Critical-point anomaly in the EPR linewidth of two-dimensional antiferromagnets

Calculations are presented which satisfactorily explain the observed temperature and angular dependence of the EPR linewidth in the two-dimensional antiferromagnets Rb₂MnF₄ and K₂MnF₄ near T_N. These calculations are based on an earlier theory by Huber for the three-dimensional systems.     

Linear magnetic birefringence in the quadratic-layer antiferromagnet K2MnF4

The linear optical birefringence of the quasi-Heisenberg antiferromagnet K₂MnF₄ has been determined between 5 and 300 K. A large magnetic contribution (LMB) is found at temperatures below 200 K. According to the earlier results with the iron-group difluorides the LMB can be related to the magnetic internal energy. Below 30 K the LMB of K₂MnF₄ obeys a T³-law. The temperature derivative of the LMB follows a course expected for the magnetic heat capacity of pronounced two-dimensional systems: a broad, rounded maximum above the three -dimensional transition point. At T_N a kink occurs in the LMB.     

Two magnon light scattering in K2MnF4

The light scattering spectra of the two dimensional Heisenberg antiferromagnet K₂MnF₄ are reported. At low temperatures a two magnon band is observed and four bands due to phonons as well.     

Temperature dependence of the ESR linewidth in KFeS2

The ESR linewidth in KFeS₂ has been measured in the range 80–340 K. The linewidth is found to decrease steadily as the temperature is increased, without any discontinuity or singularity at the transition temperature T_N=245 K. In the paramagnetic state, the region of enhanced linewidth extends further above T_N than for two-and three-dimensional compounds such as K₂MnF₄ and MnF₂.     

Critical exponents for NMR and ESR linewidths in a two-dimensional antiferromagnet

The exponent p in ΔH ∝ (T/T_N?1)^-p is derived both for NMR and ESR linewidths ΔH in a two-dimensional antiferromagnet. The approach is based on Kawasaki's dynamical equations. Comparison is made with experiments on K₂NiF₄ and K₂MnF₄.     

The magnetic order of two-dimensional anisotropic antiferromagnets

We study the two-dimensional quantum Heisenberg antiferromagnet on the square lattice with easy-axis exchange anisotropy by means of Green’s function approach within random phase and Callen’s approximations. The Néel temperature T_N, energy gap w₀ and staggered magnetization m are calculated. The theoretical predictions of T_N and w₀ for K₂NiF₄, Rb₂MnF₄, K₂MnF₄, Rb₂MnCl₄ and (CH₃NH₃)₂MnCl₄ fit well to the measured values. The power law behavior of is also investigated. The exponents β and ν for K₂NiF₄ are in excellent agreement with the experimental results.     

Pac studies of spin deviations,pressure induced hyperfine field shifts,and temperature variation of supertransferred hyperfine fields

A PAC study of¹¹¹Cd substiuted antiferromagnetic transition metal salts is reported. Supertransferred hyperfine fields at¹¹¹Cd nuclei were used for the first time to observe three effects: zero-point spin deviations, temperature variation, and pressure shift of the hyperfine field. Comparison of KNiF₃ and RbMnF₃ with their corresponding quadraticlayer fluorides K₂NiF₄ and Rb₂MnF₄ yields an estimate for the magnitude of the zeropoint spin deviation in doped antiferromagnets. The temperature dependences of the Cd hyperfine fields in RbMnF₃/Cd and MnF₂/Cd have been determined. The shift in hyperfine field observed for -MnS/Cd under external pressure further supports our model for the origin of the supertransferred hyperfine fields.This work was supported by the US Energy Research and Development Administration.     

2-D magnetism

Influence of the spatial dimensionality on magnetic ordering phenomena has been discussed. Recent work on ideal 2 dimensional magnets like Rb₂FeF₄, Rb₂MnF₄, K₂NiF₄, Rb₂Mn_xMg_1-x and K₂CuF₄, K₂Cu_xZn_{1 -x}F₄, MnSt₂ possessing planar antiferro- and ferromagnetism, respectively, has been reviewed with a view to doing planar anisotropy calculations based on the reportedly common orthorhombic structures. Existence of easy axes, easy cones in the plane and first order magnetisation processes (FOMP) in the plane and out of plane in the presence of applied magnetic field can be predicted. The results have been presented in the graphical forms as well.     

Magnetocrystalline anisotropy of MnF2 deduced from static torque

The first anisotropy constant K₁ as well as the differential susceptibility ΔX = Xχ - X∥ of MnF₂ has been determined from static torque measurements. The temperature range was for K₁ 4.2 K to 66 K and for ΔX 4.2 K to 300 K. The result for K₁ is in good agreement with values obtained from antiferromagnetic resonance experiments. In order to explain the differential susceptibility in the paramagnetic region, it was found necessary to take the g-factor anisotropy into account.     

Localized magnon gap modes in the 2-d Ising antiferromagnets K2CoF4 and Rb2CoF4

The defect (Mn²⁺,Ni²⁺,Fe²⁺) induced magnon gap modes in the layered antiferromagnets K₂CoF₄ and Rb₂CoF₄ were investigated with the methods of FIR absorption-and IR emission spectroscopy. The anisotropic exchange-parameters describing the strongly localized Mn²⁺ spin excitations far below the host lattice magnon band and the Ni²⁺ excitations in the vacinity of this band are presented. In the diluted system K₂Co_1-cMn_cF₄ localized Mn²⁺ cluster modes up to about C≈0.1 were observed. The excitation energy of these modes can only be explained by assuming an anisotropic Mn²⁺-Mn²⁺ exchange which is in contrast to the pure isomorphous system K₂MnF₄. In the spin mismatch system K₂CoF₄: Fe the magnetic moments of the isolated Fe²⁺ impurities are pulled from the plane perpendicular to the c-axis and aligned parallel to the easy axis of the magnetic crystal.     

Phase diagrams of weakly anisotropic Heisenberg antiferromagnets: III. Nonlinear excitations and random fields in quasi 2-dimensional systems

The concept of effective field-dependent anisotropy is applied to the “spinflop” transition in the quasi 2-d Heisenberg antiferromagnet with weak orthorhombic anisotropy. From the correspondence between the “spinflop” problem and the commensurate-incommensurate transtion it follows that the “spinflop” is not first order and that random fields may cause domain-wall formation. This would explain the observed broadening of the “spinflop” in K₂MnF₄. In 3-d antiferromagnets such anomalous broadening is not observed, which would agree with the critical dimensionality d_c = 2 for the random-field problem.     

Hydrostatic pressure behavior of the exchange fields in MnF2 : Fe

We have determined the pressure dependence of both impurity and host exchange fields in MnF₂ : Fe²⁺ by measuring the frequency shifts of far infrared active modes in MnF₂ : Fe²⁺, MnF₂, and ZnF₂ : Fe²⁺ at pressure up to 7.1 kbar.     

Spectroscopic properties of Fe ions at tetragonal sites—Crystal field effects and microscopic modeling of spin Hamiltonian parameters for Fe (S=2) ions in K2FeF4 and K2ZnF4

Magnetic and spectroscopic properties of the planar antiferromagnet K₂FeF₄ are determined by the Fe²⁺ ions at tetragonal sites. The two-dimensional easy-plane anisotropy exhibited by K₂FeF₄ is due to the zero field splitting (ZFS) terms arising from the orbital singlet ground state of Fe²⁺ ions with the spin S=2. To provide insight into the single-ion magnetic anisotropy of K₂FeF₄, the crystal field theory and the microscopic spin Hamiltonian (MSH) approach based on the tensor method is adopted. Survey of available experimental data on the crystal field energy levels and free-ion parameters for Fe²⁺ ions in K₂FeF₄ and related compounds is carried out to provide input for microscopic modeling of the ZFS parameters and the Zeeman electronic ones. The ZFS parameters are expressed in the extended Stevens notation and include contributions up to the fourth-order using as perturbation the spin-orbit and electronic spin-spin couplings within the tetragonal crystal field states of the ground ⁵D multiplet. Modeling of the ZFS parameters and the Zeeman electronic ones is carried out. Variation of these parameters is studied taking into account reasonable ranges of the microscopic ones, i.e. the spin-orbit and spin-spin coupling constants, and the energy level splittings, suitable for Fe²⁺ ions in K₂FeF₄ and Fe²⁺:K₂ZnF₄. Conversions between the ZFS parameters in the extended Stevens notation and the conventional ones are considered to enable comparison with the data of others. Comparative analysis of the MSH formulas derived earlier and our more complete ones indicates the importance of terms omitted earlier as well as the fourth-order ZFS parameters and the spin-spin coupling related contributions. The results may be useful also for Fe²⁺ ions at axial symmetry sites in related systems, i.e. Fe:K₂MnF₄, Rb₂Co_1−xFe_xF₄, Fe²⁺:Rb₂CrCl₄, and Fe²⁺:Rb₂ZnCl₄.     

Natural optical dichroism of A2MnX4 type crystals and mechanisms for producing intensity of the spin-forbidden d-d transitions

It has been found in a number of experiments that d-d transitions to A_1g and E_g states (in cubic notation) in tetragonal crystals Rb₂MnCl₄ and K₂MnF₄ have pronounced σ(α) polarization and the transitions to T_1g from liquid helium to room temperature. In this work the analysis of polarizable properties of different mechanisms for producing intensity of the spin-forbidden d-d transitions in A₂MnX₄ structure is carried out. It is shown that only cooperative absorption with the participation of odd vibrations can account for the observed transition polarization.     

Room-temperature photoluminescence in MnF2 under high pressure

A novel two-color photoluminescence (PL) is found in MnF₂ at room temperature (RT) under high pressure. In contrast to the low-temperature PL, the observation of room-temperature PL is unusual in transition-metal concentrated materials like MnF₂ since the de-excitation process at RT is fully governed by energy transfer to non-radiative centers. We show that room-temperature MnF₂ emissions originate in the pressure-induced cotunnite phase. Both the nine-fold Mn²⁺ coordination and the Mn–F–Mn exchange pathway inhibit exciton migration among Mn²⁺, favoring two-band PL at RT. The electronic structure and the excited-state dynamics are investigated by time-resolved emission and excitation spectroscopies under pressure. The two PL bands at 2.34 and 1.87 eV above 15 GPa are assigned to Mn²⁺ emissions arising from two distinct Mn²⁺ centers formed in the MnF₂ high-pressure phase. The microscopic origin of the two-color PL is analyzed in terms of exciton dynamics in the MnF₂ cotunnite structure.     

X-ray absorption near edge structure in some Mn compounds

The shapes of the manganese K-absorption edge in the compounds MnSO₄H₂O, MnF₂, MnCO₃, MnO and MnS have been studied using a Cauchois type X-ray spectrograph. Three peaks A, B and C observed in the main edges have been assigned the electronic transitions ls → 3d, ls → 4s and ls → 4p respectively. The transitions have been explained on the basis of the Z + 1 analogy. It has also been shown that the 3d → 4s and 4s → 4p orbital separations depend on the partial charge of an ion in its compound.     

Absorption spectra of NH4MnCl3 and NH4MnF3

The absorption spectra of NH₄MnCl₃ and NH₄MnF₃ crystals have been measured down to 10 K in the 250 to 600 nm region. The observed bands are assigned to electronic transitions from the ⁶A_1g(S) ground state to various excited levels of Mn²⁺ ions in an octahedral crystalline field. The position of the bands have been fitted within the strong crystal field scheme. Resulting parameters at room temperature are B=741, C=2990 and Dq=520 cm⁻¹ for NH₄MnCl₃ and B=800, C=3139 and Dq=694 cm⁻¹ for NH₄MnF₃. At low temperature some bands show a rich fine structure in which some phonon progressions have been identified.     

Ionic potential analysis of RHEED rocking curves from fluoride structures

Rocking curves of reflection high energy electron diffraction from MnF₂ and CaF₂(1 1 1) surfaces have been calculated with ionic scattering potentials. The potentials were derived from tabulated X-ray scattering factors and expressed in the Doyle-Turner representation. The Coulomb potential of the ions was introduced into the calculations of diffracted intensity using dynamical diffraction theory. Comparison of the calculated and measured rocking curves for CaF₂(1 1 1) surface confirmed that it is bulk terminated; the correction to volume average potential was found to be 1.1 eV (much less than when using atomic potentials). Analysis of the rocking curves from ultra-thin MnF₂ layers on CaF₂(1 1 1) indicated that MnF₂ inherits the cubic lattice of fluorides up to a thickness of three molecular layers.