Orbital Moment Determination in (MnxFe1-x)3O4 Nanoparticles

Abstract

JOURNAL OF APPLIED PHYSICS 109, 07B532 (2011) Orbital moment determination in (Mn x Fe 12x ) 3 O 4 nanoparticles V. L. Pool, 1,2,a) C. Jolley, 3,4 T. Douglas, 2,3 E. A. Arenholz, 5 and Y. U. Idzerda 1,2 Department of Physics, Montana State University, Bozeman, Montana 59717, USA Center for Bio-inspired Nanomaterials, Montana State University, Bozeman, Montana 59717, USA Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana 59717, USA Astrobiology Biogeocatalysis Research Center, Montana State University, Bozeman, Montana 59717, USA Advanced Light Source, Lawrence Berkeley National Labs, Berkeley, California 94720, USA (Presented 17 November 2010; received 24 September 2010; accepted 8 December 2010; published online 7 April 2011) Nanoparticles of (Mn x Fe 1Ax ) 3 O 4 with a concentration ranging from x ¼ 0 to 1 and a crystallite size of 14–15 nm were measured using X-ray absorption spectroscopy and X-ray magnetic circular dichroism to determine the ratio of the orbital moment to the spin moment for Mn and Fe. At low Mn concentrations, the Mn substitutes into the host Fe 3 O 4 spinel structure as Mn 2þ in the tetrahedral A-site. The net Fe moment, as identified by the X-ray dichrosim intensity, is found to increase at the lowest Mn concentrations then rapidly decrease until no dichroism is observed at 20% Mn. The average Fe orbit/spin moment ratio is determined to initially be negative and small for pure Fe 3 O 4 nanoparticles and quickly go to 0 by 5%–10% Mn addition. The average Mn moment is anti-aligned to the Fe moment with an orbit/spin moment ratio of 0.12 which gradually C decreases with Mn concentration. V 2011 American Institute of Physics. [doi:10.1063/1.3562905] I. INTRODUCTION The doping of spinel ferrite nanoparticles (c-Fe 2 O 3 and Fe 3 O 4 ) with magnetic and nonmagnetic substitutional transi- tion metals has demonstrated good control of both moment and anisotropy, 1 with magnetic behavior and dopant occu- pancy sites often quite different from the bulk behavior. One example is for the biomineralization of (Mn x Fe 1Ax ) 3 O 4 nano- particles inside protein cage structures, where Mn initially substitutes as Mn 2þ into the octahedral B-site causing the moment to decrease instead of as Mn 2þ in the tetrahedral A-site, 1 creating an enhanced moment as occurs in the bulk. 2 It is unclear whether these differences are due to the gentle synthesis conditions of biomineralization, the presence of the protein encapsulation, or the reduced dimensionality of nanoparticles. A comparison of dopant occupation sites and anisotropy energies of similar nanoparticles synthesized under different conditions would be useful. For noninteracting particles, frequency dependent ac- susceptibility measurements are a useful way to determine anisotropy energies. 1,3 A related parameter to the magneto- crystalline anisotropy energy is the elemental orbital mag- netic moment as determined from energy integration of the X-ray magnetic circular dichroism spectra (using the XMCD sum-rules). 4–7 Used predominantly for single crystal thin film geometries, this unique method for separating the orbital moment and the spin moment of each element has utility for nanoparticles, especially those found to be interacting. II. EXPERIMENTAL tein encapsulated particles (the synthesis route used in ref #1 but without inclusion of the protein). Solutions of 12.5 mM (NH 4 ) 2 Fe(SO 4 ) 2 .6H 2 O and 12.5 mM MnCl 2 were prepared using H 2 O that had been sparged with N 2 to remove dis- solved oxygen and mixed to obtain the desired [Fe 2þ ]: [Mn 2þ ] ratio. The 12.5 mM metal mixture and a deaerated 4.17 mM H 2 O 2 solution were added at a rate of 40 ml/h to a deaerated solution of 100 mM NaCl maintained at 65 C while the pH was maintained at 8.5 by addition of deaerated 100 mM NaOH using an autotitrator. Samples were centri- fuged and triple washed with de-ionized water in order to remove NaCl and unreacted metal ions. Samples used for X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) measurements were stored in an aqueous suspension and subsequently dried onto Formvar- coated TEM grids, while samples to be used for hard X-ray pair distribution function (PDF) analysis were immediately dried to powder using a vacuum lyophilizer. The XAS and XMCD measurements were conducted at beamline 4.0.2 of the Advanced Light Source of Berkeley National Laboratories simultaneously in transmission yield (using a Ga photodetector) and total electron yield (in the sample current mode). Absorption measurements were made at room temperature with the photon polarization set at 90% and an alternating applied magnetic field of 0.5 T. III. RESULTS Nanoparticles of (Mn x Fe 1Ax ) 3 O 4 were synthesized with x ¼ 0 to 1.0 by a chemical route identical to that used for pro- a) Author to whom correspondence should be addressed. Electronic mail: pool@physics.montana.edu. The Mn L 23 -edge XAS spectra for a representative sam- pling of different Mn concentrations are shown in Fig. 1. The spectra have had a linear background removed, been normalized to the integrated peak area (L 2 þ L 3 ), and were energy calibrated by comparing the peak position of a simul- taneously collected Mn 3 O 4 reference powder spectra (set to 640.05 eV). The evolution of the spectra show that as the Mn C V 2011 American Institute of Physics 0021-8979/2011/109(7)/07B532/3/$30.00 109, 07B532-1