Multilayer Co/Pd films with nanocrystalline and amorphous Co layers: Coercive force, random anisotropy, and exchange coupling of grains

Abstract

The magnetic structure of nanocrystalline and amorphous ferromagnets is determined by their random magnetic anisotropy (RMA) and results from a competition of the ordering action of exchange interactions and the disordering effect of the random local anisotropy K breaking the long-range ferromagnetic order [1]. In such materials, the ferromagnetic order is established at a characteristic length R f = R c ( H ex / D 1/2 H a ) 2 , where R c is the random anisotropy correlation radius, H ex is the exchange field, D is a numerical coefficient of the symmetry (equal to 1/15 in the case of a uniaxial anisotropy), and M s is the saturation magnetization. The resulting magnetic structure can be described in terms of the ensemble of weakly coupled magnetic blocks [1, 2] with an average dimension of 2 R f , the average block anisotropy 〈 K 〉 = K / N 1/2 = K ( R c / R f ) 3/2 , and randomly oriented unit anisotropy vectors n . To the present, researchers engaged in the magnetic properties of such materials believed that this type of magnetic structure is just what accounts for the observed values of characteristics of the magnetically soft amorphous and nanocrystalline alloys [2‐4]. It should be noted that an analysis of the behavior of the coercive force H c within the framework of the RMA model used in [2‐4] is directed mostly to the study of H c as a function of the nanograin size, H c ( R c ), thereby assuming that the values of anisotropy K ( R c ), exchange coupling A ( R c ), and saturation magnetization M s ( R c ) are constant (or their changes can be ignored). However, now there are prerequisites for undertaking complex measurements of all the parameters entering into theoretical expressions for H c ( R c , H a , A , and M s ) and revealing the corresponding dependences. In particular, the RMA and magnetic block structure can be studied using small-angle neutron diffraction [5‐7] and the measurements of magnetization curves in the region of magnetizations close to saturation [8‐10]. The exchange coupling constants are widely determined using methods based on the spin- wave resonance (SWR) [11] and Bloch’s T 3/2 law [12]. The results of our preliminary investigation of the magnetization behavior M ( H ) in multilayer Co/Pd film structures [9] showed that these films are characterized by a two-dimensional inhomogeneity of the magnetic anisotropy. In particular, it was found that the experimental curve of M ( H ) measured in the range of field strengths from 5 to 25 kOe can be described by the expression