Here we report a systematic research on effects of Fe and Cu upon properties relevant for the magnetic shape memory effect of Ni–Mn–Ga ferromagnetic shape memory alloys. Fe and Cu were identified as elements with potential synergism to increase the martensite transformation temperature of Ni–Mn–Ga magnetic shape memory
(MSM) alloys. Eighteen Ni–Mn–Ga–Fe–Cu alloys with different systematic trends in substituting the ternary elements with Cu and Fe have been investigated. We found a
method to describe the effectiveness of Ni, Mn, and Cu upon raising the martensitic transformation temperature, lowering the saturation magnetization, and varying the
Curie temperature. We find the martensite transformation temperature most influenced by the Ni content, followed by Mn, with a smaller effect of Cu. The saturation magnetization decreases with similar coefficients for Mn and Cu alloying. The Curie temperature monotonously decreases with Mn, but not Cu. The 10M martensite structure is stable for the composition Ni46.5Mn25?XGa25-X-YFe3.5CuY with X and Y range of 0–5.7, and 0.8–3.0. Used in combination with the total e/a, the elemental e/a-ratio gives
some insight into the complex behavior of quinary MSM alloys and is a useful method of analyzing MSM alloys for improved functional properties.
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Magnetization–structure–composition phase diagram mapping in Co-Fe-Ni alloys using diffusion multiples and scanning Hall probe microscopy
Transition metal alloys are essential for magnetic recording, memory, and new materials-by-design applications. Saturation magnetization in these alloys have previously been measured by conventional techniques, for a limited number of samples with discrete compositions, a laborious and time-consuming effort. Here, we propose a method to construct complete saturation magnetization diagrams for Co–Fe–Ni alloys using scanning Hall probe microscopy (SHPM). A composition gradient was created by the diffusion multiple technique, generating a full combinatorial materials library with an identical thermal history. The composition and crystallographic phases of the alloys were identified by integrated energy dispersive X-ray spectroscopy and electron backscatter diffraction. “Pixel-by-pixel” perpendicular components of the magnetic field were converted into maps of saturation magnetization using the inversion matrix technique. The saturation magnetization dependence for the binary alloys was consistent with the Slater-Pauling behavior. By using a significantly denser data point distribution than previously available, the maximum of the Slater-Pauling curve for the Co–Fe alloys was identified at ~ 32 at% of Co. By mapping the entire ternary diagram of Co–Fe–Ni alloys recorded in a single experiment, we have demonstrated that SHPM—in concert with the combinatorial approach—is a powerful high-throughput characterization tool, providing an effective metrology platform to advance the search for new magnetic materials.
more » « less- Award ID(s):
- 2004979
- NSF-PAR ID:
- 10362311
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Scientific Reports
- Volume:
- 12
- Issue:
- 1
- ISSN:
- 2045-2322
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Soft magnetic high‐entropy alloy thin films (HEATFs) exhibit remarkable freedom of material‐structure design and physical‐property tailoring, as well as, high cut‐off frequencies and outstanding electrical resistivities, making them potential candidates for high‐frequency magnetic devices. In this study, a CoCrFeNi film with excellent soft magnetic properties is developed by forming a novel core–shell structure via native oxidation, with ferromagnetic elements Fe, Co, and Ni as the core and the Cr oxide as the shell layer. The core–shell structure enables a high saturation magnetization, enhances the electrical resistivity, and thus reduces the eddy‐current loss. For further optimizing the soft magnetic properties, O is deliberately introduced into the HEATFs, and the O‐incorporated HEATFs exhibit an electrical resistivity of 237 µΩ·cm, a saturation magnetization of 535 emu cm−3, and a coercivity of 23 A m−1. The factors that determine the ferromagnetism and coercivity of the CoCrFeNi‐based HEATFs are examined in detail by evaluating the microstructures, magnetic domains, chemical valency, and 3D microscopic compositional distributions of the prepared films. These results are anticipated to provide insights into the magnetic behaviors of soft magnetic HEATFs, as well as aid in the construction of a promising material‐design strategy for these unique materials.
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Bimagnetic nanoparticles show promise for applications in energy efficient magnetic storage media and magnetic device applications. The magnetic properties, including the exchange bias of nanostructured materials can be tuned by variation of the size, composition, and morphology of the core vs overlayer of the nanoparticles (NPs). The purpose of this study is to investigate the optimal synthesis routes, structure and magnetic properties of novel CoO/NiFe 2 O 4 heterostructured nanocrystals (HNCs). In this work, we aim to examine how the size impacts the exchange bias, coercivity and other magnetic properties of the CoO/NiFe 2 O 4 HNCs. The nanoparticles with sizes ranging from 10 nm to 24 nm were formed by synthesis of an antiferromagnetic (AFM) CoO core and deposition of a ferrimagnetic (FiM) NiFe 2 O 4 overlayer. A highly crystalline magnetic phase is more likely to occur when the morphology of the core-overgrowth is present, which enhances the coupling at the AFM-FiM interface. The CoO core NPs are prepared using thermal decomposition of Co(OH) 2 at 600 °C for 2 hours in a pure argon atmosphere, whereas the HNCs are obtained first using thermal evaporation followed by hydrothermal synthesis. The structural and morphological characterization made using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), and scanning electron microscopy (SEM) techniques verifies that the HNCs are comprised of a CoO core and a NiFe 2 O 4 overgrowth phase. Rietveld refinement of the XRD data shows that the CoO core has the rocksalt (Fd3 m) crystal structure and the NiFe 2 O 4 overgrowth has the spinel (C12/m1) crystal structure. SEM-EDS data indicates the presence and uniform distribution of Co, Ni and Fe in the HNCs. The results from PPMS magnetization measurements of the CoO/NiFe 2 O 4 HNCs are discussed herein.more » « less
