Magnetic Phase Transformation Research Articles

Structural and magnetic transformations in Ni51 − x Mn36 + x Sn13 alloys

The structural and magnetic phase transitions in Ni-Mn-Sn-based alloys have been studied. The temperature dependences of the martensitic and magnetic phase transformations in Ni51 − x Mn36 + x Sn13 alloys (0 ≤ x ≤ 4) have been determined in the case where manganese atoms substitute for nickel atoms. For the studied alloys, the concentration phase diagram has been constructed and the temperature regions of the existence of the high-temperature austenite phase L21 and the low-temperature martensite phase in different magnetic states have been determined.

Pressure-magnetic field induced phase transformation in Ni46Mn41In13 Heusler alloy

The effect of hydrostatic pressure and magnetic field on the magnetic properties and phase transformation in Ni46Mn41In13 Heusler alloy was investigated. Pressure (P)-magnetic field (H)-temperature (T) phase diagram has been constructed from experimental results. In the P–T contour of the phase diagram, the slope of the austenite-martensite phase boundary line appears positive (dT/dP > 0), while it appears negative (dT/dH < 0) in the H–T contour. The results revealed that pressure and magnetic field have opposite effect on phase stabilization. The combined effect of pressure and magnetic field on martensitic transition has led to two important findings: (i) pressure dependent shift of austenite start temperature (As) is higher when larger field is applied, and (ii) field dependent shift of As is lowered when a higher pressure is applied. The pressure and magnetic field dependent shift observed in the martensitic transformation has been explained on the basis of thermodynamic calculations. Curie temperature of the phases was found to increase with pressure at a rate of 0.6 K/kbar.

Phase transitions in fluoride KFe2F6 with tetragonal tungsten bronze structure

Heat capacity, thermal dilatation, structure, Mössbauer spectra and dielectric permittivity of fluoride KFe2F6 with tetragonal tungsten bronze crystal structure were studied. The as-made sample undergoes two structural phase transitions P4/mbm (T1≈340K)→Pbam (T2≈250K)→G2 and magnetic phase transformation at Tm≈133K. Heating up to 600–700K and subsequent cooling in helium atmosphere leads to a change of phase transition temperatures and diffused anomalies in thermal expansion. The results obtained and influence of thermal prehistory of the sample on its physical properties are discussed in the context of previous studies on related fluorides KFe2F6, which sometimes suggest conflicting structural details.

Magnetic field-induced martensitic phase transformation in magnetic shape memory alloys: Modeling and experiments

In this work, a continuum based model of the magnetic field induced phase transformation (FIPT) for magnetic shape memory alloys (MSMA) is developed. Hysteretic material behaviors are considered through the introduction of internal state variables. A Gibbs free energy is proposed using group invariant theory and the coupled constitutive equations are derived in a thermodynamically consistent way. An experimental procedure of FIPT in NiMnCoIn MSMA single crystals, which can operate under high blocking stress, is described. The model is then reduced to a 1-D form and the material parameter identification from the experimental results is discussed. Model predictions of magneto-thermo-mechanical loading conditions are presented and compared to experiments.

Magnetization and Intrinsic Coercivity for τ-phase Mn54Al46/α-phase Fe65Co35Composite

We have synthesized ferromagnetic τ-phase Mn 54 Al 46 /α-phase Fe 65 Co 35 composite by annealing a mixture of paramagnetic e-phase Mn 54 Al 46 and ferromagnetic α-phase Fe 65 Co 35 particles at 650℃. The volume fraction ( f h ) of hard τ-phase Mn 54 Al 46 of the composite was varied from 0 to 1. During the annealing, magnetic phase transformation occurred from paramagnetic e-phase to ferromagnetic τ-phase Mn 54 Al 46 . The magnetization and coercivity of the composite monotonically decreased and increased, respectively, as the f h increased. These results are in good agreement with our proposed composition dependent coercivity and modified magnetization equations.

Thermomagnetic and structural analysis of as‐quenched Ni49Co1Mn37Sn13

AbstractIt is known that Co substitution in magnetic shape memory Ni‐Mn‐Sn alloys produces changes in their thermomagnetic behaviour. In this work, the Ni49Co1 Mn37Sn13 alloy was obtained by melt spinning in ribbon shape. The ribbon flakes were characterized by scanning electron microscopy (SEM), X‐ray diffraction (XRD), vibrating sample magnetometry (VSM) and differential scanning calorimetry (DSC). SEM micrographs indicate the formation of textured ribbons with columnar grains growing up perpendicular to the ribbons plane. The crystallographic structure of the sample at room temperature is the Heusler L21. DSC cyclic scans reveal the martensitic transformation on cooling and the reverse austenitic transformation on heating below room temperature. The application of a magnetic field produces a decrease in the transformation temperatures. The small amount of Co addition increases the Curie temperature of austenite. These facts suggest that the structural and magnetic phase transformation temperatures of the ribbons could be tuned to the desired functional temperature by controlling the applied magnetic field as well as the cobalt amount. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influences of the first-to-second order magnetic phase transformation on the transport properties of La0.7Ca0.3−xBaxMnO3 compounds

We studied the influence of the magnetic phase transition on the transport properties of La0.7Ca0.3−xBaxMnO3 compounds with x = 0.05, 0.075, and 0.10. Our experimental results demonstrated the ferromagnetic-paramagnetic and metal-insulator transitions taking place at temperatures TC = 265–300 K and TMI = 280–310 K, respectively, which increase with increasing Ba-doping content. While the x = 0.05 sample undergoes a first-order magnetic phase transition (FOMT), x = 0.10 undergoes a second-order magnetic phase transition (SOMT). The other sample with x = 0.075 is considered as a threshold concentration of the FOMT-SOMT transformation. Further, ρ(T) data in different temperature regions were fitted to different models. The activation energy Ep and density of states at the Fermi level N(EF) were accordingly determined. Notably, N(EF) increases while Ep decreases in the case of applying an external field. We also have found that N(EF) increases when materials transfer from the FOMT to the SOMT, which becomes smallest for the sample having the coexistence of the FOMT and SOMT (i.e., x = 0.075).

Development of martensitic transformation induced by severe plastic deformation and subsequent heat treatment in polycrystalline Ni52Mn24Ga24 alloy

The results of a study of the martensitic transformation in the polycrystalline Ni 52 Mn 24 Ga 24 alloy in different structural states are presented in the article. In the initial state in the alloy magnetic and martensitic phase transformation is observed with the following characteristic points: M S =25°C; M F =15°C; A S =35°C; A F =45°C; T C =127°C. The alloy in the initial state, after intensive plastic deformation by high pressure torsion and after the stepped annealing of the deformed material at temperatures of 400°C, 500°C and 600°C was investigated. The microstructure of the alloy was investigated by means of scanning electron microscope equipped by the detector sensitive to orientation contrast of material. Analysis of the microstructure of the alloy in the initial state shows that the average grain size is 270 µm. In the alloy after plastic deformation and subsequent annealing at 400°C, this value is 180 nm. Annealing at 500°C and 600°C leads to an increase in the average grain size up to 1.08 µm and 2.33 µm, respectively. The results of the study of the microhardness of the alloy in different structural states are presented. As a result of plastic deformation of the alloy the microhardness increases from 2.1 GPa in the initial state to 5.1 GPa in the deformed state. After annealing, a gradual decreasing of the microhardness down to 2.8 GPa is found. The temperature dependence of magnetization of the alloy in different structural states shows that as a result of severe plastic deformation the ferromagnetic order is destroyed and the martensitic transformation is suppressed. After stepped annealing at 400°C, 500°C and 600°C, there is a gradual recovery of ferromagnetic order. The martensitic transformation is observed in the alloy only after annealing at 500°C. In this case the average grain size is increased to 1 µm.

Functional Electron Microscopy for Electrochemistry Research: From the Atomic to the Micro Scale

The operation of batteries, fuel cells, and other energy storage and conversion devices is underpinned by the intricate synergy of ion transport, phase transformations, and electrochemical reactions. While electrochemical processes in well-defined molecular systems are amenable to classical chemical techniques including in situ optical spectroscopies and nuclear magnetic resonance (NMR), phase transformations and reactivity in solids and at solid interfaces remain largely an enigma. The presence of multiple types of structural defects and imperfections, competing electron and ion transport pathways, and multiple possible surface reconstruction types, combined with the directionality of ion flows and state of charge transformation fronts render these phenomena inaccessible to macroscopic interrogation techniques. At the same time, knowledge of the individual mechanisms involved in electrochemical reactivity of solids at the level of individual, atomic size structural elements is required for understanding the fundamental mechanisms of energy storage, factors controlling charge/discharge rates, degradation and failure processes, and ultimately knowledge-driven prediction and optimization of energy materials. 1-3

Dehydration-triggered magnetic phase conversion in the porous Cu(ii)Mn(iii) metal–organic framework

Dehydration of the three-dimensional Cu(II)-Mn(III) coordination network undergoes a dramatic magnetic phase transformation from a paramagnetic state to a long-range magnetic ordered phase with glassy behavior. The gas adsorption behavior and re-entrant spin glass character are uniquely apparent in the dehydrated sample.

Ion-beam-induced magnetic and structural phase transformation of Ni-stabilized face-centered-cubic Fe films on Cu(100)

Metastable face-centered cubic (fcc) Fe/Cu(100) thin films are good candidates for ion-beam magnetic patterning due to their magnetic transformation upon ion-beam irradiation. However, pure fcc Fe films undergo spontaneous transformation when their thickness exceeds 10 ML. This limit can be extended to approximately 22 ML by deposition of Fe at increased CO background pressures. We show that much thicker films can be grown by alloying with Ni for stabilizing the fcc γ phase. The amount of Ni necessary to stabilize nonmagnetic, transformable fcc Fe films in dependence on the residual background pressure during the deposition is determined and a phase diagram revealing the transformable region is presented.

Magnetostrictive and magnetocaloric effects in Mn0.89Cr0.11NiGe

The results of magnetic measurements and x-ray analysis of the Mn0.89Cr0.11NiGe alloy are presented. It is shown that the temperatures of the first-order paramagnetic structural transition from hexagonal to orthorhombic phase and paramagnetic–ferromagnetic magnetic phase transition in the orthorhombic phase can be aligned by quenching the sample. This alignment results in the change of the magnetic phase transformation order: the initially isostructural phase transition of the second order becomes a magnetostructural transition of the first order (hexagonal paramagnet–orthorhombic ferromagnet). Moreover, the character of the low-temperature ferromagnetic–antiferromagnetic transition observed in magnetic fields below 3.5 T does not change substantially. The mechanisms of the giant anisotropic magnetostrictive (up to 10%) and magnetocaloric (up to 28 J/(kg·K) upon changing the magnetic field from 0 to 5 T) effects are discussed.

Modeling and characterization of the magnetocaloric effect in Ni2MnGa materials

Magnetic shape memory alloys have great promise as magneto-caloric effect refrigerant materials due to their combined magnetic and structural transitions. Computational and experimental research is reported on the Ni2MnGa material system. The magnetic states of this system are explored using the Wang-Landau statistical approach in conjunction with the Locally Self-consistent Multiple-Scattering method. The effects of alloying agents on the transition temperatures of the Ni2MnGa alloy are investigated using differential scanning calorimetry and superconducting quantum interference device. Experiments are performed at the Spallation Neutron Source at Oak Ridge National Laboratory to observe the structural and magnetic phase transformations.

Magnetic-field-induced martensitic transformations in Ni47 − x Mn42 + x In11 alloys (with 0 ≤ x ≤ 2)

Magnetic properties and martensitic transformations in the Ni47 − x Mn42 + x In11 alloys (with 0 ≤ x ≤ 2) have been studied. The magnetic-field-induced martensitic transformation was found to be observed for all the alloys. The critical temperatures of magnetic and structural phase transformations, temperature dependences of spontaneous magnetization of austenite and martensite, and the critical field, at which the martensitic transformation occurs, have been determined based on magnetic measurements performed for the alloys under study. The spontaneous magnetization of the alloys in the martensitic state has been shown to be lower than that in the magnetic-field-induced austenitic state by a factor of six.

The Influence of Heat Treatment on the Phase Transformation Behavior and Magnetocaloric Effect of Gd<sub>5</sub>Si<sub>2</sub>Ge<sub>2</sub>

In this thesis, a study was conducted on the magnetic phase transformation behavior and magnetocaloric effect (MCE) under a low magnetic field ranging from 0-1.5 T for the casted and annealed Gd5Si2Ge2alloys. The result shows that high-heat treatment significantly influences the structural and magnetocaloric properties of Gd5Si2Ge2alloy. High-heat treatment at 1573 K is beneficial to increasing Gd5Si2Ge2-type monoclinic structure which benefits the first-order transition. The MCE of annealed Gd5Si2Ge2alloy drastically increases and the highest value of SM~18.12 J/kg·K at 282 K upon H=1.5 T which is obviously larger than that of as-casted Gd5Si2Ge2( SM~5.03 kg·K at 276 K upon H=1.5 T). The giant magnetocaloric effect is obtained for Gd5Si2Ge2alloy by high-heat treatment.

Multiple twinning and variant-variant transformations in martensite: Phase-field approach

A phase-field theory of transformations between martensitic variants and multiple twinning within martensitic variants is developed for large strains and lattice rotations. It resolves numerous existing problems. The model, which involves just one order parameter for the description of each variant-variant transformation and multiple twinnings within each martensitic variant, allows one to prescribe the twin interface energy and width, and to introduce interface stresses consistent with the sharp interface limit. A finite-element approach is developed and applied to the solution of a number of examples of twinning and combined austenite-martensite and martensite-martensite phase transformations (PTs) and nanostructure evolution. A similar approach can be developed for reconstructive, electric, and magnetic PTs.

The effect of a surface Cr film on the oxidation of SmCo-based magnetic alloy at 700°C

Abstract A 4.5 μm-thick Cr film prepared by a double glow sputtering technique was deposited on a magnetic Sm(CobalFe0.22Cu0.08Zr0.02)7.5 alloy to improve its oxidation resistance. Oxidation tests in air at 700 °C indicate that the Cr film grew a Cr2O3 scale, profoundly preventing the oxidation of the SmCo-based alloy. The coated alloy only formed a ∼2.9 μm-thick internal-oxidation-affected zone (IOAZ) for 20 h. The mechanism for the formation of this IOAZ, together with its induced alloy magnetic phase transformation, is presented.

Phase investigation in Pt supported off-stoichiometric iron-platinum thin films

The structural and magnetic phase transformation of Pt/Fe3Pt/Pt films on Si <100> substrates prepared by DC magnetron sputtering is investigated as a function of annealing temperature. Pt diffusion driven low temperature phase transformation from A1 to L10 phase is achieved at 300°C, attaining a very high coercivity of 9kOe. At 300°C, 85% L10 phase transformation is observed using the X-ray diffraction profile fitting. The estimated phase content is also further verified by fitting the demagnetization curve. The underlayer promotes the ordering at lower temperature while overlayer induces growth along (001) preferred orientation. Rutherford back scattering study reveals interlayer diffusion and confirms the desired stoichiometry for L10 phase. The presence of Pt under-overlayer provides the Pt source and further facilitates the Pt diffusion, which makes it effective in promoting the phase ordering at a lower temperature.

Structural and magnetic properties of Sm2−xMnxO3 nanoparticles

Mixed oxide Sm2−xMnxO3 (SMO), x=0.0, 0.05, 0.10, 0.15 and 0.20 were synthesized by a sol–gel process. A single phase solid solution is formed up to x=0.15 which confirmed by using X-ray diffraction, Raman spectroscopy and infrared techniques. Crystal structure and microstructure analyses were performed by Rietveld refinement. Preferential cationic distribution, over the two crystallographic sites 8b and 24d of space group Ia3¯, is found for doped samples but with different extent. The r.m.s. microstrain 〈εL2〉1/2 depends on composition x in a systematic way emphasizing the preferential distribution. Magnetization measurements show that samples with x≤0.05 have antiferromagnetic behavior, while samples with x>0.05 exhibit a weak ferromagnetic behavior with magnetic phase transformation at 15 and 16K for x=0.1 and x=0.15, respectively.

Fluctuations of the order parameter in R 0.55Sr0.45MnO3 manganites near the metal-insulator phase transition

The magnetic phase transformations induced by changes of the composition, external magnetic field strength, and temperature in manganites with a nearly half-filled conduction band in the vicinity of the metal-insulator phase transition have been investigated experimentally. It has been found that the substitution of rare-earth ions (Sm) for Nd ions with a larger ionic radius in R 0.55Sr0.45MnO3 manganites leads to a linear decrease in the Curie temperature T C from 270 to 130 K and a transformation of the second-order ferromagnetic (FM) phase transition into a first-order phase transition. The results of measurements of the alternating-current (ac) magnetic susceptibility in the (Nd1 − y Sm y )0.55Sr0.45MnO3 system indicate the existence of a Griffiths-like phase in samples with a samarium concentration y > 0.5 in the temperature range T C 0.5, the magnetization isotherms at temperatures above T C exhibit specific features in the form of reversible metamagnetic phase transitions associated with strong fluctuations of the short-range ferromagnetic order in the system of Mn spins in the high-temperature Griffiths phase consisting of ferromagnetic clusters. According to the results of measurements of the ac magnetic susceptibility in the (Sm1 − y Gd y )0.55Sr0.45MnO3 system for a gadolinium concentration y = 0.5, there is an antiferromagnetic (AFM) phase with an unusually low critical temperature of the spin ordering T N ≊ 48.5 K. An increase in the external static magnetic field at 4.2 K leads to an irreversible induction of the ferromagnetic phase, which is stable in the temperature range 4.2–60 K. In the temperature range 60 K < T < 150 K, there exists a high-temperature Griffiths-like phase consisting of clusters (correlations) with a local charge/orbital ordering. The metastable antiferromagnetic structure is retained in samples with gadolinium concentrations y = 0.6 and 0.7, but it is destroyed with a further increase in the gadolinium concentration upon the transition to the spin-glass state. The magnetization isotherm obtained with variations in the external static magnetic field in the field range ±70 kOe at 4.2 K and the temperature dependence of the ac-magnetic susceptibility χ suggest that, in the Gd0.55Sr0.45MnO3 ceramics, there is a mixed two-phase low-temperature state consisting of the quantum Griffiths phase with a characteristic divergence of χ(T) near T = 0, which was embedded in the spin-glass matrix with the spin “freezing” temperature T G ≊ 42 K. The low-temperature state with quantum fluctuations exists in the (Sm1 − y Gd y )0.55Sr0.45MnO3 system for y ≥ 0.5.