D. P. Belanger

The random field Ising model

Abstract We discuss the current status of random field systems, particularly those with Ising symmetry. Both theory and experiment agree that, in the equilibrium state, there is a transition to an ordered state in three dimensions and no such transition in two dimensions. The critical behavior in three dimensions is, however, not very well understood. More work remains to be done to understand the dynamics, both in the critical region and the low temperature phase.

Local Structure of La1 xSrxCoO3 Determined from EXAFS and Neutron Pair Distribution Function Studies

The combined local structure techniques, extended x-ray absorption fine structure and neutron pair distribution function analysis, have been used for temperatures 4< or =T< or =330 K to rule out a large Jahn-Teller (JT) distortion of the Co-O bond in La1-xSrxCoO3 for a significant fraction of Co sites (x< or =0.35), indicating few, if any, JT-active, singly occupied e_{g} Co sites exist.The combined local structure techniques, extended x-ray absorption fine structure and neutron pair distribution function analysis, have been used for temperatures 4 {le} T {le} 330 K to rule out a large Jahn-Teller (JT) distortion of the Co-O bond in La{sub 1-x}Sr{sub x}CoO{sub 3} for a significant fraction of Co sites (x {le} 0.35), indicating few, if any, JT-active, singly occupied e{sub g} Co sites exist.

Equilibrium Random-Field Ising Critical Scattering in the Antiferromagnet Fe{sub 0.93}Zn{sub 0.07}F{sub 2}

It has long been believed that equilibrium random-field Ising model (RFIM) critical scattering studies are not feasible in dilute antiferromagnets close to and below T{sub c}(H) because of severe nonequilibrium effects. The high magnetic concentration Ising antiferromagnet Fe{sub 0.93} Zn{sub 0.07} F{sub 2} , however, does provide equilibrium behavior. We have employed scaling techniques to extract the universal equilibrium scattering line shape, critical exponents {nu}=0.87{plus_minus}0.07 and {eta}=0.20{plus_minus}0.05 , and amplitude ratios of this RFIM system. {copyright} {ital 1999} {ital The American Physical Society }

The random-field specific heat critical behavior at high magnetic concentration: Fe0.93Zn0.07F2

Abstract The specific heat critical behavior is measured and analyzed for a single crystal of the random-field Ising system Fe 0.93 Zn 0.07 F 2 using pulsed heat and optical birefringence techniques. This high magnetic concentration sample does not exhibit the severe scattering hysteresis at low temperature seen in lower concentration samples and its behavior is therefore that of an equilibrium random-field Ising model system. The equivalence of the behavior observed with pulsed heat techniques and optical birefringence is established. The critical peak appears to be a symmetric, logarithmic divergence, in disagreement with random-field model computer simulations. The random-field specific heat scaling function is determined.

Order parameter criticality of the d = 3 random-field Ising antiferromagnet Fe0.85Zn0.15F2.

The critical exponent beta=0.16+/-0.02 for the random-field Ising model order parameter is determined using extinction-free magnetic x-ray scattering for Fe0.85Zn0.15F2 in magnetic fields of 10 and 11 T. The observed value is consistent with other experimental random-field critical exponents, but disagrees sharply with Monte Carlo and exact ground state calculations on finite-sized systems.

Random-field effects in d = 2 and d = 3 ising systems

Abstract The critical behavior of the d = 3 random-field Ising model (RFIM) system Fe x Zn 1- x F 2 is reviewed and compared with recent theories. An equilibrium boundary is found at T eq ( H ) > T c ( H ), the sharp phase transition; hysteresis occurs for T T eq ( H ). In the d = 2 RFIM system Rb 2 Co 0.085 Mg 0.15 F 4 , freezing occurs at a boundary T F (H) c (H)” , the destroyed phase transition. Both T eq ( H ) and T F ( H ) follow RFIM crossover scaling.

Random-field critical scattering at high-magnetic concentration in the Ising antiferromagnet Fe0.93Zn0.07F2

Abstract The high-magnetic concentration Ising antiferromagnet Fe 0.93 Zn 0.07 Fe 2 does not exhibit the severe critical scattering hysteresis at low temperatures observed in all the lower concentration samples studied. The system, therefore, provides equilibrium neutron scattering line shapes suitable for determining random-field Ising model critical behavior.

The effects of Co3O4 on the structure and unusual magnetism of LaCoO3

Bulk La(w)CoO(3) particles with w = 1.1, 1.0, 0.9, 0.8, and 0.7 were synthesized using starting materials with varying molar ratios of La(2)O(3) and Co(3)O(4). The resulting particles are characterized as LaCoO(3) crystals interfaced with a crystalline Co(3)O(4) phase. X-ray and neutron scattering data show little effect on the average structure and lattice parameters of the LaCoO(3) phase resulting from the Co(3)O(4) content, but magnetization data indicate that the amount of Co(3)O(4) strongly affects the ferromagnetic ordering at the interfaces below TC ≈ 89 K. In addition to ferromagnetic long-range order, LaCoO(3) exhibits antiferromagnetic behavior with an unusual temperature dependence. The magnetization for fields 20 Oe ⩽ H ⩽ 5 kOe is fit to a combination of a power law ((T - TC)/TC)(β) behavior representing the ferromagnetic long-range order and sigmoid-convoluted Curie-Weiss-like behavior representing the antiferromagnetic behavior. The critical exponent β = 0.63 ± 0.02 is consistent with 2D (surface) ordering. Increased Co(3)O(4) correlates well to increased ferromagnetism. The weakening of the antiferromagnetism below T ≈ 40 K is a consequence of the lattice reaching a critical rhombahedral distortion as T is decreased for core regions far from the Co(3)O(4) interfaces. We introduce a model that describes the ferromagnetic behavior of the interface regions and the unusual antiferromagnetism of the core regions.

Far infrared spectroscopy on the three-dimensional dilute antiferromagnet FexZn1−xF2

Fourier-transform infrared spectroscopy measurements have been performed on the three-dimensional dilute antiferromagnet FexZn1−xF2 with x=0.99–0.58 in the far infrared (FIR) region. The FIR spectra are analyzed taking into account the ligand field and the local exchange interaction probability with J1–J3; |J1|, |J3|≪|J2|, where J1, J2, and J3 are the nearest neighbor, second nearest neighbor, and third nearest neighbor exchange interaction constants, respectively. The concentration dependence of the FIR spectra at low temperature is qualitatively well reproduced by our analysis, though some detailed structure remains unexplained.Fourier-transform infrared spectroscopy measurements have been performed on the three-dimensional dilute antiferromagnet FexZn1−xF2 with x=0.99–0.58 in the far infrared (FIR) region. The FIR spectra are analyzed taking into account the ligand field and the local exchange interaction probability with J1–J3; |J1|, |J3|≪|J2|, where J1, J2, and J3 are the nearest neighbor, second nearest neighbor, and third nearest neighbor exchange interaction constants, respectively. The concentration dependence of the FIR spectra at low temperature is qualitatively well reproduced by our analysis, though some detailed structure remains unexplained.

Quasistationary criticality of the order parameter of the three-dimensional random-field Ising antiferromagnet Fe0.85Zn0.15F2 : A synchrotron x-ray scattering study

The critical exponent beta =0.17(1) for the three-dimensional random-field Ising model (RFIM) order parameter upon zero-field cooling (ZFC) has been determined using extinction-free magnetic x-ray scattering techniques for Fe(0.85)Zn(0.15)F2. This result is consistent with other exponents determined for the RFIM in that Rushbrooke scaling is satisfied. Nevertheless, there is poor agreement with equilibrium computer simulations, and the ZFC results do not agree with field-cooling (FC) results. We present details of hysteresis in Bragg scattering amplitudes and line shapes that help elucidate the effects of thermal cycling in the RFIM, as realized in dilute antiferromagnets in an applied field. We show that the ZFC critical-like behavior is consistent with a second-order phase transitions, albeit quasi-stationary rather than truly equilibrium in nature, as evident from the large thermal hysteresis observed near the transition.