L.M. Moreno-Ramírez

A procedure to extract the magnetocaloric parameters of the single phases from experimental data of a multiphase system

In this work, we propose a method to extract the individual parameters that describe the field dependence of magnetic entropy change of each phase in a multiphase system. This method makes use of the scaling laws of the magnetocaloric effect and can help us to determine the behavior of individual phases and to predict their critical exponents. The accuracy of the procedure is illustrated applying it to ball milled powders, in which an amorphous phase with TC around room temperature coexists with bcc-Fe type crystallites. The obtained values are in good agreement with those reported in the literature for single phase systems. The proposed method could be applied to other systems where single phase materials cannot be easily obtained in order to characterize the effect of impurities.

Ball milling as a way to produce magnetic and magnetocaloric materials: a review

Ball milling (BM) is a well-established technique for producing different materials in powder shape. Dynamical analysis of BM helps to optimize the process through simple but general relations (e.g., definition of an equivalent milling time). Concerning the field of study of magnetocaloric effect (MCE), BM is used in different ways: as a single step process (mechanical alloying), as an initial step to enhance mixing of the elements (e.g., to speed up the formation of the desired intermetallic phase) or as a final step (e.g., hydriding of La–Fe–Si). In this contribution, besides a simple description of the effects of some geometrical parameters on the power released during BM and a short review of the BM contribution to the research field of MCE, we will discuss the effect of the microstructure of the starting material and the granular shape inherent to BM on magnetic materials exhibiting MCE.

Optimal temperature range for determining magnetocaloric magnitudes from heat capacity

The determination of the magnetocaloric magnitudes (specific magnetic entropy change, Δs M, and adiabatic temperature change, ΔT ad) from heat capacity (c H) measurements requires measurements performed at very low temperatures (~0 K) or data extrapolation when the low temperature range is unavailable. In this work we analyze the influence on the calculated Δs M and ΔT ad of the usually employed linear extrapolation of c H from the initial measured temperature down to 0 K. Numerical simulations have been performed using the Brillouin equation of state, the Debye model and the Fermi electron statistics to reproduce the magnetic, lattice and electronic subsystems, respectively. It is demonstrated that it is not necessary to reach experimentally temperatures very close to 0 K due to the existence of certain starting temperatures of the experiments, the same for Δs M and ΔT ad, that minimize the error of the results. A procedure is proposed to obtain the experimental magnitudes of Δs M and ΔT ad with a minimum error from c H data limited in temperature. It has been successfully applied to a GdZn alloy and results are compared to those derived from magnetization measurements.

A New Method for Determining the Curie Temperature From Magnetocaloric Measurements

A new method is proposed for determining the Curie temperature from magnetocaloric measurements. It is based on the field dependence of the magnetic entropy change close to the Curie temperature. The main advantages over other methods are that the obtained temperature is field independent, and the process is noniterative and does not require a fitting procedure nor prior knowledge of the critical exponents of the transition. The reliability of the method is demonstrated using both simulated and experimental data for pure Ni and an Fe-based amorphous alloy.

A quantitative criterion for determining the order of magnetic phase transitions using the magnetocaloric effect

The ideal magnetocaloric material would lay at the borderline of a first-order and a second-order phase transition. Hence, it is crucial to unambiguously determine the order of phase transitions for both applied magnetocaloric research as well as the characterization of other phase change materials. Although Ehrenfest provided a conceptually simple definition of the order of a phase transition, the known techniques for its determination based on magnetic measurements either provide erroneous results for specific cases or require extensive data analysis that depends on subjective appreciations of qualitative features of the data. Here we report a quantitative fingerprint of first-order thermomagnetic phase transitions: the exponent n from field dependence of magnetic entropy change presents a maximum of n > 2 only for first-order thermomagnetic phase transitions. This model-independent parameter allows evaluating the order of phase transition without any subjective interpretations, as we show for different types of materials and for the Bean–Rodbell model.Magnetocaloric materials often perform best when their magnetic transitions are at the boundary between first- and second-order behavior. Here the authors propose a simple criterion to determine the order of a transition, which may accelerate future magnetocaloric material searches.

The influence of noise on the determination of the Curie temperature from magnetocaloric measurements

The Magnetocaloric Effect (MCE) is a topic of scientific interest due to its potential application in magnetic refrigeration systems which are called to be an energy efficient and environmental friendly alternative to the usual systems [1].

Influence of Noise on the Determination of Curie Temperature From Magnetocaloric Analysis

In this paper, we study the effect of the signal-to-noise ratio of magnetization measurements on the determination of the Curie temperature from the analysis of the magnetocaloric response. The procedure has been compared with the method of the inflection point of the magnetization versus temperature curves. Magnetization data have been simulated using the Arrott–Noakes equation of state, with the addition of different noise levels (either 1% of the measured signal or 0.3% of the measurement range). It is shown that the obtained values of the Curie temperature are more accurate in the case of the magnetocaloric procedure, although this method requires more data analysis than the inflection point method. Moreover, the field independence of the Curie temperature obtained from the magnetocaloric procedure allows us to perform a statistical analysis of the obtained values, reducing the associated error in the Curie temperature determination.