Irreversibility of field-induced magnetostructural transition in NiCoMnSb shape memory alloy revealed by magnetization, transport and heat capacity studies

The effects of magnetic field on the martensitic transition have been studied in Ni45Co5Mn38Sb12. We find a large field-induced irreversibility in this system, as revealed by the field dependence of resistivity, magnetization, and heat capacity data. At the critical temperature, the field-induced conversion of the martensitic to austenite phase is not reversible under any field variation. At this temperature any energy fluctuation induces nucleation and growth of the equilibrium austenite phase at the expense of the metastable martensitic phase and gets arrested. All these three measurements completely rule out the coexistence of austenite and martensitic phases in the irreversibility regime.Comment: 13 pages, 4 figure

Heusler 4.0: Tunable Materials

Heusler compounds are a large family of binary, ternary and quaternary compounds that exhibit a wide range of properties of both fundamental and potential technological interest. The extensive tunability of the Heusler compounds through chemical substitutions and structural motifs makes the family especially interesting. In this article we highlight recent major developments in the field of Heusler compounds and put these in the historical context. The evolution of the Heusler compounds can be described by four major periods of research. In the latest period, Heusler 4.0 has led to the observation of a variety of properties derived from topology that includes: topological metals with Weyl and Dirac points; a variety of non-collinear spin textures including the very recent observation of skyrmions at room temperature; and giant anomalous Hall effects in antiferromagnetic Heuslers with triangular magnetic structures. Here we give a comprehensive overview of these major achievements and set research into Heusler materials within the context of recent emerging trends in condensed matter physics

Pressure induced magnetic and magnetocaloric properties in NiCoMnSb Heusler alloy

The effect of pressure on the magnetic and the magnetocaloric properties around the martensitic transformation temperature in NiCoMnSb Heusler alloy has been studied. The martensitic transition temperature has significantly shifted to higher temperatures with pressure, whereas the trend is opposite with the application of applied magnetic field. The maximum magnetic entropy change around the martensitic transition temperature for Ni45Co5Mn38Sb12 is 41.4 J/kg K at the ambient pressure, whereas it is 33 J/kg K at 8.5 kbar. We find that by adjusting the Co concentration and applying suitable pressure, NiCoMnSb system can be tuned to achieve giant magnetocaloric effect spread over a large temperature span around the room temperature, thereby making it a potential magnetic refrigerant material for applications.Comment: 16 pages, 5 figure

Observation of enhanced exchange bias behavior in NiCoMnSb Heusler alloys

We report the observation of large exchange bias in Ni50-xCoxMn38Sb12 Heusler alloys with x=0, 2, 3, 4, 5, which is attributed to the coexistence of ferromagnetic and antiferromagnetic phases in the martensitic phase. The phase coexistence is possibly due to the supercooling of the high temperature ferromagnetic phase and the predominant antiferromagnetic component in the martensitic phase. The presence of exchange bias is well supported by the observation of training effect. The exchange bias field increases with Co concentration. The maximum value of 480 Oe at T=3K is observed in x=5 after field cooling in 50 kOe, which is almost double the highest value reported so far in any Heusler alloy system. Increase in the antiferromagnetic coupling after Co substitution is found to be responsible for the increase in the exchange bias.Comment: 17 pages, 8 figure

Manipulation of hybrid skyrmion dynamics by step DMI approach

The dynamic behavior of non-collinear atomic spin arrangements in a topologically protected magnetic skyrmion plays a pivotal role in potential future spintronic technologies, including racetrack memory based ultra-high-density storage devices. However, the topological nature of the skyrmion comes with an unwanted skyrmion Hall effect (SkHE) that poses a significant challenge in the practical application. Here we present a detailed micromagnetic simulation study that delves into the controlled manipulation of skyrmion dynamics through a subtle engineering of Dzyaloshinskii-Moriya interaction (DMI) in a hybrid skyrmion racetrack. In particular, we introduce a gradient variation of bulk and interfacial DMI that results into a parabolic trajectory of Skyrmion Hall angle (SkHA), thereby allowing us to find a critical DMI ratio with almost zero SkHE. Most importantly, we present a novel approach involving engineering of a racetrack with strategically placed step DMI regions that gives us a meticulous control over the size and speed of the hybrid skyrmions. The present study gives a new direction for the simultaneous realization of stable skyrmions without SkHE and increased skyrmion speed with optimized DMI engineering.Comment: 5 pages, 4 figure

Giant inverse magnetocaloric effect near room temperature in Co substituted NiMnSb Heusler alloys

The effect of Co on the structural, magnetic and magnetocaloric effect (MCE) of Ni50-xCoxMn38Sb12 (x=0,2,3,4,5) Heusler alloys was studied. Using x-ray diffraction, we show the evolution of the martensitic phase from the austenite phase. The martensitic transition temperature is found to decrease monotonically with Co concentration. Remarkable enhancement of MCE is observed near room temperature upon Co substitution. The maximum magnetic entropy change of 34 Jkg-1K-1 was achieved in x=5 at 262 K in a field of 50 kOe and a value of 29 Jkg-1K-1 found near room temperature. The significant increase in the magnetization associated with the reverse martensitic transition is responsible for the giant MCE in these compounds.Comment: 14 pages, 5 figure

Effect of Si and Ga substitutions on the magnetocaloric properties of NiCoMnSb quaternary Heusler alloys

The effect of Si and Ga substitutions on the magnetic and the magnetocaloric properties in Heusler based system Ni46Co4Mn38Sb12-xZx (Z=Si and Ga) has been studied. From the M(T) plots it is found that Si substitution stabilizes the austenite phase, whereas, Ga substitution stabilizes the martensite phase. Strong metamagnetic behaviour is observed in the M(H) isotherms for Si=0.75 and 1, whereas, such a behaviour is absent in the Ga substituted alloys. Associated with magneto-structural transition, large MCE of 58 J/kg K and 70 J/kg K is observed for x=0.75 and 1, respectively in the case of Si. Though the MCE observed in x=0.5 and 1 in the case of Ga is much lower, the MCE peak is found to be quite broad.Comment: 11 pages, 5 figures, Accepted for publication in Journal of Applied Physic