D. Bloch

The magnetic properties of FexCo1-x and FexTi1-x alloys

Abstract High pressures and high magnetic field behaviour give indications on the nature of magnetism encountered in transition metal alloys. We present experiments performed under pressure up to 7 kbar, in fields up to 150 kOe, between 1.4 K and 300 K. The results are used to discuss the occurence of ferromagnetism in Fe x Co 10 x Si and Fe x Co 1- x Ti systems. These systems exhibit two critical concentrations for the onset of ferromagnetism. Their magnetic properties are very sensitive to magnetic fields and high pressures and this allows us to characterize them as weak itinerant ferromagnets. A more localized behaviour is evidenced for the iron rich alloys and the appearance to ferromagnetism is different for cobalt rich side and for iron rich one.

HIGH MAGNETIC-FIELD BEHAVIOR OF MEM-(TCNQ)2

Abstract The charge-transfer complex MEM(TCNQ)2 is a spin-Peierls system with a non-magnetic, singlet ground state at T=0. We report high-field magnetization data which provide some evidence for a new magnetic spin-Peierls phase at fields above 190 kOe. The experimental results are compared to those for TTFCuBDT.

The low temperature magnetic behaviour of CeSn2

Abstract At 300 and 4 K, the CeSn 3 anomalous compressibility indicates a pressure induced valence change. The thermal contraction from 300 to 4 K is equivalent to that produced by a pressure of 8 kbar. High pressure susceptibility data show that the thermal expansion does not drive the “magnetic-nonmagnetic” transition of CeSn 3 .

High-pressure effects on the 2kF and 4kF transitions in the quasi-one-dimensional magnetic conductor mem-(TCNQ)2

Abstract MEM-(TCNQ)2 possesses at room pressure both an electronic transition, associated with a 4kF lattice distortion and a magnetic-to-non-magnetic transition associated with a 2kF lattice distortion. The high-pressure phase diagram, determined from neutron scattering, electrical transport, and magnetization experiments, indicates a disappearance of both the 2kF and 4kF distortions above 4 kbar.

Spin-peierls phase diagrams: Observations and models

Abstract We discuss systems displaying the spin-Peierls transition, a unique magnetoelastic phase transition which has been the subject of considerable experimental and theoretical attention recently. The first high magnetic field experiments, involving neutron scattering and magnetization measurements, have been performed. Analysis is made with reference to theoretical models.

Magnetic phase transitions and hydrostatic pressure or uniaxial stress experiments

Abstract Crystals submitted to high hydrostatic pressure or uniaxial stress have been investigated by means of neutron scattering. The techniques used are described and applications to pressure or stress induced T = 0 magnetic to nonmagnetic transitions (Pr, PrSb) and continuous to discontinuous order-disorder transitions (MnO) are given.

Weak and induced magnetism in itinerant electron systems

Abstract The occurence of ferromagnetism is described in the nickel-platinum Ni x Pt 1-x disordered alloys and in the rare earth-cobalt, TCo 2 compounds. The low temperature moments depend on chemical content, as well as on temperature, pressure or magnetic field. No simple relation exists between the paramagnetic H = 0 behaviour and the low temperature magnetic moment, whenever it exists. Magnetic moments occur continuously or discontinuously. The results are discussed within the framework of theories for weak ferromagnetism and for external field or exchange induced magnetism.

New High-Field Phenomena in Spin-Peierls Systems

The spin-Peierls (SP) transition in zero field is a spin-lattice dimeri-zation phenomenon which occurs in a quasi-1-D Heisenberg S= 1/2 antiferromag-netic chains. The chains are coupled to the 3-D phonon field of the lattice, and a magneto-elastic transition results which is the insulating analogue of the well-known Peierls transition in a quasi-1-D conductor. For T<Tsp, the magnetic chains have a dimerized (alternating) character, resulting from an underlying progressive lattice distortion, with a nonmagnetic singlet ground state and an excitation energy gap to magnetic excited states. The character of an alternating as opposed to uniform chain lattice, together with the corresponding magnetic excitations, is shown schematically in Fig. 1.