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Dielectric and magnetic properties of SrTi 1 − x Fe x O 3 were measured for a single crystal sample ( x =0.0032) and a ceramic sample ( x =0.02). Temperature dependences of the dielectric constants were analyzed on the basis of a Vendik's formula, which describes a quantum paraelectric state accurately. A small amount of Fe impurities in the single crystal does not affect the characteristic temperatures of the dielectric properties, but does affect the quality of the crystal. This change in quality causes a large change in the dielectric constant of the quantum paraelectric state. The temperature dependence of the dielectric constant of the quantum paraelectric state of the ceramic sample is different from that of the single crystal not only quantitatively, but also qualitatively. The magnetic susceptibilities obey the typical Curie law, though a deviation of the Curie law was observed below 5 K for x =0.02. Crystals with the both concentrations remain in paramagnetic states at 2.5 K. The magnetic properties of SrTi 1 − x Fe x O 3 can, in all likelihood, be explained by the orientation effect of free Fe 3 + ions. In addition, an antiferroelectric interaction suggested for EuTiO 3 by an analysis of dielectric constants based on a Barrett's formula was turned out to be unnecessary following analysis of the same data based on the Vendik's formula.

The temperature dependence of magnetic susceptibility χ (Τ), 78 59.634) decreases to a minimum for sample x=0.6. Highest resistivity (ρ) and ac susceptibility (χ) are observed for x=0.6. Metal- semiconductor transition temperature Tρ decreases gradually with Mg doping, in contrast to ferromagnetic-paramagnetic Curie temperature Tc which shows weak dependence on Mg doping level. Low temperature resistivity showed upturn in the temperature range 28–37 K. At high temperatures, variable range hopping conduction predominates with density of states N(Ef) ~3×1019−6×1019 (eV−1 cm−3). Low temperature resistivity follows ferrometallic nano-particle relation: ρ=ρo +ρ1T2 +ρ2T4.5. AC susceptibility of x=0.0 and 0.6 fit well to Curie law, samples x=0.2, 0.4, 0.8 and 1.0 are better described in the framework of Neel ferrimagnetism. Transport and magnetic susceptibility results are interpreted in terms of core-shell model where ferromagnetic core interacts anti-ferro-magnetically with ferromagnetic shell.

The dependence of magnetic properties on the thickness of few-layer thin films is investigated at finite temperature using the functional integral method for solving the Heisenberg spin model. The temperature dependence of the ultra-thin film's magnetization and Curie temperature are calculated in terms of the mean field theory and of the Gaussian spin fluctuation approximations. It has been shown that both Curie temperature and temperature interval, where the magnetization is non-zero, are strongly reduced with the thickness reduction by using the spin fluctuation approximations in comparison with the mean field results. Curie temperature dependence on the film thickness calculated numerically well agrees with the experimental data for Ni/Cu(1 0 0) and Ni/Cu(1 1 1) ultrathin films.

Size dependent Curie temperature in ferroelectric nanoparticles has been shown to originate from mechanical clamping of phase transition in particle core by a cubic surface layer. Based on a phenomenological model, the widely observed empirical relation between downshift of Curie point and particle size has been deduced by introducing an extra term of core–shell boundary energy into the free energy expression. Theoretical calculations of size dependent Curie temperature in PbTiO 3 nanoparticles are in well agreement with the literature-reported experimental data. The positive boundary free energy of nano-sized PbTiO 3 particles becomes increasingly large with size reduction, leading to a sharp drop of Curie temperature as compared to bulk single crystal.

Pressure effects on the Curie temperature and spontaneous magnetization in ferromagnetic Hf ( Fe 1 - x Co x ) 2 with the cubic Laves phase structure have been investigated under high pressures up to 1.2 GPa. An anomalously strong pressure dependence of the Curie temperature is observed around x = 0.725 . A collapse of ferromagnetic moment under pressure is also observed at x=0.6 and 0.65. These results are discussed on the basis of the spin fluctuation model of itinerant-electron metamagnetism.

Glass systems of composition (100_x) $Na_2B_4O_7 . H_2O+xMnSO_4 . H_2O$ (x = 0, 0.1, 0.3, 0.5, 0.75, 1, 1.25, 1.5, 1.75 and 2 mol%) have been studied by using electron paramagnetic resonance (EPR) and optical absorption techniques. The room temperature EPR spectra exhibit resonance signals at g \approx 2:0 with a sextet hyper.ne structure (hfs) besides two weak signals at g \approx 3:3 and 4.3. The resonance signal at g \cong 2:0 is due to Mn2+ ions in an environmentclose to octahedral symmetry, whereas the resonances at g \cong 3:3 and 4.3 are attributed to the rhombic surroundings of the Mn2+ ions. Itis observed that for x = 1:25mol% in sodium tetra borate glasses (NTB) the EPR spectrum exhibits a good resolution of allowed as well as forbidden hfs at g \approx 2:0: The Mn2+ ions in NTB glasses (with A \approx 95 G) are quite ionic in nature. The number of spins (N) participating in resonance at g \approx 2:0 has been calculated by comparing the area under the absorption curve with that of a standard of known concentration. A linear relationship is observed between log N and 1/T in accordance with the Boltzmann law. Magnetic susceptibilities (\chi) were calculated from the EPR spectra at different temperatures. The susceptibility is varying inversely with temperature in accordance with the Curie law, and the Curie constant is evaluated from the 1/ \chi versus T graph. The zero-.eld splitting parameter D has also been calculated from the allowed hyper.ne (HF) lines. The D value was found to be dependent on temperature. The optical absorption spectrum exhibits a single broad band centered around 470nm and was assigned to the $^6A_l_g(S) \rightarrow ^4T_l_g(G)$ transition. From the ultra violet absorption edges the optical bandgap and Urbach energies have been evaluated.

35 Cl nuclear quadrupole resonance was applied to study molecular dynamics in (ND4)2PtCl4. The spin-lattice relaxation time T1Q, the quadrupole resonance frequency νQ, the line width δν and the integrated intensity T∫F(ν) d ν of the resonance line F(ν) multiplied by the temperature T to compensate for the Curie law effect were measured in a wide temperature range 12.4 K K . Effects due to cation and anion mobility were clearly seen and their contributions are disentangled. Domain structure and diverse mobility of ammonium ions in the ordered phase are the basic assumptions underlying the fitting of the experimental data. An order–disorder phase transition takes place at 148 K and appears visible in all measured quantities. Unrestricted mobility of ammonium ions dominates above the phase transition. At higher temperatures, mobility of the planar PtCl42− ions, first torsional oscillations and then reorientational jumps, becomes detectable.

The ferromagnetic phase transition in the invar alloy Fe 70 Ni 30 (0.7 at% C ) with fully FCC structure was investigated by means of 3-D neutron depolarization analysis. The mean magnetic induction, magnetic anisotropy parameter and neutron depolarization were measured as a function of temperature and external magnetic field. Our experiments show the appearance of large magnetic inhomogeneities with size about 10 3 –10 4 A in the paramagnetic temperature range. The data were interpreted using the concept of a probability function ρ ( T C ) which describes the variations of the Curie temperatures over the system.

Magnetization of UNi1−xRuxAl solid solutions was measured at 4.2 K on isotropic powder samples in pulsed magnetic fields up to 26 T. Antiferromagnetism of UNiAl is suppressed completely for x=0.15 as inferred from the loss of metamagnetic transition on the high-field magnetization curve. The compounds with 0.15

LaMn 2− x Fe x Si 2 ( x = 0 , 0.05, 0.10, 0.15, 0.20) compounds were prepared by arc melting. The crystal structure and magnetic properties have been investigated by X-ray diffraction and by using a vibrating sample magnetometer, respectively. The compounds crystallize in the ThCr 2 Si 2 -type structure with the space group I4/mmm. The Curie temperature decreases gradually with the increase of Fe content. The magnetic entropy changes of the compounds, with a field change from 0 to 1.8 T, are derived from the magnetization isotherms at different temperatures. The maximal magnetic entropy change of LaMn 1.9 Fe 0.1 Si 2 reaches 1.72 J/kg K (0–1.8 T) around the Curie temperature near room temperature.

The model of the paraelectric, which is close to the point of the Mott–Hubbard instability, is shown to explain the DC resistivity and Hall effect temperature behaviour for high- T c superconductor metal oxides (HTSC). In the ground state the current is carried by a liquid of boson-like pairs of carriers in upper and lower Hubbard bands. The Mott–Hubbard instability corresponds to the order-of-lattice-constant length of the mean free path and results in the temperature insensitivity of Drude conductivity. Nearly linear on T resistivity results from the Curie law via the local (acting) electric field. Fermion-like carriers, temperature excited over the energy of boson-like pair dissociation (pseudogap), explain the temperature behaviour of Hall effect. Available data are compared with the model.

Ferromagnetic ordering in a generalization of the Hubbard model with correlated hopping to the case of twofold orbital degeneracy has been studied. An effective Hamiltonian of the model has been derived for the case of strong intra-atomic Coulomb repulsion and strong Hund's rule coupling. The energy of the system and the magnetization in the ground state have been calculated for various densities of states. The magnetization at non-zero temperature and Curie temperature of system have been obtained numerically. The important peculiarity of the concentration dependence of Curie temperature is its decrease in some region of electron concentration with increase of magnetic moment. This allows us to interpret the peculiar behavior of Curie temperature in the compound Fe 1 - x Co x S 2 with the change of conduction band filling.

The influence of disorder on the thermodynamical properties of amorphous ferromagnets near the Curie point is investigated. It is shown that for amorphous ferromagnets the Curie temperature is lower and that spontaneous magnetization increases slower with decreasing temperature than for the corresponding crystals.

A direct determination of the magnetocrystalline anisotropy field (HA) of various polycrystalline materials of the type Nd2Fe14B, Sm2Fe17Cx, Sm2Fe17Nx and R(Fe, T)12 by using the singular-point-detection technique in the temperature range from 4.2 K up to the Curie temperature is demonstrated. Some recent results on determining the critical first order magnetization process (FOMP) field (Hcr) using the same technique are also presented.

Temperature dependence of magnetic susceptibility and high field magnetization are studied for a single crystal of anti-ferromagnetic PrZn2. The paramagnetic Curie temperatures are anisotropic: θa=19.6 K, θb=2.9 K and θc=-72.5 K. Different field-induced magnetic transitions are observed along each crystal axis direction.

Magnetic and transport properties of Ce5Ge3 and La5Ge3 have been studied. The magnetic susceptibility of Ce5Ge3 above 90 K is fitted with a Curie–Weiss law with an effective paramagnetic moment of 2.66μB and a paramagnetic Curie temperature of −30.9 K. Arrott plot shows that Ce5Ge3 has a spontaneous magnetization below 6.4 K. The magnetization curve shows large hysteresis with remanent magnetic moment of 0.24μB and coercive field of 1.2 kOe at 2.0 K. The magnetization at 1.9 K reaches 0.9μB/Ce at 145 kOe and saturation behavior is not found even at this high field. The temperature dependence of the magnetic resistivity indicates that Ce5Ge3 can be classified as a dense Kondo system.

We measured the hig field magnetization process up to 30 T at 4.2 K and temperature dependence of the magnetic susceptibility in Al70Pd20Mn10 single quasicrystal. Above 5 T the magnetization showed a clear anisotropic behavior and is almost saturated in the magnetic field of 30 T. The Curie-Weiss behavior was observed in the temperature range between 40 and 150 K for the temperature dependence of susceptibility. The Curie constant, temperature-independent susceptibility and Curie temperature were determined.

Results of neutron diffraction, magnetization, specific heat and resistivity are reported for CeRu 2 Ge 2 which orders ferromagnetically at 7.9 K with a spontaneous magnetization of 1.96 ±0.02μ B along the tetragonal c axis. The anomalous behaviour of polycrystalline samples in the vicinity of the Curie temperature is not observed in sigle crystals.

The effect of dilute Gd magnetic impurities on the magnetization of the heavy fermion system Ce 1− x Gd x Al 3 with 0 ⩽ x ⩽ 0.005 for temperatures in the range 2.5 K ⩽ T ⩽ 300 K and for magnetic fields H ⩽20 kOe is reported. The Gd impurity contribution obeys a Curie law with an effective moment close to the Gd 3+ free ion value below 50 K. Evidence is presented that the addition of Gd also enhances the host CeAl 3 susceptibility by (6.9 ± 2.0)% for 0.5% Gd at 300 K.

Magnetization measurements have been performed on a Ni0.45Pt0.55 alloy in the magnetic fields up to 130 kOe and in the temperature range from 5 to 280 K . The spontaneous magnetization σS(T) follows a [1-(T/TC)2]-dependence below 15 K . In the temperature range from 15 K to the Curie temperature (41.2 K ) , however, σS(T) decreases in proportional to [1-(T/TC)4/3]. Obtained results can be explained with a new spin fluctuation theory of Takahashi.

The discovery of a ferromagnetic form of carbon (Nature 413 (2001) 716) gives a new perspective in the investigation of magnetic materials. The existence of a ferromagnetic state with the very high Curie temperature T C ≈500 K for a material with only s- and p-electrons as well as the nature of its underlying interaction are of great fundamental interest. Here we report on the observation of the ferromagnetically ordered state in a material obtained by high-pressure high-temperature treatment of the fullerene C60. It has a saturation magnetization more than four times larger than that reported previously. From our data we estimated the considerably higher value of T C ≈820 K .

The magnetic properties of 3 He nanoclusters in phase-separated solid 3 He – 4 He mixture were investigated for samples with pressures between 2.64 and 3.71 MPa and temperatures between 0.6 and 10 mK . From pulse NMR from 62 to 250 kHz , it was found that there are two components with different spin–spin relaxation times for the samples of pressure from 2.95 to 3.04 MPa . The component with longer T 2 ∗ showed large downward deviation from Curie law or decrease of magnetization at about 1.05 mK which was larger for lower field. The component with shorter T 2 ∗ showed ferromagnetic tendency. The solid-like fraction was obtained from the relative magnitudes of magnetization and it shows a sharp increase with pressure around 2.95 MPa .

We simultaneously measured the AC susceptibility and DC magnetization of the heavy fermion compound CeRu2Si2 in various small magnetic fields at temperatures down to 170 μK . The susceptibility and magnetization increased below 50mK followed by the Curie law with the small magnetic moment 0.02μB/Ce. In fields higher than 0.20 mT , the susceptibility showed the peak. The peak temperature (TM) and height shifted higher and lower with increasing the field, respectively. In addition, approaching TM, the static magnetization became a plateau in fields higher than 0.20 mT . These results reflect the spin fluctuation of the itinerant 4f electrons.

The use of a SQUID coupled to a sample by a superconducting circuit allows NMR measurements over wide bandwidths. For our recent NMR experiments on 3He monolayers we developed a sample cell that allows for indirect cooling of the grafoil without excessive eddy currents. The details of the cell design are discussed. In addition, we have found that the 13C signal from the graphite is observable in this system. The FID time constant for this signal is comparable to the field homogeneity and the amplitude follows the Curie law over our temperature range ( 15 mK >T>0.2 mK ). Since the NMR signal in 13C is observed simultaneously with the 3He signal, this signal can act as a thermometer for the graphite substrate.

Measurements of the magnetization curves in MnZnSb were made around the Curie temperature. The critical exponent β was determined to be 0.194, which is close to the value expected from the two-dimensional Ising model. On the contrary, the critical exponent δ was determined to be 3.05, which is close to the value expected from the molecular field theory and itinerant ferromagnetism.

We show that the effect of the electron-phonon interaction in the ferromagnetic state of an itinerant electron ferromagnet results in an effective magnetic field and modifies the spin splitting of the electron energy bands. From a numerical calculation with a simple model electronic structure we find that such phonon effect can be much larger than ordinarily anticipated; the effect on the saturation magnetization can be as large as ∼1μB/atom, and that on the temperature dependence of magnetization can significantly reduce the Curie temperature.

Continuous wave NMR was used to study pure3He films at coverages from below 2nd layer completion to above the ferromagnetic peak coverage at ∼2.5 layers. The films were adsorbed on Grafoil and were cooled to temperatures as low as 0.075 mK. The first layer magnetization followed Curie law very well, with a Weiss constant of less than 0.01 mK. We measured the ferromagnetic 2nd layer magnetization to temperatures far below the exchange energy (J) as a function of coverage. Preliminary analysis indicates that J did not vary strongly with coverage, and that the zero temperature 2nd layer magnetization was linear in 2nd layer density. Both of these properties are consistent with there being two phases in coexistence, only one of which is ferromagnetic.

The nuclear magnetic susceptibility of two dimensional solid helium isotopic mixtures, formed by a submonolayer film adsorbed on the surface of graphite, has been measured down to 2mK.3He concentrations in the range 1% to 16%, at total surface densities above 0.082A˚-2 were explored. At the higher concentrations there is clear evidence of a phase transition, in agreement with previous measurements. The present results suggest that this corresponds to incomplete phase separation. At lower concentrations no signature of phase separation has yet been detected. The deviations from Curie law, attributable to quantum diffusion of3He, are greatest at 0.086A˚-2.

Measurements were made of the magnetic susceptibility in the temperature range 2K–10K of twenty-nine samples of materials and components used in the construction of cryogenic instruments. The use of SQUID-based superconducting susceptometers allowed high resolution. The data was fitted to a Curie law form and the values of the coefficients were determined. The remanent magnetization at 2K of several of the samples was also measured.

It is shown in two different FeNiP alloys that magnetic scattering of neutrons at small angles persists at temperatures T well above the Curie temperature T c . This is taken as evidence of existence of zones of concentration fluctuations with higher magnetic moments that continue to produce coherent magnetic neutron scattering at T > T c of the overall alloy composition.