Charge dynamics in the half-metallic ferromagnetCrO2

Abstract

Infrared spectroscopy is used to investigate the electronic structure and charge carrier relaxation in crystalline films of CrO 2 which is the simplest of all half-metallic ferromagnets. Chromium dioxide is a bad metal at room temperature but it has a remarkably low residual resistivity (Ͻ5 ⍀ cm) despite the small spectral weight associated with free carrier absorption. The infrared measurements show that low residual resistivity is due to the collapse of the scattering rate at Ͻ2000 cm Ϫ1 . The blocking of the relaxation channels at low and T can be attributed to the unique electronic structure of a half-metallic ferromagnet. In contrast to other ferromagnetic oxides, the intraband spectral weight is constant below the Curie temperature. ͓S0163-1829͑99͒14929-4͔Chromium dioxide is one of few binary oxides that are ferromagnetic metals. Acicular powders have long been used in particulate media for magnetic recording, but key aspects of the electronic structure of CrO 2 remain unexplored. Its resistivity dc is unusual: the residual resistivity 0 can be as small 5 ⍀ cm, but above 80 K, dc becomes strongly temperature dependent and exceeds 1000 ⍀ cm above the Curie temperature T C ϭ390 K, 1,2 a value which is beyond the Ioffe-Regel limit. Hence, CrO 2 may be classified as a ''bad metal. '' 3,4 Following the original paper by Schwarz, 5 numerous band structure calculations indicate that CrO 2 is a halfmetallic ferromagnet with completely spin-polarized spin-up electrons at E F and a wide gap in the spin-down density of states ͑DOS͒. 4,6-10 Recently a 90% spin polarization at E F has been measured in CrO 2 using superconducting point contact spectroscopy. 11 Here we investigate the electronic structure and charge dynamics in CrO 2 by means of reflectance spectroscopy carried out over the frequency range from 500 GHz to near ultraviolet. Our experiments reveal an interband contribution to the complex conductivity that is in accordance with a band structure entailing a high degree of spin polarization of the electronic states at E F . A half-metallic density of states implies a temperature-and frequency-dependent intraband scattering rate. The strong suppression of the scattering rate that we observe as , T˜0 is consistent with this notion of a ͑partial͒ gap in the electronic DOS. We have also analyzed the temperature dependence of the electronic spectral weight (N e f f ). In contrast to other ferromagnetic oxides, such as the manganites, we find no variation of N e f f with temperature.We measured the near-normal incidence reflectance R( ) of crystalline films of CrO 2 grown on a TiO 2 ͓110͔ substrate. 2 Films with thickness Ͼ5 m were found to be completely opaque throughout the experimental frequency range 14-37 000 cm Ϫ1 ͑2 -4600 meV͒. Films were coated in situ with gold or aluminum in the optical cryostat and the spectrum of the metal-coated sample was used as a reference. 12 Experimental errors due to diffuse reflectance are minimized with this procedure, allowing reliable absolute values of R( ) to be obtained. ...

Evidence for two-band magnetotransport in half-metallic chromium dioxide

“…15 Such films have shown a record spin polarization of 90%, 12 and have been used in many recent studies. 13,14,16 The films were textured and polycrystalline in character, exhibiting needlelike grains aligned in the film plane. Magnetic measurements indicate a Curie temperature of 395 K and an atomic magnetic moment near 2 B , in accordance with published values.…”

“…This makes CrO 2 a good candidate for use as a spin injector and has sparked the revival of CrO 2 thinfilm growth techniques, including the original high-pressure, thermal decomposition method 2 as well as a chemical vapor deposition ͑CVD͒ technique discovered in the late 1970s. 9 A number of measurements, including Kerr effect, 10 photoemission spectroscopy, 11 Andreev reflection, 12 infrared spectroscopy, 13 Raman spectroscopy, 14 and magnetotransport, [15][16][17][18] have been recently published ͑to name a few͒.…”

Colloquium: Saturation of electrical resistivity

“…Fig. 9 compares the experimental σ(ω) at T = 300 K (Singley et al, 1999) with the large T theoretical model in Fig. 10.…”

“…If the atoms vibrate or if there is hopping between more distant neighbors, d is a weighted average of neighboring distances (Calandra and Gunnarsson, 2002). Singley et al, 1999) and the large T model in Fig. 10 (dotted line), adjusting the area to the sum rule (6).…”

Effects of 3dand 4dtransition metal substitutional impurities on the electronic properties of CrO2

“…Experimentally, its spin polarization has been shown to be very close to 100%. [9,10] Although the very high spin-polarization predicted by mean field electronic structure calculations has been experimentally confirmed, we should mention that the degree of electron correlation in CrO 2 remains a subject of debate and research [19][20][21][22][23][24][25][26][27][28][29][30]. The facts that CrO 2 is an excellent metal at low temperature [31,32], that its optical properties appear to be consistent with band theory predictions [23,30] and that its relatively low magnetocrystalline anisotropy is more consistent with weak than strong correlations [24] have been used to argue for the validity of a weakly correlated mean field picture.…”