E L E C T R O N I C S T R U C T U R E , M A G N E T I S M A N D D Y N A M I C S

S C I E N T I F I C H I G H L I G H T S

9 8 H I G H L I G H T S 2 0 2 2 I

The model includes 10 3/2 spins in a ring coupled through nearest-neighbour interactions, with axial single ion anisotropy perpendicular to the wheel s plane. The exchange coupling scheme in {Cr10} allowing to describe the experimental set consists of two semi-wheels, each with four 3/2 spins ferromagnetically coupled (JFM/kB = 2.0 K), separated by two Cr3+ ions antiferromagnetically coupled in an asymmetric way (JAF/kB = 2.0 K; J AF/kB = 0.25 K). The magnetisation curve was calculated using classical Monte Carlo methods (Figure 89c, right). The exchange coupling constants thus obtained are close to the DTF- derived values. This overall satisfying scheme is supported

by high-resolution INS, which additionally shows the S = 8 total spin state as first excited state.

The scheme of exchange coupling constants along the wheel reflects the details of the {Cr10} molecular structure. In fact, the angles of the two Cr O Cr exchange paths that connect neighbouring Cr3+ ions correlate with the exchange coupling constants. This {Cr10} wheel nicely shows that subtle changes in the molecular geometry may lead to significant changes in the exchange coupling constants, therefore producing drastic modifications in the total spin ground state.

Fig. 88: a) Structure of {Cr10(OMe)20(O2CCMe3)10} wheel. Colour

code: Cr: green; O: red; C: dark grey; H: light grey. b) Scheme of exchange coupling

constants along the {Cr10} ring.

Fig. 89: a) XANES and XMCD at Cr K edge. Top: normalised XANES, measured at 7.5 K and 17 T; bottom: black: XMCD; red: integrated XMCD, (IXMCD)4p. The vertical red dashed lines signal the lower and upper limits of integration. b) Zoom of the pre-K-edge region. Bottom: red: integrated XMCD, (IXMCD)pre-edge. c) XMCD(H) at T = 7.5 K scaled to M(H) isotherms measured with SQUID magnetometry on the same sample and calculated magnetisation curve by classical MC for a powder sample.