Magnetic interactions and properties of 3d-5d/4d nano-particles: exchange interactions mediated by non-magnetic metallic atoms – Oleg N. Mryasov

Date: Mon. April 5th, 2004, 12:30 pm-1:30 pm
Location: Rockefeller 221

The search for technological solutions for ultra high density magnetic storage devices requires to achieve thermal stability and higher signal to noise ratio for dramatically decreasing media grain size and geometrical dimensions of the field sensing elements. In this work we consider a few specific examples where it is necessary to consider atomic scale effects and correspondingly to achieve a quantitative level of understanding magnetic interactions as possible future design factors for ultra-high density magnetic storage devices. One of the possible and most researched options to advance in the magnetic recording media is to use large magneto-crystalline anisotropy (MCA) materials, possibly in their nano-particulate form. One of the technological challenges in utilizing these materials is to solve the write-ability problem using the heat-assisted magnetic recording (HAMR) approach. We present results of modeling motivated by the advances in preparation and characterization of exchange decoupled nano-particles of L₁₀ FePt, CoPt alloys and research on the HAMR recording process.

We start from the microscopic (first principles electronic structure calculations) model of magnetic interactions which enables atomic scale statistical modeling of magnetic properties of small FePt particles. We find that in these layered ferro-magnets isotropic exchange interaction parameters are strongly affected by the fact that the 5d sites magnetism is entirely due to the 3d sites exchange fields. Using a Stoner like model and a single-ion effective spin Hamiltonian, we demonstrate that induced magnetism on the Pt site and a large difference in the spin-orbit coupling constants of the 3d and 5d elements results in unusually large effective anisotropic exchange. The strength of this effective anisotropic exchange is found to be dependent on the Stoner parameter of the 5d site (I_5d), on the value of the induced magnetic moment on the 5d element (for the FePt m_5d is about 0.36 μ_B),the exchange interaction parameter J_3d-5d-ion anisotropy parameter D⁽⁰⁾. This appear to be an important ingredient to describe correctly observed magnetic properties such as critical temperature and temperature dependence of the magneto-crystalline anisotropy.

Furthermore, the model is used to study the switching behavior of the FePt nano-particles at finite temperatures. Most noticeable effects are (i) a significant deviation from the Stoner-Wohlfarth model in terms of the values of the switching fields, (ii) a strong deviation from the Neel-Brown theory for the mean switching times due to the temperature dependence of the activation energy barrier. Dynamic coercive field temperature dependence for small (~3-4nm) particles shows relatively small blocking temperatures even for high sweep rates. We demonstrate that these effects can be understood qualitatively in terms of the temperature dependence of MAE and saturation magnetization in small FePt particles.