Hydrothermal synthesis, off-axis electron holography and magnetic properties of Fe3O4 nanoparticles

The hydrothermal synthesis of Fe3O4 nanoparticles (NPs) (< 50 nm) from mixed FeCl3 / FeCl2 precursor solution at pH ~ 12 has been confirmed using complementary characterisation techniques of transmission electron microscopy and X-ray diffractometry. Off-axis electron holography allowed for visualisation of their single domain (SD) nature, as well as inter-particle interactions, with the latter attributed to explain the pseudo-SD/multi-domain behaviour demonstrated by bulk magnetic measurements.


Introduction
The ability of a rock to reliably record the geomagnetic field depends on the geometry and size of the constituent magnetic particles, whereby large or closely packed grains that have large magnetostatic interactions between them will affect their recording fidelity [1]. Generally, particles in the single domain (SD) grain size range (< 100 nm) are regarded as ideal palaeomagnetic recorders because of their strong remanence and high magnetic stability, with potential relaxation times greater than that of the age of the Universe [2]. However, rocks rarely contain ideal assemblages of mono-dispersed SD particles and techniques used to determine the magnetic domain state of remanence carriers often reveal the presence of very small superparamagnetic (SP) particles, i.e., nanometric, plus pseudosingle domain (PSD) and multi-domain (MD) particles. As the magnetic minerals in rocks commonly show broad variations in particle size, shape and spacing, synthetic analogues are considered more ideal for study.
Magnetite (Fe 3 O 4 ) is arguably the most important naturally occurring magnetic naturally mineral on Earth [3]. Understanding its grain size behaviour and recording fidelity is crucial to palaeomagnetism. Previously nano-sized Fe 3 O 4 have been prepared by various methods such as solgel processing, hydrothermal synthesis, forced hydrolysis, ball milling and microemulsion. In this paper, we consider the approach of hydrothermal synthesis (HS), which in particular, offers effective control over the size and shape of Fe 3 O 4 nanoparticles (NPs) at relatively low reaction temperatures and short reaction times, providing for well-crystallised reaction products with high homogeneity and definite composition [4]. Off-axis electron holography permits nanometre-scale imaging of magnetic induction within and around materials as a function of applied field and temperature [3,5]. In this context, the HS of Fe 3 O 4 NPs is presented, and comparison of their magnetic properties investigated using off-axis electron holography and more conventional rock magnetism technique was made.

Experimental
For the HS of Fe 3 O 4 , 0.2 ml FeCl 3 aqueous solution (45% pure FeCl 3 ; Riedel-de Haen, Germany) and 150 mg FeCl 2 powder (98.0% CoCl 2 ; Sigma-Aldrich, UK) were further diluted in 40 ml distilled water and mechanically stirred in a 125 ml Teflon-lined steel autoclave. The precursor solution was adjusted to pH ~ 12 with the drop-wise addition of aqueous NaOH solution, then sealed and inserted into a temperature controlled furnace at 200°C for 3 hours. The autoclave, once removed from the furnace, was allowed to cool down to room temperature naturally.
The synthesised reaction products were centrifuged for 5 minutes at 5000 rpm, cleaned with acetone and deposited onto single crystal silicon substrates for the purpose of structural characterisation using a PANalytical X'Pert PRO Diffractometer (CuKα radiation; θ/2θ diffraction geometry). For the purpose of transmission electron microscopy (TEM) investigation, the acetone cleaned HS suspensions were dispersed using an ultrasonic bath before deposition onto lacey carbon / copper mesh support grids (Agar Scientific Ltd, UK). Conventional bright field (BF) imaging was performed using a FEI Tecnai TEM operated at 200 kV (Centre for Electron Nanoscopy (CEN), Technical University of Denmark) and selected area electron diffraction (SAED) allowed for phase identification. For off-axis electron holography, a FEI Titan TEM fitted with a C S corrector on the condenser lens, and a Lorentz lens and biprism, was used at 300kV (CEN). For the purpose of magnetic measurements, a dry powdered sample was inserted, compressed and sealed in a plastic pellet and examined at the Institute for Rock Magnetism (I.R.M) at the University of Minnesota, Minneapolis, USA. Low-temperature hysteresis measurements from 15 -295 K (in 10 K intervals), and first-order reversal curve (FORC), at 15 K, were made using a Princeton Measurements Vibrating Sample Magnetometer (VSM). A Quantum Designs Magnetic Properties Measuring Systems (MPMS) cryogenic magnetometer was used to measure cooling and warming curves of room temperature (RT) induced saturation isothermal remanence magnetisation (SIRM) to identify lowtemperature crystallographic transitions. In addition, warming curves of SIRM at 10 K after field cooling (FC) at 2.5 T and zero-field cooling (ZFC) from 300 K to 10 K, were made.

Results
The hydrothermal processing of mixed FeCl 3 / FeCl 2 precursor solution led to the synthesis of a nanostructured Fe 3 O 4 reaction product, the evidence for which is now presented in detail. Fig. 1 provides information on the morphology, size and structure of the synthesised Fe 3 O 4 NPs. The BF TEM image of Fig. 1a presents the HS reaction product and is shown to comprise crystalline Fe 3 O 4 NPs, as identified by SAED (inset), which exhibit some faceting but are generally isotropic and < 50 nm in size (Fig. 2b). The peaks in the XRD pattern of Fig. 1c are in good agreement with the formation of Fe 3 O 4 (Joint Committee on Powder Diffraction standards (JCPDS) reference 88-0315).
The magnetic induction map of an individual Fe 3 O 4 NP (~ 45 nm) presented in Fig. 1d, determined from the magnetic contribution to the phase shift reconstructed from electron holograms, depicts the magnetic field contours to be concentrated toand flow diagonally right and upward throughthe centre of the Fe 3 O 4 NP, as well as a stray magnetic field expected for a SD particle. Fig. 1e shows magnetic induction map of a group of Fe 3 O 4 NPs, displaying the magnetic contours to again flow through the SD particles, and the expected stray magnetic field. Fig. 2 provides information on the magnetic properties of the Fe 3 O 4 NPs. The low-temperature SIRM cycling curves (Fig. 2a) displays behaviour typically reported for PSD magnetite with a Verwey transition of ~ 120 K; however, theoretical models also predict such behaviour for SD particles at this temperature [6]. The drop in the remanence at the Verwey transition suggests that part of the magnetic remanence is a carried by interacting SD and/or small PSD (vortex) domain states. Low-temperature warming of SIRM to 10K (Fig. 2b), clearly reveals the Verwey transition, and is indicative of near stoichiometric magnetite. The hysteresis parameter of coercivity (H C ) is plotted as a function of temperature (15 K to 295 K) in Fig. 2c and displays H C behaviour typical of SD or small PSD magnetite [7], i.e., on warming H C drops at 15K, until the Verwey transition, followed by a gradual drop from 16 to 13 mT on warming to room temperature; such behaviour has been previously predicted [8]. The FORC diagram of Fig. 2d was measured at 15 K after ZFC and exhibited behaviour characteristic of SD grains displaying moderate inter-particle magnetostatic interactions [9].

Discussion
This TEM and XRD investigation of the HS of mixed FeCl 3 / FeCl 2 precursor solution, at pH ~ 12, has provided evidence for the development of Fe 3 O 4 NPs that are generally isotropic and < 50 nm in size. A HS reaction temperature of 200°C was observed sufficient to promote the formation of the thermodynamically stable Fe 3 O 4 iron oxide phase as the sole reaction product, whilst it is considered that the Fe 3 O 4 NP growth mechanism may have involved the growth and dissolution of intermediate iron (oxy)hydroxide phases. Kinematically accessible amorphous Fe(OH) 3 , poorly crystalline 'hydroxychloride green rust 1': Fe 3.6 Fe 0.9 (O, OH, Cl) 9 (JCPDS 13-88) sheets and α-FeOOH nanorods are known to precipitate during HS at relatively low temperatures in iron-chloride salt solutions at pH ~ 12 [4,10], and their dissolution with increasing temperature is considered to supply elemental growth constituents for formation of the more thermodynamically stable Fe 3 O 4 .
Complementary hysteresis and thermomagnetometry combined with off-axis electron-holography measurements, provided information on the magnetic properties of the Fe 3 O 4 NPs. The magnetic induction map of an individual Fe 3 O 4 NP (Fig. 1d) demonstrates its SD behaviour, as evidenced by the uniaxial magnetic contours concentrated to the NP centre and stray magnetic field. This is reinforced by the FORC diagram that exhibits behaviour characteristic of interacting single-domain grains. However, the H c , RT-SIRM and ZFC-FC cycling data reveals behaviour more indicative of PSD/MD domain-states. Considering the HS reaction product is in a powdered form, the PSD/MD-like behaviour is suggested to be caused by inter-particle interaction of the Fe 3 O 4 NPs during measurement [11], which is supported by the visualised interactions between the SD particles shown in Fig. 1e and the FORC data (Fig. 2d).
In summary, complementary TEM and XRD have confirmed the HS of SD Fe 3 O 4 NPs (< 50 nm in diameter). Off-axis electron holography showed their SD nature and explained the PSD/MD behaviour demonstrated by the bulk magnetic measurements through visualisation of inter-particle interactions.