Barrier For Magnetization Reversal Research Articles

Unravelling the Highest Magnetic Anisotropy Among all the nd-Shells in [WCp2]0 Metallocene.

Single-molecule magnets (SMMs) with a large magnetization reversal barrier are predominated by the lanthanide systems due to their strong spin-orbit coupling (SOC). However, the transition metals have also emerged as potential contenders and the largest magnetic anisotropy has been identified for a cobalt system among any d-series-based SMMs (Bunting et al. Science 2018, 362, eaat7319). In this work, we have explored the magnetic anisotropy in highly axial ligand field systems of metallocene, having different d-subshell (3d4, 4d4, and 5d4). The wave function-based multireference methods including static and dynamic electron correlations have been employed to investigate the zero-field splitting (ZFS) parameters. Here, we report exceptionally large magnetic anisotropy for a 5d complex of [WCp2]0 with the highest energy barrier that is nearly twice as high as the previous record value for the Co complex. We have also observed that the axial ZFS parameter (D) is increasing down the group in the order of 3d < 4d < 5d, pertaining to a large SOC.

Bis-Alkoxide Dysprosium(III) Crown Ether Complexes Exhibit Tunable Air Stability and Record Energy Barrier.

High-performance and air-stable single-molecule magnets (SMMs) can offer great convenience for the fabrication of information storage devices. However, the controversial requisition of high stability and magnetic axiality is hard to balance for lanthanide-based SMMs. Here, a family of dysprosium(III) crown ether complexes possessing hexagonal-bipyramidal (pseudo-D6h symmetry) local coordination geometry with tunable air stability and effective energy barrier for magnetization reversal (Ueff) are shown. The three complexes share the common formula of [Dy(18-C-6)L2][I3] (18-C-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane; L = I, 1; L = OtBu 2 and L = 1-AdO 3). 1 is highly unstable in the air. 2 can survive in the air for a few minutes, while 3 remains unchanged in the air for more than 1 week. This is roughly in accordance with the percentage of buried volumes of the axial ligands. More strikingly, 2 and 3 show progressive enhancement of Ueff and 3 exhibits a record high Ueff of 2427(19) K, which significantly contributes to the 100 s blocking temperature up to 11 K for Yttrium-diluted sample, setting a new benchmark for solid-state air-stable SMMs.

A Self-Consistent Exact Diagonalization Approach to the Ground State Magnetic Properties of the Meridional [V(ddpd)2]3+ Complex

The present paper presents a thorough study of the ground state magnetic properties of the spin-one mononuclear nanomagnet mer-[V(ddpd)2][PF6]3, with the V3+ center exhibiting a distorted octahedral coordination. The theoretical analysis is based on a multiconfigurational, self-consistent approach that effectively parametrizes the total energy spectrum of the considered coordination complex via exact diagonalization. We provide a comprehensive discussion for the obtained zero-field and field-dependent fine structure of the ground state along with the ensuing crystal field splitting of the 3d orbitals. Furthermore, we report the results for the low-field susceptibility, magnetization and the corresponding reversal dynamics, finding good agreement with the experimental data reported in the literature. The calculations show considerable zero-field splitting and strong field-dependent orbital unquenching underlying the occurrence of a field-induced full profile magnetization reversal barrier.

Binuclear Lanthanide Complexes Based on 4-Picoline-N-oxide: From Sensitized Luminescence to Single-Molecule Magnet Characteristics

Lanthanide complexes featuring luminescent and/or single-molecule magnet (SMM) characteristic(s) are proposed candidates to develop molecule-based spintronics, memory, and quantum information processing (QIP) device architectures. Herein, we report on the luminescent and/or SMM characteristic(s) of charge-neutral binuclear lanthanide complexes─[Ln2Cl6(4-picNO)4(μ2-4-picNO)2]·2H2O (Ln3+ = Tb3+ ([Tb2]) or Dy3+ ([Dy2]); 4-picNO = 4-picoline-N-oxide). The Dy and Tb complexes show 4-picNO-sensitized luminescence with quantum yields in the solid state of 0.14% and 88%, respectively. The luminescent Dy complex is an SMM with a magnetization reversal barrier (Ueff) = 20.6(4) cm–1 under zero applied magnetic field, whereas the Tb complex is not an SMM. The facile preparation of [Ln2Cl6(4-picNO)4(μ2-4-picNO)2]·2H2O complexes showing high luminescence quantum yield (Tb3+) and SMM characteristics (Dy3+) might prove useful toward the designing of analogue luminescent-SMM hybrids suitable for technological applications.

New chain polymer [Yb(tpa)(H2O)2Co(CN)6]n· 7n H2O: synthesis, structure, and magnetic characteristics

The 3d– 4f heterometallic polymeric complex, namely [Yb(tpa)(H2O)2Co(CN)6]n·7n H2O [tpa = tris(2-pyridylmethyl)- amine], was synthesized and characterized. Its polymer structure is formed of [Yb(tpa)(H2O)2Co(CN)6] chains and crystallization water molecules with a two-capped trigonal prism Yb3+ coordination polyhedron; the Yb3+ coordination number is 8, and the coordination site is YbN6O2. Magnetic characteristics indicate that the complex exhibits the properties of a single-chain magnet with a magnetization reversal barrier(!E/kB) of 42 K.

The impact of spin-vibrational coupling on magnetic relaxation of a Co(II) single-molecule magnet.

Single-molecule magnets (SMMs) based on transition metals have appeared as enticing targets exploiting magnetic anisotropy in 3d elements. Among transition metals, Co based SMMs are very prominent as they often exhibit a high spin-reversal barrier (Ueff), owing to their large unquenched orbital angular momentum. Employing the wave function-based multireference CASSCF/NEVPT2 calculations, herein we substantiate the zero-field splitting parameters of four mononuclear Co complexes and one of them has shown potential as an SMM. The mechanism of magnetic relaxation has been studied to understand the molecular origin of the slow relaxation of magnetization. The combination of suppressed quantum tunneling of magnetization (QTM) at the ground state and the high negative D value usually manifests SMM behavior in a zero-applied magnetic field. However, mere fulfillment of these conditions ensures little about their SMM behavior, as spin-vibrational coupling often plays spoilsport by lowering the spin relaxation channels. A detailed study accounting for all the 46 vibrational modes below the first-excited state for the prospective Co(II) complex, reveals one of the vibrational modes providing a lower spin relaxation pathway. This results in the development of an SMM with a Ueff value of 239.30 cm-1, decreased by ∼81 cm-1 from the value without spin-vibrational coupling.

Tuning the Single-Molecule Magnet and Photoluminescence Properties of Binuclear Dysprosium Complexes by Light

Lanthanide-based single-molecule magnets (Ln-SMMs) showing a synergistic magnetic and photoluminescence change in response to external stimuli are very attractive due to their potential applications in information storage, spintronics, and devices. In this paper, we report two dysprosium complexes containing Dy2O2 motifs and anthracene units, namely [Dy2(SCN)4(L)2(dmpma)2(H2O)2] (1) and [Dy2(SCN)4(L)2(depma)2(H2O)2] (2), where HL stands for 2,6-dimethoxyphenol, dmpma is 9-dimethylphosphonomethylanthracene, and depma is 9-diethylphosphonomethylanthracene. Both exhibit slow magnetization relaxation at zero dc field, characteristic of SMM behavior. Interestingly, complex 2 shows two thermally activated relaxation processes, attributed to the presence of disordered anthracene groups. The effective energy barriers are 117.7 K for 1 and 48.4 and 110.8 K for 2. Under 395 nm UV light irradiation, compounds 1 and 2 undergo a [4 + 4] photocycloaddition reaction to form [Dy2(SCN)4(L)2(dmpma2)(H2O)2]n (1UV) and [Dy2(SCN)4(L)2(depma2)(H2O)2]n·nH2O (2UV) with linear chain structures, where the structural transformation of complex 2 occurs in a single-crystal-to-single-crystal (SC-SC) fashion. Synergistic changes in photoluminescence and SMM behavior were observed for both compounds before and after light irradiation. The results show that the photodimerization of anthracene groups not only inhibits the magnetic relaxation in 1 and 2 and increases the magnetization reversal barrier (172.0 K for 1UV, 120.5 K for 2UV) but also eliminates the anthracene group disorder in compound 2, leading to a significant change in the magnetic dynamics from a dual-relaxation to a single-relaxation process.

Side-Group Effect on the Slow Relaxations of {Dy2} Single-Molecule Magnets with Confined N2O6 Donors.

Deep insights into and substantial enhancement of the effective anisotropy energy barrier for magnetization reversal (Ueff) are vitally important for the technological applications of dysprosium(III)-based single-molecule magnets (Dy-SMMs). To fully refine the ligand-field effect on spin relaxation, four centrosymmetric {Dy2} entities with formula [Dy2(CH3OH)2L2(RCOO)2] (H2L = 2-hydroxy-N'-((pyridin-2-yl)methylene)benzohydrazide) have been solvothermally prepared by varying the side groups of carboxylate coligands (RCOO-, R = CF3 for 1, H for 2, CH3 for 3, and Cp2Fe for 4). Structural analyses reveal that all of the DyIII carriers in 1-4 have the same N2O6 donor environments, and the non-coordinative R groups attached to the equatorial carboxylate bridges have not substantially changed the binding ability of the shortest Dy-Ophenolate bonds located at the axial position of the ligand field. Interestingly, the side groups have monotonically decreased the zero-field Ueff barriers of these weak antiferromagnetically coupled {Dy2} analogues from 721 K down to 379 K. Further electronic structure calculations demonstrate that the main magnetic axes of 1-4 are highly dominated by these comparable Dy-Ophenolate short bonds, and the g tensors have produced gradually increased transverse components responsible significantly for the decreased Ueff barriers. Additionally, thermally assisted relaxations occur preferably through the second (for 1) and the first (for 2-4) Kramer doublets. These interesting findings afford a new side-group effect to comprehensively understand the magnetostructural relationships and advance the rational design of high-performance Dy-SMMs.

Studies of the Temperature Dependence of the Structure and Magnetism of a Hexagonal-Bipyramidal Dysprosium(III) Single-Molecule Magnet.

The hexagonal-bipyramidal lanthanide(III) complex [Dy(OtBu)Cl(18-C-6)][BPh4] (1; 18-C-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane ether) displays an energy barrier for magnetization reversal (Ueff) of ca. 1000 K in a zero direct-current field. Temperature-dependent X-ray diffraction studies of 1 down to 30 K reveal bending of the Cl-Ln-OtBu angle at low temperature. Using ab initio calculations, we show that significant bending of the O-Dy-Cl angle upon cooling from 273 to 100 K leads to a ca. 10% decrease in the energy of the excited electronic states. A thorough exploration of the temperature and field dependencies of the magnetic relaxation rate reveals that magnetic relaxation is dictated by five mechanisms in different regimes: Orbach, Raman-I, quantum tunnelling of magnetization, and Raman-II, in addition to the observation of a phonon bottleneck effect.

Low-Coordinate Dinuclear Dysprosium(III) Single Molecule Magnets Utilizing LiCl as Bridging Moieties and Tris(amido)amine as Blocking Ligands

A low-coordinate dinuclear dysprosium complex {[Dy(N3N)(THF)][LiCl(THF)]}2 (Dy2) with a double bridging ‘LiCl’ moiety and tris(amido)amine (N3N)3− anions as a blocking ligand is synthesized and characterized structurally and magnetically. Thanks to the use of the chelating blocking ligand (N3N)3− equipped with large steric –SiMe3 groups, the coordination sphere of both DyIII ions is restricted to only six donor atoms. The three amido nitrogen atoms determine the orientation of the easy magnetization axes of both DyIII centers. Consequently, Dy2 shows slow magnetic relaxation typical for single molecule magnets (SMMs). However, the effective energy barrier for magnetization reversal determined from the AC magnetic susceptibility measurements is much lower than the separation between the ground and the first excited Kramers doublet based on the CASSCF ab initio calculations. In order to better understand the possible influence of the anticipated intramolecular magnetic interactions in this dinuclear molecule, its GdIII-analog {[Gd(N3N)(THF)][LiCl(THF)]}2 (Gd2) is also synthesized and studied magnetically. Detailed magnetic measurements reveal very weak antiferromagnetic interactions in Gd2. This in turn suggests similar antiferromagnetic interactions in Dy2, which might be responsible for its peculiar SMM behavior and the absence of the magnetic hysteresis loop.

A tool to predict coercivity in magnetic materials

Magnetic coercivity is often viewed to be lower in alloys with negligible (or zero) values of the anisotropy constant. However, this explains little about the dramatic drop in coercivity in FeNi alloys at a non-zero anisotropy value. Here, we develop a theoretical and computational tool to investigate the fundamental interplay between material constants that govern coercivity in bulk magnetic alloys. The two distinguishing features of our coercivity tool are that: (a) we introduce a large localized disturbance, such as a spike-like magnetic domain, that provides a nucleation barrier for magnetization reversal; and (b) we account for magneto-elastic energy—however small—in addition to the anisotropy and magnetostatic energy terms. We apply this coercivity tool to show that the interactions between local instabilities and material constants, such as anisotropy and magnetostriction constants, are key factors that govern magnetic coercivity in bulk alloys. Using our model, we show that coercivity is minimum at the permalloy composition (Fe21.5Ni78.5) at which the alloy’s anisotropy constant is not zero. We systematically vary the values of the anisotropy and magnetostriction constants, around the permalloy composition, and identify new combinations of material constants at which coercivity is small. More broadly, our coercivity tool provides a theoretical framework to potentially discover novel magnetic materials with low coercivity.

Influence of Polymorphism on the Magnetic Properties of Single-Molecule Magnets According to the Data of EPR Spectroscopy in the Terahertz Range

Two earlier described polymorphous modifications of the cobalt(II) (pseudo)clathrochelate are studied by EPR spectroscopy in the terahertz range, where the complex exhibits the properties of a single-molecule magnet (including the record-breaking barrier for magnetization reversal at the moment). Different values of the magnetization reversal barrier found previously for the corresponding crystalline phases by the results of magnetometric measurements in an alternating magnetic field are observed in the EPR spectra in the terahertz range. A combined analysis of these two methods allows one to more precisely estimate the magnetization reversal barrier for two polymorphous modifications and also the contribution of different magnetic relaxation mechanisms to the spin dynamics. This unambiguously confirms that slight changes in the crystalline environment of the molecule of even such structurally rigid metal complexes as cobalt(II) (pseudo)clathrochelates can result in high differences in the magnetization reversal barrier, which is the key characteristic of single-molecule magnets.

Tuning the Equatorial Crystal-Field in Mononuclear DyIII Complexes to Improve Single-Molecule Magnetic Properties.

Two DyIII complexes, [Dy(bbpen-F)X] [X = Cl (1), Br (2); H2bbpen-F = N,N'-bis(2-hydroxybenzyl)-N,N'-bis(5-fluoro-2-methylpyridyl)ethylenediamine], have been synthesized that show remarkable single-molecule-magnet behavior with effective barriers of magnetic reversal of 837.7 K for 1 and 1149.7 K for 2 under zero direct-current field and hysteresis loops up to 20 K for 1 and 30 K for 2, as confirmed by magnetic properties and ab initio calculations.

A Tool to Predict Coercivity in Magnetic Materials

Magnetic coercivity is often viewed to be lower in alloys with negligible (or zero) values of the anisotropy constant. However, this explains little about the dramatic drop in coercivity in FeNi alloys at a nonzero anisotropy value. Here, we develop a theoretical and computational tool to investigate the fundamental interplay between material constants that govern coercivity in bulk magnetic alloys. The two distinguishing features of our coercivity tool are that: (a) we introduce a large localized disturbance, such as a spike-like magnetic domain, that provides a nucleation barrier for magnetization reversal; and (b) we account for magneto-elastic energy---however small---in addition to the anisotropy and magnetostatic energy terms. We apply this coercivity tool to show that the interactions between local instabilities and material constants, such as anisotropy and magnetostriction constants, are key factors that govern magnetic coercivity in bulk alloys. Using our model, we show that coercivity is minimum at the permalloy composition at which the alloy's anisotropy constant is not zero. We systematically vary the values of the anisotropy and magnetostriction constants, around the permalloy composition, and identify new combinations of material constants at which coercivity is small. More broadly, our coercivity tool provides a theoretical framework to potentially discover novel magnetic materials with low coercivity.

Remarkable Energy Barrier for Magnetization Reversal in 3D and 2D Dysprosium‐Chloranilate‐Based Coordination Polymers

Herein, we report two coordination polymers (CPs)[{Dy(Cl2An)1.5(CH3OH)}.4.5H2O]n(CP1) and [Dy(Cl2An)1.5(DMF)2]n(CP2) (where Cl2An2-= chloroanilate (2,5-dihydroxy-1,4-benzoquinone dianion))exhibiting field inducedsingle molecule magnet behavior with moderate barrier of magnetization reversible.Detailed structural and topological analysis disclose that CP1possesses three dimensional network whereas CP2has a two dimensional layered type structure. In both CPs, the magnetic measurements show weak antiferromagnetic exchange interaction between the dysprosium centers and field induced slow magnetic relaxation with the barrier of 175(9) K (CP1) and 145(7) K (CP2) for CPs1and2, respectively.Notably, thevaluesfor CP1and CP2represent remarkableenergy barrier of magnetization reversalin anymetal-chloranilate based 3D (CP1) and 2D (CP2)coordinationpolymers [Anchor] .The temperature and field dependence of relaxation time indicate the presence of multiple relaxation like direct, QTM, Raman, and Orbach processes in both the CPs. The theoreticalab initiocalculations reinforce the experimentally observed higher energy barrier in CP1as compared to CP2due to the presence of large transverse anisotropy on the ground state in the later compound. The average value of the transition magnetic moment between the computed low-lying spin-orbit states also rationalized the possible relaxation process as Orbach and Raman process through the first excited state. The BS-DFT calculations have been carried out to provide more insight into the exchange interaction.

Solvent-Induced Encapsulation of Cobalt(II) Ion by a Boron-Capped tris-Pyrazoloximate.

Boron-cross-linked cobalt(II) pseudoclathrochelate was obtained by the template reaction of 2-acetylpyrazoloxime, phenylboronic acid, and a new DMF cobalt(II) solvato complex with a decachloro-closo-decaborate dianion. As confirmed by single-crystal X-ray diffraction, this complex crystallizes with two symmetry-independent cobalt(II) pseudoclathrochelate cations, one decachloro-closo-decaborate dianion, one benzene, one dichloromethane solvent molecule, and two molecules of DMF. The latter act as pseudocapping fragments to the monocapped tris-pyrazoloximate ligands by forming N-H···O hydrogen bonds with their pyrazole groups. The CoIIN6-coordination polyhedra adopt a nearly ideal TP geometry with distortion angles φ equal to 1.22(16) and 2.58(17)° for two symmetry-independent pseudoclathrochelate cations, both containing the encapsulated cobalt(II) ion in its high-spin state (Co-N 2.115(4)-2.198(3) Å). Magnetic properties of this complex were studied both by dc-magnetometry and by solution-state NMR spectroscopy to reveal a high magnetic anisotropy, thus suggesting a large magnetic susceptibility tensor anisotropy (25.8 × 10-32 m3 at 298 K) and a large negative zero-field splitting energy (-85 cm-1). The results of magnetometry studies in the ac magnetic field suggest a single molecule magnet behavior of this TP complex with an effective magnetization reversal barrier of approximately 130 cm-1. Its pseudocapping DMF molecules that form H-bonds with tris-pyrazoloximate fragments are easy to substitute by strong H-bond acceptors, such as chloride ions and di- and tetramethylureas, thus affecting the magnetic properties of a whole pseudomacrobicyclic paramagnetic system.

Understanding a pentagonal-bipyramidal holmium(iii) complex with a record energy barrier for magnetisation reversal.

We report herein three pentagonal-bipyramidal holmium(iii) complexes sharing the formula [Ho(L)2(py)5][BPh4], where L is the alkoxide ligand. For L = (OSi(CH3)3)-, a record energy barrier for magnetization reversal (Ueff = 715(6) K) was revealed, which is due to the pure state transition of mJ = |±8〉→mJ = |±7〉→mJ = |±6〉.

High-Pressure Crystallographic and Magnetic Studies of Pseudo-D5h Symmetric Dy(III) and Ho(III) Single-Molecule Magnets.

Single-ion magnets based on lanthanide ions in pseudo-D5h symmetry have gained much attention in recent years as they are reported to possess a large blocking temperature and a large barrier for magnetization reversal. Magneto-structural correlations reveal that the axial O-Ln-O angle is an important parameter to control the barrier, and while it can be fine-tuned by chemical modification, an alternative would be to utilize hydrostatic pressure. Herein, we report the crystal structures and static magnetic properties of two air-stable isostructural lanthanide SIMs under applied pressures. The complexes exhibit pseudo-D5h symmetry around the Ln(III)-ion (Ln = Dy or Ho), which coordinates to five equatorial water molecules and two large neutral phosphonic diamide ligands along the axial direction. High-pressure single-crystal X-ray diffraction experiments revealed two phase-transitions and an increasing deviation from D5h-symmetry between ambient pressure and 3.6 GPa. High-pressure direct-current magnetic measurements of the Dy(III) compound showed large steps in the hysteresis loops near zero field, indicative of quantum tunneling of magnetization (QTM). These steps grow in size with increasing pressure, suggesting that QTM becomes progressively more active, which correlates well with the pressure-induced increased overall deviation from pseudo-D5h symmetry and decreasing axial O-Dy-O angle. A strong temperature dependence of the step size is seen at 0.3 GPa, which shows that the SMM character persists even at this pressure. To understand the origin of significant variation in the tunneling probability upon pressure, we performed a range of ab initio calculations based on the CASSCF/RASSI-SO/SINGLE_ANISO method on both Dy and Ho complexes. From the energies and magnetic anisotropy of the mJ sublevels, we find a complex variation of the energy barrier with pressure, and using a constructed geometrical parameter, R, taking into account changes in both bond angles and distances, we link the magnetic properties to the first coordination sphere of the molecules.

Slow Relaxation of Magnetization in the Cobalt‐Containing Strontium Hydroxy/Fluoro‐Apatite

Strontium phosphates with apatite structure containing cobalt and fluoride atoms in the trigonal channels were synthesized and characterized by magnetic measurements. The compounds exhibit slow magnetization relaxation with two types of relaxing centers with an energy barrier for magnetization reversal of about 55 and 200 cm–1, respectively. On the fluoride incorporation, the nature of the magnetic centers remains the same as in the initial hydroxyapatite, while the quantity of cobalt incorporated in the channels is reduced substantially. In the sample with the highest fluoride content slow relaxation behaviour almost disappears.

Insight into D6h Symmetry: Targeting Strong Axiality in Stable Dysprosium(III) Hexagonal Bipyramidal Single-Ion Magnets.

Following a novel synthetic strategy where the strong uniaxial ligand field generated by the Ph3SiO− (Ph3SiO−=anion of triphenylsilanol) and the 2,4‐di‐tBu‐PhO− (2,4‐di‐tBu‐PhO−=anion of 2,4‐di‐tertbutylphenol) ligands combined with the weak equatorial field of the ligand LN6, leads to [DyIII(LN6)(2,4‐di‐tBu‐PhO)2](PF6) (1), [DyIII(LN6)(Ph3SiO)2](PF6) (2) and [DyIII(LN6)(Ph3SiO)2](BPh4) (3) hexagonal bipyramidal dysprosium(III) single‐molecule magnets (SMMs) with high anisotropy barriers of U eff=973 K for 1, U eff=1080 K for 2 and U eff=1124 K for 3 under zero applied dc field. Ab initio calculations predict that the dominant magnetization reversal barrier of these complexes expands up to the 3rd Kramers doublet, thus revealing for the first time the exceptional uniaxial magnetic anisotropy that even the six equatorial donor atoms fail to negate, opening up the possibility to other higher‐order symmetry SMMs.