Volume 35, Issue 3, 01 March 1964
Index of content:
- MAGNETIZATION PROCESSES; THIN FILMS
35(1964); http://dx.doi.org/10.1063/1.1713531View Description Hide Description
Slowly cooled magnesium‐iron ferrites of the approximate composition MgO2FeO·4Fe2O3 have constricted hysteresis loops in a wide range of temperature. Applying a magnetothermic treatment one can induce an anisotropy which is accompanied by a square hysteresis loop. The speed of the ordering process can be varied in a certain range by variation of the temperature. The treatment of samples was performed at about 100°C allowing an exact study of the process.
During the conversion of the shape of the hysteresis loop the remanence ratio Br/Bm increases. An activation energy of 0.37 eV can be derived from the temperature dependence of this change. This is in good agreement with results of Sixtus.
More exact investigations revealed an unexpected effect; the peak induction increases with time discontinuously in two main steps while the field is kept constant. The time at which the steps occur varies with temperature. An activation energy of 1.4 eV can be derived.
Nearly the same value was calculated from the temperature dependence of an observed minimum of the magnetization energy occurring during loop transformation. The depth of this minimum can be taken as a measure of the induced anisotropy.
35(1964); http://dx.doi.org/10.1063/1.1713532View Description Hide Description
Dynamic magnetoresistancehysteresis loops were used for investigation of the magnetization reversal in cold‐rolled tapes of high‐purity nickel. The tapes were prepared by unidirectional rolling as well as by rolling with reversing tape end for end between passes. The loops were cycled in the range ±4500 Oe with a variety of frequencies f from 0.005 to 0.1 cps in the wide interval of temperatures from 297° to 4.2°K. When the magnetization field was transverse to the rolling axis, the coercive force could be expressed by Hc = A (lnf−lnf 0)/T ½, where A and f 0 are in general dependent on the reduction of the tape thickness, and vary slightly with the temperature T. Such a relation may be related to a magnetization reversal occuring through a process of reorientation of spins, which can be described as a temperature‐activated domain‐wall relaxation. The magnetoresistance loops of the samples made by unidirectional cold rolling were asymmetric when they were performed by applying simultaneously two crossed magnetic fields, one ac and the other dc. A magnetic uniaxial anisotropy induced by this rolling seems to be responsible for the observed asymmetry, a possible mechanism for this being the deformation of crystals by slips.
35(1964); http://dx.doi.org/10.1063/1.1713533View Description Hide Description
Fast flux reversal in thin (2000 Å) Permalloy films has been observed at 77° and 4°K and compared to room temperature. The inverse of the reversal time as a function of drive field with initial angle as a parameter was plotted. The switching coefficient Sw of the low‐drive region increased threefold which is inconsistent with a simple, eddy‐current‐damped domain wall motion model since the conductivity changed only 17% over the same range. The switching coefficient for the intermediate‐drive region is essentially constant to 77°K and then increases as the temperature is reduced in a manner inconsistent with an intrinsically damped nonuniform rotational mechanism. The thresholds in this region doubled, as did the anisotropy field HK between 296° and 4°K. The high‐drive region does not change with temperature consistent with a coherent rotational model.
35(1964); http://dx.doi.org/10.1063/1.1713534View Description Hide Description
For one‐dimensional plane defects, a parasitic paramagnetism is obtained if in the near vicinity of each defect there are defects of opposite sign. The two‐dimensional problem of a line defect surrounded by defects of opposite sign, is approached by assuming that the transverse magnetization vanishes on a certain circle around the defect. For such a model one obtains a magnetic hardness term, when the radius of the circle is large compared to the fall‐off distances of the disturbance in the material. For a radius much smaller than the fall‐off distances, a parasitic paramagnetism term is obtained, with apparent reduction in the saturation magnetization value.
35(1964); http://dx.doi.org/10.1063/1.1713535View Description Hide Description
Magnetic and electric properties of some single crystals of complex composition of perovskite‐type structure were investigated and coexistence of ferroelectric and antiferromagnetic properties was shown in them. It was found that single crystals YMnO3 and YbMnO3 of the new class of ferroelectrics, discovered by Bertaut and others, are also antiferromagnetics. The results of the investigation of solid solutions based on BiFeO3, which has a perovskite‐type structure, are discussed. The opinion is expressed that BiFeO3 is probably not a ferroelectric. The main results concerning the thermodynamic theory of substances which are ferroelectric and ferromagnetic simultaneously are given in this report.
35(1964); http://dx.doi.org/10.1063/1.1713536View Description Hide Description
The switching properties of polycrystalline toroids of pure YIG (yttriumirongarnet) and YIG containing 1% Dy, 3% Dy, and 4% Sm have been measured over the temperature range 4.2° to 300°K. In each of the rare‐earth‐substituted materials, the field required to switch 50% of the flux in a given time shows a peak at a temperature which is approximately 0.4 times the temperature at which a peak is observed in the ferrimagnetic resonance linewidth. It is assumed that at low switching speeds flux reversal occurs by domain wallmotion, and an explanation of the temperature dependence of the switching field is sought in a consideration of the effect of the loss mechanisms which determine the linewidth on the domain wall mobility. The field above threshold H − H 0 required for complete dissipation of magnetostatic energy in the moving wall at a given domain wall velocity V, and thus a given switching speed, should be proportional to the linewidth at the effective precession frequency applicable to domain wallmotion. For 180° domain walls this frequency is approximately Vπ/d, where d is the domain wall thickness. Measurements of the frequency dependence of the linewidth of rare‐earth‐substituted YIG have indicated that the loss mechanisms in these materials are ``slow'' relaxation processes. For slow relaxation processes the linewidth is given by the expression ΔH (T, ωτ) = F (T)ωτ/1+ω2τ2, where ω is the experimental frequency and τ is the relaxation time. H − H 0 (T, Vπτ/d) should depend in a similar way on Vπτ/d. This requires that Vπτ/d be a double‐valued function of H − H 0, the higher value of which is inaccessible. The increase of ωτ beyond unity contributes to the decrease of the linewidth at low temperature, but the decrease in the switching field at low temperature cannot be explained in this way. If the switching time chosen for reference were sufficiently short that at low temperatures Vπτ/d increases beyond unity, the required field would not be reduced but a discontinuous increase in switching speed, which is not observed, would occur at the field H − H 0 (T, Vπτ/d=1). However, the behavior of the switching field is understandable if the explicit temperature dependence of H − H 0 (T, Vπτ/d) is such as to produce the observed decrease at low temperature.
A report of this work and related topics has been submitted for publication.
35(1964); http://dx.doi.org/10.1063/1.1713537View Description Hide Description
Reversal of the net ferromagnetic moment in an antiferromagnet exhibiting weak ferromagnetism would appear qualitatively to require only small angle rotation of the sublattice moments. However, consideration of the canting mechanism responsible for the weak ferromagnetism shows that it is asymmetric in a way which requires full 180° reversal of the sublattice moments, i.e., M1×M2 retains the same direction in space. This is true for both the Dzialoshinsky‐Moriya and single‐ion anisotropy canting mechanisms. The critical values of the applied field for reversal of the magnetization and the corresponding initial modes of deviation from equilibrium have been calculated for single‐domain particles of antiferromagnetic material having either type of weak ferromagnetic moment. Of the four modes found, two have extremely high critical fields since they involve rotation in opposition of the sublattice moments at the expense of exchange energy. The two reversal modes of interest are (initially) rotations of the M i about axes parallel and perpendicular to the particle axis, principally at the expense of anisotropy energy. If the initial character of the motion is assumed to persist, the mode Ω⊥ would be a 180° rotation of m≡M1+M2 and 1≡M1−M2 in the plane of the M i and the particle axis. The mode Ω∥ would be a 180° rotation of 1 in the plane perpendicular to the particle axis while simultaneously m decreases to zero and then grows to equilibrium value in the opposite direction along the particle axis such that 12+m2 =constant. The critical fields for these two modes are given by the zero‐frequency limits of the dynamic resonance equations as pointed out by Brown. In general, the smallest critical fields will be found for the Ω⊥ mode in Dzialoshinsky‐Moriya type weak ferromagnets with small, but finite, basal plane anisotropy. The lowest |Hc | calculated is 40 Oe for αFe2O3.
35(1964); http://dx.doi.org/10.1063/1.1713538View Description Hide Description
Néel walls are presumed to exist as domain boundaries in thin films with thicknesses below 500 Å. Low‐drive switching with a superimposed transverse field Ht may take place by reverse wall motion (H t ·Mwall <0) or normal wall motion (H t ·Mwall >0). Wall velocities are determined by intrinsic damping rather than eddy current losses, for films in this thickness region. Reverse wall mobilities have been measured by interrupted pulse flux reversal experiments, in agreement with calculations based on the Landau‐Lifshitz equation and uniaxial anisotropy. With h=Ht/HK =0.1, normal wall mobility is about 20% greater than reverse wall mobility.
35(1964); http://dx.doi.org/10.1063/1.1713539View Description Hide Description
Lorentztransmission electron microscopy has been used to observe the magnetic domain structures in thin cobalt foils above room temperature. The changes in domain pattern occurring with increase in temperature in a crystal of the hcp phase are related to magnetocrystalline anisotropymeasurements, and to the rotation of the easy direction of magnetization from the  c axis to a direction in the basal plane at about 275°C. The domain pattern, in a crystal of the fcc phase, at elevated temperatures, is also in agreement with anisotropymeasurements. In a region of mixed phase there is rotation of the magnetization direction from a  direction to a  direction in the fcc areas, in disagreement with anisotropymeasurements.
35(1964); http://dx.doi.org/10.1063/1.1713540View Description Hide Description
PolycrystallineNifilms, evaporated onto single‐crystal NiO substrates, have been studied by torque measurements from 77° to 473°K in fields up to 22 kOe. The NiO films had (111) and (001) orientations, representing planes of parallel and compensated spins, respectively. After short vacuum anneals above TN , the Néel temperature, the major torque symmetry of the films on (001) NiO substrates was sin 4θ with an amplitude that decreased with increasing field. This was due to the rhombohedral distortion of NiO below TN ; a (001) crystal consists of twinned, T‐domain regions. This oriented strain field in the NiO substrate propagates into the Nifilm and produces two orthogonal uniaxial anisotropies by a strain‐magnetostriction mechanism.
The (111) NiO substrates were also twinned. After annealing above TN , the Nifilms on these substrates showed a peak in the rotational hysteresis at ∼1 kOe, and a torque component of sixfold symmetry in the moderate to high‐field range. One sample also showed rotational hysteresis that was increasing rapidly at 22 kOe. This behavior is consistent with a model involving exchange anisotropy, the motion of T‐domain walls, and a strain‐magnetostriction mechanism.
35(1964); http://dx.doi.org/10.1063/1.1713541View Description Hide Description
The spin configuration in the transition layer between an antiferromagnet and a ferromagnet is investigated. The calculation is carried out by minimizing the free energy of the system with exchange, anisotropy, magnetostatic, and Zeeman terms. The results of our calculation indicate that a domain wall is formed between the antiferromagnetic and ferromagnetic media. The configuration of the walls, although similar to that in ferromagnets, is more complex and sensitively dependent upon the relative values of the exchange constant and anisotropy for the two media as well as upon the strength and nature of the interaction across the interface. A specific example involving an oxidizedferromagneticfilm is given to illustrate the utility of our general result. It is noted that the formation of an antiferromagnetic‐ferromagneticdomain wall may influence the magnetization of and surface spin pinning in thin films.
35(1964); http://dx.doi.org/10.1063/1.1713542View Description Hide Description
The Stoner‐Wohlfarth coherent rotation model is extended to the case of biaxial anisotropy. The rotational hysteresisWr as a function of reduced field h is zero for all values of h except 0.25≤h≤1, and the maximum value of Wr/K=2.38 at h=0.25. The rotational hysteresis integral W equals 1.54. Measurements on fcc single‐crystal Co films (−K=4−6×105 ergs/cm3) prepared by epitaxial evaporation onto the (100) face of single‐crystal MgO slabs held at 400°C yield results in wide disagreement with the theory. While the expected angular symmetry of the torque is observed, the model does not correctly predict the magnitude of the torque nor the fields at which specific types of angular dependence will be observed. The rotational hysteresis begins in every case before the predicted onset and continues beyond the expected cutoff. The maximum value of Wr/K and W are found to vary from 1.0 to 1.6 and from 2.9 to 1.7, respectively. It is concluded that the magnetization reversal process in single‐crystal Co films is not coherent and that, as in the case of polycrystalline films, noncoherent rotations or wall motions must be investigated. This situation in single‐element, single‐crystal films calls for a reappraisal of the importance of compositional inhomogeneities and local crystalline anisotropy in polycrystalline alloy films.