Magnetocaloric properties and unconventional critical behavior in (Gd,Tb)6(Fe,Mn)Bi2 intermetallics

The magnetic and magnetocaloric properties of the intermetallic family (Gd,Tb)6(Fe,Mn)Bi2 have been studied from 2 K to temperatures above the respective Curie temperatures TC. The substitution of Gd by Tb (Gd6FeBi2, Gd3Tb3FeBi2, Tb6FeBi2) tunes TC in the range 350-250 K and favors the apparition of a metamagnetic transition at very low temperature (below 10 K) from a complex magnetic state to a ferromagnetic one, as well as a spin reorientation transition below Tm = 72 K. As a consequence, an important inverse magnetocaloric effect (IMCE) appears below 20 K and an interesting direct magnetocaloric effect (DMCE) appears over a wide temperature span between TC and Tm with maxima at those temperatures. The partial substitution of Fe by Mn in Tb6Fe0.5Mn0.5Bi2 shifts these effects upwards in temperature while expanding the region of the direct magnetocaloric effect between 70 and 400 K. The combination of adjoint IMCE and DMCE as well as the wide span of the latter shows that tuning this family allows to locate the magnetocaloric effect in different regions of interest. The critical behavior of the PM-FM transitions has been studied obtaining the critical exponents α, β, γ, δ and checking that the respective magnetocaloric effects also scale with the critical parameters n and δ. The transition in Gd6FeBi2 belongs to the Heisenberg universality class with deviations due to magnetocrystalline anisotropies; the critical exponents for Gd3Tb3FeBi2 (in agreement with the Mean Field model) suggest the presence of long range order magnetic interactions, while Tb6FeBi2 and Tb6Fe0.5Mn0.5Bi2 present an unconventional critical behavior aligned with long range order interactions.This work has been supported by Universidad del País Vasco UPV/EHU (project GIU19/305) and the Russian Fund for Basic Research (project N° 20-03-00209-a). A. Herrero thanks the Department of Education of the Basque Government as grantee of the programme “Programa Predoctoral de Formación de Personal Investigador No Doctor”. The authors thank for technical and human support provided by SGIker of UPV/EHU

Magnetocaloric properties, magnetic interactions and critical behavior in Ho6(Fe,Mn)Bi2 intermetallics

Four polycrystaline Fe2P-type Ho6(Fe,Mn)Bi2 intermetallic compounds (space group , No. 189, hP9) have been studied using magnetic techniques in order to explore their ability as magnetocaloric materials, and study the critical behavior of the paramagnetic (PM) to ferromagnetic (FM) transitions to obtain a deeper understanding of the range of the magnetic interactions. The obtained critical exponents β, γ and δ for the four compounds studied (Ho6MnBi2, Ho6FeBi2, Ho6(Mn0.5Fe0.5)Bi2, Ho6(Mn0.75Fe0.25)Bi2) point to long-range order interactions, as they are close to those of the Mean Field Universality class. All of the compounds show relevant magnetocaloric properties over a very broad temperature range, limited by the PM-FM transition and a spin-reorientation one, well separated in all cases. They present very high values of the refrigerant capacities (from 520 J/kg to 709 J/kg at 5 T), good magnetic entropy changes (from 3.4 to 5.7 J/(kgK) at 5 T), and a flat and wide temperature span for the working temperature range (nearly 200 K for Ho6MnBi2, 80 K for Ho6FeBi2 at 5T). The change in properties with composition proves that the magnetocaloric properties can be tuned in Fe2P-type compounds to accommodate different refrigeration applications. Finally, the magnetocaloric scaling laws have been successfully tested and universal curves for the magnetic entropy change have also been obtained in the PM-FM transition region.This work has been supported by Universidad del País Vasco UPV/EHU (GIU16/93)

In-situ nanoindentation of titania microspheres with different crystallinity

This work was supported by the Lomonosov MSU Program of Development

Tailoring the magnetocaloric, magnetic and thermal properties of Dy6(Fe,Mn)X2 intermetallics (X==Sb, Te, Bi)

[EN] The structural, magnetic, magnetocaloric (MCE) and thermal properties of seven Fe2P-type Dy6(Fe,Mn)X2 (X=Sb, Bi, Te) intermetallics (space group P 6 over line 2 m, N 189, hP9) have been experimentally studied. They present a paramagnetic to ferromagnetic transition (in the range 129-370 K), followed, as temperature decreases, by a spin-reorientation one (from 52 to 170 K) and a ground magnetic state at 2 K with anti-ferromagnetic components. This state turns into a ferromagnetic state when a magnetic field is applied. The critical exponents beta,gamma,delta related to the PM-FM transition point to long range order interactions but in most compounds their values severely deviate from the Mean Field class, presenting an unconventional critical behavior, probably due to magnetocrystalline anisotropies. This magnetic complexity has the consequence that in every intermetallic three MCE effects arise: Two direct magnetocaloric effects (DMCE) with a table-like effect in between (from 40 K to more than 400 K), with moderate values of the magnetic entropy maxima (up to 6.9 J/kgK for 140 Delta H = 5 T, with the tableau in-between being around 4 J/kgK, for Dy6FeSb2 and Dy6FeSbTe). The calculation of the Thermal Average Entropy Change allows to place the properties of two compounds (Dy6FeSb2 and Dy6FeSbTe) close to other rare earth based high entropy alloys described in literature. The seven compounds present a relevant third MCE, inverse, below 25 K, with a value as high as 17.8 J/kgK (140 Delta H = 5 T) for Dy6FeSbTe. The maximum of the magnetic entropy change at the Curie tem-perature has been shown to scale with the critical exponents found and universal curves have been built. Finally, the thermal diffusivities in the range of the DMCE have been measured, with the result that they present good values (between 1 and 3 mm2/s) to be used in real magnetocaloric refrigeration systems.This work has been supported by Universidad del Pais Vasco UPV/EHU (project GIU19/058) and the Russian Fund for Basic Research (project No 20-03-00209-a). A. Herrero thanks the Department of Education of the Basque Government as grantee of the programme "Programa Predoctoral de Formacion de Personal Investigador No Doctor". The authors thank for technical and human support provided by SGIker of UPV/EHU, specially the fruitful discussions with Dr. I. Orue

Selecting optimal R6TX2 intermetallics (R = Gd, Tb, Dy; T = Mn, Fe, Co, Ni; X = Sb, Te) for magnetic refrigeration

A complete experimental study of the physical properties playing a relevant role on magnetic refrigeration application (structural, magnetic, magnetocaloric and thermal) has been performed over nine selected Fe2P-type R6TX2 (R= Gd, Tb, Dy; T= Mn, Fe, Co, Ni; X=Sb, Te) intermetallic compounds, to work close to room temperature. Two magnetic phase transitions are present on these materials: a paramagnetic to ferromagnetic transition in the range 182-282 K and a spin reorientation transition in the range 26-76 K. As a consequence, two peaks related to a direct magnetocaloric effect (DMCE) appear in the magnetic entropy change, generating a wide table-like plateau region in between both peaks, which is required to improve the efficiency of refrigerators following an Ericsson cycle. The highest magnetic entropy peak value for μ0ΔH = 5 T is found for Tb2Dy4FeSb2, with 7.72 J/kg K around 182 K. For the same applied field the other compounds show moderate values around room temperature (2.88-4.53 J/Kg K). However, the superposition of the two peaks results in huge refrigerant capacity values, up to RCFWHM(5 T)=1103.04 J/kg in the case of Tb2Dy4FeSb2. The thermal diffusivity, effusivity conductivity and specific heat have been measured at room temperature, and the temperature dependence of the former has been obtained around the relevant magnetic phase transition region, with values in the range 1.3-2.3 mm2/s, which are good for magnetic refrigerators under high working frequencies. The study is completed with a rigorous critical behavior analisis of the second order PM-FM transition. The critical exponent γ points to long range order interactions, in general, while β values are in the range (0.59-0.90), indicating a deviation from theoretical models as a reflection of the magnetic complexity in these compounds. The critical exponents have been used to confirm the scaling relations of magnetocaloric properties, and the scaling of refrigerant capacity (RC) values in materials presenting two magnetic phase transitions is addressed, concluding that for a correct scaling of RC the magnetic entropy change peak must be considered symmetric. The role of each atom on the properties of the compounds is discussed.This work was supported by Departamento de Educación del Gobierno Vasco (project IT1430-22) and the Russian Fund for Basic Research through the project no. 20-03-00209-a, as well as by an ICDD (International Centre for Diffraction Data) (USA) grant n 05-07

Diffusion doping route to plasmonic Si/SiOx nanoparticles

International audienceSemiconductor nanoparticles (SNPs) are a valuable building block for functional materials. Capabilities for engineering of electronic structure of SNPs can be further improved with development of techniques of doping by diffusion, as post-synthetic introduction of impurities does not affect the nucleation and growth of SNPs. Diffusion of dopants from an external source also potentially allows for temporal control of radial distribution of impurities. In this paper we report on the doping of Si/SiOx SNPs by annealing particles in gaseous phosphorus. The technique can provide efficient incorporation of impurities, controllable with precursor vapor pressure. HRTEM and X-ray diffraction studies confirmed that obtained particles retain their nanocrystallinity. Elemental analysis revealed doping levels up to 10%. Electrical activity of the impurity was confirmed through thermopower measurements and observation of localized surface plasmon resonance in IR spectra. The plasmonic behavior of etched particles and EDX elemental mapping suggest uniform distribution of phosphorus in the crystalline silicon cores. Impurity activation efficiencies up to 34% were achieved, which indicate high electrical activity of thermodynamically soluble phosphorus in oxide-terminated nanosilicon

Magnetic and magnetocaloric properties of rare earth intermetallic compound Gd3Co4Ge13

Magnetic and magnetocaloric properties of polycrystalline Gd3Co4Ge13 (Cubic, Yb3Rh4Sn13-type, Space group Pm-3n, No. 223, cP40) have been studied by carrying out dc magnetization measurements in applied magnetic fields up to 140 kOe. The compound Gd3Co4Ge13 orders antiferromagnetically at 9 K (TN). The antiferromagnetism appears to be weak and with increasing applied magnetic fields, ferromagnetic interactions become dominant. This field-induced antiferromagnetic to ferromagnetic state that is marked as a change from inverse to normal magnetocaloric effect in the isothermal magnetic entropy change vs temperature plot around TN. At 2 K, the magnetization shows a tendency toward saturation in applied magnetic field and a magnetic moment of 5.3 µB per Gd3+ is obtained in 140 kOe field