Element-Specific Detection of Sub-Nanosecond Spin-Transfer Torque in a Nanomagnet Ensemble

• Published in: Nano letters Vol. 20; no. 11; pp. 7828 - 7834
• Main Authors: Emori, Satoru, Klewe, Christoph, Schmalhorst, Jan-Michael, Krieft, Jan, Shafer, Padraic, Lim, Youngmin, Smith, David A, Sapkota, Arjun, Srivastava, Abhishek, Mewes, Claudia, Jiang, Zijian, Khodadadi, Behrouz, Elmkharram, Hesham, Heremans, Jean J, Arenholz, Elke, Reiss, Günter, Mewes, Tim
• Format: Journal Article
• Language: English
• Published: American Chemical Society 11.11.2020
• Subjects: spin pumping; X-ray magnetic circular dichroism; spin-transfer torque; magnetic nanoparticles; ferromagnetic resonance
• ISSN: 1530-6984; 1530-6992
• DOI: 10.1021/acs.nanolett.0c01868

Spin currents can exert spin-transfer torques on magnetic systems even in the limit of vanishingly small net magnetization, as recently shown for antiferromagnets. Here, we experimentally show that a spin-transfer torque is operative in a macroscopic ensemble of weakly interacting, randomly magnetized Co nanomagnets. We employ element- and time-resolved X-ray ferromagnetic resonance (XFMR) spectroscopy to directly detect subnanosecond dynamics of the Co nanomagnets, excited into precession with cone angle ≳0.003° by an oscillating spin current. XFMR measurements reveal that as the net moment of the ensemble decreases, the strength of the spin-transfer torque increases relative to those of magnetic field torques. Our findings point to spin-transfer torque as an effective way to manipulate the state of nanomagnet ensembles at subnanosecond time scales.