Magnetic properties of Exchange-spring DyFe 2 /YFe 2 Superlattices by Monte Carlo Simulations
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Magnetic properties of Exchange-spring DyFe2/YFe2 Superlattices by Monte Carlo Simulations Pierre-Emmanuel Berche1, Saoussen Djedai1,2 and Etienne Talbot1 1
Groupe de Physique des Matériaux - UMR CNRS 6634, Université de Rouen BP12, 76801 Saint-Etienne du Rouvray, France
2
Laboratoire de Physique Appliquée et Théorique, Université de Tébessa, BP 12002, Tébessa, Algérie
ABSTRACT Monte Carlo simulations are used to perform an atomic scale modelling of the magnetic properties of epitaxial exchange-coupled DyFe2/YFe2 superlattices. These samples, extremely well-researched experimentally, are constituted by a hard ferrimagnet DyFe2 and a soft ferrimagnet YFe2 antiferromagnetically coupled. Depending on the layers and on the temperature, the field dependence of the magnetization depth profile is complex. In this work, we reproduce by Monte Carlo simulations hysteresis loops for the net and compound-specific magnetizations at different temperatures, and assess the quality of the results by a direct comparison to experimental hysteresis loops.
INTRODUCTION The study of exchange-coupled systems has focused an intense research activity for several years since they may exhibit interesting properties as spring magnet behavior [1]. Their potential application may concern permanent magnets [2] or data storage media [3,4] for example. These applications need a good understanding of the magnetization switching in exchange-spring structures for which the multilayer films of Laves phase REFe2 (RE = rare earth) grown by molecular beam epitaxy are very good system models [5]. DyFe2/YFe2 superlattices are heterostructures that consist of hard (DyFe2) and soft (YFe2) ferrimagnetic phases. The hard magnetic phase tends to resist to magnetization reversal in high fields owing to its high anisotropy while the soft phase possesses large magnetization. There are three magnetic interactions that couple the moments of the rare earth and transition metal sites [6,7]. For DyFe2, these are Fe-Fe interactions JFe-Fe, Dy-Fe interactions JDy-Fe and Dy-Dy interactions JDy-Dy with
J Fe − Fe >> − J Dy − Fe > J Dy − Dy .
In the case of YFe2 which is also ferrimagnetic but with a smaller induced moment essentially due to iron, its global magnetization is antiparallel to the one of DyFe2 as qualitatively shown in the figure 1(a). The simplest scenario used to describe the magnetization reversal in such a hard/soft coupled system is usually the formation of exchange springs in the soft material (figure 1(b)), before the irreversible switch of the hard material [1].
(1)
(a) B = 0 Dy
(b) B ≠ 0 Fe
Dy
m~4.3 µB /f.u
Fe
DyFe2 YFe2
m~2.8 µB /f.u YY
Fe Fe
YY
Fe Fe
Y
Fe
Y
Fe
DyFe2 YFe2
Figure 1. Schematic view of the magnetization order in DyFe2/YFe2 superlattices ((a) fundamental state without magnetic field; (b) stable state with an applied magnetic field). Two bilayers are shown in each case. However, some experimental studies have shown that this very simple description of the magnetization reversal is not valid in some circumstances of
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