Novel Ni-Co-Mn-Ti all-d-metal Heusler alloys are exciting due to large multicaloric effects combined with enhanced mechanical properties. An optimized heat treatment for a series of these compounds leads to very sharp phase transitions in bulk alloys with isothermal entropy changes of up to 38 J kg$^{-1}$ K$^{-1}$ for a magnetic field change of 2 T. The differences of as-cast and annealed samples are analyzed by investigating microstructure and phase transitions in detail by optical microscopy. We identify different grain structures as well as stoichiometric (in)homogenieties as reasons for differently sharp martensitic transitions after ideal and non-ideal annealing. We develop alloy design rules for tuning the magnetostructural phase transition and evaluate specifically the sensitivity of the transition temperature towards the externally applied magnetic fields ($\frac{dT_t}{\mu_0dH}$) by analyzing the different stoichiometries. We then set up a phase diagram including martensitic transition temperatures and austenite Curie temperatures depending on the e/a ratio for varying Co and Ti content. The evolution of the Curie temperature with changing stoichiometry is compared to other Heusler systems. Density Functional Theory calculations reveal a correlation of T$_C$ with the stoichiometry as well as with the order state of the austenite. This combined approach of experiment and theory allows for an efficient development of new systems towards promising magnetocaloric properties. Direct adiabatic temperature change measurements show here the largest change of -4 K in a magnetic field change of 1.93 T for Ni$_{35}$Co$_{15}$Mn$_{37}$Ti$_{13}$.

中文翻译：

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调整全d金属Ni-Co-Mn-Ti Heusler合金的磁热效应：结合实验和理论研究

新型 Ni-Co-Mn-Ti 全 d 金属 Heusler 合金由于具有大的多热量效应和增强的机械性能而令人兴奋。一系列这些化合物的优化热处理导致大块合金中非常急剧的相变，等温熵变化高达 38 J kg$^{-1}$ K$^{-1}$，磁场变化为2 T。通过光学显微镜详细研究微观结构和相变，分析铸态和退火样品的差异。我们将不同的晶粒结构以及化学计量（不）均匀性确定为理想和非理想退火后不同尖锐马氏体转变的原因。我们开发了用于调整磁结构相变的合金设计规则，并通过分析不同的化学计量来具体评估转变温度对外部施加的磁场 ($\frac{dT_t}{\mu_0dH}$) 的敏感性。然后我们建立了一个相图，包括马氏体转变温度和奥氏体居里温度，这取决于不同 Co 和 Ti 含量的 e/a 比。居里温度随化学计量变化的演变与其他 Heusler 系统进行了比较。密度泛函理论计算揭示了 T$_C$ 与化学计量以及奥氏体有序状态的相关性。这种实验和理论相结合的方法可以有效地开发新系统，以实现有希望的磁热特性。