The random-field specific heat critical behavior at high magnetic concentration: Fe(0.93)Zn(0.07)F2

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)F2 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.Comment: 9 pages, 4 figures, RevTeX, minor revision

d=3 random field behavior near percolation

The highly diluted antiferromagnet Mn(0.35)Zn(0.65)F2 has been investigated by neutron scattering for H>0. A low-temperature (T<11K), low-field (H<1T) pseudophase transition boundary separates a partially antiferromagnetically ordered phase from the paramagnetic one. For 1<H<7T at low temperatures, a region of antiferromagnetic order is field induced but is not enclosed within a transition boundary.Comment: 9 pages, 4 figure

Equilibrium random-field Ising critical scattering in the antiferromagnet Fe(0.93)Zn(0.07)F2

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 Tc(H) because of severe non-equilibrium effects. The high magnetic concentration Ising antiferromagnet Fe(0.93)Zn(0.07)F2, however, does provide equilibrium behavior. We have employed scaling techniques to extract the universal equilibrium scattering line shape, critical exponents nu = 0.87 +- 0.07 and eta = 0.20 +- 0.05, and amplitude ratios of this RFIM system.Comment: 4 pages, 1 figure, minor revision

Effective and Asymptotic Critical Exponents of Weakly Diluted Quenched Ising Model: 3d Approach Versus ϵ1/2\epsilon^{1/2}-Expansion

We present a field-theoretical treatment of the critical behavior of three-dimensional weakly diluted quenched Ising model. To this end we analyse in a replica limit n=0 5-loop renormalization group functions of the $\phi^4$-theory with O(n)-symmetric and cubic interactions (H.Kleinert and V.Schulte-Frohlinde, Phys.Lett. B342, 284 (1995)). The minimal subtraction scheme allows to develop either the $\epsilon^{1/2}$-expansion series or to proceed in the 3d approach, performing expansions in terms of renormalized couplings. Doing so, we compare both perturbation approaches and discuss their convergence and possible Borel summability. To study the crossover effect we calculate the effective critical exponents providing a local measure for the degree of singularity of different physical quantities in the critical region. We report resummed numerical values for the effective and asymptotic critical exponents. Obtained within the 3d approach results agree pretty well with recent Monte Carlo simulations. $\epsilon^{1/2}$-expansion does not allow reliable estimates for d=3.Comment: 35 pages, Latex, 9 eps-figures included. The reference list is refreshed and typos are corrected in the 2nd versio

Ordering in the dilute weakly-anisotropic antiferromagnet Mn(0.35)Zn(0.65)F2

The highly diluted antiferromagnet Mn(0.35)Zn(0.65)F2 has been investigated by neutron scattering in zero field. The Bragg peaks observed below the Neel temperature TN (approximately 10.9 K) indicate stable antiferromagnetic long-range ordering at low temperature. The critical behavior is governed by random-exchange Ising model critical exponents (nu approximately 0.69 and gamma approximately 1.31), as reported for Mn(x)Zn(1-x)F2 with higher x and for the isostructural compound Fe(x)Zn(1-x)F2. However, in addition to the Bragg peaks, unusual scattering behavior appears for |q|>0 below a glassy temperature Tg approximately 7.0 K. The glassy region T<Tg corresponds to that of noticeable frequency dependence in earlier zero-field ac susceptibility measurements on this sample. These results indicate that long-range order coexists with short-range nonequilibrium clusters in this highly diluted magnet.Comment: 7 pages, 5 figure