Sahu et. al. Evolution of structure and magnetization dynamics of ferromagnetic layers in multilayer structures Siddharth S. Sahu, 1 Vantari Siva, 1 P. C. Pradhan, 2 M. Nayak, 2 K. Senapati, 1 and Pratap K. Sahoo 1, a) 1) School of Physical Sciences, National Institute of Science Education and Research, HBNI, Jatni - 752050, India. 2) Indus Synchrotrons Utilization Division, Raja Ramanna Centre for Advanced Technology, Indore 452013, India (Dated: 10 July 2022) We report a study of the structural and magnetic behavior of the topmost magnetic layer in a ferromagnet-nonmagnet (Co -Au) multilayer system. Glancing angle X-ray diffraction measurements performed on a series of multilayers showed a gradually increasing degree of amorphization of the topmost magnetic layer with increasing number of bilayers. Concur- rently, the magnetic hardness and magneto-crystalline anisotropy of the top Co layer were found to decrease, as observed by magneto-optical Kerr effect measurements. This magnetic softening has been discussed in the light of Herzer’s random anisotropy model. Micromagnetic simulations of the multilayer system also corroborated these observations. Keywords: Magnetic thin films, Amorphization, Hard and soft magnet, Anisotropy, MOKE I. INTRODUCTION Ferromagnet-nonmagnet multilayers have been exten- sively studied for giant magnetoresistance (GMR) 1 , data storage 2 , exchange interactions 3 and several other appli- cations. The magnetic properties of such multilayers are dependent on competition between magneto-crystalline anisotropy and exchange interaction that dominate at higher and lower grain sizes respectively. As the av- erage grain size reduces and approaches the exchange length with increase in the degree of amorphization, ex- change interaction starts to dominate and leads to mag- netic softening. Soft magnetism, in general, is observed in alloys 4 , nanocrystalline structures 5 and composites 6 . Films grown by physical vapor deposition (PVD) tech- niques like e-beam evaporation generally involve lower temperatures and leads smoother interfaces in the de- posited films as compared to chemical vapor deposition (CVD), which is advantageous for growing multilayers of ferromagnet-nonmagnet systems. Among the strong fer- romagnetic elements, Co is particularly interesting as it has the highest Curie temperature 7 . It is known that Co has two stable phases, fcc and hcp, among which hcp phase shows large magneto-crystalline anisotropy 8 . Both theoretical 9–11 and experimental 12–14 studies on mag- netic multilayers usually proceed with the assumption that the structural and magnetic properties are uniform for all the layers. However, the fact that the top and bot- tom magnetic layers in a magnetic multilayer system have only one nearest magnetic layer as opposed to the inter- mediate layers which have two nearest magnetic layers (at the top and bottom) is expected to induce an inter-layer non-uniformity in magnetic properties and consequently, structural properties. It may be mentioned here that Co based thin films usu- ally undergo amorphization by ion irradiation 15–17 and solid state reactions 18,19 because of the high degree of ho- a) Electronic mail: pratap.sahoo@niser.ac.in mogeneity that these methods afford although they can introduce impurities. In our present study, we found a systematic deviation of the degree of amorphization, co- ercivity and magneto-crystalline anisotropy of the top- most Co layer from the rest of the Co layers in Co/Au multilayer films. Our study shows that the same can be accomplished for the top Co layer without the involving any method that would introduce additional impurities, which may be useful for fundamental studies 20 . The bi- layers of Co and Au were deposited using e-beam evap- oration and the structural changes in the top Co layer were observed when the number of bilayers was increased. Concurrently, there is also decrease in the coercivity due to which, the film slowly changes from magnetically hard to magnetically soft. II. EXPERIMENTAL DETAILS Si substrates were cleaned in acetone and then in iso- propyl alcohol to remove surface impurities and 1, 2, 5 and 10 bilayers of Co and Au thin films were deposited on them using e-beam evaporation technique. A high vacuum chamber was used for this purpose with a base pressure below 1×10 -7 mbar. The thickness of each layer was ∼5 nm, as shown by quartz crystal monitor (QCM). The rates of deposition, 0.6 nm/min and 0.4 nm/min for Co and Au respectively, were maintained throughout the deposition for all samples. The information about thickness and roughness of the films was obtained using X-ray reflectivity (XRR) and its fitting was performed using GenX software 21 which makes use of Parrat’s re- cursive method. The structural properties of Co and Au were investigated using glancing angle X-ray diffraction (GAXRD), the angle of incidence being 0.5 o . Magnetic measurements were performed by longitudinal magneto optic Kerr effect (MOKE) at room temperature. arXiv:1608.05568v4 [cond-mat.mtrl-sci] 13 Feb 2017