Magneto‐Optical Investigations of Nanostructured Materials Based on Single‐Molecule Magnets Monitor Strong Environmental Effects

Abstract

The determination of the magnetic properties of molecular magnets in environments similar to those used in spintronic devices is fundamental for the development of applications. Single-molecule magnets (SMMs) are molecular cluster systems that display magnetic hysteresis of dynamical origin at low temperature. [1,2] As they behave like perfectly monodisperse nanomagnets and show clear macroscopic quantum effects in their magnetic properties, [3] they are extremely appealing candidates for the forthcoming generation of molecular devices: they have been proposed as efficient systems for quantum computation, [4] ultra-high-density magnetic recording media, [5] and molecular spintronic systems. [6][7][8] These attractive possibilities have stimulated the creativity of chemists and materials scientists in developing several different ways of organizing such systems into addressable nanostructured materials. In particular inclusion into Langmuir-Blodgett (LB) films, [9] mesoporous silica, [10] polymeric matrices, [11] and ultrathin films [12] have been devised and studied. The most appealing approaches, both from the applicative and speculative points of view, are the functionalization and binding of such clusters on conducting surfaces [13,14] as well as their incorporation into break-junctions, [6][7][8]15] where a single molecule is directly accessible. These results have led to the creation of the first SMM-based molecular spintronic devices, [15] in which the electronic transport properties are modulated by the magnetic state of a single SMM cluster. The considerable difficulties linked to the interpretation of such results have recently stimulated much theoretical work, [6][7][8] and a number of predictions have been made. Both topological [8] and quantum tunneling [7] effects on the transport in the Kondo regime have been predicted, and several peculiar fingerprints of the SMM behavior should be apparent in transport measurements. [6,15] Interesting effects are also predicted when addressing a SMM on a surface with a tunneling current. [16] Although the influence of the surroundings on the magnetic properties of SMMs has been pointed out in several theoretical and experimental works, [17] our understanding of SMM behavior almost totally relies on measurements performed on crystalline samples. Magnetic measurements on SMMs that lie in environments similar to those of spintronic devices have not been reported up to now, mainly because of the very high sensitivity required. In this Communication we try to fill this void by using high-sensitivity instrumentation, based on magneto-optical (MO) techniques on a variety of materials.

X‐Ray Magnetic Circular Dichroism Picks out Single‐Molecule Magnets Suitable for Nanodevices

“…In fact, the spin dynamics of SMM is exceedingly sensitive to structural deformations, intermolecular interactions, and to other environmental effects, which are expected to be different for SMM hosted in a crystalline lattice, assembled into an addressable layer or incorporated in a metal-molecule-metal junction. Recently we have reported a magneto-optical investigation on the archetypal SMM, the mixed-valence Mn 12 complex, in different environments showing the disappearance of magnetic hysteresis when the clusters are organized as a SAM on gold. [12] Further studies are necessary to assess if the slow magnetization dynamics that characterizes SMM behavior is compatible with the surface environment.…”

“…Recently we have reported a magneto-optical investigation on the archetypal SMM, the mixed-valence Mn 12 complex, in different environments showing the disappearance of magnetic hysteresis when the clusters are organized as a SAM on gold. [12] Further studies are necessary to assess if the slow magnetization dynamics that characterizes SMM behavior is compatible with the surface environment. With this goal in mind we have used X-ray magnetic circular dichroism, XMCD, [13,14] at sub-Kelvin temperatures to investigate the surface magnetic properties of thin films made of two different SMM.…”

“…The 12 , [1,15] and [Fe 4 (L) 2 (dpm) 6 ], Fe 4 , [16,17] where Hdpm is dipivaloylmethane and H 3 L is a tripodal ligand derived from 2-hydroxymethyl-1,3-propanediol. Both types of clusters are characterized by a large spin value of the ground state, S ¼ 10 and S ¼ 5, respectively, and an easy-axis magnetic anisotropy which generates an energy barrier for magnetization reversal of approximately 70 and 20 K, respectively.…”

“…We carried out a bulk magnetic characterization on Mn 12 and Fe 4 samples using a vibrating sample magnetomer (vsm) and a micrometric super conducting quantum interference device (m-SQUID), respectively. The results are reported in Figure 1.…”

Electronic transport properties and orientation of individualMn12single-molecule magnets

“…8 Hence, significant effort is under way to study SMM monolayers or the individual molecules experimentally as well as theoretically. [9][10][11][12][13][14][15][16][17][18][19] For this purpose, Mn 12 acetate 20 and its derivatives are a preferred material class because the high ground-state spin ͑S =10͒ ͑Ref. 21͒ combined with a large magnetic anisotropy results in relatively high blocking temperatures ͑ϳ3.5 K͒ and very long relaxation times possibly allowing an investigation of the magnetic properties of individual molecules at experimentally accessible temperature.…”