Uniaxially anisotropic antiferromagnets in a field along the easy axis
Uniaxially anisotropic antiferromagnets in a field along the easy axis are studied with the help of ground state considerations and Monte Carlo simulations. For classical models, the XXZ model as well as variants, we analyze the role of non-collinear spin configurations of biconical or bidirectional type interpolating between the well-known antiferromagnetic and spin-flop structures. Possible experimental applications to layered cuprate antiferromagnets are discussed. Finally, results of quantum Monte Carlo simulations for the S=1/2 XXZ model on a square lattice are presented, and compared with previous findings.
Uniaxially anisotropic antiferromagnets in a field on a square lattice
Abstract.Classical uniaxially anisotropic Heisenberg and XY antiferromagnets
in a field along the easy axis on a square lattice
are analysed, applying ground state 
considerations and Monte Carlo techniques. The models are known to 
display antiferromagnetic and spin-flop phases. In the Heisenberg 
case, a single-ion anisotropy is added to the XXZ antiferromagnet, 
enhancing or competing with the uniaxial exchange anisotropy. Its 
effect on the stability of non-collinear structures of biconical 
type is studied. In the case of the anisotropic XY antiferromagnet, 
the transition region between the antiferromagnetic and spin-flop 
phases is found to be dominated by degenerate bidirectional 
fluctuations. The phase diagram is observed to resemble closely 
that of the XXZ antiferromagnet without single-ion anisotropy. 


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The Radioactive Beam Experiment REX-ISOLDE [1–3] is a pilot experiment at ISOLDE (CERN) testing the new concept of post acceleration of radioactive ion beams by using charge breeding of the ions in a high charge state ion source and the efficient acceleration of the highly charged ions in a short LINAC using modern ion accelerator structures. In order to prepare the ions for the experiments singly charged radioactive ions from the on-line mass separator ISOLDE will be cooled and bunched in a Penning trap, charge bred in an electron beam ion source (EBIS) and finally accelerated in the LINAC. The LINAC consists of a radiofrequency quadrupole (RFQ) accelerator, which accelerates the ions up to 0.3 MeV/u, an interdigital H-type (IH) structure with a final energy between 1.1 and 1.2 MeV/u and three seven gap resonators, which allow the variation of the final energy. With an energy of the radioactive beams between 0.8 MeV/u and 2.2 MeV/u a wide range of experiments in the field of nuclear spectroscopy, astrophysics and solid state physics will be addressed by REX-ISOLDE.