Gd3Ni6SiAl, Gd3Ni6SiGa, and Gd3Ni6AlGa (Ce3Ni6Si2-type, Im-3m, N 229, cI44) present a broad table-like magnetocaloric effect in the full hydrogen liquefaction range (20-77 K) with a real plateau wide enough to be competitive for refrigeration devices based on the Ericsson cycle. The characterization of the magnetocaloric variables (|dSM pk|= 6.60 J/kgK, TEC(10)= 6.44 J/kgK for Gd3Ni6SiGa, |dSM pk|= 6.05 J/kgK, TEC(10)= 6.01 J/kgK for Gd3Ni6SiAl, all at mu 0dH=5 T) state that they are among the best of table-like magnetocaloric materials in this temperature range. The physical origin of this smooth and wide table-like effect is the combination of a spin reorientation transition (which appears in Gd3Ni6XY when Si or Ga is introduced) with the paramagnetic to ferromagnetic transition, close to each other. The application of the Banerjee criterion and the evolution of the critical exponent with temperature n(T) confirm that both transitions are second order, which is extremely relevant for real applications. From the study of the magnetic properties it has been inferred that the introduction of Si induces a magnetic ground state with combined ferromagnetic and antiferromagnetic components which turns to fully ferromagnetic at very low field. The critical exponents (/3, y, S, n) have been found for the paramagnetic to ferromagnetic transitions; the values of y (from 1.22 to 1.36) suggest that the magnetic interactions are short-range, though the ensembles do not correspond to any particular universality class.

A complete experimental study of the physical properties playing a relevant role in the 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 observed for these materials: a paramagnetic to ferromagnetic transition in the range of 182-282 K and a spin reorientation transition in the range of 26-76 K. As a consequence, two peaks related to a direct magnetocaloric effect (DMCE) appear with 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-1 K-1 around 182 K. For the same applied field the other compounds show moderate values around room temperature (2.88-4.53 J kg-1 K-1). However, the superposition of the two peaks results in huge refrigerant capacity values, up to RCFWHM(5 T) = 1103.04 J kg-1 in the case of Tb2Dy4FeSb2. The thermal diffusivity, thermal effusivity, thermal conductivity and specific heat capacity 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 of 1.3-2.3 mm2 s-1, which are good for magnetic refrigerators at high working frequencies. The study is completed with a rigorous critical behavior analysis 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 of 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 exhibiting 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 in the properties of the compounds is discussed.

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. (c) 2021 The Author(s). Published by Elsevier B.V. CC_BY_NC_ND_4.0

The magnetic and magnetotransport properties of thin Heusler alloy Ni 49.7 Fe 17.4 Co 4.2 Ga 28.7 films deposited onto MgO(100) substrates are studied over a wide temperature range, which includes a martensitic transition (MT). For this composition, the MT is not accompanied by a magnetic phase transition, since the martensitic and austenitic phases are ferromagnets with similar magnetizations. The electrical resistivity does not undergo sharp changes during the MT. The magnetoresistance is negative, decreases in magnitude with increasing temperature in the range 100–250 K corresponding to the MT, and then increases to –1%. The field dependences of the Hall effect resistivity have the shape that is characteristic of homogeneous ferromagnetic alloys. The coefficients of the normal and anomalous Hall effects are determined. The anomalous Hall effect coefficient is shown to be described by the relation R s = αρ + βρ 2 , where ρ is the electrical resistivity and the second term is lower than the first, which indicates an important role of the interference impurity–phonon scattering mechanism.

A family of novel intermetallic R3CoNi with heavy rare earth ions has been synthesized (R = Tb, Dy, Ho, Er, Tm, Lu) and a study of the crystal structure of these phases performed. All the compounds adopt the rhombohedral Er3Ni2-type structure [Pearson's symbol hR45; space group R-3h (N. 148)]. A thorough investigation of their magnetic and magnetocaloric properties has been undertaken. Magnetization and ac-susceptibility measurements as a function of temperature show that the samples with Tb, Dy, Ho, Er, and Tm present a paramagnetic to ferromagnetic (PM-FM) transition at temperatures in the range 96-6 K; different reorientation transitions take place below the respective T-C, in most cases with thermomagnetic irreversibility. Thermal and magnetic measurements have been used to retrieve the set of critical exponents (alpha, beta, gamma, delta) for the PM-FM transition to assign a universality class to Tb3CoNi, Dy3CoNi, and Ho3CoNi, with the result that, in the first case, it is close to the Mean Field model, in the second one it is between the Chiral Heisenberg and the XY-Chiral, and in the third one it is close to the XY-Chiral model. Therefore, in Tb3CoNi the transition is governed by long-range order interactions, whereas in Dy3CoNi and Ho3CoNi there must be some kind of frustrated non-collinear ferromagnetism. The magnetocaloric measurements in five members of the family indicate that all of them present highly competitive magnetocaloric properties in their respective temperature ranges, with high magnetic entropy changes (from 12.8 to 18.5 J/Kg.K at kt (mu 0)Delta H = 5 T) and refrigerant capacities (from 412 to 699 J/Kg at (mu 0)Delta H = 5 T). These results assess and highlight the applicative interest of these compounds, besides suggesting the possibility of tuning the range of the operating temperature by modifying the rare earth ion. Finally, universal curves for the magnetocaloric properties have been found for Tb3CoNi, Dy3CoNi, and Ho3CoNi; the scaling of the magnetocaloric variables confirms the validity of the assigned universality classes. (C) 2021 Elsevier B.V. All rights reserved.

The influence of film thickness on the formation of twinning structure, martensitic transformation and magnetoelastic properties of epitaxial films of Ni(Co)MnSn magnetic shape memory alloy is investigated by means of ferromagnetic resonance spectroscopy, synchrotron X-ray diffraction and standard magnetic measurements. It is found that constraints from the film/substrate interface block the martensitic transformation in the 20 nm thick film. The increase of the film thickness results in a progressive stress relaxation and, as a result, the martensitic transformation becomes possible starting from 50 nm. Twinning of the films is required to conserve the films surface area. The elastic energy balance between the film/substrate interface and the twin boundaries leads to the formation of a submicron wide, stripe like, periodical structure of twins, which is of interest for spintronic or magnonic applications. The width of the twin variants increases with the film thickness growth, resulting in the dramatic modification of magnetic properties. (C) 2020 Elsevier B.V. All rights reserved.

We investigate the temperature dependences of the magnetic properties, electrical resistivity, magnetoresistance, and Hall effect resistivity, rho(H)(H), in thin films of the Heusler-type Ni47.3Mn30.6Ga22.1 (at.%) magnetic shape memory alloys epitaxially grown onto a MgO(001) substrate. The results reveal martensitic transformation at about 230 K, premartensitic transition around 285 K, and the Curie temperature of austenite around 380 K. We obtained the coefficients of normal Hall effect (NHE), R-0, and anomalous Hall effect (AHE), R-s by fitting the total Hall resistivity curves rho(H) = R0Bz + 4(pi)R(s)M(z) in several magnetic field ranges (0.1-1, 0-5, 8-16, and 0-16 kOe), using experimental magnetization data. Both coefficients R-0 and R-s strongly depend on the magnetic field. We also fit the Hall effect resistivity with the expression rho(H) = R0Bs + 4 pi RsMz + Delta rho(H) using the coefficients R-0 and R-z obtained from the high-field interval (8-16 kOe), where the last term, Delta rho(H), was considered to correspond either to the topological Hall effect or to the antiferromagnetic Hall effect. The obtained temperature dependence and magnitude of Delta rho(H) discard the presence of the skyrmions or antiskyrmions. We conclude that unconventional field dependences of the NHE and AHE coefficients are produced by the antiferromagnetic correlations and the influence of the magnetic field on the electronic structure.

Hysteresis of martensitic transformations (MTs) is a crucial factor which affects the efficiency and lifetime of actuators and magnetocaloric devices. In the present work we found a tenfold reduction of the thermal hysteresis of MT caused by increasing the MT temperature in a series of Ni(Co)-Mn-Sn thin films deposited onto MgO (001). We have observed that this evolution is accompanied by a drastic change of the temperature dependence of the lattice parameter in the temperature interval of MT. Such a behaviour can be interpreted using the concept of the critical state, where an anhysteretic behaviour is expected.

Magnetic shape memory alloys are under intensive investigation due to their unusual physical properties, such as magnetic shape memory effect, magnetic field induced superelasticity, direct and inverse magnetocaloric effect etc., promising for novel applications. One of the intriguing properties of these materials in a single phase state is a giant magnetoresistance. Here we report the remarkable results about the magnetoresistive properties of epitaxial films of Ni52.3Mn26.8Ga20.9 magnetic shape memory alloy in the temperature range of 100–370 K, well below the martensitic transformation temperature. It was found that the formation of non-collinear magnetic structure due to a nanotwinning of the film results in electron scattering on such a structure and noticeable negative magnetoresistance in the entire investigated temperature range.

1 mu m-thick polycrystalline thin films of the ferromagnetic shape memory alloy, Ni49.7Fe17.4Co4.2Ga28.7 (at.%), were sputter-deposited on MgO(001) substrates heated at 773 K. The films' structure, magnetic properties and transformation behavior have been characterized. They exhibit both forward and reverse martensitic transformations at 218 K and 232 K, respectively. Ferromagnetic resonance and x-ray diffraction measurements discerned three ferromagnetic phases: cubic L2(1)-ordered austenite, tetragonal martensite and disordered cubic gamma-phase. The latter phase showed a unique spin- waves resonance spectrum, enabling estimation of the value of the exchange stiffness constant equal to A = 0.7 x 10(-6) erg cm(-1).

We report on the fabrication and in-situ characterization of temperature-dependent electrical resistance and deflection characteristics of free-standing NiMnGa/Si bimorph cantilevers with a NiMnGa layer thickness of 200 nm and a minimum lateral width of 50 nm. The martensitic transformation in the initial NiMnGa/Si bimorph films and nanomachined NiMnGa/Si bimorph cantilevers proceeds in a wide temperature range with a hardly detectable temperature hysteresis width below 1 K. This remarkable behavior is ascribed to the internal stress in the bimorph system that exceeds the stress limit of the critical point terminating the stress-temperature phase diagram as it is known for ferromagnetic shape memory alloys. Temperature-dependent deflection characteristics reveal a competition between the bimorph effect and the shape memory effect, causing beam deflection in opposite directions. The observation of the shape memory effect strongly depends on the NiMnGa/Si thickness ratio, causing a maximum deflection change per beam length of 3% in agreement with finite element simulations.

In this work, a detailed report on the growth of elongated bar-like crystallites with a definite crystallographic orientation, forming a self-organized patterned surface layer, with a thickness of about 100 nm, onto 900 nm thick ferromagnetic shape memory alloy Ni-Mn-Ga films epitaxially grown on MgO(001) is presented. The phenomenon appears above a threshold of lattice mismatch (2%) between the film and substrate. The structural, magnetic and transformation properties of such films are described and analyzed. Particularly, the different magnetic states of the two film layers are discerned by ferromagnetic resonance. The feasibility of creating a ferromagnetic shape memory alloy with a nanoscale self patterned surface, during the very same process of film deposition, can be interesting for potential applications where control of the surface architecture would be needed. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

We report two possible routes of fabrication of large surfaces of ferromagnetic shape memory antidots with tunable pore size and center-to-center distances. By using the drop coating method, we have prepared a large area of 2D arrays (typically 1cm2) of polystyrene spheres (PS) (1.4±0.1μm diameter) on a Si substrate. We have used reactive ion etching with a gas mixture of O2(12sccm) and Ar (5sccm) to reduce the diameter of the PS spheres whereby controlling the size of pores. The film deposition was performed on a substrate heated at 500oC (route 1) and at room temperature with subsequent annealing in a furnace at 500oC for 4 hours (route 2). Route 1 proved to be promising but more work is needed to optimize it. The antidots of Ni-Mn-Ga obtained along route 2 are ferromagnetic with a Curie temperature ~100oC, and a spread martensitic transformation (between-100oC and-30oC).

The temperature evolution of the electronic structure of a Ni-Fe(Co)-Ga/MgO(100), Heusler-type, ferromagnetic shape-memory alloy thin film has been followed by a bulk-sensitive hard x-ray photoelectron spectroscopy, element-selective soft x-ray magnetic circular dichroism, and first-principles calculation. The reversible changes of the electronic states near the Fermi energy show a hysteresis associated with the martensitic phase transition (MPT), where the pseudogap opens on cooling and closes again on warming. In addition, the Ni $3d$ spin magnetic moment increases by approximately two times across the MPT, whereas the change of Fe $3d$ moment is moderate. By comparing the experimental results with the calculated spin-resolved density of states, we conclude that the band Jahn-Teller effect of Ni $3d$ and Fe $3d$ orbitals is responsible for MPT.

Ni–Mn–Ga thin films exhibiting a martensitic transformation (MT) overlapping with the Curie temperature have been sputter-deposited onto heated Si/SiNx substrates. The films had a partially oriented polycrystalline structure and the tetragonal 10M-martensitic phase undergoing a reverse MT into cubic austenite between 57 and 156°C. The MT was studied with structural, substrate curvature, magnetic and resistivity methods. The substrate imposed residual stress which changes the sign of the transformation volume strain across MT. The role of the magnetocrystalline anisotropy and its impact on magnetic properties are interpreted in terms of film texture and film stiffness.

By the ferromagnetic resonance and magnetometry measurements in a broad temperature range, it is found that the 220-fiber textured Ni51.4Mn28.3Ga20.3 (at.%) film of 100 nm thickness deposited onto NaCl(001) exhibits in the martensitic state an easy-plane magnetic anisotropy. This is in a drastic difference with the identical film of the same texture deposited onto Si(001) wafer showing out-of-plane oblique type of anisotropy [Golub et al., J. Appl. Phys. 105, 07A942 (2009)]. The results are discussed in terms of the different martensitic variant selections and stress-induced magnetic anisotropy contributions having opposite signs. (C) 2013 AIP Publishing LLC.

The temperature dependences of the lattice parameters and residual stress have been measured for a fine-grained Ni52.2Mn26.8Ga21.0 (at. %) thin film fabricated by sputter deposition onto a heated silicon wafer with SiNx buffer layer. The transformation volume strain in the film was found to be a lattice expansion during the forward martensitic transformation which is opposite to a volume contraction exhibited by bulk Ni-Mn-Ga alloys. This unusual effect can be explained by the substrate-induced residual stresses in the film and the difference in the elastic modulus of austenite and martensite.

Epitaxial Ni–Mn–Ga films have been grown onto heated substrates by sputtering. Their chemical composition depends on the sputtering argon pressure. Representative epitaxial films of Ni52.3Mn26.8Ga20.9, 0.5 μm-thick, transform martensitically at about 120 °C, accompanied by sharp changes in the lattice parameter and resistivity, and orders ferromagnetically below 98°. The observed high transformation temperature, orthorhombic martensitic structure, twinning mode and film morphology, indicate a potential multifunctional behavior of the film, such as high-temperature shape-memory effect and magnetic field actuation.