Magnetic transitions and spin dynamics in the3d1systemK2NaCrO8investigated by magnet

Abstract

We report pulsed magnetization and 23 Na NMR studies of a quantum phase transition and its associated spin dynamics in a 3d 1 system, using the alkali-metal peroxychromate K 2 NaCrO 8 . This is an example of a Cr 5+ -based S =1/ 2 and I ϳ 0 material with a clearly defined T N = 1.7 K in zero applied field ͑H͒. Its magnetization ͑M͒ shows a nonlinear increase with H, in contrast to a simple three-dimensional antiferromagnet, and saturates at H C = 7.4 T. This easily accessible H C allows for a detailed study by 23 Na NMR of the magneticfield-induced quantum phase transition from the antiferromagnetically ordered to a ferromagnetically polarized phase. The critical exponent of M͑T͒ϳ͑1−T / T N ͒ ␤ is 0.44Ϯ 0.05 at H = 5 T but shows a definitive increase to 0.53Ϯ 0.05 at 6.8 T, close to H C . This suggests that while the magnetic behavior of K 2 NaCrO 8 is largely describable by a mean-field theory, the field dependence of ␤ remains unexplained.

Magnetic field induced quantum phase transition of the $S = 1/2$ antiferromagnet $K_2NaCrO_8$

“…Cr 5+ in K2NaCrO8. The labels for the atoms is the same as used by Cage and Dalal 14 and the J1 and J2 designations are the same as used by Choi et al 18 Label Cr• • • Cr ( Å) Pathway J1…”

“…These signatures indicate the presence of low dimensional antiferromagnetic behavior and the onset of long-range antiferromagnetic ordering. More recently, Choi et al 18 performed low temperature magnetization and 23 Na NMR measurements and identified a quantum phase transition from an antiferromagnetically ordered state to a ferromagnetically polarized state at critical field H c of about 7.4 T, with β = 0.44 ± 0.05 in H = 5 T and β = 0.53 ± 0.05 in H = 6.8 T. Since mean-field theory yields β = 1/2 for magnetization, these workers concluded that the high field magnetization of K 2 NaCrO 8 is mean-field-like.…”

“…1,2,3,4,5,6,7,8,9,10,11 The purpose of the present work was to consider the curious case of the pentavalent chromium cation, Cr 5+ , an S = 1/2 (3d 1 ) ion that is not a common choice for condensed matter research. 12,13,14,15,16,17,18,19,20 Specifically, the alkali-metal peroxychromate K 2 NaCrO 8 crystallizes with orthorhombic symmetry (space group P bcm) with the lattice parameters a = 8.5883 Å, b = 7.9825 Å, and c = 9.2432 Å at 173 K. 14 The Cr 5+ ion is bonded to four peroxy (O 2 ) 2− ligands in a dodecahedral geometry, and the ions form the three dimensional network shown in Fig. 1.…”

“…1. 14,18 The data from EPR studies showed the g-tensor is anisotropic, with g z = 1.9851, g y = 1.9696, and g x = 1.9636, the electronic ground state is d z 2 , and the magnetic z-axis is along the [110] crystal direction. During studies of the static magnetic susceptibility, χ dc (H = 0.3 T, 1.8 K ≤ T ≤ 20 K), and the specific heat, C(0 ≤ H ≤ 9 T, 1.8 K ≤ T ≤ 10 K), Cage and Dalal 14 observed ordering signatures in poly-crystalline samples.…”

“…Since the bond angle is an important factor in determining the ferromagnetic versus antiferromagnetic nature of the interactions, 29,30,31 one possible exchange pathway for a given Cr 5+ • • • Cr 5+ distance are present. This analysis leads to the results given in Table I, which tabulates the most influential exchange interactions between nn (the intra-planar J 1 and J 2 ) 18 and nnn (the inter-planar J 3 and J 4 ) Cr 5+ ions, Fig. 1.…”