Session Z55: Magnetic Anisotropy, Characterization, and Imaging

Role of carbon in enhancing the magnetization of MnBi: an experimental and theoretical study

MnBi and MnBi-based materials have been investigated as prospective rare-earth-free permanent magnets with moderate energy product. We have synthesized Mn₅₅Bi₄₅ and Mn₅₅Bi₄₅C samples using arc melting and high-vacuum annealing. The room temperature x-ray diffraction patterns indicate that both Mn₅₅Bi₄₅ and Mn₅₅Bi₄₅C crystallize in the hexagonal NiAs-type crystal structure. The Rietveld analysis of the x-ray patterns shows that the amount of Bi segregation reduces from  16 wt.%  for Mn₅₅Bi₄₅ to 5 wt. % for Mn₅₅Bi₄₅C. The high-field (3T) magnetizations measured at room temperature are 61 emu/g and 66 emu/g for Mn₅₅Bi₄₅ and Mn₅₅Bi₄₅C, respectively. In addition,  the carbon containing sample shows slightly higher value of the anisotropy constant as compared to that of Mn₅₅Bi₄₅. To understand the role of C in enhancing the magnetization of MnBi, we carried out the first-principles calculations of both stoichiometric and nonstoichiometric MnBi alloys, which suggests that the increase of magnetization in Mn₅₅Bi₄₅C is due to the coating of MnBi grains with C.   

Ab initiostudy of alloying of MnBi to enhance the energy product

High energy density magnets are preferred over induction magnets for many applications, including electric motors used in flying rovers, electric vehicles, and wind turbines. However, several issues related to cost and supply with state-of-the-art rare-earth-based magnet necessities development of high-flux magnets containing low cost, earth-abundant materials. Here, we demonstrate the possibility of tuning magnetization and magnetocrystalline anisotropy of one of the candidate materials, MnBi, by alloying it with foreign elements.  By using the density functional theory in the high-throughput fashion, we consider the possibility of alloying MnBi with all possible metal and non-metal elements in the periodic table and found that MnBi-based alloys with Pd, Pt, Rh, Li, and O are stable against decomposition to constituent elements and have larger magnetization, energy product compared and magnetic anisotropy compared to MnBi  We consider the possibility of these elements occupying half and all of the available empty sites. Combined with other favorable properties of MnBi, such as high Curie temperature and earth abundancy of constituents elements, we envision the possibility of MnBi-based high-energy-density magnets.   

Magnetic and electrical transport properties of Heusler shape-memory alloy Ni2FeGa

Magnetic shape-memory materials have attracted considerable attention owing to their stability, magnetic field-induced controllability, and superior thermo-mechanical performance. These properties render them exciting application potential in robotics, electro-mechanical systems, aerospace, and medical devices. In this talk, we will report structural, magnetic, and electronic properties of a Heusler ferromagnetic shape memory alloy Ni₂FeGa. We fabricate the alloy ribbons by using a melt spinning process. The alloy possesses a well-defined γ-phase (austenite) structure at room temperature, inferred from the x-ray diffraction measurements. Moreover, the alloy ribbons undergo a first-order austenite-to-martensitic phase transition near 150 K, as indicated by the temperature-dependent electronic and magnetization measurements. We observe a 8% reduction in resistivity as the alloy ribbon transforms from the magnetically hard martensitic to the soft austenitic phase. This resistance change is large for a shape-memory alloy. We also record a negative magnetoresistance (MR) of the alloy ribbons and the MR shows a structure-phase dependence. These electronic and magnetic properties of the Ni₂FeGa alloy grants it great potential for spintronic applications.   

Giant Doping Response of Magnetic Anisotropy in MnTe

Developing simple ways to control spin states in spintronic devices is a crucial step towards increasing their functionality. MnTe is a room-temperature antiferromagnet with promising spintronic properties, including thermospintronics and magnon-based devices. Here, we show that incorporating less than 1% Li in polycrystalline MnTe results in a dramatic spin reorientation as observed by neutron diffraction. The behavior of the 0001 magnetic Bragg peak reveals a significant reorientation of the Mn spins from planar in the pure material to almost completely axial with minimal Li-doping. Temperature dependence of the magnetic peaks in Li-doped samples indicates that axial spins shift back to planar suddenly upon approaching the Néel temperature (T_N = 307 K). DFT calculations support the idea that the shift in the Fermi level caused by doping is responsible for switching the material between two competing magnetic ground states. These results pave the way for developing easy switching of magnetic states in future spintronic materials.   

Magneto-Optical Study of Ultrathin TmIG/Pt Anistropy

Ferrimagnetic insulator based spintronics has the potential to produce both low-loss technologies as well as next generation skyrmion technologies. TmIG/Pt bilayers have been a great interest for their ability to produce high speed domain walls as well as their ability to be strain and DMI engineered for room temperature skyrmion applications. In order to further the understanding of this system, the sources of anisotropy in the ultrathin film limit must be well understood. In order to overcme the difficulty of low net magnetization and sample, which limits the use of magnetometry below several nanometers of thickness, we employ a highly sensitive magneto-optical technique to study the magnetic anisotropy. We investigate the magnitudes and effects of several contributions to magnetic anisotropy as a function of film thickness.   

Phase stability and magnetic properties of ThMn$_{12}$ type rare-earth materials

We investigate the chemical and the dynamical stabilities, and magnetic properties of Ce-based ThMn$_{12}$ materials with the {\it ab initio} calculations that use the $4f$ electrons. Phonon calculations predict positive frequencies in CeFe$_{12}$ and CeFe$_{12}$N suggesting that both compounds are dynamically stable. The calculated formation energy is positive for CeFe$_{12}$, confirming that it cannot be synthesized experimentally in bulk. The formation energy is improved significantly by the addition of nitrogens to the interstitial sites. We also computed the phonon frequencies and found all positive in the substituted compounds. Electronic structure results indicate composition-dependent Ce mixed-valence behavior, which modifies magnetocrystalline anisotropy (MCA). The calculations yield a large MCA constant, $K_1$, in all compositions, especially in CeZrFe$_{11}$. While nitrogen improves the magnetic moment, it has the opposite effect in $K_1$.   

A strategic high throughput search for identifying thermodynamically stable Li based half Heusler for spintronics application

Heusler alloys are potential candidates for spintronics applications. However, it is experimentally not feasible to identify the most efficient composition pertaining to their vast compositional space. Large-scale high throughput density functional theory (DFT) based calculations may aid in this. In this work, we perform high throughput DFT calculations on alkali metal-based half Heusler alloys; LiX_pY_1-pS (X, Y = V, Cr, Mn, Fe, Co, Ni and p = 0, 0.25, 0.5, 0.75, 1). We design systematic filters to select the energetically and vibrationally favorable compositions by considering the contributions stemming from the magnetic alignments of the ions. Thereby starting with 153 alloys, we down select 26 stable magnetic compositions, of which four were ferromagnetic, and eight were ferrimagnetic with 100 % spin polarization. Following this, we search for structural anisotropy, whereby tetragonal distortion is observed for one ferromagnetic and four antiferromagnetic alloys, with the possibility of having easy-axis magnetocrystalline anisotropy. The ferromagnetic LiFe_0.5Mn_0.5S and antiferromagnetic LiFeS are found to have the most prominent easy-axis magnetocrystalline anisotropy.   

Inverted hysterons and hysteresis branch crossings: New ideas in modelling hysteresis

For nearly 100 years, it has been assumed that the ascending and descending branches of a magnetization hysteresis loop cannot cross. This assumption was largely based on the erroneous conclusion that the region of the crossed branches violates conservation of energy. It has recently been shown that the hysteresis branch crossing (HBC) is a real, reproducible, and robust phenomenon which violates no fundamental law. The existence of the HBC raises several interesting and fundamental questions about the way hysteresis has been modelled and the assumptions of various mathematical models of hysteresis; in particular, the Preisach model.

  In this talk, both experimental evidence and theoretical arguments supporting the existence of the hysteresis branch crossing will be presented. These ideas reveal the fact that a serious flaw exists in the way the Laws of Thermodynamics have been applied to hysteresis modelling. Additionally, these ideas reveal the need to consider “inverted hysterons” in both the classical Preisach model and the nonlinear Preisach model. The implications and nature of the inverted hysteron will be discussed.   

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By means of ferromagnetic resonance (FMR) technique the magnetic properties of ZnFe₂O₄ (ZFO) films grown on Si(111), MgO(100), and STO(100) were investigated. By using a model that takes into consideration the relevant contributions to the energy, we were able to extract the following parameters: effective magnetization M_eff, cubic magnetocrystalline anisotropy field, in-plane, and out-of-plane uniaxial anisotropy fields, as well as the g factor. The samples deposited on Si showed a twofold in-plane anisotropy while a fourfold magnetic anisotropy was observed for the ZFO/MgO and ZFO/SrTiO₃ samples. All samples showed a strong perpendicular uniaxial anisotropy above 1 kOe and low in-plane uniaxial and cubic anisotropy field, of the order of tenths of Oe. The FMR linewidth (ΔH) fitted from FMR data show values above 190 Oe for all samples, indicating high damping of this material to be ∼10^-2.   

Magneto Optical Sensing beyond the Shot Noise Limit

Magneto-optical sensors including spin noise spectroscopies and magneto-optical Kerr microscopies are now ubiquitous tools for materials characterization. Further improvements in sensitivity with classical resources require either increased laser power that can induce unwanted heating and perturbations or increased measurement times that are increasingly infeasible. Here, we propose a truncated nonlinear interferometric readout for low-temperature magneto-optical Kerr effect measurements that is accessible with today's quantum optical resources. We show that 10 nrad/√Hz sensitivity is achievable with optical power as small as 1 μW such that a realistic T = 83 mK can be maintained in commercially available dilution refrigerators.   

Quadratic and third-order magneto-optic Kerr effect in Ni(111) thin films with and without twinning

The longitudinal magneto-optic Kerr effect (LMOKE) as well as the quadratic MOKE (QMOKE) are important magneto-optic effects of fundamental physics but also powerful tools for thin-film sample characterization, e.g., to analyze magnetocrystalline anisotropies via vectorial magnetometry [1], to quantitatively determine the magnetooptic parameters of higher order in M [2] or to sense the crystallographic ordering in Heusler compounds [3]. We investigated sputter-deposited Ni(111) thin films and report on a strong three-fold angular dependence for LMOKE and QMOKE which cannot be explained by contributions of first and second order in M. Instead, third-order MOKE is needed to describe the observed angular dependencies [4]. We further studied Ni(111) thin films with and without two (111) phases (twinning). We observed substantial reduction of all MOKE contributions in a twinned sample compared to a non-twinned sample. This indicates that the MOKE angular dependencies truly are of crystallographic origin and not due to other, e.g., interface effects.

[1] T. Kuschel et al., J. Phys. D: Appl. Phys. 44, 265003 (2011)

[2] R. Silber et al., Phys. Rev. B 100, 064403 (2019)

[3] R. Silber et al., Appl. Phys. Lett. 116, 262401 (2020)

[4] A. V. Petukhov et al., J. Appl. Phys. 83, 6742 (1998)   

Quantitative analysis of magnetic properties in nanowires based on wide-field diamond NV magnetometry

Wide-field magnetometry based on diamond nitrogen-vacancy (NV) centers has been used to image solid-state magnetic materials, current-flowing devices, magnetic particles or nuclear spins in biomedical samples etc. Here, we use a custom-built ambient wide-field diamond magnetometer to study magnetic properties of various nanowires including Fe, Co, Fe-Au alloy, etc. From the magnitude and spatial distribution of static magnetic field around nanowires, we are able to extract magnetization of individual ferromagnet nanowires which varies depending on relative size, material composition and so on. The imaging technique can provide quantitative analysis method of studying magnetism in various nanostructures.   

Imaging of magnetization and its dynamics on the micro- and nanoscale by Scanning Transmission X-ray Microscopy

Magnetic materials on the micro- and nanoscale are addressing a wide spectrum of contemporary challenges ranging from cancer research towards spintronic and magnonic solutions for modern information technology. The synchrotron-based technique of Scanning Transmission X-ray Microscopy (STXM) allows the element-specific and spatially resolved (≥ 10 nm [1]) chemical and magnetic characterization of such materials in a wide range of X-ray absorption energies. By exploiting X‐ray Magnetic Circular Dichroism (XMCD), information on orientation and size of magnetic moments can be determined, probing the sample by circularly polarized X-rays at a photon energy set to an X-ray absorption edge of the investigated element. Time-resolved STXM (TR-STXM) allows to monitor dynamic excitations of the magnetization up to 10 GHz with  < 30 ps sampling [2]. The talk will introduce Scanning Transmission X-Ray Microscopy, with the focus on time-resolved STXM and its application to element-specifically detect magnetization dynamics on the micro- and nanometer scale. TR-STXM imaging of uniform and non-uniform resonant responses in Py/Co bilayer, and Py stripe microstructures [3] and the resonant response of Fe₃O₄ nanoparticle chains within magnetotactic bacteria Magnetospirillum Magnetotacticum [4] are shown.

  

1.         W. Chao, P. Fischer, et al. Opt. Express, 2012. 20(9): p. 9777-9783.

2.         S. Bonetti, R. Kukreja, et al. Rev. Sci. Instrum., 2015. 86(9): p. 093703-1-093703-9.

3.         S. Pile, T. Feggeler, et al. Appl. Phys. Lett., 2020. 116(7): p. 072401-1-072401-5.

4.         T. Feggeler, R. Meckenstock, et al. Phys. Rev. Res., 2021. 3(3).