Enhanced Magnetism of Fe3O4 Nanoparticles with Ga Doping

Abstract

JOURNAL OF APPLIED PHYSICS 109, 07B529 (2011) Enhanced magnetism of Fe 3 O 4 nanoparticles with Ga doping V. L. Pool, 1,a) M. T. Klem, 2,3 C. L. Chorney, 2,3 E. A. Arenholz, 4 and Y. U. Idzerda 1 Department of Physics, Montana State University, Bozeman, Montana 59715, USA Department of Chemistry and Geochemistry, Montana Tech, Butte, Montana 59701, USA Center for Advanced Supramolecular and Nano Systems, Montana Tech, Butte, Montana 59715, USA Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA (Presented 18 November 2010; received 22 October 2010; accepted 1 December 2010; published online 5 April 2011) Magnetic (Ga x Fe 1Ax ) 3 O 4 nanoparticles with 5%–33% gallium doping (x ¼ 0.05–0.33) were measured using x-ray absorption spectroscopy and x-ray magnetic circular dichroism to determine that the Ga dopant is substituting for Fe 3þ as Ga 3þ in the tetrahedral A-site of the spinel structure, resulting in an overall increase in the total moment of the material. Frequency-dependent alternating-current magnetic susceptibility measurements showed these particles to be weakly interacting with a reduction of the cubic anisotropy energy term with Ga concentration. The element-specific dichroism spectra show that the average Fe moment is observed to increase C with Ga concentration, a result consistent with the replacement of A-site Fe by Ga. V 2011 American Institute of Physics. [doi:10.1063/1.3562196] I. INTRODUCTION Gallium doped iron oxide, (Ga x Fe 1Ax ) 3 O 4 , is interesting as it exhibits an energy-dependent photoabsorption as a func- tion of Ga concentration in bulk samples. 1 This property could be advantageous in nanoparticles providing utility for applications. Additionally, Ga-based mixed oxides have shown interesting catalytic behavior, an application that nano- particles are known to be exceptional for due to their large sur- face-to-volume ratio. 2–4 Furthermore, the doping of magnetic nanoparticles can have behaviors distinct from bulk alloys, including changes in dopant site occupation and different varia- tions of moment and anisotropy with doping concentration, making them interesting from a purely scientific standpoint. Bulk (Ga x Fe 1Ax ) 3 O 4 magnetic trends have not been fully studied, but Ga is typically present as a 3þ valence ion so it is likely to be present as a 3þ valence ion given the two options in the spinel structure. 1 The effect on the magnetic properties of doping Fe 3 O 4 with other nonmagnetic transi- tion metals of similar ionic radii, such as Zn, show that Zn preferentially substitutes for Fe 3þ as Zn 2þ in the tetrahedral coordination. For bulk Zn doped Fe 3 O 4 the total moment per unit cell increases as the tetrahedral sites become occupied by the nonmagnetic transition metal. 5 These substitutional atoms no longer partially cancel the Fe moment in the octa- hedral coordination. For Zn doping the average moment of the octahedral Fe sites also slightly increases as charge neu- trality requires that some of the octahedral Fe 2þ convert to Fe 3þ . If Ga substitutes into the tetrahedral coordination as a Ga 3þ ion (rather than Zn 2þ ), it should produce a similar, though slightly reduced effect. Nanoparticles do not always follow the same trends as the bulk. Properties are often size dependent where surface effects and the relaxation of crystal lattice distortions can a) Author to whom correspondence should be addressed. Electronic mail: pool@physics.montana.edu. 0021-8979/2011/109(7)/07B529/3/$30.00 cause nanoparticles to have distinctly different magnetic properties. 6,7 In addition, nanoparticles are notoriously syn- thesis dependent, while a chemically gentle nanoparticle syn- thesis process like protein encapsulation may generate one behavior, a harsher chemical process may show an entirely different behavior. 8 In particular, encapsulation of Zn nano- particles in protein structures does result in substitution of Zn to the tetrahedral A-site, but generates a distinctly differ- ent magnetic behavior, characterized by a steep reduction in nanoparticle moment with Zn concentration, opposite the bulk behavior. 9 From simple crystal filling arguments, the reduced ion radius of Ga 3þ in comparison to Zn 2þ suggests that the Ga dopant should more strongly prefer the tetrahe- dral coordinated site than Zn, allowing for a further investi- gation of the peculiar moment behavior. II. EXPERIMENTAL PROCEDURES In this study, magnetic (Ga x Fe 1Ax ) 3 O 4 nanoparticles with 5%–33% gallium doping (x ¼ 0.05–0.33) were synthe- sized by mixing Fe(acac)3,1,2-hexadecanediol, benzyl ether, oleic acid, and oleylamine under evacuated conditions. The Fe 3 O 4 (magnetite) nanoparticles were synthesized by com- bining Fe(acac) 3 (0.5 mmol), oleic acid (1.5mmol), oleyl- amine (1.5 mmol), 1,2-hexadecanediol (2.5 mmol), and benzyl ether (5 mL) in a 50 mL roundbottom flask under vacuum. The mixture was gradually heated to 200 C and allowed to anneal for 24 h. The reaction mixture was then cooled to room temperature, and the particles were precipi- tated in ethanol, centrifuged, and dried. Gallium doped mag- netite nanoparticles, (Ga x Fe 1Ax ) 3 O 4 were synthesized by substitution of Ga(acac) 3 for the Fe(acac) 3 . TEM measure- ments determined their size to be 8.5–9 nm. Investigation of the composition, electronic structure, and magnetic properties were accomplished using x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). These measurements, performed on beamline 4.0.2 109, 07B529-1 C V 2011 American Institute of Physics