The introduction of a fluorine bridge into a single-molecule magnet strongly inhibits zero-field quantum tunneling

On July 31, 2022, the team of Professor Zheng Yanzhen of Xi’an Jiaotong University published a research report entitled “Suppression of zero-field quantum tunneling of magnetization by a fluorido bridge for a “very hard” 3d-4f single-molecule magnet” in the journal Matter.

By introducing a central fluorine bridge into a 3d-4f single-molecule magnet, this work strongly inhibits its quantum tunneling phenomenon at zero field, providing a new and powerful solution for understanding the quantum tunneling process and designing single-molecule magnets with high residual magnetic ratios. The corresponding author of the paper is Professor Zheng Yanzhen; The first authors are Ling Bokai and Zhai Yuanqi.

Three years from now, the total amount of data stored in the world is expected to exceed 200 trillion gigabytes (GB). Explosive data growth puts forward higher requirements for information storage, but due to the quantum size limitations of nanomaterials, the storage density of traditional magnetic storage materials commonly used at present is close to the limit, so it is urgent to develop new information storage materials with higher density. Among them, single-molecule magnets that use molecular scales for information storage are an effective way to address such needs.

In order to quickly read and write data, high-density magnetic materials should have a hard hysteresis loop, that is, have a large orthodontic field and zero field residual magnetization strength. For single-molecule magnets, due to the quantum tunneling effect at zero field, its residual magnetic strength is difficult to match the saturation magnetization intensity, so it is difficult to exhibit hard hysteresis. Recently, Professor Zheng Yanzhen’s team of Xi’an Jiaotong University induced ferromagnetic exchange between rare earth dysprosium ions by introducing a fluorine bridge in the trinuclear rare earth center, combined with peripheral chromium ions, and strongly inhibited the zero-field tunneling phenomenon of the 3d-4f cluster-based single-molecule magnet (DyC). At the same time, theoretical simulations also prove the role of the fluorine bridge, which provides strong support for the experimental results.

In view of the strong affinity between fluorine and rare earths, the authors use dysprosium tevalerate as a precursor to properly protect rare earth ions so that they do not generate extremely insoluble rare earth fluoride during the reaction with fluorine. The presence of a central fluorine bridge was accurately characterized using means such as XPS, EDS, and ESI-MS (Figure 1).

Figure 1: Characterization of the clusterEduct DyC structure and central fluorine bridge.

Due to the introduction of the central fluorine bridge, the structure of the hydroxyl bridge is different (Inorg. Chem. 2011, 54, 3107), DyC clusters exhibit the properties of single-molecule magnets. Through further magnetic characterization (Figures 2 and 3), it was found that the compound’s coercive field can reach 1.3 T and the residual magnetic ratio is as high as 97%, meaning that zero-field tunneling is almost completely inhibited, which is very rare in the currently reported 3d-4f single-molecule magnets. At the same time, micro-SQUID tests show that it still has a hysteresis effect at 5 K temperatures.

Figure 2: The magnetic susceptibility and magnetization intensity curves of the clustered Complex Dyscone and the magnetic moment arrangement of the ground-state dysprosium site are displayed.

Fig. 3: Clustered DyC monocrystalline Micro-SQUID magnetic test: (A) DyC monocrystalline hysteresis loop (sweep speed 8 mT/s) at 0.03−5 K temperature range; (B and C) Hysteresis loops and their derivative curves at different sweep speeds at 0.03 K; (D) Magnetic relaxation rate and fitting curve.

Magnetic theoretical calculations show that the introduction of the fluorine bridge makes the ground-state magnetic moment dy-dy ferromagnetic exchange, and the dy-Cr antiferromagnetic exchange is arranged, resulting in a large ground-state magnetic moment (10.7 μB) generated by the DyC cluster. By plotting the field-dependent Zeman split diagram (Figure 4), we find that the large spin ground state is the key to inhibiting zero-field quantum tunneling.

Figure 4: Schematic diagram of field-induced quantum tunneling and DyC Zeyman splitting.

This study successfully introduced fluorine bridges in 3d-4f clusters by prodrome method, and accurately determined the content of fluorine by multiple characterization methods, which provided a reasonable reference for the synthesis of subsequent fluorine bridges and rare earth clusters. At the same time, the work proves that the introduction of fluorine bridges is the key to the strong suppression of zero-field quantum tunneling, which is of great significance for designing to improve the relaxation time of single-molecule magnets. (Source: Science Network)

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