Antiferromagnetic spintronics

Abstract

Antiferromagnetic materials are magnetic inside, however, the direction of their ordered microscopic moments alternates between individual atomic sites. The resulting zero net magnetic moment makes magnetism in antiferromagnets invisible on the outside. It also implies that if information was stored in antiferromagnetic moments it would be insensitive to disturbing external magnetic fields, and the antiferromagnetic element would not affect magnetically its neighbors no matter how densely the elements were arranged in a device. The intrinsic high frequencies of antiferromagnetic dynamics represent another property that makes antiferromagnets distinct from ferromagnets. The outstanding question is how to efficiently manipulate and detect the magnetic state of an antiferromagnet. In this article we give an overview of recent works addressing this question. We also review studies looking at merits of antiferromagnetic spintronics from a more general perspective of spin-ransport, magnetization dynamics, and materials research, and give a brief outlook of future research and applications of antiferromagnetic spintronics.Interesting and useless -this was the common perception of antiferromagnets expressed quite explicitly, for example, in the 1970 Nobel lecture of Louis Néel.1 Connecting to this traditional notion we can define antiferromagnetic spintronics as a field that makes antiferromagnets useful and spintronics more interesting. Below we give an overview of this emerging field whose aim is to complement or replace ferromagnets in active components of spintronic devices.We recall some key physics roots of the field and first concepts of spintronic devices based on antiferromagnetic counterparts of the non-relativistic giantmagnetoresistance and spin-transfer-torque phenomena. 2We then focus on electrical reading and writing of information, combined with robust storage, that can be realized in antiferromagnetic memories via relativistic magnetoresistance and spin torque effects.3,4 Related to these topics is the research of spintronic devices in which antiferromagnets act as efficient generators, detectors, and transmitters of spin currents. This will lead us to studies exploring fast dynamics in antiferromagnets 5 and different types of antiferromagnetic materials. They range from insulators to superconductors. Here we comment also on the relation between crystal antiferromagents and synthetic antiferromagnets, with the latter ones playing an important role in spintronic sensor and memory devices.6 In concluding remarks we outline some of the envisaged future directions of research and potential applications of antiferromagnetic spintronics. Equilibrium properties and magnetic storage in antiferromagnetsThe understanding of equilibrium properties of ferromagnets has been guided by the notion of a global molecular field, introduced by Pierre Weiss.1 The theory starts from the Curie law for paramagnets with the inverse susceptibility proportional to temperature, χ −1 ∼ T . It further assumes that the externally ap...

Compensated Ferrimagnetic Tetragonal Heusler Thin Films for Antiferromagnetic Spintronics

“…[17] In recent years, antiferromagnetic spintronics has received much attention since ideal antiferromagnets do not produce stray fields and are much more stable to external magnetic fields compared to materials with net magnetization. [18][19][20][21][22] Moreover, a single layer of the compensated tetragonal thin film could replace the synthetic antiferromagnet, which is widely used in field sensing spintronic devices [1] and which has been rececntly shown to give rise to highly efficient current driven motion of domain walls in the racetrack memory device. [23] The achievement of EB up to room temperature in the present bilayer films is an important step towards practical applications.…”

Hidden orbital polarization in diamond, silicon, germanium, gallium arsenide and layered materials

“…25,26 (Because silicon has many point group symmetries, an electric current in silicon does not generate site-dependent magnetization; however, a strain can result in current-induced magnetization by breaking some symmetries. 20 ) Although silicon may not be the best material for antiferromagnetic information technology applications, it is one of the simplest and most well-known materials, a good candidate for supporting our hypothesis. The effect of strain is simulated within our tight-binding model using Harrison's universal scaling method.…”

“…Additionally, they operate much faster than ferromagnetic devices. 20 The concept of hidden orbital polarization established here should be considered in properly predicting the site-dependent magnetism because, as we have shown, the spin polarization of a Bloch state could be much smaller than the orbital polarization in many materials (e.g., see Figures 2 and 5e,f). Moreover, even in materials with weak SOC, the hidden orbital polarization can be used in antiferromagnetic information storage and processing because of the exchange interactions between localized, hidden orbital moments.…”

“…The current-induced magnetization is antiferromagnetic due to the nature of the hidden orbital and spin polarizations, and its direction depends on the direction of the current. (We note that a symmetry lowering, for example by strain, is required for diamond, Si, Ge and GaAs to generate a current-induced, site-dependent magnetization; 20 on the other hand, there are other classes of materials that do not require an additional symmetry lowering including CuMnAs 21 or Mn 2 Au, 22 which are also intrinsically antiferromagnetic.) Antiferromagnetic spintronic devices, in which a current generates sublattice-dependent spin-orbit torques and changes the magnetic state of a material, have several advantages over conventional spintronic devices based on ferromagnetism.…”

Magnon-photon coupling in antiferromagnets

“…Because of these superb properties, various aspects of antiferromagnetic spintronics have attracted significant interests in the last few years including domain wall motion, skyrmions, magnetoresistence, magnetic switching, spin pumping, spin current transport and so on. 19 However, only few works based on the classical electrodynamics were reported on the magnon-photon coupling 20,21 in AFM so far. In order to have a better understanding of the magnon-photon coupling in AFM, we would like to study the issue at the quantum level.…”