Terms Of Spin Fluctuations Research Articles

Induced by the pressure and the spin fluctuations the phase transitions in chiral itinerant ferromagnetics (for example MnSi)

In the framework of the spin-fluctuation theory, a magnetic (p-T)-diagram of strongly correlated band ferromagnets with the Dzyaloshinsky-Moriya interaction is considered. The resulting expression for the free energy functional contains pressure-dependent magnetic, electronic, and spin-fluctuation terms, which are self-consistent with each other. The conditions for the minimum of free energy and electroneutrality are considered at various pressures in the model of the electronic structure obtained for MnSi in result of GGA + U + SO calculations. A crossover of a first-order phase transition and a quantum transition is obtained, which leads to the appearance of a chiral spin liquid. The boundary point (T0, p0) at which the chiral fluid disappears is determined. The quantum transition of a ferromagnetic spin spiral into the phase of vortex spin microstructures coexistence with fixed spin chirality, which arises below the temperature of the boundary point, is considered. At higher pressures, chiral paramagnetism occurs.

Insight into the magnetism of a distorted Kagome lattice, Dy3Ru4Al12, based on polycrystalline studies

The layered compound with distorted Kagome nets, Dy3Ru4Al12, was previously reported to undergo antiferromagnetic ordering below (TN=) 7 K, based on investigations on single crystals. Here, we report the results of our investigation of AC and DC magnetic susceptibility (χ), isothermal remnant magnetization (MIRM), heat-capacity, magnetocaloric effect and magnetoresistance measurements on polycrystals. The present results reveal that there is an additional magnetic anomaly around 20 K, as though the Néel order is preceded by the formation of ferromagnetic clusters. We attribute this feature to geometric frustration of magnetism. In view of the existence of this phase, the interpretation of the linear-term in the heat-capacity in terms of spin-fluctuations from the Ru 4d band needs to be revisited. Additionally, in the vicinity of TN, AC χ shows a prominent frequency dependence and, below TN, MIRM exhibits a slow decay with time. This raises a question whether the antiferromagnetic structure in this compound is characterized by spin-glass-like dynamics. In contrast to what was reported earlier, there is a change in the sign of the magnetoresistance (MR) at the metamagnetic transition. A butter-fly-shaped (isothermal) MR loop (interestingly spanning over all the four quadrants) is observed at 2 K with distinct evidence for the magnetic phase co-existence phenomenon in zero field after travelling through metamagnetic transition field. The results on polycrystals thus provide additional information about the magnetism of this compound, revealing that the magnetism of this compound is more complex than what is believed, due to geometric frustration intrinsic to Kagome net.

ChemInform Abstract: Spin Fluctuation and Fermi Surface Instability in Ferromagnetic Superconductors

AbstractReview: emphasis on UGe2, URhGe and UCoGe; 45 refs.

Spin fluctuation and Fermi surface instability in ferromagnetic superconductors

Abstract We review the ferromagnetic superconductivity observed in the uranium based compounds, namely UGe2, URhGe and UCoGe, where the spin-triplet state is most likely realized. An unusual upper critical field H c2 , which is enhanced under a magnetic field in a certain field direction, is discussed in terms of spin fluctuations and of Fermi surface instabilities.

Pressure-decoupled magnetic and structural transitions of the parent compound of iron-based 122 superconductors BaFe 2 As 2

The recent discovery of iron ferropnictide superconductors has received intensive concern in connection with magnetically involved superconductors. Prominent features of ferropnictide superconductors are becoming apparent: the parent compounds exhibit an antiferromagnetic ordered spin density wave (SDW) state, the magnetic-phase transition is always accompanied by a crystal structural transition, and superconductivity can be induced by suppressing the SDW phase via either chemical doping or applied external pressure to the parent state. These features generated considerable interest in the interplay between magnetism and structure in chemically doped samples, showing crystal structure transitions always precede or coincide with magnetic transition. Pressure-tuned transition, on the other hand, would be more straightforward to superconducting mechanism studies because there are no disorder effects caused by chemical doping; however, remarkably little is known about the interplay in the parent compounds under controlled pressure due to the experimental challenge of in situ measuring both of magnetic and crystal structure evolution at high pressure and low temperatures. Here we show from combined synchrotron Mössbauer and X-ray diffraction at high pressures that the magnetic ordering surprisingly precedes the structural transition at high pressures in the parent compound BaFe2As2, in sharp contrast to the chemical-doping case. The results can be well understood in terms of the spin fluctuations in the emerging nematic phase before the long-range magnetic order that sheds light on understanding how the parent compound evolves from a SDW state to a superconducting phase, a key scientific inquiry of iron-based superconductors.

First-Principles Dynamical Coherent-Potential Approximation Approach to the Ferromagnetism of Fe, Co, and Ni

Magnetic properties of Fe, Co, and Ni at finite temperatures have been investigated on the basis of the first-principles dynamical CPA (Coherent Potential Approximation) combined with the LDA (Local Density Approximation) + $U$ Hamiltonian in the Tight-Binding Linear Muffintin Orbital (TB-LMTO) representation. The Hamiltonian includes the transverse spin fluctuation terms. Numerical calculations have been performed within the harmonic approximation with 4th-order dynamical corrections. Calculated single-particle densities of states in the ferromagnetic state indicate that the dynamical effects reduce the exchange splitting, suppress the band width of the quasi-particle state, and causes incoherent excitations corresponding the 6 eV satellites. Results of the magnetization vs temperature curves, paramagnetic spin susceptibilities, and the amplitudes of local moments are presented. Calculated Curie temperatures ($T_{\rm C}$) are reported to be 1930K for Fe, 2550K for Co, and 620K for Ni; $T_{\rm C}$ for Fe and Co are overestimated by a factor of 1.8, while $T_{\rm C}$ in Ni agrees with the experimental result. Effective Bohr magneton numbers calculated from the inverse susceptibilities are 3.0 $\mu_{\rm B}$ (Fe), 3.0 $\mu_{\rm B}$ (Co), and 1.6 $\mu_{\rm B}$ (Ni), being in agreement with the experimental ones. Overestimate of $T_{\rm C}$ in Fe and Co is attributed to the neglects of the higher-order dynamical effects as well as the magnetic short range order.

Spin fluctuations and the pseudogap in organic superconductors

We show that there are strong similarities in the spin lattice relaxation of non-magnetic organic charge transfer salts, and that these similarities can be understood in terms of spin fluctuations. Further, we show that, in all of the kappa-phase organic superconductors for which there is nuclear magnetic resonance data, the energy scale for the spin fluctuations coincides with the energy scale for the pseudogap. This suggests that the pseudogap is caused by short-range spin correlations. In the weakly frustrated metals k-(BEDT-TTF)_2Cu[N(CN)_2]Br, k-(BEDT-TTF)_2Cu(NCS)_2, and k-(BEDT-TTF)_2Cu[N(CN)_2]Cl (under pressure) the pseudogap opens at the same temperature as coherence emerges in the (intralayer) transport. We argue that this is because the spin correlations are cut off by the loss of intralayer coherence at high temperatures. We discuss what might happen to these two energy scales at high pressures, where the electronic correlations are weaker. In these weakly frustrated materials the data is well described by the chemical pressure hypothesis (that anion substitution is equivalent to hydrostatic pressure). However, we find important differences in the metallic state of k-(BEDT-TTF)_2Cu_2(CN)_3, which is highly frustrated and displays a spin liquid insulating phase. We also show that the characteristic temperature scale of the spin fluctuations in (TMTSF)_2ClO_4 is the same as superconducting critical temperature, which may be evidence that spin fluctuations mediate the superconductivity in the Bechgaard salts.

Spinlike Susceptibility of Metallic and Insulating Thin Films at Low Temperature

Susceptibility measurements of patterned thin films at sub-K temperatures were carried out using a scanning SQUID microscope that can resolve signals corresponding to a few hundred Bohr magnetons. Several metallic and insulating thin films, even oxide-free Au films, show a paramagnetic response with a temperature dependence that indicates unpaired spins as the origin. The observed response exhibits a measurable out-of-phase component, which implies that these spins will create 1/f-like magnetic noise. The measured spin density is consistent with recent explanations of low frequency flux noise in SQUIDs and superconducting qubits in terms of spin fluctuations, and suggests that such unexpected spins may be even more ubiquitous than already indicated by earlier measurements. Our measurements set several constraints on the nature of these spins.

High-field study of the magnetization in Ti doped ErCo2 compound

The magnetization of powdered MgCu2 structure-type intermetallic ErCo1.95Ti0.05 was studied in the temperature range of 4.2–123K in magnetic fields up to 23T. A field-cooled magnetization jump at certain temperatures (TC) corresponds to a first-order magnetic phase transition from a paramagnetic to a ferrimagnetic state indicating cobalt moment formation. We observe that TC approach asymptotically T=50K with the cooling field Hc. The magnetization above TC stays constant (temperature independent), but does not saturate even in the field of 23T. The moment per unit formula was found to be approximately 7.25μB for this field. This behavior may be attributed to domain structures with the enhanced magnetocrystalline anisotropy field with titanium impurities. Magnetization versus magnetic field curves has also been recorded for both zero-field-cooled and field-cooled cases at various temperatures below and above TC. These curves below TC show typical ferrimagnetic phase; in the intermediate temperature range of 34–50K metamagnetism was observed and the sample becomes pure paramagnetic above 50K. In addition, the resistivity measurement was performed in the temperature range of 4.2–273K. A jumplike drop in the resistivity was observed at TC. In comparison with that of ErCo2, this transition temperature shifts to higher temperature as much as 2K, consistent with the slight increase of the lattice parameter. This drop in resistivity becomes much deeper presumably due to the internal magnetoresistance within the large domains. Analysis of the resistivity data in terms of spin fluctuations including s-d electron scattering and electron-phonon interaction reveals that T2 behavior becomes dominant in the temperature range of 34–50K where the magnetization data exhibit metamagnetism. Therefore, we assert based on both magnetization and resistivity results that the spin-fluctuation effects are driving force for the cobalt moment formations.

Electrical properties of the transition metal compound MnRhP

Magnetization, electrical resistivity, thermoelectric power and magnetoresistance were measured for the ternary compound MnRhP. The magnetization measurement shows that MnRhP is a ferromagnet with a Curie temperature of T C=390 K and a magnetic moment at 4.2 K of 3.1 μ B/Mn. The electrical resistivity has a maximum near T C. Furthermore, a large negative magnetoresistance is observed near T C. Experimental results are discussed in terms of spin fluctuations.

Pressure and doping dependence of Hall effect and effective mass in V 2O 3

We present results of electrical resistivitity and Hall effect measurements on single crystalline V 2− y O 3 as a function of temperature, pressure and doping y. The pressure dependence of the Hall effect suggests an interpretation in terms of spin fluctuations, which is also consistent with the temperature dependence of the electrical resistivity ρ. If a T 2 fit is applied to ρ at low temperatures a saturation rather than a divergence of the corresponding effective mass results as the metal–insulator transition is approached.

Magnetic and electrical properties of the pseudo-binary GdCu 5− xAl x compounds

The effect of substitution of Al for Cu on the lattice parameters, and the magnetic and electrical properties of the GdCu 5− x Al x compounds has been studied. The behaviour of the compounds with low Al concentration (0≤ x≤0.5) differs markedly from that of the compounds with high Al concentration (1.0≤ x≤1.5): for x≤0.5 the compounds appear to have a complex magnetic structure, whereas for the higher Al concentration a standard two-sublattice model yields values for the intra- and intersublattice coupling parameters. For x≤0.5, below the ordering temperature, the magnetic component of the electrical resistivity decreases with increasing temperature, whereas for the higher Al concentration this component increases. These electrical properties are analysed in terms of a change in the band gap with increasing Al concentration, taking into account spin fluctuation terms.

Theoretical investigation of the magnetic behavior of substitutional Fe impurities in Zn, Cd, and Al hosts

Calculations for substitutional Fe in Al in the absence of lattice relaxation yield a large value for the magnetic moment at the impurity site. The lack of magnetic response observed experimentally has been attributed to the presence of spin fluctuations. However, more recent calculations show that the moment at the Fe impurity could vanish in the presence of lattice relaxation around the impurity, thus rendering unnecessary a description in terms of spin fluctuations. We have performed electronic-structure calculations to investigate the magnetic behavior of substitutional Fe impurities in Zn, Cd, and Al hosts as a function of lattice relaxation. In the case of Fe in Al, the theoretical results are sensitive to the lattice constant used for fcc Al. If the experimental value is used, the Fe moment decreases drastically but does not vanish at the experimentally observed relaxation values. In the case of Zn and Cd hosts, the calculated moment remains quite large for all reasonable values of relaxation. Our results indicate that a description in terms of spin fluctuations and high values of the Kondo temperature should be used for Fe in Zn and Cd and may also be appropriate to describe Fe impurities in Al.

Magnetic and electronic properties of hexairon(III) nanocluster with cyclic structure: a Mössbauer study

A molecular cluster containing a coplanar ring of iron(III) ions with spin 5/2 was investigated by Mossbauer spectroscopy. The iron spins are antiferromagnetically coupled so that the ground state has total spin S=0. Spectra in the absence of an applied magnetic field consisted of a quadrupolar doublet, the linewidth of which monotonically increased with the temperature. A quadrupolar splitting of about 0.35 mm/s was found. Calculations of the iron‐orbital electronic populations were carried out and the quadrupolar splitting was estimated. Its value was in agreement with the experimental one. In addition, the trend of the linewidth was explained in terms of isotropic spin fluctuations. Spectra in the presence of a 4.5 T longitudinal magnetic field were also collected. The hyperfine field was obtained from their fitting. Differences with respect to the hyperfine field obtained from susceptibility data were also interpreted in terms of spin fluctuations.

Pressure effect on the magnetic properties of amorphous alloys composed of icosahedral clusters

The pressure effects on the magnetic properties have been investigated for amorphous alloys prepared by high-rate DC sputtering. The pressure derivatives of magnetization and the Curie temperature are significantly large. The concentration dependences of and are similar to those of various magnetovolume and magnetoelastic properties such as the spontaneous volume magnetostriction , forced volume magnetostriction and compressibility . The pressure derivative of the Curie temperature is also large, and the linear relation of versus plot is accounted for by taking inhomogeneity into consideration. The linear plot of versus is explained in terms of spin fluctuations. In connection with the peculiar amorphous structure composed of icosahedral clusters, both and are different from those of amorphous Fe-based binary alloys. The value of determined from the pressure dependence of the spin-wave stiffness constant D is reasonable. The spin freezing temperature is increased with increasing pressure, consistent with the theory concerned with itinerant electron spin-glass systems.

High field magnetization of the spin fluctuation compounds UPt 3, U 0.1Pr 0.9In 3 and U 1− xY xAl 2

Measurements on M vs H up to 50 T in pulsed fields are reported for the spin fluctuation compounds UPt 3 (where measurements up to 35 T already exist), U 1− x Y x Al 2 and Pr 0.9U 0.1In 3. The known metamagnetic transition at 20 T in UPt 3 is observed; no further transition is observed in Upt 3 up to 50 T. Although the compounds U 0.9Y 0.1Al 2U 0.7Y 0Al 2 and Pr 0.1In, also show T 4. In T spin fluctuation terms in their specific heat with specific heat γ values spanning the range 190–670 mJ per mole UK 2, (where Upt, has γ=450 mJ per mole-K 2), no metamagnetic transitions up to 50 are observed in these systems. Possible scaling relations between B metamag and various parameters are discussed.

Magnetic properties of Pd doped with Fe: Intermediate between ferromagnetism and spinglass behavior

The electrical impedance, the ac susceptibility, and the ad magnetization of Pd doped with Fe, whose concentration is between the one for ferromagnetic order and spinglass behaviour, were investigated in the temperature range from 2 mK to 4K and at 0.3μT ≲ H ⩽2 mT. This Fe concentration is between the one for spin glass freezing and the one for ferromagnetic order. With the ac susceptibility and the dc magnetization data, we can exclude a ferromagnetic transition or a reentrant magnetic phase transition. The ac susceptibility, measured directly as well as extracted from the imaginary part of the ac impedance, shows magnetic clustering followed by spin glass (spincluster glass) freezing at 50 mK. The resistivity, obtained from the real part of the ac impedance, exhibits a very complicated temperature dependence which can be understood in terms of spin fluctuations, magnetic clustering, and electron-magnon scattering inside of the magnetic clusters.

Ferromagnetism in the ThCr 2Si 2 type phosphide LaCo 2P 2

The magnetic order of the cobalt sublattice of the ThCr 2Si 2-type phosphide LaCo 2P 2 was investigated for polycrystalline samples by neutron diffraction. A collinear ferromagnetic structure with magnetic moments of μ exp = 0.44(3) μ B / Co aligned perpendicular to the [001] axis was found. This was confirmed by a diffraction experiment with polarized neutrons, which resulted in a value of μ exp = 0.47(5) μ B . The magnetization processes of a single crystal are in agreement with the proposed magnetic structure. At 5 K a spontaneous magnetization of M s = 0.391(1) μ B / Co along the easy [100] axis was measured. The thermal variations of the spontaneous magnetization, the high-field susceptibility in the ordered magnetic phase and the paramagnetic susceptibility are interpreted in terms of spin fluctuations of which the longitudinal component is close to saturation. At 5 K the anisotropy constants k 1 = -0.195(5) X 10 6 J/ m 3 and k 2 = 0.042(4) X 10 6J/m 3 were obtained. The temp ture dependence of the energy anisotropy is unusual, which is ascribed to thermal variations of the band structure.

Thermal and Transport Properties of Sc1-xTixFe2

Low temperature specific heat and electrical resistivity have been studied for the Laves phase compounds Sc 1- x Ti x Fe 1.95 , which show the ferromagnetic to ferromagnetic (F-F) transition with increasing temperature at around x =0.7. It is shown that the electronic specific heat coefficient, γ , and the coefficient of T 2 term in the resistivity, A , show a maximum at the critical concentration for the onset of Fe moments at the 2a site. The correlation between γ and A are discussed in terms of spin fluctuations. A small but visible anomaly was observed in the specific heat curve of Sc 0.3 Ti 0.7 Fe 1.95 at 100 K due to the F-F transition. The origin of the entropy change associated with the F-F transition is also discussed.

Muon spin relaxation in ferromagnets. II. Critical and paramagnetic magnetization fluctuations

For pt.I see ibid., vol.4, p.2043 (1992). Mechanisms for the relaxation of muon spin relaxation signals in paramagnets are analysed in terms of spin fluctuations in a Heisenberg magnet with a ferromagnetic exchange interaction. Explicit predictions provided for EuO are based on the assumption that the implanted muon occupies an interstitial site of high symmetry. Because the average dipolar field at the muon site is zero, and the material is ferromagnetic, the dipolar mechanism of relaxation is not enhanced by critical spin fluctuations. In consequence, the hyperfine mechanism dominates relaxation in the critical region, even though the ratio of the hyperfine and dipolar coupling constants might be small. Calculations of relaxation rates for both mechanisms, made on the basis of coupled-mode theory, are provided for critical and paramagnetic states.