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

Abstract

The longitudinal and transverse magnetoresistance curves MR(H) and the magnetization isotherms M(H) were measured at T = 3 K and 300 K up to high magnetic fields for a microcrystalline (µc) Ni foil with grain sizes above 1 µm (corresponding to bulk Ni) and for a nanocrystalline (nc) Ni foil with an average grain size of about 100 nm. At T = 3 K, the field-induced resistivity change was quite different for the two microstructural states of Ni and the evolution of resistivity with magnetic field was also different which could be explained as arising from their very different electron mean free paths. At T = 300 K, the MR(H) curves of both the μc-Ni and nc-Ni samples were very similar to those known for bulk Ni. The MR(H) data were analyzed at both temperatures with the help of Kohler plots from which the resistivity anisotropy splitting (ΔρAMR) and the anisotropic magnetoresistance (AMR) ratio were derived, the latter values being very similar at both temperatures and for both microstructural states of Ni metal. The present high-precision MR(H) data revealed that the available theoretical models do not accurately describe the suppression of thermally induced magnetic disorder at around room temperature in high magnetic fields.

Spacer Layer Thickness Dependence of the Giant Magnetoresistance in Electrodeposited Ni-Co/Cu Multilayers

“…This picture can be well followed in Figure 12 by looking at the MR( H ) curves with increasingly smaller spacer layer thicknesses. At t Cu = 0.5 nm and 1 nm, we can even observe that the LMR( H ) curves start to increase first for very low magnetic fields as is typical for bulk FM metals [ 5 , 38 , 58 , 73 , 74 , 77 ]. This implies that, in these cases, the spacer layer is certainly discontinuous to the extent that, in a rather large volume fraction of the multilayer, the adjacent magnetic layers coalesce with each other, thus forming large contiguous ferromagnetic regions that exhibit the AMR behavior typical for bulk ferromagnets.…”

“…As the same domain magnetizations can be split into components parallel (LMR) and perpendicular (TMR) to the magnetic field direction (not to the magnetic field orientation), the TMR maximum, H p (TMR), should appear at the same magnetic field as the LMR minimum. This was demonstrated by the MR ( H ) data of a nanocrystalline Ni sample [ 77 ], in which the accuracy of the measured resistivity data and the sharp peak of the MR ( H ) curves enabled us to observe the validity of the relation H p (LMR) = H p (TMR).…”

“…The MR( H ) curve of the multilayer with t Cu = 1.72 nm reveals a typical GMR multilayer behavior in that (i) both the LMR and TMR components are negative for the whole range of the magnetic fields investigated [ 28 ]; (ii) the magnitude of TMR is larger than that of LMR due to the bulk AMR effect of the magnetic layers [ 5 , 73 , 74 ]; and (iii) a rapid saturation of the magnetoresistance at around 2.5 kOe magnetic field, which is followed by an approximately linear, slight decrease of the resistivity due to the progressive suppression of the magnetization fluctuations (spin waves) with increasing magnetic field [ 75 , 76 , 77 ]. Therefore, the Ni-Co/Cu multilayer with t Cu = 1.72 nm (and all the other multilayers with t Cu > 1.72 nm) can be considered to contain magnetic layers with fully ferromagnetic behavior without any noticeable SPM character, i.e., the observed GMR is contributed to by the GMR FM term only.…”

“…For the quantitative analysis of the MR( H ) curves, we will proceed as follows: by fitting a straight line for the MR( H ) data above the saturation field (which was about 2.5 kOe for multilayers with t Cu > 1.5 nm and about 4 kOe for thinner spacer layers), we extrapolate the measured magnetoresistance data to H = 0. This procedure yields the LMR s and TMR s values where the subscript s refers to the fact that the extrapolation was made from the magnetically saturated region [ 58 , 77 , 78 ]. The anisotropic magnetoresistance ratio is obtained as AMR = LMR s − TMR s [ 5 , 77 , 78 ].…”

“…This procedure yields the LMR s and TMR s values where the subscript s refers to the fact that the extrapolation was made from the magnetically saturated region [ 58 , 77 , 78 ]. The anisotropic magnetoresistance ratio is obtained as AMR = LMR s − TMR s [ 5 , 77 , 78 ]. In analogy with the isotropic contribution to the zero-field resistivity of ferromagnets [ 5 , 78 ], the isotropic GMR can be obtained as GMR = (LMR s + TMR s )/3 (we will take the absolute value of the resistivity change, i.e., we will have GMR > 0).…”