Influence of disorder on the structural phase transition and magnetic interactions inBa3−xSrxCr2

Abstract

The spin dimer system Ba 3−x Sr x Cr 2 O 8 is a solid solution of the triplon Bose-Einstein condensation candidates Ba 3 Cr 2 O 8 and Sr 3 Cr 2 O 8. The magnetic intradimer interaction constant J 0 in this spin system can be tuned by varying the Sr content x. Very interestingly, this variation of J 0 with x is highly nonlinear. In the present study, we show that this peculiar behavior of J 0 can be only partly explained by the changes in the average crystal structure alone. We report on neutron powder diffraction experiments to probe the corresponding structural details. Performing extended Hückel tight-binding calculations based on those structural details obtained at liquid-helium temperatures, we found that the change of the magnetic interaction constant can be well reproduced by taking into account the presence of a structural transition due to the Jahn-Teller active Cr 5+ ions. This transition, lifting the orbital degeneracy and thereby the magnetic frustration in the system, is heavily influenced by disorder in the system arising from partially exchanging Ba with Sr.

Tuning the critical magnetic field of the triplon Bose–Einstein condensation in Ba3-xSrxCr2O8

“…A similar, but less pronounced broadening of the antiferromagnetic transition has been be found in the solid solution of ungapped antiferromagnetic materials like jarosite [21]. In the case of Ba x 3-Sr x Cr 2 O 8 , we attribute this broadening to the increasing disorder that has been reported for this system [11].…”

“…Changes in the magnetic phase diagram of quantum magnets due to variations in the chemical composition have indeed been explained based on various scenarios. A partial substitution of Ba by Sr very probably leads to structural disorder [11] or even free defect spins [10] in the system. Such a chemical disorder has been suggested to lead to a Bose-Glass transition in Tl x 1-K x CuCl 3  [22] and NiCl x 2-Br x ·4SC(NH 2 ) 2 [23] (Br-DTN), thereby reducing the critical field and changing the critical exponent.…”

“…This phase transition has been shown to be gradually suppressed in the solid solution Ba x 3-Sr x Cr 2 O 8 for an intermediate Sr content x [10]. As this gradual suppression in turn strongly influences the magnetic interactions, J 0 exhibits a non-monotonous decrease from Sr 3 Cr 2 O 8 to Ba 3 Cr 2 O 8 with a minimum for J 0 around x 2 »  [11].…”

“…Alternatively, M z increases, to first approximation, virtually linearly with H-H c (T) at a fixed temperature T. First discovered in TlCuCl 3 , this effect was found to take place in a number of compounds such as Pb 2 V 3 O 9  [3], NiCl 2 ·4SC(ND 2 ) 2 [4] (DTN), BaCuSi 2 O 6  [5], Ba 3 Cr 2 O 8  [6], and Sr 3 Cr 2 O 8  [7]. Especially the latter two materials have been examined in numerous investigations, both targeting their spin systems [7][8][9][10][11] and their general electronic structure [6,[12][13][14]. Ba 3 Cr 2 O 8 and Sr 3 Cr 2 O 8 are isostructural with a three-dimensional arrangement of Cr 5+ dimers that feature a dominant intradimer interaction constant J 0 and weaker interdimer interactions J ¢.…”

Tuning magnetic spirals beyond room temperature with chemical disorder

“…For Ba and the apical oxygen, the MSD are also small in the ab plane, but very large in the c direction, signaling a static contribution < u S    2 > ∼ 0.016–0.022 Å 2 superimposed to the very weak thermal motion < u T     2 > ∼ 0.002–0.005 Å 2 . This reflects the length fluctuations of the apical distances in the bipyramids3233, confirming the large degree of Cu/Fe disorder in all samples.…”