Magnetocaloric Effect

In our previous work, we have studied structure and properties of La1−xPbxMnO3 perovskites. Variation of doping content leads to alternating structure and magnetic properties of materials. In this paper, the investigation of structure, magnetic, magnetocaloric and magnetoresistance properties of family Pr1−xPbxMnO3 (x=0.1–0.5) is presented. The grain size of samples increases with Pb content. The FC and ZFC thermomagnetic curves measured

A specific heat measuring instrument, with the capacity for the application of magnetic fields up to 9 T, resorting to microcalorimetry chips from the company Xensor Integration, has been successfully assembled and its functioning specifications are reported in the current paper.
With this instrument is it possible to perform specific heat measurements with applied magnetic fields up to 9 T in milligram samples. This offers our group the possibility to calculate the actual adiabatic temperature change of a material, as well as providing reliable and precise information on any phase transition that may be influenced by the application of a magnetic field.

The intention of this paper is to elucidate new types of heat engines with extraordinary efficiency, more specifically to eventually focus on the author's research into a temporary magnetic remanence device. First we extend the definition of heat engines through a diagrammatic classification scheme and note a paradoxical non-coincidence between the Carnot, Kelvin-Planck and other forms of the 2 nd Law, between sectors of the diagram. It is then seen, between the diagram sectors, how super-efficient heat engines are able to reduce the degrees of freedom resulting from change in chemical potential, over mere generation of heat; until in the right sector of the diagram, the conventional wisdom for the need of two reservoirs is refuted. A brief survey of the Maxwell Demon problem finds no problem with information theoretic constructs. Our ongoing experimental enquiry into a temporary magnetic remanence cycle using standard kinetic theory, thermodynamics and electrodynamics is presented – yet a contradiction results with the 2nd law placing it in the right sector of the classification diagram.

The magnetocaloric effects in RCo2, where R is a heavy rare−earth, GdAl2, GdNi5, (GdxY1-x)2Co7B3 and RFe2 (R=Y,Er)
compounds are analysed in correlation with magnetic properties. In case of ferrimagnetic compounds, when rare-earth and
transition metal sublattice magnetizations are strongly coupled, the high values of entropy change can be seen at the Curie
temperatures. When the strength of this coupling is relatively, small, there is a high variation of rare-earth sublattices
magnetizations and consequently the maxima values of the entropy changes are evidenced at lower temperatures than the
Curie points. In these cases there are large linewidth, δTm, of the ΔS vs. T dependences and consequently high values of the
specific renormalized cooling power are shown.

Thermal management technology based on the magnetocaloric effect offers several advantages over conventional gas compression cooling. The efficiency of magnetic cooling systems can be much higher than conventional gas based cooling technologies. Additionally, ozone layer depleting chemicals are not used and there is reduced noise and vibrations. Iron and manganese based magnetocaloric materials (MCM) are promising due to the challenges surrounding the use of conventional rare earth based MCM. We review the recent progress in the development of iron and manganese based MCM. The magnetic phase transitions, processing techniques, performance , as well as applications of these materials are discussed. Critical analysis to determine the critical exponents and phase transition behavior of these MCM, using modified Arrot plot, critical isotherm plots, the Kouvel-Fisher method, Landau theory and the Bean-Rodbell model, is also presented.
Keywords: Magnetocaloric materials, Magnetocaloric effect, Fe based alloys, Mn based alloys, Modeling

We report the effect of carrier doping via partial substitution of La 3 þ for Sr 2 þ on the structural, magnetic and magnetocaloric properties of Sr 2 FeMoO 6 double perovskite. Polycrystalline Sr 2 À x La x FeMoO 6 (x ¼0.0, 0.1, 0.2, 0.3) samples were prepared using the conventional solid state reaction method. Using the X-ray diffraction (XRD) analysis it was established that all the samples crystallized in a tetragonal structure with I4/mmm space group. An increase in the La doping lead to an increase in the lattice parameter 'a' and the volume of the unit cell. The lattice parameter 'c' however remained unchanged. The temperature variation in magnetization and Arrott analysis suggested a second order of ferromagnetic phase transition in all samples with Curie temperature, T C increasing from 358 K for x ¼ 0.0–365 K for x ¼0.3. A gradual increase in magnetization was also observed with the increasing La content up to x ¼ 0.2. The magnetic entropy change was calculated from the measurement of isothermal magnetization versus magnetic field at different temperatures. The tunability of magnetization and T C simply by adjusting the concentration of La and synthesis conditions makes Sr 2 FeMoO 6 an attractive material for magnetic refrigeration at desired temperature.

The efficiency of bio-molecular motors stems from reversible interactions ∼ k B T; weak bonds stabilizing intermediate states (enabling direct conversion of chemical into mechanical energy). For their (unknown) origins, we suggest that a magnetically structured phase (MSP) formed via accretion of superparamagnetic particles (S-PPs) during serpentinization (including magnetite formation) of igneous rocks comprising the Hadean Ocean floor, had hosted motor-like diffusion of ligand-bound S-PPs through its ‘template’-layers. Ramifications range from optical activity to quantum coherence. A gentle flux gradient offers both detailed-balance breaking non-equilibrium and asymmetry to a magnetic dipole, undergoing infinitesimal spin-alignment changes. Periodic perturbation of this background by local H-fields of templatepartners can lead to periodic high and low-template affinity states, due to the dipole’s magnetic degree of freedom. An accompanying magnetocaloric effect allows interchange between system-entropy and bath temperature. We speculate on a magnetic reproducer in a setting close to the submarine hydrothermal mound-scenario of Russell and coworkers that could evolve bio-ratchets.

Mn-rich Ni-Mn-Sn metamagnetic shape memory alloys exhibiting magnetostructural transformation are of a great potential as the base materials for solid-state refrigeration. With the aim of fine tuning of the transformation characteristics and improving functional properties, in the present work we have fabricated polycrystalline Ni 50-x Fe x Mn 40 Sn 10 (x = 0, 2, 4, 6, 8 at.%) melt-spun ribbons, starting from the base alloy with x = 0, which is weakly magnetic in both austenitic and martensitic phases. By exploring martensitic transformation (MT) and magnetic behaviors as a function of Fe doping and magnetic field, we have found that Fe and/or magnetic field reduce the MT temperature and Curie temperature of austenite phase, becoming closer to each other as the Fe-content increases, accompanied by an increase of the magnetic moment of austenite, magnetization jump at MT, transformation volume, and magnetic contribution, ∆S M , to the total entropy change at MT. The ribbons present moderate values of ∆S M equal to 11 J kg-1 K-1 at 5 T for x = 8, moderate thermal hysteresis (10-14 K) nearly independent of Fe doping or magnetic field, and adjustable structural and magnetic transition temperatures close to room temperature.

Structural, magnetic, magnetocaloric and magnetoresistance (MR) studies on La0.7Sr0.3Mn0.95Cu0.05O3 (No. 1) and La0.7Sr0.3Mn0.9Cu0.1O3 (No. 2) perovskites are reported. The crystal structure of the samples is rhombohedral with a change of the lattice constants depending on the Cu content. FC and ZFC thermomagnetic measurements for both compositions at low field indicate that a spin-glass-like state (or cluster glass) occurs at low temperatures and a very sharp change of magnetization around the phase-transition point. The Curie temperature, TC, does almost not depend on the content of Cu substitution. A maximum magnetic-entropy change, ΔSmax, of 1.96 and 2.07J/kgK at 13.5kOe and 350K is observed for sample No. 1 and No. 2, respectively. Therefore, they can be considered as active magnetic refrigerant materials for room-temperature applications. Electrical-resistance measurements show that both samples are metallic conductor for T<TC and semiconductor for T>TC moreover, the MR is maximal around TC.

Given the potentiality of Fe2P based alloys for magnetocaloric application, this thesis covers various offshoot material systems related to this rich family of compounds, such as the (Mn,Fe)3(Si,P), (Mn,Co)3(Si,P) and the (Fe,Co)3(Si,P). Also covered is the assembly of a microcalorimetery setup meant for the study of certain fundamental qualities of these systems, as well as an economical optimization of the (Mn,Fe)2(P,Ge) system.

The effect of hydrogenating process on the homogeneity of hydrogen absorption in the La0.8Ce0.2 (Fe1?xMnx)11.5Si1.5 compounds was investigated. It is attractive that the distribution of hydrogen atoms in the compounds becomes more uniform with increasing hydrogen pressure and annealing temperature. Homogeneous hydrides of La0.8Ce0.2(Fe0.985Mn0.015)11.5Si1.5 were obtained by annealing them at the temperature of 773 K and under the hydrogen atmosphere of 0.5 MPa. With changes of Mn content, the Curie temperatures (TC) of the hydrides of La0.8Ce0.2(Fe1-xMnx)11.5Si1.5 can be adjusted in the room temperature range from 279 to 312 K. Large magnetic entropy changes due to the itinerant-electron metamagnetic transition are obtained in the room temperature range for the hydrides of La0.8Ce0.2(Fe1-xMnx)11.5Si1.5.

A linear reciprocating magnetic refrigerator prototype was designed and built with the aid of an industrial partner. The refrigerator is based on the Active Magnetic Regenerative cycle, and exploits two regenerators working in parallel. The active material is Gadolinium in plates, 0.8 mm thick, for a total mass of 0.36 kg. The device is described and results about magnetic field and temperature span measurements are presented.

We have studied the effect of Fe substitution on magnetic and magnetocaloric properties in La0.7Sr0.3Mn1-xFexO3 (x = 0.05, 0.07, 0.10, 0.15, and 0.20) over a wide temperature range (T = 10-400 K). It is shown that substitution by Fe gradually decreases the ferromagnetic Curie temperature (Tc) and saturation magnetization up to x = 0.15 but a dramatic change occurs for x = 0.2. The x = 0.2 sample can be considered as a phase separated compound in which both short-range ordered ferromagnetic and antiferromagnetic phases coexist. The magnetic entropy change (DSm) was estimated from isothermal magnetization curves and it decreases with increase of Fe content from 4.4 J/KgK at 343 K (x = 0.05) to 1.3 J/KgK at 105 K (x = 0.2), under H = 5 T. The La0.7Sr0.3Mn0.93Fe0.07O3 sample shows negligible hysteresis loss, operating temperature over 60 K range around room temperature with refrigerant capacity of 225 J/Kg, and magnetic entropy of 4 J/kgK which will be an interesting compound for applic...

This investigation was centred on exploring the question: Do lanthanide borates offer the prospect of an alternative to gadolinium gallium garnet in respect to the magneto caloric effect? Two different forms of lanthanide borates were tested for the probability of their success in order to achieve this: GdDyBO3 and GdTbBO3, which had similar properties, yet produced various results. Through the use of high temperature synthesis, the lanthanide borates are produced, and then, using an x-ray diffraction, analysed. Overall, GdDyBO3 had a much higher probability of being able to reproduce the magneto caloric effect due to it having much larger radii of Ln3+ ions relative to its cell volume, but further testing is required for the exact measurements of its magnetic properties.

The ferromagnetically coupled cobalt ion is observed to create a magnetocrystalline anisotropy in the PrNi5−xCox structure above a critical composition of x = 2. The competition of the anisotropy energies between Co and Pr sublattices gives rise to a spin reorientation (SR) phenomenon in PrNi5−xCox compounds at a low temperature (∼150 K) which is then followed by a magnetic transition at a higher temperature. Co-doping has a strong influence on the Curie temperature, changing it from ∼60K (x = 1.95) to ∼537K (x = 3). The magnetic entropy change is associated with SR as well as a magnetic transition, and correspondingly a large full width at half maximum (δTFWHM) is obtained for this series of compounds. For example, the PrNi2.85Co2.15 compound presents δTFWHM = 166K at a 1 T field. This series therefore has an appreciable relative cooling power, which makes this material a suitable magnetic refrigerant over a large temperature span.

Harvesting low-grade thermal energy from waste heat and natural resources is an extremely promising method to partially substitute depleting fossil fuels. As one of the alternative technologies, static thermomagnetic power generation is subjected to inefficiency due to the non-regenerative power cycle. To give an insight to what degree can current technology realize regeneration and guide the direction of future research on thermomagnetic power generation, a regenerative static thermomagnetic generator was designed, modeled, constructed, and tested for the first time in this study, several parameters were systematically optimized as well. Firstly, the working principle of the studied static thermomagnetic generator and the importance of regeneration was reviewed. Then both numerical and experimental methods were used to evaluate the dependence of power output and load voltage on mean temperature, temperature difference, working frequency, and load resistance. Numerical simulation indicated that the actual regenerative effect can improve the thermal efficiency of the static thermomagnetic generator by a factor of 1 at a frequency of 0.2 Hz. In experiments, a maximum electric power of 25 nW was extracted from the static thermomagnetic generator at a frequency of 0.4 Hz. Significant discrepancies between the numerical and experimental results have been identified and explained in detail. This work also shows that the performance of the actual regenerative cycle is far from that of an ideal regenerative cycle.

Conventional and anisotropic magnetocaloric effects were studied in cubic rare earth RNi2 (
R=Nd,Gd,Tb) ferromagnetic intermetallic compounds. These three compounds are representative of small, null, and large magnetocrystalline anisotropy in the series, respectively. Magnetic measurements were performed in polycrystalline samples in order to obtain the isothermal magnetocaloric data, which were confronted with theoretical results based on mean field calculations. For the R=Tb case, we explore the crystalline electrical-field anisotropy to predict the anisotropic magnetocaloric behavior due to the rotation of an applied magnetic field of constant intensity. Our results suggest the possibility of using both conventional and anisotropic magnetic entropy changes to extend the range of temperatures for use in the magnetocaloric effect.

We have synthesized the intermetallic YPrFe17 compound by arc-melting. X-ray and neutron powder diffraction show that the crystal structure is rhombohedral with View the MathML source space group (Th2Zn17-type). The investigated compound exhibits a broad isothermal magnetic entropy change ΔSM(T) associated with the ferro-to-paramagnetic phase transition (TC ≈ 290 K). The |ΔSM| (≈2.3 J kg−1 K−1) and the relative cooling power (≈100 J kg−1) have been calculated for applied magnetic field changes up to 1.5 T. A single master curve for ΔSM under different values of the magnetic field change can be obtained by a rescaling of the temperature axis. The results are compared and discussed in terms of the magneto-caloric effect in the isostructural R2Fe17 (R = Y, Pr and Nd) binary intermetallic alloys.

We present the results of non-free parameter calculations of properties of HoAl 2 and ErAl 2 single crystals, performed with our new computation system called ATOMIC MATTERS MFA [1, 2]. A localized electron approach was applied to describe the electronic structure evolution of Ho and Er ions over a wide temperature range and estimate Magnetocaloric Effect (MCE). Thermomagnetic properties of HoAl 2 and ErAl 2 were calculated based on the fine electronic structure of the 4f 10 and 4f 11 electronic configuration of the Ho and Er ions, respectively. Our calculations yielded: magnetic moment value and direction; single-crystalline magnetization curves in zero field and in external magnetic field applied in various directions m(T,B ext); the 4f-electronic components of specific heat c 4f (T,B ext); and temperature dependence of the magnetic entropy and isothermal entropy change with external magnetic field-S(T,B ext). The cubic universal CEF parameters values used for all CEF calculations was taken from [3] and recalculated for universal cubic parameters set for the RAl 2 series: A 4 =+7.164Ka 0 and A 6 =-1.038Ka 0. Magnetic properties were found to be anisotropic due to cubic Laves phase C15 crystal structure symmetry. These studies reveal the importance of multipolar charge interactions when describing thermomagnetic properties of real 4f electronic systems and the effectiveness of an applied self-consistent molecular field in calculations for magnetic phase transition simulation.

We present the results of non-free parameter calculations of properties of TbAl 2 and GdAl 2 single crystals, performed with our new computation system called ATOMIC MATTERS MFA [1,2]. A localized electron approach was applied to describe the electronic structure evolution of Tb and Gd ions over a wide temperature range and estimate Magnetocaloric Effect (MCE). Thermomagnetic properties of TbAl 2 and GdAl 2 were calculated based on the fine electronic structure of the 4f 8 and 4f 7 electronic configuration of the Tb and Gd ions, respectively. Our calculations yielded: magnetic moment value and direction; single-crystalline magnetization curves in zero field and in external magnetic field applied in various directions m(T,B ext); the 4f-electronic components of specific heat c 4f (T,B ext); and temperature dependence of the magnetic entropy and isothermal entropy change with external magnetic field-S(T,B ext). The cubic universal CEF parameters values used for all CEF calculations was taken from [3] and recalculated for universal cubic parameters set for the RAl 2 series: A 4 =+7.164Ka 0 and A 6 =-1.038Ka 0. Magnetic properties were found to be anisotropic due to cubic Laves phase C15 crystal structure symmetry. These studies reveal the importance of multipolar charge interactions when describing thermomagnetic properties of real 4f electronic systems and the effectiveness of an applied self-consistent molecular field in calculations for magnetic phase transition simulation.

The present work intends to study and optimize the application of magnetic materials for thermal-magnetic devices. As such we have studied the series Ni2MnGa1-xBix (with x from 0 to 0.05), PrNi5-xCox (with x in the interval 1.95 to 3.00) and RNi2 (com R=Tb;Nd;Gd).
For the Ni2MnGa1-xBix series we studied the influence of the bismuth substitution in an attempt to o make the magnetic and structural transition temperatures, TC and TM, come closer and possible merge, in addition, we also study the magnetic anisotropy of this alloy. Our results have in fact increased TM and decreased TC, caused by the increase of the lattice parameter a and the increase of the electron concentration of the alloyed samples, although the studied alloying was not enough to merge the two temperatures. We found a maximum magnetic entropy change of 3.8 J/kg.K for the pure sample and 2.2 J/kg.K for the sample with the most alloying (0.05).
In the PrNi5-xCox series, due to the competition between the anisotropy energies of both the Co and Pr sub lattices this series has a spin reorientation phenomenon at low temperature (~140 K). This series presents a rich magnetic phase diagram that was constructed in the present work and some effects ratified based on the percolation theory. We also found a large magnetic entropy change peak due to the spin-reorientation process and the magnetic transition.
As far as the RNi2 series goes, our study was only limited to the production, homogeneity analyses and magnetic entropy variation calculation, for further study by Dr. Pedro J. von Ranke (State University of Rio de Janeiro) using a theoretical Hamiltonian model to simulate the magnetic properties of the RNi2 series. The TbNi2 and the GdNi2 samples have shown to be single phase while the NdNi2 does not. The theoretical model has shown to be very effective in predicting the magnetic behavior of the samples.

We present a cost-effective and robust set-up designed to measure directly the magnetic field-induced adiabatic temperature change. The system uses a piston to introduce/remove the sample to/from the magnetic field (µ0∆H up to 1.7T) created by an ordinary electromagnet. The temperature of the sample is controlled by a double pipe heat exchanger operating by the electrical heater and air flow circulation from a Dewar with liquid nitrogen to the sample holder assembly.
We have measured the adiabatic temperature change, ∆Tad, of two polycrystalline samples: Gd and Ni50Mn35In15 Heusler alloy. At the second-order magnetic phase transitions (18ºC for Gd and 42ºC for Ni50Mn35In15), ∆Tad under µ0∆H=1.7T are 3.8±0.1ºC for Gd and 1.9±0.1ºC for Ni50Mn35In15. The Heusler alloy shows an inverse magnetocaloric effect: ∆Tad is -1.5±0.1ºC on cooling and -1.6±0.1ºC on heating at the martensitic transformation temperatures of ~24ºC and ~29ºC respectively.

The influence of Ti-doping on the magnetic and magnetocaloric properties of La0.67Ba0.33Mn0.98Ti0.02O3
perovskite is investigated. La0.67Ba0.33Mn0.98Ti0.02O3 sample was prepared by ceramic route at 1400 ◦C.
It is a cubic Pm–3m single phase and exhibits a sharp ferromagnetic–paramagnetic (FM–PM) transition
at a Curie temperature TC (314 K) which is very close to room temperature. Above TC the data
follow a Curie–Weiss law with a shift between experimental and calculated effective paramagnetic
moment. The associated experimental magnetic entropy change (SM) and the relative cooling power
(RCP) have been determined. The observed field dependence of SM is explained reasonably well by
the Landau theory of second order phase transition. The maximum entropy change |Smax
M
| exhibits
a linear dependence with the applied magnetic field. |Smax
M
| and RCP are respectively 3.21 J kg−1 K−1
(21.48 mJcm−3 K−1) and 307 J kg−1 (2054 mJcm−3) at 5 T, which are about 30% of pure Gd. Our results
on the magnetocaloric effect (MCE) are compared favourably with reported values for other doped
manganites, thus concluding that our sample can be used as a magnetic refrigerant around room
temperature.