Nature © Macmillan Publishers Ltd 19988letters to natureNATURE|VOL 393|21 MAY 1998 245the upper critical field, Hc2. The square FLL, shown in the top row ofFig. 3, is stable below 2 kOe for all temperatures. We note thatalthough the magnetic order below TNis also square symmetric,other effects, including in-plane Fermi surface anisotropy16and Hc2anisotropy17, can stabilize the square FLL. As the square FLL is stablein most of the superconducting phase outside the regime ofmagnetic ordering, this symmetry certainly cannot be solely attrib-uted to the magnetic order. In the magnetically ordered state,coincident with the transition at 2 kOe, the FLL undergoes arhombic distortion, as shown in the middle of row of Fig. 3. Thetwo rhombohedral domains, rotated by 908, are each distorted by158 6 18, the same angular splitting as seen in the new magneticmodulation in this region. This symmetry remains stable between 3and 4 kOe. Above 4 kOe, the FLL undergoes a second transition intotwo hexagonal domains, also rotated by 908, shown in the bottomrow of Fig. 3. This cannot be an extension of the low-fieldrhombohedral distortion, as that would result in hexagonaldomains orientated in the [100] direction, rather than the observed[110] as is evident from Fig. 3. Instead, this change in orientationimplies a discontinuous transition. Above 4 kOe, all 12 magneticreflections are of equal intensity, giving the same number ofreflections as the FLL, albeit with a symmetry distorted from purehexagonal. The origin of the FLL symmetry transitions in relation tomagnetic transition is still an open question, as we cannot from thepresent data determine if one drives the other or vice versa.It is important to point out the difference between the symmetrytransitions reported here, and our previous reports of the low-fieldtransition (H , 1 kOe) in ErNi2B2C (ref. 10), and YNi2B2C andLuNi2B2C (ref. 11) where the transitions proceed continuouslythrough a rhombic distortion and is understood from anisotropicflux line interactions due to the high in-plane anisotropy of theFermi surface16. The low-field region was not probed in the SANSmeasurements, but Bitter decorations at 50 Oe and 4.2 K verified ahexagonal FLL. Previous studies of the interaction between magnet-ism and the superconducting FLL in ErNi2B2C showed a rotationdue to the internal field direction and a disordering due to increasedpinning9. We believe that these effects, while convincing evidence ofthe strong coupling between the two types of order, can be under-stood within the scattering time approximation. The observationsreported in here will require a deeper and more subtle under-standing.The splitting of the magnetic peaks with wavevector qmIIcan besuppressed between 4 and 10 kOe by cooling the sample in thesaturated paramagnetic state and then reducing the field into thehexagonal FLL state. Such low-temperature hysteresis is common inmagnetic systems. In HoNi2B2C, hysteresis in the metamagnetictransitions has been reported18, and in DyNi2B2C the associatedresidual moment has been seen to suppress superconductivity19.This hysteresis is not seen in the FLL. If the FLL is the determiningfactor in this problem, then this hysteresis in the split of themagnetic modulation could be ascribed to passing through a FLLsymmetry transition. 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Hedega˚rd and J. Jensen for discussions. This work wassupported by NATO. M.R.E. is supported by the Danish Research Academy, D.G.N. is supported by theRobert A. Welch