Bibliothek

Notizen: The M2,3 edge of Rh in Co-Rh alloys shows measurable x-ray magnetic circular dichroism. Such dichroism is not present, however, for analogous alloys of Co-Ru. The induced Ru magnetic moment, if any, is thus demonstrated to be significantly smaller than the induced Rh moment in otherwise similar Co alloys.

Notizen: The magnetic structures which occur in (Co/Ni81Fe19/Co)/Cu multilayer films showing giant magnetoresistance have been investigated using electron microscopy. Rather similar fine domains, with sub-μm dimensions, were found in films comprising 14 and 6 magnetic layers. Whilst the observed structure depended greatly on the magnetic history of the sample, a combination of differential phase contrast imaging and low angle diffraction allowed an estimate to be made of the extent to which neighboring magnetic layers were aligned antiparallel to each other. For both samples typically two layers were found to have parallel alignment leading to the possibility that departures from the expected antiferromagnetic behavior are more prevalent at the surfaces rather than in the bulk of the multilayer.

Notizen: The structural changes that accompany the development of GMR (giant magnetoresistance) at low ((approximately-less-than)10 Oe) fields in annealed magnetic multilayers are of current interest because of potential applications of such structures in sensors. In this paper we report a study of the development of GMR in [111]-oriented multilayers comprising ferromagnetic films of a mixture of Ag and permalloy (NixFe1−x, x∼0.8) alternating with Ag spacer films. The multilayers were grown by molecular beam epitaxy (MBE) on Pt(111) seed films on sapphire (0001) substrates at temperatures in the range 20 to 200 °C. The structure of the multilayers was investigated using x-ray diffraction and electron microscopy. For a series of multilayers grown with nominally identical ferromagnetic and spacer layer thicknesses, the magnetoresistance is found to be strongly dependent on both growth temperature and subsequent annealing temperature. The multilayers exhibited a negative magnetoresistance in the as-grown state which more than doubled when the growth temperature was increased from 20 to 100 °C. However, the highest magnetoresistance (peak 5.6%; maximum slope 0.4% per Oe) was obtained by annealing (at 400 °C) multilayers grown at 100 °C. Transmission electron microscopy studies of such multilayers showed no evidence for discontinuities or penetration of the ferromagnetic films by Ag along grain boundaries. Thus, we conclude that discontinuous or granular multilayers with complete phase separation are not necessary for GMR with low saturation fields.

Notizen: Interest in magnetic tunnel junctions (MTJ) has been increased by the recent observation of large room-temperature magnetoresistance (MR) in structures comprised of sandwiches of two ferromagnetic (FM) layers with different coercivities separated by a thin Al2O3 tunnel barrier. Using an ultrahigh vacuum dc magnetron sputtering system, a variety of MTJ structures have been explored. Junctions were fabricated directly using computer-controlled placement of successive contact masks, and by patterning using optical lithography techniques and processes, compatible with MR head structures. Structures were prepared in which one of the FM layers is exchange biased with an antiferromagnet MnFe layer. Thus, the junctions exhibit two well-defined magnetic states in which the FM layers are either parallel or antiparallel to one another. The tunnel barrier was prepared by in situ plasma oxidation of thin Al layers sputter deposited at room temperature. Using FM layers comprised of Co or permalloy (Ni81Fe19), MR values exceeding 30% were obtained at room temperature in low fields (5–10 Oe). Similar magnetic and MR properties are found in junctions ranging in size from 80×80 μm2 (contact masks) to 2×2 μm2 (lithography). The smallest junctions, of area 2×2 μm2, have resistance values of ∼5 MΩ and 25% MR. Thus, changes in resistance of ∼1 MΩ are found. Such structures may form an excellent magnetic switch. However, for many device applications, much lower resistances are required. The junction resistance can readily be varied by several orders of magnitude by varying the thickness and oxidation conditions of the Al layer as well as by varying the thicknesses and nature of the layers within the electrodes themselves. Note that it is important that the voltage drop laterally across the electrodes must be small compared to that across the tunnel barrier itself. This requires measurement of junctions with low sheet resistances in lithographically patterned devices. ©1997 American Institute of Physics.

Notizen: Magnetic tunnel junctions (MTJ), structures consisting of two ferromagnetic layers separated by a thin barrier of Al2O3, have recently been shown to exhibit large magnetoresistance (MR) at room temperature. The possible use of these structures in devices requires a thorough characterization and understanding of the transport properties of MTJs, which has motivated the present work. The junctions studied here were fabricated via dc sputtering on silicon substrates at room temperature with a series of contact masks to define the junction area (80×80 μm2), and plasma oxidation to produce the insulating barrier. A long standing problem of MTJs is the sensitivity of their MR to bias voltage. For example, in a recent work1 it was reported that the peak MR of 11% exhibited by a CoFe/Al2O3/Co junction was decreased by half at a bias voltage of 〈200 mV. In this work, we will show a significant improvement of the MR voltage dependence, with peak MR values of 20%–25% near zero bias decreasing to half these values for 300–500 mV. The voltage falloff is typically asymmetric with regard to the sign of the applied voltage. For example, the peak MR of a Co/Al2O3/Ni85Fe15 MTJ, in which the Co is exchange biased with Mn54Fe46, decreases by half at a positive bias of 300 mV but requires a negative bias of 500 mV to attain the same MR decrease. There is significant structure in the resistance versus bias voltage curves with distinct differences at high and low magnetic fields for which the moments of the Co and Ni85Fe15 layers are parallel and antiparallel, respectively. A decrease in MR at high bias levels is found in all junctions independent of their resistance and the composition of the magnetic layers. However, the detailed dependence of MR on bias voltage varies with the detailed structure of the MTJ. Results will be presented for a wide range of magnetic materials and for junction resistances varying from 1 to 10 000 Ω. Possible explanations for this phenomena will be offered. ©1997 American Institute of Physics.

Notizen: We explore the magnetic interlayer exchange coupling in single-crystalline Fe/Cr and Co/Cu wedged multilayers prepared by magnetron sputtering on two orientations of MgO—(100) and (110). Structural examination shows that high quality epitaxial films are obtained in both systems using seeded epitaxy growth techniques. Oscillatory interlayer coupling is observed in Cr-wedged samples grown on both MgO(100) and MgO(110) with similarly long oscillation periods of ∼18 A(ring). These results are comparable to earlier work for molecular beam epitaxy and sputter-deposited epitaxial Fe/Cr structures. Long-period oscillatory coupling is also observed in sputtered Co/Cu(110) for the Cu-wedged structures. Studies of the dependence of magnetic coupling on the thickness of the magnetic layers for fixed Cr layer thickness provides no evidence of any nonmonotonic dependence. © 1996 American Institute of Physics.

Notizen: The giant magnetoresistance (GMR) effect exhibited by magnetic multilayers and related structures is very sensitive to the nature of the interfaces between the magnetic and nonmagnetic layers. We have explored the dependence of GMR on the electronic character of these interfaces by inserting additional thin magnetic layers at the interfaces. Insertion of thin Co layers in Ni–Fe/Cu/Ni–Fe or thin Ni–Fe layers in Co/Cu/Cu exchange biased sandwiches leads to an increase or decrease, respectively, in the magnitude of the GMR effect; the variation is monotonic with thickness of the inserted layer. In contrast, insertion of thin layers of Fe at the Ni–Fe/Cu interfaces in Ni–Fe/Cu/Ni–Fe exchange biased sandwich structure results in a nonmonotonic variation of GMR with Fe layer thickness; for intermediate thicknesses in particular the GMR is substantially reduced. The magnetism and structure of the Fe layers is explored in related sputter-deposited single-crystalline Ni/Fe/Cu thin-film structures prepared using thin Fe/Pt seed layers grown at high temperatures on single crystalline polished wafers of MgO(100), MgO(110), and (0001)Al2O3. The magnetism of the Ni, Fe, and Cu layers is examined using x-ray magnetic circular dichroism (XMCD) and the structure of the Fe is analyzed using extended x-ray absorption fine structure (EXAFS) studies. These experiments show that the Fe moment varies nonmonotonically with thickness resulting from a structural phase transition from fcc to bcc Fe with increasing thickness.The Fe displays a very small magnetic moment for an intermediate range of Fe thickness for which the Fe structure is fcc. This range of thickness depends on the crystalline orientation of the Fe. It is within this same range of thickness that the GMR is suppressed in related exchange biased sandwich structures. Interestingly we find that such nonferromagnetic layers of fcc Fe can be used as spacer layers in magnetic multilayers of, for example, Ni/Fe. These multilayers display both antiferromagnetic interlayer coupling of the Ni layers and giant magnetoresistance, which each oscillate as a function of Fe thickness. © 1996 American Institute of Physics.

Notizen: Various uncoupled, magnetoresistive multilayer systems show quasiequilibrium resistance noise associated with ||dR(H)/dH||. The size of this 1/f noise, particularly in comparison with Johnson noise in various frequency ranges, is discussed for all the different systems. All the observed samples exhibit resistive Barkhausen noise, allowing the measurement of domain sizes very similar to those observed in coupled samples. In addition, the asymmetric hysteresis of the Barkhausen noise in uncoupled multilayers suggests the same type of hysteretic domain structure observed in antiferromagnetically coupled multilayers. © 1996 American Institute of Physics.

Notizen: The structural and magnetic properties of [111]-oriented multilayers comprising ferromagnetic films of Permalloy-silver alternating with Ag spacer films are described. The multilayers are grown by molecular-beam epitaxy on Pt(111) seed films on sapphire (0001) substrates at temperatures in the range 25–175 °C. For a series of multilayers with similar bilayer periods ((approximately-equal-to)50 A(ring)) the magnetoresistance (MR) is found to be strongly dependent on both growth temperature and subsequent annealing temperature. The multilayers exhibit a negative magnetoresistance in the as-grown state which more than doubles when the growth temperature is increased from 25 to 100 °C; however, the highest MR (peak 5.6%; maximum slope 0.4% per Oe) is obtained by annealing (at 400 °C) multilayers grown at 100 °C. The primary effects of annealing are an improvement of structural order, partial segregation of Ag from the ferromagnetic films into adjacent Ag films, a slight decrease in laminar order, and a reduction in long-wavelength roughness of the multilayer interfaces. No evidence is found for discontinuities in the magnetic layers with the highest MR.

Notizen: Uncertainties in the application of the 〈Lz〉/〈Sz〉 sum rule to experimental spectra of V, Cr, Mn, Fe, Co, and Ni are discussed. An important contribution to these uncertainties is the possible presence of dichroism due to diffuse magnetic moments, which are known to exist in Fe, Co, and Ni.