Suppression of incommensurate spin-density waves in thin epitaxial Cr(110) layers of a V/Cr multilayer

We observed a complete suppression of the incommensurate spin-density wave in thin Cr layers of a V/Cr multilayer in a temperature range from 550 K down to 2 K. The (110)-oriented V/Cr multilayer consisting of 30 nm thick Cr layers and 5 nm thick V layers was investigated by neutron and X-ray diffraction (XRD). From the XRD experiments we were able to determine that the epitaxial strain of the Cr layers in the V/Cr multilayer is about 90% larger than in earlier studied Fe/Cr(110) multilayers. That leads to a completely different magnetic phase diagram as revealed by neutron diffraction experiments. The existence of the commensurate antiferromagnetic structure in the Cr layers can be observed in the whole temperature range without a phase transition to an incommensurate spin-density wave at lower temperatures. In order to elucidate the proximity effects further we also performed experiments in an external magnetic field. Up to a field of 4 T we found no change in the magnetic structure of the Cr films whereas in earlier experiments on Fe/Cr(110) multilayers we could observe a strong perpendicular pinning of the Cr polarization to the Fe magnetization.Received: 28 August 2003, Published online: 8 December 2003PACS: 75.30.Fv spin-density waves - 75.70.-i magnetic properties of thin films, surfaces, and interfaces - 61.12.-q Neutron diffraction and scattering     

Magnetic Behaviour of Sm2Co7/Fe/Sm2 Co7, Nanocomposite Trilayers with Cr and Ti Additions

Trilayered Sm2Co7/Fe/Sm2Co7 spring exchange magnets are fabricated by dc magnetron sputtering on MgO substrates. Very thin layers （0.3-0.7 nm） of Cr and Ti are added at the interfaces of the two magnetic phases. The thickness of StucCo7 is kept at 20nm and Fe at 6nm while the thickness of Cr and Ti are varied as 0.3, 0.5, and 0.7nm. The base pressure of sputtering chamber is kept below 10^-7 Torr and Ar pressure at 3-8m Torr. The samples are characterized by x-ray diffraction （XRD） and SQUID magnetometer. We report improvement in exchange coupling of nonacomposite magnets by addition of thin layers of Cr at interfaces.     

Ferromagnetic hcp chromium in Cr/Ru(0001) superlattices

We report the first observation of a weak ferromagnetic state of Cr in Cr/Ru(0001) superlattices, based on magnetic hysteresis and corroborated by x-ray magnetic circular dichroism at the CrL(2,3) edges. In situ reflection high-energy electron diffraction, x-ray diffraction, and Cr K-edge polarized x-ray absorption investigations have shown that the Cr layers thinner than 8 angstroms adopt a slightly distorted hcp structure, accompanied by a large atomic volume expansion of up to 14% compared to the bcc packing volume. The expanded hcp structure clearly induces the observed ferromagnetism, in agreement with theory.     

Theoretical study of magnetism and electronic structure of Fe3/Crn(1 1 0) superlattices

The electronic structure and magnetism of Fe₃/Cr_n(1 1 0) (n=1, 3, 5) superlattices (SL) with varying layer thickness have been studied using the full-potential linearized augmented plane-wave (FLAPW) method within the first-principle formalism. The results show that the ferromagnetic state is the preferable phase in the ground state. The magnetic moments of the Fe layers are slightly modified by the presence of the Cr layers. The Cr magnetic moments alternate direction from layer to layer, and an antiferromagnetic coupling between Fe and Cr at the interfacial layer is seen. The magnetic moments of the Cr layers are suppressed because there is a strong hybridization between d-states of both Fe and Cr atoms. Only a small moment is found in the Cr layer. The Cr moment alignment is determined by a delicate balance between the different magnetic interaction.     

Structural and magnetic studies of Co thin films

The trend in reducing device dimension induces new physical properties and requires the development of measurement tools at the nanometer scale. This paper deals with the relation between magnetism and structure of thin films. We have chosen cobalt as a ferromagnetic layer and chromium as a bcc buffer. Magnetic and structural investigations have been led on epitaxial Co/Cr layers grown on MgO (001) substrates. The thickness of the cobalt layer varies from 0.75 to 20 nm. Investigations on the cobalt layer by EXAFS and HRTEM give evidence for a bcc or a hcp structure depending on the cobalt thickness. Magnetic measurements using SQUID indicate that the saturation magnetisation per volume unit is constant for the layers. EELS experiments have been carried out to measure any evolution in the I(L3)/I(L2) ratio for ferromagnetic layers of different thickness. We discuss the influence of structural and magnetic contributions on the evolution of the ratio with the cobalt thickness.     

Magnetism of ultrathin Cr layers and Fe/Cr multilayers studied by 119Sn probes

Cr/Sn and Fe/Cr/Sn/Cr multilayers, where monatomic Sn layers are embedded in Cr layers and Fe/Cr multilayers respectively, were prepared by means of ultrahigh-vacuum deposition technique, and the magnetic hyperfine field induced at the ¹¹⁹Sn nuclear sites was examined using conversion electron Mössbauer spectroscopy. The magnetic structures of the Cr layers are inferred from the size and direction of the magnetic hyperfine field transferred at the Sn sites.     

High-frequency properties of Fe/Cr superlattices with thin Cr layers in the millimeter-wavelength range

Microwave properties of Fe/Cr multilayer nanostructures with thin chromium layers (with thickness t _Cr < 1 nm) are analyzed. Experiments are performed by the method of penetration of microwaves in the frequency range from 26 to 38 GHz. The dependence of the transmission coefficient for microwaves on the constant magnetic field strength exhibits the microwave magnetoresistive effect and magnetic resonance. The resonance spectrum is reconstructed from measurements at various frequencies. The results of microwave measurements are analyzed together with the results of magnetic and magnetoresistive measurements.     

Reduction of magnetic moments in very thin Cr layers of Fe/Cr multilayers: evidence from 119Sn Mössbauer spectroscopy

Fe/Cr multilayers with monatomic Sn layers embedded in the Cr layers were grown epitaxially on MgO(001) substrates, and the magnetic hyperfine field at the 119Sn nuclear sites was examined using M?ssbauer spectroscopy. It was found that nonzero hyperfine field is induced at the Sn sites at room temperature and that the value reduces drastically from 10 to 2 T when the Cr layer thickness decreases from 80 to 10 A. The result indicates that the Cr layers are magnetically ordered even when the thickness is very small and that the magnetic moments of Cr become smaller as the Cr layer thickness decreases.     

Structure of sputter-deposited Pt/Fe and Cr/Fe multilayers

Conversion electron Mössbauer spectroscopy (CEMS) and X-ray diffraction (XRD) have been used to investigate the structure of Pt/Fe and Cr/Fe multilayers deposited by magnetron sputtering. The Cr/Fe samples consisted of four samples prepared under Ar sputtering pressures of 1.3, 3.0, 5.0, and 10.0 mT, all with the same multilayer structure of 3.5 nm Cr/2.5 nm Fe, repeated 35 times onto c-Si wafer substrates. The quality of the interfaces between Cr and Fe is clearly degraded with increasing sputter pressure, as seen by changes in the relative intensities of four magnetic subspectra in the CEMS and the gradual appearance of a single-line resonance similar to Fe in solution in Cr. The low-angle XRD superlattice peaks also disappear with increasing sputter pressure, while the high-angle XRD shows a tendency for loss of the preferred (110) texture. Two films of Pt/Fe were deposited epitaxially onto MgO single crystals with bilayer periods of 1.3 nm and 2.6 nm and total thickness of 300 nm each. A transition from fcc-PtFe with near-perpendicular magnetic anisotropy to a bcc-Fe/fcc-PtFe mixture with in-plane magnetic texture is observed by CEMS for the factor of two increase in bilayer period.     

Thermal stability and separation characteristics of anti-sticking layers of Pt/Cr films for the hot slumping technique

The thermal stability and separation characteristics of anti-sticking layers of Pt/Cr films are studied in this paper. Several types of adhesion layers were investigated: 10.0 nm Pt, 1.5 nm Cr + 50.0 nm Pt, 2.5 nm Cr+ 50.0 nm Pt and 3.5 nm Cr + 50.0 nm Pt fabricated using direct current magnetron sputtering. The variation of layer thickness, roughness, crystallization and surface topography of Pt/Cr films were analyzed by grazing incidence X-ray reflectometry, large angle X-ray diffraction and optical profiler before and after heating. 2.5 nm Cr + 50.0 nm Pt film exhibits the best thermal stability and separation characteristics according to the heating and hot slumping experiments. The film was also applied as an anti-sticking layer to optimize the maximum temperature of the hot slumping technique.     

多层膜［Co85Cr15/Pt］20的磁性、垂直磁记录特性和微结构的关系

采用磁控溅射法制备了性能优良的以Pt为缓冲层的［Co85₈₅Cr15_{15/Pt］2020 多层膜，研究了溅射气压对［Co8585Cr1515/Pt］2020多层膜微结构和磁性的 影响.研究结果表明，Ar溅射气压对［Co8585Cr1515/Pt］2020多层膜的微结构 、垂直磁各向异性和矫顽力有重要的影响 关键词： 溅射气压 多层膜 垂直磁各向异性 有效磁各向异性常数}     

Optical measurements of field-induced phenomena of the magnetic phase transition in quasi 2D MnS layers grown by MBE

We used a sensitive optical method to study the magnetic phase transition of antiferromagnetic MnS layers. The method is applicable for very small numbers of spins, e.g., thin single layers. We studied the optical and magnetic properties of MnS layers using the internal optical transition of the manganese 3d-shell. The temperature dependence of the Mn-emission exhibits a pronounced minimum revealing the para- to anti-ferromagnetic phase transition. The MnS layers were grown by molecular beam epitaxy, embedded between diamagnetic ZnSe cladding layers on a (100)-GaAs substrate. It was found that the Néel-temperature itself is influenced by the biaxial strain and can be changed in an external magnetic field in case of quasi 2D MnS-layers. The phase diagram reveals a weak Ising like anisotropic contribution in case of a 1.8 nm thin layer, whereas a 8.6 nm thick layer behaves still like an ideal isotropic Heisenberg system.     

Calculated angular dependence of the interlayer couplings in Fe/Cr superlattices having imperfect interfaces

We present calculations of the non-collinear magnetic structure in Fe/Cr superlattices having imperfect interfaces modeled by considering atomic steps in the Cr layers and Fe/Cr interfacial ordered compounds. The interlayer couplings are obtained directly from self-consistent tight binding band structure calculations. We show that the bilinear–biquadratic expression for the coupling energy fits nicely the calculated interlayer couplings curves.     

Time resolved magnetization dynamics of ultrathin Fe(001) films: spin-pumping and two-magnon scattering

The time-resolved magnetic response of ultrathin epitaxial Fe(001) films grown on GaAs(001) and covered by Au, Pd, and Cr capping layers was investigated by time and spatially resolved Kerr effect measurements. The magnetization was excited by an in-plane magnetic field pulse using the transient internal field generated at a Schottky barrier while the wavelength of the excitation (resonant mode) was roughly 4 microm. Each of the three cap layers affected the spin relaxation in a unique way. Au cap layers resulted in the bulk Gilbert damping of the Fe film. Pd cap layers caused an additional Gilbert damping due to spin-pump or spin-sink effects. Cr cap layers lead to a strong extrinsic damping which can be described by two-magnon scattering. In this case the strength of the extrinsic damping can be controlled by a field induced shift of the spin wave manifold with respect to the excited k vector.     

Spin-dependent resonant tunneling through quantum-well states in magnetic metallic thin films

Quantum-well (QW) states in nonmagnetic metal films between magnetic layers are known to be important in spin-dependent transport, but QW states in magnetic films remains elusive. Here we identify the conditions for resonant tunneling through QW states in magnetic films and report first principles calculations of Fe/MgO/FeO/Fe/Cr and Co/MgO/Fe/Cr. We show that, at resonance, the current increases by 1 to 2 orders of magnitude. The tunneling magnetoresistance ratio is much larger than in simple spin tunnel junctions and is positive (negative) for majority- (minority-) spin resonances, with a large asymmetry between positive and negative biases. The results can serve as a basis for novel spintronic devices.     

Design and fabrication of non-superparamagnetic high moment magnetic nanoparticles for bioapplications

We present a new type of magnetic nanoparticles for bioapplications. Multilayered nanodisks consisting of two magnetic layers separated by a non-magnetic layer with two capping layers were designed and fabricated. Two key magnetic requirements for bioapplications, a high saturation magnetic moment and a near-zero remanence, were achieved through the magnetostatic interlayer coupling between two magnetic layers. Capping layers provide functionalization sites for biomolecule attachment. A pillar-template-based synthesis method was employed for fabrication. Nanodisks with a diameter of 70 nm and a thickness of 60 nm were produced in large quantity. The magnetic characterization shows that each nanodisk possesses a magnetic moment equivalent to 100 10-nm Co nanoparticles and a near-zero remanent moment.     

Polarized neutron reflectometry of Fe/Cr/Gd superlattices

The magnetic structure of Fe/Cr/Gd superlattices is investigated using complementary methods of SQUID magnetometry and polarized neutron reflectometry. The complex magnetic behavior of the given system is caused by exchange interaction between the 3d (Fe) and 4f (Gd) layers of the ferromagnetic metals through the Cr antiferromagnetic spacer layer. It is found that a nonuniform profile of magnetization forms within the Gd layers under the influence of this interlayer interaction.     

Magnetic properties of transition metal superlattices

The magnetic coupling between Fe layers separated by spacer layers consisting of up to two atomic planes of 3d transition metal elements (Sc, Ti, V, Cr, Fe, Co, Ni and Cu) has been studied systematically by using two complimentary theories based on cluster and band structure methods. The Fe layers are found to be ferromagnetically coupled in all cases except for Cr where this coupling alternates from ferro- to anti-ferromagnetic depending on whether the spacer layers are odd or even. Furthermore the spacer layers involving Sc, Ti and V are anti-ferromagnetically coupled to Fe while Co and Ni layers are coupled ferromagnetically.     

Co-Zr/Pd多层膜的磁性与结构

Co-Zr/Pd多层膜由高频溅射方法制得．磁性合金Co-Zr层厚度固定为1.8nm，改变Pd层厚度0.5—6nm．由振动样品磁强计测量，发现随Pd层厚度增加，磁化强度发生周期性振荡变化，周期约为1nm，这是由Pd层的极化振荡引起的．经X射线衍射测得Pd层厚度超过1.3nm时，磁性合金Co-Zr层发生晶化，而厚的Co-Zr单层膜是非晶结构．X射线大角衍射图中的超晶格峰表明，在Co-Zr层和Pd层之间存在相关生长．而且还发现，随Pd层厚度增加，样品在垂直膜面方向的晶粒尺寸及fcc（111）面的面间距发生周期性 关键词：     

Spin-polarized scanning tunneling microscopy with antiferromagnetic probe tips

We have performed low temperature spin-polarized scanning tunneling microscopy (SP-STM) of two monolayers Fe on W(110) using tungsten tips coated with different magnetic materials. We observe stripe domains with a magnetic period of 50 +/- 5 nm. Employing Cr as a coating material we recorded SP-STM images with an antiferromagnetic probe tip. The advantage of its vanishing dipole field is most apparent in external magnetic fields. This new approach resolves the problem of the disturbing influence of a ferromagnetic tip in the investigation of soft magnetic materials and superparamagnetic particles.