Giant Hysteresis of Single‐Molecule Magnets Adsorbed on a Nonmagnetic Insulator

Abstract

metal electrode. We use nonmagnetic, insulating MgO, wellknown in inorganic spintronic applications, [ 17,18 ] which allows to control the electron tunneling rate over many orders of magnitude. [ 19 ] Moreover, we employ the TbPc 2 SMM [ 14,15,[20][21][22][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 1 a) 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 remanence [6][7][8][9][10] and very narrow hysteresis loops [6][7][8][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][5][6][7][8][9][10][11][12][13]26 ] as well as those of bulk samples of TbPc 2 . [ 20 ] The scanning tunneling microscopy (STM) images in Figure 1 b,c show that TbPc 2 self-assembles by forming perfectly ordered 2D islands on two monolayers (MLs) of MgO on Ag(100). In line with former results, the SMMs are adsorbed fl at on the surface (cf. discussion of our STM and X-ray linear dichroism (XLD) data below). [ 6,27 ] This excludes that the extraordinary 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 1 c 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 Supporting Information.The magnetic properties of the Tb(III) ions in the surfaceadsorbed 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 fi nd a strong remanence 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, [...

Magnetic properties of single rare-earth atoms on graphene/Ir(111)

“…In addition, it has been demonstrated that the divalent state of Tb is highly unfavorable even for freestanding dimers, i.e., a single metallic bond is sufficient to trigger its trivalent configuration [48]. Hence, a Tb atom on graphene takes on the most common trivalent configuration that is also observed for TbPc 2 molecular magnets [43,49].…”

“…The magnetic relaxation time has been reported to depend on x-ray photon flux [17,42,43]. This effect is due to the generation of secondary electrons that destabilize the magnetic quantum states.…”

Molecular single-ion magnets based on lanthanides and actinides: Design considerations and new advances in the context of quantum technologies

“…Ab-initio models have been routinely employed to predict which ligand environments allow for the best optimisation of the spin relaxation times [49][50][51], in view of fabricating molecular devices that could store and process information above cryogenic temperatures. Recent work has made significant gains in this direction, with reports of slow magnetic relaxation at temperatures as high as 100 K and magnetic blocking at 5-30 K in Dy- [32][33][34][35][36], Er- [52,53] and Tb- [54,55] SIMs. Other notable achievements are the observation of Rabi oscillations in f-element single molecules [56][57][58][59], and the detection of 'atomic clock transitions' in a Ho-SIM system [60].…”

4f occupancy and magnetism of rare-earth atoms adsorbed on metal substrates

“…In addition, the XMCD sum rules [38,39] and multiplet analysis can be used to estimate the corresponding J z values. We chose late lanthanides for their large magnetic moments coming from the parallel alignment of L and S. Additionally, they exhibit larger magnetic anisotropy and have been more intensively studied as model systems for atomic magnetic memories in single ion [40][41][42][43] and single atom magnets [17,18]. Among the series of late lanthanides, we investigated Dy, Ho, Er, and Tm as they have been found both in divalent and trivalent states in bulk compounds [44], hence, they represent ideal candidates to explore the interplay between surface binding, 4f occupancy, and strength of the crystal field.…”