Spin Pumping and Shot Noise in Ferrimagnets: Bridging Ferro- and Antiferromagnets

Abstract

A combination of novel technological and fundamental physics prospects has sparked a huge interest in pure spin transport in magnets, starting with ferromagnets and spreading to antiferro-and ferrimagnets. We present a theoretical study of spin transport across a ferrimagnet|non-magnetic conductor interface, when a magnetic eigenmode is driven into a coherent state. The obtained spin current expression includes intra-as well as cross-sublattice terms, both of which are essential for a quantitative understanding of spin-pumping. The dc current is found to be sensitive to the asymmetry in interfacial coupling between the two sublattice magnetizations and the mobile electrons, especially for antiferromagnets. We further find that the concomitant shot noise provides a useful tool for probing the quasiparticle spin and interfacial coupling.Introduction. The quest for energy efficient information technology has driven scientists to examine unconventional means of data transmission and processing. Pure spin current transport in magnetic insulators has emerged as one of the most promising candidates [1][2][3][4]. Heterostructures composed of an insulating magnet and a non-magnetic conductor (N) enable conversion of the magnonic spin current in the former to the electronic in the latter, thereby allowing for their integration with conventional electronics. In conjunction with the technological pull, these low dissipation systems have provided a fertile playground for fundamental physics [5][6][7].Commencing the exploration with ferromagnets (Fs), the focus in recent years has been shifting towards antiferromagnets (AFs) [8][9][10] due to their technological advantages [11]. While a qualitative understanding of some aspects of AFs, such as spin pumping [12,13], has been borrowed without much change from Fs, the leading order effects in several other phenomena, such as spin transfer torque [13] and magnetization dynamics [8], bear major qualitative differences. Thus, several phenomena, already known for Fs, are now being generalized for AFs [14].Although ferrimagnets (Fs) have been the subject of comparatively fewer works [7,15,16], their high potential is undoubted. The additional complexity of their magnetic structure comes hand in hand with broader possibilities and still newer phenomena. The spin Seebeck effect [17][18][19] in an F with magnetic compensation temperature has unveiled rich physics due to the interplay between the opposite spin excitations in the magnet [16]. Further studies have asserted an important role of the interfacial coupling between the magnet and the conductor [20]. While yttrium iron garnet is a ferrimagnet and has been the subject of several studies [1,3,4,[21][22][23], it is often treated as a ferromagnet on the grounds that only the low energy magnons are important [24].In this Letter, we evaluate the spin pumping current (I sz ) and the concomitant spin current shot noise [S(Ω)] in a F-N bilayer [ Fig. 1(a)], when one of the F eigenmodes is driven into a coherent sate. A two-sublattice mode...

Antiferromagnetic magnons as highly squeezed Fock states underlying quantum correlations

“…In this section, we discuss the electron-magnon coupling in an AFM/normal metal (N) bilayer with the goal of highlighting this tunability and amplification of electronmagnon coupling by exploiting the squeezing effect, as discussed in the main text. A thorough analysis of this system along with spin transport effects has been provided elsewhere 49 . We here focus on highlighting the amplification effect for an uncompensated AFM with respect to other related systems, providing mathematical expressions complementary to the intuitive physical picture discussed in the main text.…”

Magnon decay theory of Gilbert damping in metallic antiferromagnets

“…We moreover observe that both α mm and α nn are quadratic in J and g(µ). This result is shared by Gilbert damping owing to spin-pumping in insulating ferrimagnet|normal metal (NM) and AFM|NM bilayers with interfacial exchange coupling [78]. Metallic AFMs bear a close resemblance to these bilayer structures.…”

Magnon-Mediated Indirect Exciton Condensation through Antiferromagnetic Insulators

“…Much of this research focuses on interactions involving magnons or spin waves at magnetic interfaces in hybrid structures. Examples of this are spin pumping [13][14][15][16][17][18][19], spin transfer [15,[20][21][22], and spin Hall magnetoresistance [23][24][25][26][27][28] at normal metal interfaces, and magnon-mediated superconductivity [9,10]. Recently, an experiment has also demonstrated spin transport in an antiferromagnetic insulator over distances up to 80 µm [29].…”

“…which has been successfully applied to describe interactions at magnetic interfaces in similar systems [19,[57][58][59][60]. Here A L(R) k is the interface section between the left (right) fermion reservoir and the k-th (k = A, B) sublattice of the antiferromagnetic insulator.…”