Enhancement of zinc vacancies in room-temperature ferromagnetic Cr–Mn codoped ZnO nanorods synthesized by hydrothermal method under high pulsed magnetic field

Highlights

• Cr–Mn codoped ZnO nanorods were synthesized by hydrothermal method.

• High pulsed magnetic field was applied during the hydrothermal method.

• The valence state of doped elements was investigated by XPS.

• High pulsed magnetic field enhances the concentration of zinc vacancies.

• Hole doping is the key factor in Mn-doped ZnO diluted magnetic semiconductors.

Abstract

Room-temperature ferromagnetic Cr–Mn codoped ZnO diluted magnetic semiconductor was synthesized by pulse magnetic field-assisted hydrothermal method. X-ray diffraction and Raman spectra analysis reveal that all the samples have hexagonal wurtzite structure. High resolution transmission electron microscopy and Energy-dispersive spectroscopy measurements ensure that the Cr and Mn ions are incorporated into the wurtzite host matrix without any detectable impurity phase. X-ray photoelectron spectroscopy confirms that Mn and Cr ions are doped into the ZnO wurtzite host matrix with divalent states in the sample without magnetic field processing. Cr ions became trivalent states in ZnO synthesized with high pulsed magnetic field, while Mn keeps its divalent state. The presence of Cr³⁺ is attributed to hole doping in ZnO with zinc vacancies induced by the field. Magnetization measurements reveal the appearance of ferromagnetism for the magnetic field processed sample. Comparing with oxygen vacancies, zinc vacancies (hole doping) is more effectively to stabilized ferromagnetism in Mn-doped ZnO diluted magnetic semiconductors.

Introduction

Diluted magnetic semiconductors (DMSs), which refer to transition-metal (TM) ions partially substitute cations of the host semiconductor materials, have attracted considerable attention for potential applications in spin electronics and magnetic devices that allow the manipulation of both the spin and charge degrees of freedom [1], [2], [3], [4], [5]. The practical spintronic devices depend on the high Curie temperature (T_C) exceeding room temperature. The ferromagnetic properties originate as an intrinsic feature but not the second magnetic phase. ZnO is a wide band gap (E_g∼3.3 eV at 300 K) semiconductor with a large exciton binding energy (∼60 meV) [6], [7]. Moreover, well-defined doping and defect chemistries, suitability for transparent high-power high-temperature application and the ability to lase or emit spontaneously at ultraviolet wavelengths combine to make ZnO attractive in many potential device applications [8]. In the quest for materials with high T_C, TM-doped ZnO has emerged as an attractive candidate according to the theoretical studies [9], [10], [11]. In the foundations of the p-d Zener model, Dietl et al. [9] predicted that the T_C of p-type ZnO (p ≥ 3.5 × 10²⁰ cm⁻³) semiconductor doped with 5% Mn might be higher than room temperature. While p-type ZnO is difficult to attain because of the abundant native defects such as oxygen vacancies (V_O) and Zinc interstitials (Zn_i). However, Sato and Katayama–Yoshida [10] theoretically demonstrated that ZnO doped with TM atoms such as V, Cr, Fe, Co, and Ni exhibit ferromagnetic stability using first-principle calculations.

Since the first claim of room-temperature ferromagnetism in Mn²⁺: ZnO [12], high-T_C ferromagnetism has been observed in Mn²⁺: ZnO with p-type and n-type conductivity [13], [14]. Meanwhile, other controversial results have been claimed, including a low Curie temperature [15], spin-glass behavior [16], and presence of antiferromagnetism [17]. Thus, the origin of FM in DMSs is still ambiguous. Up to now, some interesting codoping experimental results were reported. Singhal et al. [18] reported that the codoping of Mn into Co doped ZnO further enhances the ferromagnetic ordering. Also, Aljawfi et al. [19] suggested that the ferromagnetism of Cr/Co codoped ZnO nanoparticles improve with increasing Cr doping. The codoping approach has drawn intensive attention due to the possibility to tailor the position and occupancy of the Fermi energy (E_f) of doped DMS. A key property common to all of the models describing ZnO DMS ferromagnetism is strong electronic coupling between the magnetic ions and charge carriers at the Fermi level [20]. Thus, codoping seems to be a potential approach to enhance the ferromagnetism in TM-doped ZnO DMSs.

Recently experimental studies have revealed that the magnetic properties depend strongly on fabrication conditions, concentration of dopants and the structure of materials. As to the fabrication condition, the TM-doped ZnO can be synthesized by chemical vapor deposition, physical vapor deposition, solid state reaction, hydrothermal, sol–gel etc. [21], [22], [23], [24], [25]. Among these methods, hydrothermal method becomes the most efficient choice for its advantages of easy control and uniform products [26]. So far, the magnetic field has been applied as an effective way to influence the crystal growth, morphology and the properties of nanomaterial, such as Cr-doped ZnO [27], Fe₃O₄ nanowires [28], Co₃O₄ nanocubes/nanospheres [29], and so on. It can be found that an external magnetic field influences significantly the growth behavior and the exchange interactions between the spins in the nanoparticles during the hydrothermal process. In view of this, the effect of pulsed magnetic field on the magnetic property of TM-codoped ZnO prepared by hydrothermal method is really expected. The present article accounts for the structural and magnetic properties of 1%Cr–1%Mn (at. %, nominal doping amount) codoped ZnO synthesized by hydrothermal method under high pulsed magnetic field.