V. V. Snegirev

Magnetic method of magnetocaloric effect determination in high pulsed magnetic fields

Abstract A contactless method of magnetocaloric effect measurements is described, based on the comparison of magnetisation curves obtained in adiabatic and isothermal processes. By this method the magnetocaloric effect of paramagnetic garnet Gd3Ga5O12 is studied in pulsed magnetic fields up to 400 kOe.

Influence of the d-electron concentration on the itinerant electron metamagnetism and ferromagnetism in M(Co1-xAlx)2 systems (M = Y, Lu): study of compounds with unvariable crystal cell parameter

Abstract Magnetic properties of the (Y 1- t Lu t )(Co 1- x Al x ) 2 system in which the crystal cell parameter was kept constant by partial substitution of lutetium for yttrium have been investigated in dependence of the aluminum concentration within the 0 ⩽ x ⩽ 0.22 interval. The concentration dependence of the critical field of the metamagnetic transition and the character of arising of ferromagnetic order are analyzed. The magnetic ( x - T ) phase diagram of (Y 1- t Lu t )(Co 1- x Al x ) 2 system is obtained. The comparison with analogous results for Y(Co 1- x Al x ) 2 and Lu(Co 1- x Al x ) 2 systems, in which the itinerant electron metamagnetism have been observed earlier, leads to the conclusion that the main factor acting on the transformation of magnetic properties of YCo 2 and LuCo 2 when aluminum replaces cobalt is the change of d-electron concentration but not the increase of the crystal cell parameter.

Two magnetic transitions and metamagnetism in the R2BaCuO5 (R = Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb) compounds

Specific heat, ac magnetic susceptibility and magnetization in static and in pulse fields were measured for polycrystalline “green phase” R 2 BaCuO 5 (R = Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb) samples. Two magnetic phase transitions were found in these compounds at temperatures T N1 and T N2 . An assumption is made that the antiferromagnetic ordering of copper and rare earth subsystem occurs at T N1 and T N2 , respectively. The Dy, Ho and Er compounds have been shown to be metamagnetic at temperatures below T N2 .

Magnetic phase transition and the corresponding magnetostriction of intermetallic compounds RMn2Ge2 (R=Sm, Gd)

The temperature dependence of lattice constants a and c of intermetallic compounds RMn2Ge2 (R = Sm, Gd) is measured in the temperature range 10-800K by using the x-ray diffraction method. The magnetoelastic anomalies of lattice constants are found at the different kinds of spontaneous magnetic transitions. The transversal and longitudinal magnetostrictions of polycrystalline samples are measured in the pulse magnetic field up to 25T. In the external magnetic field there occurs a first-order field-induced antiferromagnetism-ferromagnetism transition in the Mn sublattice, which gives rise to a large magnetostriction. The magnitude of magnetostrictions is as large as 10-3. The transversal and longitudinal magnetostrictions have the same sign and are almost equal. This indicates that the magnetostriction is isotropic and mainly caused by the interlayer Mn-Mn exchange interaction. The experimental results are explained in the framework of a two-sublattice ferrimagnet with the negative exchange interaction in one of the sublattices by taking into account the lattice constant dependence of interlayer Mn-Mn exchange interaction.

Study of ferrimagnets with negative interaction within one of the sublattices: magnetic phase diagram of Gd1−xLaxMn2Ge2 intermetallic compounds

Abstract The magnetic properties of the intermetallic compounds Gd1−xLaxMn2Ge2 with layered structure (the ThCr2Si2-type) are discussed, based on the results, obtained in Sokolov et al. (Solid State Commun 105 (1998) 289) and the present work. It is shown that the temperature dependences of magnetization of these compounds and their T–x magnetic phase diagram can be quantitatively described within the framework of the Yafet–Kittel model for the ferrimagnets with negative interaction in one of the sublattices. The exchange parameters, derived in Sokolov et al. (JETP 86 (1999) 733) from the study of the Gd1−xYxMn2Ge2 system, are used for the theoretical calculations by taking into account the distance dependence of the interlayer Mn–Mn exchange parameter.

Intersublattice f-d exchange interaction in the intermetallic Laves phase compounds Y1−tGdt(Co1−xAlx)2

Abstract Systems Y 1− t ( Co 1− x Al x ) 2 with x = 0.05, 0.07, 0.085, 0.105 and 0 ≤ t ≤ 0.2 are studied to ascertain the influence of the intersublattice f-d exchange interaction on the magnetic behaviour of itinerant metamagnets Y ( Co 1− x Al x ) 2 . The field of the metamagnetic transition decreases when the Gd-concentration is increased for the compounds with x ≥ 0.07, and then a ferrimagnetic state is stable for more Gd-substitutions. Results obtained are explained in the molecular field approximation taking into consideration the itinerant metamagnetism of the d-electron subsystem. Intersublattice molecular field coefficient λ fd is determined from both Gd-concentration dependence on the critical field of the metamagnetic transition and values of the critical field of the rise of a non-collinear magnetic structure in high external magnetic fields. The same value of λ fd = 640 kOe / μ B f . u . is obtained by different methods.

Anomalies in the Young modulus at structural phase transitions in rare-earth cobaltites RBaCo4O7 (R = Y, Tm-Lu)

The elastic properties of rare-earth cobaltites RBaCo4O7 (R = Y, Tm-Lu) have been experimentally studied in the temperature range of 80–300 K. The strong softening of the Young modulus ΔE(T)/E0 ≈ −(0.1–0.2) of cobaltites with Lu and Yb ions has been revealed, which is due to the instability of the crystal structure upon cooling and is accompanied by an inverse jump at the second-order structural phase transition. The softening of the Young modulus and the jump at the phase transition decrease by an order of magnitude and the transition temperature Ts and hysteresis ΔTs increase from a compound with Lu to that with Tm. A large softening of the Young modulus at the structural transition in Lu- and Yb cobaltites indicates that the corresponding elastic constant goes to zero, whereas this constant in Tm cobaltite is not a “soft” mode of the phase transition. It has been found that the structural phase transition in Lu- and Yb cobaltites is accompanied by a large absorption maximum at the phase transition point and an additional maximum in the low-temperature phase and absorption anomalies in Tm cobaltite is an order of magnitude smaller.

Magnetostriction of SmMn2Ge2 and GdMn2Ge2 intermetallic compounds

Longitudinal and transverse magnetostrictions of polycrystalline samples of intermetallic compounds RMn2Ge2 (R=Sm or Gd) are measured in pulsed magnetic fields up to 250 kOe. It is found that linear magnetostrictive strains of about 10−3 arise in a temperature range in which the magnetic field causes a change in the magnetic state of a manganese magnetic subsystem. The results obtained are described within the model of a two-sublattice ferrimagnet with a negative exchange interaction in the manganese subsystem in terms of a strong dependence of this interaction on interatomic distances.

f-d exchange interaction in the RCo2-type intermetallic compounds with heavy rare earths

Abstract Non-collinear magnetic structures were experimentally observed in the Al-substituted (R t Y 1− t )(Co, Al) 2 compounds by magnetization measurements in fields up to 27 T and molecular field coefficients of the f-d exchange interaction, λ fd , were determined. The f-d exchange integrals are calculated.

Transformation from a weakly to strongly ferromagnetic ground state in the Y1−tLut(Co0.88Al0.12)2 system

Abstract Magnetic characteristics of the Y1−tLut(Co0.88Al0.12)2 intermetallic system were studied. The ground state was found to change from a weakly (in Y-rich compounds) to strongly (in Lu-rich compounds) ferromagnetic within the narrow concentration range in the vicinity of t = 0.4. It is supposed that small changes of the density of d-state characteristics near the Fermi level are able to change the ground state, thus providing the abrupt change in the magnetic behaviour of the compounds by substituting Y for Lu. The anomalous Tc(t) dependence of the TmtLu1−t(Co0.88Al0.12)2 system is discussed assuming that the d-subsystem of the R(Co0.88Al0.12)2 compounds with all heavy rare earths, in which it is magnetized by the f-d exchange interaction, is similar to that of Y(Co0.88Al0.12)2.