Sub-Nanosecond Spin-Transfer Torque in an Ensemble of Superparamagnetic-Like Nanomagnets

• Published in: arXiv.org
• Main Authors: Emori, Satoru, Klewe, Christoph, Jan-Michael Schmalhorst, 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, Gunter, Mewes, Tim
• Format: Paper Journal Article
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 06.05.2020
• Subjects: Antiferromagnetism; Ferromagnetic resonance; Ferromagnetism; Physics - Mesoscale and Nanoscale Physics; Spintronics; Torque
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2005.03081

Spin currents can exert spin-transfer torques on magnetic systems even in the limit of vanishingly small net magnetization, as is the case for antiferromagnets. Here, we experimentally show that a spin-transfer torque is operative in a material with weak, short-range magnetic order -- namely, a macroscopic ensemble of superparamagnetic-like Co nanomagnets. We employ element- and time-resolved X-ray ferromagnetic resonance (XFMR) spectroscopy to directly detect sub-ns dynamics of the Co nanomagnets, excited into precession with cone angle \(\geq\)0.003\(^{\circ}\) 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 sub-ns timescales.