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P. Schio Departamento de Física, Universidade Federal de São Carlos, 13565-905 São Carlos, SP, Brazil

J.T. Matsushima Laboratório Associado de Sensores e Materiais, Instituto Nacional de Pesquisas Espaciais, CEP 12227-010, São José dos Campos, SP, Brazil

E.C. Pereira Departamento de Química, Universidade Federal de São Carlos, 13565-905 São Carlos, SP, Brazil

A.J.A. de Oliveira Departamento de Física, Universidade Federal de São Carlos, 13565-905 São Carlos, SP, Brazil

D.H. Moscaa) Departamento de Física, Universidade Federal do Paraná, 81531-990, Curitiba, PR, Brazil (Received 9 August 2015; accepted 13 April 2016)

Monte Carlo (MC) simulations of the magnetization states of disordered self-assembled arrays of particles consisting of Co87Cu13 alloy are investigated. The assemblies of magnetic particles with ellipsoidal shapes and volumes ranging from 5 to 50 lm3 exhibit densities of about 3
106 particles per mm2. Magnetization was obtained in the framework of Stoner–Wohlfarth model extended to include phenomenological contributions of second-order magnetic anisotropy and coercivity mechanism with distinct configuration of easy axes of magnetization. MC simulations for assemblies containing no more than 100 particles with negligible magnetic interaction between each other and exhibiting saturation magnetization and magnetic anisotropy constant values close to those found for cobalt in bulk are in good agreement with experimental results. We evaluate and validate our computational modeling using samples having particles with different sizes and different angular distributions of the easy axis of magnetization. A simple numerical approach with minimum of parameters was used to take into account the coercive fields of the samples. Reasonable simulation results are generated based on realistic size distributions and angular distributions of easy axis of magnetization. PACS numbers: 75.30.Gw,75.60-d,75.70-i I. INTRODUCTION

Systems of magnetic particles in the micro and nanometric size range exhibit a wide variety of experimental behavior1,2 with important technological uses mainly in the magnetic data storage technologies3–5 and in medicine.6 The Stoner–Wohlfarth (SW) model7 describes the major hysteresis loop for an isotropic system of randomly oriented and identical single-particles, but it is mostly used to study dilute noninteracting systems of randomly aligned single-domain particles presenting uniaxial Contributing Editor: Yang-T. Cheng a) Address all correspondence to this author. e-mail: mosca@fisica.ufpr.br b) Present address: Instituto de Física, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500 – Caixa Postal 15051 – CEP 91501-970 – Porto Alegre, RS, Brazil. DOI: 10.1557/jmr.2016.173

anisotropy. Only uniaxial anisotropy and the Zeeman energy are considered, resulting in a coherent magnetization rotation mechanism. SW model is also a reasonable first-order approach to describe the magnetization curves of polycrystalline materials consisting of an assembly of small noninteracting particles