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by Staff Writers
Beijing, China (SPX) Jul 11, 2014
Diluted magnetic semiconductors (DMS) have received much attention due to their potential application in spintronics, or the storage and transfer of information by using an electron's spin state, its magnetic moment and its charge.
In typical systems based on III-V semiconductors, such as (Ga,Mn)As, (In,Mn)As or (Ga,Mn)N, substitution of divalent Mn atoms into trivalent Ga (or In) sites leads to severely limited chemical solubility, resulting in metastable specimens only available as epitaxial thin films. The hetero-valence substitution, which simultaneously dopes both charges and spin, makes it difficult to individually control each quantum freedom.
Recently a group led by Professor Changqing Jin at the Institute of Physics, part of the Chinese Academy of Sciences, in Beijing, collaborated with scholar Y.J. Uemura at Columbia University in the discovery of a new DMS of bulk Li(Zn,Mn)As (termed "111" following the chemical compositions ration), where isovalent (Zn,Mn) spin doping was separated from charge control via Li concentrations, showing a Curie temperature up to Tc = 50K (Z. Den et al. Nature Communications 2, 422 (2011)).
Compared with classical diluted magnetic semiconductors such as (Ga,Mn)As, the lower Tc of the new "111" system is an obstacle for possible application.
More recently, a new ferromagnetic DMS (Ba,K)(Zn,Mn)2As2 (named "122" type following the chemical ration) system sharing the same structure with "122" type iron pnictide superconductors was reported. Via (Ba,K) substitution to dope hole carriers and (Zn,Mn) substitution to supply magnetic moments, the systems with 5-15% Mn doping exhibit ferromagnetic order with Tc up to 180 K (K. Zhao et al. NATURE COMMUNICATIONS |4: 1442 (2013)).
The ferromagnetic order, developing in the entire volume as indicated by SR results, is evidenced by the anomalous Hall effect in the ferromagnetic states.
One of the challenges to possible application for DMS is approaching Tc near room temperature. Given the fact that the Curie temperature of (Ga,Mn)As could be highly enhanced through increasing carrier density by low temperature annealing, optimizing synthesis condition may also pave the way toward further improving Tc in the (Ba,K)(Zn,Mn)2As2 system as well.
To avoid the volatility of K at high temperature, and to increase K contents in the sample and consequently increase carrier density, the mixture was heated under 650 C for 60h, a hundred degrees lower than the boiling temperature of the element potassium. This enhanced ferromagnetism with Tc at 230 K in (Ba0.7K0.3)(Zn0.85Mn0.15)2As2 DMS, which is higher than the record Tc of 200 K for (Ga,Mn)As.
The (Ba0.7K0.3)(Zn0.85Mn0.15)2As2 DMS shows spontaneous magnetization following T3/2 dependence expected for a homogeneous ferromagnet with saturation moment 1.0uB for each Mn atom.
As indicated, the carrier mediated and RKKY like interaction induced ferromagnetism could also be observed in insulating samples close to the metal-insulator transition. The resistivity curve of (Ba0.7K0.3)(Zn0.85Mn0.15)2As2, similar to that of (Ga,Mn)N, exhibits a small increase at low temperatures, due presumably to spin scattering of carriers caused by Mn dopants.
Clear signature of the ferromagnetic order is evidenced by the obvious negative magnetoresistance below Tc, which is greatly enhanced during decreasing temperature. At T=10K, an obvious hysteresis is observed in the magnetoresistance curve, showing a consistent coercive force in the M(H) curve.
In the present "122'' DMS ferromagnet (Ba0.7K0.3)(Zn0.85Mn0.15)2As2, semiconducting BaZn2As2, antiferromagnetic BaMn2As2, and superconducting (Ba,K)Fe2As2 all share the same crystal structure, with quite good lattice matching in the a-b plane (mismatch=3%). These could provide distinct advantages in attempts to generate new functional devices based on junctions of various combinations of the aforementioned DMS, superconductor, and magnetic states.
"The new DMS with decoupled spin charge doping mechanism would be promising to develop brand new spintronics," report scientists Kan Zhao, Bijuan Chen, Guoqiang Zhao, Zhen Yuan, Qingqing Liu, Zheng Deng, Jinlong Zhu and Changqing Jin, all of the Institute of Physics in Beijing, in an article titled "Ferromagnetism at 230 K in (Ba0.7K0.3)(Zn0.85Mn0.15)2As2 diluted magnetic semiconductor," published in the Chinese Science Bulletin.
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