High magnetocaloric efficiency of a NiFe/NiCu/CoFe/MnIr multilayer in a small magnetic field

S. N. Vdovichev, N. I. Polushkin, I. D. Rodionov, V. N. Prudnikov, J. Chang, and A. A. Fraerman

Phys. Rev. B 98, 014428 – Published 26 July 2018

Abstract

The isothermal magnetic entropy changes Δ $S$ (i.e., the magnetocaloric potential) are studied in ${Ni}_{80} {Fe}_{20} / {Ni}_{67} C u_{33} / {Co}_{90} {Fe}_{10} / {Mn}_{80} {Ir}_{20}$ stacks at temperatures near the Curie point of the ${Ni}_{67} C u_{33}$ spacer by applying magnetic fields of a few milli-Tesla. Such fields were sufficient for toggling magnetic moments in the soft ferromagnetic layer ( ${Ni}_{80} {Fe}_{20}$ ). It is found that this switching provides a significant enhancement of Δ $S$ in the heterostructure system with respect to that achieved in a single ${Ni}_{67} C u_{33}$ film under such weak magnetic fields. Our finding is believed to have the potential to be utilized in magnetocaloric devices (e.g., thin-film coolers) that would be based on ferromagnetic/paramagnetic/ferromagnetic heterostructures and would operate with moderate magnetic fields.

Received 18 January 2018
Revised 11 May 2018

DOI:https://doi.org/10.1103/PhysRevB.98.014428

Physics Subject Headings (PhySH)

Magnetocaloric effect

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

S. N. Vdovichev¹, N. I. Polushkin¹, I. D. Rodionov², V. N. Prudnikov², J. Chang³, and A. A. Fraerman¹

¹Institute for Physics of Microstructures of RAS, GSP-105, Nizhny Novgorod, Russia
²Lomonosov Moscow State University, Faculty of Physics, Moscow 119991, Russia
³Center for Spintronics, Post-Si Semiconductor Institute, Korea Institute of Science and Technology, Hwarangno 14-gil 5, Seongbuk-gu, Seoul 02792, Korea

Article Text (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 98, Iss. 1 — 1 July 2018

Reuse & Permissions

Access Options

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
Schematic of magnetic configurations in the $F M_{1} / PM / F M_{2} / AF$ heterostructure system, where AF is the antiferromagnet, whose role is to pin magnetic moments in the ferromagnet $F M_{2}$ , while the $F M_{1}$ is magnetically soft. As magnetizations in the $F M_{1}$ and $F M_{2}$ layers are oriented in the same direction at $H < H_{sw}$ , where $H_{sw}$ is the switching field for magnetic moments in the soft layer, the PM spacer is magnetized (low magnetic entropy) by exchange coupling to the $F M_{1}$ and $F M_{2}$ across the interfaces. However, the PM spacer becomes demagnetized (high magnetic entropy) after switching of magnetization in the $F M_{1}$ layer at $H = H_{sw}$ . In this thought experiment, the temperature is close to $T_{C}$ of the PM spacer. In the geometry of antiparallel alignment ( $H$ > $H_{sw}$ ) of magnetic moments in the $F M_{1}$ and $F M_{2}$ layers, according to Eq. (A5), the magnetization becomes zero at the spacer center. This can be explained by thermal excitations of magnetic moments inside the spacer under such a configuration of the magnetizations. Under the parallel configuration ( $H < H_{sw}$ ), magnetic moments inside the spacer are not thermally excited. Therefore, magnetization switching at $H = H_{sw}$ is expected to provide an enhanced magnetocaloric potential Δ $S$ [13, 14] by analogy with the effect of giant magnetoresistance [20, 21].
Reuse & Permissions
Figure 2
(a) Isothermal magnetization of a TaO/ ${Mn}_{80} {Ir}_{20} / {Co}_{90} {Fe}_{10}$ /(10 nm) ${Ni}_{67} C u_{33} / {Ni}_{80} {Fe}_{20}$ /substrate sample at temperatures of $T = 300$ , 250, and 5 K. In the response to an applied magnetic field ( $μ_{0} H$ ), the magnetization reversal occurs in two steps that correspond to magnetization switching in the soft ( $F M_{1} = {Ni}_{80} {Fe}_{20}$ ) and pinned ( $F M_{2} = {Co}_{90} {Fe}_{10}$ ) layers. Reversal of magnetization in the $F M_{2}$ layer is delayed due to exchange coupling of magnetic moments in this layer with those in the antiferromagnetic layer ( $AF = {Mn}_{80} {Ir}_{20}$ ) across the $F M_{2}$ /AF interface. The inset illustrates the mutual configuration of an applied field H and magnetizations in the $F M_{1}$ and $F M_{2}$ layers before switching of magnetization in the soft $F M_{1}$ layer, which occurs at $H = H_{sw}$ . (b) The switching field $H_{sw} = J (T) M_{2}$ [23] measured as a function of temperature $T$ for 10- and 21-nm-thick ${Ni}_{67} C u_{33}$ spacers.
Reuse & Permissions
Figure 3
(a),(b) Reversal of magnetization in the soft $F M_{1}$ layer under an applied magnetic field $H$ at different $T$ . The $M$ ( $H$ ) curves were taken for the samples with 21 nm (a) and 10 nm (b) ${Ni}_{67} C u_{33}$ spacers in temperature ranges of 280–170 and 300–230 K with steps of 5 K. The spacer effects on $M$ ( $H$ ) are clearly pronounced in the sample with the thicker spacer. (c) $\partial M / \partial T$ -vs- $H$ curves for the samples with 10 and 21 nm spacers at $T = 250$ and 210 K, respectively, at which Δ $S$ are maximal. (d) Δ $S$ ( $T$ ) curves evaluated by integration according to Eq. (2) for the samples with spacers of different thicknesses − 7, 10, and 21 nm. Obtained values of Δ $S$ for the heterostructure samples are compared to that quantity in a single ${Ni}_{67} C u_{33}$ film, which is retrievable by extrapolation into the low-field region ( $μ_{0} H \sim 2.0 mT$ ) of the experimental Δ $S$ ( $T$ ) dependences measured at high $μ_{0} H \geq 1 T$ and presented in Ref. [22].
Reuse & Permissions
Figure 4
Schematic of the $M$ ( $H$ ) dependence for an $F M_{1} / PM / F M_{2} / AF$ heterostructure system at two temperatures, $T_{1}$ and $T_{2}$ . The transition of the system from the state $A$ (antiparallel orientation of magnetizations $M_{1}$ and $M_{2}$ ) to the state $B$ (parallel orientation of the magnetizations $M_{1}$ and $M_{2}$ ) is triggered by lowering the temperature from $T_{1}$ to $T_{2}$ . The switching between the $A$ and $B$ states provides the redistribution of magnetization inside the spacer and thus the change in magnetic entropy.
Reuse & Permissions

Physical Review B

covering condensed matter and materials physics