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Peculiarly strong room-temperature ferromagnetism from low Mn-doping in ZnO grown by molecular beam epitaxy
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View: Figures


Image of FIG. 1.

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FIG. 1.

RHEED patterns of (a) undoped ZnO and (b) ∼ (h) Mn doped ZnO after growth taken at room temperature for Mn cell temperature of 500ºC ∼ 800ºC. The streaky patterns of all samples indicate smooth surface for all of them.

Image of FIG. 2.

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FIG. 2.

XRD patterns of (a) undoped ZnO and (b) ∼ (h) Mn doped ZnO at different Mn cell temperature. Only ZnO and sapphire related peaks are present and there is no evident secondary phase within detection limit. No evident peak shift is observed.

Image of FIG. 3.

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FIG. 3.

(a) Cross-sectional TEM image of Mn doped ZnO sample with Mn cell temperature of 800ºC, (b) Two sets of SAED patterns, belonging to the sapphire substrates and single-crystalline Mn doped ZnO thin film, respectively. (c) High resolution TEM image and selected-area FFT patterns, showing single crystalline Mn doped ZnO on sapphire.

Image of FIG. 4.

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FIG. 4.

XPS spectra of (a) undoped ZnO and (b) Mn doped ZnO with Mn cell temperature of 800ºC. (c) Zn/O signal ratio from XPS spectra of all samples, indicating minor variance. No spectra have any evident Mn signal, indicating low concentration.

Image of FIG. 5.

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FIG. 5.

SIMS spectra of Mn doped ZnO with Mn cell temperature at (a) 500ºC, (b) 600ºC, (c) 700ºC, and (d) 800ºC. All concentrations were aligned by Zn signal. Mn signals in (a) and (b) are very low and approach noise level. Signals in (c) and (d) are higher, but still within 0.1%at. This indicates that the Mn doping amount is very low and alloying is excluded.

Image of FIG. 6.

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FIG. 6.

M-H response of (a) undoped ZnO and (b) ∼ (h) Mn doped ZnO with Mn cell temperature of 500ºC ∼ 800ºC. The undoped ZnO as well as Mn cell temperature 500ºC sample show diamagnetic behavior, while ferromagnetic hysteresis loops are observed on other samples with higher Mn cell temperature. Note that linear term has been removed for the hysteresis graphs in (c) ∼ (h).

Image of FIG. 7.

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FIG. 7.

Summary of the M-H behaviors of the samples. (a) Saturation magnetization (Ms) as well as reminiscent moment at zero fields (Mr) of all samples at 300K and 10K. Ms evolves with rising Mn cell temperature while the change to Mr is less evident. (b) Coercivity field (Hc) and (c) Mr/Ms ratio of the samples. For most samples both Hc and Mr/Ms ratio follow an increasing trend with increased Mn cell temperature. The Mn 750ºC sample shows a sudden increase of both Hc and Mr/Ms ratio, which does not follow the trend.

Image of FIG. 8.

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FIG. 8.

In-plane magnetic anisotropy of Mn-doped ZnO sample. (a) Schematic of samples aligned by the observed easy-cleaving direction of sapphire substrate. Two cuts of samples with the same shape but crystal direction ∼90º apart were used. (b) M-H hysteresis and (c) fielded-cooling result of Mn cell 600ºC sample. Cut B shows increased saturation moment and reduced coercivity field. In both cases temperature cooling shows no abrupt phase change steps, supporting intrinsic DMS.

Image of FIG. 9.

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FIG. 9.

In-plane/off-plane magnetic anisotropy of Mn doped ZnO sample. (a) Schematic of the samples in this off-plane measurement in comparison to in-plane. The sample was specially cleaved into three small squares and stacked together to provide similar area of thin film, in turn similar amount of Mn doped ZnO to the in-plane sample. (b) M-H hysteresis of Mn cell 800ºC sample. Off-plane measurement shows larger saturation moment but similar coercivity field.

Image of FIG. 10.

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FIG. 10.

Temperature dependent PL of (a) undoped ZnO and (b) ∼ (e) Mn doped ZnO with Mn cell temperature of 500ºC ∼ 800ºC. Undoped ZnO sample shows donor bound exciton at ∼3.362eV, TES peak at ∼3.308eV, and its phonon replica at lower energy, indicating good quality. Similar behavior is observed in Mn 700ºC sample. The other Mn doped samples only show DX at ∼3.36eV, with broad luminescence bump at ∼3.2eV range.

Image of FIG. 11.

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FIG. 11.

Magneto resistance (MR) of (a) undoped ZnO and (b) ∼ (d) Mn doped ZnO. Undoped ZnO shows typical negative MR, which increases as temperature lowers. Mn doped films have similar MR behavior like that of undoped ZnO, but have more evident trend of positive MR. This is most evident at Mn cell 700C sample, which shows entirely positive MR at 200K.

Image of FIG. 12.

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FIG. 12.

(a) Hall effect data of undoped and Mn doped ZnO samples at 10K. It is evident that Mn 800ºC sample has an exceptionally large anomalous Hall effect, in comparison to other samples. (b) Room temperature carrier concentration data showing only a small variance in the range of 1018∼1019 cm−3.


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Strong room-temperature ferromagnetism is demonstrated in single crystalline Mn-doped ZnO thin films grown by molecular beam epitaxy. Very low Mn doping concentration is investigated, and the measured magnetic moment is much larger than what is expected for an isolated ion based on Hund's rules. The ferromagnetic behavior evolves with Mn concentration. Both magnetic anisotropy and anomalous Hall effect confirm the intrinsic nature of ferromagnetism. While the Mn dopant plays a crucial role, another entity in the system is needed to explain the observed large magnetic moments.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Peculiarly strong room-temperature ferromagnetism from low Mn-doping in ZnO grown by molecular beam epitaxy