Current magnetocaloric materials exhibit large adiabatic changes in temperature (ΔTad~10 °K) in applied magnetic fields (H) with the strength in several tens of kilo-oersted (kOe). To achieve an appreciable magnetocaloric effect (MCE) with moderate H, new magnetic materials with tailored magnetocaloric properties must be synthesized [1]. We explore the MCE in a layered system composed of a ferromagnet, NixCu1-x (x~0.7), with a relatively low Curie temperature (TC~300 °K), which is sandwiched by stronger ferromagnets, Ni80Fe20 and Co90Fe10, and exchange coupled to them across the interfaces. Switching of magnetization in the softer ferromagnetic surrounding, Ni80Fe20, exposed to H~0.1 kOe provides the isothermal magnetic entropy change (magnetocaloric potential) in the system because of magnetization redistribution in the spacer. In the limit, in which the NixCu1-x spacer can be magnetized at the interfaces up to its saturation magnetization of 4πms≈2.5 kG, the magnetocaloric potential is [2] Δs=ms2/C≈105 erg/cm3/°K (1) with C=0.4 °K being the Curie constant, whereas a separate NixCu1-x film with the same composition x exhibits such an MCE at H~100 kOe only. In real stacks with the spacer thickness ranging between 5 nm and 20 nm, we have found a significant (up to a factor of ~20) enhancement of MCE by comparison to a separate NixCu1-x film [3, 4]. However, the MCE obtained experimentally is still much lower than that expected in accordance with Eq. (1). To understand a reason for this inconsistency, we have performed simulations of the magnetization distributon across the spacer upon the basis of the phenomenological Landau theory for second-order phase transitions [4]. We find that the results of these simulations are compatible with the experimental data at realistic values of the interfacial exchange coupling constant.To further enhance the MCE in such systems, one should choose a material for the spacer, which would have a higher ms but still a low TC not far from room temperatures. This work was supported by the Russian Foundation for Basic Research (Grant No. 20-02-00356) and by the Ministry of Education and Science of Russian Federation (Grant No. 0035-2019-0022-C-01).References: [1] T. Mukherjee, et al., Magnetocaloric properties of Co/Cr superlattices, Phys. Rev. B, Vol. 79, 144406 (2009) [2] A. A. Fraerman, I. A. Shereshevskii, Magnetocaloric effect in ferromagnet/ paramagnet multilayer structures, JETP Lett., Vol. 101, p.618 (2015) [3] S.N. Vdovichev, et al., High magnetocaloric efficiency of a NiFe/NiCu/CoFe/MnIr multilayer in a small magnetic field, Phys. Rev. B, Vol. 98, 014428 (2018) [4] M.A. Kuznetsov, et al., Magnetocaloric effect in exchange-coupled strong/weak/ strong ferromagnet stacks, J. Appl. Phys., Vol. 127, 183904 (2020)

Effect of magnetic proximity on magnetocaloric properties of Ni80Fe20/NixCu1-x/Co90Fe10 stacks

Today magnetic recording is still the leading technology for mass data storage with data densities beyond 1 Tbit/in2. Pushing the recording density in the terabit regime requires new storage materials, novel recording schemes, and media designs. We propose an approach to magnetic recording upon the basis of thermally induced chemical order-disorder transitions (CODTs) in alloys of transition metals, which exhibit disorder-induced ferromagnetism. [1,2] Here we report on our simulations of CODTs and demonstrate that nanosecond laser irradiation concentrated within a nanoscale spot on the sample surface is able to induce the reversible transitions in Fe-rich FexAl1-x alloys. Figure 1 shows a design of magnetic recording media which can be realized upon that basis by using near-field optical effects and special tools, e.g., a near-field transducer (NFT) [3]. It is crucial for this approach that the bits can be written as erasable nanoscale entities in which the magnitude of magnetization differs from that of encompassing matrix. Therefore, contrary to the existing magnetic memories, the information here can be encoded in not the polarity of magnetization but in its magnitude. As a result, difficult problems in current magnetic recording technologies, e.g., thermal upsets of magnetization and cross-talks between adjacent bits, can be circumvented by using the approach we propose. This work was supported by RFBR (grant #18-02-00827_a), DFG through the SFB 767 at the University of Konstanz, by NECL and FCT support through Project Nos. NORTE-01-0145-FEDER-022096, PTDC/FIS-MAC/31302/2017, and by EXPL/IF/00541/2015, and by NSF (project TG-DMR110090).References: [1] J. Ehrler, M. He, M.V. Shugaev, et al., Laser-rewriteable ferromagnetism at thin-film surfaces. ACS Appl. Mater. Interfaces, Vol.10 (17), p.15232 (2018) [2] N. I. Polushkin, V. Oliveira, R. Vilar, et al., Phase-change magnetic memory: rewritable ferromagnetism by laser quenching of chemical disorder in Fe60Al40 alloy, Phys. Rev. Appl., Vol.10 (2), 024023 (2018) [3] W. Challener, C. Peng, A. Itagi, et al., Heat-assisted magnetic recording by a near-field transducer with efficient optical energy transfer. Nat. Photon., Vol. 3 (4), p. 220 (2009)

Chemical order-disorder transitions in intermetallic alloys for high-density magnetic recording

A4: Magneto-Elastic and Magneto-Caloric Materials I

technical paper

The 65th Annual Conference on Magnetism and Magnetic Materials (MMM 2020) was held in a virtual form. The COVID-19 pandemic, with its associated uncertainty about international travel and the ability to gather in large groups, meant that the Steering Committee decided to cancel the face-to-face meeting that had been planned in Palm Beach, Florida November 2-6, 2020.

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Nikolay Polushkin

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Effect of magnetic proximity on magnetocaloric properties of Ni80Fe20/NixCu1-x/Co90Fe10 stacks

Chemical order-disorder transitions in intermetallic alloys for high-density magnetic recording

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