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Interfacial dominated ferromagnetism in nanograined ZnO: a μSR and DFT study.

Tietze T, Audehm P, Chen YC, Schütz G, Straumal BB, Protasova SG, Mazilkin AA, Straumal PB, Prokscha T, Luetkens H, Salman Z, Suter A, Baretzky B, Fink K, Wenzel W, Danilov D, Goering E - Sci Rep (2015)

Bottom Line: Instead, the nanostructure seems to play the key role, as room temperature ferromagnetism was also found in nanograined, undoped ZnO.With molecular dynamics and density functional theory we find ferromagnetic coupled electron states in ZnO grain boundaries.Our results provide evidence and a microscopic model for room temperature ferromagnetism in oxides.

View Article: PubMed Central - PubMed

Affiliation: Max-Planck-Institute for Intelligent Systems, Heisenbergstr. 3, D-70569 Stuttgart, Germany.

ABSTRACT
Diamagnetic oxides can, under certain conditions, become ferromagnetic at room temperature and therefore are promising candidates for future material in spintronic devices. Contrary to early predictions, doping ZnO with uniformly distributed magnetic ions is not essential to obtain ferromagnetic samples. Instead, the nanostructure seems to play the key role, as room temperature ferromagnetism was also found in nanograined, undoped ZnO. However, the origin of room temperature ferromagnetism in primarily non-magnetic oxides like ZnO is still unexplained and a controversial subject within the scientific community. Using low energy muon spin relaxation in combination with SQUID and TEM techniques, we demonstrate that the magnetic volume fraction is strongly related to the sample volume fraction occupied by grain boundaries. With molecular dynamics and density functional theory we find ferromagnetic coupled electron states in ZnO grain boundaries. Our results provide evidence and a microscopic model for room temperature ferromagnetism in oxides.

No MeSH data available.


Temperature dependent SQUID magnetization curves for the fine grained (red triangles), and the coarse grained ZnO samples (blue squares).Even at RT, ferromagnetic magnetization curves with small but sizeable remanence and coercivitiy have been measured (see inset). For both sample types, SQUID loops measured at RT and 50 K show no significant difference which is an important feature identifying ferromagnetism in magnetic oxides. The supposedly nonmagnetic ZnO single crystal reference shows no significant magnetic features (black circles).
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f2: Temperature dependent SQUID magnetization curves for the fine grained (red triangles), and the coarse grained ZnO samples (blue squares).Even at RT, ferromagnetic magnetization curves with small but sizeable remanence and coercivitiy have been measured (see inset). For both sample types, SQUID loops measured at RT and 50 K show no significant difference which is an important feature identifying ferromagnetism in magnetic oxides. The supposedly nonmagnetic ZnO single crystal reference shows no significant magnetic features (black circles).

Mentions: These expectations are confirmed by the corresponding SQUID measurements presented in figure 2. All magnetization curves have been corrected for the substrate diamagnetism. In order to avoid thickness dependent effects, the magnetization curves of the thin film samples have been normalized to the corresponding thin film volume, determined from the TEM based films thicknesses and the respective sample area. The magnetization curve of the single crystal has been normalized to the single crystal volume. The magnetization curves of the FG and CG samples have been measured at RT and at 50 K in order to confirm FM and to distinguish the hysteresis loops from temperature dependent (super–) paramagnetic curves28. As expected from the large fraction of “critical” grains, the FG sample shows the highest saturation magnetization with 8.3·emu/cm3, followed by the CG sample with 1.25 emu/cm3. The single crystal shows only a very small saturation magnetization of 2·10−4 emu/cm3. The magnetization curves at RT and 50 K are quite similar, which is an important indicator for true FM and a high Curie temperature TC. Furthermore, both magnetic samples exhibit a small but sizeable remanence and coercivity at RT, as shown in the inset of figure 2.


Interfacial dominated ferromagnetism in nanograined ZnO: a μSR and DFT study.

Tietze T, Audehm P, Chen YC, Schütz G, Straumal BB, Protasova SG, Mazilkin AA, Straumal PB, Prokscha T, Luetkens H, Salman Z, Suter A, Baretzky B, Fink K, Wenzel W, Danilov D, Goering E - Sci Rep (2015)

Temperature dependent SQUID magnetization curves for the fine grained (red triangles), and the coarse grained ZnO samples (blue squares).Even at RT, ferromagnetic magnetization curves with small but sizeable remanence and coercivitiy have been measured (see inset). For both sample types, SQUID loops measured at RT and 50 K show no significant difference which is an important feature identifying ferromagnetism in magnetic oxides. The supposedly nonmagnetic ZnO single crystal reference shows no significant magnetic features (black circles).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4352909&req=5

f2: Temperature dependent SQUID magnetization curves for the fine grained (red triangles), and the coarse grained ZnO samples (blue squares).Even at RT, ferromagnetic magnetization curves with small but sizeable remanence and coercivitiy have been measured (see inset). For both sample types, SQUID loops measured at RT and 50 K show no significant difference which is an important feature identifying ferromagnetism in magnetic oxides. The supposedly nonmagnetic ZnO single crystal reference shows no significant magnetic features (black circles).
Mentions: These expectations are confirmed by the corresponding SQUID measurements presented in figure 2. All magnetization curves have been corrected for the substrate diamagnetism. In order to avoid thickness dependent effects, the magnetization curves of the thin film samples have been normalized to the corresponding thin film volume, determined from the TEM based films thicknesses and the respective sample area. The magnetization curve of the single crystal has been normalized to the single crystal volume. The magnetization curves of the FG and CG samples have been measured at RT and at 50 K in order to confirm FM and to distinguish the hysteresis loops from temperature dependent (super–) paramagnetic curves28. As expected from the large fraction of “critical” grains, the FG sample shows the highest saturation magnetization with 8.3·emu/cm3, followed by the CG sample with 1.25 emu/cm3. The single crystal shows only a very small saturation magnetization of 2·10−4 emu/cm3. The magnetization curves at RT and 50 K are quite similar, which is an important indicator for true FM and a high Curie temperature TC. Furthermore, both magnetic samples exhibit a small but sizeable remanence and coercivity at RT, as shown in the inset of figure 2.

Bottom Line: Instead, the nanostructure seems to play the key role, as room temperature ferromagnetism was also found in nanograined, undoped ZnO.With molecular dynamics and density functional theory we find ferromagnetic coupled electron states in ZnO grain boundaries.Our results provide evidence and a microscopic model for room temperature ferromagnetism in oxides.

View Article: PubMed Central - PubMed

Affiliation: Max-Planck-Institute for Intelligent Systems, Heisenbergstr. 3, D-70569 Stuttgart, Germany.

ABSTRACT
Diamagnetic oxides can, under certain conditions, become ferromagnetic at room temperature and therefore are promising candidates for future material in spintronic devices. Contrary to early predictions, doping ZnO with uniformly distributed magnetic ions is not essential to obtain ferromagnetic samples. Instead, the nanostructure seems to play the key role, as room temperature ferromagnetism was also found in nanograined, undoped ZnO. However, the origin of room temperature ferromagnetism in primarily non-magnetic oxides like ZnO is still unexplained and a controversial subject within the scientific community. Using low energy muon spin relaxation in combination with SQUID and TEM techniques, we demonstrate that the magnetic volume fraction is strongly related to the sample volume fraction occupied by grain boundaries. With molecular dynamics and density functional theory we find ferromagnetic coupled electron states in ZnO grain boundaries. Our results provide evidence and a microscopic model for room temperature ferromagnetism in oxides.

No MeSH data available.