Session M38: Magnetic Materials and Properties

Live

We have measured the Seebeck and anomalous Nernst coefficients and corresponding transverse and longitudinal thermoelectric conductivities from 2 to 400 K in thin film (∼ 10 nm) Co₂MnAl_xSi_1-x (0≤x≤1) grown by molecular beam epitaxy (MBE). A large (-14 A*m^-1K^-1 at 300 K) anomalous transverse thermoelectric conductivity α_xy^A is observed in Co₂MnAl, which is 50 times larger than α_xy^A of Co₂MnSi (0.28 A*m^-1K^-1 at 300 K), as well as a significant enhancement in the anomalous Hall conductivity (6 times larger at 300K). This enhancement is likely due to the appearance of Weyl points in the band structure of Co₂MnAl close to the Fermi level. As a result of the high sensitivity to the Weyl points near the Fermi level, we propose α_xy^A as a helpful tool to capture Weyl points that are too far from the Fermi level to produce significant effect in the anomalous Hall conductivity.   

Live

Electrically conductive metals are typically good spin conductors (e.g., Cu) or spin absorbers (e.g., Pt). We report evidence of conductive epitaxial (epi-) Cr (resistivity ~ 30 µΩ*cm) acting as a reflector of transverse spin current. Ferromagnetic resonance spin pumping measurements, performed on spin-valve-like stacks of epi-CoFe/epi-Cr/Cu/NiFe, indicate that epi-Cr blocks (reflects) an AC spin current that is polarized transverse to the CoFe and NiFe magnetization. Epi-Cr is a transverse spin-current reflector both at room temperature (where it is likely paramagnetic) and low temperature (where it is likely antiferromagnetic). The onset of this spin reflection behavior is also accompanied by a suppression of current-in-plane giant magnetoresistance. However, spin pumping measurements suggest polycrystalline Cr to be a good transverse-spin-current conductor. Our results thus suggest that the crystal structure of Cr plays a crucial role in its peculiar spin transport properties.   

Live

We report results of a computational work on the half-Heusler compound CrMnSb_(1-x)P_x. We show that the parent compound CrMnSb is nearly half-metallic, with the onset of the band gap a few meV above the Fermi energy. Moreover, although it undergoes a half-metallic transition under a uniform compression of ~1.5%, such transition is absent under epitaxial strain. The half-metallic transition could be induced by a chemical substitution of Sb with P, which results in a volume reduction of the unit cell. In particular, 50% substitution of Sb with P leads to a robust half-metallicity, with 100% spin polarization being retained at a large range of epitaxial strain. Thus, our results indicate that CrMnSb_0.5P_0.5 could be grown on different types of substrates, e.g. GaAs, without its electronic properties being detrimentally affected by biaxial strain. In addition, CrMnSb_0.5P_0.5 exhibits a fully compensated ferrimagnetic alignment, which could be potentially useful in applications where stray magnetic fields are undesirable.   

Live

Mn₂Sb and MnZnSb are characterized in a Cu₂Sb-type structure but exhibit different magnetic and transport properties. Mn₂Sb is ferrimagnetic below the T_c∼550K with magnetic moment aligned parallel to c-axis, and undergo a spin-flip transition around 240K where the magnetic moment is aligned parallel to a-b plane. Meanwhile, MnZnSb is a room temperature ferromagnet. We synthesized Mn_1+xZn_1-xSb (0 < x < 1) single crystals and studied the evolution of magnetic and transport properties. We probed two magnetic phase transitions and observed anomalous hall effect in Mn_1+xZn_1-xSb which changes sign with decreasing temperature. In addition, we have probed topological Hall effect likely to be attributed to spin texture. Our discoveries provide a new platform to study the tuning of magnetism and possibly offer a new 2D magnetic system.   

Live

In spintronics, one of the long standing questions is why the MgO barrier is almost the only option to achieve a large tunnelling magnetoresistance (TMR) ratio at room temperature (RT) but not as large as the theoretical prediction. We have developed an almost strain-free magnetic tunnel junction (MTJ) using metastable bcc Co_xMn_100-x (CoMn) ferromagnetic films to reveal the reason. We have investigated the degree of crystallisation in MTJ consisting of CoMn/MgO/CoMn in relation to their TMR ratios. Cross-sectional high resolution transmission electron microscopy reveals that almost consistent lattice constants of the MTJ layers for 66≤x≤83 with maintaining large TMR ratios of 229% at RT, confirming the soft nature of the CoMn layer with some dislocations at the MgO/Co₇₅Mn₂₅ interfaces. For x=86, the TMR ratio is found to be reduced to 142% at RT, which is partially attributed to the increased number of the dislocations at the MgO/Co₈₆Mn₁₄ interfaces and amorphous grains in the MgO barrier. Ab-initio calculations confirm the crystalline deformation stability across a broad compositional range in CoMn, proving a strain-free interface for larger TMR ratios. Further optimisation can achieve > 1,000% TRM ratio at RT.   

Live

Magnetic materials with a half-metallic band structure are desired for spin-transport-based devices. MnPtSb is one such material predicted to exhibit nearly half-metallic band structure. Our first principle calculations show that this material undergoes a half-metallic transition in Mn-rich Mn_1+xPt_1-xSb compounds at x = 0.25. Further increase of Mn content makes half-metallicity even more pronounced, as Fermi level moves well inside the minority-spin band gap, accompanied by a reduction of the unit cell volume. We have synthesized Mn_1+xPt_1-xSb (0 ≤ x < 0.5 ) compounds in cubic crystal structure using arc melting and vacuum annealing. All samples show ferrimagnetic order (in agreement with computational results) at room temperature with the Curie temperature of about 500 K. In this presentation, we will discuss the results from our theoretical and experimental investigations including electronic band structure, structural and magnetic properties of Mn_1+xPt_1-xSb compounds.   

Live

Samples of EuPd₂Si₂ has been grown in an Arc furnace and annealed at 850 C for two weeks. The XRD measurements have confirmed the sample's purity and show that it has tetragonal I4/mmm crystal symmetry with the lattice parameter values consistent with the literature. Temperature-dependent DC field magnetization measurements show a valence state transition of Eu³⁺→Eu²⁺ near T_v = 142 K. The DC M-vs-T measurements show a minor shift of ≈1 K in the T_v with 6 T applied field. Furthermore, temperature-dependent powder and single-crystal XRD measurements showed that EuPd₂Si₂ has an anisotropic thermal expansion across T_v. The unit cell parameter c expands as temperature decreases while the parameter a and unit cell volume shrinks. The effect of the high magnetic field on crystal structure and valence state transition will be discussed.   

Live

Increasingly impressive demonstrations of voltage-controlled magnetism have been achieved recently, highlighting potential for low-power data processing and storage. Magnetoionic approaches appear particularly promising, electrolytes and ionic conductors being shown capable of on/off control of ferromagnetism and tuning of magnetic anisotropy. A clear limitation, however, is that such devices either electrically tune a known ferromagnet, or electrically induce ferromagnetism from another magnetic state, e.g., antiferromagnetic. Here, we provide proof-of-principle that ferromagnetism can be voltage-induced even from a diamagnetic, i.e., zero-spin state, suggesting that useful magnetic phases could be electrically-induced in “non-magnetic” materials [1]. We use ionic-liquid-gated diamagnetic FeS₂ as a model system, showing that as little as 1 V induces a reversible insulator-metal transition, driven by electrostatic surface inversion. Anomalous Hall measurements then reveal the onset of electrically-tunable surface ferromagnetism at up to 25 K. Density-functional-theory-based modelling explains this in terms of Stoner ferromagnetism induced via filling of a narrow e_g band.
[1] Walter et al., Science Advances 6, eabb7721 (2020)   

Live

Magnetic phase transitions are one of the mainstays to understand the origin of long-range spin order in condensed matter systems. The first observation of an antiferromagnetic phase transition in the metal halides paved the way for the development of the quantum theory of superexchange and for the discovery of new magnetic ground states [1,2]. Another metal halide, chromium triiodide (CrI₃), was the first free-standing system to demonstrate that long-range magnetic phase transitions in the true two-dimensional limit were possible [3]. However, there are still unknown features in its magnetism [4] that indicate the magnetic phase diagram goes beyond the classically reported single magnetic transition at 61 K [5]. Combining muon spin rotation (μSR) and superconducting quantum interference device (SQUID) measurements, we reveal the presence of multiple magnetic phase transitions of different nature, suggesting that the ferromagnetic transition transcends the classical concept of long-range order.

References
[1] H. Kamerlingh Onnes et al., Leiden Comm., 15 (192b), 1912.
[2] P. Day et al., J. Phys. C., 9, 1976.
[3] B. Huang et al., Nature, 546, 2017.
[4] Z. Wang et al., Nat. Comm., 9, 2018.
[5] M. A. McGuire et al., ACS, 27, 2015.   

Live

Electrical control of exchange bias (EB) in a antiferromagnet/ferro(ferri)magnet heterostructure is highly desirable for spintronic applications, but is very hard to achieve since the effect is related to the spin order in both the antiferromagnet and the ferro(ferri)magnet, which electric fields have little influence over. Recently, It has been shown that ferrimagnetic order in GdCo thin films can be dynamically controlled by voltage through solid-state hydrogen gating [1], via modulation of the relative magnetic moments of the two sublattices that make up GdCo. Here we show that, when a GdCo/NiO bilayer with out of plane magnetization is loaded/unloaded with hydrogen, the expected switching of the dominant sublattice in GdCo is accompanied by a flipping of the effective EB direction as well. We demonstrate cyclical toggling of EB in heterostructures with fully shifted hysteresis loops by application of only modest voltages (<2V). Our robust but simple mechanism provides a powerful means of controlling EB that can have broad implications for spintronics.
[1]. M. Huang et al., manuscript under review   

Live

Dynamic phase transitions (DPT) in ferromagnetic systems have recently drawn experimental and theoretical attention, mainly related to the observation of anomalous fluctuations of the order parameter that do not exist in thermal equilibrium. DPTs are characterized by changes in the period averaged magnetization Q (dynamic order parameter) as a function of a time-dependent external field H(t). Accordingly, at a critical period, the system can shift between a dynamically ordered state and a disordered one and vice versa. Despite their overall success in exploring relevant dynamic behavior, so far, all experimental studies of DPTs have been limited to low T/T_c ratios only, where Tc is the Curie temperature. As a result, the impact of T/T_c onto the DPT and the general phase space behavior has not been explored, and thus experimental knowledge is still very limited to date. Therefore, here we study experimentally the dynamic magnetic behavior for a suitable series of samples that have different T/T_c ratios, to explore their role in the vicinity of the DPT by means of a magneto-optical Kerr effect based detection scheme. Our results demonstrate that the qualitative behavior and features of the dynamic phase diagram are present independent of T/T_c.   

Live

MnPt5M (M=P and As) compounds were designed and synthesized. The crystal and magnetic structure characterizations by both X-ray and neutron scattering show MnPt5M crystalizes in the tetragonal anti-CeCoIn5 layered structure with van der Waals magnetic behaviors. MnPt5P displays an antiferromagnetic ordering ~ 188 K and a spin flop transition at high magnetic field while MnPt5As is a ferromagnet with the T_c ~287 K. The hybridization of the Mn-3d and Pt-5d orbitals plays an important role in the structural stability as well as in their magnetic properties according to the electronic calculations. In addition, the presence of the 4p character of As in the HOMO of the MnPt5As material is critical to the observed ferromagnetic ordering. The lack of strong p character in P in the HOMO orbital of MnPt5P has led to a minimal orbital overlap, thus dominating the interlayer magnetic exchange interactions to give AFM ordering in MnPt5P. The discovery of MnPt5As and MnPt5P opened a tunable system to study the chemical bonding on the magnetic behavior.   

Live

Iron rhodium (FeRh) goes through an entropy-driven first-order transition from an antiferromagnetic state to a ferromagnetic state as temperature increases. The rhodium fraction x in Fe_1-xRh_x has a profound effect on its structure, transport characteristics and magnetic properties. By utilizing molecular beam epitaxy as a tool to controllably tune the chemical composition of Fe_1-xRh_x thin films, we have systematically investigated the magnetic transition as a function of x. Rhodium deficient films have a lower transition temperature T_c compared to stoichiometric film. At x=0.48, we demonstrate a room-temperature bistability in this alloy, where Fe_0.52Rh_0.48 is ferromagnetic at high temperatures down to ~280 K and antiferromagnetic at low temperatures up to ~350 K. The width of the thermal hysteresis is sufficiently narrow to enable efficient controllability, but also wide enough to robustly withstand thermal perturbations. Based on this bistability, we demonstrate the use of local laser heating to write arbitrary patterns of the ferromagnetic phase that are also erasable.

Adv. Mater. 32, 2001080 (2020)
Appl. Phys. Lett. 113, 082403 (2018)   

On Demand

Phase separation (PS) in (La,Pr,Ca)MnO3 (LPCMO) manganites is the responsible for the colossal magnetoresistance (CMR) and a complex phase diagram for these systems [1]. The PS in the LPCMO between a ferromagnetic metal (FMM) phase and an antiferromagnetic charge-ordering insulator (AFM-COI) phase could be: (i) a fluid phase separation (FPS) between FMM and AFM-COI, both with a dynamic fluid-like behavior or (ii) a static phase separation (SPS) at lower temperatures with a glassy behavior [2]. The FPS to SPS transition could be a glass-like one associated to a strain-liquid to strain-glass transition [3]. Here we report ferromagnetic resonance (FMR) and transport measurements on a LPCMO thin film across the FPS to SPS transition. Changes in the FMR signal allow to identify the possible temperature ranges for the FPS and SPS states. We confirm by micromagnetic simulations that percolation paths of FMM phase give rise to additional resonance frequencies in the FPS state, which could open the possibility of magnonic control in the LPCMO thin films.

[1] M. Uehara et. al. Nature 399, 561–564 (1999).
[2] W. Wu et al. Nat. Mater. 5, 881–886 (2006).
[3] P.A. Sharma et. al. Phys. Rev. B. 71, (2005).   

On Demand

As a pre-cursor to the formation of 2D MnGaN[1], Mn deposition on GaN(000-1) at 300 K can at first lead to a 3x3 trimer structure[2]. Collinear and non-collinear calculations based on density functional theory are carried out to elucidate the magnetic ordering of monomers, dimers and trimers of Mn on GaN(000-1) surface. We estimate the magnetic anisotropy energy (MAE), and we find that in the case of Mn dimers, the exchange magnetic coupling is the dominant interaction, and that the ground state has the Mn spins in-plane with the GaN(000-1) substrate. In the Mn trimer case, the magnetic ground state also has the Mn spins in-plane with the GaN surface, but the relative spin orientation within each trimer is non-collinear. By exploring the nature of the magnetic interaction among the Mn trimers, we find that the surface states of the substrate play a key role, involving a Ruderman-Kittel-Kasuya-Yosida (RKKY)-type interaction. To the best of our knowledge, this is the first report of an electron-mediated long-distance exchange coupling between localized magnetic moments on a GaN(000-1) surface.

[1] Y. Ma et al., Nano Letters, 18, 158 (2018)
[2] A. Chinchore, Phys Rev B 87, 165426 (2013)