ALBERT

Notes: Co/Ru hcp (0001) superlattices were grown by UHV evaporation onto mica substrates. We demonstrate the growth of epitaxially ordered Co/Ru superlattices–despite the large lattice mismatch ((approximately-equal-to)8%)–consisting of hcp (0001) Co and Ru sublayers initialized on 150-A(ring)-thick single-crystalline Ru buffer layer by using RHEED and x-ray diffraction. The magnetic properties were studied by magnetization and ferromagnetic resonance (FMR) measurements. For uncoupled Co layers, the resultant anisotropy switch from planar to axial direction (perpendicular to the film plane) with decreasing Co sublayers thickness. For Co sublayer thickness smaller than a critical one tco(approximately-equal-to)14 A(ring), the magnetization is directed perpendicular to the film plane. For small Ru interlayer thickness, large antiferromagnetic exchange coupling between the Co sublayers is observed in agreement with precious results. For a certain range of Co and Ru thicknesses, superlattices with unique magnetic parameters are obtained. Indeed, in the absence of an applied field, the magnetization is oriented along the film normal while the adjacent Co layers are coupled antiferromagnetically. For this magnetic structure, the magnetization process exhibits a hysteresis and the FMR spectra an irreversible behavior when the applied field is along the film normal.As shown by SQUID measurements, when the field is decreased from a quasi-saturated state, the layers are essentially coupled ferromagnetically, the antiferromagnetic state appearing following a spin-flip process (staircaselike on the magnetization curve) for a field smaller than a critical one. However, with increasing field, the antiferromagnetic state disappears by progressive steps suggesting that the spin reorientation process occurs by coherent rotation of the magnetization combined with spin-flip process. The occurrence of this effect is particularly spectacular on the FMR spectra. With decreasing field, a classical FMR absorption is observed due to the basical ferromagnetic state while with increasing field, this absorption is largely reduced or disappears completely due to the antiferromagnetic one. The magnitude of the hysteresis and its evolution with the direction of the applied field, the values of the critical field corresponding to the spin flip and to the field above which the magnetic state is reversible, are very well correlated on the FMR spectra and the magnetization curves.

Notes: A series of Fe/Si sandwiches have been prepared by ion beam sputtering at room temperature onto glass substrate with the following nomenclature: Glass/Si20 nm/Fe5/6 nm/Fe5 nm/Ru2 nm. Magnetization measurements have been performed at 300 K and show no evidence of antiferromagnetic exchange coupling. However, the magnetoresistance curves recorded at 300 K are very interesting and show an inverse magnetoresistance for sandwiches with Si spacer layer thicknesses between 1.2 and 1.5 nm. The resistance is smaller at zero field than at saturation. This inverse magnetoresistance is due to the superparamagnetic interfaces and finds its origin in the difference of the electronic nature of the Fe/Si interfaces and Fe/Ru interfaces. Fe1−xSix iron silicide at Fe/Si interfaces has a scattering spin asymmetry ratio (α=ρ↓/ρ↑) larger than one, whereas, Fe with Ru impurities at the Fe/Ru interfaces presents a scattering spin asymmetry ratio smaller than one. © 1999 American Institute of Physics.

Notes: We report on the magnetic and transport properties of artificially antiferromagnetically coupled CoFe/Ir/CoFe sandwiches (AAF), grown by molecular beam epitaxy on MgO(001) substrates. The sandwiches are deposited on Fe/Co/Cu/Co buffer layers and their magnetic properties are found to be strongly influenced by the anisotropy of the Fe layer. The coercive field of the AAF is HC2=600 Oe for the samples with isotropic Fe. However, when the Fe layer is anisotropic, the coercive field of the AAF is HC2=600 Oe and 400 Oe, respectively along the hard bcc Fe[110] and the easy bcc Fe[100] axes. In addition, in this second case, the rigidity of the AAF is improved. This gives rise to a sharp reversal of the magnetization vectors of the AAF and to a flat magnetization and giant magnetoresistance plateau, which is very promising for spin electronic devices. © 2002 American Institute of Physics.

Notes: A first experimental evidence of a significant tunneling magnetoresistance signal of about 5% at 300 K for a magnetic tunnel junction consisting of hard and soft magnetic layers separated by a 2 nm ZnS semiconducting barrier is reported. The samples have been grown by sputtering on Si(111) substrate at room temperature and have the following structure: Fe6 nmCu30 nmCoFe1.8 nmRu0.8 nmCoFe3 nmZnSxCoFe1 nmFe4 nmCu10 nmRu3 nm. The hard magnetic bottom electrode consists of the artificial antiferromagnetic structure in which the rigidity is ensured by the antiferromagnetic exchange coupling between two FeCo layers through a Ru spacer layer. Barrier impedance scanning microscope (BISM) measurements reveal a good homogeneity of the barrier thickness. Electric transport measurements over square tunnel elements with lateral sizes between 3 and 100 μm, exhibit a typical tunnel current–voltage variations and tunnel resistance of 2–3 kΩ μm2 with small variations which never exceed a factor of 2, which is in good agreement with the BISM results. This good reproducibility of the junctions is very promising for MRAMs and transistors applications. © 2001 American Institute of Physics.

Notes: A magnetization model is presented that is used to cover trilayers containing two magnetic layers that are exchange coupled via an intermediate nonmagnetic layer and that have different crystalline anisotropies. The interfaces are coupled to the bulk by a twisted magnetization configuration which is evaluated using the Ritz method. By minimizing the total energy, experimental magnetization curves of strongly coupled Co/Ru sandwiches can be reproduced with a good precision and with the same set of parameters in two perpendicular field directions. These physical parameters can be determined with a good reliability and are in agreement with the literature except for the bulk anisotropy of the Co layer first deposited, which is twice as large as the known bulk value. This originates in the magnetoelastic contributions due to lattice misfit and interface roughness. It is shown that the interlayer exchange coupling forces the magnetization of both layers to be along the same axis in the low-field range notwithstanding the opposite sign of the anisotropy constants in most stacks. It is also demonstrated that the differences in the orientations of the moments in one Co layer are modest and depend on the various parameters. In particular, the bulk exchange constant is a decisive parameter that makes the calculated curves close to the experimental ones. © 1996 American Institute of Physics.

Notes: Experimental evidence of inverse magnetoresistance for ferromagnetic layers separated by a Si layer is reported. A series of Fe/Si sandwiches have been prepared by ion-beam sputtering at room temperature onto a glass substrate with the following nomenclature: glass/Si20 nm/Fe5 nm/Six nm/Fe5 nm/Ru2 nm. Magnetization measurements have been performed at 300 K and show no evidence of antiferromagnetic exchange coupling. However, the magnetoresistance curves recorded at 300 K are very interesting and show a reversed magnetoresistance for sandwiches with Si spacer layer thicknesses between 1.2 and 1.5 nm. Indeed, the resistivity is smaller at zero field than at saturation. This reversed magnetoresistance is due to the superparamagnetic interfaces and finds its origin in the difference of the electronic nature of the Fe/Si interfaces and Fe/Ru interfaces. Indeed, iron silicide Fe1−ySiy at Fe/Si interfaces have scattering spin asymmetry ratios (α=ρ↓/ρ↑) larger than 1, whereas, Fe with Ru impurities at the Fe/Ru interfaces present scattering spin asymmetry ratios lower than 1. © 1998 American Institute of Physics.

Notes: The evolution of the magnetic anisotropy between room temperature and 50 K has been studied using torque magnetometry on a hcp (0001) Co (0.8 nm)/Cu (1.5 nm)/Co (0.8 nm) trilayer prepared by ultrahigh vacuum evaporation. At 300 K this sample presents an easy plane of the magnetization in the film plane, with a very small effective anisotropy constant Keff ((approximate)−4.91×105 erg/cm3). By cooling the sample, the easy magnetization direction becomes perpendicular to the film plane. Keff is positive below 288 K and increases continuously upon decreasing the temperature. At 50 K, the effective anisotropy constant reaches about 2.4×107 erg/cm3. This strong increase of the effective anisotropy upon decreasing the temperature can be explained by a strong increase of surface anisotropy term. Magnetization measurements have revealed the existence of one magnetically dead monolayer at each interface, indicating a strong intermixing in our Co/Cu interfaces at 300 K. Thus the evolution of the magnetism of the intermixed region as a function of the temperature may be at the origin of the strong increase of the effective anisotropy.© 1998 American Institute of Physics.

Notes: Hard–soft spin valve structures have been grown by molecular beam epitaxy on MgO(001) substrates. The hard magnetic layer consists of (Co50Fe50)/Ir/(Co50Fe50) artificial ferrimagnetic (AFi) system, while a Fe/Co bilayer integrated in the buffer, has been used as a soft detection layer. The Fe has been grown at 500 °C giving rise to a monocrystalline layer with a body centered cubic structure. The spin valve structure presents a progressive evolution after successive annealing steps up to 350 °C. The total giant magnetoresistance (GMR) reaches its maximum (5.3%) after annealing at 250 °C, together with a good rigidity of the hard layer and a sharp switch of the magnetic moments. Such characteristics are reduced, but still interesting, after annealing at 300 °C. For annealing at higher temperature (350 °C) the total GMR signal and the coercive field of the AFi decrease dramatically and all the stack behaves like a single magnetic layer. Rutherford backscattering measurements were performed in order to investigate the changes in the morphology of CoFe/Ir interfaces and to correlate them to the magnetotransport properties. © 2002 American Institute of Physics.

Notes: A series of Co30 Å/Ru5 Å/Co30 Å sandwiches has been prepared by ion beam sputtering on a glass substrate using different buffer layers in order to optimize the antiferromagnetic (AF) coupling strength and the giant magnetoresistance (GMR) signal. The best GMR of about 1.7% is obtained for a Co/Ru/Co sandwich deposited on the Fe50 Å/Co5 Å/Cu30 Å buffer. On the basis of the GMR and exchange coupling results, the two more interesting buffers have been chosen for a detailed analysis. Two additional series have been grown with varying Ru spacer layer thickness on these two buffers: 150 Å Ru and Fe50 Å/Co5 Å/Cu30 Å buffer layers. For the two series, oscillations in magnetoresistance and AF indirect exchange coupling have been observed as a function of the Ru thickness with the same period of about 10 Å. The first AF peak is observed at a thickness of 6 Å Ru for the series grown on Ru150 Å buffer and 4 Å for the series grown on a Fe50 Å/Co5 Å/Cu30 Å buffer layer. At this first peak, both GMR and AF coupling are largely enhanced with values JAF=−2.6 erg/cm2 and a GMR of 1.34% for the Fe50 Å/Co5 Å/Cu30 Å buffer compared to JAF=−0.75 erg/cm2 and a GMR of 0.08% for the Ru150 Å buffer. Atomic force microscopy and x-ray diffraction studies have also been performed in order to understand the origin of the observed increase in GMR and AF exchange coupling between the two series. © 2000 American Institute of Physics.

Notes: We report on the junction magnetoresistance in magnetic tunnel junctions of the hard–soft type with magnetic layers separated by a ZnS barrier. The hard magnetic bottom electrode consists of an artificial antiferromagnetic structure in which the rigidity is ensured by the antiferromagnetic exchange coupling between two FeCo layers through a Ru spacer layer. The samples were grown by sputtering on Si (111) wafers at room temperature and have the following structure: Fe6 nmCu30 nm(CoFe)1.8 nmRu0.8 nm(CoFe)3 nmZnSx(CoFe)1 nmFe4 nmCu10 nmRu3 nm. The square tunnel elements, with lateral sizes of 10, 20, 50, and 100μm, exhibit typical tunnel resistance of 2–3 kΩ μm2 and nonlinear zero field current–voltage (J–V) variation. The most interesting result is the observation of junction magnetoresistance of about 5% at room temperature with a 2 nm thick ZnS barrier. © 2001 American Institute of Physics.