Dependence of magnetic anisotropy of the La0.67Ca0.33MnO3 films on substrate and film thickness

Strain in the La_0.67Ca_0.33MnO₃ films has been tuned by varying substrate and film thickness, and its effects on magnetic anisotropy are studied based on the measurements of isothermal magnetization. Measuring the strain in the films by the out-of-plane lattice parameter (c), we found a strong dependence of the magnetic anisotropy constant (Ku) on strain. Ku decreases linearly from ∼−1.1×10⁶ erg/cm³ for c=0.763 nm to 1.2×10⁶ erg/cm³ for c=0.776 nm, corresponding to a change from tensile strain to compressive strain. Positive Ku signifies a uniaxial anisotropy with the easy axis perpendicular to the film plane, while negative Ku demonstrates an anisotropy of the easy plane character. Smaller or larger c leads a decrease or increase in Ku, which indicates the presence of other effects in addition to those associated with strain. Three distinctive processes for the magnetization are observed along the hard magnetic axis of the films on (001)SrTiO₃, suggesting a possibility of strain relaxation even in ultra-thin films.     

Magnetic properties and structure of FePt/FeMn multilayers

A systematic study of the magnetic properties by ion beam sputter-deposition system, was conducted in conjunction with the structure of FePt/FeMn multilayers fabricated onto MgO(0 0 1) substrates. Both parallel and perpendicular exchange biases were observed in the multilayers and were found to decrease drastically, as the deposition temperature is higher than 350 °C, which is evidently due to the interdiffusion at the interface. The thickness dependence study shows that the perpendicular magnetic anisotropy observed in the multilayers originates from surface anisotropy, being consistent with the decrease of perpendicular magnetic anisotropy as the deposition temperature is increased. The difference between parallel and perpendicular blocking temperatures that was clearly observed, is possibly due to the spin canting out of plane at the interface.     

Low-temperature magnetic structures in the DySi FeB-type compound 27.03.09

The low-temperature magnetic ordering of the dimorphic DySi compound has been studied at 1.5 K by neutron diffraction on two polycrystalline samples. The samples comprise various amounts of the two orthorhombic modifications: CrB-type (Cmcm Nr. 63, all atoms at 4c site: (0, y, )) and FeB-type (Pnma Nr. 62, all atoms at 4c site: (x, , z)), both order antiferromagnetically (T_N≈38 K). The CrB-type phase orders with a uniaxial structure with the wave vector q₁=(0, 0, ) requiring a doubling of the c-axis. The Dy moments point along the linear chain with the shortest distance c. At 1.5 K, the ordered moment value is 8.57(1) μ_B/Dy atom.Two symmetry independent wave vectors describe the 1.5 K magnetic ordering of the FeB-type phase: q₂=(0, , ) and q₃=(0, 0.484(1), 0.0892(1)), coexisting in form of domains. In both structures the magnetic moments are confined to the (0 0 1) plane at an angle of 2(2)° and 22(3)° from the shortest axis b, respectively. Both structures correspond to sine wave modulations. The amplitude of the q₂ wave is m_o=7.5(1) μ_B/Dy atom and that of q₃ 8.2(1) μ_B/Dy atom. The wave vector q₂ when referring to the (a, 2b, c) cell and the wave vector q=(0, 0, ) corresponds to a transversal modulation, which by a proper origin choice can be also described as an antiphase domain structure with two amplitudes. The moments point to the b-axis and are stacked in the sequence (+m_o/2, −m_o/2, −m_o, −m_o/2, +m_o/2, +m_o, …) along the c-direction, while t_b acts as an antitranslation. For the q₃ phase, the local moment value depends on the atom position in the wave. We also discuss the case where q₃ and q₂ act simultaneously in physical space.     

Competing magnetic structures in the DySi FeB-type phase diagram

The temperature magnetic phase diagrams, of the dimorphic DySi compound, have been studied in terms of wave vectors in the range 1.5-45 K, by neutron diffraction. The polycrystalline sample consists of 26% of CrB-type (Cmcm no. 63, all atoms at 4c site: (0, y, 1/4)) and of 74% of FeB-type (Pnma no. 62, all atoms at 4c site: (x, 1/4, z)). The CrB-ordering is described by the wave vector: q₁=(0, 0, 1/2) over the entire magnetically ordered regime with a uniaxial magnetic structure along the shortest axis c. The FeB-type magnetic phase diagram reveals three distinct regions of magnetic ordering below T_N and one first order transition at T₂=23.5 K (on heating). The ordering is described by two symmetry independent magnetic vectors q₂=(0, 1/2, 1/6) and q₃=(0, q_3y, q_3z) with a temperature variable length. At 1.5 K q_3y≈1/2 and q_3z≈1/11. The two phases coexist in the form of domains. They differ in the moment orientation of the q₃ phase that deviates by ∼22° from the b-axis in the (0, 0, 1) plane. The low temperature range (LT) 1.5 K—T₂ subdivides into two regions: (i) LT-1, between 1.5 K—T₁ where the relative amount of the two phases remains unchanged and in (ii) LT-2: T₁-T₂ where the amount of the incommensurate q₃ phase increases at the cost of the commensurate q₂ amplitude modulated structure which remains unchanged but fully disappears at the first order transition at T₂=23.5 K. The q₃ phase undergoes minor changes until 22 K and gets destabilised at T₂ where the q_3z component jumps from the LT value q_3z≈1/11 to the HT value ≈1/7 and the q_3y component increases from 0.484(1) to 0.495(1). (iii) The high temperature (HT) range T₂-T_N (T_N=40±1 K) is described by a single wave vector q₃. The disproportionation of the HT magnetic phase q₃ below T₂ into two coexisting distinct phases q₂, q₃ down to 1.5 K is an unusual phenomenon, to our knowledge observed for the first time. Various mechanisms are discussed.     

Coexistence of positive and negative exchange bias in CrMn/Co bilayers

Exchange-biased CrMn/Co bilayers with various thicknesses of Co sputtered onto Si(1 0 0) substrates by the RF sputtering system have been studied. Double-shifted loops have been observed with the thickness of Co layer in a narrow range and become single-shifted loops after some cycles of measurement. Those results are interpreted as the association of positive and negative exchange bias.     

Out-of-plane exchange bias and magnetic anisotropy in MnPd/Co multilayers

Magnetic and structural properties in [MnPd/Co]₁₀ multilayers deposited onto Si(1 1 1) substrates have been investigated. The dependences of anisotropy and exchange bias on the thicknesses of both MnPd and Co layers have been studied. In most of the samples, the out-of-plane magnetic anisotropy and both large out-of-plane and in-plane exchange biases have been observed at cryogenic temperature below the blocking temperature T_B≈240 K. With appropriate MnPd and Co thicknesses, we have obtained samples with a large out-of-plane exchange bias along with a large out-of-plane magnetic anisotropy. The origin of the out-of-plane magnetic anisotropy in the samples has been suggested to be due to the formation of CoPd interfacial alloys which have tensile in-plane strains, while the spin structure of the antiferromagnetic layer at the interface which is believed to be responsible for exchange bias may be the same as that of the bulk material. Also, the present study shows that the interplay between the out-of-plane magnetic anisotropy and exchange bias is evident in our multilayers and plays an important role in the out-of-plane exchange-bias mechanism.     

High Curie temperature in B-site ordered Sr2CrWO6 epitaxial thin films

Epitaxial and c-axis oriented double perovskite Sr₂CrWO₆ thin films were prepared on SrTiO₃ (100) and LaAlO₃ (100) substrates by pulsed-laser deposition. Structural, magnetic and transport properties were found to be sensitive to the gas conditions employed during the deposition. A small amount of oxygen along with Ar during the deposition was found to be essential for B-site ordering; such films displayed lattice parameters close to the bulk value and display ferromagnetic metallic behavior. The Curie temperature observed above 500 K in these films is higher than bulk Sr₂CrWO₆ samples. Films grown without oxygen were observed to have long c-parameter and no B-site ordering; they were non-magnetic and semiconducting.     

The growth of soft magnetic FeNiN thin films under different experimental procedures

FeNiN thin films with good soft magnetic properties were synthesized on Si (1 0 0) substrates at 473 K by RF magnetron sputtering. The dependence of phase structure and magnetic properties on nitrogen partial pressure, nickel concentrations, film thickness and substrate temperature were systematically investigated. The phase evolution from α-(Fe,Ni)N to ξ-(Fe,Ni)₂N with increase of nitrogen partial pressure was seen. The addition of Ni caused FeNiN films to turn from BCC structure to FCC structure. Clear reproducible striped domains appeared at the film surfaces when X_Ni=19.6%, which is explained by the high enough perpendicular anisotropy and the small stress in the film. All films show smooth surfaces and good soft magnetic properties compared to corresponding FeN compounds. The magnetic properties depended dramatically on the phase structure. Optimum soft magnetic properties with H_C of <1 Oe are obtained between 5.0%?X_Ni?10.0%.     

Magnetic phase diagram of the four-sublattice antiferromagnet Tb2O2SO4 for a particular field direction

Tb₂O₂SO₄ orders antiferromagnetically at 3.9 K in a four-sublattice structure with two of the nearest-neighbour moments antiparallel and two almost perpendicular to that of a central ion. Specific-heat and magnetization measurements were carried out and allowed to establish the phase diagram for the external magnetic field along one of the moments' direction. Starting at the Néel temperature, the boundary of the paramagnetic phase is first shifted to lower temperatures for increasing field and then it stays at constant temperature for further increase of field. The shift is caused by the reduction of the staggered field that is existing in the antiferromagnetic phase. The experimental results are corroberated to a large extent by mean-field calculations.     

The exchange bias in MnPd/Co1−xFex bilayers

A systematic study of exchange bias in MnPd/Co and MnPd/Co_1−xFe_x bilayers has been carried out. Very large unidirectional anisotropy constant of 2.2 erg/cm² and the appearance of double-shifted loops, ascribed to the coexistence of positive and negative exchange bias, have been observed. The dependence of exchange bias, unidirectional anisotropy constant and coercivity on thickness, temperature, annealing regime and Fe content has been investigated and discussed.     

Magnetic properties of FeSiB thin films displaying stripe domains

FeSiB amorphous thin films with thicknesses from 25 to 600 nm have been produced by rf sputtering on Si₃N₄ substrates. A spin reorientation transition has been observed on the as-prepared samples, as a function of thickness and temperature. Spin reorientation transition is shown to depend on the thermal treatments to which the as-prepared samples have been submitted. Static hysteresis loops obtained as a function of temperature, and magnetic force microscopy images taken at room temperature at the remanence and as a function of an applied magnetic field, have been employed to study the magnetic domain configuration of all the samples, and to see how it is affected by sample thickness, measurement temperature and annealing conditions.     

Magnetic properties of NdAu2Ge2

Physical properties of NdAu₂Ge₂, crystallising with the tetragonal ThCr₂Si₂-type crystal structure, were investigated by means of magnetic, calorimetric and electrical transport measurements as well as by neutron diffraction. The compound exhibits antiferromagnetic ordering below T_N=4.5 K with a collinear magnetic structure of the AFI-type. The neodymium magnetic moments are parallel to the c-axis and amount to 1.04(4) μ_B at 1.5 K. The observed magnetic behaviour is strongly influenced by crystalline electric field effect.     

Phase diagram and magnetic structures of the two-level singlet antiferromagnet Ho2O2SO4

The orthorhombic holmium oxisulphate orders as a two-sublattice antiferromagnet atT=3.5 K. In external fields along the crystallographica- andc-directions with large and medium-sized magnetic moment, respectively, a ferrimagnetic phase with 1/3 of the saturation magnetization is passed before the paramagnetic phase is reached. Calculations in mean-field theory reveal that for thec-direction the ferrimagnetic phase is not stable atT=0, it only exists for finite temperatures. Magnetization and susceptibility contain large contributions of van Vleck paramagnetism which at any rate have to be taken into consideration. The phase diagram for the two field directions and the magnetic structures of the different phases are established.     

Neutron diffraction studies of Tb2Rh3Si5 compound

In this work neutron diffraction studies of Tb₂Rh₃Si₅ compound are reported. The compound crystallizes in the monoclinic crystal structure of Lu₂Co₃Si₅-type. At 1.5 K an antiferromagnetic ordering with a propagation vector k=(1/2;1/2;1/2) was observed. The Tb magnetic moments of 9.8(2) μ_B form a non-collinear magnetic structure. In the vicinity of Néel temperature of 8 K a change of the magnetic ordering is evidenced. The change seems to be connected with phase transition from commensurate to incommensurate sine-wave modulation of the Tb magnetic moments.     

Magnetic and magneto-optical properties of FeRh thin films

The magnetic and magneto-optical properties of FeRh thin films epitaxially deposited onto MgO(1 0 0) substrates by RF sputter-deposition system have been investigated in conjunction with the structure. An intriguing virgin effect has been found in the M–T curves of the as-deposited FeRh thin films, which is presumably interpreted in term of a change in structural phase when heating. Also, a (negative) maximum peak of Kerr rotation at around 3.8 eV has been observed when FeRh thin films are in ferromagnetic state. The polar Kerr rotation angle is found to increase at temperatures above 100 °C, which corresponds to the antiferromagnet (AF)–ferromagnet (FM) transition of FeRh thin films.     

Surface and bulk magnetic properties of as-quenched FeNbB ribbons

The surface and bulk magnetic properties of amorphous FeNbB ribbons in as-quenched state are investigated using various non-destructive methods. The conversion electron Mössbauer spectroscopy has detected the presence of crystalline phase at both surfaces of ribbon sample while the bulk was amorphous. The coexistence of crystalline and amorphous phase was shown also in the X-ray diffraction pattern. Magnetic properties measured by bulk sensitive vibrating sample magnetometer (VSM) strongly differs from the surface characteristics investigated by magneto-optical Kerr effect (MOKE).     

Magnetic anisotropy and anisotropic ballistic conductance of thin magnetic wires

The magnetocrystalline anisotropy of thin magnetic wires of iron and cobalt is quite different from the bulk phases. The spin moment of monatomic Fe wire may be as high as 3.4 μ_B, while the orbital moment as high as 0.5 μ_B. The magnetocrystalline anisotropy energy (MAE) was calculated for wires up to 0.6 nm in diameter starting from monatomic wire and adding consecutive shells for thicker wires. I observe that Fe wires exhibit the change sign with the stress applied along the wire. It means that easy axis may change from the direction along the wire to perpendicular to the wire. We find that ballistic conductance of the wire depends on the direction of the applied magnetic field, i.e. shows anisotropic ballistic magnetoresistance. This effect occurs due to the symmetry dependence of the splitting of degenerate bands in the applied field which changes the number of bands crossing the Fermi level. We find that the ballistic conductance changes with applied stress. Even for thicker wires the ballistic conductance changes by factor 2 on moderate tensile stain in our 5×4 model wire. Thus, the ballistic conductance of magnetic wires changes in the applied field due to the magnetostriction. This effect can be observed as large anisotropic BMR in the experiment.     

Magnetic properties of amorphous Ge–Fe thin films

Thin films of Ge_100−xFe_x (x in at%) alloys, fabricated by thermal co-evaporation, have an amorphous structure at compositions x<∼40, although an unidentified crystalline phase with an FCC symmetry also exists at low Fe content. Magnetization versus temperature curves show that saturation magnetization is non-zero (1 to 2.5 emu/cm³) and remains nearly unchanged up to the highest measured temperature of 350 K. Magnetic hysteresis loops at room temperature show a typical ferromagnetic shape, complete saturation occurring by 1–2 kOe. These results may indicate ferromagnetic ordering at room temperature. No definite tendency is observed in the compositional dependence of saturation magnetization.     

Magnetic excitations in ferromagnetic semiconductors

Magnetic excitations in a series of GaMnAs ferromagnetic semiconductor films were studied by ferromagnetic resonance (FMR). Using the FMR approach, multi-mode spin wave resonance spectra have been observed, whose analysis provides information on magnetic anisotropy (including surface anisotropy), distribution of magnetization precession within the GaMnAs film, dynamic surface spin pinning (derived from surface anisotropy), and the value of exchange stiffness constant D. These studies illustrate a combination of magnetism and semiconductor physics that is unique to magnetic semiconductors.     

The magnetic spin-reorientation transitions in the RGa (R=rare earth) intermetallic compounds studied by measurements of the hyperfine interactions of the Sn probe atoms

The magnetic and electric hyperfine interactions of the probe nucleus ¹¹⁹Sn on the Ga site of the ferromagnetic rare-earth (R) gallium compounds RGa (R=Pr–Er) have been investigated by Mössbauer spectroscopy technique. For all of the compounds, the directions of the magnetic moments of the R³⁺ ions have been determined as a function of temperature in the range from 5 K to T_C. For NdGa, SmGa, HoGa, and ErGa compounds, the magnetic reorientation transitions due to the competition between the exchange interaction and the interaction with crystal field have been investigated. At high temperatures, when the electric interaction dominates, the orientation of the magnetic moments is unambiguously determined by the sign of the quadrupole moment of 4f shell of the R³⁺ ion. With decreasing temperature, the magnetic moments rotate gradually from the bc-plane toward the crystallographic a-axis. In the temperature range 5 K?T<100 K, the ferromagnetic structure of the GdGa compound is noncollinear. At 5 K the magnetic moments of the Gd³⁺ ions point in two distinct directions with respect to the crystallographic a  -axis (_θ1≈30°${\theta }_1\approx 30°$ and _θ2≈60°${\theta }_2\approx 60°$).