Eur. Phys. J. B 43, 163–174 (2005) DOI: 10.1140/epjb/e2005-00039-1 T HE EUROPEAN P HYSICAL JOURNAL B Uncompensated antiferromagnetic structure of Ho 2 Ni 2 Pb K. Prokeˇ s 1, a , E. Mu˜ noz Sandoval 2 , A.D. Chinchure 3 , and J.A. Mydosh 4 1 Hahn-Meitner-Institute, SF-2, Glienicker Str. 100, 141 09 Berlin, Germany 2 Advanced Materials Department, IPICyT, Apartado Postal 3-74,Tangamanga, 78231 San Luis Potosi, SLP, Mexico 3 Materials Research Laboratory, John F. Welch Technology Center, Bangalore - 560 066, India 4 Kamerlingh Onnes Laboratory, Leiden University, 2300 RA Leiden, The Netherlands and Max-Planck-Institute for Chemical Physics of Solids, 01187 Dresden, Germany Received 6 July 2004 / Received in final form 6 December 2004 Published online 25 February 2005 – c  EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2005 Abstract. We have studied the magnetic structure of the orthorhombic compound Ho2Ni2Pb by means of neutron diffraction in zero field and in magnetic fields up to 4.5 T. Both powder and single-crystalline samples were used. Previous bulk measurements suggest two distinct magnetic phase transitions: one at TN =7.0 K and the other at 4.8 K. Our neutron diffraction measurements, which were made in the range 1.5-20 K, showed that Ho2Ni2Pb has a collinear magnetic structure with unequal number of up and down Ho moments that are aligned parallel and antiparallel to the c axis. At the lowest temperatures the Ho moments are equal in size, each 8.3 μB in agreement with magnetization data. The magnetic structure can be described as having a 5a × b × c magnetic unit cell. Below Ts =3.0 K the structure is squared up. A smooth development of all the magnetic moment magnitudes indicates that the magnetic structure remains in principle the same over the whole temperature range, the “phase transition” around 4.8 K can be identified as an inflection point in the temperature dependence of one of the Ho moments. With increasing temperature there is a clear development towards a simple transverse sine-wave modulated magnetic structure that is established just below TN . PACS. 75.50.-y Studies of specific magnetic materials – 75.25.+z Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source X-ray scattering, etc.) – 75.50.Ee Antiferromagnetics 1 Introduction Novel magnetic compounds are currently the subject of intensive fundamental and applied research [1]. Recently, several new rare-earth (R) intermetallic compounds with the stoichiometry 2:2:1 were synthesised [2]. Rare-earth elements from Gd to Lu and Y can be combined with Ni and Pb. Detailed bulk measurements suggest that the only magnetic element in these materials is the rare-earth ion [3,4]. The last two elements do not carry any magnetic moment. The crystal structure has orthorhombic symme- try of the Mn 2 AlB 2 type and the space group Cmmm [2]. This structure type is also denoted as the AlB 2 Fe 2 in the literature [5]. All the R atoms are equivalent – they occupy the same crystallographic position 4(j). Due to the surroundings of the R atoms with non-magnetic el- ements, there exists, however, a significant magnetocrys- talline anisotropy. Multiple magnetic phase transitions ob- served in these compounds [4,6] suggest that complex a e-mail: prokes@hmi.de magnetic moment arrangements can be expected as a re- sult of competing magnetic interactions. On the basis of bulk measurements on polycrys- talline [3,7,8] and single crystalline [4] Ho 2 Ni 2 Pb samples it was suggested that two different magnetic phases exist at low temperatures in this system. Concerted magnetic susceptibility, magnetization, specific heat and electrical resistivity measurements indicated that Ho 2 Ni 2 Pb orders antiferromagnetically at T N =7.0 K, the other magnetic phase transition at T m =4.8 K is considered as being anti- ferromagnetic to ferromagnetic in nature [4,7,8]. In con- trast, Gulay et al. [3] report for Ho 2 Ni 2 Pb ferromagnetic phase transition taking place at T C =7.0 K. According to them, no other magnetic phase transition appears in this system down to 4.2 K. Thus, one encounters a rather rare case when two groups report for a magnetic substance having large magnetic moments entirely different ground states although they agree upon the value of the magnetic phase transition temperature. Magnetization measurements on single-crystalline samples suggest that low-temperature phases are very