The random field Ising model

Using x-ray and neutron scattering, we have studied the structural and magnetic properties of the single-layer manganite La{sub 1-x}Sr{sub 1+x}MnO{sub 4}(0{ =}0.45) doping. LaSrMnO{sub 4}(x=0) is an antiferromagnetic Mott insulator, and its spin-wave spectrum is well described by linear spin-wave theory for the spin-2 square-lattice Heisenberg Hamiltonian with Ising anisotropy. Upon doping, as the e{sub g} electron concentration (1-x) decreases, both the two-dimensional antiferromagnetic spin correlations in the paramagnetic phase and the low-temperature ordered moment decrease due to an increase of frustrating interactions, and Neel order disappears above x{sub c}=0.115(10). The magnetic frustration is closely related to changes in the e{sub g} orbital occupancies and the associated Jahn-Teller distortions. In the intermediate region, there exists neither long-range magnetic nor superstructural order. Short-range-correlated structural 'nanopatches' begin to form above x{approx}0.25. At high doping (x{>=}0.45), the ground state of La{sub 1-x}Sr{sub 1+x}MnO{sub 4} exhibits long-range superstructural order and a complex antiferromagnetic order, which differs frommore » that at low doping. The superstructural order is thought to arise from charge and orbital ordering on the Mn sites, and for x=0.50 we conclude that it is of B2mm symmetry. For x>0.50, the superstructural order becomes incommensurate with the lattice, with a modulation wave vector {epsilon} that depends linearly on the e{sub g} electron concentration: {epsilon}=2(1-x). On the other hand, the magnetic order remains commensurate, but loses its long-range coherence upon doping beyond x=0.50.« less

Structural and magnetic properties of the single-layer manganese oxide La 1 − x Sr 1 + x MnO 4

There exists increasing evidence that the phase diagram of the high-transition temperature (Tc) cuprate superconductors is controlled by a quantum critical point According to one distinct theoretical proposal, on decreasing the hole-carrier concentration a transition occurs to an ordered state with two circulating orbital currents per CuO2 square Below the ‘pseudogap’ temperature T*(T*>Tc), the theory predicts a discrete order parameter and two weakly-dispersive magnetic excitations in structurally simple compounds which should be measurable by neutron scattering Indeed, novel magnetic order and one such excitation were recently observed Here, we demonstrate for tetragonal HgBa2CuO4+δ the existence of a second excitation with local character, consistent with the theory The excitations mix with conventional antiferromagnetic fluctuations, which points towards a unifying picture of magnetism in the cuprates that will probably require a multi-band description The magnetic character of the cuprates is suspected by many to be involved in the emergence of unconventional superconductivity The discovery of a second distinct magnetic excitation in HgBa2CuO4 supports a multiband picture of the magnetic structure of these materials

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Two Ising-like magnetic excitations in a single-layer cuprate superconductor

Experiments in d = 2 and d = 3 random-field Ising systems are reviewed. Random fields destroy the phase transition for the d = 2 system Rb2Co x Mg1-xF4. The d = 3 Fe x Zn1-xF4 system shows a new phase transition with unusual dynamics. Extreme critical slowing down plays an essential role in the d = 3 measurements. Experiments on the related d = 3 systems Mn x Zn1-xF2 and Fe x Mg1-xC12 are also discussed.

Random-field experiments in dilute antiferromagnets

Neutron scattering and specific heat capacity measurements were made on a two-dimensional randomly mixed antiferromagnets Rb 2 Co c Ni 1- c F 2 . The experimental results show a sharp phase transition which has not been ever found in the random magnetic systems. This gives an answer to a long-lived question whether or not a sharp phase transition occurs in a homogeneously random magnetic system. The critical exponents α, β, ν and γ of Rb 2 Co c Ni 1- c F 2 is identical with those of two-dimensional Ising model.

Phase Transitions in Two-Dimensional Random Antiferromagnets Rb 2 Co c Ni 1− c F 4

Using inelastic-neutron scattering techniques, the spin wave energy dispersion relation has been investigated at three temperatures in a two-dimensional Ising-like antiferromagnet, Rb2CoF4. The results show there is relatively little dispersion across the Brillouin zone with a very large energy gap at zero wavevector. As the temperature is increased to just below TN there is relatively little renormalisation of the spin waves over the region of wavevectors examined.

https://iopscience.iop.org/article/10.1088/0022-3719/11/13/007/pdf

M T Hutchings

Papers

Spin wave excitations in a two-dimensional Ising-like antiferromagnet, Rb2CoF4