1 Unidirectional Loop Metamaterials (ULM) as Magnetless Artificial Ferrimagnetic Materials: Principles and Applications Toshiro Kodera, Senior Member, IEEE, and Christophe Caloz, Fellow, IEEE Abstract—This paper presents an overview of Unidirectional II. OPERATION PRINCIPLE Loop Metamaterial (ULM) structures and applications. Mimick- A Unidirectional Loop Metamaterial (ULM) may be seen ing electron spin precession in ferrites using loops with unidi- 1 rectional loads (typically transistors), the ULM exhibits all the as a physicomimetic artificial implementation of a ferrite in fundamental properties of ferrite materials, and represents the the microwave regime. Its operation principle is thus based only existing magnetless ferrimagnetic medium. We present here on microscopic unidirectionality, from which the macroscopic an extended explanation of ULM physics and unified description description is inferred upon averaging. of its component and system applications. Index Terms—Unidirectional Loop Metamaterials (ULM), nonreciprocity, ferrimagnetic materials and ferrites, gyrotropy, A. Microscopic Description Faraday rotation, metamaterials and metasurfaces, transistors, isolators, circulators, leaky-wave antennas. Microwave magnetism in a ferrite is based on the precession of the magnetic dipole moments arising from unpaired electron I. INTRODUCTION spins about the axis of an externally applied static magnetic Over the past decades, nonreciprocal components (isola- bias field, B0, as illustrated in Fig. 1(a), where B0k^z. This is tors, circulators, nonreciprocal phase shifters, etc.) have been a quantum-mechanical phenomenon, that is described by the have been almost exclusively implemented in ferrite tech- Landau-Lifshitz-Gilbert equation [3], [10], [33] nology [1]–[8]. This has been the case in both microwaves dm α dm = −γm × B + m × ; (1) and optics, despite distinct underlying physics, namely the dt 0 M dt purely magnetic effect (electron spin precession) in the former s case [3], [9], [10] and the magneto-optic effect (electron where m denotes the magnetic dipole moment, γ the gyromag- cyclotron orbiting) [11]–[13] in the latter case. However, netic ratio, Ms the saturation magnetization, and α the Gilbert ferrite components suffer from the well-known issues high- damping term. Equation (1) states that the time-variation rate 2 RF cost, high-weight and incompatibility with integrated circuit of m due to a transverse RF magnetic field signal, Ht ^ ^ technology, and magnetless nonreciprocity has therefore long (kt; t ? ^z), is equal to the sum of the torque exerted by B0 been consider a holy grail in this area [14], [15]. on m (directed along +φ^, φ^: azimuth angle), and a damping ^ ^ There have been several attempts to develop magnetless term (directed along −θ, θ: elevation angle) that reduces nonreciprocal components, specifically 1) active circuits [16]– the precession angle, , to zero along a circular-spherical [21], and space-time [15] 2) modulated structures [22]–[27] trajectory (conserved jmj) when the RF signal is suppressed and 3) switched structures [28] (both based on 1950ies para- (relaxation). metric (e.g. [29], [30]) or commutated (e.g. [31]) microwave Classically, magnetic dipole moments can be associated systems). All have their specific features, as indicated in Tab. I. with current loop sources, according to Ampere` law. Decom- posing a ferrite magnetic moment, m, into its longitudinal TABLE I COMPARISON (TYPICAL AND RELATIVE TERMS) BETWEEN DIFFERENT component, mz, and transverse component, mρ, as shown in m MAGNETLESS NONRECIPROCITY TECHNOLOGIES PLUS FERRITE. z Fig. 1(a), one may thus invoke the effective current loops Ieff material P bias cost noise mρ consum. and ieff as source models for the corresponding moments. ferrite yes zero magnet high N/A mρ Among these currents, only ieff matters in terms of mag- act. circ. no low DC low low mz RF arXiv:1804.08719v1 [physics.app-ph] 16 Apr 2018 switched no med. RF high high netism, since Ieff , as the source associated with Hz , does mρ modulated no med. RF med. med. not induce any precession (Footnote 2). ieff is thus the current ULM YES low DC low med one has to mimic to devise an “artificial ferrite.” This current, We introduced in 2011 [32] in a Unidirectional Loop as seen Fig. 1, has the form of a loop tangentially rotating on Metamaterial (ULM) mimicking ferrites at microwaves and an imaginary cylinder of axis z. representing the only artificial ferrite material, or metamaterial, 1 existing to date. This paper presents an overview of the ULM The adjective “physicomimetic” is meant here, from etymology, as “mim- icking physics.” and its applications reported to date. 2The longitudinal (z) component does not contribute to precession, and hence to magnetism. Indeed, since B0k^z, the z-component of m produced by RF RF RF RF RF T. Kodera is with the Department of Electrical Engineering, Meisei Univer- Hz would lead to mz ×(B0 +µ0H ^z) = [mz (B0 +µ0H )](^z×^z) = RF ^ sity, Tokyo Japan (e-mail: [email protected]). C. Caloz is with the 0, the only torque being produced by the transverse component (Ht , t 2 xy- ´ RF RF^ RF ^ Department of Electrical and Engineering, Ecole Polytechnique de Montreal,´ plane), mt × (B0 + µ0H t) = (mt B0)(t × ^z) 6= 0. In the rest of the Montreal,´ QC, H2T 1J3 Canada. text, we shall drop the superscript “RF,” without risk of ambiguity since mt, is exclusively produced by the RF signal. 2 B. Macroscopic Description Since it mimics the relevant magnetic operation of a ferrite at the microscopic level, the unit-cell particle in Fig. 1(b) must lead to the same response as bulk ferrite at the macroscopic level when repeated according to a subwavelength 3D lattice structure so as to form a metamaterial as shown in Fig. 2(a). The ULM in Fig. 2(a), just as a ferrite3, forms a 3D array of magnetic dipole moments, mi, whose average over a subwavelength volume V , ! Fig. 1. “Physicomimetic” construction of the Unidirectional Loop Metama- 1 X 1 X M = m = m ρ^ = M ρ^ (2) terial (ULM) “meta-molecule” or particle. (a) Magnetic dipole precession, V i V ρ,i ρ arising from electron spinning in a ferrite material about the axis (here z) of an i=1 i=1 externally applied static magnetic bias field, B0, with effective unidirectional mz mρ corresponds to the density of magnetic dipole moments, or current loops Ieff and ieff , and transverse radial rotating magnetic dipole mρ moment mρ associated with ieff . (b) ULM particle [32], typically (but not magnetization, as the fundamental macroscopic quantity de- exclusively [34]) consisting of a pair of broadside-coupled transistor-loaded scribing the metamaterial4. rings supporting antisymmetric current and unidirectional current wave (shown here with exaggeratedly small wavelength for the sake of visibility), with resulting radial rotating magnetic dipole moment emulating that in (a). mρ Given its complexity, the current ieff may a priori seem impossible to emulate. However, what fundamentally matters for magnetism is not this current itself, but the moment mρ, from which magnetization will arise at the macroscopic level (Sec. II-B). This moment may be fortunately also produced by a pair of antisymmetric φ-oriented currents, rotating on the (a) (b) same cylinder, which can be produced by a pair of conducting Fig. 2. ULM structures obtained by periodically repeating the unit cell with rings operating in their odd mode [35], as shown in Fig. 1(b). the particle in Fig. 1. (a) Metamaterial (3D), described by the Polder volume permeability (3a). (b) Metasurface (2D metamaterial), described by a surface If this ring-pair structure is loaded by a transistor [32], as permeability [36]. depicted in the figure, or includes another unidirectionality mechanism such as the injection of an azimuthal modula- From this point, one may follow the same procedure as in tion [34], mρ will unidirectionally rotate about z when excited ferrites [8], [37] to obtain the Polder ULM permeability tensor by an RF signal, and hence mimic the magnetic behavior of the 2 3 electron in Fig. 1(b). The structure in Fig. 1 constitutes thus µ jκ 0 the unit-cell particle of the ULM at the microscopic level. µ = 4−jκ µ 0 5 ; (3a) 0 0 µ0; One may argue there there is a fundamental difference between the physical system in Fig. 1(a) and its presumed !0!m !!m with µ = µ0 1 + 2 2 and κ = µ0 2 2 ; (3b) artificial emulation in Fig. 1(b): the ferrite material also !0 − ! !0 − ! supports the longitudinal moment mz whereas the ULM does where !0 and !m are the ULM resonance frequency (or not include anything alike. However, as we have just seen Larmor frequency) and effective saturation magnetization fre- above, particularly in Footnote 2, mz does not contribute to quency, respectively5, that will be derived in the next section. the magnetic response. It is therefore inessential and does thus As in ferrites, the effect of loss can be accounted for by not need to be emulated. So, the particle in Fig. 1(b), with its the substitution !0 !0 + jα!, where α a damping factor moment mρ is all that is needed for artificial magnetism! in (1) [8]. Does this mean that the ULM particle includes no counter- So, a ULM may really be seen as a an artificial ferrite part to the static alignment of the dipoles due to B0 (and material producing magnet-less artificial magnetism. However, producing mz) in the ferrite medium? In fact, there is a its nonreciprocity is achieved from breaking time-reversal counterpart, although mz = 0 in the ULM. The fundamental (TR) symmetry by a TR-odd current bias, originating in the outcome of the static alignment of the dipoles along z in transistor (DC) biasing, instead of a TR-odd external magnetic the ferrite is the alignment of the relevant magnetic dipoles field [14].