Low energy nuclear spin excitations in HoAl 2 investigated by high resolution neutron spectroscopy Tapan Chatterji 1 , Niina Jalarvo 2 and Andrzej Szytula 3 1 Institut Laue-Langevin, 6 rue Joules Horowitz, BP 156, 38042 Grenoble Cedex 9, France 2 J¨ ulich Centre for Neutron Science (JCNS-1), Forschungszentrum J¨ ulich, Outstation at Spallation Neutron Source (SNS), Chemical and Engineering Materials Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6475, USA 3 Institute of Physics, Jagellonian University, 30-059 Krakow, Reymonta 4, Poland (Dated: December 21, 2012) We have investigated low energy excitations in metallic HoAl2 by high resolution neutron spec- troscopy. At T = 3 K we found clear inelastic peaks in the energy loss and energy gain sides along with the central elastic peak. We interpret these inelastic peaks to be due to the transitions from hyperfine-split nuclear levels. The energy which is E= 25.03 ± 0.02 at T = 3 K, decreases contin- uously and becomes zero at TN ≈ 30 K. The intensity of the inelastic peak remains more or less constant as a function temperature before the inelastic peak merges with the central elastic peak. The energy of nuclear spin excitations seems to follow the order parameter of the ferromagnetic phase transition in HoAl2. Hyperfine interaction, Nuclear spin excitations, Ferromagnetic phase transition. PACS numbers: 75.50.Ee The study of the hyperfine interaction of the magnetic atoms in condensed matter gives very useful informa- tion about the nature of electronic magnetism and has been undertaken by various methods 1 . Of all the tech- niques for studying hyperfine interaction the high res- olution inelastic neutron scattering method 2 is proba- bly the least known. By this technique one can study the transitions between the hyperfine split nuclear levels by using cold incident neutrons with a back-scattering neutron spectrometer 2 . This technique has been so far used only in a few magnetic solids containing V, Co, Nd and Ho ions 3–16 that have larger spin dependent neu- tron scattering cross sections although there are good possibilities of studying other magnetic atoms once the new back-scattering spectrometer 17 IN16B of the Insti- tut Laue-Langevin becomes operative. Inelastic neutron scattering studies on Ho compounds has only been un- dertaken quite recently 14–16 after the construction of the back-scattering spectrometer BASIS 18 with a larger dy- namic range at the Spallation Neutron Source (SNS) in Oak Ridge, Tennessee. Here we have investigated hyper- fine interaction on a well-known rare earth intermetallic cubic ferromagnetic compound HoAl 2 . The MgCu 2 type cubic Laves phase RAl 2 (R = rare earth) compounds have attracted consider- able attention due to their intriguing fundamental properties 19 and various technological applications such as magnetostriction 20 , large magnetoresistance 21 and huge magnetocaloric effects 22–25 . Among these HoAl 2 is ferromagnetically ordered 26 below T C ≈ 30 K. It also undergoes a first-order spin reorientation transition 27–29 at about T sr ≈ 20 K. The HoAl 2 sample was prepared from 3N pure Ho and 4N pure Al. The stoichiometric amounts of the metal were melted in arc under an argon atmosphere. Next the sample was annealed at 1073 K for one weak in an evacuated quartz tube. The powder diffraction mea- surement with CuK α radiation indicates that compound crystallizes in cubic system of MgCu2-type (space group Fd ¯ 3m). The cubic lattice parameter was determined to be a =7.838 ± 0.001 ˚ A, which is in good agreement with the literature value. Magnetic measurements were per- formed in the range from 1.9 K to 300 K and in external magnetic fields up to 9 T using VSM (PPMS Quantum Design platform). From these data we find ferromagnetic transition below T C = 30 K. The small anomaly near 20 K due to the spin reorientation transition in magneti- zation vs. temperature is also observed. Above T C the reciprocal magnetic susceptibility obeys the Curie-Weiss law with the paramagnetic Curie temperature equal to 25 K and an effective magnetic moment equal to 10.75 μ B . Fig. 1 shows the inverse magnetic susceptibility of HoAl 2 vs. temperature. The solid line shows the Curie- Weiss law. Inset presents magnetization (ZFC and FC curves) at low temperature. Inelastic neutron scattering measurements were done on the BASIS back-scattering spectrometer 18 of the Spal- lation Neutron Source (SNS) of Oak Ridge National Lab- oratory, USA. Powder samples of HoAl 2 was placed in- side a flat Al sample holder which was fixed to the sam- ple stick of the closed cycle CCR refrigerator. We mea- sured the inelastic spectra from HoAl 2 in the temperature range 3-35 K. A careful examination of the inelastic spectra mea- sured at different Q values did not reveal any significant Q dependence of the inelastic peaks. We therefore inte- grated the spectra over Q leaving aside the detectors that recorded strong elastic Bragg peaks. Fig. 2 shows the Q integrated inelastic spectra of HoAl 2 at several tem- peratures. At T = 3 K we notice the presence of clear inelastic peaks at about 25 μeV on both energy loss and energy gain sides of the central elastic peak. We inter- arXiv:submit/0619324 [cond-mat.mtrl-sci] 21 Dec 2012