Quantum Tunneling Of Magnetization Research Articles

Tuning the Magnetic Interactions and Relaxation Dynamics of Dy2 Single-Molecule Magnets.

Efficient modulation of single-molecule magnet (SMM) behavior was realized by deliberate structural modification of the Dy2 cores of [Dy2(a'povh)2(OAc)2(DMF)2] (1) and [Zn2Dy2(a'povh)2(OAc)6]⋅4 H2O (2; H2a'povh = N'-[amino(pyrimidin-2-yl)methylene]-o-vanilloyl hydrazine). Compound 1 having fourfold linkage between the two dysprosium ions shows high-performance SMM behavior with a thermal energy barrier of 322.1 K, whereas only slow relaxation is observed for compound 2 with only twofold connection between the dysprosium ions. This remarkable discrepancy is mainly because of strong axiality in 1 due to one pronounced covalent bond, as revealed by experimental and theoretical investigations. The significant antiferromagnetic interaction derived from bis(μ2-O) and two acetate bridging groups was found to be crucial in leading to a nonmagnetic ground state in 1, by suppressing zero-field quantum tunneling of magnetization.

Magnetic Relaxation in Single-Electron Single-Ion Cerium(III) Magnets: Insights from Ab Initio Calculations.

Detailed ab initio calculations were performed on two structurally different cerium(III) single-molecule magnets (SMMs) to probe the origin of magnetic anisotropy and to understand the mechanism of magnetic relaxations. The complexes [Ce(III){Zn(II)(L)}2(MeOH)]BPh4 (1) and [Li(dme)3][Ce(III)(cot'')2] (1; L=N,N,O,O-tetradentate Schiff base ligand; 2; DME=dimethoxyethane, COT''=1,4-bis(trimethylsilyl)cyclooctatetraenyldianion), which are reported to be zero-field and field-induced SMMs with effective barrier heights of 21.2 and 30 K respectively, were chosen as examples. CASSCF+RASSI/SINGLE_ANISO calculations unequivocally suggest that mJ|±5/2〉 and |±1/2〉 are the ground states for complexes 1 and 2, respectively. The origin of these differences is rooted back to the nature of the ligand field and the symmetry around the cerium(III) ions. Ab initio magnetisation blockade barriers constructed for complexes 1 and 2 expose a contrasting energy-level pattern with significant quantum tunnelling of magnetisation between the ground state Kramers doublet in complex 2. Calculations performed on several model complexes stress the need for a suitable ligand environment and high symmetry around the cerium(III) ions to obtain a large effective barrier.

Spin reversal in Fe8 under fast pulsed magnetic fields

We report measurements on magnetization reversal in the Fe8 molecular magnet using fast pulsed magnetic fields of 1.5 kT s−1 and in the temperature range of 0.6–4.1 K. We observe and analyze the temperature dependence of the reversal process, which involves in some cases several resonances. Our experiments allow observation of resonant quantum tunneling of magnetization up to a temperature of ∼4 K. We also observe shifts in the maxima of the relaxation within each resonance field with temperature that suggest the emergence of a thermal instability—a combination of spin reversal and self-heating that may result in a magnetic deflagration process. The results are mainly understood in the framework of thermally-activated quantum tunneling transitions in combination with emergence of a thermal instability.

On Random Switching of Magnetization

Random switching of magnetization is studied using a continuous master equation derived for magnetization dynamics driven by a jump-noise process. This paper is concerned with numerical analysis of the Kramers-Brown approximation as well as with low temperature random magnetization switching where the so-called phenomena of macroscopic quantum tunneling of magnetization has been experimentally observed and theoretically predicted.

Nickel(II)-lanthanide(III) magnetic exchange coupling influencing single-molecule magnetic features in {Ni2 Ln2 } complexes.

Four isostructural [Ni2 Ln2 (CH3 CO2 )3 (HL)4 (H2 O)2 ](3+) (Ln(3+) =Dy (1), Tb (2), Ho (3) or Lu (4)) complexes and a dinuclear [NiGd(HL)2 (NO3 )3 ] (5) complex are reported (where HL=2-methoxy-6-[(E)-2'-hydroxymethyl-phenyliminomethyl]-phenolate). For compounds 1-3 and 5, the Ni(2+) ions are ferromagnetically coupled to the respective lanthanide ions. The ferromagnetic coupling in 1 suppresses the quantum tunnelling of magnetisation (QTM), resulting in a rare zero dc field Ni-Dy single-molecule magnet, with an anisotropy barrier Ueff of 19 K.

Organometallic Single-Ion Magnets

The single-molecule magnets (SMMs) are attracting the increasing interesting due to their potential applications in high density information storage, quantum computing, molecular spintronics, and magnetic refrigeration. This field provides scientists a possible access into the crossover of the classical and quantum world, and a wonderful model to study the fascinating magnetic properties between microscopic and macroscopic materials, such as slow magnetization relaxation and quantum tunneling effect. After the milestone discovery of the first single-molecule magnets (SMMs) Mn12ac, many new SMMs were structurally and magnetically characterized. The most studied systems are mainly traditional coordination compounds with polynuclear structures. However, for the difficulties in the control of magnetic anisotropy and exchange coupling interactions of the cluster-type molecules, Mn12ac molecule is still one of the most important SMMs with the high relaxation barrier. From 2011 [1-3], we explored an organometallic sandwich molecule, Cp*ErCOT(Cp* = pentamethylcyclopenta-dienide; COT = cyclooctatetraenide), which behaves as a single-ion magnets, into the field of molecular nanomagnets. It opened a door of SMMs to the chemists in organometallic chemistry. Recently, we found some new sandwich or half-sandwich lanthanide organometallic molecules could also show the slow relaxation of magnetization. We hope these systems can provide new understandings of slow magnetic relaxation and new clues on the design and synthesis of molecular nanomagnets. This work was supported by NSFC, the National Basic Research Program of China.

New families of hetero-tri-spin 2p-3d-4f complexes: synthesis, crystal structures, and magnetic properties.

In this work we report the synthesis, crystal structures, and magnetic behavior of 2p-3d-4f heterospin systems containing the nitroxide radical 4-azido-2,2,6,6-tetramethylpiperidine-1-oxyl radical (N3tempo). These compounds were synthesized through a one-pot reaction by using [Cu(hfac)2], [Ln(hfac)3] (hfac = hexafluoroacetylacetonate, Ln = Dy(III), Tb(III) or Gd(III)), and the N3tempo radical. Depending on the stoichiometric ratio used, the synthesis leads to penta- or trimetallic compounds, with molecular formulas [Cu3Ln2(hfac)8(OH)4(N3tempo)] (Ln = Gd, Tb, Dy) and [CuLn2(hfac)8(N3tempo)2(H2O)2] (Ln = Gd, Dy). The magnetic properties of all compounds were investigated by direct current (dc) and alternating current (ac) measurements. The ac magnetic susceptibility measurements of Tb(III)- and Dy(III)-containing compounds of both families revealed slow relaxation of the magnetization, with magnetic quantum tunneling in zero field.

Magnetic Properties of 1:2 Mixed Cobalt(II) Salicylaldehyde Schiff-Base Complexes with Pyridine Ligands Carrying High-Spin Carbenes (Scar = 2/2, 4/2, 6/2, and 8/2) in Dilute Frozen Solutions: Role of Organic Spin in Heterospin Single-Molecule Magnets

The 1:2 mixtures of Co(p-tolsal)2, p-tolsal = N-p-tolylsalicylideniminato, and diazo-pyridine ligands, DXpy; X = 1, 2, 3l, 3b, and 4, in MTHF solutions were irradiated at cryogenic temperature to form the corresponding 1:2 cobalt-carbene complexes Co(p-tolsal)2(CXpy)2, with Stotal = 5/2, 9/2, 13/2, 13/2, and 17/2, respectively. The resulting Co(p-tolsal)2(CXpy)2, X = 1, 2, 3l, 3b, and 4, showed magnetic behaviors characteristic of heterospin single-molecule magnets with effective activation barriers, Ueff/kB, of 40, 65, 73, 72, and 74 K, for reorientation of the magnetic moment and temperature-dependent hysteresis loops with a coercive force, Hc, of ∼0, 6.2, 10, 6.5, and 9.0 kOe at 1.9 K, respectively. The relaxation times, τQ, due to a quantum tunneling of magnetization (QTM) were estimated to be 1.6 s for Co(p-tolsal)2(C1py)2, ∼2.0 × 10(3) s for Co(p-tolsal)2(C2py)2, and >10(5) s for Co(p-tolsal)2(CXpy)2; X = 3b, 3l, and 4. In heterospin complexes, organic spins, carbenes interacted with the cobalt ion to suppress the QTM pathway, and the τQ value increased with increasing the Stotal values.

Exchange Interaction of Strongly Anisotropic Tripodal Erbium Single-Ion Magnets with Metallic Surfaces

We present a comprehensive study of Er(trensal) single-ion magnets deposited in ultrahigh vacuum onto metallic surfaces. X-ray photoelectron spectroscopy reveals that the molecular structure is preserved after sublimation, and that the molecules are physisorbed on Au(111) while they are chemisorbed on a Ni thin film on Cu(100) single-crystalline surfaces. X-ray magnetic circular dichroism (XMCD) measurements performed on Au(111) samples covered with molecular monolayers held at temperatures down to 4 K suggest that the easy axes of the strongly anisotropic molecules are randomly oriented. Furthermore XMCD indicates a weak antiferromagnetic exchange coupling between the single-ion magnets and the ferromagnetic Ni/Cu(100) substrate. For the latter case, spin-Hamiltonian fits to the XMCD M(H) suggest a significant structural distortion of the molecules. Scanning tunneling microscopy reveals that the molecules are mobile on Au(111) at room temperature, whereas they are more strongly attached on Ni/Cu(100). X-ray photoelectron spectroscopy results provide evidence for the chemical bonding between Er(trensal) molecules and the Ni substrate. Density functional theory calculations support these findings and, in addition, reveal the most stable adsorption configuration on Ni/Cu(100) as well as the Ni-Er exchange path. Our study suggests that the magnetic moment of Er(trensal) can be stabilized via suppression of quantum tunneling of magnetization by exchange coupling to the Ni surface atoms. Moreover, it opens up pathways toward optical addressing of surface-deposited single-ion magnets.

Quantum Tunneling of Magnetization in a Metal-Organic Framework

Resonant quantum tunneling of magnetization has been observed in a hybrid metal-organic framework where an intrinsic magnetic phase separation leads to the coexistence of long-range canted antiferromagnetic order and isolated single-ion quantum magnets. This unusual magnetic phenomenon is well interpreted based on a selective long-distance superexchange model in which the exchange interaction between transition metal ions through an organic linker depends on the position of hydrogen bonds. Our work not only extends the resonant quantum tunneling of magnetization to a new class of materials but also evokes the important role of hydrogen bonding in organic magnetism.

ChemInform Abstract: Modulating the Magnetic Relaxation of Lanthanide‐Based Single‐Molecule Magnets

AbstractReview: 52 refs.

Magnetic properties and chiral states of a trimetallic uranium complex

The magnetic properties of the triangular molecular nanomagnet [UO2L]3 (L = 2-(4-tolyl)-1,3-bis(quinolyl)malondiiminate) have been investigated through electron paramagnetic resonance spectroscopy, high-field magnetization and susceptibility measurements. The experimental findings are well reproduced by a microscopic model including exchange interactions and local crystal fields. These results show that [UO2L]3 is characterized by a non-magnetic ground doublet corresponding to two oppositely twisted chiral arrangements of the uranium moments. The non-axial character of single-ion crystal fields leads to quantum tunneling of the noncollinear magnetization in the presence of a magnetic field applied perpendicularly to the triangle plane.

Chemically Engineered Graphene-Based 2D Organic Molecular Magnet

Carbon-based magnetic materials and structures of mesoscopic dimensions may offer unique opportunities for future nanomagnetoelectronic/spintronic devices. To achieve their potential, carbon nanosystems must have controllable magnetic properties. We demonstrate that nitrophenyl functionalized graphene can act as a room-temperature 2D magnet. We report a comprehensive study of low-temperature magnetotransport, vibrating sample magnetometry (VSM), and superconducting quantum interference (SQUID) measurements before and after radical functionalization. Following nitrophenyl (NP) functionalization, epitaxially grown graphene systems can become organic molecular magnets with ferromagnetic and antiferromagnetic ordering that persists at temperatures above 400 K. The field-dependent, surface magnetoelectric properties were studied using scanning probe microscopy (SPM) techniques. The results indicate that the NP-functionalization orientation and degree of coverage directly affect the magnetic properties of the graphene surface. In addition, graphene-based organic magnetic nanostructures were found to demonstrate a pronounced magneto-optical Kerr effect (MOKE). The results were consistent across different characterization techniques and indicate room-temperature magnetic ordering along preferred graphene orientations in the NP-functionalized samples. Chemically isolated graphene nanoribbons (CINs) were observed along the preferred functionality directions. These results pave the way for future magnetoelectronic/spintronic applications based on promising concepts such as current-induced magnetization switching, magnetoelectricity, half-metallicity, and quantum tunneling of magnetization.

Influence of Intramolecular f-f Interactions on Nuclear Spin Driven Quantum Tunneling of Magnetizations in Quadruple-Decker Phthalocyanine Complexes Containing Two Terbium or Dysprosium Magnetic Centers

Nuclear spin driven quantum tunneling of magnetization (QTM) phenomena, which arise from admixture of more than two orthogonal electronic spin wave functions through the couplings with those of the nuclear spins, are one of the important magnetic relaxation processes in lanthanide single molecule magnets (SMMs) in the low temperature range. Although recent experimental studies have indicated that the presence of the intramolecular f-f interactions affects their magnetic relaxation processes, little attention has been given to their mechanisms and, to the best of our knowledge, no rational theoretical models have been proposed for the interpretations of how the nuclear spin driven QTMs are influenced by the f-f interactions. Since quadruple-decker phthalocyanine complexes with two terbium or dysprosium ions as the magnetic centers show moderate f-f interactions, these are appropriate to investigate the influence of the f-f interactions on the dynamic magnetic relaxation processes. In the present paper, a theoretical model including ligand field (LF) potentials, hyperfine, nuclear quadrupole, magnetic dipolar, and the Zeeman interactions has been constructed to understand the roles of the nuclear spins for the QTM processes, and the resultant Zeeman plots are obtained. The ac susceptibility measurements of the magnetically diluted quadruple-decker monoterbium and diterbium phthalocyanine complexes, [Tb-Y] and [Tb-Tb], have indicated that the presence of the f-f interactions suppresses the QTMs in the absence of the external magnetic field (H(dc)) being consistent with previous reports. On the contrary, the faster magnetic relaxation processes are observed for [Tb-Tb] than [Tb-Y] at H(dc) = 1000 Oe, clearly demonstrating that the QTMs are rather enhanced in the presence of the external magnetic field. Based on the calculated Zeeman diagrams, these observations can be attributed to the enhanced nuclear spin driven QTMs for [Tb-Tb]. At the H(dc) higher than 2000 Oe, the magnetic relaxations become faster with increasing Hdc for both complexes, which are possibly ascribed to the enhanced direct processes. The results on the dysprosium complexes are also discussed as the example of a Kramers system.

Macroscopic quantum tunneling of magnetization explored by quantum-first-order reversal curves

A method to study the fundamental problem of quantum double well potential systems that display magnetic hysteresis is proposed. The method, coined quantum-first-order reversal curve (QFORC), is inspired by the first-order reversal curve, based on the Preisach model for hysteresis. We successfully tested the QFORC method in the hysteresis of the Mn12Ac molecular magnet, which is governed by macroscopic quantum tunneling of magnetization. The QFORC reproduces well the experimental magnetization behavior. It is possible to separate the thermal activation and tunneling contributions from the magnetization variation, as well as associate the magnetization jumps with specific quantum transitions.

Effects of Transverse Magnetic Anisotropy on Current-Induced Spin Switching

Spin-polarized transport through bistable magnetic adatoms or single-molecule magnets (SMMs), which exhibit both uniaxial and transverse magnetic anisotropy, is considered theoretically. The main focus is on the impact of transverse anisotropy on transport characteristics and the adatom's or SMM's spin. In particular, we analyze the role of quantum tunneling of magnetization (QTM) in the mechanism of the current-induced spin switching, and show that the QTM phenomenon becomes revealed as resonant peaks in the average values of the molecule's spin and in the charge current. These features appear at some resonant fields and are observable when at least one of the electrodes is ferromagnetic.

Binuclear Phthalocyanine‐Based Sandwich‐Type Rare Earth Complexes: Unprecedented Two π‐Bridged Biradical‐Metal Integrated SMMs

Mini-magnets: Sandwich-type rare earth complexes involving two fused bis(phthalocyaninato) dysprosium(III) units, represent the first example of biradical-metal single molecule magnets (SMMs). These materials were synthesized and structurally characterized. Comparative investigation reveals the effective suppression of quantum tunneling of magnetization (QTM) by the π-bridged biradical-based antiferromagnetic interaction in the di-dysprosium-ion-based SMM.

Single-Molecule Magnetism in Three Related {CoIII2DyIII2}-Acetylacetonate Complexes with Multiple Relaxation Mechanisms

Three new heterometallic complexes with formulas of [Dy(III)2Co(III)2(OMe)2(teaH)2(acac)4(NO3)2] (1), [Dy(III)2Co(III)2(OH)2(teaH)2(acac)4(NO3)2]·4H2O (2), and [Dy(III)2Co(III)2(OMe)2(mdea)2(acac)4(NO3)2] (3) were characterized by single-crystal X-ray diffraction and by dc and ac magnetic susceptibility measurements. All three complexes have an identical "butterfly"-type metallic core that consists of two Dy(III) ions occupying the "body" position and two diamagnetic low-spin Co(III) ions occupying the outer "wing-tips". Each complex displays single-molecule magnet (SMM) behavior in zero applied magnetic field, with thermally activated anisotropy barriers of 27, 28, and 38 K above 7.5 K for 1-3, respectively, as well as observing a temperature-independent mechanism of relaxation below 5 K for 1 and 2 and at 3 K for 3, indicating fast quantum tunneling of magnetization (QTM). A second, faster thermally activated relaxation mechanism may also be active under a zero applied dc field as derived from the Cole-Cole data. Interestingly, these complexes demonstrate further relaxation modes that are strongly dependent upon the application of a static dc magnetic field. Dilution experiments that were performed on 1, in the {Y(III)2Co(III)2} diamagnetic analog, show that the slow magnetic relaxation is of a single-ion origin, but it was found that the neighboring ion also plays an important role in the overall relaxation dynamics.

The effect of uniaxial pressure on the magnetic anisotropy of the Mn12-Ac single-molecule magnet

We study the effect of uniaxial pressure on the magnetic hysteresis loops of the single-molecule magnet Mn12-Ac. We find that the application of pressure along the easy axis increases the fields at which quantum tunneling of magnetization occurs. The observations are attributed to an increase in the molecule's magnetic anisotropy constant D of 0.142(1)%/kbar. The increase in D produces a small, but measurable increase in the effective energy barrier for magnetization reversal. Density-functional theory calculations also predict an increase in the barrier with applied pressure.

Magnetization quantum tunneling and improper rotational symmetry

We examine the magnetization quantum tunneling (MQT) behavior expected for a single-molecule magnet (SMM) with improper rotational symmetry. The simplest possible realization is the [NiII(hmp)(dmb)Cl]4 cubane complex that crystallizes in the I41/a space group, resulting in S4 molecular point-group symmetry. A mapping is performed of the energy-level diagram obtained via exact diagonalization of a multi-spin Hamiltonian onto that of a giant-spin model which assumes ferromagnetic coupling and a spin S=4 ground state. The results are compared with a similar analysis for a C3 symmetric Mn3 SMM (S=6 ground state). In the even rotational case (Ni4), the time-reversal invariance associated with the spin–orbit interaction gives rise to a zero-field spin-Hamiltonian that possesses an additional mirror plane perpendicular to the S4 axis, which is not a symmetry element of the molecular point-group. This conclusion applies quite generally to any molecule with improper rotational symmetry (Sq, with q even), including the more widely studied Mn12 SMM. The combined Ni4 and Mn3 studies lead to some interesting predictions concerning MQT selection rules in molecules with even versus odd rotational symmetries. We conclude by considering a case with essentially no symmetry at all, by deliberately distorting the high-symmetry Ni4 molecule. In this case, finite gaps are found at all intersections in the energy-level diagram, indicating a complete absence of MQT selection rules.