Effective mass enhancement and spin-glass behaviour in CeCu4Mn(y)Al(1-y) compounds

We report on the magnetic ac/dc susceptibility and specific heat measurements for the CeCu(4)Mn(y)Al(1-y) series of compounds with 0 ≤ y ≤ 1. All compounds investigated crystallize in the hexagonal CaCu(5)-type structure with the space group P6/mmm. The results reveal that the frustration of the interactions dominates and leads to a spin-glass (SG) behaviour with a linear change of the freezing temperature T(f) as a function of y. The SG state has been confirmed by the frequency dependence of the ac magnetic susceptibility, the relaxation of the remanent magnetization and the split of the field-cooled-zero-field-cooled dc magnetic susceptibility. The electronic specific heat coefficient γ is enhanced for all y and increases with the Al content. Below a threshold at y ≈ 0.3, γ is large even if determined for temperatures above T(f); moreover the paramagnetic Curie-Weiss temperature θ(p) changes sign to negative. These observations indicate the possible presence of the heavy fermion (HF) state, at least below y ≈ 0.3, and the possible coexistence of the SG and HF states down to y = 0, i.e. for the CeCu(4)Al compound. A tentative magnetic phase diagram for CeCu(4)Mn(y)Al(1-y) has been constructed.     

Antiferroquadrupolar ordering in a Pr-based superconductor PrIr(2)Zn(20)

An antiferroquadrupolar ordering at T(Q)=0.11 K has been found in a Pr-based superconductor PrIr(2)Zn(20). The measurements of specific heat and magnetization revealed the non-Kramers Γ(3) doublet ground state with the quadrupolar degrees of freedom. The specific heat exhibits a sharp peak at T(Q)=0.11 K. The increment of T(Q) in magnetic fields and the anisotropic B-T phase diagram are consistent with the antiferroquadrupolar ordered state below T(Q). The entropy release at T(Q) is only 20% of Rln2, suggesting that the quadrupolar fluctuations play a role in the formation of the superconducting pairs below T(c)=0.05 K.     

High-pressure effect on elastic constants, stacking fault energy and correlation with dislocation properties in MgO and CaO

The elastic properties and the generalized-stacking-fault-energy (GSFE) in MeO (Me = Mg, Ca) under different pressures have been calculated using the first principle calculations. In the anisotropic elasticity theory approximation, by using the Foreman’s method, the core structure and Peierls stress of $\tfrac{1} {2}\left\langle {110} \right\rangle \left\{ {110} \right\}We report structural, DC magnetization, detailed linear/non-linear AC susceptibility, (with applied frequency and amplitude) isothermal and thermoremanent magnetization (TRM) behavior for RuSr₂Y_1.5Ce_0.5Cu₂O₁₀ (YRu-1222) magneto-superconductor to understand its complex magnetism. Studied sample is synthesized through the novel solid state high pressure (6?GPa) high temperature (1450?°C) (HPHT) technique. The compound is crystallized in tetragonal structure with space group I4/mmm (No.?139). DC magnetic susceptibility shows that studied YRu-1222 is magneto-superconducting with Ru spins magnetic ordering at around 110?K and superconductivity (SC) in the Cu-O₂ planes below ?~?30?K. Frequency and field dependent detailed AC magnetic susceptibility measurements confirms the spin-glass (SG) behavior with homogeneous/non-homogeneous ferromagnetic (FM) clusters in this system. Variation of cusp position with applied AC frequency follows the famous Vogel-Fulcher law, which is commonly accepted feature for spin-glass (SG) system with homogeneous/non-homogeneous ferromagnetic clusters embedded in spin-glass (SG) matrix. Above the freezing temperature (T _f ), first and third harmonics AC susceptibility analysis indicated possibility of the co-existence of spin cluster ferromagnetism with superparamagnetism (SPM). The M-H loops at low temperature exhibit the ferromagnetic behavior with rather small coercive field (H _c ) and remnant magnetization (M _r ). Summarily, the magnetic (DC and AC) susceptibility measurements and their analysis have enabled us to unearth the complex magnetism in terms of successive SG-FM-SPM transitions with temperature.     

Spin dynamics of the S = 5/2 2D triangular antiferromagnet Ba3NbFe3Si2O14

We report pulse-field magnetization, ac susceptibility, and 100 GHz electron spin resonance (ESR) measurements on the S = 5/2 two-dimensional triangular compound Ba3NbFe3Si2O14 with the Néel temperature T(N) = 26 K. The magnetization curve shows an almost linear increase up to 60 T with no indication of a one-third magnetization plateau. An unusually large frequency dependence of the ac susceptibility in the temperature range of T = 20-100 K reveals a spin-glass behavior or superparamagnetism, signaling the presence of frustration-related slow magnetic fluctuations. The temperature dependence of the ESR linewidth exhibits two distinct critical regimes; (i) ΔH(pp)(T) is proportional to (T-T(N))(-p) with the exponent p = 0.2(1)-0.2(3) for temperatures above 27 K, and (ii) ΔH(pp)(T) is proportional to (T-T*)(-p) with T* = 12 K and p = 0.8(1)-0.8(4) for temperatures between 12 and 27 K. This is interpreted as indicating a dimensional crossover of magnetic interactions and the persistence of short-range correlations with a helically ordered state.     

Effect of dipolar and exchange interactions on magnetic blocking of maghemite nanoparticles

Magnetic interparticle interactions compete with the magnetic blocking of ultrafine magnetic nanoparticles. We have prepared maghemite (γ-Fe₂O₃) nanoparticles by microwave plasma synthesis as a loose powder and in compacted form. In ZFC/FC measurements, blocking temperature of the compacted sample C is larger than that of the powder sample P. The frequency dependence of AC susceptibility of the sample C shows a large shift of blocking temperature with increasing frequency. Vogel-Fulcher law gives a large value of T₀ for the sample C. To get evidence of a possible spin-glass freezing in both samples, scaling law fitting is applied to the AC susceptibility data. The value of the exponent (zv) of the critical slowing down dynamics fits to the spin-glass regime for both samples. For the sample P, spin-glass freezing occurs on the surface of individual nanoparticles, while in the sample C surface spin-glass freezing is concomitant with a superspin-glass formation as a consequence of coupling between particles. The sample C also shows an enhancement of coercivity due to dipolar interactions among the nanoparticles. Exchange interactions are attributed only to touching nanoparticles across their interfaces. All these measurements indicate the presence of strong interparticle dipolar interactions in the compacted sample C.     

Spin-glass and magnetic blocking transitions in amorphous YFe2

Low field dc magnetic susceptibility measurements on amorphous YFe₂ show a distinct cusp-like peak at T_S.G. = 58 K. This result, together with earlier Mőssbauer and neutron scattering measurements, indicates that a true thermodynamic spin-glass transition occurs at T_S.G.. In addition, susceptibility and coercive field data are presented which strongly suggest a magnetic freezing or blocking temperature near T = 20 K. This is the first time these two magnetic phenomena have been observed in the same magnetic system.     

Cole-Cole Analysis of the Superspin Glass System Co 80 Fe 20 /Al 2 O 3

Ac susceptibility measurements were performed on discontinuous magnetic multilayers [Co 80 Fe 20 ( t )/Al 2 O 3 (3 nm)] 10 , t = 0.9 and 1.0 nm, by Superconducting Quantum Interference Device (SQUID) magnetometry. The CoFe forms nearly spherical ferromagnetic single-domain nanoparticles in the diamagnetic Al 2 O 3 matrix. Due to dipolar interactions and random distribution of anisotropy axes the system exhibits a spin-glass phase. We measured the ac susceptibility as a function of temperature 20 h T h 100 K at different dc fields and as a function of frequency 0.01 h f h 1000 Hz. The spectral data were successfully analyzed by use of the phenomenological Cole-Cole model, giving a power-law temperature dependence of the characteristic relaxation time c and a high value for the polydispersivity exponent, f , 0.8, typical of spin glass systems.     

Spin-glass-like behaviour and long-range order in dilute YTb alloys

Using neutron diffraction, we have examined the magnetic ordering of two YTb alloys whose magnetic properties are characteristic of spin-glass materials. For YTb_{5 at %} we have determined that a spiral state appears below the susceptibility cusp at 26.6 K, despite differences between field coolled and zero field cooled magnetization. No evidence for long-range order was found for YTb_{3 at %} below its cusp at 15.5 K. We conclude that irreversibility alone can not be used to characterize the spin-glass state.     

Structures and physical properties of R_2TX_3 compounds

Rare-earth compounds have been an attractive subject based on the unique electronic structures of the rare-earth elements. Novel ternary intermetallic compounds R2TX3 (R = rare-earth element or U, T = transition-metal element, X = Si, Ge, Ga, In) are a significant branch of this research field due to their complex and intriguing physical properties, such as magnetic order at low temperature, spin-glass behavior, Kondo effect, heavy fermion behavior, and so on. The unique physical properties of R2TX3 compounds are related to distinctive electronic structures, crystal structures, micro-interaction, and external environment. Most R2TX3 compounds crystallize in AlB2-type or derived AlB2-type structures and exhibit many similar properties. This paper gives a concise review of the structures and physical properties of these compounds. Spin glass, magnetic susceptibility, resistivity, and specific heat of R2TX3 compounds are discussed.     

Crystal field states in CeCu4Al

Heavy fermion CeCu₄Al compound has been studied by inelastic neutron diffraction (INS), heat capacity and magnetic susceptibility measurements. A single Crystal Electric Field (CEF) peak has been detected in the INS spectra, which may be explained by a quasi-quartet state suggested by the analysis of the Schottky anomaly contributing to the magnetic part of the specific heat. The Kondo interactions have been included in the analysis of the magnetic part employing the simplified resonance level model. The resulting Kondo temperature of about 5–10 K is somewhat larger than in the previous studies. The magnetic susceptibility confirms the CEF level scheme of the type 0–93 K and provides the values of the CEF parameters.     

Activated scaling of classical and quantum spin glasses

A model for thermally activated dynamics in disordered systems shows that the linear and nonlinear susceptibility follows a generic exponential form with a "critical rounding," chi(1) proportional to chi(3) proportional to [T ln(t/tau(0)')/K](gamma/b phi) exp - [Tt(g)(phi b)ln(t/tau(0)'/K)](nu/b) (T=temperature, t=time, K=barrier constant, t(g) = 1 - T(SG)/T, and T(SG) = transition temperature; gamma>0 for chi(3) and <0 for chi(1)). This model, also valid in the presence of resonant tunneling states at energies K(0) < K [provided that K is replaced by K(0)+2T ln (1/Gamma(0)), where Gamma(0)(2) proportional, variant tunnel splitting of a spin S=1], is potentially applicable to a wide variety of systems opening the way for the study of thermally activated quantum phase transitions. The famous spin-glass system LiHo(x)Y(1-x) seems to follow this model.     

Direct observation of spontaneous weak ferromagnetism in the superconductor ErNi2B2C

Neutron measurements show that superconducting ErNi2B2C (T(C) = 11 K) develops antiferromagnetic spin density wave magnetic order (T(N) = 6 K), which squares up with decreasing temperature yielding a series of higher-order magnetic Bragg peaks with odd harmonics. Below T(WFM) = 2.3 K where magnetization indicates a net moment develops, even-order Bragg peaks develop which low field (approximately 3 Oe) polarized beam measurements show are magnetic in origin. The data directly demonstrate the existence of a net magnetization with a periodicity of 20a, confirming the microscopic coexistence of spontaneous weak ferromagnetism with superconductivity.     

The nature of the magnetism in quasi-2D layered α-Ni(OH)2

The two layered hexagonal hydroxides of Ni are β-Ni(OH)(2) and α-Ni(OH)(2); β-Ni(OH)(2) is now known to be an antiferromagnet whereas the nature of the magnetism in α-Ni(OH)(2) is not yet well established. Here, the magnetic properties of α-Ni(OH)(2) with lattice parameters a = 3.02 ? and c = 8.6 ?, and flower-like morphology with petal thickness of approximately equal to 50 ? are reported. Temperature (2-300 K) and magnetic field (up to 65 kOe) dependence of the magnetization and ac susceptibility at f = 0.1-1000 Hz were measured. Analysis of the data yields ferromagnetic ordering in the system with T(C) is approximately equal to 16 K. In addition, a nanosize related blocking temperature T(B) = 8 K and spin-glass-like ordering of the surface spins near 3.5 K are inferred from the ac frequency and dc magnetic field dependence of these transitions. Fitting to the high temperature series and quasi-2D nature of the system is used to determine J(1)/k(B) = 4.38 K (J(2)/k(B) = 0.14 K) for the intraplane (interplane) exchange coupling between the Ni(2+) ions.     

Spin frozen in the Kondo-lattice compound: CeCuSi2

In this paper, we present the experimental results of X-ray powder diffraction, electrical resistivity, magnetic susceptibility, and specific-heat measurements as well as Ce-L_III-edge X-ray absorption spectrum of the Ce-based intermetallic compound CeCuSi₂. The results revealed that CeCuSi₂ is a Kondo-lattice compound with no superconducting or magnetic-phase transition above 0.4 K. In addition, we found spin-glass behavior in the DC susceptibility measurements. The AC susceptibility measurements and the magnetic entropy calculation also confirm the presence of the spin-glass phase. The possible formation mechanism for the spin-frozen state is also discussed in this paper.     

Hyperfine fields on59Co in ReCo2

We report⁵⁷Fe Mössbauer measurements in the diluted antiferromagnetic (AF) Fe_xZn_1?xF₂ withx=0.25 at temperatures between 4.2 and 28 K. DC susceptibility measurements show a spin-glass (SG) phase at low temperatures forx?0.3. Our Mössbauer spectra show a competitive coexistence of an SG phase and AF order. We propose an interpretation in terms of clusters in the AF order and some spins exhibiting SG behavior.     

Magnetic-field-induced transition from metastable spin glass to possible antiferromagnetic-ferromagnetic phase separation in Cd0.5Cu0.5Cr2O4

Using ac susceptibility, dc magnetization and heat-capacity measurements, we have investigated the magnetic properties of Cd_0.5Cu_0.5Cr₂O₄. Cd_0.5Cu_0.5Cr₂O₄ has an extraordinary magnetic phase including a metastable spin-glass (SG) phase at zero field, a possible phase separation scenario of AFM/FM above ∼0.5 T field, and at intermediate fields, an apparent pseudo reentrant spin-glass (RSG) plateau is observed. These phenomena are closely correlated with the pinning effect of the Cu²⁺ sublattice on the frustrated lattice.     

Observation of multiferroic properties in pyroxene NaFeGe2O6

We report the observation of multiferroicity in a clinopyroxene NaFeGe(2)O(6) polycrystal from the investigation of its electrical and magnetic properties. Following the previously known first magnetic transition at T(N1) = 13 K, a second magnetic transition appears at T(N2) = 11.8 K in the temperature dependence of the magnetization. A ferroelectric polarization starts to develop clearly at T(N2) rather than T(N1) and its magnitude increases up to ~13 μC m(-2) at 5 K, supporting the idea that the ferroelectric state in NaFeGe(2)O(6) stems from a helical spin order stabilized below T(N2). When a magnetic field of 90 kOe is applied, the electric polarization decreases to 9 μC m(-2) and T(N2) slightly increases by 0.5 K. At intermediate magnetic fields, around 28 and 78 kOe, anomalies in the magnetoelectric current, magnetoelectric susceptibility, and field derivative of magnetization curves are found, indicating field-induced spin-state transitions. Based on these electrical and magnetic properties, we provide a detailed low temperature phase diagram up to 90 kOe, and discuss the nature of each phase of NaFeGe(2)O(6).     

Size effect in the spin glass magnetization of thin AuFe films as studied by polarized neutron reflectometry

We used polarized neutron reflectometry to determine the temperature dependence of the magnetization of thin AuFe films with 3% Fe concentration. We performed the measurements in a large magnetic field of 6 T in a temperature range from 295 to 2 K. For the films in the thickness range from 500 to 20 nm we observed a Brillouin-type behavior from 295 K down to 50 K and a constant magnetization of about 0.9 micro(B) per Fe atom below 30 K. However, for the 10 nm thick film we observed a Brillouin-type behavior down to 20 K and a constant magnetization of about 1.3 micro(B) per Fe atom below 20 K. These experiments are the first to show a finite-size effect in the magnetization of single spin-glass films in large magnetic fields. Furthermore, the ability to measure the deviation from the paramagnetic behavior enables us to prove the existence of the spin-glass state where other methods relying on a cusp-type behavior fail.     

Exchange bias in bulk layered hydroxylammonium fluorocobaltate (NH?OH)?CoF?

The magnetic properties of layered hydroxylammonium fluorocobaltate (NH(3)OH)(2)CoF(4) were investigated by measuring its dc magnetic susceptibility in zero-field-cooled (ZFC) and field-cooled (FC) regimes, its frequency dependent ac susceptibility, its isothermal magnetization curves after ZFC and FC regimes, and its heat capacity. Effects of pressure and magnetic field on magnetic phase transitions were studied by susceptibility and heat capacity measurements, respectively. The system undergoes a magnetic phase transition from a paramagnetic state to a canted antiferromagnetic state exhibiting a weak ferromagnetic behavior at T(C) = 46.5 K and an antiferromagnetic transition at T(N) = 2.9 K. The most spectacular manifestation of the complex magnetic behavior in this system is a shift of the isothermal magnetization hysteresis loop in a temperature range below 20 K after the FC regime-an exchange bias phenomenon. We investigated the exchange bias as a function of the magnetic field during cooling and as a function of temperature. The observed exchange bias was attributed to the large exchange anisotropy which exists due to the quasi-2D structure of the layered (NH(3)OH)(2)CoF(4) material.     

Ferromagnetic quantum critical fluctuations in YbRh2(Si0.95Ge0.05)2

The bulk magnetic susceptibility chi(T,B) of YbRh(2)(Si(0.95)Ge(0.05))(2) has been investigated close to the field-induced quantum critical point at B(c) = 0.027 T. For B < or= 0.05 T a Curie-Weiss law with a negative Weiss temperature is observed at temperatures below 0.3 K. Outside this region, the susceptibility indicates ferromagnetic quantum critical fluctuations, chi(T) proportional, variantT-0.6 above 0.3 K. At low temperatures the Pauli susceptibility follows chi(0) proportional, variant(B-B(c))(-0.6) and scales with the coefficient of the T(2) term in the electrical resistivity. The Sommerfeld-Wilson ratio is highly enhanced and increases up to 30 close to the critical field.