The effects of surface spin on magnetic properties of weak magnetic ZnLa0.02Fe1.98O4 nanoparticles
© Xu et al.; licensee Springer. 2014
Received: 19 August 2014
Accepted: 20 September 2014
Published: 2 October 2014
In order to prominently investigate the effects of the surface spin on the magnetic properties, the weak magnetic ZnLa0.02Fe1.98O4 nanoparticles were chosen as studying objects which benefit to reduce as possibly the effects of interparticle dipolar interaction and crystalline anisotropy energies. By annealing the undiluted and diluted ZnLa0.02Fe1.98O4 nanoparticles at different temperatures, we observed the rich variations of magnetic ordering states (superparamagnetism, weak ferromagnetism, and paramagnetism). The magnetic properties can be well understood by considering the effects of the surface spin of the magnetic nanoparticles. Our results indicate that in the nano-sized magnets with weak magnetism, the surface spin plays a crucial rule in the magnetic properties.
Ferrite nanocrystals have been extensively studied due to their tunable and remarkable magnetic properties as well as catalytic properties not existing in the corresponding bulk materials [1–5]. In the fundamental research field, magnetic nanoparticles (NPs) usually serve as ideal model systems, e.g., the Stoner-Wohlfarth [6, 7] and Néel-Brown model , or to study the finite-size effect . As the size of a magnetic particle decreases, the significance of the surface spins increases, resulting in the various magnetic ordering states such as spin-glass or cluster-glass-like behavior [10–13] or weak ferromagnetism [14, 15] of the surface spins.
Compared with strong magnetic materials which have higher H c and M values, the material with weak magnetism (small values of coercivity H c and magnetization M) is a good candidate for studying the effects of surface spins on magnetic properties, because the strong anisotropy or interparticle dipolar interaction in the strong magnetic system can suppress the effects of surface spin [16, 17]. ZnFe2O4 crystallizes in the bulk in the normal spinel structure with Fe3+ ions (5 μ B moment per Fe3+ ion) occupying octahedral sites and Zn2+ ions (with zero moment) occupying tetrahedral sites. Superexchange interaction between two Fe3+ ions will have their moments aligned anti-parallel to each other which results in the antiferromagnetic (AFM) ZnFe2O4 [18, 19] with the theoretical moment being 0 μ B /f.u. For the La3+ substituted bulk ZnLa0.02Fe1.98O4 (ZLFO), the theoretical moment is 0.1 μ B /f.u., which exhibits the weak ferromagnetism. In the present work, we take ZLFO as a studying object to investigate the effects of surface spins on the magnetic properties. We first prepared ZLFO NPs via a hydrothermal method. Then, some of the NPs were diluted in the Al2O3 matrix and others were undiluted, both followed by annealing at different temperatures of 700°C, 800°C, 900°C, and 1,000°C. The magnetic measurements show that the ZLFO NPs have the weak magnetism with small M and H c values. Our results indicate that the surface spins significantly affect the macromagnetism of ZLFO NPs.
All raw materials include: iron (III) nitrate hexahydrate [Fe(NO3)3 · 6H2O, 99%], zinc nitrate hexahydrate [Zn(NO3)2 · 6H2O, 99%], lanthanum (III) acetate sesquihydrate [La(OOCCH3)3 · 1.5H2O, 99.9%], and aluminum nitrate nonahydrate [Al(NO3)3 · 9H2O], serving as the sources of metallic ions in ZLFO and Al2O3; sodium acetate trihydrate (C2H3NaO2 · 3H2O, 99%) and 1-hexadecyltrimethylammonium bromide (C19H42BrN, 99%), being used as surfactants for improving precursor’s dispersibility; ethylene glycol (C2H6O2, 99%), acting as the solvent.
Firstly, 36 mmol Zn(NO3)2 · 6H2O, 72 mmol Fe(NO3)3 · 6H2O, 7.2 mmol La(OOCCH3)3 · 1.5H2O, and 108 mmol C2H3NaO2 · 3H2O were dissolved in 300-mL anhydrous C2H6O2 with magnetic stirring. Then, 1.08 mmol C19H42BrN was added into the solution with continuous stirring at 40°C for 30 min to get a homogeneous solution. Subsequently, the solution was transferred into 50-ml Teflon-lined stainless steel autoclave and maintained at 200°C for 24 h to obtain the ZLFO NPs. The typical synthesis procedure can be shown by the following :
Zn(NO3)2 · 6H2O + 0.02 La(OOCCH3)3 · 1.5H2O + 1.98 Fe(NO3)3 · 6H2O + C2H3NaO2 · 3H2O + C2H6O2 → ZnLa0.02Fe1.98(OOCH2CH3)8 · nH2O + NaNO3.
ZnLa0.02Fe1.98(OOCH2CH3)8 · nH2O will decompose above 200°C and produce ZLFO.
After the autoclave was cooled down to room temperature naturally, the precipitate obtained was separated by centrifugation, washed with distilled water and anhydrous ethanol several times, and subsequently dried at 80°C. The obtained ZLFO NPs were divided into two parts. One is diluted in the Al2O3 matrix and the other is undiluted.
ZLFO NPs were added to the solution of Al(NO3)3 · 9H2O and ethanol under sonicating with mass ratio of ZLFO:Al2O3, being 3:2. Then, the mixture was dried at 80°C. The undiluted and diluted ZLFO were both divided equally into four parts for annealing at 700°C, 800°C, 900°C, and 1,000°C for 2 h to obtain the final samples, which are hereafter referred to as UD700, UD800, UD900, and UD1000 for undiluted samples and D700, D800, D900, and D1000 for diluted samples, respectively.
The crystal structure was characterized by X-ray diffraction analysis using an X-ray diffractometer (XRD; DX-2000 SSC) with Cu Kα irradiation (λ = 1.5418 Å) from 10° to 80° with a step of 0.02°. The magnetic measurements were carried out by Quantum Design superconducting quantum interference device (SQUID) MPMS system(PPMS EC-II) (Quantum Design, San Diego, CA, USA). High-resolution transmission electron microscopy (HRTEM) (JEOL JEM-2100, JEOL, Akishima-shi, Tokyo, Japan) was used to observe the morphology, selected area electronic diffraction (SAED), and lattice fringes.
Results and discussion
For the diluted sample D1000, as shown in Figure 1a, the diffraction peaks of ZLFO cannot be observed, indicating that ZLFO was deeply embedded in the Al2O3 matrix. Several diffraction peaks of (012), (104), and (113) facets can be attributed to the reflection of Al2O3.
As discussed above, the total moment (Mtotal) of a particle can be expressed as Mtotal = Mcore + MSSL. At 300 K, the ZLFO core is paramagnetic (PM) (with theoretical molecular moment of 0.1 μ B /f.u.). The sample annealed at low temperature (such as 700°C), as shown in Figure 5a, has the small core and the thick SSL. For the sample annealed at higher temperatures such as at 800°C and 900°C, as shown in Figure 5b, the core becomes larger and simultaneously, the SSL becomes thinner. While the sample is annealed at 1,000°C, the SSL becomes thinner or disappears, as shown in Figure 5c, and the M(H) loop behaves as PM with a linear loop shape in the field range used. Therefore, the gradual decrease in Mmax for the samples UD700, UD800, UD900, and UD1000 can be assigned to the decrease of MSSL .
The ZLFO NPs were synthesized by the hydrothermal method. Then, some of ZLFO NPs were diluted in the Al2O3 matrix through the sol–gel method and the others were undiluted. The undiluted and diluted ZLFO were finally annealed at temperatures of 700°C, 800°C, 900°C, and 1,000°C to investigate the effects of surface spin and interface effects between ZLFO and Al2O3 on the magnetic parameters and magnetic ordering states.
For the undiluted samples, with increasing the annealing temperature, the thickness of the SSL decreases and ZLFO experiences SPM and PM according to the results of hysteresis loops. The maximum magnetization, Mmax, at 2 T of ZLFO decreases with increasing the annealing temperature which can be assigned to the decrease of SSL. For the diluted samples, the surface spin and the interface effect between ZLFO NPs and the Al2O3 matrix are the dominant factors affecting the magnetic properties. Our results indicate that in the nano-sized magnets with weak magnetism, the surface spin plays a crucial rule in the magnetic properties.
This work was supported by the National Natural Science Foundation of China (Grant Nos. 11174004, 51471001, and 11204001).
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