Magnetic-nonmagnetic Transition Research Articles

Nonmagnetic-magnetic transition and magnetically ordered structure in SmS

SmS, a prototypical intermediate valence compound, has been studied by performing high-pressure nuclear magnetic resonance measurements on a $^{33}$S-enriched sample. The observation of an additional signal below 15-20 K above a nonmagnetic-magnetic transition pressure $P_{\rm c2} \approx 2$ GPa gives evidence of a magnetic transition. The absence of a Curie-term in the Knight shift near $P_{\rm c2}$ indicates that the localized character of $4f$ electrons is entirely screened and the mechanism of the magnetic ordering is not described within a simple localized model. Simultaneously, the line shape in the magnetically ordered state is incompatible with a spin density wave order. These suggest that the magnetic order in SmS may require an understanding beyond the conventional framework for heavy fermions. The fact that hyperfine fields from the ordered moments cancel out at the S site leads us to a conclusion that the ordered phase has a type II antiferromagnetic structure.

Metal-insulator and magnetic phase diagram of Ca2RuO4 from auxiliary field quantum Monte Carlo and dynamical mean field theory

Layered perovskite ruthenium oxides exhibit a striking series of metal-insulator and magnetic-nonmagnetic phase transitions easily tuned by temperature, pressure, epitaxy, and nonlinear drive. In this work, we combine results from two complementary state of the art many-body methods, Auxiliary Field Quantum Monte Carlo and Dynamical Mean Field Theory, to determine the low-temperature phase diagram of Ca$_2$RuO$_4$. Both methods predict a low temperature, pressure-driven metal-insulator transition accompanied by a ferromagnetic-antiferromagnetic transition. The properties of the ferromagnetic state vary non-monotonically with pressure and are dominated by the ruthenium $d_{xy}$ orbital, while the properties of the antiferromagnetic state are dominated by the $d_{xz}$ and $d_{yz}$ orbitals. Differences of detail in the predictions of the two methods are analyzed. This work is theoretically important as it presents the first application of the Auxiliary Field Quantum Monte Carlo method to an orbitally-degenerate system with both Mott and Hunds physics, and provides an important comparison of the Dynamical Mean Field and Auxiliary Field Quantum Monte Carlo methods.

Activated layered magnetism from bulk TiN

The unexpected properties of a uniaxially expanded TiN bulk arising from increasing the layer spacing from equilibrium are explored using a first-principles approach. We reveal an unusual nonmagnetic-magnetic transition from a TiN bulk to its monolayer. We also investigate the electronic and magnetic structures of a few TiN atomic layers. We find that the bilayer and trilayer, like the TiN bulk, are nonmagnetic poor metals. On the other hand, the monolayer TiN is found to carry a magnetic moment on its Ti atoms, and likely be a semiconductor. The unpaired electron giving rise to magnetism on Ti is primarily in the orbital perpendicular to the layers, and we find it is freed to give rise to magnetism when the layers are slightly separated. We find two different antiferromagnetic states possible on the monolayer, as well as one ferromagnetic, with one of the antiferromagnetic being the lowest energy. The exchange couplings between Ti atoms in such a monolayer are calculated to be antiferromagnetic for both the nearest-neighbor and next-nearest-neighbor sites. We also analyze the binding nature of both the monolayer and bilayer TiN by examining the predominant binding orbitals.

The Behavior of Magnetic Properties in the Clusters of 4d Transition Metals.

The current focus of material science researchers is on the magnetic behavior of transition metal clusters due to its great hope for future technological applications. It is common knowledge that the 4d transition elements are not magnetic at their bulk size. However, studies indicate that their magnetic properties are strongly dependent on their cluster sizes. This study attempts to identify magnetic properties of 4d transition metal clusters. Using a tight-binding Friedel model for the density of d-electron states, we investigated the critical size for the magnetic-nonmagnetic transition of 4d transition-metal clusters. Approaching to the critical point, the density of states of the cluster near the Fermi level is higher than and the discrete energy levels form a quasi-continuous band. Where is correlation integral. In order to determine the critical size, we considered a square shape band and fcc, bcc, icosahedral and cuboctahedral close-packed structures of the clusters. We also investigated this size dependent magnetic behavior using Heisenberg model. Taking some quantum mechanical approximations in to consideration, we determined magnetic behavior of the clusters. For practicality, we considered three clusters of transition metals (Ru, Rh and Pd) and the obtained results are in line with the results of previous studies.

Kondo-like behavior near the magnetic instability in SmB6 : Temperature and pressure dependences of the Sm valence

We report a systematic study of Sm valence in the prototypical intermediate valence compound SmB$_6$. Sm mean valence, $v_{\rm Sm}$, was measured by X-ray absorption spectroscopy as functions of pressure ($1<P<13$ GPa) and temperature ($3<T<300$ K). Pressure induced magnetic order (MO) was detected above $P_c = 10$ GPa by resistivity measurements. A shift toward localized $4f$ state with increasing $P$ and/or $T$ is evident from an increase in $v_{\rm Sm}$. However $v_{\rm Sm}$ at $P_c$ is anomalously far below 3, which differs from the general case of nonmagnetic-magnetic transition in Yb and Ce compounds. From the $T$ dependence of $v_{\rm Sm}(P,T)$, we found that $v_{\rm Sm}(P,T)$ consists of two different characteristic components: one is associated with low-energy electronic correlations involving Kondo like behavior, and the other with high-energy valence fluctuations.

Electronic states in the pressure-induced magnetically ordered phase in SmB6

We have carried out the high-pressure measurement of X-ray absorption spectroscopy on the intermediate valence compound SmB6 which shows magnetic ordering as well as an insulator-metal transition at critical pressure Pc ∼ 10 GPa. The valence of Sm atom at room temperature increases with increasing pressure, however it is far below a trivalent state at Pc. In contrast to cases of pressure-induced nonmagnetic-magnetic transition in Yb compounds, which mostly occurs in the scheme of well localized 4f electrons, the present observation suggests that electronic system in SmB6 still possesses strong delocalized characters at Pc.

Magnetic-Nonmagnetic Phase Transition with Interlayer Charge Disproportionation of Nb3 Trimers in the Cluster Compound Nb3Cl8.

We grew large single crystals of the cluster magnet Nb3Cl8 with a magnetic triangular lattice and investigated its magnetic properties and crystal structure. In Nb3Cl8, the [Nb3]8+ cluster has a single unpaired spin, making it an S = 1/2 triangular lattice anti-ferromagnet. At low temperatures, Nb3Cl8 exhibits a magnetic-nonmagnetic phase transition driven by a charge disproportionation, in which the paramagnetic [Nb3]8+ clusters transform into alternating layers of nonmagnetic [Nb3]7+ and [Nb3]9+ clusters. The observed exotic phenomenon with the strong correlation between the magnetism and structure are based on the nature of the cluster magnetism.

Substitution- and strain-induced magnetic phase transition in iron carbide

Cementite-type carbides are of interest for magnetocaloric applications owing to their temperature- or pressure-induced magnetic phase transition. Here, using first-principles calculations, we investigate the magnetism and the magnetic phase transition in iron carbide (Fe3C) with the substitution of Cr atoms at Fe sites with the strain effect. The presence of Cr atoms is found to give rise to a second-order magnetic phase transition from a ferromagnetic phase for Fe3C to a nonmagnetic phase in chromium carbide (Cr3C).While the ternary Fe2CrC and Cr2FeC compounds prefer the ferrimagnetic ground state, the magnitudes of both the Fe and Cr spin moments, which are antiparallel in orientation, decrease as x increases in Fe3-xCrxC (x = 0, 1, 2, and 3). Furthermore, the fixed spin-moment calculations indicate that the magnetization of Fe3-xCrxC compounds can be delicately altered via the strain effect and that the magnetic-nonmagnetic phase transition occurs at an early stage of Cr substitution, x = 2.

Magnetic property and possible half-metal behavior in Co-doped graphene

The magnetic property and band structures of Co-monolayer doped graphene were examined on the basis of density-functional theory. The magnetic moment of the system is closely related to the interfacial spacing. Magnetic-nonmagnetic transition would be produced by decreasing the layer distance. Although the magnetic moment can also be reduced by increasing the lattice constant, the ground states are magnetic states under tension. Besides, the increase of lattice constant greatly enlarges the direct and indirect gaps of spin-down bands near the Fermi level. With a little increase of the Fermi level or the electron density, half-metal behavior would be expectable in the Co-doped graphene under tension.

Pressure-induced phase transformation and magnetism transition in BaRuO 3: A first-principles study

Transformations of the four polymorphs and magnetic transition of BaRuO 3 under high pressure have been studied using first-principles calculations. We search for the low-enthalpy structures by relaxing four initial structural configurations under high pressure. We find that BaRuO 3 exhibits sequential structural changes from nine-layer rhombohedral (9R), four-layer hexagonal (4H), six-layer hexagonal (6H), and finally to cubic perovskite (3C) structure, consistent well with the experiments. The three hexagonal crystallographic forms (9R, 4H and 6H) of BaRuO 3 show paramagnetic states, whereas 3C–BaRuO 3 displays a ferromagnetic ground state. Along with the pressure increasing, the magnetic moment of Ru in the 3C–BaRuO 3 first come through a discontinuous drop around the critical pressure 26 GPa, then becomes smaller gradually until zero at 180 GPa. According to the theory of Moruzzi, this magnetic–nonmagnetic transition belongs to the type-III transition.

Magnetic Field Effect on Yb-Based Heavy Fermions near Magnetic-Nonmagnetic Transition

T. Mitoa,∗, T. Koyama, K. Nakagawara, T. Ishida, K. Ueda, T. Kohara, K. Matsubayashi, Y. Saigab,†, K. Munakata, Y. Uwatoko, M. Mizumaki and N. Kawamura Graduate School of Material Science, University of Hyogo, Hyogo 678-1297, Japan Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan Japan Synchrotron Radiation Research Institute, SPring-8, Sayo, Hyogo 679-5198, Japan

Behind the Quantum and Size Effects: Broken-Bond-Induced Local Strain and Skin-Depth Densified Quantum Trapping of Charge and Energy

Shrinking the size of a solid down to nanometer scale is indeed fascinating, which makes all the otherwise constant physical quantities to be tunable such as the Young’s modulus, dielectric constant, melting point, etc. The variation of size also generates novel properties that can hardly be seen in the bulk such as the conductor-insulator and nonmagnetic-magnetic transition of noble metals at the nanoscale. Although the physics of materials at the nanoscale has been extensively investigated, the laws governing the energetic and dynamic behavior of electrons at such a scale and their consequences on the tunable physical properties of nanostructures have not been well understood [C. Q. Sun, Prog Solid State Chem 35, 1-159 (2007); Prog Mater Sci 54, 179-307 (2009)]. The objective of the contribution is to update the recent progress in dealing with the coordination-resolved energetic and dynamic behavior of bonds in the low-dimensional systems with consideration of the joint effect of temperature and pressure. It is shown that the broken-bond-induced local strain and the associated charge and energy quantum trapping at the defect sites perturbs the atomic cohesive energy, electroaffinity, the Hamiltonian and the associated properties of entities ranging from point defects, surfaces, nanocavities and nanostructures. Application of the theories to observations has led to consistent understanding of the behavior of nanometer-sized materials and the interdependence of these entities as well as the means of determining the bond energy through the temperature-dependent measurements.

Switching on magnetism in Ni-doped graphene: Density functional calculations

Magnetic properties of graphenic carbon nanostructures, which are relevant for future spintronic applications, depend crucially on doping and on the presence of defects. In this paper we study the magnetism of the recently detected substitutional Ni $({\text{Ni}}_{\text{sub}})$ impurities. ${\text{Ni}}_{\text{sub}}$ defects are nonmagnetic in flat graphene and develop a nonzero-spin moment only in metallic nanotubes. This surprising behavior stems from the peculiar curvature dependence of the electronic structure of ${\text{Ni}}_{\text{sub}}$. A similar magnetic-nonmagnetic transition of ${\text{Ni}}_{\text{sub}}$ can be expected by applying anisotropic strain to a flat graphene layer.

Pressure–temperature phase diagrams of some heavy fermion systems

We have studied the pressure–temperature phase diagrams in UIr,U3P4,UP2,CeAgSb2 and CeMg2Cu9 by means of the electrical resistivity. Application of pressure suppresses the magneticordering shown at ambient pressure in all compounds. In UIr, the pressure-inducedsuperconductivity is observed in the vicinity of the magnetic–nonmagnetic transition. Inthis article, we review the pressure–temperature phase diagram and the properties of themagnetic–nonmagnetic transition in these compounds.

Thermal scanning studies of percolated Fe–Cu granular alloys

Fe–Cu alloys produced by mechanical alloying were studied with Mössbauer thermal scanning spectroscopy (MTS). This technique consists in recording Mössbauer effect absorption at a fixed energy while the temperature of the sample is changed. Hyperfine magnetic field behavior can be closely followed. Four mechanically alloyed samples Fe x Cu 100− x with iron concentration x=30, 35, 40, and 45 at.% were studied. Absorption graphs (intensity vs. temperature) are similar for all samples: intensity sharply falls with the increase of the temperature, following the collapse of the sextet lines. This collapse occurs at a very precise temperature, which coincides with a magnetic–nonmagnetic transition line in a former magnetic phase diagram. Conventional Mössbauer spectroscopy confirms the appointed phase change, displaying a doublet at temperatures above the critical.

Pressure-induced magnetic instability inCe2Rh3Ge5

A pressure-induced magnetic-nonmagnetic transition in an antiferromagnet ${\mathrm{Ce}}_{2}{\mathrm{Rh}}_{3}{\mathrm{Ge}}_{5}$ has been investigated by the measurements of the resistivity \ensuremath{\rho}, magnetic susceptibility \ensuremath{\chi}, and specific heat on a single crystal down to 0.3 K. With increasing pressure, the N\'eel temperature ${T}_{N}$ is suppressed and disappears at ${P}_{c}=0.45\mathrm{GPa}.$ For $P&gt;{P}_{c},$ the deviation from the Fermi-liquid behavior is observed in both $\ensuremath{\rho}(T)$ and $\ensuremath{\chi}(T):$ $\ensuremath{\rho}(T)={\ensuremath{\rho}}_{0}{+AT}^{n} (n\ensuremath{\cong}1.45)$ and $\ensuremath{\chi}(T)={\ensuremath{\chi}}_{0}\ensuremath{-}{\mathrm{DT}}^{m} (m\ensuremath{\cong}0.27).$ Even for $P&gt;{3P}_{c}$ both n and m hardly change, i.e., no recovery of the Fermi-liquid state. This fact is in contrast with the recovery in many of Ce-based antiferromagnets at $P&gt;{2P}_{c}.$ The magnetization hardly decreases with pressure up to 1.2 GPa, suggesting that the cerium local moments still remain even far from the quantum critical point. Moreover, the Kondo temperature ${T}_{K}$ increases weakly from 10 K at $P=0$ to 13 K at $P=0.57\mathrm{GPa}.$ These findings indicate the important role of short-range magnetic correlations in the revised Doniach phase diagram.

Magnetic properties of small Yttrium clusters

The magnetic properties of small YN clusters are studied by using a tight-binding Hubbard Hamiltonian in the unrestricted Hartree-Fock approximation. Several types of cluster geometries are considered in order to see the effects of the size and symmetry of the structures on the magnetic properties. The average magnetic moments are found to be constant over large domains of variations in the interatomic distance, a fact that can be explained by the existing closed shell electronic configurations at least for one spin direction in all our magnetic solutions. Small energy gains upon the onset of magnetization are obtained, which reveals the low stability of the magnetic solutions. Our results contradict the prediction of a magnetic-nonmagnetic transition at a large cluster size (about 90 atoms) for these kinds of systems.

Magnetic-nonmagnetic transition in fcc4d-transition-metal clusters

Size-dependent, magnetic, and structural properties of fcc ${\mathrm{Rh}}_{N}$ and ${\mathrm{Ru}}_{N}$ clusters $(N$=13, 19, 43, 55, and 79) are calculated using a tight-binding Hubbard Hamiltonian in the unrestricted Hartree-Fock approximation. Results for the magnetic moment, cohesive energy, and interatomic distance at $T=0$ are given as functions of cluster size. The magnetic-nonmagnetic transitions estimated are in agreement with experimental findings.

Phase Transitions of Ti3O5

The crystal structure and electronic state of Ti3O5, which undergoes first-order structural and magnetic–nonmagnetic transitions at 460 K on heating and 440 K on cooling, have been explored through X-ray four-circle diffraction, magnetic, and transport measurements. A successive monoclinic (C2/m)–orthorhombic transition appears at about 500 K. The crystal structure at 514 K has been determined withR=0.040 andRw=0.038 for 1027 independent reflections, where the space group isCmcmwith lattice constantsa=3.798(2) Å,b=9.846(3) Å, andc=9.988(4) Å, andZ=4. The structure is basically built up of a linkage of distorted TiO6octahedra and is of a pseudobrookite type. Each effective Ti valence is nearly equal to an average value. These characteristics are nearly consistent with magnetic and transport properties. Ti3O5in the orthorhombic phase is like a highly correlated metal.

Electronic Phase Transition Tuned by Pressure: Superconductivity and Antiferromagnetism.

Pressure introduces various types of electronic phase transitions, e. g. metal-insulator transition and magneticnon magnetic transition. In this report special attention is paid to the transitions between antiferromagnetism and superconductivity, which have been classified and the possible mechanisms behind this classification are searched for. The implications of the phase diagram of the high temperature superconducting cuprates on the plane of the doping rate, δ, and the temperature, T, are discussed based on this classification.