Ferromagnetism of Pt nanoparticles induced by surface chemisorption

The magnetic properties of different-sized Pt nanoparticles coated by alkanethiol molecules with different carbon chain lengths (1-octadecanethiol, 1-dodecanthiol, and 1-octanethiol) were systematically studied by means of magnetic measurement, x-ray magnetic circular dichroism (XMCD), x-ray absorption near-edge structure (XANES), and electron spin resonance (ESR). Furthermore, azobenzene-derivatized thiol coating of Pt nanoparticles was performed to detect the effect on magnetic characteristics of the geometrical transformation between cis and trans states induced by photoisomerization. All samples showed the ferromagnetism inherent in Pt. In all of the prepared particles Curie temperatures above 300 K were observed. The largest magnetization at 5 K was observed for the longest chain length of alkanethiol and the smallest particle size. The coercive force at 5 K increased as the chain length increased and the particle size decreased. The geometrical transformation of coated molecules barely affected the magnetization and coercive force. The increase in coercive force was accompanied by the enhanced contribution of the orbital magnetic moment. The origins of ferromagnetism and magnetic anisotropy in Pt nanoparticles are discussed in terms of the contribution from the coating molecules and downsizing effects. The appearance of ferromagnetism is mainly interpreted based on two mechanisms, i.e., the electronic band magnetism based on the Stoner criterion for ferromagnetism and the orbital ferromagnetism due to electrons captured in the atomiclike orbital on the particle surface. The chain-length and particle-size-dependent magnetic anisotropy is explained in terms of the change in angular momentum depending on the adsorption condition, especially the coverage ratio, of alkanethiol on the particle surface. This is consistent with the existence of orbital ferromagnetism in the Pt nanoparticle.

Figure 1

TEM image (a) and size distribution histogram (b) of ODT-Pt1 with $d$ = 1.9 nm.

Figure 2

Particle size distributions of ODT-Pt1, ODT-Pt2, and ODT-Pt3.

Figure 3

Optical absorption spectra of 4-(4-phenylazo-phenoxy)-butane-1-thiol (AZO) under illumination of UV and visible light.

Figure 4

Magnetization curves of ODT-Pt1 at various temperatures are shown in (a). The spontaneous magnetization, deduced from the extrapolation of high-field data to zero magnetic field, is shown as a function of temperature. The dotted curve shows the Bloch's law fitting using $\alpha$ = 2 (b). The temperature-dependent coercive force indicates a blocking temperature of ∼100 K (c).

Figure 5

XMCD spectra and the integrated MCD intensity around $L_3$ (a) and $L_2$ (b) edges of Pt. A clear MCD peak is observed only around the $L_3$ edge.

Figure 6

Pt $L_3$ edge XANES spectra of ODT-Pt1, OT-Pt, ODT-Pt3, Pt bulk, and Au bulk are shown in (a), where the Pt and Au spectra are aligned in energy on the fine structure, and normalized in intensity before the edge and at the first two features after the white line. The normalized intensity obtained by subtracting the XANES spectrum of Au from that of Pt nanoparticles is shown in (b). The integrated intensities of the white line are shown in (c), and the ratio of those for Pt nanoparticles and Pt bulk is shown in (d).

Figure 7

Hysteresis curves of OT-Pt, DT-Pt, and ODT-Pt1 at 5 K. The inset shows the coercive force plotted against the carbon number of the coating alkanethiol.

Figure 8

ESR spectra of ODT-Pt1, DT-Pt, and OT-Pt measured at room temperature.

Figure 9

Values of the $g$ factor and linewidth Δ$H$ are shown against the carbon number of coating alkanethiol, and show a similar behavior.

Figure 10

Hysteresis curves of 1-octadecanthiol-coated Pt nanoparticles with diameters of 1.9, 5.6, and 7.3 nm. The inset shows the coercive force as a function of diameter.

Figure 11

ESR spectra of ODT-Pt1 ($d$ = 1.9 nm), ODT-Pt2 ($d$ = 5.6 nm), and ODT-Pt3 ($d$ = 7.3 nm) measured at room temperature. The extra sharp signal is attributed to the remaining reactive precursor a by-product in the preparation process.

Figure 12

Values of the $g$ factor and coercive force are plotted against the diameter of Pt nanoparticles. An inversely proportional relation of coercive force to diameter is roughly observed, as shown by the solid curve.

Figure 13

Magnetic-field-dependent magnetizations of AZO-Pt1 (a) and AZO-Pt2 (b) are obtained at 5 K before and after illumination by UV light, where the enlarged figure in the vicinity of the origin is shown for AZO-Pt1. No contribution of UV light illumination was observed in either sample.