Unravelling the Elusive Antiferromagnetic Order in Wurtzite and Zinc Blende CoO Polymorph Nanoparticles

• Published in: Small (Weinheim an der Bergstrasse, Germany) Vol. 14; no. 15; pp. e1703963 - n/a
• Main Authors: Roca, Alejandro G., Golosovsky, Igor V., Winkler, Elin, López‐Ortega, Alberto, Estrader, Marta, Zysler, Roberto D., Baró, María Dolors, Nogués, Josep
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 01.04.2018
• Subjects: Antiferromagnetism; antiferromagnets; CoO wurtzite; CoO zinc blende; Cubic lattice; Ferromagnetism; Halites; incommensurate structures; Magnetic properties; Magnetism; Nanoparticles; Nanotechnology; Neutron diffraction; Neutrons; uncompensated spins; Wurtzite; Zincblende; uncompensated spins; antiferromagnets; CoO zinc blende; CoO wurtzite; incommensurate structures
• ISSN: 1613-6810; 1613-6829; 1613-6829
• DOI: 10.1002/smll.201703963

Although cubic rock salt‐CoO has been extensively studied, the magnetic properties of the main nanoscale CoO polymorphs (hexagonal wurtzite and cubic zinc blende structures) are rather poorly understood. Here, a detailed magnetic and neutron diffraction study on zinc blende and wurtzite CoO nanoparticles is presented. The zinc blende‐CoO phase is antiferromagnetic with a 3rd type structure in a face‐centered cubic lattice and a Néel temperature of TN (zinc‐blende) ≈225 K. Wurtzite‐CoO also presents an antiferromagnetic order, TN (wurtzite) ≈109 K, although much more complex, with a 2nd type order along the c‐axis but an incommensurate order along the y‐axis. Importantly, the overall magnetic properties are overwhelmed by the uncompensated spins, which confer the system a ferromagnetic‐like behavior even at room temperature. CoO nanoparticles with zinc blende and wurtzite structures are found to be antiferromagnetic below TN ≈ 225 and 109 K, respectively. Although the zinc blende phase has a conventional antiferromagnetic structure, the antiferromagentic order of the wurtzite phase is more complex with two orthogonal components (one of them incommensurate). The nanoparticles also present a large number of uncompensated spins.