Session Y40: Disordered and Novel Low Dimensional Magnetic Materials

Live

We present an investigation of the effect of randomizing exchange coupling strengths in the S = 1/2 square lattice quasi-two-dimensional quantum Heisenberg antiferromagnet (QHAF) (QuinH)₂Cu(Cl_xBr_1−x)₄●2H₂O (QuinH = Quinolinium, C₉H₈N⁺), with 0 ≤ x ≤ 1. Pulsed-field magnetization allows us to estimate an effective in-plane exchange strength in a regime where exchange fosters short-range order, while the temperature at which long-range order occurs (T_N) is found using muon-spin relaxation, allowing us to construct a phase diagram for the series. We evaluate the effectiveness of disorder in suppressing the T_N and the ordered moment size and find an extended disordered phase in the region 0.4 ≤ x ≤ 0.8 where no magnetic order occurs. The observed critical substitution levels are accounted for by an energetics-based competition between different local magnetic orders. We demonstrate experimentally that the ground-state disorder is driven by quantum effects of the exchange randomness, a feature that has been predicted theoretically and has implications for other disordered quasi-two-dimensional QHAFs.   

Live

The magnetic phase boundary of diluted hexagonal ferrites, specifically, PbF_12−xGa_xO₁₉ has attracted attention recently because of its unusual shape [Phys. Rev. 96, 020407 (2017)]. The critical temperature as a function of dilution varies as T_c ∝ (1 − x/x_c)^2/3 over the entire concentration range from x = 0 to x_c, where x_c is very close to the percolation threshold of the lattice. To explain this behavior we employ a classical percolation scenario. We perform large-scale Monte Carlo simulations to determine the magnetic phase boundary of diluted XY and Heisenberg models on simple cubic and hexaferrite lattices. Our findings are twofold. (i)In the asymptotic critical region close to x_c, the phase boundary follows T_c ∼ (1 − x/x_c )^φ with φ = 1.09(2) in agreement with the percolation theory. (ii) For smaller dilutions, φ takes smaller values but the simulation does not reproduce the 2/3 power law. We also discuss the effects of additional weak couplings which frustrate the ferrimagnetic order on the shape of the phase boundary.   

Live

We report results on solving a long outstanding problem—whether the two-dimensional spin-3/2 antiferromagnetic valence-bond model of Affleck, Kennedy, Lieb, and Tasaki (AKLT) possesses a nonzero gap above its ground state. We exploit a relation between the anticommutator and sum of two projectors and apply it to ground-space projectors on regions of the honeycomb lattice. After analytically reducing the complexity of the resultant problem, we are able to use a standard Lanczos method to establish the existence of a nonzero gap. This approach is also successfully applied to spin-3/2 AKLT models on other degree-3 semiregular tilings, namely, the square-octagon, star, and cross lattices, where the complexity is low enough that exact diagonalization can be used instead. In addition, we close the previously open cases of hybrid AKLT models on the singly decorated honeycomb and singly decorated square lattices.

Ref. Nicholas Pomata and Tzu-Chieh Wei, Phys. Rev. Lett. 124, 177203 (2020)   

Live

Recently various 2D AKLT models have been shown to be gapped, including the one on the hexagonal lattice. Here we report on a non-trivial 3D AKLT model which consists of spin-2 entities on the diamond lattice sites and one single spin-1 entity between every neighboring spin-2 site. Although the nonzero gap problem for the uniformly spin-2 AKLT models on the diamond and square lattices is still open, we are able to establish the existence of the gap for two planar lattices, which we call the inscribed square lattice and the triangle-octagon lattice, respectively. So far, these latter two models are the only two uniformly spin-2 AKLT models that have a provable nonzero gap above the ground state. We also discuss some attempts in proving the gap existence on both the square and kagome lattices. In addition, we show that if one can solve a finite-size problem of a weighted AKLT Hamiltonian and if the gap is larger than certain threshold, then the model on the square lattice is gapped in the thermodynamic limit. The threshold of the gap scales inversely with the linear size of the finite-size problem.

Ref. Wenhan Guo, Nicholas Pomata, and Tzu-Chieh Wei, arXiv:2010.03137   

Live

Magnetic materials with a high degree of spin polarization have attracted much attention due to their potential applications in spin-transport-based devices. In addition, room-temperature half-metals with a small or no magnetic moments are beneficial for nanoelectronic devices due to the absence of stray magnetic field, which otherwise interferes with the neighboring magnetic elements. We have synthesized one such predicted half-metallic alloy Ti₂MnAl in cubic Heusler structure with B2-type disorder using arc-melting and annealing. The samples show very small magnetic moment with the Curie temperature of about 305 K. In this presentation, we will also discuss the electron-transport properties of the synthesized alloys and a theoretical investigation on the effect of disorder in the magnetic properties of Ti₂MnAl.   

Live

Dilute magnetic semiconductors, achieved through substitutional doping of spin-polarized transition metals into semiconducting systems, enable experimental modulation of spin dynamics in ways that hold great promise for novel magneto-electric or magneto-optical devices. A room-temperature 2D dilute magnetic semiconductor can be achieved by doping vanadium into WS₂^[1]. Ferromagnetism peaks at an intermediate vanadium concentration of a few atomic percent and decreases for higher concentrations. The experimentally observed non-monotonicity of the magnetization as a function of the doping level can be explained by hybridization between dopant defect states that quenches the magnetic moments when they approach too closely. An effective Zeeman shift at valleys in this system corresponds to an external magnetic field of ~100 T.

[1] Zhang, Fu, et al. "Monolayer Vanadium-doped Tungsten Disulfide: A Room-Temperature Dilute Magnetic Semiconductor." arXiv preprint arXiv:2005.01965 (2020).   

Live

The ternary crystal structure R₅Co₂Ge₃ was discovered in 2017 [1]. When R is moment bearing, the low-temperature magnetic state(s) can be complex. For non-moment bearing R=La, the situation is simpler and intriguing; La₅Co₂Ge₃ is a small moment, itinerant ferromagnetic compound having a transition temperature of 3.8K and an effective moment of 0.1µ_B [2]. In order to better understand the nature and possible tunability of this ferromagnetic transition, we report the substitution of Co with Ni, Fe and Rh as well as Ge with Si. Single crystals of La₅(Co_1-xA_x)₂Ge₃ (A=Ni, Fe, Rh) and La₅Co₂(Ge_1-xSi_x)₃ were grown and characterized by magnetic and transport measurements. A comparison of the transition temperature, magnetic and transport properties will be discussed.

[1] Q. Lin, et al., Chem. Eur. J 23,10516 (2017) [2] S.M. Saunders, et al., PhysRevB 101, 214405 (2020)   

Live

The RCuAs₂ (R = rare-earth) compounds crystallize in HfCuSi₂-type tetragonal structure and compounds with moment bearing R-ions order antiferromagnetically at low temperatures. In particular, resistivity curves on polycrystal samples for R = Sm, Gd, Tb, and Dy show an unexpected resistivity minimum above their magnetic ordering temperatures [1]. To study the origin of the minimum with negligible 4f-hybridization, single crystals of RCuAs₂ have been grown and characterized by XRD, specific heat, resistivity, and magnetization measurements. Magnetization measurements for this family clearly indicate local moment behavior with a large degree of anisotropy between H || ab and H || c due to the CEF effect. All physical property measurements clearly indicate antiferromagnetic ordering at low temperatures for R = Sm - Yb. Although the resistivity value of single crystals is significantly smaller than that for polycrystalline samples, resistivity data for R = Sm, Gd, Tb, and Dy exhibit a pronounced minimum above their magnetic ordering temperatures. In this talk we will present physical property measurements for this family and discuss the possible origin of the resistivity minimum by considering the Kondo and RKKY interactions.
[1] E. Sampathkumaran, et. al, PRL 91, 036603 (2003)   

Not Participating

Colossal magnetoresistance (CMR) is a sudden drop in the electrical resistance in response to an external magnetic field – a desirable effect for magnetic sensing and recording technologies. The established paradigm of CMR is based on the manganese oxide materials (perovskite manganates) where a mixed valence of Mn³⁺/Mn⁴⁺ leads to a ferromagnetic (FM) double-exchange (DE) interaction and a dynamical Jahn-Teller (JT) distortion, which cooperatively induce CMR. Prior efforts to find different materials or mechanisms for CMR resulted in a much smaller effect. Here we show an enormous CMR at low temperatures in EuCd₂P₂ without manganese, oxygen, mixed valence, or cubic perovskite structure. EuCd₂P₂ has a layered trigonal lattice and exhibits A-type antiferromagnetic ordering at 11 K. The magnitude of CMR (10⁴ percent) in as-grown crystals of EuCd₂P₂ rivals the magnitude in optimized thin films of manganates. Our magnetization, transport, and synchrotron X-ray data suggest that strong magnetic fluctuations are responsible for this phenomenon. The realization of CMR at low temperatures without heterovalency leads to a new regime for materials and technologies related to antiferromagnetic spintronics.   

Live

We report the electronic and magnetic properties of full Heusler Pd2TiIn based on first principles
calculations. We use general potential linearized augmented planewave method to reexamine
this issue using all electron, relativistic calculations, without shape or frozen core approximations.
We find that ideal ordered Heusler Pd2TiIn remains non-magnetic. However, we do find that it is
possible to explain the magnetism seen in some experiments through disorder and in particular we
find that site disorder can lead to moment formation in this compound. In addition, we find an
alternative low energy cubic crystal structure, which will be of interest to explore experimentally.   

Live

We present a small-angle neutron scattering (SANS) analysis of the ferromagnetic (FM) alloys Ni_1-xV_x where the FM order is destroyed towards a paramagnetic phase by sufficient substitution of Ni by V. The critical temperature T_c vanishes at x_c=0.116 indicating a quantum critical point (QCP) with signatures of disorder [1]. We performed polarized SANS experiments on different Ni_1-xV_x polycrystalline samples close to x_c at NG7SANS, NCNR, NIST utilizing a ³He cell within the wave vector regime Q=0.06-1nm^-1. We could resolve magnetic scattering at different length scales within the FM state well below T_c<50K [2]. We find isotropic magnetic short-range correlations as well as anisotropic magnetic contributions that reveal large scale magnetic domains and an indication of local magnetic defects that relates to the V concentration. We show how these different contributions respond to high magnetic fields and evolve with V concentration x, characterizing a disordered FM alloy, close to the QCP.
[1] R. Wang et al, Phys. Rev. Lett. 18, 267202 (2017)
[2] A. Schroeder et al, AIP Adv.10,015036 (2020)   

Live

We present first principles calculations of Mn-Sb-Te using the Locally-Self consistent Multiple Scattering (LSMS) framework, a real space first principles code for scalable first principles density functional theory calculations of materials. Simulations of disordered systems at realistic concentrations require large cells with thousands of atoms. Traditional first principles methods scale cubically with the number of atoms and is computationally prohibitive for large system sizes. This talk will present a brief overview of the real-space formalism of LSMS, which allows first-principles calculations of O(100,000) atoms. Next, we will present results of the influence of disorder on the energetics and magnetic properties of Mn-Sb-Te systems.   

On Demand

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 with high T_c are ideal candidates for spintronic applications. Co- based Heusler alloys have recently attracted great interest because most of these alloys exhibit above mentioned criteria. Here, we present a combined experimental and theoretical study of quaternary Heusler alloys Co_1.5Y_0.5FeSi (Y = Sc,Ti,V,Cr,Mn,&Fe) to get a global overview of the electronic, magnetic and mechanical properties. All single phase alloys exhibit fcc crystal structure with a strong tendency towards L2₁ ordering. The low-temperature saturation magnetic moments agree fairly well with our theoretical results and also obey the Slater-Pauling rule, a prerequisite for half metallicity¹. All alloys are soft ferromagnets with high T_c, allowing for applications at room temperature and above. Relatively high mechanical hardness values are also observed^1,2,3.

References:
1. KC et al., Phys. Rev. Materials 3, 114406(2019)
2. Mahat et al., Journal of Alloys and Compounds, 154403 (2020)
3. Mahat et al., Journal of Magnetism and Magnetic Materials, 167398 (2020)   

On Demand

In magnonics, many researches have been conducted to study spin-waves in a thin film that has a perfect rectangular form. However, it is not always possible to achieve such an ideal rectangular structure depending on patterning techniques. For instance, a V[TCNE]_x~2 thin-film deposited by using a CVD method has taper around its edges. In this presentation, we investigate and report characteristics of dipole interaction induced spin-wave modes in a thin-film of a trapezoidal cross-section. We used a micromagnetic simulation program; MUMAX3[1] to simulate spin-wave spectra and corresponding mode profiles of the thin-film. For material parameters, we referred V[TCNE]_x~2 which is a emerging magnetic material that has low damping and narrow linewidth [2]. Our results provide understanding of how tapered edges influence spin-waves and suggest possible applications of localied edge modes.

[1] Vansteenkiste et al., AIP Adv. 4, 107133 (2014).
[2] N. Zhu, X. Zhang, I. H. Froning, M. E. Flatté, E. Johnston-Halperin, and H. X. Hong, Appl. Phys. Lett. 109, 082402 (2016)