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 ∆ρAMR 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.
Highlights
The MR(H) curves were measured to determine the anisotropic magnetoresistance (AMR) ratio and the results were analyzed together with corresponding data measured on bulk Ni metal samples
The magnitude of the AMR ratio was found to be in the range from about 2 to 2.5% for both bulk Ni and nc-Ni and did not show a systematic dependence on the grain size
The field-induced resistivity anisotropy splitting ρAMR in the magnetically saturated state was found to be proportional to the zero-field resistivity of the same samples with different grain sizes
Summary
Since the ferromagnetic elements Fe, Co and Ni as well as their alloys with each other often appear as basic ingredients of these magnetic nanostructures, it is important to have a reliable detailed knowledge of their electrical transport and magnetic properties since it appears from the literature that available data are not always satisfactorily accurate This justifies revisiting, e.g., the zero-field electrical resistivity and the magnetoresistance (MR) characteristics of these ferromagnets both in the bulk state and in their various nanostructured forms. For the electrodeposited Ni foils, the grain size could be varied by the deposition parameters between about 30 and 100 nm, and the possible influence of grain size on the electrical transport parameters could be investigated This is of interest since in the nanocrystalline state, the presence of a large amount of lattice defects such as grain boundaries represent excess scattering centers for the conduction electrons carrying the current and this leads to an increase of the zero-field resistivity [5].
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