ZnFe 2 O 4 Nanocrystals: Synthesis and Magnetic Properties Changwa Yao, ² Qiaoshi Zeng, ² G. F. Goya, ‡ T. Torres, ‡ Jinfang Liu, ²,§ Haiping Wu, ² Mingyuan Ge, ² Yuewu Zeng, ², | Youwen Wang, ², | and J. Z. Jiang* ,² International Center for New-Structured Materials (ICNSM) and Laboratory of New-Structured Materials, Department of Materials Science and Engineering, and Analysis and Testing Centre, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China, and Instituto UniVersitario de InVestigacio ´ n en Nanociencia de Arago ´ n (INA), Cerbuna 12, 50009 Zaragoza, Spain ReceiVed: April 29, 2007; In Final Form: June 18, 2007 Ferromagnetic zinc ferrite nanocrystals at ambient temperature were synthesized via the thermal decomposition of metal-surfactant complexes. Characterization measurements including transmission electron microscopy and X-ray diffraction were performed for as-synthesized ZnFe 2 O 4 particles. The sample has a relatively narrow size distribution with an average particle size of 9.8 ( 0.2 nm and standard deviation of 30%. The as- synthesized zinc ferrite nanocrystals are superparamagnetic at room temperature with a blocking temperature T B ) 68 ( 2 K and a saturation magnetization M S ) 65.4 emu‚g -1 at T ) 10 K, which are caused by the change in the inversion degree of the spinel structure. A coercive field of H C ) 102 ( 5 Oe in the blocked state indicates small particle anisotropy, although evidence of surface spin canting was inferred from magnetization data in the as-synthesized ZnFe 2 O 4 nanocrystals. Our results demonstrate that magnetic properties of magnetic particles can be largely modified by just changing particle size, which might be a useful way to design novel magnetic materials. 1. Introduction The manufacture of magnetic nanoparticles (MNPs) started many years ago, and currently there are reliable synthesis routes. However, synthesizing MNPs of a few nanometers, keeping the magnetic moment of the corresponding bulk material, is still a challenge because the high surface/volume ratio makes the surface disorder effect to be dominant. 1 Therefore, the develop- ment of synthesis methods, by which materials, having nanom- eter-sized grain size, retain the magnetic performance of the bulk materials, is still desirable. When the size of magnetic particles decreases into nanometer- sized scale, the surface area increases greatly, resulting in novel phenomena. Superparamagnetism, magnetic quantum tunneling and spin-glass-like behavior are some examples in the field of nanomagnetism. 2 These magnetic properties make magnetic nanoparticles to have many technological applications including magnetic data storage, ferrofluid, medical imaging, drug target- ing, and catalysis. 3-5 The zinc ferrite, ZnFe 2 O 4 , one of the iron- based cubic spinel series, shows striking changes in its magnetic properties by reducing the grain size to the nanometer-sized range. Bulk zinc ferrite is a completely normal spinel structure with Zn ions in the tetrahedral or A sites and Fe ions in the octahedral or B sites. Due to antiferromagnetic (AFM) super- exchange interactions between B-B ions, bulk zinc ferrite is antiferromagnetic at T N ) 10 K. However, the magnetic struc- ture of ZnFe 2 O 4 can be largely altered by developing a non- equilibrium state, i.e., redistribution of iron ions at A and B sites, as (Zn 1-x Fe x )[Zn x Fe 2-x ]O 4 , where parentheses and square brackets denote the A and B sites, respectively and the x is the inversion parameter. 6 Net magnetization at ambient temperature can be obtained in nanometer-sized (Zn 1-x Fe x )[Zn x Fe 2-x ]O 4 par- ticles. 6 Various synthesis methods, such as dry- and wet-mil- ling, 6-9 sol-gel, 10,11 co-precipitation, 12,13 microemulsions, 14 pul- sed laser deposition, 15 electrodeposition, 16 thermal solid-state reac- tion, 17,18 and ultrasonic cavitation approach, 19 have been reported to prepare nanometer-sized (Zn 1-x Fe x )[Zn x Fe 2-x ]O 4 particles. Here we report a simple method to prepare (Zn 1-x Fe x )[Zn x Fe 2-x ]O 4 nanocrystals with a relatively narrow size distribution by thermal decomposition of organometallic precursors in high boiling point solvent octyl ether, which has been used to synthesize different monodisperse nanocrystals. 20-24 We discuss the structure and magnetic data of as-synthesized (Zn 1-x Fe x )[Zn x Fe 2-x ]O 4 nanoparticles. 2. Experimental Details 2.1. Chemicals. All the chemical reagents in our experiments were used without any purification. The precursor ZnAc 2 ‚2H 2 O (99.99%) and oleic acid (90%) were purchased from Alfa Aesar company, and the other precursor Fe(CO) 5 was purchased from Sigma-Aldrich. High boiling point solvent octyl ether (99%) was purchased from Tokyo Kasei Kogyo Co. Anhydrous ethanol (C 2 H 5 OH, chromatogram grade) was purchased from Scharlau Chemie S.A. 2.2. Synthesis of Nanometer-Sized Zinc Ferrite ZnFe 2 O 4 . In a typical experiment, 0.11 g (0.5 mmol) of ZnAc 2 ‚2H 2 O and 5 mL of anhydrous ethanol were placed in a 50 mL beaker. The mixture was heated to 70 °C under strong stirring after ZnAc 2 ‚2H 2 O completely dissolved in ethanol. The mixture was injected into octyl ether-oleic acid solution (10 mL of octyl ether and 1.35 mL (4.5 mmol) of oleic acid) by a syringe at 80 °C, in which oleic acid acts as a surfactant. When the solution * To whom correspondence should be addressed. E-mail: jiangjz@ zju.edu.cn. ² ICNSM and Laboratory of New-Structured Materials, Depatment of Materials Science and Engineering, Zhejiang University. ‡ INA. § E-mail: jianglab@zju.edu.cn. | Analysis and Testing Centre, Zhejiang University. 12274 J. Phys. Chem. C 2007, 111, 12274-12278 10.1021/jp0732763 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/02/2007