Session Y51: Disordered Magnetic Materials

Electronic, structural and magnetic properties of CoFeVGe

Half-metallic Heusler alloys have attracted much attention due to their potential applications in spin-transport-based devices. We have synthesized one such predicted half-metal CoFeVGe using arc melting and high-vacuum annealing. The room temperature x-ray diffraction of the as-prepared sample exhibits cubic crystal structure without secondary phases. The sample shows a single magnetic transition at its Curie temperature of 249 K. The high field (3T) magnetization measured at room temperature is 41 emu/g. In this presentation, we will discuss effect of heat treatment on the structural and magnetic properties and also the results of our first principle calculation.   

Magnetism and magnetotransport in amorphous cobalt-germanium and amorphous cobalt-silicon thin films

Amorphous Fe_xGe_1-x and Fe_xSi_1-x exhibit ferromagnetic ordering for x≥0.4 in thin films and have an enormous intrinsic anomalous Hall effect (AHE), spin polarization, and anomalous Hall angle. While crystalline B20 phase FeGe is magnetic up to room temperature, showing a skyrmion phase, and B20 FeSi is an antiferromagnetic insulator, often described as a Kondo insulator, B20 phase CoSi is a paramagnetic semimetal hosting exotic topological states [1]. Spin-orbit coupling plays a significant role in all of these alloys and compounds. Amorphous Co_xGe_1-x and Co_xSi_1-x thin films have been grown over the range 0.4≤x≤0.7; both alloys show an onset of ferromagnetic ordering not until x≥0.6, despite density-functional theory results which suggest ferromagnetism for as low as x=0.45. Magnetotransport measurements show that the resistivity depends on x, relatively independent of the specific elements. In contrast to the iron-based alloys which have hole carriers and negative magnetoresistance (MR), the cobalt-based alloys are dominated by electron carriers and have positive MR. Magnetotransport to explore possible exotic topological states and the AHE at high x related to the density of Berry curvature in amorphous materials will be discussed.

[1] DOI: 10.1103/PhysRevLett.122.076402   

Effects of varying composition on magnetic properties of Cantor alloys

Magnetic and transport properties of CrMnFeCoNi high-entropy alloys were examined between 5 K and 300 K and in fields up to 9 T. The alloy composition was varied by adjusting the relative amount of one element while leaving equal amounts of the others, leading to compositions such as Mn_x(CrFeCoNi)_1-x. Several magnetic signatures are identified, including one associated with a ferromagnetic transition near 43 K, which is relatively independent of composition, leading to questions of whether it is an intrinsic or extrinsic (impurity) feature. Using least squares fitting for over 30 samples, the local magnetism from each element is approximated, thereby allowing the composition of a near-zero net moment alloy to be identified.   

Thermo-Remanent Magnetization (TRM) Explorations near the Critical Spin Glass Temperature

We have performed extensive TRM measurements from .8Tg to .97Tg,  in a single crystal CuMn(6%) sample, over a wide range of waiting times (300 s to 100,000 s) and magnetic fields (3G-96G). We find a collapse of both the remanent magnetization, and the effective waiting time, as Tg is approached. This collapse suggests that we are observing the effects of critical fluctuations associated with the spin glass phase transition, and we probe several different scenarios to explain the data.   

Inelastic neutron scattering study on anisotropic spin glass Fe2TiO5

Of all the spin glasses systems, Fe2TiO5 is a unique member due to its anisotropic spin glass transition along the c-axis, despite having purely isotropic spins. This has been interpreted as a longitudinal freezing prior to transverse ones in the presence of weak random anisotropy. However, our recent diffuse neutron scattering experiment [1]demonstrated that nanoscale surfboard shaped antiferromagnetic regions arise below the glass transition temperature and possess strong transverse spin fluctuations which can lead to uniaxial spin glass freezing in Fe2TiO5, implying the significant role of spin fluctuations in this system. To further understand this, we have performed an  inelastic neutron scattering study of the spin fluctuations in Fe2TiO5 single crystals and investigated the temperature- and energy- dependence of the low energy excitation spectra. These results unveil the complicated magnetic interactions in Fe2TiO5 and shed light on understanding its unique behavior from a new perspective.   

Possible half-metallic and spin-gapless semiconducting behavior in quaternary Heusler compounds Co2-xYxFeGa/Al (Y = Ti, V, Cr, Mn, Fe, and Co, x = 0.50)

The realization of physical systems whose electronic and magnetic properties can be easily manipulated by playing with valence electrons are very promising from both the fundamental and applied perspectives. Half-metallic ferromagnets are ideal candidates for spintronic applications because of their high spin polarization of charge carriers at the Fermi level. Spin-gapless semiconductors can be regarded as a combination of gapless semiconductors and half-metallic ferromagnets where the conducting electrons or holes are not only 100% spin polarized but also easily excited. The realization of spin-gapless semiconducting behavior in Heusler alloys is expected to fulfill the needs for semiconductor spintronics.  In this work, we have introduced the substitution of all low valence transition metals atoms Y = Ti, V, Cr, Mn, and Fe to Co atoms in the parent Co₂FeGa/Al system, and examined the structural, electronic, magnetic, electron transport and mechanical properties of quaternary Heusler compounds Co_2-xY_xFeGa/Al (x = 0.50), both experimentally and theoretically to get a global overview of the electronic, magnetic, electron transport and mechanical properties, promising candidates for spintronics applications. All single-phase alloys exhibit cubic crystal structure with a strong tendency towards L2₁ ordering. The low-temperature saturation magnetic moments agree fairly well with the values expected from a Slater-Pauling rule for half metals. The temperature dependence of electrical resistivity for Mn substituted alloys show tunable spin-gapless semiconducting behavior in the complete temperature regime (5K-400K). Ab initio calculations are also performed to understand the experimental findings.   

Magnetic Order in Ga-Substituted Spinel Type High Entropy Oxide (MnFeCrCoNi)3-xGaxO4

High entropy oxides (HEOs) are crystalline materials with a high degree of cation disorder and novel properties. It has been proposed that the high configurational entropy in HEOs, which can be achieved by having five or more equimolar cations with random occupancy, results in an entropy term large enough to dominate the enthalpy term at high temperatures and minimizes the Gibbs free energy resulting in a single phase material. However, the degree of configurational disorder, its role in stabilizing HEOs, and its affect on the properties remain open questions. Here we attempt to shed light on these questions using  the 3d transition metal based AB₂O₄ spinel-type HEO (s-HEO) (MnFeCrCoNi)₃O₄ as our platform.  In particular, we attempt to elucidate the relationship between chemical disorder and magnetism using a number of experimental probes, which are sensitive to different length scales, including neutron diffraction, X-ray absorption spectroscopy (XAS), and soft X-ray magnetic circular dichroism (XMCD). With XAS and XMCD, we identify preferred cation occupancy of the tetrahedral A site and octahedral B site in (MnFeCrCoNi)₃O₄ and with neutron diffraction, we characterize its ferrimagnetic ordered state below 400 K. Furthermore, we study the affect of Ga substitution on the cation A and B site occupancy and the magnetism by comparing s-HEOs (MnFeCrCoNi)_3-xGa_xO₄, with x=0.0,0.1,0.2,0.4. Finally, we comment on the role of entropy in stabilizing the s-HEOs and its affect on the magnetic order.   

Simulating spin waves in entropy stabilized oxides

The entropy stabilized oxide Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O exhibits antiferromagnetic order and magnetic excitations, as revealed by recent neutron scattering experiments. This observation raises the question of the nature of spin wave excitations in such disordered systems. Here, we investigate theoretically the magnetic ground state and the spin-wave excitations using linear spin-wave theory in combination with the supercell approximation to take into account the extreme disorder in this magnetic system. We find that the experimentally observed antiferromagnetic structure can be stabilized by a rhombohedral distortion together with large second nearest neighbor interactions. Our calculations show that the spin-wave spectrum consists of a well-defined low-energy coherent spectrum in the background of an incoherent continuum that extends to higher energies.   

Spin glass and spin dynamics in a disordered Heisenberg model for rare-earth titanates

Mott insulating rare-earth titanates can display either ferromagnetic or antiferromagnetic order at low temperatures. The nature of the magnetic ground state can be controlled by chemical substitution on the rare-earth sites, which inevitably modifies the exchange constants of the material locally, in an inhomogeneous way. To capture this effect, we propose a disordered Heisenberg model to describe the chemically substituted Mott insulating rare-earth titanates. By performing systematic Monte Carlo simulations, we identify a wide range of chemical substitutions where a spin glass with a non-zero chiral glass order parameter becomes the ground state. Moreover, a robust coexistence of the magnetically ordered state with the spin and chiral glasses is also found. The dynamic spin structure factor is computed in different regions of the model and compared with neutron scattering experiments on La-doped YTiO3.   

Understanding the anomalous magnetic heat capacity and thermopower of MnTe above the Néel temperature using cluster mean-field theory

MnTe is an antiferromagnetic (AFM) semiconductor with exotic thermoelectric properties. Experiments have shown anomalously large thermopower and magnetic heat capacity above the transition temperature T_N where the magnetic Néel order disappears. Thermopower is directly proportional to the specific heat. Hence, we study the specific heat of the system, both experimentally and theoretically. Using different approaches such as spin-wave theory (SWT), mean-field approximation (MFA), and cluster mean-field theory (CMFT), we study the effects of the thermal fluctuations below and above T_N. While SWT is reliable at low temperatures, it overestimates the values at high temperatures and fails to determine the transition temperature. MFA and CMFT show strong staggered magnetization below T_N due to the dominant AFM inter-layer interactions. The MFA shows an increasing specific heat followed by a sudden drop at T_N, different from the experimental data. However, including magnetic fluctuations through CMFT explains the transition temperature and shows a finite specific heat in the disordered phase, agreeing with the experimental results. Therefore, the enhanced specific heat in the disordered phase can be explained only by the large correlations between the nearest-neighbor inter-layer spins.