Strong Irreversibility Research Articles

Magnetic glassy phase in Zn0.85Fe0.15O diluted magnetic semiconducting nanoparticles

The authors have investigated the ground state magnetic phase and the optical band gap of chemically synthesized Zn0.85Fe0.15O diluted magnetic semiconducting nanoparticles (∼7.5nm). The temperature dependent magnetization study shows strong irreversibility along with a cusplike anomaly, which is ascribed to a freezing to a cluster glasslike magnetic ground state. This assignment is further established by Arrott-Belov-Kouvel plots along with S-like nonsaturating magnetization versus field curves. The finite size ferromagnetic clusters which are formed due to an enhanced grain surface effect in the system undergo random dipolar intercluster interactions, giving rise to strong competitive ferromagnetic and antiferromagnetic interactions, finally leading to the freezing of those clusters.

Low-temperature magnetization dynamics of oxygen-stabilized tetragonal Ni nanoparticles

A comprehensive study of the magnetization dynamics of oxygen-stabilized, tetragonal $\mathrm{Ni}\phantom{\rule{0.3em}{0ex}}(\mathrm{OS}\text{\ensuremath{-}}\mathrm{Ni})$ nanoparticles $(20\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ prepared by the borohydride reduction method is reported. Both as-prepared and air annealed (at $573\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $773\phantom{\rule{0.3em}{0ex}}\mathrm{K}$) samples have been studied using ac susceptibility, aging experiments, and field cooled (FC) and zero field cooled (ZFC) magnetization measurements as a function of temperature. The OS structure, which is a deviation from the usual face-centered-cubic (fcc) structure of Ni, arises due to the presence of interstitial oxygen atoms in the unit cell of Ni and causes it to exhibit anomalous magnetic behavior such as a very large magnetization enhancement at low temperatures. Two low-temperature magnetic transitions in close succession, manifested in the form of a sharp peak at $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and a small hump at $\ensuremath{\sim}12\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, are observed in the ZFC curve and ac susceptibility plots of the as-prepared sample. The probable nature of these transitions has been explained on the basis of a model which associates the peak at $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ with the occurrence of a $\mathrm{PM}\phantom{\rule{0.3em}{0ex}}(\mathrm{paramagnetic})\ensuremath{\rightarrow}\mathrm{FM}\phantom{\rule{0.3em}{0ex}}(\mathrm{ferromagnetic})$ transition of the oxygen-stabilized phase. Most of these newly formed FM particles block as soon as they gain internal order, yielding a strong irreversibility between the FC and ZFC branches. Some of the macromoments formed at $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, however, remain unstable down to $12\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the temperature at which they block cooperatively as shown by a critical dynamics analysis, yielding critical exponent $z\ensuremath{\nu}=9.84\ifmmode\pm\else\textpm\fi{}0.48$ and a relaxation prefactor of ${\ensuremath{\tau}}_{0}={10}^{\ensuremath{-}7}\phantom{\rule{0.3em}{0ex}}\mathrm{s}$. Aging experiments at $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for three different wait times ${t}_{\mathrm{w}}$ show wait time dependency, substantiating unequivocally the cooperative, chaotic nature of the sample magnetic dynamics. The low-temperature magnetic features displayed by the $573\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ annealed sample closely resemble those of the as-prepared one, though distinctly different features are observed in the sample annealed at $773\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. These have been explained coherently taking into account the structural changes produced upon annealing.

Magnetic properties and 155Gd Mössbauer spectroscopy of LT-GdNiIn 2

A polycrystalline, single phase sample of the low-temperature (LT) modification of GdNiIn 2 (MgCuAl 2 type, Cmcm, a = 434.5 ( 1 ) , b = 1047.6 ( 3 ) , c = 731.8 ( 2 ) pm ) was synthesized by arc-melting of the elements and subsequent annealing of the sample at 870 K. The magnetic properties of this compound were studied thoroughly by AC and DC magnetic susceptibility measurements and 155Gd Mössbauer spectroscopy. These results reveal four phase transitions: from a para- to a ferromagnetic state at T C = 60.5 ( 5 ) and three other transitions at T 1 = 19.4 ( 2 ) K , T 2 = 12.2 ( 2 ) K , and T 3 = 4.0 ( 2 ) K , respectively, which can be tentatively described as spin-reorientation transitions. LT-GdNiIn 2 shows strong thermal irreversibility between zero field cooled (ZFC) and field cooled (FC) DC magnetization.

Dynamic factor demand and irreversible adjustment in the Spanish milk sector

The accession of Spain to the European Economic Community, now the European Union, entailed major changes for the milk sector. We try to determine whether the structural adjustment processes of the firm triggered by the changing environment present irreversibilities using a adjustment costs dual model. We modify the model proposed by Chang and Stefanou (1988) in order to incorporate a theoretically more robust treatment of the zero-investment regime in quasi-fixed factors. The empirical analysis reveals that strong irreversibilities are present in the adjustment processes: speeds of adjustment are distinct in investment and disinvestment processes, and there are zero-investment regimes.

Magnetic field dependence of the reversible mixed-state properties of superconductingMgB2single crystals and the influence of artificial defects

The reversible mixed-state properties of superconducting ${\mathrm{MgB}}_{2}$ are summarized and discussed in terms of new results and evaluations, which take the two band character of the material explicitly into account. The magnetic field dependence of the reversible magnetization is analyzed within Ginzburg Landau theory which, however, fits only either at high or at low fields. The resulting parameters (e.g., characteristic fields, lengths, and anisotropy) are quite different in these two regimes. We find large anisotropies of about 4.6 at high fields and low temperatures, but values close to 1 at low fields. Finally, we present neutron irradiation experiments, where new defects, leading to strong irreversibility but also to a modification of the reversible properties, are induced.

Glassy magnetism in mechanically alloyed Fe35Cr65

A mixture of Fe and Cr powders, with the atomic concentration Fe 35 Cr 65 , was mechanically alloyed in a high-energy planetary mill for 290 h. Upon milling. the X-ray diffraction pattern stopped evolving much earlier than the hysteresis loop. The final product (milled for 290 h) showed a strong irreversibility between the field-cooled and zero-field cooled magnetisation curves below the maximum exhibited by the former at T m 170 K, accompanied by a slightly frequency-dependent peak of the AC susceptibility. These features are explained in terms of the blocking of interacting sanoclusters.

Thermochemical properties of copper forms of zeolite zsm5 containing ethylenediamine

Abstract Copper forms of synthetic zeolite ZSM5 containing ethylenediamine (en) were characterised by methods of thermal analyses - TG, DTA and DTG in the temperature range 20-1000C, in air and in argon atmosphere. Mass spectroscopy was used for the study of the released gas products of thermal decomposition. The results of thermal analyses of three Cu(en)xZSM5 zeolitic products with different composition (x depends on the mode of preparation) checked their different thermal properties. The main part of the decomposition process occurs at considerably higher temperatures than the boiling point of ethylenediamine of all three products, it proves strong bond and irreversibility of en-zeolite interaction. According to the results of the mass spectroscopy method the decomposition process in inert atmosphere is characterised by the development of a large spectrum of products with atomic mass from 28 to 178 atomic mass units, and there is a correlation between the mode of sample preparation and the spectrum of...

$\mathsf{La_{0.5}Sr_{0.5}CoO_{3}}$ : A ferromagnet with strong irreversibility

This paper presents a summary of measurements of the field and temperature dependence of the ac susceptibility, magnetic hysteresis isotherms, and the zero field cooled (ZFC) and field cooled (FC) moment. These results demonstrate that La0.5Sr0.5CoO3 is consistently and comprehensively described as a ferromagnet with strong irreversibility. Analysis of the ac susceptibility demonstrates unequivocally the occurrence of a continuous (second order) paramagnetic to ferromagnetic phase transition with mean-field-like effective exponents (although estimates for the true, asymptotic critical exponents appear precluded by the substantial anisotropy/coercivity, linked to the presence of Co ions). The behaviour of the irreversible response is analyzed within the framework of a Preisach model which decomposes the magnetizing process into a sequence of two-state Barkhausen excitations. The model is able to replicate the field and temperature dependence of the hysteresis isotherms and the FC and ZFC moments, and shows that these response functions are a product of field activated transitions over a spectrum of temperature dependent metastable state excitation barriers, with a negligible contribution from thermal fluctuations.

Magnetic properties of the perovskite compounds YFe 1– xCr xO 3 (0.5≤ x≤1)

Powder oxides with the formula YFe 1– x Cr x O 3 (0.5≤ x≤1) have been studied. The samples have been prepared using the ceramic solid state reaction in air. The X-ray diffraction spectra indicated that the samples crystallise in the perovskite structure with orthorhombic deformation. The magnetic properties of these materials are concentration and temperature dependent. Zero-field-cooled and field-cooled magnetization curves show strong irreversibilities and complex hysteresis loops. The saturation magnetization for the mixed compounds shows a unusual behaviour. The results are discussed by considering the existence of many weak ferromagnetic components induced by the random distribution of the transition metal cations in the octahedral site. This leads to frustration interactions and short-range order that smooth the phase transition.

Spin Glass Behavior of New Perovskites Ba2In2−xCoxO5 (0.5 ⩽x⩽1.70)

New oxides of general formula Ba2In2−xCoxO5 (0.5 ⩽x⩽1.70) have been synthesized as polycrystalline materials and fully characterized by means of X-rays. They are all isostructural and crystallize in a perovskite-type structure, showing cubic symmetry, space group Pm3m. The magnetic properties of these oxides have been studied by χac and χdc measurements as a function of magnetic field and temperature. In the case of x=0.5 (BaIn1.50Co0.5O5) the sample remains paramagnetic from 200 to 2 K. However, the magnetic behavior of the oxides Ba2In2−xCoxO5 (0.70⩽x⩽1.70) is rather different: χdc shows broad maxima which are shifted with the x cobalt content. Below this temperature a strong irreversibility between the zero-field-cooled (ZFC) and field-cooled (FC) curves has been detected, more pronounced as the cobalt content is higher. χac measurements also show all the common features of spin glasses, fully confirmed from the analysis of the non linear part of the χdc.

Irreversibility lines of the heavy fermion spin glass URh2Ge2

We present a study of the field dependence of the frozen-in state in the 3D Ising-like heavy fermion spin glass URh2Ge2. From DC-susceptibility experiments in field-cooled and zero-field-cooled mode along the crystallographic a and c axes of the tetragonal unit cell, we determine the crossover temperatures associated with the occurrence of weak and strong irreversibilities in the magnetic behaviour. In low fields, we observe single-ion anisotropy with a c axes susceptibility larger by a factor of 5–6 than along a, and a transition into a spin-glass state at Tf=9.38K. In higher fields, we observe two characteristic temperatures: the upper one, Tf, indicates the onset of spin freezing, while the lower one, Tirr, reflects increased irreversibility of the frozen state. From our data, we construct the B–T phase diagram, revealing anisotropy of the freezing process for fields applied along a-and c-axis, respectively, which we discuss in terms of the magnetic irreversibility in m-vector spin glasses.

Spin-glass ordering inZn1−xMnxIn2Te4diluted magnetic semiconductor

We present a study of the magnetic properties of the ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{In}}_{2}{\mathrm{Te}}_{4}$ diluted magnetic semiconductor for concentrations $0.3<~x<~1.0.$ Samples with $x<~0.5$ displayed a paramagnetic behavior down to the lowest experimental temperature (1.8 K), whereas a paramagnetic to spin-glass transition was observed for $x>0.5$ at temperatures $2.5 \mathrm{K}<~T<~3.8 \mathrm{K},$ depending on the $\mathrm{Mn}$ content. The effective magnetic moment ${\ensuremath{\mu}}_{\mathrm{eff}}=5.80(2){\ensuremath{\mu}}_{B},$ calculated from the high-temperature susceptibility, corresponds to a ${3d}^{5}$ $(S=\frac{5}{2})$ configuration for ${\mathrm{Mn}}^{2+}.$ For the sample with composition $x=0.9,$ the in-phase component of the ac susceptibility has been analyzed according to conventional power-law dynamics, obtaining a freezing temperature of ${T}_{f}=3.1(2) \mathrm{K}$ and a critical exponent $z\ensuremath{\nu}=10.3\ifmmode\pm\else\textpm\fi{}2.$ Low-field dc magnetic susceptibility data show a sharp peak at $\ensuremath{\sim}3.3 \mathrm{K},$ below which strong irreversibility is observed between zero-field-cooled and field-cooled states. Evidence of a true phase-transition phenomenon is given by the steep increase of the nonlinear susceptibility ${\ensuremath{\chi}}_{\mathrm{nl}}$ when approaching ${T}_{f}$ from above. A static scaling of ${\ensuremath{\chi}}_{\mathrm{nl}}$ yielded values $\ensuremath{\beta}=0.9(1)$ and $\ensuremath{\gamma}=3.6(4)$ for the critical exponents, and gave also the proper asymptotic behavior of the scaling function. These values are in good agreement with data reported for other spin-glasses such as ${M}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ $(M=\mathrm{Zn},$ Cd, and Hg), and constitute strong evidence of a three-dimensional spin-glass transition in ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{In}}_{2}{\mathrm{Te}}_{4}.$

Magnetic hardening and spin-glass phenomena in nanocrystalline FeNbB at low temperatures

The soft nanocrystalline alloy ${\mathrm{Fe}}_{80.5}{\mathrm{Nb}}_{7}{\mathrm{B}}_{12.5},$ containing 25 vol % of bcc Fe, displays a rapid magnetic hardening below 20 K. This interesting process is accompanied by a strong irreversibility between field- and zero-field-cooled magnetizations and by a maximum of the magnetic viscosity near 8 K, both determined in a field of 1 Oe. We investigated the magnetization dynamics in more detail by measuring the linear ac susceptibility ${\ensuremath{\chi}}^{\ensuremath{'}}+i{\ensuremath{\chi}}^{\ensuremath{''}}$ between 30 mHz and 100 kHz. The shifts of the ${\ensuremath{\chi}}^{\ensuremath{''}}$ maxima towards low temperatures with decreasing frequency reveal a critical slowing down of the characteristic time, characterized by a critical exponent $z\ensuremath{\nu}=6.9(2),$ and a collective freezing temperature of ${T}_{g}=7.5\mathrm{K}.$ The collective nature of the freezing is supported by a dynamical scaling analysis of ${\ensuremath{\chi}}^{\ensuremath{''}}(T,\ensuremath{\omega}),$ yielding for the order parameter exponent $\ensuremath{\beta}=0.40(5),$ in good agreement with results for the spin glasses. We discuss these results by assuming a frustration of the exchange in the disordered interfacial regions and by the presence of the weak magnetic Nb-rich shells around the nanocrystalline grains.

DC-magnetisation studies in the La 1− xSr xCoO 3 ( x≤0.4) system

The magnetic field and temperature dependence of DC-magnetisation in polycrystalline samples of La 1− x Sr x CoO 3 ( x≤0.4) (LSCO) perovskite system is being studied using a SQUID magnetometer. We find that for Sr-concentration, x≥0.2, a sharp increase in magnetisation is observed in the temperature regime of 200–230 K. Strong thermomagnetic irreversibility (i.e. M ZFC≠ M FC) in the entire investigated composition regime is found in this system. Furthermore, we observed a mild hysteresis and small coercivity in the M– H scans performed at various temperatures in samples with x≥0.2. The observed magnetic behaviour in these samples clearly indicates that for x≥0.2, LSCO is a soft ferromagnet. We interpret that the observed thermomagnetic irreversibility arises from large magnetocrystalline anisotropy, which at low external applied magnetic fields effectively pins the ferromagnetic (FM) domains.

Magnetic probing of tunable Eu/sup 3+/ local site in organic-inorganic nanohybrids

The magnetic properties of sol-gel derived Eu/sup 3+/-doped organic/inorganic hybrid nanocomposites were studied. These hybrids consist of poly(oxyethylene)-based chains of variable length grafted onto siloxane nanodomains by urea cross-linkages. Results are presented for two chain length series, for which the local environment of Eu/sup 3+/ ions and the urea-urea interactions induce distinct magnetic behaviors. For high Eu/sup 3+/ content strong thermal irreversibility effects are found, due to the presence of Eu/sup 3+/ magnetic interactions in the nanodomains.

Magnetism and hyperfine interactions in Gd2Ni2Mg

Abstract Gd 2 Ni 2 Mg was synthesised by reaction of the elements in a sealed tantalum tube in a high-frequency furnace. The structure was investigated by single crystal X-ray diffraction: Mo 2 FeB 2 type, P 4/ mbm , a =743.8(1), c =375.3(1) pm, wR 2 =0.0560, 256 F 2 values, 12 variables. The Gd 2 Ni 2 Mg structure is an intergrowth of distorted AlB 2 and CsCl slabs of compositions GdNi 2 and GdMg. The magnetic phase transitions were studied by AC and DC magnetic susceptibility measurements and 155 Gd Mossbauer spectroscopy. Antiferromagnetic ordering is detected at T N =49.00(5) K followed by two further transitions at T f1 =20.70(5) and T f2 =4.5(1) K, respectively. Non-vanishing second and third harmonic components of the induced voltage under application of a small AC magnetic field reveal the ferromagnetic nature of the second transition at T f1 . Below T f1 , Gd 2 Ni 2 Mg shows strong irreversibility between field cooled (FC) and zero field cooled (ZFC) DC magnetization and magnetic hysteresis behaviour.

Low-temperature magnetic hardening and exchange anisotropy in nanocrystalline FeNbCrBCu alloys

The hysteresis loops of nanocrystalline Fe 73,5Nb 4.5Cr 5B 16Cu 1 alloys have been studied in a temperature range between 1.5 and 300 K. A magnetic hardening by nearly two orders of magnitude have been observed at temperatures below ∼50 K. The remarkable shift of the hysteresis loops detected at liquid helium temperatures after the field cooling as well as the strong irreversibility between zero-field cooling and field-cooling branches of magnetization curves indicate the presence of spin-disorder phase, which influences significantly the low-temperature behavior in the investigated nanocrystalline system.

Magnetic irreversibility of discontinuous Fe/CaF 2 multilayers with thermal annealing

In this work, we discuss the magnetic properties of discontinuous Fe/CaF 2 multilayers as-deposited at room temperature and after annealing in high vacuum at 350, 450 and 550°C for 1 h. Zero-field-cooled and field-cooled magnetization were measured as a function of the magnetic field and temperature. Strong magnetic irreversibility can be explained by spatial distribution of antiferromagnetic aggregates consisting of crystalline forms of FeF 3 with the Néel temperature around 140 K.

Low-temperature magnetothermodynamics of Pr0.7Ca0.3MnO3

We present a detailed magnetothermal study of Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$, a perovskite manganite in which an insulator-metal transition can be driven by magnetic field, but also by pressure, visible light, x-rays, or high currents. We find that the field-induced transition is associated with a large release of energy which accounts for its strong irreversibility. In the ferromagnetic metallic state, specific heat and magnetization measurements indicate a much smaller spin wave stiffness than that seen in any other ferromagnetic manganite, which we explain in terms of ferromagnetism among the Pr moments. The Pr ferromagnetism also appears to influence the low temperature thermodynamic phase diagram of this material and the uniquely sensitive metastability of the insulating state.

Low-temperature magnetothermodynamics of Pr0.7Ca0.3MnO3

Abstract We present a detailed magnetothermal study of Pr0.7Ca0.3MnO3, a perovskite manganite in which an insulator-metal transition can be driven by magnetic field but also by pressure, visible light, X-rays or high currents. We find that the field-induced transition is associated with a large release of energy which accounts for its strong irreversibility. In the ferromagnetic metallic state, specific heat and magnetization measurements indicate a much smaller spin-wave stiffness than seen in any other ferromagnetic manganite, which we explain in terms of ferromagnetism among the Pr moments. The Pr ferromagnetism also appears to influence the low-temperature thermodynamic phase diagram of this material and the uniquely sensitive metastability of the insulating state.