Das kritische magnetische Verhalten von EuO

Mössbauer effect and magnetization measurements were employed in order to study the static and especially the dynamic magnetic properties of the nearly Heisenberg ferromagnet EuO near its Curie temperature,T _c=69.2 °K. The critical exponent β of the spontaneous magnetization was determined to be β=0.34±0.02. It was shown that critical slowing down of spin fluctuations takes place nearT _c with spin relaxation times between 7×10^?11 sec (T=1.01T _c) and 1.5×10^?1 sec (T=1.03T _c). The experimental values of the relaxation time were found to be in satisfactory agreement with theoretically computed ones. Just belowT _c the Mössbauer spectra exhibit relaxation effects, which are characteristic for the occurence of critical super-paramagnetism. Investigations of several samples indicated quantitatively, that critical superparamagnetism has its origin in the non ideal composition of the real crystal.     

Re-entry in ferromagnetic superconductors

The re-entry phenomenon in magnetic superconductors is studied using the generalized Ginzburg-Landau free energy introduced by Blount and Varma. The re-entry temperature T_c2 is simply that temperature at which the magnetization acts as a source of induction strong enough to destroy superconductivity. Above T_c2 ferromagnetism and superconductivity coexist. The structure is an Abrikosov vortex lattice, with ferromagnetic magnetization spreading widely around the vortex cores. Within our approximations, the phase transition at T_c2 is of second order.     

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.     

Observation of magnetization reversals during relaxation measurements on superfluid 3HeA

A reversal of the recovery of the magnetization following an rf pulse has been observed in superfluid ³HeA in 306 Oe. The time difference between the end of the rf pulse and the start of the magnetization reversal is found to be proportional to both (1 ? T/T_c)^{?$\text{1}\text{2}$} and the magnitude of the initial rotation angle θ.     

Magnetic properties of (NdxY1-x)Co2(1⩾ x ⩾ 0)

The concentration dependence of T_c and T_R (T_c magnetic ordering temperature, T_R spin reorientation temperature) of the pseudobinary system (Nd, Y)Co₂ is reported. Furthermore the influence of an external magnetic field on the spin reorientation and the magnetization is studied. The observed variation of the magnetization in the vicinity of the spin reorientation is compared with theoretical results. For the calculation a Hamiltonian with terms describing a molecular field, a cubic crystal field, and an external field is used.     

Phase transition in electronic manganite Ca0.85Sm0.15MnO3

The resistivity, the magnetic susceptibility, the magnetization, and the specific heat of electronic manganite Ca_0.85Sm_0.15MnO₃ were studied. The data obtained suggest that this compound undergoes phase transition into the insulator antiferromagnetic state at T _c ～115 K and displays negative magnetoresistance at T<T _c . A minor ferromagnetic component of 0.025µ_B in the magnetization of Ca_0.85Sm_0.15MnO₃ may be caused by the deviation of this composition from the exact stoichiometry Mn³⁺: Mn⁴⁺=1: 8. The Debye temperature Θ_D=575 K and the entropy of phase transition ΔS=5.1 J/(mol K) were derived from the temperature dependence of specific heat.     

The kinetics of relaxation of magnetization fluctuations near the phase transition from paramagnetic state to ferromagnetic state with domain structure

The equations of motion describing relaxation of magnetization fluctuations in the paramagnetic state for a uniaxial ferromagnet have been solved. In the Landau-Ginzburg functional the demagnetization energy has been taken into account. The finite dimensions of a sample have been considered. It has been shown, that a phase transition is generated by fluctuations containing information about the period of the domain structure at the phase transition point T_c¹. The obtained formulae describe T_c¹ and the period of the domain structure. The results can be used both for bulk materials and thin films.     

Thermal and magnetic properties of some YBa2Cu3O7−δ samples

Thermal and magnetic measurements have been performed on several YBa₂Cu₃O_7−δ compounds, some ones showing a large content of high T_c (93 K) superconducting phase. A jump in the specific heat ΔCp, is well evidenced at the transition allowing a determination of the ratio ΔCp/T_c ≅ 23 ± 5. mJ/ (mole Cu)K². In addition, an estimation of the γ value (≅ 11 mJ/(mole Cu). K²) has been drawn from the determination of the electronic entropy at T_c. The samples have been characterized by susceptibility, magnetization and resistivity experiments. The critical field slopes at T_c were found to be dHc₁/dT ≅ 17 Oe/K and dHc₂/dT ≅ 20 kOe/K. The results are discussed in the framework of the Ginzburg-Landau theory.     

Novel magnetic behaviour of the system ZnxCa1−xFe2O4

New magnetic system Zn_xCa_1−xFe₂O₄ (x = 0.4 to 0.6) is reported having extremely unusual properties like small ordered moments ≈ 0.13 ∼ 0.23 μ_B, large ordering temperatures T_c ≈ 700–750 K and surprising lack of hyperfine field (HF) at Fe for temperatures greater than T¹(≈ 0.25 T_c) in spite of bulk magnetization in the system. The HF appears below T¹ implying evolution of local moments far below T_c. Sharp susceptibility maxima resembling those of spin-glasses are observed for x = 0.4 and 0.5, which may be attributed to the single domain-superparamagnetic (SD-SP) transition and the overall behaviour shows a cluster spin-glass type of ordering.     

Rare earth-transition metal compound-based MOSLM for the visible spectral range

We have demonstrated a magneto-optical spatial light modulator in which functionality is realized by (i) heating up to Curie temperature (T_c) magneto-optical elements (pixels) with a semiconductor laser and (ii) application of a switching magnetic field. The pixels were made of films of amorphous rare earth-transition metal compounds (TbFe films with T_c=403 K and DyFe films with T_c=343 K) having good magneto-optical responses for wavelengths from the visible spectral range. We have found that the magnetization direction of pixels can be modulated with a laser radiation density of 5 mJ/cm² and in a switching magnetic field of 15 Oe.     

Intrinsic pinning properties of FeSe0.5Te0.5

The intrinsic pinning properties of FeSe_0.5Te_0.5, which is a superconductor with a critical temperature T_c of approximately 14 K, were studied through the analysis of magnetization curves obtained using an extended critical state model. For the magnetization measurements carried out with a superconducting quantum interference device (SQUID), external magnetic fields were applied parallel and perpendicular to the c-axis of the sample. The critical current density J_c under the perpendicular magnetic field of 1 T was estimated using the Kimishima model to be equal to approximately 1.6 × 10⁴, 8.8 × 10³, 4.1 × 10³, and 1.5 × 10³ A/cm² at 5, 7, 9, and 11 K, respectively. Furthermore, the temperature dependence of J_c was fitted to the exponential law of J_c(0) × exp(?αT/T_c) up to 9 K and the power law of J_c(0) × (1 ? T/T_c)ⁿ near T_c.     

表面与界面的绝缘铁磁理论

采用Green函数技术,我们讨论了温度从T=0直到T=T_c范围内铁磁体表面和界面的磁化强度随温度的变化情况,所得到表面磁化强度的结果与其他作者的结果一致;而在界面计算中,我们发现界面的T_c与体内的不同,并且找到了一个关系式:T_cc^A,T_c^B)。 关键词：     

Approximate calculations for the two-dimensional Ising model

A self-consistent molecular field approximation for the two-dimensional, square-lattice Ising model is used to calculate the energy and magnetization. Agreement with the exact calculations is good except near the critical temperature, which differs from the exact critical temperature by 4%. The specific heat has no anomalous behavior asT approachesT _c from above, and the magnetization follows the incorrect Weiss (T _c-T)^1/2 law asT approachesT _c from below.     

Calculation of short range magnetic order in iron

A spin fluctuation theory for itinerant electrons that includes short-range magnetic order (SRMO) is used to calculate the Curie-temperature (T_c and the temperature dependence of the magnetization and the susceptibility of bulk Fe. When spin correlations are included the Curie-temperature is reduced by 9% to T_c = 2000 K. The calculated temperature-dependence of the magnetization and the magnetic susceptibility are in excellent agreement with experimental results.     

Bulk magnetization study of a DyNi5 single crystal

Magnetization and susceptibility measurements were performed on a single crystal of DyNi₅ along the three main symmetry axes of the ortho-hexagonal cell. Below its ordering temperature (T_c = 11.6 K), b and c are respectively the easy and hard magnetization axes. The strong anisotropy originates from the crystalline electric field acting on the 4f electrons of the Dy³⁺ ions. A small magnetization is induced on nickel atoms by the applied field and the exchange interactions with the dysprosium atoms. The crystal field parameters, the molecular field coefficients and the susceptibility of nickel atoms are determined from the experimental data.     

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.     

Critical nuclear relaxation in cobalt metal

Nuclear magnetic resonance of cobalt metal was investigated in the paramagnetic and ferromagnetic states and in the critical region below T_c. The Knight shift and spin lattice relaxation times were measured in the paramagnetic phase in the solid and liquid states from 1578 K to 1825 K. The resonant frequency, spin-lattice and spin-spin relaxation times were measured in the ferromagnetic phase from room temperature to 1385 K. The main part of (T₁T)^-1 results from fluctuating orbital moments in both phases except near T_c where this process forms the background for critical spin relaxation. The critical exponents for T^-1₁ and for the magnetization in the ferromagnetic state were found to be n' = 0.96 ± 0.07 and β = 0.308 ± 0.012, respectively.     

Re-entrant magnetic behaviour in amorphous Fe-rich Fe100−x Zr x alloys

We report results of⁵⁷Fe Mössbauer measurements on the magnetically concentrated re-entrant amorphous Fe₁₀₀?xZr _x alloys (8≤x≤12) which give convincing evidence for a microscopic origin of the low-temperature magnetization anomaly atT _f<T _c. This is shown by an anomalous increase in the magnetic hyperfine field and in the relative intensities of the (?m=0)-Mössbauer lines belowT _f upon cooling. No evidence was found for an antiferromagnetic phase. The shape of the hyperfine-field distribution atT=4.2 K in zero external field and in a field of 3 T is unchanged, indicating homogeneous behaviour of all Fe moments near saturation. Ferromagnetic order is not long-ranged, but determined by exchange-coupled magnetic clusters. The cluster moment nearT _c is found to decrease strongly with increasing Fe content and extrapolates to zero atx≈96.