Magnetic susceptibility and magnetic-field behavior of CuB2O4 copper metaborate

An experimental study is reported regarding the temperature dependence of the magnetic susceptibility of a CuB₂O₄ tetragonal single crystal within the 4.2–200-K range. It has been established that the magnetic susceptibility exhibits anomalies at 21 and 10 K and depends strongly on crystal orientation in the magnetic field. A study has been carried out of the field dependences of the magnetization of CuB₂O₄ at various temperatures and crystal orientations. It is shown that for T>21 K, the crystal is in a paramagnetic state determined by Cu²⁺ copper ions with an effective magnetic moment of 1.77 μ_B. Within the 10–21 K interval, the field dependence of the magnetization is typical of a weak ferromagnet with magnetic moments of the two antiferromagnetically coupled sublattices lying in the tetragonal plane of the crystal. The spontaneous weakly-ferromagnetic moment is 0.56 emu/g at 10 K. The canting angle of the sublattice magnetic moments, determined by the Dzyaloshinski-Moriya interaction, is 0.49°. It is believed that below 10 K, the CuB₂O₄ crystal retains its easy-plane magnetic structure, but with a zero spontaneous magnetic moment.     

Weak ferromagnetism in copper metaborate CuB2O4

CuB₂O₄ single crystals have been grown and their magnetic and resonance properties have been investigated for the first time. The temperature dependence of the susceptibility was found to contain features at T=21 and 10 K. The CuB₂O₄ single crystal transformed at T=21 K to a weakly ferromagnetic state. The sharp drop in susceptibility at T<10 K is caused by a transition of the magnetic system of CuB₂O₄ to an antiferromagnetic state. The effective magnetic moment of the Cu₂₊ ion, determined from the high-temperature part of the magnetic susceptibility, is 1.77 μ _B. The room-temperature g factors are, respectively, 2.170 and 2.133 for magnetic field parallel and perpendicular to the c axis of the crystal. The antiferromagnetic resonance parameters in the weakly ferromagnetic and antiferromagnetic phases were measured. Fiz. Tverd. Tela (St. Petersburg) 41, 1267–1271 (July 1999)     

Magnetization process and principal magnetic susceptibilities in RbMnF3

The temperature dependence of χ_⊥, initialχ_p (powder), and evaluated χ_∥ for RbMnF₃ are reported for the temperature range of 4.2–106 K. A shallow minimum at $\text{T}\text{T}{}_{\mathrm{N}}\text{? 0.72}$ is observed in χ_⊥. The field dependence of the magnetization shows a smooth rotation of the spins to a normal direction above H_c ? 2.43 kOe at 4.2 K.     

Antiferromagnetic resonance and phase diagrams of gadolinium ferroborate GdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

Antiferromagnetic resonance in single crystals of rhombohedral gadolinium ferroborate GdFe₃(BO₃)₄ was studied. The frequency-field dependences of antiferromagnetic resonance over the frequency range 26–70 GHz and the temperature dependences of resonance parameters for magnetic fields oriented along the crystal axis and in the basal plane were determined. It was found that the iron subsystem, which can be treated as a two-sublattice antiferromagnet with anisotropy of the easy-plane type, experienced ordering at T=38 K. At temperatures below 20 K, the gadolinium subsystem with the opposite anisotropy sign strongly influenced the anisotropic properties of the crystal. This resulted in a spontaneous spin-reorientation transition from the easy-plane to the easy-axis state at 10 K. Below 10 K, magnetic field-induced transitions between the states were observed. Experimental phase diagrams on the temperature-magnetic field plane were constructed for fields oriented along the crystal axis and in the basal plane. A simple model was used to calculate the critical transition fields. The results were in close agreement with the experimental values measured at T=4.2 K for both field orientations.     

Transition between incommensurate phases accompanied by reversal of the magnetic structure vector in CuB<Subscript>2</Subscript>O<Subscript>4</Subscript>

The incommensurate magnetic state of copper metaborate CuB₂O₄ is studied in the temperature range 2 < T < 12 K. Competition between frustrated and non-frustrated antisymmetric exchange interactions is shown to cause the magnetic structure vector to reverse at T = 10 K.     

Anomalous magnetism and 209Bi nuclear spin relaxation in Bi4Ge3O12 crystals

The quadrupole ²⁰⁹Bi spin–spin and spin–lattice relaxation were studied within 4.2–300 K for pure and doped Bi₄Ge₃O₁₂ single crystals which exhibit, as was previously found, anomalous magnetic properties. The results revealed an unexpectedly strong influence of minor amounts of paramagnetic dopants (0.015–0.5 mol.%) on the relaxation processes. Various mechanisms (quadrupole, crystal electric field, electron spin fluctuations) govern the spin–lattice relaxation time T ₁ in pure and doped samples. Unlike T ₁, the spin–spin relaxation time T ₂ for pure and Nd-doped samples was weakly dependent on temperature within 4.2–300 K. Doping Bi₄Ge₃O₁₂ with paramagnetic atoms strongly elongated T ₂. The elongation, although not so strong, was also observed for pure and doped crystals under the influence of weak (～30 Oe) external magnetic fields. To confirm the conclusion about strong influence of crystal field effects on the temperature dependence of T ₁ in the temperature range 4.2–77 K, the magnetization vs. temperature and magnetic field was measured for Nd- and Gd-doped Bi₄Ge₃O₁₂ crystals using a SQUID magnetometer. The temperature behavior of magnetic susceptibility for the Nd-doped crystal was consistent with the presence of the crystal electric field effects. For the Gd-doped crystal, the Brillouin formula perfectly fitted the curve of magnetization vs. magnetic field, which pointed to the absence of the crystal electric field contribution into the spin–lattice relaxation process in this sample.     

New magnetic states in copper metaborate CuB<Subscript>2</Subscript>O<Subscript>4</Subscript>

The static and resonance properties of copper metaborate CuB₂O₄ were experimentally studied in a magnetic field applied in the crystal tetragonal plane. The field-induced second-order phase transition to a weakly ferromagnetic state was observed in the temperature range 10–20 K. The low-field state is characterized by the absence of spontaneous moment, and it represents, presumably, a long-period helicoid. At temperatures below 2 K, two sequential first-order phase transitions were observed. They were accompanied by jumps in resonance absorption with a hysteresis upon changing field-scan direction. These transitions can be caused by the transformation of the incommensurate spin structure into the helicoidal states with periods commensurate with the lattice translation period.     

Electrical transport properties of a quasi-one-dimensional Nb3Te4 single crystal

The electrical resistance of a linear chain metal Nb₃Te₄ were measured from 1.3 to 320 K. The residual resistance ratio $\text{R(300}\text{K}\text{)}\text{R(4.2}\text{K}\text{)}$ is about 3. Nb₃Te₄ shows an anomaly in the resistivity vs temperature at about 80 K, suggesting an occurrence of a charge-density-wave transition. The transverse and longitudinal magnetoresistance at 4.2 K are proportional to the magnetic field in the range of 2–58 kOe. In the superconducting region close to the transition temperature T_c, the critical magnetic field H_c2 is proportional to δT=T_c?T. The angular dependence of H_c2 fits well with the fluxoid model of the Ginzburg-Landau theory. The ratio of the critical fields parallel and perpendicular to the chain direction is 4.8.     

Magnetic resonance spectrum of a two-phase state in single crystals of La0.7Pb0.3MnO3 lanthanum manganite

Two absorption lines are observed over a wide temperature range below T _c in the magnetic resonance spectrum of an La_0.7Pb_0.3MnO₃ single crystal. These lines correspond to two magnetic phases in the sample. The frequency-field dependence of spectra obtained in the range of microwave radiation frequencies 10–77 GHz allows these phases to be interpreted as ferromagnetic and paramagnetic phases. The phase volume ratio depends on the temperature and the magnitude of the external magnetic field. Features in the temperature behavior of parameters of the magnetic absorption line are observed in the region of the highest magnetic resistance of the sample. The results are interpreted within the mechanism of electronic phase separation.     

Indication from NMR of Grotthuss mechanism for proton conduction in H2Sb4O11·nH2O

We have measured the second moment, the linewidth and the relaxation times T₁ and T₂ of the ¹H magnetic resonance signal from 4.2 to 380 K in the fact proton conductors H₂Sb₄O₁₁·nH₂O. Our results reveal that the high ionic conductivity of these materials is due to a Grotthuss-type proton diffusion mechanism with succession of molecular reorientations of H₃O⁺ ions or H₂O molecules and of proton jumps from H₃O⁺ to H₂O.     

Effect of frustration on the magnetization in nickel ferrite-chromites

A study is reported on the behavior of the isotherms of the magnetization σ(H) and of the longitudinal λ_‖(H), transverse λ_⊥(H), volume ω(H), and anisotropic λ_t(H) magnetostrictions measured at T=80 K in the Cu_0.4Fe_0.6[Ni_0.6Cr_1.4]O₄ and Zn_0.4Fe_0.6[Ni_0.6Cr_1.4]O₄ ferrite-chromites having a frustrated magnetic structure. It has been established that these ferrite-chromites do not undergo technical magnetization and that the growth of the magnetization with the field is accounted for by two paraprocesses of different natures.     

Influence of the ground state of the Pr<Superscript>3+</Superscript> ion on magnetic and magnetoelectric properties of the PrFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> multiferroic

The magnetic, magnetoelectric, and magnetoelastic properties of a PrFe₃(BO₃)₄ single crystal and the phase transitions induced in this crystal by the magnetic field are studied both experimentally and theoretically. Unlike the previously investigated ferroborates, this material is characterized by a singlet ground state of the rare-earth ion. It is found that, below T _N = 32 K, the magnetic structure of the crystal in the absence of the magnetic field is uniaxial (l ∥ c), while, in a strong magnetic field H ∥ c (H _cr ～ 43 kOe at T = 4.2 K), a Fe³⁺ spin reorientation to the basal plane takes place. The reorientation is accompanied by anomalies in magnetization, magnetostriction, and electric polarization. The threshold field values determined in the temperature interval 2–32 K are used to plot an H-T phase diagram. The contribution of the Pr³⁺ ion ground state to the parameters under study is revealed, and the influence of the praseodymium ion on the magnetic and magnetoelectric properties of praseodymium ferroborate is analyzed.     

Anisotropy in the magnetic susceptibility of 2H-TaSe2

The magnetic susceptibility of 2H-TaSe₂ is temperature dependent and anisotropic. The susceptibility with H parallel to the c axis is approximately 2.5 times that perpendicular. The decrease in susceptibility below the charge density wave transition at 122 K is also anisotropic with [χ₆(122 K) ? χ₆(4.2 K]?[χ_⊥(122 K) ? χ_⊥(4.2 K)] ≈ 4. Consequently, this change cannot be simply interpreted as a change in the density of states at the Fermi level, unless very anisotropic electron g values are assumed.     

Electron-phonon scattering in potassium at high magnetic fields

We have measured the temperature dependence of both the zero-field resistivity and the transverse magnetoresistance of polycrystalline potassium wires (?(300 K)/?(4.2 K)=140 to 6000) in fieldsH?35 kG and at temperaturesT?4.2 K. Our principal findings are: 1) The presence of a large magnetic fieldH=35 kG does not alter the temperature dependence of ? from that observed atH=0; below 4.2 K theT-dependent part of the resistivities,_?T (H=0) and_?T (H=35 kG), fit well to the function exp (?Θ*/T) with the same Θ*=23K. 2) Deviations from Matthiessen's rule are significantly reduced in a strong field so that the magnitude of_?T (H=0) approaches that of_?T (H=35 kG) as sample purity decreases. 3) The slope of the high-field linear magnetoresistance increases slightly (?8%) from 1.5 K to 4.2 K. We attribute the exponential temperature dependence of_?T (H) to the freezing out of electron-phonon umklapp processes as has been shown for the zero-field resistivity. The reduction in deviations from Matthiessen's rule at high fields can be understood within semiclassical theory, but the latter cannot explain the failure of_?T (H) to saturate at high fields. A proposal by Young that electron-phonon umklapp scattering may contribute aT-dependent high-field linear magnetoresistance in potassium is considered.     

Raman-Nath acoustooptical diffraction in antiferromagnet α-Fe2O3 induced by modulation of the polarizations of optical normal modes

Raman-Nath acoustooptical diffraction was experimentally revealed in easy-plane antiferromagnet α-Fe₂O₃. The diffraction is due to linear modulation of the polarizations of the optical normal modes propagating along trigonal axis C₃ of the crystal due to oscillations of the antiferromagnetism vector L caused by magnetoelastic interaction. The dependence of the diffraction parameters on a dc magnetic field H _⊥ applied in the basal plane was studied. The dependence obtained is in good agreement with the field dependence of the exchange enhancement coefficient of magnetoelastic coupling in fields higher than that at which hematite transforms to a single-domain state (H _D ～ 1.8 kOe). for a sound flux power of about 1 W/cm² in a sample, the maximum intensity of the diffracted wave at H _⊥ = H _D is about 0.9% of the input light intensity and its polarization is perpendicular to that of the incoming linearly polarized wave. The results obtained agree qualitatively with the theory of acoustooptical diffraction in antiferromagnets and confirm the antiferromagnetic mechanism of diffraction in the experimental geometry under study.     

Characteristic fields of a Bi2Sr2CaCu2Oy crystal

The field dependences of the transverse resistance of a Bi₂Sr₂CaCu₂O_y (BSCCO) layered superconducting single crystal with T _c0≥92 K are studied in a perpendicular (H⊥(ab)) pulsed magnetic field up to 50 T in a wide temperature range, 4.2–300 K. The temperature dependences of the characteristic fields identified with the “irreversibility curve” and the field corresponding to the nucleation of the superconducting phase are determined. The results obtained for the latter field are compared with the theoretical dependences.     

Specific features of magnetic phase diagram of an MnSi helimagnet

Magnetization curves of MnSi single crystals have been measured in a range of temperatures T = 5.5–35 K and magnetic field strengths H ≤ 11 kOe for H ∥ [1 1 1], [0 0 1], and [1 1 0]. Special attention has been paid to the temperature interval near T _N = 28.8 K, where MnSi exhibits a transition to the state with a long-period helical magnetic structure. Some new features in the magnetic behavior of MnSi have been found. In particular, in an intermediate temperature region above the transition (28.8 K = T _N ≤ T < 31.5 K), the dM(H)/dH curves exhibit anomalies that are not characteristic of the typical paramagnetic state. It is established that the line of the characteristic field H*(T) of this anomaly is a natural extrapolation of the temperature dependence of the field of the transition from a conical phase to an induced ferromagnetic phase observed at T < T _N. It is concluded that the properties of MnSi in the indicated intermediate region are related to and governed by those of the conical phase (rather than of the A phase). Based on these data, magnetic phase diagrams of MnSi for H ∥ [1 1 1], [0 0 1], and [1 1 0] are plotted and compared to diagrams obtained earlier by other methods.     

23Na nuclear magnetic resonance investigation of the lattice distortion in α-NaxV2O5

The full temperature dependence of the electric field gradient tensor at the Na sites has been determined by nuclear magnetic resonance (NMR) in the temperature range 8–330 K in α-Na_{x 2}O₅ (x = 0.996). Above the spin-Peierls transition (T _c = 34.7 K), only a single Na site is observed in agreement with the Pmmn space group proposed to describe this compound as the first example of a 1/4-filled ladder system. Below T_c, eight distinct quadrupolar²³Na sites are observed according to the distortion wave vector k_c = (1/2, 1/2, 1/4) previously reported. In addition, the opening of a spin gap is evidenced by a rapid drop of the magnetic hyperfine shift²³K at T_c. The results are discussed in the context of a charge-order-driven spin-Peierls transition.     

Giant magnetoresistance of MexMn1−x S (Me=Fe, Cr) sulfides

The structural, electrical, and magnetic properties, as well as the magnetoresistance of polycrystalline Me_xMn_1?x S (Me=Fe and Cr) sulfides were investigated in longitudinal magnetic fields of up to 50 kOe over the temperature range 4.2–300 K. The ferromagnetic compound Fe_xMn_1?x S (x=0.29) exhibits the giant magnetoresistance (GMR) effect with magnitude δ_H=?450% in a field of 30 kOe at 50 K. Antiferromagnetic Cr_xMn_1?x S (x=0.5) sulfide undergoes a transition to the GMR state δ_H～?25% in a field of 30 kOe at 4.2 K) in the region of antiferromagnet-ferromagnet transition (T _c ～66 K). A mechanism of the GMR in these compounds is discussed.     

Experimental study of antiferromagnetic resonance in noncollinear antiferromagnetic Mn<Subscript>3</Subscript>Al<Subscript>2</Subscript>Ge<Subscript>3</Subscript>O<Subscript>12</Subscript>

The structural and magnetic properties of the mesoporous systems based on silicon dioxide with a regular hexagonal arrangement of pores several microns in length and several nanometers in diameter, which are filled with iron compound nanofilaments in various chemical states, are studied in detail. The studies are performed using the following mutually complementary methods: transmission electron microscopy, SQUID magnetometry, electron spin resonance, Mössbauer spectroscopy, polarized neutron small-angle diffraction, and synchrotron radiation diffraction. It is shown that the iron nanoparticles in pores are mainly in the γ phase of Fe₂O₃ with a small addition of the α phase and atomic iron clusters. The effective magnetic field acting on a nanofilament from other nanofilaments is 11 mT and has a dipole nature, the ferromagnetic–paramagnetic transition temperature is in the range 76–94 K depending on the annealing temperature of the samples, and the temperature that corresponds to the change in the magnetic state of the iron oxide nanofilaments is T ≈ 50–60 K at H = 0 and T ≈ 80 K at H = 300 mT. It is also shown that the magnetization reversal of an array of nanofilaments is caused by the magnetostatic interaction between nanofilaments at the fields that are lower than the saturation field.