D. Skrzypek

Correlation between the negative magnetoresistance effect and magnon excitations in single-crystalline CuCr1.6V0.4Se4

Structural, electrical and magnetic measurements, as well as electron spin resonance (ESR) spectra, were used to characterise the single-crystalline CuCr1.6V0.4Se4 spinel and study the correlation between the negative magnetoresistance effect and magnon excitations. We established the ferromagnetic order below the Curie temperature T C ≈ 193 K, a p-type semiconducting behaviour, the ESR change from paramagnetic to ferromagnetic resonance at T C, a large ESR linewidth value and its temperature dependence in the paramagnetic region. Electrical studies revealed negative magnetoresistance, which can be enhanced with increasing magnetic field and decreasing temperature, while a detailed thermopower analysis showed magnon excitations at low temperatures. Spin–phonon coupling is explained within the framework of a complex model of paramagnetic relaxation processes as a several-stage relaxation process in which the V3+ ions, the exchange subsystem and conduction electron subsystem act as the intermediate reservoirs.

Exchange bias in the Fe/KCoF3 system: A comprehensive magnetometry study

Exchange bias was studied in the Fe/KCoF3 ferromagnet/antiferromagnet system. KCoF3 can be deposited onto single crystal of Fe, either in the polycrystalline or single crystal form, depending on growth conditions. The samples were grown by molecular beam epitaxy on Ga-terminated GaAs (100) wafers. We study effects of the crystal state of the fluoride, thickness of the Fe film, crystallographic orientation of the Fe, and temperature on exchange bias. The structures with single crystal KCoF3 show that the exchange bias is well correlated with the coercivity at low temperatures and vanishes at a temperature close to the Neel temperature. Both the magnitude of the exchange bias and the blocking temperature of the samples with the polycrystalline fluoride were significantly reduced compared to the single crystal structures. As the Fe film thickness was increased, the exchange bias decreased for all samples. In contrast, the blocking temperature remained unchanged for the samples with the single-crystal fluorid...

Temperature dependence of anisotropy in exchange biased Fe/KCoF3 system

We used molecular beam epitaxy to deposit a novel ferro-/antiferromagnet (Fe/KCoF3) system on gallium terminated GaAs (100) substrates. We varied the thicknesses of single crystal Fe (001) layers from 1.05 to 3 nm. The antiferromagnetic fluoride, with a thickness of 30 nm, was deposited either in a single-crystal or a polycrystalline form, depending on the deposition conditions. KCoF3 is an antiferromagnet with a Neel temperature of 114 K. Its cubic structure almost perfectly matches the Fe film structure. The growth was monitored by reflection high energy electro diffraction. The magnetic properties of the system were studied using ferromagnetic resonance. Uniaxial and unidirectional anisotropy fields, due to exchange bias, were measured at low temperatures in the field-cooled samples and were smaller than 45 and 72 Oe, respectively. Both anisotropy fields, unidirectional and uniaxial, decreased with increasing thickness of the Fe film or with increases in temperature. The temperature dependence of the f...

Magnetic and structural properties of KCoxMg1−xF3 compounds

The mixed crystals KCoxMg1−xF3 with 0.25 < x < 1 were studied by static susceptibility and X-ray diffraction meth in the temperature range 4.2–350 K. From these measurements, the variations of several quantities, viz. TN, t=TN(x)/TN(1), θ (Curie-Weiss temperature), μeff (effective moment), and a, c (lattice parameters) with concentration or temperature were determined. The exchange constant was evaluated using theoretical results obtained for random, dilute magnetic systeṁs with magnetic ions having nonvanishing orbital momenta. From the temperature evolution of the cubic (400) Bragg line, the values of temperature for the structural phase transition and tetragonal deformation lattice with ca < 1 were estimated. A strong correlation was found between the magnetic and structural phase transition, with the lattice deformation resulting from the magnetic properties of the crystals studied.

Dynamic and Rotatable Exchange Anisotropy in Fe/KNiF3/FeF2 Trilayers

Results from ferromagnetic resonance experiments carried out on epitaxially grown Fe/KNiF3/FeF2 trilayers are presented. Exchange coupling between the KNiF3, a weak anisotropy antiferromagnet, and the Fe leads to shifts in the resonance field of the ferromagnet. The field shifts can be described by a temperature-dependent exchange anisotropy . depends on the orientation direction of the applied field relative to the magnetic anisotropy axis, and a non-monotonic dependence on KNiF3 thickness. Three thickness regimes appear that correspond to different values of exchange bias in each region. A qualitative understanding of the basis for these three thickness regimes due to spin canting at the interfaces is presented. Our results illustrate a method to tune the value of exchange anisotropy using a combination of different antiferromagnets.

Synthesis, crystal structure, spectroscopic, and magnetic properties of a manganese(II) methoxyacetate complex [Mn(C6O6H10)(H2O)] n

A metal-organic coordination polymer, [Mn(C6O6H10)(H2O)] n (1), has been synthesized and characterized by elemental analysis, FT-IR spectroscopy, and single-crystal X-ray diffraction. Thermogravimetric analysis and EPR spectrum of the compound have also been studied. Light pink crystals of the complex are monoclinic, space group P21 /c, with a = 7.9444(16) Å, b = 9.0802(18) Å, c = 13.142(3) Å, β = 93.53(3)°, V = 946.2(3) Å3, Z = 4, and R 1 = 0.0212. The compound is mononuclear and contains six-coordinate ions bound to bi- and tridentate methoxyacetate molecules and water. Each manganese ion is connected with the neighboring manganese via carboxylate bridges forming a polymeric chain of [Mn(C6O6H10)] n and water. The 1-D manganese polymer chains are further hydrogen-bonded via the carboxyl groups and water to produce a 3-D extended network. The FT-IR spectrum from 4000 to 400 cm−1 region confirms the bonding of water. Decomposition reaction takes place in the temperature range 25–900°C in nitrogen. The temperature dependence of magnetic susceptibility reveals weak antiferromagnetic coupling interaction (J = −0.74 cm−1) between the Mn(II) sites.

Exchange bias in Fe/KNiF3 bilayers

The exchange bias effect is known to depend strongly on the condition of the ferromagnet/antiferromagnet interface. Bilayers of single-crystal Fe and polycrystalline KNiF3 were grown and exchange bias observed using superconducting quantum interference device magnetometry. Hysteresis loops after field cooling are asymmetric in the forward and reverse directions. This asymmetry disappears at 50 K, indicating a blocking temperature for the bias well below the Neel temperature of KNiF3 (250 K).

Influence of Ce substitution on the critical properties of 3D-Heisenberg Cd x Ce y Cr2Se4 ferromagnets

The critical behaviour of single-crystalline Cd x Ce y Cr2Se4 spinels investigated around the paramagnetic–ferromagnetic phase transition revealed an increase in the critical exponents from β = 0.359, γ = 1.186, δ = 4.404 at T C = 131.95 K for Cd0.96Ce0.03Cr2Se4 to β = 0.382, γ = 1.286, δ = 4.487 at T C = 133.73 K for Cd0.83Ce0.13Cr2Se4. These values are close to those predicted by the renormalization-group calculations for a three-dimensional isotropic short-range Heisenberg ferromagnet, although their increase with the Ce substitution suggests a shift towards the mean-field estimates, characteristic of isotropic long-range exchange interactions. These results are being considered in relation to the strong lowering of magnetic moment from 5.67 µB/f.u. for y = 0.03 to 3.0 µB/f.u. for y = 0.13.

Antiferromagnetic resonance in KNixMn1−xF3 crystals

The X-band AFMR spectra of single crystals of the series KNixMn1−xF3 have been measured over the range of temperatures from 4.2 K to TN. The results lead to the following conclusions: 1. (i) for x < 0.1, transition into a weak ferromagnetic phase, characteristic of KMnF3, is maintained, 2. (ii) with increasing concentration of nickel ions, the temperature range over which the observation of the X-band AFMR is feasible becomes progressively narrow, and 3. (iii) a considerable widening of the resonance lines and a simultaneous decrease in their amplitude are due to both the relaxation processes and the domain spin-flop effect occurring at relatively low external magnetic fields.

Exchange bias system of Fe/KFeF3

Epitaxial Fe/KFeF3 bilayers were grown using molecular beam epitaxy on GaAs (100). FMR measurements revealed two peaks for samples with an Fe seed layer. A peak with fourfold symmetry was identified as the main layer Fe whereas a peak with twofold symmetry appeared to be due to the seed layer Fe. The magnetocrystalline anisotropy for the fourfold peak is ∼400 Oe at 300 K whereas a large anisotropy of 1.5 kOe is associated with the seed layer. A large rotatable anisotropy of 280±20 Oe was found for a sample with Fe thickness 2.1 nm. All Fe/KFeF3 bilayers checked by superconducting quantum interference device exhibit exchange bias and enhanced coercivity. The blocking temperature of the system is close to the Neel temperature 112 K of KFeF3. One sample has polycrystalline KFeF3 and shows a much larger exchange bias and coercivity than the samples having single crystal KFeF3.