Model for temperature-dependent magnetization of nanocrystalline materials

Abstract

A magnetization model of nanocrystalline materials incorporating intragrain anisotropies, intergrain interactions, and texture effects has been extended to include the thermal fluctuations. The method relies on the stochastic Landau–Lifshitz–Gilbert theory of magnetization dynamics and permits to study the magnetic properties of nanocrystalline materials at arbitrary temperature below the Currie temperature. The model has been used to determine the intergrain exchange constant and grain boundary anisotropy constant of nanocrystalline Ni at 100 K and 298 K. It is found that the thermal fluctuations suppress the strength of the intergrain exchange coupling and also reduce the grain boundary anisotropy. In comparison with its value at 2 K, the interparticle exchange constant decreases by 16% and 42% and the grain boundary anisotropy constant decreases by 28% and 40% at 100 K and 298 K, respectively. An application of the model to study the grain size-dependent magnetization indicates that when the thermal activation energy is comparable to the free energy of grains, the decrease in the grain size leads to the decrease in the magnetic permeability and saturation magnetization. The mechanism by which the grain size influences the magnetic properties of nc–Ni is discussed.

Magnetoelastic effects and random magnetic anisotropy in highly strained ultrathin Ni nanowires epitaxied in a SrTiO3 matrix

“…The unusual thermal softening of anisotropy constitutes a key feature of the present NWs. Many studies reported on variations of anisotropy with temperature, related to a variation of physical constants, such as magnetocrystalline constants [48]. In the present work, the samples show a similar shift of the anisotropy distribution, over different temperature ranges until 150 K. This suggests a general thermally activated process and not the thermal variation of a constant that would impact the samples differently as a function of temperature.…”

Ultralow magnetostrictive flexible ferromagnetic nanowires

“…5a, we could obtain both qualitative and quantitative description of the experimental data by using typical values of saturation magnetization (M S = 1.4 MA m −1 ) and adjusting the exchange stiffness to take into account an effective room temperature exchange across the nanocrystalline structure (A ex = 17.5 pJ m −1 ). 40,41 In addition, we have tested the role of the magnetocrystalline anisotropy repeating the simulation with and without a random anisotropy model (RAM), including a grainy structure (10 nm grains) with typical intrinsic magnetocrystalline anisotropy (K = 520 kJ m −3 ) 42 and random distribution of easy axis. In both cases (with and without the use of RAM), we get the same results for both stressed and unstressed systems.…”