Session C47: Damping and Spin Polarization in Heusler Alloys

Damping in Materials for Spintronic Applications

The next generation of spintronic devices relies strongly on the development of new materials with high spin polarization, optimized intrinsic damping and tunable magnetic anisotropy. Therefore, technological progress in this area depends heavily on the successful search for new materials as well as on a deeper understanding of the fundamental mechanisms of the spin polarization, the damping and the magnetic anisotropy. This talk will focus on different aspects of materials with a low intrinsic relaxation rate. Our results are based on first principles calculations in combination with a non-orthogonal tight-binding model to predict those material properties for complex materials [1-4] which can be used for example in new spin based memory devices or logic devices. However, the intrinsic damping parameter predicted from first principle calculations does not take into account adjacent layers that are present in the final device. Spin pumping is a well-known contribution that has to be taken into account for practical applications using multilayer structures [1]. More recently a strong unidirectional contribution to the relaxation in exchange bias systems has been observed experimentally [5]. To describe this phenomenon theoretically we use the formalism of an anisotropic Gilbert damping tensor that takes the place of the (scalar) Gilbert damping parameter in the Landau-Lifshitz-Gilbert equation of motion. While for single crystals this anisotropy is expected to be small, making experimental confirmation difficult, the broken symmetry in exchange bias systems provides an excellent testing ground to study the modified magnetization dynamics under the influence of unidirectional damping. References: [1] Book chapter, ISBN: 978-981-4613-04-0, [2] Appl. Phys. Lett. 104, 22412 (2014), [3] Appl. Phys. Lett. 95, 082502 (2009), [4] Appl. Phys. Lett. 95, 022509 (2009), [5] IEEE Magn. Lett. 1, 3500204 (2010).   

Chemical-disorder effects in half-metallic Heusler alloys

The electronic structure, magnetism, and conductivity of disordered Heusler alloys are investigated theoretically and experimentally. Emphasis is on Heuslers that are predicted to be half-metallic or spin-gapless semiconducting in their fully ordered states, such as CoFeCrAl. Experimental alloys often exhibit resistivities of the order of 200 $\mu \Omega $cm, which is consistent with both dirty-metal and spin-gapless semiconducting behaviors, but a distinction can be made by comparing the residual resistivities of samples having different degrees of order. The corresponding analysis shows that careful processing is necessary to keep the alloys in the spin-gapless semiconducting regime. Several types of disorder occur in Y-type Heusler alloys, with different negative effects on half-metallicity and spin-gapless semiconductivity. A2 (or bcc) disorder is most harmful but can be limited experimentally in CoFeCrAl thin films. B2 (or CsCl) disorder is less harmful, whereas L2$_{\mathrm{1}}$ (normal Heusler) disorder, corresponding to Fe-Co solid solubility is least harmful. The residual resistivity of the partially diorderd alluys is discussed as a carrier-localization effect. --- This work is supported by DOE BES (DE-FG02-04ER46152, R.S.) and NSF DMREF (SusChEM 1436385, D.J.S.)   

Probing spin polarization in Heusler alloy thin films via point-contact Andreev reflection

The utilization of the spins of electrons has led to many important device applications. In order to further improve the spin-dependent signals in spintronic devices, incorporating half metals (i.e. materials with a spin polarization of 100{\%}) into current device designs is highly desired. Besides several confirmed examples of half metals, such as CrO$_{\mathrm{2}}$, mixed-valence manganites, etc., Heusler alloys have also been predicted to exhibit a tunable spin polarization that can reach as high as 100{\%}. In this work, we use a combinatorial fabrication method together with point-contact Andreev reflection (PCAR) measurements to determine the spin polarization in Heusler alloys. Both single-composition Co$_{\mathrm{2}}$FeAl thin films and composition-spread thin films were fabricated on Si (001) or MgO (001) substrates in a DC magnetron co-sputtering system with a base pressure below 5$\times$ 10$^{\mathrm{-7}}$ Torr. X-ray diffraction measurements indicate that the films are primarily c-axis orientated. In order to suppress the current crowding effect and also increase the junction stability during the PCAR measurements, various buffer layers and/or capping layers were adapted. Further, by using a pre-sharpened Nb tip to complete a superconductor/ferromagnet junction for each sample, a spin polarization in a range of 55{\%}-65{\%} has been obtained at low temperatures.   

Growth and characterization of half-Heusler half-metal candidate CoTi$_{1-x}$Fe$_{x}$Sb

Recent predictions suggest the semiconducting half-Heusler compound, CoTiSb, exhibits half-metallicity when substitutionally alloyed with Fe. However, to date, few studies have examined the growth of high-quality single crystal thin films of Fe doped CoTiSb. Here, we report the epitaxial growth of the substitutionally alloyed half-Heusler series CoTi$_{1-x}$Fe$_{x}$Sb by molecular beam epitaxy and the influence of Fe on the structural, electronic, and magnetic properties. CoTi$_{1-x}$Fe$_{x}$Sb epitaxial films are grown on InAlAs (001) grown on InP (001) substrates for concentrations 0$\le $x$\le $1. The films are epitaxial and single crystalline, as measured by reflection high-energy electron diffraction and XRD. For higher Fe content films a lower growth temperature is necessary to minimize interfacial reactions. Using \textit{in-situ} XPS, only small changes in the valence band are detected for Fe x$\le $0.15. Temperature dependent transport reveals thermally activated behavior for x$\le $0.5. Ferromagnetism is observed in SQUID magnetometry with a Curie temperature \textgreater 400K and a saturation magnetization of 3.4 $\mu $B/Fe atom formula unit. Ferromagnetic resonance indicates a transition from weak to strong interaction and homogeneity as Fe content is increased.   

Martensitic phase transitions in Ni2-xCrxMnGa Heusler alloys

The coupled first order magneto-crystalline transformation exhibited by ferromagnetic materials is of significant interest. The martensitic phase transformation is one such transition that is often observed in intermetallic alloys like Ni2MnGa. The material has been extensively studied for many years and it has been shown that the ferromagnetic transition temperature, TC, and the martensitic transformation temperature, TM, in this material can be precisely controlled by manipulating the stoichiometry and/or by atomic doping. In one such recent study it was shown that the partial replacement of Mn by Cr in Ni2Mn1-xCrxGa, caused an increase of TM and decrease of TC in the system. The most notable behavior was detected in the resistivity data of the materials, in the vicinity of TM, step-like drop was observed in the resistivity data and the magnitude of the drop increased dramatically with increasing Cr concentration. Considering this atypical behavior in the resistivity data of Ni2Mn1-xCrxGa system, it is interesting to investigate the transport properties of Ni2-xCrxMnGa system, where Ni (instead of Mn) is partially replaced with Cr. Therefore, we have investigated a series of Ni2-xCrxMnGa compounds by x-ray diffraction, dc magnetization, and electrical resistivity measurements. The goal was to explore the change in the nature of the martensitic transformation in the alloys caused by the Cr doping. The results show, TM decreases while TC increases with increasing Cr concentration. For x \textgreater 0.2, no martensitic transformation is observed in the materials. Interestingly, the sharp step-like drops observed in the resistivity of the Ni2Mn1-xCrxGa system was not observed in the Ni2-xCrxMnGa materials. The experimental results are discussed considering the intrinsic disorder and associated electronic structure of the system. .   

Effects of pressure and strain on spin polarization of IrMnSb

A high degree of spin polarization in electron transport is one of the most sought-after properties of a material which can be used in spintronics. An ideal candidate to exhibit highly spin-polarized current would be a room temperature half-metal, a material which behaves as an insulator for one spin channel and as a conductor for the other spin channel. Computational results on structural, magnetic, and electronic properties of a semi-Heusler compound, IrMnSb will be presented. This material has been reported to exhibit pressure induced half-metallic transition. This result is confirmed, and explained by the reduction of the exchange splitting of the spin bands consistent with the Stoner model for itinerant magnetism. It is also shown that the half-metallic transition is suppressed when instead of uniform pressure the bulk IrMnSb is exposed to biaxial strain. This suppression of half-metallicity is driven by the epitaxial strain induced tetragonal distortion, which lifts the degeneracy of the Mn 3d t$_{2g}$ and e$_{g}$ orbitals and reduces the minority-spin band gap under compressive strain, thus preventing half-metallic transition. Finally, it is demonstrated that in thin film geometry, surface states emerge in the minority-spin band gap, which has detrimental for practical applications impact on the spin polarization of IrMnSb.   

Perpendicularly magnetized Mn-based binary films compatible with semiconductor in structure and technique

Ferromagnetic films with both high perpendicular anisotropy and good compatibility with semiconductors have great potential not only in semiconductor spintronic devices, but also in high-density integration of metallic spintronic functional devices like nonvolatile MRAM on semiconductor circuits [1]. Recently, we have grown the high-quality $L$1$_{\mathrm{0}}$-MnGa and $L$1$_{\mathrm{0}}$-MnAl films on GaAs by MBE, which show giant perpendicular magnetic anisotropy [2-5]. Moreover, annealing studies revealed the thermal stability of \quad them up to at least 350C, indicating its compatible with current semiconductor industry technique [3]. Here, we will present the orbital two-channel Kondo (2CK) effect observed in ferromagnetic $L$1$_{\mathrm{0}}$-MnAl and $L$1$_{\mathrm{0}}$-MnGa, which provide the first evidence for the presence of 2CK effect in a ferromagnet [6-8]. The tunneling magnetic resistivity of $L$1$_{\mathrm{0}}$-MnGa-based perpendicularly magnetic tunnel junctions will also be mentioned. \textbf{References: }[1]~\textit{Appl. Phys.} A 111 (2013) 379; [2] \textit{Adv. Mater. }24 (2012) 4547; [3]~\textit{Appl. Phys. Lett.} 102 (2013) 132403; [4] \textit{Appl. Phys. Lett.} 102 (2013) 152405; [5] \textit{Phys. Rev.} B 89 (2014) 220406(R); [6] \textit{Nature Commun}. 7 (2016) 10817; [7] \textit{Phys. Rev. }B 93 (2016) 195112; [8] \textit{Sci. Rep.} 6 (2016) 34549   

Synthesis of low-moment CrVTiAl: a potential room temperature spin filter

The efficient production of spin-polarized currents at room temperature is fundamental to the advancement of spintronics. Spin-filter materials -- semiconductors with unequal band gaps for each spin channel -- can generate spin-polarized current without the need for spin-polarizing electrodes. In addition, a spin-filter material with zero magnetic moment would have the advantage of not producing fringing fields to interfere with neighboring components. Several quaternary Heusler compounds have recently been predicted to have spin-filter properties and Curie temperatures T$_{\mathrm{C}}$ \textgreater 1000 K[1]. In this work, CrVTiAl has been synthesized in the Y-type Heusler structure, as confirmed by X-ray diffractometry. Magnetization measurements exhibit an exceptionally small temperature-independent moment of 10$^{\mathrm{-3}} \quad \mu _{\mathrm{B}}$/f.u. up to 400 K, a result that is consistent with zero-moment ferrimagnetism. In addition, temperature dependent resistivity measurements reveal the existence of a semiconducting conduction channel. These results suggest that CrVTiAl is a promising candidate for future spintronic devices. [1] I. Galanakis, K. Ozdo\u{g}an, and E. Sa\c{s}\i ogÞlu, J. Phys. Condens. Matter 26, 086003 (2014).   

Low moment ferrimagnetism in Mn$_3$Al as probed by Polarized Neutron Reflectometry

For future spintronic devices, it is paramount to limit stray magnetic interactions which can negatively impact spin injection. A new class of materials called half-metallic antiferromagnets or compensated ferrimagnets have been proposed to remedy this problem. In this work, Mn$_3$Al thin films have shown promising room temperature low-moment ferrimagnetic magnetic properties. Epitaxial Mn$_3$Al thin films (50 nm) were grown on desorbed GaAs(001) substrates via MBE at 200 $^o$C and annealed further to temperatures between 250 - 350 $^o$C. The D0$_3$ Heusler-type phase was determined by X-ray diffraction with texturing in the [311] direction. Density functional theory, performed using VASP, indicated that the crystallographic structure of Mn$_3$Al is able to form energetically with a low magnetic moment (0.017 $\mu_B$/f.u.) despite some epitaxial distortion. SQUID magnetometry confirmed the low magnetic moment and high Curie temperature (610 K) of the structure. Polarized Neutron Reflectometry was used to determine the effect of epitaxy on the magnetic moment of Mn$_3$Al, and analysis confirms a low magnetic moment (0.11 $\mu_B$/f.u.) for the samples annealed at temperatures between 200-300 $^o$C. This analysis further suggests that the relaxation of the Mn$_3$Al at the interface.   

Spin excitation gap in epitaxial Co2FeSi thin films revealed by longitudinal resistivity and negative magnetoresistance

Heusler alloys hold great promise for spintronic applications because of their potential half-metallicity, as suggested by electronic structure calculations for certain cases, such as Co$_{\mathrm{2}}$Fe$_{\mathrm{x}}$Mn$_{\mathrm{1-x}}$Si [B. Balke \textit{et al.}, PRB \textbf{74}, 104405 (2006)]. Here we report on signatures of a minority spin gap in Co$_{\mathrm{2}}$FeSi using transport measurements. The 5-nm thick Co$_{\mathrm{2}}$FeSi thin film sample studied in this work is grown epitaxially on a GaAs (100) substrate. In addition to typical phonon and weak-localization contributions, the temperature dependence of the resistivity shows a spin-fluctuation contribution that is suppressed at low temperatures, consistent with the presence of a minority spin gap of approximately 500 K. Most significantly, the Co$_{\mathrm{2}}$FeSi shows a linear and isotropic negative magnetoresistance that increases with increasing temperature, reaching a magnitude of 0.012 $\mu \Omega $ cm T$^{\mathrm{-1}}$ at room temperature. Once the weak localization contribution at low temperature is removed, the temperature dependence of the negative magnetoresistance can be fitted using a simple model that includes a zero-field spin gap obtained from the resistivity measurement and a field-dependent contribution that can be obtained from ferromagnetic resonance measurements.   

Probing the magnetic structure of Co$_{\mathrm{2}}$Fe$_{\mathrm{x}}$Mn$_{\mathrm{1-x}}$Si thin films

We have analyzed the magnetic configuration for highly ordered epitaxial thin films across the Co$_{\mathrm{2}}$Fe$_{\mathrm{x}}$Mn$_{\mathrm{1-x}}$Si compositional series (x $=$ 0, 0.3, 0.7, 1) by x-ray circular magnetic dichroism (XMCD) and x-ray absorption spectroscopy (XAS). These measurements give the element-specific electronic structure of each film, as well as the spin and orbital moments. We will combine these results with conventional magnetometry and microscopy to explain variations from expected Slater-Pauling-like behavior that we see in material and device properties as a function of stoichiometry.