T. Veres

Ion-beam modification of Co/Ag multilayers I: Structural evolution and magnetic response

We describe the effects of 1 MeV Si+ ion-beam irradiation on a sputtered Co/Ag multilayer with layer thicknesses of 5 A for Co and 25 A for Ag, thicknesses for which the magnetoresistance is maximum in the as-deposited sample. X-ray diffraction, magnetization, and magnetoresistance measurements all point to the conclusion that the Co is initially dispersed through the Ag and segregates completely upon ion-beam bombardment. Throughout the process both Ag and Co grains maintain a high degree of texture, essentially face centered cubic (111). The magnetization behavior evolves from superparamagnetic to mixed superparamagnetic–ferromagnetic with ion dose whereas the room-temperature magnetoresistance decreases from 12% to 1.5% upon irradiation up to 5×1016 Si+/cm2. Simple models taking into account the size distribution of the Co particles have been used to analyze these phenomena in order to quantify the particle size distribution.

MeV ion irradiation of Co/Cu multilayers

We examine the effect of MeV ion-beam irradiation on the giant magnetoresistance and related interlayer magnetic coupling in sputtered Co/Cu multilayers. At ion doses higher than 1013/cm2, the resistivity of the multilayers increases noticeably, well beyond that measured for pure copper or cobalt films. This increase in resistivity of the multilayers is tentatively ascribed to ion-beam-induced interface disorder. With increasing dose, the magnetic interlayer coupling passes systematically from a mainly antiferromagnetic (AF) coupling to a ferromagnetic one and, in parallel, the giant magnetoresistance (GMR) is progressively destroyed. A linear relationship between the GMR and the volume fraction of AF coupled regions is observed up to an ion dose of 2×1014/cm2.

Ion-beam modification of Co/Ag multilayers II: Variation of structural and magnetic properties with Co layer thickness

The structural, magnetic and transport properties of rf sputtered Co/Ag multilayers with Co-layer thicknesses ranging from 1 to 14 A have been studied by a combination of x-ray diffraction, magnetic and transport measurements. The magnetoresistance at room temperature has a maximum value of more than 12% for a Co-layer thickness around 5 A. Magnetic measurements demonstrate that samples near this Co-layer thickness are in the transition region from superparamagnetic to ferromagnetic behavior. X-ray analysis indicates that, during deposition, a significant quantity of Co is dispersed throughout a highly textured Ag matrix. Upon irradiation with 1 MeV Si+ ions up to a dose of 5×1016 Si+/cm2, an initial demixing of the Co is followed by segregation into grains with the same texture as the Ag. The resulting changes in the magnetization and magnetoresistance are characterized on the basis of a log-normal distribution of the volume of the magnetic particles. As the particle sizes increase, a systematic evolutio...

Ion-beam irradiation of Co/Cu nanostructures: Effects on giant magnetoresistance and magnetic properties

We have studied the effects of ion irradiation at low doses (<5×1014 ions/cm2) on the structural properties, giant magnetoresistance (GMR), and interlayer magnetic coupling in Co/Cu multilayers. X-ray analysis combined with magnetic and resistivity measurements reveal that intermixing is promoted by ion irradiation while the periodic structure and crystallographic properties of the multilayers are not significantly altered. The GMR ratio of a multilayer decreases monotonically with ion dose. However, thermal annealing on an irradiated multilayer results in sharp recovery of the reduced GMR, and can be associated with a backdiffusion process in metastably intermixed regions. Hence, using ion irradiation and subsequent annealing, the GMR of a single multilayer can be altered reversibly over a wide range. The variation of GMR upon irradiation (or annealing) is accompanied by significant suppression (or improvement) of the antiferromagnetic interlayer coupling. The correlation between GMR and AF coupling, as ...

Structural evolution of Co/Cu nanostructures under 1 MeV ion-beam irradiation

Co/Cu multilayers with composition wavelength ranging from 2 to 10 nm have been deposited and irradiated at various doses from 1×1014 to 3×1016 ions/cm2 using 1 MeV Si+ ions. The ion-beam-induced variation in structural properties such as interfacial mixing, interface roughness, crystallographic texture, and grain size, are characterized by a variety of x-ray scattering techniques. Irradiating Co/Cu multilayers generate metastable Co–Cu alloys whose electrical and magnetic properties have been found to be very similar to the Co–Cu alloys fabricated by other nonequilibrium methods. Fitting to the low-angle x-ray reflectivity spectra using a standard optical model yields a mixing efficiency comparable to the prediction of a ballistic ion-beam mixing model, and interfacial mixing widths consistent with the values estimated from saturation magnetization measurements.

Deposition-temperature dependence of texture and magnetic properties of sputtered Ni/Fe multilayers

We present the effect of deposition temperature (Ts) on the structural and magnetic properties of sputtered Ni/Fe multilayers. As Ts increases above room temperature, a systematic change in texture from (111) to (200) and an increase in grain size are observed. Superlattice modulations are clearly visible in low-angle x-ray reflectivity measurements for all the multilayers deposited up to 250 °C. However, superlattice peaks intensities decrease with increasing Ts indicating enhanced mixing of Ni and Fe across interfaces. Nonmonotonic variations in electrical resistivity (ρs) and coercivity (Hc) with Ts are seen and explained in terms of the interplay between the increase in grain size and alloying at the interfaces. The total anisotropic magnetoresistance (AMR) as well as its field sensitivity increase with Ts.

MeV Si+ irradiation of Ni/Fe multilayers: structural, transport and magnetic properties

Abstract The modification of the structural, transport and magnetic properties of Fe(28 A)/Ni(85 A) multilayers on 1-MeV Si+ irradiation at low temperature (77 K) has been studied as a function of dose. The as-deposited multilayers exhibit a strong Ni(111)/Fe(110) texture with sharp interfaces. Upon irradiation, progressive intermixing at the interfaces is clearly observed in the low-angle X-ray reflectivity and quantified by fitting the data with a standard optical model incorporating interface roughness, intermixing and individual layer thickness fluctuations. Simultaneously, the average grain size gradually increases from ∼170 to ∼250 A with increasing ion dose up to 3×1016 ions cm−2. These changes lead to an increase in the anisotropic magnetoresistance from ∼1.4% in the as-deposited sample to ∼1.8% after irradiation with a dose of 1016 ions cm−2, and to the establishment of an in-plane easy axis for the magnetization. The effect of ion bombardment on the transport properties are explained using a model based on the Boltzmann equation incorporating the intermixing characteristics determined from the quantitative X-ray analyses.

Magnetic and magnetotransport properties of Ni/Fe multilayers

We present a study of sputtered Ni/Fe multilayers in order to examine the effect of interfacial mixing and interdiffusion on the magnetic and transport properties. The multilayer structure has been examined by low‐angle x‐ray reflectivity, which reveals interface mixing of about two monolayers. The magnetization measurements show differences in the anisotropy of multilayers for which the thickness ratio of nickel to iron is changed from 1:1 to 3:1. With the 1:1 composition, multilayers show high permeability with a longitudinal easy axis, while samples with 3:1 composition show lower permeability and a rotated easy axis. By fitting the resistivity and magnetoresistance thickness variation with a semiclassical model, we have determined an interface contribution to the resistivity and a possible contribution to the magnetoresistance.

MeV Si+ irradiation of Fe/Ni bilayers: influence of microstructural and interfacial changes on magnetic properties

Abstract The effects of microstructural changes during low-temperature MeV Si + ion-beam irradiation on the magnetic properties of Fe (500 A)/Ni (500 A) bilayers deposited onto thermally oxidized Si have been studied as a function of ion dose. The as-deposited films exhibit strong Ni(111)/Fe(011) texture with sharp interfaces. Upon irradiation, grains grow progressively within each layer but this process stops abruptly at the Ni–Fe interface. The resistivity and the anisotropic magnetoresistance are virtually unchanged by irradiation up to 10 16 ions cm −2 . Higher doses, however, lead to an in-plane unidirectional anisotropy as well as asymmetric magnetoresistance and magnetization curves which arise from interactions between the ferromagnetic layers and the magnetic oxide layers formed at the top surface and the substrate interface during ion irradiation.

Ion Beam Mixing and Thermal Demixing of Co/Cu Multilayers

X-ray reflectivity and magnetotransport studies have been used to probe the effects of ion-beam irradiation and subsequent thermal annealing on the structure and giant magnetoresistance (GMR) in Co/Cu multilayers. Low-dose ion bombardment produces interfacial mixing which is accompanied by a systematic suppression of the anti ferromagnetic (AF) coupling and the GMR. For ion doses not exceeding 5 × 10 14 ions/cm 2 , subsequent thermal annealing restores the abrupt interlayer structure as well as the GMR. The combination of low-dose ion bombardment and thermal annealing provides an ex situ technique to modify interface structure reversibly over a gnificant range.