Anomalous transport properties of the halfmetallic ferromagnets Co2TiSi, Co2TiGe, and Co2TiSn

In this work the theoretical and experimental investigations of Co2TiZ (Z = Si, Ge, or Sn) compounds are reported. Half-metallic ferromagnetism is predicted for all three compounds with only two bands crossing the Fermi energy in the majority channel. The magnetic moments fulfill the Slater-Pauling rule and the Curie temperatures are well above room temperature. All compounds show a metallic like resistivity for low temperatures up to their Curie temperature, above the resistivity changes to semiconducting like behavior. A large negative magnetoresistance of 55% is observed for Co2TiSn at room temperature in an applied magnetic field of 4T which is comparable to the large negative magnetoresistances of the manganites. The Seebeck coefficients are negative for all three compounds and reach their maximum values at their respective Curie temperatures and stay almost constant up to 950 K. The highest value achieved is -52muV/K m for Co2TiSn which is large for a metal. The combination of half-metallicity and the constant large Seebeck coefficient over a wide temperature range makes these compounds interesting materials for thermoelectric applications and further spincaloric investigations.Comment: 4 pages 4 figure

Epitaxial film growth and magnetic properties of Co_2FeSi

We have grown thin films of the Heusler compound Co_2FeSi by RF magnetron sputtering. On (100)-oriented MgO substrates we find fully epitaxial (100)-oriented and L2_1 ordered growth. On Al_2O_3 (11-20) substrates, the film growth is (110)-oriented, and several in-plane epitaxial domains are observed. The temperature dependence of the electrical resistivity shows a power law with an exponent of 7/2 at low temperatures. Investigation of the bulk magnetic properties reveals an extrapolated saturation magnetization of 5.0 mu_B/fu at 0 K. The films on Al_2O_3 show an in-plane uniaxial anisotropy, while the epitaxial films are magnetically isotropic in the plane. Measurements of the X-ray magnetic circular dichroism of the films allowed us to determine element specific magnetic moments. Finally we have measured the spin polarization at the surface region by spin-resolved near-threshold photoemission and found it strongly reduced in contrast to the expected bulk value of 100%. Possible reasons for the reduced magnetization are discussed.Comment: 9 pages, 12 figure

Ion beam induced modification of exchange interaction and spin-orbit coupling in the Co2_2FeSi Heusler compound

A Co$_2$FeSi (CFS) film with L2$_1$ structure was irradiated with different fluences of 30 keV Ga$^+$ ions. Structural modifications were subsequently studied using the longitudinal (LMOKE) and quadratic (QMOKE) magneto-optical Kerr effect. Both the coercivity and the LMOKE amplitude were found to show a similar behavior upon irradiation: they are nearly constant up to ion fluences of $\approx6\times10^{15}$ ion/cm$^2$, while they decrease with further increasing fluences and finally vanish at a fluence of $\approx9\times10^{16}$ ion/cm$^2$, when the sample becomes paramagnetic. However, contrary to this behavior, the QMOKE signal nearly vanishes even for the smallest applied fluence of $3\times10^{14}$ ion/cm$^2$. We attribute this reduction of the QMOKE signal to an irradiation-induced degeneration of second or higher order spin-orbit coupling, which already happens at small fluences of 30 keV Ga$^+$ ions. On the other hand, the reduction of coercivity and LMOKE signal with high ion fluences is probably caused by a reduction of the exchange interaction within the film material

Ferrimagnetism and disorder in epitaxial Mn(2-x)Co(x)VAl thin films

The quaternary full Heusler compound Mn(2-x)Co(x)VAl with x = 1 is predicted to be a half-metallic antiferromagnet. Thin films of the quaternary compounds with x = 0...2 were prepared by DC and RF magnetron co-sputtering on heated MgO (001) substrates. The magnetic structure was examined by x-ray magnetic circular dichroism and the chemical disorder was characterized by x-ray diffraction. Ferrimagnetic coupling of V to Mn was observed for Mn2VAl (x = 0). For x = 0.5, we also found ferrimagnetic order with V and Co antiparallel to Mn. The observed reduced magnetic moments are interpreted with the help of band structure calculations in the coherent potential approximation. Mn2VAl is very sensitive to disorder involving Mn, because nearest-neighbor Mn atoms couple anti-ferromagnetically. Co2VAl has B2 order and has reduced magnetization. In the cases with x >= 0.9 conventional ferromagnetism was observed, closely related to the atomic disorder in these compounds.Comment: 10 pages, 4 figure

Tuning of crystal structure and magnetic properties by exceptionally large epitaxial strains

Huge deformations of the crystal lattice can be achieved in materials with inherent structural instability by epitaxial straining. By coherent growth on seven different substrates the in-plane lattice constants of 50 nm thick Fe70Pd30 films are continuously varied. The maximum epitaxial strain reaches 8,3 % relative to the fcc lattice. The in-plane lattice strain results in a remarkable tetragonal distortion ranging from c/abct = 1.09 to 1.39, covering most of the Bain transformation path from fcc to bcc crystal structure. This has dramatic consequences for the magnetic key properties. Magnetometry and X-ray circular dichroism (XMCD) measurements show that Curie temperature, orbital magnetic moment, and magnetocrystalline anisotropy are tuned over broad ranges.Comment: manuscript, 3 figures, auxiliary materia

Role of defects and disorder in the half-metallic full-Heusler compounds

Half-metallic ferromagnets and especially the full-Heusler alloys containing Co are at the center of scientific research due to their potential applications in spintronics. For realistic devices it is important to control accurately the creation of defects in these alloys. We review some of our late results on the role of defects and impurities in these compounds. More precisely we present results for the following cases (i) doping and disorder in Co$_2$Cr(Mn)Al(Si) alloys, (ii) half-metallic ferrimagnetism appeared due to the creation of Cr(Mn) antisites in these alloys, (iii) Co-doping in Mn$_2$VAl(Si) alloys leading to half-metallic antiferromagnetism, and finally (iv) the occurrence of vacancies in the full-Heusler alloys containing Co and Mn. These results are susceptible of encouraging further theoretical and experimental research in the properties of these compounds.Comment: Chapter intended for a book with contributions of the invited speakers of the International Conference on Nanoscale Magnetism 2007. Revised version contains new figure

Second asymptomatic carotid surgery trial (ACST-2): a randomised comparison of carotid artery stenting versus carotid endarterectomy

Background: Among asymptomatic patients with severe carotid artery stenosis but no recent stroke or transient cerebral ischaemia, either carotid artery stenting (CAS) or carotid endarterectomy (CEA) can restore patency and reduce long-term stroke risks. However, from recent national registry data, each option causes about 1% procedural risk of disabling stroke or death. Comparison of their long-term protective effects requires large-scale randomised evidence. Methods: ACST-2 is an international multicentre randomised trial of CAS versus CEA among asymptomatic patients with severe stenosis thought to require intervention, interpreted with all other relevant trials. Patients were eligible if they had severe unilateral or bilateral carotid artery stenosis and both doctor and patient agreed that a carotid procedure should be undertaken, but they were substantially uncertain which one to choose. Patients were randomly allocated to CAS or CEA and followed up at 1 month and then annually, for a mean 5 years. Procedural events were those within 30 days of the intervention. Intention-to-treat analyses are provided. Analyses including procedural hazards use tabular methods. Analyses and meta-analyses of non-procedural strokes use Kaplan-Meier and log-rank methods. The trial is registered with the ISRCTN registry, ISRCTN21144362. Findings: Between Jan 15, 2008, and Dec 31, 2020, 3625 patients in 130 centres were randomly allocated, 1811 to CAS and 1814 to CEA, with good compliance, good medical therapy and a mean 5 years of follow-up. Overall, 1% had disabling stroke or death procedurally (15 allocated to CAS and 18 to CEA) and 2% had non-disabling procedural stroke (48 allocated to CAS and 29 to CEA). Kaplan-Meier estimates of 5-year non-procedural stroke were 2·5% in each group for fatal or disabling stroke, and 5·3% with CAS versus 4·5% with CEA for any stroke (rate ratio [RR] 1·16, 95% CI 0·86–1·57; p=0·33). Combining RRs for any non-procedural stroke in all CAS versus CEA trials, the RR was similar in symptomatic and asymptomatic patients (overall RR 1·11, 95% CI 0·91–1·32; p=0·21). Interpretation: Serious complications are similarly uncommon after competent CAS and CEA, and the long-term effects of these two carotid artery procedures on fatal or disabling stroke are comparable. Funding: UK Medical Research Council and Health Technology Assessment Programme