Magnetic structure of UPd2Si2 in a magnetic field

Magnetic structure of phase II in U(Ru(0.96)Rh(0.04))2Si2 determined by neutron diffraction under pulsed high magnetic fields

We report neutron diffraction measurements on U(Ru(0.96)Rh(0.04))(2)Si(2) single crystal under pulsed high magnetic fields up to 30 T applied along the tetragonal c axis. The high-field experiments revealed that the field-induced phase II above 26 T corresponds to a commensurate up-up-down ferrimagnetic structure characterized by the wave vector q=(2/3,0,0) with the magnetic moments parallel to the c axis, which naturally explains the one-third magnetization plateau and the substantially changed Fermi surface in phase II. This a-axis modulated magnetic structure indicates that the phase II near the hidden order phase is closely related to the characteristic incommensurate magnetic fluctuations at Q(1)=(0.6,0,0) in the pure system URu(2)Si(2), in contrast to the pressure-induced antiferromagnetic order at Q(0)=(1,0,0).

Critical point of a para-ferrimagnetic phase transition of the ANNNI model in a field

The finite field para-ferrimagnetic phase transition in the axial next-nearest-neighbor Ising (ANNNI) model is studied by using the linear perturbation real space renormalization group transformation. The method is examined in systems of ferromagnetic and antiferromagnetic Ising chains coupled by ferromagnetic interactions in the longitudinal field. As one would expect, the external field in the first case destroys the continuous phase transition and in the second shifts the critical point toward a lower temperature according to the square law. For the ferromagnetic chains coupled by the competing interchain nearest-neighbor J(1) < 0 and next-nearest-neighbor J(2) < 0 interactions, the external field changes the character of the phase transition from continuous to discontinuous. However, it has been found that for the ratio J(2)/J(1) around 0.5 there is a critical value of the field for which an isolated critical point exists. The temperature dependences of the specific heat for several values of J(2)/J(1) and the external field are presented.