Magnetotransport effects of ultrathin Ni₈₀Fe₂₀films probedin situ

Abstract

We have investigated the magnetoresistance of Permalloy (Ni80Fe20) films with thicknesses ranging from a single monolayer to 12 nm, grown on Al2O3, MgO and SiO2 substrates. Growth and transport measurements were carried out under cryogenic conditions in UHV. Applying in-plane magnetic vector fields up to 100 mT, the magnetotransport properties are ascertained during growth. With increasing thickness the films exhibit a gradual transition from tunneling magnetoresistance to anisotropic magnetoresistance. This corresponds to the evolution of the film structure from separated small islands to a network of interconnected grains as well as the transition from superparamagnetic to ferromagnetic behavior of the film. Using an analysis based on a theoretical model of the island growth, we find that the observed evolution of the magnetoresistance in the tunneling regime originates from the changes in the island size distribution during growth. Depending on the substrate material, significant differences in the magnetoresistance response in the transition regime between tunneling magnetoresistance and anisotropic magnetoresistance were found. We attribute this to an increasingly pronounced island growth and slower percolation process of Permalloy when comparing growth on SiO2, MgO and Al2O3 substrates. The different growth characteristics result in a markedly earlier onset of both tunneling magnetoresistance and anisotropic magnetoresistance for SiO2. For Al2O3 in particular the growth mode results in a structure of the film containing two different contributions to the ferromagnetism which lead to two distinct coercive fields in the high thickness regime.

Magnetoresistance measurement of tailored Permalloy nanocontacts

“…Magnetoresistance measurements ͑with a current I Ͻ 100 A͒ were carried out at 77 K in a two-probe configuration with an in-plane magnetic field ͑Ϯ100 mT͒. 34 The angle-dependent MR response of the nanocontact shows a cos 2 dependence ͓see Fig. 2͑a͔͒, as expected for AMR in bulk samples.…”

Domain-Wall Induced Large Magnetoresistance Effects at Zero Applied Field in Ballistic Nanocontacts

“…In order to keep the contacts as clean and stable as possible all subsequent steps (i.e. deposition, electromigration and MR measurements) were carried out in-situ in the same ultra-high vacuum (UHV) chamber [20] with a base pressure of 3 · 10 −10 mbar. A 24 nm thick Py film was deposited onto the sample using thermal evaporation.…”

“…Using a special device geometry, we control the spin structure and in particular the presence of a DW. Furthermore, the nanocontacts are fabricated and characterized at low temperatures in the same ultra-high vacuum (UHV) chamber without breaking the vacuum [20]. Insitu electromigration allows us to reduce the cross-section of the nanocontacts in a controlled fashion during the study [21].…”

Scaling of intrinsic domain wall magnetoresistance with confinement in electromigrated nanocontacts

“…In the experiment, the transport at the narrow-constriction is likely to be dominated by a single grain [64], while the 5% composition difference is not expected to give a significance change in the transport properties. In this configuration, the Fe atoms sit at the summit of the pseudo-cubic unit cell and the Ni atoms are at the center of the faces.…”

“…In order to characterize the magnetoresitance we perform a so-called mode-étoile measurement [63]. A magnetic field of 150 mT is applied along a given angle, θ, using a vector magnet [64] and then relaxed to zero, before measuring the two-point resistance of the contact at remanence. This process is then repeated for incrementally changing angles until the desired angular range is probed.…”