Artificial ferroic systems: novel functionality from structure, interactions and dynamics

Abstract

Lithographic processing and film growth technologies are continuing to advance, so that it is now possible to create patterned ferroic materials consisting of arrays of sub-1 μm elements with high definition. Some of the most fascinating behaviour of these arrays can be realised by exploiting interactions between the individual elements to create new functionality. The properties of these artificial ferroic systems differ strikingly from those of their constituent components, with novel emergent behaviour arising from the collective dynamics of the interacting elements, which are arranged in specific designs and can be activated by applying magnetic or electric fields. We first focus on artificial spin systems consisting of arrays of dipolar-coupled nanomagnets and, in particular, review the field of artificial spin ice, which demonstrates a wide range of fascinating phenomena arising from the frustration inherent in particular arrangements of nanomagnets, including emergent magnetic monopoles, domains of ordered macrospins, and novel avalanche behaviour. We outline how demagnetisation protocols have been employed as an effective thermal anneal in an attempt to reach the ground state, comment on phenomena that arise in thermally activated systems and discuss strategies for selectively generating specific configurations using applied magnetic fields. We then move on from slow field and temperature driven dynamics to high frequency phenomena, discussing spinwave excitations in the context of magnonic crystals constructed from arrays of patterned magnetic elements. At high frequencies, these arrays are studied in terms of potential applications including magnetic logic, linear and non-linear microwave optics, and fast, efficient switching, and we consider the possibility to create tunable magnonic crystals with artificial spin ice. Finally, we discuss how functional ferroic composites can be incorporated to realise magnetoelectric effects. Specifically, we discuss artificial multiferroics (or multiferroic composites), which hold promise for new applications that involve electric field control of magnetism, or electric and magnetic field responsive devices for high frequency integrated circuit design in microwave and terahertz signal processing. We close with comments on how enhanced functionality can be realised through engineering of nanostructures with interacting ferroic components, creating opportunities for novel spin electronic devices that, for example, make use of the transport of magnetic charges, thermally activated elements, and reprogrammable nanomagnet systems.

Terahertz emission spectroscopy of laser-induced spin dynamics inTmFeO3andErFeO3orthoferrites

“…Conversely, laser-induced spin dynamics in magnetic materials is also accompanied by emission of the THz radiation [29][30][31][32][33][34][35][36]. Here we show that THz emission spectroscopy provides novel information about ultrafast laser-induced spin dynamics in TmFeO 3 and ErFeO 3 orthoferrites which is not accessible by more common all-optical methods [10][11][12]. The power of the method is evidenced by the fact that, in addition to the expected quasi-ferromagnetic and quasi-antiferromagnetic modes of the iron sublattices, the THz emission spectroscopy enables detection of an optically excited resonance at an unexpected frequency of ß0.3-0.35 THz.…”

“…Here we employ the THz emission spectroscopy for studying the mechanisms of ultrafast optical control of magnetism in TmFeO 3 and ErFeO 3 orthoferrites. The THz spectrometer employed in our experiments was powered by an amplified Ti:sapphire laser emitting a sequence of optical pulses (800 nm wavelength, ß100 fs duration) with 1 kHz repetition frequency and is described elsewhere [27].…”

“…The THz spectrometer employed in our experiments was powered by an amplified Ti:sapphire laser emitting a sequence of optical pulses (800 nm wavelength, ß100 fs duration) with 1 kHz repetition frequency and is described elsewhere [27]. Following excitation by femtosecond optical pulses with a peak fluence of ∼10 mJ/cm 2 and focused to spot of ß1 mm radius, TmFeO 3 and ErFeO 3 demonstrate ultrafast spin-reorientation dynamics, which we resolve by monitoring THz emission signals. We show that the THz emission spectroscopy serves as an alternative and complementary tool with respect to the more conventional all-optical pump-probe spectroscopy applied to study these materials previously [10][11][12].…”

“…The abundance of magnetic phases and optomagnetic phenomena in magnetic insulators offers a wide perspective for the development of high-speed magnetic recording technologies and the custom design of spintronic devices operating at terahertz (THz) frequencies [2,3]. Moreover, the spin resonances in insulators have inherently smaller damping compared to the magnetic metals, which makes them attractive for the technologies relying on propagating spin waves [4].…”

Emergent reduced dimensionality by vertex frustration in artificial spin ice

“…Arrays of interacting, single-domain nanomagnets known as artificial spin ice have proved to be useful model systems with which to experimentally explore the microscopic nature of geometrical frustration [6][7][8] . Simple square and kagome artificial spin ice lattices have been subject to intense study because they provide unique insight into the consequences of frustrated magnetism, such as residual entropy 9,10 and magnetic charge excitations [11][12][13][14] .…”