Elsevier

Physics Letters A

Volume 378, Issue 3, 10 January 2014, Pages 276-279
Physics Letters A

Magnetic properties of Ni/Au core/shell studied by Monte Carlo simulations

https://doi.org/10.1016/j.physleta.2013.11.012Get rights and content

Highlights

  • Ni/Au core/shell has been studied using Monte Carlo simulations.

  • Blocking temperature of ferromagnetic Ni/Au core/shell is given.

  • Magnetic hysteresis cycle of ferromagnetic Ni/Au core/shell is obtained.

Abstract

The magnetic properties of ferromagnetic Ni/Au core/shell have been studied using Monte Carlo simulations within the Ising model framework. The considered Hamiltonian includes the exchange interactions between Ni–Ni, Au–Au and Ni–Au and the external magnetic field. The thermal total magnetizations and total magnetic susceptibilities of core/shell Ni/Au are computed. The critical temperature is deduced. The exchange interaction between Ni and Au atoms is obtained. In addition, the total magnetizations versus the external magnetic field and crystal filed for different temperature are also established.

Introduction

Metallic nanoparticles (NPs) or clusters, aggregates of several to millions atoms, have been emerging as a new type of important functional material [1]. They exhibit distinct properties (optical, electronic, magnetic, and so on) from those of individual atoms or their bulk counterparts due to the quantum-size and surface effects. More importantly, the properties are usually size- and structure-dependent, which have attracted tremendous research attention from either basic science or application viewpoints during the last two decades [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. The preparation of some magnetic metal nanoparticles such as iron, and nickel is relatively more difficult because they are prone to be oxidation [15]. However, although various kinds of core/shell nanoparticles have been prepared through a variety of methods, the investigation about how to control the core diameter and the shell thickness is very limited. Furthermore, because the core diameter and shell thickness of the core/shell nanoparticles has a great effect on the magnetic [4] properties, and so on, to control the dimensions of the core and shell has a significant meaning. The gold loading is very low for Au-covered Ni core/shell and a synergistic effect exits between Au and Ni in the core/shell [16], [17]. On the other hand, the Monte Carlo simulations are carried out to investigate the magnetic properties of nano-systems [18], [19], [20]. The magnetic properties of Ni/Au core/shell are studied by [21], [22]. The synthesis and super-paramagnetic properties of neodymium ferrites nanorods are established by [23]. Recently developed experimental and theoretical studies [24], [25], [26], [27] have shown that clusters of several bimetallic systems, such as AuAg, AgNi, AgPd, AgCu, present preferentially core–shell structures.

In this study, we are interested in the magnetic properties of core/shell Ni/Au (see Fig. 1) with a fixed size NAu=6 and NNi=8 sites. The thermal total magnetizations and total magnetic susceptibilities are given in Fig. 2 for a zero crystal field Δ=0 and a zero external magnetic field h=0 of Au/Ni core/shell. Total magnetizations versus the external magnetic field of Au/Ni core/shell for Δ=0 and for different temperatures are given in Figs. 3(a) and 3(b). Figs. 4(a) and 4(b) show the variation of the total magnetization versus the exchange interactions (JNiAu). The total magnetizations versus the crystal filed (Δ) for different values of the temperature of Au/Ni core/shell is given in Fig. 5 for different temperature.

Section snippets

Theoretical model

The studied system is described by the Hamiltonian of the Ising model including nearest neighbors interactions and external magnetic field:H=JNiNii,jSiSjJAuAui,jσiσjJNiAui,jSiσjh(iσi+iSi)Δ(iσi2+iSi2), where i,j stand for the first nearest neighbor sites i and j. JNiNi, JAuAu and JNiAu, are the first exchange interaction between the atoms: Ni–Ni, Au–Au and Ni–Au, respectively. The external magnetic field h is applied over all the spins of the Ni/Au in the z-direction

Monte Carlo simulations

We apply a standard sampling method to simulate the Hamiltonian given by Eq. (1). Cyclic boundary conditions on the Ni/Au core/shell lattice were imposed and the configurations were generated by sequentially traversing the lattice and making single-spin flip attempts. The flips are accepted or rejected according to a heat-bath algorithm under the Metropolis approximation (see Fig. 6 – flow chart of Monte Carlo simulations).

Cyclic conditions on the Au/Ni core/shell are used for a fixed

Results and discussions

We presented in Fig. 2 the thermal total magnetizations and total magnetic susceptibilities for a zero crystal field Δ=0 and a zero external magnetic field h=0 of Au/Ni core/shell. The sharp maximum in the total magnetic susceptibilities curves indicated a clear blocking behavior of the samples. The critical temperature increases from 6.28 to 15.88 K. The last value is comparable with those given by Refs. [15], [29], [30] (TC=16K [29]). This increasing indicating the coating enhanced the

Conclusion

The magnetic properties of ferromagnetic Au/Ni core/shell have been investigated using Monte Carlo simulations within the Ising model framework. The critical temperature of Au/Ni core/shell is obtained and comparable with those obtained by experiment results. The magnetic moment decreases when the temperature and the exchange interaction (JNiAu) decrease. This reduction in moment might stem from the small size effect, the oxidation of the Ni cores, the increased surface of the Au/Ni

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