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X-band ESR technique has been successfully applied to study S = 1/2 spin chain compound vanadyl diacetate VO(CH3COO)2. Parameter of the intrachain interactions for uniform Heisenberg antiferromagnetic chain amounts to J/kB = −167 К, while the ratio between the interchain (J′

The kagome lattice of copper ions in francisite-type compounds is quite sensitive to external stimuli, being easily affected by temperature and magnetic field. The rare-earth ions inserted between buckled layers of transition metal add new dimension to magnetism in these systems. Neodymium francisites, Cu3Nd(SeO3)2O2X (X = Cl, Br), experience long range magnetic order at TN = 34 K (Cl) or 35.2 K (Br) and spin reorientation transition at TR = 4 K (Cl) or 4.6 K (Br) marked by sharp anomalies in magnetization M, specific heat Cp, and optical spectra. The spin reorientation of Cu2+ ions is due to d-f interaction of transition and rare-earth magnetic subsystems. Under action of modest external field B

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 studies were carried out for the novel FeSe 1− x Te x superconductors ( 0 ≤ x ≤ 1 ) to investigate a behavior of the intrinsic magnetic susceptibility χ in the normal state. The magnetic susceptibility was found to increase gradually with Te content. The temperature dependencies of the magnetic susceptibility χ and its anisotropy Δ χ = χ ∥ − χ ⊥ were measured for FeSe in the temperature range 4.2–300 K, and a growth of susceptibility with temperature was revealed. For FeTe a substantial increase of χ under pressure was found. Ab initio calculations of the band structure and magnetic susceptibility have shown, that FeSe 1− x Te x systems are close to magnetic instability with dominating enhanced spin paramagnetism. The calculated paramagnetic susceptibility exhibits a strong dependence on the unit cell volume V and especially the height of chalcogen species from the Fe plane. With appropriate values of these parameters the calculations have reproduced the experimental data on χ ( T ) and χ ( P ) for FeSe and FeTe, respectively.

Nakahara et al. [Mater. Lett. 30 (1997) 163–167] have refined their X-ray diffraction patterns of Ca 2 FeMnO 5 with the brownmillerite structure, in which all the octahedral in-plane sites are occupied by Mn, and all the tetrahedral in-chain sites are occupied by Fe. Our Mossbauer study of Ca 2 FeMnO 5 is in agreement with this cationic distribution. We determine the spin orientation with respect to the crystal axes and suggest the model of layered magnetic structure. From our Mossbauer spectra and from analysis of the crystal and magnetic structures in a number of brownmillerite-related oxides we suggest for Ca 2 FeMnO 5 the mixed-spin S = 5 2 and S = 2 Ising system. The Fe 3 + and Mn 3 + moments order antiferromagnetically (AFM) both in-plane and in perpendicular direction and aligned up and down vertically in the out-of-plane direction. Although we cannot exclude a small tilt and a related small ferromagnetic component, basically, each of Fe and Mn spin subsystems generate individually a C-type sublattice and two these sublattices are superposed to give an unusual G-like type structure with two sorts of spins. Nuclear forward scattering (NFS) and nuclear inelasting scattering (NIS) techniques were used to characterize, respectively, the hyperfine interactions and phonon density of states (DOS) of 57 Fe. Comparing the vibrational properties of several brownmillerites we were able to give an approximate DOS for tetrahedral and octahedral positions.

We present thermal expansion α , magnetostriction and specific heat C measurements of TlCuCl 3 , which shows a quantum phase transition from a spin-gap phase to a Neel-ordered ground state as a function of magnetic field around H C 0 ≃ 4.8 T . Using Ehrenfest's relation, we find huge pressure dependencies of the spin gap for uniaxial as well as for hydrostatic pressure. For T → 0 and H ≃ H C 0 we observe a diverging Gruneisen parameter Γ ( T ) = α / C , in qualitative agreement with theoretical predictions. However, the predicted individual temperature dependencies α ( T ) and C ( T ) are not reproduced by our experimental data.

At isovalent substitution of Se for S, the layered chalcogenides TlCo 2 Se 2− x S x (0⩽ x ⩽2) exhibit a change of magnetic order type, from an antiferromagnetic helix at low S content to a ferromagnet at high S content. The effective magnetic moments p eff ∼2 μ B per Co, as estimated from the magnetic susceptibility at high temperatures, significantly exceed what would be the result of the Co magnetic moments μ sat ∼0.5 μ B in both the ferromagnetic and antiferromagnetic phases. This signifies the crucial role of longitudinal spin fluctuations in the magnetic behaviour of these conducting compounds. The spin fluctuations reveal themselves also in specific heat measurements, leading to a diminution of the expected anomalies at the Curie/Neel temperatures and to an excess of heat capacity at high temperatures as compared to phonon contributions.

The magnetic susceptibility and specific heat measurements were performed on YBaCo 4− x Zn x O 7 in a wide temperature range. This compound is characterized by a mixed valence of Co (Co 2+ /Co 3+ ) and has a kagome-like layered magnetic arrangement of spin moments in the P6 3 mc hexagonal crystal structure. This arrangement is known for strong frustration of magnetic interactions and prevents the formation of long range magnetically ordered ground states. However, peak-like anomalies were found in the magnetic susceptibility at the temperatures 66, 27, 15 and 3 K for x =0, 1, 2, 3, respectively. Small anomalies in the specific heat at the related temperatures were found for x =1, 2, 3, indicating the magnetic freezing of the spin system. Zn-substitution changes the ratio Co 2+ /Co 3+ which strongly influences the magnitudes and temperatures of anomalies in magnetic susceptibility and specific heat. The increase of Zn content in YBaCo 4− x Zn x O 7 leads to a rapid decrease of the magnetic contribution in the specific heat in a wide temperature range.

The YbInCu 4 exhibits a first-order phase transition associated with a transformation of the valence state of the Yb ion. The effect of alloying on the valence transition in Yb 1− x Ce x InCu 4 (0⩽ x ⩽0.2) has been investigated by specific heat C measurements in the range 5–300 K. The C vs. T dependence in YbInCu 4 reveals sharp peak at T V =39 K. This peak shifts to higher temperatures with x and its shape gradually transforms to that of continuous phase transition. The specific heat measurements have also been made for nonmagnetic isomorphous compound YInCu. The comparison of YInCu 4 data with those of Yb 1− x Ce x InCu 4 allows an estimate of the value of entropy Δ S released at the phase transition. The Sommerfeld coefficient values in the Yb 1− x Ce x InCu 4 family of compounds were found increasing with Ce concentration.

TlCuCl 3 shows a quantum phase transition from a spin-gap phase to a Neel-ordered ground state as a function of magnetic field around H C 0 ≃ 4.8 T . From measurements of the specific heat c and the linear thermal expansion α i we calculate the Gruneisen parameter Γ i ( T ) = α i / c . Close to H C 0 we find a diverging Γ i ( T → 0 ) , in qualitative agreement with theoretical predictions. However, the predicted individual temperature dependencies of α ( T ) and c ( T ) are not reproduced by our experimental data.

New mixed valence Co oxides REBaCo 4 O 7 (RE=Lu, Yb, Tm) show structural first-order phase transitions from hexagonal (P6 3 mc) to orthorhombic (Cmc2 1 or lower) system below 170, 180, and 230 K, respectively. The low-temperature phases order magnetically at 47, 75, and 105 K, respectively.

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.

The heat capacity of ferroborates RFe3(BO3)4 (R=Y, Y0.5Gd0.5, Gd, Er, Tb, Tm) were studied in the range 0.4–300 K. In TbFe3(BO3)4 at 192 K and GdFe3(BO3)4 at 156 K first-order structural phase transitions were observed. At 50% substitution of Gd for Y the transition temperature shifts to 234 K. An antiferromagnetic ordering of the Fe subsystem occurs at about 37 K in each member of this series, except TmFe3(BO3)4 ( T N ∼ 2 4 K ). Besides, in GdFe3(BO3)4 at 9 K a spin-reorientation first-order transition takes place. In the temperature range studied the rare-earth subsystem remains paramagnetic being fully polarized by the Fe subsystem. In a staggered field of the iron subsystem a Zeeman splitting of the ground state of rare-earth ions occurs, which leads to a Schottky-type anomaly and release of additional entropy.

The metaloxide pyroxene compound NaVGe 2 O 6 containing isolated edge-sharing VO 6 ( S =1) chains undergoes a transition into a long-range antiferromagnetic state at T N =16 K. The broad maximum in the temperature dependence of magnetic susceptibility at T M =26 K indicates the low-dimensional character of magnetic subsystem. Even though the antiferromagnetic ordering is accompanied by a sharp peak of specific heat, significant magnetic entropy is released above the Neel temperature.

Thermal properties of NaV2O5 reflect the complex character of the phase transition at TC∼34 K which includes the charge ordering, the lattice distortion and the spin gap formation in magnetic excitations spectrum. The specific heat shows sharp λ-type anomaly at TC, the heat conductivity strongly increases below TC, and the thermal expansion exhibits highly anisotropic behavior below TC.

In a temperature range 5–300 K the specific heat C ( T ) on a new mixed valence cobalt oxides REBaCo 4 O 7 (RE=Dy, Ho, Er, Tm, Yb, Lu) was investigated. The first-order structural phase transitions from hexagonal P 6 3 mc to orthorhombic Cmc 2 1 phase was indicated by a peak-like anomaly in C ( T ) curves at T S ∼160, 178, 224, and 280 K for RE=Lu, Yb, Tm, and Er correspondingly. The magnetic phase transitions was indicated as the changes of slope on the C ( T ) curves were found at corresponding temperatures: T N ∼50, 74, 98, and 98 K for RE=Lu, Yb, Tm, and Er, correspondingly.