S. Juszczyk

Transition from helimagnetism to ferromagnetism in CuxZn1-xCr2Se4

Abstract We present detailed results concerning temperature and field dependences of magnetization, paramagnetic susceptibility, magnetic anisotropy and neutron diffraction for CuxZn1-xCr2Se4 spinels. Particular emphasis is put on the gradual transition from helimagnetism to ferromagnetism as a function of concentration x. The transition takes place through an intermediate structure for 0.05 ≤ x ≤ 0.8 with the ferromagnetic component growing with concentration. It is suggested that a double exchange mechanism predominates as x increases and this can be regarded as the origin of the transition.

Antiferromagnetism of Zn1−xGa2x3Cr2Se4 (for 0.0⩽x⩽0.5)+

Abstract The results of extensive studies of magnetic properties for the spinels Zn1−xGa 2x 3 Cr2Se4 are summarized. They concern the isotherms of static magnetization in applied induction up to 14 T, the magnetization and its time derivative curves in pulsed fields up to 42 T, the Neel temperatures and the high-temperature susceptibility, all in the concentration range 0.0 ⩽x ⩽ 0.5. It turns out that the magnetic properties of these spinels can be explained only if we assume the existence of Cr ions in two valence states: Cr3+ and Cr2+, and the average ratio Ga:Cr2+ isabout 1:1. A hypothesis on a modification of the simple spin spiral produced by an increase of Ga composition and by a strong magnetic field is described.

Magnetic phase transitions by strong, pulsed magnetic fields at low temperatures in Cu x Zn1−x Cr2Se4

Abstract The spinels, Cu x Zn1−x Cr2Se4) with x = 0.01, 0.02, 0.025, 0.05, 0.07 have been obtained. For these spinels, as well as for x = 0.0 and 0.1, magnetization measurements were carried out using an induction method in strong, pulsed magnetic fields, in a temperature range of 2.8 K to 30 K. Three magnetic phase transitions were observed. A hypothesis on the magnetic structure of the four phases separated by the three transitions is put forward.

Helimagnetism of CuxZn1−xCr2Se4 (for 0.0 ≤ × ≤ 0.1)

Abstract The magnetization, the susceptibility and the magnetic anisotropy field of Cu x Zn1−xCr2Se4 compounds have been studied at low temperatures (down to 2.9 K) in: high magnetic stationary fields (up to 14 T), high pulsed magnetic fields (up to 25 T), medium magnetic stationary fields (up to 0.6 T). The magnetic structure of these spinels was studied by neutron powder diffraction. The magnetic properties of CuxZn1−xCr2Se4 are explained in terms of the molecular field approximation assuming the existence of 90° exchange interactions, ferromagnetic for Cr-Se-Cr between the nearest Cr ions and antiferromagnetic for Cr-Se-Se-Cr between the second-nearest Cr ions. The exchange parameters and integrals for the whole series under consideration are calculated. Taking into account the three magnetic phase transitions observed in these spinels (Juszczyk, Krok, Okonska-Kozlowska, Broda, Warczewski, Byszewski, 1981) and the neutron diffraction studies a modification of the simple spin spiral forced by a strong m...

Exchange in noncollinear Zn1−xCuxCr2Se4

Abstract The exchange interactions in the noncollinear spinel system Zn 1- x Cu x Cr 2 Se 4 have been explained on the basis of the molecular field theory, after taking into account superexchange, double exchange as well as biquadratic exchange. The data of the low-temperature susceptibility, the saturation field, the Curie and the paramagnetic Curie-Weiss temperatures, and the spin configuration angles (φ, α) as a function of copper concentration have been used in order to determine the effective exchange constants. From the Anderson-Kanamori theory of superexchange, the transfer integrals and from the molecular orbital model, the admixture and the covalent-mixing parameters as well as the electron spin transfer coefficients have been estimated. These parameters are found to be consistent with the conventional model of covalency effects and they show good quantitative agreement with much of the available experimental data.

Critical fields in Zn1−xCuxCr2Se4 and Zn1−xGasol|2x/3Cr2Se4 spinel systems

Abstract The magnetization processes in two helical spinel systems having an anisotropy energy of axial symmetry have been studied. From a comparison of both the exchange and anisotropy energy it appears, that the latter is weak and may be treated as a perturbation. Since only the exchange energy between Cr n + ions causes the appearance of the cone structure in Zn 1− x Cu x Cr 2 Se 4 for 0.05 ⩽ x ⩽ 0.7 and the anisotropy energy will stabilize only the c -axis in simple spiral and conical structures and fix the rotation plane of the magnetic moment vectors. From the magnetization process in a simple spiral structure (in the case Zn 1− x Cu x Cr 2 Se 4 for 0.0 ⩽ x ⩽ 0.025 and Zn 1− x Ga 2 x /3 Cr 2 Se 4 for 0.0 ⩽ x ⩽ 0.5) it seems that there are three transition fields separating four structures, namely, simple spiral, cone, sinusoidal oscillation (fan) and parallel alignment. In comparison similar studies of the conical structure, which appears in Zn 1− x Cu x Cr 2 Se 4 for 0.05 ⩽ x ⩽ 0. 7, indicate that the observed transitions concern the changes from cone, through the alignment in the meridian plane, then oscillation in this plane up to parallel alignment. All the transition fields between these structures are of second kind. The values of the transition fields obtained experimentally have been checked with the use of the theory of Nagamiya which describes the magnetization process of a screw spin system.

Electronic properties of certain chromium spinels

Abstract Some electronic properties of the following spinel series:Zn 1-x Cu x Cr 2 Se 4 ,Zn 1-x Ga 2x 3 Cr 2 Se 4 and Co 0.55 Cu 0 . 45Cr 2 S 4-y Se y are presented. Using Slaters ideas on non-integer occupation numbers, the numbers of 3d holes (as teh carriers of magnetism in these spinels) per Cr atom with spins up and down have been obtained. From the condition of self-consistency for the magnetic moment the number of occupied states at the Fermi level with given spin projection and the Pauli spin paramagnetic susceptibility have been calculated and compared with other terms of the total high field susceptibility. According to the Stoner-Edwards-Wohlfarth theory a set of important exchange and band parameters of these compounds was calculated for the first time.

The valence of Cu and Cr ions in Zn1−xCuxCr2Se4 spinel system

Abstract The results of extensive experimental and theoretical studies of magnetic properties for CuCr 2 X 4 (X = S, Se) spinel compounds are summarized. They concern the magnetometric, neutron and X-ray diffraction, X-ray photoelectron spectra and nuclear magnetic resonance studies, made in the different ranges of temperature. All the experimental data were interpreted within the framework of an ionic model (however, the chemical bonds between anions and cations have there the mixed ionic-covalent character, the ionic contribution is predominant). This work demonstrates the temperature dependence of the valence of the Cr and Cu ions in CuCr 2 Se 4 compound. It has been proved that the Lotgering and the Goodenough models, assuming a valence stability of these ions, fail. Evidence for the temperature dependence of the valence for these cations has been presented also in the case of the Zn 1− x Cu x Cr 2 Se 4 solid solutions. These systems may be classified as “inter-atomic” intermediate valent to be distinguished from valence fluctuating “intra-atomic” mixed valent systems.

Superexchange in helical Zn1−xGa2x3Cr2Se4

Abstract The exchange interactions in insulating, helical spinels have been explained in the basis of the Anderson-Kanamori theory of superexchange. The value of the exchange constants JBB in the first three spheres of cordinatios between the Crn+ ions via Se2- ligands have been estimated after taking into account the experimental data of the saturation field, the paramagnetic Curie-Weiss temperature and the assumption that the spin spiral angle increases linearly with increasing of Ga concentration. From the observed values of the exchange constants the trnsfer and direct integrals have been obtained which on further analysis yield the values of the covalency parameters. These parameters are found to be consistent with the chemical theory of covalency.

High field susceptibility of Zn1−xCuxCr2Se4☆

Abstract We present the results concerning field dependences of magnetization for Zn 1 - x Cu x Cr 2 Se 4 mixed spinels studied at low temperatures with the use of both high magnetic stationary fields up to 14 T and high pulsed fields up to 25 T. From the isotherms of static magnetization we have obtained the incremental high field susceptibility and from pulsed measurents the differential one. The temperature and concentration ( x ) dependences of the calculated high field susceptibility χ hf are interpreted based on the magnetic properties of the spinels under study presented in refs. [4–7]. The greatest changes of χ hf were observed in the intermediate concentration range exhibiting a conical spin structure, in which the appearance of the double-exchange interaction between Cr 3+ and Cr 4+ ions plays a main role. It drives the transition from a noncollinear magnetic structure to a ferromagnetic one with increasing copper concentration.