Article pubs.acs.org/cm
Size and Crystallinity Dependence of Magnetism in Nanoscale Iron Boride, α‑FeB Steffi Rades,†,∥ Stephan Kraemer,‡ Ram Seshadri,*,‡,§,⊥ and Barbara Albert*,† †
Eduard-Zintl-Institute of Inorganic and Physical Chemistry, Technische Universität Darmstadt, Alarich-Weiss-Str. 12, 64287 Darmstadt, Germany ‡ Materials Department, §Department of Chemistry & Biochemistry, and ⊥Materials Research Laboratory, University of California, Santa Barbara, Santa Barbara, California 93106, United States ∥ Division 6.8 Surface Analysis and Interfacial Chemistry, BAM Federal Institute for Materials Research &Testing, Unter den Eichen 44-46, 12203 Berlin, Germany
ABSTRACT: A nanoscale boride, α-FeB, with grains of variable size and crystallinity was synthesized by precipitation from solution followed by heat treatment (450 °C, 550 °C, 750 °C, 1050 °C). Analysis of transmission electron micrographs, electron diffraction, and magnetic measurements suggests superparamagnetism at room temperature for the smaller, more disordered particles of FeB, while the larger, more crystalline particles of α-FeB, with a particle size of approximately 20 nm, display open magnetic hysteresis loops and blocking. In contrast to the soft ferromagnetism of bulk β-FeB, which was synthesized by conventional solid state reaction at 1500 °C, the sample of α-FeB annealed at 1050 °C is a harder ferromagnet, possibly due to stacking faults that pin the magnetic domains; these stacking faults are apparent in the high resolution transmission electron micrographs. The changes in magnetic behavior are visible from the varying blocking temperatures (63 K, 94 K, 150 K, and >320 K from the smallest to the largest particles) and correlate with the transformations from amorphous to α-FeB and from α-FeB to β-FeB.
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ultrafine cobalt boride particles.6,7 A synthesis route starting from metal salts and sodium tetrahydroborate in solvents yields nanoscale boride powders8,9 that may turn out to be interesting magnetic materials for cancer treatment, either in hyperthermia or in boron neutron capture therapy. Such particles also recently received attention as one of the components suitable for core−shell particles or composite materials, to combine hard and soft magnetic properties.10−12 Iron borides are soft ferromagnetic substances with M:B ratios between 1:1 and 3:1. Bulk materials are obtained through high temperature solid-state reaction of the elements. For bulk monoiron monoboride, FeB, the magnetization properties were described as follows: Tc ≅ 590 K, HC = 10 Oe at 290 K, and 1.1 μB at 4 K for the high-temperature, ordered β-modification, and
INTRODUCTION The tailoring of magnetic properties via nanostructuring of solids is a highly desirable expansion of conventional synthetic procedures. Correlating particle size and disorder with magnetic properties leads to new insights into how magnetic nanomaterials can be designed. Interesting properties and possible applications of magnetic nanoparticles have been discussed by Schüth et al.1 Magnetic properties that vary significantly with the size of the nanoparticles have been described for FePt,2,3 among others. The size of the Weiss domains of a ferromagnetic nanomaterial determines whether it can be made superparamagnetic by decreasing its particle size. A very detailed and extensive overview on nanoscale borides has been given recently in ref 4. A versatile synthesis route for several nanoscale borides including FeB was developed using salt melts.5 Concerning the magnetic properties, only very few reports on nanoscale borides can be found in literature, rare examples being iron boride−silica core−shell nanoparticles or © 2014 American Chemical Society
Received: September 24, 2013 Revised: January 18, 2014 Published: January 24, 2014 1549
dx.doi.org/10.1021/cm403167a | Chem. Mater. 2014, 26, 1549−1552
Chemistry of Materials
Article
particles for the sample annealed at 1050 °C. Magnetization data was collected on a Quantum Design MPMS 5XL magnetometer at temperatures between 320 and 5 K.
Tc between 545 and 602 K, HC = 377 Oe at 290 K, and 1.09 μB at 4 K for the low-temperature, disordered α-modification.13,14 We recently described the synthesis of nanoscale particles of monoiron monoboride, FeB, via low-temperature routes, and their identification and characterization with X-ray absorption spectroscopy (XAS).15 X-ray diffraction (XRD) showed that the primary product obtained by precipitation was amorphous and that it crystallized to form α-FeB between 450 and 1050 °C. The α-modification of FeB is believed to exhibit a variant of a CrB/FeB-type structure in which B−B zigzag chains are stacked upon each other with a high degree of disorder. Hightemperature synthesis yields fully ordered β-FeB as bulk material with a so-called FeB-type structure that is related to the CrB-type. Here we have performed transmission electron microscopy (TEM) to analyze the changes in particle sizes and the increasing degree of crystallization of nanoscale amorphous FeB during heat treatment. The determination of the particle size was complemented by analysis of the full width at halfmaximum (integral breadth) of reflections in XRD data using the Scherrer formula for two of the samples. Magnetic properties of the four samples of amorphous and disordered FeB different in particle size and crystallinity were investigated and compared to magnetic data collected for a bulk β-FeB sample. Interestingly, small, superparamagnetic particles transform to a magnetically hard phase upon increasing size and crystallinity and eventually yield to a fully crystalline but softferromagnetic phase.
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RESULTS AND DISCUSSION It was shown earlier by XRD and XAS that the particles obtained by the precipitation procedure described above consist of FeB.15,18 Now, using X-ray diffraction data a restricted number of reflections was selected for particle size determination. Not all of the patterns and reflections were suitable due to strong overlap of the reflections, peak asymmetry, and a varying background (Figure 1). The mean particle sizes of the
EXPERIMENTAL SECTION
Iron borideFeBwas precipitated from iron bromide and lithium tetrahydroborate in diethylene glycoldibutylether as an amorphous powder and heated to 450 °C (in vacuo), 550 °C (in vacuo), 750 °C (argon atmosphere), and 1050 °C (argon atmosphere) as described earlier.15,18 Solid state reaction of the elements at 1500 °C was employed to produce the high-temperature modification β-FeB for use as a reference material. Particles were handled and stored under dry argon as protective atmosphere, although the samples obtained above 550 °C were air-stable according to XRD controls. Contact times with air during the transfer to the sample holder of the microscope or into the capsules for magnetic measurements were kept as short as possible (
