Spin Fluctuation Temperature Research Articles

Study of magnetic, thermodynamic and transport properties of Laves phase NdRh[formula omitted]

The results of magnetic measurements, electron spin resonance, heat capacity, resistivity, and magnetocaloric effect as well as density functional theory (DFT), are presented for the compound NdRh2 (MgCu2-type structure). This compound was synthesized at a pressure of 8 GPa and a temperature of 1700 K. The magnetic properties of the material are shown to be determined by the high-temperature (over 400 K) spin polarization of the 4d Rh electrons and by the ferrimagnetic (FiM) interaction of Rh and Nd magnetic subsystems. Concurrently, the spontaneous magnetization of the spin polarization of 4d Rh electrons is ≈0.01μB/Rh, while the Nd magnetic moment is ≈1.7μB/Nd. This leads to complicated magnetic behavior with long range FiM order at TC≲7 K at zero field and with wide temperature range of spin fluctuations TC<T≲50 K. Specific heat and resistivity data provide corroboration of the spin fluctuation regime with the spin fluctuation temperature Θsf≈28.5 K. The optimal critical parameters were identified as β=1.18±0.02, γ=0.85±0.02, and TC≈40 K from modified Arrott plotting, which are distinct from any conventional universality class. The maximum magnetocaloric effect, when the magnetic field changes from 0 to 9 T, ΔH=9 T occurs at T≈11 K and the magnetic entropy change reaches −ΔSm=7.0 J (kg K)−1. In our DFT calculations, the FiM arrangement was obtained, with values of magnetic moments of Nd and Rh. Additionally, the Fermi surface was constructed.

Tunable Spin Hall Magnetoresistance in All-Antiferromagnetic Heterostructures

We investigate the spin Hall magnetoresistance (SMR) in all-antiferromagnetic heterostructures α-Fe2O3/Cr2O3 with Pt contacts. When the temperature is ultralow (< 50 K), the spin current generated in the Pt layer cannot be transmitted through Cr2O3 (t = 4 nm), and the SMR is near zero. Meanwhile, when the temperature is higher than the spin fluctuation temperature T F (≈ 50 K) of Cr2O3 and lower than its Néel temperature T N (≈ 300 K), the spin current goes through the Cr2O3 layer and is reflected at the α-Fe2O3/Cr2O3 interface; an antiferromagnetic (negative) SMR is observed. As temperature increases higher than T N, paramagnetic (positive) SMR mainly arises from the spin current reflection at the Cr2O3/Pt interface. The transition temperatures from negative to positive SMR are enhanced with increasing Cr2O3 layer thickness, accompanied by the absence of SMR signals when t = 10 nm. Such a tunable SMR builds a bridge between spin transport and structures. It also enriches antiferromagnetic spintronics.

Intermediate Valence Behavior of Yb2Cu9Al8

Intermediate valence behavior is frequently observed in materials containing Ce, Yb, Eu, Sm, or Tm. In the current work, we report synthesis and characterization of Yb2Cu9Al8 (Th2Zn17 structure type). Its intermediate valence behavior can be described by an excitation energy Eex/kB = 319 K and a spin fluctuation temperature Tsf = 60 K. The valence state of Yb is estimated to be close to 2.04 for the low‐temperature region. The valence gradually evolves to the value of 2.80 at T = 400 K. The specific heat coefficient of γexp = 59 mJ·molYb–1·K–2 indicates a moderate effective mass enhancement, together with finite density of states at the Fermi level. The latter is also confirmed by the band structure calculations.

Local magnetism and spin fluctuation behavior of extremely dilute Fe impurity in Pd-U binary alloys: An experimental and theoretical investigation

We report the magnetic behavior of isolated Fe impurity atoms in Pd1−xUx (0 ≤ x ≤ 0.12) alloys studied through measurements of the local susceptibility (χloc) and spin relaxation time for 54Fe nuclear probe using the time differential perturbed angular distribution (TDPAD) technique. With an increase of the U concentration in Pd-U alloys, the magnetic response of Fe, exhibiting Curie-Weiss type χloc(T), reflects a steady decrease of the giant magnetic moment arising from ferromagnetic polarization of host Pd-4d band electrons. Concomitantly, we find an exponential rise in the spin fluctuation temperature TSF. The experimental observation of U induced suppression of giant magnetic moment of Fe in Pd is supported by ab intio electronic structure calculations based on density functional theory. The results presented here support the idea that sign and strength of spin polarization of host conduction band electrons play a crucial role on the spin fluctuation rate and hence moment stability.

Comparative studies of the cage systems ThFe2Al10 and UFe2Al10

We have grown single crystals of ThFe2Al10 and used them for the refinement of its crystal structure. From the specific heat data, we determined the phonon components, that is, the Debye and Einstein modes, for Th- and U-based isostructural aluminides. To approximate the Sommerfeld coefficients, we had to include the T3lnT term characteristic of spin-fluctuations originating from the Fe-sublattice. The roughly estimated spin-fluctuation temperature is about 11 K. ThFe2Al10 is a weakly temperature-dependent Pauli-paramagnet. However, at lower temperatures the ferromagnetic correlations are observed, which signals that the system drives to quantum criticality. The distinct minimum in electrical resistivity observed at about 20 K suggests a realization of a nonmagnetic two-channel Kondo-effect in ThFe2Al10, as discussed for ThAsSe [Phys. Rev. Lett. 94, 236603 (2005)], for example. The [ρ(T) – ρmin] versus T curves of ThFe2Al10, determined along the a- and c-axes, were used to subtract the phonon contributions from the corresponding ρ(T)a,c of UFe2Al10. The resulting magnetic part of the average ρm(T)av curve was then analysed in terms of the influence of the crystal field effect on the transport properties. However, to obtain agreement with the experiment, we had to take into account another effect, namely the Kondo-like one. This kind of probe has been applied for the first time in the case of uranium compounds. Based on the magnetoresistivity, we have revealed the anisotropic low-frequency vibrations of the Th atom (located in its [Al16Fe4] cage) interacting with the conduction band, the phenomenon revealed previously in the metallic UB12 [Phil. Mag. B 95, 2343 (2015)]. Furthermore, fully relativistic band structure calculations performed for ThFe2Al10 revealed its metallic-like character with a similarly large contribution of the Fe 3d electrons at the Fermi level as predicted previously for its 5f-electron analogue UFe2Al10. In addition, there are substantial differences between their Fermi surfaces.

Spin fluctuation and local magnetism of isolated Fe impurities in Pd1−xVx alloys studied by time differential perturbed angular distribution spectroscopy

The magnetic moment and spin fluctuation temperature of isolated Fe impurity atoms in Pd1−xVx (0 ≤ x ≤ 0.15) alloys have been studied by time differential perturbed angular distribution (TDPAD) technique. With increasing V content in Pd matrix, a large non-linear reduction of the local magnetic moment accompanied with an exponential increase of the spin fluctuation temperature TSF has been observed. At and beyond x = 0.12, the Fe atoms are found to be nonmagnetic. As an important new feature, TSF is observed to vary quadratically with composition dependent changes in host spin polarization.

Lattice location and magnetism of isolated Fe impurities in narrow band semiconductor InSb

Employing the time differential perturbed angular distribution method, we have measured local susceptibility and spin relaxation rate of 54Fe nuclei implanted in InSb. The magnetic response of Fe, identified to occupy the metal as well as the semi-metal atom sites, exhibit Curie-Weiss type susceptibility and Korringa like spin relaxation rate, revealing the existence of localized moments with small spin fluctuation temperature. The experimental results are supported by first principle electronic structure calculations performed within the frame work of density functional theory.

Low-temperature properties of single-crystalCrB2

We report the low-temperature properties of $^{11}$B-enriched single-crystal CrB$_{2}$ as prepared from high-purity Cr and B powder by a solid-state reaction and optical float zoning. The electrical resistivity, $\rho_{\rm xx}$, Hall effect, $\rho_{\rm xy}$, and specific heat, $C$, are characteristic of an exchange-enhanced Fermi liquid ground state, which develops a slightly anisotropic spin gap $\Delta \approx 220\,{\rm K}$ below $T_{\rm N}=88\,{\rm K}$. This observation is corroborated by the absence of a Curie dependence in the magnetization for $T\to0$ reported in the literature. Comparison of $C$ with $d\rho_{\rm xx}/dT$, where we infer lattice contributions from measurements of VB$_2$, reveals strong antiferromagnetic spin fluctuations with a characteristic spin fluctuation temperature $T_{\rm sf}\approx 257\,{\rm K}$ in the paramagnetic state, followed by a pronounced second-order mean-field transition at $T_{\rm N}$, and unusual excitations around $\approx T_{\rm N}/2$. The pronounced anisotropy of $\rho_{\rm xx}$ above $T_{\rm N}$ is characteristic of an easy-plane anisotropy of the spin fluctuations consistent with the magnetization. The ratio of the Curie-Weiss to the N$\acute{\rm{e}}$el temperatures, $f=-\Theta_{\rm CW}/T_{\rm N}\approx 8.5$, inferred from the magnetization, implies strong geometric frustration. All physical properties are remarkably invariant under applied magnetic fields up to $14\,\,{\rm T}$, the highest field studied. In contrast to earlier suggestions of local-moment magnetism our study identifies CrB$_{2}$ as a weak itinerant antiferromagnet par excellence with strong geometric frustration.

Lattice location and local magnetism of recoil implanted Fe impurities in wide and narrow band semiconductors CdTe, CdSe, and InSb: Experiment and theory

Employing the time differential perturbed angular distribution method, we have measured local susceptibility and spin relaxation rate of 54Fe nuclei implanted in III-V and II-VI semiconductors, CdTe, CdSe, and InSb. The magnetic response of Fe, identified to occupy the metal as well as the semi-metal atom sites, exhibit Curie-Weiss type susceptibility and Korringa like spin relaxation rate, revealing the existence of localized moments with small spin fluctuation temperature. The experimental results are supported by first principle electronic structure calculations performed within the frame work of density functional theory.

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.

Specific heat and magnetic susceptibility of single-crystalline ZnCr 2Se 4 spinels doped with Ga, In and Ce

The crystal structure, X-ray photoelectron spectra (XPS), dc magnetic isotherm, ac magnetic susceptibility and specific heat measurements for antiferromagnetic and semiconducting ZnCr 2Se 4 spinel diluted with Ga, In and Ce are presented. For all the studied spinels the XPS spectra exhibit the Cr 2 p 3/2 splitting of 1 eV characteristic for the 3 d 3 electron configuration of the chromium ions. A correlation between the second critical field H c2 of the helix to paramagnetic transition and the magnetic specific heat C-peak was found in (Zn 0.86Ce 0.08)[Cr 2]Se 4. This correlation weakens for (Zn 0.99)[Cr 1.92Ga 0.06]Se 4 and disappears for (Zn 0.83In 0.11)[Cr 1.73In 0.14]Se 4 as the saturation magnetic moment rapidly decreases in the sequence Ce, Ga and In. The magnetic contribution to the specific heat displays a sharp peak at T N and is maximal at the spin fluctuation temperature of 40 K, which is related to the maximum of the magnetic susceptibility at the same temperature and at 50 kOe in the spin fluctuation region, evidenced by the entropy exceeding 90% of the entropy calculated classically for the complete alignment of the Cr spins, S m = R ln(2 S + 1). These effects are considered within the cation–anion distances in octahedral sites, the cation deficiency and the spin state of 3 d Cr 3+ ions in t 2g orbital.

Specific heat and magnetic susceptibility of single-crystalline ZnCr 2− xAl xSe 4 ( x=0.15, 0.23)

A correlation between the second critical field H c2 of the helix to paramagnetic transition and the magnetic specific heat C-peak was found in ZnCr 2− x Al x Se 4 spinel single crystals with x=0.15, 0.23. The specific heat peak is anomalously sharp for all finite magnetic fields used here and this points to a first order magneto-structural transition (from cubic to tetragonal symmetry). The C( T)-peak is increasingly suppressed as the external field increases. Approaching the Neel temperature T N , a broad ac-magnetic susceptibility peak is observed for zero dc-magnetic field. That peak does not show an energy loss and thus points towards a return to a second order type of transition. The magnetic contribution to the specific heat displays a sharp peak at T N and is maximal at the spin fluctuation temperature T sf =34 K. T sf is related to the maximum of the magnetic susceptibility at T m =40 K (at 50 kOe) in the spin fluctuation region, as evidenced by the entropy exceeding 90% of the entropy calculated classically for the complete alignment of the Cr spins, (2− x) R ln(2 S+1). The X-ray photoelectron spectroscopy (XPS) data indicate that Al-substitution does not affect Cr 3+ 3d 3 electronic configuration.

Butterflylike specific heat, magnetocaloric effect, and itinerant metamagnetism in(Er,Y)Co2compounds

The field-induced first-order phase transition in $({\text{Er}}_{1\ensuremath{-}x}{\text{Y}}_{x}){\text{Co}}_{2}$ with the yttrium concentration $x=0.45$ is observed to be accompanied by a butterflylike behavior and significant irreversibility of the specific heat. The coefficient $\ensuremath{\gamma}$ of the $T$-linear specific heat decreases by $\ensuremath{\sim}48%$ under application and removal of a magnetic field up to 20 kOe. This behavior is attributed to the itinerant electron metamagnetism of $\text{Co}\text{ }3d$ electrons. The isothermal magnetic entropy change $\ensuremath{\Delta}{S}_{m}$ in ${\text{Er}}_{0.55}{\text{Y}}_{0.45}{\text{Co}}_{2}$ includes a large contribution associated with spin fluctuations induced by the $f\text{\ensuremath{-}}d$ exchange interaction in the hybridized $3d\text{\ensuremath{-}}5d$-electron subsystem. These spin fluctuations are suggested to contribute substantially to the magnetocaloric effect of the $R{\text{Co}}_{2}$ type compounds. The maximal $\ensuremath{\Delta}{S}_{m}$ value observed for ${\text{ErCo}}_{2}$ just above the Curie temperature is ascribed to the closeness of the ${T}_{\text{C}}$ value to the spin-fluctuation temperature ${T}_{\text{sf}}$ of itinerant $\text{Co}\text{ }3d$ electrons. The nonmonotonous change in $\ensuremath{\Delta}{S}_{m}$ with the Curie temperature of ${R}_{1\ensuremath{-}x}{R}_{x}^{\ensuremath{'}}{\text{Co}}_{2}$ compounds is explained by the temperature variation in the spin-fluctuation contribution to the magnetocaloric effect.

Non-Fermi-liquid behavior inCaCu3Ru4O12

We present bulk magnetic susceptibility, specific heat, transport, NMR, and NQR results of the perovskite compound ${\text{CaCu}}_{3}{\text{Ru}}_{4}{\text{O}}_{12}$. The data consistently describe a metallic state with a moderately enhanced Sommerfeld coefficient of $\ensuremath{\gamma}=92\text{ }\text{mJ}/\text{mol}\text{ }{\text{K}}^{2}$ and a spin-fluctuation temperature of about 180 K. The heat capacity below 2 K shows a logarithmic increase providing evidence for non-Fermi-liquid (NFL) behavior in ${\text{CaCu}}_{3}{\text{Ru}}_{4}{\text{O}}_{12}$. Further signatures of NFL properties are found in NQR measurements revealing deviations from a Korringa behavior of the spin-lattice relaxation rate. The results are discussed with respect to heavy-fermion and NFL properties in other transition-metal compounds and traditional $f$-electron systems.

Short-range magnetic ordering inURh0.7Ru0.3Ge

The magnetic properties of URh0.7Ru0.3Ge were investigated by means of ac susceptibility, dc magnetization, specific heat andelectrical resistivity measurements. We observed a maximum of the ac susceptibility atTmax = 2.9 K and its frequency dependence, but no spontaneous magnetization at 2 K. Analternative scenario for the observed feature, for example, spin-glass freezing,is not confirmed by dc field dependences of the ac susceptibility and dcmagnetization. The data from specific heat and electrical resistivity studies down to2 K do not suggest long-range magnetic ordering, and point to spin-fluctuationbehaviour of the alloy. We have estimated a spin-fluctuation temperature of∼27 K and Stonerenhancement factor of ∼490. The experimental data are discussed in the framework of spin-fluctuation theories.

From antiferromagnetic ordering to spin fluctuation behavior induced by hydrogenation of ternary compounds CeCoSi and CeCoGe

The ternary compounds CeCoSi and CeCoGe exhibit an antiferromagnetic ordering at 8.8 and 5.0 K, respectively. We present magnetization and specific heat measurements on these compounds and their hydrides, CeCoSiH and CeCoGeH. The hydrogen absorption reveals an original transition from antiferromagnetic to spin fluctuation behavior. CeCoSiH and CeCoGeH present a spin fluctuation temperature T sf around 130 and 15 K, respectively.

Spin fluctuations in the skutterudite compound LaFe4Sb12

We report transport, magnetic and thermodynamic properties of the skutterudite compound LaFe4Sb12. The basic features are a large magnetic susceptibility χ(T), and large electronic coefficient γ of the heat capacity. In particular, a T1.35, T1.7, and T-2/3 temperature dependence of the magnetic susceptibility χ(T), resistivity ρ(T), and Gruneisen parameter Γ(T), respectively, is found at low temperature. An overall understanding of these physical properties is achieved, assuming that LaFe4Sb12 is a non-Fermi liquid system close to a ferromagnetic quantum critical point, with a spin fluctuation temperature Tsf=50±15 K.

Field-Induced Suppression of the Heavy-Fermion State inYbRh2Si2

We report dc-magnetization measurements on YbRh2Si2 at temperatures down to 0.04 K, magnetic fields B< or =11.5 T, and under hydrostatic pressure P< or =1.3 GPa. At ambient pressure a kink at B* =9.9 T indicates a new type of field-induced transition from an itinerant to a localized 4f state. This transition is different from the metamagnetic transition observed in other heavy-fermion compounds, as here ferromagnetic rather than antiferromagnetic correlations dominate below B*. Hydrostatic pressure experiments reveal a clear correspondence of B* to the characteristic spin fluctuation temperature determined from specific heat.

Intermediate valence behaviour of the ternary cerium–nickel–phosphide Ce20Ni42P30

We have investigated the magnetic properties and the X-ray absorption spectra of the ternary compound Ce20Ni42P30, which crystallises in the hexagonal Sm20Ni41.6P30 structure type, space group P63/m. The cerium atom adopts an intermediate valence state ν=3.19. The high-temperature dependence of the magnetic susceptibility was analysed in terms of the interconfiguration fluctuation model resulting in a characteristic spin fluctuation temperature TSF=249K and an energy difference between the corresponding 4f0 and 4f1 states Eex/k=879K.