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

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.

x Mn 8 O 16 is known to have hollandite structure with unit-cell parameters a = 9.91 Å, b = 2.86 Å, c = 9.62 Å and b = 90.93° (JCPDS No. 42-1347), and the ion tunnel of which is along b-axis. To the best of our knowledge little information about this compound has been reported. Here, we report the synthesis of Na 2-x Mn 8 O 16 nanorods by a very simple molten salt method for the first time.
Exchange bias (EB) effect is observed in the materials with good ferromagnetic (FM)/antiferromagnetic (AFM) interface, such as Ni 80 Fe 20 /Ir 20 Mn 80 system [17]. The EB effect originates from the interfacial interaction between FM and AFM materials [18]. Recently, it was reported that 1D pure phase AFM nanomaterials exhibited EB effect at low temperatures, such as Co 3 O 4 nanorods [19], SrMn 3 O 6-δ nanobelts [20], CuO nanowires [21]. Since there is no FM layer in those materials, the EB effect in pure 1D AFM nanomaterials is probably related to the surface layer of the nanomaterials, which is due to the changes in the atomic coordination form a layer of disordered spins (i. e. spin glass layer) [18]. As a kind of 1D magnetic nanomaterials, the Na 2-x Mn 8 O 16 nanorods may show novel magnetic properties. Thus the magnetic properties of Na 2-x Mn 8 O 16 nanorods are explored and we find that the as-synthesized nanorods exhibit spin glass behavior and EB effect at low temperatures.

Results and discussion
The X-ray diffraction (XRD) pattern of Na 2-x Mn 8 O 16 nanorods is shown in Figure 1. The peaks can be indexed to monoclinic phase of Na 2-x Mn 8 O 16 (JCPDS No. 42-1347). No secondary phase is observed, indicating pure phase Na 2-x Mn 8 O 16 was obtained. As the Na + cation is on the small side to stabilize the 2 × 2 tunnels compared with K + cation, it is difficult to synthesize Na 2-x Mn 8 O 16 [3]. In fact, we have tried to synthesize Na 2-x Mn 8 O 16 by solid state reaction using stoichiometric amount of NaNO 3 and MnCO 3 as starting materials (suppose x = 0 in the formula Na 2-x Mn 8 O 16 ), but no Na 2-x Mn 8 O 16 phase could be obtained. In order to keep the 2 × 2 tunnel structure stable when K + cations are replaced by Na + cations, more Na + cations are needed. In the high-temperature liquid molten salt, there is a large quantity of free Na + cations. Suppose the unstable 2 × 2 tunnels formed in the molten salt first, then the Na + cations can go into the tunnels. The excess of Na + cations can guarantee there are enough Na + cations in the 2 × 2 tunnels to make the tunnels stable. Based on the above discussion, the x in Na x Mn 8 O 16 should be larger than that in K x Mn 8 O 16 . The x in K x Mn 8 O 16 is 1.5 [14], while the x in Na x Mn 8 O 16 obtained from the EDS result discussed later in this letter is 1.74, which confirms the above conclusion.
A low-magnified scanning electron microscopy (SEM) image of Na 2-x Mn 8 O 16 nanorods is shown in Figure 2a. Combing the HRTEM and SAED results, it can be concluded that the growth direction of the nanorod is along [010], which is the tunnel direction of the compound. The composition of the as-synthesized nanorods was determined by EDS. Figure 2f shows the EDS spectroscopy. The chemical components of the nanorods are Na, Mn, and O with the ratio 7.24:33.38:59.38. The ratio of O/Mn is close to 2, which is consistent with the chemical formula. The chemical formula calculated from the EDS result is Na 1.74 Mn 8 O 16 .
The magnetic properties of the Na 2-x Mn 8 O 16 nanorods were explored. Figure 3 shows the temperature-dependent magnetization curves of the nanorods in zero-field-cooled (ZFC) and field-cooled (FC) processes with an applied magnetic field of 500 Oe. The ZFC magnetization curve shows a sharp peak near 19 K (T B ) and an evident separation from the FC curve below T B , suggesting a spin-glass-like behavior at low temperatures [16,[19][20][21]. Such behavior can be attributed to uncompensated surface spins in the 1D nanostructures [16,[19][20][21]. The  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][20][21]. This is different from ordinary case, where a good AFM/FM interface is needed, such as Ni 80 Fe 20 /Ir 20 Mn 80 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 H E which is defined as H E = -(H 1 + H 2 )/2, where H 1 and H 2 are left and right coercive fields, respectively. The EB field for the FC process is about 770 Oe. The remanence asymmetry M E is defined as the vertical axis equivalent to H E . Thus the M E and remanent magnetization M r 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].
The spin-glass-like behavior of the surface can also be clearly observed for the opening in the upper right side of the FC hysteresis loop, which is shown in the upper left inset of Figure 4b. This indicates that we have a loss of magnetization during one hysteresis cycle. A similar phenomenon has been observed in Co 3 O 4 nanowires [19]. This striking experimental feature is observed here because of the large amount of measured material and due to the absence of additional ferromagnetic materials which could mask the observation of the interfacial spins behavior [19]. The EB effect induced by surface effects of the nanorods suggests that Na 2-x Mn 8 O 16 nanorods may find potential application in multifunctional spintronic devices [22].

Conclusions
In summary, single-crystalline Na 2-x Mn 8

Methods
In a typical procedure, MnCl 2 •4H 2 O and NaOH (1:2 in molar) were dissolved in distilled water, respectively. Then NaOH aqueous solution was added to MnCl 2 aqueous solution slowly with constant magnetic stirring. The precipitation was filtered and washed several times and then dried at 90°C for 24 h. After being dried, black powder was obtained. 0.1 g of the obtained black powder was mixed with 5 g NaNO 3 and ground for 20 min in an agate mortar by hand. The mixture was then placed in a corundum crucible and annealed at 550°C for 6 h. The product was collected after naturally cooling the furnace to room temperature and then washed several times with distilled water to remove residual NaNO 3 . The obtained black powder was dried at 90°C for 24 h. XRD patterns were collected using a Philips X'Pert diffractometer with Cu Kα irradiation at room temperature. For the SEM characterization, the product was pasted on a Cu sheet with conductive adhesive. A thin layer of Pt was sputtered on the sample to enhance its conductivity for the facility of SEM measurements. SEM and EDS pattern were carried out in a Hitachi-S-3400N II instrument. In further characterization, TEM images, HRTEM images, and SAED were obtained in a Philips Tecnai F20 instrument, operating at 200 kV. Magnetic properties were obtained in a superconducting quantum interference device magnetometer.