Magnetocaloric effect and multifunctional properties of Ni–Mn-based Heusler alloys

The studies of magnetocaloric properties, phase transitions, and phenomena related to magnetic heterogeneity in the vicinity of the martensitic transition (MT) in Ni–Mn–In and Ni–Mn–Ga off-stoichiometric Heusler alloys are summarized. The crystal structure, magnetocaloric effect (MCE), and magnetotransport properties were studied for the following alloys: Ni₅₀Mn_50−xIn_x, Ni_50−xCo_xMn₃₅In₁₅, Ni₅₀Mn_35−xCo_xIn₁₅, Ni₅₀Mn₃₅In₁₄Z (Z=Al, Ge), Ni₅₀Mn₃₅In_15−xSi_x, Ni_50−xCo_xMn_25+yGa_25−y, and Ni_50–xCo_xMn_32−yFeyGa₁₈. It was found that the magnetic entropy change, ΔS, associated with the inverse MCE in the vicinity of the temperature of the magneto-structural transition, TM, persists in a range of (125-5) J/(kg K) for a magnetic field change ΔH=5 T. The corresponding temperature varies with composition from 143 to 400 K. The MT in Ni₅₀Mn_50−xIn_x (x=13.5) results in a transition between two paramagnetic states. Associated with the paramagnetic austenite-paramagnetic martensite transition ΔS=24 J/(kg K) was detected for ΔH=5 T at T=350 K. The variation in composition of Ni₂MnGa can drastically change the magnetic state of the martensitic phase below and in the vicinity of TM. The presence of the martensitic phase with magnetic moment much smaller than that in the austenitic phase above TM leads to the large inverse MCE in the Ni₄₂Co₈Mn_32−yFeyGa₁₈ system. The adiabatic change of temperature (ΔT_ad) in the vicinity of TC and TM of Ni₅₀Mn₃₅In₁₅ and Ni₅₀Mn₃₅In₁₄Z (Z=Al, Ge) was found to be ΔT_ad=−2 K and 2 K for ΔH=1.8 T, respectively. It was observed that |ΔT_ad|≈1 K for ΔH=1 T for both types of transitions. The results on resistivity, magnetoresistance, Hall resistivity in some In-based alloys are discussed.     

Magnetocaloric effect of the multiphase Gd4−xTbxCo3 system

ABSTRACT

Crystal structure, microstructural chemical analysis, magnetic properties and magnetocaloric effect for ‘as cast’ and annealed Gd_4?xTb_xCo₃ (x?=?0–4) were investigated. Only Gd₄Co₃ was homogeneous and crystallised in the hexagonal Ho₄Co₃-type of crystal structure. Samples substituted with terbium proved to be a mixture of (Gd,Tb)₁₂Co₇ and (Gd,Tb)Co₂ phases. With the increase of terbium content the entropy change increased and for the samples with x?=?3, 4 reached the highest values. Even ‘as-cast’ samples exhibit pronounced values of the magnetocaloric parameters.     

Bohr–Weisskopf effect: influence of the distributed nuclear magnetization on hfs

Nuclear magnetic moments provide a sensitive test of nuclear wave functions, in particular those of neutrons, which are not readily obtainable from other nuclear data. These are taking added importance by recent proposals to study parity non-conservation (PNC) effects in alkali atoms in isotopic series. By taking ratios of the PNC effects in pairs of isotopes, uncertainties in the atomic wave functions are largely cancelled out at the cost of knowledge of the change in the neutron wave function. The Bohr–Weisskopf effect (B–W) in the hyperfine structure interaction of atoms measures the influence of the spatial distribution of the nuclear magnetization, and thereby provides an additional constraint on the determination of the neutron wave function. The added great importance of B–W in the determination of QED effects from the hfs in hydrogen-like ions of heavy elements, as measured recently at GSI, is noted. The B–W experiments require precision measurements of the hfs interactions and, independently, of the nuclear magnetic moments. A novel atomic beam magnetic resonance (ABMR) method, combining rf and laser excitation, has been developed for a systematic study and initially applied to stable isotopes. Difficulties in adapting the experiment to the ISOLDE radioactive ion beam, which have now been surmounted, are discussed. A first radioactive beam measurement for this study, the precision hfs of ¹²⁶Cs, has been obtained recently. The result is 3629.515(∼0.001) MHz. The ability of ABMR to determine with high precision nuclear magnetic moments in free atoms is a desideratum for the extraction of QED effects from the hfs of the hydrogen-like ions. We also point out manifestations of B–W in condensed matter and atomic physics. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Magnetocaloric Effect in the Vicinity of the Magnetic Compensation Temperature of Amorphous Gd–Co Ferrimagnetic Films

Physics of the Solid State - The results of the study of the magnetic and magnetocaloric properties of amorphous Gd–Co ferrimagnetic films possessing perpendicular magnetic anisotropy in a...     

Ab initio study of the composite phase diagram of Ni–Mn–Ga shape memory alloys

The magnetic and structural properties of a series of nonstoichiometric Ni–Mn–Ga Heusler alloys are theoretically investigated in terms of the density functional theory. Nonstoichiometry is formed in the coherent potential approximation. Concentration dependences of the equilibrium lattice parameter, the bulk modulus, and the total magnetic moment are obtained and projected onto the ternary phase diagram of the alloys. The stable crystalline structures and the magnetic configurations of the austenitic phase are determined.     

Texture transition in Al–Mg alloys: effect of magnesium

ABSTRACT

The evolution of deformation texture and microstructure in commercially pure Al (cp-Al) and two Al–Mg alloys (Al–4Mg and Al–6Mg) during cold rolling to a very large strain (true strain ε_t? ≈?3.9) was investigated. The development of deformation texture in cp-Al, after rolling, can be considered as pure metal or Copper-type, which is characterised mainly by the presence of Cu {112}<111>, Bs {110}<112> and S {123}<634> components. The deformation microstructure clearly indicates that deformation mechanism in this case remains slip dominated throughout the deformation range. In the Al–4Mg alloy, the initial slip mode of deformation is finally taken over by mechanism involving both slip and Copper-type shear bands, at higher deformation levels. In contrast, in the Al–6Mg alloy, the slip and twin mode of deformation in the initial stage is replaced by slip and Brass-type shear bands at higher deformation levels. Although a Copper-type deformation texture forms in the two Al–Mg alloys at the initial stage of deformation, there is a significant increase in the intensity of the Bs component and a noticeable decrease in the intensity of the Cu component at higher levels of deformation, particularly in the Al–6Mg alloy. This phenomenon indicates the possibility of transition of the deformation texture from Cu-type to Bs-type, which is concurrent with the addition of Mg. Using visco-plastic self-consistent modelling, the evolution of deformation texture could be simulated for all three materials.     

Magnetocaloric heat-pump cycles based on the AF–F transition in Fe–Rh alloys

The proposal involves a heat-pumping scheme based upon the first-order antiferromagnetism–ferromagnetism transition in FeRh alloy. Using the model S–T diagram for this alloy, the heat-pump cycles, are drawn up based on the transition latent heat absorption and emission when the transition is induced by applying magnetic field. The calculated values of heat coefficient ϕ for the cycles are ∼39 at ΔT=5 K and ∼30 at ΔT=10 K, where ΔT is the difference between the temperature surrounding and that of the heat receiver. These values are achieved using the comparatively low magnetic fields of ∼2×10⁶ A m⁻¹. The high values of ϕ, together with high value of cooling capacity, make it possible to consider Fe–Rh alloys as an effective magnetic heat-pump working body near the room temperature.     

Thermal and microstructural investigation of Cu–Al–Mn–Mg shape memory alloys

There are many studies to improve the properties of Cu–Al–Mn shape memory alloys, such as high transformation temperatures, ductility and workability. Most of them have been performed by adding a quaternary component to the alloy. In this study, the effect of trace Mg addition on transformation temperatures and microstructures of three different quaternary Cu–Al–Mn–Mg alloys has been investigated using thermal analysis, optical microscopy and XRD techniques. The transformation temperatures are within the range of 120–180 °C, and they have not changed significantly on decreasing the Mn content, replacing with Mg. The fine precipitates have been observed in the alloys with the Mg content up to 1.64 at%. Calculated entropy change and XRD analysis reveal that the alloys with high Al content have mainly 18R-type structure which could be responsible for good ductility and workability.     

The improvement of the superconducting Y–Ba–Cu–O magnet characteristics through shape recovery strain of Fe–Mn–Si alloys

Since bulk Y–Ba–Cu–O superconductors are brittle ceramics, reinforcement of mechanical properties is important for practical applications. It has been reported that bulk Y–Ba–Cu–O can be reinforced with Al or Fe based alloy ring, in that compression force acts on bulk Y–Ba–Cu–O due to a difference in thermal expansion coefficients. However, the shrinkage of the metal ring was not so large, and therefore careful adjustment of the circumference of the bulk and the metal rings was necessary. In this study, we employed Fe–Mn–Si shape memory alloy rings to reinforce bulk Y–Ba–Cu–O. The advantage of the shape memory alloy is that the shrinkage can take place on heating, and furthermore, the alloy shrinks and compresses the bulk body on cooling. Bulk Y–Ba–Cu–O superconductor 22.8 mm in diameter was inserted in a Fe–Mn–Si ring 23.0 mm in inner diameter at room temperature. Beforehand, the Fe–Mn–Si ring was expanded by 12% strain at room temperature. Then the composite was heated to 673 K. At room temperature, the Fe–Mn–Si ring firmly gripped the bulk superconductor. We then measured trapped fields before and after the ring reinforcement, and found that the trapped field was improved through the treatment.     

Electron excited Auger line shape study of ion-beam-mixed Pd–Au alloys

The electronic structure of the ion-beam-mixed Pd–Au alloys have been studied using valence band spectra of XPS and electron excited CVV–Auger spectra. To show the relationship between the electronic structure changes and the Auger spectral line shape, the data of the self-convolution of the partially weighted valence band spectra was compared with the Auger spectra of Pd–Au alloys. The Pd–Au alloy is one of the systems which both atomic and band-like contributions are evident in the Auger spectral line shape. Since the self-convolution of PDOS’s relates to the band-like part of Auger spectra, in Pd–Au alloys, the band-like structure in the Auger line shape can be classified by the self-convolution of the partially weighted valence band spectra. Finally, we found that the increase in peak size at ∼80 eV with the increase in Pd content is due to the band-like contribution in the Au N_6,7VV Auger line shape.     

Origin of the strong influence of rhodium on the Curie temperature of Pd–Fe alloys: the spin reorientation in (Pd100−xRhx)90Fe10 alloys

The hyperfine field distributions and the local spin configurations for Fe atoms in the (Pd_100−xRh_x)₉₀Fe₁₀ alloys for x=0, 10 and 20 are investigated by the Mössbauer spectroscopy technique. It was found that the anomalous behavior of T_C in these alloys is attributable to the spin reorientation in some part of Fe atoms with the formation of local antiferromagnetic spin configurations.     

Paramagnetic–ferrimagnetic transition in strong coupling paramagnetic systems: effect of cubic anisotropy interaction

The purpose of the present work is a quantitative investigation of the biquadratic exchange interaction effects on the paramagnetic–ferrimagnetic transition arising from two strongly coupled paramagnetic (1-spin) sublattices, of respective moments m and M. The free energy describing the physics of the system is of Landau type. In addition to the quadratic and quartic terms, in both m and M, this free energy involves two mixing interaction terms. The first is a lowest order coupling −CmM, where C<0 stands for the coupling constant measuring the interaction between the two sublattices. While the second, which is relevant for 1-spin systems and which traduces the dipole–dipole (or biquadratic) interaction, is of type wm²M², where w>0 is the new coupling constant. These two interactions enter in competition, and then, they induce drastic changes of the magnetic behavior of the material. The main change is that, the presence of this high order coupling tends to destroy the ferrimagnetic order of the system. We first show that the introduction of this biquadratic interaction does not affect the values of critical exponents. Also, we find that the compensation temperature (when it exists) and the compensation magnetic field are shifted to their lowest values, in comparison with the w=0 case. The Arrott-phase-diagram shape is also investigated quantitatively. We show the existence of three regimes depending on the values of w. When the latter is small, we find that the region of competition between the coupling C and the applied magnetic field H becomes more narrow under the effect of w (by competition, we mean the passage from the antiparallel state to the parallel one). While for higher values of w, this competition disappears completely, and then, the system loses its ferrimagnetic character. Kinetics of the phase transition is also examined, when the temperature is lowered from an initial value T_i to a final one T_f very close to the critical temperature T_c. As in the w=0 case, we find that kinetics is controlled by two kinds of relaxation times τ₁ and τ₂. The former is the relevant time, and is associated to long-wavelength fluctuations driving the system to undergo a phase transition. The second is a short time, which controls local dynamics. Near T_c, we show that, in particular, the longest relaxation time τ₁ becomes less important in comparison with that relative to the w=0 case. Finally, we note that the existence of two relaxation times is consistent with the predictions of a recent experiment, which was concerned with the 1/2-spin compounds Li_xNi_2−xO₂, where the composition x is close to 1.     

Comparison of the effect of anti-hyperlipidemic drugs from different groups on the phase profile of liposomal membrane–a fluorescence anisotropy study

The study compares the effect of incorporation of three different groups of anti-hyperlipidemic drugs, namely niacin, simvastatin, and fenofibrate on the phase profile of liposomal membranes of dipalmitoylphosphatidylcholine (DPPC). The fluorescence anisotropy studies, using 1,6-diphenyl-1,3,5-hexatriene as fluorescent probe, have shown that the lipophilic molecule fenofibrate changes phase behavior of DPPC liposomal membrane to a greater extent compared to the changes produced by amphiphilic simvastatin and hydrophilic niacin. This variation in effect can be attributed to the nature of the drug molecules and hence their location in different parts of the liposomal membrane. We have also calculated the changes in van’t Hoff enthalpy values in all these three cases and observed that these values decreased with increase in drug concentrations in the case of simvastatin but for fenofibrate and niacin the effect is completely the reverse. In order to get a better insight, the fraction of motionally restricted lipid molecules has been calculated.     

Relaxation in Ni–Mn–Ga ferromagnetic shape memory alloys

Evidence of relaxation has been observed in ferromagnetic Ni–Mn–Ga single crystals. The relaxation may be explained by a change in symmetry-conforming short-range ordering according to Ren and Otsuka in this off-stoichimetric ordered alloy. Martensite stabilization has also been found after martensite ageing.     

The effect of Zn on precipitation in Al–Mg–Si alloys

Effects of addition of Zn (up to 1 wt%) on microstructure, precipitate structure and intergranular corrosion (IGC) in an Al–Mg–Si alloys were investigated. During ageing at 185?°C, the alloys showed modest increases in hardness as function of Zn content, corresponding to increased number densities of needle-shaped precipitates in the Al–Mg–Si alloy system. No precipitates of the Al–Zn–Mg alloy system were found. Using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), the Zn atoms were incorporated in the precipitate structures at different atomic sites with various atomic column occupancies. Zn atoms segregated along grain boundaries, forming continuous film. It correlates to high IGC susceptibility when Zn concentration is ~1wt% and the materials in peak-aged condition.     

The effect of Cu on precipitation in Al–Mg–Si alloys

TEM investigations of two alloys isothermally heat treated at 175°C and 260°C show how Cu additions to the Al–Mg–Si system affect precipitation. Both alloys had a solute content Mg?+?Si?=?1.3 at.%, 0.127 at.% Cu, but with Mg/Si 0.8 and 1.25. Cu-containing Guinier-Preston (GP) zones and three types of Q′ precursors are identified as most common phases at peak-hardness conditions, whereas β″ accounts for maximum 30% of the total number of precipitates. The precursors have needle (L and S precipitates) or plate (C precipitate) morphologies. They consist of different arrangements of Al, Mg and Cu atoms on a grid defined by triangularly arranged Si planes parallel with and having the same period as {100} Al planes. The Si grid is composed of nearly hexagonal sub-cells of a?=?b?=?4.05?Å, c?=?4.05?Å. The Cu arrangement on the grid is often disordered in the needle precursors. The plate precursor is ordered, with a monoclinic unit cell of a?=?10.32?Å, b?=?8.1?Å, c?=?4.05?Å, γ?=?101°.     

Studying the effect of biaxial anisotropy on nonlinear magnetization oscillations accompanying the 90° pulsed magnetization process in ferrite–garnet films with easy-plane anisotropy

Time dependences of the azimuthal component of the torque T _φ(t) acting on magnetization are calculated to understand the nature of the delayed magnetization acceleration effect observed during the 90° pulsed magnetization of real ferrite–garnet films, in which biaxial anisotropy exists alongside with in-plane anisotropy. A calculation technique based on analyzing an operating point trajectory is used. Calculations show that if the effective anisotropy field H _K2 is comparable to the magnetizing pulse amplitude H _ma, abruptly ascending regions at characteristic times t* in curves T _φ(t) arise, in the limit of which nonlinear magnetization oscillations formed. The shape of these regions depends weakly on the magnetizing pulse front duration τ_f. This explains the reason of the weak dependence of the nonlinear magnetization oscillations on duration of the magnetizing pulse front. Calculations also show that the main features of the delayed acceleration effect are less clear upon an increase of the pulse amplitude: the behavior of curves T _φ(t) becomes smoother near times t*, and an increase in the pulse front duration is accompanied by a stronger drop in the intensity of magnetization oscillations.