Session H19: Spin-Order and Half-Metallicity of Magnetic Thin Films

Influence of interstitial Mn on spin order and dynamics in the room-temperature ferromagnet Mn$_{1+\delta}$Sb

Mn$_{1+\delta}$Sb is a well-known, high Curie temperature, ferromagnetic metal. It has particular importance because it, and closely related MnBi, show promise as alternatives to rare-earth-containing permanent magnets, and as magneto-optic media. To exploit these materials’ useful properties, it is desirable to tune and optimize the magnetic properties [1]. To achieve this, the magnetic interactions, and the effects of doping and defects must be understood. In Mn$_{1+\delta}$Sb the magnetic order is highly sensitive to the interstitial Mn ion content, $\delta$, suggesting a route to tune the properties [2]. However, detailed theoretical and experimental investigations of the effect of the interstitial ion, Mn2, have been lacking, probably due to a prevailing view in the literature that the Mn2 site is nonmagnetic [3,4]. We examine the magnetic state of Mn2, and its influence on the magnetic properties of Mn$_{1+\delta}$Sb. We use a combination of neutron scattering techniques alongside detailed calculations to show that the Mn2 site is in-fact magnetic, and has a dramatic impact on the magnetic dynamics in Mn$_{1+\delta}$Sb. An unusual, broad, intense feature is identified in the magnetic dynamics which cannot be explained by the long-range symmetry of the material. This reveals an area in which current theoretical/modeling techniques limit our ability to understand the magnetic excitations revealed by neutron scattering. This investigation elucidates important aspects of the behavior of Mn$_{1+\delta}$Sb, whilst highlighting requirements for future research to understand the major influence of the interstitial ion on the magnetic properties. [1] A. E. Taylor et al., Phys. Rev. B, 91, 224418 (2015). [2] T. Okita and Y. Makino, J. Phys. Soc. Jpn. 25, 120 (1968). [3] Y. Yamaguchi et al., J. Phys. Soc. Jpn. 45, 846 (1978). [4] W. Reimers et al., J. Phys. Chem. Solids 44, 195 (1983).   

Magnetic Structure and Dynamics in the Itinerant High-Temperature Ferromagnet MnBi

The high-temperature ferromagnet MnBi has been receiving much attention as a rare-earth-free permanent magnet to replace more costly rare-earth-containing magnets in applications above room temperature. This is due to MnBi containing strong Mn moments and large energy products. The synthesis of MnBi also allows for crystals that are free from interstitial Mn, allowing for the study of a more fundamental member of this family of binary Mn-based ferromagnets. In this work, we use polarized neutron diffraction to measure the magnetic moments of Mn and Bi, and find that the Bi atoms also have a magnetic moment, but 2 orders of magnitude smaller than Mn. We study their behavior through the spin reorientation that occurs at T$_S$~$\approx$~100~K, finding that both moments reorient simultaneously. We also use inelastic neutron scattering to measure the spin waves of MnBi in order to determine the magnetic exchange at low temperatures. Consistent with the strongly temperature-dependent magnetic anisotropy, we find that the spin gap is very small, and so the magnetic order arises from the strongly ferromagnetic nearest-neighbor term, but interactions up to sixth nearest neighbor are required to fully characterize the spin waves, suggesting that the Mn moments are strongly itinerant.   

Electronic and magnetic properties of ferromagnetic interfaces for spin injection applications: metallic and semiconducting cases

Robust and reliable operation of spintronic devices is determined by the quality of interfaces between magnetic and nonmagnetic materials. In order to get insights in the tuning of the magnetic properties of such interfaces we present comparative studies of two important cases relevant to applications in spin injection devices. We performed ab-initio calculations of the electronic and magnetic properties, of the ferromagnetic metallic interface of Co$_{2}$MnAl and gold, and of the interfaces of non-and of magnetic II-VI semiconductors and their quantum wells. In the case of the Heusler alloy Co$_{2}$MnAl-Au, two structural models are implemented: one with the ferromagnet slab terminated in a pure cobalt plane (Co$_{2}$-t), and the other with it terminated with a plane of MnAl (MnAl-t). The electric in-plane and averaged potential are resolved and analyzed layer by layer through the interface. We predict that both terminations are to be expected to display sensibly different spin injection performances. On the example of magnetic quantum wells of ZnSe$\slash$Zn$_{x}$Mn$_{1-x}$Te$\slash$ZnSe, we study the variations in the spin resolved density of states, and the potential energy along the junctions.   

Structural and magnetic properties of a prospective spin gapless semiconductor MnCrVAl

Recently a new class of material, spin gapless semiconductors (SGS), has attracted much attention because of their potential for spintronic devices. We have synthesized a Heusler compound, MnCrVAl, which is theoretically predicted to exhibit SGS by arc melting, rapid quenching and thermal annealing. First principles calculations are employed to describe its structural, electronic and magnetic properties. X-ray diffraction indicates that the rapidly quenched samples crystallize in the disordered cubic structure. The crystal structure is stable against heat treatment up to 650$^{o}$C. The samples show very small saturation magnetization, 0.3 emu/g, at room temperature under high magnetic field, 30 kOe. Above room temperature, the magnetization increases with increasing temperature undergoing a magnetic transition at 560$^{o}$C, similar to an antiferromagnetic-to-paramagnetic transition. The prospect of this material for spintronic applications will be discussed.   

Enhancement of ferromagnetism by Ag doping in Ni-Mn-In-Ag Heusler alloys

The effect of Ag on the structural, magnetocaloric, and thermomagnetic properties of Ni$_{\mathrm{50}}$Mn$_{\mathrm{35}}$In$_{\mathrm{15-x}}$Ag$_{\mathrm{x}}$ (x$=$ 0.1, 0.2, 0.5, and 1) Heusler alloys was studied. The magnitude of the magnetization change at martensitic transition temperature (T$_{\mathrm{M}})$ decreases with increasing Ag concentration A smaller magnetic entropy changes ($\Delta $S$_{\mathrm{M}})$ for the alloys with higher Ag concentration is observed. A shift of T$_{\mathrm{M}}$ by about 25 K to a higher temperature was detected for P $=$ 6.6 kbar with respect to ambient pressure. Large drop of resistivity is observed with the increase of Ag concentration. The magnetoresistance is dramatically suppressed with increasing Ag concentration due to the weakening of the antiferromagnetic interactions in the martensitic phase. The experimental results demonstrate that Ag substitution in Ni$_{\mathrm{50}}$Mn$_{\mathrm{35}}$In$_{\mathrm{15-x}}$Ag$_{\mathrm{x}}$ Heusler alloys suppresses the AFM interactions and enhances the FM interactions in the alloys. The possible mechanisms responsible for the observed behavior are discussed. Acknowledgement: This work was supported by the Office of Basic Energy Sciences, Material Science Division of the U.S. Department of Energy (DOE Grant No. DE-FG02-06ER46291 and DE-FG02-13ER46946).   

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

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 present our observations at the Co, Mn, and Fe L-edges to explain the significant changes in intermediate stoichiometries as compared with the parent Co$_{\mathrm{2}}$MnSi and Co$_{\mathrm{2}}$FeSi systems.   

Growth of Cr$_{\mathrm{2}}$CoGa and inverse Heusler thin films using Molecular Beam Epitaxy

Theoretical calculations have predicted the existence of inverse Heusler compounds that exhibit zero-moment magnetization while retaining their half-metallicity. These unique compounds have been labeled spin gapless semiconductors (SGS), where the density of states (DOS) can behave as a half-metal or gapless semiconductor.[1] There is a special interest for zero-moment SGS compounds since traditional antiferromagnets cannot be spin-polarized.[2] Such compounds are experimentally attractive for future spintronic devices due to their large magnetic transition temperature (400-800 K).[3] This work focuses on zero-moment inverse Heusler compounds including Cr$_{\mathrm{2}}$CoGa and Mn$_{\mathrm{3}}$Al. Thin films have been grown using MBE and their magnetic, structural, and electrical properties of these compounds have been characterized by various techniques, including XMCD and magnetometry. The atomic moments are found to be large, but significant cancellations lead to small average moments. [1] M.E. Jamer, B.A. Assaf, T. Devakul and D. Heiman, Appl. Phys. Lett. \textbf{103}, 142403 (2013). [2] M.E. Jamer, B.A. Assaf, G.E. Sterbinsky, D. Arena, L.H. Lewis, A.A. Sa\'{u}l, G. Radtke, D. Heiman, Phys. Rev. B \textbf{91}, 094409 (2015). [3] M.E. Jamer, L.G. Marshall, G.E. Sterbinsky, L.H. Lewis, D. Heiman, J. Magn. Magn. Mater. (2015).   

Large magnetoresistance induced by crystallographic defects in Fe$_x$TaS$_2$ single crystals

The search for the materials that show large magnetoresistance and the mechanisms that induce it remains challenging in both experimental and theoretical aspects. The giant magnetoresistance in one class of materials, ferromagnetic conductors, is generally attributed to the misalignments of magnetic moments, which cause spin disorder scattering. Recently, very large magnetoresistance ($>$60$\%$) was discovered in the ferromagnetic Fe-intercalated transition metal dichalcogenide, Fe$_{0.28}$TaS$_2$ [Phys. Rev. B 91, 054426(2015)]. The mechanism that led to this large magnetoresistance was suggested to be due to the deviation of Fe concentration from commensurate values (1/4 or 1/3), which caused magnetic moments’ misalignments. Here we report a study of Fe$_x$TaS$_2$ crystals with $x$ close to the commensurate values. Our results qualitatively demonstrate that crystallographic defects significantly affect magnetoresistance in Fe$_x$TaS$_2$. This provides a way to search for large magnetoresistance in more intercalated transition metal dichalcogenides.   

Development of spin-gapless semiconductivity and half metallicity in Ti$_{\mathrm{2}}$MnAl by substitutions for Al

In recent years, ever increasing interest in spin-based electronics has resulted in the search for a new class of materials that can provide a high degree of spin polarized electron transport. An ideal candidate would act like insulator for one spin channel and a conductor or semiconductor for the opposite spin channel (e.g., half metal (HM), spin-gapless semiconductor (SGS)). Here, we present the combined computational, theoretical, and experimental study of Ti$_{\mathrm{2}}$MnAl, a Heusler compound with potential application in the field of spintronics. We show that in the ground state this material is metallic, however it becomes a SGS when 50{\%} of Al is substituted with In (e.g., Ti$_{\mathrm{2}}$MnAl$_{\mathrm{0.5}}$In$_{\mathrm{0.5}})$, and a HM when 50{\%} of Al is substituted with Sn (e.g., Ti$_{\mathrm{2}}$MnAl$_{\mathrm{0.5}}$Sn$_{\mathrm{0.5}})$. Detailed study of the structural, electronic, and magnetic properties of these materials is presented.   

Antiferromagnetic-domain-dependent magnetoresistance in Pt/ Fe$_{2}$Mo$_{3}$O$_{8}$ interface

Interface between nonmagnetic metal and magnetic insulator has been extensively studied, exploiting a variety of new exotic spin transports. Among them, magnetoresistance in Pt/YIG interface attracts intense experimental and theoretical interest. The resistance of Pt layer reflects the magnetization of YIG in spite of the insulating nature of YIG, which has been explained by the spin current across the Pt/YIG interface or the magnetic proximity effect. So far, such anomalous magnetoresistance have been reported only in the interface between nonmagnetic metal and ferrimagnetic insulator. In this work, we have studied the transport properties of Pt on the antiferromagnetic insulator Fe$_{2}$Mo$_{3}$O$_{8}$. Fe$_{2}$Mo$_{3}$O$_{8}$ shows the metamagnetic phase transition under the magnetic field by which we can control the two different antiferromagnetic domains. Interestingly, transverse magnetoresistance in Pt/Fe$_{2}$Mo$_{3}$O$_{8}$ interface shows the distinct behaviors depending on the field cooling process which result in the different antiferromagnetic domains. This implies that the spin transport or proximity effect at the interface is different between two domains and can be probed by the resistance of nonmagnetic Pt.   

\textbf{Magnetic properties of Cr}$_{\mathrm{\mathbf{3}}}$\textbf{Te}$_{\mathrm{\mathbf{4}}}$\textbf{ doped with transition metals: An }\textbf{\textit{ab intio}}\textbf{ study}

We report density functional theory (DFT) study of the magnetic properties of Cr$_{\mathrm{3}}$Te$_{\mathrm{4}}$ doped with transition metals (TM) (Co, Fe, Ni, V and Mn), TM ions doped in Cr sites, Cr$_{\mathrm{3-x}}$ (TM)$_{\mathrm{x}}$Te$_{\mathrm{4}}$ (x $=$ 0.5 and 1). We performed screening of the exchange coupling interaction and magnetization modifications upon the substitution of Cr by 3d-transition metals at various Cr sites in the Cr$_{\mathrm{3}}$Te$_{\mathrm{4\thinspace }}$structure. Our calculation show that Cr$_{\mathrm{3}}$Te$_{\mathrm{4}}$ has ferromagnetic coupling and large magnetization (Magnetization per unit cell is 18.24$\mu $B). Magnetocrystalline anisotropy (MAE) of this material is also large (MAE$=$ 1.67MJ/m$^{\mathrm{3}})$. Our calculations show that the increase in interlayer spacing strengthen ferromagnetism of Cr$_{\mathrm{3}}$Te$_{\mathrm{4}}$. Doping with Mn increases Cr$_{\mathrm{3}}$Te$_{\mathrm{4}}$ magnetization, but reduces the exchange coupling energy which means reducing Curie temperature (T$_{\mathrm{c}})$. We find that doping with 3d-TM elements decreases the magnetocrystalline anisotropy energies (MAE) of Cr$_{\mathrm{3}}$Te$_{\mathrm{4}}$.   

Magnetic properties and stability of the atomic laminate Mn{\$}\textunderscore \textbraceleft 2\textbraceright {\$}GaC.

Using first-principles calculations, we predicted the thermodynamically stable magnetic Mn{\$}\textunderscore \textbraceleft 2\textbraceright {\$}GaC and subsequently synthesized it as a heteroepitaxial thin film. It belongs to a class of atomically laminated compounds with a unique combination of metallic and ceramic properties. They have a common formula $M_{n+1} AX_{n} $M{\$}\textunderscore \textbraceleft n$+$1\textbraceright {\$}AX{\$}\textunderscore \textbraceleft n\textbraceright {\$} (n $=$ 1-3), where M is an early transition metal, A is an A-group element, and A is carbon or nitrogen. Using density functional theory (DFT) and Heisenberg Monte Carlo (HMC) for a magnetic ground state search, several collinear and noncollinear low energy magnetic spin configurations have been identified, some with different symmetries compared to the non-magnetic crystal structure. Around 240 K X-ray diffraction and magnetic measurements display a sharp contraction of the lattice in the c-direction coinciding with a sharp magnetic transition. Neutron diffraction measurements displays diffraction peaks consistent with long-ranged antiferromagnetic order with a repetition distance of two structural unit cells (25 \textbraceleft $\backslash $AA\textbraceright ). This is consistent with theoretically predicted structural changes between different, close to degenerate, magnetic ground states, and it is the first unambiguous evidence of long ranged AFM order in MAX phase materials.