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We present the magnetic properties of (Y 1− x Nd x ) 2 BaNiO 5 ( x =1, 0.15) investigated by means of specific heat, magnetic susceptibility, and spectroscopic measurements. Magnetic ordering occurs at 47 K and 13 K in the compounds with x =1 and x =0.15, respectively. We estimate the magnetic contribution of the neodymium subsystem to magnetization and specific heat using temperature dependences of the splitting of ground Kramers doublet of Nd 3+ ion obtained from spectroscopic experiment. We show that both Nd–Ni and Nd–Nd interactions should be taken into account. An origin of the observed spin-glass state below ∼5 K and a contribution of the Ni chain breaks to the magnetization and specific heat are discussed.

Magnetization curves and temperature dependences of initial magnetic susceptibility of single-crystalline TbFe 3 (BO 3 ) 4 are measured both along and perpendicular to the trigonal c -axis from 2 to 350 K. For T T N =40 K, the spin-flop transition with small hysteresis and temperature-dependent magnetization jump is observed for H || c . The H – T phase diagram is constructed. The highly anisotropic character of magnetic susceptibility is ascribed mainly to the Ising behavior of the Tb 3+ ion in the trigonal crystal field. The specific heat, measured up to 3 T at H || c , is characterized by Schottky anomalies around 20 and 150 K associated with the change of occupation of excited energy levels of the Tb 3+ ion being split by f–d coupling and crystal field. All the properties and effects observed are described in the context of the unified approach, which is based on the mean-field approximation and crystal-field calculations. The main parameters of the compound are determined.

A new type of composite structure is described consisting of the todorokite-type [Mn 6 O 12 ] framework with large square tunnels accommodating columnar fragments of the rock-salt structure. The crystal structure of the new todorokite-type [SrF 0.8 (OH) 0.2 ] 2.526 [Mn 6 O 12 ] compound is solved from transmission electron microscopy, and the structure of its anhydrous form [SrF 0.8 O 0.1 ] 2.526 [Mn 6 O 12 ] is refined from X-ray powder diffraction data. The [Mn 6 O 12 ] framework consists of mutually perpendicular walls built of three edge-sharing rutile-type strings of MnO 6 octahedra delimiting large square tunnels with the size of 3 x 3 octahedra. The interior space in the tunnels is filled with rock-salt type [Sr(F,OH)] 4 columns. The structure can be interpreted as being an incommensurate composite structure with the modulation vector q 1 = γc 1 * (y = 0.63157(3)) parallel to the direction of tunnel propagation. The octahedral tunnel walls compose subsystem I with a [Mn 6 Ο 12 ] composition and a periodicity c 1 = 2.84 A, whereas the [Sr(F,OH)] 4 columns belong to subsystem II with a periodicity c 1 /γ = 4.49 A, resulting in a [Sr(F,-OH)] 4γ [Mn 6 O 12 ] composition. [SrF 0.8 (OH) 0.2 ] 2.526 [Mn 6 O 12 ] demonstrates a much larger number of cations inside the tunnels in comparison with the known synthetic and natural marine todorokites. The [SrF 0.8 -(OH) 0.2 ] 2.526 [Mn 6 O 12 ] compound shows a spin-glass behavior below T* ≈ 26 K with a dominant antiferromagnetic correlation.

The reaction of the intermetallic Gd 3 M (M = Co or Ni) compounds with hydrogen was studied. The transformation of the crystal structure of the parent Gd 3 M sample after subsequent stages of the hydrogenation was investigated by X-ray diffraction. It was found that both intermetallic compounds (Gd 3 Ni and Gd 3 Co) are thermodynamically unstable during hydrogenation decomposing into the gadolinium hydrides and the transition metal. It is very probable that, in the case of Gd 3 Ni, nickel plays the role of a catalyst in the formation of GdH 3 in the low temperature range, whereas in the case of Gd 3 Co the transition metal (Co) is rather inactive.

The ferrimagnetic compounds Ca(CuxMn3−x )Mn4O12 of the double distorted perovskites AC3B4O12 family exhibit a rapid increase of the ferromagnetic component in magnetization at partial substitution of square coordinated (Mn3+)C for (Cu2+)C. In the transport properties, this is seen as a change of the semiconducting type of resistivity for the metallic one. The evolution of magnetic properties of Ca(CuxMn3−x )Mn4O12 is driven by strong antiferromagnetic exchange interaction of (Cu2+)C with (Mn3+/Mn4+)B coordinated octahedra. The competing interactions of (Mn3+)C with (Mn3+/Mn4+)B lead to the formation of noncollinear magnetic structures that can be aligned by magnetic fields.

Polycrystalline samples of YxHo1−xNi2 solid solutions with 1≥x≥0 have been studied using X-ray diffraction analysis and magnetic, electrical resistivity, and heat capacity measurements. The X-ray diffraction performed at room temperature allowed us to reveal that the YxHo1−xNi2 solid solutions solidify with the formation of a Laves-phase superstructure corresponding to the space group F 4 3m ; no Laves-phase structure is formed. At low temperatures, the YxHo1−xNi2 solid solutions with 0.6≥x≥0 are ferromagnets. Their Curie temperatures decrease linearly from 17.6 K for HoNi2 to 4.8 K for Y0.6Ho0.4Ni2. The compounds with x≥0.8 and YNi2 do not show any long-range magnetic order. At high temperatures, all solid solutions are Curie–Weiss paramagnets. The temperature dependencies of the electrical resistivity were found to be typical of alloys exhibiting metallic conductivity. The Debye temperatures, phonon and conduction electron contributions as well as the magnetic part of heat capacity were determined from the heat capacity measurements. The magnetocaloric effect was estimated from measurements performed in a low magnetic field of 0.42 T.

The results of magnetization measurements of a Dy3Ni single crystal are presented. A sharp jump of magnetization is observed along a - and c -axis, whereas the magnetization along the b -axis increases gradually. Lack of saturation, strong anisotropy and relatively low values of the magnetic moments observed in magnetic fields up to 35 T are caused by non-collinear magnetic structure that remains even in strong magnetic fields. A magnetic phase diagram with the magnetic field applied along the main crystallographic axes is presented.

The results of measurements and analysis of initial magnetic susceptibility and specific heat of intermetallic R 3 Ni (R=heavy rare earth or Y) are presented. The comparative analysis allowed us to determine origin of magnetic anomalies as well as to define their temperatures more precisely. It is shown that strong short-range magnetic ordering remains above T N . The effect of spin freezing have been found in almost all investigated R 3 Ni. The specific splitting of the ground level multiplet leads to rather unusual magnetic entropy behavior in some of R 3 Ni compounds.

We report measurements and analysis of the initial magnetic susceptibility and specific heat in polycrystalline Dy3Ni and of magnetization processes in Dy3Ni single crystals. The measured magnetic ordering temperature is TN = 38 K and the spin-reorientation-transition and spin-freezing temperatures are Tt ~ 23–26 K and Tsf = 53 K, respectively. The field-induced magnetic transitions occur at 5.9, 8.3 and 6.4 T along the a, b and c axes, respectively, and strong magnetic anisotropy persists up to 13 T. The energy scales of the exchange interaction and field-induced magnetic structural deformation are comparable.

Specific heat measurements were performed in wide temperature range T = 2-300 K for the R 3 Co (R = Er-Gd, Y) compounds. Using values of the Sommerfeld coefficient and Debye temperature estimated for Y 3 Co compound, the Debye temperatures, phonon and conduction electron contributions as well as magnetic contribution to the specific heat were estimated for all other R 3 Co compounds.

The investigations and comparative analysis of the magnetization processes for Gd3Ni and Gd3Co single-crystal compounds are presented. The possible magnetic structures for both compounds are proposed and the features of the field-induced metamagnetic transitions are discussed on the basis of the possible magnetic structures. The anomalous behavior of the Gd3Ni compound observed above the magnetic ordering temperature is very likely caused by the appearance of the nickel atoms magnetic moment and field-induced magnetic ordering.

The investigations of magnetization processes for Er 3 Ni and Er 3 Co single crystal compounds are presented. The magnetization processes are discussed on the basis of magnetic structures determined previously by neutron diffraction for both compounds. It is shown that both ferromagnetic and antiferromagnetic exchange interactions as well as strong local ionic anisotropy of erbium atoms play a significant role in the magnetization processes forming.

Magnetization and dc-susceptibility measurements of a Gd3Ni single crystal and polycrystalline material were performed. The Gd3Ni compound crystallizes in the orthorhombic Fe3C-type structure. Below the ordering temperature, TN=100 K, the field dependence of magnetization, measured along three basic symmetry axes, exhibit one or two field induced transitions. This behavior is discussed in terms of a model that includes the influence of the magnetic field on the exchange and anisotropy energies. We also discussed the anomalous behavior of the magnetization in the paramagnetic temperature range appearing as an inflection of the field dependence of the magnetization.

The magnetization measurements of the Gd3Ni single crystal are presented. Below the ordering temperature, TN=100 K, the field dependence of the magnetization, measured along three basic symmetry axes, exhibit one or two magnetic transitions associated with ferromagnetic and antiferromagnetic components of the complex magnetic structure. The anomalous behavior in paramagnetic range of temperatures is, probably, related with field induced magnetic ordering and appearance of magnetic moments of nickel atoms.

The magnetic properties and 57Fe Mossbauer spectra of the compounds YFe11TiHx (x = 0, 1) were investigated. The magnetocrystalline anisotropy of YFe11Ti and its hydride was studied by analysing the hard and easy magnetization curves of single crystal samples in the temperature range 4.2–300 K. The spontaneous magnetization was determined in wide temperature range 4.2–650 K. It is established that at T = 4.2 K magnetic anisotropy constants K1 and K2 for YFe11TiH1 single crystal reach values of 25.8 K f.u.−1 and 0.24 K f.u.−1, respectively. The Mossbauer effect spectra of YFe11Ti and its hydride were analysed in terms of a model which takes into account the local environment of Fe atoms on three crystallographic sites (8f, 8j and 8i) and an influence of the random distribution of titanium on the 8i site. Upon hydrogenation both the hyperfine fields and the isomer shifts increase. These results are discussed in terms of the hydrogen-induced unit cell expansion and the electron charge transfer from the conduction band onto the H atoms.

Electrical resistivity and ac magnetic susceptibility measurements of R 3 Ni and their hydrides R 3 NiH x were carried out. It is shown that the conductivity of R 3 NiH x compounds with a large amount of hydrogen have semiconductor and insulator conductivity type as compared with metallic conduction in the original compounds R 3 Ni. The resistivity of the original compounds is discussed on the basis of a conduction electron scattering model. Strongly modified transport processes of the hydrides are discussed on basis of a ‘hopping’ model.

The effect of hydrogenation on the magnetic properties of rareearth–Fe/Co alloys was investigated by means of measuring the magnetization and hysteresisloops. Hydrogenation leads to an increase of positive exchange interactions in 3D-transition metalmagnetic subsystem and magnetization of the alloys. Hydrogenation effect on coercivity isdetermined by a type of the magnetization processes in a particular alloy. Various mechanisms ofphenomena were observed.

We present results on thermodynamic properties of single crystalline NdFe 3 (BO 3 ) 4 studied by means of the specific heat c p ( T ) and the magnetic susceptibility χ ( T ). At high temperatures, we observe no anomalies in c p ( T ) and χ ( T ) obeys a Curie–Weiss law. The onset of long-range antiferromagnetic order at 29 K is accompanied by anomalies in both magnetization and specific heat. A Schottky anomaly is visible in c p at ∼4 K. At ∼6 K a low-temperature anomaly is also found in χ ( T ) if a magnetic field is applied perpendicular to the c -axis. Both phenomena are related to the depopulation of the upper component of the ground Kramers doublet of Nd 3+ split in the magnetically ordered state.

Single crystals of Li x Na1 − x Cu2O2 solid solutions have been grown from the melt and crystal lattice parameters have been studied. It is found that the solid solution of this system exists in the region x ≤ 0.25. Specific heat and magnetic susceptibility are investigated on monocrystalline samples with x = 0.07, 0.14, and 0.21. Long-range magnetic order in these compounds is formed at T = 13.8, 4.1, and 14.8 K, respectively. It is found that the Neel temperature T N in these compounds increases with Li content.

Magnetization and high-field electron spin resonance (ESR) response of the s = 1 2 -Heisenberg pseudo-ladder magnet CaCu 2 O 3 is dominated at low temperatures by a few percent of ‘extra’ Cu-spin states arising due to the structural disorder. These ‘extra’ spins interact with the bulk spins and jointly order antiferromagnetically at T N = 26 K . At T T N ESR data reveal a small magnon anisotropy gap of the order of ≲ 1 meV . Substitution of 15% of nonmagnetic Zn for Cu reduces the concentration of ‘extra’ spins and results in the suppression of the magnetic order and closing of the magnon gap. These results support the scenario with a significant role of the ‘extra’ spins for stabilization of the magnetic order in this frustrated quantum magnet.

We report high field specific heat measurements on the structural S = 1 two-leg ladder compound Na 2 Ni 2 ( C 2 O 4 ) 3 ( H 2 O ) 2 at zero magnetic field and at B = 9 T . The magnetic specific heat is well described using an isolated dimer model with J = 43 K on the rungs on the spin ladder consistent with previous studies of the magnetic properties [C. Mennerich et al., Phys. Rev. B 73 (2006) 174415].

Magnetic and transport properties of double distorted perovskites CaCuMn6O12 and CaCu2Mn5O12 are studied in a range 2–300 K. The leading role in magnetism of these compounds belongs to antiferromagnetic exchange interaction of Cu2+ in square coordination with Mn3+/Mn4+ in octahedral coordination. The values of saturation magnetization indicate that Mn3+ ions in square coordination are coupled ferromagnetically with Mn3+/Mn4+ in octahedral coordination. The colossal magnetoresistance in the pellet samples is due assumingly to intergranular spin-polarized tunneling of current carriers.

Macroscopic fractal aggregates of cobalt are obtained by thermal evaporation of cobalt metal in an argon atmosphere and subsequent deposition on a silicon substrate heated to 1000 K. It is established that the fractal structure is formed by diffusion-limited aggregation of cobalt particles. The macroscopic fractal cobalt aggregates are ferromagnetic.

New data on the specific heat, thermal expansion, and magnetization of the CaMn7O12 phase require a revision of the current concepts of the sequence of phase transitions in this compound. It is found that a spin-glass phase transition occurs in CaMn7O12 at T M = 49 K, whereas the transition at T S = 89 K exhibits the features of a first-order phase transition and thereby is apparently of structural origin. In the range T M < T< T S , the CaMn7O12 compound exhibits negative thermal expansion, which is also indicative of structural changes.

Specific heat measurements have been performed on the isostructural R 3 M (R=Gd, Y and M=Co, Ni) compounds. The Debye temperatures, phonon and conduction electronic contributions as well as magnetic part of the specific heat are determined and discussed for Gd 3 Co and Gd 3 Ni compounds. Magnetocaloric effect was estimated for the Gd 3 Co compound in magnetic field of 4 T.

The magnetization processes of the Ho 3 Co single crystal along three basic symmetry axes were investigated and possible magnetic structure is suggested. Specific heat measurements of the compound have been performed at temperature range of 2–300 K. The phonon conduction electron and magnetic contributions to the total specific heat are determined and discussed. Two peaks observed on the magnetic part of the specific heat correspond to magnetic ordering transition ( T N =20 K) and Schottky anomaly ( T Sch ∼30 K).

We present the thermodynamic properties and the magnetic phase diagram of Gd${}_{2}$BaNiO${}_{5}$ investigated by means of specific heat and magnetization measurements performed along the main crystallographic axes of the single crystal. Our data imply antiferromagnetic ordering at ${T}_{\mathrm{N}}$ = 55 K and a spontaneous spin-reorientation transition at ${T}_{\mathit{SR}}$ = 24 K. Upon application of external magnetic fields, a spin-flop transition is observed at $T$ ${T}_{\mathit{SR}}$ in magnetic fields $B$ \ensuremath{\parallel} $b$ axis and at ${T}_{\mathit{SR}}$ $T$ ${T}_{\mathrm{N}}$ in magnetic fields $B$ \ensuremath{\parallel} $a$ axis. The magnetic phase diagram in Gd${}_{2}$BaNiO${}_{5}$ is established from these data. Considering Gd-Gd and Gd-Ni interactions allows estimating the magnetic contribution of the gadolinium subsystem to the magnetization and the specific heat. The Gd-Gd interaction is organized via Ni chains being therefore sensitive to the angle between Gd and Ni magnetic moments. The analysis of the experimental data shows that the behavior of the nickel subsystem differs from that of a classical antiferromagnet.

The present paper reports on the specific heat and magnetization of the YFe(3)(BO(3))(4) and ErFe(3)(BO(3))(4) single crystals. In both compounds, antiferromagnetic order of the iron spins evolves at T(N) = 38 K. The experimental data suggest that the magnetic moments are in the basal plane of the trigonal crystal for both compounds. In the magnetically ordered state the crystal is subdivided into three types of domains, the magnetic moments of the Fe(3+) ions being aligned along the a axis within each domain. For ErFe(3)(BO(3))(4), two non-equivalent magnetic positions of the Er(3+) ions in each domain are observed.

Single crystals of TbFe11Ti and its hydride with tetragonal ThMn12-type structure were obtained. Magnetostriction measurements have been made on TbFe11TiHX (x=0, 1) single crystals by the applied strain gauge method in the temperature range from 77 to 400 K and in magnetic fields up to 13 kOe. Magnetization measurement along the main crystallographic directions of the tetragonal structure have been performed on RFe11TiHx (x=0, 1) single crystals in applied high magnetic fields up to 120 kOe in the temperature range from 4.2 to 300 K. It was concluded that hydrogenation results in an increase in rare-earth anisotropy that leads to the disappearance of the spin-reorientation transition in the TbFe11TiH single crystal. Hydrogenation lead to a change in sign of the magnetic anisotropy constant K3 and the magnetostriction constant λ2α,2.

The pressure effect on magnetic properties of amorphous rare-earth basedalloys and their hydrides was investigated for the temperatures close to phase transition to spinglass like state point. It was established that the pressure decreases the temperature of transition,otherwise hydriding makes the temperature higher. Experimental results are explained by meansof strong dependence of exchange integrals on interatomic distances.

We present new x-ray and neutron-scattering measurements of stripe order in ${\text{La}}_{1.875}{\text{Ba}}_{0.125}{\text{CuO}}_{4}$, along with low-field susceptibility, thermal conductivity, and specific-heat data. We compare these with previously reported results for resistivity and thermopower. Temperature-dependent features indicating transitions (or crossovers) are correlated among the various experimental quantities. Taking into account recent spectroscopic studies, we argue that the most likely interpretation of the complete collection of results is that an unusual form of two-dimensional superconducting correlations appears together with the onset of spin-stripe order. Recent theoretical proposals for a sinusoidally modulated superconducting state compatible with stripe order provide an intriguing explanation of our results and motivate further experimental tests. We also discuss evidence for one-dimensional pairing correlations that appear together with the charge order. With regard to the overall phenomenology, we consider the degree to which similar behavior may have been observed in other cuprates and describe possible connections to various puzzling phenomena in cuprate superconductors.

The static and dynamic magnetic properties, electrical resistivity, specific heat, and magnetoresistance have been studied in ${\text{EuCu}}_{2}{\text{Si}}_{2}$ single crystals grown by a floating zone method. The magnetic susceptibility exhibits a considerable anisotropy and a steep rise below 10 K for external fields parallel to the $c$ axis but with no evident magnetic ordering in the temperature range of 2--350 K. The data imply a gradual change in the Eu valence as a function of temperature. Electron spin resonance (ESR) measurements reveal a sizeable fraction of stable ${\text{Eu}}^{2+}$ magnetic moments that interact with conduction electrons and develop quasistatic antiferromagnetic correlations on the ESR timescale. The electrical resistivity and specific heat demonstrate the presence of spin fluctuations and Kondo-like behavior, which apparently competes with the antiferromagnetic order. The analysis of experimental data enables to conclude that the remarkable diversity of the physical properties of ${\text{EuCu}}_{2}{\text{Si}}_{2}$ results from the variation of lattice parameters as well as of local crystal chemistry as a consequence of the particular preparation route employed for the growth of single crystals and polycrystals.

We present magnetization and specific heat data as well as optical spectra of $\mathrm{Nd}{\mathrm{Cr}}_{3}{(\mathrm{B}{\mathrm{O}}_{3})}_{4}$ single crystals. At ${T}_{N}=8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the magnetic and specific heat data imply long-range antiferromagnetic spin order of the ${\mathrm{Cr}}^{3+}$ subsystem. This observation is corroborated by the spectroscopic studies since in the ordered state the ${\mathrm{Nd}}^{3+}$ Kramers doublets are split by the internal staggered magnetic field. The splitting is also visible in a Schottky anomaly at the specific heat. In the spin-ordered phase, we find a staggered magnetic field at the Nd sites of $35\phantom{\rule{0.3em}{0ex}}\mathrm{kG}$ at $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The values ${\ensuremath{\lambda}}_{\mathrm{Nd}\text{\ensuremath{-}}\mathrm{Cr}}=3.9\phantom{\rule{0.3em}{0ex}}\mathrm{kG}∕{\ensuremath{\mu}}_{B}$ and ${\ensuremath{\lambda}}_{\mathrm{Cr}\text{\ensuremath{-}}\mathrm{Cr}}=29.4\phantom{\rule{0.3em}{0ex}}\mathrm{kG}∕{\ensuremath{\mu}}_{B}$ are obtained for the molecular-field constants of the Nd-Cr and the Cr-Cr interactions, respectively. The analysis of the magnetic susceptibility data points to the easy-plane type of the antiferromagnetic order. Several experimental manifestations of the low dimensionality of the title compound are mentioned.

The sequence of phase transitions and the symmetry of, in particular, the low temperature incommensurate and spin-Peierls phases of the quasi-one-dimensional inorganic spin-Peierls system TiOX (X=Br and Cl) have been studied using inelastic light scattering experiments. The anomalous first-order character of the transition to the spin-Peierls phase is found to be a consequence of the different symmetries of the incommensurate and spin-Peierls (P2(1)/m) phases. The pressure dependence of the lowest transition temperature strongly suggests that magnetic interchain interactions play an important role in the formation of the spin-Peierls and the incommensurate phases. Finally, a comparison of Raman data on VOCl to the TiOX spectra shows that the high energy scattering previously observed has a phononic origin.

We have studied the thermodynamic properties of single-crystalline $\mathrm{Tb}{\mathrm{Fe}}_{3}{(\mathrm{B}{\mathrm{O}}_{3})}_{4}$. Magnetization measurements have been carried out as a function of magnetic field (up to $50\phantom{\rule{0.3em}{0ex}}\mathrm{T}$) and temperature up to $350\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ with the magnetic field both parallel and perpendicular to the trigonal $c$ axis of the crystal. The specific heat has been measured in the temperature range $2--300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ with a magnetic field up to $9\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ applied parallel to the $c$ axis. The data indicate a structural phase transition at $192\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and antiferromagnetic spin ordering at ${T}_{N}\ensuremath{\approx}40\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. A Schottky anomaly is present in the specific-heat data around $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, arising due to two low-lying energy levels of the ${\mathrm{Tb}}^{3+}$ ions being split by $f\text{\ensuremath{-}}d$ coupling. Below ${T}_{N}$, magnetic fields parallel to the $c$ axis drive a spin-flop phase transition, which is associated with a large magnetization jump. The highly anisotropic character of the magnetic susceptibility is ascribed mainly to the Ising-like behavior of the ${\mathrm{Tb}}^{3+}$ ions in the trigonal crystal field. We describe our results in the framework of a unified approach which is based on mean-field approximation and crystal-field calculations.

From thermodynamics, local spin density approximation+Hubbard U studies and exact diagonalizations of a five-band Hubbard model on CuO2 stripes we find that Li2ZrCuO4 (Li2CuZrO4 in traditional notation) is close to a ferromagnetic critical point. Analyzing its susceptibility chi(T) and specific heat cp(T,H) within a Heisenberg model, we show that the ratio of the 2nd to the 1st neighbor exchange integrals alpha=-J2/J1 approximately 0.3 is close to the critical value alphac=1/4. Comparing with related chain cuprates we explain the rather strong field dependence of cp, the monotonic downshift of the peak of chi(T), and its increase for alpha-->alphac+0.

A new brownmillerite-type compound Sr2Al1.07Mn0.93O5 was synthesized. The crystal structure was determined using electron diffraction and high resolution transmission electron microscopy and refined from X-ray powder diffraction data (space group Imma, a = 5.4358(1) A, b = 15.6230(4) A, c = 5.6075(1) A, RI = 0.036, RP = 0.023). The structure is characterized by a disordered distribution of the tetrahedral chains in L and R configuration and a partial occupation of the octahedral position by the Mn3+ and Al3+ cations. The relationships between the crystal structures of Sr2Al1.07Mn0.93O5 and its A2B′MnO5 analogues (A = Ca, Sr, B′ = Al, Ga) and the structural reasons for the different types of tetrahedral chain ordering in brownmillerites are discussed. The temperature dependences of the magnetic susceptibility and specific heat reveal that the compound is antiferromagnetically ordered below TN = 105 K.

We report on x-ray diffraction, magnetic susceptibility, electron- spin resonance and heat- capacity studies of Co[Al_1-xCo_x]_2O_4 for Co concentrations 0 10 indicates the presence of strong spin frustration at least for x < 0.6. The frustration mechanism is attributed to competing nearest neighbor and next-nearest neighbor superexchange interactions between the A-site Co^2+ ions., Comment: 19 pages, 9 figures, 46 references

The present understanding of a novel growing class of chain cuprates with intriguing magnetic properties is reviewed. Among them, several undoped edge-shared CuO 2 chain compounds show at low temperature a clear tendency to helicoidal magnetical ordering with acute pitch angles and sometimes also to weak ferromagnetism. Our analysis is based on the isotropic 1D frustrated J 1 – J 2 Heisenberg model with ferromagnetic (FM) 1st neighbor and antiferromagnetic 2nd neighbor exchange. The achieved assignment is supported by microscopic calculations of the electronic and magnetic structure. We consider Na ( Li ) Cu 2 O 2 , LiVCuO 4 as the best studied helimagnets, Li 2 ZrCuO 4 and other systems close to a FM quantum critical point, as well as Li 2 CuO 2 with FM inchain ordering. The interplay of frustrated inchain couplings, anisotropy and interchain exchange is discussed.

We report on the pressure effects on the orbital polaron lattice in the lightly doped manganites $\mathrm{La_{1-x}Sr_xMnO_{3}}$, with $x\sim 1/8$. The dependence of the orbital polaron lattice on $negative$ chemical pressure is studied by substituting Pr for La in $\mathrm{(La_{1-y}Pr_y)_{7/8}Sr_{1/8}MnO_{3}}$. In addition, we have studied its hydrostatic pressure dependence in $\mathrm{(La_{0.9}Pr_{0.1})_{7/8}Sr_{1/8}MnO_{3}}$. Our results strongly indicate that the hopping $t$ significantly contributes to the stabilization of the orbital polaron lattice and that the orbital polarons are ferromagnetic objects which get stabilized by local double exchange processes. The analysis of short range orbital correlations and the verification of the Grueneisen scaling by hard x-ray, specific heat and thermal expansion data reinforces our conclusions.

We report on magnetization measurements of ${\mathrm{Sr}}_{14\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$, with $0\ensuremath{\leqslant}x\ensuremath{\leqslant}12$, in magnetic fields up to $16\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. The low-temperature magnetic response of the $\mathrm{Cu}{\mathrm{O}}_{2}$ spin chains changes strongly upon doping. For $x=0$, the ground state with nearly independent dimers is confirmed. A reduction of the number of holes in the chains through Ca doping leads to an additional contribution to the magnetization, which depends linearly on the magnetic field. Remarkably, the slope of this linear contribution increases with the Ca content. We argue that antiferromagnetic spin chains do not account for this behavior but that the hole dynamics might be involved.

The effect of hydrogenizing on the magnetic properties of rare earth-cobalt amorphous alloys was investigated by means of measuring the magnetization and magnetic susceptibility. The hydrogenizing leads to a Curie temperature increase and hysteresis loop changes. Various mechanisms of these phenomena were supposed.

Homometallic ferrimagnetism is quite difficult to find in Nature, and this fact makes Na2Cu5Si4O14 an interesting material due to its ferrimagnetic arrangement of Cu ions in a zigzag chain with dimers and trimers. In view of this, we developed a theoretical model for the magnetic specific heat of a homometallic ferrimagnet and then compared it with experimental data for Na2Cu5Si4O14 compound. The successful comparison validates the model and further analysis makes it possible to predict anomalous behavior of this low dimensional magnetic system, due to the crossing of the energy levels.

A novel subclass of frustrated undoped edge-shared CuO2 chain materials with intriguing magnetism is briefly reviewed. These cuprates show at low temperature a tendency to helicoidal magnetic ordering with acute pitches and in some cases also to weak ferromagnetism. In our analysis we focus on our recent theoretical and experimental studies on Na(Li)Cu2O2 and related systems. Differences and similarities in the magnetic ground states of these structurally similar localized spin-1/2 compounds are considered. The nontrivial interplay of frustrated single-chain couplings, anisotropy and interchain exchange is stressed.