Giant coercivity and high magnetic blocking temperatures for N23- radical-bridged dilanthanide complexes upon ligand dissociation

Abstract

Increasing the operating temperatures of single-molecule magnets-molecules that can retain magnetic polarization in the absence of an applied field-has potential implications toward information storage and computing, and may also inform the development of new bulk magnets. Progress toward these goals relies upon the development of synthetic chemistry enabling enhancement of the thermal barrier to reversal of the magnetic moment, while suppressing alternative relaxation processes. Herein, we show that pairing the axial magnetic anisotropy enforced by tetramethylcyclopentadienyl (Cp^Me4H) capping ligands with strong magnetic exchange coupling provided by an N₂^3- radical bridging ligand results in a series of dilanthanide complexes exhibiting exceptionally large magnetic hysteresis loops that persist to high temperatures. Significantly, reducing the coordination number of the metal centers appears to increase axial magnetic anisotropy, giving rise to larger magnetic relaxation barriers and 100-s magnetic blocking temperatures of up to 20 K, as observed for the complex [K(crypt-222)][(Cp^Me4H₂Tb)₂(μ-[Formula: see text])].

Fig. 1

Synthesis and molecular structures of radical complexes. a Synthetic scheme for 1-Ln and 2-Ln. b Structure of the non-radical N₂ ^2–-bridged complex 3-Ln, and structure of the N₂ ³⁻ radical-bridged anions in crystals of 1-Ln and 2-Ln. Dark red, red, blue, and gray spheres represent Tb, O, N, and C atoms, respectively; H atoms have been omitted and THF groups faded for clarity. Selected interatomic distances (Å) and angles (deg) for 1-Gd, 1-Tb, and 1-Dy, respectively: N–N = 1.362(9), 1.371(6), 1.374(3); mean Ln–N = 2.288(8), 2.257(4), 2.236(2); Ln···Ln = 4.344(1), 4.294(3), 4.244(1); Ln–N–N–Ln = 173.3(3), 173.4(3), 173.1(1). Selected interatomic distances (Å) for 2-Tb and 2-Dy, respectively: N–N = 1.392(9), 1.389(12); mean Ln–N = 2.221(6), 2.226(8); Ln···Ln = 4.216(1), 4.230(1); Ln–N–N–Ln = 178.3(3), 178.7(3)

Fig. 2

Temperature dependence of the χ _M T product for radical complexes. a Variable-temperature dc magnetic susceptibility data for restrained polycrystalline samples of 1-Dy (orange circles), 1-Tb (blue triangles), and 1-Gd (gray squares) collected under a 1 T applied dc field. The black line represents a fit to the data for 1-Gd, as discussed in the main text. b Variable-temperature dc magnetic susceptibility data for restrained polycrystalline samples of 2-Dy (pale blue triangles) and 2-Tb (red circles) collected under a 1 T applied dc field. Inset: plot of magnetization vs. temperature for 2-Tb during field-cooled (black circles) and zero-field-cooled (red circles) measurements displaying the thermoremanent magnetization

Fig. 3

Dynamic magnetic susceptibility data. Variable-temperature, variable-frequency in-phase (χ _M′) and out-of-phase (χ _M″) ac magnetic susceptibility data collected for 2-Tb under a zero-applied dc field from 30 to 51 K. A non-zero χ _M″ out-of-phase signal suggests the presence of an energy barrier to spin reversal. Fits of a generalized Debye function to the χ _M′ and χ _M″ data afforded the relaxation times; solid lines represent fits to the data. Low and high-frequency ac magnetic susceptibility data are shown in the Supplementary Figs. 46 and 47

Fig. 4

Arrhenius plots for radical complexes. Plots of the natural log of the relaxation time, τ (blue to red circles), vs. the inverse temperature for a 1-Tb, b 1-Dy, c 2-Tb, and d 2-Dy. Cyan circles represent data extracted from dc susceptibility measurements. Standard deviations of the relaxation times were determined from a nonlinear least-squares analysis employing the program SolverAid (Version 7) by R. de Levie (Microsoft Excel Macro, 2007); error bars are omitted as they are within the radius of the symbols. The black line corresponds to a fit of the data in the temperature range of 4–31 K for 1-Tb, 2–15 K for 1-Dy, 2–60 K for 2-Tb, and 2–18 K for 2-Dy to multiple relaxation processes (see Supplementary Methods and Supplementary Figs. 32, 39, 45, and 56)

Fig. 5

Magnetic hysteresis data for radical complexes. Plot of magnetization (M) vs. dc magnetic field (H) at an average sweep rate of 0.01 T/s for a 1-Tb from 9 to 15 K, b 2-Dy from 2 to 8 K, c 2-Tb from 14 to 24 K, d 2-Tb from 22 to 30 K. The magnetic hysteresis loops are effectively closed at 15 K for 1-Tb, 7.5 K for 2-Dy, and 30 K for 2-Tb, and are open for lower temperatures. For 2-Tb, the coercive fields observed at 22, 23, and 24 K are ~0.1, 0.06, and 0.04 T, respectively