Influence of hot pressure on the magnetoresistance of Cr O 2

In this paper, we investigate the influence of high temperature and high pressure (hot pressure) on the magnetic and transport properties of polycrystalline CrO2 samples compacted under high pressure and high temperature of up to 5GPa and 600°C, respectively. The magnetic moment increases with compacting temperature, and a metal-semiconductor transition is observed in hot-pressed samples, different from the cold-pressed samples. These results indicate that the formation of Cr2O3 at the grain boundaries of CrO2 is suppressed by hot pressure. The magnitude of low field magnetoresistance of up to 1T at 5K is enhanced first with the increase of compacting temperature and then decreased under higher compacting temperature. This result can be well explained by the change of spin-dependent tunneling at the modulated grain boundaries of CrO2 due to the transformation from Cr2O3 to CrO2 under hot pressure.


I. INTRODUCTION
As the simplest half-metallic oxide predicted by band structure calculation, 1 the magnetotransport properties of CrO 2 have drawn much attention because it has nearly perfect spin polarization ͑close to 100%͒ and higher Curie temperature T C of 395 K, which are necessary for spintronic device applications. 2,3The large low field magnetoresistance ͑MR͒ originating from the spin-dependent carrier tunneling at grain boundary has been observed in CrO 2 granular samples at low temperature, [3][4][5][6][7][8][9] but only a very small value of MR can be obtained in CrO 2 at high temperature.
Usually, there is a naturally grown Cr 2 O 3 layer on the surface of CrO 2 particles, 3,7,10 in which the higher-order hopping of a carrier is considered to be one of the main reasons for the suppressed low field MR at high temperature. 9It is obviously important to remove the Cr 2 O 3 layer for understanding the intrinsic magnetotransport property of CrO 2 .Unfortunately, Cr 2 O 3 is more stable than CrO 2 at atmospheric pressure so it is hard to eliminate this native Cr 2 O 3 layer from the surface of CrO 2 particles in air.It is well known that CrO 2 is stable under high pressure.2][13] Therefore, it is worthwhile to modulate the Cr 2 O 3 layer by high temperature and high pressure ͑hot pressure͒, but clear experimental data on the influence of hot pressure on the magnetotransport property of CrO 2 are still lacking.
In this paper, we report some preliminary results of the magnetic and transport properties of CrO 2 samples compacted under hot pressure.Hot pressure not only prevents CrO 2 from transforming into Cr 2 O 3 but also changes the original Cr 2 O 3 to CrO 2 .An enhanced low field MR and the conductance of polycrystalline CrO 2 can be obtained under suitable hot pressure conditions.

II. EXPERIMENTS
Nanosized CrO 2 powders were supplied by Dupont.They are needle-shaped particles with length of about 400 nm and an aspect ratio of about 9:1.These CrO 2 powders were pressed into a tablet and put into the chamber of a belt-type high-pressure apparatus, and compacted into bulk samples under different pressures and temperatures of up to 5 GPa and 600 °C, respectively.In the hot pressure procedure, the pressure was first increased to a desired value, followed by temperature.After holding the pressure and temperature for 30 min, the temperature was decreased to room temperature first, and then the pressure was released.The measurements of the pressure and temperature are described in Ref. 14.
X-ray diffraction ͑XRD͒ patterns were collected using a Bede D 1 XRD spectrometer with Ni-filtered Cu K␣ radiation.The surface morphology of the samples was obtained using a Hitachi S-4700 field emission scanning electron microscope.The magnetic and transport properties were measured by using the physical properties measurement system ͑PPMS͒ of Quantum Design.

III. RESULTS AND DISCUSSION
Figure 1͑a͒ gives the room temperature XRD patterns of CrO 2 samples annealed under different pressures.Obviously, Cr 2 O 3 may be introduced by annealing the CrO 2 in air at 400 °C for 20 min, 3,9 but there is not peak of Cr 2 O 3 in all hot-pressed CrO 2 samples, indicating that high pressure can suppress the formation of Cr 2 O 3 up to 600 °C.Figures 1͑b͒  and 1͑c͒ show the scanning electron microscopy ͑SEM͒ micrographs for the cold-pressed and hot-pressed CrO 2 .The cold-pressed sample still has needle-shaped nanosized particles, but only large CrO 2 grains are seen in the hot-pressed sample due to the growth of CrO 2 particles under hot pressure.These results are consistent with the changes in the XRD patterns such that the peaks of annealed CrO 2 under high pressure are sharper than the cold-pressed samples.
Figure 2 shows the isothermal magnetization curves of CrO 2 at 5 K.The saturation moments per formula unit ͑M S ͒ and coercivity ͑H C ͒ of the cold-pressed sample are about 1.7 B and 900 Oe, respectively, similar to the original CrO 2 powders. 15However, the H C of the hot-pressed CrO 2 at 5 K decreases rapidly from 900 to 40 Oe with increasing compacting temperature from room temperature to 600 °C, as shown in the inset of Fig. 2.This feature is mainly caused by the increase of grain size due to the growth of CrO 2 particles under hot pressure and is consistent with the results of XRD and SEM.The M S value of the cold-pressed CrO 2 ͑1.7 B ͒ at 5 K and 5 T is much lower than the theoretical value of pure CrO 2 ͑2 B ͒, as reported in literatures.It means that there is a large amount of Cr 2 O 3 on the surface of CrO 2 nanoparticles. 16But it is obvious from the inset of Fig. 2 that M S increases with increasing compacting temperature, and the highest value of M S ͑1.93 B ͒, close to the theoretical value, can be obtained at 2 GPa and 600 °C.This remarkable increase of M S should be attributed to the transformation from antiferromagnetic Cr 2 O 3 to ferromagnetic CrO 2 on the grain surfaces.Because these data were measured in a high magnetic field of 5 T, the contribution due to the grain growth itself can be ignored.CrO 2 is more stable than Cr 2 O 3 under high pressure, thus the Cr 2 O 3 layer on the surface of the CrO 2 particles is eventually eliminated when they grow into larger particles under hot pressure.It should also be noticed that the values of M S of 5 GPa hot-pressed samples are smaller than those at 2 GPa for the same compacting temperature, which suggests that the transformation from Cr 2 O 3 to CrO 2 is difficult under higher pressure because the amount of oxygen in the high-pressure chamber, which is necessary for oxidizing Cr 2 O 3 to form CrO 2 , decreases with increasing pressure.This result also gives strong evidence that the increase in M S arises from the transformation of Cr 2 O 3 to CrO 2 .If it were the change of grain size that is mainly responsible for the increase in M S , the M S of the 5 GPa samples would be larger than the 2 GPa samples at the same compacting temperature because the former should have larger particles than the latter.
The temperature dependence of the resistivity ͑͒ of CrO 2 in zero field is displayed in Figs.3͑a͒ and 3͑b͒.Here, is normalized to the value at 300 K.It is clear that the vs T curves of all cold-pressed samples show a semiconductor behavior in the whole temperature range, which is similar to that of original CrO 2 powders. 9However, the metalsemiconductor ͑M-I͒ transition occurs in hot-pressed samples, indicating that the resistance at the grain boundaries is weakened because much of the original intergranular barriers, such as the Cr 2 O 3 layers, have been removed under hot pressure.It is interesting that the M-I transition can be found in all 2 GPa hot-pressed samples, but only observed in the 5 GPa hot-pressed sample at compacting temperature of 600 °C.We note that all samples showing the transition behavior have higher M S over 1.85 B , and the 5 GPa samples have smaller M S than the 2 GPa samples.It therefore suggests a strong correlation between the amount of Cr 2 O 3 at the grain boundaries and the electrical conductance of the samples.Under a pressure of 2 GPa, there is enough oxygen in the high-pressure chamber for the transformation from Cr 2 O 3 to CrO 2 to occur.The transition occurs at temperatures as low as 400 °C and is easier at higher temperatures.The M-I transition is observed in all hot-pressed samples at 2 GPa.Under higher pressure of 5 GPa, the transition from Cr 2 O 3 to CrO 2 becomes more difficult and much higher temperature is needed to achieve the same.Therefore, the M-I transition only occurs in the 5 GPa hot-pressed sample at 600 °C.
Figure 4 shows the MR of 5 GPa samples as a function of magnetic field at 5 K, and the inset shows the variation of MR at 1 T and 5 K for all samples.Here, MR is defined as MR͑%͒ = ͓ H − max ͔ / H , where H and max are the resistivity of the sample in external field H and the highest resistivity value, respectively.4][5][6][7][8] As discussed earlier, because the amount of Cr 2 O 3 in the 5 GPa hot-pressed samples is much more than that of the 2 GPa hot-pressed samples, the value of MR of the former is larger than that of the latter at the same compacting temperature.However, under lower compacting temperature, the Cr 2 O 3 layer as tunneling barrier still exists at the grain boundaries in the hot-pressed samples, which may become more suitable for spin-dependent tunneling under optimum hot pressure conditions.The largest MR value of 42% is observed in the sample at 5 GPa and 400 °C.With increasing compacting temperature, the amount of the Cr 2 O 3 layer is reduced by the high pressure, and the low field MR is suppressed accordingly.

IV. CONCLUSIONS
In a high-pressure cell, not only the formation of Cr 2 O 3 at high temperature is suppressed, but the original Cr 2 O 3 layer on the CrO 2 particle surfaces has also been transformed to CrO 2 .The change in the amount of Cr 2 O 3 at the grain boundaries leads to the increase of magnetic moment and the appearance of the M-I transition in the hot-pressed CrO 2 samples, as well as changes in the magnetotransport properties of CrO 2 .As a result, the low field MR is remarkably enhanced by the hot pressure, and a large MR value of 42% was observed in the sample hot-pressed at 5 GPa and 400 °C.