F.Y. Zhang

Complex magnetic phase diagram in the hexagonal DyGa2 compound

Abstract Specific heat and neutron diffraction experiments on polycrystalline samples as well as magnetization measurements on a single crystal of the hexagonal DyGa2 compound are presented. Below TN (11 K) three antiferromagnetic phases successively appear: two collinear modulated structures for TN >T >T2 = 8.7K and T2 >T >T1 = 6.1 K, the transition at T2 corresponding to a rotation of 90° of the moments, and a simple antiferromagnetic structure below T1. Whereas susceptibility shows a huge peak at T1 and only a very small anomaly at T2 and TN, specific heat exhibits the largest anomaly at the rotation temperature. Magnetization process exhibit many transitions in the basal plane which is of easy magnetization and also along the c axis. The resulting complex field-temperature phase diagrams along the a, b and c directions of the orthohexagonal cell were carried out. Finally crystal field parameters were determined from the thermal variations of the paramagnetic susceptibility and entropy.

Crystalline electric field and high field magnetization in ErNi5 single crystal

Abstract High field magnetization measurements up to 380 kOe along the three symmetry axes of a single-crystalline sample of hexagonal ErNi 5 , especially the curves along [100] and [120] show us for the first time the strong field induced anisotropy between the two axes in the basal plane. A quantitative analysis of this experiment together with the other data such as paramagnetic susceptibility, specific heat and inelastic neutron scattering allowed us to determine unambiguously the crystalline electric field (CEF) parameters and the paramagnetic exchange coefficient. This leads to a better understanding of the CEF anisotropy in the basal plane. The Er 3+ CEF interactions are compared with those found in other RNi 5 compounds.

Field induced magnetic structures in hexagonal HoAlGa

Abstract A neutron diffraction study on a single crystal of the HoAlGa compound under magnetic field applied along the c easy magnetization direction has been performed. The different field induced structures of the multistep metamagnetic process, as well as their thermal evolution, have been determined. Whereas the component of the propagation vector in the hexagonal plane is always (1/3, 1/3), along c it is 1/2 or 1/6 at low temperature and 1/2- τ with τ = 0.02, i.e. incommensurate, at high temperature. The stability of different structures of the H - T phase diagram can be discussed in terms of exchange interactions in a compound with a huge uniaxial anisotropy.

Exchange interactions in RGa2 compounds

Abstract The magnetic properties of hexagonal RGa 2 compounds are summarized, considering recent experimental results obtained on single crystals. An analysis of the exchange interactions throughout the series as well as a qualitative description of the Fourier transform of the exchange interaction, J ( q ), are made.

Complex magnetic phase diagrams in hexagonal rare-earth compounds

Abstract We give an insight into the wealth of the complex magnetic phase diagrams of the frustrated hexagonal R(Ga 1− x Al x ) 2 compounds. The large variety of magnetic structures provides a good check for theoretical models. The field-temperature phase diagrams are determined from a wide range of experimental measurements, in particular magnetization and neutron diffraction on a single crystal.

Magnetic incommensurability in hexagonal HoGa2

Abstract Specific heat, powder neutron diffraction experiments and magnetization measurements on a single crystal of the hexagonal HoGa 2 compound are presented. Below T N = 7.6 K two collinear antiferromagnetic phases ( M∥a of the orthohexagonal cell) are observed: (i) a simple collinear antiferromagnetic structure with Q 1 =(0, 1, 0) (in the orthohexagonal cell) below T t = 6.5 K; (ii) a mixing of this phase (about 36%) and of an incommensurate amplitude modulated structure (about 64%) with Q 2 =(0.123, 1, 0) for T t T T N . Below T N smooth metamagnetic processes are observed along both directions of the easy basal plane. The second order crystal field parameter A 0 2 , deduced from the shift of the reciprocal susceptibilities along and perpendicular to c , is significantly smaller than in the other RGa 2 compounds.

Anomalous metamagnetic process of TbCo2Ge2 single crystal

Abstract Measurements of the high field magnetization up to 14 T and the magnetic susceptibility have been carried out on a single crystal of the tetragonal antiferromagnetic TbCo2Ge2 (TN = 32 K). In spite of a large uniaxial anisotropy and a simple antiferromagnetic structure with the propagation vector (0,0,1), the c-axis magnetization shows anomalous metamagnetic processes, with three or four smooth transitions at low temperatures.

Field-induced spin-flip and spin-flip transitions in PrGa2

Abstract A detailed neutron diffraction study of a single crystal of hexagonal PrGa 2 under an applied field is presented. The field is applied along the easy [100] direction of the hexagonal plane. The compound exhibits several field-induced magnetic structures and, in particular, an uncompensated long period commensurate antiphase structure defined by the propagation vector (4/27, 4/27, 0) at zero field and below T t = 3.5 K . Concerning the intermediate magnetic structures below T t , two types of magnetic phase transitions are observed: a spin-flip transition where one moment per magnetic cell turns over, and a spin-slip transition from the low field commensurate propagation vector (4/27, 4/27, 0) to another commensurate vector (1/7, 1/7, 0). Between T t and T N = 7.2 K , the structure becomes apmlitude-modulated with a propagation vector (0.148, 0.148, 0.023). Important and original magnetic domain effects are also observed due to the high degeneracy of the easy direction of the hexagonal plane in both T T t and T t T T N regions.

Magnetic phase diagram of TbGa2

Abstract Magnetization measurements on a single crystal, powder neutron diffraction experiments and specific heat on the hexagonal TbGa 2 compounds are presented. Below T N = 18.5 K, multistep metamagnetic transitions are observed when the field is applied perpendicular to the [001] hard magnetization axis. The H-T magnetic phase diagrams along the [100] and [120] directions were deduced. At any temperature below T N , the magnetic structure is described with the incommensurate propagation vector Q = ( 1 3 , 1 3 , 0.189) . From the neutron diffraction experiments three possible magnetic structures are deduced, among them a triangular structure modulated along c being the most likely.

High field induced first order magnetization process in NdCo5 single crystal

Abstract Low temperature high field magnetization measurements up to 380 kOe along the three symmetry axes of a single-crystalline sample of hexagonal NdCo 5 are presented. Along the hard [001] direction a very sharp First Order Magnetization Process (FOMP) is observed at 340 kOe. A self consistent quantitative analysis, using a phenomenological approach for the Co magnetism and a perturbing Hamiltonian for the Nd magnetism allowed us to give a satisfactory account for these experiments and of the thermal dependence of the direction of the spontaneous magnetization. The FOMP arises from the field effect on the crystal field splitting acting on Nd 3+ ions together with the Nd-Co exchange interaction and the Co anisotropy which favours the [001] axis. A range of crystal field and Nd-Co exchange parameters is proposed.