S. Wurmehl

Rational design of new materials for spintronics: Co2FeZ (Z = Al, Ga, Si, Ge)

Abstract Spintronic is a multidisciplinary field and a new research area. New materials must be found for satisfying the different types of demands. The search for stable half-metallic ferromagnets and ferromagnetic semiconductors with Curie temperatures higher than room temperature is still a challenge for solid state scientists. A general understanding of how structures are related to properties is a necessary prerequisite for material design. Computational simulations are an important tool for a rational design of new materials. The new developments in this new field are reported from the point of view of material scientists. The development of magnetic Heusler compounds specifically designed as material for spintronic applications has made tremendous progress in the very recent past. Heusler compounds can be made as half-metals, showing a high spin polarization of the conduction electrons of up to 100% in magnetic tunnel junctions. High Curie temperatures were found in Co2-based Heusler compounds with values up to 1120 K in Co2FeSi. The latest results at the time of writing are a tunnelling magnet resistance (TMR) device made from the Co2FeAl0.5Si0.5 Heusler compound and working at room temperature with a (TMR) effect higher than 200%. Good interfaces and a well-ordered compound are the precondition to realize the predicted half-metallic properties. The series Co2FeAl1- xSix is found to exhibit half-metallic ferromagnetism over a broad range, and it is shown that electron doping stabilizes the gap in the minority states for x=0.5. This might be a reason for the exceptional temperature behaviour of Co2FeAl0.5Si0.5 TMR devices. Using x-ray diffraction (XRD), it was shown conclusively that Co2FeAl crystallizes in the B2 structure whereas Co2FeSi crystallizes in the L21 structure. For the compounds Co2FeGa or Co2FeGe, with Curie temperatures expected higher than 1000 K, the standard XRD technique using laboratory sources cannot be used to easily distinguish between the two structures. For this reason, the EXAFS technique was used to elucidate the structure of these two compounds. Analysis of the data indicated that both compounds crystallize in the L21 structure which makes these two compounds suitable new candidates as materials in magnetic tunnel junctions.

Geometric, electronic, and magnetic structure of Co2FeSi: Curie temperature and magnetic moment measurements and calculations.

In this work a simple concept was used for a systematic search for materials with high spin polarization. It is based on two semiempirical models. First, the Slater-Pauling rule was used for estimation of the magnetic moment. This model is well supported by electronic structure calculations. The second model was found particularly for Co{sub 2} based Heusler compounds when comparing their magnetic properties. It turned out that these compounds exhibit seemingly a linear dependence of the Curie temperature as function of the magnetic moment. Stimulated by these models, Co{sub 2}FeSi was revisited. The compound was investigated in detail concerning its geometrical and magnetic structure by means of x-ray diffraction, x-ray absorption, and Moessbauer spectroscopies as well as high and low temperature magnetometry. The measurements revealed that it is, currently, the material with the highest magnetic moment (6{mu}{sub B}) and Curie temperature (1100 K) in the classes of Heusler compounds as well as half-metallic ferromagnets. The experimental findings are supported by detailed electronic structure calculations.

Investigation of Co2FeSi: The Heusler compound with highest Curie temperature and magnetic moment

This work reports on structural and magnetic investigations of the Heusler compound Co2FeSi. X-ray diffraction and Mosbauer spectrometry indicate an ordered L21 structure. Magnetic measurements by means of x-ray magnetic circular dichroism and magnetometry revealed that this compound is, currently, the material with the highest magnetic moment (6μB) and Curie temperature (1100K) in the classes of Heusler compounds as well as half-metallic ferromagnets.

Valence electron rules for prediction of half-metallic compensated-ferrimagnetic behaviour of Heusler compounds with complete spin polarization

In this work, a rule for predicting half-metallic compensated-ferrimagnets in the class of Heusler compounds will be described. This concept results from combining the well-known Slater–Pauling rule with the Kubler rule. The Kubler rule states that Mn on the Y position in Heusler compounds tends to a highly localized magnetic moment. When strictly following this new rule, some candidates in the class of Heusler compounds are expected to be completely compensated-ferrimagnetic but with a spin polarization of 100% at the Fermi energy. This rule is applied to three examples within the class of Heusler compounds. All discussion of the materials is supported by electronic structure calculations.

Slater-Pauling rule and Curie temperature of Co2-based Heusler compounds

A concept is presented serving to guide in the search for materials with high spin polarization. It is shown that the magnetic moment of half-metallic ferromagnets can be calculated from the generalized Slater-Pauling rule. Furthermore, it was found empirically that the Curie temperature of Co2-based Heusler compounds can be estimated from a seemingly linear dependence on the magnetic moment. As a successful application of these simple rules, it was found that Co2FeSi is, actually, the half-metallic ferromagnet exhibiting the highest magnetic moment and the highest Curie temperature measured for a Heusler compound.

Half-metallic ferromagnetism with high magnetic moment and high Curie temperature in Co2FeSi

Co2FeSi crystallizes in the ordered L21 structure as proven by x-ray diffraction and Moβbauer spectroscopy. The magnetic moment of Co2FeSi was measured to be about 6μB at 5 K. Magnetic circular dichroism spectra excited by soft x-rays were taken to determine the element-specific magnetic moments of Co and Fe. The Curie temperature was measured with different methods to be (1100±20)K. Co2FeSi was found to be the Heusler compound as well as the half-metallic ferromagnet with the highest magnetic moment and Curie temperature.

Electronic structure and spectroscopy of the quaternary Heusler alloy Co2Cr1−xFexAl

Quaternary Heusler alloys Co2Cr1 xFexAl with varying Cr to Fe ratio x were investigated experimentally and theoretically. The electronic structure and spectroscopic properties were calculated using the full relativistic Korringa-Kohn-Rostocker method with coherent potential approximation to account for the random distribution of Cr and Fe atoms as well as random disorder. Magnetic effects are included by the use of spin dependent potentials in the local spin density approximation. Magnetic circular dichroism in X-ray absorption was measured at the L2,3 edges of Co, Fe, and Cr of the pure compounds and the x = 0.4 alloy in order to determine element specific magnetic moments. Calculations and measurements show an increase of the magnetic moments with increasing iron content. Resonant (560eV - 800eV) soft X-ray as well as high resolution - high energy (� 3.5keV) hard X-ray photo emission was used to probe the density of the occupied states in Co2Cr0.6Fe0.4Al.

Powder magnetoresistance of Co2Cr0.6Fe0.4Al∕Al2O3 powder compacts

We report on the magnetotransport properties of Co2Cr0.6Fe0.4Al mixed with insulating Al2O3. The powder compacts show a maximum magnetoresistance of 88% at a saturation moment of 0.125T at 295K. Different explanations for the reversible mechanism will be discussed, such as tunneling between contiguous ferromagnetic grains, particle movement, and magnetostriction. The effect in Co2Cr0.6Fe0.4Al∕Al2O3 powder compacts is the largest room temperature magnetoresistance that has been measured until now.

Structural characterization of the Co2FeZ (Z=Al, Si, Ga, and Ge) Heusler compounds by x-ray diffraction and extended x-ray absorption fine structure spectroscopy

This work reports on the structure of Fe containing, Co2-based Heusler compounds that are suitable for magnetoelectronic applications. The compounds Co2FeZ (where Z=Al, Si, Ga, and Ge) were investigated using the x-ray diffraction (XRD) and extended x-ray absorption fine structure (EXAFS) techniques. Using XRD, it was shown conclusively that Co2FeAl crystallizes in the B2 structure whereas Co2FeSi crystallizes in the L21 structure. For compounds containing Ga or Ge, the XRD technique cannot be used to easily distinguish between the two structures. For this reason, the EXAFS technique was used to elucidate the structure of these two compounds. Analysis of the EXAFS data indicated that both compounds crystallize in the L21 structure.

Design of magnetic materials: the electronic structure of the ordered, doped Heusler compound Co2Cr1-xFexAl

The doped Heusler compounds Co2Cr1−xFexAl with varying Cr to Fe ratio x were investigated experimentally and theoretically. The electronic structure of the ordered, doped Heusler compound Co2Cr1−xFexAl (x = n/4,n = 0,1,2,3,4) was calculated using band structure calculations of different types. The ordered compounds turned out to be ferromagnetic with the small Al magnetic moment aligned antiparallel to the 3d transition metal moments. All compounds show a gap around the Fermi energy in the minority bands. The pure compounds exhibit an indirect minority gap, whereas the ordered, doped compounds exhibit a direct gap. The magnetic circular dichroism in the x-ray absorption spectra was measured at the L2,3 edges of Co, Fe, and Cr of the pure compounds and the x = 0.4 alloy in order to determine element-specific magnetic moments. Calculations and measurements show an increase of the magnetic moments with increasing iron content. The experimentally observed reduction of the magnetic moment of Cr can be explained by Co–Cr site disorder. The presence of the gap in the minority bands of Co2CrAl can be attributed to the occurrence of pure Co2 and mixed CrAl(001) planes in the L 21 structure. It is retained in structures with different order of the CrAl planes but vanishes in the X structure with alternating CoCr and CoAl planes.Doped Heusler compounds Co