Correlations between magnetic switching volume and magnetic and nanostructural properties of CoSm//Cr films (abstract)

Abstract

CoSm films with a Cr underlayer (CoSm//Cr) are promising as future high density recording media.1 It was pointed out that the grain size in media affects the media noise and thermal stability significantly. However, if small grains are exchange coupled to form larger “magnetic grains,” the size of these magnetic clusters will be important in noise considerations. Our previous works23 indicated that for CoSm film with Cr underlayer the magnetic grain size is much larger than the 50 Å nanocrystallites and is about the same size as the Cr grain of ∼240 Å in dimension. In the present work, we report our systematic studies of magnetic switching volume and its correlation with magnetic and nanostructural properties for CoSm films. Switching volumes V* were measured with the “field sweep-rate dependence of coercivity Hc,” interactions among the magnetic grains were estimated with the so-called ΔM method, and the nanostructures were investigated with atomic force microscopy and transmission electron microscopy. The thickness of CoSm layers, which were sputtered in an Ar pressure PAr=25 mT, is 300 Å for all samples. It is found that as the Cr underlayer thickness dCr increases from 0 Å (without a Cr underlayer) to 200 Å, the V* and ΔMmax decrease (Hc increases) rapidly, i.e., from V* from 9.3×10−18 to 5.7×10−18 cm3, ΔMmax from 1.3 to 0.5 (Hc from 1.4 to 2.3 kOe), respectively, and then gradually as dCr approaches 800 Å. As Ar pressure PAr of the deposition of the Cr underlayer varies from 5 to 30 mT, the V* and ΔMmax have their minima (anisotropy Ku has its maximum) at PAr≃9.5 mT. V* shows minima at dCr and PAr values where Hc and Ku have their maxima and ΔMmax has a small value. This behavior can be interpreted with a thermal activation model including interactions between magnetic grains. The relationship between crystallite grain size, magnetic grain V*, and media noise will be discussed.