ELSEVIER Physica B 211 (1995) 208-212 Magnetization studies of the Haldane gap material TMNIN G.E. Granroth a, L.-K. Chou b, W.W. Kim a, M. Chaparala c, M.J. Naughton d, E. Haanappel e A. Lacerda e, D. Rickel e, D.R. Talham b, M.W. Meisel a'* 9 a Department of Physics and the Center for Ultralow Temperature Research, University of Florida, P.O. Box 118440, Gainesville, FL 32611-8440, USA bDepartment of Chemistry, University of Florida. Gainesville, FL 32611, USA CNational High Magnetic Field Laboratory, Florida State University, Tallahassee, EL 32306-3016, USA dDepartment of Physics, State University of New York, Buffalo, NY 14260, USA eNational High Magnetic Field Laboratory, Los Alamos National Laboratory, Los Alamos, NM 87544, USA Abstract The magnetization of oriented single crystals of the Haldane gap material (CH3)aN[Ni(NO2)3] , more commonly known as TMNIN, has been measured using a capacitive platform magnetometer operating in magnetic fields up to 8 T at temperatures down to 25 inK. In addition, powdered samples have been studied by standard induction techniques in pulsed fields up to 50 T at 4 K. The results of our preliminary investigations, which probe the anisotropy of the critical magnetic field, are presented. In 1989, the high-field magnetization properties of single crystals of the Haldane gap material known as NENP, Ni(C2HsN2)2NO2(C10#), were reported [1,2]. For temperatures below the Haldane gap energy Eg = 0.41J, the non-magnetic Haldane phase yields to a magnetic high-field phase at a critical field He, where Eg ~ 14K and Hc ~ 10T for NENP. Anisotropy of Hc due to various orientations of the external magnetic field with respect to the crystalline axes arises from imperfect symmetry in the crystal. In other words, the appropriate spin Hamiltonian for the system requires terms that reflect single-ion (D) and orthorhombic (E) anisotropies and incorporates the v-factor as a tensor, namely = D ~7 tS~ 2 E ~ [(SX)2 - (S,e) 2] o ut" -J~Sl"Si+x+ ~ ,, + i i i - mZtt.g.s. (1) i * Corresponding author. Different numerical and theoretical treatments have dis- cussed the effect of an applied magnetic field on the Haldane gap [3-8]. High-field magnetization work has also been per- formed [9] on other Haldane gap materials known as NINAZ, Ni(C3HloN2)2N3(CIO4), and TMNIN, (CH3),N[Ni(NO2)3]. Using a powder TMNIN sample cooled to 0.55 K, Takeuchi et al. [9] measured Hc to be about 2.5 T while saturation magnetization was achieved at approximately 30 T. The purpose of this contribution is to present our preliminary results of high-field magnetization measure- ments that we have performed on oriented single crystals of TMNIN. Our main motivation is to establish experi- mental constraints on the size of the single-ion and or- thorhombic anisotropies. For completeness and com- parison, we will present all of our recent results of the magnetic properties on this material; however, a full discussion of the results and their implications will be presented elsewhere. 0921-4526/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0921-4526(94)00987-2