Continuous Spin Reorientation Research Articles

Magnetic anisotropy and magnetic phase diagram of a kagome antiferromagnet Fe1-xCoxSn

We investigate the anisotropic magnetic response in a kagome antiferromagnetic compound Fe1-xCoxSn (x = 0, 0.04, 0.06, 0.14). The magnetic behaviors of three distinct spin orderings, namely the planar, tilted, and axial spin states, are examined under various conditions, including doping rate, temperature, and magnetic fields. We confirm that the magnetic susceptibility of all three spin states has significant anisotropy, which depends on both the magnitude and direction of the applied magnetic field. The planar and axial states show scaling behavior in magnetic susceptibility, while the tilted state does not. Furthermore, employing the differential magnetic susceptibility, we observe that the axial spins display discontinuous spin-flop transitions, while the planar and tilted spins exhibit a continuous spin reorientation. The magnetic phase diagrams, summarizing our observations, highlight the anisotropic nature of magnetic behaviors in the Fe1-xCoxSn system. These diagrams clearly show that the magnetic responses are not overlapping and follow distinct mechanisms in their interaction with magnetic fields. We suggest that the axial state may arise due to the dopants as an obstacle within the kagome lattice, as evidenced by similar magnetic behaviors in Fe0.96Ni0.06Sn.

Nd3+ induced twofold continuous spin reorientation transition and magnetization along the b-axis in a Dy0.9Nd0.1FeO3 single crystal

An intriguing hybrid magnetic configuration Γ4+3 with non-zero spontaneous magnetization along b-axis was found in DNFO91 single crystal. Contributions of R3+ were distinguished and the relative strength can be manipulated with magnetic field.

NMR study of magnetic structure and hyperfine interactions in the binary helimagnet FeP

We report a detailed study of the ground state helical magnetic structure in monophosphide FeP by means of ${}^{31}$P NMR spectroscopy. We show that the zero-field NMR spectrum of the polycrystalline sample provides strong evidence of an anisotropic distribution of local magnetic fields at the P site with substantially lower anharmonicity than that found at the Fe site by M\"{o}ssbauer spectroscopy. From field-sweep ${}^{31}$P NMR spectra we conclude that a continuous spin-reorientation transition occurs in an external magnetic field range of 4 - 7 T, which is also confirmed by specific-heat measurements. We observe two pairs of magnetically inequivalent phosphorus positions resulting in a pronounced four-peak structure of the single crystal ${}^{31}$P NMR spectra characteristic of an incommensurate helimagnetic ground state. We revealed a spatial redistribution of local fields at the P sites caused by Fe spin-reorientation transition in high fields and developed an effective approach to account for it. We demonstrate that all observed ${}^{31}$P spectra can be treated within a model of an isotropic helix of Fe magnetic moments in the ($ab$)-plane with a phase shift of 36$^{\circ}$ and 176$^{\circ}$ between Fe1-Fe3 (Fe2-Fe4) and Fe1-Fe2 (Fe3-Fe4) sites, respectively, in accordance with the neutron scattering data.

Control of spin orientation in antiferromagnetic NiO by epitaxial strain and spin–flop coupling

A ferromagnet/antiferromagnet (FM/AFM) Fe/NiO bilayer was grown using molecular beam epitaxy on MgO(001) and Cr buffered MgO(001) substrates. X-ray linear dichroism measurements showed a dominating out-of-plane component for the NiO spins in Fe/NiO/MgO and an in-plane spin direction for NiO layers grown on the Cr buffer. Furthermore, systematic studies on the magnetic properties of Fe/NiO grown on the wedge-shaped Cr buffer revealed a continuous strain-induced spin reorientation transition from out-of-plane to in-plane NiO spin directions when the Cr thickness increased from 0 nm to 3.5 nm. The analysis of the in-plane magnetic structure of NiO in Fe/NiO/Cr showed a pronounced uniaxial anisotropy in thin AFM layers. The AFM spins are perpendicular to the Fe spins due to spin–flop interaction. These results demonstrate the feasibility of using strain and coupling with FMs to manipulate spin structures in NiO.

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.

Crystal symmetry aspects of materials with magnetic spin reorientation.

The symmetry of materials which undergo a continuous spin reorientation has been analysed. It is shown that continuous spin reorientation is possible only in materials with triclinic or monoclinic crystal structure symmetry, i.e. other symmetries - orthorhombic, tetragonal, trigonal, hexagonal and cubic - are forbidden.

Terahertz time domain spectroscopic investigation of spin reorientation transitions in HoFeO₃.

Both continuous and discontinuous spin reorientation transitions in the HoFeO₃ orthoferrite have been further investigated in this paper. Here, we use terahertz spectroscopy through the terahertz ferromagnetic and antiferromagnetic resonances to study the thermodynamics of the spin reorientation transitions in HoFeO₃ ceramics. The spin reorientations are driven by changes in the anisotropy energy, and the spin moments in and out of the (a, c) plane correspond to the continuous transitions and discontinuous transitions. We fitted the antiferromagnetic resonant frequencies, and we achieved quantitative insight on the changes in anisotropy energy. Measuring M-H (moment-magnetic field) curves, we found changes in macroscopic magnetization and verified the thermodynamics.

Magnetic and dielectric behavior in YMn1−xFexO3 (x ≤ 0.5)

The role of doping Fe on the structural, magnetic, and dielectric properties of frustrated antiferromagnet YMn1−xFexO3 (x ≤ 0.5) has been investigated. The neutron diffraction analysis shows that the structure of these polycrystalline samples changes from hexagonal phase (space group P63cm) to orthorhombic phase (space group Pnma) for x > 0.2. The frustration parameter decreases with Fe substitution. All the compounds are antiferromagnetic, and the magnetic structure is described as a mixture of Γ3 and Γ4 irreducible representation (IR) in the hexagonal phase, and the ratio of these two IRs is found to vary with Fe doping (x ≤ 0.2). A continuous spin reorientation as a function of temperature is observed in these samples. The magnetic ground state in the orthorhombic phase of the higher doped samples (x ≥ 0.3) is explained by taking Γ1 (GxCyAz) representation of Pnma setting. In YMnO3 suppression of dielectric constant ε′ is observed below TN indicative of magnetoelectric coupling. This anomalous behavior reduces in Fe doped samples. The dielectric constant is found to be correlated with the magnetic moment (M) obtained from neutron diffraction experiments and follows a M2 behavior close to TN in agreement with Landau theory.

Continuous spin reorientation transition in epitaxial antiferromagnetic NiO thin films

Fe/NiO/MgO/Ag(001) films were grown epitaxially, and the Fe and NiO spin orientations were determined using x-ray magnetic dichroism. We find that the NiO spins are aligned perpendicularly to the in-plane Fe spins. Analyzing both the in-plane and out-of-plane spin components of the NiO layer, we demonstrate unambiguously that the antiferromagnetic NiO spins undergo a continuous spin reorientation transition from the in-plane to out-of-plane directions with increasing of the MgO thickness.

Continuous spin reorientation in antiferromagnetic films

We study anisotropic antiferromagnetic one-layer films with dipolar and nearest-neighbor exchange interactions. We obtain a unified phase diagram as a function of effective uniaxial D e and quadrupolar C anisotropy constants. We study in some detail how spins reorient continuously below a temperature T s as T and D e vary.

Continuous spin-reorientation phase transition in the surface region of an inhomogeneous magnetic film

A continuous spin-reorientation transition from a uniform magnetic state with the in-plane orientation of the moments of all atomic layers to a nonuniform canted state in the surface region is considered. This transition was discovered in experiments on the divergence of magnetic susceptibility in a perpendicular magnetic field at a temperature of about 240 K, which is lower than the Curie point of gadolinium, equal to 292.5 K. These experiments were carried out on an ultrathin iron magnetic film deposited on the (0001) surface of a thin gadolinium film. It is shown that, in the vicinity of the spin-reorientation transition, the thermodynamic potential has a form characteristic of the Landau theory of second-order phase transitions. The orientation angle of the moment of the surface atomic layer with respect to the plane of the film, which is chosen as an order parameter, exhibits anomalous behavior and increases with temperature. Expressions are derived for the magnetic susceptibility of each atomic layer. It is shown that, in the vicinity of the transition, the irregular part of the magnetic susceptibility of each atomic layer exhibits behavior characteristic of the susceptibility in the Landau theory: it is less by a factor of two in the low-symmetry phase and diverges at the transition point. The regular part of the magnetic susceptibility of each atomic layer makes an additional contribution to the asymmetry of the total susceptibility in the vicinity of the transition point; this result follows from the fact that the inhomogeneous magnetic system considered is semi-infinite.

Spiral-like continuous spin-reorientation transition of Fe/Ni bilayers on Cu(100)

The magnetic domain microstructure of 0 to 3 monolayers (ML) of Fe on 7.2 and 11 ML Ni/Cu(100) films was studied by spin-polarized low-energy electron microscopy at 300 K. The 11 ML Ni layers showed perpendicular oriented stripe domains of some micrometers in width parallel to the atomic step edges of the Cu(100) substrate, whereas the magnetization of the 7.2 ML Ni film was in a canted state. For both Ni layers, perpendicular oriented domains are stabilized upon Fe deposition up to 2.5 ML Fe without changing the original Ni domain pattern significantly. Between 2.5 and 2.9 ML the domains break up into smaller stripe domains, and the magnetization of the coupled Fe/Ni bilayer continuously rotates within the oppositely oriented magnetic stripe domains into the film plane by an in-plane 74\ifmmode^\circ\else\textdegree\fi{} spiral-like motion. At 2.9 ML Fe large (10 \ensuremath{\mu}m) in-plane magnetic domains appear. From the critical Fe thickness of $2.7\ifmmode\pm\else\textpm\fi{}0.2\mathrm{ML}$ we find that the Fe/Ni interface magnetic anisotropy is ${K}_{2,\mathrm{N}\mathrm{i}\ensuremath{-}\mathrm{F}\mathrm{e}}^{\mathrm{S}}=\ensuremath{-}93\ensuremath{\mu}\mathrm{e}\mathrm{V}/\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}$ favoring an in-plane easy axis.

Magnetic anisotropies of ultrathin Fe films on Au(111) as function of iron thickness and gold capping

Iron thin films between 1.5 and 3.75 ML grown on a reconstructed Au(111) surface have been studied by in situ magneto-optic Kerr effect in the polar and longitudinal geometries. In films with Fe coverage less than 2.8 ML, we observe a perpendicular magnetic anisotropy tendency at room temperature. With further iron deposition, the film exhibits a continuous spin reorientation from out-of-plane to in-plane. Distinct changes in the magnetic properties as a function of substrate quality and gold capping layer thickness may be related to different state of the film structures.

Spin engineering in ultrathin Co0.35Pd0.65 alloy films

The easy axis of magnetization in CoxPd1−x alloy films with x=0.35 is controllably engineered by varying the thickness, tPd, of the Pd overlayers directly deposited on the alloy layers. In a Pd(50 Å)/CoPd (20 Å)/Pd (tPd) sample with a 10-Å-height step-wedge Pd layer, the easy axis smoothly changes from in-plane orientation (tPd=0 Å) through canted out of plane (0<tPd<30 Å) to perpendicular (30⩽tPd⩽60 Å). We also demonstrate that the spin switching is controllably reversible between in-plane and perpendicular orientations when the individual constituent layers of CoPd and Pd are alternately deposited. Smoothly continuous spin reorientation in a Pd (50 Å)/CoPd (30 Å)/Pd (tPd) film with increasing tPd in a broad range of 0–150 Å convincingly evidences the magnetoelastic anisotropy origin for the observed spin switching.

Higher-order magnetic anisotropies and the nature of the spin-reorientation transition in face-centered-tetragonal Ni(001)/Cu(001)

Second- and fourth-order constants ${\mathrm{K}}_{2}$ , ${\mathrm{K}}_{4}$ of the magnetic anisotropy energy in tetragonally distorted Ni(001) ultrathin films on Cu(001) are determined as a function of temperature and film thickness. We measure a positive out-of-plane constant ${\mathrm{K}}_{4\mathrm{\ensuremath{\perp}}}$ \ensuremath{\approx}+0.24 \ensuremath{\mu}eV/atom and a negative in-plane constant ${\mathrm{K}}_{4\mathrm{\ensuremath{\parallel}}}$ near room temperature. This leads to a continuous (second or higher-order) spin-reorientation transition of the easy axis from an in-plane below \ensuremath{\approx}7 ML to a perpendicular orientation for thicker films. ${\mathrm{K}}_{4\mathrm{\ensuremath{\parallel}}}$ changes sign at ${\mathrm{d}}_{\mathrm{c}\mathrm{\ensuremath{\parallel}}}$ \ensuremath{\approx}6.7 ML which results in a change of the easy axis from the [100] direction below 6.7 ML to the [110] direction above. These results are quantitatively compared to Ni(111)/W(110) for which no out-of-plane reorientation is observed.

Perpendicular anisotropy and spin reorientation in epitaxial Fe/Cu3Au(100) thin films.

The in situ magneto-optical Kerr effect in both the polar and longitudinal geometry is studied for fcc Fe films on ${\mathrm{Cu}}_{3}$Au(100) between 1.7 and 7.5 atomic layers thick. In films with Fe coverages less than 4 monolayers (ML) we observe a perpendicular magnetic anisotropy at temperatures below 300 K. At T=160 K the films exhibit a continuous spin reorientation transition from a perpendicular to an in-plane magnetization in a narrow thickness range between 3.5 and 4 ML. According to the thickness dependence of the Kerr signal the individual layers order ferromagnetically, in contrast to the findings in the Fe/Cu(100) system. Distinct changes in the magnetic properties as a function of coverage may be related to different states of the film structure and morphology.

Domain structure in single crystal ferromagnetic oxide films near the phase transitions

The observations of domain structure in single crystal ferromagnetic oxide films near phase transitions induced by temperature and by magnetic field have been carried out. The domain structure has been observed by means of the Faraday effect and by light diffraction. The following phase transition points have been studied in details: paramagnet-ferromagnet transition, Morin transition, compensation point and continuous spin reorientation points. It has been shown that in all considered cases dipolar interactions have significant influence on phase transitions and, in particular, on the magnetization distribution near the phase transition points.

On the character of the unusual morin type transition in a cobalt-substituted dysprosium orthoferrite

Magnetic measurements and Mössbauer spectroscopy revealed two types of spin reorientation in DyFe 0.9975Co 0.0025O 3 in the temperature range 77–300 K. The higher temperature transition was a continuous rotation of Fe 3+ spins from the a to the c axes of the single crystal in the region 260-145 K. The lower temperature transition was from a weakly ferromagnetic to the purely antiferromagnetic state. A peculiarity was observed with both methods. When the Fe 3+ spins, turning from the a towards the c axis, form an angle of about 20° with the latter, they do not proceed to reach c, but jump to the b axis and the crystal becomes a pure antiferromagnet. An external magnetic field of 10 kOe, H ext  c, does not change this reorientation pattern, but a smaller field along a completes the reorientation along the c axis. Mössbauer spectra indicate two hyperfine fields during the continuous spin reorientation.