Anomalous magnetic exchange in a dimerized quantum magnet composed of unlike spin species by Curley, SPM, Huddart, BM, Kamenskyi, D, Coak, MJ, Williams, RC, Ghannadzadeh, S, Schneider, A, Okubo, S, Sakurai, T, Ohta, H, Tidey, JP, Graf, D, Clark, SJ, Blundell, SJ, Pratt, FL, Telling, MTF, Lancaster, T, Manson, JL, Goddard, PA