Nature Of Spin Fluctuations Research Articles

Unconventional normal-state spin dynamics in underdoped high-Tccuprates as a fingerprint of spiral correlations of localized spins and dual localized/itinerant nature of spin fluctuations

Unconventional normal-state spin dynamics in underdoped high-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>T</mml:mi><mml:mi>c</mml:mi></mml:msub></mml:math>cuprates as a fingerprint of spiral correlations of localized spins and dual localized/itinerant nature of spin fluctuations

Study of Magnetic Properties of Layered Compound LaCoAsO and its Related Compounds

We successfully synthesized itinerant-electron ferromagnetic compound LaCoAsO and studied its magnetic properties based on the self-consistent renormalization theory of spin fluctuations and Takahashi's spin-fluctuation theory. We showed that the magnetization (M) of LaCoAsO shows the convex behavior when plotted in the form of M 2 vs H/M, while the M 4 shows the almost linear relation against H/M, which is predicted in the Takahashi's theory. From the comparison between the experimentally observed magnetic susceptibility and calculated one by using the self-consistent renormalization theory, we showed that the nature of spin fluctuations is 3-dimensional around the Curie temperature TC, while above 3TC the difference between two susceptibilities becomes more prominent with increasing temperature, indicating that 2-dimensional spin fluctuations dominate over the 3-dimensional ones in the high temperature region.

Magnetovolume effects of quasi-one-dimensional itinerant electron magnets [formula omitted

Magnetovolume effects of 3d heavy-electron compounds with linear spin chains, ( La 1 - x Y x ) Mn 4 Al 8 with x ⩽ 0.15 and = 1 , have been investigated to get information on the ground state of LaMn 4 Al 8 and the nature of spin fluctuations in this system. The negative thermal expansion observed for LaMn 4 Al 8 is suppressed by the substitution of a small amount of Y for La. Together with the field-cooled effect in the susceptibility, the magnetovolume effect suggests the development of short-range magnetic correlation in LaMn 4 Al 8 at low temperatures.

Thermal Expansion and Spin Fluctuations in Amorphous Y&amp;ndash;TM (TM=Ni, Co, Fe and Mn) Alloys

The magnetic properties of amorphous 3d metal based alloys are diverse, depending on the alloying element. Therefore, it is expected that the nature of spin fluctuations and its influence on the thermal expansion are different from one another for various alloy systems. In the present study, the nature of spin fluctuations and the thermal expansion have been investigated for amorphous Y-TM (TM=Ni, Co, Fe and Mn) alloys prepared by high-rate DC sputtering. The characteristic parameters obtained from the Arrott plots measured with a SQUID magnetometer indicate that the nature of spin fluctuations in the amorphous Y-Ni alloys is similar to that in crystalline weakly ferromagnets. With decreasing Y concentration, the nature of spin fluctuations is located in the interpolation region, being accompanied by the increase in the spontaneous volume magnetostriction. On the other hand, the spin fluctuations in the amorphous Y-Co alloys are relatively localized in the real space, compared with those of amorphous Y-Ni alloys. The amorphous Y-Fe alloys show a large spontaneous volume magnetostriction, indicating a significant variation of the amplitude of spin fluctuations. The thermal expansion coefficient of the amorphous Y-Mn alloys implies that the amplitude of spin fluctuations scarcely depends on the temperature, and the thermal variation of spin fluctuations is observed only in high Mn concentration ranges.

Slave-boson study of the t-t'-J model: Phase diagram, spin susceptibility, and Hall resistivity.

We investigate the normal-state properties of high-${\mathit{T}}_{\mathit{c}}$ cuprates in terms of the t-t'-J model using a spin-rotation-invariant slave-boson technique. The second-neighbor hopping t'0 is included in order to reproduce the Fermi surfaces of LSCO (${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$) and YBCO (${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$) type. The magnetic phase diagram of the t-t'-J model is derived within the saddle-point approximation, by taking into account also incommensurate spiral order. Compared to the pure t-J model, N\'eel order is stabilized in the low-doping region. In particular, for the case t'&gt;0, which corresponds to the electron-doped systems, e.g., ${\mathrm{Nd}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ (NCCO), the antiferromagnetic correlations are strongly enhanced near half-filling. We include fluctuation corrections and calculate the dynamic (paramagnetic) spin susceptibility \ensuremath{\chi}(q,\ensuremath{\omega}) going beyond the random-phase approximation. The instability line of the paraphase obtained from \ensuremath{\chi}(q) is in agreement with the saddle-point phase diagram. The wave-vector dependence of \ensuremath{\chi}(q) reveals the commensurate (incommensurate) nature of spin fluctuations in YBCO (LSCO). Finally, the doping dependence of the Hall resistivity ${\mathit{R}}_{\mathit{H}}$(\ensuremath{\delta}) is calculated, where the results agree surprisingly well with experiments on LSCO. For YBCO and NCCO, the t' term suffices to give the correct dependence for ${\mathit{R}}_{\mathit{H}}$(\ensuremath{\delta}).

Spin-flip scattering of low-energy electrons from magnetic surfaces near their ordering temperature: Application to Cr(110).

We examine the theory of spin-flip scattering of low-energy electrons from magnetic surfaces near their ordering temperature, to inquire if the spin-flip contribution to the diffuse background can be used to explore the nature of spin fluctuations near the surface, in this temperature regime. The spin-flip contribution to the diffuse background is related to equal-time correlation functions, which contain information about the amplitude of the near-surface spin fluctuations. These are calculated within the framework of Landau-Ginzburg theory, and their properties explored for various cases. The results are combined with a multiple-scattering-based description of the excitation process, to generate quantitative predictions for the antiferromagnetic Cr(110) surface. The calculations enable us to isolate particular scattering geometries which should be particularly favorable for the observation of surface spin fluctuations near the N\'eel temperature.

Quantitative Aspects of the Theory of Nearly Ferromagnetic Metals

Effects of spin fluctuations on the magnetic susceptibility and the specific heat of nearly ferromagnetic metals are discussed quantitatively within the long wavelength approximation, by using four parameters specifying the nature of spin fluctuations. The temperature dependence of the low temperature specific heat is discussed in detail with particular reference to Ni 3 Ga and TiBe 2 . It is pointed out that the observed T 3 In T like behavior in TiBe 2 should not be attributed to the one derived previously from the long wave-length components of spin fluctuations. The specific heat due to localized paramagnons is also discussed and the T 3 ln T like behavior is found in a certain temperature range around a characteristic temperature \(\varGamma/2\pi\), \(\varGamma\) being the damping constant of localized paramagnons. The T 3 ln T like behavior observed in UPt 3 seems to be explained primarily by this effect rather than the contribution of small q -components of spin fluctuations as postulated in previous analyses.

The nature of spin fluctuations in Fe and Ni

A recently developed cluster theory describing the coupling between quasilocal moments formed by clusters is applied to the problem of photoemission line shape. The quantitative analysis — the first of its kind — of the data shows that Ni has large short-range order.