Energy Barrier For Magnetization Reversal Research Articles

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.

An influence of the Dzyaloshinskii-Moriya interaction on the magnetization reversal process of the finite-size Co chains on Pt(664) surface

Energy barriers for magnetization reversal of the finite-size Co chains on Pt(664) surface are calculated with taking the Dzyaloshinskii-Moriya interaction into account. For the numerical calculations the geodesic nudged elastic band method is employed. It has been found that the ground states of such atomic chains are noncollinear. The magnetization reversal of short atomic chains occurs without the formation of domain walls. At the same time, there are two nonequivalent ways for the magnetization reversal of longer atomic chains. The first way is the formation of clockwise domain wall (CDW) and the second way is the formation of anticlockwise domain wall (ACDW). The second way is energetically preferable. It is shown that a metastable state corresponding to the location of ACDW in the middle of the atomic chain can appear. The variation of the parameters of the Hamiltonian shows that the magnetization reversal via the CDW formation can occur only in a certain region of the parameters. The influence of the long-range dipole–dipole interaction on the energy barriers for the magnetization reversal is also investigated. It is shown that the most of the presented results can be satisfactory explained in the framework of the XY-model. The magnetic configurations of the atomic chain near the local minima and the saddle points can be approximated with simple analytical functions.

Modulation of magnetization dynamics of an Er(III) coordination polymer by the conversion of a ligand to a radical using UV light.

Light-induced substance conversion is highly promising for creating new radical-based compounds. Herein, we report an Er(III) coordination polymer [Er(CA)(ACA)(DMF)(H2O)]n (1) and its Y(III)-diluted analogue 1@Y (H2CA = 2,5-dichloro-3,6-dihydroxy-p-quinone, HACA = 9-anthracene carboxylic acid) with the light-induced transformation of the ligand to a radical. The χMT values of light-transformed products 1a and 1a@Y are higher than those of 1 and 1@Y, respectively, due to the formation of radicals by ultraviolet light irradiation, confirmed by EPR measurement as well. The effective energy barriers for magnetization reversal (Ueff) decrease from 72 K for 1 to 67 K for 1a, and from 117 K for 1@Y to 94 K for 1a@Y. This work not only provides a new light-conversion system but also reveals the nature of photo-induced variation of magnetic properties.

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.

Influence of Dzyaloshinskii–Moriya and Dipole–Dipole Interactions on Spontaneous Magnetization Reversal Time of Finite-Length Co Chains on Pt(664) Surfaces

The spontaneous magnetization reversal of the finite-length Co chains on Pt(664) surface is investigated in the framework of the classical effective theory. The effective theory includes the Heisenberg exchange interaction, the magnetic anisotropy energy, the Dzyaloshinskii-Moriya interaction, and the dipole-dipole interaction. The geodesic nudged elastic band method is employed for calculations of the energy barriers for magnetization reversal of the finite-length Co chains. The calculation of the spontaneous magnetization reversal time shows that the dipole-dipole interaction can be neglected at the temperatures higher than 10.9 K. DMI can be neglected at the temperatures higher than 60.2 K. This means that Dzyaloshinskii-Moriya interaction can significantly influence on the magnetization reversal process at low temperatures and should be taken into account.

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.

Robotic Stepwise Synthesis of Hetero-Multinuclear Metal Oxo Clusters as Single-Molecule Magnets.

An efficient stepwise synthesis method for discovering new heteromultinuclear metal clusters using a robotic workflow is developed where numerous reaction conditions for constructing heteromultinuclear metal oxo clusters in polyoxometalates (POMs) were explored using a custom-built automated platform. As a result, new nonanuclear tetrametallic oxo clusters {FeMn4}Lu2A2 in TBA5[(A-α-SiW9O34)2FeMn4O2{Lu(acac)2}2A2] (IIA; A = Ag, Na, K; TBA = tetra-n-butylammonium; acac = acetylacetonate) were discovered by the installation of diamagnetic metal cations A+ into a paramagnetic {FeMn4}Lu2 unit in TBA7[(A-α-SiW9O34)2FeMn4O2{Lu(acac)2}2] (I). POMs IIA exhibited single-molecule magnet properties with the higher energy barriers for magnetization reversal (IIAg, 40.0 K; IINa, 40.3 K; IIK, 26.7 K) compared with that of the parent I (19.7 K). Importantly, these clusters with unique properties were constructed as designed by a step of the predictable sequential multistep reactions with the time-efficient platform.

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.

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〉.

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.

A dichlorido-bridged dinuclear Dy(iii) single-molecule magnet with an effective energy barrier larger than 600 K.

We report a dichlorido-bridged dinuclear dysprosium(iii) single-molecule magnet [Dy(Cy2N)2(μ-Cl)(THF)]2 which shows an effective energy barrier for magnetization reversal (Ueff) of ca. 623 K. This is by far the largest Ueff barrier for any chlorido-bridged lanthanide single-molecule magnet. We observe two relaxation processes with near-identical temperature dependencies, one of which disappears upon magnetic dilution. We suspect that these two processes are the isolated and coupled relaxation processes.

Phosphonate-assisted tetranuclear lanthanide assemblies: observation of the toroidic ground state in the TbIII analogue.

The reaction of LnCl3·6H2O with a multidentate flexible Schiff base ligand (LH4), H2O3PtBu and trifluoroacetic acid (tfaH) afforded a series of homometallic tetranuclear complexes, [Ln4(LH2)2(O3PtBu)2(μ2-η1η1tfa)2][2Cl] (Ln = DyIII (1), TbIII (2) and GdIII (3)). The tetranuclear lanthanide core contains two structurally different lanthanide centres, one being in a distorted trigonal dodecahedron geometry and the other in a distorted trigonal prism. Complexes 1-3 were investigated via direct and alternating current (DC and AC) magnetic susceptibility measurements. Only 1 revealed a weak single-molecule magnet (SMM) behaviour. Alternating current (ac) magnetic susceptibility measurements on 1 reveal a frequency-dependent out-of-phase signal. However, the absence of distinct maxima in the χ'' peak (within the temperature/frequency range of our experiments) prevented deduction of the experimental energy barrier for magnetization reversal (Ueff) and the relaxation time. We have carried out extensive ab initio (CASSCF + RASSI-SO + SINGLE_ANISO + POLY_ANISO) calculations on complexes 1-2 to gain deeper insights into the nature of magnetic anisotropy. Our calculations yielded only one exchange coupling parameter between the two LnIII centres bridged by the ligand (neglecting the exchange between the LnIII centres that are not proximal wrt each other). All the extracted J values indicate a weakly antiferromagnetic coupling between the metal centres (J = -0.025 cm-1 for 1 and J = -0.015 cm-1 for 2). Calculated exchange coupled Ucal values of ∼5 and ∼1 cm-1 in 1 and 2 respectively nicely corroborated the experimental observations regarding weak and no SMM characteristics. Our calculations indicated the presence of a net single-molecule toroidal (SMT) behaviour in complex 2. On the other hand, fitting the magnetic data (susceptibility and magnetization) in the isotropic cluster 3 revealed weak AFM exchange couplings of J1 = 0.025 cm-1 and J2 = -0.020 cm-1 which are consistent with those for GdIII ions.

Impact of fluoride for hydroxide substitution on the magnetic properties of a Co-based single-ion magnet imbedded in the barium apatite crystal lattice

The atomic group O–Co–O persists in the apatite channel and retains a high energy barrier for magnetization reversal.

Fine tuning of magnetization relaxation parameters of the DyO+ single ion magnet in a hydroxy/fluoro-apatite solid solution

Variation of the crystal environment of DyO+ regulates the energy barrier for magnetization reversal and the rate of quantum tunneling of magnetization.

Mononuclear Lanthanide(III)-Salicylideneaniline Complexes: Synthetic, Structural, Spectroscopic, and Magnetic Studies

The reactions of hydrated lanthanide(III) [Ln(III)] nitrates and salicylideneaniline (salanH) have provided access to two families of mononuclear complexes depending on the reaction solvent used. In MeCN, the products are [Ln(NO3)3(salanH)2(H2O)]·MeCN, and, in MeOH, the products are [Ln(NO3)3(salanH)2(MeOH)]·(salanH). The complexes within each family are proven to be isomorphous. The structures of complexes [Ln(NO3)3(salanH)2(H2O)]·MeCN (Ln = Eu, 4·MeCN_Eu, Ln = Dy, 7·MeCN_Dy; Ln = Yb, 10·MeCN_Yb) and [Ln(NO3)3(salanH)2(MeOH)]·(salanH) (Ln = Tb, 17_Tb; Ln = Dy, 18_Dy) have been solved by single-crystal X-ray crystallography. In the five complexes, the LnIII center is bound to six oxygen atoms from the three bidentate chelating nitrato groups, two oxygen atoms from the two monodentate zwitterionic salanH ligands, and one oxygen atom from the coordinated H2O or MeOH group. The salanH ligands are mutually “cis” in 4·MeCN_Eu, 7·MeCN_Dy and 10·MeCN_Yb while they are “trans” in 17_Tb and 18_Dy. The lattice salanH molecule in 17_Tb and 18_Dy is also in its zwitterionic form with the acidic H atom being clearly located on the imine nitrogen atom. The coordination polyhedra defined by the nine oxygen donor atoms can be described as spherical tricapped trigonal prisms in 4·MeCN_Eu, 7·MeCN_Dy, and 10·MeCN_Yb and as spherical capped square antiprisms in 17_Tb and 18_Dy. Various intermolecular interactions build the crystal structures, which are completely different in the members of the two families. Solid-state IR data of the complexes are discussed in terms of their structural features. 1H NMR data for the diamagnetic Y(III) complexes provide strong evidence that the compounds decompose in DMSO by releasing the coordinated salanH ligands. The solid complexes emit green light upon excitation at 360 nm (room temperature) or 405 nm (room temperature). The emission is ligand-based. The solid Pr(III), Nd(III), Sm(III), Er(III), and Yb(III) complexes of both families exhibit LnIII-centered emission in the near-IR region of the electromagnetic spectrum, but there is probably no efficient salanH→LnIII energy transfer responsible for this emission. Detailed magnetic studies reveal that complexes 7·MeCN_Dy, 17_Tb and 18_Dy show field-induced slow magnetic relaxation while complex [Tb(NO3)3(salanH)2(H2O)]·MeCN (6·MeCN_Tb) does not display such properties. The values of the effective energy barrier for magnetization reversal are 13.1 cm−1 for 7·MeCN_Dy, 14.8 cm−1 for 17_Tb, and 31.0 cm−1 for 18_Dy. The enhanced/improved properties of 17_Tb and 18_Dy, compared to those of 6_Tb and 7_Dy, have been correlated with the different supramolecular structural features of the two families. The molecules [Ln(NO3)3(salanH)2(MeOH)] of complexes 17_Tb and 18_Dy are by far better isolated (allowing for better slow magnetic relaxation properties) than the molecules [Ln(NO3)3(salanH)2(H2O)] in 6·MeCN_Tb and 7·MeCN_Dy. The perspectives of the present initial studies in the Ln(III)/salanH chemistry are discussed.

Element-specific magnetic properties of mixed 3d−4f metallacrowns

Single molecule magnets comprising rare earth metals are of high interest due to the unquenched orbital moments of the rare earth ions that result in a large energy barrier for magnetization reversal. We investigate the magnetic properties of polynuclear $3d\text{\ensuremath{-}}4f15$-MC-5 metallacrowns using x-ray magnetic circular dichroism of powder samples at a temperature of 7 K in a magnetic field of 7 T. The sum rule analysis reveals element-specific spin and orbital moments. The magnetic moments of the $3d$ transition metal Ni(II) ions are coupled antiferromagnetically to each other and contribute only little to the total molecular moment. The spin and orbital moments of the rare earth ions are unexpectedly smaller than the ionic values resulting from Hund's rules. We explain the reduction of the orbital magnetic moment by a finite magnetic anisotropy. Considering an energy functional including magnetic anisotropy and Zeeman energy the powder average reveals a magnetic anisotropy of 28 meV (340 K) in the case of Dy(III) and 7 meV (85 K) in the case of Tb(III). The spin moments agree with the ionic value, too, when the expectation values of the dipole operator are considered.

Coupling Influences SMM Properties for Pure 4 f Systems.

Increasing both the energy barrier for magnetization reversal and the coercive field of the hysteresis loop are significant challenges in the field of single-molecule magnets (SMMs). Coordination geometries of lanthanide ions and magnetic interactions between lanthanide ions are both important for guiding the magnetic behavior of SMMs. We report a high energy barrier of 657 K (457 cm-1 ) in a diamagnetic-ion-diluted lanthanide chain compound with a constrained bisphenoid symmetry (D2d ); this energy barrier is substantially higher than the barrier of 567 K (394 cm-1 ) of the non-diluted chain compound with intrachain ferromagnetic interactions. Although intrachain magnetic interaction lowers the energy barrier for magnetization reversal, it can greatly enhance the coercive fields and zero-field remanence of the hysteresis loops, which is crucial for the rational design of high-performance SMMs. Factors related to the coordination sphere of the lanthanide center, which govern the high magnetic relaxation barriers through the second excited Kramer's doublets and the magnetic interactions that affect the hysteresis loops, were revealed through ab initio calculations.

Cobalt-Based Single-Ion Magnets on an Apatite Lattice: Toward Patterned Arrays for Magnetic Memories.

Single-ion magnets (SIMs) that can maintain magnetization direction on an individual transition metal atom represent the smallest atomic-scale units for future magnetic data storage devices and molecular electronics. Here we present a robust extended inorganic solid hosting efficient SIM centers, as an alternative to molecular SIM crystals. We show that unique dioxocobaltate(II) ions, confined in the channels of strontium hydroxyapatite, exhibit classical SIM features with a large energy barrier for magnetization reversal (Ueff) of 51-59 cm-1. The samples have been tuned such that a magnetization hysteresis opens below 8 K and Ueff increases by a factor of 4 and can be further enhanced to the highest values among 3d metal complexes of 275 cm-1 when Ba is substituted for Sr. The SIM properties are preserved without any tendency toward spin ordering up to a high Co concentration. At a maximal Co content, a hypothetical regular hexagonal grid of SIMs with a 1 nm interspacing on the (001) crystal facet would allow a maximal magnetic recording density of 105 Gb/cm2.

Static and dynamic magnetic properties of the ferromagnetic coordination polymer [Co(NCS)2(py)2]n.

[Co(NCS)2(py)2]n (py = pyridine) is composed of ferromagnetic chains of Co(ii) cations connected by double NCS bridges. The chains are irregular because of two crystallographically inequivalent Co(ii) cations. The coordination polyhedron of the Co(ii) cations is a distorted octahedron built from two N and two S atoms of four equatorial NCS anions and two apical N atoms of the pyridine ligands. Magnetic and specific heat studies show that the compound undergoes a phase transition at 3.7 K to a ferromagnetic state. The determined magnetic interactions, intrachain (J/kB = 28 K) and interchain (zJ'/kB = 0.5 K), confirm the quasi 1D Ising magnetic character of [Co(NCS)2(py)2]n. Using AC magnetic susceptibility measurements the energy barriers for magnetization reversal of different relaxation processes were determined including those of the individual chains. By means of high field-high frequency ESR study the magnetic excitations were observed and explained in the frame of the Ising model in agreement with the results of the magnetic study. Ab initio calculations confirm the high magnetic anisotropy of the system and allow determination of the direction of the easy-axis. The broken symmetry DFT calculations support the ferromagnetic intrachain interactions. The energetic relations relevant for relaxations are discussed. It is concluded that the magnetic model of [Co(NCS)2(py)2]n is not a pure Ising but the transversal exchange plays a role.