N ov 2 00 8 Anisotropic magnetic diffuse scattering in an easy - plane type antiferromagnet ErNi 2 Ge 2

We report on neutron scattering studies of a rare earth intermetallic compound ErNi2Ge2. Polarized neutron scattering experiments revealed that the magnetic ordered moment m lies in ab-plane. Taking account of a lack of the third higher harmonic reflection, ErNi2Ge2 is considered to have a helical magnetic structure. The magnetic scattering profiles along the [100]∗and the [110]∗-directions are well described by the sum of Gaussian and modified-Lorentzian terms, even far below TN, indicating that short-range orders coexist with a long-range order. Interestingly, the modified-Lorentzian-type diffuse scattering is not present in the profiles along the [001]∗-direction. The anisotropy of the diffuse scattering suggests that the short-range-order consists of one dimensional long-range helices along the c-axis. Long-range interactions in magnetic systems, such as the dipolar interaction and the RKKY interaction, cause diverse magnetism because of their competing nature. For instance, Ising spin systems, such as PrCo2Si2 [1] and CeSb [2], reveal long-period magnetic soliton structures and complicated successive phase transitions in temperature (T ) and magnetic field (H). On the other hand, long-period helically modulated magnetic structures are realized in Heisenberg and XY spin systems, such as Ho [3] and MnAu2 [4]. Rare-earth intermetallic compounds RT2X2 with the tetragonal ThCr2Si2-structure (space group I4/mmm) are good examples for studying the competing nature of the long-range RKKY interaction in the Ising-type, the XY-type and the Heisenberg-type spin systems. In the RT2X2system, the magnetic anisotropy varies from the Ising-type to the XY-type systematically as magnetic rare-earth R-ions being heavier [5]. In spite of the simple crystallographic structure, the RT2X2-system exhibits rich magnetic properties, such as incommensurate magnetic structures and complicated HT phase diagrams [1, 6, 7]. ErNi2Ge2 is a candidate for the XY-type spin system in the RT2X2-system. Magnetization measurements by using a single crystalline sample revealed easy-plane type magnetic anisotropy and an antiferromagnetic phase transition at TN = 2.6 K [5]. The tetragonal ab-plane is the magnetic easy plane and the magnetization along the c-axis is less than 1/10 of the abplane magnetization. According to neutron powder diffraction experiments [8], ErNi2Ge2 has a long-period sinusoidally modulated magnetic structure with the magnetic wave vector kmag ≃ (0, 0, 0.75) and the magnetic ordered moment forms an angle 64◦ with the c-axis. This magnetic structure is inconsistent with strong easy-plane type anisotropy observed in the magnetization measurements. In ref.[8], the helical structure also arises as a possible magnetic structure. 3 Present address: Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan In this article, we report on our recent results of polarized and unpolarized neutron scattering studies by using single crystalline samples of ErNi2Ge2. Polarized neutron scattering experiments exhibit that the magnetic ordered moment in ErNi2Ge2 lies in the ab-plane. Successive phase transitions do not arise in contrast with the case of Ising-type systems as TbNi2Ge2 [6]. Instead, a novel anisotropic magnetic diffuse scattering was found in the antiferromagnetic ordered phase. Single crystalline samples of ErNi2Ge2 were grown by the Czochralski method with a tetraarc furnace. Magnetization measurements were performed by using a SQUID magnetometer MPMS (Quantum Design). Polarized and unpolarized neutron scattering experiments were performed on the triple-axis spectrometer PONTA and HQR at the JRR-3M reactor in JAEA, respectively. The polarized neutron energy E = 13.5 meV was chosen by Heusler monochromator and analyzer. The direction of the polarization vector of neutron spins P was tuned to be parallel or perpendicular to the scattering vector K by using Helmholtz coils. The polarization ratio of the neutron spins was estimated from the nuclear Bragg peak at (0, 0, 4) as being ∼ 20. The unpolarized neutron energy E = 13.5 meV was selected by PG monochromator and analyzer. A PG filter was used to eliminate higher harmonic reflections from the monochromator.