掲載種別：研究論文（学術雑誌）  

In this study, we synthesized D022 Mn3−δGa (δ = 0, 1) alloys using the spark plasma sintering method. The m(T) results confirmed the formation of the D022 phase with Tc ≃ 780 K (710 K) for Mn3Ga (Mn2Ga). The alloys exhibited hard magnetic behavior with large room temperature spontaneous magnetization of ms(80 kOe) = 1.63 (0.83) μB/f.u. and coercivity of Hc = 4.28 (3.35) kOe for Mn3Ga (Mn2Ga). The magnetic properties were investigated further up to Tc and Hc (T) analysis using the Stoner–Wohlfarth model demonstrated the nucleation mechanism for the magnetization reversal. Calculations of the electronic density of states confirmed the magnetic contribution from two symmetrically inequivalent Mn sites in a Heusler-like tetragonal phase. Density functional theory calculations also showed that the ground state of D022 Mn2Ga was achieved by the removal of Mn atoms from the full Heusler Mn3Ga structure in accordance with the half-Heusler alloy structure, which supported the experimental observations.

DOI： 10.1016/j.jpcs.2021.110050

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掲載種別：研究論文（学術雑誌）  

We present the investigation of the magnetic field dependence of magneto-caloric properties in Mn5Ge3 with a near-room temperature second order phase transition (SOPT) at Tc ≃ 297.5 K. The isothermal entropy change (∣ΔSM∣) exhibits a high-field (H ≥ 3 T) power-law behavior with exponent value of 0.58 (near TC), in corroboration with critical exponents study. The maximum entropy change (∣ΔSM∣max∣) exhibits a H2∕3 dependence with addition of spatial inhomogeneity and higher order magnetization (up to M6) terms in Landau′s theory of SOPT. The nearly identical magnetic field evolution of magnetocaloric effect and a reasonably high refrigerant capacity of 398 J/kg (at 5 T) for inexpensive and abundant Mn5Ge3 make it a promising alternative to rare-earth Gadolinium for near-room temperature magnetic refrigeration technology.

DOI： 10.1016/j.jallcom.2021.159908

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掲載種別：研究論文（学術雑誌）  

The present work investigates the static and dynamic magnetization behaviour of Co-Zn nanoferrites and optimizes the magnetocrystalline anisotropy and spin relaxation time. The CoxZn1-xFe2O4 (0 ≤ x ≤ 1) magnetic nanoparticles are synthesized by pH-controlled co-precipitation method. The Le Bail structural refinement of X-ray diffraction patterns confirms the single phase formation with negligible synthesis dependent site inversion. The room temperature static dc magnetization study shows a continuous transition from hard ferrimagnetic CoFe2O4 to soft and weakly magnetic ZnFe2O4, which has been successfully explained with Yafet-Kittel model. Furthermore, approach-to-saturation analysis gives effective magnetocrystalline anisotropy of 1.45 × 104 J/m3 and lowest anisotropy field of 1.3 kOe for x = 0.4. Next, the dynamic magnetization is studied with Electron Spin Resonance spectroscopy. The lineshape analysis gives highest g-value of 3.88 and lowest spin-relaxation time (T2) of 4.86 × 10−12 s for x = 0.4, which is in agreement with static magnetization study. The optimized magnetocrystalline anisotropy and lowest spin-relaxation time for Zn0.4Co0.6Fe2O4 make it a good candidate for use in different biomedical applications.

DOI： 10.1016/j.jmmm.2019.165737

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担当区分：筆頭著者   掲載種別：研究論文（学術雑誌）  

We present the results of a detailed investigation of magnetism in spinel chromite NiCr2O4 magnetic nanoparticles (MNPs). Compared to the bulk NiCr2O4, the finite crystallite size of about 10 nm lowers the Jahn-Teller distortion and greatly enhances the collinearity of the spin structure in MNPs with considerably reduced “frustration index” = |θCW|/Tc. This leads to (longitudinal) ferrimagnetic ordering at much higher temperature, Tc≈100 K and suppression of (transverse) antiferromagnetic ordering in MNPs (cf. Tc ≃ 65 K and Ts ≃ 30 K in the bulk); a transition to the cluster spin glass state occurs at Tg = 19.0 K. Moreover, the M-H hysteresis loops show anomalous “hour-glass” behavior at fields near Hc in the vicinity of Tg; this non-monotonous Hc (T) variation can not be accounted from the celebrated Stoner-Wohlfarth model. The present study interprets the anomalous Hc (T) behavior in the framework of magnetically interacting core-shell structure with large surface anisotropy, and points out the importance of surface effects in nanochromites compared to their counterpart ferrites.

DOI： 10.1016/j.jmmm.2018.08.037

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担当区分：筆頭著者   掲載種別：研究論文（学術雑誌）  

The complex magnetic nature of 5d layered iridate Sr2IrO4, owing to IrO6 octahedral rotation has triggered great interest in recent years. In this article, we investigated the magnetic excitations in layered canted antiferromagnet Sr2IrO4 via dc magnetization measurements and report the Griffiths phase (GP) signatures above the magnetic ordering temperature TN. The non-analytic nature of GP leads to unique critical exponent, β = 0.75(1) extracted from modified Arrott plot, in corroboration with magneto-caloric study. However, the analysis by Bray model in GP regime yields a reliable critical exponent value of β = 0.18(1), belonging to two-dimensional XYh4 universality class. The study also suggests a new picture of largely debated insulating nature of Sr2IrO4 in context of GP above TN.

DOI： 10.1016/j.jmmm.2018.07.091

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掲載種別：研究論文（学術雑誌）  

The effect of partial (1–3%) rare-earth Gadolinium doping in D022 Mn3Ga hard magnet has been investigated in this work. The Mn3Ga undergoes magnetic transition at Tc ∼ 745 K, which decreases down to Tc∼715 K upon 1% Gd-doping with introduction of mixed Mn valence states, as revealed from XPS study. The hard magnetic behavior of Mn3Ga has been observed in a wide temperature range (up to Tc). More importantly, the partial Gd-doping significantly enhances the room temperature hard magnetic properties i.e. squareness ratio (mr/ms), coercivity (Hc) and energy product (BHmax) from 0.43, 2.73 kOe and 0.39 MGOe (Mn3Ga) to 0.57, 4.25 kOe and 0.5 MGOe (Mn2.97Gd0.03Ga), respectively. The modified Stoner-Wohlfarth model and low-field minor loop analysis reveal the nucleation hardening mechanism for the magnetization reversal. The study proposes the partial rare-earth doping as a new approach to enhance the hard magnetic properties of rare-earth free hard magnets.

DOI： 10.1016/j.jmmm.2018.10.031

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担当区分：筆頭著者   掲載種別：研究論文（学術雑誌）  

The discovery of magnetism-driven ferroelectricity has recently given new hope for multiferroic devices, but very low magnetic ordering temperature as well as negligibly small electric polarization in these systems are still a challenge. Here, we report a new collinear magnetic multiferroic, Nd2CoMnO6 (TC ∼ 150 K) with a spontaneous electric polarization, PS≃ 1.3 ΜC cm-2, much higher than the other known magnetic multiferroics. The high polarization value has been associated to the intrinsic ↑↑↓↓ spin ordering-driven broken symmetry and the high degree of antisite disorder present in the system, which are confirmed through dc magnetization measurements.

DOI： 10.1088/1361-6463/aa8b57

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担当区分：筆頭著者   掲載種別：研究論文（学術雑誌）  

Single-phase multiferroics exhibiting a large low-field magnetodielectric effect (MDE) are of great interest for the practical realization of multifunctional devices, as existing multiferroics demand large magnetic fields to cause any significant MDE. Here, we report remarkably large MDE ≡ δϵ /ϵ∼ 6% at very low fields (H ≤ 5 kOe) near TC in multiferroic Bi2NiMnO6 thin film. More importantly, the insensitivity of ϵ to temperature and frequency variations, and pronounced change in ϵ near TC in zero magnetic field attribute the observed large MDE to the inherent exchange magnetoelectric interactions.

DOI： 10.1088/1361-6463/aa5948

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