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Synthesis and magnetic properties of single-crystalline Na2-xMn8O16 nanorods.

Lan C, Gong J, Liu S, Yang S - Nanoscale Res Lett (2011)

Bottom Line: The synthesis of single-crystalline hollandite-type manganese oxides Na2-xMn8O16 nanorods by a simple molten salt method is reported for the first time.The magnetic measurements indicated that the nanorods showed spin glass behavior and exchange bias effect at low temperatures.The low-temperature magnetic behaviors can be explained by the uncompensated spins on the surface of the nanorods.

View Article: PubMed Central - HTML - PubMed

Affiliation: Nanjing National Laboratory of Microstructures and Department of Physics, Nanjing University, 22 Hankou Road, Nanjing, 210093, China. sgyang@nju.edu.cn.

ABSTRACT
The synthesis of single-crystalline hollandite-type manganese oxides Na2-xMn8O16 nanorods by a simple molten salt method is reported for the first time. The nanorods were characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and a superconducting quantum interference device magnetometer. The magnetic measurements indicated that the nanorods showed spin glass behavior and exchange bias effect at low temperatures. The low-temperature magnetic behaviors can be explained by the uncompensated spins on the surface of the nanorods.

No MeSH data available.


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Magnetization as a function of magnetic field at 5 K for Na2-xMn8O16 nanorods. (a) after ZFC process; (b) after FC process with an applied magnetic field of 5 T. The inset in the lower right corner of (a) and (b) shows the magnified part of the corresponding loop in the low field ranges. The inset in the upper left corner of (b) shows the high field irreversibility of magnetization on the right-hand side.
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Figure 4: Magnetization as a function of magnetic field at 5 K for Na2-xMn8O16 nanorods. (a) after ZFC process; (b) after FC process with an applied magnetic field of 5 T. The inset in the lower right corner of (a) and (b) shows the magnified part of the corresponding loop in the low field ranges. The inset in the upper left corner of (b) shows the high field irreversibility of magnetization on the right-hand side.

Mentions: Hysteresis loops of the Na2-xMn8O16 nanorods recorded at 5 K under ZFC and FC conditions are shown in Figure 4a, and 4b, respectively. For the FC loop, the sample was cooled from room temperature under an applied magnetic field of 5 T. As can be seen in Figure 4a the hysteresis loop recorded under ZFC conditions is symmetrical, centers about the origin, and exhibits a coercive field of about 980 Oe. On the contrary, for the FC process an asymmetry magnetic hysteresis loop (Figure 4b) exhibiting shifts both in the field and magnetization axes as well as an enhanced coercivity (approximately 1,375 Oe) is observed, which indicates the existence of EB phenomenon. The EB effect can be explained on the basis of a phenomenological core-shell model where the core shows AFM behavior and the surrounding shell possesses a net magnetic moment due to a large number of uncompensated surface spins [19-21]. This is different from ordinary case, where a good AFM/FM interface is needed, such as Ni80Fe20/Ir20Mn80 system [17]. The shift to positive magnetization axis for the FC loop suggests the presence of a unidirectional exchange anisotropy interaction, which drives the FM domains back to the original orientation when the field is removed [20,21]. The strength of this anisotropy is measured by the EB field HE which is defined as HE = -(H1 + H2)/2, where H1 and H2 are left and right coercive fields, respectively. The EB field for the FC process is about 770 Oe. The remanence asymmetry ME is defined as the vertical axis equivalent to HE. Thus the ME and remanent magnetization Mr under the FC mode are about 0.071 and 0.126 emu/g, respectively. The enhanced coercivity for the FC loop is ascribed to the development of the exchange anisotropy. In the case of an AFM with small anisotropy, when the FM rotates it drags the AFM spins irreversibly, hence increasing the FM coercivity [18].


Synthesis and magnetic properties of single-crystalline Na2-xMn8O16 nanorods.

Lan C, Gong J, Liu S, Yang S - Nanoscale Res Lett (2011)

Magnetization as a function of magnetic field at 5 K for Na2-xMn8O16 nanorods. (a) after ZFC process; (b) after FC process with an applied magnetic field of 5 T. The inset in the lower right corner of (a) and (b) shows the magnified part of the corresponding loop in the low field ranges. The inset in the upper left corner of (b) shows the high field irreversibility of magnetization on the right-hand side.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211180&req=5

Figure 4: Magnetization as a function of magnetic field at 5 K for Na2-xMn8O16 nanorods. (a) after ZFC process; (b) after FC process with an applied magnetic field of 5 T. The inset in the lower right corner of (a) and (b) shows the magnified part of the corresponding loop in the low field ranges. The inset in the upper left corner of (b) shows the high field irreversibility of magnetization on the right-hand side.
Mentions: Hysteresis loops of the Na2-xMn8O16 nanorods recorded at 5 K under ZFC and FC conditions are shown in Figure 4a, and 4b, respectively. For the FC loop, the sample was cooled from room temperature under an applied magnetic field of 5 T. As can be seen in Figure 4a the hysteresis loop recorded under ZFC conditions is symmetrical, centers about the origin, and exhibits a coercive field of about 980 Oe. On the contrary, for the FC process an asymmetry magnetic hysteresis loop (Figure 4b) exhibiting shifts both in the field and magnetization axes as well as an enhanced coercivity (approximately 1,375 Oe) is observed, which indicates the existence of EB phenomenon. The EB effect can be explained on the basis of a phenomenological core-shell model where the core shows AFM behavior and the surrounding shell possesses a net magnetic moment due to a large number of uncompensated surface spins [19-21]. This is different from ordinary case, where a good AFM/FM interface is needed, such as Ni80Fe20/Ir20Mn80 system [17]. The shift to positive magnetization axis for the FC loop suggests the presence of a unidirectional exchange anisotropy interaction, which drives the FM domains back to the original orientation when the field is removed [20,21]. The strength of this anisotropy is measured by the EB field HE which is defined as HE = -(H1 + H2)/2, where H1 and H2 are left and right coercive fields, respectively. The EB field for the FC process is about 770 Oe. The remanence asymmetry ME is defined as the vertical axis equivalent to HE. Thus the ME and remanent magnetization Mr under the FC mode are about 0.071 and 0.126 emu/g, respectively. The enhanced coercivity for the FC loop is ascribed to the development of the exchange anisotropy. In the case of an AFM with small anisotropy, when the FM rotates it drags the AFM spins irreversibly, hence increasing the FM coercivity [18].

Bottom Line: The synthesis of single-crystalline hollandite-type manganese oxides Na2-xMn8O16 nanorods by a simple molten salt method is reported for the first time.The magnetic measurements indicated that the nanorods showed spin glass behavior and exchange bias effect at low temperatures.The low-temperature magnetic behaviors can be explained by the uncompensated spins on the surface of the nanorods.

View Article: PubMed Central - HTML - PubMed

Affiliation: Nanjing National Laboratory of Microstructures and Department of Physics, Nanjing University, 22 Hankou Road, Nanjing, 210093, China. sgyang@nju.edu.cn.

ABSTRACT
The synthesis of single-crystalline hollandite-type manganese oxides Na2-xMn8O16 nanorods by a simple molten salt method is reported for the first time. The nanorods were characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and a superconducting quantum interference device magnetometer. The magnetic measurements indicated that the nanorods showed spin glass behavior and exchange bias effect at low temperatures. The low-temperature magnetic behaviors can be explained by the uncompensated spins on the surface of the nanorods.

No MeSH data available.


Related in: MedlinePlus