N. Viart

Antiferromagnetism in bulk Zn1−xCoxO magnetic semiconductors prepared by the coprecipitation technique

Polycrystalline Zn1−xCoxO diluted magnetic semiconductors have been prepared by coprecipitation technique in the concentration range 0⩽x⩽0.1. Structure, composition analysis, and optical absorption measurements revealed that cobalt is incorporated into the lattice, as Co2+ substituting Zn2+ ions, forming a solid solution with wurtzite structure instead of Co precipitates. Room- and low-temperature magnetization measurements reveal a paramagnetic behavior for the Co-doped ZnO samples with a paramagnetic Co amount smaller than the nominal concentration. χT versus T evidenced that the remaining Co is antiferromagnetically coupled through oxygen. This is further supported by a simple model that shows that as the Co concentration increases the amount of nearest neighbors Co atoms increases thus giving antiferromagnetic coupling and reducing the paramagnetic contribution.

Synthesis and characterization of Co/ZnO nanocomposites: towards new perspectives offered by metal/piezoelectric composite materials

Abstract A new type of composite material, consisting of metallic cobalt nanoparticles dispersed within a piezoelectric ZnO matrix has been elaborated by a sol–gel process. Cobalt and zinc salt solutions with a cobalt to zinc atomic ratio equal to 0.1 have been spin-coated onto fused quartz substrates and films of 400 nm thickness have been obtained. An annealing of the as-deposited film in air above 500 °C has allowed ZnO to be well-crystallized and (0xa00xa02) oriented. Further heat treatments, in reducing atmosphere, were necessary to obtain the cobalt metal. ZnO being sensitive to reduction, such reducing heat treatments have been performed in a mild reducing atmosphere (H2/H2O) at a temperature not exceeding 450 °C. It is possible in such conditions to obtain Co00.1/ZnO thin films in which Co is metallic and ZnO is (0xa00xa02) oriented. The size of the cobalt particles depends on the temperature of the treatment in air prior to the reduction.

Diversity of the magnetic coupling behaviors in the CoFe2∕CoFe2O4 system

The CoFe2∕CoFe2O4 system has been considered for use as hard electrodes in spin valve devices. In this letter, we focus on the nature and intensity of the magnetic exchange coupling between the two phases. CoFe2∕CoFe2O4 and CoFe2O4∕CoFe2 bilayers were fabricated by pulsed laser deposition from a metallic CoFe2 alloy target in a vacuum and in an O2:N2 oxidizing atmosphere for the metallic and oxide layers, respectively. Depending upon the elaboration conditions, three different coupling behaviors have been observed: Ferromagnetic, biquadratic, and antiferromagnetic. The minor loop shift and the apparent coercive field of the metal observed in the first two cases are of several hundreds of Oe for a metal thickness of 10nm. Those very large loop shifts and coercive fields confirm the potential usefulness of this system in terms of applications.

Structural study of Cofe2/CoFe2O4 multilayers

Abstract The study of the CoFe 2 /CoFe 2 O 4 system is of great interest due to its potential integration in magneto-resistive devices. CoFe 2 /CoFe 2 O 4 multilayers were elaborated by plasma oxidation of sputtered CoFe 2 layers. Exclusive attention is paid, in this paper, to the study by high resolution transmission electron microscopy of the crystallographic relationships between the metal and the oxide networks. The oxide was observed to grow epitaxially from the metal with the following relationship : oxide (1xa00xa00)[1xa00xa00]xa0∥xa0metal (1xa00xa00)[1xa01xa00]. This epitaxial relationship was observed even though the metal (CoFe 2 alloy) did not always present its (1xa00xa00) face to the surface and was oxidised by the violent process that is plasma oxidation. Our conclusion that the growth of epitaxial metal/oxide multilayers by sputtering only depends on the epitaxial growth of the first deposited metal layer is of major interest.

Growth, structure and morphology of CoFe2/CoFe2O4 multilayers

Abstract We report on the possibility to prepare CoFe 2 /CoFe 2 O 4 bilayers and multilayers by deposition of a CoFe 2 layer followed by a partial oxidation in an Ar–O 2 plasma. The CoFe 2 alloy was deposited by argon sputtering onto a glass substrate and the deposition/oxidation procedure was repeated to obtain multilayered stacks. The prepared films are polycrystalline. The grains in the multilayer have an average size of ca. 500 A. The only phases detected by selected area diffraction patterns are a bcc CoFe 2 alloy and a spinel CoFe 2 O 4 phase. High resolution transmission electron microscopy images show that the ferrite and the iron–cobalt alloy layers are flat and have a constant thickness over several thousands of A. Our study of the oxidation process for varied plasma powers and oxidation durations shows that a maximum thickness of approximately 34 A can be obtained for the oxide. Despite the polycrystalline character of the film, coherent lattice fringes are observed through several bilayers, indicating not only a coherent formation of the oxide on top of the metal grains but also an epitaxial growth of the metal layers subsequently deposited onto the oxidised layers.

Synthesis and properties of Fe0-Co0/Co-magnetite composites under hydrothermal conditions

Abstract Fe0–Co0/Co-magnetite composites are synthesized in an autogeneous pressure between 100°C and 250°C in a KOH medium 0.5–14 N. X-ray diffraction and 59Co NMR spectroscopy exhibit various CoFe alloys: the ordered B2 CoFe alloy, a bcc CoFe alloy with a peculiar short range order encountered in thin films and a CoFe alloy isomorphous to α-Mn. The first occurs at low [KOH], the second is favoured by high [KOH]. The α-Mn structure is favoured for [KOH] close to 9 N, Co/Fe ratios between 0.4 and 0.5 and low temperatures of synthesis. A low [KOH] favours nucleation leading to octahedra of 150–200 nm. [KOH] of 12–14 N favours particle growth. The metal occurs in round ball-shaped particles of 10–20 nm embedded within the spinel.

Tuning the coercive field of CoFe2 in hard/soft CoFe2O4/CoFe2 bilayers

The sensitivity of magnetoresistive devices, i.e. their ability to be switched by very small fields, depends on the softness of their soft electrode. In this paper, we show the possibility to tune the coercive field of the CoFe2 alloy, commonly used as a soft electrode, from intrinsic values down to zero, by varying the pulsed laser ablation conditions of CoFe2/CoFe2O4 bilayers. This tuning possibility relies on the existence of a frustration of the spins of the CoFe2 layer originating from both the ferrimagnetic nature of the CoFe2O4 layer and the oxide/metal interface roughness.

How to obtain a magnetic hard-soft architecture by pulsed laser deposition.

In spin valve type systems, one ferromagnetic electrode must be magnetically hard to act as a reference layer while the other electrode must be magnetically soft to act as a sensor or storage layer. This magnetic hard-soft architecture can usually be obtained by four different methods: the use of two ferromagnets with different coercive fields (here CoFe(2) and Ni(80)Fe(20)), the use of an underlayer enhancing the coercive field of one of the two ferromagnets (here Ta and Ru), the use of a ferromagnet coupled to a ferrimagnet or antiferromagnet (here NiO/CoFe(2) and CoFe(2)O(4)/CoFe(2)), or the use of an artificial antiferromagnet (here CoFe(2)/Ru/CoFe(2)). We show that at least the first and the third methods seem to work with pulsed laser deposition in the thermodynamic conditions used.