Anomalous magnetic properties of 7 nm single-crystal Co3O4 nanowires

Abstract

We present a study of magnetic properties of single-crystal Co 3 O 4 nanowires with diameter about 7 nm. The nanowires expose (111) planes composed of plenty of Co 3+ cations and exhibit two Néel temperatures at 56 K (T N of wire cores) and 73 K (T N of wire shells), which are far above T N = 40 K of bulk Co 3 O 4 . This novel bahavior is attributed to symmetry breaking of surface Co 3+ cations and magnetic proximity effect. The nanowire shells show macroscopic residual magnetic moments. Cooling in a magnetic field, a fraction of the residual moments are tightly pinned to the antiferromagnetic lattice, which results in an obvious horizontal and vertical shift of hysteresis loop. Our experiment demonstrates that the exchange bias field H E and the pinned magnetic moments M pin follow a simple expression H E = aM pin with a a constant.The studies of the exchange bias was starting with the pioneering work of Meiklejohn and Bean in 1956 when studying Co particles embedded in their native antiferromagnetic oxide [1,2]. Since that time, there has been a continuously growing interest in both the fundamental physics and applications of this effect [3][4][5]. Several decades later, new phenomena, such as positive exchange bias and exchange bias in nanoscale antiferromagnetic (AFM) system, are still being uncovered in this subject [6][7][8][9][10][11][12][13][14][15][16][17].In this letter, we present a study of magnetic properties of single-crystal AFM Co 3 O 4 nanowires with diameter about 7 nm. The nanowires are selectively exposing four (111) planes composed of plenty of Co 3+ cations according to scanning tunnelling microscopy (STM) studies [18][19][20]. Remarkably, the nanowires show two characteristic order temperatures at 56 K and 73 K, which are far above the AFM order temperature of bulk Co 3 O 4 ~ 40 K. The temperature 73 K is attributed to the Néel temperature of the nanowire shell with symmetry breaking of surface Co 3+ cations. The enhancement of T N of AFM core from about 40 K to 56 K is tentatively attributed to core-shell exchange interactions, i.e., the so-called magnetic proximity effect proposed in Ref. [15,17]. The Co 3 O 4 nanowire shell shows macroscopic residual magnetic moments below 35 K. Cooling the nanowires in a magnetic field larger than 20 kOe, up to 16% of the residual moments is tightly pinned to the antiferromagnetic lattice and does not rotate in an external magnetic field, which results in an obvious horizontal (exchange bias field H E ) and vertical shift of hysteresis loop. We show that the exchange bias field H E and the size of the pinned magnetic moments M pin follow a simple expression H E = aM pin with a a constant.The bulk Co 3 O 4 has a cubic spinel structure with lattice constant of 0.8065 nm. It is a semiconductor with the band-gap of 1.5 eV [19] [22][23][24][25][26]. With decreasing the size of Co 3 O 4 , the ratio of surface-to-bulk increases and more Co 3+ cations with breaking of octahedral symmetry locate at the surface. As a result, it is expected that nanoscale Co 3 O ...

Electrostatic doping as a source for robust ferromagnetism at the interface between antiferromagnetic cobalt oxides

“…In nanostructured AFM materials10 pairing of the two mutually compensating ferromagnetic sublattices is broken due to surface rearrangement, which leads to the presence of “uncompensated” and often “frustrated” surface moments and the consequent magnetic reconstruction11. Several examples of weak ferromagnetism in nanoparticulate CoO121314 and Co 3 O 4 151617 materials have been reported in the literature based on these explanations, though the irregular shapes and the relatively wide size distributions of nanoparticles have made a detailed analysis of the origin of the observed ferromagnetic response very difficult. In contrast, nearly perfect antiferromagnetic CoO nanocrystals with octahedron shapes and different average sizes, as reported in Ref.…”

Synthesis, characterization and magnetic properties of ultrafine Co3O4 octahedra

“…The magnetic properties of nanosized Co 3 O 4 materials depend on the shape, size and crystallization conditions of the nanoparticles, [6][7][8][9][10][11][12] and are particularly sensitive to the spin state of the Co 3+ ions near the surface. 13 A variety of novel Co 3 O 4 nanostructures with variable morphologies including hollow nanospheres, 14 nanowires, 15 nanoparticles, 7 and nanotubes 6,9 have been prepared, and anomalous magnetic properties has been reported, due to their surface condition.…”

New Dimension in Magnetism and Superconductivity: 3D and Curvilinear Nanoarchitectures

“…[61] Qualitatively, in curvilinear antiferromagnetic shells one can expect additional extrinsic anisotropic and chiral terms entering the energy functional, whose strength will be proportional to the principal curvatures. The realistic models should also involve the break of antiferromagnetic ordering at the interfaces [545] and provide a route to describe curvilinear exchange-biased multilayers, where the Adv. Mater.…”

“…Nanoscale: Magnetically filled carbon nanotubes with embedded antiferromagnetically ordered structures [571,594,595] possess a high aspect ratio and support planar bends as well as twists resulting in curvilinear flat or space quasi-1D architectures, see Figure 13e. The stabilization of the long-range antiferromagnetic order is possible in quasi-1D structures: nanowires, which are grown from the antiferromagnetic compounds and core-shell structures, for example, NiO [484] or CoO [495,496,545] of different size, see Figure 13f. The latter also can be produced with antiferromagnetic core of 7 nm diameter with a ferromagnetic surface state.…”