O. Lenoble

Magnetic and structural properties of iron nitride thin films obtained by argon‐nitrogen reactive radio‐frequency sputtering

We have prepared iron nitride thin films by reactive rf sputtering of an iron target in an argon‐nitrogen plasma. The films present a wide variety of structures and compositions: α expanded bcc iron, amorphous Fe1−x‐Nx, γ’‐Fe4N, e‐Fe2–3N, and ζ‐Fe2N. Evolutions of the crystallographic structures and of the magnetic properties of the obtained phases are discussed versus the nitrogen concentration in the gaseous flow and the substrate temperature during deposition.

Tantalum oxide as an alternative low height tunnel barrier in magnetic junctions

Magnetic tunnel junctions with a barrier of tantalum oxide were prepared by plasma oxidation of sputter-deposited tantalum. They show magnetoresistance ratios of 2.5% at room temperature and 4% at low temperatures. The material exhibits low barrier heights of ∼0.4 eV. This makes it possible to substantially increase the barrier thickness, compared to a barrier of aluminum oxide. The resulting decrease of coupling between the ferromagnetic layers is easily seen. Tantalum oxide appears to be a candidate for use as a tunnel barrier of spin-dependent tunneling devices.

Magnetic properties of postoxidized Pt∕Co∕Al layers with perpendicular anisotropy

The magnetic properties of ultrathin Co layers sandwiched between Pt and Al layers are studied as a function of the Al layer oxidation time. The association of three batches of complementary experiments (extraordinary Hall effect, x-ray photoelectron spectroscopy, and tunneling magnetoresistance) allows the authors to finely characterize their samples both magnetically and chemically. The authors show that their oxidation process reduces the coercive field of ultrathin Co layers with perpendicular anisotropy (case of short oxidation time) and can even induce transition from a ferromagnetic to a superparamagnetic state (lengthy oxidation time).

Structure, magnetism and thermal stability of FeAl2O3 multilayers

Abstract We have deposited iron/alumina multilayers (ML) on sapphire wafers using r.f. magnetron sputtering. In the aim to study their thermal behaviour, MLs have been annealed under a controlled atmosphere. The phase identification has been mainly performed by a combination of classical diffractometry ( Θ 2Θ ) and grazing incidence scattering, completed by 57Fe Mossbauer spectroscopy. The superstructure of the ML has been checked by small-angle X-ray scattering. Finally, magnetic properties have been studied by room-temperature bulk magnetization. Without treatment, Fe Al 2 O 3 MLs consist of alternating amorphous alumina and polycrystalline iron. Thermal annealings at temperatures ranging between 573 and 873 K give evidence for segregation at the interfaces. We have evaluated the FeAl2O3 interdiffusion coefficient in the range of temperature from 873 to 1273 K. Above 773 K, we observe the co-existence of Fe2+ and metallic bcc and fcc Fe. Competition between interfacial and shape magnetic anisotropy is found to be dependent on the presence of fcc iron.

Magnetic and structural properties of sputtered FeN thin films

Abstract FeN thin films have been deposited by reactive rf sputtering in an N 2 Ar atmosphere onto substrates maintained at room temperature. The evolutions of the crystallographic structures and of the magnetic properties are discussed versus the nitrogen concentration in the gaseous flow.

Correlation between structural quality and magnetic properties of IrMn-based multilayers

Structural and magnetic characterizations have been performed on exchange-biased multilayers, namely, Ta∕X∕IrMn∕Y∕Ta where X and Y are Py and/or Co. In agreement with earlier calculations, magnetic hysteresis loops reveal a clear correlation between the structural quality of the IrMn layer and the variation of the surface exchange energy constant versus its thickness. Moreover, we observe a direct link between the exchange bias variation with temperature and the magnetic disorder.

Domain duplication in ferromagnetic sandwiches

In this article, we give an overview of the domain duplication process which can occur in ferromagnetic sandwiches. A brief theoretical description of the process allows us to extract the main parameters governing the effect. It is shown that even if a domain structure exists in the hard electrode, no duplication can occur for a ferromagnetic coupling below a minimum value. Then, we address also the effects of residual domains on the nucleation field of the hard electrode to reconcile theory and experiments.

Magneto-Resistance and Induced Domain Structure in Tunnel Junctions

Abstract : Magnetization reversals in sputtered Co electrodes of a magnetic tunnel junction are studied using transport measurements, magneto-optic Kerr magnetometry and microscopy. Using the tunnel magneto-resistive effect as a probe for micromagnetic studies, we first evidence the existence of an unexpected domain structure in the soft Co layer. This domain structure originates from the duplication of the domain structure of the hard Co layer template into the soft layer via ferromagnetic inter-electrode coupling. A detailed analysis of the kerr microscopy images shows that all features appearing in the variation of tunnel resistance as a function of the applied field are associated to the domain phase evolution of each electrode. By tailoring the magnetic properties of the hard Co layer, we have demonstrated that the appearance of the domain duplication is driven by the magnetic anisotropy of the hard layer. Finally, a brief theoretical description of the domain duplication process allows us to extract the main parameters governing the effect.