Magnetotransport and magnetization reversal of electrodeposited multilayer nanowires

Electrodeposited magnetic multilayer nanowires are ideal materials to study nanoscale magnetism and the giant magnetoresistance (GMR) in the current-perpendicular-to-plane (CPP) geometry. This is because the diameter of each nanowire is uniform, the surface of the nanowire is smooth, and the thickness of both the magnetic and non-magnetic layers can be varied to either larger or smaller than the spin diffusion length which is an important parameter in magnetotransport study. In addition, the aspect ratio (layer-thickness/diameter) that is related to shape anisotropy can be varied for magnetization reversal study. There has been little understanding in the magnetization reversal mechanism of multilayer nanowires, which is complicated due to the dipolar interactions between magnetic layers in each nanowire and between nanowires. The objective of this work is to study the magnetization reversal mechanism of multilayer nanowires using a vibrating sample magnetometer (VSM), where various dipolar interactions are taken into account. Although multilayer nanowires are ideal for the study of the CPP-GMR effect, there remains technical difficulty in making an electrical contact with individual nanowires for the CPP-GMR measurements. In this work, a point-contact method using a conductive plunger tip was developed in-house, that enabled us to measure the CPP-GMR of selected multilayer nanowires in an array of vertically aligned nanowires in each sample. To examine the CPP-GMR and compare the results with theoretical models, the CPP-GMR data were systematically obtained from samples with various magnetic and non-magnetic layer thicknesses.; It was found from VSM measurement that the magnetization reversal mode in electrodeposited CoNi/Cu multilayer nanowires depends on the shape and thickness of the CoNi layers where the mode in rod-shaped thick CoNi layers is different from that in disk-shaped thin CoNi layers. The reversal mode in coherent rotation or curling was determined by measuring the magnetic hysteresis loops under various directions of applied magnetic field, particularly comparing the measured coercivity data with those obtained from theoretical models. It was found that there is a transition for the magnetization reversal mode in rod-shaped CoNi layers from coherent rotation to curling with increasing angle between the field direction and the nanowires, while the reversal mode is of coherent rotation type for disk-shaped CoNi layers. When the thickness of the CoNi layers decreases to very thin regime, the layers eventually become discontinuous and form discrete islands that exhibit superparamagnetism.; The layer thickness dependence of CPP-GMR was measured for CoNi/Cu multilayer nanowires and compared with the Valet-Fert model. There are both agreement and deviation between the experimental results and theoretical prediction. When both the Cu layer thickness tCu and CoNi layer thickness tCoNi are much smaller than the spin diffusion length l of the corresponding material, i.e. tCu<< lCu and tCoNi<< lCoNi, ( DeltaR/RAP)-1/2 varies linearly with tCu, where RAP and RP are the maximum resistance attained in the GMR measurement and the resistance at saturation under higher fields, and DeltaR is the difference between RAP and RP. The obtained result is in agreement with the Valet-Fert (V-F) model. It was also found that when t Cu<< lCu and 170 nm>> tCoNi>>lCoNi, (DeltaR/R P)-1 is proportional to tCoNi , in agreement with the V-F model. Analysis of the CPP-GMR parameters shows that the interfacial spin asymmetry coefficient gamma is much larger than the bulk spin asymmetry coefficient beta, indicating that the interfacial scattering is important for the CPP-GMR. However, there is a discrepancy between the experimental data and the V-F model when tCoNi is very small, probably due to formation of thin and discontinuous magnetic layers that exhibit superparamagnetism. There is also a disagreement between the exper...