The standard material by which all materials exhibiting magnetocaloric effect are measured is Gadnolinium. In this work we are attempting to understand how nanostructuring can impact the magnetocaloric effect, to this end we have grown Gd in various thin film structures. The samples made were grown via magnetron sputtering on MgO(100) substrates. Samples of thick Gd (2000 A) were grown and sandwiched between two layers of Cr or W and annealed at increasing temperatures to study how this can perturb the magnetic and structural properties of the Gd. Another set of samples was grown in which Gd (at various thicknesses) is in a multilayer system with W. Here the purpose is to explore how changing the thickness of the Gd can change its magnetic properties. Using the appropriate Maxwell relation, the magnetic entropy change was observed to increase with increasing annealing temperature. In a 0-4T magnetic field change, the peak entropy was found to go from approximately 1.5 J/kg-K for the unannealed sample to 4.4 J/kg-K when annealed to 600°C. The multilayers were found to all have a T C near 280 K, in contrast with what is predicted by finite size scaling. This is likely due to pinholes in the W layers allowing the Gd to act as one magnetic material.

Magnetic, magnetocaloric, magnetotransport and magnetoelastic properties of Ni-Mn-X (X = In, and Ga) Heusler alloys and La-Fe-Si based rare earth compounds have been synthesized and investigated by x-ray diffraction, magnetization, strain, and electrical resistivity measurements. The phase transitions, magnetic, magnetocaloric, magnetotransport and magnetoelastic properties strongly depend on the composition of these systems. In Ni50Mn50-xInx with x = 13.5, magnetocaloric and magnetotransport properties associated with the paramagnetic martensitic to paramagnetic austenitic transformation were studied. It was shown that magnetic entropy changes (SM) and magnetoresistance (MR) associated with this transformation are larger and the hysteresis effect is significantly lower when compared to that associated with paramagnetic-ferromagnetic transitions or ferromagnetic-antiferromagnetic/paramagnetic transitions in other systems. The Hall resistivity and the Hall angle shows unusual behavior in the vicinity of the martensitic phase transition for Ni50Mn50-xInx with x = 15.2. The observed Hall resistivity and Hall angle are 50 μ*cm and , respectively. It was observed that the presence of Ge, Al and Si atoms on the In sites strongly affects the crystal structure, and the electric and magnetic behaviors of Ni50Mn35In15. It was found that the partial substitution of In atoms by Si in Ni50Mn35In15 results in an increase in the magnetocaloric effect, exchange bias and shape memory effect. In Ni50Mn35In15-xSix, the peak values of positive SM for magnetic field changes H = 5 T were found to depend on composition and vary from 82 Jkg-1K-1 for x = 1 (at T = 275 K) to 124 Jkg-1K-1 for x = 3 (at T = 239 K). The partial substitution of Ni by Co in Ni50Mn35In15 significantly improves the magnetocaloric effect and MR in the vicinity of martensitic transition. In addition, significantly large inverse SM and MR were observed at the inverse martensitic phase transitions of the Ga-based magnetic shape memory Heusler alloys Ni50-xCoxMn32-yFeyGa18. The phase transition temperatures and magnetic properties were found to be correlated with the degree of tetragonal distortion in these samples. In LaFe11.57Si1.43Bx the crystal cell parameters and Curie temperatures were found to increase linearly with increasing B concentration up to ~ 0.1 % and 9 %, respectively. It was found that the characteristics of the magnetocaloric effect of LaFe11.57Si1.43 can be adjusted by a change in B concentration in the LaFe11.57Si1.43Bx system. A study of the influence of a small substitution of Ni, Cu, Cr, and V for Fe in LaFe11.4Si1.6 revealed that the magnetic, magnetocaloric, and magnetovolume coupling constant is related to an increase in the average Fe-Fe interatomic distances, leading to a change in the d-d exchange interaction.

The attention for materials displaying high magnetocaloric effect (MCE) has grown during the past 30 years. One of the most important properties of MCE is the adiabatic temperature change ( ). The main aim of this work was to develop a method to measure the temperature change ( ) for magnetocaloric materials in a changing magnetic field.  A technique was developed where maximum reached  for Gadolinium was 1.19 K in a changing magnetic field of 1.3 T, however, this is lower value in comparison with previous studies (3.3 K in a changing magnetic field of 1 T, Bjørk, et al., 2010) which makes the developed method not sufficient enough to measure . Furthermore, finding novel materials displaying high MCE is of great interest. MnFePSiB alloys display promising MCE properties but processing method is expensive and time consuming. Therefore, a MnFePSiB compound was simply remelted several times and heat treated to enhance its properties. The MnFePSiB alloy was remelted 1, 2 and 3 times after initial casting. Melting the material 3 times resulted improvement in both the magnetic and magnetocaloric properties due to enhanced homogeneity. The material melted 3 times was further heat treated to improve its magnetic magnetocaloric properties. Heat treating the material for 5 hours at 1373K improved the magnetic entropy change more than 10 times compared to the as cast sample,  was moved closer to room temperature and maximum  of 0.71 K was obtained.

The effect of doping on the crystal structure, magnetic, magnetocaloric and transport properties of MnM′Ge (M′ = Ni, Co) intermetallic compounds and NiMnX (X = Sn, In) Heusler alloys have been studied by room temperature X-ray diffraction (XRD), differential scanning calorimetry (DSC), and magnetization measurements. The studied magnetic systems include Ni1-xCrxMnGe1.05 (0 ≤ x ≤ 0.120), Mn1-xAlxCoGe (0 ≤ x ≤ 0.05), MnCo1-xZrxGe (0.01 ≤ x ≤ 0.04), Mn1-xAgxCoGe (0.01 ≤ x ≤ 0.10), Ni50-xRxMn35Sn15 (x = 0, 1 and R = La, Pr, Sm), Ni43-xRxMn46Sn11 (x = 0, 1 and R = Pr, Gd, Ho, Er), and Ni50Mn35In15-xBix (0 ≤ x ≤ 1.5).A temperature induced first-order structural transition characterized by a change in crystal structure from high temperature austenite phase (AP) with Ni2In-type Hexagonal structure to low temperature martensite phase (MP) with TiNiSi-type orthorhombic structure was observed at T = TM (martensitic transition temperature) in some of the MnM′Ge-based compounds. The partial substitution of doping elements such as Cr, Al, Zr, and Ag resulted in a decrease in TM and at certain concentration, TM was found to decrease below / coincide with the ferromagnetic transition temperature (TC) of AP. Therefore, such system show a first-order magnetostructural transition (MST).In Ni1-xCrxMnGe1.05, a MST from antiferromagnetic (AFM) orthorhombic to ferromagnetic (FM) hexagonal phase was observed for 0.105 ≤ x ≤ 0.120. Both direct and inverse MCE were observed in this compound. The peak values of the magnetic entropy change (ΔSMpeak ) in the vicinity of TC for ΔH = 5T were found to be 4.5 J/kg K, 5.6 J/Kg K, and 5.1 J/Kg K for x = 0.105, 0.115, and 0.120 respectively. A magnetic field-induced transition from an AFM to a FM state in the martensite structure was observed in annealed Ni0.895Cr0.105MnGe1.05 melt-spun ribbons, which led to a coupled MST from a FM martensite to a PM austenite phase with a large change in magnetization. As a result of the field-induced MST, a large ΔSMpeak value of 16.1 J kg-1 K-1 (which is about a four times larger than the bulk) and Refrigeration capacity (RC-1) =144 J kg-1 at μ0∆H = 5 T was found. It was also found that the ribbon samples showed excellent magnetic reversibility that is important for application. MCE parameters, adiabatic temperature change (∆Tad) and |〖∆S〗_M |, with maximum value of ~ 2.6 K (µoH = 10 T) and 4.4 J kg-1 K-1(µo∆H = 5 T), respectively, were observed in the vicinity of TC. The ∆Tad (T) curves obtained for µoΔH = 10 T and magnetization isotherms were found to be completely reversible, which indicates the reversibility of the MCE in this system. A large temperature span (of about 61 K) and a non-saturating behavior of ∆Tad were observed at magnetic fields up to 10 T. The adiabatic temperature change was found to be a linear function of (µoH)2/3 near TC in accordance with Landau’s theory of phase transitions.In MnCoGe compounds doped with Al, Zr, and Ag, a tunable MST from the paramagnetic hexagonal to ferromagnetic orthorhombic phase was observed. The maximum ΔSM values of about 18, 7.2, and 22 J kg-1 K-1for ∆H = 5T at TM was observed for Al, Zr, and Ag doped compounds, respectively. The corresponding maximum value of RC was found to be (303, 266, and 308) JKg-1.The new compounds containing low concentration of rare earth (R) metals: Ni50-xRxMn35Sn15, Ni43-xRxMn46Sn11, with R = La, Pr, Sm, Gd, Ho, Er and Ni50Mn35In15-xBix were synthesized. The compounds crystallized in the cubic L21 austenite phase (AP) or a mixture of AP and low temperature martensitic phase (MP) at room temperature. For Ni50-xRxMn35Sn15 and Ni43-xRxMn46Sn11 alloys, TM shifted towards higher temperature with rare-earth doping, thus stabilizing the MP at higher temperature. A maximum shift in TM by ~ 60-62 K relative to the parent compound (TM = 190-195 K) was observed for the Ni49LaMn35Sn15 and Ni42PrMn46Sn11. TM shifted towards lower temperature if Bi is placed in In position in Ni50Mn35In15-xBix. A maximum shift of ~ 36 K was detected for x = 1.5. Abnormal shifts in TC and TM to higher temperatures were observed at high field for Bi concentration ≥ 0.5.The ground state magnetization decreased with the rare-earth doping and increasing Bi content. The compounds exhibit both inverse and normal magnetocaloric effects. Large values of ∆SM = 12 (Ni49PrMn35Sn15), 32 Jkg-1K-1(Ni42PrMn46Sn11), 28 Jkg-1K-1 (Ni42GdMn46Sn11), 25 Jkg-1K-1 (Ni42HoMn46Sn11), 40 J/kg K (Ni50Mn35In15) and 34 J/kg K (Ni50Mn35In15-xBix, x = 0.25) were found at TM for ∆H = 5T that can be tuned in a wide temperature range. RC values ranging from 267-336 Jkg-1 at TC, 182 -250 Jkg-1 at TM and 144-165 Jkg-1 at TC were found with ∆H = 5T for Ni50-xRxMn35Sn15, Ni43-xRxMn46Sn11, and Ni50Mn35In15-xBix, respectively. Significant magnetoresistance (MR) values of -30%, -20 % and -30% were observed in Ni49LaMn35Sn15, Ni42GdMn46Sn11, and Ni50Mn35In14.5Bi0.5 compounds, respectively, at TM and ∆H = 5T. A large exchange bias effect with HEB ~ 1.1 kOe at 10 K was observed for the Ni42PrMn46Sn11 compound in its MP. Thus, the pronounced multifunctional properties such as shape memory effects, MCE, EB, and MR make these new systems promising for the ongoing development of magnetocaloric and multifunctional technologies.

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