High-temperature molecular magnetism unlocked with X-ray dichroism

The challenge

Molecule-based magnets are promising candidates for future information technologies because they combine magnetic, optical [1], and electronic functionalities within a single, chemically tuneable material [2]. Prussian Blue analogues (PBAs), composed of metal ions bridged by cyanide ligands, are a versatile testbed for exploring multifunctional molecular systems. However, most molecular magnets order only at cryogenic temperatures, far below the threshold for practical applications, with very few known exceptions [3]. Achieving magnetic ordering above ambient temperature has long remained a key scientific challenge.

For nearly twenty years, theory has suggested that substituting chromium (Cr^III) with the heavier molybdenum (Mo^III) ion in a cyanide-bridged vanadium-chromium Prussian Blue analogue, hexacyanomolybdate (V^II-Cr^III(CN)₆ –> V^II-Mo^III(CN)₆) PBA could dramatically increase the magnetic ordering temperature [4]. Yet the idea remained untested because Mo^III cyanometallates decompose rapidly in protic solvents – including water, the standard medium for PBA synthesis. As a result, no experimental proof of this long-standing prediction was available. This study finally resolves this open question by using element-specific X-ray magnetic circular dichroism (XMCD) spectroscopy to directly probe the alignment of the magnetic moments of metal ions in a newly synthesised V^II-Mo^III(CN)₆ PBA.

 
The experiment

First, the V^II-Mo^III(CN)₆ PBA was synthesised, using mechanochemical grinding rather than solution chemistry, thereby bypassing the aqueous conditions that previously caused Mo^III decomposition. The resulting amorphous powder exhibited strong magnetism at room temperature. 

To determine the nature of the long-range magnetic order, XMCD was employed at beamline ID12. The beamline provides intense, highly circularly polarised X-rays and is one of the few facilities capable of performing XMCD at the Mo L_2,3 edges. These measurements provided element-specific magnetic signatures that helped to identify the orientation of vanadium and molybdenum magnetic moments. 

Click image to enlarge

Fig. 1: a) Variable-temperature magnetic hysteresis loops for the V^II-Mo^III(CN)₆ magnet measured using a SQUID magnetometer. b) Normalised XMCD signals at the Mo L₃-edge (green) and V K-edge (violet) at room temperature, compared with the magnetic hysteresis loop recorded at 300 K (black), confirming ferrimagnetic ordering.

The XMCD spectra revealed an antiparallel alignment of the V and Mo magnetic moments – an unequivocal fingerprint of ferrimagnetic ordering (Figure 1b). Complementary superconducting quantum interference device (SQUID) magnetometry confirmed a critical temperature exceeding 400 K, far above room temperature and among the highest ever reported for a molecule-based magnet (Figure 1a). Together, the results show that hexacyanomolybdate^III can indeed stabilise high-temperature magnetic order, validating the long-standing theoretical prediction (Figure 2).

Click image to enlarge

Fig. 2: A frame from a demonstration showing a V^II-Mo^III(CN)₆ sample being attracted to a 0.5 T electromagnet (full video available in the Supporting Information: https://doi.org/10.1002/advs.202511285).   

The impact

Demonstrating that Mo^III-based PBAs can sustain magnetic ordering well above room temperature opens an entirely new chemical landscape for molecule-based magnets. The discovery removes a key obstacle to designing cyanide-based materials that combine magnetic, optical, and electronic functions under technologically relevant conditions. 

Such robust, high-temperature molecular magnets could support advances in low-energy data storage, spintronic devices, and multifunctional materials engineered at the molecular level. More broadly, the study illustrates how combining innovative synthesis routes with element-specific X-ray techniques enables breakthroughs that were previously considered inaccessible.

 
Principal publication
Heavy Prussian Blue Analog with Magnetic Ordering above 400 K, M. Magott et al., Adv. Sci. e11285 (2025); https://doi.org/10.1002/advs.202511285

References
[1] J.J. Zakrzewski et al., Chem. Rev. 124, 5930 (2024).
[2] S. Chorazy et al., Chem. Soc. Rev. 49, 5945 (2020).
[3] S. Ferlay et al., Nature 378, 701 (1995).
[4] E. Ruiz et al., Chem. Eur. J. 11, 2135 (2005).