Investigating Exchange Bias and Coercivity in Fe3O4–γ-Fe2O3 Core–Shell Nanoparticles of Fixed Core Diameter and Variable Shell Thicknesses

Abstract

We have carried out extensive measurements on novel Fe3O4–γ-Fe2O3 core–shell nanoparticles of nearly similar core diameter (8 nm) and of various shell thicknesses of 1 nm (sample S1), 3 nm (sample S2), and 5 nm (sample S3). The structure and morphology of the samples were studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), and selected area electron diffraction (SAED). The direct current (DC) magnetic measurements were carried out using a superconducting quantum interference device (SQUID). Exchange bias and coercivity were investigated at several temperatures where the applied field was varied between 3 and −3 T. Several key results are obtained, such as: (a) the complete absence of exchange bias effect in sample S3; (b) the occurrence of nonconventional exchange bias effect in samples S2 and S1; (c) the sign-change of exchange bias field in sample S2; (d) the monotonic increase of coercivity with temperature above 100 K in all samples; (e) the existence of a critical temperature (100 K) at which the coercivity is minimum; (f) the surprising suppression of coercivity upon field-cooling; and (g) the observation of coercivity at all temperatures, even at 300 K. The results are discussed and attributed to the existence of spin glass clusters at the core–shell interface.

Investigating the Role of Shell Thickness and Field Cooling on Saturation Magnetization and Its Temperature Dependence in Fe3O4/γ-Fe2O3 Core/Shell Nanoparticles

“…Due to the broken exchange bonds [53] at the core/shell interface, the interface spins are suggested to be in disordered states. We suggest that (as shown in [37]) at the core/shell interface, spin-glass like structures [49,50] occur with amounts that increase with shell thickness. Hence, we attribute the increase of α B (increase of deviation from the original Bloch's law) with shell thickness to the increase of spin-glass structures as the shell thickness increases.…”

“…Thus, to obtain larger/smaller shell thickness, we started with larger/smaller particles (longer/shorter reaction times) and allowed for longer/shorter diffusion times. By optimizing these two parameters we were able to determine the suitable initial particle size for each diffusion time such that we obtained a fixed core for each case [37]. We found that when the reaction time is 1 h and the diffusion time is 1 h the core diameter is 8 nm and the shell thickness is 1 nm.…”

“…These results could be attributed to the existence of interface spin-glass clusters (SGCs) which occurred due to structural defects at the core/shell interface. The existence of such (SGCs) was investigated and reported in our recently published paper [37]. The spins in these SGCs are not aligned in a particular direction due to different spin interactions among the spins of each cluster and the interactions with the core and shell spins.…”

Morphology-Controlled Synthesis of Hematite Nanocrystals and Their Optical, Magnetic and Electrochemical Performance

“…For example, the hollow NCs show a size-dependent blue shift, strong visible-light and broad infrared absorption phenomenon [ 21 ]. The α-Fe 2 O 3 nanomaterials show ferromagnetic behavior and high coercivity in magnetic hysteresis measurements [ 22 , 23 ]. The rational design and synthesis of nanomaterials with different morphology have become an effective strategy to improve their optical, magnetic and electrochemical properties [ 24 ].…”

Мессбауэровские исследования структуры наночастиц Fe-=SUB=-3-=/SUB=-O-=SUB=-4-=/SUB=-/gamma-Fe-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=- типа ядро/оболочка

“…Наиболее перспективными для биомедицинских приложений являются МНЧ Fe 3 O 4 /γ-Fe 2 O 3 типа Я/О, состоящие из биологически совместимых компонентов, а именно магнетита (Fe 3 O 4 ) и маггемита (γ-Fe 2 O 3 ) [4,5]. Однако магнитные свойства, природа обменного смещения и другие эффекты в МНЧ типа Я/О полностью еще не поняты, хотя исследованиям таких МНЧ посвящено много публикаций (см., например, работы [1,4,6,7]…”