A.S. Markosyan

Physical properties of RCo2 Laves phases

The large variety of magnetic phenomena observed in the Co based Laves phases are reviewed. Following the band structure calculations it is argued that the outstanding magnetic features of the RCo2 intermetallics are intimately related to the position of the Fermi level, which is near to a local peak in N(e). This is why the Co 3d-electron system reacts sensitively either to the molecular field of the R partner element or to the changes of external parameters such as a magnetic field or pressure. Magnetic, magnetoelastic and transport measurements of RCo2 compounds and related pseudobinaries such as R(Co1-xAlx)2 with R either magnetic or nonmagnetic rare earth element are shown and discussed. The conditions for the appearance of itinerant electron metamagnetism and spin fluctuations are outlined. In particular, the influences of spin fluctuations on physical properties, e.g. the susceptibility, thermal expansion and transport phenomena, are demonstrated.

Isotropic and anisotropic magnetoelastic interactions in heavy and light RCo2 Laves phase compounds

The thermal expansion and anisotropic magnetostriction of the RCo2 Laves phases were studied in the temperature range 4-500 K using the X-ray powder diffraction method. In the heavy RCo2 the magnetic moment of the itinerant d electron subsystem derived from the magnetovolume effect was found to fit well with the magnetization curve of YCo2. A pronounced paraprocess above the metamagnetic transition has been observed when increasing the f-d exchange field. The type and temperature variation of the distortion of the cubic unit cell of the three compounds PrCo2, NdCo2 and SmCo2 have been studied in detail. The corresponding magnetostriction constants lambda 111 or lambda 100 were calculated. At 4 K the following values have been obtained: PrCo2, tetragonal distortion, easy axes (100), lambda 100=-3.4*10-3: SmCo2, rhombohedral distortion, easy axes (111), lambda 111=-4.6*10-3; NdCo2, tetragonal distortion and easy axes (100) for 42 K<T<Tc, orthorhombic distortion and easy axes (110) for T<42 K with mod lambda 111 mod =1.9*10-3 and lambda 100=-4.6*10-3.

Mn moment instability in the TbMn2 intermetallic compound

Neutron diffraction experiments have been performed on a TbMn2 single crystal and on Tb(Mn0.96Fe0.04)2 powder samples. The magnetic structure of TbMn2 is metastable poised between two structures, S1 with propagation vector (2/3 2/3 0) and S2 with propagation vector (1/2 1/2 1/2). A transition from S1 to S2 can be induced either by an applied field of 4.5 T at 25 K or by chemical pressure induced by substitution of Mn by Fe. The S2 structure has been studied in Tb(Mn0.96Fe0.04)2. The transition to this structure is accompanied by a huge rhombohedral distortion and the structure itself is notable for the coexistence of magnetic and non-magnetic manganese atoms. This peculiar feature is attributed to instability of the Mn moment combined with frustration of the Mn itinerant antiferromagnetism.

The transport properties of RCo2 compounds

The transport properties of intermetallic compounds RCo2 (R=Pr, Nd, Sm, Gd-Lu, Sc and Y) in the temperature range from 4.2 up to 1000 K were studied and different contributions were determined. In the non-magnetic compounds the spin fluctuation contribution to the electrical resistivity, rho sf varies as (T/Tsf)2, and thermal resistivity, Wsf varies as (T/Tsf), were found to follow the theoretical predictions. The spin disorder contribution arising from the scattering of conduction electrons on 4f moments was deduced and found to be proportional to the de Gennes factor. It is shown that the spin fluctuations are responsible for the low-temperature features of the temperature-dependent thermopower, whereas the high-temperature behaviour is mainly determined by the density of states function. In the paramagnetic region near the Curie temperature, the critical fluctuations cause a considerable enhancement of the electrical resistivity of the magnetic RCo2 compounds, which is manifested by pronounced upturns in rho versus T dependences.

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.

Synthesis and magnetic properties of one-dimensional ferro- And ferrimagnetic chains made up of an alternating array of 1,3-bis(N-tert-butyl-N-oxyamino)benzene derivatives and Mn(II)(hfac) 2

Abstract Bis(hexafluoroacetylacetonato) manganese(II), Mn(hfac) 2 , reacts with the bisnitroxide radicals, 5- R -1,3-bis( N - tert -butyl- N -oxyamino)benzene ( 1 R ) (R=H, Cl and Br), yielding one-dimensional (1-D) polymeric complexes of formula [Mn(hfac) 2 1 R ] n . X-ray analysis of the complexes has shown that they crystallize in the monoclinic space group P 2 1 / n . In this structure, the manganese(II) ions and 1 R molecules make up one-dimensional chains with the bisnitroxide radical serving as a bidentate ligand to Mn(II)(hfac) 2 . The 1 H complex orders antiferromagnetically at 5.5 K, while the 1 Cl and 1 Br complexes show ferrimagnetic order at 4.8 and 5.3 K, respectively. The intrachain exchange interaction parameters for a model of S =3/2 ferromagnetic chains were found to be, 2 J eff / k =23±2 K in all the compounds. J eff is the effective magnetic exchange interaction between units of NOMnNO. A change in the sign of the interchain exchange interaction is referred to as the change of the shortest exchange path, from the MnFN( tert -Bu)O ( 1 Cl and 1 Br ) to N( tert -Bu)O FN( tert -Bu)O ( 1 H )

Study of the magnetization and magnetic anisotropy of the metal-radical complex of bis(hexafluoroacetylacetonato)manganese(II) with a trisnitroxide radical:

A single crystal of the metal-radical complex , where is a trisnitroxide with a quartet ground state, was grown. The magnetization was measured along the principal crystallographic axes in the range 1.8-300 K. The compound was found to order ferrimagnetically at with collinear antiparallel alignment of the Mn and magnetic spins along the c-direction. The paramagnetic susceptibility was treated in the quantum-classical approximation by taking into account the weak positive exchange interaction between the Mn( 2) ions and one-dimensional ferrimagnetic chains, in which trimer molecules composed of one Mn( 1) and two 1/2 spins of different triradicals can be isolated. The anisotropy constants were evaluated and the anisotropy energy was estimated. Anisotropy of the paramagnetic susceptibility, which can be detected up to 55 K, was observed. The anisotropic effects are attributed both to the single-ion splitting of the Mn energy levels and the dipole-dipole interaction between the magnetic spins.

Gd substitutions in the TmCo2 Laves phase: the onset of long-range magnetic order in the itinerant subsystem

We found that in TmCo2 the molecular field does not exceed the critical value necessary to induce long-range magnetic order in the Co d-electron subsystem, which is in contrast to the other heavy RCo2 compounds. Below Tc=3.8 K a first-order magnetic phase transition at 3.4 K exists, which is due to a rearrangement of the Tm 4f magnetic moments. When substituting Tm by Gd long-range magnetic order appears in the Co d subsystem. The onset of the magnetic order in the d-electron subsystem can most clearly be seen from thermal expansion measurements. There is an increasing positive volume anomaly with increasing Gd content below Tc. The estimated magnetostriction constant lambda 111 of TmCo2 is -4.1*10-3, in agreement with the single-ion model. Pronounced discontinuities at Tc are characteristic for the transport properties (resistivity and thermopower) for TmCo2 as well as for the pseudobinary Tm1-xGdxCo2 compounds. At elevated temperatures the physical properties of all these compounds are dominated by spin fluctuations.

The magnetic phase transitions in R(Co, Al)2 compounds (R: Dy, Ho, Er)

Abstract The type of the magnetic phase transition in the R(Co1-xAlx)2 (R=Dy, Ho, Er) compounds is studied by means of magnetization, thermal-expansion and electrical-resistivity measurements. With increasing Al content, a change from a first-order to a second-order transition is observed, at x≃0.075, 0.075 and 0.025 for Er, Ho and Dy, respectively. An explanation is offered in terms of the Inoue-Shimizu model by introducing a concentration dependence of T3, the temperature at which a3(T) changes sign (a3 is the coefficient of the M4d-term in the expansion of the free energy of the d-subsystem). In fact, T3 is taken proportional to Tm, the temperature at which the susceptibility of the corresponding Y(Co, Al)2 shows a maximum.

Magnetic instability of the Co sublattice in the Ho1−xYxCo3 system

Abstract X-ray thermal expansion, magnetic and electrical resistivity measurements of the pseudo-binary Ho 1− x Y x Co 3 system with PuNi 3 -type structure have been performed in a wide temperature range. Temperature-induced itinerant-electron metamagnetism has been observed in the concentration range 0.0⩽ x ⩽0.4 where the Co sublattice is in the strong ferromagnetic state at low temperatures. The transition was found to be sharper in the diluted compounds with a lower value of the transition temperature, which is in accordance with the nature of the observed effect. The temperatures of the spin-reorientation transition have been determined from low-field magnetisation measurements and the magnetic phase diagram of this system has been constructed.