based

exclusively on the exchange interactions, which are innately stronger than dipolar or spin–orbit interactions invoked by the authors. In particular, the logical state at the gate of one transistor can be determined by the output of another transistor by exchange coupling the output drain directly to the input gate, as shown in Fig. 1. The circuit operation could, for example, be actuated and controlled thermally6, thus opening avenues for recycling waste heat from other electronic components. Although these possibilities may require more intricate theoretical analysis, future experiments in this vein seem very likely as part of a search for novel spintronic devices as well as for fundamental research. ❐

Yaroslav Tserkovnyak is in the Department of Physics and Astronomy, University of California Los Angeles, California 90095-1547, USA. e-mail: yaroslav@physics.ucla.edu

References1. Baibich, M. N. et al. Phys. Rev. Lett. 61, 2472–2475 (1988).2. Binasch, G., Grünberg, P., Sauerenbach, F. & Zinn, W. Phys. Rev. B

39, 4828–4830 (1989).3. Van Hoogdalem, K. A. & Loss, D. Phys. Rev. B 88, 024420 (2013).4. Meier, F. & Loss, D. Phys. Rev. Lett. 90, 167204 (2003).5. Trif, M., Stepanenko, D. & Loss, D. Phys. Rev. Lett.

101, 217201 (2008).6. Uchida, K. et al. Nature Mater. 9, 894–897 (2010).

Gate

Source

Hot

Drain

Cold

Gate

Source Hot

Drain Cold

1

2

Figure 1 | A simple logic circuit consisting of two transistors, 1 and 2, whose operation relies entirely on the exchange interactions. The source spin-chain may be hotter than the drain, which generates a gate-controlled spin current in each transistor. The spin flux from transistor 1 is applied to the gate of transistor 2, actuating the latter’s spin conductance. Such transistors can serve as switches and amplifiers, and can be used as building blocks for arbitrary Boolean logic gates.

© 2013 Macmillan Publishers Limited. All rights reserved