Spin-flop Field Research Articles

Spin-flop phase transitions in vdW antiferromagnet MnPSe3

Two-dimensional (2D) van der Waal (vdW) magnets are gaining significance in exploring fundamental condensed matter physics and developing cutting-edge technologies. Here, we report the structural and magnetic properties of the antiferromagnetic semiconductor MnPSe3 crystal. Intriguing magnetic behaviors and the strong anisotropy of the bulk crystal have been investigated through temperature-dependent magnetization studies along with the observation of Néel temperature at TN=70±2 K. At temperatures below TN, the bulk MnPSe3 exhibits field-induced spin-flop phase transitions with a spin-flop field (HSF) of nearly 8 kOe at 5 K. These transitions are evidenced from isothermal magnetization measurements conducted at low temperatures. Moreover, temperature-dependent enhancement in HSF was also studied. Our findings motivate the exploration of intrinsic magnetic interactions in 2D MnPSe3 and incorporating it into functional vdW heterostructures and devices.

Comparative Study of Magnetic Properties of (Mn1−xAxIV)Bi2Te4 AIV = Ge, Pb, Sn

We investigated the magnetic properties of the antiferromagnetic (AFM) topological insulator MnBi2Te4 with a partial substitution of Mn atoms by non-magnetic elements (AIV = Ge, Pb, Sn). Samples with various element concentrations (10–80%) were studied using SQUID magnetometry. The results demonstrate that, for all substitutes the type of magnetic ordering remains AFM, while the Néel temperature (TN) and spin-flop transition field (HSF) decrease with an increasing AIV = Ge, Pb, Sn concentration. The rate of decrease varies among the elements, being highest for Pb, followed by Sn and Ge. This behavior is attributed to the combined effects of the magnetic dilution and lattice parameter increase on magnetic properties, most prominent in (Mn1−xPbx)Bi2Te4. Besides this, the linear approximation of the experimental data of TN and HSF suggests higher magnetic parameters for pure MnBi2Te4 than observed experimentally, indicating the possibility of their non-monotonic variation at low concentrations and the potential for enhancing magnetic properties through doping MnBi2Te4 with small amounts of nonmagnetic impurities. Notably, the (Mn1−xPbx)Bi2Te4 sample with 10% Pb substitution indeed exhibits increased magnetic parameters, which is also validated by local-probe analyses using ARPES. Our findings shed light on tailoring the magnetic behavior of MnBi2Te4-based materials, offering insights into the potential applications in device technologies.

Peculiar crystal structure and long-range order of spin-1/2 ladders in MoOBr3

Tetragonal crystal structure of molybdenum oxytribromide, MoOBr3, consists of non-planar Mo2Br6 units joined by non-equidistant Mo-O-Mo bonds into infinite ladders, which are connected to each other by the van der Waals forces. Within unit cell, the apical Mo atoms in any given ladder are shifted along the c axis either above (+z) or below (−z) the basal ab plane of the Br atoms. Magnetic susceptibility taken at various frequencies χac(T) and magnetic fields χdc(T) as well as specific heat Cp(T) data evidence the formation of a long-range antiferromagnetic order at TN = 34 ± 1 K with largely reduced effective magnetic moments µeff = 1.3 ± 0.1 µB. First principles GGA+U calculations point to the predominance of intra-ladder ferromagnetic exchange interaction on both rungs Jrung = − 10 meV and legs Jleg = − 1 meV of the ladder, while much weaker inter-ladder antiferromagnetic exchange interaction Jinter = 0.2 meV couples the ladders. The anisotropic part of the exchange K = 0.6 meV in combination with Jinter allows estimating the spin-flop transition field µ0Hspin-flop ∼ 6 T in good correspondence with the magnetization M(H) data.

Evidence of magnetic fluctuation induced Weyl semimetal state in the antiferromagnetic topological insulator Mn(Bi1−xSbx)2Te4

We report $c$-axis transport studies on magnetic topological insulators $\mathrm{Mn}{({\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x})}_{2}{\mathrm{Te}}_{4}$. We performed systematic $c$-axis magnetoresistivity measurements under high magnetic fields (up to 35 T) on several representative samples. We find that the lightly hole- and lightly electron-doped samples, while both having the same order of magnitude of carrier density and similar spin-flop transitions, exhibit sharp contrast in electronic anisotropy and transport mechanism. The electronic anisotropy is remarkably enhanced for the lightly hole-doped sample relative to pristine $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ but not for the lightly electron-doped sample. The lightly electron-doped sample displays a giant negative longitudinal magnetoresistivity (LMR) induced by the spin-valve effect at the spin-flop transition field, whereas the lightly hole-doped sample exhibits remarkable negative LMR consistent with the chiral anomaly behavior of a Weyl semimetal. Furthermore, we find the large negative LMR of the lightly hole-doped sample extends to a wide temperature range above the N\'eel temperature $({T}_{\mathrm{N}})$ where the magnetoconductivity is proportional to ${B}^{2}$. This fact, together with the short-range intralayer ferromagnetic correlation revealed in isothermal magnetization measurements, suggests the possible presence of the Weyl state above ${T}_{\mathrm{N}}$. These results demonstrate that in the $c$-axis magnetotransport of $\mathrm{Mn}{({\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x})}_{2}{\mathrm{Te}}_{4}$, the spin scattering is dominant in the lightly electron-doped sample but overwhelmed by the chiral anomaly effect in the lightly hole-doped sample due to the presence of the Weyl state. These findings extend the understanding of the transport properties of $\mathrm{Mn}{({\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x})}_{2}{\mathrm{Te}}_{4}$.

Field-induced spin reorientation transitions in antiferromagnetic ring-shaped spin chains

Easy axis antiferromagnets are robust against external magnetic fields of moderate strength. Spin reorientations in strong fields can provide an insight into more subtle properties of antiferromagnetic materials, which are often hidden by their high ground state symmetry. Here, we investigate theoretically effects of curvature in ring-shaped antiferromagnetic achiral anisotropic spin chains in strong magnetic fields. We identify the geometry-governed helimagnetic phase transition above the spin-flop field between vortex and onion states. The curvature-induced Dzyaloshinskii-Moriya interaction results in the spin-flop transition being of first- or second-order depending on the ring curvature. Spatial inhomogeneity of the N\'eel vector in the spin-flop phase generates weak ferromagnetic response in the plane perpendicular to the applied magnetic field. Our work contributes to the understanding of the physics of curvilinear antiferromagnets in magnetic fields and guides prospective experimental studies of geometrical effects relying on spin-chain-based nanomagnets.

Competing magnetic interactions driven tuning of spin-flop field and anomalous thermal expansion in MnTi1−xMnxO3

Multiferroic MnTiO3 (MTO) is made up of alternate Mn2+ (3d5) and Ti4+ (3d0) layers along the c-axis. It undergoes a magnetic spin-flop driven ferroelectric polarization flop beyond ∼6 T below TN∼64 K. MTO also exhibits a weak negative thermal expansion along the c-axis below ∼90 K. We demonstrate that doping Mn ions at non-magnetic Ti layers acts as a very effective microscopic control parameter to systematically tune both these functional responses. Tuning of c/a lattice parameters in MnTi1−xMnxO3 leads to significant reduction in the spin-flop critical magnetic field from ∼6 T to more easily accessible fields (<2 T for x=0.05). Simultaneously, the controlled tuning of magnetic frustration due to Mn-doping at Ti sites also helps in a systematic enhancement of anomalous thermal expansion in regard to increasing the onset temperature for anomalous expansion to higher values (by ∼40 K for x=0.10) and also strengthening its magnitude (by ∼30% at 15 K for x=0.10 in comparison to x=0.00). Increase in the c/a ratio with Mn doping in MnTiO3 likely causes reduction in the effective magnetocrystalline anisotropy, which leads to a decrease in the value of the spin-flop magnetic field. Detailed temperature-dependent structural analyses shed light on the critical role of exchange striction mechanism in tuning the negative thermal expansion in doped MnTiO3.

Enhancement of charge-neutral fermionic excitations near the spin-flop transition in the magnetic Kondo material YbIr3Si7

The new Kondo material YbIr$_3$Si$_7$, similar to other Kondo insulators, has been reported to exhibit charge-neutral fermionic excitations through measurements of specific heat and thermal conductivity at low temperatures. We performed $^{29}$Si-NMR on YbIr$_3$Si$_7$ to investigate the magnetic response of charge-neutral fermions from a microscopic perspective. In low magnetic fields parallel to the $c$ axis, a single NMR peak in the paramagnetic state splits into three peaks below $T_{\rm N}$. In contrast, only a slight shift of the single NMR peak was observed in high magnetic fields. This spectral change as a function of the $c$-axis magnetic field is interpreted as spin-flop transition, at which the magnetic moments oriented along the $c$ axis (AF-I phase) are rotated to the $ab$ plane with ferromagnetic component along the $c$-axis (AF-II phase). In the vicinity of the spin-flop magnetic field $H_{\rm M}$, nuclear spin-lattice relaxation rate $1/T_1$ was found to be proportional to temperature at low temperatures, indicating the existence of charge-neutral fermions. Furthermore, a peak of $1/T_1$ vs. the $c$-axis magnetic field suggests that the charge-neutral fermions in YbIr$_3$Si$_7$ are closely related to its magnetic properties. Our findings shed light on the origin of charge-neutral fermions in insulators.

Stress-induced Néel vector reorientation in γ -FeMn antiferromagnetic thin films

The relationship between stresses and the orientation of the Néel vector were studied by varying the residual stresses in magnetron sputtered FeMn thin films by adjusting Argon working pressures. Quasistatic magnetization and AC susceptibility measurements reveal that the FeMn film with compressive stress (−27 MPa/−0.015% strain) possesses an out-of-plane Néel vector orientation with a 44 kOe spin-flop field, as contrasted to the FeMn film with tensile stress (25 MPa/0.014% strain) showing an in-plane orientation with a 34 kOe spin-flop field. An energy formulation for the films estimates a magnetostriction value of 109 ppm following an effective anisotropy of −8 kJ/m3. The film with the larger residual stress (77 MPa/0.043% strain) displayed a strain-induced phase transition from γ-FeMn to α-FeMn. These results show the dependency of the Néel vector on the stress state indicative of relatively large magnetostriction.

Determination of the tricritical point, H–T phase diagram and exchange interactions in the antiferromagnet MnTa2O6

Using the analysis of the temperature and magnetic field dependence of the magnetization (M) measured in the temperature range of 1.5 K to 400 K in magnetic fields up to 250 kOe, the magnetic field-temperature (H–T) phase diagram, tricritical point and exchange constants of the antiferromagnetic MnTa2O6 are determined in this work. X-ray diffraction/Rietveld refinement and x-ray photoelectron spectroscopy of the polycrystalline MnTa2O6 sample verified its phase purity. Temperature dependence of the magnetic susceptibility χ (=M/H) yields the Néel temperature T N = 5.97 K determined from the peak in the computed ∂(χT)/∂T vs T plot, in agreement with the T N = 6.00 K determined from the peak in the C P vs T data. The experimental data of C P vs T near T N is fitted to C P = A|T − T N|−α yielding the critical exponent α = 0.10(0.13) for T > T N (T < T N). The χ vs T data for T > 25 K fits well with the modified Curie–Weiss law: χ = χ 0 + C/(T − θ) with χ 0 = −2.12 × 10−4 emu mol−1 Oe−1 yielding θ = −24 K, and C = 4.44 emu K mol−1 Oe−1, the later giving magnetic moment μ = 5.96 μ B per Mn2+ ion. This yields the effective spin S = 5/2 and g = 2.015 for Mn2+, in agreement with g = 2.0155 measured using electron spin resonance spectroscopy. Using the magnitudes of θ and T N and molecular field theory, the antiferromagnetic exchange constants J 0/k B = −1.5 ± 0.2 K and J ⊥/k B = −0.85 ± 0.05 K for Mn2+ ions along the chain c-axis and perpendicular to the c-axis respectively are determined. The χ vs T data when compared to the prediction of a Heisenberg linear chain model provides semiquantitative agreement with the observed variation. The H–T phase diagram is mapped using the M–H isotherms and M–T data at different H yielding the tricritical point T TP (H, T) = (17.0 kOe, 5.69 K) separating the paramagnetic, antiferromagnetic, and spin-flop phases. At 1.5 K, the experimental magnitudes of the exchange field H E = 206.4 kOe and spin-flop field H SF = 23.5 kOe yield the anisotropy field H A = 1.34 kOe. These results for MnTa2O6 are compared with those reported recently in the isostructural MnNb2O6.

P31 NMR investigation of quasi-two-dimensional magnetic correlations in T2P2S6 ( T =Mn, Ni)

We report the anomalous breakdown in the scaling of the microscopic magnetic susceptibility---as measured via the $^{31}\mathrm{P}$ nuclear magnetic resonance (NMR) shift $K$---with the bulk magnetic susceptibility $\ensuremath{\chi}$ in the paramagnetic state of ${\mathrm{Mn}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$. This anomaly occurs near ${T}_{\mathrm{max}}\ensuremath{\sim}117$ K the maximum in $\ensuremath{\chi}(T)$ and is therefore associated with the onset of quasi-two-dimensional (quasi-2D) magnetic correlations. The spin-lattice relaxation rate divided by temperature ${({T}_{1}T)}^{\ensuremath{-}1}$ in ${\mathrm{Mn}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$ exhibits broad peaklike behavior as a function of temperature, qualitatively following $\ensuremath{\chi}$, but displaying no evidence of critical slowing down above the N\'eel temperature ${T}_{N}$. In the magnetic state of ${\mathrm{Mn}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$, NMR spectra provide good evidence for ${60}^{\ensuremath{\circ}}$ rotation of stacking-fault-induced magnetic domains, as well as observation of the spin-flop transition that onsets at 4 T. The temperature-dependent critical behavior of the internal hyperfine field at the P site in ${\mathrm{Mn}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$ is consistent with previous measurements and the two-dimensional anisotropic Heisenberg model. In a sample of ${\mathrm{Ni}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$, we observe only two magnetically split resonances in the magnetic state, demonstrating that the multiple-peaked NMR spectra previously associated with ${60}^{\ensuremath{\circ}}$ rotation of stacking faults is sample-dependent. Finally, we report the observation of a spin-flop-induced splitting of the NMR spectra in ${\mathrm{Ni}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$, with an onset spin-flop field of ${\ensuremath{\mu}}_{0}{H}_{\mathrm{sf}}=14$ T.

Spin-flop transition in small nanoparticles: Internal magnetic study of surface effects

In this work we study the surface effects on small antiferromagnetic (AF) nanoparticles with diameters ϕ ≤ 11 nm and Néel temperature TN ∼ 12 K under spin-flop conditions by means of Montecarlo simulations. Our model consists of spherical nanoparticles with internal AF ordering and a surface with radial anisotropy. We have considered different values ​​of surface anisotropy (SA) and studied the effect it produces as a function of the particle size. Our results show that by decreasing the size of the nanoparticle, the effective anisotropy of the system increases and with it the spin-flop field. Likewise, the influence of SA induces magnetic disorder that is manifested in the magnetization curves as a function of applied field and temperature. These effects are studied by means of some antiferromagnetic order indicators that provide information about the degree of ordering of the system as a function of SA, particle size and temperature. The spin cluster structures play crucial role on the magnetic behavior of the particle as the size is diminished. We develop a very simple model that reproduce the observed behaviors as function of the SA and particle size for low temperatures.

Identification and manipulation of spin wave polarizations in perpendicularly magnetized synthetic antiferromagnets

Interlayer exchange-coupled synthetic antiferromagnets (SAFs) have the combined advantages of both high frequency of antiferromagnets and easy detection of ferromagnets. Here, magnetic excitations are investigated by theoretical analysis and micromagnetic simulations in SAFs that consist of two identical ferromagnetic layers with perpendicular magnetic anisotropy. Different from the common in-phase acoustic mode and out-of-phase optic mode, linearly or circularly polarized spin wave modes can be excited at zero bias field by using different types of microwave magnetic fields. Once a bias magnetic field is applied along the easy-axis, left-handed (LH) and right-handed (RH) polarization modes are observed, and the resonance frequency of RH (LH) mode of the SAFs increases (decreases) linearly with the increase of bias magnetic fields until a critical spin-flop field is reached, which is in accordance with collinear antiferromagnets with easy-axis anisotropy. These simulation results agree with the theoretical derivation and provide fundamental insight into the nature of dynamic properties of the perpendicularly magnetized SAFs, which may provide new prospects for spintronic applications.

Magnon-polaron anomaly in nonlocal spin transport through antiferromagnetic insulators

We present a nonlocal spin transport theory for the coupled dynamics of magnetization and lattice vibrations in antiferromagnetic insulators. We find that magnon-polaron formation, i.e., coherently hybridized magnon and acoustic phonon modes, not only leads to anomalous features in the nonlocal spin current but also renormalizes the spin-flop transition field of the antiferromagnets. A length scale for the magnon-polaron formation below which the spin current is not affected by the lattice is also extracted from this nonlocal setup.

Magnetodielectric effect in an organic-inorganic hybrid layered perovskite (CH3NH3)2MnCl4 with canted antiferromagnetism

Herein, we report the magnetodielectric effect in a single crystal of (CH3NH3)2MnCl4, an organic–inorganic hybrid layered perovskite having a composition of A2MX4 (A = organic anion, M = metal divalent ion, X = halogen cation). Magnetic property measurements reveal that a magnetic phase transition to antiferromagnetism appears at 47 K and a spin-flop transition occurs at 3.5 T in the magnetization (M) curve. We observed a considerable maximum magnetodielectric (MD) effect of 0.4% at 8 K. Furthermore, it was found that the MD effect is proportional to M2 in low magnetic fields and the MD effect starts to saturate at the spin-flop transition field. We proposed a magnetic structure model to explain the mechanism of the peculiar step-like MD effect induced by the spin reorientation.

Exceptional field dependence of antiferromagnetic magnons in LiFePO4

Low-energy magnon excitations in magnetoelectric LiFePO$_4$ have been investigated by high-frequency high-field electron spin resonance spectroscopy in magnetic fields up to B = 58 T and frequencies up to f = 745 GHz. For magnetic fields applied along the easy magnetic axis, the excitation gap softens and vanishes at the spin-flop field of BSF = 32 T before hardening again at higher fields. In addition, for B smaller than BSF we observe a resonance mode assigned to excitations due to Dzyaloshinskii-Moriya (DM)-interactions, thereby evidencing sizable DM interaction of approx 150 micro eV in LiFePO4. Both the magnetisation and the excitations up to high magnetic fields are described in terms of a mean-field theory model which extends recent zero field inelastic neutron scattering results. Our results imply that magnetic interactions as well as magnetic anisotropy have a sizable quadratic field dependence which we attribute to significant magnetostriction.

Field-dependent Morin Transition and Temperature-Dependent Spin-flop in Synthetic Hematite Nanoparticles

Background: In nano-size α-Fe2O3 particles, the Morin transition temperature was reported to be suppressed. This suppression of the TM in nano-size α-Fe2O3 was suggested to be due to high internal strain and to the enhanced role of surface spins because of the enhanced surface to volume ratio. It was reported that for nanoparticles of diameters less than 20 nm, no Morin transition was observed and the antiferromagnetic phase disappears. In addition, annealing of samples was reported to result in both an increase of TM and a sharper transition which were attributed to the reduction in defects, crystal growth, or both. Objective: In this work, we investigated the role of applied magnetic field in TM, the extent of the Morin transition, thermal hysteresis, and the spin-flop field in synthetic α-Fe2O3 nanoparticles of diameter around 20 nm. Methods: Hematite nanoparticles were synthesized using the sol-gel method. Morphology and structural studies of the particles were done using TEM, and XRD, respectively. The XRD patterns confirm that the particles are hematite with a very small maghemite phase. The average size of the nanoparticles is estimated from both TEM images and XRD patterns to be around 20 nm. The magnetization versus temperature measurements were conducted upon heating from 5 K to 400 K and cooling down back to 5 K at several applied fields between 50 Oe and 500 Oe. Magnetization versus magnetic field measurements between -5 T and +5 T were conducted at several temperatures in the temperature range of 2-300 K. Results: We report three significant findings in these hematite nanoparticles. Firstly, we report the occurrence of Morin transition in hematite nanoparticles of such size. Secondly, we report the slight field dependence of Morin transition temperature. Thirdly, we report the strong temperature dependence of the spin-flop. Zero-field-cooled magnetization versus temperature measurements were conducted at several applied magnetic fields. Conclusion: From the magnetization versus temperature curves, Morin transition was observed to occur in all applied fields at Morin transition temperature, TM which is around 250 K with slight field dependence. From the magnetization versus magnetic field curves, spin-flop in the antiferromagnetic state was observed and found to be strongly temperature dependent. The results are discussed in terms of three components of the magnetic phase in our sample. These are the paramagnetic, soft ferromagnetic, and hard ferromagnetic components.

Magnetic phase diagram of CrPS4 and its exchange interaction in contact with NiFe

The magnetic phase diagram of the two-dimensional van der Waals magnet CrPS4 and the exchange bias effect of CrPS4 in contact with NiFe film have been investigated. Based on the magnetic measurements, we figure out the relatively low spin-flop field and spin-flip field for CrPS4, both of the spin transition phenomena are strongly affected by the temperature. The perpendicular exchange bias effect is studied in CrPS4 single-crystal flake covered with 5 nm NiFe. Meanwhile, the variation of the cooling field has a great influence on the exchange bias field and coercivity, which is mainly attributed to the competition between the Zeeman energy and the exchange coupling at the interface as well as the formation of the multi-domain state.

Resonance properties of magnetic helical structures

The dependence for the frequencies of the internal modes on the external field in spiral structures in easy-plane antiferromagnets with a small in-plane anisotropy are studied theoretically in a wide region of the fields including the spin-flop transition field. The theoretical results are compared with the known experimental data for the antiferromagnetic resonance in NdFe3(BO3)4. This comparison enabled to reconstruct the dependence for the spiral period of the magnetic structure on the external field.

Single-Ion Weak Antiferromagnetism and Spin-Flop Transition in a Two-Sublattice Ferromagnet

The ground state of a Heisenberg ferromagnet with the noncollinear single-ion anisotropy axes of two magnetic sublattices has been investigated in an external magnetic field applied in the anisotropy axes plane. Noncollinearity of the sublattice local anisotropy axes leads to a new effect called the orientational first-order spin-flop phase transition. The transition field depends on the single-ion anisotropy value and sublattice local axes orientation. An analysis of the stability of magnetic states shows that the transition is accompanied by the hysteretic field dependence of the magnetization. The dependences of the spin-flop transition field, magnetization jump, and susceptibility on the single-ion anisotropy value and axes orientation have been determined. The results obtained are used to explain the field dependence of the magnetization in the PbMnBO4 ferromagnetic crystal.

Spin flop and crystalline anisotropic magnetoresistance in CuMnAs

Recent research works have shown that the magnetic order in some antiferromagnetic materials can be manipulated and detected electrically, due to two physical mechanisms: Neel-order spin-orbit torques and anisotropic magnetoresistance. While these observations open up opportunities to use antiferromagnets for magnetic memory devices, different physical characterization methods are required for a better understanding of those mechanisms. Here we report a magnetic field induced rotation of the antiferromagnetic Neel vector in epitaxial tetragonal CuMnAs thin films. Using soft x-ray magnetic linear dichroism spectroscopy, x-ray photoemission electron microscopy, integral magnetometry and magneto-transport methods, we demonstrate spin-flop switching and continuous spin reorientation in antiferromagnetic films with uniaxial and biaxial magnetic anisotropies, respectively. From field-dependent measurements of the magnetization and magnetoresistance, we obtain key material parameters including the anisotropic magnetoresistance coefficients, magnetocrystalline anisotropy, spin-flop and exchange fields.