On the susceptibility exponent of random anisotropy magnets

The temperature dependence of the non-linear susceptibility ≈₂(T) of random anisotropy magnets in the Ising limit (speromagnets) is calculated for temperatures above the freezing temperature T_f within the framework of the correlated molecular field theory. For the effective susceptibility exponent λ_s(T) = (T?T_f)≈₂d^-1_≈2/dT a non-monotonic temperature dependence is found as for the case of spin glasses. This must be taken into account in order to obtain reliable values for the critical susceptibility exponent from experimental data.     

Pressure dependence of the Néel temperature in CeMg and CeZn

Magnetization measurements have been carried out under hydrostatic pressures up to 6 kbar and constant magnetic fields (10 kOe) in CsCl-type compounds: CeMg and CeZn. a decrease in the Néel temperature, T_N, with increasing pressure is observed in both compounds (dT_N/dp=- 0.2 and -0.17 K kbar^-1 respectively in CeMg and CeZn). The relative decrease of T_N has a similar order of magnitude to that found in other cerium compounds with Kondo-type properties.     

Influence of structural defects on magnetic properties of the submicrometer ceramic Mn3O4

A high-density single-phase submicrometer ceramic Mn₃O₄ with a grain size d ≥ 0.06 μm has been obtained by using spherically converging shock waves. The action of shock waves reduces the unit cell volume and increases the Curie temperature T _C . The shape of the temperature dependences of reciprocal susceptibility x ^?1 of a polycrystal and the submicrometer ceramic at T > T _C is hyperbolic, which is typical of ferrimagnets. It is shown that the susceptibility of the ceramic in the region of short-range order at 78 K ≤ T ≤ 300 K is larger than that of the polycrystal, while the field dependences of the magnetization are nonlinear. The bombardment by electrons with a small dose of Φ = 0.8 × 10¹⁸ cm^?2 increases the susceptibility of the submicrometer ceramic as well as that of the polycrystal. When the fluence increases to Φ = 5 × 10¹⁸ cm^?2, the susceptibility decreases, which can be explained by radiation-induced disordering and a change in the “local” exchange interactions.     

Anomalous thermopower in heavy-fermion compounds CeB<Subscript>6</Subscript>, CeAl<Subscript>3</Subscript>, and CeCu<Subscript>6 − <Emphasis Type="Italic">x</Emphasis></Subscript>Au<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

High-precision measurements of thermopower have been performed in a wide temperature range (2–300 K) for a series of cerium-based heavy-fermion compounds, including CeB₆, CeAl₃, CeCu₆, and substitutional solid solutions of the CeCu_{6 ? x} Au _x system (x = 0.1, 0.2). All compounds exhibit an unusual (logarithmic) asymptotic behavior of the temperature dependence of the Seebeck coefficient: S ∝ ?lnT. In the case of cerium hexaboride, this anomalous behavior of S(T) is accompanied by the appearance of weak-carrier-localization-mode asymptotics in the conductivity (σ(T) ∝ T ^0.39), while the paramagnetic susceptibility χ(T) and the effective mass of charge carriers m _eff(T) vary according to a power law (χ(T), m _eff(T) ∝ T ^?0.8) in the temperature interval T = 10–80 K. This behavior corresponds to renormalization of the density of states at the Fermi level. The observed anomalous behavior of thermopower in CeB₆ and other cerium-based intermetallic compounds is attributed to the formation of heavy fermions (many-body states in the metal matrix) at low temperatures.     

27Al pulsed nuclear magnetic resonance study of CeAl2

The magnetic properties and the electronic structures of a rare-earth aluminum intermetallic compound CeAl₂ are investigated by magnetic susceptibility measurements and ²⁷Al pulsed nuclear magnetic resonance (NMR) techniques. The magnetic susceptibility is strongly temperature-dependent, following a Curie–Weiss law down to ∼12 K, and shows an antiferromagnetic transition at 4 K. The ²⁷Al NMR spectra show a typical powder pattern for a nuclear spin I of 5/2 with the second-order nuclear quadrupole interaction at high temperature and an additional large dipolar broadening between the 4f electron spins of cerium and the ²⁷Al nuclear spins at low temperature. The ²⁷Al NMR Knight shift follows the same temperature dependence as the magnetic susceptibility, suggesting that the ²⁷Al NMR Knight shift originates from the transferred hyperfine field of the Ce 4f electron spins with the hyperfine coupling constant of A = +5.7 kOe/μ_B. The spin-lattice relaxation rate 1/T₁ is roughly proportional to temperature, as with most non-magnetic metals at high temperature, and then strongly temperature-dependent, increasing rapidly with a peak near the antiferromagnetic transition temperature and decreasing at lower temperature. The temperature dependence of the Korringa ratio K, however, suggests that the antiferromagnetic spin fluctuation signature, which is an enhancement in the Korringa ratio, is washed out owing to the geometrical cancellation of Ce 4f fluctuations at the Al sites.     

Low-temperature magnetic properties and susceptibility of amorphous Nd4Fe58.1Cr19.4B18.5 alloys

Low-temperature magnetic properties and the susceptibility of the amorphous Nd₄Fe_58.1Cr_19.4B_18.5 alloy were studied. The temperature dependence of magnetization exhibits T^3/2 behavior up to T/T_c=0.57. Spin-wave stiffness coefficient D=47 meV A² is much smaller than that of amorphous Fe₈₀B₂₀ alloys. The temperature dependence of the susceptibility χ₀ obeys Curie–Weiss law at T>1.5T_c. A larger effective magnetic moment per magnetic atom was obtained. The influence of Cr on low-temperature magnetic properties and the susceptibility was discussed.     

The magnetic properties of one-dimensional spin-1 ferromagnetic Heisenberg model in a magnetic field within Callen approximation

The effect of magnetic field h on the magnetic properties of the one-dimensional spin-1 ferromagnetic Heisenberg model is studied by the double-time Green’s function method. The magnetization and susceptibility are obtained within the Callen approximation. The zero-field susceptibility is as a decreasing function of the temperature T. The magnetization m increases in the whole field region, but the susceptibility maximum χ(T_m) decreases. The position T_m of the susceptibility maximum is both solved analytically and fits well to be a power law T_m∼h^γ at low fields and to be linear increasing at high fields. The height χ(T_m) decreases as a power law χ(T_m)∼h^−β with h increasing. The exponents (γ,β) obtained in our results agree with the other theoretical results. Our results are roughly in agreement with the results obtained in the experiment of Ni(OH)(NO₃)H₂O.     

<Superscript>14</Superscript>N Nuclear Magnetic Resonance and Relaxation in the Paramagnetic Region of Uranium Mononitride

The spin susceptibility of a polycrystalline sample of uranium mononitride UN is studied by measuring the ¹⁴N NMR line shift, spin–lattice relaxation rates of the nuclear spin, and static magnetic susceptibility in the temperature region of 1.5T_N < T < 7T_N A joint analysis of the results obtained has revealed the temperature dependence of the characteristic energy of spin fluctuations of the uranium 5f electrons: Γ_nmr(T) ∝ T^0.54(4) close to the dependence Γ(T) ∝ T^0.5 characteristic of concentrated Kondo systems above the coherent state formation temperature.     

Weakferromagnetism in FexNi1−xS2 (x = 0−0.075)

The effects of nonmagnetic impurities (Fe²⁺ in a low spin state) on the weakferromagnetism in NiS₂ were investigated. The weak-ferromagnetic moment decreases rapidly with Fe content (x) and almost vanishes at about x = 0.05. On the other hand, T_c remains quite constant. Above T_c, two anomalies in the temperature dependence of the susceptibility were observed except for samples with x = 0 and 0.006. The effective magnetic determined from the susceptibility above T_c is about 3 μ_B/Ni, and is independent of x. Discussions are given on the origin of weakferromagnetism and on the hysteresis of the virgin magnetization curve.     

A novel double-peaked spin-glass susceptibility-temperature response in the ternary alloy Fe69Mn26Cr5

We have studied the low-field (B ≦ 10^?2 T) d.c. susceptibility χ of the austenitic stainless-steel alloy Fe₆₉Mn₂₆Cr₅ as a function of the magnetic field B and temperature T. χ(T) shows structure, strong B dependence, and typical irreversible effects. The range of temperatures studied comprises three distinct regions. In the high-temperature region (300 K ≦ T ≦ 380 K) a blunt peak in the susceptibility is noticed at T₂ = 340 K. T₂ was not sensitive to thermal cycling. χ(T) displayed a sharp cusp at T₁ = 200 K. This peak was sensitive to the thermal history of the sample and was strongly suppressed by B. Between T₁ and T₂ a shallow valley with some hysteresis was observed. We interpret this behavior to be due to a low-temperature pure spin-glass phase, a high temperature conventional paramagnetic phase, and coexisting antiferromagnetic and spin-glass phases between T₁ and T₂.     

Enhancement of band magnetism and features of the magnetically ordered state in the CeB<Subscript>6</Subscript> compound with strong electron correlations

Precision measurements of transport and magnetic parameters of high-quality CeB₆ single crystals are performed in the temperature range 1.8—300 K. It is shown that their resistivity in the temperature interval 5 K < T < T* ≈ 80 K obeys not a logarithmic law, which is typical of the Kondo mechanism of charge carrier scattering, but the law ρ ∝ T ^?1/η corresponding to the weak localization regime with a critical index 1/η = 0.39 ± 0.02. Instead of the Curie-Weiss dependences, the asymptotic form χ(T) ∝ T ^?0.8 is obtained for magnetic susceptibility of CeB₆ in a temperature range of 15–300 K. Analysis of the field dependences of magnetization, magnetoresistance, and the Hall coefficient in the paramagnetic and magnetically ordered phases of CeB₆ and comparison with the results of measurements of Seebeck coefficient, the inelastic neutron scattering coefficient, and EPR spectroscopy lead to the conclusion that the Kondo lattice model and skew scattering model cannot be used for describing the transport and thermodynamic parameters of this compound with strong electron correlations. On the basis of detailed analysis of experimental data, an alternative approach to interpreting the properties of CeB₆ is proposed using (1) the assumption concerning itinerant paramagnetism and substantial renormalization of the density of electron states upon cooling in the vicinity of the Fermi energy, which is associated with the formation of heavy fermions (spin-polaron states) in the metallic CeB₆ matrix in the vicinity of Ce sites; (2) the formation of ferromagnetic nanosize regions from spin polarons at 3.3 K < T < 7 K and a transition to a state with a spin density wave (SDW) at T _Q ≈ 3.3 K; and (3) realization of a complex magnetic phase H-T diagram of CeB₆, which is associated with an increase in the SDW amplitude and competition between the SDW and antiferromagnetism of localized magnetic moments of cerium ions.     

Static susceptibility and longitudinal resonance frequency of superfluid 3He B

The static susceptibility and the longitudinal resonance frequency of superfluid ³He B have been measured at 18.7 bar from T_c to 0.27 T_c using continuous wave transverse NMR.     

The parallel susceptibility at 0 K of the double magnetic helix

The parallel susceptibility x^∥ at 0 K of a double magnetic helix of pitch angle ? in which two species of magnetic ions are acted on by unequal molecular fields is determined. The conditions under which x^∥ is equal to the 0 K perpendicular susceptibility x^⊥ calculated in the past are determined. Comparison is made with the powder susceptibility measurements performed on the orthorhombic compound La₂Fe_1.5Mn_0.5S₅ which displays a double magnetic helix up to T_N = 85 K.     

Monte Carlo investigation of magnetic properties of a two-dimensional classical Heisenberg magnet

The equilibrium properties of a simple quadratic lattice of classical spins with nearest-neighbor Heisenberg interactions have been examined by a Monte Carlo Method. The susceptibility was found to have a singular temperature dependence χ ∝ exp (const/T²)/T above the Stanley-Kaplan transition temperature (T_SK). A plausible argument has been presented to explain peculiar properties of the 2?d Heisenberg magnet on the basis of the observed singular behavior of the susceptibility.     

Magnetic properties and peculiarity of magnetic states in dilute antiferromagnets Mn1−x Zn x F2

The dependence of magnetic moment and susceptibility on temperature, magnetic field and frequency of some single crystals Mn_1?x Zn _x F₂ (x≈x _e=0.75—percolation limit) were experimentally investigated. Our experiments show that (Bazhan and Petrov 1984; Cowleyet al 1984; Villain 1984) in these crystals the nonequilibrium magnetic state of spinglass type with finite correlation length appears as temperature decreasesT<T in weak magnetic fields. This state is determined by fluctuation magnetic moments √nμ (wheren is the number of magnetic ions, corresponding to finite correlation length andμ the magnetic moment Mn⁺¹). In the experiments in low magnetic fields and frequencies there are no peculiarities in the magnetic susceptibility temperature dependence atT≠T _f. At temperaturesT>T _f andT<T _f magnetic susceptibility is determined by 1 $$\chi \left( {T > T_f } \right) = \frac{{N\left\langle \mu \right\rangle ^2 }}{{3k\left( {T + \theta } \right)}} = \frac{N}{n}\frac{{\left\langle {\sqrt n \mu } \right\rangle ^2 }}{{3k\left( {T + \theta } \right)}} = \chi \left( {T< T_f } \right)$$ . In strong magnetic fields and large frequencies there are peculiarities in thex(T) dependence atT=T _f. AtT<T _f and strong magnetic fieldsX(T)=x ₀ andT<T _f and at large frequenciesx(T)=x ₀+α/T. The dependences of magnetic susceptibility on the frequency are determined by the magnetic system relaxation. Calculations and comparison with experiments show that the relaxation of the investigated magnetic systems atT<T _f follows the relaxation lawM(t)=M(0) exp[?(t/τ) ^r ], suggested in Palmeret al (1984) for spin-glasses relaxation taking into account the time relaxation distributionτ ₀....τ _max in the system and its ‘hierarchically’ dynamics.     

Helium defectoscopy of cerium gadolinium ceramics Ce0.8Gd0.2O1.9 with a submicrocrystalline structure in the impurity disorder region

The concentration of impurity anion vacancies formed upon the dissociation of gadolinium-vacancy complexes has been determined using helium defectoscopy of the cerium gadolinium ceramics Ce_0.8Gd_0.2O_1.9 with a submicrocrystalline structure in the temperature range T = 740–1123 K and at saturation pressures ranging from 0.05 to 15 MPa. It has been found that the energy of dissociation of gadoliniumvacancy complexes is E _eff ^D = 0.26 ± 0.06 eV, and the energy of dissolution of helium in anion vacancies in the impurity disorder region is E ^P = ?0.31 ± 0.09 eV. The proposed mechanism of dissolution has been confirmed by the investigation of the electrical conductivity of the cerium gadolinium ceramics, as well as by the high-speed molecular dynamics simulation of the dissociation of gadolinium-vacancy complexes. It has been assumed that a decrease in the effective dissolution energy in comparison with the results of the previously performed low-temperature investigations is caused by the mutual repulsion of vacancies formed upon the dissociation of gadolinium-vacancy complexes in highly concentrated solutions of gadolinium in CeO₂ with increasing temperature.     

Spin gaps and spin dynamics in superconducting cuprates

Spin gap effects, consisting of a declining uniform susceptibility and spin paramagnetic NMR shift at low temperaturesin the normal state and associatedT ₁ behavior, are discussed and documented in several cuprate superconductors. Dynamic spin magnetism in these systems is further reviewed in the light of mean-field models, where we note that detailed results from the model by Millis, Monien, and Pines are not borne out in recent neutron data on YBa₂Cu₃O_6.92.T ₁ data on¹⁷O in La_1.85Sr_0.15CuO₄ are presented, showing consistency with neutron dynamic susceptibility data forT≧80 K, but exhibiting a strong spin gap character below 80 K which is not present in the neutron data. Data for Zn-doped YBCO withT _c≈60 K are also presented, showing strong RKKY broadening from localized moments in the planes, but no spin gap effect such as that found in theT _c=60 K oxygen-deficient phase.     

Dynamical magnetic susceptibility in Y-Ba-Cu-O near Tc

The ac susceptibility of ceramic Y-Ba₂Cu₃O_7−x is studied in the range 10²–10⁵ Hz near T_c. Measurements were done in the earth's magnetic field, given the ac field 20 mG. On the basis of the frequency dependences of the real and imaginary parts of the susceptibility the existence of a wide distribution of relaxation times is concluded.     

Environment effects on the appearance of magnetism on cerium impurities diluted in La1-xThx

Magnetic susceptibility measurements between 1.2 and 4.2 K of (La, Th) based alloys with cerium impurities are presented. Nuclear orientation experiments were also performed at very low termperatures. The appearance of magnetism is related to the number of thorium atoms surrounding each cerium. From a probabilistic model we can suggest the presence of magnetic and non-magnetic cerium ions depending on their environment when the La_1-xTh_x matrix is varied.     

Very-low-frequency ac susceptibility of the spin-glass system PrPy (y ⪅ 1)

The frequency dependence of the ac susceptibility, χ_ac, is reported for off-stoichiometric PrP_y (y ? 1) for 0.003 < f < 10⁴ Hz and for temperatures 1.4 < T < 20 K. For this spin-glass system, T_f, the temperature at which χ_ac is a maximum, reaches a minimum and constant value for f ? 0.1 Hz. The complex initial susceptibility for 0.003 < f < 0.03 Hz is presented.