Microstructure and composition design of magnetic Ni-Mn-Sn Co-sputter deposited films

"In this work, we study the effect of the substrate temperature (ST) during sputter-deposition as well as co-sputtering deposition on the fabrication of nanostructured Ni-Mn-Sn thin films. Sputtered films show Mn losses of around 10 at.% while the average grain size () increased from 30 nm to 105 nm with the increasing of ST. Mn losses compensation is proposed by co-sputtered deposition. With such a purpose a variable electrical power was applied to the radio frequency (RF) Mn cathode. By increasing the electrical power applied to the RF Mn cathode both Mn and Ni contents approach to the targeted nominal composition Ni:Mn:Sn = 50:37:13. Elemental chemical composition analyses show that the composition varied between Ni61.5Mn26.2Sn12.3 and Ni54.6Mn30.5Sn14.9 when the applied RF-power increased from 0 W to 30 W.

Magnetostructural transition and magnetocaloric effect in MnNiGe1.05 melt-spun ribbons

"Alloy ribbons of nominal composition MnNiGe1.05 were produced using the melt-spinning technique. As-quenched (aq) polycrystalline ribbons are single-phase showing the hexagonal Ni2In-type crystal structure. After thermal annealing at 1148 K, the formation of the orthorhombic TiNiSi-type crystal structure by martensitic transformation is favored. However, XRD patterns for different temperatures indicate that the phase transition from hexagonal to orthorhombic structure is incomplete. The starting and finishing temperatures for the direct and reverse martensitic transformation for aq (annealed) samples determined by DSC were MS = 264 (268) K Mf = 235 (255) K, AS = 259 (266) K, and Af = 289 (276) K. Across this structural phase transition the annealed sample undergoes a drop in magnetization giving rise to a narrow temperature dependence of the magnetic entropy change with a peak value on heating (cooling) of 5.8 (4.8) Jkg−1K−1 for a field change of 5 T.

Microstructure and magnetocaloric effect of melt-spun Ni52Mn26Ga22 ribbon

"Microstructural features and magnetocaloric properties of Ni52Mn26Ga22 melt-spun ribbons were studied. Results show that there are four types of differently oriented variants of seven-layered modulated (7M) martensite at room temperature, being twin-related one another and clustered in colonies. Due to the coupled magnetic and structural transformations between parent austenite and 7M martensite, the melt-spun ribbons exhibit a significant magnetocaloric effect. At an applied magnetic field of 5 T, an absolute maximum value of the isothermal magnetic entropy change of 11.4 J kg(-1) K-1 is achieved with negligible hysteresis losses.

Texture-induced enhancement of the magnetocaloric response in melt-spun DyNi2 ribbons

"The magnetocaloric properties of melt-spun ribbons of the Laves phase DyNi2 have been investigated. The as-quenched ribbons crystallize in a single-phase MgCu2-type crystal structure (C15; space group Fd (3) over barm) exhibiting a saturation magnetization and Curie temperature of M-S = 157 +/- 2 A m(2) kg(-1) and T-C = 21.5 +/- 1 K, respectively. For a magnetic field change of 2 T, ribbons show a maximum value of the isothermal magnetic entropy change vertical bar Delta S-M(peak)vertical bar = 13.5 J kg(-1) K-1, and a refrigerant capacity RC = 209 J kg(-1). Both values are superior to those found for bulk polycrystalline DyNi2 alloys (25% and 49%, respectively). In particular, the RC is comparable or larger than that reported for other potential magnetic refrigerants operating at low temperatures, making DyNi2 ribbons promising materials for use in low-temperature magnetic refrigeration applications.

Texture-induced enhancement of the magnetocaloric response in melt-spun DyNi2 ribbons

"The magnetocaloric properties of melt-spun ribbons of the Laves phase DyNi2 have been investigated. The as-quenched ribbons crystallize in a single-phase MgCu2-type crystal structure (C15; space group Fd (3) over barm) exhibiting a saturation magnetization and Curie temperature of M-S = 157 +/- 2 A m(2) kg(-1) and T-C = 21.5 +/- 1 K, respectively. For a magnetic field change of 2 T, ribbons show a maximum value of the isothermal magnetic entropy change vertical bar Delta S-M(peak)vertical bar = 13.5 J kg(-1) K-1, and a refrigerant capacity RC = 209 J kg(-1). Both values are superior to those found for bulk polycrystalline DyNi2 alloys (25% and 49%, respectively). In particular, the RC is comparable or larger than that reported for other potential magnetic refrigerants operating at low temperatures, making DyNi2 ribbons promising materials for use in low-temperature magnetic refrigeration applications.

Structural and magnetic characterization of the intermartensitic phase transition in NiMnSn Heusler alloy ribbons

"Phase transitions and structural and magnetic properties of rapidly solidified Ni50Mn38Sn12 alloy ribbons have been studied. Ribbon samples crystallize as a single-phase, ten-layered modulated (10M) monoclinic martensite with a columnar-grain microstructure and a magnetic transition temperature of 308 K. By decreasing the temperature, martensite undergoes an intermartensitic phase transition around 195 K. Above room temperature, the high temperature martensite transforms into austenite. Below 100 K, magnetization hysteresis loops shift along the negative H-axis direction, confirming the occurrence of an exchange bias effect. On heating, the thermal dependence of the coercive field H-C shows a continuous increase, reaching a maximum value of 1017 Oe around 50 K. Above this temperature, H-C declines to zero around 195 K. But above this temperature, it increases again up to 20 Oe falling to zero close to 308 K. The coercivity values measured in both temperature intervals suggest a significant difference in the magnetocrystalline anisotropy of the two martensite phases.

Magnetocaloric properties of rapidly solidified Dy3Co alloy ribbons

"The magnetic and magnetocaloric (MC) properties of melt-spun ribbons of the Dy3Co intermetallic compound were investigated. Samples were fabricated in an Ar environment using a homemade melt spinner system at a linear speed of the rotating copper wheel of 40 ms(-1). X-ray diffraction analysis shows that ribbons crystallize into a single-phase with the Fe3C-type orthorhombic crystal structure. The M(T) curve measured at 5mT reveals the occurrence of a transition at 32K from a first to a second antiferromagnetic (AFM) state and an AFM-to-paramagnetic transition at T-N = 43 K. Furthermore, a metamagnetic transition is observed below T-N, but the magnetization change Delta M is well below the one reported for bulk alloys. Below 12 K, large inverse MC effect and hysteresis losses are observed. This behavior is related to the metamagnetic transition. For a magnetic field change of 5 T (2 T) applied along the ribbon length, the produced ribbons show a peak value of the magnetic entropy change Delta S-M(peak) of -6.5 (-2.1) Jkg(-1) K-1 occurring close to TN with a full-width at half-maximum delta T-FWHM of 53 (37) K, and refrigerant capacity RC = 364 (83) Jkg(-1) (estimated from the product vertical bar Delta S-M(peak)vertical bar x delta T-FWHM).

Enhanced refrigerant capacity in two-phase nanocrystalline/amorphous NdPrFe17 melt-spun ribbons

"he magnetocaloric properties of NdPrFe17 melt-spun ribbons composed of nanocrystallites surrounded by an intergranular amorphous phase have been studied. The nanocomposite shows two successive second-order magnetic phase transitions (303 and 332 K), thus giving rise to a remarkable broadening (approximate to 84 K) of the full-width at the half-maximum of the magnetic entropy change curve, Delta S-M(T), with a consequent enhancement of the refrigerant capacity RC. For a magnetic field change of 2 T, vertical bar Delta S-M(peak)vertical bar = 2.1 J kg(-1) K-1 and RC = 175 J kg(-1). Therefore, the reversible magnetocaloric response together with the one-step preparation process makes these nanostructured Fe-rich alloy ribbons particularly attractive for room temperature magnetic refrigeration.

Elastocaloric and magnetocaloric effects in Ni-Mn-Sn(Cu) shape-memory alloy

"We have studied magnetocaloric and elastocaloric properties of a Ni-Mn-Sn(Cu) metamagnetic shape-memory alloy undergoing a magneto-structural transition (martensitic type) close to room temperature. Changes of entropy have been induced by isothermally applying both mechanical (uniaxial stress) and magnetic fields. These entropy changes have been, respectively, estimated from dilatometric measurements giving the length of the sample as a function of temperature at selected applied forces and magnetic fields and from magnetization measurements as a function of temperature at selected applied magnetic fields. Our results indicate that the elastocaloric effect is conventional and occurs in two steps which reflect the interplay between the martensitic and the incipient magnetic transitions. By contrast, the magnetocaloric effect is inverse and occurs in a single step that encompasses the effect arising from both transitions.

Enhanced refrigerant capacity in Gd-Al-Co microwires with a biphase nanocrystalline/amorphous structure

"A class of biphase nanocrystalline/amorphous Gd(50+5x)Al(30-5x)Co20 (x = 0, 1, 2) microwires fabricated directly by melt-extraction is reported. High resolution transmission electron microscopy and Fourier function transform based analysis indicate the presence of a volume fraction (similar to 20%) of similar to 10nm sized nanocrystallities uniformly embedded in an amorphous matrix. The microwires possess excellent magnetocaloric properties, with large values of the isothermal entropy change (-Delta S-M similar to 9.7 J kg(-1) K-1), the adiabatic temperature change (Delta T-ad similar to 5.2K), and the refrigerant capacity (RC similar to 654 J kg(-1)) for a field change of 5 T. The addition of Gd significantly alters T-C while preserving large values of the Delta S-M and RC. The nanocrystallites allow for enhanced RC as well as a broader operating temperature span of a magnetic bed for energy-efficient magnetic refrigeration.