Magnetic Phase Transitions in Substituted Spinels of Zn₁−x Cu_χCr₂Se₄; 0.0 ≤ x ≤ 0.3

Correlation between the magnetic phase transition and the electric properties in substituted spinels Zn1−xCuxCr2Se4

“…More recently, the pre-edge peaks of the X-ray absorption spectra near the Cr -K edge clearly indicated that there is the mixedvalence configuration of Cr 3+ and Cr 4+ in Zn 1-x Cu x Cr 2 Se 4 with x ≥ 0.50 at room temperature [18]. From the present investigations and the results recently obtained by Hidaka et al [14,15,18], we proposed a schematic diagram of the magnetic and electric phases in the substituted spinels of Zn 1-x Cu x Cr 2 Se 4 , as shown in Fig. 8.…”

“…The results suggested that the antiferromagnetic phases of Zn 1-x Cu x Cr 2 Se 4 occur only in the semiconducting phase (0.0 < x < 0.75) and the insulating phase (x = 0.0), in which the super- exchange interaction between localized Cr 3+ and Cr 3+ spins appears. Thus, we suspected that the antiferromagnetic to ferromagnetic phase transition in Zn 1-x Cu x Cr 2 Se 4 with 0.15 ≤ x < 0.75 occurring at about 24 K to 28 K is induced by the disappearance of the superexchange interaction, and by the predominance of the double-exchange interaction between localized Cr 3+ spin and conductive Cr 4+ spin-polaron in the Se 2-valence band [14,15]. In the high concentration of Cu ions with x ≥ 0.75, the superexchange interaction is perfectly destroyed by conductive Cr 4+ electrons, which strongly enhance the double-exchange interaction.…”

“…2. The neutron powder diffraction also suggested that there was the spin-glass semiconducting phase (SGS) with 0.0 < x < 0.15, which arises from the short-range order of Cr 3+ spins [14]. We are now in progress to measure the electric resistivity under an applied magnetic field in the metal phases and the semiconducting metal phases of Zn 1-x Cu x Cr 2 Se 4 , in order to study a correlation between the magnetic and electrical properties depending on the concentration and the mobility of the conductive Cr 4+ electrons.…”

“…It was found that there is a change of the electrical property from semiconducting to metallic behavior at about 60 K. Such an electrical phase transition was not observed by Groń et al [13]. On the other hand, Hidaka et al [14] reported that Zn 0.7 Cu 0.3 Cr 2 Se 4 showed the antiferromagnetic phase transition occurring at about 24 K. Thus, the 60 K electric phase transition arises only from the change in the electrical property. It is usually considered that the electric transition and the kink of the resistivity correlate with an amount and/or a mobility of positive carriers, holes, which are produced by the substitution of Zn ions with Cu ions in Zn 1-x Cu x Cr 2 Se 4 .…”

“…For example, Zn 1-x Cu x Cr 2 Se 4 with 0.0 < x < 0.10 are semiconducting antiferromagnets having a spiral configuration of Cr 3+ spins below about 22 K, ones with 0.10 < x < 0.20 semiconducting spin-glasses, and ones with 0.20 < x < 0.80 metallic ferromagnets having a conical spin configuration below about 380 K to 420 K, and ones with x ≥ 0.8 metallic ferromagnets having a uniaxial spin configuration below about 420 K ( [9][10][11][12][13] and references therein). More recently, it was found by neutron diffraction and magnetic susceptibility that Zn 1-x Cu x Cr 2 Se 4 with 0.15 ≤ x < 0.75 have the antiferromagnetic phase transition at about 24 K to 28 K on heating, in addition to the ferromagnetic to paramagnetic transition at about 380 K to 400 K [14,15]. Thus, the magnetic and electrical properties reported until now by many experimental and theoretical investigations suggest that Zn 1-x Cu x Cr 2 Se 4 compounds are a strongly correlated 3d-electronic system.…”