Low-temperature Ferromagnetic Phase Research Articles

Cu-doping induced tuning of magnetic properties and phase transformation in MnBi alloys

This study investigates the effect of Cu doping on the magnetic properties of MnBi alloy at sub-ambient temperatures. The Curie temperatures (Tc) were observed at 175.2 and 161.6 K for Mn52Bi45Cu3 and Mn51Bi45Cu4 alloys, respectively. The Mn55Bi45 sample demonstrates a notable saturation magnetization (Ms) of 72 emu/g, reaching 90% of the theoretical value, indicating the successful synthesis of high-purity ferromagnetic low-temperature phase MnBi. Compared to Mn55Bi45, all Cu-doped samples exhibit higher coercivity (Hc) within the temperature range of 10–200 K. In particular, the sample with 4 at. % Cu exhibits an Hc of 7.4 kOe at 10 K, which is 37 times higher than that of the MnBi sample (0.2 kOe), and it remains significantly higher than that of the 0–3 at. % Cu-doped samples, even at 200 K. Further details of this investigation are presented in this paper.

Dual-component anomalous Hall effect in a helical spin-spiral metamagnet

We report a dual-component anomalous Hall effect (AHE) in polycrystalline Fe3Ga4 thin films grown on STO (001) and Al2O3 substrates. Systematic magnetic and magnetotransport measurements reveal an AHE consisting of positive and negative contributions that coexist across a wide range of temperatures and magnetic phases. We find that both magnitudes are nearly equal in the low-temperature ferromagnetic (FM) phase, but that their relative ratio is reduced upon heating through the antiferromagnetic helical spin-spiral state where they compete with metamagnetism and topological Hall effects, maintaining finite values at least up to the high-temperature FM phase.

Temperature and Thickness Dependence of the Thermal Conductivity in 2D Ferromagnet Fe3GeTe2.

The emergence of symmetry-breaking orders such as ferromagnetism and the weak interlayer bonding in van der Waals materials offers a unique platform to engineer novel heterostructures and tune transport properties like thermal conductivity. Here, we report the experimental and theoretical study of the cross-plane thermal conductivity, κ⊥, of the van der Waals two-dimensional (2D) ferromagnet Fe3GeTe2. We observe an increase in κ⊥ with thickness, indicating a diffusive transport regime with ballistic contributions. These results are supported by the theoretical analyses of the accumulated thermal conductivity, which show an important contribution of phonons with mean free paths between 10 and 200 nm. Moreover, our experiments show a reduction of κ⊥ in the low-temperature ferromagnetic phase occurring at the magnetic transition. The calculations show that this reduction in κ⊥ is associated with a decrease in the group velocities of the acoustic phonons and an increase in the phonon-phonon scattering of the Raman modes that couple to the magnetic phase. These results demonstrate the potential of van der Waals ferromagnets for thermal transport engineering.

Concomitant interfacial spin fractal transformation and exchange bias in a magnetic shape memory alloy

Small-angle neutron scattering is used to study the exchange bias effect in Mn50Ni40Sn10. The martensitic transformation is found to be responsible for the nanometer-scale spin clusters (SCs) inside the low-temperature ferromagnetic (FM) martensite phase. The magnetic field dependences of the SCs and FM domains exhibit an asymmetry that characterizes the exchange bias. We find that the surface geometries of the two magnetic phases are strongly susceptible to magnetization changes. While the FM domains and SCs are dominated by diffuse and fractal surfaces around the coercive fields, this configuration is reversed in the saturation regime, where the FM and SC surfaces become fractal and diffuse, respectively. These geometry changes are driven by the spin reorientation inside the heterogeneous FM/SC domain walls, where exchange anisotropy arises. Our work reveals the microscopic mechanism underlying the exchange bias effect in magnetic shape memory alloys.

Anomalous magnetic entropy changes of bulk and powder Mn0.5Fe0.5-xNi1+xSi0.94Al0.06 intermetallic system

The multi-component MnTX (T = Ni, Co; X = p-block elements) intermetallic system have attracted intensive attention due to their tunable magneto-structural phase transition and associated giant magnetocaloric effect. Here, we report an experimental study on the magnetic, magnetocaloric, and electrical properties of alloys that belong to the MnTX family- Mn0.5Fe0.5-xNi1+xSi0.94Al0.06. For all values of x (0≤ x ≤0.10), the materials exhibited a first-order magneto-structural phase transition from a low-temperature ferromagnetic orthorhombic phase to a high-temperature paramagnetic hexagonal phase. The transition was accompanied by large magnetic entropy changes of up to –22 and −57 J kg−1K−1 at T = 322 K for field changes of 2 T and 5 T, respectively. Most noticeably, a sharp jump in electrical resistivity was observed at T = 320 K for x = 0.1, which confirmed that the first-order phase transition preserved the mechanical stability of the material. The alloy with x = 0.1 was further crushed into powder and the magnetic properties were compared to its bulk counterpart. The enhanced mechanical stability and nearly zero magnetic hysteresis loss along with the fact that the system is constituted of cheap, non-toxic elements make this system worthy of consideration for near-room temperature magnetic refrigeration applications.

Excitations of Ising strings on a lattice

The 3d Ising model in the low temperature (ferromagnetic) phase describes dynamics of two-dimensional surfaces — domain walls between clusters of parallel spins. The Kramers-Wannier duality maps these surfaces into worldsheets of confining strings in the Wegner’s ℤ2 gauge theory. We study the excitation spectrum of long Ising strings by simulating the ℤ2 gauge theory on a lattice. We observe a strong mixing between string excitations and the lightest glueball state and do not find indications for light massive resonances on the string worldsheet.

Magnetic coupling of guest metallocene molecules with SURMOF-2 host matrix

Metal-organic frameworks (MOFs) are crystalline and porous, molecular solids consisting of metal nodes and organic ligands. An interesting example is the MOF-2 system, where ${\mathrm{Cu}}^{2+}$ ions form antiferromagnetically coupled dimers to yield so-called paddlewheels. In the case of surface-anchored MOF-2 (SURMOF-2) systems the ${\mathrm{Cu}}^{2+}$ ions are connected via carboxylate and OH groups in a zipperlike fashion. This unusual coupling of the spin-1/2 ions within the resulting one-dimensional chains stabilizes a low-temperature ferromagnetic (FM) phase. In this study, the magnetic properties of two SURMOF-2 systems (Cu(bdc) and Cu(bpdc) with $\mathrm{bdc}=1, 4\text{-benzendicarboxylic}$ acid, $\mathrm{bpdc}=4$,4-biphenyldicarboxylic acid) were investigated using x-ray magnetic circular dichroism both in the absorption and in the scattering geometry. The presence of the FM phase in these SURMOF-2 systems is confirmed. Taking advantage of the element sensitivity of this technique. it was established that the magnetic signal originates from ${\mathrm{Cu}}^{2+}$ ions. After loading of SURMOF-2 with metallocene molecules, no remarkable changes of the magnetic properties of the host matrix were observed. However, the magnetic behavior of the guest molecules, as it turned out, is rather different. In the case of nickelocene loading, a polarization effect was found, resulting in ferromagnetic ordering of the guest molecules. However, in the case of manganocene derivatives, the polarization effect is not observed and these molecules remained in their paramagnetic state. The details of these effects are discussed.

Evolution of ground state in Cr2Te3 single crystal under applied magnetic field

Two-dimensional magnetic materials hold great promise for applications toward efficient data storage and transfer. It would be a huge advantage if their ground-state properties had strong responses against external stimulations, such as magnetic and electric fields. Here, we report several intriguing discoveries in single-crystal ${\mathrm{Cr}}_{2}{\mathrm{Te}}_{3}$. Based on comprehensive specific heat, differential scanning calorimetry, variable-temperature x-ray diffraction, and linear-thermal-expansion measurements, we find that ${\mathrm{Cr}}_{2}{\mathrm{Te}}_{3}$ has a low-temperature ferromagnetic (FM) ground-state phase, which changes to an antiferromagnetic (AFM) phase when temperature is increased to ${T}_{\mathrm{C}}$ = 160 K. This FM-AFM transition is a first-order phase transition, and the transition temperature can be further enhanced to 178 K by a moderate magnetic field. At the same time, the first-order phase transition will transform into a second-order phase transition, indicating strong spin-lattice coupling (SLC). A second-order AFM-paramagnetic phase transition emerges at ${T}_{\mathrm{N}}=181$ K. This AFM phase is gradually suppressed by the magnetic field and eventually disappears at a critical field of 0.48 T. The SLC results in a notable negative-thermal-expansion coefficient of $\ensuremath{-}17.2$ ppm/K and a remarkable magnetostriction coefficient of 44.7 ppm/T at 200 K. With the assistance of first-principles density functional theory calculations and Monte Carlo simulations, we conclude that the collinear FM, canted FM, and AFM configurations in ${\mathrm{Cr}}_{2}{\mathrm{Te}}_{3}$ depend on the temperature and applied magnetic field. Our work reveals the highly tunable magnetic phases and SLC in ${\mathrm{Cr}}_{2}{\mathrm{Te}}_{3}$, which will be helpful for developing the potential functionalities of this material.

Magnetotransport, thermophysical and magnetocaloric properties of manganite La-=SUB=-0.8-=/SUB=-Ag-=SUB=-0.1-=/SUB=-MnO-=SUB=-3-=/SUB=- in magnetic fields up to 8 T

The electrical resistance, heat capacity, thermal conductivity, thermal conductivity and magnetocaloric effect of the polycrystalline sample La0.8Ag0.1MnO3 were measured in the temperature range 77-350 K and in magnetic fields up to 8 T. It is shown that electrical resistance in the paramagnetic phase can be explained within the framework of the concept of small-radius polarons with an activation energy of EP=119 meV, and the behavior of electrical resistance in the low-temperature ferromagnetic phase suggests the existence of several scattering mechanisms. The observed large magnitude of the magnetoresistive effect at low temperatures is explained by the intergranular tunneling of conduction electrons. An anomalous abrupt change in the lattice heat capacity during the ferromagnetic-paramagnetic phase transition was detected. The increase in thermal conductivity and thermal conductivity observed below the Curie temperature TC is associated with the scattering of phonons by local Jahn-Teller distortions, which weaken during the transition to the ferromagnetic phase. The magnetocaloric effect in a magnetic field of 8 T reaches a value of Delta Delta Tad=4 K, and the value of the efficiency of magnetic cooling in the same field is 261.6 J/kg. Keywords: manganites, electrical resistance, heat capacity, thermal conductivity, thermal conductivity, magnetocaloric effect. Keywords: manganites, electrical resistance, heat capacity, thermal conductivity, thermal conductivity, magnetocaloric effect.

Suppressing antiferromagnetic coupling in rare-earth free ferromagnetic MnBi-Cu permanent magnet

Rare-earth free, ferromagnetic MnBi shows a positive temperature coefficient of coercivity from room temperature to 400 K and energy product (BH)max of 17.7 MGOe at 300 K. However, MnBi undergoes a first-order structural phase transformation from a ferromagnetic low-temperature phase (LTP) to a paramagnetic high-temperature phase at 613 K below the Curie temperature (Tc) of 716 K. The transformation is attributed to Mn diffusion into the interstitial site of LTP MnBi unit cell. Interstitial Mn antiferromagnetically couples with the Mn at lattice 2a site, lowering the magnetization. Cu-occupied bipyramidal sites are investigated as a possible means to suppress Mn diffusion into the bipyramidal sites using first-principles calculations based on the density functional theory. Saturation magnetization, magnetocrystalline anisotropy constant (K), and Tc of (Mn0.5Bi0.5)100−xCux (x = 0–33) are reported. The magnetocrystalline anisotropy changes to the out-of-plane direction (x = 13) from the in-plane direction (x = 0.0). Tc decreases gradually to 578 K at x = 33 from 716 K at x = 0.0. The calculations show a slightly lower (BH)max of 15.6 MGOe while it is expected that Cu-occupied interstitial sites will significantly suppress Mn diffusion and raise the temperature of the phase transformation.

Domain structure evolution in the ferromagnetic Kagome-lattice Weyl semimetal Co3Sn2S2

Co3Sn2S2, a Weyl semimetal that consists of layers of Kagome lattices, undergoes a transition from a high temperature paramagnetic phase to a low temperature ferromagnetic phase below 177 K. The phase transition occurs through an intermediate non-trivial magnetic phase, the so called ‘A’-phase just below the Curie temperature. The ‘A’-phase was earlier linked with a competing anti-ferromagnetic phase, a spin-glass phase and certain indirect measurements indicated the possibility of magnetic Skyrmions in this phase. We have imaged the magnetic domain structure in a single crystal of Co3Sn2S2 at different temperatures, magnetic fields and field-angles by magnetic force microscopy. At low temperatures, we observed stripe domains indicating presence of uniaxial anisotropy. Above 130 K, the domain walls become mobile and they tend to align relatively easily when the magnetic field is increased along the c-axis than in the a–b plane. Our detailed study of field-dependent domain evolution reveal that the anomalous phase below T c through which the transition happens is most probably governed by domain wall motion.

Laser synthesis of thin MnxSi1-x films (x ~ 0.5) on c- and r-Al2O3 substrates at different laser energy densities at the target

Abstract Thin films of MnxSi1-x (x ~ 0.5) were obtained by pulsed laser deposition in droplet-free mode on c- and r-Al2O3 substrates at different laser energy densities E at the target. Their structural, electrical, and magnetic properties were studied depending on the value of E and the orientation of the substrate. The films are X-ray amorphous on the substrates of both types at E 5.5 J/cm2 the high-temperature ferromagnetic phase shows in the films, and at E ≅ 4 - 5 J/cm2, the low-temperature ferromagnetic phase predominates and there is no influence of the sapphire substrate orientation. The highest Curie temperature TC reached was 330 K at E ≅ 7.4 J/cm2 for the MnxSi1-x films obtained on c- and r-Al2O3. The correlated behavior of the magnetization and the signal of diffuse X-ray radiation scattering confirms the existence of e-MnSi nanocrystallites.

Effects of Mg and Sb Substitution on the Magnetic Properties of Magnetic Field Annealed MnBi Alloys.

Rare-earth-free permanent magnets have attracted considerable attention due to their favorable properties and applicability for cost-effective, high-efficiency, and sustainable energy devices. However, the magnetic field annealing process, which enhances the performance of permanent magnets, needs to be optimized for different magnetic fields and phases. Therefore, we investigated the effect of composition on the crystallization of amorphous MnBi to the ferromagnetic low-temperature phase (LTP). The optimal compositions and conditions were applied to magnetic field annealing under 2.5 T for elemental Mg- and Sb/Mg pair-substituted MnBi. The optimum MnBi composition for the highest purity LTP was determined to be Mn56Bi44, and its maximum energy product, (BH)max, was 5.62 MGOe. The Mg-substituted MnBi exhibited enhanced squareness (Mr/Ms), coercivity (Hc), and (BH)max values up to 0.8, 9659 Oe, and 5.64 MGOe, respectively, whereas the same values for the Sb/Mg pair-substituted MnBi were 0.76, 7038 Oe, and 5.60 MGOe, respectively. The substitution effects were also investigated using first-principles calculations. The density of states and total magnetic moments of Mn16Bi15Mg and Mn16Bi15Sb were similar to those of pure Mn16Bi16. Conversely, the Sb-substituted MnBi resulted in a dramatic enhancement in the anisotropy constant (K) from a small negative value (−0.85 MJ/m3) to a large positive value (6.042 MJ/m3).

Evolution of the Ferromagnetism of MnxSi1 –x (x ≈ 0.5) Films Produced by Laser Synthesis on c- and r-Cut Sapphire Substrates as the Laser Energy Density on the Target Changes

MnxSi1 – x (x ≈ 0.5) thin films have been produced on c- and r-cut sapphire substrates by the droplet-free pulsed laser deposition method at various laser energy densities E on the target. Their magnetic, electric, and X-ray diffraction properties as a function of E and substrate orientation have been investigated. It has been established that the high-temperature ferromagnetic phase in the films at E ≥ 6 J cm–2 is more pronounced than that at E ≈ 4–5 J cm–2, when the low-temperature ferromagnetic phase dominates and there is no influence of the sapphire substrate orientation. The attained Curie temperature TC was 330 K at E ≈ 7.4 J cm–2 for the MnxSi1 – x films produced on c- and r-cut sapphire substrates. The magnetization of the MnxSi1 – x films produced at E ≥ 6 J cm–2 on c-cut sapphire is higher than that on r-cut sapphire and, conversely, lower when E ≤ 5.5 J cm–2. A change in the ratio of the diffuse signal amplitudes in the X-ray spectra for the films grown on different substrates is also observed under these conditions. Such a correlated behavior with magnetization is explained by the existence of e-MnSi nanocrystallites of optimal sizes, which, on the one hand, are responsible for the emergence of a diffuse signal in the X-ray spectra and, on the other hand, determine the high-temperature ferromagnetism of the films. The concentration of such nanocrystallites and the distribution of defects in the films are controlled by the type of substrate and the energy density on the target.

Phase investigations of manganese-bismuth alloyed in a microwave furnace

Implementation of manganese-bismuth (MnBi) alloys as high-performance permanent magnets is a challenge for physicists and engineers because the ferromagnetic low-temperature phase (LTP) is not exclusively obtained. In this work, melting powered by four commercial magnetrons of 2000–2500 W in a microwave furnace is demonstrated as a new route to alloy MnBi. Under an argon atmosphere, microwave heating transferred to pieces of broken Bi ingots and Mn flakes for 2 h gave rise to products of inhomogeneous composition and morphology. Scanning electron micrographs were classified into three regions according to morphology and elemental composition. Cubic-like clusters characterized as Mn precipitated over light solidified Bi-rich regions, and the MnBi phase was formed in homogeneous regions with a balanced composition between Mn and Bi. A ferromagnetic hysteresis loop was obtained in the ground powder with a coercivity of 40 kA/m. Subsequent annealing at 553 K under a pressure of 414 kPa for 12 h enhanced the MnBi phase with extended regions of balanced composition. It follows that the coercivity was increased to 60 kA/m. However, remanent magnetization was slightly reduced. This MnBi alloyed by microwave radiation can be further used in rare-earth-free magnets.

Change in magnetic properties of La2MnCoO6 in composite with CaCu3Ti4O12

The magnetic properties of double perovskite La2MnCoO6 (LCMO) by making a composite with ordered double perovskite CaCu3Ti4O12 (CCTO) have been investigated. The composites were prepared by mixing the prior synthesized pure phases of LCMO and CCTO at different mole ratio. We observed that a new low temperature ferromagnetic phase (TC ~ 110 K) with lower magnetization value evolved at the expense of a high temperature ferromagnetic phase (TC ~ 228 K). The latter ferromagnetism is associated with ordering of Mn4+ and Co2+ ions. The new magnetic phase could be correlated to vibronic superexchange interaction between Mn3+–O–Co3+ cations, which would establish a ferromagnetic state. The genesis of this magnetic phase can be attributed to the local structural modulation of LCMO across the grain boundary in contact with CCTO. The lower magnetic moments and low temperature transitions have been attributed to the atomic disorder antiferromagnetic interactions (Mn4+–O–Mn4+ and Co2+–O–Co2+) akin to parent LCMO.

Enhanced Magnetic Properties in Ar+-Ion Beam-Bombarded MnBi Thin Films

MnBi is a promising ferromagnetic material for applications at elevated temperatures due to its high Curie temperature, sizable magnetocrystalline anisotropy, and increasing coercivity with temperature. However, fabrication of MnBi thin films with ferromagnetic low-temperature phase (LTP) is conventionally difficult due to the peritectic Mn-Bi reaction and Mn oxidization. In this work, in situ Ar+-ion beam bombardment was introduced to modify the microstructure and magnetic properties of (Mn/Bi)n superlattice. Structural characterization revealed formation of MnBi LTP in the as-deposited ion-beam-bombarded samples. Vacuum annealing further promoted growth of MnBi LTP grains with c-axis orientation, leading to remarkably improved perpendicular magnetic anisotropy. Ion-beam bombardment resulted in a 200% increase in the saturation magnetization of the annealed MnBi thin film. This advancement is attributed to the enhanced interdiffusion and reaction of Mn and Bi in the ion-beam-bombarded thin films. A stability test after 150 days in air revealed a remarkable reduction in ferromagnetism due to MnBi decomposition. This work provides an effective approach for fabrication of high-quality MnBi thin films.

Insights into phase transitions and magnetism of MnBi crystals synthesized from self-flux

To effectively synthesize high purity ferromagnetic low temperature phase (LTP) MnBi with optimal microstructure is still a challenge that needs to be overcome for the system to reach its full potential. Here, the phase transitions and magnetic properties of MnBi crystals are reported. The phase transition between the low and high temperature structure of MnBi was systematically investigated at different heating/cooling rates using in situ synchrotron radiation X-ray diffraction. The material crystallizes in a layered hexagonal structure giving a platelike microstructure. The magnetic characterization of the crystals reveal that the saturation magnetization varies from 645 kA/m at 50 K to 546 kA/m at 300 K. Magnetization measurements also show that the sample upon heating becomes non-magnetic and transforms to the high temperature phase (HTP) at ∼640 K, and that it regains ferromagnetic properties and transforms back to the LTP at ∼610 K upon subsequent cooling.

Modification of the structure and magnetic properties of ceramic La2CoMnO6 with Ru doping

Effects of Ru doping on the structure and magnetic properties of La2CoMn1−xRuxO6 ceramics with different compositions (x = 0, 0.1, 0.2, 0.3, 0.5, 0.75, 1.0) have been investigated using a variety of complementary techniques (X-ray powder diffraction and photoelectron spectroscopy, electron microscopy, and magnetometry). Phase-pure polycrystalline samples were prepared by the conventional solid-state reaction method including high pressure (3000 kg/cm2) treatment of the original mixture of reagents. X-ray diffraction analyses of the samples accompanied with Rietveld refinement suggested a change in the crystal structure upon doping: for 0 ≤ x ≤ 0.3 the structure of the samples is predominantly disordered orthorhombic (s.g. Pnma) which transformed to ordered monoclinic (s.g. P21/n) for 0.5 ≤ x ≤ 1. This suggests that Ru doping induces (partial) cation order in the B-sublattice. Magnetic measurements indicated a low temperature ferromagnetic phase, and a positive value of the Curie–Weiss temperature (θcw) was derived for all samples of composition x ≤ 0.75. However, the Curie temperature (TC) and θcw decreased with increasing Ru doping. These changes in TC upon doping can be correlated to the changes in the nearest neighbor and next nearest neighbor exchange interactions. The sample with x = 1 orders antiferromagnetically at approximately 25 K.

Band structure evolution and the origin of magnetism in (Ga,Mn)As: From paramagnetic through superparamagnetic to ferromagnetic phase

The authors investigate the development of magnetic phases in (Ga,Mn)As -- the canonical dilute ferromagnetic semiconductor -- in a thoroughly prepared set of high-quality layers grown by molecular beam epitaxy. The Mn content in the (Ga,Mn)As solid solution extends from the extremely diluted (0.0007% Mn) to the medium Mn content of 1.6%. The authors trace simultaneously the evolution of magnetic and electronic properties of (Ga,Mn)As by comprehensive magnetic (SQUID magnetometry) optical (Raman scattering and photoreflectance) and ARPES measurements. The onset of a low-temperature ferromagnetic phase in (Ga,Mn)As is revealed for 1.6% Mn, the composition just slightly above the one displaying purely superparamagnetic behavior (0.9% Mn). No signature of a Mn-related impurity band has been identified in superparamagnetic and ferromagnetic (Ga,Mn)As.