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MAGNETIC SYSTEMS 2009Magnetic structure of the half magnetization plateau phase in CdCr 2 O 4CdCr 2 O 4 belongs to the well-known family of cubic Crbasedspinels ACr 2 O 4 (A=Hg, Cd, and Zn) which have attractedmuch attention because of the highly frustrated pyrochlorelattice formed by their magnetic Cr 3+ (S = 3/2)ions. Furthermore, due to the direct overlap of t 2g orbitalsof neighboring Cr 3+ ions (3d 3 ), the spin Hamiltonianhas dominant isotropic antiferromagnetic nearest neighborinteractions. The resulting strong frustration suppressesthe system from ordering down to a much lower temperaturethan the Curie-Weiss temperature Θ CW . In case ofCdCr 2 O 4 , the system remains paramagnetic up to T N =7.8 K, far below |Θ CW | = 88 K. The ordered state is accompaniedby a cubic to tetragonal structural transition, and isnot a simple collinear antiferromagnet, but an incommensurate(IC) helical magnetic order with a single characteristicwave vector of Q m = (0,δ,1) or (δ,0,1) where δ ∼ 0.09[Ueda et al. Phys. Rev. Lett. 94, 047202 (2005); Chunget al., Phys. Rev. Lett. 95, 247204 (2005)]. Upon applicationof an external magnetic field, CdCr 2 O 4 undergoes aphase transition into a half-magnetization plateau phase atH c1 = 28 T suggesting that each tetrahedron has three upand one down spins (3:1 constraint). Under this restriction,two spin arrangements, one with the rhombohedralR3m symmetry and one with the cubic P4 3 32, are possible,depending on the sign of the next nearest neighbor interaction.Thanks to the recent combination of a 30 T portable miniaturepulsed magnet and the world highest flux neutronsource of the Institut Laue-Langevin (ILL) [Yoshii et al.,Phys. Rev. Lett. 103, 077203 (2009)], we have succeededin following the field dependence of selected magneticBragg reflections, which allowed to distinguish betweenthese two possible magnetic structures. The experimentwas carried out on the thermal neutron tripleaxisspectrometer IN22. The single crystal (a thin plate(∼ 4 × 4 × 0.2 mm 3 ) of ∼ 40 mg) was mounted with the[111] and [110] axes in the horizontal scattering plane. Thepulsed measurements were performed more than 100 timesat each reflection to obtain reasonable statistics.Figure 112(a) shows the time dependence of the elastic neutronscattering intensity measured at the IC magnetic peakof (1.0675, -1.0125, 0.0275) at 2.5 K. The peak intensitygradually decreases to background level and then remainszero between 3 and 4.6 ms (H > 28 T) after which the intensityincreases back to the intermediate level but not tothe original intensity because of magnetic domain orientation.In order to find out where the magnetic intensity ofthe IC peak was transferred to, we performed similar measurementsat a commensurate Q = (1,-1,0) position. Asillustrated in figure 112(b), when a magnetic field was applied,no signal was initially observed at (1,-1,0) for 3 msat which point the intensity suddenly increased due to thefirst-order nature of the field-induced phase transition. Thecommensurate magnetic intensity remained non-zero overexactly the same range of time (and field) over which theIC magnetic signal went down to zero. Our results indicatethat as CdCr 2 O 4 enters the half magnetization plateaustate, the magnetic structure changes from the IC spiral to acommensurate collinear spin structure with Q m = (1,0,0).Figure 112: Time dependence of the magnetic field (solid redlines) and neutron counts (filled circle) measured at (1.0675,-1.0125, 0.0275) and (1,-1,0) reflections at T = 2.5 K.Figure 113: Magnetic field dependence of the peak intensity ofthe (2,-2,0) reflections measured at T = 2.5 K with the ascending(filled circles) and descending (open circles) field.To distinguish between the two possible models, we alsoperformed similar pulsed field measurements at (2,-2,0) atwhich point the R3m structure should produce magneticBragg scattering while the P4 3 32 structure would not. AtH = 0 T, nuclear Bragg intensity is observed at (2,-2,0). Asshown on figure 113, the (2,-2,0) intensity does not changeas the system enters the half-magnetization phase. Thus, weconclude that the half-magnetization spin state of CdCr 2 O 4has the P4 3 32 spin structure.F. Duc, P. Frings, B. Vignolle, G.L.J.A. RikkenH. Nojiri, K. Ohoyama, S. Yoshii (Institute for Materials Research, Tohoku University, Japan), M. Matsuda (JAEA,Tokai, Japan), L-P. Regnault (CEA, DRFMC-SPSMS-MDN, Grenoble)84