Magnetic reversal dynamics of NiFe-based artificial spin ice: Effect of Nb layer in normal and superconducting state

摘要

Square arrays of artificial spin ice (ASI) constituting weakly interacting NiFe nano-islands, with length ∼312 nm, width ∼125 nm, thickness ∼20 nm, and lattice constant ∼570 nm, were fabricated on Nb thin film and on thermally grown 300 nm SiO_2 on silicon. Detailed investigations of magnetic force microscopy (MFM) at room temperature, and magnetization M(H) loops and relaxation of remanent magnetization (M_r) at various temperatures were carried out in two in-plane field geometries, namely, parallel ("P"-parallel to the square lattice) and diagonal ("D"- 45° to the square lattice). The magnetic response of the ASI samples shows striking difference for insulating (SiO_2), metallic (Nb, T > 6.6 K) and superconducting (Nb, T < 6.6 K) bases, and the field geometry. For instance, with the Nb base in the normal metallic state (T > 6.6 K), (1) in "P" geometry the M(H) loops are found to be more "S" shaped in comparison with that for SiO_2 base; (2) the ratio of magnetic vertex population of Type II to Type III vertices extracted from MFM studies in "P"("D") geometry is ∼1:1.1(1.2:1) that changed for the SiO_2 base to ∼2.1:1 (4: 1). However, the NiFe-ASI on both metallic Nb and SiO_2 bases exhibit a highly athermal decay of magnetization, and the % change in M_r in about two hours at T = 10 K (300 K) lies in a range of ∼1.07-1.80 (0.25-0.62). With Nb base in superconducting state (T < 6.6 K), the M(H) loops not only look radically different from those with SiO_2 and metallic Nb as bases but also show significant difference in "P" and "D" geometries. These results are discussed in terms of inter-island magnetostatic energy as influenced by field geometry, presence of metallic Nb base and competing vortex pinning energy of superconducting Nb base.