R. M. Roshko

Modelling the irreversible response of magnetically ordered materials: A Preisach-based approach

We present a general theoretical framework, based on the Preisach model of hysteresis, for describing irreversible phenomena in magnetically ordered materials, which includes the effects of the critical ordering temperature TC. The model is based on the premise that the free energy landscape can be decomposed into an ensemble of bistable Barkhausen elements, each characterized by two energy barriers which are defined by local anisotropy and interaction fields. The free energy landscape has an explicit temperature dependence which originates from the critical growth of the spontaneous moment below the critical temperature TC, and thermal overbarrier fluctuations are responsible for relaxing the system towards equilibrium. The theory identifies certain fundamental characteristic energies which play a primary role in determining the principal features of the magnetic response. The model is able to replicate a broad spectrum of behaviour observed experimentally in the field and temperature dependence of magnetic response functions such as the field cooled and zero-field cooled moment and the major hysteresis loop, in a wide variety of magnetic materials, including very specific experimental anomalies and trends, and provides a rigorous theoretical framework which quantifies the interpretation of the experimental data.

Comparisons of scaling predictions with an SK-like model in the ferromagnetic regime

The authors present a model calculation which not only indicates the modification of the region of validity of the scaling-theory predictions by a broadened effective field distribution but which also reproduces substantially the systematics of such behaviour as reported experimentally for PdMn.

A mean field criterion for identifying a reentrant phase transition

Abstract We present numerical simulations of the differential susceptibility in the reentrant regime of a simple Ising mean field model which yield a singular anomaly in the nonlinear components of the susceptibility in the vicinity of the reentrant transition temperature, where asymptotic expansions are invalid. These simulations bear a striking resemblance to an anomaly which has recently been observed in measurements of the nonlinear frequency dependent susceptibility of a reentrant ( Pd Fe)Mn alloy, and reproduce many of the systematics of the experimental data, over a comparable range of reduced fields and temperatures. The calculations provide theoretical justification for regarding the experimental anomaly as a manifestation of critical behaviour associated with a reentrant F-SG phase transition.

Critical behavior in two reentrant NiMn ferromagnets

The real and imaginary components of the low‐field dynamic susceptibility of two reentrant NiMn ferromagnets containing 22.5 and 23 at. % Mn have been measured as a function of temperature in static magnetic fields up to 43.3 Oe. The real component χ’ exhibits a triple‐peaked structure. A critical analysis confirms the highest temperature component of this structure as a manifestation of the critical fluctuations associated with the paramagnetic–ferromagnetic transition. The remaining double‐peaked structure is localized in the vicinity of the reentrant transition and is accompanied by a complementary structure in the imaginary component χ‘. A correlation is established between the reentrant susceptibility peaks and the reentrant phase boundaries predicted by vector spin models for bond disordered systems.

The non-linear susceptibility of an arbitrary spin Ising model with competing interactions using an effective field approach

Abstract We present the predictions of an effective field treatment of an arbitrary spin Ising model with a Gaussian distribution of exchange coupling strengths, with emphasis on the region j T SG .

A PREISACH MODEL WITH A TEMPERATURE AND TIME-DEPENDENT REMANENCE MAXIMUM

We present a simple physical model which is capable of generating a maximum in the field dependence of both the isothermal remanent magnetization and the thermoremanent magnetization, with temperature and time-dependent systematics which are identical to those observed experimentally in spin glasses. The model uses an output dependent (moving) Preisach formalism to calculate the response to an applied field of a collection of interacting two-level subsystems with intrinsic anisotropy barriers, which are allowed to relax to equilibrium by thermal over-barrier activation. The maxima occur when the distribution of subsystem anisotropies has an output dependence which reduces the mean activation barrier in proportion to the magnetization induced in the system, so that the thermal relaxation processes become progressively more effective as the system approaches saturation.

Spurious re-entrant behaviour in a system with short-range order

Abstract The a.c. susceptibility of three samples of a Ni76Mn24 alloy has been measured as a function of temperature and magnetic field at two widely different excitation freauencies, and under a variety of auenching conditions. While rapid auenching yields a canonical spin glass, a reduced auenching rate generates an apparently typical re-entrant ferromagnet with seauential transitions. For the latter samples, the superposition of finite static magnetic fields reveals a distinct peak in the vicinity of the lower transition, which is systematically suppressed in amplitude and temperature, while magnetic isotherms exhibit a strongly-temperature-dependent anomalous structure in the nonlinear susceptibility. There is also evidence for extremely subtle structure in the neighbourhood of the upper transition, but no well-defined critical peaks. We conclude that, while the lower transition is definitely correlated with the onset of a spin-glass phase, the high-temperature portion of the re-entrant profile is a c...

Crossover between equilibrium and nonequilibrium dynamics in a re‐entrant CrFe ferromagnet

Measurements of the low field static magnetization and thermoremanent decay of a Cr‐17 at. % Fe spin glass and a re‐entrant Cr‐22 at. % Fe ferromagnet are presented. The thermoremanent relaxation within the pure spin glass phase exhibits nonequilibrium effects and is describable by the product of a power law and a stretched exponential superposed on a constant term. In the ferromagnet, a crossover is observed between two distinct dynamic regimes: a high temperature, power law regime, with negligible aging effects, which is essentially coincident with the ferromagnetic phase, and a low temperature, stretched exponential regime, with nonequilibrium, age‐dependent behavior, which is correlated with the re‐entrant phase.

The low-field magnetic response of a re-entrant Ni Mn alloy

Abstract The a.c. susceptibility of a re-entrant Ni77·5Mn22·5 alloy has been measured as a function of temperature, 15 K < T < 250 K, and magnetic field, 0 < Ha < 704 Oe, at excitation frequencies, ω, of 16 and 2400 Hz. While the zero-field susceptibility exhibits typical re-entrant characteristics, the application of small static magnetic fields reveals a triple-peaked structure in the temperature dependence of the susceptibility. The high-temperature peak exhibits a systematic suppression in amplitude and upward shift in temperature which is typical of ferromagnetic critical peaks. A critical analysis is performed which yields estimates for the Curie temperature, Tc, and the effective exponents γ∗ and δ∗. The remaining binary structure is localized in the vicinity of the re-entrant transition. Both peaks are suppressed in amplitude and shifted downward in temperature with increasing field. The upper peak displays the reversible behaviour characteristic of the Gabay-Toulouse (GT) transition, while the lo...

Ageing and memory in the relaxation dynamics of collections of two-level subsystems

The relaxation dynamics of collections of interacting, thermally activated, two-level subsystems are shown to be characterized by nonequilibrium ageing and memory effects which are analogous to those observed in magnetic systems where randomness and frustration yield a collectively frozen magnetic state with spin-glass-like correlations. Numerical simulations of these nonequilibrium relaxation effects are presented and compared with measurements performed on assemblies of nanodimensional magnetic particles, and their physical origins, as well as their relationship to genuinely collective nonequilibrium relaxation phenomena, are discussed within a theoretical framework based on the Preisach model of hysteresis.