INSIGHTS | PERSPECTIVES
sciencemag.org SCIENCEGRAPHIC: C. BICKEL/SCIENCEitinerant magnetic excitations (magnons) in
ferromagnets can only transport in one di-
rection of angular momentum. By contrast,
in antiferromagnets, because of the presence
of opposite spins, magnetic excitations with
opposite chiralities can transport angular
momenta with opposite signs. Thus, in a way,
spin transport by magnons in ferromagnets
resembles charge transport in metals (with
just one type of charge carrier, electrons),
whereas spin transport in antiferromagnets
resembles charge transport in semiconduc-
tors, where two complementary carriers
(electrons and holes) can coexist.
The current work by Vaidya et al. dem-
onstrates experimentally both of these as-
pects (see the figure). They observed coher-
ent spin pumping for frequencies of up to
395 GHz, which is one to two orders of mag-
nitude higher than corresponding typical
measurements with ferromagnetic materi-
als. Furthermore, using differently circular-
polarized subterahertz radiation for the
excitation of the dynamics in the antiferro-
magnetic manganese difluoride, Vaidya et al.
demonstrated the preferential pumping of
different directions of angular momentum
into an adjacent conducting platinum layer.
This finding underscores that antiferromag-
nets may be much more versatile than fer-
romagnets with respect to using magnetic
excitations for information processing.
Similar results were also recently ob-
served for a different antiferromagnet,
chromium sesquioxide, by Li et al. ( 5 ). Their
measurements were performed at only a
single frequency of 240 GHz, which only al-
lowed for the coherent excitation of magnon
modes with a single chirality. However, in
their measurements, incoherently excited
magnons with opposite chirality from heat-
ing contributed to the spin pumping sig-
nals. Thus, they were able to distinguish co-
herent and incoherent contribution to the
spin-current injection.
Given that the work from Vaidya et al.
and Li et al. demonstrates unambiguously
the connection between coherent magne-
tization dynamics in antiferromagnets and
concomitant spin and charge currents, do
reciprocal effects also exist? Namely, can
direct-current electric charge currents, in
analogy to similar effects in ferromagnets,
modulate the dynamics of antiferromagnets,
so that magnetic damping can be reduced or
even result in autonomous, sustained oscil-
lations at terahertz frequencies? This capa-
bility would provide new perspectives not
only for using antiferromagnets in ultrafast
and energy-efficient information technolo-gies but also for integrating antiferromag-
nets into new terahertz applications, such
as narrow-bandwidth tunable emitters and
detectors. Thus, clear experimental demon-
stration of electrically driven coherent mag-
netization dynamics in antiferromagnets is
the next milestone in the quest for practical
applications of antiferromagnets. jREFERENCES AND NOTES
1. D. C. Ralph, M. D. Stiles, J. Magn. Magn. Mater. 320 , 1190
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- P. Vaidya et al., Science 368 , 160 (2020).
- J. Li et al., Nature 578 , 70 (2020).
- A. Hoffmann, IEEE Trans. Magn. 49, 5172 (2013).
- E. Saitoh, M. Ueda, H. Miyajima, G. Tatara, Appl. Phys.
Lett. 88 , 182509 (2006). - O. Mosendz et al., Phys. Rev. Lett. 104 , 046601 (2010).
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(2015). - A. D. Kent, D. C. Worledge, Nat. Nanotechnol. 10 , 187
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Nat. Phys. 14 , 220 (2018).
1 2. P. W a d l e y et al., Science351, 587 (2016).
10.1126/science.aba8388
Spin pumping
Terahertz radiation pumps spin currents into
adjacent metals, which get converted into charge
currents and concomitant charge voltages
through spin Hall efects.
Spin torque driven magnetization dynamics
Charge currents are converted to spin current through
spin Hall eCects. Injection into antiferromagents can
excite magnetization dynamics that can drive
terahertz emission.Terahertz radiation input Vo l t a g e Terahertz radiation emission An applied
charge current
Ve- e-IAF
MetalSpinElectron pathPALEONTOLOGYRafting on a
wide and
wild ocean
By Marc GodinotS
cientists first met with skepticism the
notion that small mammals crossed
large oceanic barriers to populate
faraway lands. However, progress in
phylogenetics during the 1980s forced
researchers to admit that the excel-
lent North American fossil record showed
no relatives of South American caviomorph
rodents or platyrrhine (New World) mon-
keys, and that their closest relatives lived
on the Afro-Arabian landmass during the
Eocene epoch ( 56 to 34 million years ago).
Therefore, to reach South America, these
animals would have had to cross the South
Atlantic Ocean—which probably was more
than 1500 to 2000 km wide during this pe-
riod. On page 194 of this issue, Seiffert et
al. ( 1 ) report on fossils, from Santa Rosa in
Amazonian Perú, that provide evidence of a
third mammalian lineage of African origin
that briefly appeared in South America in
the early Oligocene (35 to 32 million years
ago): a n ow-extinct parapithecid anthro-
poid monkey (genus: Ucayalipithecus).
Crossing an ocean on a raft is an excep-
tional feat, especially for small mammals,
some of which lack the capacity t o endure
seasonal food shortages. Such a crossing
also necessitates the accessibility of big veg-
etation-containing rafts (produced by tropical
storms), favorable oceanic currents, a drop
in sea level, and, possibly, island stopovers
on emerged parts of the mid-Atlantic ridge
(see the figure). Indeed, only two mammalian
groups crossed the Atlantic Ocean during a
time period of more than 50 million years,
from the Eocene to the present. Scientists
have wondered whether the crossings were
one and the same event for both groups.
For several decades, fossil evidence has
suggested that rodents were present in theEcole Pratique des Hautes Etudes, Museum National
d’Histoire Naturelle, Centre National de la Recherche
Scientifique, and Paris 6 University, Paris, France.
Email: [email protected]During the Oligocene,
now-extinct monkeys crossed
the treacherous ocean from
Africa to South America
136 10 APRIL 2020 • VOL 368 ISSUE 6487
Spin pumping with antiferromagnets
In an antiferromagnet (AF), terahertz irradiation excites magnetization dynamics at high frequencies because
of additional exchange energy involved with the relative canting between the two spin directions.