The results of the investigation of the paraprocess in various ferro- and ferrimagnetics carried out at the Department of General Physics for Natural Science Faculties and the Laboratory for Problems of Magnetism of Moscow University are summed up. Specific features of the paraprocess and the attendant effects (the magnetocaloric effect, magnetostriction, and magnetoresistance) in ferrimagnetics as compared to ferromagnetics are revealed. The character of the paraprocess and the attendant effects in antiferromagnetics are discussed. Examples are given to illustrate the application of the data obtained for solving a number of problems in the physics of magnetoordered substances.
Show AbstractThe compositions $Ga_{2/3}Cr_2S_4, Cu_{1/2}Me_{1/2}Cr_2Se_4$ ($Me=In, Ga$), $Cu_{2/3}Ge_{1/3}Cr_2S_4$, and solid solutions of the type $xCuCr_2S_4—(1—x)Ga_{2/3}Cr_2S_4$ ($0<x<0,4$) and $xCuCr_2Se_4—(1—x)Cu_{0,5}Me_{0,5}Cr_2Se_4$ ($Me = In, Ga$; $0<x<0,1$) are shown to possess magnetic properties that are typical of spin glasses. These features include (i) a maximum in the initial susceptibility (at a definite temperature $Т_f’$, (ii) the dependence of low-temperature magnetic properties of a sample on its thermomagnetic prehistory and on time, and (iii) the lack of spontaneous magnetization (as established by the Belov-GoryagaArrott method). There are several experimental facts suggesting the existence of a spin glass-paramagnetic phase transition: (i) the validity of several static and dynamic scaling relationships and of the Almeida-Thouless condition and (ii) the presence of a giant maximum of negative magnetoresistance.
Show AbstractIt is shown that Fe-Ti oxides of rocks undergo complex evolution in the course of geologic time which affects their magnetic properties. In some cases there is a nonlinear relationship between natural remanent magnetization and geomagnetic field $H_g$ resulting from strong internal interaction in multiphase ferrimagnetic minerals, including those that determine the self-reversal of their magnetization. Consideration for nonlinear magnetization processes in rocks is required to increase the accuracy of paleomagnetic data.
Show AbstractMagnetization and magnetic susceptibility under applied hydrostatic pressures of up to 10 kbar were studied for R-Fe amorphous alloys and crystalline compounds, where R is a heavy rare-earth metal. The results are discussed in terms of the molecular field theory. The difference in exchange integral versus applied pressure relationships between R-Fe amorphous alloys and crystalline compounds are explained by the difference in the short-range order. Hydrostatic pressure induces a noncollinear magnetic structure in $Y_2Fe_{17}$ and $Er_2Fe_{17}$ crystals and transforms two paramagnetism-asperomagnetism-reversible spin-glass phase transitions into a single paramagnetism-spin-glass phase transition in the amorphous $Y_{19}Fe_{81}$ alloy. This is explained by a reversal of the ratio between the positive and negative exchange interactions within the iron subsystem.
Show AbstractMagnetic and magnetoelastic properties of a series of rare-earth zircons exhibiting the cooperative Jahn-Teller (JT) effect have been systematically studied. Typical anomalies of magnetic susceptibility, magnetization, and magnetic anisotropy in the basal plane have been observed, which are caused by the JT interaction and the cooperative JT effect. It is shown that the magnetoelastic properties of rare-earth zircons become much more pronounced due to JT correlations. The behavior of these characteristics in crystals differs qualitatively depending on whether the JT correlations are suppressed or enhanced by the magnetic field.
Show AbstractMagnetization, magnetostriction, rotation moments, thermal expansion, Young's modulus, and internal friction were measured in rare-earth orthoferrites (orthochromites) to study spontaneous and induced magnetic phase transitions in them. The measurements were conducted in fields not exceeding 60 kOe, in the 1.7-140 K temperature range. New orientation phase transitions are found, experimental phase (H, T)-diagrams are constructed, and microscopic mechanisms of the phase transitions are discussed.
Show AbstractThe effect of a magnetic field H<110 kOe on the electric polarization of single-crystal samples of metastable ferroelectric rare-earth metal molybdates $Тb_1(МоO_4)_3, Gd_2(MoO_4)_3$, and $TbGd(MoO_4 )_3$ was studied at 78 K. The experiments showed that at 78 K the applied magnetic field changes the electric polarization and the ferroelectric domain structure of $TbGd(MoO_4)_3$ and $Tb_2(MoO_4)_3$. Effects of two types were observed: a) a magnetic field-induced deflection of the spontaneous polarization vector from its equilibrium orientation (rotation processes), and b) a field-induced nucleation and growth of domains with opposite electric polarization directions in initially single-domain samples (displacement processes). It is most likely that the observed effects are related to the properties of rare-earth metal ions.
Show AbstractThe results of a comprehensive investigation of magnetic and electric properties of $LaMnO_3$-based magnetic semiconductors with Mn ions replaced by ions of 3d metals (Fe, Co, and Ni) are presented. Also discussed are the available literature data on manganites with variously replaced La and Mn ions. The conclusion is drawn that the manganites studied are magnetic semiconductors with a narrow forbidden zone, whose magnitude is comparable with that of the exchange interaction energy. It is shown that the effect of the exchange energy on the structure of energy bands of current carriers results in (a) a correlation of the magnetic and electric properties, (b) anomalous behavior of kinetic effects, (c) a metal-semiconductor transition in the ferromagnetic region at a given temperature upon variation of the composition, and (d) a metal-semiconductor transition in the region of the Curie point.
Show AbstractThe data obtained in the studies on the temperature dependences of magnetization, magnetostriction, thermal expansion, magnetic anisotropy constant, and magnetoresistance for the $CoFe_2O_4$ and $MnFe_2O_4$ spinel ferrites are presented. The curves display anomalous behavior (maxima, inflections, etc.) below the Curie temperatures. The data suggest that ferrites with the spine! structure have several low-temperature structural and magnetic phase transitions. By low-temperature phase transitions we mean phase transitions at temperatures below the Curie point.
Show AbstractShifts of hyperfine fields are measured by the method of spin echo on the nuclei of gadolinium and yttrium in quasi-binary and ternary Laves phases containing impurity atoms of terbium and samarium. The analysis of the results obtained suggests the presence of negative 5d polarization in the neighborhood of gadolinium atoms. A new mechanism of intersublattice exchange, which is a version of hybrid exchange, is proposed.
Show AbstractA great body of experimental data on the magnetic and anisotropic properties of yttrium iron garnets doped with $Ce^{3+}$, $Sm^{3+}$, $Nd^{3+}$, and $Tm^{3+}$ ions have been considered. The data obtained are compared with the available theoretical concepts concerning anisotropy and magnetostriction of ferrimagnetics. The possible nature of magnetic anisotropy in these compounds is suggested. It is shown that for all the ferrites studied, the magnetic anisotropy at small concentrations of rare-earth ions can be explained within the framework of the single-ion model, whereas at high concentrations the two-ion model must be used because the anisotropic exchange interaction between pairs of magnetic ions must be taken into account.
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