Self‐Assembly and Magnetism of Mn₁₂ Nanomagnets on Native and Functionalized Gold Surfaces

Abstract

Progress in spin electronics and data storage strongly relies on properties of highly integrated nanostructures with characteristic sizes of only a few atoms. In this respect, a particularly promising approach consists of exploiting advances in coordination chemistry which make it possible to synthesize small clusters with a well-defined number of transition metal ions that behave like identical tiny magnets. [1,2] Various forms of such single-molecule magnets (SMMs) exhibit magnetic bistability at low temperature, which makes them very attractive as elementary units for high-density data-storage applications and in the design of quantum computers.[3] SMMs also exhibit fascinating features related to their borderline behavior between classical and quantum physics: the magnetic hysteresis behavior is superposed by field-dependent features due to quantum relaxation of the magnetization between opposite spin states. These systems are characterized by their blocking temperature, the temperature below which the magnetization does not fluctuate in direction anymore but is blocked on the time scale of the experiment. The highest blocking temperature (T B ) reported so far belongs to the Mn 12 family, whose canonical structure is represented schematically in Figure 1. [4] In these systems, T B » 3 K as measured in SQUID (superconducting quantum interference device)experiments. Besides their comparatively slow relaxation times of magnetization, SMMs also possess distinct advantages over nanoscale metal clusters, such as perfect size calibration and good solubility in organic solvents. A tremendous challenge then consists of assembling monolayers of periodically arranged SMMs on a substrate where they can be addressed easily by near-field techniques. Although a few remarkable strategies aimed at the deposition of Mn 12 -based molecules on surfaces have already been developed, [8±11] detailed topographic information on molecular order is missing. Furthermore, while the magnetic properties of Mn 12 SMMs are reasonably well understood in their crystallized form, only one example has addressed the issue of magnetism in reduced dimensionality, namely by means of a Langmuir±Blodgett stacking of SMMs, [12] and there is virtually no magnetic study of monolayers on surfaces.The surface-science approach adopted in our work encompasses scanning tunneling microscopy (STM) studies in ultrahigh vacuum (UHV) on well-defined crystallographic surfaces. This route is very rewarding since it is meant to provide fundamental information on the organization of atomic [13] and supramolecular [14] clusters on surfaces. A considerable advantage of this approach is to make entities accessible to a detailed study by local, near-field probes, such as STM or atomic force microscopy (AFM) and related spectroscopy, [15] leading to innovating experiences. [16]

Investigation of the stability of Mn12 single molecule magnets

“…However, experiments on the magnetic properties of individual Mn 12 molecules require the ability to deposit and address intact molecules on a surface. With respect to this, there have been several approaches to graft Mn 12 molecules on different surfaces [8][9][10][11][12][13][14]. However, the conservation of the structural, electronic, and magnetic properties of Mn 12 SMMs upon deposition on surfaces could not be unambiguously confirmed so far.…”

Single-Molecule Magnets: A New Approach To Investigate the Electronic Structure of Mn12 Molecules by Scanning Tunneling Spectroscopy

“…For STS investigations, Mn 12 complexes must be grafted to an appropriate surface. Recently, several different approaches to this goal have been reported, such as direct deposition of Mn 12 complexes via sulfur containing ligands, [16][17][18][19] bonding of Mn 12 complexes to functionalized Au(111) or Si(100) surfaces via ligand-exchange reaction, [20][21][22] 23 In a recent study 24 the direct deposition of a Mn 12 complex via sulfur-containing ligands on the bare Au(111) surface was compared with the bonding of the same Mn 12 complex to a functionalized Au(111) surface via ligand exchange reaction. In the case of direct deposition the complete destruction of Mn 12 clusters due to the strong gold-sulfur interaction was revealed.…”

Self-assembled aggregates formed by single-molecule magnets on a gold surface

“…Recently, there have been successful attempts to deposit various single-molecule magnets on surfaces using solution-based deposition methods. This can be achieved either by functionalizing the cluster so that it may be grafted onto an Au(111) surface 13,14,[20][21][22][23][24] or prefunctionalizing both the substrate and the molecule to encourage the formation of adsorbed layers [25][26][27] . Deposition in UHV using pulsed laser deposition 28 , matrix-assisted pulsed laser evaporation 29 , a pulsed vacuum spray technique 30 and a local mechanical method 31 has also been demonstrated.…”

“…The magnetic core is protected by an organic shell that may be altered by conventional wet chemistry techniques such as ligand exchange [12][13][14] and direct synthesis from the appropriate carboxylate reagents 15 . Mn 12 (acetate) 16 exhibits magnetic properties, such as hysteresis cycles, magnetic anisotropy originating from a purely molecular basis 10,16 , quantum magnetic tunnelling effects 17,18 and also has potential for molecular information storage devices 11,19 .…”