Room-temperature magnetoresistance of nanocrystalline Ni metal with various grain sizes

Abstract

The room-temperature magnetoresistance (MR) characteristics of nanocrystalline (nc) Ni metal with various grain sizes (between 30 and 100 nm) are investigated in this work for the first time. The nc-Ni foils were produced by electrodeposition and the results are compared with data measured on coarse-grained (bulk) pure Ni metal samples prepared by cold-rolling and annealing. The MR(H) curves measured in magnetic fields up to H 9 kOe are analyzed in detail to determine the anisotropic magnetoresistance (AMR) ratio. The magnitude of the AMR ratio was found to be around 2.5% for bulk Ni and in the range from about 2 to 2.5% for the nc-Ni samples, the latter data not exhibiting a systematic dependence on the grain size. On the other hand, the field-induced resistivity anisotropy splitting ρ AMR in the magnetically saturated state of the nc-Ni series was found to be proportional to the zero-field resistivity of the same samples with different grain sizes. The slope of this proportionality relation provided an AMR ratio of 2.4% for all nc-Ni samples, matching well the value for the bulk Ni samples. Thus, the AMR ratio for polycrystalline Ni metal seems to be fairly independent of the microstructural features. This also means that the AMR ratio is an inherent characteristic of the Ni metal matrix and it remains the same even if the matrix resistivity changes (e.g., by introducing grain boundaries) without noticeably modifying the electronic density of states at least in the vicinity of the Fermi level.

High-field magnetoresistance of microcrystalline and nanocrystalline Ni metal at 3 K and 300 K

“…The zero-field electrical resistivity has already been extensively studied for nc-Ni [18][19][20][21][22][23]. The investigation of the field dependence of the resistivity of nc-Ni has received much less attention [22,24]. In the detailed work by Madduri and Kaul [22], the field dependence was measured in high magnetic fields (up to H 90 kOe) in a wide temperature range (from T 1.8 K to 300 K) in a configuration where the measuring current and the applied magnetic field were perpendicular to each other.…”

“…In the detailed work by Madduri and Kaul [22], the field dependence was measured in high magnetic fields (up to H 90 kOe) in a wide temperature range (from T 1.8 K to 300 K) in a configuration where the measuring current and the applied magnetic field were perpendicular to each other. In a recent paper [24], we have reported and analyzed room-temperature MR data on a series of nc-Ni samples with various grain sizes in a limited magnetic field range (up to H 9 kOe) with the magnetic field and current either parallel (longitudinal MR LMR) or perpendicular (transverse MR TMR) to each other. The zero-field values of the resistivity were determined by extrapolation to H 0 from the magnetically saturated (monodomain) state for both the LMR and TMR components (ρ Ls and ρ Ts , respectively) in a manner as described in Ref.…”

“…25. From these data [24], we have then derived parameters characterizing the AMR of ferromagnetic metals [4][5][6]25] by determining the resistivity anisotropy splitting ρ AMR ρ Ls − ρ Ts and the anisotropic magnetoresistance ratio defined as AMR ρ AMR /ρ is . The normalizing factor in the latter expression is the isotropic resistivity ρ is (1/3)ρ Ls + (2/3)ρ Ts , the quantity ρ is being usually very close to the resistivity ρ 0 measured in the absence of a magnetic field in the demagnetized state of a ferromagnetic specimen [4].…”

“…As an extension of our previous room-temperature study in a limited magnetic field range [24], the major purpose of the present work was to investigate the MR behavior of a nc-Ni and a bulk Ni sample at both low temperature (3 K) and high temperature (300 K) up to high magnetic fields. It was expected from this comparison to reveal the significance of the influence of a large density of grain boundaries on the MR characteristics.…”

“…In addition, Appendix A will be devoted to a discussion of the field dependence of the resistivity in the magnetically saturated (monodomain) state of Ni metal at T 300 K in view of previous theoretical considerations [22,34]. An analysis of the correlation between ρ AMR and ρ o , by using also the current low-temperature data, will be performed in Appendix B similarly to our recent work on a series of nc-Ni samples with various grain sizes [24].…”