Field-induced Spin-reorientation Transitions Research Articles

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.

Disappearance of spiral spin order and field induced spin reorientation transition in Fe[formula omitted]Cr[formula omitted]VO[formula omitted] (0.1[formula omitted][formula omitted][formula omitted]0.3) multiferroic: 57Fe Mössbauer spectroscopic studies

Here we report a progressive suppression of spiral spin order and evidence for disappearance of spin reorientation transition (SRT) in Fe1−xCrxVO4 (0.1 ≤x≤ 0.3) studied by low temperature and high magnetic field (LTHM) Mössbauer spectroscopy (MS). Variation in the area ratio of lines 2 and 3 (A23) with external magnetic field confirms the presence of antiferromagnetic (AFM) ordering for all the compositions at the lowest temperatures (5 K). Field induced spin reorientation transition is being observed in both triclinic (x = 0.10) and mixed phase (x = 0.175) systems, evidenced by the change in the sign of quadrupole shift (QS) value at low temperatures, where as the SRT is absent in the monoclinic phase (x = 0.20 and 0.30). MS line width (Γ) values indicate the presence of spiral spin order in T-phase, partial suppression in mixed phase (T + M) and finally complete suppression for the M-phases.

Spin structure at zero magnetic field and field-induced spin reorientation transitions in a layered organic canted antiferromagnet bordering a superconducting phase

We attempted to assign the spin structure of a layered organic antiferromagnet, $\ensuremath{\kappa}$-(d8-BEDT-${\mathrm{TTF})}_{2}\mathrm{Cu}[\mathrm{N}{(\mathrm{CN})}_{2}]\mathrm{Br}$, which is a key material located closest to the Mott boundary at ambient pressure among this family of compounds, by investigating its macroscopic magnetization thoroughly, motivated by a recent successful assignment of the spin structure of an isostructural material, $\ensuremath{\kappa}$-(BEDT-${\mathrm{TTF})}_{2}\mathrm{Cu}[\mathrm{N}{(\mathrm{CN})}_{2}]\mathrm{Cl}$ [BEDT-TTF and d8-BEDT-TTF are bis(ethylenedithio)tetrathiafulvalene and its deuterated molecule, respectively]. We measured the isothermal magnetization after careful choice of the measurement temperatures and cooling speed at around 80 K, so that the magnetism of the antiferromagnetic phase can be effectively extracted. Consequently, we observed hysteresis loops signifying ferromagnetism and steplike behavior when the magnetic field applied parallel to the crystallographic $b$ and $a$ axes was swept, respectively. The possible spin structure consistent with these results was discussed in terms of probable interactions between the spins, such as exchange interactions and the Dzyaloshinskii-Moriya interaction. Eventually, we asserted that $\ensuremath{\kappa}$-(d8-BEDT-${\mathrm{TTF})}_{2}\mathrm{Cu}[\mathrm{N}{(\mathrm{CN})}_{2}]\mathrm{Br}$ has a spin structure with the easy axis being the $c$ axis and the net canted moment parallel to the $b$ axis, which is surprisingly different from that of $\ensuremath{\kappa}$-(BEDT-${\mathrm{TTF})}_{2}\mathrm{Cu}[\mathrm{N}{(\mathrm{CN})}_{2}]\mathrm{Cl}$. We suggested that this difference originates from the difference of the sign of the interlayer interaction between the two materials. We also elucidated the overall picture of the magnetization processes of this material under the magnetic fields parallel to the three principle axes, which are also in contrast to those of $\ensuremath{\kappa}$-(BEDT-${\mathrm{TTF})}_{2}\mathrm{Cu}[\mathrm{N}{(\mathrm{CN})}_{2}]\mathrm{Cl}$. In particular, the spin-reverse transition at which half of the spins rotate by ${180}^{\ensuremath{\circ}}$ was not induced by the $b$-axis magnetic field, as in the case of $\ensuremath{\kappa}$-(BEDT-${\mathrm{TTF})}_{2}\mathrm{Cu}[\mathrm{N}{(\mathrm{CN})}_{2}]\mathrm{Cl}$, but by the $a$-axis magnetic field. Finally, numerical simulations and magnetic symmetry analysis enabled us to confirm the validity of the spin structures proposed for the two antiferromagnets under zero and high magnetic fields.

Magnetolattice coupling, magnetic frustration, and magnetoelectric effect in the Cr-doped FeVO4 multiferroic material and their correlation with structural phase transitions

A detailed account of magnetolattice coupling, magnetic frustration, and magnetoelectric effects in ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}{\mathrm{VO}}_{4}$ ($x=0\ensuremath{-}1.0$) studied by temperature-dependent synchrotron x ray diffraction (SXRD), temperature- and magnetic-field-dependent dielectric permittivity $\ensuremath{\varepsilon}$, and magnetization measurements are presented in this paper where progressive Cr doping leads to structural transitions from triclinic (T)---monoclinic (M)---orthorhombic (O) symmetries. SXRD data shows an intricate relationship between magnetic, ferroelectric, and lattice degrees of freedom in these systems. ${\mathrm{FeVO}}_{4}$ reaches a magnetically ordered state with two successive antiferromagnetic orderings at ${\mathrm{T}}_{N1}$ (21.85 K) and ${\mathrm{T}}_{N2}$ (15.65 K), having collinear and noncollinear natures, respectively, as evidenced in DC magnetization measurements. Progressive ${\mathrm{Cr}}^{3+}$ incorporation at the ${\mathrm{Fe}}^{3+}$ site in ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}{\mathrm{VO}}_{4}$ shifts these transitions to higher temperatures in T phase ($x=0.0$ and 0.10). At $x=0.175$ [in (T $+$ M) phase], these transitions become closer to each other. Beyond this concentration, a single broad antriferromagnetic transition is observed in M $(x=0.20\ensuremath{-}0.30$) and O ($x=0.90\ensuremath{-}1.0$) phases. A nonlinear behavior in isotherm M-H curves below ${\mathrm{T}}_{N2}$ indicates field-induced spin-reorientation transitions at higher magnetic field. In dielectric permittivity $\ensuremath{\varepsilon}$ a sharp peak at ${\mathrm{T}}_{N2}$ in T and near M regions with a minimal suppression because of applied magnetic field is found and no such peak is observed in far M phase. A discontinuity evidenced in electromagnetic susceptibility indicates magnetoelectric effect at the polar to nonpolar transition regions. The structural incongruence in progressive transformation from T to M to O symmetries plays a vital role in controlling the nature of magnetic interactions. Our results indicate a strong correlation between structural transitions, magnetolattice coupling, magnetic frustrations, and magnetoelectric effect in ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}{\mathrm{VO}}_{4}$.

Electric-field control of spin orientation of manganocene: An insight into molecule-substrate interactions.

The manipulation of spin orientations in molecular nanomagnets assembled on surfaces is essential for the development of memory devices. These properties are dominated by interactions with the substrate. Here, we show that individual manganocene molecules deposited on Cu(111) exhibit different easy magnetization directions in an applied electric-field due to different contact geometries. Using Hubbard-U corrected density-functional theory to describe strong correlation effects and a non-self-consistent diagonalization method to treat spin-orbit coupling, we demonstrate that the field-induced spin reorientation transition occurs in the standing-up molecule in both high-spin (HS) and low-spin states, while the transition only occurs in the HS state for the flat-lying molecule. We propose plausible mechanisms in terms of charge polarization at the interface as well as modifications of the electronic states near the Fermi level E F. We show that the molecule largely preserves its arrangement of 3d orbitals in the standing configuration due to the "insulating layer" (bridging ligand), whereas direct contact of the Mn ion with the substrate in the lying configuration induces an orbital degeneracy around E F, thus preventing the electrical modulation of magnetic anisotropies.

A2MnXO4 Family (A = Li, Na, Ag; X = Si, Ge): Structural and Magnetic Properties.

Four new manganese germanates and silicates, A2MnGeO4 (A = Li, Na) and A2MnSiO4 (A = Na, Ag), were prepared, and their crystal structures were determined using the X-ray Rietveld method. All of them contain all components in tetrahedral coordination. Li2MnGeO4 is orthorhombic (Pmn21) layered, isostructural with Li2CdGeO4, and the three other compounds are monoclinic (Pn) cristobalite-related frameworks. As in other stuffed cristobalites of various symmetry (Pn A2MXO4, Pna21 and Pbca AMO2), average bond angles on bridging oxygens (here, Mn-O-X) increase with increasing A/X and/or A/M radius ratios, indicating the trend to the ideal cubic (Fd3̅m) structure typified by CsAlO2. The sublattices of the magnetic Mn2+ ions in both structure types under study (Pmn21 and Pn) are essentially the same; namely, they are pseudocubic eutaxy with 12 nearest neighbors. The magnetic properties of the four new phases plus Li2MnSiO4 were characterized by carrying out magnetic susceptibility, specific heat, magnetization, and electron spin resonance measurements and also by performing energy-mapping analysis to evaluate their spin exchange constants. Ag2MnSiO4 remains paramagnetic down to 2 K, but A2MnXO4 (A = Li, Na; X = Si, Ge) undergo a three-dimensional antiferromagnetic ordering. All five phases exhibit short-range AFM ordering correlations, hence showing them to be low-dimensional magnets and a magnetic field induced spin-reorientation transition at T < TN for all AFM phases. We constructed the magnetic phase diagrams for A2MnXO4 (A = Li, Na; X = Si, Ge) on the basis of the thermodynamic data in magnetic fields up to 9 T. The magnetic properties of all five phases experimentally determined are well explained by their spin exchange constants evaluated by performing energy-mapping analysis.

Study of spin-phonon coupling and magnetic field induced spin reorientation in polycrystalline multiferroic [formula omitted

The present work reports the preparation of polycrystalline multiferroic GdFeO3 (GdFO) and characterization with x-ray diffraction (XRD), magnetization, temperature dependent Raman spectroscopy, temperature and magnetic field dependent F57e Mössbauer spectroscopy measurements. The sample is found to be phase pure from Rietveld refinement of XRD pattern. The Mössbauer spectra measured in the presence of external magnetic field show the signatures of field induced spin reorientation transition, which are corroborated by magnetization measurements. From the temperature dependent variation of internal hyperfine field, Neel transition temperature (TN,Fe) of 672.5 ± 0.2 K and critical exponent (β) of 0.333 ± 0.003 is obtained. Temperature dependent (300–760 K) Raman spectroscopy measurements show the signatures of spin-phonon coupling and local structural re-arrangement across TN,Fe.

Magnetoelectric coupling and spin reorientation in BiFeO3

We have studied the electric properties of multiferroic BiFeO3 (BFO) using the transverse Ising model in terms of pseudo-spin variables S with S= 7/2 and the Green's function method. Mechanisms of magnetoelectric (ME) couplings and electric field-induced spin-reorientation (SR) transition in BFO (71∘ in-plane and 109∘ out-of-plane switching) are examined. It is established that the spontaneous polarization Ps is responsible for the origin of an incommensurate non-collinear magnetic phase below TN. The relevance of two types of ME mechanisms is shown: the first one is quadratic with respect to the magnetic spins and the ferroelectric variables, whereas a second one defines a ME interaction which is induced from the appearance of a spontaneous polarization in BFO, called antisymmetric ME interaction. When an external magnetic field is applied they compete against each other and as a result the spontaneous polarization Ps is renormalized. The extraordinary polarization below TN is numerically calculated and its dependency on the easy-axis anisotropy K and the external magnetic field is examined. The type of magnetic SR transition under the influence of the electric field is determined. The critical values of E for different switching processes are numerically calculated.

Unusual effects of Ho3+ ion on magnetic properties of YFe0.5Cr0.5O3

The temperature-induced dual magnetization reversal and field-induced spin reorientation transition are observed in Y0.95Ho0.05Fe0.5Cr0.5O3. The Ho3+ dopant improves the sensitivity of the magnetization switching and weak ferromagnetism of YFe0.5Cr0.5O3. Based on the experimental results, we suggest that the ferrimagnetic ground state and the magnetization reversal mainly result from the antiferromagnetic coupling of weak ferromagnetic moments of Fe–O–Fe and Cr–O–Cr with that of Fe–O–Cr and Ho–O–Fe/Cr interactions. The fact that the magnetization reversal of Y0.95Ho0.05Fe0.5Cr0.5O3 under a lower magnetic field makes it attractive candidates for the magnetic dual sensor devices.

The influence of spontaneous and field-induced spin reorientation transitions on the magnetocaloric properties of HoZn and ErZn

We report a theoretical investigation on the magnetocaloric properties of the cubic CsCl-type HoZn and ErZn compounds. Several anomalies in the magnetocaloric quantities, ΔST and ΔTS, are observed due to spontaneous and/or field-induced spin reorientation transitions in these compounds. In HoZn, a discontinuity in the isothermal entropy change and in the adiabatic temperature change around T1 = 23 K is ascribed to the spontaneous reorientation transition. Under a magnetic field variation from 0 up to 2 T in the 〈110〉 and 〈100〉 directions, an almost table-like behavior in ΔST is predicted between T1 and TSR1. The peak around the ferromagnetic–paramagnetic transition temperature in the magnetocaloric quantities shows a dependence on the direction of the applied field. For μ0ΔH = 2 T, it reaches 11.9 J/kg K (magnetic field along the 〈111〉 direction) and 7.9 J/kg K (magnetic field in the 〈100〉 direction). In ErZn there is also a dependence of ΔST and ΔTS on field direction. From the analysis of the spin reorientations in both compounds we have built spin reorientation diagrams that summarize their temperature and field dependence. Our theoretical approach is based on a model Hamiltonian that includes exchange, crystal field, and quadrupolar interactions.

Field induced spin reorientation transition in epitaxial La 0.5Sr 0.5CoO 3 films

Strained epitaxial La 0.5Sr 0.5CoO 3 films are grown on LaAlO 3 substrate. Structural, electrical, and magnetic measurements were carried out. Out of plane lattice parameter of the film undergoes compressive strain and the coercivity is enhanced. The zero field cooled (ZFC) magnetization curve for a field applied parallel to the film plane shows a jump, which suggests a spin reorientation transition (SRT), while ZFC magnetization for a field applied perpendicular to the film plane is featureless. This jump in magnetization is shifted to higher temperatures when the magnetic field is reduced. The SRT is attributed to the strain in the film.

The influence of spontaneous and field induced spin reorientation transitions on the magnetocaloric properties in rare earth intermetallic compounds: Application to TbZn

We report a theoretical investigation on the magnetocaloric properties of the cubic CsCl-type TbZn compound. Two successive peaks in the magnetocaloric quantities are observed and attributed to different types of phase transitions. For the magnetic field applied in the ⟨110⟩ direction, the first peak is ascribed to a spontaneous first-order spin reorientation transition (SRT) at T1=63 K, and the second one to the ferroparamagnetic phase transition. The application of an external magnetic field of 2 T along this direction leads to a tablelike behavior in the magnetocaloric quantities (ΔST and ΔTS) as a consequence of two successive second-order SRTs at TSR1=71 K and at TSR2=160 K. Applying a magnetic field of 5 T suppress the flat behavior but a high refrigeration capacity of 352 J/kg is predicted in a wide temperature range from 62 to 258 K. When the magnetic field is applied along the ⟨100⟩ direction an inverse magnetocaloric effect is observed in the temperature range below T1. The system was studied throughout a Hamiltonian that takes into account the Zeeman, exchange, and crystal field interactions.

Low-temperature magnetic phase diagram ofHoFe3(BO3)4 holmium ferroborate: a magnetic and heat capacity study

We present the results of the magnetic and heat capacity study of a magnetic phase diagram of aHoFe3(BO3)4 single crystal. Two magnetic phase transitions are found in the low-temperature region.The transition from the paramagnetic to easy-plane antiferromagnetic state occurs atTN = 37.4 K and is independent of an applied magnetic field. The sharp heat capacity peaks andmagnetization jumps corresponding to the spontaneous and field-induced spin-reorientationtransitions between the easy-axis and easy-plane states are observed below 4.7 K. Also, theadditional heat capacity peaks, which can be attributed to the Schottky anomalieswith the field-dependent characteristic temperatures, are found. According tothe magnetic and thermal measurement data, the magnetic phase diagrams ofHoFe3(BO3)4 for the magnetic field parallel and perpendicular to the crystal axis are constructed.

Single-crystal growth of trigonal DyFe3(BO3)4 and study of magnetic properties

Fluxes with a wide stability range of trigonal DyFe3(BO3)4 were selected by the successive-approximation method using direct phase probing in the Bi2Mo3O12-B2O3-Dy2O3-Fe2O3 system. Crystallization parameters of the fluxes were determined. The conditions for single-crystal growth were recommended using both spontaneous crystal nucleation and seeded growth according to the Kyropoulos method. An analysis of the measured temperature and field dependences of magnetization of crystals in fields parallel or perpendicular to the threefold axis led to the conclusion that DyFe3(BO3)4 undergoes a phase transition to an ordered easy-axis state and is characterized by a field-induced spin-reorientation transition with the predominant alignment of Dy3+ spins along the applied field.

The giant anisotropic magnetocaloric effect in DyAl2

We report on calculations of the anisotropic magnetocaloric effect in DyAl2 using a model Hamiltonian including crystalline electrical field effects. The anisotropic effect is produced by the rotation of a constant magnetic field from the easy to a hard magnetic direction in the crystal and is enhanced by the first order nature of the field induced spin reorientation transition. The calculated results indicate that for a field with modulus of 2 T rotating from a hard to the easy direction, the isothermal magnetic entropy (ΔSiso) and adiabatic temperature (ΔTad) changes present peak values higher than 60% the ones observed in the usual process, in which the field direction is kept constant and the modulus of the field is varied.

Enhanced magnetoelectric effect in nanostructured magnetostrictive thin film resonant actuator with field induced spin reorientation transition

We report the use of a magnetic instability of the spin reorientation transition type to enhance the magnetoelectric sensitivity in magnetostrictive-piezoelectric structures. We present the theoretical study of a clamped beam resonant actuator composed of a piezoelectric element on a nonpiezo substrate actuated by a magnetostrictive layer. The experiments are made on a polished 150μm thick 18×3mm2 lead zirconate titanate (PZT) plate glued to a 50μm silicon plate and coated with a giant magnetostrictive nanostructured Nx(TbCo25nm∕FeCo5nm) layer. Another set of experiments was done with PZT substrate only. Results agree with analytical theory.

Low-temperature magnetic properties of the TbPd and TbPd 3 compounds

Field dependences of the magnetization M( H) for the TbPd and TbPd 3 compounds were measured at 4.2–70 K in magnetic fields up to 120 kOe. For both samples, no saturation is reached in the magnetic fields. At 4.2 K, the field dependence of the magnetization of TbPd is characterized by a slight hysteresis and two anomalies observed at 15 and 60 kOe, which can be related to field-induced spin-reorientation transitions. As the temperature increases, both anomalies (low- and high-field) smooth out progressively and disappear at 10 and 40 K, respectively. The temperatures of the magnetic order–disorder transition for TbPd and TbPd 3 were determined using a Belov–Arrotte proceduce as well as from measured temperature dependences of the magnetic susceptibility (for TbPd) and heat capacity (for TbPd 3); they were found to be equal to ∼50 and ∼9 K, respectively.

Theory of field-induced spin reorientation transition in thin Heisenberg films

We consider the spin reorientation transition in a ferromagnetic Heisenberg monolayer with a second order single ion anisotropy as a function of temperature and external field. Up to now analytical methods give satisfying results only for the special case that the external field is aligned parallel to the easy axis of the crystal. We propose a theory based on a generalization of the Callen decoupling, which can be used for arbritrary direction of the external field. Excellent agreement between our results and Quantum Monte Carlo data is found for the field induced reorientation at finite temperatures. Additionally, we discuss the temperature dependence of the transition in detail.

Non-collinear magnetic structures and first-order magnetization process in rare-earth systems

First-order field-induced spin reorientation transitions in multisublattice intermetallic compounds are considered within an anisotropic Heisenberg model, only leading-order anisotropy constants being included. The exchange interactions both between the rare-earth (RE) and transition metal sublattices and within the RE sublattice play an important role in the model. The latter interaction results in the occurrence of metastable states corresponding to a non-collinear magnetic structure. The possible FOMP scenarios for R 2Fe 17, HoFe 11Ti, and TbMn 6Sn 6 systems are discussed.

A new mechanism of first-order magnetization in multisublattice rare-earthcompounds

First-order field-induced spin-reorientation transitions in multisublatticeintermetallic compounds are considered within an anisotropic Heisenberg model.Unlike in previous works, only leading-order anisotropy constants of thesublattices are included. The exchange interactions both between the rare-earth(RE) and transition metal (TM) sublattices and within the RE sublatticeplay an important role in the model. The latter interaction results in theoccurrence of metastable states. One of the minima corresponds to thenon-collinear magnetic structure where the RE subsystem is divided into twosublattices, and their moments deviate in opposite directions from theTM moment direction. The possible first-order magnetization processscenarios for R2Fe17, HoFe11Ti, and TbMn6Sn6 systems are discussed.