Synthetic ferrimagnets with thermomagnetic switching

Interlayer exchange coupling in strong/weak/strong ferromagnetic multilayers is investigated as a function of external magnetic field and temperature, with the focus on the magnetization switching near the Curie transition in the spacer composed of a diluted ferromagnet of concentration paramagnetic in the bulk. The effect of an externally applied reversing magnetic field on the width of the thermomagnetic transition is studied experimentally and explained theoretically as a result of the interplay between the proximity-induced exchange and the Zeeman effects in the system. Of high potential for applications should be the ability to switch one of the ferromagnetic outer layers using magnetic field, temperature, or a combination of the two.

Figure 1

Magnetization distribution in ${\mathrm{F}}_1$/f/${\mathrm{F}}_{\text{2}\text{pin}}$ Curie switch at high ($T>T_{\text{C}}^{\text{f}}$) and low ($T<T_{\text{C}}^{\text{f}}$) temperatures. The upper panel illustrates the configuration of the magnetic moments $M_1$ and $M_2$ of the layers at below and above the Curie temperature; the arrows indicate the direction of the magnetic moments of the layers.

Figure 2

Theoretical magnetization distribution in the spacer (f) of a ${\mathrm{F}}_1$/f/${\mathrm{F}}_{\text{2}\text{pin}}$ Curie switch for parallel ($\phi =0$) and antiparallel ordering of the magnetic moments of the outer layers ($\phi =\pi$). The temperature in this illustration is chosen such that the interlayer exchange from the induced ferromagnetism is “pinched off” in the center of the spacer.

Figure 3

Magnetization loops of ${\mathrm{F}}_1$/${\mathrm{Ni}}_x{\mathrm{Cu}}_{100-x}$/${\mathrm{F}}_{\text{2}\text{pin}}$, measured at $T=300$ K and $T=5$ K. (a) $x=35$ at.% Ni; (b) $x=40$ at.% Ni; (c) $x=45$ at.% Ni; (d) $x=52$ at.% Ni.

Figure 4

(a) Exchange fields $H_{\text{coupl}}$ and $H_{\text{eb}}$ and (b) their difference $\Delta H_{\text{ex}}$ for ${\mathrm{F}}_1$/${\mathrm{Ni}}_x{\mathrm{Cu}}_{100-x}$/${\mathrm{F}}_{\text{2}\text{pin}}$ with $x=35$, 40, 45, and 52 at.% Ni, measured at $T=300$ K and $T=5$ K.

Figure 5

(a) Magnetization minor loops and (b) its temperature dependence for ${\mathrm{F}}_1$/${\mathrm{Ni}}_{35}{\mathrm{Cu}}_{65}$/${\mathrm{F}}_{\text{2}\text{pin}}$ measured in three characteristic applied fields $H_{\text{b}}$.

Figure 6

Magnetic properties of ${\mathrm{F}}_1$/${\mathrm{Ni}}_{40}{\mathrm{Cu}}_{60}$/${\mathrm{F}}_{\text{2}\text{pin}}$ Curie switch. (a) Minor magnetization loops for $T=5$ K and $T=300$ K. The dashed lines show the characteristic biasing fields $H_{\text{b}}$ used in (b)–(d). (b) Magnetization versus temperature measured in various $H_{\text{b}}$. (c) First derivative of $M\left(T\right)$ curves shown in (b). (d) Position $T_{\text{tr}}$ and half width $d{T}_{\text{tr}}$ of the Curie switching shown in (c).

Figure 7

(a) Magnetization versus temperature measured in optimal biasing fields; (b) temperature $T_{\text{tr}}$ and half width $d{T}_{\text{tr}}$ of the Curie transition for trilayers ${\mathrm{F}}_1$/${\mathrm{Ni}}_x{\mathrm{Cu}}_{100-x}$/${\mathrm{F}}_{\text{2}\text{pin}}$ with $x=35$, 40, 45, and 52 at.% Ni.

Figure 8

Measured temperature dependence of the switching field in ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$(10 nm)/${\mathrm{Ni}}_{40}{\mathrm{Cu}}_{60}$(6 nm)/${\mathrm{Co}}_{90}{\mathrm{Fe}}_{10}$(5 nm)/${\mathrm{Mn}}_{80}{\mathrm{Ir}}_{20}$(12 nm), fitted using the analytical result (13) with the following parameters: ${\chi }_0=C/T_0=0.016$, $m_0/M_1=0.1$, $d/\delta =3$, and $L/\delta =5$.

Figure 9

(a) The two components of the spacer magnetization at low temperature of 5 K, with the outer layers ${\mathrm{F}}_1$ and ${\mathrm{F}}_2$ set in to the antiparallel alignment. The component along the easy axis, $m_x$ (direction of the exchange pinning), has the structure of Bloch wall type, with the spins rotating in the film plane. The perpendicular in-plane component, $m_y$, is maximum in the center of the spacer ($z=0$). Solid lines show the fits using the analytical theory. (b) As the temperature is increased well above the $T_{\text{C}}^{\text{f}}$ of the spacer (here to 300 K), the spacer becomes paramagnetic in the middle, $m_y$ becomes zero everywhere in f, and $m_x$ rapidly decays away from the interfaces and is always along the easy axis.