SCIENCE sciencemag.org
ent consequences for histone interactions.
The use of the same monoamine mol-
ecule as a neurotransmitter and a histone
modification in the same cells illustrates
that evolution proceeds by molecular tin-
kering, using available odds and ends to
make innovations ( 13 ). These findings
also raise exciting questions for future
research. It is unclear what regulates se-
rotonylation and dopaminylation and their
removal from histone H3. Because TGM2
concentrations are regulated and its ac-
tivity is calcium (Ca2+)–dependent ( 9 , 10 ),
it will be important to examine the role of
synaptic and neuronal activity. What is the
role of substrate availability? It is unclear
whether changes in intranuclear concen-
tration of dopamine or serotonin matter.
Cocaine increases extracellular concen-
trations of dopamine by blocking the do-
pamine transporter, but the subsequent
changes in intracellular dopamine concen-
tration in VTA neurons are not known. A
decrease due to reuptake inhibition could
hypothetically decrease dopaminylation
after cocaine self-administration. However,
it would be expected to also occur when
rats received cocaine passively, suggesting
the existence of more integrated regula-
tion. It will also be interesting to examine
the effects of antidepressant drugs on se-
rotonylation. Is there a role of H3K5ser in
the delayed clinical effects of these drugs
that act by preventing serotonin reuptake
or degradation? Additionally, the similari-
ties and differences of dopaminylation and
serotonylation on epigenetic regulation are
important to understand. Future work will
tell whether monoaminylation of H3Q5 is
a curious and isolated epigenetic modifi-
cation limited to a single histone residue
in monoamine-rich cells or whether it is a
more general mechanism. j
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ACKNOWLEDGMENTS
The author is supported by Inserm, Sorbonne Université,
Agence Nationale de la Recherche (ANR-16-CE16-0018
and ANR-19-CE16-0020 ); Laboratory of Excellence Bio-Psy
(Investissements d’Avenir, ANR-11-IDEX-0004-02); and
Fondation pour la Recherche Médicale.
10.1126/science.abb3533SPINTRONICSSpin pumping gathers speed
Coherent spin pumping from an antiferromagnet into a
metal occurs at ~400 gigahertz
By Axel HoffmannT
he discovery of giant magnetoresis-
tance in the mid-1980s demonstrated
that magnetic structure can change
electric resistivities substantially.
Theoretical predictions and experi-
mental demonstration^ of spin transfer
torques in the 1990s demonstrated the in-
verse effect of injecting spin-polarized elec-
tric currents into ferromagnetic metals that
could modify magnetic states ( 1 ). In general,
injection of spin currents into magnetically
ordered systems can excite magnetization
dynamics ( 2 ), and magnetization dynamics
can induce spin currents in adjacent materi-
als through spin pumping ( 3 ). Coherent spin
pumping effects have now been observed
with antiferromagnetically ordered materi-
als, as reported on page 160 of this issue by
Vaidya et al. ( 4 ) and by Li et al. ( 5 ). These
results provide new opportunities for us-
ing spintronics phenomena at ultrafast time
scales and considerably higher frequencies,
exceeding by several orders of magnitude the
limitations of ferromagnetic materials.
These findings add to the rich interplay be-
tween magnetic structures and their dynam-
ics with electronic structures. These effects
have revolutionized information technologies
because they allow transient electrical signals
to be stored as nonvolatile magnetic states.
Thus, giant magnetoresistance was har-
nessed to create the sensitive magnetic field
sensors that enabled tremendous advances
in magnetic data storage, and injected spin-
polarized electric currents into ferromag-
netic metals were harnessed to switch states
in nonvolatile magnetic memory devices.
Magnetization dynamics in magnetic
heterostructures (for example, interfaces
of magnetic materials with nonmagnetic
metals) can exhibit an enormous variety
of complex phenomena given the ubiq-
uity of nonlocal effects. Namely, the time-
dependent magnetization of a ferromagnet
can pump angular momentum into adja-
cent materials. This spin pumping can gen-
erate spin accumulations and spin currents
whose spatial and temporal distributions
are governed by spin relaxation processesand can readily exceed the dimensions of
contemporary electronic devices.
The resultant spin currents are typically
detected indirectly through either their
influence on the magnetization dynamics
(where spin currents give rise to additional
dissipation) or the conversion of spin cur-
rents into electric-charge currents. This
latter phenomenon is known in the bulk
of materials as the inverse spin Hall effect
( 6 ). Indeed, one of the earliest electric de-
tections of spin Hall effects was through
spin pumping ( 7 ). Spin pumping has been
instrumental in detecting spin currents and
quantifying spin Hall effects ( 8 ) and forms
the basis for many contemporary energy-
efficient spintronics applications ( 9 ).
For ferromagnetically ordered materi-
als, the connection between magnetization
dynamics and electric charge currents has
already been extensively studied and has
been incorporated into modern informa-
tion technologies ( 10 )—for example, in spin
transfer torque magnetic random-access
memory. For antiferromagnetic materials,
the applied impact of similar spintronics ef-
fects is just starting to emerge ( 11 ), driven
by the recent discovery of staggered (Néel)
spin-orbit torques in metallic antiferromag-
nets that enable the electric manipulation
of antiferromagnetic order ( 12 ).
In ferromagnets, individual magnetic mo-
ments (spins) are aligned parallel, resulting
in a net magnetic moment, but in antifer-
romagnets, spins point in alternating direc-
tions so that the net magnetization vanishes.
This alternating magnetic structure has a
profound impact on magnetization dynam-
ics. In ferromagnets, the dynamics is gener-
ally dominated by the crystalline anisotropies
and applied magnetic fields, which results in
fundamental excitation frequencies of the or-
der of a few gigahertz and requires substan-
tially higher energies for manipulating mag-
netization states at times <1 ns. By contrast,
magnetization dynamics in antiferromagnets
invariably involves strong exchange interac-
tions because they require the relative cant-
ing between adjacent spins, which results in
frequency dynamics closer to the terahertz
regime and may enable much faster manipu-
lation of magnetization.
Another distinction is that for the dy-
namics in ferromagnets, the magnetization
always precesses with the same chirality, soDepartment of Materials Science and Engineering,
University of Illinois at Urbana-Champaign, Urbana, IL
61801, USA. Email: [email protected]10 APRIL 2020 • VOL 368 ISSUE 6487 135