The Bloch point in uniaxial ferromagnets as a quantum mechanical object
© Shevchenko and Barabash; licensee Springer. 2014
Received: 4 December 2013
Accepted: 28 February 2014
Published: 19 March 2014
Quantum effects such as tunneling through pinning barrier of the Bloch Point and over-barrier reflection from the defect potential of one have been investigated in ferromagnets with uniaxial strong magnetic anisotropy. It is found that these phenomena can be appeared only in subhelium temperature range.
KeywordsQuantum tunneling The bloch point Domain walls Vertical bloch lines uniaxial magnetic film Quantum depinning Magnetic field Potential barrier Ferromagnetic materials
Mesoscopic magnetic systems in ferromagnets with a uniaxial magnetic anisotropy are nowadays the subject of considerable attention both theoretically and experimentally. Among these systems are distinguished, especially domain walls (DWs) and elements of its internal structure - vertical Bloch lines (BLs; boundaries between domain wall areas with an antiparallel orientation of magnetization) and Bloch points (BPs; intersection point of two BL parts) . The vertical Bloch lines and BPs are stable nanoformation with characteristic size of approximately 102 nm and considered as an elemental base for magnetoelectronic and solid-state data-storage devices on the magnetic base with high performance (mechanical stability, radiation resistance, non-volatility) . The magnetic structures similar to BLs and BPs are also present in nanostripes and cylindrical nanowires [3–6], which are perspective materials for spintronics.
It is necessary to note that mathematically, the DW and its structural elements are described as solitons, which have topological features. One of such features is a topological charge which characterized a direction of magnetization vector reversal in the center of magnetic structure. Due to its origin, the topological charges of the DW, BL, and BP are degenerated. Meanwhile, in the low temperature range (T < 1 K), vector reversal direction degeneration can be lifted by a subbarier quantum tunneling. Quantum magnetic fluctuations of such type in DWs of various ferro- and antiferromagnetic materials were considered in [7–11]. The quantum tunneling between states with different topological charges of BLs in an ultrathin magnetic film has been investigated in .
Note that in the subhelium temperature range, the DWs and BLs are mechanically quantum tunneling through the pining barriers formed by defects. Such a problem for the case of DW and BL in a uniaxial magnetic film with strong magnetic anisotropy has been investigated in  and , respectively. Quantum depinning of the DW in a weak ferromagnet was investigated in article . At the same time, the BPs related to the nucleation [16–18] definitely indicates the presence of quantum properties in this element of the DW internal structure, too. The investigation of the abovementioned problem for the BP in the DW of ferromagnets with material quality factor (the ratio between the magnetic anisotropy energy and magnetostatic one) Q > > 1 is the aim of the present work. We shall study quantum tunneling of the BP through defect and over-barrier reflection of the BP from the defect potential. The conditions for realization of these effects will be established, too.
Quantum tunneling of the Bloch point
where ϕ = arctg M y /M x are the components of the vector . In this case, a distribution of the magnetization along the axis OY has the Bloch form: sinθ = ch−1(y/Δ), where θ is the polar angle in the chosen coordinate system.
It is noted that it is the area which mainly contributes to mBP = Δ/γ2 (γ is the gyromagnetic ratio) - the effective mass of BP . It is natural to assume that the abovementioned region of the DW is an actual area of BP.
where M S is the saturation magnetization.
where H c is the coercive force of a defect, d is the coordinate of its center, , D is the barrier width.
It is reasonable to assume that the typical change of defect field is determined by a dimensional factor of given inhomogeneity. It is clear that in our case, and hence D ~ Λ. Note also that the abovementioned point of view about defect field correlates with the results of work , which indicate the dependence of coercive force of a defect on the characteristic size of the DW, vertical BL, or BP.
where and ℏ is the Planck constant.
where h c = H c /8M S , ω M = 4πγM S .
Substituting into the expressions (7) and (8), the numerical parameters corresponding to uniaxial ferromagnets: Q ~ 5–10, Δ ~ 10−6 cm, 4πM S ~ (102 − 103) Gs, H c ~ (10 − 102) Oe  (see also articles [20, 21], in which the dynamic properties of BP in yttrium-iron garnet were investigated), γ ~ 107 Oe−1 s−1, for ϵ ~ 10−4 − 10−2, we obtain B ≈ 1–30 and T c ~ (10−3 − 10−2) К.
The value obtained by our estimate B ≤ 30 agrees with corresponding values of the tunneling exponent for magnetic nanostructures , which indicate the possibility of realization of this quantum effect. In this case, as can be seen from the determination of the BP effective mass, in contrast to the tunneling of the DW and vertical BL through a defect, the process of the BP tunneling is performed via the ‘transfer’ of its total effective mass through the potential barrier.
where p is momentum, m is the quasiparticle mass, and F is the force acting on it.
Setting the abovementioned parameters of the ferromagnets and defect into Equation 11, it is easy to verify that this relationship is satisfied, that in turn indicates the appropriateness of use of the WKB approximation in the problem under consideration.
The analysis of this expression shows that at 10−2 ≤ h c ≤ 10−1, ϵ ~ 10−4 − 10−2 and α ~ 10−3 − 10−2. The obtained result indicates that at the consideration of the BP quantum tunneling process, the effect of breaking force can be neglected.
Note also that the mechanism of breaking force has been investigated in the work  and is associated with the inclusion of relaxation terms of exchange origin in the Landau-Lifshiz equation for magnetization of a ferromagnet .
Results and discussion
The over-barrier reflection of the Bloch point
In the above, it was mentioned that tunneling of DW and vertical BL is carried out via sub-barrier transition of small parts of the area of DW or the length in case BL. In this case, both DW and vertical BL are located in front of a potential barrier at a metastable minimum that makes possible the process of their tunneling. At the same time, the BP depinning occurs via ‘transmission’ through the potential barrier instantly of entire effective mass of the quasiparticle. This result indicates that the presence of a metastable minimum in the interaction potential of BP with a defect (in contrast to DW or BL) is not necessary. Moreover, it means that there exists a possibility of realization for BP of such quantum effect as over-barrier reflection of a quasiparticle from the defect potential. In this case, the velocity at which BP ‘falls’ on the barrier may be determined by a pulse of magnetic field applied to the BP. And, as we shall see bellow, the potential of interaction between the BP and a defect has a rather simple form. Obviously, the effect is more noticeable in the case when the BP energy is not much greater than the height of the potential barrier U0.
where v = ∂z0/∂t is the BP velocity, χ(1 − t/T) is the Heaviside function, H0 is the amplitude, and T is the pulse duration.
Note that the study, performed for time (or with taking into account the value of the magnetization decay ω M t < < 102 − 103), allows us to neglect the effect on the process of the braking force
where in accordance with the formula (2), the height of the potential barrier is U0 = π2Λ2ΔM S H c .
Note that phenomenological expression for defect-effective field H d (see formula (4)) follows from the series expansion of the potential U d (z0) near the inflection point. It was at this point that there is maximum field of defect. It is natural to assume that if BP has overcome the barrier in this point, then the tunneling process is probable in general.
where , and are the roots of the equation EBP − U d (z0) = 0.
where the parameter ϵ′ = (EBP − U0)/EBP < < 1 (recall that we consider the case when the energy EBP close to U0).
Substituting into the expressions (15) and (17), the ferromagnet and defect parameters, at ϵ′ ≥ 5 × 10−5 we obtain R ≤ 0.1, which is in accordance with criterion of applicability of Equation 15 (see ).
Note that from Equations 15 and 16, it follows that R → 0 at U0 → 0, i.e., we obtain a physically consistent conclusion about the disappearance of the effect of over-barrier reflection in the absence of a potential barrier.
Based on the obvious relation, and the numerical data, given above, we determine τ, the characteristic time of interaction of BP with the defect 0.6 ≤ ω M τ ≤ 2.3. It is easy to see that τ satisfies the relation ω M τ < ω M t ~ 10 − 102, which together with an estimate for R indicates on the possibility of the quantum phenomenon under study. In this case, the analysis of expressions (13) and (14) shows that the amplitude of a pulsed magnetic field is H0 ~ 4π(M S H c )1/2/ω M T < 8M S , which is consistent with the requirement for values of the planar magnetic fields applied to DW in ferromagnets .
An estimate of the expression (19) shows that K. Such values of are in the same range with critical temperatures for processes of quantum tunneling of DW , vertical BL  and BP through a defect. This fact indicates the importance of considering the effect of over-barrier reflection of BP in the study of quantum properties of these magnetic inhomogeneities.
It is shown that in the subhelium temperature range, the Bloch point manifest themselves as a quantum mechanical object. Thus, the BP may tunnel through the pining barrier formed by the defect and over-barrier reflection from the defect potential. In this case, since the over-barrier reflection of the BP and sub-barrier tunneling of the BP occur in pulse and permanent magnetic fields, respectively, the practical possibility to study these quantum phenomena separately exists. Moreover, the experimental realization of the mentioned phenomena can be the basis for the creation of new methods of diagnostic of ferromagnetic materials and sensitive methods for studying an internal structure of their DWs.
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