40èmes Journées des Actinides & 2nd Workshop on Actinide Targets

High-field metamagnetism in UCo2Si2

29 Mar 2010, 11:05

5m

503/1-001 - Council Chamber (CERN)

poster Strongly correlated behaviours, superconductivity, quantum criticality Introductions to Posters

Speaker

Yurii Skourski (Hochfeld-Magnetlabor Dresden, FZ Dresden-Rossendorf, D-01314 Dresden, Germany)

Description

UCo2Si2 belongs to a wide group of UT2X2 compounds, where T is a late d metal and X is Si or Ge. They exhibit a large variety of magnetic states starting from antiferromagnetic (AF) ordering for UCr2Si2, ferrimagnetic (UNi2Si2) and ferromagnetic (F) structures (UCu2Si2) through Pauli paramagnets (UFe2Si2) to compounds, which become superconducting inside an AF state (URu2Si2) [1-3]. UCo2Si2 has a tetragonal ThCr2Si2 crystal structure and orders AF below TN = 83-85 K. The magnetic structure (from powder neutron diffraction) consists of F basal-plane layers of U moments of MU = 1.42 B oriented parallel to the c axis, which are coupled in a simple sequence ++ (AF type-I structure) in the same direction [2]. Magnetic moment is carried only by U atoms. UNi2Si2 exhibits in addition to the AF type I structure, a ferrimagnetic and an incommensurate AF phase. In the ground state, it exhibits ++ sequence with longitudinally modulated amplitude of the magnetic moment resulting in a spontaneous moment of 0.53 B, 1/3 of MU [4]. The ++ phase is also observed at high magnetic fields in UPd2Si2 (with an AF type-I structure in zero field) [5]. It was interesting to check whether UCo2Si2 exhibits this phase at high magnetic fields as well. In this work we indeed observed such transition. The investigated single crystal was grown by Czochralski method in a tri-arc furnace. The x-ray powder-diffraction analysis confirmed the tetragonal body-centered ThCr2Si2-type crystal structure with lattice parameters a = 392.1 pm, and c = 963.9 pm in agreement with literature. The x-ray Laue patterns showed the high quality of the crystal. The magnetization curves were measured in pulsed fields up to 60 T applied along the c and a axes using a non-destructive pulsed magnet with pulse duration of 25 ms. The magnetization signal was detected by integrating the voltage induced in a pick-up coil surrounding the sample. The absolute values of the magnetization were calibrated from steady-field measurements up to 14 Tesla. For the fields applied along the c axis, the metamagnetic transition is observed at the critical field 0Hcr = 45 T (Fig. 1). It is very sharp but has a small hysteresis (0Hcr = 0.16 T). The analogous transition in UPd2Si2 shows huge hysteresis of more then 15 T [5]. The transition in UCo2Si2 is characterized by a magnetization jump of M = 0.52 B. M roughly corresponds to 1/3 of MU = 1.42 B [2]. Therefore, we can suppose that the high-field state is ferrimagnetic with the ++ arrangement along the c direction. At much higher fields another transition to a fully polarized state can be expected. The magnetization curve measured along the a axis shows no transition and is linear up to the highest fields. The a-axis susceptibility of 3.7x10-3 B/T per U atom is a typical value for the hard-axis magnetization of U intermetallic compounds independent of crystal structure and type of magnetic ground state and reflects mostly the Pauli paramagnetism of the conduction electrons [1]. The transition for H || c is still very sharp at 20 K. At higher temperatures, it becomes considerably wider and might loose the first-order character above 80 K (Fig. 2). The transition field 0Hcr determined as maximum in the derivative dM/dH (Fig. 3) decreases with increasing temperature. At 80 K the small maximum at 21 T in dM/dH still indicates the metamagnetic transition. M, Hcr, and Hcr decrease monotonously with increasing temperature and vanish at TN. In the H-T phase diagram of UCo2Si2 in fields applied along the c axis (Fig. 4), the diamonds correspond to TN(H) taken from Ref. 3. Fig. 1. Magnetization curves measured along the principal axes at 1.4 K. Fig. 2. Magnetization curves measured along the c axis at different temperatures. Fig. 3. Field dependence of the differential susceptibility dM/dH measured along the c axis at different temperatures. Fig. 4. Temperature dependence of the magnetization gain M at the transition, the width of hysteresis 0Hcr and H-T phase diagram of UCo2Si2 in fields applied along the c axis. This work is a part of the research program AVOZ 10100520 financed by the Academy of Sciences of the Czech Republic and was supported by grant 202/09/0339 of Czech Science Foundation and by EuroMagNET under the EU contract 228043. References [1] V. Sechovský and L. Havela, in Handbook of Magnetic Materials, edited by K.H.J. Buschow (North Holland, Amsterdam, 1998), Vol. 11, p. 1 and references therein. [2] L. Chelmicki et al., J. Phys. Chem. Solids 46 (1985) 529. [3] M. Mihalik et al., J. Phys. Soc. Japan 76 (2007) 54. [4] L. Rebelsky et al., Physica B 180-181 (1992) 43. [5] T. Honma et al., J. Phys. Soc. Japan 67 (1998) 1017.

Presentation materials

Formatted Abstract

Abstract_38_skourski.pdf