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The expected energy transition to hydrogen gas as a greener energy vector has revived the interest in magnetic refrigeration at the cryogenic range, specifically between 20 and 80 K, with the vision to develop a new generation of hydrogen gas liquefiers. From the materials science point of view, the search for magnetocaloric materials containing mainly non-critical elements with a significant response in that temperature range, together with good cyclability and stability, is a challenging task. Given the increasing interest of the research community on this topic, we aim to establish a comprehensive catalog of the magnetocaloric compounds characterized so far, to be used as a starting point for further research. For this purpose, a systematic outlook of the state of the art is presented here, with the analysis and classification of more than 400 cryogenic magnetocaloric materials, divided into five large families according to their physicochemical properties. Moreover, we provide detailed information about their magnetocaloric properties, magnetic behavior, and transition characteristics together with criticality, which will facilitate the future search for optimal compounds.

RENiX2 compounds, where RE = rare-earth element and X = p-block element, have been highly regarded for cryogenic magnetocaloric applications. Depending on the elements, they can crystallize in CeNiSi2-type, NdNiGa2-type, or MgCuAl2-type crystal structures, showing different types of magnetic ordering and thus affect their magnetic properties. Regarding the magnetocaloric effect, MgCuAl2-type aluminides show larger values than those of the CeNiSi2-type silicides and the NdNiGa2-type gallides due to the favored ferromagnetic ground state. However, RENiGa2 gallides can crystallize in either NdNiGa2- or MgCuAl2-type structures depending on the RE element. In this work, we select heavy RE (HRE) elements for exploring the microstructure, magnetic ordering and magnetocaloric performance of HRENiGa2 (HRE = Dy, Ho or Er) gallides. They all crystallize in the desired MgCuAl2-type crystal structure which undergoes a second-order transition from ferro- to para-magnetic state with increasing temperature. The maximum isothermal entropy change (∣∆Sisomax∣) values are 6.2, 10.4, and 11.4 J kg−1 K−1 (0–5 T) for DyNiGa2, HoNiGa2, and ErNiGa2, respectively, which are comparable to many recently reported cryogenic magnetocaloric materials. Particularly, the excellent magnetocaloric properties of HoNiGa2 and ErNiGa2 compounds, including their composite, fall in the temperature range that enables them for the in-demand hydrogen liquefaction systems.

All-d-metal Ni(Co)-Mn-Ti Heusler alloys show high magnetocaloric/barocaloric effects ascribed to the occurrence of a martensitic transformation together with excellent mechanical properties. However, high magnetic fields are needed to fully drive the transformation and to obtain their maximum responses. To further tune the martensitic transition and the associated magnetocaloric response, we systematically investigate the role of partial Mn substitution by Fe or Cr on the parent composition Ni36Co14Mn35Ti15. On the one hand, Cr doping increases the entropy change of the transformation but causes a tighter overlap of both martensitic and Curie transitions. This significantly reduces the magnetization difference between austenite and martensite and, consequently, strongly decreases the magnetocaloric response. On the other hand, Fe doping reduces the entropy change of the transformation and separates both martensitic and Curie transitions while keeping the magnetization difference among both phases. These two combined features reduce the magnetic field needed to completely drive the martensitic transformation and leads to higher and broader isothermal entropy change peaks for moderate magnetic field changes, reaching up to 25% enhancement for 2 T when compared to the undoped alloy.

Cryogenic magnetic refrigeration is a highly efficient and environmentally friendly technique for gas liquefaction. However, refrigerant materials undergoing large magnetocaloric response at the interesting cryogenic range are dominated by critical elements (mainly rare-earth elements) which impedes practical applicability of such refrigeration systems. Therefore, there is a need for dedicated investigations on optimization of magnetocaloric response at cryogenic range by utilizing compositions that are rare-earth free. In this work, we synthesize the mechanically stable and rare-earth free, all-d-metal Ni35Co15Mn35Ti15 Heusler alloys and investigate the role of varying Mn:Ni ratio on the magnetostructural and magnetocaloric properties of the alloy system. The results of the microstructural characterization indicate homogenous composition for the investigated alloy series. As the Mn:Ni ratio increases from 1.01 to 1.10, the martensitic transition shifts from near-room temperature down to cryogenic region (120–140 K) while the magnetization of the austenitic phase remains unaltered. Isothermal entropy change as high as ∼ 13 J kg−1 K−1 at 1.5 T is achieved for the sample with the highest Mn:Ni ratio at the temperature region for natural gas liquefaction, which significantly surpasses the values previously reported in the literature for similar alloys. In addition to large magnetocaloric response, the martensitic transformation falls in an interesting temperature of the cryogenic region, paving the way for various low-temperature magnetocaloric applications.

Optimizing the performance of magnetocaloric materials is facilitated by understanding the thermomagnetic transitions they undergo, including the order of these transitions and their strength. Those exhibiting strong first-order phase transitions (FOPT) are accompanied by large heating and cooling responses but with relatively small cyclic responses, while materials with second-order (SOPT) character exhibit moderate heating and cooling responses. However, the lack of hysteresis could partially compensate for the lower magnitudes with a more cyclic response. One way to effectively maximize the cyclic response, combining the advantages of FOPT and SOPT, is to fine tune the transition towards the borderline of FOPT-SOPT, which can minimize hysteresis. For the well-known La(Fe,Si)13 family, it is challenging to identify and/or evaluate the critical point where FOPT crossovers to SOPT based on conventional techniques. To address these ambiguities, in this work, we apply the field dependence exponent n criteria to a series of lowly hysteretic and high-performance La(Fe,Mn,Si)13H magnetocaloric materials with compositions close to the critical one. Even if the sample with the lowest hysteresis resembles characteristics of SOPT, it is evidently identified as undergoing FOPT from the n criteria: (1) existence of n > 2 overshoot and (2) n at the transition temperature, ntransition, is 0.37. This proximity to the critical composition (ntransition=0.4) further explains the low hysteresis observed. This FOPT character of the series is confirmed by temperature-dependent 57Fe Mössbauer spectrometry studies, fitting the hyperfine field to the Bean-Rodbell model instead of the usual Brillouin function. As it is a zero-field method, the confirmation by Mössbauer spectrometry gives further strength to the n-criterion.

Magnetocaloric refrigeration has remained a promising alternative to conventional refrigeration for the last few decades. The delay in reaching the market is significantly based on materials' related issues, such as hysteresis/reversibility, mechanical stability, or formability. This perspective paper shows the current trends in magnetocaloric materials research, highlighting the families of alloys and compounds that are gaining attention in the recent years. It also includes an overview of novel approaches that can be used to analyze these properties that could improve the applicability of magnetocaloric materials.

In most cases, substitution studies that aim to optimize magnetic properties are performed at the magnetic atomic site. However, in the case of MnB, magnetic substitutions at the Mn site significantly decrease the once promising magnetocaloric and magnetic properties. This study employs computationally directed search to optimize the magnetocaloric properties of MnB where partial substitutions of boron atoms (Mn50B50−xSix and Mn50B50−xGex where x = 3.125, 6.25, and 12.5) reveal new compounds with a greater magnetocaloric effect than pure MnB at the same Curie temperature. These new compounds were obtained by arc melting the pure elements and further characterized. The computationally driven screening process is based on density functional theory calculations that do not require large databases of known compounds. This work demonstrates that using simple computational screening procedures to search for new magnetocaloric materials with improved properties can be done quickly, cost-effectively, and while maintaining reliability.

The highly regarded Fe2P-based magnetocaloric materials are usually fabricated by ball milling, and require an additional extended annealing treatment at high temperatures (at temperatures up to 1423 K for several hours to days). In this work, we show that fabricating Mn1.3Fe0.6P0.5Si0.5 into the form of microwires attained 82.1 wt.% of the desired Fe2P phase in the as-cast state. The microwires show a variable solidification structure along the radial direction; close to the copper wheel contact, Fe2P phase is in fine grains, followed by dendritic Fe2P grains and finally secondary (Mn,Fe)5Si3 phase in addition to the dendritic Fe2P grains. The as-cast microwires undergo a ferroto para-magnetic transition with a Curie temperature of 138 K, showing a maximum isothermal magnetic entropy change of 4.6 J kg−1 K −1 for a magnetic field change of 5 T. With further annealing, a two-fold increase in the maximum isothermal magnetic entropy change is found in the annealed microwires, which reveal 88.1 wt.% of Fe2P phase.

Additive manufacturing could be an excellent way of shaping magnetocaloric heat exchangers in magnetic refrigerators. However, the metal additive manufacturing techniques present the serious limitation that the melting of a magnetocaloric material can cause its transformation and the loss of functionality. Fused deposition modeling using polymer-based composite filaments is presented as a promising alternative as temperatures are low enough to preserve the magnetocaloric material. To prove this claim, a polymer-based composite filament containing 55 wt% of (La,Ce)(Fe,Mn,Si)13–H magnetocaloric fillers has been manufactured using custom-made polymer capsules as the feedstock for the extrusion. Both adiabatic temperature change and isothermal entropy change have been characterized for the fillers, as-prepared filaments and as-printed parts, indicating that the magnetocaloric material functionality is not altered along the whole process. Printing resolution is comparable to the raw PLA filament.

The search for high-entropy alloys (HEAs), a new class of materials, with high magnetocaloric perfor- mance can address the open question about their potential in functional applications. HEAs exist in a vast compositional space but magnetocaloric HEAs typically exhibit modest magnetocaloric response as they undergo second-order magnetic phase transitions. In this work, through a property-directed search ap- proach, FeMnNiGeSi HEAs with varying Ge/Si ratio are found in the large HEA space to exhibit magneto- structural first-order phase transition. Isothermal entropy change as large as 13 J kg -1 K -1 (for 2.5T) is achieved, which is the largest reported to date for magnetocaloric HEAs. When compared to conventional high-performance magnetocaloric materials, our work is observed to be comparable to many of the con- ventional systems, closing the gap between magnetocaloric HEAs versus conventional magnetocaloric ma- terials for the first time. This opens a new path for the search of functional materials with the optimal mechanical properties of HEAs.

We propose a combination of the Kolmogorov–Johnson–Mehl–Avrami nucleation and growth theory and the Bean–Rodbell model to describe the field-induced transition in LaFe11.6Si1.4 alloys. The approach is applied to a set of bulk samples undergoing first-order transitions produced by different routes and including doping effects. The kinetic analysis of both magnetization and demagnetization processes reveals a nucleation and three-dimensional interface-controlled growth for these alloys. Introducing the kinetic process between the metastable and stable solutions of the Bean–Rodbell model, the field dependence of the magnetization/demagnetization processes, including magnetic hysteresis for different magnetic field sweeping rates, is better reproduced than with the pure model.

First Order Reversal Curve (FORC) distributions of magnetic materials are a well-known tool to extract information about hysteresis sources and magnetic interactions, or to fingerprint them. Recently, a temperature variant of this analysis technique (Temperature-FORC, TFORC) has been used for the analysis of the thermal hysteresis associated with first-order magnetocaloric materials. However, the theory supporting the interpretation of the diagrams is still lacking, limiting TFORC to a fingerprinting technique so far. This work is a first approach to correlate the modeling of first-order phase transitions, using the Bean–Rodbell model combined with a phenomenological transformation mechanism, with the features observed in experimental TFORC distributions of magnetocaloric materials. The different characteristics of the transformations, e.g., transition temperatures, symmetry, temperature range, etc., are correlated to distinct features of the distributions. We show a catalogue of characteristic TFORC distributions for magnetocaloric materials that exhibit some of the features observed experimentally.

Resumen En este trabajo se presentan un conjunto de simulaciones de elementos finitos con el objetivo de modelar la respuesta de un sistema de control de la temperatura basado en ferrofluidos. Concretamente estudiaremos los diodos térmicos magnéticamente activados, dispositivos que se basan en la modificación de la conductividad térmica del sistema mediante el movimiento del ferrofluido controlado por campos magnéticos. En este caso, aplicando campo magnético, el ferrofluido actúa como puente térmico entre dos zonas en las que queremos establecer contacto térmico (estado encendido del diodo), mientras que retirando el campo magnético se aíslan térmicamente dichas zonas (estado apagado del diodo). Mediante las simulaciones realizadas (que considerarán las condiciones encendida y apagada del dispositivo) se comprueba la viabilidad de diversas configuraciones a fin de mejorar la respuesta del dispositivo, considerando como objetivos a optimizar: 1) la diferencia de conductividad entre los estados apagado y encendido del dispositivo, y 2) el tiempo de respuesta del mismo. Partiendo de una configuración convencional, en la cual es dispositivo consta de un orificio cilíndrico para albergar el ferrofluido, ampliamos el estudio al caso de varios orificios cilíndricos (3, 5 y 8 orificios) y rectangulares orientados perpendicularmente y paralelamente al flujo de calor suministrado (en todos los casos la cantidad de ferrofluido a usar se ha mantenido constante). Encontramos que la configuración de 5 orificios cilíndricos, y la configuración de ranuras orientadas perpendicularmente al flujo de calor, son las configuraciones que nos proporcionan una mejor respuesta. Además, se pone de manifiesto que la formación de caminos para la propagación térmica son un factor importante a la hora de analizar el funcionamiento del sistema. Se ha de mencionar que parte de este trabajo incluía la elaboración de un dispositivo experimental mediante técnicas de impresión 3D. Sin embargo, su real

In the present work, using Heusler Ni49+xMn36-xIn15 (with x = 0 and 0.5) alloys, it is shown that the choice of the appropriate measurement protocol (erasing the prior state of the sample in between experiments) in DTad first shot characterization is crucial for obtaining reliable results. Unlike indirect measurements, for which incorrect protocols produce overestimates of the characteristics of the material, erroneous direct measurements underestimate DTad in the region close to its first order phase transition. The error in DTad is found to be dependent on the temperature step used, being up to ~40% underestimation, including a slight shift in its peak temperature.

This thesis focuses on the analysis of the relation between thermomagnetic phase transitions and magnetocaloric response of materials. First, various experimental factors that can a ect the measurements of the magnetocaloric e ect and can potentially cause misleading interpretation or artifacts are studied with the aid of experimental data and numerical calculations. These are sample geometry, compositional inhomogeneities, low temperature limitation in heat capacity measurements and measurement protocols in direct techniques. After that, it is shown that the analysis of the magnetic eld dependence of the magnetocaloric e ect is useful for obtaining information of thermomagnetic phase transitions, such as Curie temperature, critical exponents (even in biphasic systems), order of the phase transition and critical composition for which rst order phase transition becomes second order. These analyses were performed in well know materials such as Gd, Gd5Si2Ge2, La(Fe,Si)13, Ni-Mn-In Heusler alloys and Fe-, Co-, Ni-amorphous and nanocrystalline alloys. Next, various possible avenues to optimize the magnetocaloric behavior by tapping on the characteristics of rst and second order phase transition type systems were investigated. For second order phase transition type, the coe cient of refrigerant performance was optimized as a function of the amount of the various phase fractions in Gd-GdZn composite, while an enhanced magnetic eld responsiveness is achived in nanostructured Gd/Ti multilayers. For rst order phase transition type, reduction of hysteresis and enhancement of rst order character is achieved in Ni- and Cr-doped La(Fe,Si)13 compounds, respectively., Esta tesis se centra en el análisis de las relaciones entre las transiciones de fase termomagnéticas y la respuesta magnetocalórica de los materiales. En primer lugar,se han estudiado diferentes factores experimentales que pueden afectar a la medida e interpretacióndel efecto magnetocalórico, como son: la geometría de la muestra, la presencia de inhomogeneidades composicionales, el truncado de datos calorimétricos a bajas temperaturas y los protocolos de medida para la caracterización directa del efecto magnetocalórico. Posteriormente, se ha puesto de manifiesto que el análisis de la dependencia del efecto magnetocalórico con el campo magnético sirve para obtener información de las transiciones de fase termomagnéticas tales como la temperatura de Curie, los exponentes críticos y el orden de la transición, así como la composición crítica a la queel sistema transita de una transición de fase deprimer orden a una desegundo orden. Estos análisis se han llevado a cabo en materiales ampliamente conocidos tales como Gd, Gd5Si2Ge2, La(Fe,Si)13, aleaciones Heusler Ni-Mn-In y aleaciones amorfas y nanocristalinas basadas en Fe, Co y Ni. Finalmente, se han investigado varias rutas para la optimización de las propiedades magnetocalóricas de los materiales aprovechando las características de las transiciones de primero y segundo orden. Para las transiciones de segundo orden, se ha optimizado la capacidad de refrigeración en función de la fracción de fasesen compuestos Gd-GdZn.Para multicapas nanoestructuradas de Gd/Ti, se ha encontrado una mejor respuesta al campo magnético. Para las transiciones de primer orden, en compuestos La(Fe,Si)13 dopados con metales de transición,se ha reducido la histéresis asociada a la transición mediante el dopado con Ni y se ha aumentado el carácter de primer orden de la transición mediante la adición de Cr. Para estos estudios se han utilizado los métodos de análisis previamente tratados.

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.

In this work, we study the effect of Ni substitution on the magnetocaloric properties of La(Fe,Si)13 compounds. Sample quality has been optimized by a combination of induction melting and suction casting techniques, which allowed to shorten the annealing time by an order of magnitude and expand the existing range of LaFe11.6-xNixSi1.4 phase with cubic NaZn13-type structure up to x ¼ 0.4. According to our Density Functional Theory calculations, Ni addition does not significantly alter the crystal structure (finding that Ni atoms occupy preferably 96i positions) as well as the total magnetic moment; both predictions are in agreement with experimental results. With increasing of Ni concentration, the tran- sition temperature increases and the order of the phase transition changes from first to second-order type. In addition, we show that the magnetic field dependence of magnetocaloric effect enables a clear analysis of the order of phase transition even for compositions near the critical point, surpassing the accuracy of conventionally used techniques for determining the order of magnetic phase transitions (e.g. Banerjee's criterion).

The magnetocaloric effect and its most straightforward application, magnetic refrigeration, are topics of current interest due to the potential improvement of energy efficiency of cooling and temperature control systems, in combination with other environmental benefits associated to a technology that does not rely on the compression/expansion of harmful gases. This review presents the fundamentals of the effect, the techniques for its measurement with consideration of possible artifacts found in the characterization of the samples, a comprehensive and comparative analysis of different magnetocaloric materials, as well as possible routes to improve their performance. An overview of the different magnetocaloric prototypes found in literature as well as alternative applications of the magnetocaloric effect for fundamental studies of phase transitions are also included.

Availability of a restricted heat capacity data range has a clear influence on the accuracy of calculated magnetocaloric effect, as confirmed by both numerical simulations and experimental measurements. Simulations using the Bean-Rodbell model show that, in general, the approximated magnetocaloric effect curves calculated using a linear extrapolation of the data starting from a selected temperature point down to zero kelvin deviate in a non-monotonic way from those correctly calculated by fully integrating the data from near zero temperatures. However, we discovered that a particular temperature range exists where the approximated magnetocaloric calculation provides the same result as the fully integrated one. These specific truncated intervals exist for both first and second order phase transitions and are the same for the adiabatic temperature change and magnetic entropy change curves. The effect of this truncated integration in real samples was confirmed using heat capacity data of Gd metal and Gd5Si2Ge2 compound measured from near zero temperatures.

In this work, the field dependence of the magnetocaloric effect of Gd bulk samples has been enhanced through nanostructuring of the material. Nanostructuring consists in multilayers preparation by alternative rf-sputtering deposition of Gd layers and Ti spacers onto glass substrates. The results obtained for the multilayers were compared to those obtained for the Gd bulk. Assuming a power law for the field dependence of the magnetic entropy change (ΔSM ∝ Hn ), higher field dependences close to the transition in a wider temperature range are obtained for the multilayer material (n = 1.0) with respect to the bulk counterpart (n = 0.78). The effect of a Curie temperature distribution in the multilayer material (due to variations of the layer thickness) has been studied through numerical simulations to explain the observed field dependence of the magnetocaloric effect, obtaining a remarkable agreement between experiments and results.

The magnetocaloric effect of La(Fe,Si)13 samples deteriorates with the presence of secondary phases. However, it is highly challenging to produce single phase samples by conventional procedures and purification requires long annealing time. We propose grinding and particle size selection of non-optimal starting samples as procedure to enhance the magnetocaloric response. In this study a starting multiphase LaFe11.8Si1.2 ingot was grinded and sieved to select three different particle size ranges. X-ray diffraction and Mossbauer spectrometry reveal that all samples mainly contain fcc-La(Fe,Si) € 13 and bccFe(Si) phases. Microstructural and magnetic results show that the fcc-La(Fe,Si)13 phase fraction increases for samples with the smallest average particle size, increasing their magnetocaloric response in a factor larger than three with respect to that of the bulk sample.

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.

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.

The broad compositional range in which transition metal (TM) based amorphous alloys can be obtained, yields an easily tunable magnetocaloric effect (MCE) in a wide temperature range. In some TM-based alloys, anomalous behaviors are reported, as a non-monotonous trend with magnetic moment (e.g. FeZrB alloys). Moreover, in certain Cr-containing Vitroperm alloys anomalously high values of the magnetic entropy change were published. In this work, a systematic study on MCE response of Cr-containing amorphous alloys of composition Fe74-xCrxCu1Nb3Si15.5B6.5 (with x=2, 8, 10, 12, 13, 14 and 20) has been performed in a broad Curie temperature range from 100 K to 550 K. Curie temperature and magnetic entropy change peak of the amorphous alloys decrease with the increase of Cr content at rates of-25.6 K/at% Cr and-54 mJ kg-1 K-1/at% Cr, respectively, following a linear trend with the magnetic moment in both cases. The presence of nanocrystalline phases has been considered as a possible cause in order to explain the anomalies. The samples were nanocrystallized in different stages, however, the magnetocaloric response decreases as crystallization progresses due to the large separation of the Curie temperatures of the two phases.

The magnetocaloric effect (MCE) has been measured for an amorphous mechanically alloyed (MA) Co62Nb6Zr2B30. It is demonstrated that sample shape must be taken into account for a correct interpretation of the results, especially to correctly determine the field dependence of magnetic entropy change and refrigerant capacity. However, although correcting the influence of the demagnetizing factor allows us to recover the theoretically predicted behavior for these field dependencies, they differ from those obtained for a rapidly quenched (RQ) amorphous alloy of the same composition. This deviation is assigned to a broader Curie temperature distribution in MA alloys with respect to RQ alloys. Theoretical simulations based on the Arrott-Noakes equation of state and a Gaussian distribution of TC support this hypothesis.

Amorphous samples of nominal composition Co62Nb6Zr2B30 have been prepared using mechanical alloying (MA) and rapid quenching (RQ) techniques. Differences appear in Curie temperature and the phases developed after crystallization. Refrigerant capacity is enhanced 20% in the MA-sample with respect to that of RQ-sample. Neglecting the demagnetizing factor of powder samples significantly affects the exponent n characterizing the field dependence of the maximum magnetic entropy change

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 T C 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.