The current status and prospects of research on ferromagnetism in semiconductors are reviewed The question of the origin of ferromagnetism in europium chalcogenides, chromium spinels and, particularly, in diluted magnetic semiconductors is addressed The nature of electronic states derived from 3d of magnetic impurities is discussed in some details Results of a quantitative comparison between experimental and theoretical results, notably for Mn-based III-V and II-VI compounds, are presented This comparison demonstrates that the current theory of the exchange interactions mediated by holes in the valence band describes correctly the values of Curie temperatures T_C magnetic anisotropy, domain structure, and magnetic circular dichroism On this basis, chemical trends are examined and show to lead to the prediction of semiconductor systems with T_C that may exceed room temperature, an expectation that are being confirmed by recent findings Results for materials containing magnetic ions other than Mn are also presented emphasizing that the double exchange involving hoping through d states may operate in those systems

Ferromagnetic semiconductors

A series of copper-doped zinc oxide films were grown by pulsed-laser ablation. Films grown under conditions that produced n-type ZnO were nonmagnetic while those grown under conditions that produced p-type were ferromagnetic with a Curie temperature above 350 K. The magnetic moment per copper atom decreased as the copper concentration increased. An explanation for this result is proposed based on the distance between nearest-neighbor copper atoms.

Room-temperature ferromagnetism in Cu-doped ZnO thin films

Ferromagnetic insulators that exhibit strong ferromagnetism at the atomic level are believed to be suitable for magnetic dielectric barriers in spintronic devices and solid-state qubits in quantum computing. Here a giant magnetic moment of $6.1{\ensuremath{\mu}}_{B}∕\mathrm{Co}$ and a high Curie temperature ${T}_{C}$ of $790\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ are observed in $(4\phantom{\rule{0.3em}{0ex}}\mathrm{at}.\phantom{\rule{0.2em}{0ex}}%)$ Co-doped $\mathrm{ZnO}$ films, which is not carrier mediated, but co-exists with the dielectric state. Direct current reactive magnetron co-sputtering is used to grow ${\mathrm{Zn}}_{0.96}{\mathrm{Co}}_{0.04}\mathrm{O}$ dilute magnetic insulator on ${\mathrm{LiNbO}}_{3}$ (104) substrates at considerably low growth temperature $(\ensuremath{\sim}200\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$, which is significant for complementary metal oxide semiconductor technology. X-ray photoelectron spectroscopy and x-ray-absorption spectroscopy reveal a solid solution of cobalt in $\mathrm{ZnO}$, where Co is in the $2+$ state substituting for Zn. A supercoupling mechanism in terms of bound magnetic polarons is proposed to discuss the ferromagnetism in the dielectric ground state of $\mathrm{Co}:\mathrm{ZnO}$, which would lead to different consideration for the origin of giant magnetic moment and high-temperature ferromagnetism in transition-metal doped oxides.

Giant magnetic moment in an anomalous ferromagnetic insulator: Co-doped ZnO

We report the synthesis of nominal 2 and 5 at.% Mn-doped ZnO nanocrystalline particles by a co-precipitation method. Rietveld refinement of x-ray diffraction data revealed that Mn-doped ZnO crystallizes in the monophasic wurtzite structure and the unit cell volume increases with increasing Mn concentration. DC magnetization measurements showed ferromagnetic ordering above room temperature with Hc~150 Oe for nominal 2 at.% Mn-doped ZnO nanoparticles annealed at 675 K. A distinct ferromagnetic resonance (FMR) signal was observed in the EPR spectra of the 2 at.% Mn-doped ZnO nanoparticles annealed at 675 K. EPR measurements were used to estimate the number of spins participating in ferromagnetic ordering. Of the total Mn present in the 2 at.% Mn ZnO lattice, 25% of the Mn2+ ions were responsible for ferromagnetic ordering, whereas nearly 5% of the Mn2+ ions remained uncoupled (isolated spins). A well resolved EPR spectrum of 5% Mn-doped ZnO samples annealed at 875–1275 K (g = 2.007, A = 80 G, D = 210 G and E = 15 G) confirmed that Mn was substitutionally incorporated into the ZnO lattice as Mn2+. On increasing the temperature of annealing beyond 1075 K an impurity phase emerges in both the 2 and 5 at.% Mn-doped ZnO samples, which has been identified as a variant of (Zn1−XMn(II)X)Mn(III)2O4 with Tc~15 K. Our results indicate that the observed room temperature ferromagnetism in Mn-doped ZnO can be attributed to the substitutional incorporation of Mn at Zn-sites rather than due to the formation of any metastable secondary phases.

https://iopscience.iop.org/article/10.1088/0957-4484/17/5/020/pdf

Magnetism in Mn-doped ZnO nanoparticles prepared by a co-precipitation method

For the over-lithiated-oxides (OLOs), a composite of layered Li2MnO3 and LiMO2 (M = Mn, Co, Ni), the Li2MnO3 part is not stable after the 1st charge–discharge cycle and partly transforms into layered LiMnO2, which in practice indicates that the phase used is actually a mixture of both Li2MnO3 and LiMnO2. In the present work, the influence of 10 cationic (Mg, Ti, V, Nb, Fe, Ru, Co, Ni, Cu, and Al) and 2 anionic (N and F) dopants on the phase stability, redox potential, ionic and electronic conductivity of both Li2MnO3 and LiMnO2 is investigated in detail using density functional theory. The calculations show that all the cationic dopants and F can be thermodynamically stable in the layered structures. The redox potential of both oxides is quite sensitive to some of the dopants, like V, Nb, and Ru, due to the appearance of gap states introduced by those dopants. The Jahn–Teller effect has a strong influence on the Li vacancy diffusion behavior in both LiMnO2 and its doped phases. Li vacancy diffusion behavior in Li2MnO3, including both interlayer and intralayer pathways, is relatively more complex and some dopants like Mg, Ti, Nb, and Ru can decrease the barriers of the diffusion paths. The calculations also show the evidence of hole polaron formation in LiMnO2 and electron polaron formation in Li2MnO3 which should be the reason why these phases have low electronic conductivities. Based on these findings, possible ways to improve the electronic conductivity through the doping process are discussed.

Ab initio study of doping effects on LiMnO2 and Li2MnO3 cathode materials for Li-ion batteries

The bound magnetic polaron (BMP) is the characteristic collective state of diluted magnetic semiconductors. Isolated BMP are well understood, but their interactions are only beginning to be explored. Recent polaron magnetization experiments on p‐ZnMnTe suggest a ferromagnetic polaron‐polaron interaction, in contrast to the invariably antiferromagnetic impurity exchange interaction in conventional semiconductors. To investigate this question theoretically, we have developed a simplified model of polaron pairs whose central feature is competition between the usual, antiferromagnetic, virtual‐hopping interaction, and the loss of carrier‐magnetic ion exchange energy, by intermediate ions, when the polaron moments are antiferromagnetically aligned. The model is sufficiently simple that its partition function can be calculated in detail. With reasonable parameters, it predicts a ferromagnetic polaron‐polaron interaction at low temperatures.

P. A. Wolff

Papers

Polaron‐polaron interactions in diluted magnetic semiconductors