Martensitic transformation and magnetic properties of manganese-rich Ni-Mn-In and Ni-Mn-Sn Heusler alloys

In der vorliegenden Arbeit wurden sowohl die martensitische Umwandlung als auch die magnetischen Eigenschaften Mangan-reicher Ni50Mn50-xSnx- und Ni50Mn50-yIny-Legierungen mit 5 at.-%<x(y)<25 at.-% untersucht. Hierzu wurden Kalorimetrie-Messungen, Röntgen- und Neutronenbeugungsexperimente, Magnetisierungs- und Dehnungsmessungen an polykristallinen Proben durchgeführt. Es zeigt sich, dass Legierungen nahe den stöchiometrischen Zusammensetzungen Ni50Mn25Sn25 bzw. Ni50Mn25In25 strukturell nicht umwandeln, während Legierungen mit x<15 at.-% Zinn bzw. y<16 at.-% Indium eine martensitische Umwandlung aufweisen. Die Umwandlungstemperaturen steigen dabei mit abnehmendem Zinn- (Indium-) Gehalt jeweils linear an. Als Tieftemperaturphasen treten sowohl modulierte als auch modulierte Kristallstrukturen auf. In Legierungen nahe den stöchiometrischen Zusammensetzungen dominieren ferromagnetische Wechselwirkungen. Legierungen nahe der binären Legierung Ni50Mn50 ordnen antiferromagnetisch. In einem schmalen Konzentrationsbereich zwischen 13 at.-%<x<15 at.-% Zinn bzw. 15 at.-%<x<16 at.-% Indium kommt es zur Koexistenz zwischen ferromagnetischer Ordnung und martensitischer Umwandlung. Dabei treten interessante magnetoelastische Effekte auf. Die Legierung Ni50Mn34In16 zeigt einen Magnetfeld-induzierten strukturellen Phasenübergang, bei dem durch Anlegen eines externen Magnetfeldes im martensitischen Zustand der Austenit stabilisiert wird. Der Beweis für diesen strukturellen Phasenübergang wurde durch Neutronenbeugungsexperimente im Magnetfeld geführt. Die Umwandlungstemperaturen dieser Legierung zeigen große Magnetfeldabhängigkeiten. Mit Hilfe von DSC-Messungen,M(T)-Untersuchungen und temperaturabhängigen Dehnungsmessungen wurden Änderungen der Ms-Temperatur bis zu -11 K/Tesla gemessen. Solche starken Temperaturabhängigkeiten, bei denen zudem die L21-Phase stabilisiert wird, wurden bisher in keiner Heusler-Legierung beobachtet. Da während der Umwandlung eine Volumenänderung erfolgt, treten zudem reversible Magnetfeld-induzierte Dehnungen von 0.12 % auf. Des Weiteren wurden an den Legierungen Ni50Mn35Sn15, Ni50Mn37Sn13, Ni50Mn34In16, Ni51,5Mn33In15,5 und Ni50Mn35In15 bemerkenswerte magnetokalorische Eigenschaften festgestellt. Diese Legierungen zeigen einen inversen magnetokalorischen Effekt, bei dem sich die Proben bei Anlegen eines Magnetfeldes unter adiabatischen Bedingungen abkühlen. Die Entropieänderungen DeltaS sind, bedingt durch das Vorzeichen von deltaM(T)=deltaT, immer positiv und nehmen die bisher größten gemessenen Werte von bis zu +23 JK-1kg-1 (Ni50Mn35In15) bei Raumtemperatur an.In the present work, the martensitic transition and the magnetic properties of Manganese rich Ni50Mn50-xSnx and Ni50Mn50-yIny alloys with 5 at%<x(y)<25 at% were investigated. Calorimetry, X-ray and neutron diffraction, magnetization, and strain measurements were performed on polycrystalline samples. It was shown that alloys close to the stoichiometric composition Ni50Mn25Sn25 and Ni50Mn25Sn25 do not exhibit a structural transition on lowering of the temperature, whereas alloys with x<15 at% Tin and y<16 at% Indium transform martensitically. The structural transition temperatures increase linearly with decreasing Tin or Indium content. The crystal structures of the low temperature martensite are modulated as well as unmodulated. Alloys with compositions close to stoichiometry are dominated by ferromagnetic interactions, whereas those close to the binary composition Ni50Mn50 order antiferromagnetically. Ferromagnetic order and structural instability coexist in a narrow composition range between 13 at%<x<15 at% and 15 at%<x<16 at% for Ni50Mn50-xSnx and Ni50Mn50-yIny respectively. As a consequence, interesting magnetoelastic effects are observed. The Ni50Mn34In16 alloy shows a magnetic field-induced structural transition, whereby application of an external magnetic field in the martensitic state stabilizes the high temperature L21 structure. Evidence for this was given by neutron diffraction experiments in external magnetic fields. Moreover, the structural transition temperatures of this alloy show large magnetic field dependencies. By use of calorimetry, M(T), and strain measurements, changes in Ms up to -11 K/Tesla are observed. Such large values have, until now, not been observed in Heusler alloys. Since during transformation the volume changes reversibly, magnetic field-induced strains of about 0.12 % appear. Additionally, the alloys Ni50Mn35Sn15, Ni50Mn37Sn13, Ni50Mn34In16, Ni51,5Mn33In15,5, and Ni50Mn35In15 show remarkable magnetocaloric properties. The entropy change DeltaS, which is determined by the sign of deltaM(T)=deltaT, is positive in all cases. The absolute values of DeltaS reach up to +23 JK-1kg-1 (Ni50Mn35In15) at room temperature, which are, up to now, the largest values ever obtained in Heusler type alloys. This means that these alloys exhibit an inverse magnetocaloric effect, whereby the samples cool by adiabatic application of an external magnetic field

Effect of Co and Fe on the inverse magnetocaloric properties of Ni-Mn-Sn

At certain compositions Ni-Mn-$X$ Heusler alloys ($X$: group IIIA-VA elements) undergo martensitic transformations, and many of them exhibit inverse magnetocaloric effects. In alloys where $X$ is Sn, the isothermal entropy change is largest among the Heusler alloys, particularly in Ni$_{50}$Mn$_{37}$Sn$_{13}$ where it reaches a value of 20 Jkg$^{-1}$K$^{-1}$ for a field of 5T. We substitute Ni with Fe and Co in this alloy, each in amounts of 1 at% and 3 at% to perturb the electronic concentration and examine the resulting changes in the magnetocaloric properties. Increasing both Fe and Co concentrations causes the martensitic transition temperature to decrease, whereby the substitution by Co at both compositions or substituting 1 at% Fe leads to a decrease in the magnetocaloric effect. On the other hand, the magnetocaloric effect in the alloy with 3 at% Fe leads to an increase in the value of the entropy change to about 30 Jkg$^{-1}$K$^{-1}$ at 5T.Comment: 5 pages, 7 figures. Accepted for publication in the Journal of Applied Physic

Magnetization easy-axis in martensitic Heusler alloys estimated by strain measurements under magnetic-field

We study the temperature dependence of strain under constant magnetic-fields in Ni-Mn based ferromagnetic Heusler alloys in the form Ni-Mn-$X$ ($X$: Ga, In, Sn, Sb) which undergo a martensitic transformation. We discuss the influence of the applied magnetic-field on the nucleation of ferromagnetic martensite and extract information on the easy-axis of magnetization in the martensitic state.Comment: 3 pages, 3 figures. Accepted for publication in Applied Physics Letter

Magnetic superelasticity and inverse magnetocaloric effect in Ni-Mn-In

Applying a magnetic field to a ferromagnetic Ni$_{50}$Mn$_{34}$In$_{16}$ alloy in the martensitic state induces a structural phase transition to the austenitic state. This is accompanied by a strain which recovers on removing the magnetic field giving the system a magnetically superelastic character. A further property of this alloy is that it also shows the inverse magnetocaloric effect. The magnetic superelasticity and the inverse magnetocaloric effect in Ni-Mn-In and their association with the first order structural transition is studied by magnetization, strain, and neutron diffraction studies under magnetic field.Comment: 6 pages, 8 figures. Published in the Physical Review

Magnetic superelasticity and inverse magnetocaloric effect in Ni-Mn-In

Applying a magnetic field to a ferromagnetic Ni$_{50}$Mn$_{34}$In$_{16}$ alloy in the martensitic state induces a structural phase transition to the austenitic state. This is accompanied by a strain which recovers on removing the magnetic field giving the system a magnetically superelastic character. A further property of this alloy is that it also shows the inverse magnetocaloric effect. The magnetic superelasticity and the inverse magnetocaloric effect in Ni-Mn-In and their association with the first order structural transition is studied by magnetization, strain, and neutron diffraction studies under magnetic field.Comment: 6 pages, 8 figures. Published in the Physical Review

Phase diagram of Fe-doped Ni-Mn-Ga ferromagnetic shape-memory alloys

We have studied the effect of Fe addition on the structural and magnetic transitions in the magnetic shape memory alloy Ni-Mn-Ga by substituting systematically each atomic species by Fe. Calorimetric and AC susceptibility measurements have been carried out in order to study the magnetic and structural transformation properties. We find that the addition of Fe modifies the structural and magnetic transformation temperatures. Magnetic transition temperatures are displaced to higher values when Fe is substituted into Ni-Mn-Ga, while martensitic and premartensitic transformation temperatures shift to lower values. Moreover, it has been found that the electron per atom concentration essentially governs the phase stability in the quaternary system. However, the observed scaling of transition temperatures with $e/a$ differs from that reported in the related ternary system Ni-Mn-Ga.Comment: 8 pages, 8 figures. Accepted for publication in the Physical Review

Martensitic transitions and the nature of ferromagnetism in the austenitic and martensitic states of Ni-Mn-Sn alloys

Structural and magnetic transformations in the Heusler-based system Ni0.50Mn0.50¿xSnx are studied by x-ray diffraction, optical microscopy, differential scanning calorimetry, and magnetization. The structural transformations are of austenitic-martensitic character. The austenite state has an L21 structure, whereas the structures of the martensite can be 10M , 14M , or L10 depending on the Sn composition. For samples that undergo martensitic transformations below and around room temperature, it is observed that the magnetic exchange in both parent and product phases is ferromagnetic, but the ferromagnetic exchange, characteristic of each phase, is found to be of different strength. This gives rise to different Curie temperatures for the austenitic and martensitic states

Lattice dynamics and phonon softening in Ni-Mn-Al Heusler alloys

Inelastic and elastic neutron scattering have been used to study a single crystal of the Ni$_{54}$Mn$_{23}$Al$_{23}$ Heusler alloy over a broad temperature range. The paper reports the first experimental determination of the low-lying phonon dispersion curves for this alloy system. We find that the frequencies of the TA$_2$ modes are relatively low. This branch exhibits an anomaly (dip) at a wave number $\xi_{0} ={1/3}\approx 0.33$, which softens with decreasing temperature. Associated with this anomalous dip at $\xi_{0}$, an elastic central peak scattering is also present. We have also observed satellites due to the magnetic ordering.Comment: 6 pages, 6 figures. Accepted for publication in the Physical Review

Cooling and heating by adiabatic magnetization in the Ni50_{50}Mn34_{34}In16_{16} magnetic shape memory alloy

We report on measurements of the adiabatic temperature change in the inverse magnetocaloric Ni$_{50}$Mn$_{34}$In$_{16}$ alloy. It is shown that this alloy heats up with the application of a magnetic field around the Curie point due to the conventional magnetocaloric effect. In contrast, the inverse magnetocaloric effect associated with the martensitic transition results in the unusual decrease of temperature by adiabatic magnetization. We also provide magnetization and specific heat data which enable to compare the measured temperature changes to the values indirectly computed from thermodynamic relationships. Good agreement is obtained for the conventional effect at the second-order paramagnetic-ferromagnetic phase transition. However, at the first order structural transition the measured values at high fields are lower than the computed ones. Irreversible thermodynamics arguments are given to show that such a discrepancy is due to the irreversibility of the first-order martensitic transition.Comment: 5 pages, 4 figures. Accepted for publication in the Physical Review

Temperature and magnetic field dependences of the elastic constants of Ni-Mn-Al magnetic Heusler alloys

We report on measurements of the adiabatic second order elastic constants of the off-stoichiometric Ni$_{54}$Mn$_{23}$Al$_{23}$ single crystalline Heusler alloy. The variation in the temperature dependence of the elastic constants has been investigated across the magnetic transition and over a broad temperature range. Anomalies in the temperature behaviour of the elastic constants have been found in the vicinity of the magnetic phase transition. Measurements under applied magnetic field, both isothermal and variable temperature, show that the value of the elastic constants depends on magnetic order, thus giving evidence for magnetoelastic coupling in this alloy system.Comment: 7 pages, 5 figures. Accepted for publication in Physical the Review