A. Garnier

Magnetic properties of tetragonal TbRu2Ge2 and TbRu2Si2

Abstract A detailed study of the magnetic properties of TbRu 2 Ge 2 and TbRu 2 Si 2 single crystals is presented, from specific heat and magnetization measurements. Both compounds exhibit very similar features, in particular a multistep metamagnetic behaviour at low temperature along the [001] easy direction, and the occurrence of a zero-field phase transition at a temperature far below the Neel temperature. The complex H-T magnetic phase diagrams are deduced, a situation typical of long-period commensurate or incommensurate systems.

Magnetic properties of tetragonal DyRu2Si2

A detailed study of the magnetic properties of the tetragonal rare earth compound DyRu2Si2 is presented. Magnetization measurements on a single crystal as well as specific heat and resistivity measurements on a polycrystal show features which are typical of a long-period commensurate system in the presence of a huge uniaxial anisotropy. Multistep metamagnetism occurs below the Neel temperature TN=29 K, in particular at low temperatures. In the low-field low-temperature region, a new magnetic phase has been discovered with an anomalous behaviour.

Mixed magnetic phases in tetragonal

A careful analysis of neutron diffraction experiments under magnetic field performed on a single crystal of tetragonal is presented. It shows that the zero-field-low-temperature magnetic phase is not the natural compensated antiphase structure proposed so far for the Q = (3/13, 0, 0) propagation vector, but a mixed phase where the moment of two of the 26 Tb sites of the unit cell vanishes. The magnetic structures of the first three field induced phases of the multistep metamagnetic process agree with previous analysis whereas the fourth field induced phase is not the non-mixed proposed one but a mixed one with two zero moments as in zero field. Contrary to previous interpretations, the transition at 4.7 K observed by specific heat and susceptibility corresponds to the change of the magnetic structure from the mixed low-temperature one to a slightly modulated non-mixed one. Numerical simulation allowed us to account for the main properties observed and emphasizes the crucial role of the crystal field.

Giant magnetic anisotropy in tetragonal GdRu2Ge2 and GdRu2Si2

Abstract A detailed magnetic study of the tetragonal compounds GdRu2Ge2 and GdRu2Si2 is presented, including heat capacity, susceptibility and magnetization measurements on single crystals. Despite the isotropic character of the S-state Gd3+ ion, the compounds exhibit a huge anisotropy within their antiferromagnetic range, i.e. below TN = 33 and 47 K, respectively. Both compounds present several different magnetic phases in zero field, and strong metamagnetic field-induced transitions, giving rise to complex H-T magnetic phase diagrams, a situation usually typical of nondashS rare earth compounds. This behaviour suggests a strong anisotropy of the exchange interactions.

Magnetic Phase Diagram and Field Induced Magnetic Structure in DyCo2Si2

The H - T magnetic phase diagram and field induced magnetic structure in tetragonal DyCo 2 Si 2 with a two-step metamagnetic process have been determined by magnetization and neutron diffraction experiments using a single crystal as well as specific heat experiment on a polycrystalline sample. Three magnetic phases exist below T N =23 K. A simple antiferromagnetic structure (phase I), with the propagation vector Q 1 =(0, 0, 1) and moments along the c -axis, is stable below 21 K under low field. The intermediate field induced magnetic structure (phase II) is characterized by two additional propagation vectors: i) Q 0 =(0, 0, 0) and ii) Q 2 =(1/2, 1/2, 1/2) which is a sub-harmonics of (1, 1, 1) (≡ Q 1 ) and consists of ferromagnetic (111) planes with the sequence of + + + -. It has a resultant magnetization of half the saturation value, i.e. 5.0 µ B /f.u., at low temperature. Between T N and 21 K, an antiferromagnetic phase (phase III) with an unknown structure appears.

Double-Q magnetic structures and strong planar anisotropy in tetragonal ErRu 2 Ge 2 and ErRu 2 Si 2

Abstract.Single crystal magnetization measurements and powder neutron diffraction on tetragonal ErRu2Ge2 as well as anisotropy of the paramagnetic susceptibility and specific heat measurements on ErRu2Si2 are presented. Besides the huge crystal field contribution to the uniaxial anisotropy, which favors the basal plane, a strong in-plane anisotropy is evidenced. From these features and neutron diffraction experiments it is shown that magnetic structures of these materials are double-Q and accordingly non-colinear below their Néel temperature (5.2 and 6.0 K for Ge and Si based compounds, respectively). The magnetic structures induced during the metamagnetic processes are discussed.

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.

Magnetic properties of singlet ground states in RM2X2 compounds

Abstract A simple survey of the magnetic properties of new tetragonal RM 2 X 2 compounds is given. Particular attention is paid to the interaction of Crystal Field in the case of nonmagnetic singlet ground states. A number of physical effects which result from this situation are discussed. In particular, the nature of this type of ground state is proved to be indispensable for the existence of amplitude magnetic structures at 0 K and for the most amazing of mixed magnetic phases, in which Rare Earth ions with two magnetic states are involved. Evidence of this behaviour through several examples in such compounds as PrNi 2 Si 2 , TbNi 2 Si 2 , TbRu 2 Si 2 and TbRu 2 Ge 2 from the joint analysis of different magnetic properties will be presented.

Magnetic properties of tetragonal DyNi2Si2

Abstract A detailed study of the magnetic properties of the tetragonal compound DyNi 2 Si 2 is presented. This compound exhibits two successive antiferromagnetic phases below its Neel temperature T N = 6 K, as shown by specific heat and neutron diffraction experiments. Moreover, from magnetization measurements on a single crystal, a multistep behaviour has been found along the [001] easy direction, leading to a complex magnetic H - T phase diagram which includes 5 different magnetic phases.

Comparative study of the complex magnetic properties of RRu2X2 (R = rare earth; XSi,Ge)

Abstract The magnetic properties of RRu 2 X 2 compounds are characterized by long period commensurate and/or incommensurate structures, anisotropic multistep metamagnetic processes, and complex field-temperature phase diagrams. These properties originate from the competition between antagonistic RKKY interactions in the presence of magnetocrystalline anisotropy.