arXiv subjects

Takafumi D. Yamamoto

Publications and source records attributed to Takafumi D. Yamamoto.

18 recordsLinked to original sources

arXiv2026

Role of Gd 5d-Au 5d hybridization for the magnetic properties of Au-Al-Gd quasicrystal approximants

We have studied the electronic states of magnetic Au-Al-Gd quasicrystal approximants, which exhibit different magnetic ground states at low temperatures, by orbital-selective soft and hard x-ray electron spectroscopies in the paramagnetic phase. Our results reveal that the valence electronic states vary among the Au-Al-Gd approximants with different magnetic ground states, whereas the Gd 4f electrons are localized in all of them. In particular, we find that the Gd 5d states are hybridized with the Au 5d states around the Fermi level, providing information about a local pathway for the intersite exchange interaction between the localized Gd 4f electrons. This hybridization strength varies among these samples. Our findings bring an orbital-resolved microscopic picture to the magnetism of rare-earth-based quasicrystals and approximants.

cond-mat.str-el↗

arXiv2025

Highly anisotropic magnetic phase diagram of the ferromagnetic rare-earth diboride HoB$_{2}$

Rare-earth (RE) compounds have been of enormous interest in condensed matter physics as a platform for the exploration of interesting physical phenomena. Here, we report the successful single crystal growth of HoB$_{2}$, which exhibits a paramagnetic (PM)-ferromagnetic (FM) phase transition at 15 K and another phase transition at 11 K, and the discovery of a highly anisotropic magnetic phase diagram of this FM diboride. Magnetization measurements suggest that the ferromagnetically ordered moments of Ho$^{3+}$ ions are oriented at a direction tilted by 50 degrees from the ab-plane at 2K, which rotate largely toward the ab-plane direction upon application of a magnetic field, but hardly toward the c-axis direction. Heat capacity and electrical resistivity measurements clearly demonstrate that the low-temperature phase transition at 11 K occurs even under high magnetic fields along the ab-plane direction, whereas it disappears immediately by magnetic fields along the c-axis direction. Moreover, the field-induced crossover phenomenon between FM and PM phases near 15 K is found to be more promoted when applying a magnetic field along the ab-plane direction. The resulting magnetic phase diagram reveals that in-plane magnetic anisotropy is predominant in the present system, which contradicts the previous report of a spin reorientation phenomenon toward the c-axis direction between 11 K and 15 K. Taken together, the present findings suggest the presence of strong competition between in-plane and out-of-plane magnetic anisotropies in HoB$_{2}$, giving rise to the unique FM spin arrangement.

cond-mat.str-el↗

arXiv2025

Anomalous magnetic response in the Au-Al-Gd 1/1 quasicrystal approximant

The magnetic response of the Tsai-type 1/1 Au-Al-Gd approximant crystals (ACs) was quantitatively investigated in terms of the magnetic entropy change ($ΔS_{M}$) for different magnetic ground states. A comprehensive $ΔS_{M}$ map over a wide electron concentration range has been established, demonstrating the detailed variation of $ΔS_{M}$ across the entire magnetic phase diagram in the Tsai-type 1/1 ACs. Near the boundaries of the ferromagnetic (FM) phase, a clear deviation from the mean-field theory (MFT) was observed in both the Curie temperature ($T_{C}$) and magnetic field ($H$) dependences of the maximum magnetic entropy change ($ΔS_{M}^{max}$). Contrary to general expectations, a high $ΔS_{M}^{max}$ (7.2 J/K mol-Gd under a 5 T field variation), even comparable to those of candidate materials for low-temperature magnetic refrigeration, was obtained within the antiferromagnetic (AFM) region near the FM / AFM phase boundary. The unexpected enhancement of $ΔS_{M}^{max}$ toward the AFM region under high magnetic fields indicates an anomalous magnetic response in the present Tsai-type 1/1 AC, which is presumably associated with the breakdown of the MFT. The present finding suggests that tuning the magnetic ground state across the phase boundary is an effective strategy to enhance $ΔS_{M}$, even in general rare-earth intermetallic compounds.

cond-mat.mtrl-sci↗

arXiv2023

Critical behavior and magnetocaloric effect in Tsai-type 2/1 and 1/1 quasicrystal approximants

Stable Tsai-type quinary 1/1 and 2/1 approximant crystals (ACs) with chemical compositions Au56.25Al10Cu7In13Tb13.75 and Au55.5Al10Cu7In13Tb14.5, respectively, exhibiting ferromagnetic (FM) long-range orders were successfully synthesized and studied for their magnetic properties and magnetocaloric effect. The 1/1 and 2/1 ACs primarily differ in their long-range atomic arrangement and rare earth (RE) distribution, with the latter approaching quasiperiodic order while still preserving periodicity. Analyses based on the scaling principle and Kouvel-Fisher (KF) relations suggested mean-field-like behavior near Curie temperatures in both compounds. From magnetization measurements and the Maxwell equation, a magnetic entropy change of -4.3 and -4.1 J/K mol Tb were derived under a magnetic field change of 7 T for the 1/1 and 2/1 ACs, respectively. The results indicated a prominent role of intra-cluster magnetic interactions on critical behavior and magnetic entropy of the Tsai-type compounds.

cond-mat.str-el↗

arXiv2022

Tunable magnetic and magnetocaloric properties by thermal annealing in ErCo2 atomized particles

Processing magnetocaloric materials into magnetic refrigerants with appropriate shapes is essential for the development of magnetic refrigeration systems. In this context, the impact of processing on the physical properties of magnetocaloric materials is one of the important issues. Here, we investigate the crystallographic, magnetic, and magnetocaloric properties of gas-atomized particles of the intermetallic compound ErCo$_{2}$, a giant magnetocaloric material for low-temperature applications. The results demonstrate that the physical properties of ErCo$_{2}$ are significantly changed by atomization and subsequent thermal annealing. In the as-atomized particles, the magnetic transition temperature increases from 34 to 56 K and the phase transition changes from first order to second order. The thermal annealing shifts the transition temperature back to the original one and restores the first-order phase transition characteristic. The changes in magnetic properties are closely related to those in crystallographic properties, suggesting the importance of the magneto-structural coupling. The magnetic entropy change $-ΔS_{M}$ of the particles can be tuned in size, shape, and peak temperature depending on the annealing conditions. The peak value of $-ΔS_{M}$ varies in the range of 9--33 J kg$^{-1}$ K$^{-1}$ for a magnetic field change of 0--5 T. All the ErCo$_{2}$ atomized particles have magnetocaloric properties comparable or superior to other promising candidates for low-temperature magnetic refrigerants.

cond-mat.mtrl-sci↗

arXiv2022

Magnetic, thermal, and magnetocaloric properties of the holmium trialuminide HoAl$_{3}$ with polytypic phases

We investigate the magnetic, thermal, and magnetocaloric properties of the intermetallic HoAl$_{3}$ compounds with two different crystal structures. The room-temperature equilibrium trigonal phase HoAl$_{3}$ undergoes an antiferromagnetic (AFM) transition at the Neel temperature $T^{\rm tri}_{\rm N} =$ 9.8 K. The AFM ordering of the compound is strong against the magnetic field, so that inverse magnetocaloric effect (MCE) is found even under a magnetic field change of 0-50 kOe. The high-pressure cubic phase HoAl3, prepared by a rapid-solidification process, is antiferromagnetically ordered below $T^{\rm cub}_{\rm N} =$ 15 K. Magnetic and specific heat measurements reveal that this long-range AFM state becomes unstable as the temperature drops and then it is replaced by a magnetic glassy state below a spin freezing temperature $T_{\rm f} =$ 11 K. The successive changes in magnetic state result in complicated temperature and field dependence of the MCE at low temperatures.

cond-mat.mtrl-sci↗

arXiv2022

Effect of Non-stoichiometry on Magnetocaloric Properties of HoB2 Gas-Atomized Particles

We fabricate gas-atomized particles by inductively melting electrode rods of HoB2-x (x = -0.3, 0, 0.3, and 1.0) and investigate the effect of non-stoichiometry on the phase fraction, microstructure, and physical properties. Shifting the stoichiometric ratio of the electrode rod to the B (Ho)-rich side increases HoB4 (Ho) phase in the resulting atomized particles. Even if the atomized particles contain 15-20 weight percent (wt.%) of the impurity phase, the influence of which on the physical properties is less severe: the maximum value of the magnetic entropy change is only reduced by 10% compared to HoB2.0 particles. We further find that the ductile Ho phase exists so as to fill the space between the brittle HoB2 phases in the atomized particles, which may be beneficial to the mechanical properties of the particles. Our findings suggest that it would be better to use the Ho-rich electrode rods than the stoichiometric ones to produce HoB2-x particles with more suitable properties as a magnetic refrigerant for magnetic refrigeration systems.

cond-mat.mtrl-sci↗

arXiv2022

Magnetocaloric particles of the Laves phase compound HoAl2 prepared by electrode induction melting gas atomization

Processing magnetocaloric materials into magnetic refrigerant particles is an essential issue in developing high-performance magnetic refrigerators. Here, we succeed in stably producing magnetocaloric particles of the promising material HoAl2 by a newly devised method based on electrode induction melting gas atomization process. The particle size range is on the order of submillimeter, which is suitable for practical refrigeration systems. The resulting particles with less contamination have good morphological, magnetic, and magnetocaloric properties: (i) almost spherical shapes with few internal pores, (ii) a sharp ferromagnetic transition around 30 K, and (iii) a large magnetocaloric effect comparable to the bulk counterpart. These features suggest the HoAl$_{2}$ gas-atomized particles have the potential of use as a magnetic refrigerant. The presented method can be applied not only to HoAl2 but also to other brittle magnetocaloric materials with high melting points, facilitating the production of various magnetic refrigerants needed to develop magnetic refrigerators for hydrogen liquefaction.

cond-mat.mtrl-sci↗

arXiv2022

Gas-atomized particles of giant magnetocaloric compound HoB2 for magnetic hydrogen liquefiers

The processing of promising magnetocaloric materials into spheres is one of the important issues on developing high-performance magnetic refrigeration systems. In the present study, we achieved in producing spherical particles of a giant magnetocaloric compound HoB2 by a crucible-free gas atomization process, despite its high melting point of 2350 C. The particle size distribution ranges from 100 to 710 micrometers centered at 212-355 micrometers with the highest yield of 14-20wt% of total melted electrode, which is suitable for magnetic refrigeration systems. The majority of the resulting particles are mostly spherical with no contamination during the processing, while unique microstructures are observed on the surface and inside. These spherical particles exhibit sharp magnetic transitions and huge magnetic entropy change of 0.34 J cm^{-3} K^{-1} for a magnetic field change of 5 T at 15.5 K. The high sphericality and the high magnetocaloric performance suggest that the HoB2 gas-atomized particles have good potential as magnetic refrigerants for use in magnetic refrigerators for hydrogen liquefaction.

cond-mat.mtrl-sci↗

arXiv2022

Adiabatic temperature change in ErAl2/metal PIT wires: A practical method for estimating the magnetocaloric response of magnetocaloric composites

We report the adiabatic temperature change in ErAl2 magnetocaloric wires fabricated by a powder-in-tube (PIT) process. The adiabatic temperature change of the PIT wires is found to be determined by not only the volume fraction of ErAl2 core but also the magnitude relationship between the specific heat of the ErAl2 core and the metal sheath. We propose a quantitative analysis method for calculating the temperature and core volume fraction dependence of adiabatic temperature change in the PIT wire, whose formula is applicable to also various magnetocaloric composites, useful to estimate the magnetocaloric response prior to fabrication.

cond-mat.mtrl-sci↗

arXiv2022

High-pressure synthesis of superconducting Sn$_{3}$S$_{4}$ using diamond anvil cell with boron-doped diamond heater

High-pressure techniques open exploration of functional materials in broad research fields. An established diamond anvil cell with a boron-doped diamond heater and transport measurement terminals has performed the high-pressure synthesis of a cubic Sn$_{3}$S$_{4}$ superconductor. X-ray diffraction and Raman spectroscopy reveal that the Sn$_{3}$S$_{4}$ phase is stable in the pressure range of P>5 GPa in a decompression process. Transport measurement terminals in the diamond anvil cell detect a metallic nature and superconductivity in the synthesized Sn$_{3}$S$_{4}$ with a maximum onset transition temperature of 13.3 K at 5.6 GPa. The observed pressure-T$_{c}$ relationship is consistent with that from the first-principles calculation. The observation of superconductivity in Sn$_{3}$S$_{4}$ opens further materials exploration under high temperature and pressure conditions.

cond-mat.supr-con↗

arXiv2020

Effect of Dy substitution in the giant magnetocaloric properties of HoB$_{2}$

Recently, a massive magnetocaloric effect near the liquefaction temperature of hydrogen has been reported in the ferromagnetic material HoB$_{2}$. Here we investigate the effects of Dy substitution in the magnetocaloric properties of Ho$_{1-x}$Dy$_{x}$B$_{2}$ alloys ($\textit{x}$ = 0, 0.3, 0.5, 0.7, 1.0). We find that the Curie temperature ($\textit{T}$$_{C}$) gradually increases upon Dy substitution, while the magnitude of the magnetic entropy change |$Δ\textit{S}_{M}$| at $\textit{T}$ = $\textit{T}_{C}$ decreases from 0.35 to 0.15 J cm$^{-3}$ K$^{-1}$ for a field change of 5 T. Due to the presence of two magnetic transitions in these alloys, despite the change in the peak magnitude of |$Δ\textit{S}_{M}$|, the refrigerant capacity ($\textit{RC}$) and refrigerant cooling power ($\textit{RCP}$) remains almost constant in all doping range, which as large as 5.5 J cm$^{-3}$ and 7.0 J cm$^{-3}$ for a field change of 5 T. These results imply that this series of alloys could be an exciting candidate for magnetic refrigeration in the temperature range between 10-50 K.

cond-mat.mtrl-sci↗

arXiv2020

Demonstration of Electric Double Layer Gating under High Pressure by the Development of Field-Effect Diamond Anvil Cell

We have developed an approach to control the carrier density in various material under high pressure by the combination of an electric double layer transistor (EDLT) with a diamond anvil cell (DAC). In this study, this EDLT-DAC was applied to a Bi thin film, and here we report the field-effect under high pressure in the material. Our EDLT-DAC is a promising device for exploring unknown physical phenomena such as high transition-temperature superconductivity (HTS).

cond-mat.mtrl-sci↗

arXiv2020

Magnetic entropy change of ErAl2 magnetocaloric wires fabricated by a powder-in-tube method

We report the fabrication of ErAl2 magnetocaloric wires by a powder-in-tube method (PIT) and the evaluation of magnetic entropy change through magnetization measurements. The magnetic entropy change of ErAl2 PIT wires exhibits similar behavior to the bulk counterpart, while its magnitude is reduced by the decrease in the volume fraction of ErAl2 due to the surrounding non-magnetic sheaths. We find that another effect reduces the magnetic entropy change of the ErAl2 PIT wires around the Curie temperature, and discuss its possible origin in terms of a correlation between magnetic properties of ErAl2 and mechanical properties of sheath material.

cond-mat.mtrl-sci↗

arXiv2018

Dynamical coupling of dilute magnetic impurities with quantum spin liquid state in the S = 3/2 dimer compound Ba3ZnRu2O9

We have investigated the dilute magnetic impurity effect on the magnetic properties of a quantum spin liquid candidate Ba3ZnRu2O9 and a spin gapped compound Ba3CaRu2O9. The magnetic ground state of each compound stands against 2% substitution of magnetic impurities for Zn or Ca. We have found that the magnetic response of these impurities, which behave as paramagnetic spins, depends on the host materials and the difference of the two manifests itself in the Weiss temperature, which can hardly be explained by the dilute magnetic impurities alone in the case of Ba3ZnRu2O9. We consider a contribution from the Ru5+ ions which would appear only in the substituted Ba3ZnRu2O9 and discuss a possible physical meaning of the observed Weiss temperature.

cond-mat.str-el↗

arXiv2017

Magneto-thermopower in the Weak Ferromagnetic Oxide CaRu0.8Sc0.2O3: An Experimental Test for the Kelvin Formula in a Magnetic Material

We have measured the resistivity, the thermopower, and the specific heat of the weak ferromagnetic oxide CaRu0.8Sc0.2O3 in external magnetic fields up to 140 kOe below 80 K. We have observed that the thermopower Q is significantly suppressed by magnetic fields at around the ferromagnetic transition temperature of 30 K, and have further found that the magneto-thermopower ΔQ(H, T) = Q(H, T) - Q(0, T) is roughly proportional to the magneto-entropy ΔS(H, T) = S(H, T)-S(0, T).We discuss this relationship between the two quantities in terms of the Kelvin formula, and find that the observed ΔQ is quantitatively consistent with the values expected from the Kelvin formula, a possible physical meaning of which is discussed.

cond-mat.str-el↗

arXiv2016

Ferromagnetic Cluster Glass Phase Embedded in a Paramagnetic and Metallic Host in Non-uniform Magnetic System CaRu_{1-x}Sc_{x}O_{3}

We have investigated both static and dynamic magnetic properties of polycrystalline CaRu$_{1-x}$Sc$_{x}$O$_{3}$ system in order to clarify the role of Sc ions as a disorder for magnetic ordering. We have observed typical features of a ferromagnetic cluster glass state below around 40 K: (i) a broad, frequency-dependent peak in the ac magnetic susceptibility, (ii) a slow relaxation of the magnetization, and (iii) a continuous increase in the dc magnetic susceptibility in field cooling process. The composition dependence of characteristic parameters for the cluster glass state suggests that chemical segregation can hardly explain the clustering mechanism. We propose a possible picture that the ferromagnetic clusters are distributed uniformly and form the glassy state embedded in the paramagnetic and metallic host of CaRuO$_{3}$.

cond-mat.str-el↗

arXiv2014

Non-uniform magnetic system driven by non-magnetic ion substitution in CaRu1-xScxO3:Two-component analysis

We have studied magnetic and transport properties in polycrystalline CaRu1-xScxO3 for 0 =< x =< 0.20 in order to clarify the substitution effects of a non-magnetic trivalent ion. We find that a ferromagnetic transition with Tc = 30 K is observed in Sc-substituted samples. The composition dependence of the Curie-Weiss temperature implies that the magnetic susceptibility has a paramagnetic contribution with negative theta and a ferromagnetic contribution with positive theta. The field dependence of magnetization at 2 K is also understood as a summation of the ferromagnetic and paramagnetic components. These results suggest that CaRu1-xScxO3 is a non-uniform magnetic system. The relationship between the ferromagnetic ordering and the transport properties is also discussed.

cond-mat.str-el↗