P.E. Brommer

Metamagnetism, giant magnetoresistance and magnetocaloric effects in RCo2-based compounds in the vicinity of the Curie temperature

Abstract Magnetisation and magnetoresistance isotherms were measured for a number of (R,R′)Co2, (R,Y)Co2 and R(Co,Si)2 (R,R′=rare earth) compounds. A metamagnetic transition is observed just above the Curie temperature (TC) of compounds having a first order phase transition, i.e. ErCo2-, HoCo2-, and DyCo2-based ones. Both 4f- and 3d-sublattice magnetic moments contribute to a sharp change of the magnetisation at this transition. The concurring suppression of the magnetoresistance can be considered to be due to quenching of spin fluctuations. In addition, the magnetic entropy change ΔSm is estimated from the magnetisation data by using a Maxwell equation. The resulting giant magnetocaloric effects are discussed in terms of the 4f(R)-localised spin and the 3d(Co)-spin fluctuations as well as the nature of the phase transition.

The magnetic behaviour of rare-earth—transition metal compounds

Abstract The basic exchange interactions will be discussed for a variety of (R, T) compounds, in particular RT 2 (R: rare earth, or Y; T: transition metal). A magnetic phase diagram for (R, Y)Co 2 compounds is derived. The combined effect of the volume and the molecular field caused by the rare-earth spins is demonstrated on R(Co, Fe) 2 , (R, Y)Co 2 , R(Co, Al) 2 and R(Co, Cu) 2 , with an emphasis on the metamagnetic transition in the 3d-subsystem.

A generalization of the Inoue-Shimizu model

Abstract Inoue and Shimizu [1] give the free-energy expansion for a system consisting of a set of identical local (rare-earth) magnetic moments interacting with an itinerant electron subsystem (Co2). In this paper the free-energy expansion is given for a generalized system consisting of an arbitrary number of interacting magnetic contributions. This generalization is necessary for a discussion of the type of magnetic transition in quasi-binary RE-Co2 compounds such as TbxHo1−xCo2.

Electronic and magnetic properties of ErxY1-xCo2 compounds

Ferromagnetic compounds of the series ErxY1-xCo2 (x=1, 0.8, 0.7, 0.6) have been investigated in specific-heat, thermal-expansion, magnetisation, AC susceptibility and resistivity experiments. The magnetic moment on the Co atoms decreases with decreasing Er content, as does the Curie temperature. The character of the magnetic transition changes from first order to second order around x=0.7. This change is compared with the similar one observed in other RExY1-xCO2 compounds with RE-Gd, Tb, Dy and is discussed in terms of the Inoue-Shimizu model by taking into account the strong temperature dependence of the magnetic susceptibility of the 3d electrons. The spontaneous volume magnetostriction at zero temperature, the magnetic contribution to the specific heat and the magnetic entropy below the magnetic ordering temperature are determined. The experimental results for these quantities confirm the induced character of the cobalt magnetic moment in these compounds.

Magnetic phase transitions in (Nd, Dy)Co2 and (Pr, Dy)Co2 compounds

Abstract In a series of (Nd, Dy)Co 2 and (Pr, Dy)Co 2 compounds, the type of the magnetic phase transition was studied by magnetization and electrical-resistivity measurements. In both systems, a cchange of type was observed from second order (for NdCo 2 and PrCo 2 ) to first order (for compounds containing more than about 20% Dy). The results are discussed in the Inoue-Shimizu model, generalized in such a way that the presence of both light and heavy rare-earth atoms is taken into account. The volume dependence of the parameter a 3 (i.e. of the free-energy contribution a 3 M 2 Co /4) is supposed to play a decisive role in the determination of the order of the magnetic transition.

The magnetic phase transitions in (Tb, Ho)Co2 and (Tb, Y)Co2 compounds

Abstract The type of the magnetic phase transitions in a series of TbxHo1−xCo2 and TbxY1−xCo2 compounds is studied by means of magnetization, electrical-resistivity and specific-heat measurements. In both systems the TC values decrease with decreasing Tb content. At about x = 0.7 the type of the magnetic transition changes from second order to first order. An explanation is offered in terms of the Inoue-Shimizu model by taking into account the strong temperature dependence of the magnetic susceptibility of the 3d itinerant electrons.

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.

The magnetic properties of Ni3Al under high pressures

Abstract The magnetic parameters of the perfectly ordered Ni 3 Al compound are derived from magnetovolume experiments on ferromagnetic stoichiometric and non-stoichiometric compounds. These results are discussed in the Stoner model for itinerant electron magnetism. A comparison is made with the recent band structure calculations on Ni 3 Al by Hackenbracht and Kubler.

A study of magnetic coupling in GdT2Si2 compounds (T=transition metal)

Abstract The magnetic coupling in the GdT 2 Si 2 (T=transition metal) antiferromagnetic compounds is studied by means of DC susceptibility and high-field free-powder (HFFP) measurements. In most compounds, the experimental results can be described in the framework of a two-sublattice model without anisotropy. From this analysis, an intrasublattice exchange coupling constant, n 11 , and an intersublattice exchange coupling constant, n 12 , are obtained. The exchange couplings are discussed in terms of the change of the lattice parameters, the volume V and the filling of the transition metal electron band. The apparently reduced value of the moment per Gd atom in the forced ferromagnetic configuration is attributed to a reduction of the on-site 5d-moment and a possible negative polarisation of the other non-4f electrons.

A rule for counting neighbours in rare-earth transition metal compounds

Abstract In (R, T) compounds (R: rare earth; T: 3d transition metal) such as R 2 T 17 , R 2 T 14 B, and so on, the exchange interaction between a rare-earth spin (operator) S R and the neighbouring 3d spins can be written as -2 J RT z RT S R ·〈 S T 〉, where 〈 S T 〉 is a site average of the expectation value of the transition metal spins in the molecular field approximation. z RT is the number of nearest T neighbours of an R-atom. A more precise definition is given in the main text. Analogously, z TR , the average number of nearest R neighbours of a T atom can be defined. We stress that a consistent definition must lead to the relation z RT N R = z TR N T for a stoichiometric compound containing N R rare-earth atoms and N T transition metal atoms per formula unit. Appropriate definitions are discussed for more intricate cases, in particular for off-stoichiometric compounds.