Mn Spins Research Articles

Magnetization enhancement in B-doped Heusler alloys Fe2MnSi1−xBx (x = 0–0.4)

A series of Fe2MnSi1−xBx (x = 0–0.4) Heusler alloys was synthesized and investigated. The variation of lattice parameters with B content suggests that B prefers entering the substitutional site of Fe2MnSi when the amount is small. But when B content is high, it enters the interstitial site. A spin reorientation is observed at ~54 K in Fe2MnSi, which moves to low temperatures with increasing B content and disappears when x = 0.3. Both the Curie temperature TC and saturation magnetization Ms increase with the substitution of B for Si. The Ms gets closer to the calculated total spin moment Mt. All this suggests that the dope of B can enhance the ferromagnetic character of Fe2MnSi1−xBx. For the high B content, B-doping expands the lattice and enlarges the distance between magnetic atoms. This leads to the increase of Fe and Mn spin moments. The substitution of B for Si has an ordering-effect for Fe2MnSi lattice by forming a strong covalent bonding and relieves the possible Fe-Mn disorder. This effect also contributes to the increase of Ms.

Evolution of cation and spin orders in the double–double-double perovskite series CaxMn2−xFeReO6

Variations of perovskite superstructure type and magnetic properties across the high-pressure ${\mathrm{Ca}}_{x}{\mathrm{Mn}}_{2\ensuremath{-}x}{\mathrm{FeReO}}_{6}$ series have been investigated by powder neutron-diffraction, x-ray magnetic circular dichroism, and magnetization measurements. Monoclinic $P{2}_{1}/n\phantom{\rule{0.16em}{0ex}}{A}_{2}B{B}^{\ensuremath{'}}{\mathrm{O}}_{6}$ double perovskite solid solutions with rocksalt-type ordering of $B/{B}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{Fe}/\mathrm{Re}$ cations but no $A$-cation order are discovered to exist only close to the end members $0lxl0.17$ and $1.73lxl2$. Compositions in the range $0.74lxl\ensuremath{\sim}1.1$ adopt a tetragonal $P{4}_{2}/n\phantom{\rule{0.16em}{0ex}}A{A}^{\ensuremath{'}}B{B}^{\ensuremath{'}}{\mathrm{O}}_{6}$ double-double perovskite phase based on ${\mathrm{CaMnFeReO}}_{6}$, which has both columnar $A/{A}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{Ca}/\mathrm{Mn}$ and rocksalt $B/{B}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{Fe}/\mathrm{Re}$ cation orders. Two-phase coexistence of double and double-double perovskites occurs over wide regions between these limits. All samples are ferrimagnetic with Curie temperatures near 500 K. Low-temperature Mn spin order is also observed in the Mn-rich double perovskites and the double-double perovskites. Spin reorientation transitions observed across all of the double perovskites are shown to result from $5{d}^{2}\phantom{\rule{4pt}{0ex}}{\mathrm{Re}}^{5+}$ orbital ordering, but this does not occur in the double-double perovskites. The Ca-rich double perovskites show significant magnetic coercivity at low temperatures.

Modulation of magnetic damping in antiferromagnet/CoFeB heterostructures

The modulation of antiferromagnetic (AFM) material and thermal annealing treatment on the magnetic damping of various AFM/CoFeB (CFB) samples have been systematically studied with the time-resolved magneto-optical Kerr effect. It is found the saturated magnetic damping factor αs increases considerably after introducing a thin MnIr or MnPt AFM layer. As a thin Al spacer of tAl = 0–2 nm is inserted, αs is found to decrease rapidly, reaching nearly the same value as that of the single CFB film at tAl = 2 nm. The result suggests that the AFM layer is not a good spin sink material and the surprisingly strong decrease in the damping factor is mainly attributed to the reduced direct exchange coupling between CoFe and Mn spins at the AFM/FM interface. Moreover, in spite of the exchange bias effect occurring or not, a similar monotonic increasing trend of αs with the increasing AFM layer thickness is observed for the as-deposited and annealed AFM/CFB samples, indicating that the enhanced magnetic damping at an elevated annealing temperature is mainly related to the increased interface roughness and atomic diffusion. These findings provide deeper insights into the role of the AFM/FM interface in magnetization dynamics, which will be helpful for future spintronic applications.

Ground-state magnetic structure of Mn3Ge

We have used spherical neutron polarimetry to investigate the magnetic structure of the Mn spins in the hexagonal semimetal Mn$_3$Ge, which exhibits a large intrinsic anomalous Hall effect. Our analysis of the polarimetric data finds a strong preference for a spin structure with $E_{1g}$ symmetry relative to the $D_{6h}$ point group. We show that weak ferromagnetism is an inevitable consequence of the symmetry of the observed magnetic structure, and that sixth order anisotropy is needed to select a unique ground state.

Nanoscale degeneracy lifting in a geometrically frustrated antiferromagnet

The local atomic and magnetic structures of the compounds $A$MnO$_2$ ($A$ = Na, Cu), which realize a geometrically frustrated, spatially anisotropic triangular lattice of Mn spins, have been investigated by atomic and magnetic pair distribution function analysis of neutron total scattering data. Relief of frustration in CuMnO$_2$ is accompanied by a conventional cooperative symmetry-lowering lattice distortion driven by N\'eel order. In NaMnO$_2$, however, the distortion has a short-range nature. A cooperative interaction between the locally broken symmetry and short-range magnetic correlations lifts the magnetic degeneracy on a nanometer length scale, enabling long-range magnetic order in the Na-derivative. The degree of frustration, mediated by residual disorder, contributes to the rather differing pathways to a single, stable magnetic ground state in these two related compounds. This study demonstrates how nanoscale structural distortions that cause local-scale perturbations can lift the ground state degeneracy and trigger macroscopic magnetic order.

Impact of vacancy defects on optoelectronic and magnetic properties of Mn-doped ZnSe

Based on first principles calculations, we have investigated the electronic, magnetic and optical properties of Mn-doped ZnSe with and without vacancy defects. We find that the incorporation of Mn at Zn site changes the non-magnetic ground state of ZnSe to magnetic ground state with total magnetic moment 5 μB. From the investigation of magnetic coupling, it is found that the interaction between Mn spins in Mn-doped ZnSe is AFM which can be explained in term of super-exchange mechanism. The effect of vacancy defects such as Zn or Se vacancy on magnetic coupling between Mn spins were also analyzed and we found that Zn-vacancy promotes double exchange interaction which stabilizes the ferromagnetic state and room temperature ferromagnetism is expected while Se-vacancy doesn’t change magnetic ground state from AFM to FM states. The magnetic interactions in Mn-doped ZnSe system with and without Zn vacancy were explained using phenomenological band structure model. Moreover, optical absorptions for all systems were investigated at their stable magnetic states and we found that Mn doping causes the red shifted in absorption edge which is due to the spin forbidden d-d transitions (4T1-6A1) in Mn ions. The new peak in low energy range is related to the acceptor states introduced by Zn-vacancy in Mn-doped ZnSe system. The Se-vacancy generates peaks in visible region which is attributed to the donor states caused by Se-vacancy. Finally, the d-d transition (4T1-6A1) in FM and AFM coupled Mn ions were also studied and we found that the d-d transition peaks in AFM and FM coupled Mn ions are blue and red shifted respectively. Similarly, the blue and red shift in the energy bandgap of ZnSe are also observed in AFM and FM coupled Mn ions which support the agreement between the experimental and theoretical observations.

Oxygen over stoichiometry in the 2H-perovskite related structure: the route to a large family of cation deficient Ising chain oxides Sr1+y[(Mn1−xCox)1−z□z]O3

Considering the incommensurability of the Co–Mn spin chain oxides, the concept of oxygen over stoichiometry associated with cationic vacancies is demonstrated and quantified, allowing the design of fragmented Ising chain magnet derivatives.

Two closed ATP- and ADP-dependent conformations in yeast Hsp90 chaperone detected by Mn(II) EPR spectroscopic techniques

Hsp90 plays a central role in cell homeostasis by assisting folding and maturation of a large variety of clients. It is a homo-dimer, which functions via hydrolysis of ATP-coupled to conformational changes. Hsp90's conformational cycle in the absence of cochaperones is currently postulated as apo-Hsp90 being an ensemble of "open"/"closed" conformations. Upon ATP binding, Hsp90 adopts an active ATP-bound closed conformation where the N-terminal domains, which comprise the ATP binding site, are in close contact. However, there is no consensus regarding the conformation of the ADP-bound Hsp90, which is considered important for client release. In this work, we tracked the conformational states of yeast Hsp90 at various stages of ATP hydrolysis in frozen solutions employing electron paramagnetic resonance (EPR) techniques, particularly double electron-electron resonance (DEER) distance measurements. Using rigid Gd(III) spin labels, we found the C domains to be dimerized with same distance distribution at all hydrolysis states. Then, we substituted the ATPase Mg(II) cofactor with paramagnetic Mn(II) and followed the hydrolysis state using hyperfine spectroscopy and measured the inter-N-domain distance distributions via Mn(II)-Mn(II) DEER. The point character of the Mn(II) spin label allowed us resolve 2 different closed states: The ATP-bound (prehydrolysis) characterized by a distance distribution having a maximum of 4.3 nm, which broadened and shortened, shifting the mean to 3.8 nm at the ADP-bound state (posthydrolysis). This provides experimental evidence to a second closed conformational state of Hsp90 in solution, referred to as "compact." Finally, the so-called high-energy state, trapped by addition of vanadate, was found structurally similar to the posthydrolysis state.

Observation of drastic changes in the magnetic response of epitaxial TbMnO3 thin films upon Al-doping

The influence of Al3+ doping on the ferromagnetic response of epitaxial TbMn1-xAlxO3 (x = 0, 0.1) thin films (~100 nm) was studied experimentally. TbMnO3 and TbMn0.9Al0.1O3 films (~100 nm thin) were epitaxially grown on (001)-SrTiO3 substrates by means of the high-pressure oxygen DC magnetron sputtering technique. X-ray diffraction patterns clearly showed that both films were epitaxial, with the c-axis oriented in the (00ℓ) direction. X-ray photoelectron spectroscopy analysis confirmed that the nominal valence of the Mn ions in the TbMnO3 film is 3+. Although a small shake-up peak was observed in the Mn 2p edge of the TbMn0.9Al0.1O3 film, depth-profiling experiments showed that the presence of this peak was only a surface effect. Anomalous ferromagnetism was observed in the pristine TbMnO3 films, which seemed to be caused by coupling between the magnetization and the epitaxial strain, due to the lattice mismatch between the film and substrate. Interestingly, drastic changes in the ferromagnetic response of the TbMnO3 films were observed upon Al3+ substitution at the Mn3+ positions (chemical pressure). Although Al3+ and Mn3+ ions are isovalent, the smaller size of the Al3+ ions brings about further microstructural strain, which clearly influenced the ferromagnetism observed in the TbMnO3 films. The introduction of Al3+ ions into the TbMnO3 lattice resulted in a shift of the (00ℓ) reflections to lower angles as compared with those of the TbMnO3 films. The TbMnO3 films showed a well-defined transition at ~42 K, which corresponds to the magnetic ordering temperature from the paramagnetic phase to the sinusoidal antiferromagnetic structure of the Mn spins. The antiferromagnetic transition shifted to lower temperatures for the TbMn0.9Al0.1O3 thin films. The experimental results demonstrated that the magnetic response of the single-phase spin-driven multiferroic TbMnO3 can effectively be tuned by both epitaxial strain and chemical pressure at the Mn site.

Formation and magnetic properties of hexagonal DO19 phase in Fe2MnGa: An experimental and theoretical investigation

The formation and magnetic properties of Fe2MnGa hexagonal DO19 phase were investigated in detail. Hexagonal Fe2MnGa can be stabilized by furnace cooling, while the FCC phase can be obtained by quenching from high temperature. Strong ferromagnetic character is identified in Fe2MnGa DO19 phase. Its saturation magnetic moment Ms at 5 K is as large as 4.40 μB/f.u. and Curie temperature TC is 563 K. First-principles calculations indicate that Fe and Mn atoms tend to occupy the 6h sites randomly in the DO19 lattice. The DO19 type Fe2MnGa shows a normal ferromagnetic metal character in its electronic structure. Similar to the experimental results, the calculated total magnetic moment is also quite large and mainly comes from the contributions of Fe and Mn spin moments. Both of them are larger than 1.8 μB and are parallel coupled. These properties are quite different from the antiferromagnetic character in FCC Fe2MnGa.

Commensurate to incommensurate magnetic phase transition in honeycomb-lattice pyrovanadateMn2V2O7

We have synthesized single crystalline sample of Mn$_2$V$_2$O$_7$ using floating zone technique and investigated the ground state using magnetic susceptibility, heat capacity and neutron diffraction. Our magnetic susceptibility and heat capacity reveal two successive magnetic transitions at $T_{N1} =$ 19 K and $T_{N2} =$ 11.8 K indicating two distinct magnetically ordered phases. The single crystal neutron diffraction study shows that in the temperature ($T$) range 11.8 K $\le T \le$ 19 K the magnetic structure is commensurate with propagation vector $k_1 = (0, 0.5, 0)$, while upon lowering temperature below $T_{N2} =$ 11.8 K an incommensurate magnetic order emerges with $k_2 = (0.38, 0.48, 0.5)$ and the magnetic structure can be represented by cycloidal modulation of the Mn spin in $ac-$plane. We are reporting this commensurate to incommensurate transition for the first time. We discuss the role of the magnetic exchange interactions and spin-orbital coupling on the stability of the observed magnetic phase transitions.

Ubiquitous first-order transitions and site-selective vanishing of the magnetic moment in giant magnetocaloric MnFeSiP alloys detected by Mn55 NMR

We report on a study on a representative set of FeP-based MnFePSi samples by means of Mn NMR in both zero and applied magnetic field. The first-order nature of the magnetic transition is demonstrated by truncated order parameter curves with a large value of the local ordered moment at the Curie point, even at compositions where the transition appears second order from magnetic measurements. No weak ferromagnetic order could be detected at Si-poor compositions showing the kinetic arrest phenomenon, but rather the phase separation of fully ferromagnetic domains from volume fractions where Mn spins are fluctuating. The more pronounced decrease of the ordered moment at the $3f$ sites than at the $3g$ sites on approaching ${T}_{C}$, characteristic of the mixed magnetism of these materials, is shown to be driven by the drop of the $3f$ spin density instead of enhanced spin fluctuations. The temperature-driven $3f$ spin extinction is demonstrated to evolve into a truly nonmagnetic state of the $3f$ Mn ions well above ${T}_{C}$, in agreement with theoretical models and in contrast with previous experiments who detected just a partial moment quenching. Besides ``normal'' $3f$ Mn ions undergoing a magnetic to spinless state transition, NMR in Mn-rich compositions detects the disproportionation at $3f$ sites of a significant minority Mn fraction with negligible hyperfine couplings, which retains its diamagnetic character independent of temperature. Such a diamagnetic fraction qualitatively accounts for the reduced average $3f$ moment previously reported at large Mn concentrations.

Tetramer orbital ordering and lattice chirality in MnTi2O4

Using density-functional theory calculations and experimental investigations on structural, magnetic and dielectric properties, we have elucidated a unique tetragonal ground state for MnTi2O4, a Ti^{3+} (3d^1)-ion containing spinel-oxide. With lowering of temperature around 164 K, cubic MnTi2O4 undergoes a structural transition into a polar P4_1 tetragonal structure and at further lower temperatures, around 45 K, the system undergoes a paramagnetic to ferrimagnetic transition. Magnetic superexchange interactions involving Mn and Ti spins and minimization of strain energy associated with co-operative Jahn-Teller distortions plays a critical role in stabilization of the unique tetramer-orbital ordered ground state which further gives rise to lattice chirality through subtle Ti-Ti bond-length modulations.

Interplay between spin dynamics and crystal field in the multiferroic compound HoMnO3

In the multiferroic hexagonal manganite HoMnO3, inelastic neutron scattering and synchrotron based THz spectroscopy have been used to investigate the spin waves associated to the Mn order together with Ho crystal field excitations. While the Mn order sets in first below 80 K, a spin reorientation occurs below 37 K, a rare feature in the rare earth manganites. We show that severalHo crystal field excitations are present in the same energy range as the magnons, and that they are all affected by the spin reorientation. Moreover, several anomalous features are observed in the excitations at low temperature. Our analysis and calculations for the Mn spin waves and Ho crystal field excitations support Mn-Ho coupling mechanisms as well as coupling to the lattice affecting the dynamics.

Room-Temperature Low-Field Colossal Magnetoresistance in Double-Perovskite Manganite.

We have discovered room-temperature low-field colossal magnetoresistance (CMR) in an A-site ordered NdBaMn_{2}O_{6} crystal. The resistance changes more than 2 orders of magnitude at a magnetic field lower than 2T near 300K. When the temperature and magnetic field sweep from an insulating (metallic) phase to a metallic (insulating) phase, the insulating (metallic) conduction changes to the metallic (insulating) conduction within 1K and 0.5T, respectively. The CMR is ascribed to the melting of the charge and orbital ordering. The entropy change which is estimated from the B-T phase diagram is smaller than what is expected for the charge and orbital ordering. The suppression of the entropy change is attributable to the loss of the short-range ferromagnetic fluctuation of Mn spin moments, which is an important key of the high temperature and low magnetic field CMR effect.

Ab initio study of optoelectronic and magnetic properties of Mn-doped ZnS with and without vacancy defects

The effect of vacancy defects on optoelectronic and magnetic properties of Mn-doped ZnS have been systematically investigated using first principle approaches. A single Mn substitution at Zn site induces a spin-polarized ground state in pure ZnS with total magnetization 5 . Our results for magnetic coupling show that the coupling between Mn spins in pure Mn doped ZnS is antiferromagnetic under the super-exchange mechanism. The existence of native defects has a great influence on the magnetic ground state of Mn-doped ZnS. In particular, a p -type defect such as Zn vacancy play a crucial role in stabilizing ferromagnetic ground state while n-type defect, such as S vacancy, has no effect on the magnetic ground state i.e. the interaction between two Mn spins with S-vacancy remain antiferromagnetic. Furthermore, optical properties such as dielectric functions, absorption coeffiecients, reflectivity and transmissitivity for Mn doped systems with and without vacancy defect were also studied, and we found that an absorption peak was obtained in the infrared region which is attributed to the defect states introduced by Zn vacancy in the system. In a S-vacancy defect system, the peaks in the near infrared and visible region are due to donor states introduced by S vacancy defect and these peaks are produced by electrons flipping from a spin up state to a spin down state. Finally, we also correlated the magnetic interactions with the d–d optical transition in pure Mn-doped ZnS and found that the d–d transitions during optical absorptions are red shifted and blue shifted in FM and AFM coupled Mn ions pair, which is in good agreement with the experimental observations. This study may help to understand the behavior of optical and magnetic properties of DMS under vacancy defects.

Theoretical Study of the Stability and Electronic Properties of HgMnTe Alloys

Herein, preliminary theoretical results for the structural and electronic properties of Hg1−xMnxTe alloys in the zincblende structure using density functional theory calculations are presented. The results suggest that there is a strong dependence of the electronic structure of the alloys on the exchange correlation functional used. When using the generalized gradient approximation, the evaluated data show that this material behaves as a semimetal in almost all Mn concentration range, in disagreement with the experimental data. However, with the hybrid Heyd–Scuseria–Ernzerhof functional calculations, results show that the band gap of these alloys opens for the diluted Mn concentration range 0.05 < x < 0.2, consistent with the experimental observations. Moreover, when considering the magnetic ordering of the Mn spins in the alloy, the results show that, in this material, the lower total spin configurations is energetically more favorable.

Magnetic and electrical transport signatures of uncompensated moments in epitaxial thin films of the noncollinear antiferromagnet Mn3Ir

Noncollinear antiferromagnets, with either an L12 cubic crystal lattice (e.g., Mn3Ir and Mn3Pt) or a D019 hexagonal structure (e.g., Mn3Sn and Mn3Ge), exhibit a number of phenomena of interest to topological spintronics. Among the cubic systems, for example, tetragonally distorted Mn3Pt exhibits an intrinsic anomalous Hall effect (AHE). However, Mn3Pt only enters a noncollinear magnetic phase close to the stoichiometric composition and at suitably large thicknesses. Therefore, we turn our attention to Mn3Ir, the material of choice for use in exchange bias heterostructures. In this letter, we investigate the magnetic and electrical transport properties of epitaxially grown, face-centered-cubic γ-Mn3Ir thin films with (111) crystal orientation. Relaxed films of 10 nm thickness exhibit an ordinary Hall effect, with a hole-type carrier concentration of (1.500 ± 0.002) × 1023 cm−3. On the other hand, TEM characterization demonstrates that ultrathin 3 nm films grow with significant in-plane tensile strain. This may explain a small net magnetic moment, observed at low temperatures, shown by X-ray magnetic circular dichroism spectroscopy to arise from uncompensated Mn spins. Being of the order of 0.02 μB/atom, this dominates electrical transport behavior, leading to a small AHE and negative magnetoresistance. These results are discussed in terms of crystal microstructure and chiral domain behavior, with spatially resolved XML(C)D-PEEM supporting the conclusion that small antiferromagnetic domains, <20 nm in size, with differing chirality account for the absence of observed Berry curvature driven magnetotransport effects.

Carrier and spin dynamics of high-density exciton magnetic polarons in Cd0.8Mn0.2Te

We investigated the carrier and spin dynamics of high-density exciton magnetic polarons (HD-EMPs) in Cd0.8Mn0.2Te based on the measurement of their time-resolved photoluminescence (PL) spectra and polarization states, and the utilization of photo-induced Faraday rotation techniques. The PL from the HD-EMPs were collected in a forward scattering configuration, and was observed as a pulsed emission of a few picoseconds duration, exhibiting a blue-shift with time evolution. The blue shift originated from the refractive-index dispersion of the sample. By excluding the influence of the refractive-index dispersion on the time profile, it was revealed that the ultra-short pulsed emission with a time width smaller than 1 ps was initially radiated with a time delay of ~2.4 ps after photoexcitation. From the results of time evolution of the polarization states, it is concluded that the exciton–Mn spin interactions occurs immediately after the excitation, which causes the Mn ion spins to align to follow the spin states of photoexcited excitons. The alignment of the Mn ion spins through the formation of the HD-EMPs was significantly faster than that of the localized EMP. On the other hand, the time evolution of the photo-induced Faraday rotation showed two decay components attributed to spin relaxations of the excitons and Mn ions within the HD-EMP. The observation of the Faraday rotation signal due to the Mn ion spins further confirms that these spins were aligned by the photo-excited spin-aligned excitons. Our findings suggest a novel mechanism for the effective optical control of spins in a semimagnetic semiconductor, which is associated with a multi-exciton system and its localized state.

Martensitic transition and magnetic structure in Zn-doped Heusler alloy Mn2NiGa: A theoretical approach

The phase stability, martensitic transition and magnetism of Zn-doped Heusler alloys Mn2NiGa1-xZnx (x = 0, 0.25, 0.5, 0.75, 1) were investigated by first-principles calculations. In Mn2NiGa1-xZnx, Zn atom plays a similar role like main-group element Ga and also occupies the D site. A possible reason for this is the closed 3d-shell of Zn. The austenite-martensite energy difference ΔEM in Mn2NiGa1-xZnx increases rapidly with the doping of Zn, suggesting that the martensitic transformation temperature can be elevated with only a little Zn doping in Mn2NiGa. This can be meaningful for the development of new magnetic shape memory alloys. The variation of ΔEM can be explained by comparing the DOS structure of the austenite and martensite: The Jahn-Teller instability of Mn2NiGa1-xZnx austenitic phase is enhanced with increasing Zn content. An increase of the c/a ratio is also observed at the same time. Zn-doping does not change the magnetic structure of Mn2NiGa1-xZnx, they are always ferrimagnets within the range studied. The magnetic properties of these alloys are mainly characterized by the antiparallel coupled Mn spin moments.