Superparamagnetism and spin-glass like state for the MnFe2O4 nano-particles synthesized by the thermal decomposition method

MnFe₂O₄ nano-particles with an average size of about 7 nm were synthesized by the thermal decomposition method. Based on the magnetic hysteresis loops measured at different temperatures the temperature-dependent saturation magnetization (M_S) and coercivity (H_C) are determined. It is shown that above 20 K the temperature-dependence of the M_S and H_C indicates the magnetic behaviors in the single-domain nano-particles, while below 20 K, the change of the M_S and H_C indicates the freezing of the spin-glass like state on the surfaces. By measuring the magnetization–temperature (M–T) curves under the zero-field-cooling (ZFC) and field-cooling procedures at different applied fields, superparamagnetism behavior is also studied. Even though in the ZFC M–T curves peaks can be observed below 160 K, superparamagnetism does not appear until the temperature goes above 300 K, which is related with the strong inter-particle interaction.     

Unusual magnetic hysteresis behavior of oxide spinel MnCo2O4

Magnetic hysteresis behavior of the oxide spinel MnCo₂O₄ has been studied at different temperatures below its T_c≈184 K. Normal hysteresis behavior is observed down to 130 K whereas below this temperature the initial magnetization curve, at higher magnetic fields, lies outside the main loop. No related anomaly is observed in the temperature variation of magnetization or coercivity. However, the anisotropy field overcomes the coercivity below 130 K. The unusual magnetic hysteresis behavior of MnCo₂O₄, at low temperatures, may be associated with irreversible domain wall movements due to the rearrangement of the valence electrons.     

Structural and magnetic studies of Fe2O3/SiO2 granular nanocomposites

Fe₂O₃/SiO₂ nanocomposites were synthesized by mechanical alloying, using Fe and SiO₂ powders as precursors. After 340 h milling, the sample essentially consists of hematite and amorphous silica. TEM images show hematite particles embedded in and surrounded by an amorphous silica matrix. A broad size distribution—5–50 nm—of hematite particles is found, and other group of very small—2–3 nm—unidentified particles are observed. Room temperature Mössbauer spectra show a paramagnetic doublet, which may correspond to a non-crystalline phase in the sample (probably the small unidentified particles), and a sextet corresponding to hematite. Magnetic properties were investigated by measuring hysteresis curves at different temperatures (5–300 K) and by zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves (10 mT). The hysteresis loops were well fitted by a ferromagnetic contribution. No evidence of Morin transition is found down to 5 K.     

Spin-glass behaviour of novel ternary uranium aluminide U3Co4+ x Al12− x (x = 0.55)

Experimental results on dc and ac susceptibility, magnetization and magnetic relaxation, specific heat, electrical resistivity and magnetoresistivity up to 8?T are reported for the novel ternary uranium aluminide U₃Co_4.55Al_11.45. The temperature dependence of the dc susceptibility shows a cusp at a characteristic temperature T _f?=?8–10?K that depends weakly on the applied magnetic field. The observed pronounced difference between the ZFC and FC magnetizations, as well as the decay in the remanent, both give evidence that a highly irreversible, frozen state is formed below T _f which is reminiscent of spin-glass behaviour. The real and imaginary parts of the ac susceptibility show that the corresponding T _f peaks are only slightly dependent on frequency. Electrical resistivity measured at zero and in fields up to 8?T indicates that the Kondo-like state becomes dominant at temperatures above T _f.     

The spin-glass state in Y2Co2/3Mo4/3O7 and Y2Fe2/3Mo4/3O7 complex oxides

The magnetic properties of Y₂ B _2/3Mo_4/3O₇ complex oxides (B = Co, Fe) were studied in the temperature range 2–300 K. At low temperatures, these compounds exhibit spin-glass properties with freezing temperatures T _f=26 and 33 K, respectively, and typical features in the magnetic hysteresis and in the dependences of the real part of the dynamic magnetic susceptibility on temperature and ac magnetic field frequency. Above T _f, the static magnetic susceptibility of the samples studied depends on the applied magnetic field, which is tentatively assigned to the presence of metallic cobalt and/or yttrium orthoferrite YFeO₃ introduced in the course of sample preparation. __________ Translated from Fizika Tverdogo Tela, Vol. 47, No. 12, 2005, pp. 2182–2188. Original Russian Text Copyright ? 2005 by Bazuev, Korolev.     

FeTe as a candidate material for new iron-based superconductor

Tetragonal FeSe is a superconductor with a transition temperature T_c of 8 K and shows a huge enhancement of T_c with applying pressure. Tetragonal FeTe has a structure very analogous to superconducting FeSe, but does not show superconducting transition. We investigated the pressure effect of resistivity on FeTe. The resistivity at room temperature decreased with increasing pressure. An anomaly in resistivity around 80 K shifted towards a lower temperature with increasing pressure.     

Heavy-fermion behavior and spin-glass freezing in Si-stabilized amorphous alloys based on UPt3

U-Pt-Si and U-(Pt, Pd)-Si alloys have been prepared in the amorphous state by splat-cooling. The average nearest-neighbor distance is the same as the U-Pt distance in UPt₃. The low-temperature behavior of these alloys (with composition (U_0.25Pt_0.75)_1–xSi_x, x=0.18, 0.20, 0.25; and (U_0.25(Pt_1–yPd_y)_{0.75 0.8}Si_0.2,y=0.05 and 0.1) has been studied by specific heatC, magnetization and resistivity measurements in the temperature range 1.5 K to 20 K and for magnetic fields up to 6 T. Some samples were investigated down to 0.1 K and the specific heat for one sample was measured in fields up to 22 T. With ₀ =C/T| _T0 between 0.25 and 0.35 J/moleUK², the alloys can be classified as heavy-fermion materials. As opposed to UPt₃, the alloys exhibit spin-glass behavior with a concomitant decrease of ₀. The magnetization exhibits typical spin-glass features (e.g., an irreversibility line). An inverse relationship between ₀ andT _max (temperature ofC/T maximum which is proportional to the spin-glass freezing temperature), irrespective of the Pd content, is observed. The behavior can be qualitatively described by a coexistence of heavy-fermion behavior and spin-glass freezing.     

Magnetic properties for the Cu2MnSnSe4 and Cu2FeSnSe4 compounds

Magnetic susceptibility χ measurements in the range from 2 to 300 K were carried out on samples of the Cu₂FeSnSe₄ and Cu₂MnSnSe₄ compounds. It was found that Cu₂FeSnSe₄ was antiferromagnetic showing ideal Curie-Weiss behavior with a Néel temperature T_N of about 19 K and Curie-Weiss temperature θ=−200 K, while for Cu₂MnSnSe₄ the behavior was spin-glass with a freezing temperature T_f of about 22 K and Curie-Weiss temperature θ=−25 K. The spin-glass order parameter q(T), determined from the susceptibility data, was found to be in agreement with the prediction of conventional spin-glass theory.     

Enhanced temperature-independent magnetoresistance below the metal-insulator transition temperature of epitaxial La0.2Nd0.4Ca0.4MnO3 thin films

Epitaxial La_0.2Nd_0.4Ca_0.4MnO₃ thin films have been deposited at 800°C on LaAlO₃ substrate using pulsed laser deposition technique. The structural and magnetotransport properties of the films have been studied. The sharp peak in the temperature dependence of the resistance corresponding to metal-to-insulator transition (T _p) has been observed at a temperature of T _p=82 K, 97 K and 110 K for 0 Oe, 20 kOe and 40 kOe magnetic fields, respectively. The film exhibits a large nearly temperature-independent magnetoresistance around 99% in the temperature regime below T _p. The zero field-cooled (ZFC) and field-cooled (FC) magnetization data at 50 Oe shows irreversibility between the ZFC and FC close to the ferromagnetic transition temperature T _c=250 K. The ZFC temperature data of the film displays ferromagnetic behavior for higher temperature regime T _c=250 K>T>T _p=82 K, and a decrease in magnetization with decreasing temperature up to 5 K below 82 K exhibiting a sort of antiferromagnetic behavior in the low temperature regime (T<82 K=T _p=T _N).     

Effect of hydrostatic pressure on the static susceptibility of the spin-glass systems Ag:Mn,Au:Mn,Cu:Mn and Au:Fe

Previous studies of the magnetic susceptibility of a Ag-3 at.% Mn spin-glass under high pressure are extended to include the systems Au-3at.% Mn, Cu-3at.% Mn and Au-3at.% Fe. In all four alloys the spin-glass freezing temperatureT ₀ increases with pressure, yielding the volume dependences lnT ₀/ lnV=–3.6±0.4, –4.1±0.6, –2.1±0.4 and –1.64±0.25, respectively. These results give information about the nature of the interaction mechanisms responsible for the spin-glass freezing phenomena. The pressure dependence of the Curie temperature of ferromagnetic Au-20at.% Fe was also studied.Dedicated to B. Mühlschlegel on the occasion of his 60th birthday     

Inhomogenous magnetic ground state in CeAgGa

CeAgGa crystallizes in CeCu₂ Imma structure with Ag and Ga atoms randomly distributed at 8h sites. The magnetic and transport properties of the orthorhombic CeAgGa compound have been obtained from the analysis of ac magnetic susceptibility χ _ac , magnetization M vs. magnetic field, specific heat C and electrical resistivity ρ. The results provide evidence for the formation of a spin-glass state with a freezing temperature T _f = 5.1 K. The randomness in the Ce-Ce magnetic exchange interactions seem to arise from a statistical distribution of Ag and Ga atoms on a crystallographic site of the CeAgGa crystal lattice. The results provide also evidence for the formation of ferromagnetic-like order at the temperature T _C ≈ 3.6 K. Band structure calculations for a disordered system give magnetic moment similar to saturation moment obtained from magnetization measurements, however, its calculated value is insensitive on Ga/Ag off-stoichiometry in the 8h position. Complex behavior of CeAgGa Kondo-lattice compound is discussed in terms of interplaying Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions, Kondo effect and structural disorder.     

Magnetic field and temperature-induced first-order transition in Gd5(Si1.5Ge2.5): a study of the electrical resistance behavior

Magnetic field (0–4 T) and temperature dependencies (4.2–320 K) of the electrical resistance of Gd₅(Si_1.5Ge_2.5), which undergoes a reversible first-order ferromagnetic↔paramagnetic phase transition, have been measured. The electrical resistance of Gd₅(Si_1.5Ge_2.5) indicates that the magnetic phase transition can be induced by both temperature and magnetic field. The temperature dependence of the electrical resistance, R(T), for heating at low temperatures in the zero magnetic field has the usual metallic character, but at a critical temperature of T_cr=216 K the resistance shows a 20% negative discontinuity due to the transition from the low-temperature high-resistance state to the high-temperature low-resistance state. The R(T) dependence for cooling shows a similar but positive 25% discontinuity at 198 K. The isothermal magnetic field dependence of the electrical resistance from 212T224 K indicates the presence of temperature-dependent critical magnetic fields which can reversibly transform the paramagnetic phase into the ferromagnetic phase and vice versa. The critical magnetic fields diagram determined from the isothermal magnetic field dependencies of the electrical resistance of Gd₅(Si_1.5Ge_2.5) shows that the FM↔PM transition in zero magnetic field on cooling and heating occurs at 206 and 213 K, respectively. The full isothermal magnetic filed hysteresis for the FM↔PM transition is 2 T, and the isofield temperature gap between critical magnetic fields is 7 K.     

Acoustic susceptibility of an insulating spin-glass

High frequency acoustic measurements have been performed at temperatures around the freezing pointT _c in a cobalt insulating spin-glass. Assuming a Waller coupling mechanism between the spins and the acoustic wave, the data are interpreted in the framework of a dynamical model which takes into account the modulation of the reaction field. As a result, at the freezing temperature the characteristic relaxation time has no anomaly while the relevant susceptibility presents a maximum. The peculiar frequency dependence (as ^1/2) atT _c of the complex elastic constant is experimentally put in evidence.Associated with the Centre National de la Recherche Scientifique (UA 789)     

Electrical and magnetic properties of ε-Fe3N nanoparticles synthesized by chemical vapor condensation process

ε-Fe₃N nanoparticles synthesized by chemical vapor condensation (CVC) are covered with shells of disordered Fe₃O₄ phase, as observed by a transmission electron microscopy. The zero-field cooling and field cooling temperature dependence of magnetization, ac susceptibility as a function of frequency, magnetic hysteresis loops, and the temperature dependence of resistivity of the ε-Fe₃N nanoparticles are systematically studied. The results indicate the existence of complex magnetic properties, such as superparamagnetic behavior, exchange bias, magnetic dipole interaction, and the possible coexistence of ferromagnetic and spin-glass-like states and/or disordered surface spins of the shells at low temperatures. The temperature dependence of resistivity ρ(T) for compacted ε-Fe₃N nanoparticles in a temperature range of 110 K< T< 300 K can be well described by the mechanism of fluctuation-induced tunneling conduction, while that below 110 K can be ascribed to conducting electrons scattered by localized magnetic moments and impurity as well as the influence of freezing of spin-glass-like moments and/or disordered surface spins of the shells.     

Magnetism in a UNi2/3Rh1/3Al single crystal

We report a magnetization, magnetostriction, electrical resistivity, specific heat and neutron scattering study of a UNi_2/3Rh_1/3Al single crystal, a solid solution of an antiferromagnet UNiAl and a ferromagnet URhAl. The huge uniaxial magnetic anisotropy confining the principal magnetic response to the c axis in the parent compounds persists also for the solid solution. The magnetization curve at 1.6 K has a pronounced S shape with an inflection at 12 T. The temperature dependence of magnetic susceptibility exhibits a maximum around 10 K and is magnetic history dependent at lower temperatures where the resistivity increases linearly with decreasing temperature. The low-temperature ρ(T) anomaly is removed in a magnetic field applied along c, which yields a large negative magnetoresistance amounting to m46 zin 14T (at 2 K). The C/T values exhibit a minimum around 12 K and below 8 K they become nearly constant (about 250 mJ mol^?1 K^?2), which is strongly affected by magnetic fields. Neutron scattering data confirm a non-magnetic ground state of UNi_2/3Rh_1/3Al. The bulk properties at low temperatures are tentatively attributed to the freezing of U magnetic moments with antiferromagnetic correlations. The additional intensities detected on top of nuclear reflections in neutron diffraction in a magnetic field applied along c are found to be proportional to the field-induced magnetization, which reflects field-induced ferromagnetic coupling of U magnetic moments. This scenario is corroborated also by finding low-temperature magnetostriction data that also scale with the square of magnetization.     

Magnetic properties for the Mn2GeTe4 compound

Measurements of magnetic susceptibility χ, in the temperature range from 2 to 300 K, and of magnetization M vs. applied magnetic field B, up to 5 T, at various temperatures were made on polycrystalline samples of the Mn₂GeTe₄ compound. It was found that Mn₂GeTe₄ has a Néel temperature T_N of about 135 K, shows mainly antiferromagnetic behavior with a very weak superimposed ferromagnetic component that is attributed to spin canting. Also, the magnetic results suggest that a possible spin-glass transition takes place at T_f≈45 K. The spin-glass order parameter q(T), determined from the susceptibility data, was found to be in agreement with the prediction of conventional spin-glass theory. The M vs. B results indicated that bound magnetic polarons (BMPs) occur in the compound, and that the effects from BMPs disappear at approximately 80 K. The M vs. B curves were well fitted by a Langevin type of equation, and the variation of the fitting parameters determined as a function of temperature. Using a simple spherical model, the radius of the BMP in the material was found to be about 27 Å; this value is similar to the effective Bohr radius for an acceptor in the II-IV-V₂ and I-III-VI₂ ternary semiconductor compounds.     

Spin-glass-like ordering in the spinel ZnFe2O4 ferrite

In the present study, spin-glass-like ordering has been observed in the spinel ZnFe₂O₄ ferrite. Field cooled (FC) and zero-field cooled (ZFC) DC magnetizations display divergence at low temperature, which indicates a frozen state with the freezing temperature of T_f=21 K. Frequency dependence of AC susceptibility measurement was performed on the sample. It shows a peak at around T_f, with the peak position shifting as a function of driving frequency, indicating a spin-glass-like transition of the sample. The sample shows a typical spin-glass behavior with a manifestation of non-equilibrium dynamics of the spin glass, such as aging, rejuvenation and memory effects. These experimental findings indicate that Zn-ferrite exhibits a spin-glass-like phase at low temperature and it is not canted antiferromagnetic.     

Sr2CoO4-δ薄膜中的自旋玻璃态的磁性质研究

用脉冲激光沉积方法(PLD)在铝酸镧衬底上制备了c取向的高氧空位含量的锶钴氧薄膜.X射线衍射分析表明薄膜单一取向且没有明显杂相.原位的高气压反射式高能电子衍射仪(RHEED)监测显示,薄膜为层状生长.通过对薄膜磁化强度随温度、磁场及时间的变化曲线进行测量,发现零场冷曲线上可能存在两个特征温度:T_f和T_a.T_f为对应玻璃态的冻结温度而T_a对应少量的不缺 关键词： 自旋玻璃 超交换相互作用 双交换相互作用 脉冲激光沉积     

Exchange bias effect in multiferroic Eu0.75Y0.25MnO3

The exchange bias phenomenon has been investigated in multiferroic Eu_0.75Y_0.25MnO₃. The material shows a weak ferromagnetism with cone spin configuration induced by external magnetic field below 30 K. Consequently, the electric polarization coming from the cycloid spin order below 30 K can be suppressed by external magnetic fields. The magnetic hysteresis loops after cooling in a magnetic field exhibit characteristics of exchange bias below the spin glassy freezing temperature (T_g)∼16 K. The exchange bias field, coercivity field, and remanent magnetization increase with increasing cooling magnetic field. The exchange bias effect is ascribed to the frozen uncompensated spins at the antiferromagnetism/weak ferromagnetism interfaces in the spin-glass like phase.     

Magnetic properties of BiFeO3 micro-cubes synthesized by microwave agitation

In this article, we present results of field cooled (FC) and zero field cooled (ZFC) magnetization measurements and investigation of aging and memory effect in bismuth ferrite (BFO) multiferroic micro-cubes obtained by means of simple microwave synthesis procedure. It is found that difference between FC and ZFC magnetizations appear at the temperature of freezing of ferromagnetic domain walls. The decay of the magnetic moment versus time described by power-law relation and the absence of memory effect are caused by domain growth mechanism rather than by the spin-glass phase. The negligible value of remnant magnetic moment indicates that BFO compound exhibits low concentration of ferromagnetic domains and can be close to ferromagnetic to spin-glass transition.