The magnetization and magnetic susceptibility of Bridgman-grown Sn_{1-x}Gd_{x}Te with values of x up to 0.09 have been measured over a temperature range from 2 to 300 K and in magnetic fields up to 5.5 T. The magnetic susceptibility data followed the Curie-Weiss relation with a small Curie temperature that indicated a weak antiferromagnetic coupling among Gd ions. The magnetic field dependence of the magnetization was fitted to a modified Brillouin function with parameter values that agreed fairly well with those from Curie-Weiss plots. The value of the exchange parameter was larger than in Pb_{1-x}Gd_{x}Te. The samples were p-type with carrier concentrations up to 1.3 x 10^{21} cm^{-3}. The ferromagnetic or spin-glass phase due to the RKKY interaction was not observed.
Antiferromagnetic interlayer exchange coupling in semiconductor EuS-PbS-EuS ferromagnetic trilayers grown on PbS (001) substrates with ultrathin (0.6-1.2 nm) nonmagnetic PbS spacers is studied by SQUID magnetometry and model calculations. Analysis of the experimentally observed magnetic field and temperature dependence of the magnetization of EuS-PbS structures reveals a rapid decrease in the interlayer coupling energy with increasing temperature indicating a temperature dependence of the microscopic coupling mechanism acting in these all-semiconductor ferromagnetic/nonmagnetic multilayers.
Antiferromagnetic interlayer coupling between ferromagnetic layers of EuS via nonmagnetic PbS spacer layer was experimentally studied in EuS-PbS wedge multilayers grown on KCl (001) substrates with EuS thickness of 6 nm and PbS thickness varying in the wedges in the range 0.3-6 nm (i.e. n=1-20 monolayers). Measurements of magnetic hysteresis loops of EuS-PbS multilayers performed in the temperature range 5-30 K by superconducting (SQUID) and magneto-optical magnetometers revealed a rapid increase in saturation magnetic field in multilayers with PbS spacer thinner than about 1.5 nm. It shows a monotonic increase in interlayer coupling strength with a decreasing PbS spacer thickness, in qualitative agreement with 1/2^n dependence predicted theoretically for semiconductor magnetic superlattices.
Electric conductivity, Hall effect and magnetic susceptibility of Pb_{1-x-y} Sn_{y}Gd_{x}Te mixed crystals with 0.13 ≤ y ≤ 0.93 and 0.001 ≤ x ≤ 0.04 were experimentally studied over the temperature range 4K ≤ T ≤ 300 K. The incorporation of Gd ions into the Pb_{1-y}Sn_{y}Te matrix results in semi-metallic n-type conductivity of the crystals with y < 0.6. For crystals with y > 0.6 one observes only semi-metallic p-type conductivity. We present a model explaining these results in terms of the Sn composition dependence of the location of Gd^{2+/3+} level with respect to the band edges of PbSnGdTe.
Magnetic interactions between nanoparticles of magnetite Fe_3O_4 and α-Fe formed during calcination of ferrihydrite in H_2 at temperatures 533 ÷ 713 K were studied. The detailed phase analysis describes evolution of contents of magnetite and α-Fe. Explanation of an anomalous course of magnetic moment characterized by a local maximum at an early stage of formation of nanoparticles is discussed.
Magnetic, transport and structural properties of bulk crystals of Sn_{1-x}Gd_{x}Te with Gd content 0.002 < x < 0.09 and varying carrier concentrations obtained by an isothermal annealing were studied in the temperature range T = 1.5 - 80 K. We found the effect of resonant increase in antiferromagnetic spin-spin exchange interactions in the crystals with 0.025 ≤ x ≤ 0.05. No effect was found in crystals either with higher (x > 0.05) or with lower (x < 0.025) Gd concentration. The observed Gd composition dependence of the magnetic and transport properties of SnGdTe can be explained in a proposed model relating these experimental properties to the Gd composition induced shift of the position of Gd^{3+/2+} level with respect to the top of the valence band of SnGdTe.
Magnetic properties of semiconductor EuS(t)-PbS(d)-EuS(t) ferromagnetic trilayers (t=30÷300Å and d=7.5÷70Å) grown on n-type monocrystalline PbS (100) substrate were studied by SQUID magnetometry and ferromagnetic resonance technique yielding, in particular, the dependence of the ferromagnetic Curie temperature on the thickness of the EuS layer. Structural parameters of layers were examined by X-ray powder diffraction analysis. A high structural quality of the substrate and the multilayer was verified by the measurements of the X-ray rocking curve width indicating the values of the order of 100 arcsec and by atomic force microscopy revealing the presence on the cleft PbS surface regions practically flat in the atomic scale over the area of 1×0.1μm^2.
Magnetic susceptibility has been measured in Pb_{1-x-y}Sn_{y}Mn_{x}Se with x values up to 0.02 and y values up to 0.05. The measurements were carried out using a SQUID system over a temperature range from 5 to 250 K. The susceptibility followed the Curie-Weiss relation with a small paramagnetic Curie temperature that indicated a weak antiferromagnetic exchange interaction between Mn ions. We analyzed the results together with our previously published data on high-field magnetization in this material. A reasonable agreement of the exchange parameters obtained from the low-field susceptibility and high-field magnetization data was obtained.
The magnetic susceptibility of Bridgman-grown Sn_{1-x}Eu_{x}Te with nominal values of x up to 0.095 was measured over a temperature range from 2 to 385 K. The samples were p-type with hole concentrations up to 1.5 × 10^{21} cm^{-3}. The susceptibility data above 50 K followed the Curie-Weiss relation with a small Curie temperature. At about 10 K a small cusp in susceptibility was observed in samples with the higher range of x values. The data indicate a presence of both antiferromagnetic and ferromagnetic exchange interactions, with some paramagnetic contributions from charged defects. There is also evidence of Eu going into the lattice both as Eu^{2+} and Eu^{3+} ions.
Kerr magnetometry was employed to study the temperature dependence of magnetization and magnetic hysteresis loops in ferromagnetic EuS-PbS semiconductor multilayers in the temperature range T=3-35 K at low magnetic fields H≤150 Oe. For EuS-PbS/KCl(100) structures with ultrathin non-magnetic PbS spacer of 1~nm, we observed a maximum on the temperature dependence of magnetization at low fields H≤30 Oe. For higher fields, we found for these structures a regular mean-field-like increase in magnetization with decreasing temperature. The same regular behavior was also found for EuS-PbS/KCl structures with thicker PbS spacer, as well as for all EuS-PbS/BaF_2(111) multilayers independently of spacer thickness. For qualitative interpretation of these findings, we consider two magnetic contributions to the total energy of EuS-PbS multilayers: the Zeeman energy and the antiferromagnetic interlayer exchange coupling between ferromagnetic EuS layers via diamagnetic PbS spacer.
The magnetic properties of Ni(II) and Cu(II) complexes were measured. In the case of Ni(II) samples strong enhancement of the magnetic susceptibility below 23 K was observed. The model of structural transition was proposed to explain this behavior.
We tried to repeat the observation of the ferromagnetic response in proton and He-irradiated carbon made by the group of Esquinazi et al. We used He^+ and H^+ beams focused on graphite sample. The amount of charge deposited in the sample was comparable to the amount of charge used by Esquinazi. Magnetic measurements were performed in SQUID magnetometer. The magnetization of the samples before and after irradiation was compared. We did not observe any ferromagnetic enhancement of magnetization of our irradiated samples. Even if experiment was not the same as Esqinazi's one, we can exclude some of the mechanisms of ferromagnetism proposed by Esquinazi.
Temperature and magnetic field dependence of magnetization of EuS-SrS multilayers grown epitaxially on KCl (001) substrate is experimentally studied by superconducting magnetometry technique. In these lattice-matched semiconductor heterostructures EuS layers are ferromagnetic quantum wells whereas SrS layers are nonmagnetic spacer barriers. The multilayers composed of EuS layers with thickness 3.5-5 nm and SrS layers (thickness 0.5-10 nm) exhibit ferromagnetic transition at 17 K. In the multilayers with ultrathin SrS spacers (0.5-1 nm) a nonmonotonic temperature dependence of magnetization as well as a characteristic switching in magnetic hysteresis loops is observed. These experimental findings are explained considering antiferromagnetic interlayer coupling between ferromagnetic EuS layers via nonmagnetic SrS spacers. The strength of this coupling is determined based on model magnetization calculations.
The study of transport and of magnetic properties of Pb_{1-x}Ge_{x}Te:Yb mixed crystals (0 ≤ x ≤ 0.04) is reported. It is shown that Yb forms a donor state resonant with the PbTe valence band. The donor state position may be tuned (shifted relative to the energy gap) by admixture of Ge. The properties of the Yb ion in the Pb_{1-x}Ge_{x}Te matrix makes the system unique from the point of view of magnetic properties. It is demonstrated that the change of the conductivity type from p to n induces transitions from the paramagnetic state to the diamagnetic one.
The magnetization of p-type Sn_{1-x}Gd_{x}Te with x up to 0.045 and the hole concentration, p, varying from 2.7 to 8.3×10^{20} cm^{-3} has been measured in magnetic fields up to 27 T, at the temperatures 4.2 and 1.3 K. The data were fitted to a magnetization equation with single-ion and pair terms. From comparison of the exchange parameters determined from the high-field magnetization with those previously obtained from the high-temperature magnetic susceptibility it was found that in samples with p>5×10^{20} cm^{-3} the exchange was of a short-range type, while in samples with a lower carrier concentration the long-range exchange mechanism was observed.
Magnetic and structural properties of EuS-SrS semiconductor multilayers were studied by SQUID and magneto-optical Kerr effect magnetometry techniques and by X-ray diffraction method. The multilayers composed of monocrystalline, lattice matched ferromagnetic EuS layers (thickness 35-50Å) and nonmagnetic SrS spacer layers (thickness 45-100Å) were grown epitaxially on KCl (001) substrates with PbS buffer layer. Ferromagnetic transition in EuS-SrS multilayers was found at the Curie temperature T_c=17 K. The multilayers exhibit only weak in-plane magnetic anisotropy with [110] easy magnetization axis. Coercive field of EuS-SrS multilayers shows a linear increase with decreasing temperature. Magneto-optical mapping of magnetic hysteresis loops of the multilayers revealed good spatial homogeneity of their magnetic properties.
The magnetization and magnetic susceptibility of Bridgman-grown Cd_{1-x}Co_{x}Se with values of x up to 0.04 have been measured over a temperature range from 2 to 390 K and in magnetic fields up to 23 T. A pair-exchange value J/k_{B} of about -3 K was obtained from the magnetic field dependence of the magnetization. This exchange is probably due to the next-nearest-neighbor interaction. The high-temperature susceptibility data indicated a presence of the Curie-Weiss like paramagnetism and the temperature independent Van Vleck type paramagnetism.
Magnetic properties of IV-VI semimagnetic (diluted magnetic) semiconductors with Mn or with Gd can be controlled by changing the electronic parameters such as carrier concentration, Fermi level position or density of states at the Fermi level. The examples of these effects are the carrier concentration induced paramagnet-ferromagnet and ferromagnet-spin glass transitions in PbSnMnTe and the Fermi level position controlled Gd-Gd exchange interactions in SnGdTe.
Temperature dependence of current-voltage I-V characteristics and resistivity is studied in ferromagnetic PbS-EuS semiconductor tunnel structures grown on n-PbS (100) substrates. For the structures with a single (2-4 nm thick) ferromagnetic EuS electron barrier we observe strongly non-linear I-V characteristics with an effective tunneling barrier height of 0.3-0.7 eV. The experimentally observed non-monotonic temperature dependence of the (normal to the plane of the structure) electrical resistance of these structures is discussed in terms of the electron tunneling mechanism taking into account the temperature dependent shift of the band offsets at the EuS-PbS heterointerface as well as the exchange splitting of the electronic states at the bottom of the conduction band of EuS.
Magnetic susceptibility, electron paramagnetic resonance and transport properties of Sn_{1-x}Gd_{x}Te with 0.04 < x < 0.07 and hole concentrations in the range from 0.7 × 10^{20} to 16 × 10^{20} cm^{-3} were investigated. After annealing of the Sn_{1-x}Gd_{x}Te samples with x < 0.05 in Sn vapor their hole concentration decreased from 5 × 10^{20} cm^{-3} to about 3 × 10^{20} cm^{-3} and their paramagnetic Curie temperature increased a few times. In samples with x > 0.05 no significant change in the magnetic properties was observed after annealing, even at lower hole concentrations. The results can be explained by assuming that an indirect exchange interaction, 4f-5d-band electrons, is responsible for the coupling among Gd ions.
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