Session Z22: Disordered Magnetic Materials

Glassy behavior of anisotropic magnetoresistance in the antiferromagnet Fe1/3NbS2

Antiferromagnets have attracted attention as candidate material for designing faster and more robust memory devices. Recent work has shown that the magnetic order in the antiferromagnet Fe_1/3NbS₂ can be switched with low current densities [1], which may be enabled by<!--EndFragment --> the coupling between the spin glass and antiferromagnetic order [2]. The coexistence of spin glass and antiferromagnetic order is often verified with magnetometry [2, 3], but rarely with transport measurements.

We show that anisotropic magnetoresistance (AMR) can probe glassy behavior in Fe_1/3NbS₂. We observe that below the spin-glass freezing temperature the AMR differs with the presence and the direction of the cooling field. We attribute such history-dependent AMR to the coupling between the antiferromagnetic and spin-glass orders, where the resistance changes with the relative orientation of antiferromagnetic order to the spin glass moments. Our observation shows the prospect of AMR as a sensitive probe of coexisting magnetic orders.

[1] N. L. Nair et al., Nat. Mater. 19, 153 (2020)

[2] E. Maniv et al., Sci. Adv. 7, 1 (2021)

[3] E. Maniv et al., Nat. Phys. 17, 525 (2021)   

Entropy changes and the magnetocaloric effect in anisotropic magnetic materials

The magnetocaloric effect, the heating or cooling of magnetic materials upon magnetic field variation,  is usually characterized by the entropy change in an isothermal process  and the temperature change in an adiabatic process. The behavior of curves of the magnetocaloric quantities as a function of temperature strongly depends on the magnetic interaction inside the material. For example, for isotropic materials, these curves exhibit a sharp peak around the magnetic ordering temperature and fade away outside this temperature region. On the other hand, in anisotropic materials these curves present anomalies such as the table-like effect, a structure with two peaks, the inverse effect etc.

In this work, we theoretically discuss the magnetic properties and the magnetocaloric effect of anisotropic magnetic materials with two or more nonequivalent sites. For this purpose, we consider a model Hamiltonian of interacting magnetic moments with extra terms to account for the anisotropy and the nonequivalence of the sites in the crystalline lattice. In the first step, we perform a systematic analysis in terms of the model parameters to understand the physics behind the different behaviors observed in the curves of the caloric quantities. Afterwards, we apply the model to calculate the magnetocaloric quantities in the compounds RCu2Cd where R stands for rare earth element. Our obtained results of the entropy change are in reasonable agreement with the available experimental data.     

Oral: Magnetic Structure Evolution in the Entropy-Stabilized Perovskite Oxide (Y0.2La0.2Tb0.2Pr0.2Nd0.2)MnO3

Large configurational disorder in entropy-stabilized single-phase materials can lead to unique functional properties that deviate from traditional alloy mixing rules. Recently, it has been shown that entropy-stabilization can be applied to specific sublattices of transition metal oxide structures, adding to the already rich composition-structure-property phase space found in this class of materials. In particular, previous work on entropy-stabilized oxides (ESOs) has shown the formation of colossal dielectric constants, enhanced magnetic exchange couplings, and mixed phase magnetic structures. In this study, we have focused on the latter and combined bulk magnetometry and temperature-dependent neutron diffraction to better understand the magnetic structure formed in an ESO perovskite, with an emphasis on how long-range magnetic order forms despite large disorder of a cation sublattice. Specifically, we present here results on the A-site alloyed perovskite, (Y0.2La0.2Tb0.2Pr0.2Nd0.2)MnO3, or (5A)MnO3. Our magnetometry data show two sharp phase transitions as well as the presence of exchange bias at low temperature, suggesting a mixed phase magnetic ground state consistent with previous reports. Neutron powder diffraction shows clear long-range antiferromagnetic ordering below 60 K and Reitveld refinement of the diffraction data reveals that the two phase transitions can be attributed to ordering on the Mn and A-site sublattices, and that the dominant low-temperature magnetic ordering is Pn'ma'. Finally, we compare the remarkable similarities between the magnetic structure evolution in our (5A)MnO3 to NdMnO3 to highlight the insensitivity of magnetic structures to extreme disorder on the A-site of the perovskite lattice.   

Oral: Theory of magnetic interactions and spin textures in amorphous FeGe

Magnetic properties and topological spin textures such as skyrmions are both fundamentally intriguing and relevant to novel information storage and advanced logic technologies. Topological spin textures have long been studied and considered mostly to form due to Dzyaloshinskii-Moriya interaction (DMI) caused by global symmetry-breaking or stabilized by spin frustration induced by alternating higher-order Heisenberg exchange interactions. Recently, experimental observation of chiral spin textures, such as helical spins and skyrmions, was reported for amorphous FeGe thin films [1].  However, there is yet to be a theoretical explanation or model to explain the magnetic properties and topological spin textures found in amorphous systems. How are topological spin textures formed in an amorphous system? Is DMI or spin frustration important? How does magnetic property differ in amorphous systems with respect to their crystalline counterparts? Here, we provide answers to these questions by performing first-principles calculations, ab-initio-based molecular dynamics, and Monte Carlo simulations on amorphous FeGe. Our results show that the magnetic exchange coupling parameters can be much stronger in amorphous FeGe compared to the crystalline value and we find nano-metric-sized topological spin-texture called anti-Skyrmion. Furthermore, anti-Skyrmions were found to form by the magnetic exchange coupling parameters rather than DMI or single-ion-anisotropy. Our results give first hints toward finding topological spin textures in any amorphous magnetic system without a heavy element.    

[1] Streubel, et al., Chiral Spin Textures in Amorphous Iron–Germanium Thick Films. Adv. Mater. 2021, 33, 2004830.   

Ferrimagnetism in Cr-substituted Mn4–xCrxAl11

We report on the flux growth, x-ray diffraction, magnetic measurements, and density functional theory calculations of the new mixed-site compound Mn_4–xCr_xAl₁₁ (x = 1.74). This compound is isostructural with binaries Mn₄Al₁₁ and Cr₄Al₁₁, and experiences site-mixing at both transition metal sites, although at different Mn:Cr ratios: 0.64:0.36 at one site and 0.49:0.51 at the other site. Magnetic measurements show that it is a ferrimagnet unlike the Mn₄Al₁₁ binary displaying one-dimensional magnetic character. The DFT calculations indicate that Mn_2.26Cr_1.74Al₁₁ is possibly a half-metal.   

Disorder, dipolar, and quadrupolar physics of the square lattice oxyhalides DyOX (X=Cl,Br,I)*

"Rare-earth ions are an important ingredient in frustrated magnetism due to their propensity for anisotropic magnetization distributions. Due to this and their crystal field splitting allowing for an effective spin-½ degrees of freedom, they are commonly investigated in the search for exotic magnetic phases of matter. The Dysprosium Oxy-halides (DyOX, X=Cl,Br,I) have come under recent investigation as layered Van der Waals materials [1], with interplanar spacing being a function of the halide's ionic radius. This, paired with Dysprosium's large magnetic moment and single-ion quadrupolar behavior, leads DyOX to be an exciting platform to study specific aspects of quantum magnetism that are generally difficult to probe. This talk will present several forms of data: neutron diffraction and scattering, thermomagnetic measurements, and simulation results to elucidate their magnetic properties. The results systematically show two ordering transitions, yet dramatically different magnetic structures across the family of compounds. Of particular interest is a disordered magnetic structure that arises with increased interplanar spacing. We will compare these compounds' magnetic properties, as well as the origin of these novel behaviors."

[1] Tian, Congkuan, Feihao Pan, Le Wang, Dehua Ye, Jieming Sheng, Jinchen Wang, Juanjuan Liu, et al. "DyOCl: A Rare-Earth Based Two-Dimensional van Der Waals Material with Strong Magnetic Anisotropy." Physical Review B 104, no. 21 (December 7, 2021): 214410. https://doi.org/10.1103/PhysRevB.104.214410.   

Understanding the Magnetic Structure of Spin Glass Zinc Manganese Telluride

Zinc Manganese Telluride (Zn,Mn)Te has attracted interest as a dense spin glass system. To gain a better understanding of the local magnetic correlations and the transition into the spin glass state, we performed neutron diffraction experiments on a powder sample of Zn_0.5Mn_0.5Te. We observe strong magnetic diffuse scattering features that persist well above the freezing temperature. Unexpectedly, sharp magnetic Bragg peaks also develop at low temperature and coexist with the diffuse scattering, pointing to an unusual ground state supporting both short- and long-range magnetic correlations. We present an analysis of the magnetic structure of Zn_0.5Mn_0.5Te utilizing both the reciprocal space data and the real space magnetic pair distribution function. We also introduce an algorithm to filter out the Bragg peaks and isolate the diffuse scattering signal. The analysis clarifies the nature of the short- and long-range magnetic correlations in (Zn,Mn)Te and provides a template for similar studies of magnetic systems with complex and short-range correlations.   

Magnetic and lattice thermodynamics in bulk rock salt high-entropy oxides

High entropy oxides feature high configurational entropy related to site occupancy disorder and exhibit several interesting functional properties, such as colossal dielectric constants, superionic conductivity, and low thermal conductivity. We will report on neutron scattering studies of the magnetic structure and dynamics in the prototypical (Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O[1]  and the new (Mg_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O [2] rock salt high entropy oxides featuring divalent 3d transition metals.[1] At low temperature, neutron diffraction reveals long-range magnetic order with (1/2,1/2,1/2) propagation vector, an order that survives the extreme configurational disorder, with short range order subsisting even at room temperature. The phase transition is continuous over an extended temperature range. The insights from neutron scattering will be presented in the light of Mössbauer spectroscopy,[2] calorimetry,[1-3] and muon spin resonance.[3]   

Tunable magnetic properties of Cantor Alloy CrMnFeCoNi studied via Muon Spin Relaxation

High-entropy alloys, or alloys made up of five or more elements in roughly equal proportions, exhibit exceptional properties not seen in other materials. Many of these alloys are known to be magnetic, raising the possibility of unusual magnetic properties driven by the extreme chemical disorder of high-entropy alloys. However, a comprehensive understanding of the magnetism in these materials has not yet been established. Here, we report muon spin relaxation (μSR) experiments to examine the magnetic properties of the well-known Cantor alloy CrMnFeCoNi. We tested five different samples of CrMnFeCoNi with various processing treatments and compositions. We found that the temperature and uniformity of the magnetic phase transition depends strongly on the preparation and stoichiometry of the sample. We also observed a strong variation of the spin dynamics across all samples. These results demonstrate that the magnetic properties of CrMnFeCoNi can be effectively tuned via composition and mechanical treatments, laying a solid foundation for further investigation of the magnetic properties of high-entropy materials.   

FeGaB Alloys for Magnetoelastic Coupling at Microwave Frequencies

Magnetoelastic coupling is important for many magnetoelectric applications such as devices based on surface acoustic waves. Fe₈₀Ga₂₀ is a high-performance magnetoelastic material due to its strong magnetoelastic response in the disordered A2 phase. In this work, we explored amorphous phases by adding boron into magnetron sputtered Fe₈₀Ga₂₀ films. Fe₈₀Ga₂₀ thin films with boron concentration from 2% to 16% were synthesized by co-sputtering from three targets and the compositions were characterized with X-ray photoemission spectroscopy. Structural characterization by X-ray diffraction indicated the structure of the films transitioning from crystalline to amorphous around 10% boron content. The coercivity, obtained from magnetization hysteresis loops, decreased rapidly from 75 Oe with 2% B to 4 Oe with 16% B, with the most rapid decrease right around the 10% B structure transition region. In addition to high magnetostriction, low damping is also desired to minimize energy loss in magnetic system. Using ferromagnetic resonance, we determine a low damping around 3×10^-3 in B-doped FeGa films. The magnetostriction constant of the B doped FeGa films was also characterized by measuring cantilever deflection in a rotating magnetic field.   

Magnetic properties of the possible altermagnet candidate YbFe4Al8

YbFe₄Al₈ has been studied in the past in polycrystalline form with claims of high-temperature superconductivity [1] and more recently reports of anomalous, low-field diamagnetism due to the interaction between the moment of Yb and the effective moment of the canted Fe spins [2]. This system has also been suggested to be a promising candidate for an altermagnet [3]. Motivated by this, we synthesized single crystals of YbFe₄Al_8, which manifest a tetragonal ThMn₁₂-type structure and studied their anisotropic magnetic properties. The magnetism is highly sensitive to magnetic field and temperature and shows a series of transitions. This is supported by resistivity and specific heat results.  No sign of diamagnetism has been observed in our low-field data. Anisotropic H-T phase diagram will be presented and discussed to show the evolution of magnetism. Characterization by single-crystal X-ray diffraction has also been done to examine the possibility of structural transition in this system.

 

[1] Drulis, H. et al., Solid State Communications, 123 (2002)

[2] Andrzejewski, B. et al., Phys. Stat. Sol. (b), 243 (2006)

[3] Libor Šmejkal –private communication.

    

RuO2: a puzzle to be solved

Altermagnetism is a topic that has lately been gaining attention and the RuO2 compound is among one of the most studied altermagnetic candidates. However, the survey of available literature on RuO2 properties suggests that there is no consensus about the magnetism of this material, especially about the size of the ordered moments. By performing density functional theory calculations, we show that the stoichiometric RuO2, for reasonable values of Hubbard U in DFT+U, is not, or only very weakly magnetic, in agreement with spin-polarized neutron scattering experiments. We further argue that Ru vacancies can actually aid the formation of a magnetic state in RuO2, and that may be the reason why other, indirect probes are consistent with sizeable magnetic moment. This. in turn. suggests that a characterization of the amount of Ru vacancies in experimental samples might help to resolve of the controversy between the different experimental results.   

Single Crystal Synthesis and Characterization of R4Ni2InGe4 (R = Gd, Tb)

Rare-earth (R) germanides are known for their diverse physical properties such as complex magnetic ordering (e.g., RT₂Ge₂ [1]). R₄T₂InGe₄ (T = Fe, Co, Ni, Ru, Rh, Ir) was the first reported variant of quaternary compound belonging to this family. They manifest a monoclinic structure and have been synthesized via arc-melting [2]. Although they have been known for almost a decade, few of their properties have been examined. To start our exploration of the quaternary rare earth germanides, we grew single crystals of Gd₄Ni₂InGe₄ and Tb₄Ni₂InGe₄ from R-Ni-In-Ge melt, extending the study on previously reported single crystal R₄Ni₂InGe₄ (R = Dy, Ho, Er, and Tm) series grown out of In flux [3]. The samples were characterized by powder X-ray diffraction, resistivity and magnetization measurements.

[1] Szytula, A et al., (1994). Handbook of Crystal Structures and Magnetic Properties of Rare Earth Intermetallics. CRC Press.

[2] Anton O. Oliynyk et al., Inorg. Chem. 2015, 54, 6, 2780−2792

[3] James R. Salvador et al., Inorg. Chem. 2006, 45, 18, 7091−7099