A systematic study of temperature dependence of magnetic inelastic neutron scattering for itinerant antiferromagnet Mn(12%Ge) is presented. At low temperatures the experimental data are well described in terms of damped spin waves. This description becomes insufficient at least 150 K below T_{N}. In the temperature range 200-400 K the integrated intensity of distribution for energy transfer 13 THz increases by one third and the shape of the deconvoluted neutron scattering cross-section changes. These changes suggest that ca. 200 K below T_{N} an additional contribution to the inelastic neutron scattering starts to appear and that its origin is different from spin waves.
We present the experimental study of magnetocaloric effect on powder sample [Cu(phen)(H_2O)_2]SO_4 (phen = phenantroline = C_{12}H_8N_2) in the temperature range from nominally 0.2 K to 4 K and in magnetic fields up to 2 T. The values of total entropy were extracted from the experimental data obtained by adiabatic demagnetization. The comparison was performed with the temperature dependence of the entropy for S=1/2 paramagnet calculated at selected magnetic fields. The deviations observed below 0.6 T were attributed to internal field mediated by exchange coupling. In addition, the temperature response observed in [Cu(phen)(H_2O)_2]SO_4 revealed very short relaxation time below 1 K. The comparison of the behavior observed at the end of the demagnetization in [Cu(phen)(H_2O)_2]SO_4 and other systems suggests that the magnetocaloric effect can serve as an alternative tool for studying slow magnetic relaxation.
We report on magnetothermodynamic properties of single crystal KEr(MoO_4)_2 which were investigated from nominally 0.4 K to 20 K in magnetic fields up to 4 T. Using the available specific heat data the diagram of temperature and magnetic field dependence of the total entropy was constructed. Predictions for temperature dependences of the relative temperature variations during the adiabatic demagnetization and the entropy variation during the isothermal demagnetization were calculated from the specific heat data. The obtained results suggest that unlike common refrigerants, in KEr(MoO_4)_2 the quantities describing magnetic cooling remain nearly constant in the temperature range 2-10 K.
TmB_4 is a frustrated system based on the Shastry-Sutherland lattice that exhibits complex magnetic properties. In this contribution the magnetic field B vs. temperature T phase diagram of TmB_4 has been studied by ultrasensitive AC calorimetry in the temperature range between 2.9 and 12 K and in magnetic fields up to 8 T. Apart from already known phases our measurements have recognized several new phase transitions suggesting that the phase diagram of TmB_4 is even more complex and deserves further studies.
The magnetic ordering in ErFe_{0.3}Ge_{2} and ErNi_{0.65}Ge_{2} compounds crystallizing in the orthorhombic structure of the CeNiSi_{2}-type has been investigated by neutron diffraction and magnetic measurements. The Er magnetic moments have been found to order antiferromagnetically below 2.5 K in ErFe_{0.3}Ge_{2} and 2.3 K in ErNi_{0.65}Ge_{2}. The magnetic structure of the former compound can be described by the propagation vector k = (0.044(1), 0, 0.384(1)). The Er magnetic moments are aligned along the a-axis and alternate with the sequence ++- in the unit cell. At 1.5 K they are equal to 5.9(1) μ_{B}. In contrast, the magnetic unit cell of ErNi_{0.65}Ge_{2} has been established to be equal to the chemical one. The magnetic moments in this compound are arranged in a collinear manner pointing along the a-axis with the sequence +-+-. The Er moment value measured at 1.5 K is 2.90(8) μ_{B}.
Dysprosium monotelluride is a metallic substance with one valence electron per formula unit belonging to the conduction band. It crystallizes in the defected NaCl-type structure (a= 0.6070 nm) with cation deficiency in the metal sublattice. AC and DC susceptibilities exhibit a maximum at 15 K and a second maximum at higher temperatures which position is field dependent.
The synthesis of polycrystalline Mn_{1-x}Gd_{x}Se solid solutions is carried out by solid state reaction method followed by quenching from the temperature of 1370 K. The X-ray diffraction studies realized at 300 K revealed that the structure of the single phase samples in the 0 < x < 0.15 concentration range is identified on base a face centered cubic crystal cell of Fm3m space group. The heating of the solid solutions to 900 K does not affect on the magnetic susceptibility as the dependences is identical to the measurements in the "heating-cooling" regime. Comparing the research results of magnetic properties of the Mn_{1-x}Gd_{x}Se solid solutions with those of Mn_{1-x}Gd_{x}S solid solutions, we can conclude that substitution of manganese ions by gadolinium in manganese selenide lead to more changes in the basic magnetic characteristics than in manganese sulfide.
The low-temperature magnetic, thermal and magnetocaloric properties of the half-Heusler compound DyNiSb were studied on polycrystalline samples. The temperature variations of the magnetization and the heat capacity revealed a phase transition from paramagnetic to antiferromagnetic state at the Néel temperature T_{N} = 3.1 K. The compound exhibits normal and inverse magnetocaloric effect with the isothermal magnetic entropy change reaching 5.2 J/(kg K) at 4.8 K for a magnetic field change of 3 T. The estimated refrigerant capacity is about 58 J/kg.
A formula for the temperature dependent magnetoelectric susceptibility of Cr_{2}O_{3} is obtained using the method of renormalized spin waves of Nagai and compared with the formula in random phase approximation. The three coefficients a^{g}, a^{LS}, and a^{J} which respectively represent the change in the g-factor, in the crystal field splitting and in the exchange integral due to the electric field parallel to the easy axis, are determined by comparing the theoretical with experimental susceptibility curves, as well as the antiferromagnetic resonance shift.
The phase H-P-β diagram of an easy-plane strongly anisotropic antiferromagnet is studied. It is shown that in such a system a realization of the phase with the nonmagnetic order parameter, the so-called quadruple phase, is possible. The realization of this phase is caused by several purely quantum effects, moreover the mechanism of its appearance differs in principle from that of the antiferromagnet or ferromagnet. The possibility of the disappearance of the angular phase (the so-called quadruple ferromagnetic phase) is also shown.
High precision measurements of the Seebeck coefficient S(T) were carried out on the single crystals of RB_{12} (R = Ho, Er, Tm, Lu) at temperatures 2-300 K. It was shown that the effects of phonon drag result from vibrations of rare earth ions (ħω_E≈10-33 meV) in the rigid framework structure of the B_{12} clusters and determine the main contribution to thermopower at intermediate temperatures (30-300 K). The correlated behavior of transport parameters favors the appreciable enhancement of spin fluctuations in the sequence of magnetic compounds (HoB_{12}-TmB_{12}) when approaching to the valence instability state in YbB_{12}. The giant increase in S(T) detected in the vicinity of the Néel temperature T_N for HoB_{12}, ErB_{12}, and TmB_{12} seems to result from the density of states renormalization caused by antiferromagnetic ordering.
High precision measurements of magnetoresistance Δρ/ρ = f(T,H) and magnetization M(T,H) were carried out on single crystals of rare-earth dodecaboride ErB_{12} at temperatures in the interval 1.8-30 K in magnetic fields up to 70 kOe. The high accuracy of the experiments allowed us to perform numerical differentiation and analyze quantitatively the behavior of the derivative d(Δρ/ρ)/dH = f(T,H) and of the magnetic susceptibility χ(T,H) = dM/dH in paramagnetic and magnetically ordered (antiferromagnetic, T_N ≈ 6.7 K and T_M ≈ 5.85 K) phases of ErB_{12}. It was shown that negative magnetoresistance anomalies observed in present study in paramagnetic state of ErB_{12} may be consistently interpreted in the framework of a simple relation between resistivity and magnetization -Δρ/ρ ~ M^2.
The new class of magnetic materials with low temperature magnetic relaxation was studied. It was evidenced that the crystal DyAlO_{3} with antiferromagnetically ordered high anisotropy rare-earth ions showed an exponential magnetic relaxation. For the first time low temperature magnetic relaxation was investigated by means of magnetostriction and rare-earth optical absorption spectra instead of standard magnetization measurements. The possible mesoscopic nature of tunneling process was suggested.
We have studied the temperature dependence of the lattice parameters and the influence of spin anisotropy on the electron paramagnetic spectra of Cu(tn)Cl_2, an S=1/2 quasi-two-dimensional spatially-anisotropic triangular-lattice Heisenberg antiferromagnet. The variation of the resonance fields with temperature reflects the presence of an easy-plane exchange anisotropy with J_{z}/J_{x,y}<1 and g-factor anisotropy, g_{z}/g_{x,y}>1.
Magnetocaloric studies of a two-dimensional antiferromagnet Cu(tn)Cl_{2} (tn=1,3-diaminopropane=C_3H_{10}N_2) have been performed by adiabatic magnetization and demagnetization measurements, in the temperature range from 0.2 to 4 K and magnetic fields up to 2 T. The compound represents an S=1/2 spatially anisotropic triangular-lattice antiferromagnet. The magnetocaloric measurements were focused at the identification of the phase transition to the magnetically ordered state which was not indicated in the previous specific heat studies. Furthermore, the interplay of the magnetic-field induced easy-plane anisotropy and the intrinsic spin anisotropy present in the studied system should manifest in low magnetic fields. The obtained results of the magnetocaloric experiments of Cu(tn)Cl_{2} indicate a double crossover from the normal to inverse magnetocaloric effect (MCE). The first crossover from the normal to inverse MCE occurring at about 0.3 K can be attributed to the competition of the aforementioned anisotropies. The second crossover from the inverse to normal MCE observed at about 2.2 K might be ascribed to the formation of spin vortices stabilized by the easy-plane anisotropy introduced by magnetic field.
Ternary thallium lanthanide dichalcogenides TlLnX_{2} (X = S, Se, or Te; Ln = lanthanide, except Ce and Pr) crystallize in the rhombohedral structure of α-NaFeO_{2} type ($R\overline{3}m$). Their crystal lattice consists of the layers of Ln^{3+} ions separated by three layers of the non-magnetic ions (-Ln-X-Tl-X-Ln-). The magnetization was measured in the field range 0-14 T. The molecular field constants λ_{m} were estimated by fitting the Brillouin function to the experimental magnetization plots. The difference between the λ_{m} values for the thallium gadolinium sulphide and the selenide corresponds to the different character of Gd-S and Gd-Se bonds and gives rise to the different J_{1} and J_{2} exchange integrals.
We deal with spin-wave propagation in two antiferromagnetically aligned sublayers forming one bilayer film owing to the antiferromagnetic exchange coupling assumed to exist at the film interface. We get the following picture: as the spin wave traverses the interface, the creation of a spin-reversal in the one sublayer becomes an annihilation in the other sublayer. This feature is expressed by the non-conventional type of normalization condition of the spin-wave amplitudes.
Nonlinear dynamics of an antiferromagnet in continuum approximation on the basis of the Heisenberg Hamiltonian for two sublattices with biquadratic exchange interaction in each sublattice and single ion anisotropy is considered. Exact particular solution, describing the stationary kink movement, including the region v ≥ c, is found.
Magnetism of borides which contain the B_{12} icosahedra as a structural building block are attracting increasing interest since they exhibit unexpectedly strong magnetic interactions, despite being magnetically dilute f-electron insulators. The magnetic behavior among the different compounds has also been found to be diverse. The f-electron dependence of magnetic B_{12} icosahedra borides is compared, and found to be different from conventional mechanisms. The TbB_{25} system is also investigated further and the transition is assigned to a typical antiferromagnetic transition.
We have investigated the effect of hydrostatic pressure on magnetic properties of TbNiAl, crystallizing in hexagonal ZrNiAl-type structure. TbNiAl orders antiferromagnetically below T_{N}=45 K and undergoes further magnetic phase transition to another AF phase at T_{1}=23 K. The magnetic field of B_{c} ≅ 0.3 T applied along the c-axis at 2 K leads to the transition to ferromagnetic order. By applying the hydrostatic pressure, both T_{N} and T_{1} remain almost unaffected whereas B_{c} shows a strong increase. The hydrostatic pressure stabilizes the antiferromagnetic state which can be related to development of structural parameters.
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