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Phototransferred thermoluminescence (PTTL) of Al2O3:C,Mg induced by 470 nm blue, 525 nm green, and 870 nm infrared light is reported. Although its conventional TL glow curve measured during heating to 600 °C has seven peaks (labeled I–VII), only peak II is reproduced under phototransfer, except when the sample is preheated to 180 °C when peak IV also re-appears. The dependence of PTTL intensity on the duration of illumination for both PTTL peaks is analyzed using phenomenological and kinetic models as systems of acceptor and donors, with the number of the latter determined by experiment. The intensity of PTTL increases with the temperature of illumination with an activation energy of thermal assistance of either 0.015 ± 0.002 or 0.047 ± 0.002 eV when either 470 or 525 nm light is used but decreases at elevated temperatures with an activation energy of thermal quenching equal to 0.51 ± 0.02 eV independent of illumination wavelength. The long term behavior of time-response profiles for a system of one acceptor and a single donor, as an exemplar, as studied by stability theory, shows an unstable critical point. Mechanisms for the PTTL are discussed and the emission is ascribed to electron centers in the oxygen sub-lattice.

We describe a first-order phase transition in an isotropic non-Heisenberg ferromagnet induced by a magnetic field with a fourth-order spin exchange interaction at temperatures above the Curie point. The field behavior of the magnetization is analyzed and the temperature dependences of the critical fields of the stability of the paramagnetic and ferromagnetic phases as well as the critical field of the phase transition between them are determined. It is shown that this first-order phase transition has a finite amplitude of the magnetization jump and can occur in small magnetic fields. An H–T phase diagram is presented. It is found that the amplitude of the phase jump during the magnetic field-induced transition from the paramagnetic to the ferromagnetic phase can exceed the magnitude of the entropy change of an isotropic Heisenberg ferromagnet by two orders of magnitude when it is magnetized at the Curie point. It is shown that the expansion of the free energy model up to the 6th power in magnetization can only qualitatively describe the entropy behavior in the phase transition between the paramagnetic and ferromagnetic phases.

Phototransferred thermoluminescence (PTTL) from BeO is reported. The conventional TL glow curve has five peaks, I–V, at 50, 182, 283, 437, and 535 °C when measured at 1 °C s−1 The dominant one is peak II. Only the first three reappear under phototransfer. Electron traps for peaks IV and V weakly contribute to the PTTL. The PTTL at peak III, supposedly associated with these weak donors, is counterintuitively intense. All PTTL peaks are affected by light-induced fading, thermal quenching, and all but peak II by thermal fading. The activation energy for thermal quenching was found as 0.56±0.02, 0.53±0.05, and 0.94±0.07eV using peaks I, II, and III, respectively. Electronic processes relevant in the PTTL of BeO are discussed. PTTL time-response profiles are analyzed using systems of coupled linear differential equations as well as by theoretical modeling.

Numerical calculations of magnetization reversal loops of nanogranular films having perpendicular anisotropy in an inclined magnetic field at finite temperature, for film “quality parameters” both less than, and greater than, one. The calculations are performed with consideration of the film's demagnetizing field in the two-level approximation for the energy of the granule. It is found that the hysteresis of the transition fields between states with uniformly directed and non-uniformly directed magnetic moments for all granules narrows with increasing temperature and field deviation angle from the normal to the film. As the field deviation increases this hysteresis vanishes, and there is a transition from the uniformly magnetized state of the granules to the equilibrium non-homogeneous state without hysteresis. In the case of large field deflection angles, at finite temperature the magnetization consists of three regions. At the center of the magnetization curve there is a hysteresis region in which the m...

We present a theoretical description of a quantum phase transition in a dimerized antiferromagnet, induced by an external magnetic field. The description of such transitions is constructed by determining the minimum of the Lagrange function which depends on the parameters of the ground-state crystal wavefunction. We determine that during a crystal transition from the singlet state of the dimer to the state with a noncollinear spin orientation, an order-order type phase transition occurs. We show that in the case of a single-axis character of the exchange anisotropy, the location of the quantum phase transition critical point is affected by the spin-spin interactions inside the dimer. Additionally, analogous to the classical Neele antiferromagnets, single-axis type anisotropic spin-spin interactions between dimers, shift the point of the spin-flip transition from the noncollinear antiferromagnetic phase to the ferromagnetic phase.

A Monte-Carlo (MC) method is used to study the magnetization of an ensemble of Stoner-Wohlfarth particles with interparticle interactions of a ferromagnetic type. A modification of the procedure for the MC calculation is proposed to speed up the calculations. The magnetization of the ensemble is studied at different temperatures and for different ratios of the exchange field of the interparticle interaction and the uniaxial magnetic anisotropy field of the particles. It is shown that Neel relaxation has a significant influence on the coercive properties of the ensemble if the interparticle exchange field does not exceed the anisotropy field of the particles. It is also found that, at low temperatures, the coercive properties of Stoner-Wohlfarth particles with exchange interactions are related to the loss of stability of the orientation of the average magnetic moment vector of the particles. At high temperatures, the coercivity of the particle ensemble is associated with the complete loss of stability of t...

Two series of nanosized cobalt spinel ferrites CoFe2O4 are synthesized from metal salts using high-energy ball milling with the addition of NaCl as a growth agent (series CFO-NaCl), and without (CFO Series). The particle properties are characterized using atomic force microscopy, as well as magnetic and calorimetric measurements. It is shown that the average sizes of the nanoparticles were ∼5.6 and ∼10.3 nm for CFO and CFO-NaCl series, respectively. We performed magnetostatic measurements and determined the parameters that are required to analyze the magnetic state and remagnetization processes of the nanoparticles. It is shown that the blocking temperature is ≈160 K for CFO samples and ≈300 K for the CFO-NaCl series. It was concluded that at 293 K the CFO series particles exhibit a superparamagnetic state, whereas the CFO-NaCl series are in the blocked state. The specific loss power that is scattered by the synthesized nanoparticle ensembles placed in an alternating magnetic field, is measured experiment...

It was found that the anomalous magnetorheological effect observed in initially unstructured magnetoisotropic magnetoactive elastomers, i.e., the growth of their effective shear modulus in an external magnetic field by almost two orders of magnitude, is a result of the uniaxial magnetic anisotropy induced by the magnetic field. An additional magnetoelastic contribution to the mechanical stress created by the induced magnetic anisotropy counteracts the shear and increases the effective shear modulus of the magnetoactive elastomer when the latter is magnetized. Numerical estimates made for the magnitude of magnetorheological effect are in good agreement with known experimental data.

It is shown that the first-order isostructural magnetic phase transition between antiferromagnetic phases with different magnitudes of the antiferromagnetism vector induced by an external magnetic field in Ising antiferromagnets can be related to entropy. It is found that, depending on the temperature, the entropy jump and the corresponding heat release can change their signs at the phase transition point. In the low-temperature region of the metamagnetic first-order phase transition the entropy jump is positive, while the entropy jump is negative near the triple point for isostructural magnetic phase transitions.

An influence of demagnetizing field on a process of equilibrium magnetization of nanogranular films with perpendicular anisotropy in a tilted magnetic field is studied theoretically. An effect of the canting and flipping of granules magnetic moments on magnetization curves is described. It is shown that in a tilted field the transition from an inhomogeneous state to the state with a uniform orientation of magnetic moments takes place in a critical way.

The quantum phase transition induced by an external magnetic field in a dimerized spin crystal from its singlet ground state to an antiferromagnetic ordered state is examined under the assumption that the exchange interaction is antiferromagnetic, both within dimers and between them. It is shown that during this transition the magnetic polarization of the ions within a dimer and, therefore, of the dimers as a whole, is critical. The orientations and projections of the spin vectors in the magnetically ordered phase depend on the applied magnetic field, while the magnetic susceptibility of this essentially quantum phase remains (similarly to the case of a classical Neel antiferromagnet) constant as the spins of the magnetic sublattices undergo reorientation.

The temperature (4.5–550K) and angular (in the film plane) dependences of the coercivity field for physically nonpercolated nanogranular (CoFeB)x–(SiO2)1−x films with an oriented intraplane granule anisotropy have some unusual properties compared to those owing to blocking of thermally activated reorientation of the magnetic moments of the granules. At temperatures above the blocking temperature Tb≈350–370K, the dependence of the coercivity on the measurement duration is weak compared to that below Tb. Its angular (in the film plane) dependence differs qualitatively from that expected with blocking of superparamagnetic particles. This behavior is explained by the appearance of superferromagnetic ordering of the granule magnetic moments at T>Tb owing to interactions among them. As the temperature is reduced below 100K, the coercive field Hc for magnetization in the easy direction increases significantly more than expected for a blocked superparamagnetic ensemble. Here Hc≠0 for magnetization along the hard ...

An analysis is made of the various protocols of magnetostatic measurements for an ensemble of uniaxial single-domain particles oriented with easy axes along the magnetic field. It is shown that an important role in the magnetization reversal process is played by the continuous sweep of the magnetic field or temperature with time in making various measurements. A new characteristic feature in the temperature dependence of the magnetization rate obtained from the magnetization reversal loop, corresponding to the blocking temperature, is found. It is found that the values of the blocking temperature Tb determined from different magnetostatic measurements are not identical. It is shown that a comparison of the results obtained by these methods requires matching of the effective sweep times for each type of magnetostatic measurement. It is found that taking the measurement time into account in measurements in a constant field for changing temperature modifies the dependence of Tb on the measurement field. It i...

Temperature and field investigations of the magnetic properties of multilayer films, consisting of a sequence of alternating magnetic layers of iron and the rare-earth terbium separated by layers of nonmagnetic gold, are performed. The samples were obtained by electron-beam deposition. The field dependence of the magnetic moment of a film with H∥n are analyzed to determine the characteristics of the uniaxial anisotropy. It is shown that a nonuniform uniaxial anisotropy results in nonlinearity in the magnetization of the film in H∥n. The process of magnetization of a film in H⊥n, when the transition to a single-domain state occurs only as a result of the motion of domain walls, is described. It is shown on the basis of the data on the field dependences of the magnetic moment at low temperatures that the magnetic moments of the Fe and Tb layers are oriented oppositely with respect to one another. A compensation point is observed in the temperature dependence of the magnetization; this indicates the presence...

An attempt is made at a theoretical interpretation of the observed collapse of the magnetic state in solid oxygen under pressure. The basic premise rests on a growing role of the biquadratic intermolecular exchange with increasing pressure, leading to instability of the initial antiferromagnetic state.

The magnetic ordering of an easy-plane Van Vleck magnet with a large anisotropy of the easy-plane type and an antiferromagnetic exchange interaction between spins is considered in the case when the ground state of the integer-spin ions is a singlet and the antiferromagnetic phase is realized only in an external magnetic field. It is found that the phase transformation of the singlet (paramagnetic) phase to the antiferromagnetic phase is a quantum phase transition belonging to the magnetic phase transitions of the displacive type. It is shown that the Landau theory of phase transitions is applicable to such a transformation, with the order parameter being the polarization of the singlet state. Nonstandard manifestations of the paraprocess converting the second-order magnetic phase transition of the displacive type to a first-order transition are noted. The phase diagram is constructed; it differs from those of classical antiferromagnets in that the antiferromagnetic phase on it is surrounded only by the pa...

We study the formation of antiferromagnetic magnetoelastic domains in easy-plane antiferromagnets of the iron-group dihalides, in which the infinite degeneracy of the spatial orientation of the antiferromagnetic vector in the basal plane is lifted on account of the spontaneous magnetostriction. In these crystals the domains differ from each other not only by the direction of the antiferromagnetic vector L in them but also by the related directions of the principal axes of the spontaneous magneostrictive strain. The system of antiferromagnetic domains turns out to be identical to the system of elastic domains. It is shown that the processes of magnetization and the induced striction in a magnetic field in the multidomain antiferromagnetic state are interconnected. Data on the field dependence of the induced magnetostriction in the multidomain state of the easy-plane antiferromagnets CoCl2 and NiCl2 are presented and analyzed. It is shown that although the magnetostriction in the cycles of imposition and re...

A simple interpretation is proposed for the series of magnetization jumps observed in two-sublattice Ising antiferromagnets with single-ion anisotropy of the easy-plane type in a longitudinal external field.

An analysis is made of the experimental data on the magnetic-field dependence of the magnetostriction and magnetization of the NiCl2 crystal at its transition from the multidomain antiferromagnetic state to a uniform state. It is shown that their field dependence is determined by the mean orientation of the domains, which is characterized by a domain coalignment parameter. That parameter is used to give a phenomenological description of the multidomain state of the antiferromagnet NiCl2.

For the example of a system with a ferromagnetic exchange interaction between ions with spins S=1, it is shown that the methods of the phenomenological theory based on the use of a Landau potential can be used for a description of processes of ordering and magnetization of magnets with a large single-ion anisotropy.

Experimental data from measurements of the induced magnetostriction of the easy-plane two-sublattice layered antiferromagnet NiCl2 in the homogeneous and multidomain states at T=4.2 K are analyzed. It is shown that the intersublattice magnetoelastic interactions are dominant in NiCl2. The induced magnetostriction of the crystal in the single-domain state is due to canting of the spins of the sublattices toward the magnetic field direction. The induced magnetostriction in the multidomain state is due to a rearrangement of the domain structure and is independent of the small in-plane anisotropy. In both cases the values of the induced magnetostriction are directly proportional to the square of the value of the external magnetic field.

The features of the transformation of the multidomain state of an easy-plane antiferromagnet in an external magnetic field are investigated in the case of a magnetoelastic origin of the multidomain state. Experimental data are presented on the field dependence of the induced magnetostriction of the easy-plane two-sublattice antiferromagnet CoCl2 at different temperatures. In the discussion of the experiments and in the theoretical description we use the approximation of a continuous distribution of domains in the easy plane. It is shown that the orientational distribution of the domains upon the introduction of a magnetic field depends only on the magnitude and direction of the field and is independent of temperature. These features of the distribution of domains can be explained by a matching of their equilibrium magnetostriction with the elasticity of defects wherein the elasticity of the domains compensates the elasticity of the defects.

A structure is proposed for the magnetoelastic antiferromagnetic domains in easy-plane antiferromagnets of the iron-group dihalides. The proposed structure is made up of a triad of regions with 120° rotation of the antiferromagnetic vector on passing between adjacent regions. This domain structure may be stabilized by defects which are isotropic in the easy plane and which preserve the orientational degeneracy of the directions of the antiferromagnetic vector in the easy plane.

An analysis is made of the experimental magnetic-field curves of the anisotropic (in the easy-plane) magnetostriction of the antiferromagnet CoCl2 at different temperatures in the ordered phase. It is shown that the field dependence of the magnetostriction in a single-domain antiferromagnetic state remains proportional to the square of the applied field over the entire temperature range; this behavior corresponds well to the classic (Neel) scheme of collapse of the sublattice spins (spin flip). The temperature dependences of the intrasublattice and intersublattice contributions to the magnetoelasticity are proportional to each other. The temperature dependence of the spin-flip field obtained from the magnetostriction data agrees with that obtained previously from observations of the antiferromagnetic resonance (AFMR). The temperature dependence of the spontaneous anisotropy of the magnetostriction, obtained by extrapolating the magnetostriction in the single-domain state to zero field, is close to M∼T4.

The magnetic ordering model is considered for a single-lattice spin subsystem with S=1 and a large biquadratic exchange whose energy has the same order of magnitude as the bilinear exchange interaction energy. The expression for free energy is obtained in the microscopic approximation, and the phase states are defined and analyzed. The Landau potential is represented as a power expansion in the model microscopic free energy. This expression is used to describe the peculiarities of the temperature dependences of the magnetic susceptibility and magnetoelastic deformation.

A theory is proposed for the two-step phase transition from the singlet to a ferromagnetic state in systems of the ABX3 type. It is shown that the transition is due to a substantial rearrangement of the single-ion spectrum. The critical fields and magnetic susceptibility are calculated.

It is shown that the spin Hamiltonian of the spin–spin interactions, written with the use of effective spinons defined on the two lowest doublets of magnetic ions with a partially frozen orbital moment, has a non-Heisenberg form. The Heisenberg contribution is represented by spin–spin terms containing three powers of ion spin projections and is anisotropic and comparable to the bilinear spin–spin interaction.

Magnetic and magneto-transport properties of granular ferromagnetic Cox-(Al2O3)1-x films with a large range of cobalt concentrations x are investigated. The films with x below the percolation threshold xp demonstrate the presence of oriented uniaxial anisotropy of the granules with the anisotropy field Ha ∼ 2 kOe and easy anisotropy axis perpendicular to the film plane. It is shown that for the values of x close but below xp, field dependencies of the magnetoresistance in the magnetic fields applied in the film plane, possess an anomalous field dependence with a positive magnetoresistance and a maximum in the range of field magnitudes 0

Ferromagnetic resonance (FMR) in the blocked state of nanogranular magnetic films with perpendicular anisotropy of granules is considered for the magnetic field, which is perpendicular to the film plane, for the field strengths corresponding to the unsaturated magnetic state of the films. It is shown that in this case FMR response of the film is the sum of contributions from two subensembles of granules with the magnetic moments oriented “up” and “down” with respect to the film plane. These subensembles are coupled through the common demagnetization field of the film. It is established that FMR signals registered with the help of microwave detector direct current and a component of the detector current at the frequency of radiospectrometer magnetic field high frequency modulation are essentially different. It is shown that the distinction arises due to the fact that total magnetization of each of the subensembles and the film as a whole are practically not modulated by the high frequency modulation of the...

An anomalous angular dependence of the coercive field of physically nonpercolated granular CoFeB–SiO2 films with oriented anisotropy of the granules, where the presence of the superferromagnetic phase has been established earlier, is observed experimentally at temperatures above the blocking temperature. The angular dependence of the coercive field is similar to that typically observed in the case of magnetization reversal through an inhomogeneous magnetic state of single granules in the ensemble. As the temperature becomes lower than the blocking temperature, the angular dependence changes to that typical of the magnetization reversal through a coherent rotation of granules’ magnetic moments. It is shown theoretically that the anomaly is determined by a singularity of the angular dependence of the critical field causing the absolute loss of the superferromagnetic phase stability in the ensemble of weakly interacting ferromagnetic nanoparticles.

We reveal the low-temperature anomaly in the temperature and angular dependencies of the coercivity in granular (CoFeB)x–(SiO2)1−x films with oriented in-plane anisotropy. Namely, at T

Structural, electric, and magnetic properties of the bulk polycrystalline samples with nominal composition La0.7Ca0.3−xNaxMnO3 (x=0−0.10) have been investigated in the work. As follows from the results of chemical and x-ray structural analyses, the increase in x brings about the rise in the number of structural vacancies in both lanthanum and oxygen sublattices. The influence of the variation of the vacancy number on peculiar features of the transition from ferromagnetic metallic state to paramagnetic state with activated conductivity has been studied in detail. The conclusion is made that the rise in the number of structural vacancies gives rise not only to the enhancement of magnetic inhomogeneity and broadening of the magnetic transition, but also to the increase in the temperature of magnetic transition. It is demonstrated that the changes in magnetic parameters also result in the increase of the temperatures of resistivity and magnetoresistance peaks as well as in the change of the character of the t...

The studies of Fe∕Au∕Tb multilayers (mls) prepared by e-beam evaporation are presented. We found that for mls obtained by seven reiterations of the layer sequence: ⋯∕24AFe∕17.5AAu∕36ATb∕17.5AAu∕⋯ only the Fe layers are ferromagnetic (FM) at T>100K, while Tb layers are paramagnetic. At T

A thermodynamic analysis of the influence of non-Heisenberg interionic spin-spin interactions on the temperature dependence of the NMR frequency of a NiCl2 crystal is carried out.