In this work we present results obtained by the first-order reversal-curve (FORC) method for finding the effect of pressure on a spin transition. The FORC data were recorded by diffuse reflectivity measurements for the spin-transition compounds [Fe(btr)2(NCS)2]\ifmmode\cdot\else\textperiodcentered\fi{}H2O and [Fe0.6Zn0.4(btr)2(NCS)2]\ifmmode\cdot\else\textperiodcentered\fi{}H2O (btr = 4,4$'${}-bis-1,2,4-triazole) under constant pressure in the range 1\char21{}1600 bars. The joint distributions in coercivity-bias coordinates, obtained by the FORC method, were expressed in energy gap $\Delta${} and interaction parameter J coordinates for a complete discussion of the pressure effect in terms of physical parameters, including their average values, distribution widths, and $\Delta${}-J correlation parameter. Pressure increases both $\Delta${} and J, as expected. Pressure has a negligible effect on the distribution widths but sizably decreases the correlation parameter value of the diluted system, an unexpected feature which suggests that the diluted system has multiple-domain behavior with pressure-dependent domain size. This deduction is supported by inspection of the samples using optical microscopy at room temperature. Simulations of the metal distribution in a two-dimensional lattice are performed in order to estimate the pressure-induced change in domain size.

Complex transverse susceptibility is a recent proposed method for the determination of anisotropy and volume distributions in particulate magnetic media. So far, only thermal fluctuations and rate-dependent damped dynamics of the magnetic moment have been identified as reasons for the existence of the imaginary transverse susceptibility. In this paper, we apply a more general approach to derive the complex transverse susceptibility, and we show that the hysteresis phenomenon is the most general concept behind the existence of complex transverse susceptibility. In this paper, the physical origins of the imaginary part of transverse susceptibility are analyzed: rate-independent hysteresis, viscous-type rate-dependent hysteresis, and thermal relaxation effect origin. The rate-independent origin is an intrinsic contribution to the imaginary transverse susceptibility and cannot be neglected because it is a zero-temperature effect.

The first-order reversal curve (FORC) diagram method is one of the most successful characterization techniques used to characterize complex hysteretic phenomena not only in magnetism but also in other areas of science like in ferroelectricity, geology, archeology, in spin-transition materials, etc. Because the definition of the FORC diagram involves a second-order derivative, the main problem in their numerical calculation is that the derivative of a function for which only discrete noise-contaminated data values are available magnifies the noise that is inevitably present in measurements. In this paper, we present the doFORC tool for calculating FORC diagrams of noise scattered data. It can provide both a smooth approximation of the measured magnetization and all its partial derivatives. Even if doFORC is mainly dedicated to FORC diagrams’ computation, it can process a general set of arbitrarily distributed two-dimensional points. doFORC is a free, portable application working on various operating systems, with an easy to use graphical interface, with four regression methods implemented to obtain a smooth approximation of the data which may then be differentiated to obtain approximations for derivatives. In order to perform the diagnostics and goodness of fit, doFORC computes residuals to characterize the difference between the observed and predicted values, generalized cross-validation to measure the predictive performance, two information criteria to quantify the information that is lost by using an approximate model, and three degrees of freedom to compare different amounts of smoothing being performed by different smoothing methods. Based on these, doFORC can perform automatic smoothing parameter selection.