* Corresponding author. Tel.: #49-892891-4661; fax: #49- 892895-4661. E-mail address: pyka@physik.tu-muenchen.de (N.M. Pyka). Physica B 292 (2000) 191}193 The momentum space distribution of the low-energy magnetic response in Nd  Ce  CuO  at T)0.1 K and up to 5.7 T N.M. Pyka*, M. Loewenhaupt, A. Metz, W. Schmidt Technical University Munich, ZBE FRM-II, D-85747 Garching, Germany Institut fu ( r Angewandte Physik (IAPD), TU-Dresden, D-01062 Dresden, Germany Institut fu ( r Festko ( rperforschung, FZ-Ju ( lich, D-52425 Ju ( lich, Germany Institut Laue-Langevin, F-38042 Grenoble, France Received 25 February 2000 Abstract The momentum space distribution of the magnetic response of Nd spins in a single crystal of Nd  Ce  CuO  has been measured with inelastic neutron scattering as a function of an applied magnetic "eld along [0 1 0]. The observed q-widths along [1 0 0] and [0 0 1] of the tetragonal lattice are rather large in the low-"eld region up to 2 T, which corresponds to the heavy-fermion phase. A drastic decrease in q-width occurs to higher "elds. The q-width of the magnetic response at (0 0 2) along the [1 0 0] direction changes by a factor two, while in the [0 0 1] direction and around (0 0 3) the decrease is 30% to 40%. Nevertheless, all observed q-widths remain signi"cantly broadened up to 5.7 T.  2000 Elsevier Science B.V. All rights reserved. Keywords: Heavy fermions; Magnetic correlations; Short-range order In pure Nd  CuO  low-energy spin waves with E(1 meV are observed, which are due to transi- tions within the ground state of Nd [1]. Replac- ing Nd ions by Ce shifts these excitations to lower energies, mainly due to a reduction of the Cu-staggered "eld acting on the Nd spins [2]. With increasing Ce doping the Nd spin waves lose the long-range correlation and a broad inelastic mag- netic response is observed [3,4]. In addition, at high doping, x'0.1 in Nd  Ce  CuO  , heavy- fermion behavior was observed [5,6], which is caused by a hybridization of Nd-4f electrons with Cu-3d band electrons and can be detected with neutron scattering as Q-independent quasielastic scattering [7]. At a Ce doping of x"0.15 this quasielastic scattering is removed by the applica- tion of a magnetic "eld of 2 T [8]. For "elds above 3.7 T, a partially aligned phase of Nd spins is for- med [9] and in the following this is referred to as the high-"eld phase. On a x"0.15 single crystal, the same as used in the present experiment, two branches of magnetic excitations have been ob- served in the c-direction [7], starting at the - points (0 0 2) and (0 0 3) at 0.13 and 0.24 meV energy transfer. In this paper, we present the magnetic "eld dependence of the momentum space distribution of the inelastic magnetic response of Nd spins in Nd  Ce  CuO  around (0 0 2) and (0 0 3). 0921-4526/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 0 ) 0 0 4 7 8 - 6