Magnetism of TbPc2 on ferromagnetic iron oxide surface

Realization of stable spin states in surface-supported magnetic molecules is crucial for their applications in molecular spintronics, memory storage or quantum information processing. In this work, we studied the surface magnetism of dimetallo-azafullerene Tb2@C79N, showing a broad magnetic hysteresis in a bulk form. Surprisingly, monolayers of Tb2@C79N exhibited a completely different behavior, with the prevalence of a ground state with antiferromagnetic coupling at low magnetic field and a metamagnetic transition in the magnetic field of 2.5-4 T. Monolayers of Tb2@C79N were deposited onto Cu(111) and Au(111) by evaporation in ultra-high vacuum conditions, and their topography and electronic structure were characterized by scanning tunneling microscopy and spectroscopy (STM/STS). X-ray photoelectron spectroscopy (XPS), in combination with DFT studies, revealed that the nitrogen atom of the azafullerene cage tends to avoid metallic surfaces. Magnetic properties of the (sub)monolayers were then studied by X-ray magnetic circular dichroism (XMCD) at the Tb-M4,5 absorption edge. While in bulk powder samples Tb2@C79N behaves as a single-molecule magnet with ferromagnetically coupled magnetic moments and blocking of magnetization at 28 K, its monolayers exhibited a different ground state with antiferromagnetic coupling of Tb magnetic moments. To understand if this unexpected behavior is caused by a strong hybridization of fullerenes with metallic substrates, XMCD measurements were also performed for Tb2@C79N adsorbed on h-BN|Rh(111) and MgO|Ag(100). The co-existence of two forms of Tb2@C79N was found on these substrates as well, but magnetization curves showed narrow magnetic hysteresis detectable up to 25 K. The non-magnetic state of Tb2@C79N in monolayers is assigned to anionic Tb2@C79N- species with doubly-occupied Tb-Tb bonding orbital and antiferromagnetic coupling of the Tb moments. A charge transfer from the substrate or trapping of secondary electrons are discussed as a plausible origin of these species.

Probing the magnetic moments of [MnIII6CrIII]3+ single-molecule magnets—A cross comparison of XMCD and spin-resolved electron spectroscopy

Bisphthalocyaninato terbium complexes show a long magnetization relaxation time at relatively high temperatures---which makes them very interesting as magnets at single-molecule level. Their technological exploitation, however, requires the addressing of the individual molecules, therefore the deposition of single-molecule magnets on surfaces is a topic of great interest as the interaction with the substrate can play a crucial role in the definition of the molecule properties. In this work we investigate the electronic and magnetic properties of anionic and neutral forms of a bis(phthalocyaninato)terbium derivative deposited on graphite by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism, performed at low temperature and high magnetic field at the ${M}_{4,5}$ edge of Tb. We were able to reproduce the experimental spectra by means of multiplet calculations and to validate the applicability of sum rules to the present case. Sum rules were then used for determining the orbital and spin moments of thick (several monolayers) and of thin films (submonolayer range). Calculations of spectra as a function of the molecule orientation with respect to the impinging x-ray beam, allowed us to ascertain the adsorption geometry of molecules. For both compounds, molecules stay essentially flat when adsorbing as thin film on graphite. This result is also confirmed by scanning probe microscopy, which also finds a very interesting ordered arrangement for the molecules of the neutral form. In the thick film of the neutral compound the molecules keep the same orientational order, arranging almost flat as well. On the contrary, in the thick film of the anionic compound their orientation appears to be random. The origin of this different behavior can be related to the hindrance of the counterion moiety and/or to the different solvent used for each compound. Finally, the comparison of the magnetization values and their dependence on the external magnetic field and temperature suggest that the magnetic properties of molecules are preserved when adsorbed onto the graphite surface.

A detailed report on the X-ray Magnetic Circular Dichroism (XMCD) investigation of monolayers of Mn12-based single molecule magnets (SMMs) deposited on gold Au(111) is presented. A semi-quantitative analysis of data is provided in order to extract chemical and magnetic information on Mn ions, by comparison with XMCD on bulk samples. This work points that XMCD is a key-tool for the characterization of SMMs-based nanostructured systems. XMCD surface sensitivity and element-specificity will play a fundamental role in the identification of good candidates for SMMs based devices.

Subsequent to the similar [MnIII6CrIII]3+ single-molecule magnets (SMM), the recently studied [FeIII6CrIII]3+ structural type adsorbed thin films prepared on Si and gold-coated glass substrates have been experimentally studied by means of spin-polarized electron spectroscopy (SPES) and X-ray magnetic circular dichroism (XMCD) at the Fe L3,2 edge using circularly-polarized synchrotron radiation. The results are cross-compared to the corresponding data obtained from the recently published measurements with Mn-based SMM [1], also in terms of the local spin and orbital magnetic moments obtained. Furthermore, [FeIII6CrIII]3+ single crystals have been experimentally studied by means of magnetometry and X-ray diffraction.

We present here a study of the x-ray magnetic circular dichroism (XMCD) at the $K$ edge of the transition metal on rare-earth $(R)$ transition-metal $(T)$ intermetallics. The analysis of the $T\text{ }K$-edge XMCD in the $R{T}_{2}$ compounds $(T=\text{Fe},\text{Co})$ reveals that, when $R$ is magnetic, there is a rare-earth contribution to these spectra which is as intense as to dominate the overall shape and sign of the XMCD signal. As a result, for a given $R$, the XMCD signal recorded in $R{\text{Fe}}_{2}$ is very similar to that of $R{\text{Co}}_{2}$ despite the magnitude of the $\text{Co}\text{ }3d$ magnetic moment is quite different from that of Fe in these compounds. The study of ${\text{XMCD}}_{R}$ as a function of the rare earth itself suggests that the rare-earth contribution to the $T\text{ }K$-edge XMCD has an orbital origin and that its magnitude is related to the orbital component of the magnetic moment, ${L}_{4f}$, instead of the total magnetic moment. Moreover, despite no significant variation in the signals is found when Fe is changed by Co, the amplitude of the signals decreases remarkably as Fe or Co are diluted by nonmagnetic Al. Since aluminum substitution affects only slightly the magnitude of the individual ${\ensuremath{\mu}}_{T}$ and ${\ensuremath{\mu}}_{R}$ magnetic moments but strongly reduces the exchange interaction, this points out that ${\text{XMCD}}_{R}$ shows also a dependence on the strength of the $R\text{\ensuremath{-}}T$ interaction. Therefore, our results suggest that the behavior of ${\text{XMCD}}_{R}$ can be accounted for in terms of a ``molecular fieldlike'' (with ${B}_{RT}\ensuremath{\propto}{n}_{RT}{L}_{R}$) model.

Nanoscale magnetic materials have been attracting enormous interests during the past few decades due to the rapid development of high-density magnetic storage devices. A breakthrough in nanomagnetism is the discovery of single molecule magnets (SMMs), which exhibit slow relaxation and quantum tunneling of the magnetization resulted from a molecular-based blocking anisotropy, and thus have potential applications in high-density data storage, molecular spintronics and quantum computing. Since the report of Mn12-complex as the first SMM by Sessoli et al. in 1993, a large number of SMMs based on organometallic coordination compounds have been synthesized. Compared to the traditional bulk magnets, the magnetic properties of SMMs are determined by their instrinsic spin structures resulting in slow magnetic relaxation. SMM is characterized by a bi-stable magnetic ground state and a certain spin-reversal barrier ( U eff). Additionally, hysteresis loops can be observed below the blocking temperature ( T B). Therefore, SMMs with excellent magnetic properties always have high T B and U eff. U eff depends strongly on spin ground state ( S ) and magneto-crystalline anisotropy ( D ), and can be expressed with the formula U eff =| D|S 2. For transition metal-based SMMs, their low magneto-crystalline anisotropies ( D ) result in relative low U eff. In recent years, lanthanide based SMMs have been studied intensively and become the most promising high-performance SMMs, owning to their strong single ion anisotropy. Encapsulating metal atoms or metallic clusters into fullerene cages generate a special class of fullerenes, the so-called endohedral metallofullerenes (EMFs), for which the encapsulation of metal atoms endow EMFs more fascinating properties compared to empty fullerenes. For instance, EMFs are paramagnetic when the entrapped metal ions have unpaired electrons. Furthermore, EMFs were able to show excellent SMM properties when the encapsulated clusters induce appropriate ligand-fields. EMFs with lanthanide open up a new avenue in the field of SMMs in 2012 when single molecule magnetism was proved for the metal nitride clusterfullerene (NCF) DySc2N@ I h-C80. Fullerene cages are stable in air and thus they can protect the magnetic ions from ambient conditions, and the negatively charged nonmetallic atom(s) within the cluster and the negatively charged carbon atoms of fullerene cages provide magnetic anisotropy which is necessary for single molecule magnetism. Moreover, the molecular structures of EMFs can be modulated by varying the encapsulated metal ions, metal cluster and cage isomer; hence the research on EMF-SMMs has been developed rapidly since 2012. Up to now, metal nitride clusterfullerenes (NCFs), metal carbide clusterfullerenes (CCFs), metal oxide clusterfullerenes (OCFs), metal sulfide clusterfullerenes (SCFs), metal cyanide clusterfullerenes (CYCFs) and dimetallofullerenes (di-EMFs) have been confirmed as SMMs. In this paper, we present an exhaustive review on all kinds of EMF-SMMs reported to date including their 1D, 2D and 3D assemblies. We mainly discuss the influences of the types and the numbers of the encapsulated metal ions and non-metal atoms on the magnetic properties of EMF-SMMs. Finally, the prospects and challenges of EMF-SMMs are also discussed.

Temperature ($T$) and magnetic field ($H$) dependence of the magnetic properties in metamagnetic UCoAl have been investigated using a soft x-ray magnetic circular dichroism (XMCD). In order to extract element-specific magnetic properties at the U and Co sites, the XMCD experiment has been performed at the U 4$d$-5$f$ (${N}_{4},5$) and Co 2$p$-3$d$ (${L}_{2},3$) absorption edges, respectively. Directions of magnetic moments at the U and Co sites have been determined from shapes of the XMCD spectra. The directions of the total magnetic moments at the U and Co sites are parallel to the $H$ direction ($c$ axis), but the direction of the spin magnetic moment at the U site is opposite to that at the Co site. The XMCD intensities at both the U and Co sites at $T=5.5$ K increase steeply at $H=0.77$ T (${H}_{\mathrm{m}}$), corresponding to the metamagnetic transition. The XMCD intensities do not saturate, even in the field-induced ferromagnetic state above ${H}_{\mathrm{m}}$. In addition, the ratio of the increase of the XMCD intensity at the Co site is smaller than that at the U site. From comparison of the $H$ dependence of the XMCD intensities at $T=25$ and 5.5 K, we found that the magnetic behavior of the Co atom has a stronger $T$ dependence than that of the U atom.

The electronic and magnetic states of Mn2.97Co0.03GaC are studied by soft X-ray photoelectron spectroscopy (SXPES) and X-ray magnetic circular dichroism (XMCD) in the Mn 2p→3d excitation region. On the basis of the XMCD results, the contribution of the orbital magnetic moment to the Mn 3d magnetic moment is evaluated by the XMCD sum rules. The spectra obtained by SXPES show a clear change in electronic states near the Fermi level across the first-order ferromagnetic to antiferromagnetic phase transition. The experimental results are consistent with the theoretically obtained electronic structures of the two phases.

The controlled deposition, characterization and manipulation of single molecule magnets (SMMs) on surfaces is one of the crucial points to be addressed with regard to their possible implementation as functional units in future electronic and spintronic devices. Profound understanding of molecule-molecule and molecule-substrate interactions is required as well as unraveling their effect on the molecular electronic and magnetic properties. Local measurement techniques like scanning tunneling microscopy (STM) together with ensemble-averaging methods like X-ray absorption spectroscopy (XAS) have been proven to set up an appropriate frame to study these materials. The majority of these studies deal with SMMs that exhibit rather simple structures with mostly only one magnetic ion. The situation gets more complicated when it comes to larger polynuclear compounds that can be quite fragile with respect to surface deposition or not easy to organize on surfaces due to their bulky ligand shell. Here, we provide an overview of our results on successful deposition of polynuclear SMMs on functional surfaces by employing the electrospray ion beam deposition method. For two prototypes in the field, Mn12-ac and Fe4H, we obtain highly ordered submonolayers on functional surfaces and elucidate the electronic coupling to the respective substrates using scanning tunneling spectroscopy (STS). New results for Mn12-ac on graphene/Ir(111) and for Fe4H on Au(111) are compared to previous studies on a decoupling graphene layer. X-ray magnetic circular dichroism (XMCD) measurements on submonolayers of uniformly aligned Fe4 molecules on both substrates reveal its robust magnetism, showing magnetic anisotropy values similar to bulk.

Recent advances on the organization and characterization of [Mn12] single molecule magnets (SMMs) on a surface or in 3D are reviewed. By using nonconventional techniques such as X-ray magnetic circular dichroism (XMCD) and scanning tunneling microscopy (STM), it is shown that [Mn12]-based SMMs deposited on a surface lose their SMM behavior, even though the molecules seem to be structurally undamaged. A new approach is reported to get high-density information-storage devices, based on the 3D assembling of SMMs in a liquid crystalline phase. The 3D nanostructure exhibits the anisotropic character of the SMMs, thus opening the way to address micrometric volumes by two photon absorption using the pump-probe technique. We present recent developments such as µ-SQUID, magneto-optical Kerr effect (MOKE), or magneto-optical circular dichroism (MOCD), which enable the characterization of SMM nanostructures with exceptional sensitivity. Further, the spin-polarized version of the STM under ultrahigh vacuum is shown to be the key tool for addressing not only single molecule magnets, but also magnetic nano-objects.

In this thesis, the magnetic properties of self-assembled 3d and 4d metal nano-structures supported on surfaces have been investigated. The atomic coordination within the nano-structures was found to profoundly affect important quantities such as the magnetic moment and the magnetic anisotropy. The use of thin, atomically flat insulating Xe spacers of 1-15 monolayers (ML) thickness allowed for a study of coordination effects in the two limits of strong and weak coupling with an underlying metal substrate. The systems were characterized by surface-sensitive methods, based on synchrotron radiation (X-ray magnetic circular dichroism, and X-ray scattering/diffraction) and variable temperature scanning tunneling microscopy (VT-STM). The VT-STM was developed and implemented during this PhD work. First, the magnetism of Rh nano-structures on a Xe buffer layer has been investigated. Rh is non-magnetic in bulk but shows a finite magnetic moment upon reducing cluster sizes to below 100 atoms. Within this work a small, non-zero magnetic moment was found for Rh nano-structures situated on Xe. The effect of intra-cluster Rh-Rh coordination was observed to affect both the spin and orbital part of the magnetic moment, leading to strongly oscillating values at smallest cluster sizes. Further, the analysis of the spectroscopic data suggests an interpretation for the absence of magnetism in directly deposited Rh on Ag(100) that is based on the formation of a kinetically promoted Ag-Rh alloy. Second, the buffer layer assisted growth (BLAG) was studied for sub-monolayer Co nano-clusters on Ag(111) and Pt(111) surfaces. The observation of the cluster formation process in the very early stages of BLAG revealed the paramount importance of the substrate in determining both magnetism and structural properties of the nano-clusters. On Ag(111), a weakly interacting substrate, the clusters form on the buffer layer independently from the metal substrate and show no magnetic anisotropy at this stage. As soon as the Xe is desorbed by sample annealing an in-plane anisotropy forms, as a consequence of the contact with the substrate. X-ray scattering and diffraction data support this interpretation and also show that in the limit of a single monolayer of Xe on Ag(111) the BLAG is a 'simple' atomic diffusion process, with a very high mobility of Co atoms on Xe. On a thick Xe buffer layer instead, due to a lower Xe-Xe binding energy and to the higher surface energy of Co compared to Xe, the deposition of Co provokes a re-arrangement of the Xe atoms. On the other hand, on Pt(111) the BLAG process fails to ensure a cluster formation process ontop the buffer layer and independent of the metal substrate. Here in fact, electric dipolar interactions occurring between Co atoms and the substrate through the Xe layer, are strong enough to destroy the Xe ML order and bring the Co atoms in direct contact with the Pt(111) before Xe atoms are thermally desorbed. This complex process becomes evident from VT-STM investigations and by the occurrence of perpendicular magnetic anisotropy right after Co deposition on the Xe ML/Pt(111). In a detailed discussion it is shown that magnetic properties like magnetic anisotropy and orbital/spin moments are strongly entangled with their morphology. Both morphology and magnetism are determined by the interaction with the environment. This opens the way to more complex systems, where the interaction with the medium is tuned such as to gain nano-structures with a pre-defined structure and function. Third, the knowledge about the cluster-substrate interactions during BLAG was exploited to build highly ordered arrays of Co nano-structures on a patterned template substrate. In this case the hexagonal boron nitride (h-BN) nanomesh on Rh(111) was used. These systems have been employed to study the effect of hybridization of the Co d band with capping layers such as Pt, Au, Al2O3 and MnPt on the magnetic moment of Co. It was found that in all these cases Co clusters have no remanence, due to the small size and weak coupling with the h-BN atoms. However, it could be shown that capping the clusters strongly influence the clusters magnetization, in a non-trivial way.

The discovery of materials capable of storing magnetic information at the level of single molecules and even single atoms has fueled renewed interest in the slow magnetic relaxation properties of single-molecule magnets (SMMs). The lanthanide elements, especially dysprosium, continue to play a pivotal role in the development of potential nanoscale applications of SMMs, including, for example, in molecular spintronics and quantum computing. Aside from their fundamentally fascinating physics, the realization of functional materials based on SMMs requires significant scientific and technical challenges to be overcome. In particular, extremely low temperatures are needed to observe slow magnetic relaxation, and while many SMMs possess a measurable energy barrier to reversal of the magnetization ( Ueff), very few such materials display the important properties of magnetic hysteresis with remanence and coercivity. Werner-type coordination chemistry has been the dominant method used in the synthesis of lanthanide SMMs, and most of our knowledge and understanding of these materials is built on the many important contributions based on this approach. In contrast, lanthanide organometallic chemistry and lanthanide magnetochemistry have effectively evolved along separate lines, hence our goal was to promote a new direction in single-molecule magnetism by uniting the nonclassical organometallic synthetic approach with the traditionally distinct field of molecular magnetism. Over the last several years, our work on SMMs has focused on obtaining a detailed understanding of why magnetic materials based on the dysprosium metallocene cation building block {Cp2Dy}+ display slow magnetic relaxation. Specifically, we aspired to control the SMM properties using novel coordination chemistry in a way that hinges on key considerations, such as the strength and the symmetry of the crystal field. In establishing that the two cyclopentadienyl ligands combine to provide a strongly axial crystal field, we were able to propose a robust magneto-structural correlation for understanding the properties of dysprosium metallocene SMMs. In doing so, a blueprint was established that allows Ueff and the magnetic blocking temperature ( TB) to be improved in a well-defined way. Although experimental discoveries with SMMs occur more rapidly than quantitative theory can (currently) process and explain, a clear message emanating from the literature is that a combination of the two approaches is most effective. In this Account, we summarize the main findings from our own work on dysprosium metallocene SMMs, and consider them in the light of related experimental studies and theoretical interpretations of related materials reported by other protagonists. In doing so, we aim to contribute to the nascent and healthy debate on the nature of spin dynamics in SMMs and allied molecular nanomagnets, which will be crucial for the further advancement of this vibrant research field.

Magnetoelectric (ME) multiferroic materials have unique advantages in low-power and high-density information storage, because they can simultaneously display ferroelectricity and ferromagnetism. However, research on how to construct air-stable high-performance ME single-molecule magnets (SMMs) is nonexistent. Herein, by introducing homochirality while reducing molecular symmetry, two double-decker Dy(III) enantiomers adopting the polar space group P2₁ and exhibiting excellent thermal stability were obtained. They displayed zero field SMM behavior with an anisotropy barrier (Ueff) of ca. 100 cm⁻¹. This work establishes a rational chemical design strategy for crystallizing SMMs in polar space groups and elucidates the direction for future research, that is, engineering small-size high-performance SMMs.

Endohedral fullerenes are perfect nanolaboratories for the study of 4f electron magnetism. The substitution of a diamagnetic scandium atom in Dy2ScN@C80 with magnetically isotropic gadolinium decreases the stability of a given magnetization that is imposed by the Dy2 unit and demonstrates Gd to accelerate the reaching of thermal equilibrium. X-ray magnetic circular dichroism at the M4,5 edges of Gd and Dy shows that Gd also affects the magnetic ground state. The Gd magnetic moment follows the sum of the external and the dipolar magnetic field of the two Dy ions and compared to Dy2ScN@C80 a lower exchange barrier is found between the ferromagnetic and the antiferromagnetic Dy configuration. The Arrhenius equilibration barrier as obtained from superconducting quantum interference device magnetometry is more than one order of magnitude larger, though a much smaller prefactor imposes the faster equilibration in Dy2GdN@C80. This sheds light on the importance of angular momentum balance and symmetry in magnetic relaxation [1].[1] Gadolinium as a single atom catalyst in a single molecule magnet, Aram Kostanyan, Christin Schlesier, Rasmus Westerstrom, Jan Dreiser, Fabian Fritz, Bernd Buchner, Alexey A. Popov, Cinthia Piamonteze, Thomas Greber, arXiv:2009.12259 September (2020).The figure shows the Dy2GdN@C80 molecule where the two Dy 4f9 and the Gd 4f7 orbitals of the paramagnetic entities are shown in red. Note the orientation of the Jz=15/2 Dy 4f9 orbitals towards the N3- ligand. Picture by Ari P. Seitsonen. Figure 1