arXiv subjects

Hiroyuki Nakamura

Publications and source records attributed to Hiroyuki Nakamura.

18 recordsLinked to original sources

arXiv2024

Unusual strong ferromagnetism in site-ordered cubic Laves phase compound LuInCo$_4$ with Co-pyrochlore lattice

We have successfully synthesized single crystals of the site-ordered cubic (C15b) Laves phase compound LuInCo$_4$ with the Co-pyrochlore sublattice for the first time. LuInCo$_4$ undergoes a ferromagnetic transition at 306 K and has a saturation moment of 3.43 $μ_{\rm{B}}$/f.u. at 5 K. The strongly ferromagnetic nature was verified by DFT calculations, suggesting that Co-3$d$ flat bands near the Fermi level induce the spin polarization. The magnetization is isotropic above $\sim$100 K, saturates most easily in the [100] direction at low temperatures. In this anisotropic ferromagnetic state, the magnetization undergoes a metamagnetic transition in the [111] direction. Our results suggest that LuInCo$_4$ is a new strong but unusual itinerant electron ferromagnet, which deserves further study as a pyrochlore metal.

cond-mat.str-el↗

arXiv2024

Competing magnetic correlations and uniaxial anisotropy in (Fe$_{1-x}$Mn$_{x}$)$_2$AlB$_2$ single crystals

We have succeeded for the first time in synthesizing single crystals of nanolaminated borides (Fe$_{1-x}$Mn$_{x}$)$_2$AlB$_2$ in the entire Fe-Mn composition range using the Al self-flux method, and have established $T$-$x$, $H$-$T$ and three-dimensional $H$-$T$-$x$ magnetic phase diagrams from the results of magnetization measurements. The ferromagnetic correlation of Fe$_2$AlB$_2$ is weakened with the Mn substitution, whereas the antiferromagnetic correlation of Mn$_2$AlB$_2$ is enhanced with the Fe up to $x=0.65$. The spin direction in the magnetic ordered states changes from the $a$ to the $b$ axis with increasing Mn concentration and temperature. At $x$ = 0.31-0.46, there are three magnetic phases; ferromagnetic, antiferromganetic, and intermediate phases in between. At $x$ = 0.65 and 0.74, a spin-flop-like metamagnetic transition was observed at a finite field parallel to the spin direction. These observations indicate that in (Fe$_{1-x}$Mn$_{x}$)$_2$AlB$_2$ the ferromagnetic and antiferromagnetic correlations coexist and the uniaxial magnetic anisotropy competes between the $a$ and $b$ axes.

cond-mat.mtrl-sci↗

arXiv2024

Site-selective cobalt substitution in La-Co co-substituted magnetoplumbite-type ferrites: $^{59}$Co-NMR and DFT calculation study

The La-Co co-substituted magnetoplumbite-type (M-type) ferrites $A$Fe$_{12}$O$_{19}$ ($A$ = Ca, Sr and Ba, ion sizes Ca$^{2+}$ $<$ Sr$^{2+}$ $<$ Ba$^{2+}$) with Co compositions around 0.2 have been subjected to $^{59}$Co-NMR. The results show that Co occupies the 4f$_1$, 2a and 12k sites, and that the smaller the $A$ ion, the more Co tends to occupy the 4f$_1$ minority spin site, which is effective in enhancing both uniaxial anisotropy and magnetisation. First-principles total energy calculations based on density functional theory (DFT) of undoped $A$Fe$_{12}$O$_{19}$ and a supercell ($2 \times 2 \times 1$ of the unit cell) in which 1/96 of Fe$^{3+}$ is replaced by Co$^{2+}$ were performed to predict the stable structure and Co occupancy sites. The results show that regardless of $A$, Co is most stable when it occupies the 4f$_1$ site, followed by the 2a and 12k sites with energy differences on the order of 100 meV, and Co practically does not occupy the 2b and 4f$_2$ sites. As the $A$ ion becomes smaller, the energy difference when Co occupies each Fe site tends to increase, and the Co occupancy of the 4f$_1$ site also increases. The site selectivity of Co can be roughly explained as a result of the difference in uniaxial strain along the $c$-axis associated with the difference in $A$. However, the influence of the $A$ ion differs between the R and S blocks and the local strain also has a secondary effect on the Co distribution. Based on these results, the guidelines for improving the performance (anisotropy and magnetisation) of La-Co co-substituted M-type ferrite magnets with a limited amount of Co can be summarised as follows: It is effective to select as small $A$ ions as possible and to post-anneal at low temperature or cool slowly to concentrate Co at the 4f$_1$ site in tetrahedral coordination.

cond-mat.mtrl-sci↗

arXiv2023

Novel slow dynamics of phase transition in the partially ordered frustrated magnet DyRu2Si2

DyRu2Si2 is a frustrated magnet to exhibit multiple magnetic phase transition in zero and finite magnetic fields. We investigated and characterized the phase transition between the partially-ordered antiferromagnetic phases at zero field by ac susceptibility measurements. Detailed ac susceptibility measurements reveal the novel critical dynamics of the phase transition; extremely slow dynamics with the relaxation time in the order of 10-100 msec, speed-up of the dynamics on cooling indicating its non-thermally activated origin and growing of the ferromagnetic correlations towards the phase transition temperature. On the basis of these findings, we propose the novel phase transition process, namely, the spontaneous striped-arrangement of the precedently emergent "belt-like" ferromagnetic spin texture.

cond-mat.str-el↗

arXiv2023

Doping Dependence of the in-Plane Transition in Co$_3$Sn$_2$S$_2$

In Co$_3$Sn$_2$S$_2$ two transitions are observed, the main one to a ferromagnetic state at $T_C = 174$ K and the second one, involving in-plane components at $T_P = 127$ K. We follow their doping dependence as Sn is replaced with In, which causes a reduction of $T_C$ and $T_P$. Importantly, both transitions follow the same doping dependence, indicating a single energy scale involved with both processes.

cond-mat.str-el↗

arXiv2023

Structural, Optical and Single-domain Magnetic Features of the Noncollinear Ferrimagnetic Nano-spinel Chromites ACr$_2$O$_4$ (A = Ni, Co, and Mn)

Spinel chromites ACr$_2$O$_4$ with inherent magnetic geometrical frustration usually exhibit a noncollinear ferrimagnetic ground state when A are magnetic ions, with possibly crystallite-size dependent intriguing magnetic features. Here, we report single-domain magnetic properties of ACr$_2$O$_4$ (A = Ni, Co, and Mn) nanocrystals, with an average crystallite size of 18, 15 and 10 nm, exhibiting an optical energy gap of 2.87, 3.05 and 2.9 eV, respectively. The temperature dependence of magnetization indicates the main bulk magnetic transitions with a commonly coexisting spin-glass-like state and finite-size effects on the noncolinear ferrimagnetic transitions. An anomaly observed at Ts = 15, 24 and 10 K is attributed to the bulk magnetic transition to a canted antiferromagnetic state in NiCr$_2$O$_4$ and incommensurate spiral orders in CoCr$_2$O$_4$ and MnCr$_2$O$_4$ NCs, respectively. A further bulk magnetic transition to a commensurate spiral order is observed for CoCr$_2$O$_4$ NCs at a lock-in temperature Tl = 5 K much lower than that reported using bulk samples, while it is completely suppressed in the MnCr$_2$O$_4$ NCs. Finite-size effects and single-domain magnetic behaviors indicated by anomalous temperature-dependences of the coercive field and the hysteresis-loop squareness, mainly driven by a magnetocrystalline anisotropy, are discussed in comparison to results reported using bulk counterparts.

cond-mat.mtrl-sci↗

arXiv2023

Emergent SU(3) magnons and thermal Hall effect in the antiferromagnetic skyrmion lattice

Complexity of quantum phases of matter is often understood by the underlying gauge structures, as was recognized by the $\mathbb{Z}_2$ and U(1) gauge theory description of spin liquid in frustrated magnets. Anomalous Hall effect of conducting electrons can intrisically arise from U(1) gauges expressing the spatial modulation of ferromagnetic moments or from SU(2) gauges representing the spin-orbit coupling effect. Similarly, in insulating ferro and antiferromagnets, the magnon excitations can contribute to anomalous transports by feeling the U(1) and SU(2) gauges arising from the features of ordered moments or interactions. In this work, we report the emergent higher rank SU(3) gauge structure in the magnon transport based on the thermal conductivity measurements of MnSc$_2$S$_4$ in an applied field up to 14\,T. The thermal Hall coefficient takes a substantial value when the material enters a three-sublattice antiferromagnetic skyrmion phase, which is confirmed by the large-scale spin wave theory. The excited magnons are dressed with SU(3) gauge field, which is a mixture of three species of U(1) gauge fields originating from the slowly varying magnetic moments on these sublattices.

cond-mat.str-el↗

arXiv2022

Terahertz spin-to-charge current conversion in stacks of ferromagnets and the transition-metal dichalcogenide NbSe$_2$

Transition-metal dichalcogenides (TMDCs) are an aspiring class of materials with unique electronic and optical properties and potential applications in spin-based electronics. Here, we use terahertz emission spectroscopy to study spin-to-charge current conversion (S2C) in the TMDC NbSe$_2$ in ultra-high-vacuum-grown F|NbSe$_2$ thin-film stacks, where F is a layer of ferromagnetic Fe or Ni. Ultrafast laser excitation triggers an ultrafast spin current that is converted into an in-plane charge current and, thus, a measurable THz electromagnetic pulse. The THz signal amplitude as a function of the NbSe$_2$ thickness shows that the measured signals are fully consistent with an ultrafast optically driven injection of an in-plane-polarized spin current into NbSe$_2$. Modeling of the spin-current dynamics reveals that a sizable fraction of the total S2C originates from the bulk of NbSe$_2$ with the same, negative, sign as the spin Hall angle of pure Nb. By quantitative comparison of the emitted THz radiation from F|NbSe$_2$ to F|Pt reference samples and the results of ab-initio calculations, we estimate that the spin Hall angle of NbSe$_2$ for an in-plane polarized spin current lies between -0.2% and -1.1%, while the THz spin-current relaxation length is of the order of a few nanometers.

cond-mat.mes-hall↗

arXiv2021

Squeezing Néel-type Magnetic Modulations by Enhanced Dzyaloshinskii-Moriya interaction of $4d$ Electrons

In polar magnets, such as GaV$_4$S$_8$, GaV$_4$Se$_8$ and VOSe$_2$O$_5$, modulated magnetic phases namely the cycloidal and the Néel-type skyrmion lattice states were identified over extended temperature ranges, even down to zero Kelvin. Our combined small-angle neutron scattering and magnetization study shows the robustness of the Néel-type magnetic modulations also against magnetic fields up to 2 T in the polar GaMo$_4$S$_8$. In addition to the large upper critical field, enhanced spin-orbit coupling produces a variety of modulated phases with sub-10 nm periodicity and a peculiar distribution of the magnetic modulation vectors. Thus, our work demonstrates that non-centrosymmetric magnets with $4d$ and $5d$ electron systems are ideal candidates to host highly compressed magnetic spirals and skyrmions.

cond-mat.str-el↗

arXiv2021

Architecture of nanoscale ferroelectric domains in GaMo4S8

Local-probe imaging of the ferroelectric domain structure and auxiliary bulk pyroelectric measurements were conducted at low temperatures with the aim to clarify the essential aspects of the orbitally driven phase transition in GaMo4S8, a lacunar spinel crystal that can be viewed as a spin-hole analogue of its GaV4S8 counterpart. We employed multiple scanning probe techniques combined with symmetry and mechanical compatibility analysis to uncover the hierarchical domain structures, developing on the 10-100 nm scale. The identified domain architecture involves a plethora of ferroelectric domain boundaries and junctions, including primary and secondary domain walls in both electrically neutral and charged configurations, and topological line defects transforming neutral secondary walls into two oppositely charged ones.

cond-mat.mtrl-sci↗

arXiv2020

Crystal growth and metallic ferromagnetism induced by electron doping in FeSb$_2$

In order to study the metallic ferromagnetism induced by electron doping in the narrow-gab semiconductor FeSb$_2$, single crystals of FeSb$_2$, Fe$_{1-x}$Co$_x$Sb$_2$ ($0 \le x \le 0.5$) and FeSb$_{2-y}$Te$_y$ ($0 \le y \le 0.4$), were grown by a simplified self-flux method. From powder x-ray diffraction (XRD) patterns, wavelength-dispersive x-ray spectroscopy (WDX) and x-ray Laue diffraction, pure and doped high-quality single crystals, within the selected solubility range, show only the orthorhombic $Pnnm$ structure of FeSb$_2$ with a monotonic change in lattice parameters with increasing the doping level. In consistence with the model of nearly ferromagnetic small-gap semiconductor, the energy gap of FeSb$_2$ Pauli paramagnet gradually collapses by electron doping before it closes at about $x$ or $y$ = 0.15 and subsequent itinerant electron anisotropic ferromagnetic states are observed with higher doping levels. A magnetic phase diagram is established and discussed in view of proposed theoretical scenarios.

cond-mat.mtrl-sci↗

arXiv2020

Unconventional critical behaviors at the magnetic phase transition of Co3Sn2S2 kagome ferromagnet

Co3Sn2S2 has generated a growing interest as a rare example of the highly uniaxial anisotropic kagome ferromagnet showing a combination of frustrated-lattice magnetism and topology. Recently, via precise measurements of the magnetization and AC susceptibility we have found a low-field anomalous magnetic phase (A-phase) with very slow spin dynamics that appears just below the Curie temperature (T_C). The A-phase hosts high-density domain bubbles after cooling through T_C as revealed in a previous in-situ Lorentz-TEM study. Here, we present further signatures of the anomalous magnetic transition (MT) at T_C revealed by a study of the critical behaviors of the magnetization and magnetocaloric effect using a high-quality single crystal. Analyses of numerous magnetization isotherms around T_C (177 K) using different approaches (the modified Arrot plot, Kouvel-Fisher method and magnetocaloric effect) result in consistent critical exponents that do not satisfy the theoretical predictions of standard second-order-MT models. Scaling analyses for the magnetization, magnetic entropy change and field-exponent of the magnetic entropy change, all consistently show low-field deviations below TC from the universal curves. Our results reveal that the MT of Co3Sn2S2 can not be explained as a conventional second-order type and suggest an anomalous magnetic state below T_C.

cond-mat.str-el↗

arXiv2017

Dynamic scaling analysis of the long-range RKKY Ising spin glass Dy$_{x}$Y$_{1-x}$Ru$_{2}$Si$_{2}$

Dynamic scaling analyses of linear and nonlinear ac susceptibilities in a model magnet of the long-rang RKKY Ising spin glass (SG) Dy$_{x}$Y$_{1-x}$Ru$_{2}$Si$_{2}$ were examined. The obtained set of the critical exponents, $γ$ $\sim$ 1, $β$ $\sim$ 1, $δ$ $\sim$ 2, and $zν$ $\sim$ 3.4, indicates the SG phase transition belongs to a different universality class from either the canonical (Heisenberg) or the short-range Ising SGs. The analyses also reveal a finite-temperature SG transition with the same critical exponents under a magnetic field and the phase transition line $T_{\mbox{g}}(H)$ described by $T_{\mbox{g}}(H)$ $=$ $T_{\mbox{g}}(0)(1-AH^{2/ϕ})$ with $ϕ$ $\sim$ 2. The crossover exponent $ϕ$ obeys the scaling relation $ϕ$ $=$ $γ+ β$ within the margin of errors. These results strongly suggest the spontaneous replica-symmetry-breaking (RSB) with a {\it non- or marginal-mean-field universality class} in the long-range RKKY Ising SG.

cond-mat.dis-nn↗

arXiv2017

Low-field anomalous magnetic phase in the kagome-lattice shandite Co3Sn2S2

The magnetization process of single crystals of the metallic kagomé ferromagnet Co3Sn2S2 was carefully measured via magnetization and AC susceptibility. Field-dependent anomalous transitions in the magnetization indicate a low-field unconventionally ordered phase stabilized just below TC. The magnetic phase diagrams in applied fields along different crystallographic directions were determined. The magnetic relaxation process studied in frequencies covering five orders of magnitude from 0.01 to 1000 Hz indicates characteristic relaxation times of several seconds at the boarders of the anomalous phase. Our results arise a lot of questions about the nature of Co3Sn2S2 magnetic states.

cond-mat.mtrl-sci↗

arXiv2014

Novel Magnetic Chiral Structures and Unusual Temperature Hysteresis in the Metallic Helimagnet MnP

We have reinvestigated the magnetic properties of the classical metallic helimagnet MnP by magnetization and neutron scattering experiments. Our neutron scattering results indicate that the previously reported magnetic structure in the low-temperature (LT) helimagnetic phase (T < 47K) should be modified to an alternately tilted helimagnetic structure pro- duced by the Dzyaloshinsky-Moriya interaction. In the intermediate temperature (IT) range between the LT helimagnetic phase and the high-temperature (HT) ferromagnetic phase along the c-axis, 47K < T < 282K, we have found a weak ferromagnetic behavior along the b-axis. Surprisingly, the IT weak ferromagnetic phase has two different states, namely, the large magnetization (LM) and small magnetization (SM) states. The SM state emerges with cooling from the paramagnetic phase above 292 K via the HT ferromagnetic phase and LM state emerges with warming from the LT helimagnetic phase. The weak ferromagnetism along the b-axis and the unusual temperature hysteresis in the IT phase can be understood by assuming a spontaneous formation of the stripe structure consisting of alternately arranged HT ferromagnetic and LT helimagnetic domains.

cond-mat.str-el↗

arXiv2011

Anomalous slow dynamics in the metallic helimagnet Gd_{1-x}Y_{x}

Ac-suseptibility measurements were performed in the metallic helimagnet Gd_{1-x}Y_{x} alloy. A remarkable increase of the imaginary part of the ac-suseptibility was observed in the temperature range of the helimagnetic phase. Moreover, a strong nonlinearity of the magnetization was observed at paramagnetic-helimagnetic transition temperature T_{N}. On the other hand, These anomalous behavior were not observed in similar rare-earth helimagnets Ho and Ho_{1-x}Y_{x}. It strongly suggests that the weak magnetic anisotropy of the Gd-moments is responsible for the anomalous slow dynamics in the helimagnetic phase and for the strong nonlinearity of the magnetization at T_{N}. The slow dynamics may result from a chiral-domain motion, or a long-time variation of the period of the helimagnetic structure.

cond-mat.str-el↗

arXiv2011

Critical Phenomena in Long-Range RKKY Ising Spin Glasses

We have investigated critical phenomena in spin glasses RxY1-xRu2Si2 (R = Dy, Tb, Gd). These compounds, where the magnetic moments of rare-earth ions interact by the long-range Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction via conduction electrons, has uniaxial magnetic anisotropy. The separation of the zero-field-cooled and field-cooled magnetization was found only along the c-axis in all compounds, and hence, they are classified into the long-range Ising spin glass. The magnetic anisotropic energies in these compounds are different from each other in two orders of magnitude, from 330 K to 1.8 K, however, the critical exponents are similar. It clearly indicates a presence of the universality of the long-range RKKY Ising spin glasses.

cond-mat.dis-nn↗

arXiv2010

Interplay between quantum criticality and geometrical frustration in Fe3Mo3N with stella quadrangula lattice

In the eta-carbide-type correlated-electron metal Fe3Mo3N, ferromagnetism is abruptly induced from a nonmagnetic non-Fermi-liquid ground state either when a magnetic field (~14 T) applied to it or when it is doped with a slight amount of impurity (~5% Co). We observed a peak in the paramagnetic neutron scattering intensity at finite wave vectors, revealing the presence of the antiferromagnetic (AF) correlation hidden in the magnetic measurements. It causes a new type of geometrical frustration in the stellla quadrangula lattice of the Fe sublattice. We propose that the frustrated AF correlation suppresses the F correlation to its marginal point and is therfore responsible for the origin of the ferromagnetic (F) quantum critical behavior in pure Fe3Mo3N.

cond-mat.str-el↗