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. 2016 Aug 1;7(8):5181-5191.
doi: 10.1039/c6sc00279j. Epub 2016 Apr 13.

An air-stable Dy(iii) single-ion magnet with high anisotropy barrier and blocking temperature

Sandeep K Gupta  1 , Thayalan Rajeshkumar  1 , Gopalan Rajaraman  1 , Ramaswamy Murugavel  1
Affiliations

An air-stable Dy(iii) single-ion magnet with high anisotropy barrier and blocking temperature

Sandeep K Gupta et al. Chem Sci. .

Abstract

Herein we report air-stable Dy(iii) and Er(iii) single-ion magnets (SIMs) with pseudo-D5h symmetry, synthesized from a sterically encumbered phosphonamide, t BuPO(NHiPr)2, where the Dy(iii)-SIM exhibits a magnetization blocking (TB) up to 12 K, defined from the maxima of the zero-field cooled magnetization curve, with an anisotropy barrier (Ueff) as high as 735.4 K. The Dy(iii)-SIM exhibits a magnetic hysteresis up to 12 K (30 K) with a large coercivity of ∼0.9 T (∼1.5 T) at a sweep rate of ∼0.0018 T s-1 (0.02 T s-1). These high values combined with persistent stability under ambient conditions, render this system as one of the best-characterized SIMs. Ab initio calculations have been used to establish the connection between the higher-order symmetry of the molecule and the quenching of quantum tunnelling of magnetization (QTM) effects. The relaxation of magnetization is observed via the second excited Kramers doublet owing to pseudo-high-order symmetry, which quenches the QTM. This study highlights fine-tuning of symmetry around the lanthanide ion to obtain new-generation SIMs and offers further scope for pushing the limits of Ueff and TB using this approach.

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Figures

Scheme 1
Scheme 1. Synthesis of the seven-coordinate complexes 1–3.
Fig. 1
Fig. 1. (a) Molecular structure of 1. Lattice water molecule and most of the H-atoms have been omitted for clarity. The H-atoms of the water molecules are hydrogen bonded to the three iodide anions and two lattice phosphonic diamide ligands. (b) Polyhedron showing D5h symmetry around DyIII ion.
Fig. 2
Fig. 2. Experimental and ab inito CASSCF computed temperature dependence of the χMT product at 1000 Oe for 1 and 2. Red and blue hollow circles correspond to the experimental magnetic susceptibility data for 1 and 2. The solid lines are the computed magnetic susceptibilities. The intermolecular interaction is assumed to be –0.02 cm–1 in the calculations.
Fig. 3
Fig. 3. (a) Out-of-phase (χM′′) component (inset: in-phase component) of the frequency dependent (0.1–1464 Hz) ac susceptibility measured in the temperature range of 11–40 K in an oscillating ac field of 3.5 Oe and zero applied dc field for 1. (b) Out-of-phase (χM′′) component of the temperature dependent ac susceptibility in an oscillating ac field of 3.5 Oe and zero applied dc field for 1. (Inset) Plot of the relaxation time τ (logarithmic scale) versus T–1 obtained; the dashed blue line corresponds to the fitting of the Orbach relaxation process and the solid black line represents the best fitting to the multiple relaxation process for 1.
Fig. 4
Fig. 4. (a) Plot of zero field-cooled (red) and field-cooled (black) magnetization vs. temperature for 1. (b) The field-dependent magnetization data for 1 were collected at a sweep rate of 0.0018 T s–1 sweeping the field from +2 T to –2 T and back to +2 T in the temperature range 1.8–12.0 K. (Inset; expansion of the M–H curve at 12.0 K.)
Fig. 5
Fig. 5. (a) In-phase (χM′) and (b) out-of-phase (χM′′) component of the ac susceptibility measured in the frequency range of 0.1–1464 Hz in an oscillating ac field of 3.5 Oe under an applied dc field of 2000 Oe for complex 2. (c) Plot of the relaxation time τ (logarithmic scale) versus T–1; the dashed blue line corresponds to fitting of the Orbach relaxation process and the solid black line represents the best fitting to the multiple relaxation process.
Fig. 6
Fig. 6. Plot of the relaxation time τ (logarithmic scale) versus T–1 for 1′; the solid red line corresponds to fitting of the Orbach relaxation process.
Fig. 7
Fig. 7. Electronic structures and energy levels for 1 and 2. (a) and (b) CASSCF computed gzz orientation of the ground state KD of complexes 1 and 2. (c) and (d) Possible relaxation pathways in 1 and 2. The black line indicates the KDs as a function of magnetic moments. Red lines represent QTM via ground state, KDs/TA-QTM via first and second excited states. Pink dashed lines show possible Orbach process.
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