© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 5195 wileyonlinelibrary.com COMMUNICATION Giant Hysteresis of Single-Molecule Magnets Adsorbed on a Nonmagnetic Insulator Christian Wäckerlin, Fabio Donati, Aparajita Singha, Romana Baltic, Stefano Rusponi, Katharina Diller, François Patthey, Marina Pivetta, Yanhua Lan, Svetlana Klyatskaya, Mario Ruben, Harald Brune, and Jan Dreiser* Dr. C. Wäckerlin, Dr. F. Donati, A. Singha, R. Baltic, Dr. S. Rusponi, Dr. K. Diller, Dr. F. Patthey, Dr. M. Pivetta, Prof. H. Brune, Dr. J. Dreiser Institute of Physics (IPHYS) École Polytechnique Fédérale de Lausanne (EPFL) Station 3, CH-1015 Lausanne, Switzerland E-mail: jan.dreiser@psi.ch Dr. Y. Lan, Dr. S. Klyatskaya, Prof. M. Ruben Institute of Nanotechnology Karlsruhe Institute of Technology (KIT) D-76344 Eggenstein-Leopoldshafen, Germany Prof. M. Ruben Institut de Physique et Chimie des Matériaux (IPCMS) Université de Strasbourg F-67034 Strasbourg, France Dr. J. Dreiser Swiss Light Source Paul Scherrer Institut (PSI) CH-5232 Villigen, Switzerland DOI: 10.1002/adma.201506305 metal electrode. We use nonmagnetic, insulating MgO, well- known in inorganic spintronic applications, [17,18] which allows to control the electron tunneling rate over many orders of mag- nitude. [19] Moreover, we employ the TbPc 2 SMM [14,15,20–23] as a model system. In the neutral molecule, the Tb(III) ion exhibits an electronic spin state of J = 6. It is sandwiched between two phthalocyanine (Pc) macrocycles (cf. schematic view in Figure 1a) hosting an unpaired electron delocalized over the Pc ligands. The easy-axis-type magnetic anisotropy imposes an energy barrier of ≈65 meV for magnetization reversal, [23] which is largest within the whole series of lanthanide-Pc 2 SMMs. [14,15] On nonmagnetic conducting substrates, only vanishing rema- nence [6–10] and very narrow hysteresis loops [6–9] were observed, much smaller than in bulk measurements, [20] illustrating the disruptive effects of the surface. We note that the adsorption of TbPc 2 on (anti)ferromagnetic materials represents a different situation because of the magnetic exchange interaction with the substrate. [24,25] In those cases, the SMMs were not shown to exhibit slow relaxation of magnetization. Rather, the hysteresis is linked to the one of the magnetic substrates, i.e., it is not an intrinsic property of the SMMs. Overall, the detailed knowledge on TbPc 2 makes it an ideal candidate to test if a tunnel barrier can boost the magnetic properties of surface-adsorbed SMMs. In this communication we show that the magnetic remanence and hysteresis opening obtained with TbPc 2 on MgO tunnel barriers outperform the ones of any other surface-adsorbed SMM [4–13,26] as well as those of bulk samples of TbPc 2 . [20] The scanning tunneling microscopy (STM) images in Figure 1b,c show that TbPc 2 self-assembles by forming per- fectly ordered 2D islands on two monolayers (MLs) of MgO on Ag(100). In line with former results, the SMMs are adsorbed flat on the surface (cf. discussion of our STM and X-ray linear dichroism (XLD) data below). [6,27] This excludes that the extraor- dinary magnetic properties observed in this study are due to upstanding molecules having their macrocycles perpendicular to the surface, which would lead to a reduced interaction of the Tb(III) ion with the surface. The high-resolution image in Figure 1c reveals eight lobes per molecule, reminiscent of the staggered conformation of the two phthalocyanine ligands. [27] Islands with the identical molecular assembly are formed by TbPc 2 adsorbed directly onto Ag(100), as shown in the Sup- porting Information. The magnetic properties of the Tb(III) ions in the surface- adsorbed SMMs are determined by X-ray magnetic circular dichroism (XMCD) measurements at the M 4,5 (3 d → 4 f ) edges of Tb. For sub-MLs of TbPc 2 on MgO we find a strong rema- nence larger than 40% of the saturation magnetization sat M and Single-molecule magnets (SMMs) [1] are very promising for molecular spintronics [2] and quantum information processing, [3] because of their magnetic bistability and the quantum nature of their spin. The first step toward devices based on SMMs is their adsorption onto electrode surfaces. [4,5] However, this step already represents a serious obstacle as it severely compromises the magnetic remanence. [6–13] Here, we solve this problem by introducing a tunnel barrier between the SMMs and the metal electrode. For TbPc 2 SMMs [14,15] on nonmagnetic, insulating MgO on Ag(100) we demonstrate record values of the magnetic remanence and the hysteresis opening, outperforming any pre- viously reported surface adsorbed SMMs. The two key properties of a magnet relevant to devices are large remanence and wide hysteresis opening. Achieving these goals represents a largely unresolved challenge for SMMs adsorbed at surfaces. Current strategies are to exploit weak adsorption, e.g., on graphite, [8,9] or decoupling from the sur- face by long chemical linkers [4,5,10] or bulky ligands. [11,12] While some of the approaches were successful in achieving a sizeable butterfly-like hysteresis opening, [4,5,10–12] so far all attempts to enhance the vanishingly small magnetic remanence of SMMs in contact with surfaces have failed. [4–13] Consequently, the mag- netic remanence of surface-adsorbed SMMs lags far behind the benchmark set by bulk samples, [16] which are, however, not useful for device applications. Here we introduce an entirely different strategy, namely, the insertion of a tunnel barrier between the SMMs and the Adv. Mater. 2016, 28, 5195–5199 www.advmat.de www.MaterialsViews.com