Spin Fluctuations In Systems Research Articles

Constitution, physical properties and thermodynamic modeling of the Hf-Mn system

The Hf-Mn system is of a long-time interest due to the intermetallic Laves phase HfMn2, a hydrogen storage material. Although this system has been experimentally investigated by several authors and critical reviews and thermodynamic modelling have been performed, there is still a lack of reliable information, particularly as the phase "HfMn" (sometimes labelled as "Hf3Mn2" or "Hf2Mn") is suspected to be oxygen stabilized. This work includes a thorough investigation of the Hf-Mn phase equilibria employing diffusion zones, thermal analysis, powder and single crystal X-ray analyses, analytical electron microscopy as well as physical property studies of the Laves phase (magnetic susceptibility, specific heat, electrical resistivity and mechanical properties). The phase near “HfMn” was shown (TEM, WDX electron microprobe data, X-ray single crystal analysis) to be an oxygen stabilized phase with the formula Hf3+xMn3−xO1−y (defect η-W3Fe3C type). Properties such as magnetic susceptibility/magnetization; 2–300 K, specific heat (2–1100 K), electrical resistivity (2–300 K) classify HfMn2 as a metallic spin-fluctuation system with itinerant paramagnetism, originating from 3d states at Mn-sites and local moment paramagnetism of antisite Mn-atoms at Hf-sites. Mechanical properties (elastic moduli from density functional theory (DFT) and nanoindentation as well as hardness) group the Laves phase among rather hard and brittle intermetallics. DFT modeling revealed that Hf3+xMn3−x is thermodynamically unstable, but significant gains in enthalpy of formation arise from the inclusion of oxygen atoms, stabilizing the η phase. All phase diagram and DFT data together with the former literature information were used for the thermodynamic CALPHAD-type modelling of the Hf-Mn system.

Theoretical prediction of a highly responsive material: Spin fluctuations and superconductivity in FeNiB2 system

By analyzing the Fe-Ni-B compositional diagram, we predict an energetically and dynamically stable FeNiB2 compound. This system belongs to the class of highly responsive state of materials, as it is very sensitive to the external perturbations. This state is also characterized by a high level of spin fluctuations, which strongly influence possible magnetic long- and short-range orders. Furthermore, we demonstrate that these antiferromagnetically dominating fluctuations could lead to the appearance of spin mediated superconductivity. The obtained results suggest a promising avenue for the search of strong spin fluctuation systems and related superconductors.

Ti8(TixMn1−x)6Mn39 (‘TiMn∼4’): a metallic spin fluctuation system

The crystal structure of Ti8(TixMn1−x)6Mn39, x = 0.187, was obtained from x-ray single-crystal diffraction data, confirming it to have rhombohedral symmetry (space group ; ahex = 1.100 70(2) nm, chex = 1.944 11(4) nm; RF = 0.0293) and isotypism with the prototype Mo0.38Cr0.16Co0.46 (the so-called R-phase). On the basis of electron probe micro-analyser results and structure determination, the homogeneity region of the phase TiMn∼4 was determined for temperatures in the range 800 °C < T < 1200 °C and is in between 16.0 at.% Ti and 20 at.% Ti. Various physical properties, determined in the temperature range from ∼2 K to room temperature, characterize the compound with composition TiMn4.26 as a metallic spin fluctuation system, evidenced from a T3lnT dependence of the heat capacity in combination with large values of the electronic Sommerfeld constant of the order of 140 mJ mol−1 K−2. The occurrence of a small anomaly in the heat capacity and magnetization data around 10 K is attributed to a scenario involving spin freezing phenomena, since a fraction of the order of 10% of all Mn–Mn distances within the unit cell are above a critical distance, where Mn atoms carry a spontaneous magnetic moment.

Role of nitrogen in AlNxMn3: A density functional theory study

Recently, we successfully synthesized the nitrogen-deficient manganese antiperovskites AlNxMn3 with the very high ferromagnetic Curie temperatures TC up to 818 K [Lin et al., Appl. Phys. Lett. 98, 092507 (2011)]. In order to figure out the role of nitrogen for the magnetism, in the present work, we performed a theoretical investigation on AlNxMn3 through the first-principles calculation based on density functional theory. The results show that AlNxMn3 have the ferromagnetic ground states, and the total magnetic moments in a cell are enhanced with decreasing the nitrogen concentration. Based on the calculations of band structure and density of states, we showed the influence of nitrogen concentration on magnetism for AlNxMn3: The decreasing of nitrogen concentration from ideal AlNMn3 moves the spin-down bands towards the high energy remarkably, which enhances the exchange splitting energy ΔEex; on the other hand, nitrogen deficiency reduces the Mn-N hybridizations, which makes the 3d electrons of Mn tend to occupy the spin-up states. The nitrogen deficiency reduces the Mn-N hybridization, which narrows Mn-d bands and enhances the spin polarization. Furthermore, we estimated TC of AlNxMn3, and found the calculated TC can be scaled by spin fluctuation temperature TSF, suggesting AlNxMn3 belong to the spin fluctuation system.

Coherent energy scale revealed by ultrafast dynamics of UX3(X=Al, Sn, Ga) single crystals

Temperature dependence of relaxation dynamics of UX$_{3}$ (X = Al, Ga, Sn) compounds is studied using time resolved pump-probe technique in the reflectance geometry. UGa$_{3}$ is an itinerant antiferromagnet, while UAl$_{3}$ and USn$_{3}$ are spin fluctuation systems. For UGa$_{3}$, our data are consistent with the formation of a spin density wave SDW gap as evidenced from the quasidivergence of the relaxation time $\tau$ near the N\'{e}el temperature $T_{N}$. For UAl$_{3}$ and USn$_{3}$, the relaxation dynamics shows a change from single exponential to two exponential behavior below a particular temperature, suggestive of coherence formation of the 5\textit{f} electrons with the conduction band electrons. This particular temperature can be attributed to the spin fluctuation temperature $T_{sf}$, a measure of the strength of Kondo coherence. Our $T_{sf}$ is consistent with other data such as resistivity and susceptibility measurements. The temperature dependence of the relaxation amplitude and time of UAl$_{3}$ and USn$_{3}$ were also fitted by the Rothwarf-Taylor model. Our results show ultrafast optical spectroscopy is sensitive to c-\textit{f} Kondo hybridization in the \textit{f}-electron systems.

Study of spin fluctuations in Ni3±xAl1∓x using point contact Andreev reflection spectroscopy

We report point contact Andreev reflection (PCAR) spectroscopy studies on Ni3±xAl1∓x with composition range spanning the ferromagnetic to paramagnetic phase boundary. PCAR studies performed using Nb tip as counterelectrode reveal that the superconducting quasiparticle lifetime (τ) and superconducting energy gap (Δ) decrease with increasing spin fluctuation in the normal metal electrode. Our study reveals that PCAR could be a useful probe to study spin fluctuations in systems that are on the verge of magnetic instability.

IsomorphousPrT2B2C(T=Co,Ni,Pt)single crystals: Structural, transport, and magnetic properties

The full substitution of Ni by Co and Pt enclosed in the same crystalline structure $(I4∕mmm)$ of Pr-based compounds have been investigated. The structural characteristics, magnetic properties, and magnetoresistance of $\mathrm{Pr}{\mathrm{Co}}_{2}{\mathrm{B}}_{2}\mathrm{C}$, $\mathrm{Pr}{\mathrm{Ni}}_{2}{\mathrm{B}}_{2}\mathrm{C}$, and $\mathrm{Pr}{\mathrm{Pt}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ single crystals were studied to determine the reason for the absence or presence of superconductivity in those compounds. An antiferromagnetic transition was found for $\mathrm{Pr}{\mathrm{Co}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ at about $8.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, where the easy magnetization axis is along the $c$-crystallographic direction whereas in $\mathrm{Pr}{\mathrm{Ni}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ the easy magnetization was at the $ab$ plane. In $\mathrm{Pr}{\mathrm{Ni}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ a small magnetization without traces of saturation is found when it is measured up to $18\phantom{\rule{0.3em}{0ex}}\text{Tesla}$ of applied magnetic field. This behavior suggests a screening effect as a result of a moderate hybridization between the conduction band and Pr ions. Superconductivity occurs in $\mathrm{Pr}{\mathrm{Pt}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ at about $6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The Pt-Pt distance and internal bonding angles are similar to those observed in the superconducting Ni-based compounds. Magnetoresistance measurements show positive and quadratic behavior $(\ensuremath{\Delta}\ensuremath{\rho}∕{\ensuremath{\rho}}_{0}\ensuremath{\sim}a{H}^{2})$ suggesting a spin fluctuation system. In the $\mathrm{Pr}{\mathrm{Pt}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ compound the electronic mean free path is smaller than the BCS coherence length $(\ensuremath{\iota}⪡\ensuremath{\epsilon})$ suggesting a dirty type II superconductor.

Effect of H insertion on the magnetic, electronic, and structural properties of CeCoSi

Contrary to other cerium equiatomic intermetallic systems ($\mathrm{Ce}TX$ with $T=\text{transition}$ metal, $X=p$ metal) which become magnetically ordered upon hydrogen insertion, magnetic measurements characterize an antiferromagnet for CeCoSi and a spin fluctuation system for its hydride CeCoSiH. The presented results of electrical and magnetic properties of the silicide CeCoSi demonstrate the existence of an antiferromagnetic transition at ${T}_{N}=8.8(2)\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. For the purpose of further assessing the experimental findings, the electronic and magnetic sructures of CeCoSi and CeCoSiH are self-consistently calculated within the local spin density functional LSDF theory. The analysis of the electronic structures and of the chemical bonding properties lead to suggest that the chemical effect of hydrogen prevails over the cell expansion effect which would enhance the magnetization otherwise. Spin orbit coupling effects on cerium are examined. A potential antiferromagnetic structure with a propagation along the $c$ axis is proposed for CeCoSi ground state.

Photoinduced second harmonic generation of LaFe4Sb12 near spin fluctuated critical points

AbstractThe temperature dependence of the resistivity, the Seebeck coefficient and photoinduced second harmonic generation (PISHG) are studied near the quantum critical point in the skutterudite compound LaFe4Sb12, possessing increased spin fluctuations. We observed a large maximum of the PISHG at a temperature of about 15 K. The PISHG signal increases substantially below 35 K. We found a correlation between the temperature dependences of PISHG, resistivity and Seebeck coefficient. We proposed a phenomenological explanation for the occurrence of the PISHG signal in LaFe4Sb12 implying strong spin fluctuations exist in this system, which may present some interest for the study of other spin fluctuation systems. Physical insight into the phenomenon observed is grounded in the participation of anharmonic electron–phonon and electron–paramagnon interactions stimulated by inducing light in the interactions with the photoexcited dipole moments. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Magnetic and transport properties of the mixed-valent compoundYbRh2Ga

We report the measurements of specific heat, electrical resistivity, magnetic susceptibility, Hall coefficient and thermoelectric power on a ternary compound of ${\mathrm{YbRh}}_{2}\mathrm{Ga},$ which crystallizes in the triple-hexagonal ${\mathrm{Na}}_{3}\mathrm{As}$-type structure with the space group ${P6}_{3}\mathrm{cm}.$ ${\mathrm{YbRh}}_{2}\mathrm{Ga}$ remains in a paramagnetic state down to 1.2 K. The magnetic susceptibility exhibits a broad maximum around 90 K for $H\ensuremath{\parallel}c$ or 170 K for $H\ensuremath{\perp}c,$ respectively. Such maximum is a characteristic of the Ce- and Yb-based mixed-valent compounds. The temperature dependence of electrical resistivity shows an S-type behavior. The electronic specific heat coefficient is 36.6 ${\mathrm{m}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{K}}^{2}$, which is nine times larger than that of ${\mathrm{YRh}}_{2}\mathrm{Ga}$ (3.9 ${\mathrm{m}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{K}}^{2}$) with no $4f$ electron. The thermoelectric power $(\ensuremath{\Delta}T\ensuremath{\parallel}c)$ shows a broad minimum at about 120 K with a relatively large negative value, \ensuremath{\sim}-36 \ensuremath{\mu}V/K, which is typical for Yb-based spin-fluctuation systems. The Hall coefficient $(J\ensuremath{\parallel}c,H\ensuremath{\perp}c)$ exhibits a maximum around 60 K; at higher temperatures it varies with temperature in a manner suggestive of skew scattering. All of the measurement results indicate that ${\mathrm{YbRh}}_{2}\mathrm{Ga}$ is a typical mixed-valent system. The anisot-ropy observed in ${\mathrm{YbRh}}_{2}\mathrm{Ga}$ indicates anisotropic hybridization between localized $4f$-electron and conduction elections.

Magnetoresistance in RCo2 spin-fluctuation systems

Abstract The effect of the spin fluctuations on the field and temperature dependence of the magnetoresistance in ScCo2 and LuCo2 was studied. The experimental data where explained assuming two competing mechanisms determining the magnetoresistance of these substances. One is the ‘normal magnetoresistance’ caused by the influence of the Lorentz force on conduction electron trajectories. The other is due to the suppression of the spin fluctuations caused by an external magnetic field. This interplay give rise to a pronounced drop of the magnetoresistance towards the lower temperature range.

Molecular Spin System and Neutron Scattering

Excellent results of neutron scattering techniques in a field of molecular magnetism were briefly reviewed. Polarized neutron diffraction gave detailed information of electron spin density distributions in the crystals of organic magnets. This information is crucial for clarifying the mechanism of organic magnet. Magnetic scattering of neutron directly revealed the parameter of magnetic anisotropy of magnetic cluster molecules, which is an origin of single-molecule magnet. Some proposals of neutron scattering experiment on spin fluctuation system in connection with a research project of “Application of Molecular Spins” and hope for J-PARC were mentioned.

Possible high-Tcsuperconductivity mediated by antiferromagnetic spin fluctuations in systems with Fermi surface pockets

We propose that if there are two small pocket-like Fermi surfaces, and the spin susceptibility is pronounced around a wave vector {\bf Q} that bridges the two pockets, the spin-singlet superconductivity mediated by spin fluctuations may have a high transition temperature. Using the fluctuation exchange approximation, this idea is confirmed for the Hubbard on a lattice with alternating hopping integrals, for which $T_c$ is estimated to be almost an order of magnitude larger than those for systems with a large connected Fermi surface.

Pressure studies on (Ce 1− xU x)Al 2

CeAl 2 and UAl 2 are considered as archetypal systems representing a magnetically ordered Kondo lattice and a spin fluctuation system, respectively. Here we present our resistivity measurement of (Ce,U)Al 2 alloys under pressures. Hydrostatic pressure causes a substantial increase of Kondo temperatures T K for the magnetically ordered side of Ce-rich materials, and is found to suppress completely the magnetic order of the 30% U doped sample above a critical pressure of 5.5 kbar. Spin fluctuations dominant in the U-rich side become suppressed and the U-rich systems are driven towards a simple metallic regime with pressure.

Magnetoresistance in Y(Ni 1− xCo x) 5 around the critical concentration for the onset of ferromagnetism

Resistivity measurements with and without a magnetic field (magnetoresistance) have been used to investigate the pseudobinary Y(Ni 1− x Co x ) 5 system. The aim was to confirm the existence of spin fluctuations mainly at the concentration where the long-range magnetic order appears ( x=0.3) since from the temperature dependence of the susceptibility it has been claimed that YNi 5 is a spin fluctuation system (Coldea et al., J. Magn. Magn. Mat. 157–158 (1996) 627). None of the obtained results indicate that spin fluctuations are significant for the properties, neither for YNi 5 nor for the pseudobinaries around the onset of magnetism.

Suppression of the enhanced paramagnetic thermal expansion of YMn 2 by Fe substitutions

The thermal expansion coefficient α of YMn 2 just above its Néel temperature ( T N = 90K) is about 45 × 10 −6 K −1, it takes a value of 53 × 10 −6 K −1 at room temperature and passes through a maximum around 350 K. This unusual behaviour of α(T) and the higher value (if compared with other known spin fluctuation systems such as YCo 2) has been attributed to electronic instabilities of the Mn ions. In the present investigation it is shown that - when changing the environment of an individual Mn ion by substituting Mn by Fe — α decreases very quickly with increasing Fe content. This has been considered as a proof that the electronic instabilities of the Mn ions depend very sensitively on the surroundings.

Temperature-dependent properties of Y 3Ni

We present susceptibility, thermal expansion, thermopower and magnetoresistance measurements of Y 3Ni from which follows that spin fluctuations play only a minor role compared to other known spin fluctuation systems such as UAl 2 or YCo 2.

Non-Fermi-liquid behavior and spin fluctuations in dopedUAl2

Using the canonical spin-fluctuation system UAl{sub 2} as a starting point, via negative chemical pressure (doping with Y) we have expanded d{sub U-U} in a system known to be near the Hill limit of f-electron localization, and characterized the samples via resistivity, magnetic susceptibility, and specific-heat measurements. All system parameters, including magnetic susceptibility, specific heat {gamma} ({equivalent_to}C/Tlim{sub T{r_arrow}0}), and spin-fluctuation temperature, behave monotonically. For U{sub 1{minus}x}Y{sub x}Al{sub 2}, 0.30{le}x{le}0.70, spin-glass behavior is found with T{sub f}{approx_equal};5.1{plus_minus}0.5 K. This spin-glass behavior weakens (T{sub f} sinks, smaller magnetic signature, no specific-heat anomaly) for x{ge}0.75 while, at the same time, the spin-fluctuation T{sup 3}lnT term also gradually disappears from the specific heat. For x{ge}0.875, a non-Fermi-liquid (nFl) lnT term is found in the low temperature C/T. This new, perhaps equilibrium, ground state persists upon further dilution of the U ions with Y. Thus, we report on the evolution of nFl behavior in the neighborhood of a spin-glass ground state but, indeed, directly out of a yet weaker form of magnetism than heretofore reported, that of spin fluctuations. {copyright} {ital 1997} {ital The American Physical Society}

Magnetic behavior in UTSi 2 (T = Fe, Co and Ni) compounds

New ternary uranium intermetallics UTSi 2 where T = Fe, Co and Ni, crystallizing with the orthorhombic CeNiSi 2-type structure, have been investigated by means of magnetization, magnetic susceptibility, electrical resistivity and magnetoresistivity measurements. UNiSi 2 is a ferromagnetically ordered ( T c = 95 K) Kondo lattice with rather well localized 5 f-electrons. UCoSi 2 is probably a spin fluctuation system, while UFe 0.8Si 2 exhibits features of a weakly temperature dependent paramagnet. A large variety in the magnetic properties within the UTSi 2 series is discussed in terms of increasing strength of the 5 f-3 d hybridization on going in the transition elements row from Ni to Co to Fe.

Magnetic behaviour of U1-xYxAl3 and U1-xLaxAl3 compounds

Magnetic measurements were performed on U1-xRxAl3 with R = La or Y in the temperature range 4(77)-700 K and fields up to 7 T. UAl3 is a spin fluctuation system. When uranium is substituted by Y or La the effective magnetic moments decrease together with the absolute magnitude of θ. This suggests a gradual quenching of the spin fluctuations with increasing R content.