Curie Temperature Of Austenite Research Articles

Interacting magnetic cluster‐spin glasses and strain glasses in Ni–Mn based Heusler structured intermetallics

Magnetic Ni–Mn based Heusler intermetallics show complex magnetic behavior in connection with martensitic transformations (see, for instance, the phase diagram of Ni–Co–Mn–Sn on the right‐hand side). The cubic austenitic phase at high temperature shows long‐range ferromagnetic order which can considerably be weakened by the appearance of competing antiferromagnetic interactions which are induced by Mn excess and chemical disorder. With decreasing temperature a martensitic/magnetostructural transformation takes place from cubic to non‐modulated/modulated tetragonal/monoclinic or orthorhombic structure, where long‐range ferromagnetic order can no longer be maintained, leading to superparamagnetic behavior. At still lower temperatures superparamagnetism changes to superspin glass because of strong competition of ferromagnetic and antiferromagnetic interactions and chemical disorder. In addition, disorder and local structural distortions can lead to strain glass in austenite, as observed for some non‐magnetic martensitic systems. The magnetic intermetallics are of technological importance in view of their functional properties involving magnetic shape‐memory and exchange‐bias effects as well as magnetocaloric effects. The ‘ferroic cooling’ is of particular relevance since it avoids the use of ozone‐depleting and greenhouse chemicals compared with conventional fluid‐compression technology.Experimental phase diagram of for . Here, is the Curie temperature of austenite; at the system transforms to paramagnetic martensite and at to superparamagnetic martensite (SPM) and then to superspin‐glass martensite (SSG) at . The possible strain‐glass phases (labeled by question marks) are predicted because of kinetic arrest phenomena and local distortions associated with the magnetostructural transition and ergodicity breaking by field‐cooling/zero‐field‐cooling experiments.

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)

Effects of Cu on the martensitic transformation and magnetic properties of Mn50Ni40In10 alloy

The structures, the martensitic transformations, and the magnetic properties are studied systematically in Mn50Ni40−xCuxIn10, Mn50−xCuxNi40In10, and Mn50Ni40In10−xCux alloys. The partial substitution of Ni by Cu reduces the martensitic transformation temperature, but has little influence on the Curie temperature of austenite. Comparatively, the martensitic transformation temperature increases and the Curie temperature of austenite decreases with the partial replacement of Mn or In by Cu. The magnetization difference between the austenite phase and the martensite phase reaches 70 emu/g in Mn50Ni39Cu1In10; a field-induced martensite-to-austenite transition is observed in this alloy.

Giant magnetocaloric effect in isostructural MnNiGe-CoNiGe system by establishing a Curie-temperature window

An effective scheme of isostructural alloying was applied to establish a Curie-temperature window in isostructural MnNiGe-CoNiGe system. With the simultaneous accomplishment of decreasing structural-transition temperature and converting antiferromagnetic martensite to ferromagnetic state, a 200 K Curie-temperature window was established between Curie temperatures of austenite and martensite phases. In the window, a first-order magnetostructural transition between paramagnetic austenite and ferromagnetic martensite occurs with a sharp jump in magnetization, showing a magnetic entropy change as large as −40 J kg−1 K−1 in a 50 kOe field change. This giant magnetocaloric effect enables Mn1−xCoxNiGe to become a potential magnetic refrigerant.

Magnetic influence on the martensitic transformation entropy in Ni-Mn-In metamagnetic alloy

We study the martensitic transformation (MT) of metamagnetic shape memory alloy Ni50Mn34.5In15.5 in the magnetic fields up to 12 T. The observed dependence of the MT temperature, Tm, on the field is highly nonlinear. As far as magnetization change, ΔM, remains field-independent, a depart from linearity of Tm(H) function is attributed to a decrease of the transformation entropy, ΔS. This decrease correlates with the parameter (TC-Tm), controlled by magnetic field, where TC is the Curie temperature of austenite, and with the dependence of ΔS on the width of the MT temperature interval deduced from a ferroelastic model of MT.

Entropy Changes in Ferromagnetic Shape Memory Alloys

Several features of the entropy change S related to the martensitic transformation (MT) in metamagnetic alloys are discussed. In these alloys a change in magnetic order is concomitant with the MT, as it occurs between ferromagnetic austenite and non-magnetic (weakly magnetic) martensite. In this case it has been shown that S strongly decreases as the MT temperature range go far below the Curie temperature of austenite. The behavior of S can be understood considering the different signs of the lattice and magnetic contributions to the total entropy change. It has been shown that the so called kinetic arrest of the MT is directly related to the decrease of S, which in the limit S → 0 leads to the suppression of the driving force for the transformation.

Effect of ageing on the structural and magnetic transformations and the related entropy change in a Ni–Co–Mn–Ga ferromagnetic shape memory alloy

The effect of thermal treatments on the structural and magnetic transitions in a Ni 42Co 8Mn 32Ga 18 alloy has been studied. The selected composition shows coincidental Curie temperature of the parent phase and martensitic transformation to paramagnetic martensite, thus leading to enhanced magnetization difference between austenite and martensite. Quenching raises the martensitic transformation temperatures, which decrease upon post-quench ageing although an increase is observed for prolonged ageing time. This behaviour is mostly attributed to quenched-in atomic order and its improvement during ageing, the final increase being imputed to a process of elimination of defects occurring at the same time than ordering or to an early stage of precipitation. As the austenite Curie temperature increases with ageing, the structural and magnetic transitions shift in a different way modifying their relative position. On its turn, the transformation entropy change decreases with increasing ageing time, and its evolution can be correlated with the different magnetic order of the austenite when it undergoes the martensitic transformation, as it is modified by ageing.

Martensitic transformation and magnetic properties in high-Mn content Mn 50Ni 50− xIn x ferromagnetic shape memory alloys

A series of Mn 50Ni 50− x In x ( x = 9.75, 10, 10.25, 10.5 10.75, and 11) ferromagnetic shape memory alloys with Mn content as high as 50 at.% were prepared. The martensitic transformation (MT), magnetocaloric effect, and magnetoresistance in Mn 50Ni 50− x In x alloys were investigated. With x increasing from 9.75 to 11, the MT temperature decreased from 270 to 110 K and the Curie temperature of austenite remains relatively constant. Large positive magnetic entropy change and negative magnetoresistance were observed around MT temperatures in these alloys. Large magnetic entropy change and magnetoresistance would be ascribed to the temperature and magnetic field-induced MT in Mn 50Ni 50− x In x alloys.

Heat capacity of the Ni50Mn37(In0.2Sn0.8)13 alloy

Heat capacity of the Heusler-type Ni50Mn37(In0.2Sn0.8)13 alloy has been studied in external magnetic fields up to 3 T. The alloy has the austenite Curie temperature TCA≈290 K, the martensitic phase transformation temperature TM≈220–240 K, and the martensite Curie temperature TCM≈185 K. The martensitic phase transition is accompanied by a considerable change of both magnetization and resistivity of the sample. It was found that upon the first-order martensitic transformation the heat capacity demonstrate hysteresis, both temperature and peak values being different. The maximum value of the heat capacity near TM on cooling strongly differs from that on heating. In magnetic field the peaks of the heat capacity shift to lower temperatures with the rate ΔTM/ΔB≈2.5 K/T. The difference in the peak values of ΔC is caused by either overheating or overcooling the sample during the measurement of each point and the release of the latent heat of the martensitic phase transformation. The estimations of magnetocaloric effect (MCE) from the magnetic field effect on the heat capacity and direct measurements clearly demonstrate that the transition is accompained by a negative (MCE) at TM. The hysteresis observed leads to a decrease of the (MCE) in magnetic refrigeration cycle.

Magnetic and magnetocaloric properties in melt-spun and annealed Ni 42.7Mn 40.8Co 5.2Sn 11.3 ribbons

The Ni 42.7Mn 40.8Co 5.2Sn 11.3 ribbons were prepared by melt spinning. After heat treatment, the martensitic transformation temperature and Curie temperature of austenite of the annealed ribbons increased remarkably. Inverse and direct magnetocaloric properties were investigated in the melt-spun and annealed ribbons. The effective refrigerant capacities for these ribbons were discussed in this paper.

Investigation of the intermediate phase and magnetocaloric properties in high-pressure annealing Ni–Mn–Co–Sn alloy

The Ni–Mn–Co–Sn alloy is prepared by high-pressure annealing method. Besides the enhanced martensitic transformation temperature and the Curie temperature of austenite, an intermediate phase above the martensitic transformation is observed in this alloy. As a result, two successive magnetic entropy changes with the same sign are obtained around room-temperature, corresponding to the martensitic transformation and intermediate phase transition, respectively. The origin of the intermediate phase for high-pressure annealing Ni–Mn–Co–Sn alloy is discussed.

Ageing effects on structural and magnetic transformations in a Ni–Co–Mn–Ga alloy

Partial substitution of Ni by Co in Mn-rich Ni–Mn–Ga alloys has been found to modify the magnetic ordering of the phases, improving in this way the possibility to obtain large magnetization difference between austenite and martensite, an essential requirement to induce the martensitic transformation by application of a magnetic field. Particularly, Ni50−xCoxMnyGa50−y alloys undergo, for Co content below x=9, structural transformation between ferromagnetic austenite and paramagnetic martensite, thus leading to enhanced magnetization difference values.The martensitic transformation temperatures as well as the martensite and austenite Curie temperatures depend on composition, but significant changes can be brought about by selected thermal treatments. In this work, the composition is chosen as Ni42Co8Mn32Ga18 in order to obtain concurrent martensitic transformation and austenite Curie temperature, and the effect of quench and subsequent ageing on the structural and magnetic transitions is studied. Aside from the monotonic transformation temperatures change, which is mostly attributed to atomic ordering taking place upon post-quench ageing, the results show the effect of the relative position of the structural and magnetic ordering reactions on the transformation entropy change.

Reverse magnetostructural transformation in Co-doped NiMnGa multifunctional alloys

We studied the composition dependence of the structural and magnetic properties of Co-doped Ni–Mn–Ga alloys around the Mn-rich composition Ni50Mn30Ga20. By varying the Co and Mn content we have been able to tune the critical temperatures. In particular, in a suitable composition range, the Curie temperature of martensite is lower than Curie temperature of austenite and lower than martensitic transformation temperature, giving rise to a paramagnetic gap between magnetically ordered martensite and austenite and to the occurrence of a reverse magnetostructural transformation.

Thermodynamic, kinetic, and magnetic properties of aNi54Fe19Ga27magnetic shape-memory single crystal

Ferromagnetic Heusler alloys exhibiting martensitic transformations are known to change their shape in an external magnetic field. Magnetization, electric resistance, and specific heat as a function of temperature are examined in ${\mathrm{Ni}}_{54}{\mathrm{Fe}}_{19}{\mathrm{Ga}}_{27}$ single crystal. Structural transition appears as sharp anomaly in these dependencies. This points to an avalanchelike character of martensitic transformation. The jump in resistivity at the structural phase transition and the lower density of states at the Fermi level in the martensite phase supports the hypothesis of the Jahn-Teller origin of the martensitic transformation. Magnetic measurements show that transformation to the martensitic phase is accompanied by the increase of spontaneous magnetization and an increase of magnetocrystalline anisotropy. Magnetization increase is due to different Curie temperatures of austenite and martensite. These were determined from critical behavior using Arrott plot. Additional analysis of magnetic behavior indicates ferrimagnetic ordering in this nonstoichiometric compound. Intrinsic properties of the compound are analyzed with respect to both of the actuation modes possible in magnetic shape-memory alloys. However, neither a magnetically induced martensite transformation nor a magnetically induced reorientation of variants has been observed.

Phase transitions inNi2+xMn1−xGawith a high Ni excess

Ferromagnetic shape memory alloys Ni$_{2+x}$Mn$_{1-x}$Ga were studied in the range of compositions $0.16 \le x \le 0.36$. Experimental phase diagram, constructed from differential scanning calorimetry, transport and magnetic measurements, exhibits distinctive feature in a compositional interval $0.18 \le x \le 0.27$, where martensitic and magnetic transitions merge in a first-order magnetostructural phase transition ferromagnetic martensite $\leftrightarrow$ paramagnetic austenite. Observed in this interval of compositions a non-monotonous behavior of the magnetostructural phase transition temperature was ascribed to the difference in the exchange interactions of martensitic and austenitic phase and to the competition between increasing number of valence electron and progressive dilution of the magnetic subsystem which occur in the presence of a strong magnetoelastic interaction. Based on the experimental phase diagram, the difference between Curie temperature of martensite $T_C^M$ and Curie temperature of austenite $T_C^A$ was estimated. Influence of volume magnetostriction was considered in theoretical modeling in order to account for the existence of the magnetostructural phase transition over a wide range of compositions.

The Effect of Order on the Martensitic Transformation in Fe3Pt

ABSTRACTThe lattice parameter and Curie temperature of austenite in Fe3Pt increase sharply on ordering for times up to 2 hr at 650° C. These effects, together with a rapid decrease in the martensitic transformation temperatures, are interpreted in terms of increased short-range order. The onset of thermoelastic reversibility of the martensitic transformation is associated with the development of a significant volume fraction of long-range ordered domains.