Hard-axis Loop Research Articles

Angle-Dependent Inverted Hysteresis Loops in an Exchange-Biased [Co/Pt]5/IrMn Thin Film

The magnetization reversal processes in an exchange-biased [Co/Pt]5/IrMn multilayer thin film have been studied experimentally to analyze the existence of inverted hysteresis loops (IHLs). The magnetic properties of the sample have been investigated in detail by using vibrating sample magnetometer (VSM), magneto-optical Kerr effect (MOKE), and planar Hall effect (PHE) methods. L-MOKE measurements exhibited IHLs at some angles, while VSM data indicated conventional hysteresis loops. The existence of IHLs is explained by the competition between magnetic anisotropies within the sample. Furthermore, both L-MOKE and PHE data exhibited jumps around the hard axis loops due to the incoherent magnetization reorientations of Co layers at relatively low magnetic fields.

Epitaxially grown BaM hexaferrite films having uniaxial axis in the film plane for self-biased devices

Barium hexaferrite (BaM) films with in-plane c-axis orientation are promising and technically important materials for self-biased magnetic microwave devices. In this work, highly oriented BaM films with different thickness and an in-plane easy axis (c-axis) of magnetization were grown on a-plane single-crystal sapphire substrates by direct current magnetron sputtering. A procedure involving seed layers, layer-by-layer annealing was adopted to reduce the substrate-induced strains and allow for the growth of thick (~3.44 μm) films. The epitaxial growth of the BaM film on sapphire was revealed by high-resolution transmission electron microscopy with dislocations being observed at the film-substrate interface. The orientation was also verified by X-ray diffraction and more notably, polarized Raman scattering. The magnetic properties and ferromagnetic resonant frequencies were experimentally characterized by a vibrating sample magnetometry and a frequency-swept ferromagnetic resonant flip-chip technique, respectively. The micron-thick BaM films exhibited a large remanence ratio of 0.92 along in-plane easy axis and a small one of 0.09 for the in-plane hard axis loop measurement. The FMR frequency was 50.3 GHz at zero field and reached 57.9 GHz under a magnetic field of 3 kOe, indicating that the epitaxial BaM films with strong self-biased behaviors have good electromagnetic properties in millimeter-wave range.

Unexpected Width of Minor Magnetic Hysteresis Loops in Nanostructures

We study the coercive fields and phase stability of magnetic nanostructures in hard axis loops and periodic rotational external magnetic fields over wide ranges of field amplitudes depending on the particle size and shape. For this purpose, we use a finite difference code with a gradient descent energy minimization algorithm. Our numerical simulations show that magnetic nanostructures can, in certain field amplitude regions, exhibit minor magnetic hysteresis loops with higher width than their major counterpart as well as variations in the coercive field for consecutive external field periods. The origin of the variations is chaotic behavior of the underlying magnetization processes leading to different residual domains after each field cycle.

Highly strain-sensitive magnetostrictive tunnel magnetoresistance junctions

Tunnel magnetoresistance (TMR) junctions with CoFeB/MgO/CoFeB layers are promising for strain sensing applications due to their high TMR effect and magnetostrictive sense layer (CoFeB). TMR junctions available even in submicron dimensions can serve as strain sensors for microelectromechanical systems devices. Upon stress application, the magnetization configuration of such junctions changes due to the inverse magnetostriction effect resulting in strain-sensitive tunnel resistance. Here, strain sensitivity of round-shaped junctions with diameters of 11.3μm, 19.2μm, 30.5μm, and 41.8μm were investigated on macroscopic cantilevers using a four-point bending apparatus. This investigation mainly focuses on changes in hard-axis TMR loops caused by the stress-induced anisotropy. A macrospin model is proposed, supported by micromagnetic simulations, which describes the complete rotation of the sense layer magnetization within TMR loops of junctions, exposed to high stress. Below 0.2‰ tensile strain, a representative junction with 30.5μm diameter exhibits a very large gauge factor of 2150. For such high gauge factor a bias field H=-3.2kA/m is applied in an angle equal to 3π/2 toward the pinned magnetization of the reference layer. The strain sensitivity strongly depends on the bias field. Applying stress along π/4 against the induced magnetocrystalline anisotropy, both compressive and tensile strain can be identified by a unique sensor. More importantly, a configuration with a gauge factor of 400 at zero bias field is developed which results in a straightforward and compact measuring setup.

The Role of Magnetoelastic Field Related to Underlayers on Magnetic Properties of FeCo Thin Films

In this work, the influence of the magnetoelastic field on the magnetic properties of FeCo thin film is analyzed by the micromagnetic model. Formula of magnetoelastic energy related to the stress between the FeCo and the underlayer are derived to analysis the role of stress on soft magnetic properties. The cubic anisotropy determines the symmetry of the loops. For the magnetoelastic field, the main term Hma has significant influence on the easy-axis coercivity and the cross term Hmb affects the slope of the hard-axis loop. Good in-plane anisotropy of FeCo thin film, which can be obtained by using certain underlayers (such as Cu, NiFe, CoFe), is corresponding to low value of Hma and Hmb. The magnetization 4πMs at amorphous grain boundary has great effect on the properties of FeCo film.

Anisotropy measurements of FePt thin films

We report on anisotropy measurements of FePt thin films prepared at different temperatures and find that the anisotropy increases with increasing chemical ordering. Within measurement accuracy, we conclude that the second-order anisotropy constant K2 is negligibly small compared to the first-order anisotropy constant K1. We assert that the presence of a small hard-axis hysteresis causes spatial fluctuations of the field direction within the sample, which changes the shape of the hard-axis loop, such that it is misinterpreted as a K2 in the Sucksmith–Thompson formalism.

Micromagnetics of Hard Axis Loops for Composite Media

Hard axis loops are measured for composite magnetic films, consisting of a hard magnetic layer ferromagnetically exchange coupled to a soft magnetic layer. A quasi-analytic spin chain model is able to fully explain the experimental findings. The agreement between theory and experiment demonstrates the presence of a nonhomogeneous magnetization state in the composite films.

Controlling magnetic anisotropy in epitaxial FePt(001) films

Epitaxial equiatomic Fe50Pt50 thin films with a variable order parameter ranging from 0 to 0.9 and Fe100−xPtx thin films with x ranging from 33 to 50 were deposited on MgO (001) substrates by dc sputtering. A seed layer consisting of nonmagnetic Cr (4nm)∕Pt (12nm) was used to promote the crystallinity of the magnetic films. The crystal structure and magnetic properties were gauged using x-ray diffraction and magnetometry. The magnetic anisotropy can be controlled by changing the order parameter. For Fe100−xPtx films, the increase in Fe composition leads to an increase in coercivity in the hard axis loop and causes a loss of perpendicular anisotropy.

Anisotropy-graded media: Magnetic characterization

The concept of exchange-coupled media (each grain having a soft end whose exchange field helps to switch a hard end) has recently been generalized to allow a continuous gradation of anisotropy from soft to hard. We have recently shown that the “figure of merit” for such media ξ=2Eb∕μ0MsHsw, proportional to the ratio of the energy barrier Eb to the switching field Hsw, cannot exceed 4 for any anisotropy profile K(r). In the thin-wall limit (exchange constant A⪡KL2), it can be made to approach 4 by choosing a graded anisotropy K(z)∝z2. In developing such a medium, it is important to be able to experimentally probe the anisotropy distribution. In this paper, we study one method for doing this, the hard axis loop. In the absence of exchange, the second derivative of this loop gives the distribution directly; we show that even in the presence of realistic exchange, this remains approximately true and the anisotropy distribution can be extracted from the hard axis loop.

Magnetic anisotropy and crystalline texture in BaO(Fe2O3)6 thin films deposited on GaN∕Al2O3

Ba O ( Fe 2 O 3 ) 6 (BaM) thin films were deposited by pulsed laser deposition on GaN∕Al2O3 substrates. A pole figure obtained from the (006) reflection indicated that ∼81% of the film volume had the c axis tilted less than 5° from the film normal. A low anisotropy field was inferred from vector coil vibrating sample magnetometer (VVSM) measurements. The reduction in Ha from literature values and a two-step switching of the easy axis magnetization is postulated to result from interdiffusion and misalignment effects. To alleviate interdiffusion and to improve the c-axis alignment, experiments were repeated with lower deposition temperatures, thinner films, and MgO buffer layers. The features of the hysteresis loop due to two-step switching and the in-plane coercivity were reduced while the anisotropy field (Ha) was larger. Films deposited with MgO buffer layers are observed to have single-step switching of the easy axis magnetization, larger anisotropy fields, and sharp ferromagnetic resonance (FMR) peaks. Films with MgO buffer layers were determined to have anisotropy fields Ha=1.57T by FMR and Ha∼1.5–1.6T as determined from the difference in the saturation fields for the easy and hard axis loops.

Investigation of magnetization reversal in micron-sized stripes of epitaxial-grown (1 1 0) Laves phase DyFe 2 films

A large-scale striped array of width 3.4 μm has been fabricated in 400-nm-thick epitaxial (1 1 0) DyFe 2 films with Laves phase structure using a UV direct write system. The magnetic and magneto-optical properties of the sample were characterized by a vibrating sample magnetometer and magneto-optical Kerr magnetometry, respectively. The close easy-axis [−1 1 0] direction, confirmed by VSM, was chosen for patterning. The MO Kerr loops reveal that an additional anisotropy can be induced by the shape of the stripes, which can obviously enhance the coercivity of the close easy-axis [−1 1 0] loop. The magnitude of this additional anisotropy was determined to be 4.2 × 10 5 erg/cm 3, which is not sufficient to change the coercivity of the hard-axis loop due to a large intrinsic anisotropy along the hard-axis [0 0 1] direction.

Out-of-plane exchange coupling between epitaxial Ni (50 Å) and NiO (600 Å) bilayers

We have investigated the exchange coupling between an epitaxial Ni (50 Å) film with an out-of-plane magnetic easy axis and a NiO (600 Å) film by polar magneto-optic Kerr-effect measurements. The temperature dependences of exchange field HE for both as-deposited and field-cooled states exhibit the same blocking temperature TB∼130 °C. The exchange field HE for the field-cooled state is lower than that for the as-deposited state. The hard-axis in-plane loop shows a much smaller value of HE. No coercivity enhancement is observed. The data are suggestive of linear coupling across the ferromagnet/antiferromagnet interface.

Study of the reversal mechanism in longitudinal hard disk media: The role of K2/K1

The validity of the coherent rotations mechanism of reversal in CoCrPtB longitudinal hard disk media is probed. The proposed method is based on a comparison between the K2/K1 ratios (K1 and K2 being the first and second order anisotropy constants) measured by a hard axis loop method, and that estimated using in-plane isothermal remanent magnetization and dc-demagnetization processes in the hypothesis of coherent reversal. The disagreement between the two values of K2/K1, noticed for all the investigated samples, indicates an incoherent reversal mechanism. The origin of this mechanism is believed to be the grain coupling. The high values of the switching field at π/2 angle between the applied field and the easy axis HSW(ψ=π/2), measured by hard axis loop, can be larger than the writing field. This may explain the experimentally noticed degradation of signal/noise ratio with increasing HSW(ψ=π/2).

Field-driven reorientation in ultrathin ferromagnetic films with uniaxial anisotropy

The field-driven spin reorientation transition in uniaxial in-plane magnetized films is discussed by means of phase diagrams which are obtained from the stability analysis of the thermodynamical potential. A classification of the field induced reorientation is carried out concerning the anisotropy axes in films grown on vicinal and twofold low-index surfaces. It is shown that hard-axis loops with magnetization flop are directly related to the reorientation in field. In the framework of the general theory the hard-axis loops can be precisely analyzed.

Unicrystal Co–alloy media on Si(110)

Unicrystal Co–alloy/Cr/Ag films, which exhibit a single, in-plane easy axis orientation, were epitaxially grown on hydrofluoric acid etched Si(110) single crystal substrates by sputter deposition for the purpose of the systematic study of Co–alloy magnetocrystalline anisotropy. The orientation relationship was studied by x-ray θ/2θ diffraction, pole figure φ scan, and transmission electron microscopy, and it was determined to be Co(101̄0)[0001]‖Cr(112)[11̄0]‖Ag(110)[001]‖Si(110)[001]. The φ scan also showed two twin-related orientations of Cr grains. The easy axis hysteresis loops had a square shape, while the hard axis loops showed zero openness. The uniaxial anisotropy constants K1 and K2 of the unicrystal Co and CoCrTa films were determined from torque and hard axis hysteresis loop measurements.

High frequency properties of Fe–Cr–Ta–N soft magnetic films

High frequency magnetic properties of a new, soft magnetic Fe–Cr–Ta–N alloy film have been investigated. Thin films with a composition of Fe-4.6% Cr-0.2% Ta-7.4% N (in atomic %) were prepared by reactive sputtering in a nitrogen-containing atmosphere. The films, most likely nanocrystalline, exhibit excellent soft magnetic properties in the as-deposited condition without any post heat treatment, e.g., Hc as low as 1.2 Oe (95.3 A/m) and 4πMs∼20 kG. The easy-axis M–H loop is square. The hard-axis loop is linear and closed, with the anisotropy field Ha=20–100 Oe (1.59–7.95 kA/m). By virtue of their high 4πMs and relatively high Ha, these soft magnetic films exhibit high permeability and low loss in the GHz frequency range with the undesirable ferromagnetic resonance interference suppressed to beyond 2 GHz.

Fe-Cr-N soft magnetic thin films

Soft magnetic properties of new Fe-Cr-N and Fe-Cr-Ta-N alloy films have been investigated. Thin films with compositions in the range of Fe-2∼8% Cr-0∼1% Ta-5∼15% N (in at. %) were prepared by reactive sputtering in a nitrogen-containing atmosphere. The films, most likely nanocrystalline, exhibit excellent soft magnetic properties in the as-deposited condition without any post heat treatment, e.g., Hc∼1–2 Oe (79.4–158.8 A/m) and 4πMs∼15–20 kG. The easy-axis M-H loop is square. The hard-axis loop is linear and closed, with the anisotropy field Ha=20–60 Oe (1.59–4.77 kA/m). The combination of high 4πMs and relatively high Ha in these films is conducive to the suppression of the undesirable ferromagnetic resonance (FMR) interference up to the GHz frequency range.

Fe-Cr-Hf-N and Fe-Cr-Ta-N soft magnetic thin films

We have investigated the soft magnetic properties of new Fe-Cr-Hf-N and Fe-Cr-Ta-N alloy films. Thin films with compositions in the range of Fe-2/spl sim/8%Cr-0/spl sim/1%Hf (or Ta)-5/spl sim/10%N (in atomic %) were prepared by reactive sputtering in a nitrogen-containing atmosphere. The films, most likely nanocrystalline, exhibit excellent soft magnetic properties in the as-deposited condition without any post-deposition heat treatment, e.g., H/sub c//spl sim/1-2 Oe (79.4-158.8 A/m) and 4/spl pi/M/sub s//spl sim/15-20 kG. The magnetic properties are highly anisotropic in the film plane as the easy-axis M-H loop is square and the hard-axis loop is linear and closed, with the anisotropy field H/sub a/=20-60 Oe (1.59-4.77 kA/m). The films exhibit excellent high frequency behavior even in the GHz frequency range which is attributed to the combination of high 4/spl pi/M/sub s/ and relatively high H/sub a/.

Fe-Cr-N Based Nanocrystalline Soft Magnetic Thin Films

ABSTRACTWe have investigated the soft magnetic properties of new Fe-Cr-Ta-N and Fe-Cr-Hf-N alloy films. Thin films with compositions in the range of Fe-2∼8%Cr-0∼l%Ta (or Hf)-5∼10%N (in atomic %) were prepared by reactive sputtering in a nitrogen-containing atmosphere. The films, most likely nanocrystalline, exhibit excellent soft magnetic properties in the as-deposited condition without any post-deposition heat treatment, e.g., Hc∼l-2 Oe and 4πMs∼15–20 kG. The magnetic properties are highly anisotropie in the film plane as the easy-axis M-H loop is square and the hard-axis loop is linear and closed, with the anisotropy field Ha=20–100 Oe. The films exhibit excellent high frequency behavior even in the GHz frequency range which is attributed to the combination of high 4πMs and relatively high Ha.

Hysteresis reduction in NiFeCo/Cu multilayers exhibiting large low-field giant magnetoresistance

Giant magnetoresistance (GMR) has been observed in Ni66Fe16Co18/Cu multilayered uniaxial magnetic thin films prepared by dc magnetron sputtering. Both easy and hard axis loops saturate at very low applied fields. The MR ratio reaches ∼80% of its saturation value in less than 30 Oe. A maximum MR ratio of ∼10% has been observed in the as-deposited state in a structure containing only ten bilayers. Typical hysteresis values for magnetoresistance loops are ∼4 Oe in the as-deposited state. A gradual decrease of this hysteresis has been observed with annealing. An increase in the saturation field is also observed with annealing. Short-term annealing increases the magnitude of the magnetoresistance ratio; prolonged annealing causes a decrease. A field sensitivity of about 0.3% per Oe has been observed and dc magnetoresistance minor loops exhibit no measurable decrease in sensitivity.