Zinc blende and wurtzite CoO polymorph nanoparticles: Rational synthesis and commensurate and incommensurate magnetic order

Abstract

On the nanoscale, CoO can have different polymorph crystal structures, zinc blende and wurtzite, apart from rock salt, which is the stable one in bulk. However, the magnetic structures of the zinc blende and wurtzite phases remain virtually unexplored. Here we discuss some of the main parameters controlling the growth of the CoO wurtzite and zinc blende polymorphs by thermal decomposition of cobalt (II) acetylacetonate. In addition we present a detailed neutron diffraction study of oxygen deficient CoO (CoO 0.70−0.75) nanoparticles with zinc blende (∼15 nm) and wurtzite (∼30 nm) crystal structures to unravel their magnetic order and its temperature evolution. The magnetic order of the zinc blende nanoparticles is antiferromagnetic and appears at the Néel temperature T N ∼ 203 K. It corresponds to the 3-rd type of magnetic ordering in a face-centered cubic lattice with magnetic moments aligned along a cube edge. The magnetic structure in the wurtzite nanoparticles turned out to be rather complex with two perpendicular components. One component is incommensurate, of the longitudinal spin wave type, with the magnetic moments confined in the ab-plane. In the perpendicular direction, this magnetic order is uncorrelated, forming quasi-two-dimensional magnetic layers. The component of the magnetic moment, aligned along the hexagonal axis, is commensurate and corresponds to the antiferromagnetic order known as the 2-nd type in a wurtzite structure. The Néel temperature of wurtzite phase is estimated to be ∼ 109 K. The temperature dependence of the magnetic reflections confirms the reduced dimensionality of the incommensurate magnetic order. Incommensurate magnetic structures in nanoparticles are an unusual phenomenon and in the case of wurtzite CoO it is probably caused by structural defects (e.g., vacancies, strains and stacking faults).

Spin Splitting in Systems Described by Magnetic Rod Groups

“…The Mulliken charge value of +1.34 on the Co atom demonstrates considerable O p/Co d hybridization in the valence band, in agreement with the results of LSDA+ U calculations . The value of the magnetic moment of 2.69μ B on the Co atom obtained in our calculations agrees reasonably with the experimental value of 2.8μ B , which was found in wurtzite CoO with the magnetic moments aligned along the c -axis …”

Exchange Bias and Coercivity in Vicinity of Magnetic Compensation Point in GdFeCo Amorphous Film

“…The Néel temperature of CoO in rock salt structure is 293 K, lower than near room temperature. While in the presence of oxygen deficiency, CoO x may be in the zinc blende or wurtzite crystal structures with Néel temperature higher than room temperature [37]. In the vicinity of the compensation point, the bias field is enhanced greatly and exhibits a multiple peak variation.…”

Revisiting the Decomposition Process of Tetrahydrate Co(II) Acetate: A Sample’s Journey through Temperature

“…Moreover, I.V. Golosovsky et al, [22] studied the main parameters determining the growth of either hexagonal or cubic polymorphs (solvents or surfactants, heating rate, reflux temperature or time . .…”

“…Nanocrystals of wurtzite-CoO were isolated for the first time from the thermolytic decomposition of cobalt acetylacetonate in a high-temperature solvent (benzyl ether) [21]. By using the same precursor, I. Golosovsky et al analyzed some of the parameters that determine the formation of wurtzite or Zn-blende polymorphs; they also performed a detailed magnetic characterization of both CoO nanoparticles [5,22].…”