Magnetic remanence in single atoms

Physical Review Letters, 2018

We use spin-polarized scanning tunneling microscopy to demonstrate that Ho atoms on magnesium oxide exhibit a coercive field of more than 8 T and magnetic bistability for many minutes, both at 35 K. The first spontaneous magnetization reversal events are recorded at 45 K for which the metastable state relaxes in an external field of 8 T. The transverse magnetic anisotropy energy is estimated from magnetic field and bias voltage dependent switching rates at 4.3 K. Our measurements constrain the possible ground state of Ho single atom magnets to either Jz = 7 or 8, both compatible with magnetic bistability at fields larger than 10 mT.

Nature, 2017

The highest-density magnetic storage media will code data in single-atom bits. To date, the smallest individually addressable bistable magnetic bits on surfaces consist of 5-12 atoms 1,2 . Long magnetic relaxation times were demonstrated in molecular magnets containing one lanthanide atom , and recently in ensembles of single holmium (Ho) atoms supported on magnesium oxide (MgO) 12 . Those experiments indicated the possibility for data storage at the fundamental limit, but it remained unclear how to access the individual magnetic centers. Here we demonstrate the reading and writing of individual Ho atoms on MgO, and show that they independently retain their magnetic information over many hours. We read the Ho states by tunnel magnetoresistance and write with current pulses using a scanning tunneling microscope. The magnetic origin of the long-lived states is confirmed by single-atom electron paramagnetic resonance (EPR) 15 on a nearby Fe sensor atom, which shows that Ho has a large out-of-plane moment of (10.1 ± 0.1) µB on this surface. In order to demonstrate independent reading and writing, we built an atomic scale structure with two Ho bits to which we write the four possible states and which we read out remotely by EPR. The high magnetic stability combined with electrical reading and writing shows that single-atom magnetic memory is possible.

Nano Letters

Single atom magnets offer the possibility of magnetic information storage in the most fundamental unit of matter. Identifying the parameters that control the stability of their magnetic states is crucial to design novel quantum magnets with tailored properties. Here we use X-ray absorption spectroscopy to show that the electronic configuration of dysprosium atoms on MgO(100) thin films can be tuned by the proximity of the metal Ag(100) substrate onto which the MgO films are grown. Increasing the MgO thickness from 2.5 to 9 monolayers induces a change in the dysprosium electronic configuration from 4f 9 to 4f 10. Hysteresis loops indicate long magnetic lifetimes for both configurations, however, with a different field-dependent magnetic stability. Combining these measurements with scanning tunneling microscopy, density functional theory, and multiplet calculations unveils the role of the adsorption site and charge transfer to the substrate in determining the stability of quantum states in dysprosium single atom magnets.

Advanced materials (Deerfield Beach, Fla.), 2016

TbPc2 single-molecule magnets adsorbed on a magnesium oxide tunnel barrier exhibit record magnetic remanence, record hysteresis opening, perfect out-of-plane alignment of the magnetic easy axes, and self-assembly into a well-ordered layer.

Surface Science, 2009

Magnetic impurities in solids cause manifold changes in their macroscopic properties, such as anomalous low temperature resistance due to Kondo screening, reduction of the superconducting transition temperature due to local suppression of the order parameter, they create magnetic signatures in semiconductors, and lead to inelastic spin excitations in tunnel junctions. In the present paper we review what has been learnt about these effects from a surface science approach. Placing the magnetic impurities at well defined adsorption sites on single crystal surfaces makes their effect on the host, as well as their own magnetic properties better accessible to experiments, and also better understandable since the atomic environment of the impurity is exactly known lending comparison with theory more direct. After an introduction we discuss X-ray magnetic circular dichroism measurements which are spatially averaging and therefore report on ensemble properties. One of the recent progresses achieved in surface science is the preparation of well defined ensembles, such as surfaces with only single adatoms, each of them in an identical atomic environment and with sufficient mutual distance to exclude interactions. Due to this approach we can now determine the electronic configuration of individual adatoms, their hybridization with the host, and quantify their spin and orbital moments, as well as the spin-orbit induced magnetocrystalline anisotropy, which can be orders of magnitude larger than thin film and bulk values. In the second part we review recent progress in revealing the magnetic properties of individual atoms with the scanning tunneling microscope (STM). With this technique the spatial extent of the Kondo screening cloud and of subgap excitations in the superconductor quasiparticle density of states became apparent. We outline the first pioneering experiments measuring transport through reversible atomic point contacts containing magnetic atoms and measurements using the subgap features caused in superconducting STM tips to detect the magnetism of individual atoms. We then describe experiments using inelastic spin excitation spectroscopy to pin down the magnetic ground state and anisotropy energy of magnetic impurities. We continue with spin-polarized STM experiments reporting magnetization curves of individual magnetic adatoms and finish by a description of the most recent spin-excitation experiments revealing the necessary anisotropy environment for a high spin impurity to display the Kondo effect.

Nature, 2010

A fundamental step towards atomic-or molecular-scale spintronic devices has recently been made by demonstrating that the spin of an individual atom deposited on a surface 1 , or of a small paramagnetic molecule embedded in a nanojunction 2 , can be externally controlled. An appealing next step is the extension of such a capability to the field of information storage, by taking advantage of the magnetic bistability and rich quantum behaviour of single-molecule magnets 3-6 (SMMs). Recently, a proof of concept that the magnetic memory effect is retained when SMMs are chemically anchored to a metallic surface 7 was provided. However, control of the nanoscale organization of these complex systems is required for SMMs to be integrated into molecular spintronic devices 8,9 . Here we show that a preferential orientation of Fe 4 complexes on a gold surface can be achieved by chemical tailoring. As a result, the most striking quantum feature of SMMs-their stepped hysteresis loop, which results from resonant quantum tunnelling of the magnetization 5,6 -can be clearly detected using synchrotron-based spectroscopic techniques. With the aid of multiple theoretical approaches, we relate the angular dependence of the quantum tunnelling resonances to the adsorption geometry, and demonstrate that molecules predominantly lie with their easy axes close to the surface normal. Our findings prove that the quantum spin dynamics can be observed in SMMs chemically grafted to surfaces, and offer a tool to reveal the organization of matter at the nanoscale.

Journal of the American Chemical Society, 2004

The largest single-molecule magnet (SMM) to date has been prepared and studied. Recrystallization of known [Mn12O12(O2CCH2Bu t )16(H2O)4] (1; 8Mn III , 4Mn IV ) from CH2Cl2/MeNO2 causes its conversion to [Mn30O24(OH)8(O2CCH2Bu t )32(H2O)2(MeNO2)4] (2; 3Mn II , 26Mn III , Mn IV ). The structure of 2 consists of a central, near-linear [Mn4O6] backbone, to either side of which are attached two [Mn13O9(OH)4] units. Peripheral ligation around the resulting [Mn30O24(OH)8] core is by 32 Bu t CH2CO2 -, 2 H2O, and 4 MeNO2 groups.

Journal of the American Chemical Society, 2005

The structural characterization of complexes [Mn II 4Mn III 22(pdol)12(OCH3)12(O)16(N3)6] (1) and [Mn II 4-Mn III 22(pdol)12(OCH3)12(O)16(OH)2(H2O)(OCH3)3]‚ClO4‚5CH3OH (2), where pdol 2is di-2-pyridyl methanediol, reveals that each has a metallacryptand shell that encapsulates a manganese oxide core. Variabletemperature direct current magnetic susceptibility measurements on 2 indicate a paramagnetic ground state that results from an overall antiferromagnetic interaction in the cluster, with T values decreasing from 300 K (51.2 cm 3 K mol-1) to 2 K (19.8 cm 3 K mol-1). Variable-temperature alternating current magnetic susceptibility measurements imply that both 1 and 2 behave as single-molecule magnets. Fitting the frequency-dependent out-of-phase magnetic susceptibility to the Arrhenius equation yields an effective energy barrier, Ueff, to magnetization relaxation of 16.5 (0.7 K (11.5 (0.5 cm-1) for 1 and 36.2 (2.0 K (25.1 (1.4 cm-1) for 2. The larger value for 2 is in agreement with the lower molecular symmetry, larger magnetoanisotropy, and higher ground spin state of 2 compared to those of 1. This observation suggests a new strategy for increasing the blocking temperatures in high-nuclearity manganese clusters.

ACS Nano, 2016

The organization of single-molecule magnets (SMMs) on surfaces via thermal sublimation is a prerequisite for the development of future devices for spintronics exploiting * To whom correspondence should be addressed

Inorganic Chemistry, 2000

The preparation, X-ray structure, and detailed physical characterization are presented for a new type of singlemolecule magnet [Mn 4 (O 2 CMe) 2 (pdmH) 6 ](ClO 4 ) 2 (1). Complex 1‚2MeCN‚Et 2 O crystallizes in the triclinic space group P1 h, with cell dimensions at 130 K of a ) 11.914(3) Å, b ) 15.347(4) Å, c ) 9.660(3) Å, R ) 104.58(1)°, ) 93.42(1)°, γ ) 106.06(1)°, and Z ) 1. The cation lies on an inversion center and consists of a planar Mn 4 rhombus that is mixed-valent, Mn III 2 Mn II 2 . The pdmHligands (pdmH 2 is pyridine-2,6-dimethanol) function as either bidentate or tridentate ligands. The bridging between Mn atoms is established by either a deprotonated oxygen atom of a pdmHligand or an acetate ligand. The solvated complex readily loses all acetonitrile and ether solvate molecules to give complex 1, which with time becomes hydrated to give 1‚2.5H 2 O. Direct current and alternating current magnetic susceptibility data are given for 1 and 1‚2.5H 2 O and indicate that the desolvated complex has a S ) 8 ground state, whereas the hydrated 1‚2.5H 2 O has a S ) 9 ground state. Ferromagnetic interactions between Mn III -Mn II and Mn III -Mn III pairs result in parallel spin alignments of the S ) 5 / 2 Mn II and S ) 2 Mn III ions. High-frequency EPR spectra were run for complex 1‚2.5H 2 O at frequencies of 218, 328, and 436 GHz in the 4.5-30 K range. A magnetic-field-oriented polycrystallite sample was employed. Fine structure is clearly seen in this parallel-field EPR spectrum. The transition fields were least-squares-fit to give g ) 1.99, D ) -0.451 K, and B 4°) 2.94 × 10 -5 K for the S ) 9 ground state of 1‚2.5H 2 O. A molecule with a large-spin ground state with D < 0 can function as a single-molecule magnet, as detected by techniques such as ac magnetic susceptibility. Out-of-phase ac signals ( ′′ M ) were seen for complexes 1 and 1‚2.5H 2 O to show that these complexes are single-molecule magnets. A sample of 1 was studied by ac susceptibility in the 0.4-6.4 K range with the ac field oscillating at frequencies in the 1.1-1000 Hz range. A single peak in ′′ M vs temperature plots was seen for each frequency; the temperature of the ′′ M peak varies from 2.03 K at 995 Hz to 1.16 K at 1.1 Hz. Magnetization relaxation rates were evaluated in this way. An Arrhenius plot gave an activation energy of 17.3 K, which, as expected, is less than the 22.4 K value calculated for the thermodynamic barrier for magnetization direction reversal for an S ) 8 complex with D ) -0.35 K. The 1‚2.5H 2 O complex with an S ) 9 ground state has its ′′ M peaks at higher temperatures.