Magnetization dynamics below of EuO and EuS

Between 4.2 K and the Curie temperatures of the cubic Heisenberg ferromagnets EuS and EuO, their homogeneous dynamic susceptibilities have been investigated by means of a broad-band reflectometer operating from 0.1 GHz to 40 GHz. For internal magnetic fields larger than the anisotropy fields H A ( T ) of both materials, their static susceptibilities exhibit a -divergence, which reveals quantitatively the dominance of dipolar-anisotropic spin-wave fluctuations. displays a Lorentzian shape the damping frequency of which obeys scaling in terms of .The scaling function agrees quantitatively with work by Frey and Schwabl [#!FS88!#] for dipolar Heisenberg ferromagnets at temperatures above T_c. Building upon their approach, the resonance frequency of the Lorentzian can be related to a memory effect in the damping determined by the large value of the relaxation rate of the longitudinal magnetization fluctuations . For EuS, this relation is substantiated directly by inelastic neutron scattering. All these features reveal the hitherto uncovered importance of the dipolar anisotropic fluctuations below T_c of ferromagnets. Received: 4 March 1998 / Accepted: 12 May 1998     

Low temperature heat capacity and magnetic study of the Al80Mn20 icosahedral alloy

Low temperature heat capacity and magnetization measurements are reported for the Al₈₀Mn₂₀ alloy in the quasicrystalline icosahedral phase. The heat capacity, which was measured for temperatures ranging from 0.5 to 5.0 K and magnetic fields up to 11.7 kOe, shows a broad magnetic contribution around 1.0 K. The linear electronic contribution does not indicate an anomalously high density-of-states at the Fermi energy as predicted theoretically for quasicrystalline systems. The d.c. magnetization, which was measured from 2.0 to 300 K and with magnetic fields up to 50 kOe, indicates an effective number of one 11 μ_B localized magnetic moment for approximately every 100 Mn atoms. The low field susceptibility follows the Curie-Weiss law for temperatures ≧ 10 K, while a spin-glass-like ordering is observed at low temperatures.     

Setup for studying nonlinear magnetic properties of high-temperature superconductors with the aid of magnetization harmonics

A setup for measuring cophasal and quadrature components of higher harmonics of an electromotive-force signal of the response of a high-temperature superconductor makes it possible to study nonlinear magnetic properties of superconductors in variable magnetic fields of up to 1 kOe and constant magnetic fields of up to 10 T in the temperature range of 5?C300 K. This setup was used to measure the temperature dependences of the absolute values of the real and imaginary parts of the first and third harmonics of the magnetization of textured Yba₂Cu₃O_{7 ? x} polycrystalline samples in the temperature range of 77?C220 K at various values of variable and constant magnetic fields. An analysis of resulting data made it possible to reveal the presence of different dynamical modes of the magnetic flux in YBa₂Cu₃O_{7 ? x} that were dominant in different temperature ranges. The nonlinearity of the magnetization of YBa₂Cu₃O_{7 ? x} (the appearance of higher harmonics) was observed up to temperatures in the range of T = 103?C112 K, which were substantially higher than the temperature of the transition of this compound to a superconducting state. The observed feature in the magnetization of YBa₂Cu₃O_{7 ? x} was associated with the emergence of a pseudogap state in this compound.     

Specific features of the formation of the ground state in PrB6

The temperature dependences of the thermal conductivity and magnetization of a PrB₆ single crystal in magnetic fields of 0–14 T have been measured. An analysis of the data amassed has revealed that, apart from the well-known magnetic phase transitions at T ～ 7 and ～4 K, there appears a spontaneous magnetization with a relatively small magnetic moment in PrB₆ at temperatures below 20 K, which is initiated by the splitting of the ground state, apparently, due to dynamic structure distortions.     

Novel behavior in the M–H and field cooled (FC) curves of the MnFe2O4 nanoparticles synthesized by the coprecipitation method

MnFe₂O₄ nanoparticles were prepared by a coprecipitation chemical method. The average size of the obtained nanoparticles was about 30 nm. The hysteresis measured at T=300 K clearly shows ferromagnetic order at room temperature while that measured at T=450 K shows superparamagnetic behavior. The difference in the magnetization curves in the field increasing cycle and field decreasing cycle at higher temperatures (450 K or higher) is very unusual. In this case, a hysteresis in magnetization in higher magnetic fields with an opening up of the magnetization curve was observed. In this work, the effect of temperature and time of application of the applied field on the magnetization behavior was studied.     

Room temperature ferromagnetism and magnetotransport properties of the layered manganite system La1.2Ba1.8Mn2−xRuxO7 (0≤x≤1.0)

The dc magnetization and ac susceptibility measurements on two dimensional layered manganite La_1.2Ba_1.8Mn₂O₇ samples reveal the occurrence of ferromagnetism above room temperature with ferromagnetic (FM) to paramagnetic (PM) transitions at 338 K. The bifurcation temperatures shown by the zero-field cooled (ZFC) and field cooled (FC) dc magnetization curves at high temperatures shift towards lower temperatures as the applied field is increased from 100 to 2500 Oe. The data are suggestive of a large magnetic anisotropy due to the strong competing ferromagnetic and antiferromagnetic interactions resulting in a spin-glass-like state. Ru doping is found to enhance the ferromagnetism and metallicity of the system in a remarkable way. The magnetoresistance (MR) values obtained are very high and about 40% even at 260 K for the undoped sample.     

Magnetic coupling between Gd and Pr ions and magnetocaloric effect in Gd0.5Pr0.5Al2 compound

In this work, we report the theoretical and experimental investigations on the magnetic and magnetocaloric properties for Gd_0.5Pr_0.5Al₂ compound in different magnetic fields. The magnetization features indicate that Gd_0.5Pr_0.5Al₂ is ferrimagnetic at low temperatures. We also present data from X-ray magnetic circular dichroism (XMCD) experiments for this compound, with which we have confirmed that the magnetic moments of the Pr ions are antiparallel to the magnetic moments of the Gd ions. The magnetocaloric parameters, ΔT_S and ΔS_T, were obtained from calorimetric data and both curves present normal and inverse magnetocaloric effect. A theoretical model for ferrimagnetic coupling, including the crystalline electrical field anisotropy, was used to describe the ΔT_S and ΔS_T experimental results.     

Magnetization anomalies in melt-spun Ni–Mn–Ga ribbons

Temperature and field dependencies of the magnetization of melt-spun ribbons of Ni_45.7Mn_37.2Ga_17.1 (T_C<T_A) and Ni_54.3Mn_20.5Ga_25.2 (T_C>T_A), where T_A and T_C are the reverse martensitic transformation and Curie temperatures, respectively, have been studied in broad ranges of magnetic fields (±30 kOe) and temperatures (10–350 K). It was found that the magnetic and thermomagnetic properties have a number of peculiarities and anomalies. Particularly, the magnetization values measured at both low and high magnetic fields increase significantly after annealing. Low-temperature anomalies of magnetization suggest the presence of the superparamagnetic behavior and/or phenomena related to the concurrent exchange interactions in the as-spun state. The obtained results can be explained by the influence of concentration inhomogeneities and atomic disordering in the rapidly quenched ribbons, which can be reduced by annealing.     

High-Field and Multifrequency ESR in the Two-Dimensional Triangular Lattice Antiferromagnet NiGa2S4

We report the experimental results of electron spin resonance in high magnetic fields up to about 53 T on the quasi-two-dimensional triangular lattice antiferromagnet NiGa₂S₄. The temperature dependence of the resonance field at 584.8 GHz shows a steep change below about 30 K, indicating a development of the short-range correlation. The frequency dependence of the resonance field at the lowest temperature for H∥c is explained by one of the helical resonance modes. These experimental results suggest that the short-range order is well developed at low temperatures, and the resonance mode is described by a conventional spin wave theory.     

Antiferromagnetic versus spin-glass like behavior in MnIn2S4

The low-temperature magnetic properties of MnIn₂S₄ have been studied using AC magnetic susceptibility and magnetization experiments. High-temperature susceptibility fits indicate the presence of antiferromagnetic interactions. Low-field magnetization data show a peak at 5.6±0.1 K, below which strong irreversibility is observed between zero-field-cooled (ZFC) and field-cooled (FC) cycles suggesting that the observed peak corresponds to a spin-glass-like transition instead of the antiferromagnetic one previously reported. Further evidence of this magnetic state comes from AC susceptibility data at different frequencies. The in-phase component χ′(T) exhibits the behavior expected of spin glasses, i.e. a shift of the cusp to higher temperatures for higher frequencies.     

Magnetic properties and charge ordering in Pr0.75Na0.25MnO3 manganite

The structural and magnetic properties of Pr_0.75Na_0.25MnO₃ have been investigated experimentally. At room temperature, the compound shows paramagnetic characteristic. Along with decreasing temperature, a peak appears in the magnetization versus temperature curve around 220 K. To clarify whether this peak is associated with the ordering arrangement of Mn³⁺ and Mn⁴⁺ ions, electron diffraction experiments were carried out below and above 220 K respectively. Only basic Brag diffraction spots can be observed at high temperatures, however, superlattice diffraction appears below 220 K. This provides direct evidence for the existence of charge ordering in Pr_0.75Na_0.25MnO₃. We find the Mn³⁺ and Mn⁴⁺ cations form zigzag chains in a-c plane by analyzing the diffraction patterns. Combining with the magnetization measurements and the results of electron spin resonance, we confirm the antiferromagnetic phase and ferromagnetic component coexist in Pr_0.75Na_0.25MnO₃ below 120 K.     

Synthesis and magnetic properties of Cu3B2O6 single crystals

The temperature dependence of the magnetic susceptibility of Cu₃B₂O₆ single crystals grown by spontaneous crystallization from a melt consisting of a mixture of CuO and B₂O₃ and the behavior of their magnetization are investigated in magnetic fields up to 55 kOe. A broad susceptibility maximum is observed near 39 K, and a sharp drop in susceptibility is observed at T<10 K. The paramagnetic Néel temperatures for all orientations of the magnetic field in the crystal investigated are negative, attesting to the predominantly antiferromagnetic character of the exchange interactions. The effective magnetic moment of the Cu²⁺ ion is anisotropic and lies in the range from 1.054μ _B to 1.545μ _B. The magnetization depends linearly on magnetic field at T>10 K, whereas at temperatures below 10 K a discontinuity is observed at fields of the order of 40 kOe. At room temperature, electron magnetic resonance characterized by an almost isotropic g factor (g=2.165) is detected at 36.22 GHz. The exchange interactions in Cu₃B₂O₆ are analyzed on the basis of the Goodenough-Kanamori rules. The possibility of the establishment of a singlet magnetic state in the crystal is analyzed. Fiz. Tverd. Tela (St. Petersburg) 41, 677–679 (April 1999)     

Antiferromagnetic resonance and phase diagrams of gadolinium ferroborate GdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

Antiferromagnetic resonance in single crystals of rhombohedral gadolinium ferroborate GdFe₃(BO₃)₄ was studied. The frequency-field dependences of antiferromagnetic resonance over the frequency range 26–70 GHz and the temperature dependences of resonance parameters for magnetic fields oriented along the crystal axis and in the basal plane were determined. It was found that the iron subsystem, which can be treated as a two-sublattice antiferromagnet with anisotropy of the easy-plane type, experienced ordering at T=38 K. At temperatures below 20 K, the gadolinium subsystem with the opposite anisotropy sign strongly influenced the anisotropic properties of the crystal. This resulted in a spontaneous spin-reorientation transition from the easy-plane to the easy-axis state at 10 K. Below 10 K, magnetic field-induced transitions between the states were observed. Experimental phase diagrams on the temperature-magnetic field plane were constructed for fields oriented along the crystal axis and in the basal plane. A simple model was used to calculate the critical transition fields. The results were in close agreement with the experimental values measured at T=4.2 K for both field orientations.     

Effect of illumination on magnetization and microwave absorption of La1−xCaxMnO3 (x<0.2) films

Light-induced changes of the hysteresis loops of magnetization and microwave absorption are investigated in low-doped La_1−xCa_xMnO₃ (x<0.2) thin films. The width of the hysteresis loops decreases clearly under illumination with visible or near-infrared light at temperatures below 50 K. The microwave conductivity has a minimum value at magnetic fields corresponding to the magnetization reversal and is shifted towards weaker fields under illumination. These effects show complex nonexponential time evolution and dependence on strength of the magnetic field. The results can be explained by assuming that small ferromagnetic metallic regions exist within the insulating ferromagnetic phase of the sample, and that these regions are expanded by optically induced charge transfer between Jahn–Teller split e_g states of neighboring Mn³⁺ ions. Decrease of the Mn³⁺ XPS core level spectrum is observed in the samples under illumination with a HeNe laser.     

Anomalous magnetization of CsFeCl3 appearing at the applied magnetic fields exceeding 33 T

The magnetization of a singlet-ground-state magnet CsFeCl₃ at 4.2 and 1.3 K is measured by the pulsed magnetic field up to 40 T applied parallel to the crystal c-axis. In both temperatures, a large linear increase in the magnetization appears between 4 and 12 T, which is consistent with the magnetization observed using the static magnetic fields. Above 12 T, the magnetization increases gradually up to 33 T. A metamagnetic transition appears around 34 T suggesting a new magnetic structure. The possible models are discussed.     

Antiferromagnetic resonance in ferroborate NdFe3(BO3)4

The AFMR spectra of the NdFe₃(BO₃)₄ crystal are measured in a wide range of frequencies and temperatures. It is found that by the type of its magnetic anisotropy the compound is an “easy-plane” antiferromagnet with a weak anisotropy in the basal plane. The effective magnetic parameters are determined: anisotropy fields H_a1=1.14 kOe and H_a2=60 kOe and magnetic excitation gaps Δν₁=101.9 GHz and Δν₂=23.8 GHz. It is shown that commensurate-incommensurate phase transition causes a shift in resonance field and a considerable change in absorption line width.At temperatures below 4.2 K nonlinear regimes of AFMR excitation at low microwave power levels are observed.     

Spin glass like behavior in Cd0.42Mn0.58In2S4

The magnetic behavior of the diluted magnetic semiconductor Cd_0.42Mn_0.58In₂S₄ has been study by dc magnetization and ac susceptibility experiments. Zero field cooled and field cooled measurements reveal irreversibility below T_irr=2.60±0.15 K. Ac susceptibility data, performed as a function of the temperature and the frequency, confirm the spin-glass like behavior of the material with T_f=2.75±0.15 K. High temperature susceptibility data follow a typical Curie-Weiss law with θ=−74±1 K which suggests predominant antiferromagnetic interactions. The randomness of the magnetic ions, necessary to explain the magnetic behavior of the material, has been determined by X-ray powder diffraction experiments.     

Specific features of magnetic and transport properties of La1−x Mn1+x O3 manganites

The magnetic and transport properties of La_1?x Mn_1+x O₃ manganites with excess manganese are studied. It is shown that magnetic and charge ordering heavily depends on the superstoichiometric manganese content, magnetic field, and pressure. The magnetoresistive effect (MRE) is enhanced as the manganese concentration increases. In addition to the paramagnet-ferromagnet transition, the temperature dependences of the magnetization exhibit anomalies at low temperatures in samples with x=0.1–0.4. The magnetization decreases at T<45 K in fields H<0.2 kOe and increases as H changes from 0.2 to 10 kOe. An analysis shows that the features observed at low temperatures are most probably related to the transition from the ferromagnetic state to the canted spin structure in clusters of mixed-valence manganese ions. The temperature dependences of the magnetization and resistivity remain unchanged as the pressure increases. It is demonstrated that the Curie and metal-dielectric transition temperatures shift to higher values as the manganese concentration increases under pressure. The temperature of the MRE peak increases under pressure, while the MRE decreases.     

Anisotropic and large low-field magnetic entropy change in a La4/3Sr5/3Mn2O7 single crystal

We have studied the magnetocaloric effect (MCE) in a bilayered La_4/3Sr_5/3Mn₂O₇ single crystal with applied field along both ab-plane and c-direction. Due to the quasi-two-dimensional structure, the crystal exhibits a strong anisotropy in the MCE. The difference of magnetic entropy change between two crystallographic directions depends on external magnetic fields and has a maximum of 2 J/kg K. A large low-field magnetic entropy change, reaching 3.2 J/kg K for a magnetic field change of 15 kOe, is observed when the applied field is along ab-plane. This large low-field magnetic entropy change is attributed to the rapid change of magnetization in response to external magnetic fields in the easy magnetizing plane.