Out-of-Plane and In-Plane Magnetization Behavior of Dipolar Interacting FeNi Nanoislands around the Percolation Threshold

Abstract

Magnetic properties of inhomogeneous nanoisland FeNi films were studied by SQUID magnetometry. The FeNi films with nominal thickness ranging from 0.6 to 2.0 nm were deposited by rf sputtering on Sitall glass substrates and covered by a protecting Al2O3 layer on the top. The SQUID data indicate pronounced irreversibility behavior for the out-of-plane temperature-dependent magnetization response (measured at H≃100 Oe) using zero-field cooling (ZFC) and field-cooled warming (FCW) after the applied dc magnetizing field Hm≃2 T for the FeNi samples with nominal thickness 1.1 nm ≲d≲1.8 nm, below the percolation threshold. The positive difference between the FCW and ZFC data identifies two irreversibility temperature scales, TB≈50 K and T⁎≈200 K, which can be associated with the superparamagnetic and superferromagnetic behavior in inhomogeneous nanoisland FeNi films, respectively. However, above the film percolation threshold, we observed a crossover from the out-of-plane to in-plane magnetization orientation. Here, the in-plane FCW-ZFC difference implies negative remanent magnetization response in the temperature range TB≲T≲T⁎. The observed magnetization properties can be associated with the presence of the superferromagnetic phase in self-assembled clusters of quasi-2D metallic magnetic FeNi nanoislands.

Collective magnetic response of inhomogeneous nanoisland FeNi films around the percolation transition

“…Beyond conventional spin-magnetism of Fe 21 Ni 79 permalloy, which is known to saturate at low magnetic fields, we discovered that the out-of-plane magnetization response of the discontinuous FeNi films exhibits paramagnetic-like behavior in the range of investigated magnetic fields up to 7 T. The anomalous out-of-plane magnetization response quickly decreases on approaching the film percolation threshold due to the nanoisland coalescence. For the same nominal thickness range of the inhomogeneous nanoisland FeNi films on the Sitall (TiO 2 ) substrate, the out-of-plane SFM magnetization behavior was found in our recent study (Stupakov et al 2016). Moreover, the high-field magnetization response exhibits remarkable anisotropy for the studied nanoisland FeNi film samples, where the in-plane response is significantly suppressed.…”

“…In addition, the temperature dependence of their dc conductivity suggests the existence of a percolation transition for this critical FeNi film thickness (Sherstnev 2014;Boltaev et al 2017). Below the percolation transition at the critical thickness d c ≃ 1.8 nm, the FeNi layer has a discontinuous structure (Stupakov et al 2016), where lateral sizes of the nanoislands are 5-30 nm, and the distance between them is less than 5 nm (as schematically illustrated by Fig. 1).…”

“…Transitions into a superferromagnetic (SFM) phase were discovered in two-dimensional (2D) self-assembled (Kleemann et al 2001;Rancourt and Daniels 1984;Miu et al 2015;Stupakov et al 2016) or regularly structured NP arrays (Russier 2001;Sugawara et al 1997;Cowburn et al 1999). The collective states are characterised by the observed magnetic moment hysteretic-like irreversibility behaviour, induced by an external magnetizing field, which persists essentially above the blocking temperature T B .…”

“…Here, by using SQUID magnetometry, we study high-field (up to 7 T) low-temperature (T ≃ 10 K) magnetization response of the inhomogeneous nanoisland FeNi film samples with the nominal film thickness varying from 0.6 to 2.5 nm, across the physical percolation threshold at the d c ≃ 1.8 nm (Stupakov et al 2016;Sherstnev 2014;Boltaev 2017), in the planar and perpendicular geometry of an applied dc magnetic field. The nanoisland FeNi films were grown by rf sputtering deposition onto Sitall glass substrates.…”

“…4(a,b)). The substrate sample showed unsaturated, Langevin-like, magnetization field dependence, which can be associated with the presence of magnetic impurities and/or defects pertaining the used Sitall glass material (Kovaleva et al 2016). We fitted the in-plane substrate magnetization with the Langevin function, Fig.…”