Field Dependence of Magnetic Disorder in Nanoparticles

Abstract

The performance characteristics of magnetic nanoparticles toward application, e.g., in medicine and imaging or as sensors, are directly determined by their magnetization relaxation and total magnetic moment. In the commonly assumed picture, nanoparticles have a constant overall magnetic moment originating from the magnetization of the single-domain particle core surrounded by a surface region hosting spin disorder. In contrast, this work demonstrates the significant increase of the magnetic moment of ferrite nanoparticles with an applied magnetic field. At low magnetic field, the homogeneously magnetized particle core initially coincides in size with the structurally coherent grain of 12.8(2) nm diameter, indicating a strong coupling between magnetic and structural disorder. Applied magnetic fields gradually polarize the uncorrelated, disordered surface spins, resulting in a magnetic volume more than 20% larger than the structurally coherent core. The intraparticle magnetic disorder energy increases sharply toward the defect-rich surface as established by the field dependence of the magnetization distribution. In consequence, these findings illustrate how the nanoparticle magnetization overcomes structural surface disorder. This new concept of intraparticle magnetization is deployable to other magnetic nanoparticle systems, where the in-depth knowledge of spin disorder and associated magnetic anisotropies are decisive for a rational nanomaterials design.

Mechanism of magnetization reduction in iron oxide nanoparticles

“…In the work of Herlitschke et al 28 no magnetic core-shell structure was reported for spherical particles with diameter of 7.4 nm and σ of a lognormal size distribution of 0.057 and the reduction of the magnetization was entirely ascribed to random orientations of atomic moments homogeneously distributed in the particle. A recent study by Zákutná et al 91 on cobalt ferrite nanoparticles with mean diameter of 14.1 nm and lognormal size distribution of σ = 0.031 revealed a magnetically disordered surface region of 0.7 nm thickness. Since the particles used there were composed of cobalt ferrite the results are only partially comparable to pure ferrite nanoparticles.…”

Embracing Defects and Disorder in Magnetic Nanoparticles

“…Using magnetic SANS, a significant reduction of surface spin disorder was found upon cooling to low temperatures of 10 K. [ 116 ] More recently, a strong field‐dependence of surface spin disorder was revealed, expressed by a gradual polarization of initially disordered surface spins even beyond the structurally coherent grain size (Figure 5h). [ 117 ] The field dependence of the spatial distribution of surface spin disorder ultimately gives access to the spatially resolved disorder energy toward the particle surface. [ 117 ]…”

“…[ 117 ] The field dependence of the spatial distribution of surface spin disorder ultimately gives access to the spatially resolved disorder energy toward the particle surface. [ 117 ]…”

“…In contrast, noninteracting ferrite nanoparticles of similar size reveal random surface spin disorder without a traceable coherent transversal magnetization component. [ 117 ] In consequence, a strong impact of interparticle interactions on spin disorder and spin canting is evident and needs to be considered for applications of spin disorder toward magnetic heating.…”

“…Indeed, polarized SANS has revealed significant contributions of spin disorder even in the particle interior, [ 121 ] expressed as a reduced local magnetization in the particle core. [ 113,116,117 ] In several reports, the spin disorder in the particle interior has even been found to dominate surface spin disorder. [ 113,122 ] Such a reduced magnetization has been attributed to canting of Fe moments in both tetrahedral and octahedral sites, revealed by a combination of NMR and Mössbauer spectroscopies, and related to reversed moments and frustrated topology.…”