Magnetic behavior in the incommensurate phase ∼LaCrS3:: effect of the partial substitution of Cr by Ti on the magnetic properties

Dilution of the magnetic interactions between Cr³⁺ ions by Ti³⁺ ions was observed in the CrS₂ layer of the misfit-layer compound ∼LaCrS₃. Pure ∼LaCrS₃ has complex magnetic properties which are reminiscent of spin glass behavior. This magnetic behavior comes from both the modulated character of the structure and the magnetic frustration of the planar-antiferromagnetic-triangular network of Cr³⁺ ions. Thus, there is a large hysteresis between the zero field cooled and the field cooled magnetic susceptibility curves below the transition temperature (≈75 K). Formation of a solid solution ∼LaCr_1−xTi_xS₃ by the addition of Ti³⁺ ions results in the decrease of transition temperature up to a doping level of x≈0.5, where the transition is no longer observed. The magnetic behavior of the phase with x≈0.5 is similar to that of several random exchange antiferromagnetic compounds.     

Spin dynamics and internal motion in magnetically dilute manganites probed by EPR

EPR spectra of a series of single crystal samples of LaGa_1?xMn_xO₃ manganites were studied in a broad range of temperatures (20–300 K) and compositions (0.02≤x≤1). Evolution of the spectra under the action of exchange interactions of Mn³⁺ ions leads to the formation of a single Lorentzian line for x≥0.2. The results confirm the transition from antiferromagnetic to ferromagnetic spin ordering during diamagnetic dilution of the manganite with gallium, beginning with x=0.8. For x=0.1, the EPR spectra exhibit an anomalous broadening and splitting on cooling of the samples, which is interpreted as manifestations of a thermoactivated internal motion with a characteristic energy of about 50 meV. The results are compared with published data on the nuclear spin-lattice relaxation of gallium. The model of thermoactivated reorientations of the electron orbitals of Mn³⁺ ions subject to the Jahn-Teller effect is discussed.     

Magnetic phase diagram of the Bi<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript>x</Subscript>MnO<Subscript>3</Subscript> manganites

The magnetic and elastic properties of the Bi_1-xCa_xMnO₃ manganites are studied. The phase transformations revealed are ferromagnet-spin glass (x≥0.15) and spin glass-charge-ordered antiferromagnet (x≥0.25). The ferromagnetic state is characterized by ordering of the Mn³⁺d _x ²-y orbitals. It is suggested that thespin glass state originates from local static Jahn-Teller distortions. The antiferromagnetic charge-ordered and the spin-glass disordered phases coexist in samples with 0.25<x<0.32, which may be due to the charge order-disorder phase transformation being martensitic in character. The magnetic phase diagram is constructed.     

Structural study of Pr1−xCaxMnO3 and Y1−xCaxMnO3 perovskites

The structural properties of Prx_1?xCa_xMnO₃ and Y_1?xCa_xMnO₃ perovskite systems were studied by X-ray and magnetic susceptibility measurements. The differences in the structural properties of both systems are attributed to the differences of tolerance factors, i.e. of ionic radii of the ions present and the results are interpreted as being due to simultaneous action of steric effect, orbit-lattice interactions of Mn³⁺ ions and Coulomb interactions among 3d electrons. Their individual contributions depending on the compositions are analyzed. Besides the usual cooperative Jahn-Teller effect existing in PrMnO₃ a new type of transformation was found which is connected, under condition of Mn³⁺ ? Mn⁴⁺ valency migration, with the formation of local Jahn-Teller distortions. It is characterized by the equality of v_hop and v_phon at a critical temperature T_crit; it has been found that T_crit depends only very weakly on the composition. A correpsonding O'→O change in the yttrium system is, however, rather continuous which is explained by a strong influence of the cooperative buckling effect. An analysis of the magnetic data points to the presence of double exchange in the praseodymium system at high temperatures, while at low temperatures this mechanism applies only in the region, where cooperative Jahn-Teller effect exists, i.e. at x0.4.     

Electron spin resonance study of the dynamic magnetic susceptibility of CuO, Cu1−x Zn x O, and Cu1−x Li x O single crystals

Results are presented of studies of the dynamic magnetic susceptibility of CuO, Cu_1?x Zn _x O (x ≈ 1.5%), and Cu_1?x Li _x O (x ≈ 1%) single crystals. The orientational dependence of the ESR spectra was investigated at room temperature. The results for CuO are analyzed using a model of a quasi-one-dimensional antiferromagnet (S = 1/2) with anisotropic exchange interaction between Cu²⁺ spins in the chains and exchange coupling between the chains allowing for one-dimensional spin diffusion and spinon excitations. The estimated line width is of the same order of magnitude as the experimental data. Substituting Cu with Zn scarcely alters the spin dynamics of the Cu²⁺ ions, as in weakly diluted magnets. Lithium doping substantially increases the ESR line width and this is attributed to excess holes forming rapidly relaxing spin complexes with copper ions.     

Tunable exchange bias in La1.5Sr0.5CoMnO6 double perovskite doped with nonmagnetic Ga ions

The crystal structure, electronic structure, and magnetic behaviors of nonmagnetic Ga ions doped double perovskite La_1.5Sr_0.5CoMnO₆ single phase crystals have been investigated. Different from the traditional magnetic dilution effect of nonmagnetic doping, Ga doping in La_1.5Sr_0.5CoMnO₆ enhances the ferromagnetic (FM) exchange interaction of Co³⁺-O-Mn³⁺. Moreover, both conventional and spontaneous exchange bias (EB) effects can be tuned by modulating the Ga doping content, which is accompanied by the variation of the Co^3+/4+ and Mn^3+/4+ and the effective magnetic moment. The EB field and magnetization can be improved by nonmagnetic Ga³⁺ doping with content lower than 0.2. The evolution of conventional and spontaneous EB effects in La_1.5Sr_0.5Co_1-xGa_xMnO₆ can be understood in terms of the unidirectional interface anisotropic coupling between FM/anti-FM, and/or FM/spin glass, which is affected by antisite disorder, spin glass, and the uncompensated coupling between Co and Mn.     

Observation of mixed-phase behavior in the Mn-doped cobaltite La0.7Sr0.3Co1−xMnxO3 (x=0-0.5)

Studies on La_0.7Sr_0.3Co_1−xMn_xO₃ (x=0-0.5) compounds evidence that the interaction between Mn and Co ions in this system is antiferromagnetic super-exchange and not ferromagnetic (FM) double-exchange (DE). As a result, antiferromagnetism and magnetic glassiness develop steadily with increasing Mn content and the system becomes a spin glass at x∼0.1. Analyses of high-field magnetization data indicate that the system consists of two major phases: a metallic FM phase which magnetically saturates in rather low field, and an insulating non-FM phase which has a linear dependence of magnetization on magnetic field. In the low doping regime, the fraction of the non-FM component expands with temperature at the expense of the FM phase and becomes maximal at T_C. Ferromagnetism reappears in highly doped (x≥0.2) compounds due to the presence of DE interaction between the Mn ions. The small volume fraction of the FM phase derived from the M(H) data in high-field region supports the coexistence of insulating and FM behaviors in the highly doped samples.     

Magnetic phase transitions in Nd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Emphasis Type="Italic">x</Emphasis>MnO<Subscript>3</Subscript> manganites

The magnetic properties of manganites of the Nd_1?xCa_xMnO₃ system with x≤0.15 have been studied. It is shown that, in the 0.06≤x≤0.1 interval, the results can be interpreted using a model according to which the concentrational transition from a weakly ferromagnetic (WFM) state (x=0) to a ferromagnetic (FM) state (x>0.15) proceeds via a mixture of the exchange-coupled FM and WFM phases. In the vicinity of T=9 K, samples with 0.06≤x≤0.1 exhibit a spontaneous magnetic phase transition involving reorientation of the magnetization vectors of the WFM and the exchange-coupled FM phases. In the temperature interval between 5 and 20 K, a sample with the composition Nd^0.92Ca^0.08MnO^2.98 exhibits metamagnetic behavior. Magnetic phase diagrams in the H?T and T?x coordinates are presented. The appearance of the spin-reorientation transitions is explained in terms of the magnetic analog of the Jahn-Teller effect with allowance for the fact that, according to the neutron diffraction data, the magnetic moments of neodymium ions in the FM phase are parallel to the magnetic moments of manganese ions.     

Magnetic and electrical properties of the ZnGeAs<Subscript>2</Subscript>: Mn chalcopyrite

Doping of the ZnGeAs₂ semiconductor with manganese has produced compositions with spontaneous magnetization and high Curie temperatures of up to 367 K for the composition 3.5 wt% Mn. Their magnetic properties are characteristic of spin glasses at temperatures T < T _S and magnetic fields H < 11 kOe. In stronger fields, the spin glass state transforms into a phase with a spontaneous magnetization 4–5 times weaker than that to be expected under ferromagnetic ordering of all Mn ions. This is obviously a singly-connected ferromagnetic phase containing regions with frustrated bonds. The frustrated regions and the spin glass phase have inclusions of noninteracting ferromagnetic clusters, because these regions and the spin glass phase at low temperatures exhibit a strong increase in the magnetization M, with the dependence M(T) being described by the Langevin function. Measurements of the electrical resistivity ρ and the Hall effect have revealed that, for T < 30 K, the resistivity ρ of compositions with 1.5 and 3.5 wt % Mn is higher that at 30 K, which makes superexchange dominant and gives rise to the onset of the spin glass state. The nonuniform distribution of Mn ions in the spin glass phase accounts for the existence of isolated ferromagnetic clusters, their ferromagnetism being generated by carrier-mediated exchange. As the temperature increases still more, the increase in the mobility occurs faster than the decrease in the concentration, thus promoting an enhancement of the carrier-mediated exchange and growth of the ferromagnetic clusters in size, which at T = T _S come in contact. This signifies a transition from a multiply-to a singly-connected ferromagnetic phase, which contains microregions with frustrated bonds.     

自旋玻璃的重入行为及CoxZn1-x(FeyCr1-y)2O4尖晶石系统的磁相图

本文研究了Co_xZn_1-x(Fe_yCr_1-x2O₄尖晶石系统的磁性,测量了不同成份样品的低频弱场交流磁化率与低场直流磁化强度的温度关系。根据实验结果,给出了该系统可能的磁相图。发现在该系统中相当宽的成份范围内,都存在自旋玻璃的重入现象。同时发现,自旋玻璃的重入温度随磁性离子浓度增加而增加。这些行为是磁性离子浓度含量较高和多种磁性离子共存系统的共同特性。还讨论了自旋玻璃重入行为 关键词：     

Saturation and forced volume magnetostriction of Fe-rich FeZr amorphous alloys

AC susceptibility, saturation and forced volume magnetostriction were studied on iron-rich zirconium amorphous alloys a-Fe_100-xZr_x (8 at% ≤ x ≤ 12 at%). The experimental results are discussed in relation to the re-entrant spin glass state. It is found that the transverse magnetostriction accompanies a remarkable magnetic aftereffect in the spin glass phase, and that the forced volume magnetostriction shows an apparent peak at the ferromagnet to spin glass transition where the ac susceptibility has a cusp.     

Synthesis,structure and magnetic properties of layered manganate Sr4Mn3−xCrxO10 (x=0, 0.2)

The manganates Sr₄Mn_3−xCr_xO₁₀ (x=0 and 0.2) have been synthesized by solid state reaction. Powder X-ray diffraction analysis shows orthorhombic symmetry with space group Cmca for both compounds. The magnetic susceptibility measurements show an antiferromagnetic transition at 192 and 176 K for x=0 and 0.2, respectively. The magnetic susceptibility data were estimated using a model based on spin exchange antiferromagnetic interactions in isolated (Mn⁴⁺) trimer; a paramagnetic contribution due to the chromium ions was added in the case of Cr-doped materials.     

Spin glass phase transition in Hg1−kMnkTe semimagnetic semiconductors

Low field interband Faraday rotation and a.c. magnetic susceptibility are investigated in Hg_1?kMn_kTe and Hg_1?k?xMn_kTe semimagnetic semiconductors (k?0.5). The spin glass phase transition is experimentally manifested by characteristic cusps in χ(T) and kinks in θ(T). The transition temperatures obtained from both types of experiments are well correlated which enable us to determine the phase diagram for Hg_1?kMn_kTe system. The spin glass phase transition occurs at low temperature in quaternary alloys than in Hg_1?kMn_kTe alloys with identical Mn composition. This implies that the antiferromagnetic interaction between Mn²⁺ ions due to the virtual valence to conduction band transitions plays a significant role in the mechanism responsible for the spin glass formation.     

Noncollinear cluster ferromagnetism in lanthanum manganite perovskites with an excess of manganese

A theoretical substantiation is provided for realization of the spin structure in a weakly doped subsystem based on LaMnO₃, which is observed in compounds with an excess of manganese, (La_1?y Ca_y)_{1? x} Mn_1+x O₃, where y=0 and 0.3, x=0–0.4. It is proved experimentally that samples with x>0.1 exhibit an anomalous behavior of magnetization at T<45 K. The magnetization decreases in fields H<100 Oe and increases for H>200 Oe. It is assumed that this is associated with the emergence of a canted phase in clusters of manganese ions with difference valences. A theoretical analysis indicates that a competition between double and superexchange interaction is in principle possible in such clusters, since the charge carrier concentration in them is considerably lower than in the host matrix. It is shown that the inclusion of quantum properties of spin leads to a considerable modification of the thermodynamic behavior of a magnet with collectivized electrons. The results of analysis are compared with analogous results in the de Gennes classical theory. The possibility of formation of a state with a canted magnetic sublattice in the weakly doped subsystem in the low-temperature region is substantiated on the basis of calculations. An analysis of the thermodynamic behavior of the weakly doped subsystem based of LaMnO₃ taking into account quantum-mechanical properties of spin shows that relaxation phenomena determined, to a considerable extent, by the relation of parameters of intra-and interplanar indirect exchange, as well as the electron transport energy, can take place in the region of phase transition to the canted state. The microscopic parameters of interactions are estimated quantitatively. The results of calculations are in qualitative agreement with experimental data.     

Microphase separation and magnetic Jahn-Teller polarons in LaSrAl1−xCuxO4−δ

An EPR study of the solid solutions LaSrAl_1−x Cu_xO₄, which are isostructural to La₂CuO₄, shows that microphase separation of the structure occurs already at small copper concentrations (x≈0.01). A phase enriched with Cu appears along with a phase of La₂AlO₄, which contains isolated CuO₆ centers. It is established that the nature of the states and the deformations of the CuO₆ centers is determined by internal Jahn-Teller factors. When x⩽0.1, EPR signals are detected for new dynamic centers, which are identified as CuO₆ Jahn-Teller centers with a hole delocalized among the four in-plane oxygen ions (the total spin S=1). When 0.1⩽x<1, local CuO ₆ ⁻ hole centers transform into magnetic Jahn-Teller polarons, which include five or more CuO₆ fragments, in the copper phase of the structure. Their transformations and the conditions for observing them are discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 622–628 (April 1998)     

Phase transitions in crystals with competitive Jahn-Teller subsystems

A statistical thermodynamic model of phase transformations caused by the cooperative Jahn-Teller effect in spinels containing two types of Jahn-Teller cations has been proposed. It has been shown that the specific features of the phase diagrams of compounds similar to Cu_{1 ? x} Ni _x Cr₂O₄, such as the presence of one orthorhombic and two anti-isostructural tetragonal phases, are associated with the competition between orderings of the subsystems of tetrahedra distorted due to the Jahn-Teller effect. The degree and character of the ordering of each subsystem of the solid solution have been determined. The main factors responsible for the thermodynamic properties and phase diagrams, in particular, the conditions for splitting of an isolated critical point into two triple points, have been investigated.     

Optical and magnetic properties of laser-deposited Co-doped ZnO thin films

We report the optical and magnetic properties of laser-deposited Zn_1−xCo_xO (x=0.06-0.3) thin films with no intentional electrical carrier doping. The analysis of the high-temperature magnetization data provides an unambiguous evidence that antiferromagnetic superexchange interaction is the dominant mechanism of the exchange coupling between Co ions in Zn_1−xCo_xO alloy, yielding the value of the effective exchange integral J₁/k_B to be about −27 K. The low-temperature magnetization data reveals a spin glass transition in Zn_1−xCo_xO alloy for the Co content x>0.15, giving the value of the spin freezing temperature T_f to be ∼8 and ∼12 K for x=0.2 and 0.25, respectively. Optical spectra analysis shows a linear increase of the band gap E_g with the increase of the Co content following E_g=3.231+1.144x eV.     

Itinerant electron theory of transition metal compounds of mixed valency: La1−xSrxMnO3

We use a simplified model to calculate the electronic band structure of compounds of the type of La_1?xSr_xMnO₃. The model includes two nonequivalent sites for the transition metal ions as well as strong Coulomb and exchange interactions between d electrons. Using the resulting band structure we discuss the correlation between magnetic order and conductivity in these compounds.     

Magnetic phase diagram of diluted spinel Zn1−xCuxCr2Se4 system

By using mean field theory, we have evaluated the nearest-neighbour and the next-neighbour super-exchange J₁(x) and J₂(x), respectively, for Zn_1−xCu_xCr₂Se₄ in the range 0?x?1. The intraplanar and the interplanar interactions are deduced. High-temperature series expansions are derived for the magnetic susceptibility and two-spin correlation functions for a Heisenberg ferromagnetic model on the B-spinel lattice. The calculations are developed in the framework of the random phase approximation. The magnetic phase diagram is deduced. A spin glass phase is predicted for intermediate range of concentration. The results are comparable with those obtained by magnetic measurements. The critical exponents associated with the magnetic susceptibility (γ) and the correlation lengths (ν) have been deduced. The values are comparable to those of the 3D Heisenberg model, and are insensitive to the dilution x.