Characterization of the natural barriers of intergranular tunnel junctions : Cr 2 O 3 surface layers on CrO 2 nanoparticles

Cold-pressed powder compacts of CrO2 show large negative magnetoresistance (MR) due to intergranular tunneling. Powder compacts made from needle-shaped nanoparticles exhibit MR of about 28% at 5 K. Temperature dependence of the resistivity indicates that the Coulomb blockade intergranular tunneling is responsible for the conductance at low temperature. In this letter we report direct observation and characterization of the microstructure of the intergranular tunnel barriers, using transmission electron microscopy, x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS). A very thin native oxide layer with a thickness of 1–3 nm on the surface of CrO2 powders has been observed. The composition and crystal structure of this surface layer has been determined to be Cr2O3 by XPS and XRD. The dense and uniform Cr2O3 surface layers play an ideal role of tunnel barriers in the CrO2 powder compacts.

Cold-pressed powder compacts of CrO 2 show large negative magnetoresistance ͑MR͒ due to intergranular tunneling.Powder compacts made from needle-shaped nanoparticles exhibit MR of about 28% at 5 K. Temperature dependence of the resistivity indicates that the Coulomb blockade intergranular tunneling is responsible for the conductance at low temperature.In this letter we report direct observation and characterization of the microstructure of the intergranular tunnel barriers, using transmission electron microscopy, x-ray diffraction ͑XRD͒, and x-ray photoelectron spectroscopy ͑XPS͒.A very thin native oxide layer with a thickness of 1-3 nm on the surface of CrO 2 powders has been observed.The composition and crystal structure of this surface layer has been determined to be Cr 2 O 3 by XPS and XRD.The dense and uniform Cr 2 O 3 surface layers play an ideal role of tunnel barriers in the CrO 2 powder compacts.© 2000 American Institute of Physics.͓S0003-6951͑00͒00344-2͔ 3][4] The high degree of spin polarization of the electrons at the Fermi level suggests that CrO 2 could be an ideal material for the magnetic tunneling devices.6][7][8] Manoharan et al. 5 reported a 30% negative MR of cold-pressed CrO 2 powder sample at 4.2 K. Coey et al. 6 also reported a ϳ30% MR of cold-pressed CrO 2 powder compacts and a 50% MR of a dilute composite sample at 5 K.
The conduction mechanism of the above CrO 2 structures is due to the spin dependent intergranular tunneling affected by the Coulomb gap, 6 and the grain boundaries are believed to play the role of the tunnel barriers of the intergranular tunneling.The grain boundaries are suspected to be made of a native Cr 2 O 3 layer on the surfaces of CrO 2 , 6,7,9 since it is the most stable phase of CrO x . 2,6,10It is suggested in Ref. 6 that the boundary layer can be the thermodynamically stable oxide Cr 2 O 3 .Huang and Cheong 7 also suggest that the Cr 2 O 3 at the interface act as a tunnel barrier.However, this intergranular tunnel barrier has not been carefully examined before and its nature is not precisely known.It is also possible that the grain boundary, or surface layer, is made of amorphous CrO 2 , 2,11 or even compounds like CrOOH. 12It is obviously important to learn the microstructure of the grain boundaries and surface layers, and to understand why they work extremely well as the tunnel barriers.
In this work, we have studied the microstructure of the surface layers of the CrO 2 particles using transmission electron microscopy ͑TEM͒, and x-ray diffraction ͑XRD͒ and x-ray photoelectron spectroscopy ͑XPS͒.We report direct observation by TEM of a 1-3 nm thick native oxide layer on the surface of the CrO 2 single crystal powders, which has been characterized as crystalline Cr 2 O 3 .This layer is believed to play the role of tunnel barrier in the intergranular tunneling effect in the CrO 2 powder compacts.
Samples used in our experiments were made from CrO 2 powders supplied by DuPont.The powders have been analyzed by XRD and TEM.They are single crystal needleshaped particles with length of about 400 nm and an aspect ratio of about 9:1.The coercivity is about 600 Oe at room temperature and about 1000 Oe at Tϭ5 K. Cold-pressed CrO 2 compacts were made under a pressure of 5 ϫ10 8 N/m 2 .The electron transport properties were measured using a Quantum Design physical properties measurement system ͑PPMS͒ in magnetic fields from Ϫ5 to 5 T over the temperature range from 5 to 300 K.
The resistivity of the cold-pressed CrO 2 powder compact is about 0.1 ⍀ cm. Figure 1 shows the negative MR at 5 K, where the MR is defined as (R H ϪR Hϭ0 )/R Hϭ0 .It reaches about 28% in a field of 5 T. The inset of Fig. 1 shows the low field MR and magnetic hysteresis loop which exhibit clear correlation between the two.Figure 2 shows the resistance R as a function of temperature T, and in the inset ln R is plotted against T 1/2 .The resistance R has been normalized to the value at Tϭ300 K.One can see that ln R is linear to 1/T 1/2 at low temperature, which is typical of intergranular tunneling.The spin dependent tunneling across the interface between a͒ Also at Dept. of Applied Physics, Jiao Tong University, Shanghai, People's Republic of China, electronic mail: jdai@uno.edub͒ Electronic mail: jtang@uno.edutwo neighboring CrO 2 particles is understood as intergranular tunneling with a Coulomb gap. 6The resistance as a function of temperature can be expressed as [13][14][15] Rϰexp͑⌬/T ͒ 1/2 , ͑1͒ where ⌬ is proportional to the Coulomb charging energy E c and tunnel barrier thickness s.In our CrO 2 sample, ⌬ determined from the slope of Fig. 2 inset is found to be about 6 K, which is in reasonable agreement with the value estimated from the particle size and barrier thickness. 13The data in Fig. 2 fit Eq. ͑1͒ very well and imply the intergranular tunneling is the major mechanism of the conductance at low temperature.
TEM analysis has been performed to characterize the microstructure of the single crystal CrO 2 particles as shown in Figs.3͑a͒-3͑d͒.Figure 3͑a͒ shows the CrO 2 particles of needle shape with a diameter of ϳ50 nm.A very thin surface layer can be seen on each particle in the TEM image with higher magnification as shown in Fig. 3͑b͒.Figure 3͑c͒ is a high resolution lattice image of the CrO 2 particle with ͓001͔ axis along its length, showing clearly the thin layer on the surface of the single crystal lattice image of CrO 2 .By means of XPS, one can identify the composition of this surface layer to be Cr 2 O 3 , the details of which will be discussed later.The corresponding electron diffraction pattern of the CrO 2 particle is shown in Fig. 3͑d͒ and the ring-shaped background is from the Cr 2 O 3 surface layer.The diffuse electron diffraction pattern of Cr 2 O 3 is because the native oxide layer is very thin ͑1-3 nm͒ and mostly of low degree of crystallinity.
XPS measurements were performed in a separate UHV chamber with a base pressure of 10 Ϫ10 Torr.CrO 2 powders and standard Cr 2 O 3 powders were cold pressed and mounted on sample holders.The Cr 2 O 3 sample is used as a standard for comparison of the Cr 2p peak with the CrO 2 sample.As shown in Fig. 4, the similarity of the Cr 2p peak shapes between CrO 2 and Cr 2 O 3 suggests that the Cr 3ϩ dominates in the near surface region ͑ϳ10 ML͒ of the CrO 2 particles.One should notice that XPS is sensitive within 10-20 Å into the particle surface.Our result indicates that the 1-3 nm thick surface layer on the CrO 2 particles is not CrO 2 but a Cr 3ϩ compound.The result strongly supports the expectation that a Cr 2 O 3 native oxide layer exists on the CrO 2 surface 6,7,9 due to the fact that Cr 2 O 3 is the most stable phase of chromium oxides. 10The high quality as seen from the TEM images and good insulating properties of Cr 2 O 3 make it an ideal tunneling barrier in the spin dependent intergranular tunnel junctions of CrO 2 powder compacts.
Crystalline phases of Cr 2 O 3 can be observed within the surface layer in the high-resolution TEM images, which implies the structure of the Cr 2 O 3 surface layer is mostly crystalline as opposed to amorphous.Considering the 1-3 nm thickness of the Cr 2 O 3 layer and the CrO 2 particle size, one can roughly estimate that there is about 5% of Cr 2 O 3 in our ''pure'' CrO 2 powder samples.The CrO 2 samples were analyzed by x-ray diffraction through a detailed scan, in which very small scan steps and long scan time were used.A weak but very clear signal of Cr 2 O 3 has been observed confirming the existence of Cr 2 O 3 crystalline phase as is shown in Fig. 5.This XRD result further suggests that the surface layer of Cr 2 O 3 is mostly made of crystalline Cr 2 O 3 .
Another experiment has been performed to adjust the amount of Cr 2 O 3 in our sample.The CrO 2 powders were annealed in air at 400 °C for 20 min.XRD shows about 20%-25% of CrO 2 has transformed into Cr 2 O 3 , which is estimated from the intensity of the XRD peaks as shown in the inset of Fig. 6.This value is approximately equal to the amount of Cr 2 O 3 measured by weight change of the sample.TEM shows that some CrO 2 transforms to Cr 2 O 3 particles of spherical shape.Cold-pressed compacts have been made using the annealed powders.Due to the annealing, change in intergranular tunneling is expected.The resistivity increases to about 2 ⍀ cm, and the MR ratio of the annealed sample increases to about 33% and the slope in the ln R vs 1/T 1/2 is increased as shown in Fig. 6.The ⌬ value in Eq. ͑1͒ is increased from 6 to 30 K accordingly.][15] The annealing increases the tunnel barrier thickness probably via one of two ways: by thickening the Cr 2 O 3 surface layer and by precipitating more Cr 2 O 3 in the interface region.
In conclusion, we have studied the microstructure of the natural barrier layer of the spin dependent intergranular tun-neling in CrO 2 powder samples.Direct observation of the barrier by TEM is reported.The barrier is a thin layer of dense and uniform crystalline Cr 2 O 3 on the surface of the single crystal CrO 2 powders.The composition and crystal structure of the layer is confirmed by XRD and XPS techniques.The concentration of Cr 2 O 3 and the thickness of the Cr 2 O 3 surface layer can be increased by heating in air.Enhancing the properties of the oxide layer, for example, changing the thickness or impurity level of this Cr 2 O 3 layer, may be helpful to obtain more desirable magnetotransport properties for applications.

FIG. 1 .
FIG. 1. MR of the cold-pressed CrO 2 powder compact at Tϭ5 K. Inset shows the low field MR and hysteresis loop.

FIG. 5 .
FIG. 5. X-ray diffraction pattern of the CrO 2 powders.Inset shows the detailed scan over the range 23°Ͻ2Ͻ26°and 32°Ͻ2Ͻ35°.Peaks corresponding to the ͑012͒ and ͑104͒ reflections of Cr 2 O 3 are shown.FIG. 6. ln R as a function of 1/T 1/2 at low temperature for CrO 2 sample ͑᭺͒ and the sample after heating at 400 °C for 20 min ͑᭹͒.R has been normalized to the value at Tϭ300 K. Inset shows the XRD of the heated sample showing about 20% -25%Cr 2 O 3 .